Volume 506, Number 2, November I 2009
|Page(s)||711 - 727|
|Section||Galactic structure, stellar clusters, and populations|
|Published online||27 August 2009|
Star formation history of Canis Major R1*
I. Wide-Field X-ray study of the young stellar population
Universidade de São Paulo, IAG, Departamento de Astronomia, Brazil e-mail: email@example.com
2 Laboratoire d'Astrophysique de Grenoble, Université Joseph Fourier-CNRS, France
3 Instituto Nacional de Pesquisas Espaciais, Divisão de Astrofísica, São José dos Campos, SP, Brazil
4 Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
5 Astrophysikaliches Institut Potsdam, Germany
Accepted: 28 July 2009
Aims. The CMa R1 star-forming region contains several compact clusters as well as many young early-B stars. It is associated with a well-known bright rimmed nebula, the nature of which is unclear (fossil HII region or supernova remnant). To help elucidate the nature of the nebula, our goal was to reconstruct the star-formation history of the CMa R1 region, including the previously unknown older, fainter low-mass stellar population, using X-rays.
Methods. We analyzed images obtained with the ROSAT satellite, covering ~5 sq. deg. Complementary VRI photometry was performed with the Gemini South telescope. Colour-magnitude and colour-colour diagrams were used in conjunction with pre-main sequence evolutionary tracks to derive the masses and ages of the X-ray sources.
Results. The ROSAT images show two distinct clusters. One is associated with the known optical clusters near Z CMa, to which ~40 members are added. The other, which we name the “GU CMa” cluster, is new, and contains ~60 members. The ROSAT sources are young stars with masses down to ~ 0.5 , and ages up to 10 Myr. The mass functions of the two clusters are similar, but the GU CMa cluster is older than the cluster around Z CMa by at least a few Myr. Also, the GU CMa cluster is away from any molecular cloud, implying that star formation must have ceased; on the contrary (as already known), star formation is very active in the Z CMa region.
Key words: stars: pre-main sequence / X-rays: stars / infrared: stars / ISM: clouds
Based in part on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).
© ESO, 2009
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