Abstract
Abstract We are undertaking the first systematic infrared (IR) census of R Coronae Borealis (RCB) stars in the Milky Way, beginning with IR light curves from the Palomar Gattini IR (PGIR) survey. The PGIR is a 30 cm J-band telescope with a 25 deg2 camera that is surveying 18,000 deg2 of the northern sky (δ > −28°) at a cadence of 2 days. We present PGIR light curves for 922 RCB candidates selected from a mid-IR color-based catalog. Of these 922, 149 are promising RCB candidates, as they show pulsations or declines similar to RCB stars. The majority of the candidates that are not RCB stars are either long-period variables (LPVs) or RV Tauri stars. We identify IR color-based criteria to better distinguish between RCB stars and LPVs. As part of a pilot spectroscopic run, we obtain NIR spectra for 26 of the 149 promising candidates and spectroscopically confirm 11 new RCB stars. We detect strong He i λ10830 features in the spectra of all RCB stars, likely originating within high-velocity (200–400 km s−1) winds in their atmospheres. Nine of these RCB stars show 12C16O and 12C18O molecular absorption features, suggesting that they are formed through a white dwarf merger. We detect quasiperiodic pulsations in the light curves of five RCB stars. The periods range between 30 and 125 days and likely originate from the strange-mode instability in these stars. Our pilot run results motivate a dedicated IR spectroscopic campaign to classify all RCB candidates.
Highlights
The R Coronae Borealis (RCB) stars form a distinct class of variable stars (Clayton 2012)
Most RCB stars are thought to be remnants of white dwarf (WD) mergers (Webbink 1984; Clayton 2012); some could originate from a final helium shell flash in an evolved low-mass star (Iben et al 1996)
In the first part of this search, we focus on Palomar Gattini IR (PGIR) J-band light curves for objects in a preexisting catalog of candidate Galactic RCB stars (Tisserand et al 2020)
Summary
The R Coronae Borealis (RCB) stars form a distinct class of variable stars (Clayton 2012). From a campaign to identify Galactic RCB stars using mid-IR colors, Tisserand et al (2020) estimated the total number to be ≈380–550 This suggests a lower WD merger rate of ∼10−3 yr−1, which is consistent with theoretical estimates made from population synthesis via the DD channel (Ruiter et al 2009; Karakas et al 2015). The high-cadence, long-baseline photometric observations are ideal to flag RCB stars (from their declines) These can be further followed up spectroscopically to study their chemical compositions. The extinction due to dust is significantly lower in the IR compared to optical wavelengths This makes the PGIR an ideal instrument to conduct a systematic search for Galactic RCB stars.
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