Search for the magnetic field of the O7.5 III star xi Persei

Cyclical wind variability is an ubiquitous but as yet unexplained feature among OB stars. The O7.5 III(n)((f)) star xi Persei is the brightest representative of this class on the Northern hemisphere. As its prominent cyclical wind properties vary on a rotational time scale (2 or 4 days) the star has been already for a long time a serious magnetic candidate. As the cause of this enigmatic behavior non-radial pulsations and/or a surface magnetic field are suggested. We present a preliminary report on our attempts to detect a magnetic field in this star with high-resolution measurements obtained with the spectropolarimeter Narval at TBL, France during 2 observing runs of 5 nights in 2006 and 5 nights in 2007. Only upper limits could be obtained, even with the longest possible exposure times. If the star hosts a magnetic field, its surface strength should be less than about 300 G. This would still be enough to disturb the stellar wind significantly. From our new data it seems that the amplitude of the known non-radial pulsations has changed within less than a year, which needs further investigation.Comment: 2 pages, 6 figures, contributed poster at IAU Symposium 259 "Cosmic Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife, Spain, November 3-7, 200

On the Mass-Loss Rates of Massive Stars in the Low-Metallicity Galaxies IC 1613, WLM and NGC 3109

We present a spectroscopic analysis of VLT/X-Shooter observations of six O-type stars in the low-metallicity (Z ~ 1/7 Z\odot) galaxies IC 1613, WLM and NGC 3109. The stellar and wind parameters of these sources allow us, for the first time, to probe the mass-loss versus metallicity dependence of stellar winds below that of the Small Magellanic Cloud (at Z ~ 1/5Z\odot) by means of a modified wind momentum versus luminosity diagram. The wind strengths that we obtain for the objects in WLM and NGC 3109 are unexpectedly high and do not agree with theoretical predictions. The objects in IC 1613 tend towards a higher than expected mass-loss rate, but remain consistent with predictions within their error bars. We discuss potential systematic uncertainties in the mass-loss determinations to explain our results. However, if further scrutinization of these findings point towards an intrinsic cause for this unexpected sub-SMC mass-loss behavior, implications would include a higher than anticipated number of Wolf-Rayet stars and Ib/Ic supernovae in low-metallicity environments, but a reduced number of long-duration gamma-ray bursts produced through a single-star evolutionary channel.Comment: 9 pages, 3 figures; accepted for publication in The Astrophysical Journal Letter

The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM and NGC 3109

Massive stars likely played an important role in the reionization of the Universe, and the formation of the first black holes. Massive stars in low-metallicity environments in the local Universe are reminiscent of their high redshift counterparts. In a previous paper, we reported on indications that the stellar winds of low-metallicity O stars may be stronger than predicted, which would challenge the current paradigm of massive star evolution. In this paper, we aim to extend our initial sample of six O stars in low-metallicity environments by four. We aim to derive their stellar and wind parameters, and compare these to radiation-driven wind theory and stellar evolution models. We have obtained intermediate-resolution VLT/X-Shooter spectra of our sample of stars. We derive the stellar parameters by fitting synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic fitting algoritm. We compare our parameters to evolutionary tracks and obtain evolutionary masses and ages. We also investigate the effective temperature versus spectral type calibration for SMC and lower metallicities. Finally, we reassess the wind momentum versus luminosity diagram. The derived parameters of our target stars indicate stellar masses that reach values of up to 50 $M_{\odot}$. The wind strengths of our stars are, on average, stronger than predicted from radiation-driven wind theory and reminiscent of stars with an LMC metallicity. We discuss indications that the iron content of the host galaxies is higher than originally thought and is instead SMC-like. We find that the discrepancy with theory is lessened, but remains significant for this higher metallicity. This may imply that our current understanding of the wind properties of massive stars, both in the local universe as well as at cosmic distances, remains incomplete.Comment: Accepted for publication in Astronomy and Astrophysics. 10 pages, 8 figure

On the origin of the difference between the runaway velocities of the OB-supergiant X-ray Binaries and the Be/X-ray Binaries

The recent finding by Chevalier & Ilovaisky (1998) that OB-supergiant X-ray binaries have relatively large runaway velocities whereas Be/X-ray binaries have low runaway velocities, provides confirmation of the current models for the formation of these two types of systems. These predict a difference in runaway velocity of an order of magnitude. This difference basically results from the variation of the fractional helium core mass as a function of stellar mass, in combination with the conservation of orbital angular momentum during the mass transfer phase that preceded the formation of the compact object in the system. This combination results into: (i) Systematically narrower pre-supernova orbits in the OB-supergiant systems than in the Be-systems, and (ii) A larger fractional amount of mass ejected in the supernovae in high-mass systems relative to systems of lower mass. Regardless of possible kick velocities imparted to neutron stars at birth, this combination leads to a considerable difference in average runaway velocity between these two groups. The observed low runaway velocities of the Be/X-ray binaries confirm that in most cases not more than 1 to 2Msun was ejected in the supernovae that produced their neutron stars. This, in combination with the --on average-- large orbital eccentricities of these systems, indicates that their neutron stars must have received a velocity kick in the range 60 - 250 km/s at birth.Comment: reduced abstract, 13 pages, accepted by A&

A dearth of short-period massive binaries in the young massive star forming region M17: Evidence for a large orbital separation at birth?

The formation of massive stars remains poorly understood and little is known about their birth multiplicity properties. Here, we investigate the strikingly low radial-velocity dispersion measured for a sample of 11 massive pre- and near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive star forming region M17 to obtain first constraints on the multiplicity properties of young massive stellar objects. Methods: We compute the RV dispersion of synthetic populations of massive stars for various multiplicity properties and we compare the simulated sigma_rv distributions to the observed value. We specifically investigate two scenarios: a low binary fraction and a dearth of short-period binary systems. Results: Simulated populations with low binary fractions (f_bin = 0.12_{-0.09}^{+0.16}) or with truncated period distributions (P_cutoff > 9 months) are able to reproduce the low sigma_rv observed within their 68%-confidence intervals. Parent populations with f_bin > 0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level. Both constraints are contrast with the high binary fraction and plethora of short-period systems found in few Myr-old, OB-type populations. To explain the difference, the first scenario requires a variation of the outcome of the massive star formation process. In the the second scenario, compact binaries must form later on, and the cut-off period may be related to physical length-scales representative of the bloated pre-main-sequence stellar radii or of their accretion disks. Conclusions: If the obtained constraints are representative of the overall properties of massive young stellar objects, our results may provide support to a formation process in which binaries are initially formed at larger separations, then harden or migrate to produce the typical (untruncated) power-law period distribution observed in few Myr-old OB binaries.Comment: 5 pages; Accepted for publication in Astronomy and Astrophysics Letter

The mass of the very massive binary WR21a

We present multi-epoch spectroscopic observations of the massive binary system WR21a, which include the January 2011 periastron passage. Our spectra reveal multiple SB2 lines and facilitate an accurate determination of the orbit and the spectral types of the components. We obtain minimum masses of $64.4\pm4.8 \ M_{\odot}$ and $36.3\pm1.7 \ M_{\odot}$ for the two components of WR21a. Using disentangled spectra of the individual components, we derive spectral types of O3/WN5ha and O3Vz~((f*)) for the primary and secondary, respectively. Using the spectral type of the secondary as an indication for its mass, we estimate an orbital inclination of $i=58.8\pm2.5^{\mathrm{o}}$ and absolute masses of $103.6\pm10.2 \ M_{\odot}$ and $58.3\pm3.7 \ M_{\odot}$, in agreement with the luminosity of the system. The spectral types of the WR21a components indicate that the stars are very young (1$-$2 Myr), similar to the age of the nearby Westerlund 2 cluster. We use evolutionary tracks to determine the mass-luminosity relation for the total system mass. We find that for a distance of 8 kpc and an age of 1.5 Myr, the derived absolute masses are in good agreement with those from evolutionary predictions.Comment: 7 pages, 4 figures; accepted for publication in MNRA

The Primordial Binary Population in OB Associations

For understanding the process of star formation it is essential to know how many stars are formed as singles or in multiple systems, as a function of environment and binary parameters. This requires a characterization of the primordial binary population, which we define as the population of binaries that is present just after star formation has ceased, but before dynamical and stellar evolution have significantly altered its characteristics. In this article we present the first results of our adaptive optics survey of 200 (mainly) A-type stars in the nearby OB association Sco OB2. We report the discovery of 47 new candidate companions of Sco OB2 members. The next step will be to combine these observations with detailed simulations of young star clusters, in order to find the primordial binary population.Comment: 2 pages, 1 figure, poster paper to appear in proceedings of IAU Coll. 191 "The environments and evolution of binary and multiple stars

First constraint on cosmological variation of the proton-to-electron mass ratio from two independent telescopes

A high signal-to-noise spectrum covering the largest number of hydrogen lines (90 H2 lines and 6 HD lines) in a high redshift object was analyzed from an observation along the sight-line to the bright quasar source J2123$-$005 with the UVES spectrograph on the ESO Very Large Telescope (Paranal, Chile). This delivers a constraint on a possible variation of the proton-to-electron mass ratio of Dmu/mu = (8.5 \pm 3.6_{stat} \pm 2.2_{syst}) x 10^{-6} at redshift z=2.059$, which agrees well with a recently published result on the same system observed at the Keck telescope yielding Dmu/mu = (5.6 \pm 5.5_{stat} \pm 2.9_{syst}) x 10^{-6}. Both analyses used the same robust absorption line fitting procedures with detailed consideration of systematic errors.Comment: Accepte

RCW36: characterizing the outcome of massive star formation

Massive stars play a dominant role in the process of clustered star formation, with their feedback into the molecular cloud through ionizing radiation, stellar winds and outflows. The formation process of massive stars is poorly constrained because of their scarcity, the short formation timescale and obscuration. By obtaining a census of the newly formed stellar population, the star formation history of the young cluster and the role of the massive stars within it can be unraveled. We aim to reconstruct the formation history of the young stellar population of the massive star-forming region RCW 36. We study several dozens of individual objects, both photometrically and spectroscopically, look for signs of multiple generations of young stars and investigate the role of the massive stars in this process. We obtain a census of the physical parameters and evolutionary status of the young stellar population. Using a combination of near-infrared photometry and spectroscopy we estimate ages and masses of individual objects. We identify the population of embedded young stellar objects (YSO) by their infrared colors and emission line spectra. RCW 36 harbors a stellar population of massive and intermediate-mass stars located around the center of the cluster. Class 0/I and II sources are found throughout the cluster. The central population has a median age of 1.1 +/- 0.6 Myr. Of the stars which could be classified, the most massive ones are situated in the center of the cluster. The central cluster is surrounded by filamentary cloud structures; within these, some embedded and accreting YSOs are found. Our age determination is consistent with the filamentary structures having been shaped by the ionizing radiation and stellar winds of the central massive stars. The formation of a new generation of stars is ongoing, as demonstrated by the presence of embedded protostellar clumps, and two exposed jets.Comment: 18 pages, 10 figures, accepted for publication in Astronomy & Astrophysic