PHYS/ASTR Colloquium: "The Shadow the Scientists program and two recent highlights from my research group" - Prof. Raja GuhaThakurta (U.C. Santa Cruz)

The Shadow the Scientists program and two recent highlights from my research group: distant RR Lyrae stars in the Milky Way halo and weak CN stars in M31, M33, and the LMC

First, at the height of the pandemic, a couple of my UCSC colleagues and I started the Shadow the Scientists (StS) program. A few times a month, StS offers free online experiential learning sessions for students at any/all academic levels, educators, and other enthusiasts. During these sessions, the participants eavesdrop via Zoom on researchers while they are actively conducting research. StS started in astronomy (astronomers using some of the world’s most powerful tele- scopes) but we have expanded the program to include other scientific disciplines such as oceanography/paleoclimatology, solar physics, stem cell biology, telescope engineering, tropical forest ecology, and volcanology. Our hope is to ultimately ex- pand StS beyond science. The StS YouTube channel contains recordings of past StS sessions. Here is a brief StS promotional video.

Next, my talk will touch upon two recent research highlights from my group: (1) the extent and structure of the Milky Way’s stellar halo, and (2) the discovery of unexpected surface chem- istry in red supergiant stars in three Local Group galaxies. We have used CFHT MegaCam time-series photometry imag- ing 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 analy- sis 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 spectro- scopic 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⊙ stars in the core He burning phase of stellar evolution) con- tain 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.