175 research outputs found
The VLT-FLAMES Survey of Massive Stars: Observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large Array
Multi-Element Spectrograph (FLAMES) instrument in fields centered on the
clusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), and NGC 2004
and the N11 region in the Large Magellanic Cloud (LMC). A further 14 stars were
observed in the N11 and NGC 330 fields using the Ultraviolet and Visual Echelle
Spectrograph (UVES) for a separate programme. Spectral classifications and
stellar radial velocities are given for each target, with careful attention to
checks for binarity. In particular we have investigated previously unexplored
regions around the central LH9/LH10 complex of N11, finding ~25 new O-type
stars from our spectroscopy. We have observed a relatively large number of
Be-type stars that display permitted Fe II emission lines. These are primarily
not in the cluster cores and appear to be associated with classical Be-type
stars, rather than pre main-sequence objects. The presence of the Fe II
emission, as compared to the equivalent width of H, is not obviously
dependent on metallicity. We have also explored the relative fraction of Be- to
normal B-type stars in the field-regions near to NGC 330 and NGC 2004, finding
no strong evidence of a trend with metallicity when compared to Galactic
results. A consequence of service observations is that we have reasonable
time-sampling in three of our FLAMES fields. We find lower limits to the binary
fraction of O- and early B-type stars of 23 to 36%. One of our targets
(NGC346-013) is especially interesting with a massive, apparently hotter, less
luminous secondary component.Comment: 35 pages, 17 figures (some reduced in size). Replacement copy,
includes an erratum on the final page. A copy with full res. & embedded
figures is at http://www.roe.ac.uk/~cje/flamesMC.ps.g
A survey of diffuse interstellar bands in the Andromeda galaxy: optical spectroscopy of M31 OB stars
We present the largest sample to-date of intermediate-resolution blue-to-red
optical spectra of B-type supergiants in M31 and undertake the first survey of
diffuse interstellar bands (DIBs) in this galaxy. Spectral classifications,
radial velocities and interstellar reddenings are presented for 34 stars in
three regions of M31. Radial velocities and equivalent widths are given for the
5780 and 6283 DIBs towards 11 stars. Equivalent widths are also presented for
the following DIBs detected in three sightlines in M31: 4428, 5705, 5780, 5797,
6203, 6269, 6283, 6379, 6613, 6660, and 6993. All of these M31 DIB carriers
reside in clouds at radial velocities matching those of interstellar Na I
and/or H I. The relationships between DIB equivalent widths and reddening
(E(B-V)) are consistent with those observed in the local ISM of the Milky Way.
Many of the observed sightlines show DIB strengths (per unit reddening) which
lie at the upper end of the range of Galactic values. DIB strengths per unit
reddening are found (with 68% confidence), to correlate with the interstellar
UV radiation field strength. The strongest DIBs are observed where the
interstellar UV flux is lowest. The mean Spitzer 8/24 micron emission ratio in
our three fields is slightly lower than that measured in the Milky Way, but we
identify no correlation between this ratio and the DIB strengths in M31.
Interstellar oxygen abundances derived from the spectra of three M31 H II
regions in one of the fields indicate that the average metallicity of the ISM
in that region is 12 + log[O/H] = 8.54 +- 0.18, which is approximately equal to
the value in the solar neighbourhood
The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars
We have previously analysed the spectra of 135 early B-type stars in the LMC
and found several groups of stars that have chemical compositions that conflict
with the theory of rotational mixing. Here we extend this study to Galactic and
SMC metallicities with the analysis of ~50 Galactic and ~100 SMC early B-type
stars with rotational velocities up to ~300km/s. The surface nitrogen
abundances are utilised as a probe of the mixing process.
In the SMC, we find a population of slowly rotating nitrogen-rich stars
amongst the early B type core-hydrogen burning stars, similar to the LMC. In
the Galactic sample we find no significant enrichment amongst the core
hydrogen-burning stars, which appears to be in contrast with the expectation
from both rotating single-star and close binary evolution models. However, only
a small number of the rapidly rotating stars have evolved enough to produce a
significant nitrogen enrichment, and these may be analogous to the non-enriched
rapid rotators previously found in the LMC sample. Finally, in each metallicity
regime, a population of highly enriched supergiants is observed, which cannot
be the immediate descendants of core-hydrogen burning stars. Their abundances
are, however, compatible with them having gone through a previous red
supergiant phase. Together, these observations paint a complex picture of the
nitrogen enrichment in massive main sequence and supergiant stellar
atmospheres, where age and binarity cause crucial effects. Whether rotational
mixing is required to understand our results remains an open question at this
time, but could be answered by identifying the true binary fraction in those
groups of stars that do not agree with single-star evolutionary models
(abridged).Comment: Accepted paper - 86 pages with tables and figure
The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution
Rotation has become an important element in evolutionary models of massive
stars, specifically via the prediction of rotational mixing. Here, we study a
sample of stars, including rapid rotators, to constrain such models and use
nitrogen enrichments as a probe of the mixing process. Chemical compositions
(C, N, O, Mg and Si) have been estimated for 135 early B-type stars in the
Large Magellanic Cloud with projected rotational velocities up to ~300km/s
using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including
rotational mixing, have been generated attempting to reproduce these
observations by adjusting the overshooting and rotational mixing parameters and
produce reasonable agreement with 60% of our core hydrogen burning sample. We
find (excluding known binaries) a significant population of highly nitrogen
enriched intrinsic slow rotators vsini less than 50km/s incompatible with our
models ~20% of the sample). Furthermore, while we find fast rotators with
enrichments in agreement with the models, the observation of evolved (log g
less than 3.7dex) fast rotators that are relatively unenriched (a further ~20%
of the sample) challenges the concept of rotational mixing. We also find that
70% of our blue supergiant sample cannot have evolved directly from the
hydrogen burning main-sequence. We are left with a picture where invoking
binarity and perhaps fossil magnetic fields are required to understand the
surface properties of a population of massive main sequence stars.Comment: ApJL. 10 pages, 1 figure. Updated to match accepted versio
B-type supergiants in the SMC: Rotational velocities and implications for evolutionary models
High-resolution spectra for 24 SMC and Galactic B-type supergiants have been
analysed to estimate the contributions of both macroturbulence and rotation to
the broadening of their metal lines. Two different methodologies are
considered, viz. goodness-of-fit comparisons between observed and theoretical
line profiles and identifying zeros in the Fourier transforms of the observed
profiles. The advantages and limitations of the two methods are briefly
discussed with the latter techniques being adopted for estimated projected
rotational velocities (\vsini) but the former being used to estimate
macroturbulent velocities. Only one SMC supergiant, SK 191, shows a significant
degree of rotational broadening (\vsini 90 \kms). For the remaining
targets, the distribution of projected rotational velocities are similar in
both our Galactic and SMC samples with larger values being found at earlier
spectral types. There is marginal evidence for the projected rotational
velocities in the SMC being higher than those in the Galactic targets but any
differences are only of the order of 5-10 \kms, whilst evolutionary models
predict differences in this effective temperature range of typically 20 to 70
\kms. The combined sample is consistent with a linear variation of projected
rotational velocity with effective temperature, which would imply rotational
velocities for supergiants of 70 \kms at an effective temperature of 28 000 K
(approximately B0 spectral type) decreasing to 32 \kms at 12 000 K (B8 spectral
type). For all targets, the macroturbulent broadening would appear to be
consistent with a Gaussian distribution (although other distributions cannot be
discounted) with an half-width varying from approximately 20 \kms
at B8 to 60 \kms at B0 spectral types.Comment: 4 figures, 8 pages, submitted to Astronomy and Astrophysic
The most rapidly rotating He-strong emission line star: HR7355
Using archival spectroscopic and photometric data, we searched for massive
stars with Balmer-emission consistent with magnetically confined circumstellar
material. HR 7355 is a formerly unknown He-strong star showing Balmer emission.
At V=6.02 mag, it is one of the brightest objects simultaneously showing
anomalous helium absorption and hydrogen emission. Among similar objects, only
sigma Ori E has so far been subjected to any systematic analysis of the
circumstellar material responsible for the emission. We argue that the
double-wave photometric period of 0.52d corresponds to the rotation period. In
tandem with the high projected equatorial velocity, v sin i=320 km/s, this
short period suggests that HR 7355 is the most rapidly rotating He-strong star
known to date; a class that was hitherto expected to host stars with slow to
moderate rotation only.Comment: 4 pages with 2 figures. Accepted for publication as Research Note by
Astronomy and Astrophysic
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