A spectroscopic study of the Scorpio-Centaurus association

Abstract

Rotational velocities as well as hydrogen and helium line intensities of one hundred and twelve members of the Scorpio-Centaurus association have been derived. For stars withM _v<0·0, the distribution of rotational velocities of both the upper Scorpius subgroup and the upper Centaurus -Lupus subgroup are similar and closely resemble those of the field stars. Stars withM _v>0·0, all of which are found in the dense upper Scorpius region, rotate much faster than their counter-parts amongst field stars, the Pleiades and Alpha-Persei cluster members.

The measured equivalent width ofH _γ for 77 stars provide a distance modulus of 6·0 ± 0·09 magnitudes for the association. Evolutionary effects in the derived hydrogen line intensities are found between the two subgroups. The hydrogen-line intensities at all spectral types in the upper Centaurus-Lupus subgroup are systematically smaller than those of members in the upper Scorpius subgroup.

Analysis of high dispersion spectra of five members of the association yield a helium abundance ofN _He/N _H=0·096 ± 0·004. Along with data available in the literature, the mean helium abundance of thirteen stars of this association is found to be 0·098 ± 0·004 by number. For the two main subgroups of this association, we derive a value of 0·105 ± 0·001 for the upper Centaurus-Lupus group from three stars and 0·096 ± 0·005 for the upper Scorpius group from ten stars.

Within the capabilities of our present methods, there seems to be no difference between the two subgroups as far as rotational velocities and initial helium abundance are concerned, a fact that must be borne in mind in the interpretation of the difference found between the two subgroups from photoelectric photometry.