Three Puzzles in Star and Planet Formation

[Music] [Applause] [Music] [Applause] [Music] everybody welcome to today's CFA colloquium I'm very happy that Caitlyn credit has come back to visit us many of us remember Caitlyn finally of course from when she was an ICC postdoc here from 2010 to 2012 although it seems so much more but anyway Caitlyn you know I could summarize her research by basically saying if it's if it's like a disc and it could make planets or stars or break apart Caitlyn would like to figure out why and how okay that's pretty much my simplest summary of Caitlyn's research and the reason I have to give such an overall summary is because she's worked on many different forms of that problem and today I think she's only going to talk about three of them but before before I let her start I should tell you that she's written a really wonderful annual reviews article very recently called gravitational instabilities and circumstellar disks which is a more eloquent way to say what I just said and that she went to Barnard undergraduate and I first met you when you were in Toronto with Chris Matzner and then she's worked with lots of other people in addition to Chris even when she was a graduate student you won the Plaskett Prize didn't you yeah she won the prize for the best thesis in Canada essentially um and then she was an ICT postdoc here and then she went to Jilla as Hubble Fellow and then she went to Arizona where she's now on the faculty doing wonderful things that she's about to tell us about so thank you very much all right Alyssa thank you for that wonderful introduction um how's the volume I'm okay I'm not screaming at you too much they promised me they would turn it down up there if I was screaming because I get really excited so as always I said I'm gonna talk to you about a few different problems today and and I will make a disclaimer upfront that well I hope to have a cheerful demeanor I'm kind of gonna be a little bit of a downer today in the sense that when I say three puzzles I kind of mean I'm going to talk about three things where we found some problems but we haven't quite got the solution yet so you can think of this as an inspiration for some of the students to you know do better than we have done and come up with some answers and so maybe in a couple years I can come back again and tell you about three answers to long-standing puzzles in star and planet formation so the three things I want to focus on today are represented by these images and while this is going to be a theoretical talk I will try to keep you entertained with some nice pictures along the way I'm gonna start out and spend a fair bit of time talking about the puzzle of close binary formation I alluded to this a little bit in my ITC lunch talk but I'll talk about what we do know about binary information and where we have some really open questions I'll then move on to talk a little bit about planet disk interaction and in particular I'm going to focus on the puzzle that is these asymmetric features that we keep finding in protostellar discs it seems every time we upgrade our resolution abilities with Alma we find more and more things that look messy and you know not like nice smooth power laws or Exponential's in the last part of my talk I'm gonna focus on a local binary analog that is the Pluto Charon system hopefully you've had a chance to talk to your local expert here on this Scot Kenyon he's working on solving some of the problems which I haven't haven't been able to so come back again in a couple years and he can tell you the answers okay so the first part binary formation so why should you care about binary formation I'd say there there are many many reasons the first of which you know it keeps being relevant to say this it's like right so LIGO keeps finding these massive black hole black hole binaries and for some fraction of the theory community their existence has been a puzzle but what they should really indicate to you is that models of close binary formation are poorly understood across the entire stellar-mass range if we want to understand the origin of these systems and of the new on star neutron star binaries that LIGO was built to find if they exist we need to understand binary formation globally if you don't care about gravitational waves and LIGO and you just care about the numbers well 50% of solar-type stars are in binary systems and it's more for higher math stars now I know Charlie might tell you well everything's an M dwarf you know maybe we shouldn't care but I still think the the rest of the stellar mass function is interesting this is a really nice plot from a great summary paper of binary statistics that was written by a former graduate student here max Moe and Roseanne - Stefano that just came out this year if you want to know anything about binaries this is really the best resource on it by far and what they're showing years that is a function of primary mass the multiplicity for mass ratios above zero point one is just this strong climbing function of mass when this number gets above 1 what that means is on average a star will have more than one companion and so during the rest of this talk I will be talking about both binaries and in many cases triple star systems where there's a whole lot more fun things that can happen if you're a planet person of course binaries impact planet formation the fact that binaries are not an unusual feature for stars that host planets means that you can't just throw them out of your planet formation models and if there are any explosion types in the audience of course type 1a supernova rates are a strong function of our understanding of binary stellar evolution and that's something else that this this this great guy max Moe has been working on so keep an eye out for some really fantastic papers from him ok so what do we know about binary information here's what I'm going to argue that we've actually done a pretty good job and solved some things so I would argue that there are two dominant modes of and I will admit here wide binary formation fill them into a core fragmentation and disk fragmentation so the I the idea behind filament or core fragmentation is that on very large scales within the giant molecular cloud you have these turbulent structures and sometimes on a scale of you know a thousand or maybe 10,000 au instead of getting just one over density you get to over densities maybe even three or four that are relatively close together they're still gravitationally bound and they will in spiral to form a relatively wide binary that's what's illustrated in these cartoons over here and the second mode is this idea of disk fragmentation which seems to work better for higher mass stars and in this case we just have to be a little bit more patient we wait for one star to form first and in some cases if the in fall rates on to that proto stellar disk are sufficiently high you can build up enough mass in that protostellar disk to become gravitationally unstable and form a secondary mass and because of the scales that are involved the mass in the disk the temperatures in the disc and the in fall from the background cloud these objects tend to form stellar mass or at least brown dwarf mass objects not planets so this is a really good way we think to make massive stellar binaries and Anna Rosen who's a new arrival here to the ITC has done some nice work on on these disks as well so you should talk to her about that so lest you think I am just a you know hopeful theorist what I think has been really exciting in the last couple years is that we actually have observational evidence for some of these things so this is a really beautiful result that came out in nature a couple of years ago with kymia pinata and still oftener to others who have CFA backgrounds where they use jvla observations to find a gravitationally bound young quadruple system in the Perseus molecular cloud and so the structures that we see and the masses that we see and the separations that we see are all really consistent with this picture of large-scale fragmentation into a multiple system now I thought this was great but I was admittedly even more excited when just last year John Tobin and I and a whole bunch of other collaborators saw this amazing image in our dataset and idomeneus cuz I'm really close it doesn't look let's bill looks pretty good up here so I thought if and I just like fell off my chair this to me looks like the smoking gun of a fragmenting gravitationally unstable disc or you have two relatively massive objects and here these each have dynamically estimated massive about a half to one solar mass you've got this maybe one or two on spiral structure and then this bright blob out here which looks like it's dominating the mass budget is really just big is just bright because it is colder and more diffused and probably a recently born fragment I can I can help your eye if you want to see the theorist picture of this let me rotate it for you now you can see that we you know predicted is exactly the structure uh you know six years before its detection now this was from a set of very simplified models so there's no reason to expect this kind of morphological agreement other than the general statement that when you get this instability you tend to form these kind of systems relatively frequently I don't know what happened to this guy in the image he got lost somehow so to be fair we've done better than just you know comparing pretty pictures we actually made a relatively simple one-dimensional model that showed that if we do our best estimate at what the surface density profile and the temperature profile and the rotation profile is in this disk it does in fact agree with simple models for gravitational instability what I'm showing here is just the radius in the disk and this is an azimuthal average which if you looked at that previous image you might be somewhat skeptical of and then each of these curves represent sort of standard quantities you might want to calculate the tumor is Q parameter the surface density or the temperature or how quickly it cools and again all of them basically give you the same result that we should expect this disc to the unstable fragmenting and making massive objects this is just one of the moment maps that shows you the rotation where you can clearly see that the center of mass is around these two objects here and this bright thing really is a low-mass object so those are two sort of taste studies what we now can say wide binary formation seems to happen this fragmentation seems to happen but we have some other population-wide evidence as well that there might be these two months the first one I will emphasize that I have underlined and starred this word hint so before the observers get all up in arms about these error bars this is not conclusive but I think it's kind of interesting to see that if we look at very young class zero and class one objects at higher resolution that had done before this is again work led by John Tobin that we can resolve binaries and we see a bunch of binaries on these scales of ten thousand au and down to a thousand au and then we see this other little bin pop up right around one hundred of you where we might expect to see disk fragmentation and there appears to be a deficit in between in the very youngest sources now again the error bars are big the numbers are small time will tell if this trend bares itself out but I think it's an interesting demonstration that at the very early time for you would expect to see this imprint of small-scale and large-scale maybe it's there there's more evidence though that I think is is somewhat more conclusive coming at later times and this is again from this really nice work by Maximo and Roseanne where when they have done a careful job looking at binary statistics and going beyond just how many stars have binary as a function of mass but trying to understand how does the mass ratio distribution scale with separation how does the mass ratio distribution depend on primary mass they found something really interesting so if we just look at this bottom panel here what this slope is telling you is about the mass ratio distribution of different type stars as a function of separation and what this line down here shows you minus two point three five is the mass ratio distribution you would expect for binary stars if it was by random pairing from the IMF so if I make binaries by just drawing two stars from my IMF bag and throwing them together this should be here the whole time instead what we see is that it closer separations the slope is very very different from that it's much closer to a uniform distribution in mass and at very large separations we start to approach this random pairing from the IMF this is exactly what we would expect from these two different modes joining together then at small scales where the objects are maybe sharing a mass reservoir in a disk we should expect that the star knows about its hanyan the mass because they can share material when we go to these very large separations or things are just forming from two random turbulent fluctuations in the same filamentary structure we shouldn't expect the process to be substantially different than single star formation drawing from the IMF so again this is not conclusive you know we certainly don't see two peaks solar-type stars seem to behave a little bit differently from all the rest of the stars in the sample there's a lot of mysteries still here but I think it's starting to push towards an answer okay so now here's where the problem comes in I just told you a nice story about how great I think we've solved binary formation but of course I was only talking about wide binary formation this core fragmentation picture maybe works on scales of thousands to ten thousand au this fragmentation is hard pressed to go to scales much less than 30 au more like hundred au scales and then there are these guys which are admittedly rare maybe only two percent of the overall binary population for solar-type stars but they're on a very very different scale orders of magnitude smaller and we need to find a way to take these primordial fragmentation models and get things all the way down to these very close separations where they can then go on to seed all sorts of interesting Astrophysical phenomena so you could think of maybe handful of ways that you might make this so maybe I've just lied to you and ignored the Institut formation mechanisms we could have a hydrodynamic migration picture or maybe a dynamic migration picture and what I would argue to you that is that the Institut picture is pretty well ruled out at this point one easy way to understand that is it is that if you calculate the contraction of the initial protostar Halle Larson 1969 you all maybe remember from a star formation class that there's that phase where you form that first hydrostatic core well a lot of these binaries live inside that first hydrostatic core so it's pretty hard to figure out how you could get fragmentation inside that structure and there's been a lot of work on this over the years to suggest that it's really unlikely for hydrodynamic migration you could imagine a couple different ways to do this you could either imagine that you have two stars in the same disk that are interacting and exchanging energy and angular momentum with the gas and migrating inwards or you could imagine you have two separate discs each with their you know own host star that also interact and somehow dissipate energy and anchor momentum and I'll talk a little bit about both of those in a minute and then finally dynamic migration what I mean here are things that don't involve gas so things that can involve either violent instabilities between three or more stars or perhaps something that's a little bit more long time scale like a cause I live of oscillation where eccentricity X and excitation plus tides can bring things in to very very close separations so let's look at each of these pictures in a little bit more detail so we've done a fair bit of work now on looking at how either disk disk interactions might evolve binary orbits this is also work that was led by a former CFA graduate student Diego muñoz or how binaries can evolve due to interactions with their external circumvent disk and while there's a lot more work to be done on this the short answer is that it's very difficult to remove quite enough energy and angular momentum just by interaction with the disk because the disk is a finite mass and angular momentum reservoir one of the more popular models is this dynamical model of Cozine lid of oscillations plus tides and you've probably had colloquy about this before so I'm not gonna go into the details of how this mechanism works but the basic picture is you have a triple star system that is hierarchical the four under certain conditions for sufficiently high relative inclinations between the orbits the eccentricity of the inner orbit can get excited and then if it gets excited to have a small enough peri Center tithes can begin to circularize the orbit and bring in the star two periods of only a few days and this has been for the last decade or so the standard model for how we make close binaries and it's a very very compelling model there was this really paper by den father King Scott remain in 2007 where they showed that if you started with this initial distribution of inner binaries and they picked sensible distributions for the tertiary companion in each of these cases that they could shift these wide binaries quote wide binaries from periods of you know tens to hundreds of days down into this peak where tidal circularization has created these very short orbits now the problem with this is that their initial distribution kind of cheated in the sense that they already made a lot of closed binaries so if I draw where those other two mechanisms I talked about operate there all the way out here off the edge of the plot this is where we think the binaries start so starting with a bunch of objects filling in here it's not clear we've solved the whole problem we have to make this sub population before we can do this step there's another problem that only became I think a parent very recently and that's the time scale problem so in previous models because this cause eyelid off oscillation is a secular time scale instability where it means that you have to wait many many orbits to excite eccentricities and then many many orbits for tidal circularization to happen we expect this process to take Giga years and most previous models allowed their simulations to run for Giga years but here again máximo and Roseanne came to the rescue because when they went back and they did a really careful analysis of the close binary fraction they found it by five million years all the close binaries were basically there at least 50 percent of the close binaries already exist after five million years so if you're five if you're close binary formation method takes a Giga here I think you're how to lock the fact that we have circumbinary planet also is a hint because we know that planet-forming disks only live for a few million years so if you have a 10-day orbit and you've got a disk that forms planets around it you have to get those binaries onto the tenant and they orbit within about a million years now there's some good news here and this is what Max and I have been looking at in this recent paper first of all if you try to make a young star is undergo closed eyelid of oscillations and have very high eccentricity orbits with very short peri centers you get a little bit of a bonus for two reasons one pre main sequence stars are big they have radii of several stellar radii so you get a boost in how efficient your tides are secondly if you start out with your binaries on wide orbits all the way out here then the only systems that can undergo tidal circularization are ones which reach eccentricities of like 0.99 and what that means is that the standard tidal models that people use the so called equilibrium tide models are a little iffy because as you will remember right tidal forces scale very strongly with separation so almost all of the title evolution happens at peri Center and almost nothing happens the rest of the orbit and so what we think might be a better model in this case is something called a dynamical tide which is basically like stellar capture so if you remember the old models of taking two stars that are unbound and throwing them at each other and occasionally they would they would get captured that's kind of the picture you might want to have in your head for the formation of clothes binaries on the pre main sequence so what this plot here is showing is that in the standard bottle it takes Giga years to undergo these eccentricity oscillations that eventually leads to tidal circularization if you start out with an extremely eccentric system with big stellar radii and you use this dynamical tide model you can get very very rapid shrinkage within less than a mega year now for the experts in the room you might be thinking gosh it seems like a bad idea to combine dynamical models with secular models you're probably right I don't know how to do it any better and so this is something that needs a little bit more work but nevertheless we think that there's something interesting and worth exploring here so what about that other Dyne Chemical model this this seems to be maybe somewhat promising we should also consider the instability model and now again I'm going back to a movie that I made when I was a postdoc here because I think it's a nice illustration of what instability looks like so these are just three stars that have gotten too close together and their orbits are unstable you can see they undergo many Close Encounters and you might think ah well this solves the problem I can just take wide triple systems kick one guy out and make a nice close by area if you take the time to do a simple order of magnitude calculation of course what you will find is that as messy and violent as that looked to go from the typical separations we think we start out at down to these 10-day orbital periods you need to remove basically 99% of the orbital energy and if you take typical mass ratio distributions and you take three stars and you kick one guy out you remove about half of the energy and that's simply not enough so it's very difficult on its own for this kind of mechanism to create a bunch of close binaries if everything starts out wide and so we think maybe what you need is a combination and so what I'm showing here is a plot from our paper showing where we started to do a series of sort of population calculations where we said okay what if I start out with reasonable initial conditions based on formation models and then try to make some close binaries so this is period in days this is the companion' frequency per log P this is the overall expected distribution you should get out at the end of the day we're interested in this 2% closed binary fraction with periods less than 10 days and we say okay we can imagine starting out with populations that look like this for the inner companion in the binary and then the outer companion assuming that say these may be forming disks and these form in cores and when we evolve all of these models and consider both cozy little oscillations plus dynamical tides when we consider what happens for systems that begin on dynamically unstable orbits we can't quite fit the data we can't quite make enough close binaries so here again you see the initial distributions for the triples you can see in red and blue the systems that were progenitors over here and the final systems over here so red systems here show the inner progenitor and the outer progenitor that make this distribution over here in five million years and the blue line shows you what you get after five beginning years of evolution and of course what we need to get to up here is about two percent so evolving for longer times helps you a little bit but at the end of the day we're only recovering maybe half of the close binaries from this combination of models and so our conclusions are that we probably need a combination of migration mechanisms so we need to somehow figure out how these are these kind of hydrodynamic mechanisms can operate in concert with these dynamical mechanisms to give us the full complement of binary systems that we need and this is like I said still kind of an unsolved problem we're gonna need to do a lot more work to figure out exactly how much of the process we can accomplish at this stage to see what we get is our initial conditions for cosine and dynamical instability so this is this is ongoing work I'll say that I expect we'll have some very good data to help us in the coming years because Gaea is going to give us an unprecedented Lee large sample of perfect constrained orbits for a whole bunch of triple systems in the early to mid 2020s so we'll have an answer but we may have to wait another five years okay I'm gonna move on now to another puzzle which I alluded to at the beginning and that's disk asymmetries so this is a beautiful compilation of Alma data from the inca van Romero published in 2016 and you see just these array of interesting features I mean we've really come a long way in our observations of disks and now you can basically find any kind of shape you might imagine what I want to focus on are these very lopsided images here where it looks like we have a whole bunch of flux coming out one side of the disc and nothing coming out on the other so there's been a popular explanation for these systems which is the Roz B wave instability so for the hydrogen emesis in the room you can trigger the Roz B wave instability when you have an extremum in an in your inverse for tensity for 10 cities just vorticity divided by density so that's the formal definition if you are imagining that you're having this instability arise in a capillary and protostellar disc what you need is a density bump that's a little bit easier to think about so if you have a very sharp density bump in your nice smooth capillary and disk you can generate this instability and what this instability does is it causes the gap edge to start to break up and I'll show you a movie of this in a second and you get these vortices that merge into this one big honkin lopsided picture that is a sinc single M equals one vortex so here's what it looks like these are simulations run in the 2d hydrodynamics code Fargo these snapshots are taken once per orbit how do you get that density bump that we need you put in a planet this is a very popular thing to do again in discs right if you see a structure in a dish you wouldn't it be fun if it told you there was a planet there and this is the game we all like to play and you can see it does a very nice job of creating that asymmetry that is now orbiting with the outer disk edge it's nice and bright this looks great right we have explained this nice crazy-looking feature and it means that every time we in it see an asymmetry we've got a 5 jupiter-mass planet that would be fantastic right lots of fun everyone likes planets ok as I told you at the outset I'm not here to be a cheerleader there's a problem I'm gonna show you the same movie but now I've done something different I've taken two into account the fact that if I want to make up Jupiter or a5 jupiter-mass planet that takes time right they don't form instantaneously in fact they take thousands or tens of thousands or millions of orbits to form so here's what happens if I do that now we're moving through time a little bit faster you can see that the instability is still generated but it looks very very different now I should point out sorry I forgot to mention this is the dust on the right so we've done ah gas and dust simulations as well and so what you're seeing here are dust grains that are gathering in these traps and of course what you're observing with Alma is the dust I'll talk a little bit more about this in a second so what you see here though is in this case where we've allowed the planet to grow at a more realistic albeit still pretty fast rate because there are some computational limitations here you get something it looks very very different at the end of the day it's elongated it occupies more than half the disk it just doesn't have that same characteristic that we saw in some of those early images and again we think this is probably the more realistic case so you can see again here this is the dust on the right and and it's occupying more than half of the axis and it's in this very confined region ok so fast growth which is what we saw we needed in that first picture is really fundamentally inconsistent with vortex formation in the forest first place so you know maybe it doesn't seem like a crazy thing to say oh well of course it takes planets a little while to form they don't form instantly but it's even worse than that because there's another requirement for this instability to arise and that's that the disk has to have a very low effective viscosity or in other words it needs to have very little turbulence be very very laminar and the reason for that is that in order to trigger the instability in the first place you need that dancing it about to be really really steep right you need it to be really fun and effective viscosities turbulence what does that do it's smooth things out so to have the right initial conditions in the first place I need low turbulence low viscosity nice smooth laminar flow well if we take that limit for the disk we can calculate in it at least a simple steady-state accretion model what the mass flux rate is through that disk and the less turbulent I make my disk the slower I expect it to accrete so if I calculate the accretion rate I would expect in a disk that has low enough turbulence or viscosity to allow these instability to rise in the first place I find that it should take me a million orbits to grow a jupiter-mass planet so the simulations I'm going to show you now we don't approach a million orbits because we can't run a simulation for that long but the trend isn't good so what we're showing here is growth time in planet orbits versus the lifetime of the vortex also measured in planet orbits up here on the top I'm showing that for different distances measured in AU where a growth time of ten to the five years corresponds just to give you some reference so it depends obviously on the orbit on the semi-major axis because that's how you convert between orbital periods and physical timescales what you can see is that for different effective viscosities and different math cases as you approach longer growth times the vortex gets weaker and the life time declines precipitously the reason there aren't a full sampling of points here isn't because we had limited computational resources in fact it's because when you go to much longer growth times especially for this case the vortex never forms at all and again when it does it is weaker and has a very very different character so this is showing it to you in the gas I showed you the movies before I just want to emphasize that when we do simulations now we can't just look at the gas we have to look at the dust and dust doesn't behave identically to gas because it's a pressure less fluid so in these simulations we've now gone back and we've put in dust this is with a code written by Xiao Han Xu and so you put in a secondary fluid which doesn't experience pressure we include gas drag because of course the gas rotates in a slightly sub capillary velocity because there's a pressure gradient which counteracts the force of gravity and so dust particles move at the capillary and velocity or they try to so there's this headwind and that causes them to experience a drag force and that changes how they move in the disk and we also include a diffusion term and so if we take these models for how the dust behaves and we run them in a wide range of wavelengths we can then convulsion to see what these simulations would actually look like and on the left side I'm showing you one of the vortices that we would expect for these quickly going unrealistic planets and on the right side I'm showing you what we would expect for these slowly growing planets still in the optimistic case where it only took a thousand orbits to grow which is still probably too short anyway and what you can see is that the picture is very different so this is just a warning for you know if you want to fit one of these very concentrated asymmetries with some planet model it really matters how the planet grows in the disk because it strongly affects the type of instability that you trigger so I'm not yet convinced that even these are realistic explanations I have never left nevertheless put on the other on the on this slide two examples that might be more consistent with this slowly growing planet case these things that occupy more than 50 percent of the disk azimuth but I'll just conclude this part of the talk by saying that I think we have to be cautious when we use these numerical models to try to reproduce the simulations first of all because numerical constraints push us to considering kind of crazy unphysical things like Jupiter's that grow in ten orbits again you can think of this as taking your nice smooth disc taking a hammer and whacking it right it shouldn't be a big surprise that you get an answer that's a little bit funny for some of the numerical experts in the room I'll tell you the problem is a little bit worse than that it's not just that you might worry that that's a little unphysical there are also other very strange things that happen so if you change the gravitational softening length in your disk in one of these fast growing cases the lifetime of the vortex changes by a factor of seven ah it doesn't change monotonically as you go from one side of the softening length to the other if you pick some fixed value so there's all sorts of funny things that arise when you basically hammer your disk with a rapidly growing planet I should also point out that you might be concerned about these simulations because they're isothermal we don't include this a a crucial heating that the planet might generate so this might counteract some of the effects that we found and increase the amplitude of that density bump for a slowly growing planet so it's worth considering in future models how that might change things but nevertheless I will just end by this part of the talk by saying we need to be very cautious in interpreting these asymmetries I'm not convinced we have a good answer yet okay so that's the second part of my talk and now to be totally schizophrenic let's jump back into the solar system and talk about the Pluto Charon system I sort of call this part of the talk of an excuse to show New Horizons data because I think it's absolutely phenomenal I was really excited when this started to come out I think this has just been really really amazing there's been some some fantastic work you know we got to write a paper with the word cryovolcanism in it I thought that was pretty fun um but but let me just remind you of some of the reasons why this mission was phenomenal you know I think in this day and age sometimes we can get complacent about space missions and space travel so I just thought I like to show the sort of trajectory of the of New Horizons have it passed by the Pluto Charon system so here's Pluto and Charon as you might recall it has four circumbinary moons Styx Kerberos Nix and Hydra and they have periods of so six days for Charon Styx is 20 Nix is 24 cameras is 32 Hydra is 38 and new horizons traveled after you know seven years they're going through the solar system in between all of these objects they sort of threaded this needle and you know they had all these practices to sort of make sure they could adjust their trajectories at the last minute and they told me that it was you know within a few seconds of their predicted arrival times at each of these different places so it's a really phenomenal engineering task I think just just really really cool stuff and obviously the images that have come back are amazing well let's talk a little bit about some of the physics of this system so this is looking at it face on this is a pre New Horizons image with HST where you can again see Pluto and Charon and then the four circumbinary moons now if you want to understand how these things form again you should really talk to Scott here because he's done some of the most sophisticated work on this to date it's been a real long-standing problem what we do think we understand is how the inner big binary formed itself we think that the origin of that binary is the giant impact scenario not so dissimilar from the earth moon system we think that to partially differentiated bodies smashed into each other and the remnant of that created Charon one of the neat things about it is that this model predicts that the material that gets fluffed off after this collision should be pretty icy and if we think the moon's formed out of that remnant material they should also be icy and actually when I was at the CFA we wrote a paper that suggested that the masses of those moons were consistent with icy albedo's and and both follow-up work with HST and New Horizons has actually confirmed that that our dynamical mass predictions for these things were right it doesn't mean we've explained how they formed but we at least think we know what they are so we think that the binary itself formed this way initially on a much closer orbit than it is today probably eccentric from the collision and then it tidally evolved outwards to become tidally locked on a six-day almost perfectly or perfectly circular orbit to the best of our ability to measure it so why is this a problem why is it hard to form these moons out of that remnant material so if we look at where the moons are today we run into this problem of basic stability now for those of you who are at my ITC lunch talk you might remember I talked about one of the fun things about planets in binary systems isn't that you take a binary and you try to put a planet around it if you put it too close that time dependent potential those strong kicks will ultimately make the planet leave the system you can't make a nice stable capillary in orbit if you put the planet too close to the binary what is too close mean somewhere between the three to one and the four to one period ratio with the binary so because Charon and Pluto had their orbital ball and started out much more eccentric we can ask where were orbits stable around the binary in the past so on the x axis here we have time in years this black line shows sharon's evolving semi-major axis and this shows its evolving outer radius for different eccentricity models so most of the collisional models suggest that for different orbital eccentricities it had excursions out to this is in units of ten to the three kilometers so 40 or 50 times ten to the three kilometers so this is the evolution and website that's the black line down here and what these color curves are showing is where the stability boundary would have been for Styx Nix Kerberos and Hydra and so the fact that these blue lines cross these pink lines show that for most of the extensions ex eccentric evolution models for Charon all of the moons except for maybe Hydra at some point should have been driven unstable by the tidal evolution of the pluto Charon system so it's very difficult to see how they could form where they are today or even closer in without being destabilized by the eccentric orbital evolution of the inner system so again talk to Scott who's trying to figure out how to solve this part of problem I'm gonna do something a little bit easier I'm gonna start from the hypothesis that okay we have this it threw off the material it landed in the debris disc and then we can do the simple thing of modeling how that disc and debris evolved Wow Pluto Charon evolved I'm not gonna try to form these exact moons I'm just gonna ask a simpler question which is where should this debris have gone both in the Pluto Charon system and in the rest of the solar system so what we think based on some simple physical calculations is that most of the materials should be ejected some of it will just go unstable by coming too close to the binary some of it might collide with one of the two objects but we think that should be a small fraction now just because it's a small fraction doesn't mean it's uninteresting and again we can show with some very simple physics that very little little of it will collide with Pluto because it's the more massive body in the system but some of it should collide with Sharon now the reason this is interesting is it what do we do with bodies in the solar system that have craters right we count them exactly you count them and you measure them and then what do you do with those numbers ages right so people have done that the other thing you do is you infer something about the population of stuff that hit it right and so um what you may not be familiar with it was one of the big goals of New Horizons was to look at the surfaces of Pluto and Charon which were thought to be 4 Giga years old they were thought to be totally inactive dead bodies okay we knew that there was atmospheres but they were thought to be pretty inactive and dead and then we were just gonna go count the craters measure their sizes and back out the size distribution of the Kuiper belt and that's really cool right if you learn about the size distribution of the Kuiper belt you learn about the most primordial bodies in the solar system you can say something about plant information about planetesimals formation so that's great but if you had this disk of material that's contaminating your signal you might have a problem so we wanted to ask how contaminated should that signal be so we ran a series of simulations four different Pluto Charon evolution models where we just track the evolution of the debris so you can see here a face on picture of the system where these are again all test particles they're not interacting with each other and they don't have any mass but what we can do at a later time is go back and convolve this model with any distribution for the debris that you like this is the edge on projection here where you can see that the objects are being scattered out to large radius if we zoom out even more to the hill radius we can see many of the bodies being ejected into the solar system this is a histogram showing where the bodies are being lost from as a function of distance from the center of mass of the Pluto Charon binary these pink lines indicate the locations of the current moons in the system so it's not obvious for example that there's a lot a huge amount of stuff at them it turns out that all of the moons live very near mean motion resonances with the Pluto Charon binary so they're basically in this almost resonant chain of four to five four to one five to one and three two one four to one five one six to one got that right yeah and and and of course none of them are actually in the residence which is another puzzle that nobody has has totally figured out yet but you can see over the course of the simulation we start to carve some interesting structures in the disk and again we haven't tried to make the actual moons yet we're just trying to see where the stuff goes so what can we learn from studying these collisions and ejections so for a given debris size distribution we can predict again the number of craters that have a disc origin which allows us to constrain what else might be coming from the Kuiper belt and we can also look at these bodies that were ejected which was the majority of them and see whether or not they stay in the solar system so first let's look at the things that collide is a function of disk properties so on the y axis here we have the particle size distribution that's the the the index Q on the x-axis we have the disk surface density distribution for reference for protoplanetary disks you expect say this kind of surface density distribution for probe the proto lunar disk the model is for something a little bit steeper and the nominal expectation for the Kuiper belt would be right here and so what the 2d histogram underneath shows is how many colliders we would expect with a radius bigger than 300 meters the reason we've chosen 300 meters is because that's what we expect to create craters large enough to be observable by new horizons on the surface that calculation is non-trivial because it depends a little bit on the surface properties of the thing that you're hitting so you need to know kind of the properties of really cold ice in the outer solar system to get this right and so you can see that depending on exactly the type of properties with the disk these two different panels show you what happens when you allow Charon to migrate outwards and this is when you put it it basically its initial step its current separation but with a little bit of eccentricity you get anywhere from a few tens to a few hundred thousand objects that might collide with the disk we expect for reasonable parameters that the number is maybe something around hundreds to thousands of craters in principle could have been produced on the surface from this debris disk what about those ejected bodies so this is showing the solar system scale of the picture where we have semi-major axis in the solar system on the x axis and for each of these different sub panels we have cases where we have taken the Pluto Charon system taking the bodies that we know are lost from it and we've injected them at different points we say okay I know where things are when they leave the hill radius of Pluto Charon now I'm gonna take the Pluto system pop it into the outer solar system including Jupiter and Uranus and Neptune I can include the migration of Neptune outwards which we thinks we think sweeps Pluto and the rest of the resonant Kuiper belt objects outward with it and I can ask where does this stuff go does it get ejected from the solar system does it collide with Jupiter and the answer is yes both of those things happen but the more interesting things that happens is that it gets caught as you might expect in mean motion resonances with Neptune just like Pluto itself because when things get ejected from the hill radius of the Pluto Charon system the velocities are relatively low compared to the orbital velocity of the Pluto Charon system and so it has chemical dynamic similar dynamical features to the Pluto Charon system itself on the left side I'm showing it the case where we align the ejections so the disk of the the debris disc with the Pluto Charon heliocentric orbit and this is the case where it's misaligned you may know that the entire system is actually kind of tilted on its side and almost 90 degrees to its own orbit also funny you know I'm not sure there's a really great explanation for that yet either but in both cases you see that many of these objects are trapped in the mean ocean resonances and the ones that I'm particularly interested in are these ones that are in the 3 to 2 along with Pluto and what this pink color bar here is showing you is that the the ratio of the ejection velocity to the escape velocity and the fact that these are all the same color tells you that they have low relative velocities with the Pluto Charon system why should you care this means that we might have a collisional family kind of like how man now these are probably small bodies it's not clear how small but what this suggests is that if you had a massive disk of material that formed as part of the Pluto Charon forming impact there should be evidence of it trapped in the 3 to 2 along with Pluto that if you have big enough telescope or something in space and somebody's willing to give you the time to look for it there might well be a collisional family of other objects and what this could do is give us more constraints on the properties of that disk if we could look at the distribution of albedo's the distribution of sizes we might get some more hints as to how the Pluto Sharon system formed so I'll wrap up this part by just saying that the conclusions that we've reached are that even though there's a possibility that some small number of craters on Sharon's surface could have arisen from this debris disc so far we haven't really found any so the crater countering experts have gone and looked at the surfaces and as you may know from news articles on the the new horizon system there was another issue with the two bodies which is that their services were not really primordial there was lots of evidence of some geologic activity less so on Sharon than on Pluto when they looked they haven't found a bunch of small craters this is really cool for Kuiper belt object models because it tells us that we think that the initial population of planetesimals was big and it also maybe tells us that there was an upper limit on the amount of mass in this primordial Pluto Charon disk that could have contributed to a population of has yet undetected small craters I also mentioned that I think we might be able to find a Pluto Charon collisional family from this ejecta this has not yet begun I think LSS team might be able to do it I think it's gonna be a little bit tricky depending on the observing cadence so maybe we have to wait for a JWST or maybe even GMT I'm not sure so I will finish up with a little bit more gratuitous advertising so I want to point out that we're organizing another conference on star and planet formation in Arizona this March we had one in 2015 it was a lot of fun it's going to be at the biosphere 2 Center yes that biosphere not the bio dome that was the bad polish or movie but it's a really cool venue for conferences it's a small group we strongly encourage submissions from graduate students and postdocs we have subsidized the registration fees everybody stays together and houses it's great as you can see from the background slides Arizona is beautiful that time of year it is cold and miserable in Boston I lived here I know don't try to tell me otherwise so come out and see some nice wildflowers and water and greenery and then lastly I'll just say we are also having a steward theory prize fellowship postdoc this year so if you have excellent graduate students or you are an excellent graduate student or postdoc and you're interested in coming to Arizona we have theorists working on all manner of topics from star formation plant information black holes gr supernova whoo I'm missing some galaxies evolution without a whole lot of cool stuff going on and there's a lot of observers to talk to as well it's a big group one of the biggest groups outside the CFA in the country I would say so come check us out all right I will thank you for your attention and take any questions density in the places where these binaries are forming not highly enough to get a lot of random binary binary interactions you gotta go to globular clusters even the core of Orion which is like kind of the four stars for the cubic parsec that sort of marginal rating expects to get a lot of close interactions they should be number four the typical star forming cluster this is similar to Scott Adams did a few years ago is that you probably expect one encounter within a few hundred eight of you so it's hard to get a lot of help in hardening binaries the way you wouldn't apply me to a cluster professor Turner yeah he's been going to that you know that you know you know in globular as you do have title capture can't you have you know doubles and triples doing all kinds of things to each other forming systems that we see today is special systems so I was kind of hoping that there might be an analog yeah I think it's not average the cellar densities are just too low even early on I mean they get less dense with time there sure right as the gas goes away the clusters become unbound but you still don't end up there blonde another buck your density team generally compared to the start story regions in the in the realistic you probably give you a better number on that than I can okay so not not very high maturity okay things that are already bound in high multiplicity you're going to do a lot better shouldn't new horizons have seen your debrief article - no probably not right you'd have to mean they're not going to necessarily be next door right there little man there will have to be it's in the same orbit it should have a relatively low relative velocity but it doesn't have to be adjacent to it's extremely lucky if it could be smoothly people had a lot of time would even agency to make sure that New Horizons wasn't gonna do anything because that would be crazy right it's like a meteor sized body you know it's moving it you know bullet speeds and we're just totally obliterated the mission so they had this whole thing when they turn on the camera just before they went in and you know made sure that they're gonna be there but they had done all these real actually where they like sequester to all the new horizon scenes in these rooms for like 72 hours and then they stimulated how they would be happy to separate if they didn't seem to breathe I was a part of that [Music] okay can you put one of those gloopy five pictures back up yeah so this B five system it shows you a few people here my favorite we did this paper there's another vehicle development yes I'm going to talk about why not anyway there's this huge doesn't matter to me no it's not remember this acute element okay that goes to parsec scale that I'm really connects the whole thing and as you know in this paper it says a digit time scale only like forty thousand meters okay for this little fragmentation or whatever you want to call it that happen so we actually don't think that it's really fragmentation that there was ever like up to LA determinate but then instead it's more like cell is picture which we've had a few slides yeah where there's all this filament you think the rap and that the reason that the large-scale filament still looks like the small-scale things because it just hasn't had time in the chain so my question is how far could that go but if you have that kind of filament reads kind of almost appreciate stream stuff going into the scales of the next picture where you know you want some perturbation but you sort of go to the flames I Hannibal forget about the gas but what if there actually were still these kind of gas streams a much finer scale for this yeah so we've looked at that we've done simulations of the disk phase where we start with turbulent initial conditions and you have sort of elementary structures feeding the disk and and I'll show that some nice pictures of that in our talk earlier today and what you see is that the main effect is it you end up shifting the angular momentum vector of the disk over time so it can Bob a little bit or you can make the disks a little bit smaller so it it might in some ways and give it instability if you make this too small but then contribute it because it's kind of hammering hard on one side rather than the other so it had some of it but I don't think it changes the qualitative picture that you rain material on quickly enough so you can still cause the presentation to happen right well we were very surprised to have those structures yes that's true a whole other story right so I just I just what I'm thinking is that there's something even more structure hidden in fact there might yes that I mean and that's that's an almond the sheriff's every time we look at higher resolution but more and more substructure I hope there's not too many more objects and map them in your view are there any polarization measurements but when you're talking about the one side of disks you were using the five jupiter-mass planets that's not even a very diagnostic we could have a hundred jupiter-mass ground floor right you could within you can see it right so this is the problem that's like and in fact even five supermassive now our direct imaging surveys for exoplanets have gotten so good but at the separations of some of these asymmetries we probably would have seen a young by Jupiter mass object because they're very bright at that face because they're either still creating so you've been some protections of these objects at like H alpha as well and to our best ability to detect there's not a whole lot of things that are a few Jupiter masses that separations of several tens to hundreds a of a view from stars they're just not there so in fact thinking it is five two hundred is going in the wrong direction we probably should make it one Jupiter mass and then all the problems I talked about getting even worse so it's really hard to make those structures without some pretty you know fine-tuning of the model just a failure to see those is that starting to worry is it starting to worry me no I'm sure we'll come up with some other creative one of the results from couple areas that kind of certain binary planets are pretty much sitting around the shortest binaries so does that just remain well I'd say that's a unsurprising conclusion because I think and people have not shown this that it's the very closest binaries were made by : a solution that's very hard to maintain a planet around that system in the first place if you manage to have the plan is there form and then have the oscillations start then the planet will end up getting either kinks to iron clinicians or ejected from the system entirely and if you wanted to have the binary form first the problem is that the process of undergoing the oscillations probably would have gotten rid of that disk material around it because it would have been causing the electricity to grow and shrink information so I think it might be our different planets path leads back as well but what I would argue is that what I think it tells us is that there must be some other part of the story to make some of these close binaries that's giving us both these very very short period things and these slightly longer period things with plant that doesn't involve cosine of oscillation that allows us to keep that disk one thing I'm wondering in these regions now go into the question of the aren't they perhaps into the snow objects like planets anything do you need to rely on something that's actually part of your system to give you the initial asymmetry opening of that would give you something I think it's hard to get something that would be long-lived enough that we would expect to see so many from just a flyby and there's some evidence you know what so there's good evidence from the observation that those asymmetries are in fact pressure bumps of some kind because we can look and see how the articles are concentrated in the function of size by probing different wavelengths with Alma and you see the exact the trend that you would expect where the smaller particles are more concentrated in the center and the larger things are around which is what you'd expect based on the dragon timescales so it's intrinsic will choice their stuff it looks like you have a pressure maximum they got that is trapping dust that part seems pretty good it's just what caused that it's not clear whether or not it's a bore type it's pretty hard to measure the velocity so yeah I think Shawn had to leave but you should ask him we may have a new hypothesis out great okay well a few of us are going to dinner and this one or two more spots possibly and so or accessible please tell me afterwards and if you like tell me please just come up here [Applause] [Music] [Music]

Channel: CfA Colloquium

Views: 8,276

Rating: 4.8222222 out of 5

Keywords: CfA, Center for Astrophysics, Harvard, Smithsonian

Id: TdZxfTvzd0A

Length: 66min 0sec (3960 seconds)

Published: Thu Sep 28 2017