1,248 research outputs found
The IACOB project: A grid-based automatic tool for the quantitative spectroscopic analysis of O-stars
We present the IACOB grid-based automatic tool for the quantitative
spectroscopic analysis of O-stars. The tool consists of an extensive grid of
FASTWIND models, and a variety of programs implemented in IDL to handle the
observations, perform the automatic analysis, and visualize the results. The
tool provides a fast and objective way to determine the stellar parameters and
the associated uncertainties of large samples of O-type stars within a
reasonable computational time.Comment: 8 pages, 2 figures, 1 table. Proceedings of the "GREAT-ESF Stellar
Atmospheres in the Gaia Era Workshop
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
Observational evidence for a correlation between macroturbulent broadening and line-profile variations in OB Supergiants
The spectra of O and B supergiants are known to be affected by a significant
form of extra line broadening (usually referred to as macroturbulence) in
addition to that produced by stellar rotation. Recent analyses of high
resolution spectra have shown that the interpretation of this line broadening
as a consequence of large scale turbulent motions would imply highly supersonic
velocity fields in photospheric regions, making this scenario quite improbable.
Stellar oscillations have been proposed as a likely alternative explanation. As
part of a long term observational project, we are investigating the
macroturbulent broadening in O and B supergiants and its possible connection
with spectroscopic variability phenomena and stellar oscillations. In this
letter, we present the first encouraging 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 supergiants with
spectral types ranging from O9.5 to B8.Comment: 8 pages, 3 figures, accepted for publication in ApJ
Astrophysical parameters and orbital solution of the peculiar X-ray transient IGR J00370+6122
BD+6073 is the optical counterpart of the X-ray source IGR J00370+6122, a
probable accretion-powered X-ray pulsar. The X-ray light curve of this binary
system shows clear periodicity at 15.7 d, which has been interpreted as
repeated outbursts around the periastron of an eccentric orbit. We obtained
high-resolution spectra of BD+6073 at different epochs. We used the FASTWind
code to generate a stellar atmosphere model to fit the observed spectrum and
obtain physical magnitudes. The synthetic spectrum was used as a template for
cross-correlation with the observed spectra to measure radial velocities. The
radial velocity curve provided an orbital solution for the system. We have also
analysed the RXTE/ASM and Swift/BAT light curves to confirm the stability of
the periodicity. BD +6073 is a BN0.7 Ib low-luminosity supergiant located at an
approximate distance of 3.1 kpc, in the CasOB4 association. We derive
Teff=24000 K and log gc=3.0, and chemical abundances consistent with a
moderately high level of evolution. The spectroscopic and evolutionary masses
are consistent at the 1 sigma level with a mass of 15 solar masses. The
recurrence time of the X-ray flares is the orbital period of the system. The NS
is in a high eccentricity (e=0.56) orbit, and the X-ray emission is strongly
peaked around orbital phase 0.2, though the observations are consistent with
some level of X-ray activity happening at all orbital phases. The X-ray
behaviour of IGR J00370+6122 is reminiscent of intermediate SFXTs, though its
peak luminosity is rather low. The orbit is somewhat wider than those of
classical persistent supergiant X-ray binaries, which, combined with the low
luminosity of the mass donor, explains the low X-ray luminosity. IGR
J00370+6122 will likely evolve towards a persistent supergiant system,
highlighting the evolutionary connection between different classes of
wind-accreting X-ray sources.Comment: Accepted for publication in A&
Investigation of the fluid behavior of asphaltenes and toluene insolubles by high-temperature proton nuclear magnetic resonance and rheometry and their application to visbreaking
The fluid behavior of asphaltenes at elevated temperatures impacts coke formation in a number of hydrocarbon conversion processes, including visbreaking and delayed coking. In this study, the asphaltenes from a number of sources, namely, a vacuum residue, a petroleum source rock (Kimmeridge clay) bitumen obtained by hydrous pyrolysis, and bitumen products from a sub-bituminous coal and pine wood obtained by thermolytic solvent extraction using tetralin, have been characterized using high-temperature proton nuclear magnetic resonance (1H NMR), and the results correlated with those from small-amplitude oscillatory shear rheometry. Further for comparison, the coke (toluene insolubles) obtained from visbreaking the vacuum residue was also characterized. All of the asphaltenes became completely fluid by 300 °C, with hydrogen being completely mobile with coke formation, identified as a solid phase, not occurring to a significant extent until 450 °C. Extremely good agreement was obtained between high-temperature 1H NMR and rheometry results, which confirmed that the asphaltenes were highly fluid from 300 °C, with initial signs of resolidification being observed at temperatures of around 450 °C. During softening, extremely good correlations between fluid hydrogen and phase angle were obtained as the asphaltenes softened. The toluene insolubles however did contain some fluid material; thus, it cannot be regarded as strictly solid coke, but clearly, with increasing temperature, the fluid material did convert to coke. Under actual process conditions, this fluid material could be responsible for coke adhering to reactor surfaces
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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