98 research outputs found
The metallicity dependence of WR winds
Wolf-Rayet (WR) stars are the most advanced stage in the evolution of the
most massive stars. The strong feedback provided by these objects and their
subsequent supernova (SN) explosions are decisive for a variety of
astrophysical topics such as the cosmic matter cycle. Consequently,
understanding the properties of WR stars and their evolution is indispensable.
A crucial but still not well known quantity determining the evolution of WR
stars is their mass-loss rate. Since the mass loss is predicted to increase
with metallicity, the feedback provided by these objects and their spectral
appearance are expected to be a function of the metal content of their host
galaxy. This has severe implications for the role of massive stars in general
and the exploration of low metallicity environments in particular. Hitherto,
the metallicity dependence of WR star winds was not well studied. In this
contribution, we review the results from our comprehensive spectral analyses of
WR stars in environments of different metallicities, ranging from slightly
super-solar to SMC-like metallicities. Based on these studies, we derived
empirical relations for the dependence of the WN mass-loss rates on the
metallicity and iron abundance, respectively.Comment: 5 pages, 4 figures, to be published in the Proceedings of the IAU
Symposium No. 329 "The lives and death-throes of massive stars
Wolf-Rayet stars in the Small Magellanic Cloud: I. Analysis of the single WN stars
Wolf-Rayet (WR) stars have a severe impact on their environments owing to
their strong ionizing radiation fields and powerful stellar winds. Since these
winds are considered to be driven by radiation pressure, it is theoretically
expected that the degree of the wind mass-loss depends on the initial
metallicity of WR stars. Following our comprehensive studies of WR stars in the
Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates
for all seven putatively single WN stars known in the SMC. Based on these data,
we discuss the impact of a low-metallicity environment on the mass loss and
evolution of WR stars. The quantitative analysis of the WN stars is performed
with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical
properties of our program stars are obtained from fitting synthetic spectra to
multi-band observations. In all SMC WN stars, a considerable surface hydrogen
abundance is detectable. The majority of these objects have stellar
temperatures exceeding 75 kK, while their luminosities range from 10^5.5 to
10^6.1 Lsun. The WN stars in the SMC exhibit on average lower mass-loss rates
and weaker winds than their counterparts in the Milky Way, M31, and the LMC. By
comparing the mass-loss rates derived for WN stars in different Local Group
galaxies, we conclude that a clear dependence of the wind mass-loss on the
initial metallicity is evident, supporting the current paradigm that WR winds
are driven by radiation. A metallicity effect on the evolution of massive stars
is obvious from the HRD positions of the SMC WN stars at high temperatures and
high luminosities. Standard evolution tracks are not able to reproduce these
parameters and the observed surface hydrogen abundances. Homogeneous evolution
might provide a better explanation for their evolutionary past.Comment: 18+12 pages; 22+8 figures; accepted for publication in A&
2dF-AAOmega spectroscopy of massive stars in the Magellanic Clouds: The north-eastern region of the Large Magellanic Cloud
We present spectral classifications from optical spectroscopy of 263 massive
stars in the north-eastern region of the Large Magellanic Cloud. The observed
two-degree field includes the massive 30 Doradus star-forming region, the
environs of SN1987A, and a number of star-forming complexes to the south of 30
Dor. These are the first classifications for the majority (203) of the stars
and include eleven double-lined spectroscopic binaries. The sample also
includes the first examples of early OC-type spectra (AAOmega 30 Dor 248 and
280), distinguished by the weakness of their nitrogen spectra and by C IV 4658
emission. We propose that these stars have relatively unprocessed CNO
abundances compared to morphologically normal O-type stars, indicative of an
earlier evolutionary phase. From analysis of observations obtained on two
consecutive nights, we present radial-velocity estimates for 233 stars, finding
one apparent single-lined binary and nine (>3sigma) outliers compared to the
systemic velocity; the latter objects could be runaway stars or large-amplitude
binary systems and further spectroscopy is required to investigate their
nature.Comment: Accepted by A&
The Wolf-Rayet stars in the Large Magellanic Cloud: A comprehensive analysis of the WN class
Aims: Following our comprehensive studies of the WR stars in the Milky Way,
we now present spectroscopic analyses of almost all known WN stars in the LMC.
Methods: For the quantitative analysis of the wind-dominated emission-line
spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By
fitting synthetic spectra to the observed spectral energy distribution and the
available spectra (ultraviolet and optical), we obtain the physical properties
of 107 stars. Results: We present the fundamental stellar and wind parameters
for an almost complete sample of WN stars in the LMC. Among those stars that
are putatively single, two different groups can be clearly distinguished. While
12% of our sample are more luminous than 10^6 Lsun and contain a significant
amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate
the luminosity range between log (L/Lsun) = 5.3...5.8. Conclusions: While the
few extremely luminous stars (log (L/Lsun) > 6), if indeed single stars,
descended directly from the main sequence at very high initial masses, the bulk
of WN stars have gone through the red-supergiant phase. According to their
luminosities in the range of log (L/Lsun) = 5.3...5.8, these stars originate
from initial masses between 20 and 40 Msun. This mass range is similar to the
one found in the Galaxy, i.e. the expected metallicity dependence of the
evolution is not seen. Current stellar evolution tracks, even when accounting
for rotationally induced mixing, still partly fail to reproduce the observed
ranges of luminosities and initial masses. Moreover, stellar radii are
generally larger and effective temperatures correspondingly lower than
predicted from stellar evolution models, probably due to subphotospheric
inflation.Comment: 17+46 pages; 10+54 figures; v2: typos corrected, space-saving layout
for appendix C, published in A&
Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars
Massive stars are the key agents of feedback. Consequently, quantitative
analysis of massive stars are required to understand how the feedback of these
objects shapes/ creates the large scale structures of the ISM. The giant HII
region N206 in the Large Magellanic Cloud contains an OB association that
powers a X-ray superbubble, serving as an ideal laboratory in this context. We
obtained optical spectra with the muti-object spectrograph FLAMES at the
ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra
from the HST, IUE, and FUSE archives. Detailed spectral classifications are
presented for our sample Of-type stars. For the quantitative spectroscopic
analysis we use the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The
physical parameters and nitrogen abundances of our sample stars are determined
by fitting synthetic spectra to the observations. The stellar and wind
parameters of nine Of-type stars are used to construct wind momentum,luminosity
relationship. We find that our sample follows a relation close to the
theoretical prediction, assuming clumped winds. The most massive star in the
N206 association is an Of supergiant which has a very high mass-loss rate. Two
objects in our sample reveal composite spectra, showing that the Of primaries
have companions of late O subtype. All stars in our sample have an evolutionary
age less than 4 million years, with the O2-type star being the youngest. All
these stars show a systematic discrepancy between evolutionary and
spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen
enrichment shows a clear correlation with increasing projected rotational
velocities. The mechanical energy input from the Of stars alone is comparable
to the energy stored in the N206 superbubble as measured from the observed
X-ray and H alpha emission.Comment: Accepted for the pubblication in Astronomy & Astrophysic
Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars
Context. Metal-poor massive stars are supposed to be progenitors of certain
supernovae, gamma-ray bursts and compact object mergers, potentially
contributing to the early epochs of the Universe with their strong ionizing
radiation. However, they remain mainly theoretical as individual spectroscopic
observations of such objects have rarely been carried out below the metallicity
of the SMC.
Aims. This work aims at exploring what our state-of-the-art theories of
stellar evolution combined with those of stellar atmospheres predict about a
certain type of metal-poor (0.02 Z) hot massive stars, the chemically
homogeneously evolving ones, called TWUIN stars.
Methods. Synthetic spectra corresponding to a broad range in masses (20-130
M) and covering several evolutionary phases from the zero-age
main-sequence up to the core helium-burning stage were computed.
Results. We find that TWUIN stars show almost no emission lines during most
of their {core hydrogen-burning} lifetimes. Most metal lines are completely
absent, including nitrogen. During their core helium-burning stage, lines
switch to emission and even some metal lines (oxygen and carbon, but still
almost no nitrogen) show up. Mass loss and clumping play a significant role in
line-formation in later evolutionary phases, particularly during core
helium-burning. Most of our spectra are classified as an early O type giant or
supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning
phase.
Conclusions. An extremely hot, early O type star observed in a
low-metallicity galaxy could be the outcome of chemically homogeneous evolution
and therefore the progenitor of a long-duration gamma-ray burst or a type
Ic supernova. TWUIN stars may play an important role in reionizing the Universe
due to their being hot without showing prominent emission lines during the
majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres
A combined HST and XMM-Newton campaign for the magnetic O9.7 V star HD 54879: towards constraining the weak-wind problem of massive stars
Context: HD 54879 (O9.7 V) is one of a dozen O-stars for which an organized
atmospheric magnetic field has been detected. To gain insights into the
interplay between atmospheres, winds, and magnetic fields of massive stars, we
acquired UV and X-ray data of HD 54879 using the Hubble Space Telescope and the
XMM-Newton satellite. In addition, 35 optical amateur spectra were secured to
study the variability of HD 54879. A multiwavelength (X-ray to optical)
spectral analysis is performed using the Potsdam Wolf-Rayet (PoWR) model
atmosphere code and the xspec software.
Results: The photospheric parameters are typical for an O9.7 V star. The
microturbulent, macroturbulent, and projected rotational velocities are lower
than previously suggested (<4 km/s). An initial mass of 16 and an
age of 5 Myr are inferred from evolutionary tracks. We derive a mean X-ray
emitting temperature of [K] and an X-ray luminosity of
[erg/s]. Short- and long-scale variability is seen in
the H-alpha line, but only a very long period of yr could be
estimated. Assessing the circumstellar density of HD 54879 using UV spectra, we
can roughly estimate the mass-loss rate HD 54879 would have in the absence of a
magnetic field as . The
magnetic field traps the stellar wind up to the Alfv\'en radius >
, implying that its true mass-loss rate is . Hence, density enhancements around magnetic stars
can be exploited to estimate mass-loss rates of non-magnetic stars of similar
spectral types, essential for resolving the weak wind problem.
Conclusions: Our study confirms that strongly magnetized stars lose little or
no mass, and supplies important constraints on the weak-wind problem of massive
main sequence stars.Comment: Accepted for publication in A&A on Aug. 9, 2017, 12 + 1 pages, 15
figures. Paper replaced due to typos and missing acknowledgment
A rare early-type star revealed in the Wing of the Small Magellanic Cloud
Sk 183 is the visually-brightest star in the N90 nebula, a young star-forming
region in the Wing of the Small Magellanic Cloud (SMC). We present new optical
spectroscopy from the Very Large Telescope which reveals Sk 183 to be one of
the most massive O-type stars in the SMC. Classified as an O3-type dwarf on the
basis of its nitrogen spectrum, the star also displays broadened He I
absorption which suggests a later type. We propose that Sk 183 has a composite
spectrum and that it is similar to another star in the SMC, MPG 324. This
brings the number of rare O2- and O3-type stars known in the whole of the SMC
to a mere four. We estimate physical parameters for Sk 183 from analysis of its
spectrum. For a single-star model, we estimate an effective temperature of
46+/-2 kK, a low mass-loss rate of ~10^-7 Msun yr^-1, and a spectroscopic mass
of 46^+9_-8 Msun (for an adopted distance modulus of 18.7 mag to the young
population in the SMC Wing). An illustrative binary model requires a slightly
hotter temperature (~47.5 kK) for the primary component. In either scenario, Sk
183 is the earliest-type star known in N90 and will therefore be the dominant
source of hydrogen-ionising photons. This suggests Sk 183 is the primary
influence on the star formation along the inner edge of the nebula.Comment: Accepted by ApJ, 10 pages, 7 figures, v2 after proof
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