The Canis Major Star Forming Region

نویسنده

  • Jane Gregorio-Hetem
چکیده

The shape of the main arc formed by the Canis Major clouds has been suggested to result from a supernova explosion possibly triggering the recent star formation activity. The presence of dozens of OB stars and reflection nebulae forms the CMa OB1/R1 associations. More than a hundred emission line stars are found in this region, including the famous Z CMa, a binary system containing a Herbig Be star and a FUor companion. Several embedded infrared clusters with different ages are associated with the CMa clouds. The main characteristics of the region in terms of cloud structure, stellar content, age of associated young clusters, distance, and X-ray emission are presented in this chapter. Some of the arguments in favor and against the hypothesis of induced star formation are discussed in the last section. 1. The Canis Major Associations The star-forming region in Canis Major is characterized by a concentrated group of early type stars, which was first identified as a stellar association by Ambartsumian (1947). The angular size of 4 in diameter was defined by Markarian (1952), who estimated a distance of 960 pc, based on the identification of 11 probable members. Ruprecht (1966) established the approximate boundaries: 222 < l < 226 and −3.4 < b < +0.7 of the CMa OB1 association. He suggested that NGC 2353, a young cluster of age ∼ 12.6 Myr (Hoag et al. 1961) and with a distance of ∼1315 pc (Becker 1963), constitutes the main stellar concentration of CMa OB1. More recently, Fitzgerald et al. (1990) estimated an age of 76 Myr for NGC 2353 and a distance of 1.2 kpc, indicating that the cluster is unrelated to CMa OB1 despite their similar distances. Clariá (1974a,b) studied the space distribution of O and B stars, based on UBV photometry obtained for 247 stars. The estimated E(B-V) color excess confirms the existence of a group of young OB stars together with excited gas and obscuring matter, which belong to the CMa OB1 association. Clariá used these data to derive a distance of 1.15 kpc and an age of 3 Myr. The CMa R1 association is defined by a group of stars embedded in reflection nebulae located within the boundaries of the OB1 association. It was first identified by van den Bergh (1966), who listed ten stars distributed in the −3.4 < b < −2 range. Racine (1968) reported photometric and spectroscopic observations of the CMa R1 members and suggested that the reflection nebulae seem to be nearer than the CMa OB1 association. Clariá (1974b) compares the distance moduli that he obtained with those values estimated by Racine. The evaluation of the discrepancies led to the conclusion of the physical relation of the CMa OB1/R1 associations.

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تاریخ انتشار 2008