Hot Subdwarf All Southern Sky Fast Transit Survey with the Evryscope

Abstract

We have conducted a survey of candidate hot subdwarf stars in the southern sky searching for fast transits, eclipses, and sinusoidal like variability in the Evryscope light curves. The survey aims to detect transit signals from Neptune size planets to gas-giants, and eclipses from M-dwarfs and brown dwarfs. The other variability signals are primarily expected to be from compact binaries and reflection effect binaries. Due to the small size of hot subdwarfs (R ≈ 0.2R ), transit and eclipse signals are expected to last only ≈twenty minutes, but with large signal depths (up to completely eclipsing if the orientation is edge on). With its 2-minute cadence and continuous observing Evryscope is well placed to recover these fast transits and eclipses. The very large field of view (8150 sq. deg.) is critical to obtain enough hot subdwarf targets, despite their rarity. We identified ≈11,000 potential hot subdwarfs from the 9.3M Evryscope light curves for sources brighter than m g = 15. With our machine learning spectral classifier, we flagged high-confidence targets and estimate the total hot subdwarfs in the survey to be ≈1400. The light curve search detected three planet transit candidates, shown to have stellar companions from follow-up analysis. We discovered several new compact binaries (including two with unseen degenerate companions, and several others with potentially rare secondaries), two eclipsing binaries with M-dwarf companions, as well as new reflection effect binaries and others with sinusoidal like variability. The hot subdwarf discoveries identified here are spectroscopically confirmed and we verified the Evryscope discovery light curve with TESS light curves when available. Four of the discoveries are in the process of being published in separate followup papers, and we discuss the followup potential of several of the other discoveries.

A search for variable subdwarf B stars in TESS full frame images – I. Variable objects in the southern ecliptic hemisphere

“…We detected significant flux variations in 1807 objects, out of which 28 are classified as sdBs, 2 as subdwarfs (sd), 77 as non-sdBs, and 1619 are not spectroscopically classified. To identify sdB objects we used the sdB database Geier (2020), the LAMOST spectroscopic survey (Lei et al 2018(Lei et al , 2019aLuo et al 2019), the Evryscope survey (Ratzloff et al 2020) and the Simbad database (Wenger, M. et al 2000). The large square pixels, 21 arcsec on side, cause serious issues in crowded regions of the sky, since an optimal aperture may contain neighboring objects.…”

“…This figure contains candidates for reflection binaries (a sdB and a low mass main sequence companion) and ellipsoidal binaries (a sdB and a white dwarf). One of the eclipsing binaries, Gaia DR2 2969438206889996160, which has just recently been published by Ratzloff et al (2020), is an HW Vir system (Wood et al 1993; Baran et al 2018). Figure 3 includes 14 variable sdB stars that show low amplitude variation that are detectable in amplitude spectra.…”

Space observations of AA Doradus provide consistent mass determinations. New HW-Vir systems observed with TESS

“…The lowresolution NTT/EFOSC survey spectrum taken in June 2015 confirms that the primary is a sdB star, with T eff ∼ 30 kK, log 𝑔 = 5.6 and no detectable helium. It is reported as a new HW-Vir system dubbed EVR-CB-003 in the recent Evryscope survey (Ratzloff et al 2020), with a promise of a forthcoming discovery paper. It was observed with TESS in Sectors 11 and 12, and owing to the brightness of this object (G = 13.5), the light curve is of excellent precision (middle panel in Figure 10).…”

Physical Properties of 29 sdB+dM Eclipsing Binaries in Zwicky Transient Facility

“…Such stars have been studied continuously and extensively over the last few decades, and their number is increasing (see e.g. Kilkenny et al (1978); Menzies & Marang (1986); Drechsel et al (2001); Østensen et al (2007); For et al (2010); Barlow et al (2013); Schaffenroth et al (2014); Kupfer et al (2015); Almeida et al (2017); Ratzloff et al (2019a); Koen (2019); Ratzloff et al (2020); Sahoo et al (2020a); Baran et al (2021)). The orbital periods of HW Vir-type stars are easily obtained from LCs, while the physical parameters such as mass and radius of sdB and the companion stars can be determined based on radial velocity measurements and LC analysis.…”