substellar objects (brown dwarfs)

I spent the day at the NSBP / NSHP meeting in San José. My favorite session of the day was the morning astro session, which was entirely about brown dwarfs. I learned a lot in a very short time. Caprice Phillips (UCSC) introduced the session with an introduction to the scientific and technical questions in play. She put a lot of emphasis on using binaries and clusters to put detailed abundance ratios onto substellar objects. This was what I expected: I thought (walking in to this session) that all known abundance ratios for brown dwarfs were from such kinds of studies. I learned different (keep reading).

Gabriel Munoz Zarazua (SFSU) followed by showing spectra from M-dwarfs, brown dwarfs, and Jupiter. It definitely looks like a sequence. He does spectral fitting (what they call, in this business, retrievals). It looks like he is getting very good, somewhat precise, abundance ratios for the photospheres of substellar objects! I asked more about this in the question period, and apparently I am way behind the times (Emily Rauscher, Michigan, helpfully pointed this out to me): Now brown-dwarf photosphere models are so good, they can be used to measure abundances, and pretty well.

I also learned in this session (maybe from Jorge Sanchez, ASU, or maybe from Efrain Alvarado, SFSU) that there is a very strong mass–abundance relation in the Solar System. That is, we don't expect, if brown dwarfs form the way planets do, that the detailed abundances of the brown dwarfs will match exactly the detailed abundances of the primary stars. But now we are really in a position to test that. Sanchez showed that we can get, from even photometry, abundances for substellar objects in the Milky Way halo. Again, totally new to me! And he finds metallicities at or below −3. Alvarado showed data on an amazing system J1416, which is an L–T binary with no stellar companion. Apparently it is the only known completely substellar binary.

a likelihood for our Phi-M radio

There are AM radios and FM radios and (if you are a nerd) PCM radios. But Abby Shaum (CUNY) and I have built a Phi-M radio, which demodulates phase variations in a carrier signal. We (with Keaton Bell, CUNY) are using it to find binary companions and planets around stars that show coherent pulsation modes in their photometry. Today I wrote down a noise model for the output of our demodulator. It isn't completely trivial. But it's good, because we can make a likelihood function for fitting our companions. Our model will end up being a limit of the more general model called Maelstrom by Dan Hey (Hawai'i).

calibrating Gaia

I was honored today by being invited to a meeting of the group at Cambridge (UK) that extracts and calibrates the low-resolution ESA Gaia XP spectra. The model is bilinear: Each source is represented as a linear sum of basis functions, and each observation of each source has an expectation which is that linear sum multiplied by a convolution kernel, which is also represented as a sum of components. These components are smooth functions of position in the device and wavelength. It's a very nice system! I went through it all with them and said what I would have done differently (which is not much, I have to admit).

simulating data for phase and frequency modulation

Abby Shaum (CUNY) and I have been working on phase demodulation for binary detection and characterization, using coherent oscillation modes in stellar light curves. We are taking a pure signal-processing approach, which is lightweight and fast, such that we could automatically apply it to everything in Kepler or TESS. We also want to do frequency modulation for incoherent modes (which somehow our people think won't work; won't it?).

Today we discussed how to build fake data to fully test our systems. In the coherent case, this is easy! In the incoherent case this is harder. We discussed simulating it by drawing from a Gaussian Process. And we discussed simulating it by forward modeling a stochastically forced, damped harmonic oscillator.

time dependences in ultra-short-period planets

I had a great conversation with Noah Sodickson (high schooler) about ultra-short planets in the NASA Kepler data. He has been filtering the light curves, running astronomers'-favorite “box least squares” and plotting the folded light curves. When he does this in quarters (meaning, cutting the data into 90-day chunks), he can see small variations in the transit shapes. These have been attributed to various things. My goal is to figure out a way to model these variations without averaging the data. Binning is sinning, after all. Sodickson can see that everything we see is a strong function of how we filter the light curves, so we have to think pretty hard about that.

infrared-excess stars

My day started with a call with Gaby Contardo (SISSA) and Trevor David (Flatiron) about Contardo's project to find stars in ESA Gaia and NASA WISE that have infrared excesses. These stars should be young or dust-enveloped or host aliens! We are trying to phrase this problem as a prediction problem: How well can we predict infrared brightness from Gaia (visible) information, and are there stars with significant excess infrared? The answer seems to be yes: The histogram of differences between predicted and observed skews nicely to infrared-excess. Now: Are any of these known objects? And can we rediscover (say) star-forming regions?

Gaia quasar redshift distributions

Today I worked with Kate Storey-Fisher on the ESA Gaia quasar sample. We looked at the redshift distribution as a function of magnitude and as a function of color cuts or selection. Right now we are getting an odd angular correlation function for the sample, which we think is because our random catalog (or completeness model) is wrong in important but subtle ways. Interestingly, there are probably terms coming from both the dust map (extinction) and from the Gaia internal completeness (scanning law) and maybe both contribute enough to change the answer? But it sure looks like dust dominates.

Gaia quasar redshifts

Today Kate Storey-Fisher (NYU) arrived in Heidelberg and we worked on improving the ESA Gaia DR3 quasar redshifts, using NASA WISE and IRAS information. We were trying to reproduce experiments performed by Hans-Walter Rix (MPIA) last week. It looks like we can get clean-ish samples, where more than 90 percent of the redshifts are correct (as compared to ground-based spectroscopic surveys), provided that we stick to bright quasars. The question is: Is this good enough for our cosmological goals? We discussed how to empirically evaluate the selection function, which is our next task.

radio reboot

[Somehow this blog keeps failing. I will try to get back into it, but no promises! I apologize to my loyal reader.]

Today I met with Abby Shaum (IPAC) who worked with me a few years ago making a phase demodulator to find stellar companions. The idea is that if a star is broadcasting a coherent (or even incoherent) asteroseismic or pulsation mode, and if the star is orbiting a companion, the kinematics of the orbit will be imprinted on phase and frequency modulations of the carrier frequency. Like a radio! Indeed we built a signal-processing method that looks just like a radio demodulator. Today we discussed how to reboot this project and write a paper for the refereed literature.

what is a transmission function?

Mike Blanton (NYU) and I agree and disagree on almost everything about fundamental astronomy. As my loyal reader knows, I am writing something on how apparent magnitudes work, and also absolute, bolometric, reddening-corrected, and so on. The apparent magnitude of star depends (among other things) on a filter bandpass or a transmission function. There are two possible definitions of this. One is the fraction of light (as a function of wavelength or frequency) that makes it through the system, from the top of the atmosphere to stimulating the detector. The other is the mean contribution to the total counts read out by the detector of a photon (of a particular wavelength or frequency) impinging on the top of the atmosphere. These might sound similar—they are identical when your detector is a photon counter. But they aren't identical if your detector is, say, a bolometer. I struggled with how to simply communicate all this today.

infrared excesses for planet-hosting stars

Gaby Contardo (Flatiron) and I went to the Gaia EDR3 Archive to make use of its matched catalogs, matching up Gaia, 2MASS, and WISE. We are looking at very short-period planet hosts, which might show interesting photometric deviations. We took one host star, and then found many other stars with similar photometry in the visible. Do they agree in the infrared? It looks like maybe there is a tiny discrepancy? But the power will come from doing many, not just one.

why do we still use magnitudes?

As my loyal reader knows, I am writing a piece on magnitudes, distance moduli, color indices, color excesses, bolometric corrections, and so on. Today I sent a copy of the draft to my excellent colleague Mike Blanton (NYU) for comments. He came back with lots! One of the main points he made is an elucidation of why (in his opinion) magnitudes make sense to be using even in this day and age: The physical quantities of interest about a (say) star are its bolometric or total luminosity, and its detailed (high-resolution) spectrum. The thing we can observe, photometrically, is neither of these. So we integrate photons in a bandpass. The magnitude system is a way of summarizing the choices we make when we do that, and reminds the theorist or interpreter that our measurements are complex integrals of the things of interest. That's a good argument. Prior to this feedback, my answer to the “why?” question was about precise relative measurement, which isn't so relevant in many contemporary contexts.

Gaia DR3 plans: much better distances; unicorns

Katie Breivik (Flatiron) and I convene a weekly meeting at Flatiron for NYC-area astronomers to discuss and work on things we are going to do with the ESA Gaia DR3 and SDSS-V data. Today I showed people what Price-Whelan (Flatiron), Eilers (MIT), and I are planning on doing with the 10⁸ (!) very low-resolution Gaia BpRp spectra, which will be released in DR3 this summer: We plan on making a much-improved version of this method for estimating spectrophotometric distances to stars but covering the whole HR diagram and with better overall data and precision (we hope). In the meeting today we discussed ways to validate our distances (which is hard!) and we discussed what interesting side things might appear as part of the project. One category, that Dalcanton (Flatiron) liked, is this: Since our method involves comparing stars to other stars in their neighborhoods in spectrophotometric space, we will automatically and unintentionally find unicorn stars that are unlike any others. Those will be interesting for follow-up.

the bolometric correction of Vega is not zero

Wow I went Inside Baseball (tm) today on magnitudes. Magnitudes were originally designed to be purely observational, purely relative measurements, relative to a standard star. But then once you go off-course and define absolute magnitudes (gasp) and then even farther off and define bolometric magnitudes (double gasp), the system is no longer relative to anything! It depends on (imprecise) distance estimates and (wrong) stellar models. So you get many absurdities, like that Vega is not zero in any Vega-relative absolute or bolometric magnitude system. I think maybe we've lost our way! And not because magnitudes are obtuse; apparent magnitudes and colors make lots of sense! We lost our way when we tried to extend this to cover luminosities of various kinds.

the magnitude system: go big or go home

I spent some time on the weekend and today writing in my various writing projects. Actually, I am obliged to do this: I have a two-paragraph-per-day New Year's resolution. One thing I am (stupidly) writing is a note about how apparent magnitudes work, in order to (ultimately) explain the bolometric correction to physicists. This latter concept is confusing because when you measure through a limited band, you don't see the whole stellar spectrum, and yet the bolometric correction can be positive or negative or zero. Today as I was writing in this document, I realized that I have to do it all: Absolute magnitudes, distance moduli, color excesses, and so on, if I am going to reach my intended audience. I expanded the outline of the paper and I feel good about where it's going. That said, shouldn't I be doing research?

other people's code

I spent much of the day understanding Other People's Code (tm). One piece of code is legacy code from Aaron Dotter (Harvard) that computes magnitudes from flux densities. I was looking at it to confirm my frantic writing of yesterday. Another piece of code is the code by Weichi Yao (NYU) that implemented our group-equivariant machine learning. I was looking at it to see if we can modify it to impose units equivariance (dimensional scaling symmetries). I think we can, and I confirmed that it Just Works (tm).

magnitudes are (logarithmic) ratios of signals

A couple of weeks ago I had intense arguments with Belokurov (Cambridge) and Farr (Stony Brook) about the definition of a photometric bandpass and a photometric magnitude. And a few months ago I had long conversations with Breivik (Flatiron) about the bolometric correction. For some reason these things took over my mind today and I couldn't stop myself from starting a short pedagogical note about how magnitudes are related to spectral energy distributions. It's not trivial! Indeed, the integral isn't the integral you naively think it might be, because most photometric systems count photons (they don't integrate energy). People often say that a magnitude is negative-2.5 times the log of a flux. But that's not right! It is negative-2.5 times the log of a ratio of signals measured in two experiments.

Dr Steven Mohammed

Today I had the great pleasure to serve on the PhD defense committee for Steven Mohammed (Columbia). Mohammed worked on the post-GALEX-misson GALEX data in the Milky Way plane. We took the data specially as part of the Caltech/Columbia takeover of the mission at the end of the NASA mission lifetime. Mohammed and my former student Dun Wang built a pipeline to reduce the data and produce catalogs. And Mohammed has done work on photometric metallicities and the abundances in the Milky Way disk using the data. It is a beautiful data set and a beautiful set of results.

making age estimates out of abundances

Today Trevor David (Flatiron) showed me some amazingly precise and detailed dependences of abundance ratios on stellar age. The idea is: Different stars formed in different moments in the chemical-enrichment history of the Milky Way, and so the abundance ratios give the stellar ages in detail. The abundances he has are from Brewer and Fischer, and the ages he has are from various sources, including especially isochrone ages. We discussed the following problem:

Given that you don't believe any age estimates in detail, and given that any abundance measurements and age estimates are noisy and biased, what is the best way to build usable abundance–age relationships that can be used as alternative clocks (alternative to isochrones, stellar rotation, asteroseismology, and C–N dredge-up, for examples)? We settled on a few ideas, most of which involve building a low-dimensional hypersurface in the space of abundances and age, and then fitting for adjustments or corrections to different age systems.

Gaia BP-RP spectral fitting

Hans-Walter Rix and I discussed with Rene Andrae (MPIA) and Morgan Fouesneau (MPIA) how they are fitting the (not yet public) BP-RP spectra from the ESA Gaia mission. The rules are that they can only use certain kinds of models and they can only use Gaia data, nothing external to Gaia. It's a real challenge! We discussed methods and ideas for verifying and testing results.