List of Past LMA/CARMA Seminars : 01-Sep-2015 to 31-Dec-2015

Date:   Thu 05-Nov-2015
Speaker:   Dr. Jeff Bary (Colgate University)
Title:  Using Large Spots to Explain the Spectral Type/Color Anomaly for Young Sun-Like Stars

Million-year old sun-like stars are active formation sites for planetary systems . As a result, these objects are important targets for those of us interested i n understanding how proto-planetary disks evolve into planetary systems. In ord er to trace the evolution of such systems, we must observe stars at different ag es and stages of development. Evolutionary models predict how quickly a contrac ting protostar will move on the HR diagram as it approaches the main sequence. Placement of a young star on the HR diagram clearly depends on our ability to de termine its spectral type and surface temperature. For nearly two decades, an a nomaly between optical and infrared colors and spectral types on the order of th ree or four spectral classes have been observed for many T Tauri stars, but has received little attention in the field. In this talk, I will show that large, s tar spots with filling factors as high as 40-50% can account for most of the ano malies using a straightforward spotted star model. In addition to better or mor e confidently constraining the ages of T Tauri stars, the results of this study also will be valuable to those interested in characterizing the near-infrared ex cesses associated with these stars as a means of understanding the evolution of the dust in proto-planetary disks.

Date:   Thu 19-Nov-2015
Speaker:   Dr. Jonathan C. Tan (University of Florida)
Title:  Inside-Out Planet Formation

The Kepler-discovered systems with tightly-packed inner planets (STIPs), typical ly with several planets of Earth to super-Earth masses on well-aligned, sub-AU orbits may host the most common type of planets in the Galaxy. They pose a great challenge for planet formation theories, which fall into two broad classes : (1) formation further out followed by migration; (2) formation in situ from a disk of gas and planetesimals. I review the pros and cons of these classes, before focusing on a new theory of sequential in situ formation from the inside-out via creation of successive gravitationally unstable rings fed from a continuous stream of small (~cm-m size) "pebbles," drifting inward via gas drag. Pebbles first collect at the pressure trap associated with the transition from a magnetorotati onal instability (MRI)-inactive ("dead zone") region to an inner MRI-active zone. A pebble ring builds up until it eith er becomes gravitationally unstable to form an Earth to super-Earth-mass planet directly or induces gradual planet formation via core accretion. The planet continues to accrete until it becomes massive enough to isolate itself from the accretion flow via gap opening. The process repeats with a new pebble ring gathering at the new pressure maximum associated with the retreating dead-zone boundary. I discuss the theory's predictions for planetary masses, relative mass scalings with orbital radius, and minimum orbital separations, and their comparison with observed systems. Finally I speculate about potential causes of diversity of planetary system architectures, i.e. STIPs versus Solar System analogs.

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