Peter Scherbak (Caltech)

• Title: Rapid Binary Mass Transfer: Circumbinary Outflows and Angular Momentum Losses across Mass Transfer Rates
• Abstract: High rates of stable mass transfer (MT) occur in some binary star systems, producing luminous transients and driving circumbinary outflows. We perform hydrodynamical simulations of a donor star transferring mass to a point-mass accretor across a range of mass ratios, in order to quantify the mass, angular momentum, and energetics of the resulting accretion disk and outflows. We conduct two suites of simulations that differ in their treatment of energy losses: (1) adiabatic, appropriate for the highest MT rates, and (2) including approximate radiative cooling. By varying the orbital separation, we probe MT rates from 10^{-5} to 10^{-1} Msun/yr. Mass transferred from the donor forms an accretion disk, with strong equatorial outflows through the outer Lagrange point (L2) occurring for MT rates > 10^{-3} Msun/yr, while MT remains mostly conservative at lower rates. In all cases, outflowing gas carries approximately the specific angular momentum of L2, decreasing moderately with increasing accretor-to-donor mass ratio. The gas cooling luminosity and temperature both scale with MT rate, reaching L ~ 10^{5} Lsun in the most extreme cases, with contributions from both the disk and circumbinary outflow. Such luminous MT-driven transients are expected to be rare due to the high MT rate required, but generate significant optical emission from the accretion disk and the outflow. If the outflowing material becomes unbound, it may supply circumstellar material that later interacts with ejecta from a supernova or stellar merger.

Saugata Barat (MKI)​

• Title: Exoplanets Through Time: Tracing the Early Evolution of Sub-Neptunes
• Abstract: 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.