Variable Stars and Quasars in the Canada-France-Hawaii Telescope Legacy Survey Deep Fields

Studies of the radial stellar density profile and substructure of the Milky Way’s stellar halo provide a window into our galaxy’s accretion history. RR Lyrae are arguably the most reliable tracers of the Milky Way’s stellar halo for two reasons: (1) their periodic pattern of photometric variation makes them relatively easy to identify; (2) they are excellent standard candles. AGNs/quasars are the most significant source of contamination when identifying faint RR Lyrae. The Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) is a deep, multi-epoch (~300 epochs per filter), multi-band (ugriz filters) photometric survey of four one-square degree patches of sky. The 1-σ photometric precision is 0.2 mag at u = 22.6, g = 23.6, r = 23.2, i = 22.5, and z = 21.2. Its depth and exquisite cadence allow for accurate identification of RR Lyrae as well as quantifying degrees of incompleteness and contamination in other time domain surveys. In this work we analyze the CFHTLS data to: (1) identify candidate variable objects; (2) classify variable objects. We 5-σ clip the data to remove outliers, which are mostly systematic errors in photometry associated with cosmic ray hits. Since most objects are non-variable, we fit polynomials to the ridge line of log(σ) versus median ugriz magnitude to find the measurement error. We sum the intrinsic RMS of the 5 bands for each object weighted by the measurement error in each band. Objects are categorized as “marginal”, “intermediate” or “extreme” variable candidates based on their weighted intrinsic variance, defined as the difference between the observed variance and the variance associated with measurement error. The Lomb-Scargle periodogram is used to create phase-folded light curves, and their period, amplitude and shape will be used to classify variables.

This research was supported in part by the NSF. CD, TS and JZ conducted this research under the auspices of the Science Internship Program (SIP) at the UC Santa Cruz. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to work with observations from this mountain.