Today I visited the group of Saskia Hekker (HITS). We discussed many things asteroseismological! We discussed:
It was my pleasure to be a part of the PhD committee for Sebastian Ratzenböck (Vienna), who wrote a dissertation in computer science but as applied to astrophysics. He had three advisors, in statistics, in computer science, and in astronomy, and he beautifully bridged the three worlds. His research was on finding members of stellar clusters, and on finding new stellar clusters. He showed (pretty convincingly, I think) that star-forming regions break up into many individual star-forming events with different ages and different kinematics. One of his conclusions is that all star formation happens in clusters or groups! He also made a nice technical advance, which was to build a tool to select clustering hyper-parameters in the space of physical quantities one cares about, instead of in the space of arbitrarily-defined clustering-method parameters. It was a great thesis, a beautiful defense, and a fun time drinking afterwards. Congratulations Dr Ratzenböck!
Early in the morning I spoke with Ana Bonaca (Harvard) about the amazing velocity data she has taken for stars in the GD-1 stellar stream in the Milky Way halo. As my loyal reader knows, this stream has a spur of stars off the main branch that are consistent with being perturbed away by a massive perturber that flew by. Now she has precise velocity information about stars in the main body of the stream and in the spur. Contrary to our naive predictions, the stream and spur have very similar velocities. But the spur appears to be far lower in velocity dispersion. Is this real? And is this what we expect? We didn't predict it in our theoretical paper on the subject, but then again we didn't look! I can see some arguments that it might be true. Bonaca also sees many other things in the data, like that the GD-1 stream membership is improved dramatically when we have metallicity information.
Today at Flatiron, Adrian Price-Whelan (Princeton) gave a very nice talk about using stars to infer things about the dark matter in the Milky Way. He drew a very nice graphical model, connecting dark matter, cosmology, and galaxy formation, and then showed how we can infer properties of the dark matter from stellar kinematics (positions and velocities). As my loyal reader knows, this is one of my favorite philosophical questions. Anyway, he concentrated on stellar streams and showed us some nice results on Ophiucus and GD-1 (some of which have my name on them).
The weekly Stars Meeting at Flatiron was great today. Ben Pope (NYU) showed us some terrific systematic effects in the NASA TESS data, which leads to spurious transit detections if not properly tracked. Some of these probably relate to Earthshine in the detector, about which I hope to learn more when Flatiron people return from the TESS meeting that's on in Baltimore right now.
In that same meeting, Price-Whelan showed us evidence that stars lower on the red-giant branch are more likely to have close binary companions (from APOGEE radial-velocity data). This fits in with an engulfment model (as red giants expand) but it looks like this effect must work out to pretty large orbital radius. Which maybe isn't crazy? Not sure.
And Jason Curtis (Columbia) showed amazing stellar-rotation results from TESS data on a new open cluster that we (Semyeong Oh et al) found in the ESA Gaia data. He can show from a beautifully informative relationship between rotation period and color (like really tight) that the cluster is extremely similar to the Pleiades in age. Really beautiful results. It is clear that gyrochronology works well for young ages (I guess that's a no-brainer) and it is also clear that it is way more precise for groups of stars than individuals. We discussed the possibility that this is evidence for the theoretical idea that star clusters should form in clusters.
At lunch we had a discussion (inspired by Bertrand Goldman, MPIA) about the expected shapes of open clusters. I think they should be elongated along their orbits. There was some back and forth but this made me more confident: Once the clusters start to disperse, they should distort through orbital phase-frequency differences. I proposed a simple test of this. But I'm more interested in the point that this should help us find new kinds of (maybe older) clusters!
In the afternoon, Amina Helmi (Kapteyn) showed up and Bonaca (Harvard), Price-Whelan (Princeton), and I discussed many things with her. We discussed the question of when and how stellar streams in the Milky Way halo constrain purely local properties of the Galaxy. Does this result (from Bonaca) depend on the potential being time-dependent? I think it does. Helmi didn't disagree but is optimistic that we can handle the time dependence.
We also discussed the lack of tidal tails around globular clusters: Is it surprising that only Palomar 5 has these tails? Price-Whelan has looked at a few of the most likely clusters in Gaia, and nada. This led to (or was part of) a longer discussion of the statistics of streams: How many will there be and how many do we expect?
At Stars group meeting, I spoke about Ana Bonaca and my new paper looking at the information content of cold stellar streams in the Milky-Way halo. It is a huge document, with lots of results, but my absolute favorite is this: As we make the potential model for the Milky Way more flexible, each stream constrains each potential parameter less well. This is the issue with information studies: They depend strongly on the model flexibility! But something cool happens in the limit of very flexible potential model: Each stream appears to end up constraining the local acceleration field, local to the current position (not past position) of the stream. This has lots of consequences: One is that if this is true, we can just model each stream independently, in a flexible potential, and then interpolate the acceleration constraints they deliver with a flexible or non-parametric model as an interpolator! That would make stream fitting more tractable than it is now, not less (and most other ideas we have are computationally impossible at present).
In the discussion, Vasily Belokurov (Cambridge) suggested that we might get more information—and more global information—if we modeled the density of stars along the stream. He is reacting to the point that the Bonaca stream model is a stream-track model, not a full six-dimensional distribution function. Belokurov might be right; we should add something like this to the paper.
After I spoke, Jackie Faherty (AMNH) got us really excited about what Gaia has done and will do for nearby moving groups of young stars (like open clusters). She believes that several of the “connected components” in the Oh et al paper are new, previously undiscovered young clusters, and that Gaia DR2 might find hundreds of new members, going down the main sequence! That's amazing. I hope it's true.
Group meetings were fun today. In Gaia DR2 prep meeting, I worked with Megan Bedell (Flatiron) to get some plots ready for Gaia DR2. That is, we planned what we will plot the moment that the data release happens. The goal is to look at physical and kinematic properties of exoplanet host stars.
In Stars meeting, JJ Hermes (UNC) showed some incredible WD lightcurves, which appear to come from white dwarfs that have quadrupolar temperature distortions on their surfaces, rotating. There appears to be a common sub-type of white dwarfs that show evidence of magnetism and strong surface temperature variations. We discussed things to do with Gaia and other data sources.
John Brewer (Yale) showed hot-off-the-presses results on chemical-abundance variations within Praesepe. This cluster has some really strange properties, like an amazingly low velocity dispersion. But he finds a chemical-abundance variation in iron but also elements ratioed to iron. This is in qualitative disagreement with work I have done with Melissa Ness (Columbia), so there is something to work out there. We discussed critical tests of his methods and results.
Today was the first day of Preparing for TESS, organized by Dan Foreman-Mackey (Flatiron) and others. It is organized like the #GaiaSprint in that it is a hack week, starting with pitches and dedicated to getting stuff done. The crew pitched some great ideas on day one and then hacked. I am trying to work on algorithmic approaches to efficient radial-velocity follow-up.
Melissa Ness (Columbia) and Megan Bedell (Flatiron) started an interesting project to follow up anomalous stars in an open cluster: Do the stars with element-abundance anomalies also show anomalies in the time domain or in asteroseismology? Many other projects are working towards obtaining cleaned or calibrated light curves, although my heart sang when various people (notably Rodrigo Luger at UW) pointed out that we don't want to de-trend, we want to have a model that explains every light curve as a combination of spacecraft and stellar variability (and planets).
Ana Bonaca (Harvard) arrived in town for a week of hacking on our stream-information project. She spent today getting more streams in to the analysis. The point of the project is not to model each stream in detail, but rather to examine, using Fisher Information, the information that each stream (or any combination of streams) brings to the measurement of gravitational-potential parameters. We worked also on paper scope and our original goal (way long ago) of constraining the mass and orbit of the LMC.
Many great things happened at the meeting today; way too many to mention. Steinmetz showed how good the RAVE-on results are, and nicely described also their limitations. Korn showed an example of an extremely underluminous star, and discussed possible explanations (most of them boring data issues). Brown explained that with a Gaia mission extension, the parameter inference for exoplanet orbit parameters can improve as a huge power (like 4.5?) of mission lifetime. That deserves more thought. Gerhard explained that the MW bar is a large fraction of the mass of the entire disk! Helmi showed plausible halo substructure and got me really excited about getting ready for Gaia DR2. In the questions after her talk, Binney claimed that galaxy halos don't grow primarily by mergers, not even in theory! Hobbs talked about a mission concept for a post-Gaia NIR mission (which would be incredible). He pointed out that the reference frame and stellar positions require constant maintenance; the precision of Gaia doesn't last.
One slightly (and embarrassingly) frustrating thing about the talks today was that multiple discussed open clusters without noting that we found a lot ourselves. And several discussed the local standard of rest without mentioning our value. Now of course I (officially) don't mind; neither of these are top scientific objectives for me (I don't even think the LSR exists). But it is a Zen-like reminder not to be attached to material things (like citations)!
I broke my own rules and left #AstroHackWeek to catch up with #LennartFest. The reason for the rule infraction is that the latter meeting is the retirement celebration of Lennart Lindegren (Lund) who is one of the true pioneers in astrometry, and especially astrometry in space and at scale. My loyal reader knows his influence on me!
Talks today were somewhat obscured by my travel exhaustion. But I learned some things! Francois Mignard (Côte d'Azur) gave a nice talk on the reference frame. He started with an argument that we need a frame. I agree that we want inertial proper motions, but I don't agree that they have to be on a coordinate grid. If there is one thing that contemporary physics teaches us it is that you don't need a coordinate system. But the work being done to validate the inertial-ness of the frame is heroic, and important.
Floor van Leewen (Cambridge) spoke about star clusters. He hypothesized—and then showed—that proper motions can be as informative about distance as parallaxes, especially for nearby clusters. This meshes with things Boris Leistedt (NYU) and I have been talking about, and I think we can lay down a solid probabilistic method for combining these kinds of information responsibly.
Letizia Capitanio (Paris) reminded us (I guess, but it was new to me) that the Gaia RVS instrument captures a diffuse interstellar band line. This opens up the possibility that we could do kinematic dust mapping with Gaia! She also showed some competitive dust maps based on Gaussian Process inferences.
In Stars group meeting, Ruth Angus (Columbia) showed her catalog of rotation periods in the Kepler and K2 fields. She has a huge number! We discussed visualizations of these that would be convincing and also possibly create new scientific leads.
Also in Stars group meeting, Arash Bahramian (MSU) spoke about black holes in globular clusters. He discussed how they use simultaneous radio and X-ray observations to separate the BHs from neutron stars: Radio reveals jet energy and X-ray reveals accretion energy, which (empirically) are different for BHs and NSs. However, in terms of making a data-driven model, the only situations in which you are confident that something is a NS is when you see X-ray bursts (because: surface effect) and the only situations in which you are confident that something is a BH is when you can see a dynamical mass substantially greater than 1.4 Solar masses (because: equation of state). He highlighted some oddities around the cluster Terzan 5, which is the globular cluster with the largest number of X-ray sources, and also an extremely high density and inferred stellar collision rate. This was followed by much discussion of relationships between collision rate and other cluster properties, and also some discussion of some individual x-ray sources.
[In non-research news: Today I became an employee of the Flatiron Institute, as a new group leader within the CCA! Prior to today I was only in a consulting role.]
Today, Kathryn Johnston (Columbia) organized a “Local Group on the Local Group” meeting at Columbia. Here are some highlights:
Lauren Anderson (Flatiron) gave an update on her data-driven model of the color–magnitude diagram of stars. This led to a conversation about which features in her deconvolved CMD are real? And are there too many red-clump stars given the total catalog size?
Steven Mohammed (Columbia) showed our GALEX Galactic-Plane survey data on the Gaia TGAS stars. The GALEX colors look very sensitive to metallicity and possibly other abundances. The audience suggested that we look at the full dependences on metallicity and temperature and surface gravity to see if we can break all degeneracies. This led to more discussion of the use of the Red Clump stars for Galactic science.
Adrian Price-Whelan (Columbia) presented a puzzle about the Galactic globular cluster system, which he has been thinking about. Are the distant clusters accreted? The in-situ formation hypothesis is unpalatable (it had to be many clusters at early times; should be many thin streams); the accreted hypothesis over-produces the smooth component of the stellar halo (unless dwarf galaxies had far more GCs per unit stellar mass in the past). These problems can be resolved, but only with strong predictions.
Yong Zheng (Columbia) spoke about the gaseous Magellanic stream and associated (or plausibly associated) high-velocity clouds. Many of the challenges in interpretation connect to the problem that we don't know where the gas is along the line of sight. She showed really nice data on something called Wright’s Cloud. For this huge structure—and for the stream as a whole—there is little to no associated stellar component.
Nicola Amorisco (Harvard) Showed theoretical simulations of the accreted part of the MW (and MW-like-galaxy) halo, with the goal of finding stellar-halo observables that strongly co-vary with the assembly history of the dark-matter halo. Both theory and observations suggest large scatter in halo properties at Milky-Way-like masses, and much less scatter at higher masses (because of central-limit-like considerations). His results are promising for understanding the MW assembly history.
Glennys Farrar (NYU) spoke about the MW magnetic field, using rotation measures and CMB to constrain the model. She showed UHECR deflections in the inferred magnetic field, and also discussed implications of her results for electron and cosmic-ray diffusion. There are also tantalizing implications for the synchrotron spectrum and CMB component separation. One interesting comment: If her results are right for the scale and amplitude of the field, there are serious questions about origin and generation; is it primordial or generated on scales much larger than the galaxy?
In Stars group meeting, Lauren Anderson (Flatiron) showed our toy example that demonstrates why our method for de-noising the Gaia TGAS data works. That led to some useful conversation that might help us explain our project better. I didn't take all the notes I should have! One idea that came up is that if there are two populations, one only seen at very low signal-to-noise, then that second population can easily get pulled in to the first. Another is the question of the circularity of the reasoning. Technically, our reasoning is circular, but it wouldn't be if we marginalized out the hyper-parameters (that is, the parameters of our color–magnitude diagram).
Also in the Stars meeting, Ruth Angus (Columbia) suggested how we might responsibly look for the differences in exoplanet populations with stellar age. And Semyeong Oh (Princeton) and Adrian Price-Whelan (Princeton) described their very successful observing run to follow up the comoving stellar pairs. Preliminary analyses suggest that many of the pairs (which we found only with transverse information) are truly comoving.In Cosmology group meeting, Jeremy Tinker discussed the possibility of using halo-occupation-like approaches to determine how the globular cluster populations of galaxies form and evolve. This led to a complicated and long discussion, with many ideas and issues arising. I do think that various simple scenarios could be ruled out, making use of some kind of continuity argument (with sources and sinks, of course).
I spent some time hidden away working on multiplying and integrating Gaussians. I am doing lots of algebra, completing squares. I have the tiniest suspicion that there is an easier way, or that all of the math I am doing has a simple answer at the end, that I could have seen before starting?
At stars group meeting, David Spergel (Flatiron) was tasked with convincing us (and Price-Whelan and I are skeptics!) that the Milky Way really does have spiral arms. His best evidence came from infrared emission in the Galactic disk plane, but he brought together a lot of relevant evidence, and I am closer to being convinced than ever before. As my loyal reader knows, I think we ought to be able to see the arms in any (good) 3-d dust map. So, what gives? That got Boris Leistedt (NYU), Keith Hawkins (Columbia), and me thinking about whether we can do this now, with things we have in-hand.
Also at group meeting, Semyeong Oh (Princeton) showed a large group-of-groups she has found by linking together co-moving pairs into connected components by friends-of-friends. It is rotating with the disk but at a strange angle. Is it an accreted satellite? That explanation is unlikely, but if it turns out to be true, OMG. She is off to get spectroscopy next week, though John Brewer (Yale) pointed out that he might have some of the stars already in his survey.
Today was the second day of The Galactic Renaissance. Two scientific themes of the day were globular-cluster star abundance patterns, and stellar models that account for 3-d and non-thermal-equilibrium (NLTE) effects. On the former, it was even suggested by one speaker that the existence of chemical-abundance variations of certain kinds might be part of the definition of a globular cluster! There are some extreme cases, and various claims that the most extreme examples might be the stripped centers of ancient accreted galaxies!
On the stellar modeling front, there were impressive demonstrations from Frebel (MIT), Bergemann (MPIA), and Thygesen (Caltech) that improving the realism of the physical inputs to stellar models improves their precision and their accuracy. Thygesen did a very nice thing of using (relatively cheap) 1-D models to inform functional forms for interpolation across grid points of a (relatively expensive) 3-D model grid. That got me interested in thinking about physics-motivated or physics-constrained interpolation methods, which could have value in lots of domains.
In a session about Judy's scientific and intellectual life, Steve Shectman (OCIW) described what the world was like in 1967, when Judy Cohen (Caltech) started graduate school. It was a time of optimisim, disruption, and violence. This resonated with things I know about Cohen, because she and I used to discuss the historical context of her origins as an astronomer back when I was a graduate student.
Another highlight of the day was a discussion with Kim Venn (Victoria) and Matt Shetrone (Texas) about persistence effects that damage a significant fraction of spectra in a significant fraction of APOGEE exposures. We discussed the trade-offs between correction and avoidance, and what it might take to fix the problem.
Over dinner, I and others delivered tributes to Judy Cohen. She really has had an amazing scientific impact, and also been a wonderful person, and had a big influence on me. She also said nice things about me in her own speech!
I had great Gaia DR1 and Gaia Sprint brain-storming sessions today with Adrian Price-Whelan and Dan Foreman-Mackey. Price-Whelan proposed that we “run back the clock” on the TGAS stars to see if any pairs emerge from disruption events in the past. I commented that if we go to short times in the past, you need neither distances nor radial velocities: The angular motions suffice. It must be that as the time scale gets longer, distances and radial velocities matter more and more; it would be nice to see this come in continuously. We left that as a conceptual to-do, but we worked out a plan that ought to work in general. This project has a lot in common with the Kinematic Consensus projects that Hans-Walter Rix (MPIA) and I have worked on in the past.
I described to Foreman-Mackey some of my ideas about a data-driven model of the color-magnitude diagram. I have this idea of transforming the space to one in which the noise model is Gaussian, but that kind of thing always makes me feel dirty. We discussed the possibility that I could make my model only one-dimensional (in the parallax direction) by cutting the data into tiny color boxes. That's crazy, but the Gaia Sprint is a hack week, where experiments reign and we want to make things work. So maybe I will write that down this weekend.
Today I spent some research time pair-coding (yet more) with Price-Whelan the fully marginalized likelihood ratio test we are using for our comoving-stars project. There are subtleties! Every time we look at the code we find bugs and think-os. I am reminded of the point that we generally find bugs continuously and with no sign of slowing until we are happy that the code seems to pass our unit and functional tests, at which point we stop looking. That doesn't mean the code is correct! A corollary: Almost no-one has ever wasted time testing code. It turns out that one of the most troublesome parts of this project is units conversion. Not surprising, really, when we have AU, pc, km/s, mas/yr, the inverse squared of these (in inverse variance matrices) and many, many more.
Today Kathryn Johnston (Columbia) convened the first of (we hope) many Local-Group group meetings up at Columbia, with people present from NYU, CCA, Columbia, and Princeton. Johnston reported on things she learned at the recent meeting in Paris. Of particular interest to her—and everyone, apparently—was work by Yuan-Sen Ting (Harvard), building on experiments by Anna Y. Q. Ho (Caltech), showing that it is possible to get detailed stellar abundances, even without huge covariances, out of low-resolution spectra. The point is that as resolution decreases, the information per line gets worse, but you also (usually) get more spectral coverage, and this (mostly) compensates. This could have a huge impact on the future of stellar astrophysics.
I spent time today pair-coding (with Adrian Price-Whelan, Princeton) the analytic marginalized likelihood that Semyeong Oh (Princeton) and Price-Whelan and I have been working on. We found a couple bugs and by the end of the screen-sharing video call (yes, that's the way we do it), we had a marginalized likelihood ratio that seems to be delivering very good answers, and fast! Very excited.
The research day ended with a great astrophysics seminar at NYU by Zoltan Haiman (Columbia, NYU) about fast growth of black holes in the early Universe. He has found a spherically symmetric, steady-state, achievable accretion process that is (much) faster than Eddington, using the same assumptions (essentially). I need to think about it and understand it better. The Eddington limit is one of the most secure, robust, and well-tested arguments in all of astrophysics!
In the morning I met with Kelle Cruz (CUNY) and Ellie Schwab (CUNY), to discuss the statistics component of their project to measure and model radio emission from brown dwarf stars. We worked through a mixture model, in which some are emitting in the radio and some aren't, and how we could do inference in that model.
Having yesterday written math for the comoving stars paper with Adrian Price-Whelan (Princeton) and Semyeoung Oh (Princeton), today I wrote a draft title and abstract. My view is that projects should more-or-less start with a title and abstract, in part because these are the most important parts of the paper, and in part because it helps guide work towards the true critical path.
The research day ended with a Physics Colloquium by Andrea Ghez (UCLA). She talked about the stellar orbits in the Galactic Center and their demonstration of the existence of a black hole there. She showed that (in principle) the black hole was discovered in the 1980s, but the discoverers were very circumspect and conservative. There are lots of remaining puzzles and projects, with existing data and new data. As I said yesterday, this is a very fruitful context for thinking about new engineering challenges.
