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تسجيل مستخدم جديدThe VLT-FLAMES Tarantula Survey XXV. Surface nitrogen abundances of O-type giants and supergiants
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Theoretically, rotation-induced chemical mixing in massive stars has far reaching evolutionary consequences, affecting the sequence of morphological phases, lifetimes, nucleosynthesis, and supernova characteristics. 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 $15,M_odot$ by analysing their surface nitrogen abundances. Using stellar and wind properties derived in a previous VFTS study, 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 $v_mathrm{e}sin i >200,mathrm{km s^{-1}}$. We analysed the nitrogen abundance as a function of projected rotation rate $v_mathrm{e}sin i$ and confronted it with predictions of rotational mixing. The upper limits on the nitrogen abundance of the rapidly rotating stars are not in apparent violation with theoretical expectations. However, 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 $log,g_c = 3.75$ 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.
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The Tarantula region in the Large Magellanic Cloud 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-l
oss properties. 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. We apply an automated method for quantitative spectroscopic analysis of O stars combining the non-LTE stellar atmosphere model {{sc fastwind}} with the genetic fitting algorithm {{sc pikaia}} to determine the following stellar properties: effective temperature, surface gravity, mass-loss rate, helium abundance, and projected rotational velocity. We present empirical effective temperature versus spectral subtype calibrations at LMC-metallicity for giants and supergiants. 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 Brott et al. (2011) and K{o}hler et al. (2015). Except for five stars, the helium abundance of our sample stars is in agreement with the initial LMC composition. The aforementioned five stars present moderate projected rotational velocities (i.e., $v_{mathrm{e}},sin,i,<,200,mathrm{km,s^{-1}}$) and hence do not agree with current predictions of rotational mixing in main-sequence stars. Adopting theoretical results for the wind velocity law, we find modified wind momenta for LMC stars that are $sim$0.3 dex higher than earlier results. [Due to the limitation of characters, the abstract appearing here is slightly shorter than that in the PDF file.]
We present a number of notable results from the VLT-FLAMES Tarantula Survey (VFTS), an ESO Large Program during which we obtained multi-epoch medium-resolution optical spectroscopy of a very large sample of over 800 massive stars in the 30 Doradus re
gion of the Large Magellanic Cloud (LMC). This unprecedented data-set has enabled us to address some key questions regarding atmospheres and winds, as well as the evolution of (very) massive stars. Here we focus on O-type runaways, the width of the main sequence, and the mass-loss rates for (very) massive stars. We also provide indications for the presence of a top-heavy initial mass function (IMF) in 30 Dor.
Detailed spectral classifications are presented for 352 O-B0 stars in the VLT-FLAMES Tarantula Survey, of which 213 O-type are of sufficient quality for further morphological analysis. Among them, six subcategories of special interest are distinguish
ed. (1) Several new examples of the earliest spectral types O2-O3 have been found. (2) A group of extremely rapidly rotating main-sequence objects has been isolated, including the largest $vsin i$ values known, the spatial and radial-velocity distributions of which suggest ejection from the two principal ionizing clusters. (3) Several new examples of the evolved, rapidly rotating Onfp class show similar evidence. (4) No fewer than 48 members of the Vz category, hypothesized to be on or near the ZAMS, are found in this sample; in contrast to the rapid rotators, they are strongly concentrated to the ionizing clusters, supporting their interpretation as very young objects, as do their relatively faint absolute magnitudes. (5) A surprisingly large fraction of the main-sequence spectra belong to the recently recognized V((fc)) class, with C III emission lines of similar strength to the usual N III in V((f)) spectra; there are also six objects with very high-quality data but no trace of either mission feature, presenting new challenges to physical interpretations. (6) Five spectra with morphologically enhanced nitrogen lines have been detected. Absolute visual magnitudes have been derived for each star with individual extinction laws, and composite HRDs provide evidence of the multiple generations present in this field. Associations with X-ray sources are noted. Further analyses of this unique dataset underway will provide new insights into the evolution of massive stars and starburst clusters.
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
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The Tarantula Survey is an ambitious ESO Large Programme that has obtained multi-epoch spectroscopy of over 1,000 massive stars in the 30 Doradus region of the Large Magellanic Cloud. Here we introduce the scientific motivations of the survey and giv
e an overview of the observational sample. Ultimately, quantitative analysis of every star, paying particular attention to the effects of rotational mixing and binarity, will be used to address fundamental questions in both stellar and cluster evolution.
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