Adam Burgasser: Cold Stars, New Neighbors: Discovering Brown Dwarfs

so now that everybody's thoroughly caffeinated and sugared we're going to continue on with our program with Adam Berg a sir who got his bachelor's degree from University of California San Diego and then went down the coast to Caltech to get his master's in PhD in physics with a minor in planetary science then spent his postdoc years as first a Hubble fellow then Spitzer fellow of Hubble fellow UCLA and in Spitzer fellow at American Museum of Natural History and he was faculty at MIT for a while and has since come on back home to UCSD so without further ado Edinburgh yes sir thank you very much thank you I get that response just for my introduction just be great talk so how is everyone's brains full yeah yeah what okay so I'm gonna put a little bit more in your brains we're gonna go a little bit off maybe off on a tangent here because this is these are gonna be objects I'm gonna talk about brown dwarfs these are objects that are debatably somewhere between star planets and stars and we still argue about where that middle road is but I'll try to convince you that in fact we can consider them both and we can look at them both and in many ways they provide some our best constraints of looking at the atmospheres of the kind of planets that we're now finding through Kepler through all these other different satellites I wanted to start though a little bit back and take us up into the sky this is a free program if you're ever interested in having a nice sort of stellarium that you show in classes actually called Stellarium you can download it offline it's totally free and it's very useful and this is actually what it's what Santa Barbara looking to the West is going to look like in about four hours all right and so we've got the Setting Sun and we've got these two nice bright little stars here which turn out not to be stars they're actually Venus and Mercury you can't see Juno up there and of course as we if we start time moving a little bit faster we'll get the Sun setting they'll become very very bright stars in the sky and they're moving right they're moving across the sky and of course as we let we let things go by it gets darker some more stars come by let this go a little bit faster just so it goes by and everything is moving okay so any you know any one of us 10,000 years ago would have seen this up in the sky would have seen the Stars and the planets swinging by on their daily cycle all right let's go back whoo do far well I'm a little trigger-happy here I guess okay it's the coffee okay so so that's what's look going to look like at about twenty to eight so I you know I c'n courage you to go out and look in the sky at that time now with us a layer we can actually do something really fun and we can turn off the atmosphere and we can turn off the ground so we can see what it look like in space so anyway to reduce the lights a little bit one can probably not with the Sun all right that's actually that's great that's fine I took the earth away I'm that powerful so now if you're watching on a daily cycle you would see these stars moving across the sky you would see these other bright things moving across the sky and if you were a careful observer and as humans had more time to look up the sky and keep track of things and actually write things down they would notice that if they looked at the same time of night every night laughter which he this is if they went back the next night oops they would find that the stars are in the same position but those planets have moved relative to the Stars so I'm stepping through by one sad Ariel day which essentially the time it takes for the stars go right back to the same position of the sky and if you're a careful observer and you did this every night you would notice that a few things in the sky change position all right including the Sun and here comes Jupiter so daily we see the stars moving across the sky multiple days months weeks years we would see other things moving across the sky let's go back and see where do Pater's areas okay and you could track these patterns over years and you would find that there certainly we can divide the whole universe into two kinds of objects things that move and things that don't all right stellar things which stay put oh this is a great time remember this date April 24 2011 everything lined up here's the problem of course it's in the middle of the day all right we'll go on with that all right so so the the pointer is that for for thousands of years thousands of years we have been able to divide our universe up into sort of two oh now I can't get to my other thing we've been able to divide our universe up into sort of two classes of objects planets which move across the sky and stars that don't and so there's this cultural distinction that we have between these two types of objects and there's a problem with my position there we go okay so there's a cultural distinction we mean between these two types of object if you sit and you get you know a bunch of guys together in underwear and watch the sky for a while you would find that you could break up everything in the sky but these two classes so for thousands of years we've done this and now as we've gotten more smart in our physics and been able to sort of actually figure out what's actually going on with these stars what makes them work we've still been able to divide those objects into two different classes in many ways so if we look at a star like the Sun we know of course that admits all of this light across its entire surface all right and that light is it ultimately coming from fusion energy in its core it's an energy producer in fact it produces all the energy that we really use here on earth at some level so so that's one part of the physics of the table you can of course planets at least in the optical the light that we see is light this reflected back at us from the Sun so again least at optical wavelengths we don't see them generating that light they're reflecting the light back again a very clear distinction between those two objects if we look at them in the sky we find that stars are really found all over the place they found as isolated objects is their own individual systems our Sun at some level is an individual star its own individual system and there are many many stars that are like that whereas when we look for planets and now that we're actually able to detect planets and other other solar systems so this is a picture of the HR 8799 system which was just resolved about a year ago right it's not a plaid star this is just the artifact of subtracting out the extremely bright light from That star to reveal these very faint planets but again we see these planets is orbiting around other stars and a lot of our planet detection methods is based on that assumption that planets are things that we find in orbit around other stars so you can go on with that assumption as long as you want but of course if you're a very skeptical type of scientist you might ask well are there stars that don't generate enough light to fuse I don't have enough energy to fuse hydrogen and don't generate that really bright optical light that we see from the Sun and are there planets that are actually in orbit around other stars all right how far can we really push those definitions and so based on the title talk the answer is of course yes and they are brown dwarfs and that's what I'm going to talk about is these class of objects that really don't fit into these very nice classes of stars and planets that we've had for thousands of years so I'm gonna break up my talk today and give you my my goal was sort of give you more of a flavor of the astrophysics of brown dwarfs what they are why we actually classify them as something different and how we actually came to find them what is the physics of actually finding these objects and at the end I'll give a little bit sort of a brief overlay of some of the exciting things have been happening in the field but I want to make sure you go home with some idea of the physics of these objects because you are physics teachers and I think that's probably what you're should but also maybe you can take some of this back home house so so the first thing the first question I ask are there stars that don't fuse hydrogen so we have to go back and actually understand why stars do fuse hydrogen and the simple answer is that the inside of stars are extremely hot all right their Sun is about 15 million degrees in this core and at those extremely high temperatures the protons electrons are now stripped apart from each other there are plasma and those protons have enough energy to actually smack together and get close enough together to overcome the Coulomb barrier the electrostatic repulsion and actually fuse and it's that energy of fusion that energy released from those processes that give the star all the energy that we need that come from its surface all right now notice that you know the core is about 15 million degrees the surface of the Sun is only about six thousand degrees Kelvin there's a huge difference between surface in core now how did that difference come about well it's the same physics that goes on when we look at a waterfall and we try to reproduce James Jules classic and I should say failed experiments to measure the difference of the temperature between the top of water on the bottom have how many of you have assigned this kind of problem in your classroom right how many of you know that this has actually been measured and successfully done yeah I don't think it has but I'm willing to try and I'd like to try it this place this is a beautiful waterfall it happens to be almost exactly 200 meters tall and if you believe that the gravitational potential energy releases the water falls down is converted into heat enter the bottom here you would expect to measure something like a half a degree difference okay just based on very simple physics gravitational potential energy release now that's 200 meters the Sun all right much much bigger seven times ten to the ten centimetres seven times and the eight meters all right so that is a much bigger difference in in gravitational potential from the top to the bottom and as Deborah mentioned earlier what we believed in how stars form is a storm from the contraction of large gas clouds and as they contract they're literally giving up their gravitational potential energy and they're giving it up to heat they're giving it up to heat and also radiation from the surface the heat from the friction of that gas moving together and release in that gravitation potential energy and that's enough to heat the car up to the point where it can conduct those fusion reactions okay so so we can get some basic idea of that of the physics of that we just look at sort of our basic equations for that we have GM squared over R as our potential energy for gravitational potential and that's converted again into heat and also radiation but notice that potential depends on the mass of whatever thing is contracting divided by its effective radius so if we have something that's more massive we don't have to squeeze it as much right releasing just as much potential energy by squeezing it less if we have a lot more mass and at some level this explains why we look at stars from very very low mass to very very big mass that there is a sort of size range going through there the idea is that you don't have to shrink a big massive star as much to heat that core to start those nuclear reactions okay now this is a very simplistic view and there's other physics going on in here but essentially you can get something that is really big only if you're able to heat it fast enough to start those nuke reactions in the core if you have just very very li'l mass like the little m dwarfs down here at the bottom they have to shrink more and more and more and keep going and keep going until they finally get to that temperature where hydrogen ignition actually occurs right so so big mass big size little mass little size and so you could say well let's keep going on this end how much how low in mass can we go and still shrink things down can we string cast are down to the size of say a Cadillac and start fusing in the in the core well turn that we can't although it'd be cool if we could much better for laboratory experiments that way there's a quantum mechanical limit on how much we can actually squeeze the star together to get that bad temperature up to the point where we've released all that potential energy into heat energy and start new reactions and that limit is based on the Pauli exclusion principle and essentially is that we can't squeeze in this case electrons close enough until they actually are sort of supported they actually can't put into the same position the same place the same reason you can't put two shirts in the same place same time which is always a problem when packing clothes all right it's that prevention of overlapping the quantum states of those electrons that sort of halts that contraction all right and this was thought about in the 1960s again looking at the experiment how low of a massive star can we make and how small can we make it when it was realized we run into this fundamental limit so this is a plot from one of the earliest plots looking at brown dwarfs this is a plot from shieff kumar who currently is actually a professor in university of virginia and i'll tell a little bit more him and a little bit but this is a plot from his seminal paper 1963 looking at how a star would collapse as you go in terms of density and temperature so a star would start as something that's big and puffy so very low density and very low temperature this is the core temperature and as it collapses it gets more dense and it gets hotter again those patek that potential energy is released and heats the core so a star would follow this trajectory up here and hopefully it would just keep going to the point where it's able to start hydrogen fusion and that threshold is about three million degrees Kelvin I mentioned that the Sun Center is about 15 million degrees Kelvin so the Sun is actually able to fuse not just hydrogen but also other elements but the minimum that you need to start your fusing hydrogen is about three million degrees Kelvin now this other line here is sort of the barrier between when you have a gas that's sort of normal gas and a gas that's degenerate that's supported by this Jannetty pressure you can't squeeze it down any further and it turns out that as you go to lower lower masses again you have to compress to higher higher densities to get all the energy out and you run into that limit and if you run into that limit before you've actually started hydrogen fusion the star stops contracting and it's held in place and you no longer have that potential energy to heat the core any further so a star that starts like that and in fact that mass threshold is about 7% of the Sun a star that starts at that very low mass never gets the point where it fuses hydrogen it doesn't make it as a star it's a failed star it's a very negative connotation but all right but that's essentially what happens and in fact because it's still radiating at energy away it's not like it can just wait longer for that potential energy to come by if it's already missed that's it's missed the boat all right so it's never gonna fuse hydrogen alright so 1963 that's when that idea came about that there will be these stars that don't fuse hydrogen because they're very low mass now that has a few consequences first of all it has a consequence on how the star involves over time so this is another plot showing the effective temperature which is essentially the surface temperature of the star there's no real surface on the star but it's essentially where we start to see most of the photons coming from the star it's a photosphere of the star it's the temperature at that photosphere so when I say the Sun has a temperature of 6,000 Kelvin that's the temperature at its photosphere all right so this is a plot of temperature versus time for some models of these very low mass stars in fact these are models from Adam burrows up here and looking at very low masses now you can't see the numbers but I'll tell you that this is sort of about 1/10 of a solar mass at the very top here and then going down to lower masses this is about 5% of a solar mass you go all the way down here to about 1% of a solar mass and you notice a pattern here the things that are in the blue region sort of come to a point where they equivalent we have exactly the same temperature over long periods of time and that sort of the normal evolution of a star it's producing enough energy to make up for the radiation that's releasing from its surface it's a radiative equilibrium so it stays at a roughly the same temperature now modulo some of the you know James talked about the young faint star so there is some change in temperature that's very small but thirty percent compared to the range here is very very small difference effectively a star that has enough mass a fuse hydrogen is going to stay at the same temperature for most of the lifetime but if you don't have enough energy you're still reading energy way you get cooler and cooler over time so these are stars that not only can't fuse hydrogen they're stars that change over time right they get cooler and they get get lower and lower luminosity and their temperatures are actually getting very low so the scale here here's five hundred Kelvin a thousand Kelvin five hundred Kelvin is something I can do in my oven at home all right so it's a star that's in my kitchen just bigger okay how bigger well it turns out that that's size where the brown dwarf contracts to is about a tenth of a solar radius that's the minimum size roughly that a star can be all right and then again it's supported by degeneracy pressure now this is just a sort of info plots you don't have to read all the information here but I wanted to focus on what the conditions here in the core again in order to get hot it has to compress more and more and more to the point where it just can't compress any more because of quantum mechanics but it gets to pretty high densities tend to a thousand grams or kilogram or grams per cubic centimeter just for comparison LED is about eleven right so this is orders of magnitude potentially look more dense than lead pressures that are ten to eleven bars all right our atmosphere is one bar center of Jupiter is about ten to the five bars like ten yeah ten to five bars so this is much much more dense than that 10 to 5/10 6/10 7/10 seven thank you still smaller all right very very high pressures very very high densities exotic states of material can exist in here in fact States mature that we don't even understand because we can't do laboratory experiments to actually know what happens to hydrogen at those kind of conditions right it's probably some kind of metallic state it might be some kind of crystalline state alright we don't really know we don't have laboratory measurements to tell us what happens had those extreme regimes right so very bizarre interiors here but still it's an object that we can consider as a star all right it's just star with very very high density and changes with time all right so that's the basic physics of these objects I want to talk now a little bit more about how we actually found them in the first place a lot of that theory was worked out again starting the 1960s and through the 70s and as particularly as atmospheric models got better in the 90s 80s and 90s and and more recently we've got a very good theoretical picture of what we think a brown dwarf should look like but of course as we heard earlier today one always has to be careful what the theorists tell us because it might be that reality is much more different than that now so there's motivation to look for brown dwarfs and that reason because there's sort of an odd exotic object let's see if we can actually see them but there was another reason to look for them and the fact that they could be dark matter all right so this is so you know bat you know dark matter has been known for you know something like 50 60 70 years but of course we still to this day don't know what it's made out of but around the time that you know people were looking at brown dwarfs as another state of object another type of star they realized that well look these things have mass so they qualify for the matter parts and they get very cold over time that seems to qualify them for the dark parts and if you get enough of these dark massive things in the sky it may be that they're enough there to make up for this dark matter and there's actually beyond that sort of very simple argument there's even more motivation for this when we look up into the sky and we count the number of stars particularly stars nearest to the Sun we find that most of those stars are these red stars these very low temperature and low mass stars and in fact around the 1960s and 70s it was believed that the number of stars as a function of mass as you go to lower and lower masses just kept going higher and higher and if you just drew a line which astronomers love to do particularly in regions where we don't know anything about we would find that you have exactly enough mass to make up dark matter okay so a lot of motivation to look for these things and in fact it rains around that time that this that the term brown dwarfs because I've no I was gonna get questioned about this the term brown dwarfs actually came in the beam there were many suggestions for the names of brown dwarfs alright black dwarfs dark stars failed stars sub stars in fridge dwarfs we'll talk about why that's important one super Jupiter is very positive alright we still use that when we write NASA proposals I mentioned chief Kumar Kumar was a big proponent of calling these of course Kumar stars sorry the person who actually came up with the word brown dwarfs was Jill tarter say me then we know who Jill Turner's yep so he's she's she's the director of SETI Institute's just as you moved on to greener pastures from brown dwarfs looking for life but the time her thesis project was actually trying to understand what the properties of brown dwarfs would look like particularly atmospheres and it turned out at a time I mean for a couple of reasons a it's very hard to bottle the atmospheres of brown dwarfs particularly you know back in the 1970s when we don't have our supercomputers but also the fact that brown dwarfs change with time means that it's very hard to assign a color to this entire class of objects we can call red dwarfs red dwarfs because they're all red alright that's easy yellow dwarfs are all yellow yeah okay white dwarfs well we'll talk about that later but brown dwarfs can be all kinds of colors because they change with time so you know her suggest is we don't know what color they are let's call them Brown cuz Brown is sort of every color okay that's good turns out the actual color of brown dwarfs we'll talk about a little later is actually purple that would have been more exciting but that's fine okay so all this excitement to look for brown dwarfs starting from the 1960s believing that their dark matter motivation to look for these the missing mass of the universe how many discoveries are made in the 30 years after that theoretical discovery you got it zero right alright so it took until 1995 for this object to be found least a two to nine B this is perhaps the only brown dwarf anyone could name off the top of their head it's the most famous it's technically people argue whether there's a first one but this is the first one that really every astronomer agreed has to be a brown dwarf and it was found as a companion to this much brighter star here this star actually turns out to be one of these faint M dwarfs so you can see how much fainter this little little star is okay this is a Hubble image by the way in fact this image is also sitting up at the Americans in natural history this is one of their displays big displays huge monster image of this of this system because this is really an iconic photo for the field of brown dwarfs the real first discovery of a thing that that is one of these missing matter things and when you took we took a spectrum of it or when astronomers took a spectrum in fact the person who took one of the first spectra of this object is going to be here this week a very amazing thing is shown so this is showing that again the spectrum the breakup of the light is a function of wavelength so brightness on this axis and wavelength on this axis and this turns out to be in the near infrared region here is the spectrum of this gleets a 2 to 9 B object here is the spectrum of the moon Titan Wow all right that's kind of amazing in fact the reason that it's so much structure in here is that this part of the region of the the spectrum of the star is absorbed out by a molecule called methane all right we've heard of methane before one of our potential or definite greenhouse gases happens to be sitting in the spectrum of this brown dwarf ok so phenomenally this star which we think is about thousand Kelvin has a mass maybe about 4% of the Sun looks just like a moon all right so odd connections between stars and planets here all right overlapping in terms of science now why did it take 30 years to find this one little object well sure as many of you have been hopefully you know successfully teaching your students I try I'm not so successful both the time as explaining how the spectra of things of different temperature work the blackbody spectrum all right if we look at the blackbody spectrum for different temperatures we start at 5500 Kelvin 3,000 Kelvin thousand Kelvin of course as we move to longer and longer wavelengths more sorry we've go to cooler and cooler temperatures more of the light is coming out at longer and longer wavelengths so a star like the Sun right 5,500 6,000 Kelvin peaks in the yellow part of the spectrum all right Peaks in the visible band this happens that we evolved to have eyes that look in the dizzle Bank because we're around the star that peaks in the visible band and dwarfs are peeking more in the red region here but when we get down to a temperature like that object the light is all coming out at wavelengths longer than the visible in the infrared regime and it was really because of that that we you know because most of light is coming out at these longer wavelengths we couldn't see them with our traditional photographic plates or CCD cameras that were in use from 1960s up through the 1980s they just weren't showing their light in those wavelengths now I like to skip this one right so so it took until really the 1980s so you know early technology for looking at infrared light was basically little blocks of lead that you put behind the telescope LED sulfide or LED solenoid to actually detect the infrared radiation and there's very very coarse technologies we manage just putting a big block of metal by a telescope is probably not going to give you a very accurate measure of anything all right but indeed the first infrared surveys were done with little blocks of lead behind the telescope step up to 1980s as a better technology come out in different crystals to detect infrared light came about we actually started having real infrared detectors real things that look like see CDs but are sensitive to infrared lights okay there's a huge jump in technology from stuff like this and of course today we have cameras that have all bunch of these things this is a camera for the Vista Survey that's just been started out in South America it has a 67 million pixel infrared camera okay little blocks of lead 67 million pixel camera right so we've made big jumps in our technology and we required those big jumps in technology to actually detect the light from these very faint objects now infrared is a great regime I would consider myself more of an infrared astronomer that a brown dwarf astronomer because I like the things that happen the infrared all right you get the stuff like if you look at the moon in the visible and the moon in the thread very different stuff going on there all right we see very very bright craters this is actually slightly off shifted from each other different features show up and wavelengths if we look in clusters in the infrared and the visible we see these dark dark bands of clouds these are the clouds that actually our making are going to make those stars in brown dwarfs that we're going to look at later on but to peer through them if we look in the infrared much later on right very long graduate thesis to look for stars that are forming here it's only ten million years I mean really look in the infrared we can actually see through some of those dust clouds and see those stellar nurseries right the infrared light is penetrating through those clouds all right very different universes are in there all right look at galaxies same thing very different structures that we see we compare visible to infrared light even we look at people all right we saw an image of the the Las Cumbres group in the infrared here are animals in the infrared there's a great if you're ever interested in teaching anything about infrared science cool cosmos has a great set of resources for infrared light right so the whole point is that there is a whole different universe out there when we look at different wavelengths and indeed for brown dwarfs the whole universe was in infrared wavelengths we weren't seen in the optical we needed to go to the infrared so in the late 90s there were surveys that were commissioned such as the 2mass to micron all-sky survey that looked at the entire sky at these near-infrared wavelengths and this is a picture of the telescope of the Sloan Digital Sky Survey it uses traditional CCD cameras but it also included lights at the very very near in fragrance of those cameras so it was still picking up some of that very very red and near-infrared light and it's these surveys that are really responsible for the large number of brown dwarfs that we have today and just to give an example of what it's like to look for one of these objects this is a visible light image Traverse sort of inverse scale so the stars are black and the sky is white little easier in the eyes but just random patch of sky not sarena patch to the sky and here's a same image in the near-infrared ok if I go back and forth here you see more stars invisible less stars in the infrared but importantly if I go really fast we see something coming into view right in the middle okay oops right so it's a Dark Star it's an object that didn't show up in those visible wavelength photographic plates or digital CCD photographic plates it shows up in the near-infrared these are exactly the kind of objects that that we're looking for so again as I said these big surveys started finding these objects you know we started making news we can see that we're already having sort of a bad publicity thing with the name brown dwarfs poorly understood poorly named all right clearly not the best term for a new object but today so this all happened in early 2000s today we have at least you know we're starting to get to the point where we have about a thousand of these objects known all right this this is a webpage that I maintain with a couple of colleagues of mine this on a couple classes of objects that I'll talk about a second but we have at least 70 50 of 750 of those know where 10 years ago they were really none right or a few so this field is really exploded with lots discoveries now is there there's another field that exploded with lots of discoveries that is the exoplanet field all right and it turns out there's an interesting little connection between the discoveries of brown dwarfs and discovery of exoplanets in the there was one conference in 1994 that happened in Italy where the first detection of of that object that showed that Gliese tatoos 9b was announced and another object was announced there as well and we have a guess on what that object was yeah 51 pay you got it but you have an institute know that all right so the same conference was the first detection of an extrasolar planet was also the first detection of brown dwarf what a great year that was for our field right but both fields have now really exploded and in part again it has to do with with advances in technology for the brown dwarfs has really been advances in yearn for detecting technology and in for planets as Debra was talking about today there's a lot of advances in spectroscopic methods there are advances in sort of detection to rapid transit section is Alan and talked about earlier so there's these are both driven by technology development alright the more advanced we get with our detection technology the more of the sky that we're finding more of these interesting objects that we're finding okay so that's the that's sort of how these objects were found now that we have literally hundreds and of these things we are starting to learn more about their physical properties and what they're actually made out of so that's gonna be focused third part of the talk here now you know we had the we had James I mentioned this how we actually want to find habitability in planets it's important to look at the spectra to look for the signatures of these molecules in the atmosphere that might indicate that life is on the on the planet we also are very interested in the spectra of objects less so about looking for life but more about understanding what the atmospheres of these objects look like these are you know these are new objects and we want to know what's actually going on in their atmospheres and so these are the kind of spectra that we see this is extremely complicated cuz this is kind of everything we know about brown dwarfs spectroscopically in one slide so let me walk through a little bit here it's not that much right these are the spectra of four objects three of them are brown dwarfs and one is the spectrum of the planet Jupiter alright part of it is reflectance of the Sun and part of it in fact in the mid infrared here the long wavelengths is actually intrinsic emission from the planet Jupiter so I mentioned that Jupiter most of the light we see is reflected from the Sun in fact if we go out to these longer wavelengths Jupiter is itself a glowing body it produces its own light at those long wavelengths okay now stepping through here you can see that each of these spectra which is color-coded has slightly different shapes all right at the very top one here which is a which is an M type dwarf which has a temperature of about 3,000 Kelvin and surface is pretty smooth it's a pretty smooth curve you might even argue it's kind of close to a blackbody curve although it's not quite there are little divots in here right these little features over here and the very optical end little lines and the top but a pretty pretty smooth overall when we go down in temperature to something that's 1700 Kelvin we start seeing more features stronger features look at this this is on a logarithmic scale so to go from the bottom of this to the top of it it's a factor of a hundred all right turns out this is the absorption from an atom from potassium not one atom a bunch of atoms but they're all potassium all right the really strong features now we go into even cooler something that looks like that Gliese r22 9b object and we see really crazy structure now people I show this they say oh this must been really faint you had a lot Noi's kind of a messy spectrum you know sorry but this is all real this is all structured real structure and the spectrum of this object and it comes from absorption from a lot of molecules in the atmosphere and notice that this spectrum which is a brown dwarf again looks a lot like a planet spectrum of Jupiter right so crossing over that divide between stars and planets now the first order what we astronomers do with these spectra we use them as sort of fingerprints to classify these objects just like an ornithologist will go out and find lots of different birds and classify those groups of birds because that's sort of your first step as a you know to make a ruler to measure and compare things together you actually need to group them together so you can make those comparisons to understand the physical objects you have sort of set your scale so astronomers do have a ruler and that's the spectral classification system and as a result of these discoveries of brown dwarfs we've actually added two new spectral types to our standard classification scheme and potentially a third although they haven't been found yet so these are the new types the M dwarfs are the lowest mass stars so those actually existed before brown dwarfs were found but it turns out because brown dwarfs cool over time if you go to very very young ages very early on when they've still got some of that formation energy some of that formation heat they're actually still fairly hot and they still follow the M dwarf classes but these next two classes the elder Orff's and the T dwarfs are two new classes of stars that have been found only because we found brown dwarfs and they're very interesting so you know the L dwarfs have temperatures roughly around 1300 to 2100 Kelvin and their atmospheres they're very molecule rich I'll show in a second and importantly they have these really interesting clouds of dirt in their atmospheres I'll talk about that in a second the next class of the T dwarfs these are the coldest known brown dwarfs this is in fact the class of objects that I did my PhD thesis research on and they have lots of gases including water methane and ammonia we just heard about water methane and ammonia as important greenhouse gases in the atmosphere of Earth and potentially other planets there's lots of that greenhouse gas in these brown dwarfs turns out now there's another class that has yet to be discovered but of course we get ahead of ourselves we've already got a name for it and these are the Y dwarfs and possibly these objects maybe things that have water clouds in the atmosphere so now we're starting to get to the things that really look like planets that well we think we might want to go although a water cloud around a jupiter-mass thing is probably not a great vacation spot but at least it's interesting because it's going to be something that looks very much in terms of clouds very much like the earth now you might say well that's kind of an odd sequence MLT why doesn't really follow much of any order and of course if you're familiar with the stellar classification system it doesn't really follow any kind of order all right there are good reasons why these letters were chosen and I don't have much time to go into in great detail it turns out there's only a few letters left right we've used them for other things like double use or wolf-rayet stars okay that's out all right V what a great letter V can't use it because of V magnitude is very important so so there's only a few letters left and that's the order they end up when it's a great time to like come up with great mnemonics of course they may have a good mnemonic for when you teach the seller Coski yeah yeah [Applause] they may have any other ones so I think about this a lot I don't know why all right here's a recent one Obama's bailout of federal government killer much luck to you here's from watching late-night TV okay so so I like I've been collecting these because I give you know whenever I teach a class on on stars or you know the universe or astronomy something like that I always do this as an assignment that they have to come up with a new mnemonic and I get some great ones so much I can't share in public but I get some really great ones okay so so those are the three new classes of stars what do they actually mean physically well let's go back to this plot that shows the temperature of a star over time all right all these funny lines that go across here for different masses and we define these types roughly you know these types are based on the spectrum of the star and the spectrum is determined by the chemicals that are present in the star and those chemicals are determined in part in large part by the temperature of the star also the pressure but temperature is a big big factor in it and so if we put down really simply the areas and temperature where these three objects lie notice something interesting if I follow a line that goes through kind of the middle of this plots it starts off at well it's say about a hundred million years that's something that looks like an M dwarf and as I wait and wait and wait and wait so about let's say three hundred four hundred million years it starts to look like an L door and if I keep waiting only a couple billion years that long it might look like something like a T door they keep waiting like that's gonna turn out to be something that looks like a wider so this sequence of spectra and some level trace the evolution of one of these brown dwarfs with time now this has a very interesting historical precedent because the original motivation for the stellar classification scheme before we understood that it was nuclear fusion that was powering these stars was that you start out as an O star very hot and you just cool with time so the g RG dwarf an average age star just happens to be in the average part of its lifespan an event it's gonna cool down to something like an M dwarf okay sounds like it makes a lot of logical sense of course we know now it's not a single star cooling the time it's determined by mass but now thank you that a zero or two okay thank you so so we know now that that's not the sequences and evolutions of a star makes but that's the origin of our terms early type in late type all right late type stars are cool stars because very early on it was the late stages of a revolution but with brown dwarfs that works out we can actually talk about M dwarfs cooling to L dwarfs cooling the T dwarfs okay so we've somehow come all the way back to the 1890s somehow which is great eighteen nineties were pretty cool okay so so that's sort of where those spectral types fit on the evolution of a brown dwarf and it's not like I can say an elder or I know has a mass of this it might be more massive and older or it might be less massive than younger they're mixed up but the spectral types do trace the evolution of a particular brown dwarf alright so going back to the spectra I mention these various chemicals in here and here's just a laundry list of all the chemicals that are in the gas molecules that we see in the atmospheres of the objects and sort of been a funny but Venn diagram but you can see there's lots of them here all right we've got metal hydrides a weak aluminum hydride magnesium hydride calcium hydride we we get to cooler temperatures we get all those chemicals that form hydro sorry those are ox sorry hydrides yes and hydrates over here iron hydride chromium hydride lots of really cool chemicals in this out spectra but of course in the t dwarfs again we have some of the most interesting molecules ammonia methane and water alright those same molecules that we're looking for in these planets and how do we know in detail that we find these here well this is a spectra in the mid infrared range so 6 to 14 microns way beyond what we can see with our eyes and this is from the Spitzer Space Telescope and again you see this sort of funny structure in here in these spectra well again those are all due to different molecules so we've got water coming in here we've got methane here and our most recent discovery molecule is ammonia gas in the spectrum of the brown dwarf right ammonia gas in a star that's pretty cool now I mentioned that some objects some of these elders also have clouds and it's because the reason that these molecules are in these atmosphere is because they've cooled to the point where you know the chemical equilibrium allows their formation if it was any hotter they would break up into their constituent parts but it's now cool to the point where you can have a stable molecule well you can also have cool enough temperatures to form solid materials as well all right so here based on the chemical equilibrium models have been done for brown dwarfs here are some of the kind of chemicals that we would expect in l-type dwarfs and they're kind of interesting substances right aluminum oxide enstatite right these things I can pick up off the ground are floating around in the atmospheres of these objects right here's my favorite molten iron what a terrible rainstorm that would be right but they're there and how do we know we're there we can actually detect it again with the Spitzer Space Telescope these are spectra of some al Dorf this little region in here we see a very faint absorption feature and the absorption feature we believe is due to silicates in the atmosphere hard particles they're actually present in the atmosphere and those hard particles just like the hard particles of water Rome atmosphere probably form clouds and we actually do see evidence of that as well if we watch a given star over a few hours I give him browned over a few hours we notice occasionally a variation in the light you got it okay we know it's a variation in the light and that variation even changed at the time sometimes we see a very strong variation but a few days later with the same source it might be very flat and what we believe we're seeing is that it's something like a cloudy object like something like Jupiter for example that is rotating and if it's got any features those features are rotating in and out of view so it's like seeing a really bright spot come in and disappear might explain this variation and of course the fact that it goes away suggests that those clouds themselves are changing over time and changing fairly rapidly okay so so there are all these these features of brown dwarfs that look just like planets they've got the same molecules all right methane water and ammonia and they've got clouds and so you know we're talking about looking at the meteorology of stars well how about two are some more like stars well there's a few things for example we see brown dwarfs are magnetically active this is some radio data that was taken recently that shows that the radio emissions from brown dwarfs are sometimes seen and there seem to be fairly regular we don't see this kind of emission in anything but a pulsar right pulsar is a completely different object than a brown dwarf and a planet alright pulsar is the extremely condensed neutron star that's formed after a very large mass of stars exploded essentially and yet somehow this little tiny little faint guy has very similar radio emission and it's a signature that it has magnetic fields and it also in particularly it may have a very strong magnetic pole which is coming again in and out of you spinning in and out of you through the rotation right so that's that's more of a stellar like feature another style like features when we look at the Galactic orbits of some of these objects right we can now because again because technology is caught up we were able to measure the positions of these objects very accurately and also their velocities right so velocities across the sky and velocities toward an away from a same Doppler method that Deborah is using for her work and by combining those things we can make a prediction of what the orbit of these individual objects are so recently we've done this and this is a plot of two of those orbits this white circle is what the Sun is doing around the galaxy right this object is plunging from the where we are now because that's where we measured it straight into the center of the galaxy and coming back out again looks like a little spirograph right and then this green object is again where we are today because that's where we're measuring it but it probably started off something like 70,000 parsecs away from the center of the galaxy before it came to us right the galaxies visible like galaxies right here this is where this object has been sitting for part of its lifetime that most of its life time it's way outside the galaxy so this is not something you effect from a planet right not even the planet that got kicked out it from its star this is an extremely fast-moving object and in fact an object that may not even be part of our own galaxy may have been accreted from another dwarf galaxy that's an that's a that's in our immediate area so something that you know we look at in terms of planets in terms of atmospheres has a very unusual star like property in the fact that it's a star that's come from literally outside the galaxy here's another thing that brown dwarfs look like stars they have their own planetary systems this is a recent discovery made by micro lensing team looking again looking for variations in the stellar light of a background star I think actually this is so the background star let's say imagine is way over there all right on the other side of the wall I like when people look over that's really funny all right we're looking at that point and we see this change in the light over time this dramatic change in the light over time and not only do we see a big change in light we see this little divot at the very peak of it right that big change of light is the result of a brown dwarf passing from That star this little divot is that brown dwarfs planet and how big as a planet about three earth masses all right amazing right something that tiny that we're detecting based on the sort of magnification in this micro lensing events right the primary something like six tenths of solar mass so we know that's a brown dwarf it has the right mass or brown dwarf and it has something that's like a planet in orbit around it so all these hallmarks of both planets and both stars all right the atmospheres are very planetary like whereas a lot of their motions and their you know systems that they have and their magnetic fields are very star like and so it's a very interesting field because we really approach it from different directions because there are features from both sides of it so I'll just leave you with this last thought is that we now know all right we didn't know this fifteen years ago but we now know brown dwarfs exist in substantial numbers in the galaxy and in fact they exist in almost every region the galaxy that we look we see them in the immediate vicinity of our Sun we see them in young star clusters as I showed that big orbit we see them coming in from out and make halo of the galaxy and just passing by and saying hi all right nice to see you see you later all right all corners the galaxy's where we find these brown dwarfs and they filled this gap that we've had for thousands of years literally all of human history we've had this clique gap between stars and planets and that gap is gone right these objects fill that entire gap and they show properties of both and in particular from the perspective of a planet like atmospheres they provide a very unique look at what planet atmospheres really look like we've been talking about and today about actually detecting and looking at the atmospheres these hide exoplanets but it's difficult to do that because they're near these bright stars these things are just out in the open and we can say them as long as we want and get all kinds of information about the atmospheres of objects that that look like exercise planets and I'll leave you with that and I'll be happy to take some questions [Applause] all right questions let's pretend I saw is there any estimate of the percentage of mass of the missing mass that brown dwarfs might represent so excellent question the question was what what missing masses brown dwarfs actually make up in order to make up dark matter they would have to be something like 20,000 brown dwarfs for every star in the galaxy now that's a lot that's it's a long job all right and it turns out you know we haven't obviously we haven't found every brown dwarf in the galaxy we've only really seen the ones that are closest to the Sun but by measuring the number of those objects and correcting for you know the fact that we're probably missing some that are fainter and stuff like that there's probably as many brown dwarfs as there are stars in the galaxy roughly order of magnitude and that doesn't make up for dark matter at all it means it's only something like 15% of the stellar mass is probably brown dwarfs so they're definitely not dark matter that even close you mentioned magnetic fields and you also showed a slide that seemed to suggest maybe liquid metallic hydrogen yeah yeah yeah does that imply there's convection in a layer like that does that tell you there's a strong temperature gradient and does that then tell you something about energy transport and chemistry inside the star so one thing we at least from the theoretical modeling we know is that the interior brown dwarfs are probably fully convective which is actually very interesting because one of the tests for example finding a brown dwarf is looking for an element called lithium and lithium is destroyed in the same nuclear reactions that that change hydrogen into helium so the fact that lithium all the lithium in an object passes through the core at some points if you have an object that is fusing hydrogen we find that there is no lithium in the atmosphere it's completely gone totally burned away whereas if we do find lifting atmosphere that means it's never all that lithium has passed successfully through the core without being destroyed so that's one of our tests of brown dwarfs is look for this lithium ion so the first answer question is yes they're fully convective and we have plenty of evidence to show that that's the case now what that has to do with the generation of magnetic fields we have no doubt that these objects have magnetic fields our problem is actually detecting that magnetic Admission when we look at stellar magnetic fields we're typically looking at x-ray emission or emission in the H alpha line or calcium hydride calcium H&K lines these high-energy lines that really come from the corona and chromaspheres03 this is this is this is still a bit of a mystery is how they are showing this magnetic emission without showing these other signatures that we normally see and how to actually make the fields in the first place this is a very different regime than the kind of fields that we see in the Sun for example it's probably more like the kind of fields in Jupiter produces a question how do you define the difference between a brown dwarf and a planet because I've heard two definitions one is based on a brown dwarf is big enough to burn deuterium and the other is that brown dwarfs formed by gravitational collapse and planets by core accretion so that's that is the million dollar question that gets people and heated debates and stuff like that I'm looking over to Adam right now so you're right there are two definitions one that I would say the one one that it's based that's more of a physical physics definition and that's that we would divide sort of arbitrarily divided brown dwarfs and planets based on the fact that a planet would be something that doesn't fuse anything so it turns out I talked about brown dwarfs don't fuse hydrogen in the cores but can't accuse other elements they can fuse deuterium in fact they can fuse to tearing them down to very low masses about thirteen Jupiter massive thirteen or fourteen Jupiter masses and so you could define a planet sort of after the fact and say a planet's got to be something that doesn't fuse anything at all in its core and thirteen to madrassas is where that that boundary would lie and that's great because that means that all the planets in our solar system qualify as planets which you kind of want because culturally again we have this definition between planets and stars that we've had with us for thousands of years we've run into trouble though is that we see we frequently see another reason way we think about planets is whether they formed around stars is this secondary formation make secondary formation outputs of star formation that they form in these discs that we talked about today and that doesn't necessarily have anything to do with the final mass of the object and today we see systems that have very massive planets around them plants are even very close orbits around them that are more massive than thirteen Jupiter masses so are those brown dwarf star systems are they planet star systems and have that form we also see free-floating objects that we believe are three or five Jupiter masses that fall below that that mass criteria but they're off by themselves so that they get kicked out of their planetary system or that they just form that way so it's very difficult so yes what might where I stand on the fence I stand on the formation scenario which is a very dangerous one because we don't actually see the formation I kind of know what brown dwarf when I see it let's put it that way when I meet a brown dwarf I know but I think the complication is that we are in this realm where you know it was very easy before we found brown dwarfs stars and planets were totally different things we didn't have to worry about it and it's very clear that we're in a regime now where there is a continuum and that continuum may overlap in ways that we don't quite understand and I think it from a physicists that's very interesting from someone who wants to classify things in the great boxes very challenging yeah great question though okay so I don't really understand how very close binary star systems form as opposed to wide binaries but in any case there is a continuum for our sorry there are four gravitationally bound systems there are binary star systems and then there are star planet systems but there really are not star brown dwarf systems and maybe you were touching on that when I was thinking about my question but why why don't the brown dwarfs form and those bounces - yes so Deborah's referring to this thing that maybe some of you have heard of the sort of brown dwarf desert is that term that that has been heard before all right so what when we look at you know when when the planet hunters look for planets at this radial velocity technique that ever has been using should be able to find brown dwarfs really easy because brown dwarfs are massive and so they should really yank that star around very hard right so that should be easy to find and it turns out that they're not a lot of them in closed orbits around stars there there are a lot of binary star systems all right that are very closely separated and there we're now finding many many star planet systems very closely separated but there seems to be a gap in mass where if you got a number of planets and then very few brown dwarfs and then lots of stars so the question is why is that and I think people still debate that there is an argument there's a question of whether that you know one of the reasons one of the arguments for how you get a planet a jupiter-mass planet in too close to the star is that there's some kind of migration process that it might start out more like five au where Jupiter is but for some reason and migrates in towards the star and it may be that that migration process preferentially moves brown dwarfs right into the star right throws it right into the star incorporation I actually don't know the status of what that what the theory is explaining that that's the last service or if I understood about it I don't know if there's a real explanation for it at this point right yeah binary stars yeah well the binary stars formed out of their own separate collapse then you know one thing we see about very close binary stars is they tend to be equal mass right so they're they're preferentially seem to be about the same mass as opposed to a big massive star a little master right we see this in young systems we see this in old systems particularly in high mass systems so it may be that when you get to very close things that collapse out of the same cloud that the mass is sort of shared between them right one doesn't get everything they kind of split it evenly right there's a competitive accretion process between those so it'd be hard to form a brown dwarf in that kind of scenario because you know it would just be more massively fall into the brown dwarf it would just become a star whereas if you form something like a planet that collapses out of the disk there's not enough masks usually in the disks to actually form something as big as a brown dwarf which is another reason why worry about brown dwarfs being in close orbits so it may be that that's just a different formation process and you're seeing two different formation processes play them out putting themselves out with nothing in the middle so right here in front in the back could you give me a very quick status report on the atom burg Astor endowment is explan that well actually the recipient of the atom Burgas or endowed chair physics is probably going to have to give that up because he's about to get tenure that's insight is inside jokes oh sorry any more questions I'm hoping the answer to this is you don't know because that will mean I won't didn't miss something but where in the world would a brown dwarf come extra galaxies whizzing by to say hello I don't know thank you it's a good question I really don't know we've just we only realized we only found this object within last year and we're still trying to figure out where it came from so I'm a little fuzzy on the brown dwarf formation process could you just detail it a little bit one more time sure actually I don't think I talked about that much so it's it's understandable they would be fuzzy about it so there's there is a prop one of the big problems in browner West physics days how you actually make these things you know we talked about very hand wavy that they're probably formed like stars because you know we see stars we see brown dwarfs in the same cluster they seem to be in the same distribution on the sky so it kind of makes sense that they probably form something like stars it actually turns out to be very hard to make something as low mass as a brown dwarf when when that cloud of gas is collapsing down gravity is pushing in right collapsing things down but there's also thermal pressure in the gas that's trying to push out and you need to have enough mass or high enough density of gas to make that gas cloud collapse down and it turns out whatever your final mass is the lower that final mass is the denser you need your initial cloud to be okay so something like a usual molecular cloud forms a mass something like the Sun once two one solar mass pretty easily but something like a brown dwarf which is only 7% of a solar mass it actually can't do it it's not dense enough so you need to get much much denser clumps to make those things now the problem is if you have a dense clump there's lots of gas around so you make make a brown dwarf but then it just grabs more gas around it right kind of like we're talking about with the accretion in a binary system yeah well so that your name is a different question that's happening inside the core it I have to remember what my my reactions are it's probably going to form lithium it's combining with hydrogen so yeah so but let me let me follow up on the formation thing so so we actually have a problem on how to form brown dwarfs and there's a lot of sort of simulations are going on to try to actually make brown dwarfs and it turns out it looks like what's very important is that that things don't form from gas balls to one star right it's not in isolation there are dynamics there are things that are interacting there are things flying and there's gas coming in with different angular momentum and it's that dynamic process that may allow a very dense pocket of gas to form may a brown dwarf and then stop accreting and that's still an open question how to how that works there's many many theories actually on how to how to make brown dwarfs yes is there any data to support that the migrating brown dwarfs merge together and then can start burning hydrogen because they accumulate enough mass ah so if you have two brown dwarfs that kind of thing fine yet combined and it becomes a regular star is there any evidence for them so there's no there's really no direct evidence of that happening I would suspect that that's a very rare occurrence we very I don't I can't even think off the top my head of any evidence of two stars combining to make another one star or that being caught in the process and I may be wrong I don't know if anyone else's it's heard of that but so so so we don't have any evidence of that happening however that it's certainly a possibility and there was this great paper by Fred Adams back in 2002 where he looked at what the universe would look like what our galaxy would look like you know a hundred billion years from now when all of the stars that we see today have burned out have exceeded their their main-sequence lifetimes and what you would end up with are black holes neutron stars white dwarfs and brown dwarfs because they don't die because they're not burning anything right and and one of his arguments in that paper was that you know after gazillions and in zillions of years the only way to make a star because all the gases are dissipated is through the collision of brown dwarfs and you could do that frequently enough where you'd have something like 50 stars in the galaxy at any one time and they're just formed from brown dwarf collisions so so but that's very theoretical and it's not something we've seen yeah alright so I think that's the end of the question section we have just about run out of time so let's thank Adam again [Applause] and next up on the

Channel: Kavli Institute for Theoretical Physics

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Keywords: kavli institute for theoretical physics, kitp, ucsb, uc santa barbara, cold stars, brown dwarf stars, ucsd, physics lecture, astrophysics, astronomy

Id: 5FiRwoRnnLE

Length: 62min 57sec (3777 seconds)

Published: Tue Dec 26 2017