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High-energy emission from early-type stars and their surroundings

Research project PX/8/SS/06 (Research action PX)

Persons :

  • Dr.  RAUW Grégor - Université de Liège (ULG)
    Coordinator of the project
    Financed belgian partner
    Duration: 1/1/2005-31/12/2007

Description :

Observations in the high-energy domain are important to address several open questions related to massive stars and their surroundings:

- What is the chemical composition of evolved massive stars? Thanks to its high spectral resolution, we can use the RGS onboard XMM-Newton to measure the strength and shape of individual lines of chemical elements (including O, Ne, S, Fe, Mg,…) that cannot be studied easily at other wavelengths (Kahn et al. 2001, A&A 365, L312). Deriving accurate abundances of these elements through RGS observations allows to test the predictions of the most recent stellar evolution models and to assess the importance of stellar rotation in the evolution of the most massive stars (Meynet & Maeder 2000, A&A 361, 101). In addition, a thorough analysis of the line profiles will allow us to constrain the location of the X-ray emitting plasma within the stellar wind.

- What are the mass loss rates of massive stars? Though this question has been a research topic for several decades, it is still far from solved. Our team has started observing a sample of massive binaries in different evolutionary stages. In massive binary systems, the collision of the stellar winds results in a shock region where the plasma is heated to temperatures of several million Kelvin, thereby emitting copious amounts of X-rays. Measuring both, the strength of this emission, as well as the absorption that it suffers from the surrounding wind allows us to determine the mass loss rates of the binary components (see e.g. De Becker et al. 2004, A&A 416, 221; Sana et al. 2004, MNRAS 350, 809). This is done comparing the observed X-ray spectra to simulations with state of the art hydro-dynamical calculations. Further insight into the physics of stellar winds (including the wind driving mechanisms, see Henley et al. 2003, MNRAS 346, 773) is expected from high resolution RGS (with XMM) and XRS (with Astro-E2; we have submitted a proposal to this effect) observations that allow to resolve for the first time the profiles of individual lines in the X-ray spectra of colliding wind binaries. We have started several projects in this context.

- How do the most massive stars form? X-ray observations of several very young open clusters by our group reveal a concentration of lower-mass pre-main sequence (PMS) stars surrounding the massive OB stars (e.g. Rauw et al. 2003, A&A 407, 925). Comparing the ages of these PMS objects with those of the OB stars suggests that the lower mass objects are significantly older. Such a situation is expected if massive stars form through a combination of accretion and physical collisions (Bonnell et al. 1998, MNRAS 298, 93). In this case, the rejuvenation of the massive stars after each merger would make these objects appear much younger. Since X-ray observations (supplemented by ground-based follow-up observations) are the best way to identify these PMS objects, we intend to observe more open clusters with XMM-Newton and Chandra to check whether this is a general feature. The results of this study will eventually provide strong constraints on the massive star formation models as well as on the impact of high-energy radiation on star formation regions.

- What is the nature of the unidentified gamma-ray sources in OB associations? Is this emission produced in the atmospheres of “normal” massive stars, i.e. are the relativistic particles accelerated in hydro-dynamical shocks either in the winds of single stars or more likely in the colliding wind region of massive binaries (Rauw 2004, in “Cosmic Gamma-ray Sources”, Kluwer Acad. Publ., p.105)? In this case, a multi-wavelength study (such as our ongoing campaign including gamma-rays, X-rays, optical light and radio waves) will allow to measure the magnetic fields of these stars and to derive strong constraints on the acceleration mechanisms of cosmic particles. Alternatively, these unidentified gamma-ray sources could also be associated with radio-quiet pulsars, micro-quasars… or a new category of high-mass interacting binaries. In addition, to the observations with the IBIS instrument onboard INTEGRAL (mainly obtained in the INTEGRAL core programme), we will organise follow-up campaigns in the optical and X-rays to constrain the nature of these sources.

Satellite(s) or flight opportunity(ies):

- XMM-Newton

Field of research:

Space Sciences: High-Energy Astrophysics

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