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Astrophysics Luncheon Seminar

Disentangling Stellar and Planetary Features in Exoplanet Transmission Spectra
Presented by Benjamin Rackham
University of Arizona

Monday, November 5, 2018
12:00 noon in 169-336

Abstract
Transmission spectra are powerful probes of exoplanet atmospheres, but they are also subject to spectral features introduced by the transit light source (TLS) effect. This phenomenon imprints on transit depths the contrast between the emergent spectrum of the transit chord (the true light source for the transmission measurement) and the out-of-transit disk-averaged stellar spectrum (the necessarily assumed light source). In this talk, I will summarize a series of studies exploring the TLS effect in F to M dwarfs. I use suites of model rotating photospheres to determine spot and faculae covering fractions for typical stellar activity levels, with which I calculate the corresponding TLS signals in transmission spectra. I find that transit depth changes due to the TLS effect can be comparable to or even an order of magnitude larger than those expected for transiting exoplanets. TLS signals are more pronounced in smaller and cooler main sequence stars - the same stars that have been the focus of many transit studies, given their favorable planet-to-star radius ratios. The TLS effect can mimic or mute H2O features from planetary atmospheres in M-dwarf systems and TiO/VO features in active late-G- and K-dwarf systems. I will discuss the spectral characteristics and scales of TLS signals for F to M spectral types and present transmission spectra from three systems that show evidence for TLS contamination: WASP-19 (G8V), GJ 1214 (M4.5V), and TRAPPIST-1 (M8V). I will discuss recent efforts to constrain stellar and planetary contributions to transit depths via retrieval analyses. These and other robust methods of disentangling stellar and planetary features in transits will be crucial to interpretations of high-precision transmission spectra from JWST and future missions.

JPL Contact: Renyu Hu (4-6090)

About the Speaker
Ben Rackham is a postdoctoral research associate at the University of Arizona. He received his PhD in 2018 from the University of Arizona, where he was advised by Dániel Apai. He is interested in studying exoplanetary atmospheres and stellar photospheres through transmission spectroscopy, with a focus on enabling unique identifications of stellar and planetary signals in future high-precision observations.


SVCP Astrophysics


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