47 research outputs found
The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries
The initial distribution of spin rates of massive stars is a fingerprint of
their elusive formation process. It also sets a key initial condition for
stellar evolution and is thus an important ingredient in stellar population
synthesis. So far, most studies have focused on single stars. Most O stars are
however found in multiple systems. By establishing the spin-rate distribution
of a sizeable sample of O-type spectroscopic binaries and by comparing the
distributions of binary sub-populations with one another as well as with that
of presumed single stars in the same region, we aim to constrain the initial
spin distribution of O stars in binaries, and to identify signatures of the
physical mechanisms that affect the evolution of the massive stars spin rates.
We use ground-based optical spectroscopy obtained in the framework of the
VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial
rotational velocities (\vrot) for components of 114 spectroscopic binaries in
30 Doradus. The \vrot\ values are derived from the full-width at half-maximum
(FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we
derive based on previous line analysis methods applied to single O-type stars
in the VFTS sample. The overall \vrot\ distribution of the primary stars
resembles that of single O-type stars in the VFTS, featuring a low-velocity
peak (at \vrot < 200 kms) and a shoulder at intermediate velocities (200 <
\vrot < 300 kms). The distributions of binaries and single stars however
differ in two ways. First, the main peak at \vrot \sim100 kms is broader and
slightly shifted toward higher spin rates in the binary distribution compared
to that of the presumed-single stars. Second, the \vrot distribution of
primaries lacks a significant population of stars spinning faster than 300 kms
while such a population is clearly present in the single star sample.Comment: 16 pages, 16 figures, paper accepted in Astronomy & Astrophysic
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&
Rotational properties of the O-type star population in the Tarantula region
The 30 Doradus (30\,Dor) region in the Large Magellanic Cloud (also known as
the Tarantula Nebula) is the nearest massive starburst region, containing the
richest sample of massive stars in the Local Group. It is the best possible
laboratory to investigate aspects of the formation and evolution of massive
stars. Here, we focus on rotation which is a key parameter in the evolution of
these objects. We establish the projected rotational velocity, ,
distribution of an unprecedented sample of 216 radial velocity constant
() O-type stars in 30\,Dor observed in
the framework of the VLT-FLAMES Tarantula Survey (VFTS). The distribution of
shows a two-component structure: a peak around 80 and a high-velocity tail extending up to 600 .
Around 75% of the sample has 0 200
with the other 25% distributed in the high-velocity tail. The presence of the
low-velocity peak is consistent with that found in other studies of late-O and
early-B stars. The high-velocity tail is compatible with expectations from
binary interaction synthesis models and may be predominantly populated by
post-binary interaction, spun-up, objects and mergers. This may have important
implications for the nature of progenitors of long-duration gamma ray bursts.Comment: 4 pages, 1 figure. Conference proceedings article: Massive stars:
from alpha to Omega, 10-14 June 2013, Rhodes, Greec
Rotational velocities of single and binary O-type stars in the Tarantula Nebula
Rotation is a key parameter in the evolution of massive stars, affecting
their evolution, chemical yields, ionizing photon budget, and final fate. We
determined the projected rotational velocity, , of 330 O-type
objects, i.e. 210 spectroscopic single stars and 110 primaries in
binary systems, in the Tarantula nebula or 30 Doradus (30\,Dor) region. The
observations were taken using VLT/FLAMES and constitute the largest homogeneous
dataset of multi-epoch spectroscopy of O-type stars currently available. The
most distinctive feature of the distributions of the
presumed-single stars and primaries in 30 Dor is a low-velocity peak at around
100\,. Stellar winds are not expected to have spun-down the
bulk of the stars significantly since their arrival on the main sequence and
therefore the peak in the single star sample is likely to represent the outcome
of the formation process. Whereas the spin distribution of presumed-single
stars shows a well developed tail of stars rotating more rapidly than
300\,, the sample of primaries does not feature such a
high-velocity tail. The tail of the presumed-single star distribution is
attributed for the most part -- and could potentially be completely due -- to
spun-up binary products that appear as single stars or that have merged. This
would be consistent with the lack of such post-interaction products in the
binary sample, that is expected to be dominated by pre-interaction systems. The
peak in this distribution is broader and is shifted toward somewhat higher spin
rates compared to the distribution of presumed-single stars. Systems displaying
large radial velocity variations, typical for short period systems, appear
mostly responsible for these differences.Comment: 6 pages, 3 figures, Proceedings IAU Symposium No. 307, 2014, 'New
windows on massive stars: asteroseismology, interferometry, and
spectropolarimetry
The VLT-FLAMES Tarantula Survey: XXV. Surface nitrogen abundances of O-type giants and supergiants
Context. Theoretically, rotation-induced chemical mixing in massive stars has far reaching evolutionary consequences, affecting the
sequence of morphological phases, lifetimes, nucleosynthesis, and supernova characteristics.
Aims. Using a sample of 72 presumably single O-type giants to supergiants observed in the context of the VLT-FLAMES Tarantula
Survey (VFTS), we aim to investigate rotational mixing in evolved core-hydrogen burning stars initially more massive than 15 M� by
analysing their surface nitrogen abundances.
Methods. Using stellar and wind properties derived in a previous VFTS study we computed synthetic spectra for a set of up to
21 N ii-v lines in the optical spectral range, using the non-LTE atmosphere code FASTWIND. We constrained the nitrogen abundance
by fitting the equivalent widths of relatively strong lines that are sensitive to changes in the abundance of this element. Given the quality
of the data, we constrained the nitrogen abundance in 38 cases; for 34 stars only upper limits could be derived, which includes almost
all stars rotating at 3e sin i > 200 km s−1
.
Results. We analysed the nitrogen abundance as a function of projected rotation rate 3e sin i and confronted it with predictions of rotational
mixing. We found a group of N-enhanced slowly-spinning stars that is not in accordance with predictions of rotational mixing
in single stars. Among O-type stars with (rotation-corrected) gravities less than log gc = 3.75 this group constitutes 30−40 percent
of the population. We found a correlation between nitrogen and helium abundance which is consistent with expectations, suggesting
that, whatever the mechanism that brings N to the surface, it displays CNO-processed material. For the rapidly-spinning O-type stars
we can only provide upper limits on the nitrogen abundance, which are not in violation with theoretical expectations. Hence, the data
cannot be used to test the physics of rotation induced mixing in the regime of high spin rates.
Conclusions. While the surface abundances of 60−70 percent of presumed single O-type giants to supergiants behave in conformity
with expectations, at least 30−40 percent of our sample can not be understood in the current framework of rotational mixing for single
stars. Even though we have excluded stars showing radial velocity variations, of our sample may have remained contaminated by postinteraction
binary products. Hence, it is plausible that effects of binary interaction need to be considered to understand their surface
properties. Alternatively, or in conjunction, the effects of magnetic fields or alternative mass-loss recipes may need to be invoked
The VLT-FLAMES Tarantula Survey
Context. The Tarantula region in the Large Magellanic Cloud (LMC) contains the richest population of spatially resolved massive O-type stars known so far. This unmatched sample offers an opportunity to test models describing their main-sequence evolution and mass-loss properties.
Aims. Using ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to determine stellar, photospheric and wind properties of 72 presumably single O-type giants, bright giants and supergiants and to confront them with predictions of stellar evolution and of line-driven mass-loss theories.
Methods. We apply an automated method for quantitative spectroscopic analysis of O stars combining the non-LTE stellar atmosphere model fastwind with the genetic fitting algorithm pikaia to determine the following stellar properties: effective temperature, surface gravity, mass-loss rate, helium abundance, and projected rotational velocity. The latter has been constrained without taking into account the contribution from macro-turbulent motions to the line broadening.
Results. We present empirical effective temperature versus spectral subtype calibrations at LMC-metallicity for giants and supergiants. The calibration for giants shows a +1kK offset compared to similar Galactic calibrations; a shift of the same magnitude has been reported for dwarfs. The supergiant calibrations, though only based on a handful of stars, do not seem to indicate such an offset. The presence of a strong upturn at spectral type O3 and earlier can also not be confirmed by our data. In the spectroscopic and classical Hertzsprung-Russell diagrams, our sample O stars are found to occupy the region predicted to be the core hydrogen-burning phase by state-of-the-art models. For stars initially more massive than approximately 60 M⊙, the giant phase already appears relatively early on in the evolution; the supergiant phase develops later. Bright giants, however, are not systematically positioned between giants and supergiants at Minit ≳ 25 M⊙. At masses below 60 M⊙, the dwarf phase clearly precedes the giant and supergiant phases; however this behavior seems to break down at Minit ≲ 18 M⊙. Here, stars classified as late O III and II stars occupy the region where O9.5-9.7 V stars are expected, but where few such late O V stars are actually seen. Though we can not exclude that these stars represent a physically distinct group, this behavior may reflect an intricacy in the luminosity classification at late O spectral subtype. Indeed, on the basis of a secondary classification criterion, the relative strength of Si iv to He i absorption lines, these stars would have been assigned a luminosity class IV or V. Except for five stars, the helium abundance of our sample stars is in agreement with the initial LMC composition. This outcome is independent of their projected spin rates. The aforementioned five stars present moderate projected rotational velocities (i.e., νesini < 200kms-1) and hence do not agree with current predictions of rotational mixing in main-sequence stars. They may potentially reveal other physics not included in the models such as binary-interaction effects. Adopting theoretical results for the wind velocity law, we find modified wind momenta for LMC stars that are ~0.3 dex higher than earlier results. For stars brighter than 105 L⊙, that is, in the regime of strong stellar winds, the measured (unclumped) mass-loss rates could be considered to be in agreement with line-driven wind predictions if the clump volume filling factors were fV ~ 1/8 to 1/6
The VLT-FLAMES Tarantula survey VIII. Multiplicity properties of the O-type star population
Context. The Tarantula Nebula in the Large Magellanic Cloud is our closest view of a starburst region and is the ideal environment to investigate important questions regarding the formation, evolution and final fate of the most massive stars.
Aims. We analyze the multiplicity properties of the massive O-type star population observed through multi-epoch spectroscopy in the framework of the VLT-FLAMES Tarantula Survey. With 360 O-type stars, this is the largest homogeneous sample of massive stars analyzed to date.
Methods. We use multi-epoch spectroscopy and variability analysis to identify spectroscopic binaries. We also use a Monte-Carlo method to correct for observational biases. By modeling simultaneously the observed binary fraction, the distributions of the amplitudes of the radial velocity variations and the distribution of the time scales of these variations, we constrain the intrinsic current binary fraction and period and mass-ratio distributions.
Results. We observe a spectroscopic binary fraction of 0.35 ± 0.03, which corresponds to the fraction of objects displaying statistically significant radial velocity variations with an amplitude of at least 20 km s-1. We compute the intrinsic binary fraction to be 0.51 ± 0.04. We adopt power-laws to describe the intrinsic period and mass-ratio distributions: f(log 10P/d) ~ (log 10P/d)π (with log 10P/d in the range 0.15−3.5) and f(q) ~ qκ with 0.1 ≤ q = M2/M1 ≤ 1.0. The power-law indexes that best reproduce the observed quantities are π = −0.45 ± 0.30 and κ = −1.0 ± 0.4. The period distribution that we obtain thus favours shorter period systems compared to an Öpik law (π = 0). The mass ratio distribution is slightly skewed towards low mass ratio systems but remains incompatible with a random sampling of a classical mass function (κ = −2.35). The binary fraction seems mostly uniform across the field of view and independent of the spectral types and luminosity classes. The binary fraction in the outer region of the field of view (r > 7.8′, i.e. ≈117 pc) and among the O9.7 I/II objects are however significantly lower than expected from statistical fluctuations. The observed and intrinsic binary fractions are also lower for the faintest objects in our sample (Ks > 15.5 mag), which results from observational effects and the fact that our O star sample is not magnitude-limited but is defined by a spectral-type cutoff. We also conclude that magnitude-limited investigations are biased towards larger binary fractions.
Conclusions. Using the multiplicity properties of the O stars in the Tarantula region and simple evolutionary considerations, we estimate that over 50% of the current O star population will exchange mass with its companion within a binary system. This shows that binary interaction is greatly affecting the evolution and fate of massive stars, and must be taken into account to correctly interpret unresolved populations of massive stars
An excess of massive stars in the local 30 Doradus starburst
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