binary stars with periods of exactly one year

On Friday, Kareem El-Badry (Caltech) gave a seminar about looking for (and finding!) stars in binary orbits around dark or much darker companions, like black holes, neutron stars, and white dwarfs. He showed results that involve ESA Gaia astrometry, where he noted that the Gaia Mission has no sensitivity to periods right at (or within an inverse mission-length frequency difference of) one-year periods (inverse year frequencies). After the talk I objected that these are not exactly degenerate; El-Badry said that the inferences blow up there.

I spent some time on the weekend thinking about this point, and I now understand it: There is a particular one-year orbit that a star can have (around a darker companion) such that the photocenter of the system makes a motion that is identical to the apparent parallax motion. Thus there is an exact degeneracy between the parallax and a certain one-year orbit.

Does that mean that we can't measure orbits at one year (or, for that matter, parallaxes)? No, it does not. After all, the parallax ellipse has a particular celestial (angular) shape and phase. But it might require some kind of reparameterization of orbits near one-year periods. I think I know how to do that. Should we find the missing binaries? (Oh and by the way, this degeneracy means that, in a strict frequentist sense, Gaia can't measure parallaxes at all without additional information.)

finding emission lines (and other oddities) in hot stars

I showed my robust spectral decomposition (dimensionality reduction) and residuals to the MPIA Binaries group today. There was much useful feedback (including that my H-gamma was actually H-delta; embarassing!). One comment was that the model isn't truly a causal separation between star and lines, so there will be some mean lines in the star model; lines aren't entirely outliers. That's true! The group suggested that I iterate to remove stars with lines from the training set.

After the meeting, I implemented some of that, but problems like this have a pathology: If you carefully remove stars with high residuals at some wavelength, then the training data will be deficient, or low, at that wavelength. And then the model will go lower, and then more stars will have excess at that wavelength and: Disaster. So when I implemented, I required a 2-sigma deviation, and I removed both high and low outliers. I don't know if this will work, but I am testing now.

SPHEREx data

Dustin Lang (Perimeter) and I spoke today about many things, but the conversation got de-railed when Lang showed me his visualizations of the brand-new NASA SPHEREx data. Oh. My. Goodness. First of all, the data are being released daily, before the team has done its analysis, so that anyone in the world can do anything with it. Talk about open! Talk about international! Talk about everything I believe about astrophysics! Also, the data are well documented, high in signal to noise, and released with good and useful metadata. This is killer. Of course Lang has already made a viewer for it and can do all comparisons to the DESI data. Get in touch with him if you want tools.

should I write a book?

Is it research to have a set of conversations about whether to write a book on data analysis? Hans-Walter Rix (MPIA) thinks I should put together my arXiv-only submissions plus a lot more into a book about data analysis. His point of view is that the most important thing is how to convert an ill-posed question about the Universe into a well-posed operation on data.

robust matrix factorization

There is a very nice algorithm and set of methods called “Robust PCA,” originating in a paper by Candès. This method makes use of ideas from convex optimization to simultaneously learn a low-rank representation of the data plus a sparse representation for the outliers. This kind of situation comes up in astronomy all the time.

Way back, Tsalmantza and I made a replacement for PCA called HMF that deals with heteroskedastic data (variable error bars or variable data weights) and also missing data; we used it to build low-rank models of quasar spectra. This weekend I built a robust version of HMF (Robust HMF maybe?) that uses ideas from iteratively reweighted least squares to mimic the algorithm behind Robust PCA. It works! And it works well. Unfortunately, right now the investigator has to tune the rank of the low-rank part and also the soft outlier cutoff used in the IRLS. I would love to figure out principled ways to choose both. If you want to follow along, development is happening here for now.

coherent oscillator injection and recovery

Coherent oscillators—astronomical sources that pulse or oscillate in a phase-stable way over long timescales—have been useful astrophysical tools. For two examples: Stably pulsing pulsars were used to discover gravitational radiation (and are being used to find the stochastic background). Delta-scuti star asteroseismic modes were used to find orbital companions. This led undergrad Nana Miller (NYU) and me and others to look for all the coherent modes we can find among all the stars in the NASA Kepler Mission data. We have have technology to find modes, and we have technology to test for coherence. Now we have to do injection–recovery tests to estimate our detection limits. Today I delivered a simple plan for doing injections.

Our plan is to produce a catalog, not of stars but of modes, every one of which has a coherence time that is longer than the lifetime (4 years) of the Kepler Mission. Then: What do we use them for?

stellar twins vs synthetic stellar twins

In the Milky Way meeting at MPIA today, a bit of a discussion broke out about using stellar twins, inspired by work by Yuan-Sen Ting (OSU). The idea is: If you have two stars with very similar overall metallicity, and very similar temperature and surface gravity, then it should be possible to measure accurate element abundnace anomalies between the two stars, even in the absence of an extremely accurate spectral synthesis code.

My view, which does not contradict this point, is that an even better way to use this stellar-twin idea is to synthesize a twin for every star, using stars that are similar in (either) parameters or else spectral space. After all, an interpolation to your target star should more accurately represent it than even the most similar individual comparison star. That idea, fundamentally, is the main idea behind The Cannon.

possible Trojan planet?

In group meeting last week, Stefan Rankovic (NYU undergrad) presented results on a very low-amplitude possible transit in the lightcurve of a candidate long-period eclipsing binary system found in the NASA Kepler data. The weird thing is that (even though the period is very long) the transit of the possible planet looks just like the transit of the secondary star in the eclipsing binary. Like just like it, only lower in amplitude (smaller in radius).

If the transit looks identical, only lower in amplitude, it suggests that it is taking an extremely similar chord across the primary star, at the same speed, with no difference in inclination. How could that be? Well if they are moving at the same speed on the same path, maybe we have a 1:1 resonance, like a Trojan? If so, there are so many cool things about this system. It was an exciting group meeting, to be sure.

The Cannon and El Cañon

At the end of the day I got a bit of quality time in with Danny Horta (Flatiron) and Adrian Price-Whelan (Flatiron), who have just (actually just before I met with them) created a new implementation of The Cannon (the data-driven model of stellar photospheres originally created by Melissa Ness and me back in 2014/2015). Why!? Not because the world needs another implementation. We are building a new implementation because we plan to extend out to El Cañon, which will extend the probabilistic model into the label domain: It will properly generate or treat noisy and missing labels. That will permit us to learn latent labels, and de-noise noisy labels.

APOGEE spectra as a training set

I spent a lot of the day building a training set for a machine-learning problem set. I am building the training set out of the SDSS-V APOGEE spectra, which are like one-dimensional images for training CNNs and other kinds of deep learning tasks. I wanted relatively raw data, so I spent a lot of time going deep in the SDSS-V data model and data directories, which are beautiful. I learned a lot, and I created a public data set. I chose stars in a temperature and log-gravity range in which I think the APOGEE pipelines work well and the learning problem should work. I didn't clean the data, because I am hoping that contemporary deep learning methods should be able to find and deal with outliers and data issues. If you want to look at my training set (or do my problem set), start here.

getting the absolutely rawest APOGEE data

I spent time today (at the bar!) understanding the data model and directory structure for the raw, uncalibrated APOGEE data. The idea is that I want to do a real-data example for my paper with Casey (Monash) on combining spectra, and I want to get back to the raw inputs. I also might use these spectra for a problem set in my machine-learning class. The code I wrote is all urllib and request and re, because I think it is necessary to read directories to understand the data dependencies in the survey. Is that bad?

Putting aside my concerns: The coolest thing about this project is that the SDSS family of projects (currently SDSS-V) puts absolutely every bit of its data on the web, in raw and reduced form, for re-analysis at any level or stage. That's truly, really, open science. If you don't believe me, check out this this code that spelunks the raw data. It's all just URL requests with no authentication!

Galactic cartography

Neige Frankel (CITA) and I discussed measurements of the age and metallicity gradients in the Milky Way today. In my machine-learning world, I am working on biases that come in when you use the outputs of regressions (label transfer) to perform population inferences (like mean age as a function of actions or radius). We are gearing up to do a fake but end-to-end simulation of how the Milky Way gets observed, to see if the observed Galaxy looks anything like (what we know in this fake world to be) the truth.

auto-encoder for calibration data

Connor Hainje (NYU) is looking at whether we could build a hierarchical or generative model of SDSS-V BOSS spectrograph calibration data, such that we could reduce the survey's per-visit calibration overheads. He started by building an auto-encoder, which is a simple, self-supervised generative model. It works really well! We discussed how to judge performance (held-out data) and how performance should depend on the size of the latent space (I predict that it won't want a large latent space). We also decided that we should announce an SDSS-V project and send out a call for collaboration.

[Note added later: Contardo (SISSA) points out that an autoencoder is not a generative model. That's right, but there are multiple definitions of generative model; only one of which is that you can sample from it. Another is that it is a parameterized model that can predict the data. Another is that it is a likelihood function for the parameters. But she's right: We are going to punk parts of the auto-encoder into a generative model in the sense of a likelihood function.]

Terra Hunting Fall Science Meeting, day 3

Today was a delight! In a working session, Clark Baker (Cambridge) gave a beautiful, conceptual and concrete description of how an echelle spectrograph works and the blaze and the resolution and etc. My favorite moment was the aha! moment I had when he described the Littrow condition. This was followed by Alicia Anderson (Cambridge) explaining how the data reduction proceeds. Then she and Federica Rescigno (Exeter) helped us install the data-reduction software for the ESO instruments (ESPRESSO, HARPS-N, etc) and we started reducing raw echelle data.

Before all this there was a wide-ranging discussion of measuring 3-point functions of radial-velocity time series data. This was inpired by the question: Is a Gaussian process a good model for these data? I hope this turns into a project or set of projects.

Terra Hunting Fall Science Meeting, day 2

So many good things happened in the meeting today! Highlights were presentations by Niamh O'Sullivan (Oxford) Ben Lakeland (Exeter) who showed amazing results running models of stellar variability on data from the Sun. O'Sullivan can see that the sun goes through many different phases of spots, granulation, and super-granulation. She finds these by fitting Gaussian processes of certain forms. Related: Suzanne Aigrain (Oxford) showed that even in very gappy data, the GP fits are unbiased, whereas naive use of periodograms is biased!

Lakeland showed that super-granulation can in principle be modeled in the Solar time series, and maybe the tiniest hint that when he corrects for super-granulation well, the RV variability might be even lower than at times at which there is no super-granulation in play at all. Does super-granulation suppress other kinds of variability?

I'm very optimistic—between Liang yesterday, Zhou's work at Flatiron, and these presentations—that we will be able to mitigate many difficult sources of stellar variability. I was inspired to outline a conceptual paper on why or how this is all going to work.

double periodogram

Cole Johnston (Leuven) is in New York this week. We discussed the problem of finding oscillation modes in the photometry of stars in the presence of a large, binary-induced periodicity. What he kind-of wants is a simultaneous fitting of a flexible periodic function plus a periodogram. We did some experiments (very promising!) and discussed the elements that will come together to make this all happen. The final method will look like a double fourier transform, in which one frequency grid gets the periodic part, and the other grid gets the rest of the modes and noise.

radical papers I want to write (or will never write)

I have to finish my NSF proposal with Mike Blanton (NYU), so naturally I am in procrastination mode. Here are three papers I wish I would write. Maybe I should post them on my ideas blog:

Occam's Razor is wrong: This paper, co-authored with Jennifer Hill (NYU), would be about the fact that, in the real, observed world, the simplest explanation is always wrong or at least incomplete.

Causation is just causality: This paper, maybe co-authored with David Blei (Columbia) or Bernhard Schölkopf (MPI-IS) or Hill, shows that you don't need to have free will in order to have cogent causal explanations of data. That is, you don't need to phrase causality in terms of predictions for counter-factual experiments that you might have chosen to do.

You don't ever want evidence: This paper shows that any time you are computing the Bayesian evidence—what I call the fully marginalized likelihood (fml)—you are doing the wrong integral and solving the wrong problem. For both practical and theoretical (principled) reasons.

data augmentation

A highlight of my day was a colloquium by Renée Hložek (Toronto) about cosmology and event detection with the LSST/Rubin. Importantly (from my perspective), she has run a set of challenges for classifying transients, based on simulations of the output of the very very loud LSST event-detection systems. The results are a bit depressing, I think (sorry Renée!), because (as she emphasized), all the successful methods (and none were exceedingly successful) made heavy use of data augmentation: They noisified things, artificially redshifted things, dropped data points from things, and so on. That's a good idea, but it shows that machine-learning methods at the present day can't easily (or ever?) be told what to expect as an event redshifts or gets fainter or happens on a different night. I'd love to fix those problems. You can almost think of all of these things as group operations. They are groups acting in a latent space though, not in the data space. Hard problems! But worthwhile.

predicting spectra from spectra

Saakshi More (NYUAD) came into my office during office hours today to ask about possible data science projects in physics. I pitched to her predicting ESA Gaia RVS spectra from Gaia XP spectra, and vice versa. Has anyone done that? In one direction, you have to predict high resolution detail from low-resolution input; in the other direction, you have to predict a wide wavelength range from narrow input. It seems like perfect for something like a linear auto-encoder (at least for a small patch of the color–magnitude diagram; non-linear for a large patch). Later in the day I talked to Gaby Contardo and she said: If you want to go simple, how about nearest neighbor? Good idea!

toning down my language

I spent travel time (at airports and on airplanes) working on the title, abstract, and introduction of the forthcoming paper with Andy Casey (Monash) about combining visit spectra into mean spectra. This was mainly about me changing the tone from “You are all doing it wrong!” to a tone more like “Here's a way to think about it, and the consequences thereof.” After all, no method is the best for all situations and cases. Our method is best for situations where the individual visit spectra are barely sampled or under-sampled.