bi-linear-ish models

Before my day got ruined by a deadly SDSS-V Advisory Council meeting, I worked with Emily Griffith (Colorado) on a data-driven 2-process model for nucleosynthesis. This model is amplitudes (2 per star) times process yield vectors (2 per element). In this sense it is like a matrix factorization. But it involves a log-sum-exp (rather than just a matrix multiply), so it is mildly nonlinear. It can still be optimized the same way and it is well behaved. In some sense, I realized, it is very like The Cannon in form. But different! We failed to fully implement before I turned into a (very unhappy) technocrat.

SDSS-V Science Festival, day 2

On day two of the Collaboration meeting, I talked to Emily Griffith (Colorado) about data-driven models of nucleosynthetic processes. We were inspired by this paper, on which Griffith is an author. The paper builds an empirical two-process enrichment model based on the observed morphology of the [Fe/Mg] vs [Mg/H] plane. We discussed how to make this model into a full (but constrained) latent-variable model. I am interested in moving it towards causal inference, but we could also look at third processes, anomalous stars, anomalous elements, calibration issues, and so on. We wrote down math and started to write code.

six-quark state?

Today was a great blackboard talk at CCPP by Glennys Farrar (NYU) about a possible six-quark state in QCD. She has been thinking about this for a decade or so, because it might have implications for dark matter and issues in QCD. Today she focused on the latter: There are terms in the g−2 calculation for the muon that can be estimated either with lattice QCD or by integrating some observed branching ratios from experiment. These two methods disagree, and the observational method disagrees (more strongly) with the g−2 measurement. But Farrar shows that if there is a long-lived 6-quark state, it can potentially affect the QCD calculation (implicitly) but would be evaded by the branching-ratio measurements (because it would evade all event triggers). Her model requires some good luck with QCD parameters and bound states, but if that luck holds, she can pull dark matter into the standard model and solve some precision-measurement issues! After her talk we discussed a bit about just how hard lattice QCD is. It's absurd!

black holes and nucleosynthesis

Today Selma de Mink (Harvard) gave a great and energizing Astrophysics Seminar at NYU. She talked about many things related to the extremely massive-star progenitors of the estremely massive black holes being observed in merger by LIGO. One assumption of her talk, which is retrospectively obvious but was great, is that the vast majority of LIGO events should be first-generation mergers. A second merger is very unlikely, dynamically. But that wasn't her point: Her point was that the masses that LIGO sees will constrain how very massive stars evolve. In particular, she showed that there is a strong prediction of a mass gap: There can't be black holes formed by stellar evolution in the mass range 45 to 150 solar masses. The physics is all about pair-instability supernovae from very low-metallicity stars. But the details of this black-hole mass gap depend on some nuclear reaction rates, so she concludes that LIGO will make nucleosynthetic measurements! The LIGO data probably already do. It's a new world!

Gaia halo stars; neutron lifetime

In the morning, Lauren Anderson (Flatiron) and I discussed Gaia DR2 projects. First we talked about things we could do with David Blei (Columbia) and his group, who have variational methods for extremely large inferences of the types we would like to do. We drew some graphical models (and posted them on twitter). Then we looked at halo red giants selected by parallax and color. Sagittarius shows up beautifully, and now it is time to start to look at other features. The data are incredibly rich.

Alberto Sirlin (NYU) gave the brown-bag talk, on the neutron lifetime. He showed that the neutron lifetime and a certain coupling are related, and showed that measurements of each, and their combination, are consistent, for at least some measurements. There are interesting puzzles though: Some kinds of lifetime measurements disagree with other kinds, and there was a step change in the coupling measurements in 2002-ish. So there are hints of new physics, but also a consistent no-new-physics story. He also showed that the simplest new-physics scenarios are not sensible. The neutron lifetime is important for many things, but especially big-bang nucleosynthesis.

exploding white dwarfs

Abi Polin (Berkeley) came through NYU this week. Today she delivered a great seminar on explosions of white dwarfs. She is looking at different ignition mechanisms, and trying to predict the resulting supernovae spectra and light curves. This modeling requires a huge range of physics, including gastrophysics, nuclear reaction networks, and photospheres (both for absorption and emission lines). The current models have serious limitations (like one-d, which she intends to fix during her PhD), but they strongly suggest that type Ia supernovae (the ones that are created by white-dwarf explosions) do seem to come from a narrow mass range in white-dwarf mass. If you go too high in mass, you over-produce nickel. If you go too low in mass, you under-produce nickel and get way under-luminous. In addition to the NYU CCPP crew, Surabh Jha (Rutgers) and Armin Rest (STScI) were in the audience, so this talk was followed by a lively lunch! Jha suggested that the narrow mass range implied by the talk could also help with understanding the standard-candle-ness of these explosions.

57 elements; research meetings

Today the astro seminar was given by Or Graur (CfA). He spoke about various discoveries he and collaborators have made in type Ia supernovae. For me, the most exciting was the discovery of atomic-mass-57 elements, which he can find by looking at the late-time decay: The same way we identify the mass-56 elements from timing supernovae decays at intermediate times, he finds the mass-57 elements. The difference is that they are at much later times (decay times in the years). He pointed out a caveat, which is that the late-time light curve can also be affected by unresolved light echoes. That's interesting and got me thinking (once again) about all the science related to light echoes that might be under the radar right now.

I hosted today my first-ever undergraduate research meeting. I got together undergraduates and pre-PhD students who are interested in doing research, and we discussed the Kepler and APOGEE data. My plan (and remember, I like to fail fast) is to have them work together on overlapping projects, so they all have coding partners but also their own projects. With regular meetings, it can fit into schedules and become something like a class!

nucleosynthesis and stellar ages

Benoit Coté (Victoria & MSU) came to NYU for the day. He gave a great talk about nucleosynthetic models for the origin of the elements. He is building a full pipeline from raw nuclear physics through to cosmological simulations of structure formation, to get it all right. There were many interesting aspects to his talk and our discussions afterwards. One was about the i-process, intermediate between r and s. Another was about how r-process elements (like Eu) put very strong constraints on the rate at which stars form within their gas. Another was about how we have to combine nucleosynthetic chemistry observations with other kinds of observations (of, say, the PDMF, and neutron-star binaries, and so on) to really get a reliable and true picture of the nucleosynthetic story.

Late in the afternoon, I met with Ruth Angus (Columbia) to further discuss our project on cross-calibrating (or really, self-calibrating) all stellar age indicators. We wrote down some probability expressions, designed a rough design for the code, and discussed how we might structure a Gibbs sampler for this model, which is inherently hierarchical. We also drew a cool chalk-board graphical model (in this tweet), which has overlapping plates, which I am not sure is permitted in PGMs?

precise ages from imprecise indicators; QCD dark matter

In the morning, I met with Ruth Angus (Columbia) to discuss the ages of stars. We brainstormed all possible age estimates for stars, and listed some limitations and epistemological properties. In addition to the usuals (rotation, activity, isochrone fitting, asteroseismology, and so on), we came up with some amusing options.

For example, the age of the Universe is a (crude, simple, very stringent) age estimate for every single star, no matter what. It is a very low-precision estimate, but it is unassailable (at the present day). Another odd one is the separation of comoving pairs. In prinicple every co-moving pair provides an age estimate given the relative velocity and relative position, with the proviso that the stars might not be co-eval. This is a good age estimate except when it isn't, and we only have probabilistic information about when it isn't.

We then wrote down the basic idea for a project to build up a hierarchical model of all stellar ages, where each star gets a latent true age, and every age indicator gets latent free parameters (if there any). Then we use stars that overlap multiple age indicators to simultaneously infer all free parameters and all ages. The hope—and this is a theme I would like to thread throughout all my research—is that many bad age indicators (and they are all bad for different reasons) will, when combined, produce precise age estimates nonetheless for many stars.

At lunch-time, Glennys Farrar (NYU) gave an energizing black-board talk about a dark-matter candidate that exists within QCD, made up of a highly symmetric state of six quarks. QCD is a brutal theory, so it is hard to compute the properties of this state, or its stability, but Farrar laid out some of the conditions under which it is a viable dark-matter candidate. It is very interesting phenomenologically if it exists, because it has a non-trivial cross-section for scattering off of atomic nuclei, and it could be involved in baryogenesis or the matter–anti-matter asymmetry.

#ken75, day 4

Today was day 4 of Galactic Archaeology and Stellar Physics. The day ended with my summary talk, which was unfair, scoldy, and mean and which was shouted (by me) over slides available here. Those slides will be incomprehensible without the things I said alongside them, but they will give you a sense of what themes I assembled (in real time) from the talks. A few non-representative highlights from today:

Binney kicked off the day with a discussion of analytic modeling of galaxies. His talk contained many valuable insights. For example, he showed that very simple distribution functions (in action space) can nonetheless create very non-trivial distributions in configuration space. He gave his usual—but excellent—argument for working in action–angle space: It is a consistent, continuous, conjugate coordinate system in which inference is possible. He also showed that high-quality modeling of the nearby Solar neighborhood can make good predictions for the position—velocity relationshiops for a larger patch of the Galaxy; that is, good modeling makes for highly predictive theories. At the end of his talk, he was asked about a radical dark-matter model, and he answered in terms of researcher utility, which was music to my ears.

Wegg showed an extremely good model of the mass function and microlensing optical depth towards the bulge, and constrained the dark-matter fraction. His results rule out a strong cusp in the dark matter, which is consistent with other things that were said at the meeting.

Grillmair and Carlberg talked about cold stellar streams. Grillmair showed marginal evidence for many more streams, consistent with a steep mass function in such objects. He name-checked our work on chaos and stream fanning. Carlberg shocked me by saying that the stellar streams are shorter than expected in theory. I just straight- up disagree with that, but maybe he is right when you take the full complement of streams together.

Côté brought us back to the subject of nucleosynthesis. He showed that there are many competing nucleosynthetic models that can produce the same data, but if you look on the inside, they imply very different things about the latent (physical) parameters. He breaks some of these by looking to LIGO and the rate of neutron-star mergers, which are probably involved in r-process. I loved the connections he drew between stellar chemical abundances, nuclear physics, and gravitational wave astronomy.

#ken75, day 2

Today was day 2 of Galactic Archaeology and Stellar Physics. Again, a great day; here only a few highlights:

Else Starkenburg gave a review on first stars. There is no true first star known—nothing with primordial abundances—but there is one at or near −7 (that is, 7 orders of magnitude below Solar. This star, like many extremely metal-poor stars, is low in iron but very high in carbon relative to iron. That is a mystery, with many conceivable solutions. Starkenburg spoke about binary-star (mass transfer) ideas. The talk left me wondering: Do we know what a primordial-abundance star would look like? Afterwards, Schlaufman argued to me that we do, at least pretty accurately.

This was followed by a bunch of other low-metallicity star talks. DaCosta, Venn, and Schlaufman all spoke about searches for extremely metal-poor stars using clever photometric techniques. This ties in with my comment yesterday that we might be able to do a lot of Galactic Archaeology science with photometric surveys (possibly backed up by spectroscopy for calibration or training). It also bodes extremely well for Gaia Bp–Rp narrow-band photometry, which will be laden with stellar information.

Stello and Huber gave talks about asteroseismology. In Stello's review, I learned that the brightness variations are temperature variations (or really temperature–size variations), not pure size variations. This surprised me, and then was immediately obvious: The atmosphere reacts adiabatically to fast changes. He also very clearly connected the mode properties to the stellar properties, and explained the important point that dwarfs, subgiants, and red giants have different physics connecting their seismic modes to their masses, ages, and bolometric luminosities. Huber compared existing asteroseismology to Gaia data, showing that there is a consistent story, but also showing that for almost all asteroseismic targets, the asteroseismology will provide more precise distances than even end-of-mission parallaxes.

There were a set of nucleosynthesis and supernovae yield talks. My personal highlight here was a talk by Hampel about neutron capture physics. She starts with the observation that between s-process and r-process there is a whole range of neutron densities, and at different densities, you get different abundance yields. She then used real stellar data to measure the neutron density for an intermediate neutron density between s and r, calling it i. This talk stood out among the nucleosynthesis talks for its containing (like Stello's talk on asteroseismology), clearly explained fundamental physics.

nuclear composition of UHECRs

Today Michael Unger (Karlsruhe) told us over lunch about ultra-high energy cosmic rays from Auger. There are many mysteries, but it does look like the composition moves to higher-Z nuclei as you go to higher energies, or at least that's my read. He told us also about a very intriguing extension to Auger which would make it possible to distinguish protons from iron in the ground detectors; if that became possible, it might be possible to do cosmic-ray imaging: It is thought that the cosmic magnetic fields are small enough that protons near the GZK cutoff should point back to their sources. So far this hasn't been possible, presumably because the iron (and other heavy elements) have charge-to-momentum ratios too large; they get heavily deflected by the magnetic fields they encounter.

Blandford

The highlight of a low-research day was a visit from Roger Blandford (KIPAC), who gave the Physics Colloquium on particle acceleration, especially as regards ultra high-energy particles. He pointed out that the (cosmic) accelerators are almost the opposite of thermal systems: They put all the energy very efficiently into the most energetic particles, with a steep power-law distribution. He made the argument that the highest energy particles are probably accelerated by shocks in the intergalactic media of the largest galaxy clusters and groups. This model makes predicitions, one of which is that the cosmic rays pretty-much must be iron nuclei. In conversations over coffee and dinner we touched on many other subjects, including gravitational lensing and (separately) stellar spectroscopy.

KIPAC@10, day 2

Today was the second day (I missed the first) of the KIPAC@10 meeting at KIPAC at SLAC. There was a whirlwind of talks on compact objects and galaxy evolution, too many to summarize, but some highlights for me were the following:

Steiner (UW) showed neutron-star mass and radius measurements and discussed their implications for the properties of matter at extreme density. He showed some very noisy likelihood functions (ish) in mass–radius space, one per measured neutron star (and there are 8-ish with measurements) and tried to draw a curve through them. I have opinions about how to do that and he seems to be doing it right; each time we tried to discuss this over coffee something interrupted us.

Perna (Colorado) talked about magnetars; I hadn't appreciated how extremely short-lived these stars must be; their lifetimes are measured in kyr, which is not a unit you see every day. Romani (Stanford) made a pitch that Fermi-discovered gamma-ray pulsars are the bees knees. He didn't show folded light-curves but apparently there are now hundreds where you can see the periodicity in the (sparse) Fermi data. Tomsick (Berkeley) showed some outrageously awesome NuSTAR data, making me want to hear much more about that mission. It's PI is my old friend from graduate school, Fiona Harrison (Caltech), to drop a name.

Cordes (Cornell) talked about pulsar timing and gravitational radiation, a subject on which I have opinions (from a precision measurement perspective). He, like is common in that business, concentrated on the stochastic gravitational wave background; I would like to hear or think more about coherent source detection. It is usually easier! Along those lines, at one point Blandford (KIPAC) asked Aarons (Berkeley) if physical models of pulsar emission were likely to help in measurements of pulsar timing. Aarons didn't commit either way, but I think the answer has to be yes. Indeed, I have suggested previously that modeling the emission almost has to improve the measurements.

Stark (Arizona) showed very nice new data on galaxies at extremely high redshifts. He noted that almost every result at redshifts beyond six depends entirely on photometric redshifts. That's true, but is it a concern? I guess it is because there could be interloping lower-redshift objects (or stars) having a big effect on the conclusions. Kriek (Berkeley) and Lu (KIPAC), in separate talks, showed that it is difficult to explain the evolution of galaxies in sizes and stellar populations with simple models of star formation and merging. Also, Kriek called into question the star-formation-rate estimates people have been using, which is interesting; she finds a factor-of-two-ish range in the mistakes that could be being made, and this is the same order of magnitude as the amplitude of the variation in specific star-formation rate with galaxy mass. She didn't claim that there is an error there.

In the discussions at lunch, Stuart Lynn (Adler) pitched an idea from David Harris (NAS) that we start a journal of short contributions. Marshall was all over that; it might get launched tomorrow in the unconference session.