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 fNL 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!


reconstructing the initial conditions

Today was a 21-cm cosmology meeting at Flatiron. Unfortunately I could only do the morning. Ue-Li Pen (CITA) spoke about reconstruction using a simplified dynamics. I suggested that anything that could be done with simplified dynamics could be done better with machine learning. I think I can even prove this, since the machine learning could be trained on the residuals away from the simplified dynamics reconstruction! In his talk, however, he mentioned this incredible Wang et al paper that does full reconstruction of the initial conditions for the entire SDSS Main Sample volume! This gives me hope for the future of cosmology.


small planets are all rocky?

At lunch today, Angie Wolfgang (PSU) gave a talk at the CCA on hierarchical inference of small and rocky exoplanet population properties. She made a nice set of arguments for the hierarchical Bayesian methodology, which was preaching to the converted (but good). She showed her results on exoplanet compositions and H/He envelopes, both of which are impressive, and then she went on to look at parametric and non-parametric fitting of the mass–radius relationship at small radii. She excludes zero mass scatter at fixed radius at all except the smallest radii. There is a consistent story emerging that the very smallest planets are indeed (pretty much) all rocky.

I had some back-and-forth with Megan Bedell (Chicago) about derivatives of our spectral model with respect to parameters. She has this all correct but I recommended parameterization changes, and the whole time, in the background, Dan Foreman-Mackey (UW) was saying things like “you should never take your own derivatives” and “you should use Theano”. I ignored him, probably at my peril.



Andy Casey (Cambridge) organized a call today in which we discussed his project to re-factor the ESO HARPS archive of high-resolution stellar spectra into a more useful form. We discussed our needs for this new archive for our own personal science projects. The idea is to have the schema and work on the archive be directly related to short-term science goals, and then hope that it will be useful for many other goals.


map-making, self-calibration, stellar rotation

In my stars group meeting at CCA, Ruth Angus (Columbia) told us about her work to replace standard methods for determining stellar rotation with a probabilistic model, based on Gaussian Processes with a quasi-periodic kernel. This seems to work extremely well! There are some pesky outlier stars, even in simulated data, in which all methods (including Angus's best) seem to get the wrong answer for the stellar rotation; these are interesting for further investigation.

In my cosmology group meeting, I got fully taken to school. First Brian Keating (UCSD) told us about self-calibration of CMB polarimetry. It turns out that you can't self-calibrate for some of the most important science. That is, your signal might be in exactly the modes to which the self-calibration is orthogonal! That's bad. And an aspect of self-calibration that I haven't thought about before. He discussed many crazy and creative ways to do absolute calibration of polarimetry devices; none of them look good enough (or cheap enough) at this point.

Then Colin Hill (Columbia) told us about map-making things he is working on in CMB data. I got all crazy because he is only considering linear combinations of observed data to (say) produce the thermal S-Z map (from, say, Planck data). But then he pointed out (correctly) that all least-squares methods return linear combinations of the data! Oh duh! All L2-like methods return linear combinations of the data. So then we went on to think about combined L1 and L2 methods that could permit him to open up his model space without enormously over-fitting. At the end of the discussion I had a job to do: Write down the largest class of convex map-making methods I can, given what I know about L1 and L2.

In between group meetings Cameron Hummels (Caltech) talked about open-source codes he is building that take simulation outputs from cosmological hydro simulations and predict observables, especially those that relate to the inter-galactic medium and circum-galactic medium. We talked a lot about the differences between resolution effects and sub-grid physics choices, which are confusingly inter-related.


Spitzer Oversight, day 2

Today was the second day of the Spitzer Oversight Committee meeting. Far and away the highlight for me was a presentation by Sean Carey (IPAC) about the overall health of the spacecraft and the imaging instrument. He showed the photometric throughput of the system as a function of time, which has been amazingly constant (sub-percent level), and yet showing a consistent and repeatable trend (of less than one mmag per year, ish?). He showed the bad pixel count, which has risen linearly with time, but only to a few hundred (and many of those are nonetheless still calibrated and useful pixels). He showed the astrometric wobble and drifts, associated with spacecraft thermal events. These are substantial, but changing, as the spacecraft goes through more and more extreme solar-angle events to downlink its data to the Deep Space Network. Spitzer is in an Earth-trailing orbit, so as time goes on, it has to point at worse and worse Sun angles to send its data home.

The latter point is very interesting: Carey showed that the batteries are still just as good as new, but that the spacecraft draws 11 A (yes, 11 Amps at 300 Volts!) on average (don't ask why), and when it is in downlink to Earth, the solar panels are not getting enough insolation to cover it. This leads to non-trivial scheduling in which the spacecraft must point near-orthogonal to the Sun vector for a while (hours) after downlink. This complexity is handled without issues by the non-trivial scheduling systems, and the overall spacecraft health is excellent. The mission can run until March 2019, when it is cut off, both by funding and by these Sun-angle issues. Carey also showed the long-term performance of the solar panels. This also declines linearly with time, but is easily within spec to keep the mission running (barring any severe micrometeorite hit). Knock wood!

It has been a great two days, with an absolutely great team, working on an absolutely great mission.


Spitzer Oversight, day 1

Today was the first of two days at the Spitzer Science Center, where I am (for the 8th year in a row) helping to advise the Spitzer mission as part of the Spitzer Oversight Committee, chaired by Mike Hauser (STScI, retired). The mission is in its last years, funded to continue observing to March 2019, then closing out for a year-ish afterwards, with no opportunity for further extension. Its lifetime is not set by money alone, however: Geometric constraints on its downlink to the Deep Space Network and its power and insolation and thermal needs during that downlink take the spacecraft out of safe operating conditions in 2019.

One of the things that was discussed today was synergies with JWST and TESS. There will be more than a year in which TESS and Spitzer are simultaneously flying. This creates a lot of interesting opportunities. I made a mental note to discuss this with some of my more ambitious exoplanet people. The JWST Early Release Science call has been released, and the calibration and photometry things we have learned (over many years) in the Spitzer, Kepler, and SDSS contexts (and Euclid and LSST contexts) could form the basis of a great ERS proposal. Let's not wait for five to ten years to figure out how to do the best possible science with JWST! Again, if I am going to go there, I need to assemble the right team.

Before the start of the meeting, I had lunch with Kat Deck (Caltech). We had a wide-ranging conversation, which included exoplanets, proto-planetary disks, the theory of planet formation, and gravitational radiation backgrounds and sources. On the latter, I really have to get involved; there are so many simple questions at the intersection of theory and data analysis.


transits of swarms of debris

I met with Ellie Schwab (CUNY) and Kelle Cruz (CUNY) to discuss Schwab's model of stellar activity in low-mass stars. We checked her MCMC sampling diagnostics and worked out how to make her model more general. It is a mixture model, with an active and inactive population of stars, mixed.

I met with Caroline Kaler (NYU) to get her started looking at the Kepler data. Inspired by my visit to Rochester this week, I have her looking at the Boyajian Star. I have a crazy thought that we might be able to use the smoothness (or not) to limit (or measure) the number of bodies contributing to the light-curve events, if those events are multi-object transits.


detailed abundance trends with binarity

Taisiya Kopytova (ASU) arrived in New York today for two days to work on the detailed chemical abundances of giant stars in APOGEE that host substellar and low-mass stellar companions. We have cast this problem as a very simple comparison between the companion-hosting population and a control sample. The control sample is carefully selected to be identical in stellar parameters. It looks like she has a result, and the result is interesting. It isn't a large effect, however. We discussed how to show it, convincingly, and how to describe it, accurately.

Lauren Anderson (Flatiron) and I continued to debate how to put spectroscopic labels onto Gaia TGAS stars without spectroscopy, and how to (before that) de-noise the labels themselves, which we are getting from the RAVE-on catalog of Andy Casey et al. We are a bit confused about how principled to be in the latter de-noising. It seems crazy to proceed without doing it, but building a full hierarchical model seems like overkill when the main point is just that stars on the main-sequence must have large surface gravities!



I spent the day at the University of Rochester, where I gave the Physics Colloquium. I spoke about data-driven models. Before my talk, I had many interesting and valuable conversations with faculty and students. One highlight was work that Alice Quillen (Rochester) is doing on tidal dissipation. She is building mechanical models of solid bodies (think: planets) to parameterize tidal dissipation and look at tidal locking mechanisms, and spin–orbit resonances and dynamics.

Another highlight was a long conversation with Eva Bodman (Rochester) who (among other things) has been looking at extra-solar comets in the Kepler data. We discussed things she has done, but also the low-hanging fruit for future work on comets around other stars. She has built a model of the strange behavior of the Boyajian Star in terms of a (bizarre, huge) comet population; this made me think that there are lots of things we might do with comet population models, or other models of swarms of debris.


foregrounds and optimization

It was a low-research day today. But I did get in a short and valuable discussion of CMB foregrounds with Boris Leistedt (NYU). The approach I want to pursue is to make a latent-variable model, which posits a set of scalar fields, and nonlinear functions that convert them into (high resolution) maps, that are compared to the data through the relevant beams. I think this will (almost provably) beat current approaches. I also had some conversations with Bedell about optimization. We are trying to fit for stellar spectra and radial velocities, and (as usual) we are finding that out-of-the-box optimizers don't work well!


better exoplanet searches through chemistry

Today I had the great privilege of spending the day with the group of Karin Öberg (CfA) at Harvard and also the NG Next team. Öberg's group is doing so many great things, related to astronomical observations of proto-planetary disks and also real lab experiments on ices and solid-state chemistry relevant to interstellar and accretion-disk physical conditions. Here are some highlights:

Ellen Price (CfA) showed a consistent chemical model in which they evolve the molecular contents of gas as it orbits in the evolving accretion disk. This is based on a NLTE chemical model built by Ilse Cleeves (CfA). She can see big changes as gas crosses the snow line (or various snow lines for different species). Edith Fayolle (CfA) showed absolutely incredible ALMA observations they have (with Cleeves and also Ryan Loomis, CfA) of proto-planetary disks around young stars. In these observations, there is so much, I could fill a whole separate set of blog posts: They see various kinds of organics that weren't expected to be formed in abiotic conditions. They also can image the disk in two spatial dimensions and the radial velocity dimension in thousands of chemical species. This is unprecedented detail on a disk, and also unprecedented information about molecules in these conditions. We discussed ways we could simultaneously model all of this and make very sensitive measurements of what is going on at various laces in the disk. As part of this discussion, Öberg and I discussed the problem that there isn't a good out-of-the-box imaging pipeline for ALMA, in part because different users with different targets have very different priors and goals.

But then we switched to lab stuff! Mahesh Rajappan (CfA) described the Öberg-lab experimental setups, in which they can deposit ices, including multilayer things, and then radiate them or heat them, to measure solid-state chemistry processes directly. Jennifer Bergner (CfA) is doing lab experiments to find and measure configurational rate constants for chemical processes in ices. These rates relate to the processes by which molecules find one another and reorient to permit solid-state reactions to take place. She was working in particular on O+CH4 to CH3OH. One theme of the day's conversations is that the organic chemistry of proto-planetary disks is seriously complex and contains everything that is needed for life (we think).

At the end of the day we discussed research synergies. I think the biggest is in building consistent models of the thousands of molecules, in the dynamical disk. One incredible idea is that a forming gas-giant planet should be hot (gravitational or accretion energy); this could affect the local chemistry in the disk: We could see the thermal signature of a forming planet in molecular species! That's a great goal for the near future. Öberg's group (and especially Cleeves and Loomis) have the data in hand, or coming when ALMA gets to their targets.


stars, disruption, photometric redshifts

Today began with a meeting about GALEX, where Steven Mohammed (Columbia) showed that there is great metallicity information in the overlap of GALEX and Gaia, and we discovered that something must be seriously wrong with the astrometry in our re-calibration of the data.

Andy Casey (Cambridge) organized a phone meeting in which a bunch of us discussed possible scientific exploitation of the data in the ESO HARPS archive, which contains thousands of stars, each of which has tens to thousands of epochs, each of which is signal-to-noise of hundred-ish, and resolution of 100,000. Incredibly huge amounts of data. Huge. Casey asked each of us to describe low-hanging fruit, and take on short-term tasks. One thing we might do is re-factor the archive into something more directly useful to investigators.

Sjoert Van Velzen (JHU) gave the astrophysics seminar about tidal disruption events. He has a great set of results, starting from search and discovery, going through theory and models, and continuing on to multi-wavelength follow-up. The most intriguing result is that the TDEs are amazingly over-represented in post-starburst (E+A) galaxies (which I used to work on). It is hard to imagine any origin for TDEs that would so strongly concentrate them into these environments. It makes me wonder whether the things they are seeing aren't TDEs at all?

After the seminar, Boris Leistedt (NYU) posted to the arXiv our new paper on photometric redshifts. The idea is that we use what we know about Doppler Shift and bandpasses and calibration of photometry, but let the galaxy SEDs themselves be inferred, latent variables. This combines the best properties of machine-learning methods (that is, flexibility, non-parametrics) with the best properties of template-based methods (that is, regularization to physically realizable models, a generative model, and interpretability). It seems to work very well!



It's job season and my head is only just above water! Adam Riess (JHU) gave a nice colloquium at NYU today about the distance scale, and the comparison between the distance ladder and the cosmic microwave background.