The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way

The picture of the young stellar groups in the Canis Major-Puppis-Vela (215 degrees < l < 275 degrees) section of the Milky Way is studied and updated utilizing uvby beta photometry of intrinsically luminous OB stars. We use all data from the literature to create a sample with 98 per cent completeness to 9.5 mag.

The very dense low reddened OB association CMa OB1 is confirmed at a distance of 0.99 (+/- 0.05 s.e.) kpc. Towards Puppis the brightest intrinsically luminous stars do not reveal Pup OB1 and Pup OB2. In the same direction, we separate two small groups, previously related to the association surrounding NGC 2439 at 3.5-4.5 kpc. The first one contains four highly reddened B-type supergiants situated in front of the cluster at 1.03 (+/- 0.14 s.e.) kpc - much closer to the Sun than has been estimated before. The second one lies north-west from the cluster at 3.2 (+/- 0.23 s.e.) kpc according to our estimate. In the direction to Vela, the bright OB stars are apparently embedded in a dust cloud and spread out between 0.3 and 2.5 kpc, forming clumps over this distance range.

In general, the prominent apparent young structures delineated by the brightest intrinsically luminous OB stars in the directions of Canis Major and Vela are some 20-25 per cent closer to the Sun than has previously been thought. This is in agreement with the Hipparcos results for the Galactic OB associations, and is highly likely to be caused by the overestimation of the spectroscopic distances used in the previous studies.