393 research outputs found
The little-studied cluster Berkeley 90. II. The foreground ISM
Context: Nearly one century after their discovery, the carrier(s) of Diffuse
Interstellar Bands is/are still unknown and there are few sightlines studied in
detail for a large number of DIBs. Aims: We want to study the ISM sightlines
towards LS III +46 11 and LS III +46 12, two early-O-type stellar systems, and
LS III +46 11 B, a mid-B-type star. The three targets are located in the
stellar cluster Berkeley 90 and have a high extinction. Methods: We use the
multi-epoch high-S/N optical spectra presented in paper I (Ma\'iz Apell\'aniz
et al. 2015), the extinction results derived there, and additional spectra.
Results: We have measured equivalent widths, velocities, and FWHMs for a large
number of absorption lines in the rich ISM spectrum in front of Berkeley 90.
The absorbing ISM has at least two clouds at different velocities, one with a
lower column density (thinner) in the K I lines located away from Berkeley 90
and another one with a higher column density (thicker) associated with the
cluster. The first cloud has similar properties for both O-star sightlines but
the second one is thicker for LS III +46 11. The comparison between species
indicate that the cloud with a higher column density has a denser core,
allowing us to classify the DIBs in a sigma-zeta scale, some of them for the
first time. The LS III +46 12 sightline also has a high-velocity redshifted
component.Comment: Accepted for publication in A&
Oxygen and silicon abundances in Cygnus OB2: Chemical homogeneity in a sample of OB slow rotators
Cygnus OB2 is a rich OB association in the Galaxy which has experienced
intense star formation in the last 20-25 Myr. Its stellar population shows a
correlation between age and Galactic longitude. Exploring the chemical
composition of its stellar content we will be able to check the degree of
homogeneity of the natal molecular cloud and possible effects of
self-enrichment processes. Our aim is to determine silicon and oxygen
abundances for a sample of eight early-type slow rotators in Cygnus OB2 in
order to check possible inhomogeneities across the whole association and
whether there exists a correlation of chemical composition with Galactic
longitude. We have performed a spectroscopic analysis of a sample of late O and
early B stars with low rotational velocity, which have been chosen so as to
cover the whole association area. We have carried out an analysis based on
equivalent widths of metal lines, the wings of the H Balmer lines and FASTWIND
stellar atmosphere models to determine their stellar fundamental parameters as
well as the silicon and oxygen surface abundances. We derive a rather
homogeneous distribution of silicon and oxygen abundances across the region,
with average values of 12+log(Si/H)=7.530.08 dex and
12+log(O/H)=8.650.12 dex. We find a homogeneous chemical composition in
Cygnus OB2 with no clear evidence for significant chemical self-enrichment,
despite indications of strong stellar winds and possible supernovae during the
history of the region. Comparison with different scenarios of chemical
enrichment by stellar winds and supernovae point to star forming efficiencies
not significantly above 10%. The degree of homogeneity that we find is
consistent with the observed Milky Way oxygen gradient based on HII regions. We
also find that the oxygen scatter within Cygnus OB2 is at least of the same
order than among HII regions at similar Galactocentric distance.Comment: 15 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
The spectroscopic Hertzsprung-Russell diagram of Galactic massive stars
The distribution of stars in the Hertzsprung-Russell diagram narrates their
evolutionary history and directly assesses their properties. Placing stars in
this diagram however requires the knowledge of their distances and interstellar
extinctions, which are often poorly known for Galactic stars. The spectroscopic
Hertzsprung-Russell diagram (sHRD) tells similar evolutionary tales, but is
independent of distance and extinction measurements. Based on spectroscopically
derived effective temperatures and gravities of almost 600 stars, we derive for
the first time the observational distribution of Galactic massive stars in the
sHRD. While biases and statistical limitations in the data prevent detailed
quantitative conclusions at this time, we see several clear qualitative trends.
By comparing the observational sHRD with different state-of-the-art stellar
evolutionary predictions, we conclude that convective core overshooting may be
mass-dependent and, at high mass (), stronger than previously
thought. Furthermore, we find evidence for an empirical upper limit in the sHRD
for stars with between 10000 and 32000 K and, a strikingly large
number of objects below this line. This over-density may be due to inflation
expanding envelopes in massive main-sequence stars near the Eddington limit.Comment: 5 pages, 2 figures, 1 table; accepted for publication in A&A Letter
PN fast winds: Temporal structure and stellar rotation
To diagnose the time-variable structure in the fast winds of central stars of
planetary nebulae (CSPN), we present an analysis of P Cygni line profiles in
FUSE satellite far-UV spectroscopic data. Archival spectra are retrieved to
form time-series datasets for the H-rich CSPN NGC 6826, IC 418, IC 2149, IC
4593 and NGC 6543. Despite limitations due to the fragmented sampling of the
time-series, we demonstrate that in all 5 CSPN the UV resonance lines are
variable primarily due to the occurrence of blueward migrating discrete
absorption components (DACs). Empirical (SEI) line-synthesis modelling is used
to determine the range of fluctuations in radial optical depth, which are
assigned to the temporal changes in large-scale wind structures. We argue that
DACs are common in CSPN winds, and their empirical properties are akin to those
of similar structures seen in the absorption troughs of massive OB stars.
Constraints on PN central star rotation velocities are derived from
Fast-Fourier Transform analysis of photospheric lines for our target stars.
Favouring the causal role of co-rotating interaction regions, we explore
connections between normalised DAC accelerations and rotation rates of PN
central stars and O stars. The comparative properties suggest that the same
physical mechanism is acting to generate large-scale structure in the
line-driven winds in the two different settings.Comment: Accepted for publication in MNRAS; 10 pages, 5 figure
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
Lucky Spectroscopy, an equivalent technique to Lucky Imaging. Spatially resolved spectroscopy of massive close visual binaries using the William Herschel Telescope
CONTEXT: Many massive stars have nearby companions whose presence hamper
their characterization through spectroscopy. AIMS: We want to obtain spatially
resolved spectroscopy of close massive visual binaries to derive their spectral
types. METHODS: We obtain a large number of short long-slit spectroscopic
exposures of five close binaries under good seeing conditions, select those
with the best characteristics, extract the spectra using multiple-profile
fitting, and combine the results to derive spatially separated spectra.
RESULTS: We demonstrate the usefulness of Lucky Spectroscopy by presenting the
spatially resolved spectra of the components of each system, in two cases with
separations of only ~0.3". Those are delta Ori Aa+Ab (resolved in the optical
for the first time) and sigma Ori AaAb+B (first time ever resolved). We also
spatially resolve 15 Mon AaAb+B, zeta Ori AaAb+B (both previously resolved with
GOSSS, the Galactic O-Star Spectroscopic Survey), and eta Ori AaAb+B, a system
with two spectroscopic B+B binaries and a fifth visual component. The systems
have in common that they are composed of an inner pair of slow rotators orbited
by one or more fast rotators, a characteristic that could have consequences for
the theories of massive star formation.Comment: Accepted for publication in A&A, 7 page
The IACOB project. V. Spectroscopic parameters of the O-type stars in the modern grid of standards for spectral classification
The IACOB and OWN surveys are two ambitious complementary observational
projects which have made available a large multi-epoch spectroscopic database
of optical high resolution spectra of Galactic massive O-type stars. As a first
step in the study of the full sample of (more than 350) O stars surveyed by the
IACOB/OWN projects, we have performed the quantitative spectroscopic analysis
of a subsample of 128 stars included in the modern grid of O-type standards for
spectral classification. We use semi-automatized tools to determine the set of
spectroscopic parameters that can be obtained from the optical spectrum of
O-type stars. We also benefit from the multi-epoch character of the surveys to
perform a spectroscopic variability study of the sample, accounting for
spectroscopic binarity and variability of the main wind diagnostic lines. We
provide a general overview of the stellar and wind parameters of this reference
sample, and updated recipes for the SpT\,--\,Teff/log g calibrations for
Galactic O-type stars. We evaluate our semi-automatized analysis strategy with
40 stars from the literature, and find a good agreement. The agreement
between the synthetic spectra associated with fastwind best fitting models and
the observed spectra is good for most targets, but 46 stars present a
particular behavior of the wind diagnostic lines that cannot be reproduced by
our grid of spherically symmetric unclumped models. These are potential targets
of interest for more detailed investigations of clumpy winds and/or the
existence of additional circumstellar components. Last, our variability study
has led to the detection of signatures of spectroscopic binarity in 27\% of the
stars and small amplitude radial velocity variations in the photospheric lines
of another 30\%. Additionally, 31\% of the investigated stars show variability
in the wind diagnostic lines.Comment: 20 pages, 18 figures, accepted for publication in Astronomy &
Astrophysic
The IACOB project. VI. On the elusive detection of massive O-type stars close to the ZAMS
The apparent lack of massive O-type stars near the zero-age main sequence (at
ages < 2 Myr) is a topic widely discussed. Different explanations for this
elusive detection have been proposed, but no firm conclusions have been reached
yet. We reassess this empirical result benefiting from the high-quality
spectroscopic observations of >400 Galactic O-type stars gathered by the IACOB
and OWN surveys. We used temperatures and gravities from a iacob-gbat/fastwind
spectroscopic analysis to locate our sample in the Kiel and spectroscopic HR
diagrams. We evaluated the completeness of our sample of stars, observational
biases using information from the Galactic O star catalog (GOSC), systematics
of our methodology, and compare with other recent studies using smaller samples
of Galactic O-type stars. We base our discussion on the spectroscopic HR
diagram to avoid the use of uncertain distances. We performed a detailed study
of the young cluster Trumpler-14 as an example of how Gaia cluster distances
can help to construct the associated classical HR diagram. The apparent lack of
massive O-type stars near the ZAMS with masses between 30 and 70 Msol persist
even when spectroscopic results from a large, non-biased sample of stars are
used. We do not find correlation between the dearth of stars and observational
biases, limitations of our methodology, or the use of spectroscopic HR diagram
instead of the classical one. Investigating the efficiency of mass accretion
during the formation process we conclude that an adjustment of the accretion
rate towards lower values could reconcile the hotter boundary of detected
O-type stars and the theoretical birthline. Last, we discuss that the presence
of a small sample of O2-O3.5 stars found closer to the ZAMS might be explained
taking into account non-standard star evolution (e.g. binary interaction,
mergers, or homogeneous evolution).Comment: 20 pages, 15 figures, accepted for publication in Astronomy &
Astrophysic
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