The first session of participant talks is chaired by PSU graduate student Taran Esplin and will focus on characterization of planet hosting stars.
Accessing the fundamental properties of young stars (Ian Czekala, Harvard Smithsonian CfA, @iczekala)
Talking abuot two techniques for measuring young stars and their protoplanetary disks. What are stellar properties of near solar mass stars before they hit the main sequence? How do we find this out? Stars start out above and to the right of the MS, and stars of different masses take different paths and take different amounts of time to travel from their initial positions to the MS. Lower mass stars take the longest time to hit the MS from their initial positions.
Technique 1: protoplanetary disk radio intereferometers. 3D structure model to get temperature, density, and velocity as a function of stellar mass. Then imaging across a CO line reveals the kinematic fingerprint of the star. This can dynamically weigh single stars in their “teenage” years.
Technique 2: using stellar spectroscopy to get the stellar mass. Get a spectrum of a star and you can usually get pretty good info on the effective stellar temperature and stellar radius. But, right now we only really use parts of the spectrum that we are very familiar with. What happens if we can fit an entire large chunk of the spectrum? With a more complex spectral model to use in fitting we need to be more careful with the statistical methods we use to guarantee a good fit (need to be more careful than a simple chi^2!). Essentially, do the residuals between your model and your data resemble white noise? If not, you may need to query a covariant noise matrix to model your noise residuals more carefully.
In the future, we can combine dynamical masses from ALMA/SMA. The current sample is about 20 stars, and they hope to calibrate the early HR diagram.
Defining the Range of Chemistry for Exoplanet Interiors (John M. Brewer, Yale)
While we know of a lot of exoplanets, there are very few of them where we know their masses accurately. The problem is actually getting an accurate stellar surface gravity, which affects estimates of radius and temperature, since the parameters are degenerate.
They use Spectroscopy Made Easy (SME) to get a better handle on surface log(g) of the planet. They use over 7000 lines to get this better estimate. By comparing to stars that have asteroseismic surface gravities they can test whether their SME procedure works better than their previous procedure. SME does a substantially better jobs, excepting a few stars that are rapid rotatrs, which mess up the comparison.
There doesn’t seem to be any trends in derived log(g) with other parameters, meaning that the SME method works well across a wide range of stars (excepting rapid rotating stars). However, they want to now use this to get stellar compositions, and they do this using asteroid spectra to test the accuracy of the methods (this assumes that we know the composition of the Sun).
For asteroids, they test their method using the ratio of carbon to oxygen (C/O), since that ratio is highly dependent on initial Solar composition. In their stellar sample, they find very few high C/O ratio stars (very few diamond planets, boo).
Exoplanet properties require accurate knowledge of the host stars, and the stellar log(g) is a key parameter to characterizing the star. Look out for a catalog to be published with all these parameters soon
Broadening our Horizons on Short-Period Stellar and Substellar Companions with APOGEE (Nicholas Troup, UVA)
Serendipitous science from APOGEE. About 1/2 of stars are in binary (or higher order systems) and on average every star has 1 planet. Companions can be stellar binaries, planets, or brown dwarfs. Brown dwarfs are the “missing link” between stars and planets. A strange phenomenon is the “brown dwarf desert”, which means that there is a lack of BD companions within 5 AU of a solar-type star. This is strange, because we have lots of Hot Jupiter planets very close to their stars and at near-BD masses.
APOGEE was meant as a galactic structure survey, but has been very useful for exoplanet discovery and characterization. The APOGEE RV Companion Survey takes stellar spectra and can pull out stellar parameters and abundances in the APOGEE samples. Their spectral model is exceptionally good at fitting their APOGEE spectra to get stellar properties. They can then measure radial velocities and search for planets. They have to go through a rigorous false positive analysis to weed out the “not planet planetary-like signals” from the real planets.
About half of their stellar sample is giant stars, which really haven’t been searched for companions all that much. They have a galactic distribution of stars, looking both inwards and outwards from the galactic core. While their sample is mainly in the thin disk, there are a few thick disk and halo stars, as well as globular cluster and open cluster stars. After first analysis they have about half of their sample as stellar or BD companions, and the other half are potentially planetary companions. Using their methods they hope to “map the shores” of the BD desert, and build a galactic map of companion frequency.
Re-characterization of a gravity-darkened and precessing planetary system PTFO 8-8695 (Shoya Kamiaka, University of Tokyo)
This system is a T-Tauri star + hot Jupiter. The transits that were observed in 2009 and then in 2010 don’t have the same shapes, and they are trying to figure out why that is.
The star is rotationally deformed - that is, it’s rotating so fast that it more closely resembles a football than a soccer ball, and the central band of the star is gravity darkened, while the poles are brightened. There is also a precession of the orbital axis angle relative to the stellar spin axis, called “nodal precession”. Together, these two phenomena can explain the time-variable transit lightcurves. Since this system fits this scheme so well, it’s an ideal benchmark case for this model. Previous work in this area does not favor such a synchronized state (the two components of the model varying in a synchronized way) with the serious misalignment found in PTFO 8-8695.
Their method of characterizing the synchronicity of the nodal precession and the gravity darkening is expected to unveil the properties of younger or hotter stars, which are known to be more rapidly rotating than older or cooler ones.
Next session is the first of our Planetary Atmospheres talks. Stay tuned!