My talk will cover two topics: (1) the extent and structure of the Milky Way’s stellar halo, and (2) the discovery of unexpected surface chemistry in red supergiant stars in three Local Group galaxies.
We have used CFHT MegaCam time-series photometry imaging from the Next Generation Virgo Cluster Survey (NGVS) to identify some of the most distant RR Lyrae candidates in the halo of the Milky Way, all the way out to a Galactocentric distance of 300 kpc (all in the foreground of the Virgo cluster, of course). Results will be presented from ongoing analysis of Keck ESI spectra of RR Lyrae candidates in our NGVS dataset and those discovered in the Dark Energy Survey and Pan-STARRS-1.
In the course of our team’s extensive Keck DEIMOS spectroscopic survey of the resolved stellar populations of the disks of the Andromeda (M31) and Triangulum (M33) galaxies, we have discovered that red supergiants (short-lived 5–10 M_sun stars in the core He burning phase of stellar evolution) contain a weak version of the CN molecular spectral absorption feature at ~8,000 Angstrom that is prominent in the spectra of carbon stars. Most of the rest of the spectrum of a weak CN star resembles that of a normal O-rich star. At this point in time, we don’t have a good understanding of the astrophysical processes that give rise to the weak CN phenomenon. This same weak CN absorption feature is also present in CTIO 4-m and Hydra spectra of red supergiants in the Large Magellanic Cloud.
