Brown Dwarf Variability and implications for Exoplanets - Tyler Robinson (SETI Talks)

all right hello everybody and welcome to the Sean suit colloquium we don't have the usual movie star MC today so I'm substituting um like to introduce Tyler Robinson he's going to be talking about brown dwarf variability and its implications for exoplanet habitability or for exoplanets in general um Tyler got his bachelor's degree from the University of Arizona and his PhD from University of Washington where he studied um all all kinds of habitability of exoplanets he's done modeling um and observations of exoplanets and how we might detect habitable planets out there by looking at the planets in our own solar system including the Earth and seeing what they might look like from afar um so today I think he's going to talk more about the brown dwarf end um where he's been doing lately which is modeling atmospheric modeling of brown dwarfs um and I will just let Tyler start right away welcome thanks stuck [Applause] sure good afternoon everybody so I'm tyr I'm a MPP fellow at Nasa as so just around the corner here uh and so I'm working with Mark Marley who's here he's he's in the back he's my adviser and so a lot of these tools that I've been using uh and a lot of the models that you'll see showing up kind of towards the end are are things that he spent a career developing so I I definitely owe a thanks to to him um so where I want to start actually is with this amazing looking structure uh which is in the constellation ukus and so this is the r oyuki um uh uh uh Cloud forming comp or Star formation complex it's a molecular cloud is is what this is um and so this is a false color image uh taken by the Spitzer Space Telescope uh which looks at infrared wavelengths and which allows you to peer through all of the dust and muck that occurs in Star forming regions in molecular clouds to see through to uh uh light sources that are inside of the cloud so you can see these little reddish things in there um and so each of those sources is actually a star that is in the process of forming right now um as we speak uh and so current counts put the number of forming stars in This Cloud upwards of several hundred to 400 things that are forming inside of this Cloud uh and by studying those objects that are in the process of forming we know that uh the This Cloud complex has been undergoing star formation for about 100,00 years to a million years which sounds really long but on Galactic time scales that's actually essentially just yesterday um but those time scales are long compared to say human lifetime which means we have to turn to models and simulation uh if we kind of want to watch the process of star formation unfold um and so these are these are hydrodynamical simulations from uh uh Matthew bate um that show on the on the left is uh density column density of material and then on the right is temperature uh in 3D simulations of of star formation uh and so what we know from simulations like these as well as a wealth of studies of uh star forming regions in the Galaxy uh is that star formation leads to uh the formation uh or the birth of a very wide range of masses of objects and so I'm actually going to take us so you can actually see you know different size things that are occurring in in in these simulations but what I'm going to do is take us back to uh row oyuki um as soon as the simulation stops there we go uh and so this is actually data from a a region in that star forming uh Cloud um showing uh on the on the y- axis here so the vertical axis is the log of the number so this would be 10 and this would be 100 and then on the x- axis here uh is the mass of the thing that is that is forming so this would be something that is 10 times the mass of a sun uh of our sun this would be something that is the mass of the Sun and this would be something that is 1/10th the mass of the Sun so this is just a count in in Mass bins of the stuff that's forming inside of R Fuki um and so what you see is uh at the high end of the mass scale so stuff that is more massive uh than than the Sun so the be STS the a stars and the f- stars or what will become ba and f- stars um there are relatively few of of those kinds of stars forming uh and then as you head to lower Mass things the G type uh Stars which is what our sun is down to the K stars and the M Stars uh you see that there are a wealth of of those objects forming um and so actually the most prevalent thing that gets formed as a result of of star formation are actually stars that are less massive than the sun these are M dwarfs um and so M dwarfs actually extend down to about 8% the mass of the Sun or 80 times the mass of Jupiter um which is kind of what this bar is right here but what I want you to notice uh is that uh the universe doesn't really care so much about uh the weird nomenclature and the bins that we place on how we divide up Stars uh and the universe continues to form stuff right on down past uh what we would call M dwarfs and so you can see that there is still stuff forming uh down here at at lower masses and so what that stuff is is it's the realm of the brown dwarves and so here's uh a schematic where sizes are to scale uh of of bonafide stars like the sun uh and like this star here gisa 229 which is a a higher mass m dwarf um down into the brown dwarves uh which kind of fill in the regime between uh the M dwarfs and uh things that we would recognize as planets like Jupiter and so uh Brown doors actually span a really wide range of temperatures uh and so the highest temperatures we have a category of brown dwarves called the L dwarves uh and so those span about 1300 Kelvin to 2,000 Kelvin uh and so they are they are hot by our standards but by Stellar standards quite cool um at lower temperatures than the L dwarves we then have a category of brown dwarfs called the tea dwarfs uh that span about 700 Kelvin to 1300 Kelvin uh and then only very recently we now have observational evidence for uh a new class of brown dwarfes that are even cooler than the t do which which are called wi dwarves uh that are temperatures below 700 Kelvin uh and so again by Stellar standards or by star standards these are very uh very cool uh temperatures okay so you're going to hear L dwarfs and T dwarfs throughout this presentation and so I just kind of wanted to put it in perspective uh and remember that Lars are things that are that are uh cooler than M dwarfs and we're talking about 1300 to 200000 Kelvin T dwars we're talking 700 to 1300 Kelvin uh and the other thing that I want to point out on this slide uh is that again the sizes are to scale and so I want you to note that for the brown dwarves uh even though their masses span from about a Jupiter Mass up to about 80 Jupiter masses uh their sizes are all about the same so they're all about a Jupiter radi and so they're all about the same size as Jupiter so when you picture these things in your mind picture something that's cool uh it's glowing deep red uh about the size of Jupiter um uh uh spanning a range of temperatures from something like Jupiter up to a couple thousand Kelvin so with that with that picture in mind now I want to I want to return to uh the Ruki Cloud complex uh this is from a press release where uh some authors uh searched through the cloud complex for things that are brown dwarfish uh in their in their uh Mass uh and so all of the little circles that you see here are brown dwarves or things that will become Brown dwarves detected uh in the rof Yuki Cloud complex and you see if you count them there's kind of several tens of them in that in that image uh and so what we we know from studies like this is that uh when our galaxy goes about forming stars that it forms about one brown dwarf for every five stars uh and so that means that there's actually lots and lots and lots of of brown dwarfs out there but what makes a brown dwarf different from a star uh and so to that I actually want to turn to um some some schematics uh and so what I'm going to show you here it's kind of a cartoon of the temperature at the inside of say a star or uh a brown dwarf um as it's in the process of forming and then running along the x-axis down here is time so the furthest left we're talking a time scale of of a million years which is picking up kind of where Ro Ruki is right now uh and then extending up to 100 million years and then 10 10 billion years all the way over at the at the far right now uh for an M dwarf a Bonafide star um you start off relatively cool uh in your core uh and then as the exterior of the star radiates to space and cools you undergo Kelvin hel Holt contraction uh which means that the core actually heats up uh and eventually the core gets hot enough that uh hydrogen Fusion ignites and then your core temperature stabilizes and you become a star um now let's contrast that to Brown dwarves which are the little engines that couldn't um and so because they are lower Mass than the M dwarves uh they start off slightly cooler on this track uh and they also undergo Kelvin H hold's contraction uh but they never quite get hot enough inside of their core to ignite core hydrogen fusion um which means that then they are faded to uh cool over the course of tens of billions of years so they just get cooler and cooler and cooler or fainter and fainter and fainter okay so that's what's going on in the cores of these objects and this is what distinguishes a star that's doing core hydrogen Fusion from uh a brown dwarf um something else that's interesting and needs to be pointed out is now let's move to the outside of the star so we're going to talk about the effective temperature so the temperature of the outside of the star essentially uh and so here's that that same M dwarf track uh starting off young it starts off hot and then it cools uh and then eventually hydrogen fusion and its core ignites and it stabilizes with regards to its effective temperature and the amazing thing about M dwarfs is then that they glow with that effective temperature for tens to 100 billion years so they're they're amazing little furnaces um contrast that to what I'll call a high mass brown dwarf so let's say that this is something that's like 60 or 70 times the mass of of Jupiter um it like we just discussed on the previous slide doesn't get to do core hydrogen Fusion so it just cools over time which is what I'm showing you here uh and then I'm going to throw up also what is a a low mass brown dwarf so let's say this is something like 10 times the mass of Jupiter uh it does the same thing doesn't do core hydrogen Fusion just cools over time but here we see uh something that makes brown dwarfs a little tricky uh which is that if You observe a brown dwarf and say you measure it it's effective temperature so you have a data point here on this on this y AIS um and you draw a line over to the to the right from that if you have no other information you don't know if that brown dwarf that you have observed is just a young low mass object that's still hot with this residual energy of formation or uh if that object is actually a higher Mass brown dwarf that's older and so has cooled over time uh and so it takes actually extra information to say about the gravity surface gravity of that object to to break that degeneracy okay so that's how Brown dwarves are distinguished from stars um so now I want to talk with you a little bit about why they're cool in kind of a colloquial sense um so i' I've just convinced you that brown dwarfs are are cool in a temperature sense and so to put it in perspective um I'm showing you here temperatures as you head down into uh the atmospheres and Interiors of a different of different kinds of Worlds uh and so this up here would be the say the top of the atmosphere and then this is headed down in uh so here's uh the sun uh which has an effective temperature of about 5,800 Kelvin so it's hot uh moving down the temperature scale you get to the m dwares which have effective temperatures in ballpark of several thousand Kelvin uh and then the L dwars are shown here this is characteristic 1800 Kelvin and then down to the T dwarf which is 1,000 Kelvin uh and then the wide dwars would fill in the space in here and then you'd have something with Jupiter even further down um point being is that the L dwarfs and the T dwarfs and the Y Dwarfs that spectral sequence is actually a temperature sequence uh and so I want you to keep in mind that when I'm talking about L dwarfs T dwarfs uh and the little bit that I might say about wi dwarfs is I'm really talking about things that are hotter cool and and cooler um and so the interesting thing that starts to happen in atmospheres as you allow them to cool uh is that uh molecules start to form and so what I'm going to show you here are actually Spectra of uh Brown dwarfs and actually some of the lowest mass m dwarfs uh and so on the the y- axis is just brightness uh and Spectra that I'm about to show you have all been offset from from one another for for clarity uh and then on the x-axis here is the wavelength that we're that we're looking at and so those wavelengths are in the near infrared and so we go to the near infrared to look at Brown dwarves uh because uh their effective temperatures are in the regime where most of their light is coming out uh at near infrared wavelengths and so that's the best place to go to to detect their light so what I'm showing you here are so here are the the lowest mass m dwarf so this is an M6 dwarf and this is an M8 dwarf uh and then you transition into the brown dwarf so here are some L dwarfs down here again all offset from one another and so in the atmospheres of M dwarves um you can see The Wiggles here and you can see the Wiggles here uh the temperature conditions are right such that hydrogen and oxygen can finally come together and form stable water vapor molecules um so you can you kind of have to squint to see those uh but as you head down into the Lors which are again cooler and cooler uh you see that you get these very deep bands of uh water vapor absorption and I think that that's really amazing that a molecule that's so common in our atmosphere here on Earth is also uh prevalent uh in the atmospheres of of of these strange kinds of Worlds and then adding the T dwarfs to the picture so what I've done to you now is that plot that I just showed you got moved over to the left and now over here we're looking at the end of The L dwarf sequence and then down into the T dwarf sequence uh we see that the water vapor bands still stay very strong but the other thing that you see creeping in uh say focus on this region right here uh as you head down is you start to get methane that forms in these atmospheres CH4 uh and so uh the thermal conditions in the T dwarf atmospheres are appropriate for methane to be stable there and that's actually characteristic of the of the T dwarfs is that you see uh methane absorption bands in their atmospheres could you just for a second what the temperatures are once again yes so the the the L dwarves will span uh so this is about 2,000 Kelvin uh down to about 1300 Kelvin and then 1300 Kelvin uh down to about 700 Kelvin so the other thing that happens uh as a result of things getting cooler and we have experience with this here on our planet uh is that if you take a gas and you cool it uh and you have volatiles in that gas uh eventually they condense out to form condensates um and so I think what is really astounding about the atmospheres of of the brown dwarves is that indeed they do have condensates in them but they are very otherw worldly kinds of condensat so there are clouds in these atmospheres but not clouds like anything you and I would would really uh recognize so in the portions of the atmosphere that we can best probe for for Lors with with observations uh we know that forming in those atmospheres are clouds of magnesium silicate so things like rocks condense out uh and also uh Licor and iron will uh will condense out so there will be Iron clouds there too which is just amazing um if you move to the T dwars which are again cooler so deeper in their atmosphere those same clouds form because you have to go deeper into the atmosphere to get to the same kinds of appropriate temperatures for uh the the the Rock and the iron clouds uh but higher up in these atmospheres the the conditions are appropriate for essentially things that look like salt clouds forming uh so something like say potassium chloride that that is indicated there um and so that then leads to the kind of crazy cool artist concepts of what the atmospheres of of brown dors may may look like um where you have say clouds over some hot roiling surface down here uh and these clouds are raining out say uh salts or iron or something like that and then some some some Liberties were taken maybe with the lightning but of course maybe there could be lightning in in these atmospheres um so clouds form these atmospheres now um we know from looking at solar system worlds that when you have clouds uh you have um you have a variation of brightnesses across the the surface of of that world and so what I'm showing you here is is Jupiter and so over uh on the left is Jupiter in reflected light uh and you can see this deep red band right here that when you go to longer wavelengths where you're now looking at Jupiter in the light that it is emitting this is not reflected light anymore um that that band is actually a place where you get to see relatively deep down into the atmosphere and so you see the band pops out here too and because you get to see deep down into the atmosphere you're seeing down to where the temperatures are warmer uh and so that band glows brightly uh and it also grows uh glows brightly over here at even longer wavelengths um and so then you could imagine that uh say as Jupiter's bands come and go over long time scales or even over the time scale of a rotation of Jupiter which is 10 hours uh if you have different Cloud features over the over the the surface of of Jupiter um that if you're watching its brightness as it rotates you're going to see variation in its brightness that comes from there being Cloud structures on Jupiter um and so the thing that I want you to remember and and I'll highlight this in a in a second uh is that when we're talking about Brown dwarves uh we're not talking about looking at these things in reflected light so we are talking about brown dwarf emitting its own its own radiation and so the the better model for thinking about Brown dwarves are these images over here on the right where you're looking at Jupiter uh in its own IM light um so Doug said that uh my background is actually in in the pale blue dot and so I actually want to take just a moment and tie this discussion of variability into uh the world that we call home uh teres with a with a capital T um and so let's see seven years ago now so in in 2008 NASA repurposed the Deep Impact spacecraft uh and so one of the things that they had it do was on several occasions it turned around and it took data for our planet the pale blue dot um so we happened on one of those occasions to catch a Transit of the of the Moon across the Earth's disc um and so on those three occasions we got light curves so that is measurements of brightness as a function of of time uh and we stared for a full 24 hours so we got a full rotation of the planet uh underneath the underneath the spacecraft uh and so I want to show you some of uh some of these light curves um and so what I'm showing you here is normalized brightness is what is on the the Y AIS um as a function of time so the observations start here uh and then proceed over the course of 24 hours but because the planet is rotating underneath you or underneath the spacecraft from from this perspective um again the observations start here and then what that means is that the west longitude is increasing with time uh so so time and longitude are are synonymous there um um and so each of the different colors here was a different band that uh the uh the Deep Impact spacecraft had on board for for making these observations these are Broadband filters they're 100 nanometers wide so they're pretty fat um and so we had uh observations at UV wavelengths I guess near UV wavelengths blue green red redder uh and then kind of into the into the near infrared there uh and you see that there's actually a lot of structure uh in in those light curves and and what we know from uh modeling's observations is that most of that structure comes from cloud complexes uh uh in the earth's in the Earth's atmosphere and so focusing on this blue light curve that you see running through here um there are actually three distinct Peaks so there's a peak there there's a peak there and there's also a peak there uh then in this case uh we know from comparing these observations to essentially weather maps uh that those Peaks correspond to uh thick bright clouds that were forming near the earth's equator um and had actually punched high up into the atmosphere so they're also quite cool um and so what we can do now is look at the Earth in infrared wavelength so this is in reflected light which I said isn't a particularly good model for brown dwars because Brown dwars were looking them in their thermal emission um but the uh epoxy mission that reused the Deep Impact spacecraft also had measured thermalite from from the earth and so here's here's that exact same uh blue light uh light curve that I just showed you again with the One Two Three Peaks and this black line now is Earth at longer wavelengths where it's dominated primarily by emitted light uh and you see that those peaks in that light curve actually correspond to troughs in the uh emitted light and that's because like I said that those clouds were cold and cold things aren't very good emitters uh and so you actually have a deficit of light emitted from the earth when you have something cold a cold cloud in view um and so actually in the earth's thermal light curve uh it achieves its peak uh here at where zero is in west longitude which is actually when the Sahara rotates into view is when Earth appeared uh brightest in its in those infrared wavelengths so all of this is just to orient you with um uh with regards to your thinking that you're you're probably used to thinking of clouds as being bright and so if you see a light curve uh that uh indates that something is that a brown dwarf is dark you would you would it's counterintuitive to think that that is that that is actually that's where the cloud is but when you're looking at something in in thermal light uh clouds prevent you from seeing the Deep hot parts of the atmosphere so it's actually the light curves are low uh when you happen to have say lots of cloud in them so that's that's the the counterintuitive bit that I want you to to tuck away uh for for the the rest of the presentation okay so that was the terrestrial perspective and actually came from uh some of mine dissertation work at the University of Washington okay um so the first brown dwarf was the I guess the first unequivocal brown dwarf was announced in 1995 uh within about a week of the first uh exoplanet Bonafide uh exoplanet also being announced which was a hot Jupiter um so Brown dwarves and exoplanets have had uh a very similar trajectory with regards to to study uh and lots of kind of uh fruitful exchange between between the two Fields um so shortly after 1995 uh it was already clear that these worlds probably had clouds on their atmosphere and so it wasn't too far-fetched to then think about taking observations that would hunt for variability in the light curves of these things that would indicate the clouds are say coming and going in their atmospheres or that you have a patchy surface where there are some thick clouds and some Thin clouds and as that brown dwarf rotates uh it appears dimmer and brighter at different times uh and so a lot of people tried for this and again I told you the first uh Bron dwarf was 95 so then in 9 n uh was was kind of a a first attempt um that that claimed a detection of of variability by tiny and trolley um and so what I'm showing you here is uh essentially you can think about this as being brightness uh variations on the y- axis and then this is time over the course of about two hours uh on the x-axis and so in the background here is a comparison star that should have been non-variable um and you can see that it actually has pretty large error bars and it looks pretty messy but it is consistent with roughly uh a straight line uh and then these thick data points here that have smaller a bars uh is a is an object that's kind of bounding the m to L transition so it's not entirely clear if it's uh say a very very low mass m dwarf or or an L dwarf um and so you can see that over the course of the observations that it does appear to to brighten um but this really wasn't the um hole in one that everyone was kind of hoping for especially since what you would like to see is you'd like to see it say uh if this is say a a cloud that's causing it to appear darker um you'd like to see it return over a full cycle over a full rotation to uh to this same point that would indicate that you'd actually just observed a full rotation of of the object um and so the fact that you're just seeing something brightening here um isn't isn't super convincing uh and also shortly thereafter uh Nakajima at all um published some results for a much cooler object uh so this is something that's in the mid T dwarf frame so this is around a th000 Kelvin um and so here they're showing you brightness on this axis as a function of wavelength so now we have Spectra uh so this is again in the near INF Fred which is where we look for for for brown dwarves uh and so as you head off to the left here the reason my brightness is decreasing as you're heading into a water vapor absorption feature um and so you have two different Spectra uh on this curve uh and you might look at that and say oh well they definitely observe variability look it's much brighter in these observations than it is in this one um but they've kind of tricked you and that they've intentionally offset these two if you did overplot them without removing this plus a constant here um they would sit right on top of one another uh and so the argument uh in in this paper was that actually some of these features do appear to look slightly different from one observation to the next um but again it's not entirely clear if that's noise uh and so it wasn't again super convincing um so it took almost a decade for someone to finally knock it out of the park uh and so this was the observation that finally knocked it out of the park this was a observation by igal uh at all in 2009 uh of an early T dwarf um and so this is again looking in the in the near INF fored this is a big fat broad filter uh spanning 3 microns or 300 nanometers from 1.1 to 1.4 microns and so that bounds a couple of water vapor bands um and so this is normalized brightness on on the Y AIS and so this is the T dwarf up here and then this is a comparison star down here that should be non-variable uh and so this was a groundbased observation uh and so there are air mass effects which means that at different times in the observation you're looking through different Columns of air so that the uh if you look at the comparison s it actually appears slightly brighter uh here than it did over here and that's just an atmospheric effect because you're doing ground base observing uh but if you look at the brown dwarf superimposed on on top of that is what is very clearly uh variability um and so if you look at the scale of that variability it's kind of peak to trough about 10% which from studies of Jupiter is pretty consistent with what you would expect at least given Jupiter the numbers that I showed you for Earth Peak to trough were 10 to 20% um so it's not like this is uh out of line with what you might expect for thermal variability uh but then the other neat thing that you can do with these observations because they kind of repeat is you can pull out um you can pull out a rotation period And so this guy is rotating in about 2 and a half hours which is really surprising so Jupiter so this is something the size of Jupiter Jupiter rotates in in 10 hours uh and so this is rotating four times faster than than Jupiter does um but the other thing that you can see here is that uh the light curve does not repeat perfectly and so that is evidence for say maybe Cloud features evolving uh on this world or uh another way to put that is that maybe this is evidence for weather uh occurring occurring on this world um ariga all had observations over the course of actually multiple nights uh and so here um spending about five separate nights they have observations of of that same object where the light curve actually shows a great deal of uh complexity um enough complexity that we we certainly haven't been able to to disentangle it yet but that seems like that this is something this is probably weather happening on this planet um which is which is quite amazing okay um so that was in 2009 and since then brown dwarf variability Studies have uh kind of exploded um and so uh what I want to show you now are actually some really notable and exciting cases of of weird things that brown dwarfs have done while we've been watching them who knows what they've done while we haven't been watching them um so this was a study published by uh Jackie rigan just a few years ago um of a of an early T dwarf so a t1.5 dwarf so this is again kind of near that that bounding temperature for the T dwars 1300 Kelvin um so these were observations taken over the course of several nights uh this is groundbased uh and these were some of these are actually different wavelengths so this is a broad filter at 1.2 microns this is a broad filter at 2.2 microns back to 1.2 and then at 1.6 so these are all slightly different wavelengths but what I really want to draw your attention to is this first nights worth of observations um so here we have normalized brightness on this axis and then this is time and hours on this axis and so you see that after the observations start the brown dwarf achieves some Maximum brightness and then starts to drop off pretty steeply uh and that when the observations end it still hasn't actually achieved its minimum brightness uh but the range and brightnesses that we that that Jackie did manage to capture there uh was actually Peak to trough about 30% uh so that's a huge amount of of variability uh and it likely uh more than that because again they didn't they didn't capture the the trough of of of that uh decrease in in variability and so this is currently the the record holder from the most amount of variability that's been seen in in in a brown dwarf um and then a very a very a different case that I think is just as interesting just as intriguing so this is a a midt dwarf so again we're talking about a th000 Kelvin here um these are observations from HST U which helps because then you you don't have to peer through the Earth's atmosphere um and so these again are light curves so brightness is a function of time this is normalized brightness uh on the Y AIS and this is time uh normalized by the rotation period so this guy rotates in 1.4 hours which is even more surprising um and so if you look at the top two light curves uh so this one spans 1.2 2 to 1.3 microns this SP 1.5 to about 1.6 microns um you see that they have a peak to trough uh variation of uh 2 or 3% uh which isn't that much by the 30% standard that I that I just showed you um but if we go to this final panel down here which is an range of wavelengths that's centered on a water vapor absorption band um you see that the uh variability is actually flipped from one another uh and so when the brown dwarf was at its brightest uh in these other two bands uh when you look in that water band The Brown dorf is actually at its dimmest uh and so there's actually some very complicated stuff going on here with regards to brightness variations um not only happening brightness varying as a function of time but also varying as a function of of of wavelength um which is very intriguing so uh brightness variability uh studies brown dwarf variability studies uh have actually gotten to the point where you're starting to do statistics on them uh and so folks are now looking at uh the variability of brown dwarfs uh in different bins across different spectral types and so here's the brown dwarf uh spectral sequence running through here and then the corresponding kind of temperatures running up here um and so each of these different uh data points is a brown dwarf that's been studied and he's either had or had not had variability measured but the interesting cases are the ones that have uh the dash lines and so these were observations where the brown dwarf was observed on one occasion to have large variabilities say 10% in this case uh and then someone came back later and made another measurement and only found an upper limit so didn't find any evidence for variability so it also seems like variability is something that can turn on and turn off uh in in some of these worlds um which again is probably pointing to to weather on these planets maybe storms outbreaking or something like that um and then in a really uh amazing result I guess um Ian Crossfield just last year um using um essentially the the Doppler technique to to his Advantage uh and looking at the Doppler shift in in lines uh and splotches on the surface of a brown dwarf as that splotch rotates towards you and then away from you um was able to produce a map of brightness over the surface of a nearby brown dwarf uh and so I'm showing you this map here um so time starts here and then as it rotates around time is progressing uh this way uh the the the dark colors are where it's relatively cool on the surface of that brown dwarf uh and the bright colors are where it's relatively warm uh and so you definitely see at least splotchiness Hot patches and cool patches uh on the surface of of this brown dwarf um and maybe that's an argument for uh say uh Cloud patterns here and say lack of of cloud patterns here it's not entirely clear at this point so um the reason it's not entirely clear is that we're um still kind of sorting out exactly what is causing or what could be causing the the the brown dwarf um variability uh and so a few different authors have taken a stab at this uh and we certainly don't have the final picture pieced together yet um so Daniel P University of Arizona and his team have essentially um taken some techniques from the the Stellar literature uh specifically star spot models where you take a a hot uh sphere uh and then you paint onto it splotches that are cooler and then you spin that thing and you try to reproduce the light curve that you've measured for uh a brown dwarf um and so these results have shown that indeed if you if you paint two or three cool splotches onto a onto a sphere and rotate it that that you can reproduce the light curves uh and and I guess what's what's really weth noting is that if you look at these pictures uh down here which is a different which is a sphere that he's rotating through different angles that that picture that they have in their mind actually looks kind of qualitatively at least like what we're what we're seeing here in the the Crossfield at all observations um but the the the setback to using these these starspot models is that um they don't actually get at the physics of what's causing uh the hot and the cool patches um they're just providing you essentially a map of where hot and cool patches might be on the on the surface of the brown dwarf to reproduce uh the known light curve so it's just fitting the observations um there's been only a few studies now of um of actually the physics that could be leading to uh variability in in brown dwarf atmospheres um so so most recently uh XI Jang um working with Adam showman also at the University of Arizona did uh some circulation models of brown dwarfs and different regimes uh and so they had some free parameters and those free parameters were things like how fast they spun that that brown dwarf uh they had a free parameter for the characteristic time scale at which they introduced uh convective noise essentially and then had a free parameter for the time scale at which they allowed radiative cooling to occur and so depending on how they set those different knobs they could get U brown dwarf circulations that were in two different regimes and so what I'm showing you here are circulations on two different kinds of brown dwarves that they dreamt up in their in their um simple models uh and these are temperature variations and so these are cool patches and these are Hot patches and so if they set the parameters in a certain way they could actually get jets that occurred near the equator um or if they set the parameters in some other way they got something that just looked like a a a pot of boiling water it's just a roiling surface um and so then if you imagine spinning this thing and measuring its brightness as a function of time they show that uh you could actually get things that look like the light curves that we observe for for brown dwarves uh and that uh as as say these uh these uh convective plumes come and go or uh as the the Jets shrink and widen or uh you get turbulence around those Jets um they show that you can actually get an in you can get additional structure in the light curves that also looks something like what we observe with with brown dwares um these simulations don't yet include radiation um so they're not actually producing a modeled spectrum of their of their Brown dwarves uh which means that it's kind of difficult to compare that to the brightnesses that we're actually measuring for brown dwarves so right now it's just kind of qualitative that they can say that if they take this sphere that has hot and cool surfaces and spin it that they can get something that looks like hot and cool variations that have been observed for for brown dwarves um Mark and I um took a slightly different tag um and so we actually were using 1D simulations where the one dimension is just the vertical through the atmosphere of of that brown dwarf uh to study uh variability um and so we we picked out a a specific case so this was a mid T dwarf this was actually uh the the T dwarf that Esther Boley at all studied that had the the variations uh and brightness that occurred both as a function of wavelength and as a function of time we thought that that observation was so intriguing that we would try to go after figuring out some way to to model it um and so we we took some of Mark's tools uh for simulating temperatures in the atmosphere of of of a brown dwarf uh and what we did is we introduced a thermal pertubation deep in the atmosphere um we didn't really say what would be causing that thermal pertubation it could be a cloud uh that forms then dissipates and introduces a thermal pulse uh into the atmosphere and we had that thermal pertubation uh occurring at some characteristic time scale which is just what this toore p is here so I'm just going to show you two cases of of something that that came out of these these simulations uh and so what I'm this this curve actually is is the is the Baseline uh temperature profile down through the atmosphere of that brown dwarf so that's kind of the the standard uh case um before we introduce pertubations um and so what this movie is actually going to show you is down through that same range of pressures through the brown dwarf atmosphere uh variations in temperature as a function of time so it's going to be a movie um uh and how those pertubations propagate upwards through the atmosphere and I like watching it because it's kind of mesmerizing um and so uh the brown dwarf is is convective down here and so the thermal pertubation travels pretty efficiently up through the convective Zone uh but uh in the upper portions of the atmosphere the only way that energy can be communicated through radiation uh which occurs on slower time scales and so it actually takes about 100 hours for the thermal pulse that happens down here to make it up to the top of the atmosphere which means that uh the top at top of the atmosphere is out of sink uh with what's occurring in in the Deep atmosphere um which is kind of what's been observed for the the the the the blesl at all brown dwarf where they had uh a light curve that was out of sync at certain wavelengths um and so if you actually if you go and you use the model and simulate the Spectrum so given that temperature profile that's varying as a function of time you can actually create a spectrum of brightness variations so these are variations in brightness on this axis and this is wavelength on this axis so the're near infrared and so as the movie goes on it's going to be time playing um so you can see that uh over here at these shorter wavelengths these are two locations where you can see deep down into the atmosphere they're on opposite ends of of a water band and this atmosphere we assume to be cloud-free um and so you these wavelengths you see uh down to near where the uh thermal perturbation is being introduced um but then at other wavelengths you don't get to see as deep into the atmosphere you see higher up uh and so then you get wiggling that is out of sync with the wavelengths where you see deep into the atmosphere which is similar to what bla blainley at all observed now the the complication is or what came out of this uh study is that really the characteristic time scale for driving these thermal perturbations up through the atmosphere of a t dwarf is more like 100 hours um which is longer than the time scales that that uh variability is currently being observed at so most people tend to study Brown dwarfs over the course of a rotation which is a couple of hours so we have lots of data for how Brown DWS are variable over a couple of hours we have limited data for how Brown DWS are variable over say 10 hours um and we have almost no information about how Brown do might be variable over say 100 hours um which is the the characteristic time scale that kind of came out of this this study with between uh Mark and I okay so what about exoplanets so hopefully you've all been very excited about bror because they're just cool and interesting worlds but uh I'll do the obligatory exoplanet tiin because I'm also very interested in in exop Planet so I've shown you a lot of brown dwarf Spectra over the course of of this um of this talk and so here again I'm showing you brightness uh is a function of wavelength uh for a brown dwarf that's um kind of in the the midt range so this is an effective temperature about 900 Kelvin um I've labeled a lot of the features here so these are the strong water vapor absorption features that we talked about before and then the methane also starting to creep in um but what I kind of took for granted while I was going through this talk was exact well was how uh high quality quity these observations are signal to noise on these things is very high you don't see lots of of of noise uh in the Spectra in fact I don't really have to plot aor bars on them because we have such good data for them um let's compare that to the emission Spectra that we have for uh characteristic exoplanets and so this is hd189733b this is the Prototype hot Jupiter um and so here I'm showing you brightness ratio to the brightness of a toast star uh so this is a measure of the planet's brightness this essentially um is a function of wavelength again through a portion of the near infrared uh and you can see just how big these air bars are uh you can also see it's very difficult to spot what are clearly absorption features in these observations uh the problem being that it's very difficult to get good data for exoplanet because exoplanets are sitting right next to a really bright star whereas Brown doors can just occur in the field uh and so you don't have to compete with the brightness of of a nearby star to get uh good data for for the brown dwarf um and so a take on point from that is that brown dwarfs probe a very similar set to atmospheric physics the uh temperature here for this hot Jupiter is 1100 Kelvin which sits within the regime of the temperatures that you get for brown dwarves uh so the the physics are uh common um but for brown dwarves you get really high signal noise data whereas for exoplanets you don't get high signal noise data so they're are great laboratory for understanding the kinds of things that you might expect to see for exoplanets or the kinds of processes that are probably happening for exoplanets um and then another point so this is the the famous uh directly imaged exoplanet system uh around the star HR 8799 so HR 8799 uh is more massive than the sun it's about 1.5 times the mass of the Sun uh it is uh a relatively well it's a relatively Young Star which means it's formed recently and so these planets planets uh B C D and E are um all still hot with their residual energy of of formation um each of those planets is ballpark about seven times the mass of Jupiter um so they're they bigin um uh but recently observations with the with the newly commissioned uh Gemini Planet imager um along with models developed by uh Mark and so here we are actually applying uh Mark's brown dwarf models to exoplanets you can actually already start to see the the exchange between the two um have indicated that at least for uh planets C and d uh that models that include patchy clouds are the best at reproducing the uh the observations and so now there's evidence for patchy clouds in the atmospheres of of exoplanets like these um and so I think it's safe to say uh both from our experience in the solar system uh and from what we're now learning about some of the directly image exoplanets uh that we're going to see pchy clouds there and as a result they're also going to be variable uh and so studying BR dwarf variability is now our leg up uh to being able to understand exoplanet variability when those observations start to uh come down the pipeline okay so uh just to to wrap it up I want to say a little bit about where this is going to go I told you that uh observations of brown door variability are to the point where we're starting to do statistics on them um so those observations are uh coming along quite nicely uh and where kind of the holdup is is is in the the modeling aspect of it we still don't have a very coherent story that I can tell you about uh how uh these uh variations being caused what weather is like on Brown dwarves what might be causing clouds to come and go are these things like big convective storms that are forming inside the atmosphere of brown dwarfs or are we talking about a world that has a thick cloud cover and maybe just due to uh advection every now and then you get slightly thicker cloud cover here and slightly thin cloud cover over there um and so there's going to be a lot of work on on circulation uh on this uh in in the future um uh and so I showed you work from uh uh uh XI and Adam showman and showman and collaborators have been working on producing the first models of uh circulation on Brown doors but I told you that those models don't yet have radiation in them and so to actually be able to tie them back to the observations that's something that needs to be added uh and so um fortunately um showman and Jonathan Fortney and Mark Marley have already kind of at least from the exoplanet perspective built tools that allow radiation to be um incorporated into the kinds of simulations that I'm showing you here um so so there's kind of clear path forward for being able to get radiation into these and then being able to compare them more directly to the to the observations um the 3D models that I'm showing you down here don't include clouds the variability models that I showed you that had the the snake likee wiggling curves that I like to to stare at for hours on end um those also didn't include clouds and the problem being that uh the cloud model that it's kind of universally used or or widely used in in the community um is a steady state model it's not Dynamic uh and so we're it's clear now that we're seeing evidence for dynamic clouds in these atmospheres U but our our preferred tool for studying clouds uh currently isn't isn't ready for the for the task of studying Dynamic clouds and so there's going to be uh work in in the in the near future um and and this is actually part of my my next post talk is to develop tools uh to be able to do Dynamic clouds uh in both 1D and in and in 3D models uh and then of course once you have those tools uh kind of on your on your tool belt uh you'll be ready for when the observations of variability and exoplanet start to come down the pipeline um which I'm guessing is going to be in the next 5 to 10 years and so the timelines there actually sync up pretty well okay uh with that I want to thank support from uh NASA and the Oakridge Phil universities and say thank you uh to everyone for coming and and I'm happy to take your questions we'll take questions please wait for the microphone so that people online can hear what you're asking thank you um over here hi so uh do you think that the James web Space Telescope will be very useful for uh since it's like focuses on infrared on the temperature uh mappings of brown dwarfs uh so so James web is going to be extremely extremely useful I mean it it operates at uh exactly the right kind of wavelengths where you need to be uh to be studying BR dwarves and it's uh it's such a huge aperture it's such a great light bucket uh that we're really going to be getting great data for for for brown dwarves yep it's going to be it's a it's going to be a whole new ballpark once we once we have James web looking at these things can you make a few comments perhaps on the relationship between the modeling you're doing these brown dwarf systems and the weather models for the planet Earth [Music] um so no one has uh attempted to make the the bridge yet from from Earth weather models to to to brown dwarf uh circulation models um so the the 3D simulations that I that I was showing you are um by the standards of the general circulation models that we used to say study climate change on Earth um are are are kind of quite primitive um and reason being that it's not exactly a straightforward task to Port over the Earth circulation models to the to the Bron dwarf regime uh Bron dwarfs don't have surfaces the Earth has a surface um very different temperature regimes which means that the kinds of molecules that are providing absorption or providing capacity are different the kinds of clouds that are forming are very different um and so the the approach has been uh instead of going through the headache of of trying to adapt a model that may not be perfectly suited to to doing Brown dwars is to actually build the tools from the ground up and and start aresh when when studying the the brown dwarf circulation um which I think is probably the the right approach um you can tend to get yourself in trouble if you take a UR ECM and and Take It Outside The regime of where it's most comfortable uh every now and then you forget oh I forgot to set the rotation rate in this one portion of the code over here and so that code thought that the planet was rotating at 24 hours U but actually I told it that the brown dwarf was rotating at 1.4 hours and now it's all goofed up um so it's it's probably best to to start simple and and add complexity like uh like showman and his colleagues are doing Tyler I have sort of a related question while I'm walking over here a lot of the variability in stars is driven by oscillations rather than rotation do you is that expected in brown dwarfs also uh as far as I know no uh that it's it's it's not going to be an oscillation thing that it's it's going to be it's going to be some kind of weather it's going to be tied to circulation in clouds um kind of a two-part question I didn't realize that um Brown the LT and Y Brown dwarfs were all the same size I know the masses were different but um any idea what causes them to collapse to the same size as a Jupiter size that's the first part yep there's um uh so the I'm not a a brown DFS interior person but the the more physics minded folks who do brown DF Interiors there's a uh it's I believe it's an electron degeneracy and so it's something similar that's happening with with white dwarves um that fixes their size that also fixes the size of of of a brown dwarf okay and the second part is um is that also true for um planets that are two three six Jupiter size do they since the the brown dwarfs are all the same size are those also the same size uh yes so things that are in ballpark Mass range of of Jupiter planets that are in the ballpark Mass range of Jupiter are are going to have about the same radius of Jupiter except I mean there's some Noti cases so for the hot Jupiters um they there's a lot of conversation in the literature about them being or bloated uh and so there's actually some mechanism that's actually causing them to be larger than than they should be um but I mean if you look at Saturn Saturn size uh even though it's it's much more much less massive than than uh Jupiter is its size is not dramatically smaller than than than Jupiter so y that same process operates for the for the gas giants Jupiter and Saturn of the solar system uh the first qu I have two questions the first is um did you present uh the the first half of this in a National Geographic magazine or no somebody else presenting my work for me then I'm sorry what somebody else must be presenting my work for me then because I've seen the first half and some magaz yeah yeah I mean the the I think the the what what I laid out in the beginning is kind of the motivation for why we would want to study bror variability and and kind of motivating the models that uh folks including myself have put together is um the the motivation is is is pretty Universal so yeah I'm sure the arguments of star formation occurring down to very low masses uh but the low mass things having interesting chemistry in physics is it's a it's a universal argument and the second one was that uh very beautiful simulation you showed at the beginning did that include the effects of uh dark matter um probably not I don't know if the effects of dark matter are important at those kinds of of size scales yeah okay any more questions I have I have one more um Kepler as part of K2 observed row of yucus in its second campaign and um I wonder if there are any brown dwarfs that have been identified where you could get like curves if there are any bright enough invisible um I I don't know the answer to that so I guess we're going to have to chat about whether or not we could dig some brown door variability out of out of cap that would be exciting it's a neat idea any other questions anybody let's thank Tyler again thank you very much we have as part of our SE colloquium series we get the obligatory SEI mug to present to Tyler if he can drink his coffee or water out of now and thank you again thank you everybody for coming thank you thanks

Channel: SETI Institute

Views: 11,785

Keywords: brown drawfs, gas giants, sub-stellar objects, Exoplanet (Celestial Object Category), Atmospheric Sciences (Field Of Study), Astronomy (Field Of Study)

Id: 738cg3j37mE

Length: 53min 45sec (3225 seconds)

Published: Tue May 12 2015