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Exoplanets

Yale Astronomy

Exoplanets Alumni

Matt Giguere

Matt Giguere

Matt Giguere completed his PhD in the Astronmy Department at Yale in Fall 2015 (graduating in Spring 2016). He went on to study in the Insight program and is now a data scientist for a startup company in the Bay Area. Matt worked on three major projects as a grad student.

Matt commisioned the CHIRON spectrograph that was built by our team (Tokovinin et al. 2013) for the 1.5-m CTIO telescope and carried out a Doppler survey with this instrument. Matt built the software pipelines to extract and wavelength calibrate the spectra, He collaborated on the complex scheduling software, and developed quality control pages to catch problems (calibration exposures, sufficient SNR, problems with the exposure meter) the morning after the observations. Matt was awarded a NASA Earth and Space Science Fellowship (NESSF) to carry out a planet search with CHIRON.

Matt also took over the "Next 2000" or N2K planet search program at Keck. He developed a software console that permitted the fitting of up to 7 Keplerian signals in a radial velocity data set. The planet-fitting console (KFME for Keplerian Fitting Made Easy) was written using an IDL widget interface. Once the Levenberg-Marquart fitting algorithm finds a good fit, the user can run a bootstrap Monte Carlo program to derive parameter errors. The console will generate publication-ready plots of Keplerian fits to the data (it is possible to subtract signals from other planets out and show each Keplerian signal individually, either in time series or phase-folded plots) and publication-ready LaTeX data tables for all of the fitted planetary signals. Two of Matt's publications (Giguere et al 2012, 2014) stem from the N2K project.

Matt tackled the very difficult and ambitious project of distinguishing stellar photospheric velocities from Keplerian Doppler shifts. He began his investigations by following up on a 1996 paper by David Gray that claimed a precision of 3K in measuring relative stellar effective temperatures by looking at adjacent spectral lines with different responses to temperature. Such temperature variations might be seen in stars with cool spots. Gray used a few line pairs to characterize the temperature sensitivity of the line depth ratio technique. Matt extended this significantly; he ran the SME (Spectroscopy Made Easy) spectral synthesis code, stepping through 5K changes in temperature to empirically quantify the response of spectral lines. He then identified a few hundred line pairs and showed that (with the noiseless synthetic temperatures) he could recover relative temperature variations of a few degrees for a given star. Testing this on the CHIRON data, he found that the stability of the instrument and the SNR were not adequate to reach the precision needed, even for stars like Epsilon Eridani with ~1% spot coverage.

To continue the study of the impact of star spots on radial velocity measurements, we took over operations of the Canadian MOST satellite for two months and obtained nightly observations of Epsilon Eridani with the CHIRON spectrograph. Matt simultaneously modeled the spot signal in the MOST and CHIRON data (Giguere et al. 2016). He then searched the spectra for a feature that correlated with the time series photometric data and found that emission of the H-alpha line core followed the photometric variations. This allows us to have a proxy for space-based photometry in the very spectra that we analyze for Keplerian Doppler shifts and inspired a new wave of statistical analysis techniques at Yale.

Matt was also co-founder of Planet Hunters, a citizen science project. He and his wife, Haven, have a dog (Lyra) and became the proud parents (of Aurilia) in Oct 2013.

Jack Moriarty

Jack Moriarty

Jack completed his PhD in Spring 2016 in the Astronomy Dept at Yale and has gone on to a postdoctoral fellowship at MIT working with Sarah Ballard.

His dissertation represents a substantive contribution to the theory of planet formation. Moriarty (Moriarty & Fischer 2015) studies the formation of rocky planets via pebble accretion, a method for accelerating the growth of planetesimals to avoid the so-called meter-size problem: planetesimals that are of order a meter in size do not clear a gap in the disk since they are traveling at Keplerian velocities in a pressure supported gas and dust disk, they experience a head wind that causes them to lose energy and spiral into the star. By pre-assembling disk material into centimeter-sized objects, accretion proceeds quickly to gap-clearing dimensions. This is an innovative application of theory to a problem that has stymied theoreticians for decades.

The next section of his dissertation studies small planets detected with the Kepler mission. These objects are very different from planets in the solar system and puzzling in that a series of planets are often found in close orbits (e.g., 3-, 5-, 8-day orbits, but then no planets in wider orbits. (Moriarty & Ballard 2016, submitted) try to assess whether this is an artifact of relative inclination (non-coplanarity) of the planetary orbits. This is one of the most interesting questions about the population of planets detected with the Kepler spacecraft.

Moriarty rounds out his thesis dissertation with a study of the chemical evolution of protoplanetary disks (Moriarty, Madhusudhan & Fischer 2014). This bears on the interior composition and structure of small rocky planets. For example, Madhusudhan et al (2012) found that the interior composition of 55 Cnc e was likely to be dominated by high pressure the diamond ). Moriarty studied the chemical evolution of protoplanetary disks using disk models (pressure, temperature) that tag planetesimals with likely compositions from equilibrium chemistry and then allow for orbital migration in the disk. This work shows that for stellar C/O ratios greater than 0.6, significant carbon enhancement is likely and that the interior structure of planets that form around these stars will be very different from the silicate magnesium interior of Earth. By accounting for temporal evolution of the disk structure, Jack Moriarty revises the threshold C/O ratio for carbon-rich planets downward from previous estimates of 0.8 to 1.0.

Jack also developed a radial velocity precision simulation program to evaluate the effects of various spectrometer design choices on the overall precision and accuracy of radial velocity measurements, and to influence future designs accordingly.

Jack loves bicycling and baking bread. He and his wife, Erin, have a wonderful dog (Maple).

 

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