Cosmography of OB stars in the solar neighbourhood



Volume 584, December 2015
Article Number A26
Number of page(s) 13
Section Galactic structure, stellar clusters and populations
Published online 16 November 2015

1 Centro de Astrobiología, INTA-CSIC, Depto Astrofísica, ESAC Campus, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain e-mail:
2 Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria


We construct a 3D map of the spatial density of OB stars within 500 pc from the Sun using the Hipparcos  catalogue and find three large-scale stream-like structures that allow a new view on the solar neighbourhood. The spatial coherence of these blue streams and the monotonic age sequence over hundreds of parsecs suggest that they are made of young stars, similar to the young streams that are conspicuous in nearby spiral galaxies. The three streams are 1) the Scorpius to Canis Majoris stream, covering 350 pc and 65 Myr of star formation history; 2) the Vela stream, encompassing at least 150 pc and 25 Myr of star formation history; and 3) the Orion stream, including not only the well-known Orion OB1abcd associations, but also a large previously unreported foreground stellar group lying only 200 pc from the Sun. The map also reveals a remarkable and previously unknown nearby OB association, between the Orion stream and the Taurus molecular clouds, which might be responsible for the observed structure and star formation activity in this cloud complex. This new association also appears to be the birthplace of Betelgeuse, as indicated by the proximity and velocity of the red giant. If this is confirmed, it would solve the long-standing puzzle of the origin of Betelgeuse. The well-known nearby star-forming low-mass clouds, including the nearby T and R associations Lupus, Cha, Oph, CrA, Taurus, Vela R1, and various low-mass cometary clouds in Vela and Orion, appear in this new view of the local neighbourhood to be secondary star formation episodes that most likely were triggered by the feedback from the massive stars in the streams. We also recover well-known star clusters of various ages that are currently cruising through the solar neighbourhood. Finally, we find no evidence of an elliptical structure such as the Gould belt, a structure we suggest is a 2D projection effect, and not a physical ring.

1. Introduction

Because of their relatively short lifetimes, massive O and B stars are a good tracer of recent star formation in the Milky Way and nearby galaxies. It was noted very early on that O and B stars are not distributed randomly on the sky (Herschel 1847Gould 1879). Following the early canonical work of Eddington (1914)Kapteyn (1914)Charlier (1926), and Pannekoek (1929)Blaauw (1964) analysed the spatial and velocity distributions of OB stars in the solar neighbourhood to identify and study the content of nearby stellar groups. At the end of the last century, the accuracy and whole-sky coverage of the Hipparcos mission led to a major improvement in the definition and characterization of nearby OB associations and clusters, which profoundly changed our knowledge and understanding of the solar vicinity (e.g. Figueras et al. 1997de Zeeuw et al. 1999Platais et al. 1998Subramaniam & Bhatt 2000Hoogerwerf 2000de Bruijne 2000Branham 2002Elias et al. 2006a,b).

There are two predominant methods for identifying stellar groups in the literature. The first consists of searching for concentrations in 2D projections of the position and/or velocity spaces. Starting with John Herschel and Benjamin Gould’s original observations, OB stars were grouped in a “belt” on the projected sky; this is known today as the Gould belt. As precise photometric or parallactic distance measurements were accumulating, the searches were performed in all possible 2D projections of their XYZ Cartesian galactic coordinates (e.g. Charlier 1926Stothers & Frogel 1974Westin 1985Elias et al. 2006b,and references therein). At the beginning of the twentieth century, the advent of precise proper motion measurements (and to a lesser extent, radial velocities) opened the search space to velocities. When the mean proper motion of a group is sufficiently high to distinguish it from the quasi-random velocity distribution of disk stars, its co-moving members are identified in 2D vector-point diagrams. Whenever radial velocities were also available, the search for co-moving stars was often made in 2D projections of their Cartesian galactic velocities (U vs. VV vs. W, and U vs. W, e.g. Eggen 1984Torres et al. 2006Elias et al. 2006a, and references therein). But 2D projections are incapable of describing all the features of a 3D space, even if multiple projections are considered simultaneously. Important structures can be lost, hidden in the projection, while artificial structures can appear.

The second main procedure used to identify stellar groups relies on the convergent point method originally developed by Charlier that continued to be refined over the years (e.g. de Bruijne 1999Galli et al. 2012). Because of perspective effects, groups of co-moving stars tend to move towards a common convergence point on the sky, while unrelated stars will move towards random directions. If a co-moving group exists, the paths defined by its members will intersect at the convergence point and the distribution of intersection points will be denser in that direction. The convergent point method has been widely and successfully used and led to the discovery or confirmation of most well-known OB associations and clusters listed in the extensive inventory of de Zeeuw et al. (1999). But it suffers from a major bias towards groups with high tangential motion. When the motion of a stellar group is mostly radial, the convergent point coincides with the projected geometric centre of the group. The discovery of such a group is often impeded because the typical relative error on the proper motion of current surveys is usually too large. This bias will in particular affect the identification of stellar groups at large heliocentric distances or located towards the solar apex or antapex. We show in the course of the present study the surprising consequences in the Orion and Vela star-forming complexes.

Finally, both methods can be affected by the presence of companions. The orbital motion of long-period binaries can add a significant non-linear component to the apparent motion and alter the proper motion measurements. This effect is particularly relevant for OB stars because of their high multiplicity rate. It did not affect the discovery of large conspicuous groups of OB stars whose co-motion will statistically dominate (de Zeeuw et al. 1999). But it might prevent the discovery of smaller and sparser groups of OB stars by shuffling the velocity measurements of a significant fraction of their members and diffusing the group coherence in the convergent point diagram. Therefore velocities should be used with caution when blindly searching for small groups of OB stars, and in particular in the subsequent assignment of membership probabilities to individual stars.

In the present study, we use the Hipparcos catalogue to revisit the cosmography of OB stars in the solar neighbourhood. Because of the drawbacks mentioned above, we focus on the 3D spatial distribution using modern full 3D data analysis and interactive visualization techniques instead of 2D projections, and refrain from using velocities as a discovery criterion for stellar groups.

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OB star members of the new foreground group in the Orion stream. The symbol size is inversely proportional to the cube of the distance to enhance the perception of depth. Background photograph courtesy of Rogelio Bernal Andreo2.
OB star members of the new foreground group in the Orion stream. The symbol size is inversely proportional to the cube of the distance to enhance the perception of depth. Background photograph courtesy of Rogelio Bernal Andreo.