Abstract
Abstract Dwarf carbon (dC) stars, main-sequence stars showing carbon molecular bands, are enriched by mass transfer from a previous asymptotic-giant-branch (AGB) companion, which has since evolved to a white dwarf. While previous studies have found radial-velocity variations for large samples of dCs, there are still relatively few dC orbital periods in the literature and no dC eclipsing binaries have yet been found. Here, we analyze photometric light curves from DR5 of the Zwicky Transient Facility for a sample of 944 dC stars. From these light curves, we identify 34 periodically variable dC stars. Remarkably, of the periodic dCs, 82% have periods less than two days. We also provide spectroscopic follow-up for four of these periodic systems, measuring radial velocity variations in three of them. Short-period dCs are almost certainly post-common-envelope binary systems, because the periodicity is most likely related to the orbital period, with tidally locked rotation and photometric modulation on the dC either from spots or from ellipsoidal variations. We discuss evolutionary scenarios that these binaries may have taken to accrete sufficient C-rich material while avoiding truncation of the thermally pulsing AGB phase needed to provide such material in the first place. We compare these dCs to common-envelope models to show that dC stars probably cannot accrete enough C-rich material during the common-envelope phase, suggesting another mechanism like wind-Roche lobe overflow is necessary. The periodic dCs in this paper represent a prime sample for spectroscopic follow-up and for comparison to future models of wind-Roche lobe overflow mass transfer.
Highlights
Carbon (C) stars are those that show molecular absorption bands of C, such as C2, CN and CH, in their optical spectra (Secchi 1869)
In this paper we report on a unique sample of close binary dwarf carbon (dC) — implicating them as post-common envelope binaries (PCEBs) and likely pre-CVs — discovered from their periodic photometric variability in the Zwicky Transient Facility
Even if the detected Zwicky Transient Facility DR5 (ZTF) periods arise from ellipsoidal variations and represent half the orbital period, such short periods are surprising for dC stars, which require significant accretion from a thermally pulsing (TP)-asymptotic giant branch (AGB) C giant to turn them into the C-enriched dwarfs we see today
Summary
Carbon (C) stars are those that show molecular absorption bands of C, such as C2, CN and CH, in their optical spectra (Secchi 1869). Most blue straggler stars are found in wide binaries with periods of order 1000 days, consistent with expectations of mass transfer from an AGB star onto a main-sequence companion (Chen & Han 2008; Gosnell et al 2014), which leaves a CO-core WD remnant (Paczynski 1971). The longest dC periods that are of order 10s of years likely experienced only standard Roche-lobe overflow (RLOF) or wind-RLOF (WRLOF) These periods are consistent with other types of post-mass-transfer systems, such as the blue straggler stars. Margon et al (2018) conducted a search for periodic dCs using the Palomar Transient Factory (PTF; Law et al 2009; Rau et al 2009), finding just one periodic dC They clearly highlighted the potential for large photometric surveys to find periodic dCs, dCs with short periods that should have experienced the strongest phases of CE mass transfer.
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