Mass transfer from the donor of GRS 1915+105

Free access article

Issue		A&A Volume 388, Number 3, June IV 2002


Page(s)		936 - 939
Section		Formation, structure and evolution of stars
DOI		http://dx.doi.org/10.1051/0004-6361:20020610

A&A 388, 936-939 (2002)
DOI: 10.1051/0004-6361:20020610

Mass transfer from the donor of GRS 1915+105

O. Vilhu

Observatory, Box 14, 00014 University of Helsinki, Finland

(Accepted 21 February 2002 / Accepted 15 April 2002 )

Abstract
A scenario for a periodic filling and emptying of the accretion disc of the microquasar GRS1915+105 is proposed, by estimating the mass transfer rate from the donor and comparing it with the observed accretion rate onto the primary black hole. The mass of the Roche-lobe-filling donor ( $1.2 \pm 0.2 ~ M_{\odot}$ ), the primary black hole mass ( $14 \pm 4 ~ M_{\odot}$ ) and the binary orbital period of 33.5 d (Greiner et al. 2001b) predict for the donor spectral type and K-magnitude around K6 III and -2.6, respectively. The He-core of 0.28 $M_{\odot}$ of such a giant leads to evolutionary expansion along the giant branch with a conservative mass transfer rate of $\dot{M}_{\rm d} = (1.5 \pm 0.5 ) \times 10^{-8}~ M_{\odot}$ /year. On the other hand, the average observed accretion rate onto the primary is ten times larger: $\dot{M}_{\rm obs} = 2.0 \times (\eta/0.1)^{-1}$ ( d/12.5 kpc) $^2\times 10^{-7}~ M_{\odot}$ /y, where $\eta$ is the efficiency of converting accretion into radiation. We propose a duty cycle with (5-10)( $\eta$ /0.1) per cent active ON-state. The timescale of the (recurrent) OFF-state is identified as the viscosity time scale at the circularization radius (14 $R_{\odot}$ ) and equals $t_{\rm visc} = 370 (\alpha$ /0.001) ^-4/5 years, where $\alpha$ is the viscosity parameter in the $\alpha$ -prescription of a classical disc. If the viscosity at the outer edge of the disc is small and $\eta$ is close to the maximum available potential energy (per rest mass energy) at the innermost stable orbit, the present activity phase may still last another 10-20 years. We also discuss other solutions allowing a broader range of donor masses (0.6-2.4 $M_{\odot}$ ).

Key words: stars: binaries: close -- stars: individual: GRS 1915+105

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