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Astron. Astrophys. 361, 60-62 (2000)

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Radio continuum observations of possible B-type stars in the halo of M 31 *

J.V. Smoker 1, F.P. Keenan 1, M.J. Marcha 2, D. Watson 3 and M.J. Irwin 4

1 Astrophysics and Planetary Science Division, Department of Pure and Applied Physics, The Queen's University of Belfast, University Road, Belfast, BT7 1NN, UK
2 Observatorio Astronomico de Lisboa, Tapada da Ajuda, 1300 Lisboa, Portugal
3 Department of Experimental Physics, University College Dublin, Dublin 4, Ireland
4 Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK

Received 14 March 2000 / Accepted 8 May 2000

Abstract

We present Very Large Array (VLA) 5 GHz continuum observations of six point sources towards the halo of M 31, which have featureless optical spectra and magnitudes ranging from V=21.5-22.2, in order to determine whether these objects are BL Lacs. No radio emission coincident with the optical positions is detected to a 5[FORMULA] noise level of between 0.08 and 0.11 mJy. The resulting upper limits to the two-point radio-to-optical spectral indexes from 5 GHz to 2500 Å ([FORMULA]) are less than 0.30 for all of the sample. These observations make it highly likely that these objects are not BL Lacs, and hence strengthens the case that they are normal B-type stars in the halo of M 31.

Key words: galaxies: BL Lacertae objects: general – galaxies: halos – galaxies: individual: M 31 – radio continuum: general – stars: early-type

* Based on observations taken at the National Radio Observatory which is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation, USA

Send offprint requests to: J.V. Smoker (j.smoker@qub.ac.uk)

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000
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