Stars, Planets, and ISM Seminar

Saugata Barat, MIT

Exoplanets Through Time: Tracing the Early Evolution of Sub-Neptunes

Demographic studies have revealed a sub-Neptune ‘radius valley’. One of the leading hypothesis to explain the radius valley is the ‘gas-dwarf’ scenario, in which sub-Neptunes are born with H/He rich puffy atmospheres and undergo significant evolution (mass loss, thermal contraction), which is expected to significantly alter their atmospheric properties. However, it is unknown how the primordial atmospheres of sub-Neptunes look like: 

What is the nature and composition of primordial sub-Neptune atmospheres? How diverse are the atmospheric properties right after formation? How do their atmospheres compare with their mature counterparts? 

Young transiting planets (< 100Myr old) represent the earliest phase in the lifetime of exoplanets and are ideally suited to address these questions. In this talk we will present the first observations of the atmospheres of young transiting planets (10-20 million years old) with the Hubble Space Telescope and the James Webb Space Telescope. We measure their mass, atmospheric composition and internal entropy using transmission spectroscopy. We compare them with each other, as well as with the mature sub-Neptunes to understand their formation history and impact of evolutionary mechanisms on their atmospheric composition.  Our findings challenge the predictions of standard core-accretion planet formation theory. We explore new ideas about the internal structure of these planets which could potentially reconcile them with their mature counterparts.

Kaleo Toguchi-Tani, The University of Texas at Austin

Unveiling the Sagittarius Dwarf Spheroidal Galaxy Core with Gaia DR3

The Sagittarius dwarf spheroidal galaxy provides us with the unique opportunity to study an ongoing Galactic cannibalistic event between our Milky Way Galaxy and a satellite dwarf galaxy. We present the first major membership star catalog of the Sagittarius dwarf core (≈200,000 sources) and Messier 54 (≈2000 sources) with positions, proper motions, and parallaxes from Gaia DR3, supplemented with metallicities from the Apache Point Observatory Galactic Evolution Experiment (APOGEE). We isolate the Sagittarius dwarf core (3.7 core radii) and Messier 54 (0.125 core radii) using literature positions. Using evolutionary sub-samples separated within a color-magnitude diagram, we analyze the structures and substructures of the Sagittarius core and infer its positional relationship with Messier 54 within 5D phase space. A sample of Milky Way stars from a similar galactic latitude was used to identify contaminants and member stars from the core of Sagittarius and Messier 54 using a machine learning Gaussian Mixture Model in proper motion space. We present the derived proper motion, parallaxes, and metallicities for these evolutionary sub-samples while demonstrating the precision of our sample using red clump standard candles. We find a distance modulus for the Sagittarius core and Messier 54 of (m-M)₀ = 17.002_-0.042^+0.038 mag and (m-M)₀ = 17.011_-0.039^+0.038 mag, corresponding to a heliocentric distance of d = 25.14_-0.48^+0.45 kpc and d = 25.25^+0.48_-0.45 kpc respectively. With red clump distance analysis, our results imply there is no separation between the Sagittarius core and Messier 54.