The stellar composition of the star formation region CMa R1 - I. Results from new photometric and spectroscopic classifications

A new photometric and spectroscopic survey of the star formation region (SF R) CMa R1 is described. In a sample of 165 stars brighter than 13th mag, 88 stars were found to be probable members of the SFR. They are defined as ea rly-type stars with E(B-V) greater than or equal to 0.16 mag, which corresp onds to a distance of about 1 kpc. 74 of the probable members are B stars. 19 stars are possibly associated with an IRAS point source. We derive a mos t probable distance of 1050 +/- 150 pc to the association. It appears that about 80 candidate members are pre-main-sequence stars with ages lower than 6 million years, while the main sequence extends over 6.0-7.6 mag, which i s consistent with star formation starting about 8 million years ago and con tinuing until at least half a million years ago. Two bright B stars in the association (GU CMa and FZ CMa) seem to be much older and probably do not o riginate from the same star formation episode. The star formation efficienc y appears to increase roughly monotonically with time up to half a million years ago. From our data, we conclude that only a minor fraction of the sta rs has been created through the scenario suggested by Herbst & Assousa, in which the members of CMa R1 form by compression of ambient material by a su pernova shock wave. An extensive search for candidate members with H alpha emission did not reveal new Herbig Ae/Be candidates, so that the number of stars in this class seems to be limited to four: Z CMa, LkH alpha 218, LkH alpha 220 and possibly HD 53367.