something in astronomy is wrong

Today I chatted with my old friend Phil Marshall (SLAC) about various things. Well actually I ranted at him about catalogs and how their use is related to the way they are made. He was sensible in reply. He suggested that we write something, and maybe also develop some guidance for the NSF LSST developer community. His recommendation was to create some examples that are simple but also that connect obviously to research questions in the heads of potential readers. Easier said than done! I said that any such paper needs a good title and abstract. He pointed out that this is true of every paper we ever write! Okay fine.


Are there young, alpha-rich stars?

I asked this question in Data Group meeting: With Emily Jo Griffith (Colorado) and I have a data-driven nucleosynthetic story for essentially every red-giant-branch star in the SDSS-IV APOGEE survey. Since the parameters of this model relate to the build-up of elements over time, they might be used to indicate age. We matched to the NASA Kepler asteroseismic sample and indeed, our nucleosynthetic parameters do a very good job of predicting ages.

On the RGB, age is mass, and the asteroseismology gives you masses, not ages. There are some funny outliers: Stars with large masses, which means young ages, but with abundances that strongly indicate old ages. Are they young or old? I am betting that they are old, but they’ve undergone mass transfer, accretion, or mergers. If I’m right, what should we look for? The Data Group (plus visitors) suggest looking for binarity, for vertical action (indicating age), for ultraviolet excess (indicating white dwarf companion), for abundance anomalies, and Gaia RUWE. Will do! My money is that all these stars are actually old.



Today I went to the L2G2 (Local Local Group Group) meeting at Columbia. This meeting started with a dozen of us around a table and is now 50 people packed into the Columbia Astronomy Library! A stand-out presentation was by Grace Telford (Rutgers), who showed beautiful spectroscopy of low-metallicity O stars. From their spectral features and (in one case) surrounding H2 region, she can calculate their production of ionizing photons. This is all very relevant to the high redshift universe and reionization. Afterwards, Scott Tremaine (IAS) argued that The Snail could be created by random perturbations, not just one big interaction.


two-dimensional disk?

I had a great set of conversations today with Griffiths and Eisner about the relationship between the idea that the abundance patterns in stars in the disk look very two-dimensional and the idea that the formation of stars in the disk might depend only on birth radius and birth time. If these two things are related, it creates all sorts of new projects for measuring stellar ages and for reconstructing the formation history of the disk.


scope of a paper

Emily J Griffith (Colorado) and I have been working on a two-process (or really few-process) model for the creation of the elements, fit to the abundances measured in the APOGEE survey. Our big conversation this week has been about the scope for our first paper: We have so many results and ideas we don’t know how to cut them into papers. Today we made a tentative scope for paper one: we’ll explain the model, deliver a huge catalog of abundance information, and demonstrate the usefulness for practitioners of Galactic archaeology. Then, later, we can actually do that archaeology!


a blackboard talk on orbital torus imaging

I gave the brown bag talk (chalk only) today at the NYU Center for Cosmology and Particle Physics. I spoke about torus imaging—using moments of the abundance distribution to measure or delineate the orbits in the Galaxy. I focused on the theory of dynamics and what it looks like if you can insert new invariants. The questions were great, including hard ones about non-equilibrium structures, radial migration, and chaos. All these things matter! Talking at a board in front of a skeptical, expert audience is absolutely great for learning about one's own projects, communication, and thinking.


causal structure in ML

Today I had the honor on serving on the PhD advising committee of Yan Liang (Princeton), who is designing her PhD project. She is adding causal structure to an autoencoder such that it can separate stellar variability-induced radial-velocity signals from exoplanet-induced signals in extreme precision radial-velocity data. Her method design is novel, and tests suggest that it might work. The committee recommended adding even more causal structure and physics knowledge (more is probably always better, provided that it isn’t incorrect)! As my loyal reader knows, I think this is the frontier for machine learning in the natural sciences: adding causal structure.


stars don't orbit at their guiding radii!

A subject of conversation all week has been about stellar orbits in the Milky Way disk, driven by some visualizations made by Adrian Price-Whelan (Flatiron). We often describe the azimuthal action or the z component of the angular momentum (L_Z or J_Phi) of a disk star in terms of the guiding radius, or the radius of a circular orbit of the same azimuthal action. The idea is: If the star has radial action, it will oscillate around the guiding radius as it orbits. Wrong!! If the vertical action is comparable to or larger than the radial action (and that’s typical), the star will orbit outside the guiding radius, always. The trivial picture is simply incorrect.