Direct detection of exoplanet host star companionγCep B and revised masses for both stars and the sub-stellar object

Abstract

Context. The star γ Cep is known as a single-lined spectroscopic triple system at a distance of 13.8 pc, composed of a K1 III-IV primary star with V = 3.2 mag, a stellar-mass companion in a 66-67 year orbit (Torres 2007, ApJ, 654, 1095, and a substellar companion with M p sin i = 1.7 M Jup that is most likely a planet (Hatzes et al. 2003, ApJ, 599, 1383. Aims. We aim to obtain a first direct detection of the stellar companion, to determine its current orbital position (for comparison with the spectroscopic and astrometric data), its infrared magnitude and, hence, mass. Methods. We use the Adaptive Optics camera CIAO at the Japanese 8 m telescope Subaru on Mauna Kea, Hawaii, with the semitransparent coronograph to block most of the light from the bright primary γ Cep A, and to detect at the same time the faint companion B. In addition, we also used the IR camera Ω Cass at the Calar Alto 3.5 m telescope, Spain, to image γ Cep A and B by adding up many very short integrations (without AO). Results. γ Cep B is clearly detected on our CIAO and Ω Cass images. We use a photometric standard star to determine the magnitude of B after PSF subtraction in the Subaru image, and the magnitude difference between A and B in the Calar Alto images, and find an average value of K = 7.3 ± 0.2 mag. The separations and position angles between A and B are measured on 15 July 2006 and 11 and 12 Sept. 2006, B is slightly south of west of A. Conclusions. By combining the radial velocity, astrometric, and imaging data, we have refined the binary orbit and determined the dynamical masses of the two stars in the γ Cep system, namely 1.40 ± 0.12 M for the primary and 0.409 ± 0.018 M for the secondary (consistent with being a M4 dwarf). We also determine the minimum mass of the sub-stellar companion to be M p sin i = 1.60 ± 0.13 M Jup .

Disc-protoplanet interaction

“…Their semi-major axis was 20 au and the chosen eccentricity was 0.4. These values are close to those of γ Cephei provided by Neuhäuser et al (2007) and were also used in the study of . Our initial disc mass was 0.01 M and we used an alpha viscosity prescription, where α has a value of 5 × 10 −3 .…”

“…We investigated the evolution of particles embedded in a protostellar disc by using orbital parameters similar to the wellknown γ Cephei system (Neuhäuser et al 2007). Introducing our hydrodynamical 2D code, which is suitable to compute several thousands of self-gravitating particles that interact gravitationally with a viscous and radiative circumprimary disc within a binary star system, we presented a series of tests that validated our program (see the Appendix A).…”

“…This suggests a significant influence of the secondary star on the circumprimary protoplanetary disc. Planets have been discovered in mid-separation binaries, with semi-major axes of about 20 au, for example, Gl86 (Santos et al 2000;Lagrange et al 2006), HD 41004 (Zucker et al 2004), γ Cephei (Hatzes et al 2003;Neuhäuser et al 2007), and the recently discovered HD 196885 (Chauvin et al 2011) and α Centauri B (Dumusque et al 2012), which has led to a growing interest in understanding planetary formation processes in general. In mid-separation binary star systems, the secondary and the heavily truncated and possibly distorted disc causes the formation and evolution of planets throughout several stages of the planet-forming process.…”

The formation of massive planets in binary star systems

“…In our studies we have taken the data for γ Cep from Hatzes et al (2003). The more recent data by Neuhäuser et al (2007) only slightly change the dynamical status.…”

On the RZ Draconis substellar circumbinary companions

“…Other types of hierarichical star systems reside in so-called S-type (Schwarz et al 2011) configurations where one body is orbiting one component of a binary pair. Several examples of such systems have been reported in the literature (Neuhäuser et al 2007;Chauvin et al 2007).…”