systemic

A recent e-mail from Bruce Gary prompted me to pay a return visit the Exoplanet Transit Database (ETD) which is maintained by the variable star and exoplanet section of the Czech Astronomical Society. I came away both impressed and inspired. The ETD is really leveraging the large, fully global community of skilled small-telescope photometric observers.

There are hundreds of citizen scientists worldwide who have demonstrated the ability to obtain high-quality light curves of transiting extrasolar planets. I’ve developed many contacts with this cohort over the past decade through the Transitsearch.org project, and small-telescope observers played a large role in the discovery of the two longest-period transits, HD 17156b, and HD 80606b.

Once a particular planet has been found to transit, there is considerable scientific value in continued monitoring of the transits. Additional perturbing planets can cause the transit times to deviate slightly from strict periodicity, and a bona-fide case of such transit timing variations (TTVs)  has become something of a holy grail in the exoplanet community. A perturbing body will also produce changes in the depth and duration of transits as a consequence of changes in the orbital inclination relative to the line of sight. Moreover, for favorable cases, a large moon orbiting a transiting planet can produce TTVs detectable with a small telescope from the ground.

New transiting planets are being announced at a rate of roughly one per month. The flow of fresh transits continuously improves the odds that systems with detectable TTVs are in the catalog, but also makes it harder for any single observing group (e.g. the TLC project) to stay on top of all the opportunities.

The Exoplanet Transit Database maintains a catalog of all publicly available transit light curves. At present, there are 1113 data sets distributed over 58 transiting planets. The ETD site provides a facility for photometric observers to upload their data, and also provides online tools for observation scheduling and automated model fitting. Simply put, this is a groundbreaking resource for the community.

The ETD also provides concise summaries of the state of the data sets. Light curves are divided into five quality bins, depending on the noise level, the cadence, and the coverage of the photometry:

It’s interesting to go through the summary reports for each of the transiting planets. Here’s the current plot of predicted and observed transit times for Gliese 436b, the famously transiting hot Neptune:

The data show no hint of transit timing variations. (So what’s up with that e?)

In other cases, however, there are hints that either the best-fit orbital period needs adjustment, or that, more provocatively, the TTVs are already being observed. TrES-2 provides an intriguing example:

In sifting through the database, it looks like XO-1, CoRoT-1, Hat-P-2, OGLE-TR-10, OGLE-TR-132, OGLE-TR-182, TrES-1, TrES-3, and WASP-1 are all worthy of further scrutiny.

Over the past year, as a result of Stefano Meschiari’s efforts, the Systemic Console (latest version downloadable here) has been evolving quite quickly behind the scenes. Stefano and I are working on a paper which illustrates how the console can be used to solve the TTV inverse problem through the joint analysis of radial velocity and transit timing data. In the meantime, it’s worth pointing out that the ETD database lists transit midpoints in HJD for all of the cataloged light curves. These midpoints can easily be added to the .tds files that come packaged with the console.

Israelian et al’s Nature paper on the planet-stellar lithium correlation (featured in last week’s post) caused quite a stir in the community. The depletion of lithium in the atmosphere of a solar-type star seems to be a prerequisite for the presence of a detectable planetary system. Here’s the paper’s plot again, this time, with Alpha Cen A added for effect.

Had Israelian et al.’s paper come out a decade ago, much of the ensuing hubub would have focused on the fact that low lithium abundance is an effective signpost to planetary systems. Nowadays, though, mere detection of new planets is passé. Everyone knows there are tons of planets out there. Focus is shifting to finding the lowest-mass (and preferably transiting) planets around the brightest M, K, and G main sequence stars in the Sun’s neighborhood. There is a short, highly select, list of worlds that have been, and will eventually be, followed up to great advantage with HST, Warm Spitzer, and JWST.  All of the Sun’s most alluring stellar neighbors are under heavy and ongoing scrutiny, and in fact,  it’s these particular stars (in the form of the HARPS GTO list) that enabled discovery of the planet-lithium correlation.

So planet-finding utility aside, the intense interest in the planet-lithium effect stems from the fact that it’s guaranteed to be imparting an important clue to the planet-formation process.

With over 400 planets known, clear populations are starting to emerge. It’s remarkable that the strength of the lithium-planet correlation seems to be largely independent of the masses and periods of the planets themselves. The mass-period diagram for planets, on the other hand, shows that there are at least three distinct concentrations of planet formation outcomes:

It’s important to keep in mind that Israelian et al.’s correlation holds over only a very narrow range of stellar temperature. The M-dwarfs (Gliese 581, Gliese 876), the K-dwarfs (HD 69830, Alpha Cen B), and the F-dwarfs (Upsilon Andromedae) all fall outside the band of utility. This dovetails nicely with standard models of stellar evolution that suggest the amount of Lithium depletion in stars with masses very close the the Sun (that is, stars falling in the narrow effective temperature range of the above plot) depends sensitively on both the efficiency of convection and also on rotational mixing. That is, the stars that show the lithium-planet effect, are exactly the stars where subtle differences in properties seem to generate a big effect on lithium abundance.

After writing last week’s post, I got an e-mail from Jonathan Irwin (of MEarth fame) who makes several interesting points:

The low lithium could be more of a coincidence resulting from the long-lived circumstellar disks that are presumably needed to form planets.

Mediation of the stellar rotation rates by long-lived disks is thought to be responsible for generating the wide dispersion in rotation rates observed in open clusters around 100Myr age, and there have been suggestions (e.g. Denissenkov et al.’s paper that appeared on astro-ph 2 weeks ago) that the slowly-rotating stars evolve developing some degree of decoupling of the rotation rates of their radiative core and convective envelope, whereas the rapidly-rotating stars evolve more like solid bodies.

Bouvier (2008) has suggested that the shear at the radiative convective boundary resulting from this could mix lithium into the interior more efficiently, and thus could result in lower lithium for stars that were slow rotators, preserving evidence of their rotational history even though the final rotation rates all converge by the solar age.  Some evidence for this last part exists in the form of a correlation between rotation and lithium in young open clusters such as the Pleiades.

A hypothesis along these lines seems quite appealing to me. As long as a protoplanetary disk is present, and as long as its inner regions are sufficiently ionized, then there’ll be a connection between the stellar magnetic field and the magnetic field of the disk. To a (zeroth) degree of approximation, the equations of ideal MHD allow us to envision the situation as consisting of a rapidly rotating star connected to a slower-rotating disk by lot of weak rubber bands. The net effect will be to slow down the stellar rotation to bring it into synch with the rotation at the inner edge of the disk.

Trying to sound like a tough-guy, I stressed the importance of predictions in last weeks post. If Irwin’s hypothesis is correct, then the formation of the Mayor et al. 2008 planet population is associated with disks that contain lots of gas, even in regions interior to R~0.1 AU. I’d thus expect that the “super Earths” are actually “sub Neptunes”, and that we can expect considerable H-He envelopes for the majority of these planets.

Another speculative prediction concerns the stars that aren’t depleted in lithium. In Irwin’s picture, these stars had short-lived disks and lost their gas relatively rapidly. This shouldn’t hinder the formation of terrestrial planets, but one would expect that the final configurations of the rocky planets would sport higher eccentricities, as there was little or no gas to damp the orbits down during the final stages of terrestrial planet accretion (see this paper for more on this).

We’re back on line after a skin-crawling attack that exploited the WordPress installation to rebrand the oklo.org name as synonomous with the latest in spamware. I noticed the problem yesterday morning, and took the site offline. Buried in the WordPress .php scripts, I found a piece of code that looked like this:

Luckily, the MySQL database seems to have been unaffected, so I did an rm -rf * and started from scratch with the latest WordPress.

It’s been a rather apocalyptic-themed week: Russian hackers attack oklo.org, the University of California is disentigrating under the weight of repeated budget cuts, and on Tuesday, I went to Los Angeles to film a segment for a History Channel episode describing how the Earth would fare in the sudden absence of human presence. My particular interview focused on what would happen to the geostationary satellites over a timescale of weeks to months to years. The filming was done at an abandoned hospital, which was one of the creepiest places I’ve ever seen.

Diamond prospecting proceeds through the identification of indicator minerals such as specific forms of garnet. The garnets can be traced upstream to the Kimberlite pipes. The Kimberlite pipes contain the sparkling gemstones.

Planet prospecting can be done in similar fashion. If you want to jump-start a new planet search, it’s wise to observe metal-rich stars. Stars with more than twice the Sun’s metal abundance are roughly five times more likely than average to harbor one or more planets in the readily detectable hot Jupiter and Eccentric Giant categories. Histogrammed data from Exoplanet.eu shows the metallicity correlation quite nicely:

The metallicity correlation can be readily interpreted in the context of the core-accretion paradigm for giant planet formation. In this picture, nascent planets reach the stage of rapid gas accretion when their rocky-icy cores grow to somewhere in the neighborhood of ten Earth masses. The speed with which a core can be assembled in a protoplanetary disk is a very sensitive function of the density of solid material (e.g. ices and dust) in the disk. The density of solids, in turn, scales with metallicity.

If one explains the planet-metallicity correlation with the core-accretion theory, several predictions follow almost immediately. One expects that low-mass stars will show a paucity of readily detectable giant planets, and that high-mass stars will have a larger fraction of giant planets. Observationally, both of these trends have been shown to hold.

A less-well-known prediction is that one also expects that stars with high oxygen (and by proxy, silicon) abundances relative to iron will also show increased planet fractions at given metallicity. Sarah Dodson-Robinson showed this was true as part of her Ph.D. Thesis. Here’s the the key diagram from her paper on the topic:

A very interesting paper came out in Nature this week which shows an equally compelling, but significantly harder-to-understand abundance correlation. Garik Israelian, and colleagues that include members of the Geneva Team, write (italics are mine):

Here we report Li abundances for an unbiased sample of solar-analogue stars with and without detected planets. We find that the planet-bearing stars have less than one per cent of the primordial Li abundance, while about 50 per cent of the solar analogues without detected planets have on average ten times more Li.

Here’s the graphic from their paper. The filled red circles are planet-bearing stars. Downward arrows indicate that the measurement is an upper limit, and in all likelihood lies at a lower value. Note also, that the y-axis has a logarithmic scale, which de-emphasizes the strength of the effect. To the eye, it’s clear that the lithium abundances of the planet-bearing stars are quite low:

The effect is dramatic, and yet its origin is mysterious and seems to have gone unpredicted. It’s the best sort of scientific puzzle. Lithium is a rather fragile element, and undergoes nuclear fusion in a star when the temperature reaches ~2.5 million degrees. Lithium depletion in the atmosphere of a star can thus be taken as evidence that the gas that’s currently at the surface has, at one point, been mixed far down enough into the star for the lithium to have burned. This implies that the base of the star’s convective envelope has dipped further into the star than the 2.5 million degree isotherm. (The hot F-type stars on the far right of the diagram have very thin convective envelopes nearly right from the start, and so have been unable to burn their lithium.)

So it seems that somehow, the presence of a planetary system (and even one as wimpy as our own solar system) is enough to alter the evolution of the stellar convective envelope. This, in turn, likely has something to do with angular momentum transfer mediated by planets, but quite frankly the story isn’t very clear. Certainly, there will be papers that explain the effect, and certainly, they are being cranked out even as I write, but unless they make specific, testable, and preferably startling predictions, I’d advise taking them with a grain of lithium chloride.

Grant-proposal season puts a crimp on one’s style. Despite many interesting developments in the field over the past few weeks, I haven’t had time to write. I’m glad that’ll change shortly.

We’re also very close to getting upgraded versions of the systemic backend and a new Transitsearch-related project on line. In the interim, here’s a link to the old transitsearch.org candidates page. I have it running on our server here at UCO/Lick, and it’s updated every 10 minutes. This information should also soon be available at JPL’s NStED site.