EDP Sciences
Free access
Volume 484, Number 3, June IV 2008
Page(s) L31 - L34
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200809614
Published online 06 May 2008

A&A 484, L31-L34 (2008)
DOI: 10.1051/0004-6361:200809614


A possible solution for the lack of EHB binaries in globular clusters

Z. Han

National Astronomical Observatories / Yunnan Observatory, the Chinese Academy of Sciences, Kunming 650011, PR China
    e-mail: zhanwenhan@hotmail.com

Received 20 February 2008 / Accepted 27 April 2008

Context. The binary fraction among extreme horizontal branch (EHB) stars in Galactic globular clusters (GCs) is an order of magnitude lower than the binary fraction among their counterparts, field hot subdwarfs. This casts serious doubt on their formation channels.
Aims. In this Letter, I explain the difference between the field and the cluster EHB stars with the binary model of Han et al. (2002, MNRAS, 336, 449; 2003, MNRAS, 341, 669) for the formation of EHB stars.
Methods. With the binary population synthesis code of Han et al. (2002, 2003), I follow the evolution of simple stellar populations resulting from single star bursts (note that Han et al. 2002, 2003, adopted a constant star formation rate over the last 15 Gyr for the production of field EHB stars), and obtain EHB stars at different stellar population ages.
Results. I found that the binary fraction among EHB stars decreases with the stellar population age. The fraction of EHB binaries with orbital periods P < 5 d is ~2.5% for a stellar population of 10 Gyr from the standard simulation set.
Conclusions. The binary model of Han et al. (2002, 2003) is able to explain the lack of EHB binaries in globular clusters. I also propose that the precise determination of the physical parameters of close EHB binaries in GCs can lead to the strictest constraint on common-envelope ejection efficiency.

Key words: stars: horizontal-branch -- binaries: close -- stars: subdwarfs -- globular clusters: general

© ESO 2008

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access.

An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account.
    In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.