Studies of the LMC stellar content. II. Distribution of supergiants, correlations with gas and basic stellar structure.
Abstract
Studies of the LMC Stellar Content H.Distribution of Supergiants, Correlations with Gas and Basic Stellar Summary. From main catalogues already published, we have set up a comprehensive list of supergiant stars in the Large Magellanic Cloud. From their positions, we obtained the apparent distribution centro d which is found to be located at cc = 5h20m0, = - 68020 (1975), as the inclination of equatorial plane i =250+20, in agreement with usual values. The LMC has been surveyed inside a radius of 4 degrees on plates taken with the ESO objective-prism (dispersion 125 A, at Hy). We have classified most of the supergiants, using LMC stars already classified on slit spectrograms as secondary standards. The mean accuracy is estimated to be 0.15 spectral class. We -thus obtained' spectral types for 1471 stars among which we found 834 OB 2, 229 B 2.5-B 6, 344 B 7-A 9 and 64 F -G supergiants. We have completed this sample with 76 WR stars and 22 cepheids (log P >1) which mean photographic magnitudes are brighter than 13.5. The large-scale study of LMC supergiants distributed in six spectral groups shows that OB 2 and WR stars are mainly settled in six concentrations corresponding, for the largest ones, to Shaphey's constellations strongly extended. Jn the whole, the extreme LMC Population I may be shared out amongst eight large regions which draw an apparent complex structure. Inside these regions, the ratios of early to late-type stars are different, giving good evidence of a variable rate of stellar formation in the LMC. In spite of the uncertainty concerning the overall distribution of cosmic dust, we could estimate the dust- to-gas ratio 15. 1021 atom , showing the LMC to be deficient by a factor of nine with respect'to the Galaxy. Furthermore, in the comparison between spatial distributions of supergiants, ionised and neutral gas, we obtained the rates of correlation, which are found not to be constant. On an average, OB 2 stars are relatively illcorrelated (n =0.43) to neutral hydrogen: It seems'4.iiat these stars must be formed in regions showing a gradient in the gaseous density rather than in regions where it exists a maximum density; oppositely, WR stars are strongly associated with H I and H ii (n= 1.70 and 1.39 respectively) and the correlation between ionised and neutral gas (n =1.62) seems to be the most representative of the bright part of the luminosity function. Moreover, comparison with the rate of correlation obtained for some nearby galaxies shows that this rate is probably not a universal constant. Finally comparison between the LMC stellar structure as determined from our sample and de Vaucouleurs and Freeman's model of irregular barred spiral galaxies shows that the central part of the LMC disc is made up of two well-defined stellar arms merging from east end of the bar, one pseudo arm likely seen as superimposed on it, and several large bodies among which we find with less evidence the stellar counterpart of the proposed model. Key words: Large Magellanic Cloud supergiant stars spectral classification interstellar absorption H I regions H n regions
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- August 1976
- Bibcode:
- 1976A&A....51...31M
- Keywords:
-
- Galactic Structure;
- Hydrogen Clouds;
- Interstellar Gas;
- Magellanic Clouds;
- Star Distribution;
- Supergiant Stars;
- B Stars;
- Galactic Evolution;
- Hot Stars;
- Molecular Absorption;
- O Stars;
- Spatial Distribution;
- Spectrum Analysis;
- Spiral Galaxies;
- Stellar Luminosity;
- Stellar Spectra;
- Astrophysics