1,953 research outputs found
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
Fabrication of Large Domain YBa2Cu3O(x) for Magnetic Suspension Applications
Large domain YBa2Cu3O(x) levitators have been fabricated using a seeded melt processing technique. Depending upon the seed, either a single or five domained sample can be obtained. The grain boundaries separating each domains in the five domain levitator are found to be 90 degrees. Similar levitation forces can be observed for single and five domained samples. After thermal cycling, however, a small decrease in the levitation force of the five domain levitator was observed as a function of thermal cycles while nearly no change in force was observed in the single domain levitator. Finally, it is shown that both, single and five domain YBCO, behave similarly as a function of sample thickness
The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud: spectroscopy, orbital analysis, formation, and evolution
Massive Wolf-Rayet (WR) stars dominate the radiative and mechanical energy
budget of galaxies and probe a critical phase in the evolution of massive stars
prior to core-collapse. It is not known whether core He-burning WR stars
(classical WR, cWR) form predominantly through wind-stripping (w-WR) or binary
stripping (b-WR). With spectroscopy of WR binaries so-far largely avoided due
to its complexity, our study focuses on the 44 WR binaries / binary candidates
of the Large Magellanic Cloud (LMC, metallicity Z~0.5 Zsun), identified on the
basis of radial velocity variations, composite spectra, or high X-ray
luminosities. Relying on a diverse spectroscopic database, we aim to derive the
physical and orbital parameters of our targets, confronting evolution models of
evolved massive stars at sub-solar metallicity, and constraining the impact of
binary interaction in forming them. Spectroscopy is performed using the Potsdam
Wolf-Rayet (PoWR) code and cross-correlation techniques. Disentanglement is
performed using the code Spectangular or the shift-and-add algorithm.
Evolutionary status is interpreted using the Binary Population and Spectral
Synthesis (BPASS) code, exploring binary interaction and chemically-homogeneous
evolution.
No obvious dichotomy in the locations of apparently-single and binary WN
stars on the Hertzsprung-Russell diagram is apparent. According to commonly
used stellar evolution models (BPASS, Geneva), most apparently-single WN stars
could not have formed as single stars, implying that they were stripped by an
undetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing
(e.g., during the red supergiant phase) are strongly underestimated in standard
stellar evolution models.Comment: accepted to A&A on 10.05.2019; 69 pages (25 main paper + 44
appendix); Corrigendum: Shenar et al. 2020, A&A, 641, 2: An unfortunate typo
in the implementation of the "transformed radius" caused errors of up to
~0.5dex in the derived mass-loss rates. This has now been correcte
PoWR grids of non-LTE model atmospheres for OB-type stars of various metallicities
The study of massive stars in different metallicity environments is a central
topic of current stellar research. The spectral analysis of massive stars
requires adequate model atmospheres. The computation of such models is
difficult and time-consuming. Therefore, spectral analyses are greatly
facilitated if they can refer to existing grids of models. Here we provide
grids of model atmospheres for OB-type stars at metallicities corresponding to
the Small and Large Magellanic Clouds, as well as to solar metallicity. In
total, the grids comprise 785 individual models. The models were calculated
using the state-of-the-art Potsdam Wolf-Rayet (PoWR) model atmosphere code. The
parameter domain of the grids was set up using stellar evolution tracks. For
all these models, we provide normalized and flux-calibrated spectra, spectral
energy distributions, feedback parameters such as ionizing photons, Zanstra
temperatures, and photometric magnitudes. The atmospheric structures (the
density and temperature stratification) are available as well. All these data
are publicly accessible through the PoWR website.Comment: 12 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
The extreme O-type spectroscopic binary HD 93129A
HD 93129A was classified as the earliest O-type star in the Galaxy (O2~If*)
and is considered as the prototype of its spectral class. However,
interferometry shows that this object is a binary system, while recent
observations even suggest a triple configuration. None of the previous spectral
analyses of this object accounted for its multiplicity. With new
high-resolution UV and optical spectra, we have the possibility to reanalyze
this key object, taking its binary nature into account for the first time. We
aim to derive the fundamental parameters and the evolutionary status of HD
93129A, identifying the contributions of both components to the composite
spectrum. We analyzed UV and optical observations acquired with the Hubble
Space Telescope and ESO's Very Large Telescope. A multiwavelength analysis of
the system was performed using the latest version of the Potsdam Wolf-Rayet
model atmosphere code. Despite the similar spectral types of the two
components, we are able to find signatures from each of the components in the
combined spectrum, which allows us to estimate the parameters of both stars. We
derive , kK, and for the primary Aa, and , kK, and for the secondary Ab. Even when accounting for the
binary nature, the primary of HD 93129A is found to be one of the hottest and
most luminous O stars in our Galaxy. Based on the theoretical decomposition of
the spectra, we assign spectral types O2~If* and O3~III(f*) to components Aa
and Ab, respectively. While we achieve a good fit for a wide spectral range,
specific spectral features are not fully reproduced. The data are not
sufficient to identify contributions from a hypothetical third component in the
system.Comment: 16 pages, 20 figures, accepted for publication in Astronomy &
Astrophysics, typos corrected by language edito
The earliest O-type eclipsing binary in the Small Magellanic Cloud, AzV 476: A comprehensive analysis reveals surprisingly low stellar masses
CONTEXT: Massive stars at low metallicity are among the main feedback agents in the early Universe and in present-day star forming galaxies. When in binaries, these stars are potential progenitors of gravitational-wave events. Knowledge of stellar masses is a
prerequisite to understanding evolution and feedback of low-metallicity massive stars. AIMS: Using abundant spectroscopic and photometric measurements of an outstandingly bright eclipsing binary, we compare its dynamic, spectroscopic, and evolutionary mass estimates and develop a binary evolution scenario. METHODS: We comprehensively studied the eclipsing binary system, AzV 476, in the Small Magellanic Cloud (SMC). The light curve and radial velocities were analyzed to obtain the orbital parameters. The photometric and spectroscopic data in the UV and optical were analyzed using the Potsdam Wolf-Rayet (PoWR) model atmospheres. The obtained results are interpreted using detailed binaryevolution tracks including mass transfer. RESULTS: AzV 476 consists of an O4 IV-III((f))p primary and an O9.5: Vn secondary. Both components have similar current masses
(20 M_{\odot} and 18 M_{\odot}) obtained consistently from both the orbital and spectroscopic analysis. The effective temperatures are 42 kK and 32 kK, respectively. The wind mass-loss rate of log(M˙ /(M_{\odot}yr^{-1})) = −6.2 of the primary is a factor of ten higher than a recent
empirical prescription for single O stars in the SMC. Only close-binary evolution with mass transfer can reproduce the current stellar
and orbital parameters, including orbital separation, eccentricity, and the rapid rotation of the secondary. The binary evolutionary
model reveals that the primary has lost about half of its initial mass and is already core helium burning. CONCLUSIONS: Our comprehensive analysis of AzV 476 yields a consistent set of parameters and suggests previous case B mass transfer. The derived stellar masses agree within their uncertainties. The moderate masses of AzV 476 underline the scarcity of bright massive stars in the SMC. The core helium burning nature of the primary indicates that stripped stars might be hidden among OB-type populations
Stellar wind properties of the nearly complete sample of O stars in the low metallicity young star cluster NGC 346 in the SMC galaxy
CONTEXT: Massive stars are among the main cosmic engines driving the evolution of star-forming galaxies. Their powerful ionising
radiation and stellar winds inject a large amount of energy in the interstellar medium. Furthermore, mass-loss (M˙ ) through radiatively
driven winds plays a key role in the evolution of massive stars. Even so, the wind mass-loss prescriptions used in stellar evolution
models, population synthesis, and stellar feedback models often disagree with mass-loss rates empirically measured from the UV
spectra of low metallicity massive stars. AIMS: The most massive young star cluster in the low metallicity Small Magellanic Cloud galaxy is NGC 346. This cluster contains
more than half of all O stars discovered in this galaxy so far. A similar age, metallicity (Z), and extinction, the O stars in the NGC 346
cluster are uniquely suited for a comparative study of stellar winds in O stars of different subtypes. We aim to use a sample of O stars
within NGC 346 to study stellar winds at low metallicity METHODS: We mapped the central 10 of NGC 346 with the long-slit UV observations performed by the Space Telescope Imaging
Spectrograph (STIS) on board of the Hubble Space Telescope and complemented these new datasets with archival observations.
Multi-epoch observations allowed for the detection of wind variability. The UV dataset was supplemented by optical spectroscopy
and photometry. The resulting spectra were analysed using a non-local thermal equilibrium model atmosphere code (PoWR) to
determine wind parameters and ionising fluxes. RESULTS: The effective mapping technique allowed us to obtain a mosaic of almost the full extent of the cluster and resolve stars in its
core. Among hundreds of extracted stellar spectra, 21 belong to O stars. Nine of them are classified as O stars for the first time. We
analyse, in detail, the UV spectra of 19 O stars (with a further two needing to be analysed in a later paper due to the complexity of
the wind lines as a result of multiplicity). This more than triples the number of O stars in the core of NGC 346 with constrained wind
properties. We show that the most commonly used theoretical mass-loss recipes for O stars over-predict mass-loss rates. We find that
the empirical scaling between mass-loss rates (M˙ ) and luminosity (L), M˙ ∝ L^{2.4}, is steeper than theoretically expected by the most
commonly used recipes. In agreement with the most recent theoretical predictions, we find within M˙ ∝ Z
α
that α is dependent upon
L. Only the most luminous stars dominate the ionisation feedback, while the weak stellar winds of O stars in NGC 346 and the lack
of previous supernova explosions in this cluster restrict the kinetic energy input
The Shortest-period Wolf-Rayet binary in the Small Magellanic Cloud: Part of a high-order multiple system
SMC AB 6 is the shortest-period (6.5d) Wolf-Rayet (WR) binary in the Small
Magellanic Cloud, and is therefore crucial for the study of binary interaction
and formation of WR stars at low metallicity. The WR component in AB 6 was
previously found to be very luminous (logL=6.3[Lsun]) compared to its reported
orbital mass (8Msun), placing it significantly above the Eddington limit.
Through spectroscopy and orbital analysis of newly acquired optical data taken
with UVES, we aim to understand the peculiar results reported for this system
and explore its evolutionary history.
Results: We find that AB 6 contains at least four stars. The 6.5d period WR
binary comprises the WR primary (WN3:h, star A) and a rather rapidly rotating
early O-type companion (O5.5 V, star B). Static N and He lines suggest the
presence of an emission line star (O5.5 I(f), star C). Finally, narrow
absorption lines portraying a long-term radial velocity variation show the
existence of a fourth star (O7.5 V, star D). Star D appears to form a second
140d period binary together with a fifth stellar member, which is a B-type
dwarf or a black hole. It is not clear that these additional components are
bound to the WR binary. The WR star is found to be less luminous than
previously thought (logL = 5.9[Lsun]) and, adopting 41Msun for star B, more
massive (18Msun). Correspondingly, the WR star does not exceed the Eddington
limit. We derive the initial masses of 60 and 40Msun for stars A and B and an
age of 3.9 Myr for the system. The WR binary likely experienced nonconservative
mass transfer in the past supported by the relatively rapid rotation of star B.
Conclusion: Our study shows that AB 6 is a multiple -- probably quintuple --
system. This finding resolves the previously reported puzzle of the WR primary
exceeding the Eddington limit and suggests that the WR star exchanged mass with
its companion in the past.Comment: 15 pages, 16 figures, accepted to A&A on the 30.4.2018. Comments
welcome
Stellar wind properties of the nearly complete sample of O stars in the low metallicity young star cluster NGC346 in the SMC galaxy
Massive stars are among the main cosmic engines driving the evolution of
star-forming galaxies. Their powerful ionising radiation and stellar winds
inject a large amount of energy in the interstellar medium. Furthermore,
mass-loss () through radiatively driven winds plays a key role in the
evolution of massive stars. Even so, the wind mass-loss prescriptions used in
stellar evolution models, population synthesis, and stellar feedback models
often disagree with mass-loss rates empirically measured from the UV spectra of
low metallicity massive stars. The most massive young star cluster in the low
metallicity Small Magellanic Cloud galaxy is NGC346. This cluster contains more
than half of all O stars discovered in this galaxy so far. A similar age,
metallicity (), and extinction, the O stars in the NGC346 cluster are
uniquely suited for a comparative study of stellar winds in O stars of
different subtypes. We aim to use a sample of O stars within NGC346 to study
stellar winds at low metallicity. We mapped the central 1" of NGC346 with the
long-slit UV observations performed by the Space Telescope Imaging Spectrograph
(STIS) on board of the {\em Hubble Space Telescope} and complemented these new
datasets with archival observations. Multi-epoch observations allowed for the
detection of wind variability. The UV dataset was supplemented by optical
spectroscopy and photometry. The resulting spectra were analysed using a
non-local thermal equilibrium model atmosphere code (PoWR) to determine wind
parameters and ionising fluxes. The effective mapping technique allowed us to
obtain a mosaic of almost the full extent of the cluster and resolve stars in
its core. Among hundreds of extracted stellar spectra, 21 belong to O stars.
Nine of them are classified as O stars for the first time. We analyse, in
detail, the UV spectra of 19 O stars... (continued)Comment: 33 pages, 40 figure
- …