A new sdO+dM binary with extreme eclipses and reflection effect

TL;DR

This paper reports the discovery and detailed analysis of a rare, totally-eclipsing binary system with an sdO hot subdwarf and a strongly irradiated red dwarf companion, characterized by extreme eclipses and reflection effects.

Contribution

It presents the first detailed spectroscopic and photometric characterization of a new sdO+dM binary with extreme eclipses and irradiation effects, expanding understanding of such systems.

Findings

The binary has an orbital period of approximately 0.1873 days.

The sdO star has a temperature of about 55,000K and high surface gravity.

The red dwarf companion is heavily irradiated, with surface features dominated by emission lines.

Abstract

We report the discovery of a new totally-eclipsing binary (RA=06:40:29.11; Dec=+38:56:52.2; J=2000.0; Rmax=17.2 mag) with an sdO primary and a strongly irradiated red dwarf companion. It has an orbital period of Porb=0.187284394(11) d and an optical eclipse depth in excess of 5 magnitudes. We obtained two low-resolution classification spectra with GTC/OSIRIS and ten medium-resolution spectra with WHT/ISIS to constrain the properties of the binary members. The spectra are dominated by H Balmer and He II absorption lines from the sdO star, and phase-dependent emission lines from the irradiated companion. A combined spectroscopic and light curve analysis implies a hot subdwarf temperature of Teff(spec) = 55 000 +/- 3000K, surface gravity of log g(phot) = 6.2 +/- 0.04 (cgs) and a He abundance of log(nHe/nH) = -2.24 +/- 0.40. The hot sdO star irradiates the red-dwarf companion, heating its substellar point to about 22 500K. Surface parameters for the companion are difficult to constrain from the currently available data: the most remarkable features are the strong H Balmer and C II-III lines in emission. Radial velocity estimates are consistent with the sdO+dM classification. The photometric data do not show any indication of sdO pulsations with amplitudes greater than 7mmag, and Halpha-filter images do not provide evidence of the presence of a planetary nebula associated with the sdO star.