8 research outputs found
On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking into account
the effects induced by the fast rotation on their fundamental parameters. All
program stars were observed in the BCD spectrophotometric system in order to
minimize the perturbations produced by the circumstellar environment on the
spectral photospheric signatures. This is one of the first attempts at
determining stellar masses and ages by simultaneously using model atmospheres
and evolutionary tracks, both calculated for rotating objects. The stellar ages
() normalized to the respective inferred time that each rotating star can
spend in the main sequence phase () reveal a mass-dependent
trend. This trend shows that: a) there are Be stars spread over the whole
interval 0 \la \tau/\tau\_{\rm MS} \la 1 of the main sequence evolutionary
phase; b) the distribution of points in the () diagram indicates that in massive stars (M \ga
12M\_{\odot}) the Be phenomenon is present at smaller
age ratios than for less massive stars (M \la 12M\_{\odot}). This
distribution can be due to: ) higher mass-loss rates in massive objets,
which can act to reduce the surface fast rotation; ) circulation time
scales to transport angular momentum from the core to the surface, which are
longer the lower the stellar mass.Comment: 18 pages, 6 figures, A&A, in pres
Emission-line stars discovered in the UKST H-alpha survey of the Large Magellanic Cloud; Part 1: Hot stars
We present new, accurate positions, spectral classifications, radial and
rotational velocities, H-alpha fluxes, equivalent widths and B,V,I,R magnitudes
for 579 hot emission-line stars (classes B0 - F9) in the Large Magellanic Cloud
which include 469 new discoveries. Candidate emission line stars were
discovered using a deep, high resolution H-alpha map of the central 25 deg2 of
the LMC obtained by median stacking a dozen 2 hour H-alpha exposures taken with
the UK Schmidt Telescope. Spectroscopic follow-up observations on the AAT,
UKST, VLT, the SAAO 1.9m and the MSSSO 2.3m telescope have established the
identity of these faint sources down to magnitude R~23 for H-alpha (4.5 x
10^-17 ergs cm^2 s^-1 Ang). Confirmed emission-line stars have been assigned an
underlying spectral classification through cross-correlation against 131
absorption line template spectra covering the range O1 to F8. We confirm 111
previously identified emission line stars and 64 previously known variable
stars with spectral types hotter than F8. The majority of hot stars identified
(518 stars or 89%) are class B. Of all the hot emission-line stars in classes
B-F, 130 or 22% are type B[e], characterised by the presence of forbidden
emission lines such as [SII], [NII] and [OII]. We report on the physical
location of these stars with reference to possible contamination from ambient
HII emission. Along with flux calibration of the H-alpha emission we provide
the first H-alpha luminosity function for selected sub-samples after correction
for any possible nebula or ambient contamination. We find a moderate
correlation between the intensity of H-alpha emission and the V magnitude of
the central star based on SuperCOSMOS magnitudes and OGLE-II photometry where
possible. Cool stars from classes G-S, with and without strong H-alpha
emission, will be the focus of part 2 in this series.Comment: 24 pages (main paper) 36 figures, 6 tables; Appendix Tables: 22
pages, MNRAS, 201