Hot subdwarf binaries from the MUCHFUSS project

Abstract

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions like massive white dwarfs (M > 1.0 M ), neutron stars, or stellar-mass black holes. The existence of such systems is predicted by binary evolution theory, and recent discoveries indicate that they exist in our Galaxy. We present orbital and atmospheric parameters and put constraints on the nature of the companions of 12 close hot subdwarf B star (sdB) binaries found in the course of the MUCHFUSS project. The systems show periods between 0.14 and 7.4 days. In nine cases the nature of the companions cannot be constrained unambiguously whereas three systems most likely have white dwarf companions. We find that the companion to SDSS J083006.17+475150.3 is likely to be a rare example of a low-mass helium-core white dwarf. SDSS J095101.28+034757.0 shows an excess in the infrared that probably originates from a third companion in a wide orbit, which makes this system the second candidate hierarchical triple system containing an sdB star. SDSS J113241.58−063652.8 is the first helium deficient sdO star with a confirmed close companion. This study brings to 142 the number of sdB binaries with orbital periods of less than 30 days and with measured mass functions. We present an analysis of the minimum companion mass distribution and show that it is bimodal. One peak around 0.1 M corresponds to the low-mass main sequence (dM) and substellar companions. The other peak around 0.4 M corresponds to the white dwarf companions. The derived masses for the white dwarf companions are significantly lower than the average mass for single carbon-oxygen white dwarfs. In a T eff -log g diagram of sdB+dM companions, we find signs that the sdB components are more massive than the rest of the sample. The full sample was compared to the known population of extremely low-mass white dwarf binaries as well as short-period white dwarfs with main sequence companions. Both samples show a significantly different companion mass distribution indicating either different selection effects or different evolutionary paths. We identified 16 systems where the dM companion will fill its Roche Lobe within a Hubble time and will evolve into a cataclysmic variable; two of them will have a brown dwarf as donor star. Twelve systems with confirmed white dwarf companions will merge within a Hubble time, two of them having a mass ratio to evolve into a stable AM CVn-type binary and another two which are potential supernova Ia progenitor systems. The remaining eight systems will most likely merge and form RCrB stars or massive C/O white dwarfs depending on the structure of the white dwarf companion.

The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project

“…Geier et al (2011a) discussed the target selection and the follow-up strategy. Detailed analyses of sdB binaries discovered in the course of this project are given in Geier et al (2011b) and Kupfer et al (2015). Three eclipsing systems have been discovered, two of them being the first sdBs with brown dwarf companions (Geier et al 2011c;Schaffenroth et al 2014).…”

“…Spectroscopic follow-up, criterion for variability and RV catalogue During our follow-up campaign we obtained medium resolution (R = 1200−7700), time-resolved spectroscopy using WHT/ISIS, CAHA3.5 m/TWIN, ESO-NTT/EFOSC2, SOAR/Goodman, Gemini/GMOS, and INT/IDS (see Table 1, Geier et al 2011b;Kupfer et al 2015) and measured the RVs as described above.…”

“…Removing those non-detections we end up with a sample of 177 stars, which show RV variability with probabilites between 95% and 99.9% (see Table 2). Orbital solutions were already derived for 22 close binary sdB systems (see Kupfer et al 2015, and references therein).…”

“…Up to now the orbital parameters of 142 close binaries have been measured. Most of the solved systems have hydrogen-rich sdB primaries, but this sample also contains 46 hydrogen-rich sdOB and sdO stars (see Kupfer et al 2015 and references therein, but see also the discussion in Sect. 4.5).…”

“…Most of the known companions of sdBs in RV variable close binary systems are white dwarfs or A&A 577, A26 (2015) late-type main sequence stars, but substellar companions like brown dwarfs have been found as well (see Kupfer et al 2015, and references therein). Those systems were most likely formed after a common envelope (CE) and spiral-in phase, which also provides an explanation for the required mass-loss on the RGB.…”