Substellar Companions and the Formation of Hot Subdwarf Stars

Abstract

We give a brief review over the observational evidence for close substellar companions to hot subdwarf stars. The formation of these core helium-burning objects requires huge mass loss of their red giant progenitors. It has been suggested that besides stellar companions substellar objects in close orbits may be able to trigger this mass loss. Such objects can be easily detected around hot subdwarf stars by medium or high resolution spectroscopy with an RV accuracy at the km s −1 -level. Eclipsing systems of HW Vir type stick out of transit surveys because of their characteristic light curves. The best evidence that substellar objects in close orbits around sdBs exist and that they are able to trigger the required mass loss is provided by the eclipsing system SDSS J0820+0008, which was found in the course of the MUCHFUSS project. Furthermore, several candidate systems have been discovered.

The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars

“…Measuring the radial velocity by fitting synthetic models, we selected only stars faster than ±100 km s −1 . For most of the known sdB binaries, the RV semi-amplitudes are below 100 km s −1 (Geier et al 2010b). Hence the radial velocity of a typical sdB binary of the Galactic disc will rarely exceed 100 km s −1 in absolute value and these stars are consequently excluded in MUCHFUSS.…”

“…1 shows a flowchart of the target selection method. We selected sdO/B candidates by colour (g-r<0.1 and u-g<0.4, see Geier et al 2010b) and pre-classified their spectra by visual inspection. Measuring the radial velocity by fitting synthetic models, we selected only stars faster than ±100 km s −1 .…”

“…This observational cut was introduced by Brown et al (2007) to distinguish HVS from halo stars by their origin. More information on MUCHFUSS and the process of target selection is presented by Geier et al (2010b).…”

“…Motivated by the discovery of US 708, we embarked on a project to identify a sample of population II hot subdwarfs and study their kinematics from radial velocity and proper motion. We make use of the MUCHFUSS survey (Geier et al 2010b), which searches for close binaries with high radial-velocity variations. The search strategy also provides targets that are not close binaries but travel through space at high RV without variations.…”

Pulsations and planets: The asteroseismology‐extrasolar‐planet connection

“…On the low-mass end, it can be argued to comprise the V391 Pegasi planet and, possibly, further secondary sub-stellar companions. Four companions with unusually high masses have also been reported; according to Geier et al (2009bGeier et al ( , 2010Geier et al ( , 2008, the massive compact companions found from radial velocity variations must at least be heavy white dwarfs or in two cases even neutron stars or black holes.…”