27 research outputs found
X-ray spectral diagnostics of activity in massive stars
X-rays give direct evidence of instabilities, time-variable structure, and
shock heating in the winds of O stars. The observed broad X-ray emission lines
provide information about the kinematics of shock-heated wind plasma, enabling
us to test wind-shock models. And their shapes provide information about wind
absorption, and thus about the wind mass-loss rates. Mass-loss rates determined
from X-ray line profiles are not sensitive to density-squared clumping effects,
and indicate mass-loss rate reductions of factors of 3 to 6 over traditional
diagnostics that suffer from density-squared effects. Broad-band X-ray spectral
energy distributions also provide mass-loss rate information via soft X-ray
absorption signatures. In some cases, the degree of wind absorption is so high
that the hardening of the X-ray SED can be quite significant. We discuss these
results as applied to the early O stars zeta Pup (O4 If), 9 Sgr (O4 V((f))),
and HD 93129A (O2 If*).Comment: To appear in the proceedings of IAU 272: Active OB Star
Chandra and Suzaku observations of the Be/X-ray star HD110432
We present an analysis of a pointed 141 ks Chandra high resolution
transmission gratings observation of the Be X-ray emitting star HD110432, a
prominent member of the gamma Cas analogs. The Chandra lightcurve shows a high
variability but its analysis fails to detect any coherent periodicity up to a
frequency of 0.05 Hz. The analysis of the Chandra HETG spectrum shows that, to
correctly describe the spectrum, three model components are needed. Two of
those components are optically thin thermal plasmas of different temperatures
(kT~8-9 and 0.2-0.3 keV respectively). Two different models seem to describe
well the third component. One possibility is a third hot optically thin thermal
plasma at kT=16-21 keV with an Fe abundance Z~0.3Zo, definitely smaller than
for the other two thermal components. Alternatively, the third component can be
described by a powerlaw with a photon index Gamma=1.56. In either case, the
Chandra HETG spectrum establishes that each one of these components must be
modified by distinct absorption columns. The analysis of a non contemporaneous
25 ks Suzaku observation shows the presence of a hard tail extending up to at
least 33 keV. The Suzaku spectrum is described with the sum of two components:
an optically thin thermal plasma at kT ~ 9 keV and a very hot second plasma
with kT ~33 keV or, alternatively, a powerlaw with photon index Gamma=1.58. The
analysis of the Si XIII and S XV He like triplets present in the Chandra
spectrum point to a very dense (n_e ~ 10^13 cm^-3) plasma located either close
to the stellar surface (r<3R_*) of the Be star or, alternatively, very close (r
~1.5R_WD) to the surface of a (hypothetical) WD companion. We argue, however,
that the available data supports the first scenario.Comment: 13 pages, 21 Figures. Accepted for publication in Ap
The [O III] Veil: Astropause of Eta Carinae's Wind?
We present narrowband images of eta Carinae in the light of [O III] 5007
obtained with HST/WFPC2, as well as a ground-based image in the same emission
line with a larger field of view. These images show a thin veil of [O III]
emission around eta Car and its ejecta, confirming the existence of an
oxygen-bearing ``cocoon'' inferred from spectra. This [O III] veil may be the
remnant of the pre-outburst wind of eta Car, and its outer edge probably marks
the interface where eta Car's ejecta meet the stellar wind of the nearby O4
V((f)) star HD303308 or other ambient material -- i.e., it marks the
``astropause'' in eta Car's wind. This veil is part of a more extensive [O III]
shell that appears to be shaped and ionized by HD303308. A pair of HST images
with a 10 yr baseline shows no proper motion, limiting the expansion speed away
from eta Car to 12pm13 km/s, or an expansion age of a few times 10^4 yr. Thus,
this is probably the decelerated pre-outburst LBV wind of eta Car. The [O III]
morphology is very different from that seen in [N II], which traces young knots
of CNO-processed material; this represents a dramatic shift in the chemical
makeup of material recently ejected by eta Car. This change in the chemical
abundances may have resulted from the sudden removal of the star's outer
envelope during eta Car's 19th century outburst or an earlier but similar
event.Comment: 11 pages, 4 figs. Figs 1 and 3 in color. Accepted to AJ, October 200
Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs
We report the first discovery of argon in hot evolved stars and white dwarfs.
We have identified the ArVII 1063.55A line in some of the hottest known
(Teff=95000-110000 K) central stars of planetary nebulae and (pre-) white
dwarfs of various spectral type. We determine the argon abundance and compare
it to theoretical predictions from stellar evolution theory as well as from
diffusion calculations. We analyze high-resolution spectra taken with the Far
Ultraviolet Spectroscopic Explorer. We use non-LTE line-blanketed model
atmospheres and perform line-formation calculations to compute synthetic argon
line profiles. We find a solar argon abundance in the H-rich central star
NGC1360 and in the H-deficient PG1159 star PG1424+535. This confirms stellar
evolution modeling that predicts that the argon abundance remains almost
unaffected by nucleosynthesis. For the DAO-type central star NGC7293 and the
hot DA white dwarfs PG0948+534 and REJ1738+669 we find argon abundances that
are up to three orders of magnitude smaller than predictions of calculations
assuming equilibrium of radiative levitation and gravitational settling. For
the hot DO white dwarf PG1034+001 the theoretical overprediction amounts to one
dex. Our results confirm predictions from stellar nucleosynthesis calculations
for the argon abundance in AGB stars. The argon abundance found in hot white
dwarfs, however, is another drastic example that the current state of
equilibrium theory for trace elements fails to explain the observations
quantitatively.Comment: Accepted for publication in A&
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
Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate
We present analysis of both the resolved X-ray emission line profiles and the
broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the
Chandra HETGS. This star is among the earliest and most massive stars in the
Galaxy, and provides a test of the embedded wind shock scenario in a very dense
and powerful wind. A major new result is that continuum absorption by the dense
wind is the primary cause of the hardness of the observed X-ray spectrum, while
intrinsically hard emission from colliding wind shocks contributes less than
10% of the X-ray flux. We find results consistent with the predictions of
numerical simulations of the line-driving instability, including line
broadening indicating an onset radius of X-ray emission of several tenths
Rstar. Helium-like forbidden-to-intercombination line ratios are consistent
with this onset radius, and inconsistent with being formed in a wind-collision
interface with the star's closest visual companion at a distance of ~100 AU.
The broadband X-ray spectrum is fit with a dominant emission temperature of
just kT = 0.6 keV along with significant wind absorption. The broadband wind
absorption and the line profiles provide two independent measurements of the
wind mass-loss rate: Mdot = 5.2_{-1.5}^{+1.8} \times 10^{-6} Msun/yr and Mdot =
6.8_{-2.2}^{+2.8} \times 10^{-6} Msun/yr, respectively. This is the first
consistent modeling of the X-ray line profile shapes and broadband X-ray
spectral energy distribution in a massive star, and represents a reduction of a
factor of 3 to 4 compared to the standard H-alpha mass-loss rate that assumes a
smooth wind.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 12 pages, 10 figures (incl. 5 color
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&
Toward Understanding Massive Star Formation
Although fundamental for astrophysics, the processes that produce massive
stars are not well understood. Large distances, high extinction, and short
timescales of critical evolutionary phases make observations of these processes
challenging. Lacking good observational guidance, theoretical models have
remained controversial. This review offers a basic description of the collapse
of a massive molecular core and a critical discussion of the three competing
concepts of massive star formation:
- monolithic collapse in isolated cores
- competitive accretion in a protocluster environment
- stellar collisions and mergers in very dense systems
We also review the observed outflows, multiplicity, and clustering properties
of massive stars, the upper initial mass function and the upper mass limit. We
conclude that high-mass star formation is not merely a scaled-up version of
low-mass star formation with higher accretion rates, but partly a mechanism of
its own, primarily owing to the role of stellar mass and radiation pressure in
controlling the dynamics.Comment: 139 pages, 18 figures, 5 tables, glossar
A Library of Theoretical Ultraviolet Spectra of Massive, Hot Stars for Evolutionary Synthesis
We computed a comprehensive set of theoretical ultraviolet spectra of hot,
massive stars with the radiation-hydrodynamics code WM-Basic. This model
atmosphere and spectral synthesis code is optimized for computing the strong P
Cygni-type lines originating in the winds of hot stars, which are the strongest
features in the ultraviolet spectral region. The computed set is suitable as a
spectral library for inclusion in evolutionary synthesis models of star
clusters and star-forming galaxies. The chosen stellar parameters cover the
upper left Hertzsprung-Russell diagram at L >~ 10^2.75 Lsun and T_eff >~ 20,000
K. The adopted elemental abundances are 0.05 Zsun, 0.2 Zsun, 0.4 Zsun, Zsun,
and 2 Zsun. The spectra cover the wavelength range from 900 to 3000 {\AA} and
have a resolution of 0.4 {\AA}. We compared the theoretical spectra to data of
individual hot stars in the Galaxy and the Magellanic Clouds obtained with the
International Ultraviolet Explorer (IUE) and Far Ultraviolet Spectroscopic
Explorer (FUSE) satellites and found very good agreement. We built a library
with the set of spectra and implemented it into the evolutionary synthesis code
Starburst99 where it complements and extends the existing empirical library
towards lower chemical abundances. Comparison of population synthesis models at
solar and near-solar composition demonstrates consistency between synthetic
spectra generated with either library. We discuss the potential of the new
library for the interpretation of the rest-frame ultraviolet spectra of
star-forming galaxies. Properties that can be addressed with the models include
ages, initial mass function, and heavy-element abundance. The library can be
obtained both individually or as part of the Starburst99 package.Comment: ApJS (in press); 90 pages, 33 figures, 7 table
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