190 research outputs found
Atmospheric NLTE-Models for the Spectroscopic Analysis of Blue Stars with Winds. II. Line-Blanketed Models
We present new or improved methods for calculating NLTE, line-blanketed model
atmospheres for hot stars with winds (spectral types A to O), with particular
emphasis on a fast performance. These methods have been implemented into a
previous, more simple version of the model atmosphere code FASTWIND
(Santolaya-Rey et al.1997) and allow to spectroscopically analyze rather large
samples of massive stars in a reasonable time-scale, using state-of-the-art
physics.
We describe our (partly approximate) approach to solve the equations of
statistical equilibrium for those elements which are primarily responsible for
line-blocking and blanketing, as well as an approximate treatment of the
line-blocking itself, which is based on a simple statistical approach using
suitable means for line opacities and emissivities. Furthermore, we comment on
our implementation of a consistent temperature structure.
In the second part, we concentrate on a detailed comparison with results from
those two codes which have been used in alternative spectroscopical
investigations, namely CMFGEN (Hillier & Miller 1998) and WM-Basic (Pauldrach
et al. 2001). All three codes predict almost identical temperature structures
and fluxes for lambda > 400 A, whereas at lower wavelengths a number of
discrepancies are found. Optical H/He lines as synthesized by FASTWIND are
compared with results from CMFGEN, obtaining a remarkable coincidence, except
for the HeI singlets in the temperature range between 36,000 to 41,000 K for
dwarfs and between 31,000 to 35,000 K for supergiants, where CMFGEN predicts
much weaker lines. Consequences due to these discrepancies are discussed.Comment: 30 pages incl. 20 figures, accepted by A&
Self-consistent modelling of line-driven hot-star winds with Monte Carlo radiation hydrodynamics
Radiative pressure exerted by line interactions is a prominent driver of
outflows in astrophysical systems, being at work in the outflows emerging from
hot stars or from the accretion discs of cataclysmic variables, massive young
stars and active galactic nuclei. In this work, a new radiation hydrodynamical
approach to model line-driven hot-star winds is presented. By coupling a Monte
Carlo radiative transfer scheme with a finite-volume fluid dynamical method,
line-driven mass outflows may be modelled self-consistently, benefiting from
the advantages of Monte Carlo techniques in treating multi-line effects, such
as multiple scatterings, and in dealing with arbitrary multidimensional
configurations. In this work, we introduce our approach in detail by
highlighting the key numerical techniques and verifying their operation in a
number of simplified applications, specifically in a series of self-consistent,
one-dimensional, Sobolev-type, hot-star wind calculations. The utility and
accuracy of our approach is demonstrated by comparing the obtained results with
the predictions of various formulations of the so-called CAK theory and by
confronting the calculations with modern sophisticated techniques of predicting
the wind structure. Using these calculations, we also point out some useful
diagnostic capabilities our approach provides. Finally we discuss some of the
current limitations of our method, some possible extensions and potential
future applications.Comment: 15 pages, 8 figures; accepted for publication in MNRA
Atmospheric NLTE-models for the spectroscopic analysis of blue stars with winds : II. Line-blanketed models
We present new or improved methods for calculating NLTE, line-blanketed model atmospheres for hot stars with winds (spectral types A to O), with particular emphasis on fast performance. These methods have been implemented into a previous, more simple version of the model atmosphere code FASTWIND (Santolaya-Rey et al. 1997) and allow us to spectroscopically analyze large samples of massive stars in a reasonable time-scale, using state-of-the-art physics. Although this updated version of the code has already been used in a number of recent investigations, the corresponding methods have not been explained in detail so far, and no rigorous comparison with results from alternative codes has been performed. This paper intends to address both topics.
In particular, we describe our (partly approximate) approach to solve the equations of statistical equilibrium for those elements that are primarily responsible for line-blocking and blanketing, as well as an approximate treatment of the line-blocking itself, which is based on a simple statistical approach using suitable means of line opacities and emissivities. Both methods are validated by specific tests. Furthermore, we comment on our implementation of a consistent temperature structure.
In the second part, we concentrate on a detailed comparison with results from two codes used in alternative spectroscopical investigations, namely CMFGEN (Hillier & Miller 1998) and WM-Basic (Pauldrach et al. 2001). All three codes predict almost identical temperature structures and fluxes for λ > 400 Å, whereas at lower wavelengths a number of discrepancies are found. Particularly in the HeII continua, where fluxes and corresponding numbers of ionizing photons react extremely sensitively to subtle differences in the models, we consider any uncritical use of these quantities (e.g., in the context of nebula diagnostics) as unreliable. Optical H/He lines as synthesized by FASTWIND are compared with results from CMFGEN, obtaining a remarkable coincidence, except for the HeI singlets in the temperature range between 36 000 to 41 000 K for dwarfs and between 31 000 to 35 000 K for supergiants, where CMFGEN predicts much weaker lines. Consequences of these discrepancies are discussed.
Finally, suggestions are presented as to adequately parameterize model-grids for hot stars with winds, with only one additional parameter compared to standard grids from plane-parallel, hydrostatic models.Facultad de Ciencias AstronĂłmicas y GeofĂsica
Mass-loss rates of Very Massive Stars
We discuss the basic physics of hot-star winds and we provide mass-loss rates
for (very) massive stars. Whilst the emphasis is on theoretical concepts and
line-force modelling, we also discuss the current state of observations and
empirical modelling, and address the issue of wind clumping.Comment: 36 pages, 15 figures, Book Chapter in "Very Massive Stars in the
Local Universe", Springer, Ed. Jorick S. Vin
The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
The quantitative study of the physical properties and chemical abundances of
large samples of massive blue stars at different metallicities is a powerful
tool to understand the nature and evolution of these objects. Their analysis
beyond the Milky Way is challenging, nonetheless it is doable and the best way
to investigate their behavior in different environments. Fulfilling this task
in an objective way requires the implementation of automatic analysis
techniques that can perform the analyses systematically, minimizing at the same
time any possible bias.
As part of the ARAUCARIA project we carry out the first quantitative
spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55
at 1.94 Mpc away. By applying the methodology developed in this work, we derive
their stellar parameters, chemical abundances and provide a characterization of
the present-day metallicity of their host galaxy.
Based on the characteristics of the stellar atmosphere/line formation code
FASTWIND, we designed and created a grid of models for the analysis of massive
blue supergiant stars. Along with this new grid, we implemented a spectral
analysis algorithm. Both tools were specially developed to perform fully
consistent quantitative spectroscopic analyses of low spectral resolution of
B-type supergiants in a fast and objective way.
We present the main characteristics of our FASTWIND model grid and perform a
number of tests to investigate the reliability of our methodology. The
automatic tool is applied afterward to a sample of 12 B-type supergiant stars
in NGC55, deriving the stellar parameters and abundances. The results indicate
that our stars are part of a young population evolving towards a red supergiant
phase. The derived chemical composition hints to an average metallicity similar
to the one of the Large Magellanic Cloud, with no indication of a spatial trend
across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&
Atmospheric NLTE-models for the spectroscopic analysis of blue stars with winds : II. Line-blanketed models
We present new or improved methods for calculating NLTE, line-blanketed model atmospheres for hot stars with winds (spectral types A to O), with particular emphasis on fast performance. These methods have been implemented into a previous, more simple version of the model atmosphere code FASTWIND (Santolaya-Rey et al. 1997) and allow us to spectroscopically analyze large samples of massive stars in a reasonable time-scale, using state-of-the-art physics. Although this updated version of the code has already been used in a number of recent investigations, the corresponding methods have not been explained in detail so far, and no rigorous comparison with results from alternative codes has been performed. This paper intends to address both topics.
In particular, we describe our (partly approximate) approach to solve the equations of statistical equilibrium for those elements that are primarily responsible for line-blocking and blanketing, as well as an approximate treatment of the line-blocking itself, which is based on a simple statistical approach using suitable means of line opacities and emissivities. Both methods are validated by specific tests. Furthermore, we comment on our implementation of a consistent temperature structure.
In the second part, we concentrate on a detailed comparison with results from two codes used in alternative spectroscopical investigations, namely CMFGEN (Hillier & Miller 1998) and WM-Basic (Pauldrach et al. 2001). All three codes predict almost identical temperature structures and fluxes for λ > 400 Å, whereas at lower wavelengths a number of discrepancies are found. Particularly in the HeII continua, where fluxes and corresponding numbers of ionizing photons react extremely sensitively to subtle differences in the models, we consider any uncritical use of these quantities (e.g., in the context of nebula diagnostics) as unreliable. Optical H/He lines as synthesized by FASTWIND are compared with results from CMFGEN, obtaining a remarkable coincidence, except for the HeI singlets in the temperature range between 36 000 to 41 000 K for dwarfs and between 31 000 to 35 000 K for supergiants, where CMFGEN predicts much weaker lines. Consequences of these discrepancies are discussed.
Finally, suggestions are presented as to adequately parameterize model-grids for hot stars with winds, with only one additional parameter compared to standard grids from plane-parallel, hydrostatic models.Facultad de Ciencias AstronĂłmicas y GeofĂsica
Quantitative spectroscopy of Galactic BA-type supergiants. I. Atmospheric parameters
BA-type supergiants show a high potential as versatile indicators for modern
astronomy. The focus here is on the determination of accurate and precise
atmospheric parameters for a sample of 35 Galactic BA-type supergiants. Some
first applications include a recalibration of functional relationships between
spectral-type, intrinsic colours, bolometric corrections and effective
temperature, and an exploration of the reddening-free Johnson Q and Str\"omgren
[c_1] and beta-indices as photometric indicators for effective temperatures and
gravities of BA-type supergiants. An extensive grid of theoretical spectra is
computed based on a hybrid non-LTE approach. The atmospheric parameters are
derived spectroscopically by line-profile fits to high-resolution and high-S/N
spectra obtained at various observatories. Ionization equilibria of multiple
metals and the Stark-broadened H and the neutral He lines constitute our
primary indicators for the parameter determination, supplemented by
(spectro-)photometry. Data on Teff, logg, helium abundances, microturbulence,
macroturbulence and rotational velocities are presented. The interstellar
reddening and the ratio of total-to-selective extinction towards the stars are
determined. Our empirical spectral-type-Teff scale is steeper than reference
relations, the stars are significantly bluer, and bolometric corrections differ
significantly from established literature values. Photometric
Teff-determinations based on the reddening-free Q-index are found to be of
limited use for studies of BA-type supergiants because of large errors of
typically +-5%+-3% (1sigma statistical, 1sigma systematic), compared to a
spectroscopically achieved precision of 1-2%. The reddening-free [c_1]-index
and beta on the other hand are found to provide useful starting values for
further analyses, with uncertainties of +-1%+-2.5% in Teff, and +-0.04+-0.13dex
in log g. [abriged]Comment: 18 pages, 18 figures; A&
SN 2006oz: rise of a super-luminous supernova observed by the SDSS-II SN Survey
We study SN 2006oz, a newly-recognized member of the class of H-poor,
super-luminous supernovae. We present multi-color light curves from the SDSS-II
SN Survey, that cover the rise time, as well as an optical spectrum that shows
that the explosion occurred at z~0.376. We fitted black body functions to
estimate the temperature and radius evolution of the photosphere and used the
parametrized code SYNOW to model the spectrum. We constructed a bolometric
light curve and compared it with explosion models. The very early light curves
show a dip in the g- and r-bands and a possible initial cooling phase in the
u-band before rising to maximum light. The bolometric light curve shows a
precursor plateau with a duration of 6-10 days in the rest-frame. A lower limit
of M_u < -21.5 can be placed on the absolute peak luminosity of the SN, while
the rise time is constrained to be at least 29 days. During our observations,
the emitting sphere doubled its radius to 2x10^15 cm, while the temperature
remained hot at 15000 K. As for other similar SNe, the spectrum is best modeled
with elements including O II and Mg II, while we tentatively suggest that Fe
III might be present. We suggest that the precursor plateau might be related to
a recombination wave in a circumstellar medium (CSM) and discuss whether this
is a common property of all similar explosions. The subsequent rise can be
equally well described by input from a magnetar or by ejecta-CSM interaction,
but the models are not well constrained owing to the lack of post-maximum
observations, and CSM interaction has difficulties accounting for the precursor
plateau self-consistently. Radioactive decay is less likely to be the mechanism
that powers the luminosity. The host galaxy, detected in deep imaging with the
10 m GTC, is a moderately young and star-forming, but not a starburst, galaxy.
It has an absolute magnitude of M_g = -16.9.Comment: Contains minor changes (of editorial nature) with respect to v1 in
order to match the published version. The abstract has been modified to fit
the arXiv space requirements. 11 pages, 8 figures, 3 table
Center-of-Mass Properties of the Exciton in Quantum Wells
We present high-quality numerical calculations of the exciton center-of-mass
dispersion for GaAs/AlGaAs quantum wells of widths in the range 2-20 nm. The
k.p-coupling of the heavy- and light-hole bands is fully taken into account. An
optimized center-of-mass transformation enhances numerical convergence. We
derive an easy-to-use semi-analytical expression for the exciton groundstate
mass from an ansatz for the exciton wavefunction at finite momentum. It is
checked against the numerical results and found to give very good results. We
also show multiband calculations of the exciton groundstate dispersion using a
finite-differences scheme in real space, which can be applied to rather general
heterostructures.Comment: 19 pages, 12 figures included, to be published in Phys. Rev.
The B-Supergiant Components of the Double-Lined Binary HD1383
We present new results from a study of high quality, red spectra of the
massive binary star system HD 1383 (B0.5 Ib + B0.5 Ib). We determined radial
velocities and revised orbital elements (P = 20.28184 +/- 0.0002 d) and made
Doppler tomographic reconstructions of the component spectra. A comparison of
these with model spectra from non-LTE, line blanketed atmospheres indicates
that both stars have almost identical masses (M_2/M_1 = 1.020 +/- 0.014),
temperatures (T_eff = 28000 +/- 1000 K), gravities (log g = 3.25 +/- 0.25), and
projected rotational velocities (V sin i < 30 km/s). We investigate a number of
constraints on the radii and masses of the stars based upon the absence of
eclipses, surface gravity, stellar wind terminal velocity, and probable
location in the Perseus spiral arm of the Galaxy, and these indicate a range in
probable radius and mass of R/R_sun = 14 - 20 and M/M_sun = 16 - 35,
respectively. These values are consistent with model evolutionary masses for
single stars of this temperature and gravity. Both stars are much smaller than
their respective Roche radii, so the system is probably in a pre-contact stage
of evolution. A fit of the system's spectral energy distribution yields a
reddening of E(B-V)=0.55 +/- 0.05 and a ratio of total-to-selective extinction
of R=2.97 +/- 0.15. We find no evidence of H-alpha emission from colliding
stellar winds, which is probably the consequence of the low gas densities in
the colliding winds zone.Comment: Accepted for publication in ApJ: 20 pages, 5 figure
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