The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analog of large star-formation events in the distant universe. We determined the recent formation history and the initial ...mass function (IMF) of massive stars in 30 Doradus on the basis of spectroscopic observations of 247 stars more massive than 15 solar masses (Formula: see text). The main episode of massive star formation began about 8 million years (My) ago, and the star-formation rate seems to have declined in the last 1 My. The IMF is densely sampled up to 200 Formula: see text and contains 32 ± 12% more stars above 30 Formula: see text than predicted by a standard Salpeter IMF. In the mass range of 15 to 200 Formula: see text, the IMF power-law exponent is Formula: see text, shallower than the Salpeter value of 2.35.
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Context.
It has recently been proposed that LB-1 is a binary system at 4 kpc consisting of a B-type star of 8
M
⊙
and a massive stellar black hole (BH) of 70
M
⊙
. This finding challenges our current ...theories of massive star evolution and formation of BHs at solar metallicity.
Aims.
Our objective is to derive the effective temperature, surface gravity, and chemical composition of the B-type component in order to determine its nature and evolutionary status and, indirectly, to constrain the mass of the BH.
Methods.
We use the non-LTE stellar atmosphere code
FASTWIND
to analyze new and archival high-resolution data.
Results.
We determine (
T
eff
, log
g
) values of (14 000 ± 500 K, 3.50 ± 0.15 dex) that, combined with the
Gaia
parallax, imply a spectroscopic mass, from log
g
, of 3.2
+2.1
−1.9
M
⊙
and an evolutionary mass, assuming single star evolution, of 5.2
+0.3
−0.6
M
⊙
. We determine an upper limit of 8 km s
−1
for the projected rotational velocity and derive the surface abundances; we find the star to have a silicon abundance below solar, and to be significantly enhanced in nitrogen and iron and depleted in carbon and magnesium. Complementary evidence derived from a photometric extinction analysis and
Gaia
yields similar results for
T
eff
and log
g
and a consistent distance around 2 kpc.
Conclusions.
We propose that the B-type star is a slightly evolved main sequence star of 3–5
M
⊙
with surface abundances reminiscent of diffusion in late B/A chemically peculiar stars with low rotational velocities. There is also evidence for CN-processed material in its atmosphere. These conclusions rely critically on the distance inferred from the
Gaia
parallax. The goodness of fit of the
Gaia
astrometry also favors a high-inclination orbit. If the orbit is edge-on and the B-type star has a mass of 3–5
M
⊙
, the mass of the dark companion would be 4–5
M
⊙
, which would be easier to explain with our current stellar evolutionary models.
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The VLT-FLAMES Tarantula Survey Dunstall, P R; Dufton, P L; Sana, H ...
Astronomy and astrophysics (Berlin),
8/2015, Volume:
580
Journal Article
Peer reviewed
Open access
We investigate the multiplicity properties of 408 B-type stars observed in the 30 Doradus region of the Large Magellanic Cloud with multi-epoch spectroscopy from the VLT-FLAMES Tarantula Survey ...(VFTS). We use a cross-correlation method to estimate relative radial velocities from the helium and metal absorption lines for each of our targets. Objects with significant radial-velocity variations (and with an amplitude larger than 16 km s super(-1)) are classified as spectroscopic binaries. We find an observed spectroscopic binary fraction (defined by periods of <10 super(3,5)d and mass ratios >0.1) for the B-type stars, (obs) = 0.25 + or - 0.02, which appears constant across the field of view, except for the two older clusters (Hodge 301 and SL639). These two clusters have significantly lower binary fractions of 0.08 + or - 0.08 and 0.10 + or - 0.09, respectively. Using synthetic populations and a model of our observed epochs and their potential biases, we constrain the intrinsic multiplicity properties of the dwarf and giant (i.e. relatively unevolved) B-type stars in 30 Dor. We obtain a present-day binary fraction f sub(B)(true) = 0.58 + or - 0.11, with a flat period distribution. Within the uncertainties, the multiplicity properties of the B-type stars agree with those for the O stars in 30 Dor from the VFTS.
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We present an analysis of high resolution VLT-FLAMES spectra of 61 B-type stars with relatively narrow-lined spectra located in 4 fields centered on the Milky Way clusters; NGC 3293 and NGC 4755 and ...the Large and Small Magellanic cloud clusters; NGC 2004 and NGC 330. For each object a quantitative analysis was carried out using the non-LTE model atmosphere code TLUSTY; resulting in the determination of their atmospheric parameters and photospheric abundances of the dominant metal species (C, N, O, Mg, Si, Fe). The results are discussed in relation to our earlier work on 3 younger clusters in these galaxies; NGC 6611, N11 and NGC 346 paying particular attention to the nitrogen abundances which are an important probe of the role of rotation in the evolution of stars. This work along with that of the younger clusters provides a consistent dataset of abundances and atmospheric parameters for over 100 B-type stars in the three galaxies. We provide effective temperature scales for B-type dwarfs in all three galaxies and for giants and supergiants in the SMC and LMC. In each galaxy a dependence on luminosity is found between the three classes with the unevolved dwarf objects having significantly higher effective temperatures. A metallicity dependence is present between the SMC and Galactic dwarf objects, and whilst the LMC stars are only slightly cooler than the SMC stars, they are significantly hotter than their Galactic counterparts.
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Rotation has become an important element in evolutionary models of massive stars, specifically via the prediction of rotational mixing. Here we study a sample of stars, including rapid rotators, to ...constrain such models and use nitrogen enrichments as a probe of the mixing process. Chemical compositions (C, N, O, Mg, and Si) have been estimated for 135 early B-type stars in the Large Magellanic Cloud with projected rotational velocities up to similar to 300 km s-1 using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including rotational mixing, have been generated attempting to reproduce these observations by adjusting the overshooting and rotational mixing parameters and produce reasonable agreement with 60% of our core hydrogen burning sample. We find (excluding known binaries) a significant population of highly nitrogen-enriched intrinsic slow rotators (vsiu unk km s-1) incompatible with our models ( similar to 20% of the sample). Furthermore, while we find fast rotators with enrichments in agreement with the models, the observation of evolved (logg < 3.7 dex) fast rotators that are relatively unenriched (a further similar to 20% of the sample) challenges the concept of rotational mixing. We also find that 70% of our blue supergiant sample cannot have evolved directly from the hydrogen-burning main sequence. We are left with a picture where invoking blnarity and perhaps fossil magnetic fields is required to understand the surface properties of a population of massive main-sequence stars.
We present an analysis of high-resolution FLAMES spectra of approximately 50 early B-type stars in three young clusters at different metallicities, NGC 6611 in the Galaxy, N 11 in the Large ...Magellanic Cloud (LMC) and NGC 346 in the Small Magellanic Cloud (SMC). Using the tlusty non-LTE model atmospheres code, atmospheric parameters and photospheric abundances (C, N, O, Mg and Si) of each star have been determined. These results represent a significant improvement on the number of Magellanic Cloud B-type stars with detailed and homogeneous estimates of their atmospheric parameters and chemical compositions. The relationships between effective temperature and spectral type are discussed for all three metallicity regimes, with the effective temperature for a given spectral type increasing as one moves to a lower metallicity regime. Additionally the difficulties in estimating the microturbulent velocity and the anomalous values obtained, particularly in the lowest metallicity regime, are discussed. Our chemical composition estimates are compared with previous studies, both stellar and interstellar with, in general, encouraging agreement being found. Abundances in the Magellanic Clouds relative to the Galaxy are discussed and we also present our best estimates of the base-line chemical composition of the LMC and SMC as derived from B-type stars. Additionally we discuss the use of nitrogen as a probe of the evolutionary history of stars, investigating the roles of rotational mixing, mass-loss, blue loops and binarity on the observed nitrogen abundances and making comparisons with stellar evolutionary models where possible.
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Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small Magellanic Cloud has been combined with that for 116 targets from the VLT-FLAMES Survey of Massive Stars. Spectral ...classification yields a sample of 47 O-type and 287 B-type spectra, while radial-velocity variations and/or spectral multiplicity have been used to identify 45 candidate single-lined (SB1) systems, 17 double-lined (SB2) systems, and one triple-lined (SB3) system. Atmospheric parameters (Teff and log g) and projected rotational velocities (ve sin i) have been estimated using TLUSTY model atmospheres; independent estimates of ve sin i were also obtained using a Fourier Transform method. Luminosities have been inferred from stellar apparent magnitudes and used in conjunction with the Teff and ve sin i estimates to constrain stellar masses and ages using the BONNSAI package. We find that targets towards the inner region of NGC 346 have higher median masses and projected rotational velocities, together with smaller median ages than the rest of the sample. There appears to be a population of very young targets with ages of less than 2 Myr, which have presumably all formed within the cluster. The more massive targets are found to have lower projected rotational velocities consistent with previous studies. No significant evidence is found for differences with metallicity in the stellar rotational velocities of early-type stars, although the targets in the Small Magellanic Cloud may rotate faster than those in young Galactic clusters. The rotational velocity distribution for single non-supergiant B-type stars is inferred and implies that a significant number have low rotational velocity (≃10% with ve < 40 km s−1), together with a peak in the probability distribution at ve≃ 300 km s−1. Larger projected rotational velocity estimates have been found for our Be-type sample and imply that most have rotational velocities between 200–450 km s−1.
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ABSTRACT
We present results from the B-type binaries characterization (BBC) programme, a multi-epoch spectroscopic study of 88 early B-type binary candidates in the 30 Doradus region of the Large ...Magellanic Cloud (LMC). From radial-velocity analysis of 29 observational epochs, we confirm the binary status of 64 of our targets, comprising 50 SB1 and 14 SB2 B-type binaries. A further 20 systems (classified as SB1*) show clear signs of periodicity but with more tentative periods. Orbital solutions are presented for these 84 systems, providing the largest homogeneous sample to date of the binary properties of early B-type stars. Our derived orbital-period distribution is generally similar to those for samples of more massive (O-type) binaries in both the LMC and the Galaxy. This similarity with the properties of the more massive O-type binaries is important as early B-type stars are expected to account for the majority of core-collapse supernovae. Differences in the period distributions of the different samples start to increase above 4 d, and are also present between the earliest (B0-0.7) and later-type (B1-2.5) systems within the BBC sample, although further study is required to understand if this is an observational bias or a real physical effect. We have examined the semi-amplitude velocities and orbital periods of our sample to identify potential candidates that could hide compact companions. Comparing with probability distributions of finding black hole companions to OB-type stars from a recent theoretical study, we have found 16 binaries in the higher probability region that warrant further study.
The VLT-FLAMES Tarantula Survey Ramirez-Agudelo, O H; Simon-Diaz, S; Sana, H ...
Astronomy and astrophysics (Berlin),
12/2013, Volume:
560
Journal Article
Peer reviewed
The 30 Doradus (30 Dor) region of the Large Magellanic Cloud, also known as the Tarantula nebula, is the nearest starburst region. It contains the richest population of massive stars in the Local ...Group, and it is thus the best possible laboratory to investigate open questions on the formation and evolution of massive stars. Using ground-based multi-object optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to establish the (projected) rotational velocity distribution for a sample of 216 presumably single O-type stars in 30 Dor. The presence of a sizeable population of fast rotators is compatible with recent population synthesis computations that investigate the influence of binary evolution on the rotation rate of massive stars. Even though we have excluded stars that show significant radial velocity variations, our sample may have remained contaminated by post-interaction binary products. That the high-velocity tail may be populated primarily (and perhaps exclusively) by post-binary interaction products has important implications for the evolutionary origin of systems that produce gamma-ray bursts.
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