The main part of my research on evolved stars began with my postdoctoral position at Space Telescope Science Institute. I worked with Margaret Meixner, the Principal Investigator of the Surveying the Agents of a Galaxy's Evolution (SAGE) team, modeling the photometry and spectroscopy of the stars identified in the Spitzer Space Telescope SAGE surveys of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) galaxies as Asymptotic Giant Branch (AGB) and red supergiant (RSG) stars, to determine their mass loss rates. My colleagues and I created a grid of radiative transfer models known as the Grid of Red supergiant and Asymptotic giant branch ModelS (GRAMS). This work began by determining optimal dust properties to use for models of Carbon-rich (C-rich) AGB stars (Srinivasan et al 2010) and Oxygen-rich (O-rich) AGB stars (Sargent et al 2010). After determining optimal dust properties, we created the C-rich (Srinivasan et al 2011) and O-rich (Sargent et al 2011) parts of GRAMS. These grids were then used to determine the mass loss from AGB and RSG stars in the Large Magellanic Cloud (Riebel et al 2012). I also study the Spitzer Infrared Spectrograph (IRS) spectra from AGB and RSG stars in the LMC and elsewhere to constrain the properties of the stardust grains produced in the outflows from these stars, such as grain size, composition, etc. In addition, I have also observed emission from CO gas in outflows from OH/IR stars in the Galactic bulge using the Submillimeter Array (SMA), and the research was published recently (Sargent et al 2013). I am also involved in Herschel Space Observatory studies of circumstellar material around evolved stars, with the HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE; PI: M. Meixner) science team. All these studies are part of the overall SAGE and HERITAGE goal to explore the life-cycle of matter in the LMC and SMC.
Star- and Planet-Formation:
Regarding star- and planet-formation, my research focuses on studying how protoplanetary disks around young stars evolve toward planetary systems. Most of my graduate research (Sargent et al 2006; Sargent et al 2009a; Sargent et al 2009b) at the University of Rochester involved modeling the dust emission features seen in Spitzer-IRS spectra of T Tauri stars (TTSs); i.e., Class II Young Stellar Objects. I worked on this research with Bill Forrest (my graduate advisor at the University of Rochester), Dan Watson, and the IRS_Disks Spitzer Guaranteed Time Observing (GTO) team. I am searching for signatures of protoplanetary disk evolution, such as dust processing (processing of interstellar medium type dust into other kinds of dust) and grain growth. I have also modeled the dust emission in Spitzer-IRS spectra of debris disks, systems that are beyond the protoplanetary disk stage, at a point when planetesimals have already formed but are colliding with each other and producing debris. This work was in collaboration with Christine Chen, and I modeled the dust emission from 5 debris disks: HR 3927, eta Crv, HD 113766, HR 7012, and eta Tel
(Chen et al 2006). Currently, I am studying the emission and absorption features from water vapor and other gases in the 5-7.5 micron wavelength region of Spitzer-IRS spectra of TTS's. I am generally interested in all processes involved in forming a young star and the planetary system around it.
My interest in massive stars, specifically B[e] supergiant stars, began with a collaboration with Joel Kastner at Rochester Institute of Technology, while I was still a graduate student. Joel had obtained Spitzer-IRS spectra of B[e] supergiant stars in the LMC. Their infrared spectra resembled those of disks around T Tauri stars, with emission from crystalline silicates, Polycyclic Aromatic Hydrocarbons (PAHs), amorphous interstellar-medium-like dust, etc (Kastner et al 2006). However, these stars can be quite massive - up to 100 solar masses or more! As such, they are short-lived stars. But what is the origin of their circumstellar dust disks? How do they fit into stellar evolution and the bigger picture?
For my publication list, query me in NASA ADS.