480 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 IACOB project: I. Rotational velocities in Northern Galactic O and early B-type stars revisited. The impact of other sources of line-broadening
Stellar rotation is an important parameter in the evolution of massive stars.
Accurate and reliable measurements of projected rotational velocities in large
samples of OB stars are crucial to confront the predictions of stellar
evolutionary models with observational constraints. We reassess previous
determinations of projected rotational velocities (vsini) in Galactic OB stars
using a large, high quality spectroscopic dataset, and a strategy which account
for other sources of broadening appart from rotation affecting the diagnostic
lines We present a versatile and user friendly IDL tool, based on a combined
Fourier Transform (FT) + goodness of fit (GOF) methodology, for the
line-broadening characterization in OB-type stars. We use this tool to (a)
investigate the impact of macroturbulent and microturbulent broadenings on
vsini measurements, and (b) determine vsini in a sample of 200 Galactic OB-type
stars, also characterizing the amount of macroturbulent broadening (\vmacro)
affecting the line profiles. We present observational evidence illustrating the
strengths and limitations of the proposed FT+GOF methodology for the case of OB
stars. We confirm previous statements (based on indirect arguments or smaller
samples) that the macroturbulent broadening is ubiquitous in the massive star
domain. We compare the newly derived vsini with previous determinations not
accounting for this extra line-broadening contribution, and show that those
cases with vsini< 120 km/s need to be systematically revised downwards by ~25
(+/-20) km/s. We suggest that microturbulence may impose an upper limit below
which vsini and \vmacro\ could be incorrectly derived by means of the proposed
methodology as presently used, and discuss the implications of this statement
on the study of relatively narrow line massive stars.Comment: Accepted for publication in A&A (19 pages, 15 figures, 6 tables).
Tables A1-A5 will be make available in the final edited version of the paper
(or under request to SS-D
The Cocoon Nebula and its ionizing star: do stellar and nebular abundances agree?
(Abridged) Main sequence massive stars embedded in an HII region should have
the same chemical abundances as the surrounding nebular gas+dust. The Cocoon
nebula, a close-by Galactic HII region ionized by a narrow line B0.5 V single
star (BD+46 3474), is an ideal target to perform a detailed comparison of
nebular and stellar abundances in the same Galactic HII region. We investigate
the chemical content of O, N and S in the Cocoon nebula from two different
points of view: an empirical analysis of the nebular spectrum and a detailed
spectroscopic analysis of its ionizing B-type star using state-of-the-art
stellar atmosphere modeling. By comparing the stellar and nebular abundances,
we aim to indirectly address the long-standing problem of the discrepancy found
between abundances obtained from collisionally excited lines (CELs) and optical
recombination lines in photoionized nebulae. We collect spatially resolved
spectroscopy of the Cocoon nebula and a high resolution optical spectrum of its
ionizing star. Standard nebular techniques are used to compute the physical
conditions and gaseous abundances of O, N and S. We perform a self-consistent
spectroscopic abundance analysis of BD+46 3474 based on the atmosphere code
FASTWIND to determine the stellar parameters and Si, O, and N abundances. The
Cocoon nebula and its ionizing star, located at a distance of 800+-80 pc, have
a very similar chemical composition as the Orion nebula and other B-type stars
in the solar vicinity. This result agrees with the high degree of homogeneity
of the present-day composition of the solar neighbourhood as derived from the
study of the local cold-gas ISM. The comparison of stellar and nebular CELs
abundances in the Cocoon nebula indicates that O and N gas+dust nebular values
are in better agreement with stellar ones assuming small temperature
fluctuations, of the order of those found in the Orion nebula.Comment: Accepted for publication in A&A. 13 pages, 7 tables and 6 figure
OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A
Our aim is to find and classify OB stars in Sextans A, to later determine
accurate stellar parameters of these blue massive stars in this low metallicity
region .
Using UBV photometry, the reddening-free index Q and GALEX imaging, we built
a list of blue massive star candidates in Sextans A. We obtained low resolution
(R 1000) GTC-OSIRIS spectra for a fraction of them and carried out
spectral classification. For the confirmed O-stars we derive preliminary
stellar parameters.
The target selection criteria and observations were successful and have
produced the first spectroscopic atlas of OB-type stars in Sextans A. From the
whole sample of 18 observed stars, 12 were classified as early OB-types,
including 5 O-stars. The radial velocities of all target stars are in agreement
with their Sextans A membership, although three of them show significant
deviations. We determined the stellar parameters of the O-type stars using the
stellar atmosphere code FASTWIND, and revisited the sub-SMC temperature scale.
Two of the O-stars are consistent with relatively strong winds and enhanced
helium abundances, although results are not conclusive. We discuss the position
of the OB stars in the HRD. Initial stellar masses run from slightly below 20
up to 40 solar masses.
The target selection method worked well for Sextans A, confirming the
procedure developed in Garcia \& Herrero (2013). The stellar temperatures are
consistent with findings in other galaxies. Some of the targets deserve
follow-up spectroscopy because of indications of a runaway nature, an enhanced
helium abundance or a relatively strong wind. We observe a correlation between
HI and OB associations similar to the irregular galaxy IC1613, confirming the
previous result that the most recent star formation of Sextans A is currently
on-going near the rim of the H\,{\sc I} cavity
Fundamental parameters of massive stars in multiple systems: The cases of HD17505A and HD206267A
Many massive stars are part of binary or higher multiplicity systems. The
present work focusses on two higher multiplicity systems: HD17505A and
HD206267A. Determining the fundamental parameters of the components of the
inner binary of these systems is mandatory to quantify the impact of binary or
triple interactions on their evolution. We analysed high-resolution optical
spectra to determine new orbital solutions of the inner binary systems. After
subtracting the spectrum of the tertiary component, a spectral disentangling
code was applied to reconstruct the individual spectra of the primary and
secondary. We then analysed these spectra with the non-LTE model atmosphere
code CMFGEN to establish the stellar parameters and the CNO abundances of these
stars. The inner binaries of these systems have eccentric orbits with e ~ 0.13
despite their relatively short orbital periods of 8.6 and 3.7 days for
HD17505Aa and HD206267Aa, respectively. Slight modifications of the CNO
abundances are found in both components of each system. The components of
HD17505Aa are both well inside their Roche lobe, whilst the primary of
HD206267Aa nearly fills its Roche lobe around periastron passage. Whilst the
rotation of the primary of HD206267Aa is in pseudo-synchronization with the
orbital motion, the secondary displays a rotation rate that is higher. The CNO
abundances and properties of HD17505Aa can be explained by single star
evolutionary models accounting for the effects of rotation, suggesting that
this system has not yet experienced binary interaction. The properties of
HD206267Aa suggest that some intermittent binary interaction might have taken
place during periastron passages, but is apparently not operating anymore.Comment: Accepted for publication in A&
The chemical composition of the Orion star forming region: stars, gas and dust
We present a summary of main results from the studies performed in the series
of papers "The chemical composition of the Orion star forming region". We
reinvestigate the chemical composition of B-type stars in the Orion OB1
association by means of state-of-the-art stellar atmosphere codes, atomic
models and techniques, and compare the resulting abundances with those obtained
from the emission line spectra of the Orion nebula (M42), and recent
determinations of the Solar chemical composition.Comment: 5 pages, 4 figures, 2 tables. Poster contribution to the proceedings
of the LIAC2010 conference "The multi-wavelength view of hot, massive stars
The ionizing radiation from massive stars and its impact on H ii regions: results from modern model atmospheres
We present a detailed comparison of the ionizing spectral energy distributions (SEDs) predicted by four modern stellar atmosphere codes, tlusty, cmfgen, wm-basic and fastwind. We consider three sets of stellar parameters representing a late O-type dwarf (O9.5 V), a mid-O-type (O7 V) dwarf and an early O-type dwarf (O5.5 V). We explore two different possibilities for such a comparison, following what we called evolutionary and observational approaches: in the evolutionary approach, one compares the SEDs of stars defined by the same values of Teff and log g; in the observational approach, the models to be compared do not necessarily have the same Teff and log g, but produce similar H and He i-ii optical lines. We find that there is a better agreement, in terms of Q(H0), the ratio Q(He0)/Q(H0) and the shape of the SEDs predicted by the four codes in the spectral range between 13 and 30 eV, when models are compared following the observational approach. However, even in this case, large differences are found at higher energies. We then discuss how the differences in the SEDs may affect the overall properties of surrounding nebulae in terms of temperature and ionization structure. We find that the effect over the nebular temperature is not larger than 300-350 K. Contrarily, the different SEDs produce significantly different nebular ionization structures. This will lead to important consequences on the establishment of the ionization correction factors that are used in the abundance determination of H ii regions, as well as in the characterization of the ionizing stellar population from nebular line ratio
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&
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