Astron. Astrophys. 356, 517-528 (2000)
We have investigated several blue stragglers and main sequence
stars from the following open clusters: NGC 3496, NGC 6475,
NGC 6633 and IC 2602. For all of these stars a detailed
spectroscopic study, based on the LTE assumption, has been performed
for the first time. The main results are as follows:
1). Our present knowledge about NGC 3496 does not allow a
confident classification of the three stars S 43, S 115 and
S 117 as blue stragglers.
2). All investigated blue stragglers showed low projected
rotational velocities, while investigated B-A main sequence stars
appeared to be the fast rotators in accordance with their spectral
types (the case of S 115 and S 117 is disputable).
3). We have discovered two new He-weak stars (one is a blue
straggler) and obtained a quantitative estimate of the helium content
for the already known He-weak blue straggler HD 162374. For the
quantitative analysis of the He-weak stars, atmosphere models were
calculated with decreased helium content.
4).All the investigated middle-to-late B stars showed a subsolar
carbon abundance. These abundances have been checked by NLTE
5). The severe helium deficiency found for some stars and the
moderate-to-strong carbon underabundances could be explained as a
result of atomic diffusion in the stellar atmospheres. However, three
stars with apparent helium/carbon deficiencies possess rather high
projected rotational velocities (
70 km s-1 - 135
6). The silicon abundance in our program stars shows a clear
dependence upon the helium content. In the helium deficient stars this
element is increased, while its abundance is solar (or slightly
subsolar) for the He-normal stars.
7). The iron-group elements do not show any significant deviations
from the solar abundances in either blue stragglers or main sequence
8). Generally, based on their chemical compositions, the blue
stragglers stars are not different from main sequence stars. This
conclusion is made for a very restricted sample of blue stragglers and
requires further investigation.
To summarize, one can note that any theory which attempts an
exhaustive explanation of the blue straggler phenomenon should be able
to account for the two important features of these enigmatic stars: a)
their small projected rotational velocities; b) the similarities of
their chemical compositions with those of ordinary cluster and
galactic field stars.
© European Southern Observatory (ESO) 2000
Online publication: April 10, 2000