40 research outputs found
The VLT-FLAMES Tarantula Survey V. The peculiar B[e]-like supergiant, VFTS698, in 30 Doradus
We present an analysis of a peculiar supergiant B-type star (VFTS698/Melnick
2/Parker 1797) in the 30 Doradus region of the Large Magellanic Cloud which
exhibits characteristics similar to the broad class of B[e] stars. We analyse
optical spectra from the VLT-FLAMES survey, together with archival optical and
infrared photometry and X-ray imaging to characterise the system. We find
radial velocity variations of around 400 km/s in the high excitation Si IV, N
III and He II spectra, and photometric variability of ~0.6 mag with a period of
12.7 days. In addition, we detect long-term photometric variations of ~0.25
mag, which may be due to a longer-term variability with a period of ~400 days.
We conclude that VFTS698 is likely an interacting binary comprising an early
B-type star secondary orbiting a veiled, more massive companion. Spectral
evidence suggests a mid-to-late B-type primary, but this may originate from an
optically-thick accretion disc directly surrounding the primary.Comment: 18 pages, 14 figures and 8 tables. Table 8 to be published onlin
The VLT-FLAMES Tarantula Survey XXII. Multiplicity properties of the B-type stars
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) are
classified as spectroscopic binaries. We find an observed spectroscopic binary
fraction (defined by periods of 0.1) for the B-type
stars, f_B(obs) = 0.25 +/- 0.02, which appears constant across the field of
view, except for the two older clusters (Hodge 301 and SL 639). These two
clusters have significantly lower fractions of 0.08 +/- 0.08 and 0.10 +/- 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_B(true) = 0.58 +/- 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.Comment: Accepted by A&
The VLT-FLAMES Tarantula Survey XXIII. Two massive double-lined binaries in 30 Doradus
Aims. We investigate the characteristics of two newly discovered short-period, double-lined, massive binary systems in the Large
Magellanic Cloud, VFTS 450 (O9.7 II–Ib + O7::) and VFTS 652 (B1 Ib + O9: III:).
Methods. We perform model-atmosphere analyses to characterise the photospheric properties of both members of each binary (denoting the “primary” as the spectroscopically more conspicuous component). Radial velocities and optical photometry are used to estimate the binary-system parameters.
Results. We estimate Teff = 27 kK, log g = 2.9 (cgs) for the VFTS 450 primary spectrum (34 kK, 3.6: for the secondary spectrum); and Teff = 22 kK, log g = 2.8 for the VFTS 652 primary spectrum (35 kK, 3.7: for the secondary spectrum). Both primaries show surface nitrogen enrichments (of more than 1 dex for VFTS 652), and probable moderate oxygen depletions relative to reference LMC abundances. We determine orbital periods of 6.89 d and 8.59 d for VFTS 450 and VFTS 652, respectively, and argue that the primaries must be close to filling their Roche lobes. Supposing this to be the case, we estimate component masses in the range ∼20–50 M⊙.
Conclusions. The secondary spectra are associated with the more massive components, suggesting that both systems are high-mass analogues of classical Algol systems, undergoing case-A mass transfer. Difficulties in reconciling the spectroscopic analyses with the light-curves and with evolutionary considerations suggest that the secondary spectra are contaminated by (or arise in) accretion disks
The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars
Aims: Projected rotational velocities (\vsini) have been estimated for 334
targets in the VLT-FLAMES Tarantula survey that do not manifest significant
radial velocity variations and are not supergiants. They have spectral types
from approximately O9.5 to B3. The estimates have been analysed to infer the
underlying rotational velocity distribution, which is critical for
understanding the evolution of massive stars.
Methods: Projected rotational velocities were deduced from the Fourier
transforms of spectral lines, with upper limits also being obtained from
profile fitting. For the narrower lined stars, metal and non-diffuse helium
lines were adopted, and for the broader lined stars, both non-diffuse and
diffuse helium lines; the estimates obtained using the different sets of lines
are in good agreement. The uncertainty in the mean estimates is typically 4%
for most targets. The iterative deconvolution procedure of Lucy has been used
to deduce the probability density distribution of the rotational velocities.
Results: Projected rotational velocities range up to approximately 450 \kms
and show a bi-modal structure. This is also present in the inferred rotational
velocity distribution with 25% of the sample having \ve100\,\kms
and the high velocity component having \ve\,\kms. There is no
evidence from the spatial and radial velocity distributions of the two
components that they represent either field and cluster populations or
different episodes of star formation. Be-type stars have also been identified.
Conclusions: The bi-modal rotational velocity distribution in our sample
resembles that found for late-B and early-A type stars. While magnetic braking
appears to be a possible mechanism for producing the low-velocity component, we
can not rule out alternative explanations.Comment: to be publisged in A&
The VLT-FLAMES Tarantula Survey XIX. B-type Supergiants - Atmospheric parameters and nitrogen abundances to investigate the role of binarity and the width of the main sequence
TLUSTY non-LTE model atmosphere calculations have been used to determine
atmospheric parameters and nitrogen (N) abundances for 34 single and 18 binary
B-type supergiants (BSGs). The effects of flux contribution from an unseen
secondary were considered for the binary sample. We present the first
systematic study of the incidence of binarity for a sample of BSGs across the
theoretical terminal age main sequence (TAMS). To account for the distribution
of effective temperatures of the BSGs it may be necessary to extend the TAMS to
lower temperatures. This is consistent with the derived distribution of mass
discrepancies, projected rotational velocities (vsini) and N abundances,
provided that stars cooler than this temperature are post RSG objects. For the
BSGs in the Tarantula and previous FLAMES surveys, most have small vsini. About
10% have larger vsini (>100 km/s) but surprisingly these show little or no N
enhancement. All the cooler BSGs have low vsini of <70km/s and high N abundance
estimates, implying that either bi-stability braking or evolution on a blue
loop may be important. A lack of cool binaries, possibly reflects the small
sample size. Single star evolutionary models, which include rotation, can
account for the N enhancement in both the single and binary samples. The
detailed distribution of N abundances in the single and binary samples may be
different, possibly reflecting differences in their evolutionary history. The
first comparative study of single and binary BSGs has revealed that the main
sequence may be significantly wider than previously assumed, extending to
Teff=20000K. Some marginal differences in single and binary atmospheric
parameters and abundances have been identified, possibly implying non-standard
evolution for some of the sample. This sample as a whole has implications for
several aspects of our understanding of the evolution of BSGs. Full abstract in
paperComment: 21 pages, 15 figures, 11 table
The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds
Aims. We compare the predictions of evolutionary models for early-type stars
with atmospheric parameters, projected rotational velocities and nitrogen
abundances estimated for a sample of Be-type stars. Our targets are located in
4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11
region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346.
Methods. Atmospheric parameters and photospheric abundances have been
determined using the non-LTE atmosphere code tlusty. Effective temperature
estimates were deduced using three different methodologies depending on the
spectral features observed; in general they were found to yield consistent
estimates. Gravities were deduced from Balmer line profiles and
microturbulences from the Si iii spectrum. Additionally the contributions of
continuum emission from circumstellar discs were estimated. Given its
importance in constraining stellar evolutionary models, nitrogen abundances (or
upper limits) were deduced for all the stars analysed. Results. Our nitrogen
abundances are inconsistent with those predicted for targets spending most of
their main sequence life rotating near to the critical velocity. This is
consistent with the results we obtain from modelling the inferred rotational
velocity distribution of our sample and of other investigators. We consider a
number of possibilities to explain the nitrogen abundances and rotational
velocities of our Be-type sample.Comment: 14 pages, 9 figures, submitted to A&
The VLT-FLAMES Tarantula Survey XV. VFTS 822: A candidate Herbig B[e] star at low metallicity
We report the discovery of the B[e] star VFTS 822 in the 30 Doradus star-forming region of the Large Magellanic Cloud, classified by optical spectroscopy from the VLT-FLAMES Tarantula Survey and complementary infrared photometry. VFTS 822 is a relatively low-luminosity (log L = 4.04 ± 0.25 L⊙) B8[e] star. In this Letter, we evaluate the evolutionary status of VFTS 822 and discuss its
candidacy as a Herbig B[e] star. If the object is indeed in the pre-main sequence phase, it would present an exciting opportunity to
spectroscopically measure mass accretion rates at low metallicity, to probe the effect of metallicity on accretion rates
The VLT-FLAMES Tarantula Survey XVIII. Classifications and radial velocities of the B-type stars
We present spectral classifications for 438 B-type stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud. Radial velocities are provided for 307 apparently single stars, and for 99 targets with radial-velocity variations which are consistent with them being spectroscopic binaries. We investigate the spatial distribution of the radial velocities across the 30 Dor region, and use the results to identify candidate runaway stars. Excluding potential runaways and members of two older clusters in the survey region (SL 639 and Hodge 301), we determine a systemic velocity for 30 Dor of 271.6 ± 12.2 kms-1 from 273 presumed single stars. Employing a 3σ criterion we identify nine candidate runaway stars (2.9% of the single stars with radial-velocity estimates). The projected rotational velocities of the candidate runaways appear to be significantly different to those of the full B-type sample, with a strong preference for either large (≥345 kms-1) or small (≤65 kms-1) rotational velocities. Of the candidate runaways, VFTS 358 (classified B0.5: V) has the largest differential radial velocity (−106.9 ± 16.2 kms-1), and a preliminary atmospheric analysis finds a significantly enriched nitrogen abundance of 12 + log (N/H) ≳ 8.5. Combined with a large rotational velocity (ve sin i = 345 ± 22 kms-1), this is suggestive of past binary interaction for this star
The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
The quantitative study of the physical properties and chemical abundances of
large samples of massive blue stars at different metallicities is a powerful
tool to understand the nature and evolution of these objects. Their analysis
beyond the Milky Way is challenging, nonetheless it is doable and the best way
to investigate their behavior in different environments. Fulfilling this task
in an objective way requires the implementation of automatic analysis
techniques that can perform the analyses systematically, minimizing at the same
time any possible bias.
As part of the ARAUCARIA project we carry out the first quantitative
spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55
at 1.94 Mpc away. By applying the methodology developed in this work, we derive
their stellar parameters, chemical abundances and provide a characterization of
the present-day metallicity of their host galaxy.
Based on the characteristics of the stellar atmosphere/line formation code
FASTWIND, we designed and created a grid of models for the analysis of massive
blue supergiant stars. Along with this new grid, we implemented a spectral
analysis algorithm. Both tools were specially developed to perform fully
consistent quantitative spectroscopic analyses of low spectral resolution of
B-type supergiants in a fast and objective way.
We present the main characteristics of our FASTWIND model grid and perform a
number of tests to investigate the reliability of our methodology. The
automatic tool is applied afterward to a sample of 12 B-type supergiant stars
in NGC55, deriving the stellar parameters and abundances. The results indicate
that our stars are part of a young population evolving towards a red supergiant
phase. The derived chemical composition hints to an average metallicity similar
to the one of the Large Magellanic Cloud, with no indication of a spatial trend
across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&
The VLT-FLAMES Tarantula Survey I: Introduction and observational overview
The VLT-FLAMES Tarantula Survey (VFTS) is an ESO Large Programme that has
obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30
Doradus region of the Large Magellanic Cloud (LMC). Here we introduce our
scientific motivations and give an overview of the survey targets, including
optical and near-infrared photometry and comprehensive details of the data
reduction. One of the principal objectives was to detect massive binary systems
via variations in their radial velocities, thus shaping the multi-epoch
observing strategy. Spectral classifications are given for the massive
emission-line stars observed by the survey, including the discovery of a new
Wolf-Rayet star (VFTS 682, classified as WN5h), 2' to the northeast of R136. To
illustrate the diversity of objects encompassed by the survey, we investigate
the spectral properties of sixteen targets identified by Gruendl & Chu from
Spitzer photometry as candidate young stellar objects or stars with notable
mid-infrared excesses. Detailed spectral classification and quantitative
analysis of the O- and B-type stars in the VFTS sample, paying particular
attention to the effects of rotational mixing and binarity, will be presented
in a series of future articles to address fundamental questions in both stellar
and cluster evolution.Comment: Accepted by A&A, 52 pages (main body: 19 pages, supplementary tables:
33 pages), v3: two classifications updated to match a parallel pape