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Astron. Astrophys. 341, 163-173 (1999)


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A search for star formation in the translucent clouds MBM7 and MBM55 *

T. Hearty 1, L. Magnani 2, J.-P. Caillault 2, R. Neuhäuser 1, J.H.M.M. Schmitt 3 and J. Stauffer 4

1 Max-Planck-Institut für Extraterrestrische Physik, D-85740 Garching, Germany (e-mail: thearty@xray.mpe.mpg.de)
2 Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA
3 Hamburger Sternwarte Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
4 Harvard/Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA

Received 12 May 1998 / Accepted 24 September 1998

Abstract

The star formation capability of two molecular clouds at high galactic latitude ([FORMULA]b[FORMULA] [FORMULA] [FORMULA]) is investigated. Possible pre-main sequence stars in and around the translucent clouds MBM7 and MBM55 have been identified via their X-ray emission by inspecting ROSAT All-Sky Survey observations of the clouds and environs and ROSAT pointed observations of the high-density cores within the clouds. Follow-up optical spectroscopy of the stellar X-ray sources with V [FORMULA] 15.5 mag was conducted with the 1.5-m Fred Lawrence Whipple Observatory telescope to identify standard signatures of pre-main sequence stars (LiI [FORMULA]6708 Å absorption and H[FORMULA] emission). We found 11 stars which have lithium equivalent widths, W(Li), above our detection threshold. Three of the stars with lithium also have weak H[FORMULA] emission. Relative ages for the stars with lithium are estimated by their position on an W(Li) vs. [FORMULA] diagram. A calibration derived from data for several clusters with known ages indicates the stars are older than the translucent high-latitude clouds. This conclusion is supported by a comparison with theoretical evolutionary tracks of the stars from our sample for which we have distance measurements from Hipparcos. We find it is unlikely that any of the X-ray active, lithium-rich stars we identified have formed in the clouds in question. Theoretical and observational arguments support this conclusion and render unlikely the possibility that low-extinction translucent clouds are the sites of star formation.

Key words: ISM: clouds – stars: formation

* Table 3 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr/pub/cats/J/A+A/341/163 (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/341/163.

Send offprint requests to: T. Hearty

© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998

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