229 research outputs found
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
Stellar spectroscopy far beyond the Local Group
Multi-object spectroscopic observations of blue supergiants in NGC 3621, a
spiral galaxy at a distance of 6.7 Mpc, carried out with the ESO VLT and FORS
are presented. We demonstrate the feasibility of quantitative stellar
spectroscopy at distances approaching a ten-fold increase over previous
investigations by determining chemical composition, stellar parameters,
reddening, extinction and wind properties of one of our targets, a supergiant
of spectral type A1 Ia located in the outskirts of NGC 3621. The metallicity
(determined from iron group elements) is reduced by a factor of two relative to
the sun in qualitative agreement with results from previous abundance studies
based on H II region oxygen emission lines. Reddening and extinction are E(B-V)
= 0.12 and Av = 0.37, respectively, mostly caused by the galactic foreground.
Comparing stellar wind momentum and absolute V magnitude with galactic and M31
counterparts we confirm the potential of the wind momentum-luminosity
relationship as an alternative tool to estimate extragalactic distances.Comment: 9 pages, 4 figures, 1 table, accepted for publication in The
Astrophysical Journal Letter
The EDIBLES Survey. VIII. Band profile alignment of diffuse interstellar bands
Context: There have been many attempts to identify families of diffuse
interstellar bands (DIBs) with perfectly correlating band strengths. Although
major efforts have been made to classify broadly based DIB families and
important insights have been gained, no family has been identified with
sufficient accuracy or statistical significance to prove that a series of
selected DIBs originates from the same carrier. This can be attributed in part
to the exclusive use of equivalent widths to establish DIB families.
Aims: In a change of strategy, we search for DIBs that are highly correlated
in both band strength and profile shape. This approach increases the chance of
correlating DIBs being members of one family and originating from the same
carrier molecule. We also search for correlations between DIB profile families
and atomic interstellar lines, with the goal of further chemically constraining
possible DIB carriers.
Methods: We adapted the well-known method of time-series alignment to perform
a spectral alignment; that is, DIB alignment. In a second step, we analysed the
alignment results using a clustering analysis. This method required a
statistically significant data set of DIB sight lines. The ESO Diffuse
Interstellar Bands Large Exploration Survey (EDIBLES) data were perfectly
suited for this application.
Results: We report eight DIB families with correlating strengths and
profiles, as well as four previously unreported DIBs in the visual range, found
using DIB alignment. All profile family members show Pearson correlation
coefficients in band strength higher than 0.9. In particular, we report the
6614 - 6521 AA DIB pair, in which both DIBs show the same triple-peak
substructure and an unprecedented band strength Pearson correlation coefficient
of 0.9935. The presented approach opens up new perspectives that can guide the
laboratory search for DIB carriers.Comment: 53 pages, 53 figures, Accepted for publication in Astronomy &
Astrophysic
Non-LTE Model Atmospheres for Late-Type Stars II. Restricted NLTE Calculations for a Solar-Like Atmosphere
We test our knowledge of the atomic opacity in the solar UV spectrum. Using
the atomic data compiled in Paper I from modern, publicly available, databases,
we perform calculations that are confronted with space-based observations of
the Sun. At wavelengths longer than about 260 nm, LTE modeling can reproduce
quite closely the observed fluxes; uncertainties in the atomic line data
account fully for the differences between calculated and observed fluxes. At
shorter wavelengths, departures from LTE appear to be important, as our LTE and
restricted NLTE calculations differ. Analysis of visible-near infrared Na I and
O I lines, two species that produce a negligible absorption in the UV, shows
that observed departures from LTE for theses species can be reproduced very
accurately with restricted (fixed atmospheric structure) NLTE calculations.Comment: 13 pages, 11 figures, to appear in Ap
CRIRES-POP: A library of high resolution spectra in the near-infrared
New instrumental capabilities and the wealth of astrophysical information
extractable from the near-infrared wavelength region have led to a growing
interest in the field of high resolution spectroscopy at 1-5 mu. We aim to
provide a library of observed high-resolution and high signal-to-noise-ratio
near-infrared spectra of stars of various types throughout the
Hertzsprung-Russell diagram. This is needed for the exploration of spectral
features in this wavelength range and for comparison of reference targets with
observations and models.
High quality spectra were obtained using the CRIRES near-infrared
spectrograph at ESO's VLT covering the range from 0.97 to 5.3 mu at high
spectral resolution. Accurate wavelength calibration and correction for of
telluric lines were performed by fitting synthetic transmission spectra for the
Earth's atmosphere to each spectrum individually. We describe the observational
strategy and the current status and content of the library which includes 13
objects. The first examples of finally reduced spectra are presented. This
publication will serve as a reference paper to introduce the library to the
community and explore the extensive amount of material.Comment: accepted for publication in A&A; see also the project webpage
http://www.univie.ac.at/crirespo
Atmospheric velocity fields in tepid main sequence stars
The line profiles of the stars with v sin i below a few km/s can reveal
direct signatures of local velocity fields (e.g. convection) in stellar
atmospheres. This effect is well established in cool main sequence stars, and
has been detected and studied in three A stars. This paper reports observations
of main sequence B, A and F stars with two goals: (1) to identify additional
stars having sufficiently low values of v sin i to search for spectral line
profile signatures of local velocity fields, and (2) to explore how the
signatures of the local velocity fields in the atmosphere depend on stellar
parameters such as effective temperature T_eff and peculiarity type.
For stars having T_eff below about 10000 K, we always detect local
atmospheric velocity fields indirectly through a non-zero microturbulence
parameter, but not for hotter stars. Among the A and F stars in our sample
having the sharpest lines, direct tracers of atmospheric velocity fields are
found in six new stars. The velocity field signatures identified include
asymmetric excess line wing absorption, deeper in the blue line wing than in
the red; line profiles of strong lines that are poorly fit by computed
profiles; and strong lines that are broader than they should be for the v sin i
values deduced from weak lines. These effects are found in both normal and Am
stars, but seem stronger in Am stars.
These data still have not been satisfactorily explained by models of
atmospheric convection, including numerical simulations.Comment: Acepted for publication by Astronomy and Astrophysic
Search for Pairs of Isolated Radio Pulsars - Components in Disrupted Binary Systems
We have developed a method for analyzing the kinematic association of
isolated relativistic objects - possible remnants of disrupted close binary
systems. We investigate pairs of fairly young radio pulsars with known proper
motions and estimated distances (dispersion measures) that are spaced no more
than 2-3 kpc apart. Using a specified radial velocity distribution for these
objects, we have constructed 100-300 thousand trajectories of their possible
motion in the Galactic gravitational field on a time scale of several million
years. The probabilities of their close encounters at epochs consistent with
the age of the younger pulsar in the pair are analyzed. When these
probabilities exceed considerably their reference values obtained by assuming a
purely random encounter between the pulsars under consideration, we conclude
that the objects may have been gravitationally bound in the past. As a result,
we have detected six pulsar pairs (J0543+2329/J0528+2200,
J1453-6413/J1430-6623, J2354+6155/J2321+6024, J1915+1009/J1909+1102,
J1832-0827/J1836-1008, and J1917+1353/J1926+1648) that are companions in
disrupted binary systems with a high probability. Estimates of their kinematic
ages and velocities at binary disruption and at the present epoch are provided
The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae
We have developed a radiative transfer code, CMFGEN, which allows us to model
the spectra of massive stars and supernovae. Using CMFGEN we can derive
fundamental parameters such as effective temperatures and surface gravities,
derive abundances, and place constraints on stellar wind properties. The last
of these is important since all massive stars are losing mass via a stellar
wind that is driven from the star by radiation pressure, and this mass loss can
substantially influence the spectral appearance and evolution of the star.
Recently we have extended CMFGEN to allow us to undertake time-dependent
radiative transfer calculations of supernovae. Such calculations will be used
to place constraints on the supernova progenitor, to place constraints on the
supernova explosion and nucleosynthesis, and to derive distances using a
physical approach called the "Expanding Photosphere Method". We describe the
assumptions underlying the code and the atomic processes involved. A crucial
ingredient in the code is the atomic data. For the modeling we require accurate
transition wavelengths, oscillator strengths, photoionization cross-sections,
collision strengths, autoionization rates, and charge exchange rates for
virtually all species up to, and including, cobalt. Presently, the available
atomic data varies substantially in both quantity and quality.Comment: 8 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Subaru and Swift observations of V652 Herculis: resolving the photospheric pulsation
High-resolution spectroscopy with the Subaru High Dispersion Spectrograph, and Swift ultraviolet photometry are presented for the pulsating extreme helium star V652 Her. Swift provides the best relative ultraviolet photometry obtained to date, but shows no direct evidence for a shock at ultraviolet or X-ray wavelengths. Subaru has provided high spectral and high temporal resolution spectroscopy over six pulsation cycles (and eight radius minima).
These data have enabled a line-by-line analysis of the entire pulsation cycle and provided a description of the pulsating photosphere as a function of optical depth. They show that the photosphere is compressed radially by a factor of at least 2 at minimum radius, that the phase of radius minimum is a function of optical depth and the pulse speed through the photosphere is between 141 and 239 km s−1 (depending how measured) and at least 10 times the local sound speed. The strong acceleration at minimum radius is demonstrated in individual line profiles; those formed deepest in the photosphere show a jump discontinuity of over 70 kms−1 on a time-scale of 150 s. The pulse speed and line profile jumps imply a shock is present at minimum radius. These empirical results provide input for hydrodynamical modelling of the pulsation and hydrodynamical plus radiative transfer modelling of the dynamical spectra
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