84 research outputs found
Massive main-sequence stars evolving at the Eddington limit
Context. Massive stars play a vital role in the Universe, however, their evolution even on the main-sequence is not yet well understood.
Aims. Because of the steep mass-luminosity relation, massive main-sequence stars become extremely luminous. This brings their envelopes very close to the Eddington limit. We analyse stellar evolutionary models in which the Eddington limit is reached and exceeded, explore the rich diversity of physical phenomena that take place in their envelopes, and investigate their observational consequences.
Methods. We use published grids of detailed stellar models, computed with a state-of-the-art, one-dimensional hydrodynamic stellar evolution code using LMC composition, to investigate the envelope properties of core hydrogen burning massive stars.
Results. We find that the Eddington limit is almost never reached at the stellar surface, even for stars up to 500 M⊙. When we define an appropriate Eddington limit locally in the stellar envelope, we can show that most stars more massive than ~40 M⊙ actually exceed this limit, in particular, in the partial ionisation zones of iron, helium, or hydrogen. While most models adjust their structure such that the local Eddington limit is exceeded at most by a few per cent, our most extreme models do so by a factor of more than seven. We find that the local violation of the Eddington limit has severe consequences for the envelope structure, as it leads to envelope inflation, convection, density inversions, and, possibly to, pulsations. We find that all models with luminosities higher than 4 × 105L⊙, i.e. stars above ~40 M⊙ show inflation, with a radius increase of up to a factor of about 40. We find that the hot edge of the S Dor variability region coincides with a line beyond which our models are inflated by more than a factor of two, indicating a possible connection between S Dor variability and inflation. Furthermore, our coolest models show highly inflated envelopes with masses of up to several solar masses, and appear to be candidates for producing major luminous blue variable eruptions.
Conclusions. Our models show that the Eddington limit is expected to be reached in all stars above ~40 M⊙ in the LMC, even in lower mass stars in the Galaxy, or in close binaries or rapid rotators. While our results do not support the idea of a direct super-Eddington wind driven by continuum photons, the consequences of the Eddington limit in the form of inflation, pulsations and possibly eruptions may well give rise to a significant enhancement of the time averaged mass-loss rate
The VLT-FLAMES Tarantula Survey XVI. The optical+NIR extinction laws in 30 Doradus and the photometric determination of the effective temperatures of OB stars
Context: The commonly used extinction laws of Cardelli et al. (1989) have
limitations that, among other issues, hamper the determination of the effective
temperatures of O and early B stars from optical+NIR photometry. Aims: We aim
to develop a new family of extinction laws for 30 Doradus, check their general
applicability within that region and elsewhere, and apply them to test the
feasibility of using optical+NIR photometry to determine the effective
temperature of OB stars. Methods: We use spectroscopy and NIR photometry from
the VLT-FLAMES Tarantula Survey and optical photometry from HST/WFC3 of 30
Doradus and we analyze them with the software code CHORIZOS using different
assumptions such as the family of extinction laws. Results: We derive a new
family of optical+NIR extinction laws for 30 Doradus and confirm its
applicability to extinguished Galactic O-type systems. We conclude that by
using the new extinction laws it is possible to measure the effective
temperatures of OB stars with moderate uncertainties and only a small bias, at
least up to E(4405-5495) ~ 1.5 mag.Comment: Accepted for publication in A&A. Revised version corrects language
and fixes typos (one of them caught by David Nicholls). Figure 4 has poor
quality due to the size restrictions imposed by arXi
The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence
The evolution and fate of very massive stars (VMS) is tightly connected to
their mass-loss properties. Their initial and final masses differ significantly
as a result of mass loss. VMS have strong stellar winds and extremely high
ionising fluxes, which are thought to be critical sources of both mechanical
and radiative feedback in giant Hii regions. However, how VMS mass-loss
properties change during stellar evolution is poorly understood. In the
framework of the VLT-Flames Tarantula Survey (VFTS), we explore the mass-loss
transition region from optically thin O to denser WNh star winds, thereby
testing theoretical predictions. To this purpose we select 62 O, Of, Of/WN, and
WNh stars, an unprecedented sample of stars with the highest masses and
luminosities known. We perform a spectral analysis of optical VFTS as well as
near-infrared VLT/SINFONI data using the non-LTE radiative transfer code CMFGEN
to obtain stellar and wind parameters. For the first time, we observationally
resolve the transition between optically thin O and optically thick WNh star
winds. Our results suggest the existence of a kink between both mass-loss
regimes, in agreement with recent MC simulations. For the optically thick
regime, we confirm the steep dependence on the Eddington factor from previous
theoretical and observational studies. The transition occurs on the MS near a
luminosity of 10^6.1Lsun, or a mass of 80...90Msun. Above this limit, we find
that - even when accounting for moderate wind clumping (with f = 0.1) - wind
mass-loss rates are enhanced with respect to standard prescriptions currently
adopted in stellar evolution calculations. We also show that this results in
substantial helium surface enrichment. Based on our spectroscopic analyses, we
are able to provide the most accurate ionising fluxes for VMS known to date,
confirming the pivotal role of VMS in ionising and shaping their environments.Comment: Accepted for publication in A&A, 19 pages, 14 figures, 6 tables, (74
pages appendix, 68 figures, 4 tables
Summary of IAU GA SpS 5 II: Stellar and Wind Parameters
The development of infrared observational facilities has revealed a number of massive stars in obscured environments throughout the Milky Way and beyond. The determination of their stellar and wind properties from infrared diagnostics is thus required to take full advantage of the wealth of observations available in the near and mid infrared. However, the task is challenging. This session addressed some of the problems encountered and showed the limitations and successes of infrared studies of massive stars
The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136
VFTS 682 is located in an active star-forming region, at a projected distance
of 29 pc from the young massive cluster R136 in the Tarantula Nebula of the
Large Magellanic Cloud. It was previously reported as a candidate young stellar
object, and more recently spectroscopically revealed as a hydrogen-rich
Wolf-Rayet (WN5h) star. Our aim is to obtain the stellar properties, such as
its intrinsic luminosity, and to investigate the origin of VFTS 682. To this
purpose, we model optical spectra from the VLT-FLAMES Tarantula Survey with the
non-LTE stellar atmosphere code CMFGEN, as well as the spectral energy
distribution from complementary optical and infrared photometry. We find the
extinction properties to be highly peculiar (RV ~4.7), and obtain a
surprisingly high luminosity log(L/Lsun) = 6.5 \pm 0.2, corresponding to a
present-day mass of ~150Msun. The high effective temperature of 52.2 \pm 2.5kK
might be explained by chemically homogeneous evolution - suggested to be the
key process in the path towards long gamma-ray bursts. Lightcurves of the
object show variability at the 10% level on a timescale of years. Such changes
are unprecedented for classical Wolf-Rayet stars, and are more reminiscent of
Luminous Blue Variables. Finally, we discuss two possibilities for the origin
of VFTS 682: (i) the star either formed in situ, which would have profound
implications for the formation mechanism of massive stars, or (ii) VFTS 682 is
a slow runaway star that originated from the dense cluster R136, which would
make it the most massive runaway known to date.Comment: 5 pages, 5 figures, accepted by A&A Letter
The VLT-FLAMES Tarantula Survey IV: Candidates for isolated high-mass star formation in 30 Doradus
Whether massive stars can occasionally form in relative isolation or if they
require a large cluster of lower-mass stars around them is a key test in the
differentiation of star formation theories as well as how the initial mass
function of stars is sampled. Previous attempts to find O-type stars that
formed in isolation were hindered by the possibility that such stars are merely
runaways from clusters, i.e., their current isolation does not reflect their
birth conditions. We introduce a new method to find O-type stars that are not
affected by such a degeneracy. Using the VLT-FLAMES Tarantula Survey and
additional high resolution imaging we have identified stars that satisfy the
following constraints: 1) they are O-type stars that are not detected to be
part of a binary system based on RV time series analysis; 2) they are
designated spectral type O7 or earlier ; 3) their velocities are within 1\sigma
of the mean of OB-type stars in the 30 Doradus region, i.e. they are not
runaways along our line-of-sight; 4) the projected surface density of stars
does not increase within 3 pc towards the O-star (no evidence for clusters); 5)
their sight lines are associated with gaseous and/or dusty filaments in the
ISM, and 6) if a second candidate is found in the direction of the same
filament with which the target is associated, both are required to have similar
velocities. With these criteria, we have identified 15 stars in the 30 Doradus
region, which are strong candidates for being high-mass stars that have formed
in isolation. Additionally, we employed extensive MC stellar cluster
simulations to confirm that our results rule out the presence of clusters
around the candidates. Eleven of these are classified as Vz stars, possibly
associated with the zero-age main sequence. We include a newly discovered W-R
star as a candidate, although it does not meet all of the above criteria.Comment: 14 pages, 13 figures, 5 tables; Accepted for publication by A&
Recent star formation at low metallicities. The star-forming region NGC 346/N66 in the Small Magellanic Cloud from near-infrared VLT/ISAAC observations
Context. The emission nebula N66 is the brightest H II region in the Small Magellanic Cloud (SMC), the stellar association NGC 346 being located at its center. The youthfulness of the region NGC 346/N66 is well documented by studies of the gas and dust emission, and the detection in the optical of a rich sample of pre-main sequence (PMS) stars, and in the mid- and far-IR of young stellar objects (YSOs). However, a comprehensive study of this region has not been performed in the near-IR that would bridge the gap between previous surveys. <BR /> Aims: We perform a photometric analysis on deep, seeing-limited near-IR VLT images of the region NGC 346/N66 and a nearby control field of the SMC to locate the centers of active high- and intermediate-mass star formation by identifying near-IR bright objects as candidate stellar sources under formation. <BR /> Methods: We use archival imaging data obtained with the high-resolution camera ISAAC at VLT of NGC 346/N66 to construct the near-IR color-magnitude (CMD) and color-color diagrams (C-CD) of all detected sources. We investigate the nature of all stellar populations in the observed CMDs, and we identify all stellar sources that show significant near-IR excess emission in the observed C-CD. We, thus, select the most likely young stellar sources. <BR /> Results: Based on their near-IR colors, we select 263 candidate young stellar sources. This sample comprises a variety of objects, such as intermediate-mass PMS and Herbig Ae/Be stars and possible massive YSOs, providing original near-IR colors for each of them. The spatial distribution of the selected candidate sources indicates that they are located along the dusty filamentary structures of N66 seen in mid- and far-IR dust emission and agrees very well with that of previously detected candidate YSOs and PMS stars. <BR /> Conclusions: Our study provides an original accurate set of near-IR colors for candidate young stellar sources. This provides significant information about the star formation process in NGC 346/N66, but does not establish the types of these objects, which requires the construction of complete spectral energy distributions for individual sources from multiwavelength data. This would be an important follow-up study to that presented here. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program ID 063.I-0329.Table 1 is available in its entirety only in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A href="http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A56">http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A56</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
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