22 research outputs found
Temperature, gravity and bolometric correction scales for non-supergiant OB stars
Context. Precise and accurate determinations of effective temperature and
surface gravity are mandatory to derive reliable chemical abundances and
fundamental parameters like distances, masses, radii, luminosities of OB stars.
Aims. Atmospheric parameters recently determined at high precision with several
independent spectroscopic indicators in NLTE are employed to calibrate
photometric relationships. Methods. Temperatures and gravities of 30
calibrators are compared to reddening-independent quantities of the Johnson and
Stroemgren photometric systems. We also examine the spectral and luminosity
classification of the star sample and compute bolometric corrections. Results.
Calibrations of temperatures and gravities are proposed for various photometric
indices and spectral types. Effective temperatures can be determined at a
precision of ~400 K for luminosity classes III/IV and ~800 K for luminosity
class V. Surface gravities can reach internal uncertainties as low as ~0.08 dex
when using our calibration to the Johnson Q-parameter. Similar precision is
achieved for gravities derived from the beta-index and the precision is lower
for both atmospheric parameters when using the Stroemgren indices c1 and [u-b].
Our uncertainties are smaller than typical differences among other methods in
the literature, reaching values up to ~2000 K for temperature and ~0.25 dex for
gravity, and in extreme cases, ~6000 K and ~0.4 dex, respectively. A parameter
calibration for sub-spectral types is also proposed. We present a new
bolometric correction relation to temperature based on our empirical data.
Conclusions. The photometric calibrations presented here are useful tools to
estimate effective temperatures and surface gravities of non-supergiant OB
stars in a fast manner. We recommend to use these calibrations as a first step,
with subsequent refinements based on spectroscopy (abridged).Comment: 14 pages, 11 figures, accepted to be published in A&
Present-day cosmic abundances. A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution and interstellar dust models
Aims. A sample of early B-type stars in OB associations and the field within
the solar neighbourhood is studied comprehensively. Present-day abundances for
the astrophysically most interesting chemical elements are derived. Methods.
High-resolution and high-S/N spectra of early B-type stars are analysed in
NLTE. Atmospheric parameters are derived from the simultaneous establishment of
independent indicators, from multiple ionization equilibria and the hydrogen
Balmer lines. Results. Teff is constrained to 1-2% and logg to less than 15%
uncertainty. Absolute values for metal abundances are determined to better than
25% uncertainty. The synthetic spectra match the observations reliably over
almost the entire visual spectral range. Conclusions. A present-day cosmic
abundance standard is established. Our results i) resolve the discrepancy
between a chemical homogeneous local gas-phase ISM and a chemically
inhomogeneous young stellar component, ii) facilitate the amount of heavy
elements locked up in the interstellar dust to be constrained precisely:
carbonaceous dust is largely destroyed inside the Orion HII region, unlike the
silicates, and that graphite is only a minority species in interstellar dust -,
iii) show that the mixing of CNO-burning products in the course of massive star
evolution follows tightly the predicted nuclear path, iv) provide reliable
present-day reference points for anchoring Galactic chemical evolution models
to observation, and v) imply that the Sun has migrated outwards from the inner
Galactic disk over its lifetime from a birthplace at a distance around 5-6 kpc
from the Galactic Centre; a cancellation of the effects of Galactic chemical
evolution and abundance gradients leads to the similarity of solar and
present-day cosmic abundances in the solar neighbourhood, with a telltaling
signature of the Sun's origin left in the C/O ratio. (ABRIDGED)Comment: 20 pages, 11 figures, published in A&A. A complete version (57 pages,
17 figures) including online material can be downloaded from
http://www.sternwarte.uni-erlangen.de/~nieva/8158.pd
A non-LTE spectral analysis of the 3He and 4He isotopes in the HgMn star kappa Cancri
We present a pilot study on non-local thermodynamic equilibrium (NLTE)
line-formation computations for the isotopes 3He and 4He in the
mercury-manganese star kappa Cancri. The impact of NLTE effects on the
determination of isotopic abundances and the vertical stratification of helium
in the atmosphere is investigated. Modern NLTE line-formation computations were
employed to analyse a high-resolution and high signal-to-noise ratio
ESO-VLT/UVES spectrum of kap Cnc. The atmospheric parameters were determined
from fitting the hydrogen Balmer lines and the spectral energy distribution.
Multiple HeI lines were investigated, including HeI 4921A and 6678A, which show
the widest isotopic splits. Half of the observed HeI lines in the spectrum of
kap Cnc show significant NLTE strengthening, the effects are strongest in the
red lines HeI 5875A and HeI 6678A. NLTE abundances from individual HeI lines
are up to a factor of about 3 lower than LTE values. Helium is found to be
stratified in the atmosphere of kap Cnc. While the LTE analysis indicates a
step-like profile of the helium abundance, a gradual decrease with height is
indicated by the NLTE analysis. A 3He/4He ratio of about 0.25-0.30 is found.
With the available data it cannot be decided whether the two isotopes follow
the same stratification profile, or not. This work implies that NLTE effects
may be ubiquitous in the atmospheres of HgMn stars and may have a significant
impact on abundance determinations and the interpretation of the vertical
abundance stratification of elements.Comment: 7 pages, 9 figure
Improving stellar parameter and abundance determinations of early B-type stars
In the past years we have made great efforts to reduce the statistical and
systematic uncertainties in stellar parameter and chemical abundance
determinations of early B-type stars. Both the construction of robust model
atoms for non-LTE line-formation calculations and a novel self-consistent
spectral analysis methodology were decisive to achieve results of unprecedented
precision. They were extensively tested and applied to high-quality spectra of
stars from OB associations and the field in the solar neighbourhood, covering a
broad parameter range. Initially, most lines of hydrogen, helium and carbon in
the optical/near-IR spectral range were reproduced simultaneously in a
consistent way for the first time, improving drastically on the accuracy of
results in published work.By taking additional ionization equilibria of oxygen,
neon, silicon and iron into account, uncertainties as low as ~1% in effective
temperature, ~10% in surface gravity and ~20% in elemental abundances are
achieved - compared to ~5-10%, ~25% and a factor ~2-3 using standard methods.
Several sources of systematic errors have been identified when comparing our
methods for early B-type stars with standard techniques used in the nineties
and also recently (e.g. VLT-FLAMES survey of massive stars). Improvements in
automatic analyses are strongly recommended for meaningful comparisons of
spectroscopic stellar parameters and chemical abundances ('observational
constrains') with predictions of stellar and galactochemical evolution models.Comment: 6 pages, 2 figures. To appear in "Hot And Cool: Bridging Gaps in
Massive Star Evolution", C. Leitherer, Ph. D. Bennett, P. W. Morris & J. Th.
van Loon, eds., APS Conf. Se
Fundamental properties of nearby single early B-type stars
Fundamental parameters of a sample of 26 apparently slowly-rotating single
early B-type stars in the solar neighbourhood are presented and compared to
high-precision data from detached eclipsing binaries (DEBs). The data are used
to discuss the evolutionary status of the stars in context of the most recent
Geneva grid of models. Evolutionary masses plus radii and luminosities are
determined to better than typically 5%, 10%, and 20% uncertainty, respectively,
facilitating the mass-radius and mass-luminosity relationships to be recovered
with a similar precision as derived from DEBs. Good agreement between
evolutionary and spectroscopic masses is found. Absolute visual and bolometric
magnitudes are derived to typically 0.15-0.20mag uncertainty. Metallicities are
constrained to better than 15-20% uncertainty and tight constraints on
evolutionary ages of the stars are provided. Signatures of mixing with
CN-cycled material are found in 1/3 of the sample stars. Typically, these are
consistent with the amount predicted by the new Geneva models with rotation. A
few objects are possibly the product of binary evolution. In particular, the
unusual characteristics of tau Sco point to a blue straggler nature, due to a
binary merger. The accuracy and precision achieved in the determination of
fundamental stellar parameters from the quantitative spectroscopy of single
early B-type stars comes close (within a factor 2-4) to data derived from DEBs.
However, significant systematic differences with data from the astrophysical
reference literature are found. Masses are about 10-20% and radii about 25%
lower then the recommended values for luminosity class V, resulting in the
stars being systematically fainter than assumed usually, by about 0.5mag in
absolute visual and bolometric magnitude. (abstract abridged)Comment: 11 pages, 11 figure
A new method for an objective, -based spectroscopic analysis of early-type stars
A precise quantitative spectral analysis - encompassing atmospheric parameter
and chemical elemental abundance determination - is time consuming due to its
iterative nature and the multi-parameter space to be explored, especially when
done "by eye". A robust automated fitting technique that is as trustworthy as
traditional methods would allow for large samples of stars to be analyzed in a
consistent manner in reasonable time. We present a semi-automated quantitative
spectral analysis technique for early-type stars based on the concept of
minimization. The method's main features are: far less subjective than
typical "by eye" methods, correction for inaccurate continuum normalization,
consideration of the whole useful spectral range, simultaneous sampling of the
entire multi-parameter space (effective temperature, surface gravity,
microturbulence, macroturbulence, projected rotational velocity, radial
velocity, elemental abundances) to find the global best solution, applicable
also to composite spectra. The method is fast, robust and reliable as seen from
formal tests and from a comparison with previous analyses. Consistent
quantitative spectral analyses of large samples of early-type stars can be
performed quickly with very high accuracy.Comment: 32 pages, 4 figures, Astronomy and Astrophysics, accepte
A cosmic abundance standard: chemical homogeneity of the solar neighbourhood and the ISM dust-phase composition
A representative sample of unevolved early B-type stars in nearby OB
associations and the field is analysed to unprecedented precision using NLTE
techniques. The resulting chemical composition is found to be more metal-rich
and much more homogeneous than indicated by previous work. A rms scatter of
~10% in abundances is found for the six stars (and confirmed by six evolved
stars), the same as reported for ISM gas-phase abundances. A cosmic abundance
standard for the present-day solar neighbourhood is proposed, implying mass
fractions for hydrogen, helium and metals of X=0.715, Y=0.271 and Z=0.014. Good
agreement with solar photospheric abundances as reported from recent 3D
radiative-hydrodynamical simulations of the solar atmosphere is obtained. As a
first application we use the cosmic abundance standard as a proxy for the
determination of the local ISM dust-phase composition, putting tight
observational constraints on dust models.Comment: 4 pages, 2 figures. Accepted for publication in ApJ Letter
Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars
It is generally accepted that the atmospheres of cool/lukewarm stars of
spectral types A and later are described well by LTE model atmospheres, while
the O-type stars require a detailed treatment of NLTE effects. Here model
atmosphere structures, spectral energy distributions and synthetic spectra
computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid
method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE
are compared. Their ability to reproduce observations for effective
temperatures between 15000 and 35000 K are verified. Strengths and weaknesses
of the different approaches are identified. Recommendations are made as to how
to improve the models in order to derive unbiased stellar parameters and
chemical abundances in future applications, with special emphasis on Gaia
science.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics:
Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er