77 research outputs found
Minimizing Strong Telluric Absorption in Near Infra-red Stellar Spectra
We have obtained high resolution spectra (R = 25000) of an A star over
varying airmass to determine the effectiveness of telluric removal in the limit
of high signal to noise. The near infra-red line HeI at 2.058 microns, which is
a sensitive indicator of physical conditions in massive stars, supergiants, HII
regions and YSOs, resides among pressure broadened telluric absorption from
carbon dioxide and water vapor that varies both in time and with observed
airmass.
Our study shows that in the limit of bright stars at high resolution,
accuracies of 5% are typical for high airmass observations (greater than 1.9),
improving to a photon-limited accuracy of 2% at smaller airmasses (less than
1.15). We find that by using the continuum between telluric absorption lines of
a ro-vibrational fan a photon-limited 1% accuracy is achievable.Comment: 14 pages, 7 figures. Accepted for publication in PAS
The spectral variations of the O-type runaway supergiant HD 188209
We report spectral time series of the late O-type runaway supergiant HD 188209. Radial velocity variations of photospheric absorption lines with a possible quasi-period of âŒ6.4 d have been detected in high-resolution echelle spectra. Night-to-night variations in the position and strength of the central emission reversal of the Hα profile occurring over ill-defined time-scales have been observed. The fundamental parameters of the star are derived using state-of-the-art plane-parallel and unified non-LTE model atmospheres, the latter including the mass-loss rate. The derived helium abundance is moderately enhanced with respect to solar, and the stellar masses are lower than those predicted by the evolutionary models. The binary nature of this star is not suggested either from Hipparcos photometry or from radial velocity curves
Spectral Modelling of Star-Forming Regions in the Ultraviolet: Stellar Metallicity Diagnostics for High Redshift Galaxies
The chemical composition of high redshift galaxies is an important property
which gives clues to their past history and future evolution and yet is
difficult to measure with current techniques. In this paper we investigate new
metallicity indicators, based upon the strengths of stellar photospheric
features at rest-frame ultraviolet wavelengths. By combining the evolutionary
spectral synthesis code Starburst99 with the output from the non-LTE model
atmosphere code WM-basic, we have developed a code that can model the
integrated ultraviolet stellar spectra of star-forming regions at metallicities
between 1/20 and twice solar. We use our models to explore a number of spectral
regions that are sensitive to metallicity and clean of other spectral features.
The most promising metallicity indicator is an absorption feature between 1935
A and 2020 A, which arises from the blending of numerous Fe III transitions. We
compare our model spectra to observations of two well studied high redshift
star-forming galaxies, MS1512-cB58 (a Lyman break galaxy at z = 2.7276), and
Q1307-BM1163 (a UV-bright galaxy at z = 1.411). The profiles of the
photospheric absorption features observed in these galaxies are well reproduced
by the models. In addition, the metallicities inferred from their equivalent
widths are in good agreement with previous determinations based on interstellar
absorption and nebular emission lines. Our new technique appears to be a
promising alternative, or complement, to established methods which have only a
limited applicability at high redshifts.Comment: 18 pages, 12 figures, accepted for publication in the Astrophysical
Journa
VLT spectroscopy of blue supergiants in IC 1613
We present multi-object spectroscopy of young, massive stars in the Local
Group galaxy IC 1613. We provide the spectral classification and a detailed
spectral catalog for 54 OBA stars in this galaxy. The majority of the
photometrically selected sample is composed of B- and A-type supergiants. The
remaining stars include early O-type dwarfs and the only Wolf-Rayet star known
in this galaxy. Among the early B stars we have serendipitously uncovered 6 Be
stars, the largest spectroscopically confirmed sample of this class of objects
beyond the Magellanic Clouds. We measure chemical abundances for 9 early-B
supergiants, and find a mean oxygen abundance of 12+log(O/H)=7.90 +/- 0.08.
This value is consistent with the result we obtain for two HII regions in which
we detect the temperature-sensitive [OIII]4363 auroral line.Comment: 17 pages, accepted for publication in the Astrophysical Journa
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
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
Fundamental Properties of O-Type Stars
We present a comprehensive analysis of high-resolution, far-UV HST/STIS,
FUSE, and optical spectra of 17 O stars in the SMC. Our analysis is based on
NLTE metal line-blanketed model atmospheres calculated with our NLTE code
TLUSTY. We systematically explore the sensitivity of various UV and optical
lines to different stellar parameters. We have obtained consistent fits of the
UV and the optical spectrum to derive the effective temperature, surface
gravity, surface composition, and microturbulent velocity of each star. Stellar
radii, masses, luminosities and ages then follow. Similarly to more limited
recent studies, we derive cooler temperatures than the standard Teff
calibration of O stars. We propose a new calibration between the spectral type
and effective temperature based on our results from UV metal lines as well as
optical hydrogen and helium lines. For stars of the same spectral subtype, we
find a general good agreement between Teff determinations obtained with TLUSTY,
CMFGEN, and FASTWIND models. We derive ionizing luminosities that are smaller
by a factor of 3 compared to luminosities inferred from previous standard
calibrations. The chemical composition analysis reveals that the surface of
about 3/4 of the program stars is moderately to strongly enriched in nitrogen,
while showing the original helium, carbon, and oxygen abundances. Our results
support the new stellar evolution models that predict that the surface of fast
rotating stars becomes N-rich during the main sequence phase because of
rotationally-induced mixing. Most stars exhibit the ``mass discrepancy''
problem. This discrepancy too is a result of fast rotation which lowers the
measured effective gravity. Our study thus emphasizes the importance of
rotation in our understanding of the properties of massive stars. (abridged)Comment: Submitted to Astrophysical Journal; 69 page
Broad Balmer Wings in BA Hyper/Supergiants Distorted by Diffuse Interstellar Bands: Five Examples in the 30 Doradus Region from the VLT-FLAMES Tarantula Survey
Extremely broad emission wings at HÎČ and Hα have been found in VLT-FLAMES Tarantula Survey data for five very luminous BA supergiants in or near 30 Doradus in the Large Magellanic Cloud. The profiles of both lines are extremely asymmetrical, which we have found to be caused by very broad diffuse interstellar bands (DIBs) in the longward wing of HÎČ and the shortward wing of Hα. These DIBs are well known to interstellar but not to many stellar specialists, so that the asymmetries may be mistaken for intrinsic features. The broad emission wings are generally ascribed to electron scattering, although we note difficulties for that interpretation in some objects. Such profiles are known in some Galactic hyper/supergiants and are also seen in both active and quiescent Luminous Blue Variables (LBVs). No prior or current LBV activity is known in these 30 Dor stars, although a generic relationship to LBVs is not excluded; subject to further observational and theoretical investigation, it is possible that these very luminous supergiants are approaching the LBV stage for the first time. Their locations in the HRD and presumed evolutionary tracks are consistent with that possibility. The available evidence for spectroscopic variations of these objects is reviewed, while recent photometric monitoring does not reveal variability. A search for circumstellar nebulae has been conducted, with an indeterminate result for one of them
Analysis of Four A-F Supergiants in M31 from Keck HIRES Spectroscopy
The first stellar abundances in M31 are presented, based on Keck I HIRES
spectroscopy and model atmospheres analyses of three A-F supergiants, 41-2368,
41-3712, and A-207. We also present the preliminary analysis of a fourth star,
41-3654. We find that the stellar oxygen abundances are in good agreement with
those determined from nebular studies, even though the stars do {\it not} show
a clear radial gradient in oxygen. The uncertainties in the stellar abundances
are smaller than the range in the nebular results, making these stars ideal
objects for further studies of the distribution of oxygen in M31. We show that
the stars can be used to study the abundance distributions of other elements as
well, including iron-group and heavier elements.
The A-F supergiants also provide direct information on the metallicity and
reddening of nearby Cepheid stars. We have examined the metallicity and
reddening assumptions used for Cepheids within 1' of our targets and noted the
differences from values used in the literature.Comment: Accepted for publication in the ApJ, October 2000 (23 pages, 4
tables, 11 figures
The Physical Properties and Effective Temperature Scale of O-type Stars as a Function of Metallicity. I. A Sample of 20 Stars in the Magellanic Clouds
We have obtained HST and ground-based observations of a sample of 20 O-type
stars in the LMC and SMC, including six of the hottest massive stars known
(subtypes O2-3) in the R136 cluster. In general, these data include (a) the HST
UV spectra in order to measure the terminal velocities of the stellar winds,
(b) high signal-to-noise, blue-optical data where the primary temperature- and
gravity-sensitive photospheric lines are found, and (c) nebular-free H-alpha
profiles, which provide the mass-loss rates. The line-blanketed non-LTE
atmosphere code FASTWIND was then used to determine the physical parameters of
this sample of stars. We find good agreement between the synthetic line
profiles for the hydrogen, He I, and He II lines in the majority of the stars
we analyzed; the three exceptions show evidence of being incipiently resolved
spectroscopic binaries or otherwise spectral composites. One such system is
apparently an O3 V+O3 V eclipsing binary, and a follow-up radial velocity study
is planned to obtain Keplerian masses. Although we did not use them to
constrain the fits, good agreement is also found for the He I
and He II lines in the near-UV, which we plan to exploit in
future studies. Our effective temperatures are compared to those recently
obtained by Repolust, Puls & Herrero for a sample of Galactic stars using the
same techniques. We find that the Magellanic Cloud sample is
3,000-4,000K hotter than their Galactic counterparts for the early
through mid-O's. These higher temperatures are the consequence of a decreased
importance of wind emission, wind blanketing, and metal-line blanketing at
lower metallicities.Comment: Accepted for publication in the Astrophysical Journal. A postscript
version with the figures embedded can be found at
ftp://ftp.lowell.edu/pub/massey/haw.p
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