The History of Low-Mass Star Formation in the Upper Scorpius OB Association

Thomas Preibisch & Hans Zinnecker

Astronomical Journal, 117, 2381, (1999)


We use a large sample of about 100 low-mass pre-main sequence (PMS) stars in the Upper Scorpius OB association to explore the star formation history and the IMF of this association. Upper Scorpius is an ideal target for such a study, because the star formation process there is finished. The PMS stars have recently been found in a spatially unbiased wide field survey of X-ray selected stars in a 160 square-degree area, covering the Upper Scorpius association nearly completely. Following the optical characterization of these PMS stars, we present a new HR diagram for this association.

We perform a detailed analysis of the HR diagram, taking proper account of the uncertainties and the effects of unresolved binaries, and derive ages and masses for the PMS stars. We find that the low-mass PMS stars have a mean age of about 5 Myr and show no evidence for a large age dispersion. This agrees very well with the age of 5 - 6 Myr previously found for the massive stars and shows that low-mass and high-mass stars are coeval and co-spatial and thus have formed together.

We conclude that the star formation process in Upper Scorpius was probably triggered by the shock wave of a supernova explosion in the nearby Upper Centaurus-Lupus association. After a short burst of very high star formation activity, which lasted only for a few Myr, star formation in Upper Scorpius was halted, probably by the strong winds and the ionizing radiation of the numerous massive stars which dispersed the molecular cloud.

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You might be also interested in the previous paper about the young stars in Upper Scorpius, which was the basis of this work.

Some nice images of the Upper Scorpius association can be found here .

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