A census of massive stars in NGC 346. Stellar parameters and rotational velocities

Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small Magellanic Cloud has been combined with that for 116 targets from the VLT-FLAMES Survey of Massive Stars. Spectral classification yields a sample of 47 O-type and 287 B-type spectra, while radial-velocity variations and/or spectral multiplicity have been used to identify 45 candidate single-lined systems, 17 double-lined systems, and one triple-lined system. Atmospheric parameters (T$_eff$ and log$g$) and projected rotational velocities ($v_e$sin$i$) have been estimated using TLUSTY model atmospheres; independent estimates of $v_e$sin$i$ were also obtained using a Fourier Transform method. Luminosities have been inferred from stellar apparent magnitudes and used in conjunction with the T$_eff$ and $v_e$sin$i$ estimates to constrain stellar masses and ages using the BONNSAI package. We find that targets towards the inner region of NGC 346 have higher median masses and projected rotational velocities, together with smaller median ages than the rest of the sample. There appears to be a population of very young targets with ages of less than 2 Myr, which have presumably all formed within the cluster. The more massive targets are found to have lower $v_e$sin$i$ consistent with previous studies. No significant evidence is found for differences with metallicity in the stellar rotational velocities of early-type stars, although the targets in the SMC may rotate faster than those in young Galactic clusters. The rotational velocity distribution for single non-supergiant B-type stars is inferred and implies that a significant number have low rotational velocity ($\simeq$10\% with $v_e$<40 km/s), together with a peak in the probability distribution at $v_e \simeq$300 km/s. Larger projected rotational velocity estimates have been found for our Be-type sample and imply that most have rotational velocities between 200-450 km/s.Comment: Accepted by A&

Neuropilin-1 Controls Endothelial Homeostasis by Regulating Mitochondrial Function and Iron-Dependent Oxidative Stress.

The transmembrane protein neuropilin-1 (NRP1) promotes vascular endothelial growth factor (VEGF) and extracellular matrix signaling in endothelial cells (ECs). Although it is established that NRP1 is essential for angiogenesis, little is known about its role in EC homeostasis. Here, we report that NRP1 promotes mitochondrial function in ECs by preventing iron accumulation and iron-induced oxidative stress through a VEGF-independent mechanism in non-angiogenic ECs. Furthermore, NRP1-deficient ECs have reduced growth and show the hallmarks of cellular senescence. We show that a subcellular pool of NRP1 localizes in mitochondria and interacts with the mitochondrial transporter ATP-binding cassette B8 (ABCB8). NRP1 loss reduces ABCB8 levels, resulting in iron accumulation, iron-induced mitochondrial superoxide production, and iron-dependent EC senescence. Treatment of NRP1-deficient ECs with the mitochondria-targeted antioxidant compound mitoTEMPO or with the iron chelator deferoxamine restores mitochondrial activity, inhibits superoxide production, and protects from cellular senescence. This finding identifies an unexpected role of NRP1 in EC homeostasis

Fe XI emission lines in a high resolution extreme ultraviolet spectrum obtained by SERTS

New calculations of radiative rates and electron impact excitation cross sections for Fe XI are used to derive emission line intensity ratios involving 3s^23p^4 - 3s^23p^33d transitions in the 180-223 A wavelength range. These ratios are subsequently compared with observations of a solar active region, obtained during the 1995 flight Solar EUV Research Telescope and Spectrograph (SERTS). The version of SERTS flown in 1995 incorporated a multilayer grating that enhanced the instrumental sensitivity for features in the 170 - 225 A wavelength range, observed in second-order between 340 and 450 A. This enhancement led to the detection of many emission lines not seen on previous SERTS flights, which were measured with the highest spectral resolution (0.03 A) ever achieved for spatially resolved active region spectra in this wavelength range. However, even at this high spectral resolution, several of the Fe XI lines are found to be blended, although the sources of the blends are identified in the majority of cases. The most useful Fe XI electron density diagnostic line intensity ratio is I(184.80 A)/I(188.21 A). This ratio involves lines close in wavelength and free from blends, and which varies by a factor of 11.7 between N_e = 10^9 and 10^11 cm^-3, yet shows little temperature sensitivity. An unknown line in the SERTS spectrum at 189.00 A is found to be due to Fe XI, the first time (to our knowledge) this feature has been identified in the solar spectrum. Similarly, there are new identifications of the Fe XI 192.88, 198.56 and 202.42 A features, although the latter two are blended with S VIII/Fe XII and Fe XIII, respectively.Comment: 21 pages, 9 gigures, accepted for publication in the Astrophysical Journa

The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution

Rotation has become an important element in evolutionary models of massive stars, specifically via the prediction of rotational mixing. Here, we study a sample of stars, including rapid rotators, to constrain such models and use nitrogen enrichments as a probe of the mixing process. Chemical compositions (C, N, O, Mg and Si) have been estimated for 135 early B-type stars in the Large Magellanic Cloud with projected rotational velocities up to ~300km/s using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including rotational mixing, have been generated attempting to reproduce these observations by adjusting the overshooting and rotational mixing parameters and produce reasonable agreement with 60% of our core hydrogen burning sample. We find (excluding known binaries) a significant population of highly nitrogen enriched intrinsic slow rotators vsini less than 50km/s incompatible with our models ~20% of the sample). Furthermore, while we find fast rotators with enrichments in agreement with the models, the observation of evolved (log g less than 3.7dex) fast rotators that are relatively unenriched (a further ~20% of the sample) challenges the concept of rotational mixing. We also find that 70% of our blue supergiant sample cannot have evolved directly from the hydrogen burning main-sequence. We are left with a picture where invoking binarity and perhaps fossil magnetic fields are required to understand the surface properties of a population of massive main sequence stars.Comment: ApJL. 10 pages, 1 figure. Updated to match accepted versio

Iron abundances from optical Fe III absorption lines in B-type stellar spectra

The role of optical Fe III absorption lines in B-type stars as iron abundance diagnostics is considered. To date, ultraviolet Fe lines have been widely used in B-type stars, although line blending can severely hinder their diagnostic power. Using optical spectra, covering a wavelength range ~ 3560 - 9200 A, a sample of Galactic B-type main-sequence and supergiant stars of spectral types B0.5 to B7 are investigated. A comparison of the observed Fe III spectra of supergiants, and those predicted from the model atmosphere codes TLUSTY (plane-parallel, non-LTE), with spectra generated using SYNSPEC (LTE), and CMFGEN (spherical, non-LTE), reveal that non-LTE effects appear small. In addition, a sample of main-sequence and supergiant objects, observed with FEROS, reveal LTE abundance estimates consistent with the Galactic environment and previous optical studies. Based on the present study, we list a number of Fe III transitions which we recommend for estimating the iron abundance from early B-type stellar spectra.Comment: 3 figures and 8 tables. Table 3 is to be published online only (included here on last page). Accepted for publication in MNRA

B-type supergiants in the SMC: Rotational velocities and implications for evolutionary models

High-resolution spectra for 24 SMC and Galactic B-type supergiants have been analysed to estimate the contributions of both macroturbulence and rotation to the broadening of their metal lines. Two different methodologies are considered, viz. goodness-of-fit comparisons between observed and theoretical line profiles and identifying zeros in the Fourier transforms of the observed profiles. The advantages and limitations of the two methods are briefly discussed with the latter techniques being adopted for estimated projected rotational velocities (\vsini) but the former being used to estimate macroturbulent velocities. Only one SMC supergiant, SK 191, shows a significant degree of rotational broadening (\vsini $\simeq$ 90 \kms). For the remaining targets, the distribution of projected rotational velocities are similar in both our Galactic and SMC samples with larger values being found at earlier spectral types. There is marginal evidence for the projected rotational velocities in the SMC being higher than those in the Galactic targets but any differences are only of the order of 5-10 \kms, whilst evolutionary models predict differences in this effective temperature range of typically 20 to 70 \kms. The combined sample is consistent with a linear variation of projected rotational velocity with effective temperature, which would imply rotational velocities for supergiants of 70 \kms at an effective temperature of 28 000 K (approximately B0 spectral type) decreasing to 32 \kms at 12 000 K (B8 spectral type). For all targets, the macroturbulent broadening would appear to be consistent with a Gaussian distribution (although other distributions cannot be discounted) with an $\frac{1}{e}$ half-width varying from approximately 20 \kms at B8 to 60 \kms at B0 spectral types.Comment: 4 figures, 8 pages, submitted to Astronomy and Astrophysic

The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars

We have previously analysed the spectra of 135 early B-type stars in the LMC and found several groups of stars that have chemical compositions that conflict with the theory of rotational mixing. Here we extend this study to Galactic and SMC metallicities with the analysis of ~50 Galactic and ~100 SMC early B-type stars with rotational velocities up to ~300km/s. The surface nitrogen abundances are utilised as a probe of the mixing process. In the SMC, we find a population of slowly rotating nitrogen-rich stars amongst the early B type core-hydrogen burning stars, similar to the LMC. In the Galactic sample we find no significant enrichment amongst the core hydrogen-burning stars, which appears to be in contrast with the expectation from both rotating single-star and close binary evolution models. However, only a small number of the rapidly rotating stars have evolved enough to produce a significant nitrogen enrichment, and these may be analogous to the non-enriched rapid rotators previously found in the LMC sample. Finally, in each metallicity regime, a population of highly enriched supergiants is observed, which cannot be the immediate descendants of core-hydrogen burning stars. Their abundances are, however, compatible with them having gone through a previous red supergiant phase. Together, these observations paint a complex picture of the nitrogen enrichment in massive main sequence and supergiant stellar atmospheres, where age and binarity cause crucial effects. Whether rotational mixing is required to understand our results remains an open question at this time, but could be answered by identifying the true binary fraction in those groups of stars that do not agree with single-star evolutionary models (abridged).Comment: Accepted paper - 86 pages with tables and figure

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 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&