how did the Solar System form?

I saw a very nice talk today by Philippine Griveaud (MPIA) about how the Solar System formed. The idea is that the giant planets formed in an accretion disk. Their formation opened gaps and caused migration (first Type I and then Type II, if you must know :). That migration pulled them into a resonant chain. That is, if the giant planets formed the way we think they formed, they must have been in a resonant chain. But they aren't in such a chain now; what gives?

The idea is that when the gas is expended (or blown out by winds), the remaining planetestimals (think: asteroids, comets, Kuiper Belt objects) interact with the planets such that they get moved from orbit to orbit and eventually ejected. These dynamical interactions break the resonant chain, migrate the giant planets to their current locations, and scatter rocks and ice balls into the interstellar regions.

It was a great talk, but also led to a lot of interesting questions, such as: How does this all fit in with the formation of the rocky planets? And how does this square with our observations (growing rapidly, apparently) of interstellar asteroids? Oh and: How does all this connect to observations of debris disks, which I now (officially) love.

time-domain astronomy in Gotham

Today, Tyler Pritchard (NYU) and I assembled a group of time-domain-interested astrophysicists from around NYC (and a few who are part of the NYC community but more far-flung). In a two-hour meeting, all we did was introduce ourselves and our interests in time domain, multi-messenger, and Vera Rubin Observatory LSST, and then discuss what we might do, collectively, as a group. Time-domain expertise spanned an amazing range of scales, from asteroid search to exoplanet characterization to stellar rotation to classical novae to white-dwarf mergers with neutron stars, supernovae, light echoes, AGN variability, tidal-disruption events, and black-hole mergers. As we had predicted in advance, the group recognized a clear opportunity to create some kind of externally funded “Gotham” (the terminology we often use for NYC-area efforts these days) center for time-domain astrophysics.

Also, as we predicted, there was more confusion about whether we should be thinking about a real-time event broker for LSST. But we identified some themes in the group that might make for a good project: We have very good theorists working, who could help on physics-driven multi-messenger triggers. We have very good machine-learners working, who could help on data-driven triggers. And we have lots of non-supernovae (and weird-supernova) science cases among us. Could we make something that serves our collective science interests but is also extremely useful to global astrophysics? I think we could.

finding outer Solar System objects in TESS

Ben Montet (UNSW) put up the bat symbol for a project to find outer Solar System objects using NASA TESS data. It was the scale of problem I wanted: A stack of postage stamps from eleanor and a reflex angular velocity vector (really parallax vector) for Sedna. The issue is all those other pesky stars, and asteroids on closer orbits! I did some exploratory data analysis, in which I detrended each image using a PCA of all the images. But at the same time, others did pixel-based detrending like in CPM and others did more traditional image differencing with a reference image. It's nice: In all methods, Sedna sticks out beautifully! The question is: What methods will be best for finding objects further out in the Solar System? These are harder because they are fainter, but also because they don't move as far relative to the stars in a month!

#GaiaSprint, day 2

After playing with visualization yesterday, Christina Eilers (MPIA) and I got the idea that perhaps the radial-velocity variations we see in the Milky Way disk might indicate density variations. In particular, does the radial-velocity field converge on high-density regions in the disk (spiral arms, say) and diverge on low-density regions (inter-arm gaps, say)? Sarah Pearson (Flatiron) came to our rescue with a nice visualization of the density and velocity fields, in which she could smoothly go from showing one to the other. And indeed, our intuitions were justified, at least qualitatively.

In the evening check-in, Paolo Tanga (Côte d'Azur) showed some beautiful results on the ESA Gaia coordinate systems relative to other catalogs. He calls these differences "zonal corrections" for historical reasons! I asked him how he knows which of the coordinate systems is best, and he said: In the best frame, the asteroids will travel on calculable trajectories. (I would say gravitational trajectories, but for asteroids, radiation pressure and other forces are relevant too!) So the best coordinate system will be Newtonian in the Solar System! Of course given frame dragging, and strictly speaking, Newtonian for the Solar System will not be Newtonian for the Galaxy! I asked about that and it led to some discussion with Larry Widrow (Queen's). I have much to say about all this, but I'm not yet ready to say it out loud.

actions useful? and stars meeting

In the morning I met with Gus Beane (Flatiron) to discuss his use and understanding of actions in empirical work on the Milky Way, following up on the blow-up of last week. We discussed the point that small issues with the Galactocentric coordinate system could totally mess any action calculation, even if the actions make sense, and the point that there are many possible galaxies we might live in for which the actions don't even make sense. We vowed to move the conversation / argument going on at Flatiron towards the question of what we are trying to achieve with these calculations. Are they just orbit labels? Or are they quasi-invariants? Or are we using them to match up stars that are far apart?

Stars Meeting was a whirlwind of interesting things! Kim Bott (UW) told us about polarimetry for exoplanet discovery and characterization. Evan Bauer (UCSB) told us about accretion signatures on white dwarfs and how they have probably been mis-interpreted (but in a way that makes accretion more important!). And Suroor Gandhi (NYU) told us about relationships between ages, abundances, and actions in the Milky Way that suggest that there are interesting relationships at all ages, and at all abundances. Hard to summarize, but there are lots of things to think about in there.

phase-space volume; Oort dynamics

The research highlights of the day were a call with Matt Buckley (Rutgers) and a Physics Colloquium by Scott Tremaine (IAS). In the former, we discussed the design of a first paper about Buckley's work on measuring phase-space volumes of bound and disrupting dynamical objects in the Milky Way halo. He has some great results! But we don't understand the sensitivities to noise yet, or the in-practice issues of making robust measurements. And I mean “robust” here in the statistical inference sense.

In the latter, Tremaine answered most of the questions we formulated a few weeks ago about the origin and properties of the Oort cloud. My loyal reader may know that I am suspicious about many of the things that are said about the Oort cloud, but Tremaine showed numerical results that seem to back up most of the lore. He then switched to talking about interstellar asteroid 'Oumuamua. Aside from the usual loose talk of aliens, Tremaine said something remarkable: The pre-Solar-System velocity vector of the object is very close to current consensus on the Local Standard of Rest (something else of which I doubt the existence). Tremaine noted that it might conceivably represent an amazingly accurate measurement of the LSR! Too early to tell yet.

planetesimals!

Today Michele Bannister (Belfast) gave a great talk about the outer Solar System. She was very clear that her observations do not rule out in any way the existence of Planet 9. But they do discredit every single shred of evidence in its favor! And she gave many other mechanisms that could explain the same data. That is, there really doesn't seem to be any reason to believe that there is an unknown planet hanging out in the outer Solar System. Lots of what she said relies on the following theoretical observation: When a planetesimal is perturbed by a massive body on an orbit interior to its perihelion, it tends to preserve its perihelion but change its semi-major axis. And the same but opposite when the massive body is outside it's aphelion. All planetesimal migration scenarios must respect these constraints.

Before that, Kate Storey-Fisher (NYU) and I had a long conversation in which we re-discovered our confusions about the differences between the continuous Fourier transform (which never exists in any real-data context) and the discrete Fourier transform (which is what's appropriate when the data are treated as a patch of a periodic function. We got confused and then un-confused, but I am still somewhat confused!

the very local neighborhood

Today Jackie Faherty (AMNH) gave the astro seminar at NYU. She got us fired up about Gaia even before her talk, at lunch, where she said that on April 25 the curtains would finally open and we would get to see the Milky Way for the first time! Her seminar didn't disappoint: She pointed out that of the five closest stars to the Sun, three were discovered in 2014! And it appears that the Solar Neighborhood still has lots of secrets for us to discover. She also showed us a star that passed within 60,000 AU of the Sun some 70,000 years ago. That's interesting! If it disturbed comets onto elliptical orbits, we won't see their infall for a few million years! (Just a free-fall argument there.) That observation, combined with things people have found in Gaia DR1, suggests that we have a close encounter like that about once per million years.

circumbinary planets, next-gen EPRV

The Gaia DR2 workshop and Stars Group meeting were both very well attended! At the former, Price-Whelan (Princeton) showed us PyGaia, a tool from Anthony Brown's group in Leiden to simulate the measurement properties of the Gaia Mission. It is really a noise model. And incredibly useful, and easy to use.

In the Stars meeting, so many things! Andrew Mann (Columbia) spoke about the reality or controversies around Planet 9, which got us arguing also about claims of extra-solar asteroids. Kopytova (ASU) described her project to sensitively find chemical abundance anomalies among stars with companions, and asked the audience to help find ways that true effects could be scooped. Her method is very safe, so it takes a near-conspiracy, I think, but Brewer (Yale) disagreed. Veselin Kostov (Goddard) talked about searching for circumbinary planets. This is a good idea! He has found a few in Kepler but believes there are more hidden. It is interesting for TESS for a number of reasons, one of which is that you can sometimes infer the period of the exoplanet with only a short stretch of transit data (much shorter than the period), by capitalizing on a double-transit across the binary.

Didier Queloz (Cambridge) was in town for the day. Bedell (Flatiron) and I discussed with him next-generation projects for HARPS and new HARPS-like instruments. He is pushing for extended campaigns on limited sets of bright stars. I like this idea for its statistical and experimental-design simplicity! But (as he notes) it is hard to get the heterogeneous community behind such big projects. He has a project to pitch, however, if people are looking to buy in to new data sources. He, Bedell, and I discussed what we know about limits to precision in this kind of work. We aren't far apart, in that we all agree that HARPS (and its competitors) are extremely well calibrated machines, much better calibrated than the end-to-end precision obtained.

#AAS227, day 4; AAS Hack Day

Today was the fourth annual AAS Hack Day (#hackaas) at #AAS227, organized by Kelle Cruz (CUNY), Meg Schwamb (Taiwan), and myself, and sponsored by the LSST Corporation and Northrop Grumman. We had a huge crowd: About fifty people and the staff had to bring in extra tables, chairs, and power strips. The hacks varied enormously in scope and category; here are just a few that stood out:

AAS meeting conflicts

Adrian Price-Whelan (and a bit Scott Idem and me) used some vector-of-words methods from previous AAS Hack Days to look at schedule issues in the AAS 227 program. He found pairs of oral sessions that were scheduled in conflict that contain talks with abstracts that are close in word space. The idea was to predict which sessions led to the largest number of complaints to the AAS about scheduling, and also provide prototypes of tools that might be used to make scheduling better in the future.

gender and questions in AAS oral sessions

Mehmet Alpaslan and a team including Hack-Day veteran Jim Davenport looked at new data on oral session question-askers and speakers and chairs, finding (as we learned at earlier meetings) that men ask more questions than women, but also finding that the gender of the speaker seems to be correlated with the gender of the question-asker. The data are barely understood at present, being only days old.

crowd-sourcing the old literature reference graph

In some twitter activity prior to the meeting, we discovered that old papers have poor citation and reference information, because the references were often in footnotes, formatted inconsistently, and OCR-ed badly. Brooke Simmons taught the AAS Hack Day participants how to build prototype Zooniverse projects for crowd-sourcing, and Brendan Wells used that knowledge to build a project to solve this old-reference problem. Love that collaboration, which was un-imagined prior to the Hack Day!

glassdome: glassdoor for astronomy

Ellie Schwab and friends started to build a site where people of all different ranks and seniorities could openly or anonymously review their home institutions, and comment on salary and other often-private things. Originally the project started as anonymous, but evolved to more encouraging of open and transparent reviewing as the day went on.

finding asteroids with Kepler

Geert Barentsen arrived with the retrospectively obvious point that the Kepler satellite is awesome for finding asteroids: It spends (in its K2 mode) half of its time looking inside the Earth's orbit, so it is great for finding Earth-crossing and inner asteroids. It also has great cadence and sensitivity. He assembled a great team and started to look. Science! Also on the science with Kepler tip, Jennifer Cash and Lucianne Walkowicz started work extracting photometry from full-field images.

death to Jet

Timothy Pickering propagated the new matplotlib non-Jet colormaps to plotly.js. This is God's work, as it permits web-plotting gurus to benefit from the latest research in visual perception of continuous data. In case you haven't been paying attention, Hack Days are a great time for people to bond over their hatred of the Jet colormap, but Pickering also reminded us of the research that shows that it leads to misconceptions about the data, fails in black-and-white printing, and is bad for people with vision impairments.

exoplanetary systems in WWT

David Weigel, after reminding us that World-Wide Telescope has gone open source and is now a project of the AAS, showed us how he put a known exoplanet system into the software. The plan is to get them all in there and then make possible tours and activities around exoplanet discovery and science.

fabric poster upcycling

Ashley Pagnotta and company brought a sewing machine to #hackaas. It turns out that it makes sense these days to print your poster on fabric not paper! This is because fabric printing is now very cheap, and you can pack a fabric poster trivially in your luggage. Check it out. But Pagnotta and colleagues brought patterns and skills and turned posters into infotaining clothing. Insane.

More (crowd-sourced but incomplete) notes are available here. Thanks to our sponsors and everyone who came, and see you next year!

faint asteroids; statistical significance in dollar units

In a day shortened by health issues, I did get in a good conversation with David Schlegel (LBL), Aaron Meisner (LBL), and Dustin Lang on asteroid detection below the “plate limit”. That is, if we have multi-epoch imaging spread out over time, and we want to find asteroids, do we have to detect objects in each individual exposure or frame and then line up the detections into orbits, or can we search without individual-image detections? Of course the answer is we don't have to detect first, and we can find things below the individual-image detection limits. Meisner has even shown this to be true for the WISE data. We discussed how to efficiently search for faint, Earth-crossing (or impacting) asteroids.

I had lunch with luminary David Donoho (Stanford); we discussed a very clever of idea of his regarding significance thresholds (like five-sigma or ten-sigma): The idea is that a five-sigma threshold is only interesting if it is unlikely that the investigator would have come to this threshold by chance. As computers grow and data-science techniques evolve, it is easier to test more and more hypotheses, and therefore accidentally find (say) five-sigma results. Really the question should be: How expensive would it be to find this result by chance? That is, how much computation would I have to do on a “null” data set to accidentally discover a result of this significance? If the answer is “$5000 of Amazon EC2 time” then the result isn't really all that significant, even if it is many sigma! If the answer is “a billion dollars“ it is, probably, significant. We expanded on this idea in a number of directions, including what it would take to keep such calculations (translations of significance into dollars) up-to-date, and how to get this project funded!

UV colors of asteroids

I plotted the UV–optical colors of the minor planets we have found with GALEX. They are very, very red; redder even than the Sun by quite a bit. No surprise there. Are there any minor planets in there that are previously unknown? If so, we get to name them, right?

stars, galaxies, and minor planets

I spent most of the productive part of the day confirming that our statistical counterpart associations between GALEX and SDSS match the detailed figures published by the GALEX team here and here. They do, although we have more outliers, in part because we are being too liberal with the input catalogs. The thought of figuring out and applying conservative cuts drove me to outlining the GALEX but not SDSS paper and improving the output of the code I wrote to automatically compare the GALEX orphans against the Minor Planet Checker.

minor planets in GALEX

After getting annoyed with some of the output of my statistical counterpart association stuff, I went back to looking at the no-SDSS GALEX sources, especially the minor planets. I hope to have a full list by the end of the day tomorrow.

minor planets, hacking

I hacked a script-run query to the minor planet center so I can automatically check whether or not my GALEX-only (no SDSS) sources are known minor planets. This required some of Lang's web foo (much appreciated). Some of them are clearly bright, known minor planets, and some of them aren't, but I don't yet have any evidence that I have discovered any new Solar System objects. As a commenter on an earlier post noted, the point is to measure the ultraviolet albedos of the known minor planets we have caught with GALEX; this is much easier than discovery, and more likely to produce something I can publish!

minor planets, black-hole orbits

In our work on sources that show up in GALEX imaging but not SDSS imaging, Schiminovich and I have found large numbers of minor planets. This surprised me, but GALEX has huge coverage! Interestingly, GALEX has time-tagged photons, so you can get (minimal) proper motion information for fast-moving sources straight out of the GALEX time stream direcctly. My next job is to figure out which of these minor planets are already known. It should be all of them, but there is no reason not to check!

At pizza lunch, Gabe Perez-Giz (Columbia) gave a very nice talk about test-particle orbits around black holes. He (with Janna Levin) has found that periodic orbits are very much easier to analyze in many ways than non-periodic orbits; this would be sophistry except that he showed that for any non-periodic orbit there is an arbitrarily similar periodic orbit. This is completely obvious once someone (Gabe in this case) does an enormous amount of completely non-obvious work.