Tom R. Marsh

Publications (5)3.37 Total impact

• Article: The orbital period distribution of subdwarf-B binaries

  Luisa Morales-Rueda, Pierre F.L. Maxted, Tom R. Marsh
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  ABSTRACT: We present the results of a 3.5 year long campaign to measure orbital periods of subdwarf-B (sdB) star binaries. We directly compare our observed orbital period distribution with that predicted by using binary population synthesis. Up to now, most of our systems seem to have been formed through two of the formation channels discussed by Han et al. (2003, MNRAS 341, p. 669), i.e. the first and the second common envelope ejection (CE) channels. At this point, thanks to the long baseline of our observations, we are starting to detect also very long orbital period systems. These have probably come from a complete different formation path, the first stable Roche Lobe overflow (RLOF) channel in which the first mass transfer phase is stable. This channel is expected to lead to the formation of very wide binaries with typical orbital periods ranging from 1 month to 1 year.
  Astrophysics and Space Science 05/2004; 291(3):299-306. · 1.69 Impact Factor
• Article: Companions to sdB binaries−degenerate or non-degenerate?

  Pierre F.L. Maxted, Luisa Morales-Rueda, Tom R. Marsh
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  ABSTRACT: We report the results of an on-going programme to collect lightcurves for EHB binaries with orbital periods P ≲ 1 d. Degenerate and non-degenerate companions are clearly distinguished by the presence or absence of a reflection effect in these binaries. The amplitude of the reflection effect can be combined with other data to provide an estimate of the mass of the companion star. We find that the fraction of sdB binaries in our sample which have non-degenerate companions is 0.080.06 (1−σ error). These non-degenerate companions have very low masses (≲ 0.1M⊙). This property is not predicted by existing population synthesis models.
  Astrophysics and Space Science 05/2004; 291(3):307-314. · 1.69 Impact Factor
• Source

  Available from: Zhanwen Han

  Article: The origin of sdB stars (II)

  Zhanwen Han, Philipp Podsiadlowski, Pierre F. L. Maxted, Tom R. Marsh
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  ABSTRACT: We have carried out a detailed binary populations synthesis (BPS) study of the formation of subdwarf B (sdB) stars and related objects (sdO, sdOB stars) using the latest version of the BPS code developed by Han et al.(1994, 1995a, 1995b, 1998, 2001). We systematically investigate the importance of the five main evolutionary channels in which the sdB stars form after one or two common-envelope (CE) phases, one or two phases of stable Roche-lobe overflow (RLOF) or as the result of the merger of two helium white dwarfs (WD) (see Han et al. 2002, Paper I). Our best BPS model can satisfactorily explain the main observational characteristics of sdB stars, in particular their distributions in the orbital period - minimum companion mass diagram and in the effective temperature - surface gravity diagram, their distributions of orbital period, log (g theta^4), and mass function, their binary fraction and the fraction of sdB binaries with WD companions, their birthrates and their space density. We obtain a Galactic formation rate, a total number in the Galaxy, the intrinsic binary fraction for sdB stars. We also predict a distribution of masses for sdB stars that is wider than is commonly assumed and that some sdB stars have companions of spectral type as early as B. The percentage of A type stars with sdB companions can in principle be used to constrain some of the important parameters in the binary evolution model. We conclude that (a) the first RLOF phase needs to be more stable than is commonly assumed; (b) mass transfer in the first stable RLOF phase is non-conservative, and the mass lost from the system takes away a specific angular momentum similar to that of the system; (c) common-envelope ejection is very efficient. Comment: 26 pages, 27 figures, accepted for publication in MNRAS
  01/2003;
• Source

  Available from: Zhanwen Han

  Article: The Origin of Subdwarf B Star (I): the Formation Channels

  Zhanwen Han, Philipp Podsiadlowski, Pierre L. F. Maxted, Tom R. Marsh, Natasha Ivanova
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  ABSTRACT: Subdwarf B (sdB) stars (and related sdO/sdOB stars) are believed to be helium core-burning objects with very thin hydrogen-rich envelopes. In recent years it has become increasingly clear from observational surveys that a large fraction of these objects are members of binary systems. To better understand their formation, we here present the results of a detailed investigation of the three main binary evolution channels that can lead to the formation of sdB stars: the common envelope (CE) ejection channel, the stable Roche lobe overflow (RLOF) channel and the double helium white dwarfs (WDs) merger channel. We obtained the conditions for the formation of sdB stars from each of these channels using detailed stellar and binary evolution calculations where we modelled the detailed evolution of sdB stars and carried out simplified binary population synthesis simulations. The observed period distribution of sdB stars in compact binaries strongly constrains the CE ejection parameters. We also present the distribution of sdB stars in the $T_{\rm eff}$ - $\log g$ diagram, the Hertzsprung-Russell diagram and the distribution of mass functions. Comment: 20 pages, 23 figures, accepted for publication in MNRAS
  06/2002;
• Source

  Available from: Zhanwen Han

  Article: The Formation of Subdwarf B Stars

  Zhanwen Han, Philipp Podsiadlowski, Pierre F. L. Maxted, Tom R. Marsh, Natasha Ivanova
  279:285.