3. Stars and star clusters
In order to obtain information on the distribution of early type stars and on interstellar absorption in this direction, the OB star distances and the visual absorption coefficient have been estimated within an area ranging from l = to and from b = -5:O34 to +5:O86 using data (spectral type and UBV photometry) from the literature (Wramdemark 1980, Deutschman et al. 1976, Savage et al. 1985, Grillo et al. 1992, Garrison et al. 1977, Schild et al. 1983). For young star clusters we choose a slightly wider range of longitude, from l = to ; their distances and corresponding were taken directly from the literature (Pandey et al. 1989, Janes & Adler 1982, Hron 1987, Battinelli & Capuzzo-Dolcetta 1991).
For each star, the absolute magnitude has been estimated using the - spectral type relation of Schmidt Kaler (1983) and Humphrey & Mc Elroy (1984). The spectral type found in the literature was preferably used for the deduction. For stars without known spectral type, it has been estimated from UBV photometry and H measurements (Schmidt Kaler 1983). Stars with neither spectral type nor H photometry, were arbitrarily supposed to be main sequence stars. Their adopted luminosity thus being a minimum value, the distances of these stars are minimum distances.
The smoothed distribution of (= 3.2 ) as a function of distance displayed on Fig. 2 shows that there is no distinct absorption zone within 4 kpc.
One can see on Fig. 3 that the maximum number of stars lie between 2 and 4 kpc suggesting that the line of sight crosses an arm there. This idea has already been invoked by Mc Cuskey (1983) in order to explain the maximum of the OB star distribution around 2.5 kpc within a side area centered at l = and b = .
We can note that some stars lie between 4 and 6 kpc. With a wider latitude range, Wramdemark (1980) found some stars with distances greater than 4.5 kpc. One can divide these stars into two groups: the ones with distances between 4.6 and 6.9 kpc and the other ones with distances between 8.6 and 15.9 kpc. This second group can be unambiguously identified with farthest part of the Carina arm. This fact well agrees with the observation of HII regions beyond the solar circle discussed in Sect. 5.4.
As a conclusion, the distribution of early type stars in the l = direction, clearly shows the crossing of the Carina arm at its nearest and farthest parts, but also reveals a bulk of stars at about 6 kpc. We will see below how this peculiar star distribution can be connected with the ionized emissions detected.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998