S. Simón-Díaz

The IACOB project - I. Rotational velocities in northern Galactic O- and early B-type stars revisited. The impact of other sources of line-broadening

Context. Stellar rotation is an important parameter in the evolution of massive stars. Accurate and reliable measurements of projected rotational velocities in large samples of OB stars are cruci al to confront the predictions of stellar evolutionary mode ls with observational constraints. Aims. We reassess previous determinations of projected rotational velocities (v sin i) in Galactic OB stars using a large, high quality spectroscopic dataset, and a strategy which account for other sources of broadening appart from rotation affecting the diagnostic lines Methods. We present a versatile and user friendly IDL tool — based on a combined Fourier Transform (FT)+ goodness of fit (GOF) methodology — for the line-broadening characterization in OB-type stars. We use this tool to (a) investigate the impact of macroturbulent and microturbulent broadenings on v sin i measurements, and (b) determine v sin i in a sample of∼ 200 Galactic OB-type stars, also characterizing the amount of macroturbulentbroadening (vm) affecting the line profiles. Results. We present observational evidence illustrating the strengths and limitations of the proposed FT+GOF methodology for the case of OB stars. We confirm previous statements (based on ind irect arguments or smaller samples) that the macroturbulent broadening is ubiquitous in the massive star domain. We compare the newly derived v sin i in the case of O stars and early-B Supergiants and Giants (where the effect of macroturbulence is found to be larger) with previous determinations not accounting for this extra line-broadening contribution, and show that those cases with v sin i≤ 120 km s −1 need to be systematically revised downwards by∼ 25 (± 20) km s −1 . We suggest that microturbulence may impose an upper limit below which v sin i and vm could be incorrectly derived by means of the proposed methodology as presently used, and discuss the implications of this statement on the study of relat ively narrow line massive stars. Conclusions. An investigation of impact of the revised v sin i distributions on the predictions by massive star evolution ary models is now warranted. Also, the reliability of v sin i measurements in the low v sin i regime, using a more precise description of the intrinsic profiles used for the line-broadening analysis, needs to be f urther investigated.

The VLT-FLAMES Tarantula Survey - XII. Rotational velocities of the single O-type stars

Context. 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. Aims. 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 sample is large enough to obtain statistically significant information and to search for variations among subpopulations – in terms of spectral type, luminosity class, and spatial location – in the field of view. Methods. We measured projected rotational velocities, ν_esini, by means of a Fourier transform method and a profile fitting method applied to a set of isolated spectral lines. We also used an iterative deconvolution procedure to infer the probability density, P(ν_e), of the equatorial rotational velocity, ν_e. Results. The distribution of νesini shows a two-component structure: a peak around 80 kms^(-1) and a high-velocity tail extending up to ~600 kms^(-1). This structure is also present in the inferred distribution P(ν_e) with around 80% of the sample having 0 < ν_e ≤ 300 kms^(-1) and the other 20% distributed in the high-velocity region. The presence of the low-velocity peak is consistent with what has been found in other studies for late O- and early B-type stars. Conclusions. Most of the stars in our sample rotate with a rate less than 20% of their break-up velocity. For the bulk of the sample, mass loss in a stellar wind and/or envelope expansion is not efficient enough to significantly spin down these stars within the first few Myr of evolution. If massive-star formation results in stars rotating at birth with a large portion of their break-up velocities, an alternative braking mechanism, possibly magnetic fields, is thus required to explain the present-day rotational properties of the 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.

The chemical composition of the Orion star forming region - III. C, N, Ne, Mg, and Fe abundances in B-type stars revisited

Context. Early B-type stars are invaluable indicators of elemental abundances of their birth environments. In contrast to the surrounding neutral interstellar matter (ISM) and H ii regions, their chemical composition is unaffected by depletion onto dust grains and the derivation of different abundances from recombination and collisional lines. In combination with ISM or nebular gas-phase abundances, they facilitate the otherwise inaccessible dust-phase composition to be constrained. Aims. We determine precise abundances of C, N, Mg, Ne, and Fe in early B-type stars in the Orion star-forming region to: a) review previous determinations using a self-consistent quantitative spectral analysis based on modern stellar atmospheres and recently updated model atoms; b) complement our previous results for oxygen and silicon; and c) establish an accurate and reliable set of stellar metal abundances to constrain the dust-phase composition of the Orion H ii region. Methods. A detailed, self-consistent spectroscopic study of a sample of 13 narrow-lined B0 V-B2 V stars in Ori OB1 is performed. High-quality spectra obtained with FIES at the NOT are analysed using both a hybrid non-local thermodynamic equilibrium (nonLTE) method (i.e., classical line-blanketed LTE model atmospheres and non-LTE line formation) and line-profile fitting techniques, validating the approach by comparison with previous results obtained using line-blanketed non-LTE model atmospheres and a curveof-growth analysis. Results. The two independent analysis strategies provide consistent results for basic stellar parameters and the abundances of oxygen and silicon. The extended analysis to C, N, Mg, Ne, and Fe finds a high degree of chemical homogeneity, with the 1σ-scatter typically being 0.03−0.07 dex around the mean for the various elements. The present-day abundances of B-type stars in Ori OB1 are compatible at similar precision with cosmic abundance standard values as recently established from early-type stars in the solar neighbourhood and also with the Sun.

The chemical composition of the Orion star forming region II. Stars, gas, and dust: the abundance discrepancy conundrum

Aims. We re-examine the recombination/collisional emission line (RL/CEL) nebular abundance discrepancy problem in the light of recent high-quality abundance determinations in young stars in the Orion star-forming region. Methods. We re-evaluate the CEL and RL abundances of several elements in the Orion nebula and estimate the associated uncertainties, taking into account the uncertainties in the ionization correction factors for unseen ions. We estimate the amount of oxygen trapped in dust grains for several scenarios of dust formation. We compare the resulting gas+dust nebular abundances with the stellar abundances of a sample of 13 B-type stars from the Orion star-forming region (Ori OB1), analyzed in Papers I and III of this series. Results. We find that the oxygen nebular abundance based on recombination lines agrees much better with the stellar abundances than the one derived from the collisionally excited lines. This result calls for further investigation. If the CEL/RL abundance discrepancy were caused by temperature fluctuations in the nebula, as argued by some authors, the same kind of discrepancy should be seen for the other elements, such as C, N and Ne, which is not what we find in the present study. Another problem is that with the RL abundances, the energy balance of the Orion nebula is not well understood. We make some suggestions concerning the next steps to undertake to solve this problem.

OBSERVATIONAL EVIDENCE FOR A CORRELATION BETWEEN MACROTURBULENT BROADENING AND LINE-PROFILE VARIATIONS IN OB SUPERGIANTS*

The spectra of O and B supergiants (Sgs) are known to be affected by a significant form of extra line broadening (usually referred to as macroturbulence) in addition to that produced by stellar rotation. Recent analyses of high-resolution spectra have shown that the interpretation of this line broadening as a consequence of large-scale turbulent motions would imply highly supersonic velocity fields in photospheric regions, making this scenario quite improbable. Stellar oscillations have been proposed as a likely alternative explanation. As part of a long-term observational project, we are investigating the macroturbulent broadening in O and B Sgs and its possible connection with spectroscopic variability phenomena and stellar oscillations. In this Letter, we present the first encouraging results of our project, namely, firm observational evidence for a strong correlation between the extra broadening and photospheric line-profile variations in a sample of 13 Sgs with spectral types ranging from O9.5 to B8.

The VLT-FLAMES Tarantula Survey. XIV. The O-type stellar content of 30 Doradus

Detailed spectral classifications are presented for 352 O-B0 stars in the VLT-FLAMES Tarantula Survey ESO Large Programme, of which 213 O-type are judged of sufficiently high quality for further morphological analysis. Among them, six subcategories of special interest are distinguished. (1) Several new examples of the earliest spectral types O2-O3 have been found, while a previously known example has been determined to belong to the nitrogen-rich ON2 class. (2) A group of extremely rapidly rotating main-sequence objects has been isolated, including the largest vsini values known, the spatial and radial-velocity distributions of which suggest ejection from the two principal ionizing clusters NGC 2070 and NGC 2060. (3) Several new examples of the evolved, rapidly rotating Onfp class show similar evidence, although at least some of them are spectroscopic binaries. (4) No fewer than 48 members of the Vz category, hypothesized to be on or near the zero-age main sequence, are found in this sample; in contrast to the rapid rotators, they are strongly concentrated to the ionizing clusters and a newly recognized region of current and recent star formation to the north, 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, although a comparable number of the latter are also present, as well as six objects with very high-quality data but no trace of either emission feature, presenting new challenges to physical interpretations. (6) Two mid-O Vz and three late-O giant/supergiant spectra with morphologically enhanced nitrogen lines have been detected. Absolute visual magnitudes have been derived for each star with individual extinction laws, and composite Hertzsprung-Russell diagrams provide evidence of the multiple generations present in this field. Spectroscopic binaries, resolved visual multiples, and possible associations with X-ray sources are noted. Astrophysical and dynamical analyses of this unique dataset underway will provide new insights into the evolution of massive stars and starburst clusters.

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 (ve sini) 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 km s −1 and show a bi-modal structure. This is also present in the inferred rotational velocity distribution with 25% of the sample having 0 ≤ ve ≤ 100 km s −1 and the high velocity component having ve ∼ 250 km s −1 . 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.

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.

The ARAUCARIA project: Grid-based quantitative spectroscopic study of massive blue stars in NGC 55

Context. 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 natur e 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. Aims. As part of the ARAUCARIA project we carry out the first quantit ative spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC 55 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. Methods. Based on the characteristics of the stellar atmosphere/line formation codefastwind, 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 resol ution of B-type supergiants in a fast and objective way. Results. We present the main characteristics of ourfastwind 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 NGC 55, deriving the stellar parameters, Si , C , N , O and Mg abundances. The results indicate that our stars are part of a young population evolving towards a red supergiant phase. For half of the sample we find a remarkab le agreement between spectroscopic and evolutionary masses, whilst for the rest larger discrepancies are present, but still wit hin the uncertainties. 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. Conclusions. The consistency between the observed spectra and our stellar models supports the reliability of our methodology. This objective and fast approach allows us to deal with large samples in an accurate and more statistical way. These are two key issues to achieve an unbiased characterization of the stars and their host galaxies.

The VLT-FLAMES Tarantula Survey XVI. The optical and 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, ApJ, 345, 245) have limitations that, among other issues, hamper the determination of the effective temperatures of O and early B stars from optical and near-infrared (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 and 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 and 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. 5m ag.