Calendar

Mar
29
Thu
Colloquium (Joint with Physics): Jim Fuller (Host: Ethan Dederick)
Mar 29 @ 4:00 pm – 5:00 pm
Colloquium (Joint with Physics): Jim Fuller (Host: Ethan Dederick) @ Gardiner Hall 230

Surprising Impacts of Gravity Waves

Jim Fuller, Caltech

Gravity waves are low frequency fluid oscillations restored by buoyancy forces in planetary and stellar interiors. Despite their ubiquity, the importance of gravity waves in evolutionary processes and asteroseismology has only recently been appreciated. For instance, Kepler asteroseismic data has revealed gravity modes in thousands of red giant stars, providing unprecedented measurements of core structure and rotation. I will show how gravity modes (or lack thereof) can also reveal strong magnetic fields in the cores of red giants, and I will demonstrate that strong fields appear to be common within “retired” A stars but are absent in their lower-mass counterparts. In the late phase evolution of massive stars approaching core-collapse, vigorous convection excites gravity waves that can redistribute huge amounts of energy within the star. I will present preliminary models of this process, showing how wave energy redistribution can drive outbursts and enhanced mass loss in the final years of massive star evolution, with important consequences for the appearance of subsequent supernovae.
Apr
6
Fri
Colloquium PhD Thesis Defense: Sten Hasselquist
Apr 6 @ 3:15 pm – 4:15 pm
Colloquium PhD Thesis Defense: Sten Hasselquist @ BX102

Colloquium Title

Sten Hasselquist, NMSU

Abstract

Sep
21
Fri
Colloquium: Dave Thilker (Host: Rene Walterbos)
Sep 21 @ 3:15 pm – 4:15 pm
Colloquium: Dave Thilker (Host: Rene Walterbos) @ BX102

Fresh Perspectives on Star Formation from LEGUS, the Legacy ExtraGalactic Ultraviolet Survey

David Thilker, Johns Hopkins University

The Legacy ExtraGalactic Ultraviolet Survey (LEGUS) was a Cycle 21 Large Treasury HST program which obtained ~parsec resolution NUV- to I-band WFC3 imaging for 50 nearby, representative star-forming Local Volume galaxies, with a primary goal of linking the scales of star formation from the limit of individual stars, to clusters and associations, eventually up through the hierarchy to giant star forming complexes and galaxy-scale morphological features.

I will review the basics of the survey, public data products and science team results pertaining to clusters and the field star hierarchy.  I will then describe work to optimize photometric selection methods for massive main sequence O star candidates and LBV candidates, in the former case establishing a means to statistically constrain the fraction of O stars in very isolated locales.  I will introduce new ideas on how to quantify the complex spatio-temporal nature of hierarchical star formation using multi-scale clustering methods. The first steps of this work have yielded a landmark OB association database for 36 LEGUS target fields (in 28 of the nearest available galaxies), with tracer stellar populations selected and interpreted uniformly.  I will finish with discussion of a pilot HST program to demonstrate remarkably increased survey efficiency of WFC3 UV imaging enabled by use of extra-wide (X) filter bandpasses.  Such efficiency is required as we move beyond LEGUS and begin to rigorously explore low surface brightness star-forming environments where canonical results for the IMF and cluster formation efficiency are increasingly called into question.

 

Oct
19
Fri
Colloquium: Sanchayeeta Borthakur (Host: Kristian Finlator)
Oct 19 @ 3:15 pm – 4:15 pm
Colloquium:  Sanchayeeta Borthakur (Host: Kristian Finlator) @ BX102

Understanding How Galaxies Reionized the Universe

 Sanchayeeta Borthakur, Arizona State University

Identifying the population of galaxies that was responsible for the reionization of the universe is a long-standing quest in astronomy. While young stars can produce large amounts of ionizing photons, the mechanism behind the escape of Lyman continuum photons (wavelength < 912 A) from star-forming regions has eluded us. To identify such galaxies and to understand the process of the escape of Lyman continuum, we present an indirect technique known as the residual flux technique. Using this technique, we identified (and later confirmed) the first low-redshift galaxy that has an escape fraction of ionizing flux of 21%. This leaky galaxy provides us with valuable insights into the physics of starburst-driven feedback. In addition, since direct detection of ionizing flux is impossible at the epoch of reionization, the residual flux technique presents a highly valuable tool for future studies to be conducted with the upcoming large telescopes such as the JWST.

Oct
4
Fri
Colloquium: Raja GuhaThakurta (Host: Rene Walterbos)
Oct 4 @ 3:15 pm – 4:15 pm
Colloquium: Raja GuhaThakurta (Host: Rene Walterbos) @ BX102

The SPLASH Survey of the Andromeda Galaxy

Raja Guhathurkurta, University of California, Santa Cruz

Our nearest large spiral galaxy neighbor, the Andromeda galaxy (M31),
and its dwarf satellites, offer a panoramic yet detailed view of
galaxy formation and evolution in our astronomical backyard. This
system also serves as an excellent laboratory for the study of stellar
populations because the stars are all practically at the same distance
from us. I will present results from the SPLASH (Spectroscopic and
Photometric Landscape of Andromeda’s Stellar Halo) survey, the
backbone of which was a large Keck DEIMOS spectroscopic survey of
evolved stars in M31. Most of the SPLASH spectroscopic targets in
M31’s disk were selected from the PHAT (Panchromatic Hubble Andromeda
Treasury) survey, a wide-field 6-filter Hubble Space Telescope mosaic
image of a portion of the disk of M31. The talk will cover a range of
science topics including: Local Group dynamics, structure/substructure
and metallicity of M31’s stellar halo, satellite tidal interactions,
disk/halo interface, dynamical heating of the disk, and rare stellar
populations.