The Search for Cool Brown Dwarfs with WISE

okay we're gonna get started it's my great pleasure to introduce Mike pushing from the University of Toledo Mike got his undergraduate degree across the river at BU and then he went to University of Hawaii for graduate school which is where I knew him and then he went on it's a Spitzer fellow to NASA Ames and the University of Arizona and then had brief post oxidants back at the University of Hawaii at JPL and since then he's been a professor at the University of Toledo and Mike's an expert infrared spectroscopic spectroscopy who works on very low-mass stars and brown dwarfs and he's going to talk about his work with the wise telescope observing the coolest brown dwarfs the wide where's Mike take away thank you thank you for inviting me I very much enjoyed my my day here talking to people it's a obviously it's a vibrant community and I really I really enjoyed myself so today I'm gonna tell you about some of the work I've been doing looking for the absolute coldest brown dwarfs with the wise satellite these are the people that are involved with this work the people in red are the sort of core members of the wise brown dwarfs science team it was led by David Kirkpatrick for many years I was a postdoc under him at JPL but there's a lot of other names on here the theorists that we work with mark Marley dia Simone Carolyn Morley they do a lot of our theory work Adam Schneider is a postdoc who works with me and much of the the data that I will show you today he's been working on we have a large HST program to follow up a lot of these objects and I'll also be showing work from other people who are doing this we're not the only ones and so I put competitors in quotes because technically they are but a lot of them I actually work with so they're really colleagues as well trying to create the University of Austin Kevin lumen at Penn State sandi Leggett Gemini and David Pinfield and Jackie Farah Diaz so today these are the three things I'd like to talk to you about I'm gonna do some brown dwarf basics and and sort of motivate why it is that we're what is what I'm going to tell you today I'm gonna talk to you about wise and the discovery in the early characterization of the Y dwarfs we're still sort of in the very early phases of trying to understand these objects and then I'll tell you something about some of the constraints for putting on the low mass field mass function alright so the first part is sort of brown dwarf basics and motivation the defining characteristic of a brown dwarf of course is that it does not fuse hydrogen in scores and so it no it in no time in its life reaches an equilibrium configuration where the surface radiative losses are balanced by some energy being generated right it never reaches a main sequence and so brown dwarfs simply cool as they age so this is a figure from Adam Rose's review article showing the theoretical evolution of various objects of various masses as a function of time so we have effective temperature here so hot 4000 degrees the sun's up here somewhere all the way down to 100 200 degrees Kelvin and time so we have a million years we have a billion years we have ten billion years you start up here at two hundred times the mass of Jupiter Jupiter's like a thousandth the mass of the Sun so this is point two solar masses and then you work your way down and what you see is that if you are a point two solar mass star you reach a main-sequence your temperature and luminosity stabilizes and you do this for like 20 Hubble times right before you run out of hydrogen if you are something slightly lower mass that still happens and then eventually you reach a point here which is about 80 times the mass of Jupiter where you never reach that equilibrium configuration and you simply cool off over time and the temperatures just fall and so we can sort of nicely divided this into stars appear and brown dwarfs down here some people like to make a distinction about planets happening at 13 Jupiter masses because deuterium is infused and for proposal purposes I use that as well but for our purposes they're just cold things brown dwarfs so you'll notice the sort of temperatures right we're talking at you know maybe a billion years we're talking thousands of degrees 2,000 degrees down to you know several hundred degrees and so because of those low temperatures brown dwarfs have very molecular rich atmospheres so this is sort of a cartoon diagram showing you the log of the abundance relative to hydrogen over here versus the atomic number and various atoms that we talked about are sort of floating over here and then in in bubbles here we have some of the things that can form from these atoms and so you know carbon is in CO or methane nitrogen is in ammonia or n2 oxygen which I probably forgot to put on here is in water primarily and then you have refractory things like magnesium and silicon and aluminum and they form these these complex molecules I mean complex from from our point of view but they're actually dust grains forced to write enstatite so you get not only molecules but you get solid things and this chemistry actually evolves in time as either you move through the sequence of brown dwarfs or you just take a brown dwarf and you let it cool off over time right so what this is is the temperature pressure profiles for of various objects of various masses so we have the run of temperature and pressure within an atmosphere so on the Left axis here we have the pressure and bars so this is the upper atmosphere where it's low pressure as you go into the atmosphere you come down and then we have temperature going from 100 degrees out to 10,000 degrees and so if you go into an atmosphere you go down into the right so here's the curve for the Sun here it is for an M dwarf an elder or a T dwarf a wide dwarf and Jupiter and I'll tell you what these letters mean in a second but here's the temperatures that go with them right the dots here show you the photosphere roughly speaking where the light that you see emerges from and so as you go to colder temperatures you see they become higher pressure and lower temperatures right so molecules start to take over there are two important chemical transitions that have to do with the carbon and nitrogen chemistry so here's an equally abundance curve so if your total if you're on this dotted line either to the right your carbon is primarily in CO and if you're to the left of this diagram your carb is primarily in methane and so right away you can say okay well L&M doors probably have a lot of CO and T dwarfs probably have a lot of methane and I'll show you that in a second and that's true the same thing happens in nitrogen you can go from being primarily diatomic over to ammonia as you go to the to the coolest temperatures and so you have all of these molecules and so they're going to dominate what the emergent spectrum of these objects look like so here's a sequence of model spectra going from 3500 Kelvin down to 400 Kelvin in black and then the corresponding Planck function with the same effective temperature as shown in color right and so obviously as you go to colder temperatures the Planck function shifts to longer wavelengths but you can immediately see that these spectra are distinctly not plunking right they look very much not like a Planck function and so when you see things like this people sort of sometimes have a hard time thinking about what does that really mean well this is the sort of overall Planck function and so you can see in the near infrared you're way super plunking whereas at other wavelengths your sub plunking right and so the spectrum is very different than a star and you can see the chemistry changing you've got water and Co in the hottest regions the hottest objects and then as you go to colder regions the methane kicks in and then if you go to even colder objects the ammonia kicks in over here and then we hope we see it down here someday there's another level of complication which is the dust so in gray here are where the are the condensation curves from various species so if you tur the right of the curve you're in the gas phase if you're to the left of the curve you're in the solid phase and so if you're colder than this line here then you will form al2o3 or iron liquid iron or potassium chloride clouds or water clouds or ammonia cool dust regularly condensed out of the gas and form this in the solid phase and when that happens these particles now feel the gravitational field they will gravitationally setta mate in out of the atmosphere and it will form clouds at the intersection points of the photo spheres sorry intersection point of the temperature pressure profile in the condensation point so these clouds form here this these kind of clouds form here and then you've got water and ammonia clouds over here and so immediately you can see well looks like El dwarf the clouds seem to form right in the photosphere so there probably can have an impact on what the spectrum looks like these particular clouds intersect all the way down here the T dwarfs right and the photosphere is kind of like up here so those clouds probably don't matter but those gloves probably do all right so you have this evolving chemistry with more and more clouds forming so here's sort of a cartoon of that right it's just sort of a summary as you go from M 2 L to T down to say Jupiter you pick up more and more and more and more clouds ok so you've got chemistry you've got molecules and you've got dust and condensates the clouds have a really big impact on what the spectrum actually looks like so here's a model of a brown dwarf at 1300 Kelvin the black is the plunk function and then the two models show a model where you are clear you have no clouds and then a model where you're cloudy all right and they look completely different I mean yes the overall shape is sort of right but the but the blue clear one has much more jagged molecular bands the molecular bands are much deeper whereas when you have a lot of dust in the atmosphere it sort of tries to push it more towards a plum function right and so you know this is something we have to contend with if we want to infer the physical properties of these objects right you can't just go on oh there's a really deep molecular band here so it's got to be cold well maybe not right it can be just as cold but if you've got a lot of dust that band is gonna be much weaker so sort of the summary of this part is you know brown dwarfs have cool molecular rich atmospheres and they evolve as they as they cool off so the first brown dwarf that was discovered and I'll say first in quotes was Gliese a 2 to 9 B and actually I think the the anniversary of the actual first day that Ben Oppenheimer viewed it at Palomar actually at the telescope was like two weeks ago or something so it was like 20 years ago so this was the first one that the Astronomy community said yes you finally got one the brown dwarf community will fight over what the real one is but this was the one where everyone said yes yes you have won and the reason why is because it's spectrum was so distinct right so here's its spectrum compared to other things that we knew so this is in the near-infrared one to two and a half microns here's what our Sun looks like or a star like our Sun it's basically applying function with some weak atomic lines one of the coldest stars known at the time was VB 10 it's got some water and some some co over here um and here's two to nine be right completely different it has the deep methane absorption bands and in order to be in order to be cold enough to have meth you have to be sub stellar right and that was the that was the linchpin of this argument and that's what that's what really made this one stick and you might also although it's fortuitous it kind of looks like the spectrum of Jupiter right the reason why they look like that is not the same physics but it looks similar which was another thing that helped sort of push the point so after the discovery to mass Sloane kicked in and literally hundreds of these things were found and with spectra that looks so different than anything else we had to add two new spectral types to the spectral sequence the l and the teen dwarfs so the elves are typically classified in the in the red optical from 6,000 to 1000 here's an M dwarf you see lots of titanium and vanadium oxide that's the hallmark feature of an M dwarf as you go to the later types those go away titanium vanadium or refractory they turn into dust they turn into clouds you lose them from the gas phase you lose the opacity you lose the band heads right you start to pick up hydrants and weird things like rubidium and cesium right who ever thought you'd see rubidium and cesium in a star also the potassium doublet here becomes absurdly pressure broadened right this is due to the fact that you're looking so deep into the atmosphere that the pressure broadening is literally hundreds of hundreds of angstroms wide and so the potassium line actually controls a lot of what's going on on this red wing and then of course in the near infrared you pick up them the methane which is the teen were so2 t9b was the sort of archetype of the t dwarfs so this is where this these things land on this sort of temperature diagram so that the L Dorf sort of go from 2300 to 1400 Kelvin and the T dwarfs go from like 1400 to maybe 500 Kelvin or so and so you know the question for a long time was is there anything colder than that out there right and so the you know it sort of is it T dwarfs all the way down or you know is there some things there's something else beyond that that we can talk about and this isn't just stamp collecting right it's not just a question of who gets ad a letter on to the DeMorgan Keenan scheme there's physics here as to why we want to study these things the first is that these atmospheres can tell us a lot about ultra cool atmospheric physics right and so I've already shown that it's a very complex environment where you have molecular formation you have condensate formation you have to worry about how those condensates fall out of the atmosphere how their how the clouds how the mass is distributed in the cloud what the particle size distribution is there's a lot physics that goes on here and these atmospheres are nearly identical to gas giant exoplanet atmospheres and so it is infinitely easier to study an isolated brown dwarf sitting in the field than it is to go and look at a Jupiter sitting next to some bright wonking G star right we want to do it for other reasons but just to study atmospheres it's just easier to do it here and to motivate the cold ones this is a simulation that was done a couple years ago for G pi which is a Gemini planet imager which is actually on its first commissioning run which is wonderful this is an instrument that was built by Bruce McIntosh it's on Gemini looking for planets so the the rows here are just various selection criteria of their of their of the planets they would expect to find given their underlying assumptions so don't worry about the rows the point here is to look over here at the effective temperature distribution and you can see here that in the black curve they're all peaking around 400 Kelvin right so if we want to actually understand what it is that they're finding when they go out and they find some objects sitting around some star they're gonna go and use models and say oh it's this mass it's this you know it has this radius you'd like to think that those models have been you know tested a little bit and brown dwarfs in particular these cold brown dwarfs or the right way to do this before that happens the other thing is the space density or what we call a mass function of cold brown dwarfs it can constrain brown dwarfs theories at least in principle so the problem with forming a brown dwarf from our sort of standard jeans collapse is that you have to have extremely high densities right for the for the cloud to collapse in form of a brown dwarf and that's a difficult thing to do and so there are various ways you get around that you may talk about turbulent flows collapsing or maybe you let stars form like normal and then they get ripped away from their gas cloud so they get cut off from their accretion and that makes it a brown dwarf or maybe you just have a direct collapse of a disk and then it gets tossed out right so there's all these other ideas about how you would form a brown dwarf and these have to predict how many you would make right and so if we can go out and observe how many there are out there we can then in principle choose between these various models I sort of all set all that so I guess I'll keep going so let's let's do let's talk of a why so we've talked about you know the the what brown dwarfs are why we would want to go find these cold things and so let's go find them so the way we're doing this is with the wide field Infrared Survey Explorer this was NASA mission that was launched in 2009 late 2009 the p i-- is Ned right at at UCLA normally I would assume most of you probably heard of what wise for years I was sort of giving like an intro to wise but I figured it's been around enough that I won't go into too much detail basically it's an all-sky survey at three point six and four point five twelve and twenty two microns and we call these w1 w2 w3 and w-4 this is sort of like Spitzer one in two and like iris one and two the people who know what IRS is the the latest data release is called all-wise and it consists of about eighteen thousand two degree by two degree Atlas tiles that had Co added all of the data together that were taken and a source catalog of about 750 million sources the beauty of all-wise is that it also includes apparent measurement apparent motion measurements it turns out that wise was actually able to scan the sky twice in its mission at the two shortest wavelength so ever you want to W two because it can operate those channels can operate it ambient temperature in space you don't need to be cooled by the the cryogen and so we have a six-month baseline and so we can actually measure the proper motion not proper Masari the apparent motion and of course a pair of motion is proper motion and parallax and if you're a nearby you can't distinguish those two with with two points so we call it a parent motion so this is wonderful you can go in and look for things that are moving on the sky here's a nice all sky image from wise right one of these beautiful three color images three point four or 12 and 22 you see all the starlight and all the dust here so the reason why why is is is so good at finding these it's not just a function of the fact that it's looking at MIT infrared wavelengths which is where the Planck function peaks of these things it's that it was tuned specifically defined cold brown dwarfs and so if you look where the two that the bands are and wise you can see that W 1 is set about here and W 2 is set about here so in the coldest brown dwarfs say down here maybe a thousand or even four hundred Kelvin right there's very little flux coming out here this is due to a big methane ban a fundamental but methane ban and then right here there's an opacity hole there are no molecules that absorb at those wavelengths and so you have all this light trying to get out in the moon for red with nothing to get in the way and so it all pours out at five microns and so by looking at w1 minus w2 a color or flux ratio of these wavelengths you can look for things that are very very red right as opposed to stars which look more like this which are very very blue right and so that's what this sort of shows here this is a sequence going from M down to 2y if you will the cold things so they look very blue here because you're just basically doing a Raleigh Gene's tail and then as you move through the elves and into the T's they start to look green right because we've color-coded blue green red this is sort of slightly more quantitative way of looking at it w1 minus w2 versus w2 minus w3 so it's three point five four point five four point five twelve this is where all things that I don't care about live like stars and galaxies and interesting things over here and this is where the at the time T doors because we didn't have Y dorsal and they made this figure we thought would live so sort of two in greater right and this is exactly where we're finding these things so the predictions were not not too far off so the the one that really got our attention was this object here Y is 1828 2650 I think you can see it right it sort of sits right in the middle there it's this nice little green thing here it has a w1 minus w2 of greater than four because of that methane band at three microns we don't get a lot of light out there and it's not like this is a massive telescope where we're integrating for hours so you get a you've got a lot of upper limits in that channel so it's at least four but probably more than it's Jay - age is 0.7 which is actually very red for a cold object and I'll show you that in a second and it's Jay of 23 and a half so this is faint right this is not you're not doing this from your backyard and you're not even doing this from irt at Freud in Hawaii it's so faint that we need a just T to get a spectrum of it and so here is it's it's HST spectrum we published an initial one in 2001 this was it was actually contaminated because of a woops III is a slit the spectrograph and so you don't get the roll angle just perfectly some annoying star off the field or just out of on the edge of the field will put its spectrum on your spectrum right and if your galaxy person who cares throw that galaxy out and you have 20 other redshifts if you're a star person you want that one object right and that's the problem so we had to go back and get a better spectrum and this is it and this is we're working on this now and so it's shown here in black and then on top of it in red is shown the spectrum of 89 so this is the one of the latest t wharfs known at the time right it's a really famous object and you can see that they don't look the same right 18:28 is not a t dwarf and specifically this pattern of sort of Y down J up and H down is consistent through the whole team works right and that is just not the case here the peaks are roughly the same heights and units of that should say F lambda there and so based on that remember we had his contaminated spectrum so it was hard to fit a model on that but we knew that even with the contaminated spectrum those Peaks were roughly the same and the fact that it's color is insanely red this j - w - is like 9 we put a limit of like 300 Kelvin on the subject right which is the temperature of the air in this room so we were very excited and we had a wide Wharf right because it did not look like a teen worth it was a different object than a teen worth so why you might wonder when we use the word why this is why that's a lot of why's sorry I didn't plan that actually so this is a this is a table from David Kirkpatrick 1999 paper when he defined the L sequence and it has to be the only table in science where the first column is the letters but there it is he went through each of the letters and said can we use it for a new spectral class right and for a star's obviously for a you can't because we have a stars right in oh stars you can't because we have oh stars and what he showed was that there were he thinks that there were four letters that we can that we can actually use we can use H LT and why and I don't remember what the rationale for L was I think initially Eduardo Martinez said it for lithium because we could see lithium in these objects and then I think it just stuck and then the next one we figured well it's got to be colder so we'll do T and so the next one is why and that's why why right so it's not any interesting thing and yes it's hard when you try to say a white dwarf or a wide warf and we know that but this is we've been that our community has been saying why dwarfs for for ten years now so it was gonna be a really hard thing to try to change that currently we know of about 23 spectra 23 Y dwarfs 20 of which are spectroscopically confirmed and I'm sort of being a little bit sly on that because you can't know it's a wide orphan so you see it's spectrum but there are other reasons why we think these are so here's the here's the 20 21 of them that we have discovered primarily by our group but by a couple by other groups as well and then we have two over here that were that are it have to be Y Dwarfs they're cold enough they're faint enough that they have to be why dwarfs but they're just too faint to get a spectrum up we can't even do it with HST unless you want to give us like a hundred orbits for one object which I'd love to do but I don't think I'm gonna get that through the tack the first one is a WD Oh 806 it was discovered by Kevin lumen it's a companion to a white dwarf star it's a J of like 25 right 18 20 it was twenty three and a half and it was like fifteen orbits to get that spectrum right so do the math and it's effective temperatures like 300 Kelvin 250 Kelvin the other one that is really quite exciting is why so 855 this one was just recently discovered by Kevin it's the fourth closest star system to the Sun so it's like two parsecs we still don't have a limit we somewhat of a detection at j24 if someone claims a detection and we think it's like a 250 Kelvin brown dwarfs so these clearly have to be wide oh so we're sort of at 23 sort of the numbers so you compare that to the thousands of L&T Dwarfs we have right we're nowhere near having the statistics we have as we do for L&T dwarfs here's the sort of why dwarf sequence as we have it right now it's rather short this is work that my my postdoc has been doing so these are all HST spectra and so you have the sort of team the t8 t9 and then we go y0 y1 and then we have this 1828 here which we're saying is sort of greater than or equal to y2 so could be y3 it could be y4 until we get more objects to fill the sequence in we just don't know it's it's the the width of the so so the the difference here is that Jay band narrows so this is the Jay band right here and if you look this goes down down and keeps getting narrower and I didn't plot it but if I were to put them on top of each other normalized to Jay yes yes this one's a problem child we'll get to this problem child in a bit yes absolutely I agree so the other thing is that the Y J and H peak seem to move towards unity so here it's that sort of yjh and as you go down they get closer and closer together and then the peak of the Y band which is this thing here seems to shift towards the blue until we get this sort of very square looking shape and I'll talk more about that in a little bit the colors seem to transition at the at the T Y boundary so this is a Y minus J so one versus one point two microns for suspect row type and you can see that you know the T's are sort of just hanging out at one point too and then all of a sudden things change rather dramatically right at this point and the same thing happens here in in J minus H they sort of get bluer and then they Plateau and you might argue there's a turnaround although I wouldn't necessarily say that there's a lot of scatter in this data and the errors are quite large but at the very least it plateaus right this drop is clearly stopping here at this boundary and the reason why we think that's happening is the formation of these clouds the NA 2s and the potassium chloride clouds when you take away potassium right from the gas phase you lose the opacity from potassium and that doublet down at 7700 angstroms or so goes away and now you can see deeper into the atmosphere more light comes out so the colored turns right more blue instead of being very steeply read and then the reason why the near-infrared is doing this is because the increased opacity from the clouds so it's not the clouds not only muck with the chemistry and so that's what's going on here but they also add their own a pasady right these particles absorb they scatter and they change what's happening in the spectrum right and so that's what's causing this flattening here wide Orff's are are cool they're less than 500 Kelvin I won't show you the fits to the to the model atmosphere fits they're not pretty and I'll show you that in a minute there's one that's really not pretty and I'll get into a little bit more detail about that but it the consensus using various models and various techniques is that these things are very cold and so this is sort of our take on this here's the T dwarfs and then as you move down to colder temperatures they get colder and if you want to know why this is happening I can explain that later but sort of look from t6 down they just sort of collapsed down here and like I said this is this is people come as conclusion using photometry using other techniques and other spectroscopy why dorks are very faint I've already sort of mentioned how faint they were from an apparent magnitude and absolute magnitude they're very faint especially the near-infrared so this is work by Trent Dupree and Adam Krauss they went out and got Spitzer imaging to do parallaxes of these objects and they have their absolute magnitude versus spectral type plots of Y jhk 3.6 and 4.5 and you can see that these things are very very faint 22 for an absolute magnitude right you might notice there's 18 28 all right so this sort of happens here and then why wait a minute why is that up here right yet again another sign of a problem with this object and you might even argue that over in the edge band right if this is dropping down why is this one so high same thing here so it looks like this object is a little bit over luminous why doors also have very very red near to mid infrared colors so this is the J minus w2 so this is one point two microns versus five microns so a big lever arm right and what you see is you know as you move through the M's and the LS things happen chemistry clouds and then as you move into the T's it just goes extremely extremely red right and we're up to sort of like ten eleven twelve for some of these cold objects in particular I want to focus on these last two guys and I'd put the break here because of the spectra type problem that we're having and these guys don't even have spectra so we can't really type them so be careful about inferring too much about this this trend they could be over here or over here but I want to focus on these guys because these are the ones that are sort of the outliers right there above sort of ten and as I mentioned these are sort of the three ones that we think are lower than that 300 Kelvin right or at least there's some indications of it in particular that this one everyone agrees is this one everyone agrees who is not everyone agrees with this one and so you know they're they're faint they have very very red colors and I'll just point out what I pointed out a second ago which is that why is 1828 appears over luminous right and all of these in all of these features I'm sorry in all of these and all of these figures so what are the possible ways around this over luminous problem oh yeah one possibility is that it's just hotter right so depleting Krauss which would argue that actually no it's just hot it's like 600 Kelvin it's not 300 Kelvin it's 600 governor how do they do this well they went out and they built a spectral energy distribution they know its distance they can get a Beaulieu metric luminosity sort of if they use atmospheric models to fill in 50% of the flux but that's what we're working with in our field so they have a bulla metric luminosity using stefan-boltzmann if you knew the radius you could get the effective temperature right so the problem is what is the radius the beauty of brown dwarfs is is that due to their equation of state due to the competing effects of sort of Coulomb forces and electron degeneracy the mass radius relation is basically flat across the entire brown dwarf regime for oldish brown dwarfs and so we can just say that the radii is Jupiter because Jupiter sort of at the tail end of this and even if you account for some slop in that you still get a very hot temperature of like 600 Kelvin so maybe this is just a hot one right maybe it's a it's a tea Dorf but something is really we are going on in the near-infrared possibly sandy Leggett has suggested that it could be a binary you know the first thing you think of when you see something over luminous in a sequence is well we'll just make it a binary right but in some of the sequences that we see it's not three-quarters of a magnitude it's like two magnitudes all right which is you know that's a lot more than just a binary so that one it's still out there but we don't know and then the third thing is maybe some new physics and I don't mean news and you know we're discovering new physics we're just applying you know we're just not using all of physics to understand these things and so maybe it is that this thing is just an oddball and we find more of them we'll see a sequence and our precondition our preconceived notions of what it should be will change and it will just fit the sequence very nicely this happened in the L and the t dwarf sexually we would have thought they would have evolved a certain way in color-magnitude space and they didn't and we had to say well our models don't work for that and that's what it is right and so maybe that's what's going on here so we don't know so but we have the data and so let's see what the models do with this object so here is all the data we have for this object so here's why is 1828 there's your spectrum and we have two spitzer points here so this is three point six and four point five microns right so that's our spectral energy distribution for this source not not so not so impressive so the way that what we want to do is compare atmospheric models that is do these data and see how well they fit and so what we did is we've been using the Morley at all models these are the latest generation from the from the Marley group they have a range of temperatures and gravities we assume solar module isset e they have various clouds they have various ways of dealing with the clouds don't worry about that and then this will sort of come back in a second we assume equilibrium chemistry and I'll explain why that's important in a second in order to compare models which are one dimensional model atmospheres right so the the emergent flux at the surface of a star you have to multiply by the radius divided by the distance squared to to compare directly to observations at Earth so we have this sort of scale factor here and the way the way I've been doing this is sort of a Bayesian way where you start out with a likelihood function which is the probability of your data given say the atmospheric parameters and the radius and the distance and your uncertainties you have some priors on that knowledge what you think those would be so maybe for example the radius is like Jupiter roughly speaking with us with a narrow bar you know the distance to within you know parsec you you push that through Bayes theorem and out pops what you really want which is the probability of your model given the data that you've observed right so we fit this in a couple of ways trying to figure out what's going on with this source so let's imagine instead of having a prior on Rd let's just say we don't know Rd it's just a scale factor it's just some unknown scale factor right so if I fit for that scale factor in this process this is this is what I get I get an effective temperature of 275 Kelvin that's cold the data and the model is sort of kind of okay right I mean it has bumps where it should but it's not the best fit it misses very badly over here in the methane it's very hard to have a lot of flux coming out at this wavelength when you're this cold because all that methane is sitting there and this is a real sticking point which I don't understand and then this is not so bad the issue that you might see one big issue is this wonking thing over here right that's ammonia in the models right and for a long time we have been hoping to see ammonia in these data and we still have yet to see it there is a little feature that sits in our data that we thought could be ammonia but it turns out it's not at the right wavelength and we've scratched our heads hoping that we made a mistake and it would shift over and lineup and we could announce we have ammonia than your infrared but we just can't get there so we don't see ammonia and the reason why these models have so much ammonia is because they're assuming equilibrium chemistry it turns out that there's mixing in the atmospheres of brown dwarfs right clouds require three dimensions to form and so you can actually take say for example diatomic nitrogen rich gas in the deep interior of the atmosphere mix it up into the upper atmosphere where it wants to turn into ammonia in equilibrium right but if you mix it faster than you can convert it into ammonia you'll end up with an overabundance of ammonia sorry an overabundance of diatomic nitrogen and an under abundance of ammonia right and so you turn mixing on and the ammonia bands get weaker and so we think that if we do that that has not been included in these models yet that this will go away and it will fit better the other issue is is that the ammonia really impacts what's going on on the left and the right side of the J band here and the H band and so maybe the we'll get better here as well and so I'm pushing Carolyn pretty hard to try to get this incorporated into her models although it's probably not gonna happen so the summary that the problem is okay so maybe maybe it's not such a bad fit maybe maybe the near-infrared is another clue from chemistry and maybe the 3.6 is just a bad data point so here's here's a sort of summary of it that scale factor gives you a value of 0.3 our Jupiter over parsecs and we know the distance so that means it has to have a radius of about 3.3 Jupiter's 3 point 3 Jupiter radii that's too big all right I told you that the mass the mass radius relation is sort of flat that's just unphysical at this point right so we might have had a good fit but it's unphysical so that's not so useful and even if and it's not even a good fit because chi-squared is like 10,000 and i have new i have like 200 degrees of freedom right so I don't know I mean I know what that p-value is right it's it's epsilon and epsilon is really close to zero so now let's imagine I fit with R and D so now I put a prior of Jupiter radius plus or minus like a little bit and then I know the distance to it and I fit that and this is what I get right so I get the same sort of mess going on down here it's 300 Kelvin similar surface gravity same problems over here but now I get nowhere near these these spitzer channels right and this is this underlies this this problem with this object right when we first had this object we had we had no distance information to it right all we had was the color the shape of the spectrum and then this this big whopping color and so that's why we came up with 300 cuz we were looking sort of at this information right Trent Dupree who came up with the hotter temperature is primarily worried about this because he's doing luminosities right and this is where all the energy is coming out so he's getting a hotter temperature based on that point and this is the problem we cannot simultaneously fit these two things and so you know do my delusional thoughts at night err is well maybe there's some dust cloud around it right have to be a really big dust cloud would be emitting but you know maybe there's some plunk function doing this that's messing this up or I don't know what right I mean maybe there's just maybe there's other physics that are going on in here that we don't understand yet but if we do this so let's even assume this is a good fit which it of course is not out of this probability distribution you get the wrong distance right so you give it a prior which has you know eleven plus or minus two parsecs but you know it gets out to six parsecs and that's where the probability is ruching Peaks so that's not right that's it's too far it's too close right so we get the wrong distance so even if you buy that crappy fit you don't get the right distance and we know that so that's wrong another possibility that I tried was to do sandy like it's a binary star I use the the radii from from DD of Simone's evolutionary models it's not quite a good comparison because those models are not self consistent with the atmospheric models but it's what we got so if I did that I just built a bunch of binaries with the model atmospheres and tried to fit it I could do it I would show you what it looks just as bad as the other ones 250 and 300 but again we get an unphysical distance and also an unphysical gravity so a lot of G of 5 you can't even evolve a brown dwarf there you can't be 250 Kelvin and a log to you 5 you can make a model atmosphere there right because the gravity just sets the pressure all right so I can do that ready to transfer but I can't start a brown dwarf off and let it evolve to that temperature in gravity so now I'm not only getting the wrong distance but the wrong gravity right so binary at least with these models in this method doesn't seem to work so is there a way out of this and a paper just came out on Astro pH that I'm really praying is the way out of this which is this recent paper by sandy Leggett she's been getting a lot of infrared photometry from Gemini on Neri so here is a absolute w2 versus j minus w2 diagram right so this is that one where that object is really red and then here's oh wait five five and there's oh eight oh six so these are these three red ones and for a long time we sort of had these objects here not well I should say and then we had this one and this is why we say it's over luminous right because if you assume that this is some continuum that's way over luminous but her new photometry is saying well maybe this is just a plateau here right maybe there's something going on here right where the clouds are not behaving like the models suggest right there's nothing that says that the clouds have to behave like Carolyn's model they don't behave that way at the ltte transition in fact we still don't understand how a brown dwarf goes from the elves to the T dwarfs we think it has something to do with the clouds either having holes poked in them or they're just disappearing by some unknown physical mechanism but just a regular set of clouds will not reproduce that transition so maybe that's what's happening here maybe this is really what's happening in the U and you know in the universe and we just need to fix our models maybe there's another another thing going on and that's what I'm hopeful for because we'll learn something right just like we learned something at the LTTE transition so this object has been annoying but hopefully it's it's pointing the way to something new about the clouds happening down at these very very cold temperatures all right so the last piece of the talk I want to talk about is the the constraints on the mass function so as I didn't show you earlier but I will show you now the mass function of course you know summarizes how molecular clouds turn their gas into stars right and so if you want to understand process of star formation you have to be able to at least reproduce that right because that's the sort of basic thing here's what we know about the mass function in a very very schematic way so it's you know so this goes from point O oh one so this is like Jupiter down here up to 10 solar masses and then just think of this as log of the number of objects roughly speaking if you parameterize this this curve is power law so DN DM is M to the Alpha you see here we have this sort of constant slope this is known as the saltpetre slope there seems to be this break around roughly a solar mass and so it turns over and the question becomes what happens down here a lot of work in star formation regions that suggest it turns over and it it most certainly does but we'd like to do it in the field as well right it'd be nice to make sure that everything is everything is consistent that we've got at our eyes and crossed our T's and so you know what is the slope down here and can we figure out what the minimum mass of star formation is assuming that brown dwarfs form by fragmentation of molecular clouds which might not be the case so in order to do this we need to go out and collect a volume limited sample of these things so this was in David Kirkpatrick 2012 paper so here's the 8 parsec sample of objects M Dwarfs Wow there's a lot of them out there right and then there's not a lot of brown dwarfs and so you may have seen maybe 10 years ago this figure that Robert hurt mate with the scale right the scales of justice with like the solar neighborhood stars on one hand and the brown dwarfs on the other hand and they sort of had equal you know equal masses and that's just not that that figures not right anymore right we know now that there aren't that many brown dwarfs out there so we sort of have this volume limited sample we pushed it to a little bit longer but this was just to show you how few brown dwarfs there are compared to stars the ratio is like 6 to 1 ish and this is of course incomplete because we're still looking for them we're still trying to measure distances to them we haven't gotten parallaxes for all these things that's a very hard thing to do and the issue further besides not having a volume limited sample for all of these things yet is how do you measure the mass of a brown dwarf right it's not like you've got a sound like you've got some you know planet that you can watch going around these things to get the mass typically for stars you can go out and observe a luminosity and convert it to a mass with a mass luminosity relation right and the reason why that works is because you have a main sequence right and so here's the same sort of figure but now it's Bowl metric luminosity so here's a you know a point to solar mass star so if I went out and measured it and it had a luminosity of that I could say it's a point to solar mass star well with a brown dwarf I measure a luminosity of say 10 to the minus 5 well it could be a 1 jupiter-mass object at a million years or it could be a 60 jupiter-mass object at 10 billion years all right I don't you know they don't come with age dates and it's hard enough to age hitting stars right we have zero ability to do that with brant well not zero epsilon ability to do that with brown dwarfs so how do we get around that we forward model a population of brown dwarfs and then we compare it to what we see so you start out with an assumed mass function so some power law we assume some formation history say uniform over the age of the Milky Way it turns out it's relatively insensitive to whether it's uniform or soak a stick or some other exponential decay we then simulate by drawing randomly from these things a population of brown dwarfs with a certain mass and age right we then run we take those values and we say okay we have evolutionary models let's assume we know something about how these things change in bulk we can then get a simulated population of effective temperature and gravity right and then we ask do is that what we see right and if it's what we see then we have some confidence that all of this is right this is what it looks like when you run one of these simulations right so here we are effective temperature so twenty-five hundred Kelvin down to four three two one hundred Kelvin two hundred Kelvin roughly and now I've got it in sort of slightly weird units but number per hundred Kelvin within ten parsecs right so you know not a lot actually here and what you see is that the mass function if you assume that the mass function over the entire mass range of brown dwarfs is just a power lies single power law it's it divert the the various power lies diverge the most at the lowest temperatures right so it's most easy to pick out between alpha minus 1 of 0 and one out here than it is down here right you don't want to do it down here because they all overlap whereas you have a shot of doing it over here right and this was to show that in principle if we had enough objects you might be able to start picking off the the minimum mass but we're nowhere near that in fact we're nowhere near really getting the shape of the mass function right so here is here's what we have so far so this is sort of zoomed in on that upper right hand corner the colors are the same so alphas plus 1 0 and minus 1 and here's where the Y's data is pointing to things right now so this is sort of roughly complete this is probably incomplete at this point because we know it's incomplete cuz we're still looking for parallax's for these things and so it looks like to us that you know yeah we could probably rule this out we're not gonna have this many brown dwarfs but a simple power law doesn't seem to really fit the data even if you even if you leave this here and then just Jack this up in some you know uniform way it doesn't really fit any of these any of these models very well and so it's very possible that you know we have to have a more complex model for how brown dwarfs form and in fact other people would argue that it isn't fragmentation of molecular clouds because it's off by if you know whatever this one is at two three four four and this is you know starting in upwards of ten and then this is upper limits so in this one's gonna go up and this one's gonna go up and the shape is wrong right so alpha equals zero sort of flat and in turns over they will in yes that's right they will go up because well I can't guarantee because I don't know parallaxes but photometric lee we have objects that we think are within a volume that this will impact and so it's just a question of actually getting the parallax's to put them at the distance we think they're at we can do this photo metrically and we can put more of them in there but we don't want to do that because by narrating other issues we don't you know we want to get the parallax so we know this is going to go up it's not it's not that we run out of things to find we have a huge parallax program and I'll talk about that just a second to find these things so it will it will change yes I agree I agree that's why I think a single power-law is not the right answer it's also very coarse right I mean and and this is a hard game right this forward modeling assumes a lot of things these evolutionary models are mostly right but not perfectly right this is a this is a hard thing to do right so I mean you know it's sort of gross huge statements that we're making we're not gonna have this mini brown dwarfs right we're just you know you're not going to get up here from these guys we're pretty secure in that am I going to tell you what the actual slope of the mass function is from this data probably not yes yes though the black curve is data yeah that's the models are the curves they have the same colors as this so I'll lead you to zoom in over here No so we're plotting this isn't this this isn't a mass function we're plotting a temperature distribution so over here you could have an old object in a young object that the Alpha is how you start it right you you generate masses and ages you generate a table of masses and ages then you use those in evolutionary models to convert from mass and age to temperature and gravity yeah we assume a uniform age right and it's relatively insensitive to whether we do at exponential decay or stochastic star formation events yeah so it's a it's a temperature distribution because we can't measure the mass right it's the only way around it at this point so we still have things to do on this mostly getting parallax is to really fill this in and then we can really start started talking about this and Pablo Krupa just came and gave a talk to us and he has some very interesting ideas about brown dwarfs are actually a separate mass function and we're just seeing them sort of overlapped right which is a whole new thing that I'm gonna have to go think about so this is in no way an answered answered problem so just looking to the future obviously for example we want to measure the parallel axis to these things to start filling out this volume limited sample so that we can understand and these objects also we can start filling in those color absolute magnacube color diagrams to see really is there that flattening I this morning at the lunch talk I talked up a little bit about whether of these objects we these things have clouds they're patchy you spin it up and an integrated light you're gonna see variability right and by looking for variability you can then in principle infer things about what's going on on the surface of these objects and so I have a large Spitzer program doing that and then this thing we're sort of we're kind of excited about now is the fact that Weis has been turned back on the asteroid folks at NASA are worried about us getting hit and Weis is a very good asteroid finder and so the planetary division of NASA funded turning wise back on for another three years which is great the problem for us is that we are looking for very fine things and we like co-edit images but asteroid people don't care about co-edit images because they want to see things move and so there's no money to do the to do the co adding and so all-wise is sort of it was the end it was the six-month baseline now we have like a five-year baseline so in principle we could we could find things that are moving much much slower which would be fabulous but we don't have the infrastructure we've tried and I shouldn't say we the wise people have tried multiple times to find funding for that and every time the the panel comes backs is wonderful science no money sorry it turns out David hog and his collaborators actually wrote some code to go ad wise images and they'll just you can take it and go at all the data and make Atlas images just like all wise and so we're gonna be starting to do that we're gonna Co out all the the fourth year data for this thing and then blink images to look for things that are moving and the hope is just like heaven lumen found that very very nearby object by its proper motion that we will find new missing members of the solar neighborhood by their proper motion so that's what we'll be doing for the next couple years okay thank you most complete knowledge what should be going on and there's a white why it's because the issue with that groups models is that in order to do itself consistently you have to take into account the chemistry as you generate the temperature pressure profile of the atmosphere because it can change what's happening in the atmosphere if the opacities are changing all of the non-equilibrium chemistry work that has been done by that group previously is they it's a post-processing so they generate temperature pressure profiles in equilibrium and then when they run the ready to transfer to generate the spectrum they tweak the chemistry and they convince themselves that it doesn't impact the emergent spectrum dramatically and so it's okay but that these cold temperatures it matters and it is not a simple thing to get that into the code and it's it's the way they do their radio transfer they use k coefficients which means you have to generate these things from there literally tens of billions of molecular lines and you have to do this at all ten it's a lot of work and the motivation is not there yet to do that and she doesn't want to spend her time doing that when she could be doing other cool things even though i think it's important in a we all think it's important of it she has a career run and so she wants to do something else if she can do something else so I don't know honestly when that will ever get done it kind of worries me because the Marley group is a very prolific group in this community they have I would argue some of the best models out there and so this is unfortunately a hole in their in their product but they are severely manpower limit and you know they need to find someone who's going to do this and they just don't have that yet so that's why I got pessimistic presence of having seen a brown more immediate effect of our debride Eve's does understand the other question is in the effects of the rotation because these guys should have a few hours on this paper yes okay so the first question no in order for it to be cold enough to be hidden so that we don't see it obviously the shape of the spectrum is just too insane to do that and I should note that sandy I believe when she built her buyer if he did it with data right so she was trying to find by individual objects that she thinks are individual and merging them to make this object I was doing to buy the models and the models just can't get there so no is the short version they just they're just not plunk enough to do it and just adding you know a 250 Kelvin thing down there cuz it has a lot of 4.5 my kind of flex that's true but in a relative sense it's all the way down from say 300 Kelvin object right because we're still talking about you know its energy conservation so no on that front the rotation that's a great question and I have no idea what that would do to the spectrum the only we do see evidence for variability at the mid-infrared we see it in photometric bands and spitzer know so that's one of the things that we're working on now it's that levels of a few percent so we do see variability there what would it look like in the near infrared I really couldn't tell you mostly because we don't understand we don't understand what's happening to make it variable right it probably has something to do with the clouds being patchy or not homogeneous or the thicknesses change but that's something we're still trying to work out I can say that for the hotter objects we do see spectral variability for Daniel pi at at Arizona the lunar planetary is LPL has an HST program where he's getting student - Illinois time series spectra of L&T dwarfs and he sees the spectral the spectra very actually it's not dramatic in the sense that it goes from looking like an L and it turns into a tea or something like that but the water bands change and some of the atomic lines change and depending on how you bend it up you can say juice you've J and H you'll see things out of phase right so the variability in that J van will have one phase and the variability yet another wavelength of a different phase which is suggesting that we know when we look at different wavelengths were actually seeing two different layers of the atmosphere right just like an atomic line you're seeing the different layers you work it well on the atomic line it's way worse when it comes to molecules so you're seeing different layers and the fact that they're they're not in phase suggests that the variability they're not talking to each other what's happening down here is happening independent of up here because they're out of phase with each other so it does change what the spectrum looks like if I had a spectrograph that could do it with this I would expect to see the change but that's not gonna happen any time soon so I don't know that we'll ever know the answer to that for these guys so you showed us some of the spectra of the Y's yeah I mean okay you know something like half century of working classification it still seems kind of perverse to me to measure a width of this thing and then put it in this box and say alright we're gonna round it off in this column I want and just disparate eyes this continuous measurement yeah is is there a better way I will in a second are you heartily for no but I have people who disagree with me been Oppenheimer Rebecca Oppenheimer at the Museum another history very much opposed to special classification and thinks it's a waste of time I would argue quite strongly against that and for the following reasons first you know what we're after is physical properties alright no one wanted cares about letters right what we want is the temperatures and the gravities and their radio problem is is that nobody measures that directly if we measured it directly that's fine right so if you have an interferometer your back yard you can measure it go for it I don't care what the spec of it because I don't know the radiuses but until that happens right you have to use models to do that which means that if I derive the radius of this object and I say it's you know 0.9 Jupiter masses I have to put a date on it I have to put a model grid on it I have to put other things on it and then the next person to publish it has to come along and say okay well I changed you know I changed this little bit in the model and so it's a you know you're later and it's this and then you have this long list of various radii with various tags and try to figure it out and so what I would argue the thing to do is call it something and say that's what it is and then you let the model slide around underneath it all right and a y1 is always a y1 and the temperature changes as the models get better right and so that's one of the reasons why you I would argue you do special presentation because it's a and and other reasons it's a shorthand way of talking about things when I talk of a round of astronomers and I say a t4 that means something to them right there's a median property of a t4 right and then it's a great way of looking for outliers you know if you have this is what a t4 looks like and then you plot a spectrum on there and it kind of looks like a t4 but it's not really you're gonna learn something about that then you can say t4 peculiar it's someone we go oh ok so it looks like a key for not really you know and we'll say you know and I know you know if you look at some of the classification papers it gets a little insane with all the letters and symbols out there but they mean something right they have value I would already so no I don't think classification should go the way of dinosaurs I think it's a very useful thing I think the reason why NK is stood for this time is because it's still valuable right we you know I would imagine the temperatures of a stars are still changing all right and so an a0 is in a 0 and it will always be in a 0 all right and then someday when we have infinite knowledge about the universe well know what the temperature urban a zero is on average and then we can stop using spectral types that's why that's my argument I understand your charts correctly the surface gravity of these things is much much lower than say Jupiter latest and it's higher so ok it's already got me on multiple ground course here the Sun is that is it three I think a lot of you freezin right somebody remind me to three oh I think it's three where's it for 4.4 ok so a border or higher for the hotter mass for the hotter once and then for these colder ones yeah they're sort of a border the Sun and lower but if you go to the higher me if you go to the higher mass hotter drug works they can get up to five five point five depending on where you are so it's sort of a range of four to five and a half depending on the temperature I think for the cluster's gives you the best idea about well how it's very because one you get rid of the agency absolutely I think I think part of the trouble with that of course is then converting those two masses right you've got extinction you've got to worry about you know all of these infer excess detail yes Oh from that point of view yes I yeah okay I can see that that's right absolutely and I didn't say this but you know I think it's it's sort of it's not a scientific review he is assigned a big reason I think it's important to know our solar neighbors right the fact that Kevin Newman found two objects in the last two years that were two parsecs away that's I know I give public talks I start off with the Hubble Deep Field but I say you know we can stare to the edge of the universe and yet if you look at the solar neighborhood we've added two new members in two years right so don't think that just because we can look deep we know everything about the universe is sort of my argument so you know I think I think it is important to look so you could go to these places right you know like crystal and go there instead of a black hole I think it's important to find me but I don't put it in science talk because it's really yes is there any hope of separating out variability over the surface at one time from temporal variations by for example getting the period and then looking at the same phase repeatedly in principle that's true that's possible in cross field actually did Doppler imaging of lumen 16 a and B so one of Kevin's binaries is a binary LMT torque that's more than ones at two parsecs so it's really nearby and it's really bright from a brown dwarf point of view and so he actually did high-resolution cave in spectroscopy like 30 years ago 67 on that was cry rezzed on the VLT so really high resolution and the Doppler imaging and actually has a surface map of the very of the the structure of B of the B component and it was in nature of science that came member which one it is but you know you can go down you can go by the little cube thing right that you can put together and make up maybe a primer or surface oh yes except that's the one right and I don't know that anyone has thought about the issue for the what you suggested was how does it evolve on longer timescales right and for example for these wide works we've already found that things change on months timescale so you'd have to do this sort of very rapidly and get a lot of telescope time to do this right I mean you have like an eight hour period yes that's right that's right yeah but once you know the period you could do that but in order to to see it multiple times you have to get data multiple times and if the rotation period is 10 hours that's a lot of spitzer time right and we already I think we got our lottery I know they're gonna give us more time more that much more time but yeah in principle that could that could happen practice of good practical it's I don't know that I can tell you what the underlying averaging out is but I know that when Adam Berg asker first ran these kinds of simulations in 2004 he did different star-formation history to get exponential decay and he did so casting and he found that those were the dominant thing controlling the shapes of what the of what the mass function has looked like and that's as far as I'll go with that because I didn't I I don't know it details I don't know I mean I recoded it myself but I don't know I haven't tried different masks I haven't tried different age distributions let me put it that way I took his work at face value and just assumed that I could ignore that for the time

Channel: CfA Colloquium

Views: 8,046

Rating: 4.8769231 out of 5

Keywords: Brown Dwarf (Celestial Object Category), Astrophysics (Field Of Study), Astronomy (Field Of Study)

Id: wbklGIjt5Kw

Length: 63min 37sec (3817 seconds)

Published: Thu Nov 20 2014