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Astron. Astrophys. 360, 1011-1018 (2000)


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Reconstructing the evolution of double helium white dwarfs: envelope loss without spiral-in

G. Nelemans 1, F. Verbunt 2, L.R. Yungelson 1,3 and S.F. Portegies Zwart *  4

1 Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands (gijsn@astro.uva.nl)
2 Astronomical Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands (verbunt@astro.uu.nl)
3 Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatnitskaya Str., 109017 Moscow, Russia (lry@inasan.rssi.ru)
4 Department of Astronomy Boston University, 725 Commonwealth Avenue, Boston, MA 01581, USA (spz@komodo.bu.edu)

Received 8 February 2000 / Accepted 8 June 2000

Abstract

The unique core-mass - radius relation for giants with degenerate helium cores enables us to reconstruct the evolution of three observed double helium white dwarfs with known masses of both components.

The last mass transfer phase in their evolution must have been a spiral-in. In the formalism proposed by Webbink (1984), we can constrain the efficiency of the deposition of orbital energy into the envelope to be [FORMULA], for an envelope structure parameter [FORMULA]. We find that the two standard mass transfer types (stable mass transfer and spiral-in) are both unable to explain the first phase of mass transfer for these three binaries.

We use a parametric approach to describe mass transfer in low-mass binaries, where both stars are of comparable mass and find that the orbital characteristics of the observed double helium white dwarfs can be well reproduced if the envelope of the primary is lost with [FORMULA]1.5 times the specific angular momentum of the initial binary. In this case no substantial spiral-in occurs.

Key words: stars: binaries: close – stars: evolution – stars: mass-loss – stars: white dwarfs

* Hubble Fellow

Send offprint requests to: Gijs Nelemans

© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000

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