Time-Resolved Spectroscopic RR Lyrae Monitoring Program

RR Lyrae (RRL) are useful for two kinds of astrophysical studies: (1) galactic structure and dynamics; and (2) physics of stellar pulsations. These stars vary dramatically in lumosity and volume, with regular periods (on the order of 6-18 hours). RRab type stars produce a characteristic light curve, while RRc stars vary roughly sinusoidally. These curves make RRL reliably identifiable in large surveys. RRL have a very shallow relationship between period and time-averaged I-band luminosity, making them great standard candles. This allows for distance measurement with up to ~3% precision.

These benefits come with complications. RRL pulsations are rapid enough that their line-of-sight velocity varies as a function of pulsation phase. The instantaneous photospheric velocity is, in general, different from the center-of-mass velocity of the star. Furthermore, different layers inside RRL expand or contract at different rates. The result is variation in measured velocity for different absorption lines (for a given phase), making RRL difficult to use as kinematical tracers.

Our project has compiled a database of high-resolution, time-resolved RRL spectra, along with their apparent magnitude, pulsation period, amplitude (per band), and RR type for 32 individual stars. Each target has been observed multiple times, some with over 15 exposures. Most observations were conducted using the DEIMOS and ESI spectrographs on the Keck II telescope on Mauna Kea in Hawaii. ESI provides spectral resolution of up to R≈13,000, while DEIMOS provides spectral resolution of up to R≈6000 and wide spectral coverage.

Raw data was analyzed using software to calculate doppler shift (line of sight velocity), absorption line width, shape, and depth (equivalent width) as a function of pulsation phase for different absorption lines (e.g., Balmer lines, metal lines).

Going forward, our project intends to continue monitoring current targets, as well as to expand the database to include a more representative set of RRL types (namely RRc).

This research was funded in part by the National Science Foundation. RD and YZ carried out this research under the auspices of the Science Internship Program (SIP) at the University of California Santa Cruz.