492 research outputs found
Cold gas in hot star clusters: the wind from the red supergiant W26 in Westerlund 1
The massive red supergiant (RSG) W26 in Westerlund 1 is one of a growing
number of RSGs shown to have winds that are ionized from the outside in. The
fate of this dense wind material is important for models of second generation
star formation in massive star clusters. Mackey et al. (2014) showed that
external photoionization can stall the wind of RSGs and accumulate mass in a
dense static shell. We use 1D R-HD simulations of an externally photoionized
wind to predict the Halpha and [NII] emission arising from photoionized winds
both with and without a dense shell. We analyse spectra of the Halpha and [NII]
emission in the environment around W26 and compare them with predicted
synthetic emission. Simulations of slow winds that are decelerated into a dense
shell show strongly limb-brightened line emission, with line radial velocities
that are independent of the wind speed. Faster winds (>22 km/s) do not form a
dense shell, have less limb-brightening, and the line radial velocity is a good
tracer of the wind speed. The brightness of the [NII] and Halpha lines as a
function of distance from W26 agrees reasonably well with observations when
only the line flux is considered. The radial velocity disagrees, however: the
brightest observed emission is blueshifted by ~25 km/s relative to the radial
velocity of the star, whereas a spherically symmetric wind has the brightest
emission at zero radial velocity. Our results show that the bright nebula
surrounding W26 must be asymmetric; we suggest it is confined by external ram
pressure from the wind of the nearby supergiant W9. We obtain a lower limit on
the nitrogen abundance within the nebula of 2.35 times solar. The line ratio
strongly favours photoionization over shock ionization, and so even if the
observed nebula is pressure confined there should still be an ionization front
and a photoionization-confined shell closer to the star.Comment: 12 pages plus appendices, accepted for publication in Astronomy &
Astrophysics; abstract shortened to fit arXiv limit
Open cluster Dolidze 25: Stellar parameters and the metallicity in the Galactic Anticentre
The young open cluster Dolidze 25, in the direction of the Galactic
Anticentre, has been attributed a very low metallicity, with typical abundances
between and dex below solar. We intend to derive accurate cluster
parameters and accurate stellar abundances for some of its members. We have
obtained a large sample of intermediate- and high-resolution spectra for stars
in and around Dolidze 25. We used the FASTWIND code to generate stellar
atmosphere models to fit the observed spectra. We derive stellar parameters for
a large number of OB stars in the area, and abundances of oxygen and silicon
for a number of stars with spectral types around B0. We measure low abundances
in stars of Dolidze 25. For the three stars with spectral types around B0, we
find dex (Si) and dex (O) below the values typical in the solar
neighbourhood. These values, even though not as low as those given previously,
confirm Dolidze 25 and the surrounding H II region Sh2-284 as the most
metal-poor star-forming environment known in the Milky Way. We derive a
distance kpc to the cluster (kpc).
The cluster cannot be older than Myr, and likely is not much younger.
One star in its immediate vicinity, sharing the same distance, has Si and O
abundances at most dex below solar. The low abundances measured in
Dolidze 25 are compatible with currently accepted values for the slope of the
Galactic metallicity gradient, if we take into account that variations of at
least dex are observed at a given radius. The area traditionally
identified as Dolidze 25 is only a small part of a much larger star-forming
region that comprises the whole dust shell associated with Sh2-284 and very
likely several other smaller H II regions in its vicinity.Comment: Accepted for publication in Astronomy \& Astrophysics. 16 pages, 12
figure
Metallicity dependence of turbulent pressure and macroturbulence in stellar envelopes
Macroturbulence, introduced as a fudge to reproduce the width and shape of
stellar absorption lines, reflects gas motions in stellar atmospheres. While in
cool stars, it is thought to be caused by convection zones immediately beneath
the stellar surface, the origin of macroturbulence in hot stars is still under
discussion. Recent works established a correlation between the
turbulent-to-total pressure ratio inside the envelope of stellar models and the
macroturbulent velocities observed in corresponding Galactic stars. To probe
this connection further, we evaluated the turbulent pressure that arises in the
envelope convective zones of stellar models in the mass range 1-125 Msun based
on the mixing-length theory and computed for metallicities of the Large and
Small Magellanic Cloud. We find that the turbulent pressure contributions in
models with these metallicities located in the hot high-luminosity part of the
Hertzsprung-Russel (HR) diagram is lower than in similar models with solar
metallicity, whereas the turbulent pressure in low-metallicity models
populating the cool part of the HR-diagram is not reduced. Based on our models,
we find that the currently available observations of hot massive stars in the
Magellanic Clouds appear to support a connection between macroturbulence and
the turbulent pressure in stellar envelopes. Multidimensional simulations of
sub-surface convection zones and a larger number of high-quality observations
are necessary to test this idea more rigorously.Comment: Accepted A&A, 8 p
The IACOB spectroscopic database of Galactic OB stars
We present the IACOB spectroscopic database, the largest homogeneous database
of high-resolution, high signal-to-noise ratio spectra of Northern Galactic
OB-type stars compiled up to date. The spectra were obtained with the FIES
spectrograph attached to the Nordic Optical Telescope. We briefly summarize the
main characeristics and present status of the IACOB, first scientific results,
and some future plans for its extension and scientific exploitation.Comment: 2 pages. Poster contribution to the proceedings of the IAU272 "Active
OB stars: structure, evolution, mass loss and critical limits
The upper sequence of young open clusters is shaped by binary interaction
The upper main-sequence of young star clusters displays a spread in colour that is very difficult to explain in terms of single star evolution. In recent years, it has been interpreted as a combination of the effects of rotation and mass transfer in multiple systems on the evolution of individual stars.
I will present the results of a large spectroscopic survey of massive stars in open clusters in the 10-100 Ma age range, confirming that blue supergiants are in most cases far too massive for the corresponding cluster turn-off. We have used a large grid of FASTWIND models to derive stellar parameters, finding that a substantial fraction of blue supergiants in clusters have masses equivalent to two turn-off stars.
Combined with the Gaia results on membership, these results suggest that most stars at the top of cluster sequences are the product of binary interaction. If the masses derived are to be believed, many blue supergiants are the results of mergers
Is macroturbulence in OB Sgs related to pulsations?
As part of a long term observational project, we are investigating the
macroturbulent broadening in O and B supergiants (Sgs) and its possible
connection with spectroscopic variability phenomena and stellar oscillations.
We present the first results of our project, namely firm observational evidence
for a strong correlation between the extra broadening and photospheric
line-profile variations in a sample of 13 Sgs with spectral types ranging from
O9.5 to B8.Comment: 2 pages, 1 figure. Poster contribution to the proceedings of the
IAU272 "Active OB stars: structure, evolution, mass loss and critical limits
Recommended from our members
Resolving stellar populations with integral field spectroscopy
High-performance instruments at large ground-based telescopes have made
integral field spectroscopy (IFS) a powerful tool for the study of extended objects
such as galaxies, nebulae, or even larger survey fields on the sky. Here, we discuss
the capabilities of IFS for the study of resolved stellar populations, using the
newmethod of point-spread-function-fitting crowded field IFS, analogous to the
well-established technique of crowded field photometry with image sensors.We
review early pioneering work with first-generation integral field spectrographs,
the breakthrough achieved with the multiunit spectral explorer (MUSE) instrument
at the European Organisation for Astronomical Research in the Southern
Hemisphere (ESO)Very Large Telescope, the remarkable progress accomplished
with MUSE in the study of globular clusters, and first results on nearby galaxies.
We discuss the synergy of integral field spectrographs at 8–10 mclass telescopes
with future facilities such as the extremely large telescope (ELT)
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