Knutson, calibration, hot stars

Heather Knutson (Caltech) arrived for a week of hacking on exoplanet and brown-dwarf spectroscopy. She has a number of things she has brought for our consideration. But the one that seems to be sticking is the inadequacy of her theory-driven or physical tellurics model. It has systematic residuals. We are going to explore options for tweaking the model using a data-driven fit to the residuals. This is a structure that I would like to try also for The Cannon: Instead of making a data-driven model for the stellar spectra, we could make a data-driven model for the residuals of the spectra away from best-fit models. And the parameters for the physics-driven model and the data-driven model could be tied together (or not) in various clever ways. So much idea.

At lunch, Anthony Pullen (NYU) gave a great talk about foreground mitigation in line-intensity mapping experiments. He went through all the kinds of auto-correlations, cross-correlations, and de-correlations that can be done to remove or mitigate foregrounds. The talk reminded me of many conversations I have had over my life about self-calibration, which led me to think about whether we could replace the cross-correlation parts of his model with a kind of self-calibration. Worth thinking about!

Late in the day, Benjamin Pope (NYU) and I came up with a good plan for looking at hot stars in Kepler. We could look at modeling them as a mixture of asteroseismic modes, spacecraft systematics, and planets. And then probably find nothing! But find nothing better than it has been found before. I like that kind of project.

star spots and exoplanets; mapping the disk

Today in MPIA/LSW Stars Meeting Néstor Espinoza (MPIA) gave a nice presentation about how star spots (cool spots) and faculae (hot spots) on stellar surfaces make it difficult to simply extract an exoplanet transit spectrum from differences between in-transit and out-of-transit spectra of the star. Some of the issues are extremely intractable: Even spectral monitoring of the star might not help in certain geometries. But we did agree that space-based spectral monitoring could do a lot towards understanding the issues. He showed that some of the transit-spectrum results in the literature are likely wrong, too. One conclusion: Gaia low-resolution spectrophotometry as a function of transit epoch at Gaia DR4 or thereabouts might have a lot to say here! And I also thought: SPHEREx!

After weeks of writing, today I finished the zeroth draft (yes, it isn't even close to being ready for anything) of the paper about our spectrophotometric parallax model for luminous red giant stars with Eilers (MPIA). I will get it into a state that I can share it with the APOGEE team this week.

And Eilers made maps of kinematic evidence of non-axi-symmetry in the Milky Way disk and radial abundance gradients, using our luminous red giants. We have lots of issues of interpretation, but there are a lot of things here. In my spare brain cycles I figured out a way that we could use Eilers's results to calibrate the variations of the inferred stellar abundances as a function of effective temperature and surface gravity: We can see that the data have issues.

SPHEREx workshop, day 2

I got up at 0530 and looked at the participants and schedule for the SPHEREx workshop. I realized that I had prepared precisely the wrong talk yesterday! So I threw away my slides and made completely new slides. It was rushed. I forgot things. But it was still an improvement. I switched from saying things about scientific goals to saying things about technical improvements or extensions that could make the project more capable in respects that would serve the needs of (among other things) stellar science.

I then headed in to the workshop; I could only make it to the second day. I learned so much today. I can't do it justice. Here are some random facts: A lot could be learned about exoplanets if we could get bolometric fluxes for the stars.
I knew this already, I guess, but the prospects for SPHEREx here are excellent, if the project can deliver absolutely calibrated flux densities. There is a mass–metallicity relationship inside the Solar System! The Solar System contains Trojan satellites/asteroids around Neptune, not just Jupiter! There is no model for the zodiacal light in the Solar System that matches the observations to the level of precision that an infrared survey would need to remove or avoid it. The zodiacal light is consistent with being made up of ground up asteroids and evaporated comets! ALMA has observed many debris disks around nearby stars; some of these are angularly huge. The poster child is Fomalhaut, which has a thin, elliptical ring. It's a crazy thing. I learned these things from a combination of Dan Stevens (OSU), Jennifer Burt (MIT), Carey Lisse (JHU), and Meredith MacGregor (Harvard), but that's just a tiny sampling.

At the end of the day there was discussion of calibration, led by Doug Finkbeiner (CfA) and me. I very much enjoy the technical challenges for SPHEREx and the enthusiasm of the team taking them on.

slides prep

It was a very low-research day! But on the train to Boston, I prepared slides for a short talk at a meeting at Harvard about the SPHEREx mission concept. I wrote about how this cosmology mission (line intensity mapping and large-scale structure) might revolutionize our knowledge of stars in the Milky Way.

stars, planets, SPHEREx, and black-hole dark matter

In the last stars group meeting of the year, we had special guests John Brewer (Yale, AMNH) talking about the chemical abundances of stars hosting planets, Ellie Schwab (CUNY) talking about magnetic activity in low-mass stars and brown dwarfs, and Jackie Faherty (AMNH) talking about searches for long-period companions to solar-like stars. Brewer killed the diamond-planet hypothesis that was so cool a few years ago. Ue-Li Pen (CITA) commented to Schwab that 21-cm surveys (see yesterday's post) will and even already do have time-domain radio observations of thousands to millions of stars. And Faherty showed that searches for long-period companions have been incredibly productive, even though they haven't led to exoplanet discoveries (yet).

In the last cosmology group meeting of the year, we had special guests Roland de Putter (Caltech) talking about the observing plans for SPHEREx and Yacine Ali-Haimoud (JHU) talking about black-hole dark matter (my favorite theory of dark matter). SPHEREx performs a very cleverly designed 0.75-5 micron all-sky low-res spectral survey of every point on the sky. It will get redshifts for hundreds of millions of sources, with small photometric-redshift uncertainties. He talked about primordial non-linearity; the survey will get limits or a detection of f_NL of <1. The audience was very interested in foregrounds, including Milky-Way stars, and even the zodiacal light in the Solar System.

Ali-Haimoud spoke about 2-body and 3-body effects in a black-hole theory of dark matter to get rates for LIGO. With careful re-analysis, he revises (heavily) the Ricotti et al 2008 limit on BHs as a dark-matter candidate and greatly weakens the constraints from CMB spectral distortions and anisotropies. But in the end he was very careful not to endorse black holes as a dark-matter canadidate. I'm stoked nonetheless!