We perform a study of the spatial and kinematical distribution of young open clusters in the solar neighborhood, discerning between bound clusters and transient stellar condensations within our sample. Then, we discriminate between Gould Belt (GB) and local Galactic disk (LGD) members, using a previous estimate of the structural parameters of both systems obtained from a sample of O-B6 Hipparcos stars. Using this classified sample we analyze the spatial structure and the kinematic behavior of the cluster system in the GB. The two star formation regions that dominate and give the GB its characteristic inclined shape show a striking difference in their content of star clusters: while Ori OB1 is richly populated by open clusters, not a single one can be found within the boundaries of Sco OB2. This is mirrored in the velocity space, translating again into an abundance of clusters in the region of the kinematic space populated by the members of Ori OB1, and a marginal number of them associated to Sco OB2. In the light of these results we study the nature of the GB with respect to the optical segment of the Orion Arm, and we propose that the different content of star clusters, the different heights over the Galactic plane and the different residual velocities of Ori OB1 and Sco OB2 can be explained in terms of their relative position to the density maximum of the Local Arm in the solar neighborhood. Although morphologically intriguing, the GB appears to be the result of our local and biased view of a larger star cluster complex in the Local Arm, that could be explained by the internal dynamics of the Galactic disk.
Deep Dive into Hierarchical Star Formation: Stars and Stellar Clusters in the Gould Belt.
We perform a study of the spatial and kinematical distribution of young open clusters in the solar neighborhood, discerning between bound clusters and transient stellar condensations within our sample. Then, we discriminate between Gould Belt (GB) and local Galactic disk (LGD) members, using a previous estimate of the structural parameters of both systems obtained from a sample of O-B6 Hipparcos stars. Using this classified sample we analyze the spatial structure and the kinematic behavior of the cluster system in the GB. The two star formation regions that dominate and give the GB its characteristic inclined shape show a striking difference in their content of star clusters: while Ori OB1 is richly populated by open clusters, not a single one can be found within the boundaries of Sco OB2. This is mirrored in the velocity space, translating again into an abundance of clusters in the region of the kinematic space populated by the members of Ori OB1, and a marginal number of them associa
The Gould Belt (GB) was first discovered by John Herschel (1847) and Benjamin Gould (1879) as a system of bright stars inclined with respect to the plane of the Milky Way. For more than a century, many studies have been devoted to describing its structure and its kinematical behavior, as well as to proposing a reliable global scenario that would account for its origin (for an extensive review on the subject, see Pöppel 1997, 2001and Grenier 2004). Today it is considered that, in the scope of the most recent theories of hierarchical star formation (Efremov 1978(Efremov , 1995;;Elmegreen et al. 2000;
⋆ E-mail: felias@astroscu.unam.mx (FE); emilio@iaa.es (EJA) Elmegreen 2006), the GB is likely to be our closest giant star forming complex (Comerón 2001).
The stellar component of this complex takes the shape of a planar distribution of bright and young OB stars inclined with respect to the Galactic plane (Lesh 1968;Stothers & Frogel 1974;Westin 1985). Most of the young OB associations in the solar neighborhood are known to be part of the GB (Blaauw 1965;de Zeeuw et al. 1999;Elias et al. 2006a, hereafter Paper I). Also, a system of young, lowmass stars, detected by cross-matching X-ray and optical Hipparcos (Perryman et al. 1997) based catalogs, appears to be associated with the GB (Guillout et al. 1998).
As we should expect from a giant ongoing star forming complex, the local interstellar medium is prominently asso-2 F. Elias, E.J. Alfaro and J. Cabrera-Caño ciated with the GB. The works by van den Bergh (1966) on reflection nebulae, by Sandqvist (1977) on dark clouds and, more recently, by Gaustad & Van Buren (1993) on maps of infrared emission, have found a spatial distribution of the dark clouds of interstellar dust compatible with the pattern shown by the stellar component of the GB. Tomita (1986Tomita ( , 1987) ) and the study of the CO molecule by Taylor et al. (1987) seem to confirm this inclination for the local molecular clouds.
Neutral hydrogen in the solar neighborhood has also been related to the GB after the discovery of Lindblad’s “feature A” (Lindblad 1967;Lindblad et al. 1973), interpreted as a ring of gas with an expansion movement (Olano 1982;Elmegreen 1982). After Lindblad’s work, the giant molecular clouds were found to be related to the most prominent OB star associations (Sancisi et al. 1974;Kutner et al. 1977;de Geus 1992). A full map of the CO molecule over the sky later confirmed that most of these clouds within 1 kpc from the Sun follow the GB pattern (Dame et al. 1993).
Then, if the GB is a star formation complex composed of both young stars and associations and interstellar material, we would expect to find a population of young open clusters following the GB pattern. This is obvious if we consider the concept of young star cluster in its widest sense, i.e., representing the dense inner regions of the hierarchical structure of young star fields (Elmegreen 2006). Nonetheless, in this study we want to distinguish between a young cluster understood as a mere stellar condensation, and a gravitationally bound system (that is, as a condensation that “has had sufficient time and gravitational self-attraction to get mixed by stellar orbital motions”, as put by Elmegreen 2006). Our work focuses on the analysis of the cluster system, and on how these objects distribute and behave in comparison with the GB defined by the massive stars.
Since the first systematic studies that led to a discrimination between the GB and the LGD (Stothers & Frogel 1974;Westin 1985) there has been a great improvement in the number of cataloged open clusters, as well as in the availability of their astrometric and physical data. Surprisingly, it is not possible to find in the scientific literature any work devoted to the study of the open clusters membership to the GB and their distribution within this system before 2006. Thus, for many years, it has been accepted that the GB did not contain a significant population of bound clusters. Only very recently Piskunov et al. (2006), in their analysis of the Galactic open cluster population, discovered an open cluster complex (OCC) that they associate with the GB. Although they find this OCC as a density peak within the spatial distribution of clusters in the solar neighborhood, they estimate OCC membership probabilities by kinematical methods, through the analysis of the tangential velocities.
In two previous papers (Paper I; Elias et al. 2006b, hereafter Paper II) we have studied the spatial distribution and the kinematic properties of the OB stars and associations in the GB. Our line of work thus leads in a logical way to the study of the distribution of young open clusters in the GB and their kinematic properties. Our analysis will be centered in the comparison of the GB morphology as obtained from the distribution of massive stars and clusters. This will represent another step in the understanding of the nature of the GB, and will also contribute to the knowledge
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