Context. Massive stars, although being important building blocks of galaxies, are still not fully understood. This especially holds true for Wolf-Rayet (WR) stars with their strong mass loss, whose ...spectral analysis requires adequate model atmospheres. Aims. Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods. For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results. We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10 super(6)L sub(middot in circle) and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/L sub(middot in circle)) = 5.3 ... 5.8. Conclusions. While the few extremely luminous stars (log (L/L sub(middot in circle)) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/L sub(middot in circle)) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M sub(middot in circle). This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation.
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Multiplicity is one of the most fundamental observable properties of massive O-type stars and offers a promising way to discriminate between massive star formation theories. Nevertheless, companions ...at separations between 1 and 100 milliarcsec (mas) remain mostly unknown due to intrinsic observational limitations. At a typical distance of 2 kpc, this corresponds to projected physical separations of 2-200 AU. The Southern MAssive Stars at High angular resolution survey (SMASH+) was designed to fill this gap by providing the first systematic interferometric survey of Galactic massive stars. We observed 117 O-type stars with VLTI/PIONIER and 162 O-type stars with NACO/ Sparse Aperture Masking (SAM), probing the separation ranges 1-45 and 30-250 mas and brightness contrasts of deltaH < 4 and deltaH < 5, respectively. Taking advantage of NACO's field of view, we further uniformly searched for visual companions in an 8" radius down to deltaH = 8. This paper describes observations and data analysis, reports the discovery of almost 200 new companions in the separation range from 1 mas to 8" and presents a catalog of detections, including the first resolved measurements of over a dozen known long-period spectroscopic binaries. Excluding known runaway stars for which no companions are detected, 96 objects in our main sample (delta < 0degrees; H < 7.5) were observed both with PIONIER and NACO/SAM. The fraction of these stars with at least one resolved companion within 200 mas is 0.53. Accounting for known but unresolved spectroscopic or eclipsing companions, the multiplicity fraction at separation rho < 8" increases to functionof sub(m) = 0.91 + or - 0.03. The fraction of luminosity class V stars that have a bound companion reaches 100% at 30 mas while their average number of physically connected companions within 8" is functionof sub(c) = 2.2 + or - 0.3. This demonstrates that massive stars form nearly exclusively in multiple systems. The nine non-thermal radio emitters observed by smash+ are all resolved, including the newly discovered pairs HD 168112 and CPD-47degrees 2963. This lends strong support to the universality of the wind-wind collision scenario to explain the non-thermal emission from O-type stars.
Context. It is generally agreed upon that stars form in open clusters (OCs) and stellar associations, but little is known about structures in the Galactic OC population; whether OCs and stellar ...associations are born isolated or if they prefer to form in groups, for example. Answering this question provides new insight into star and cluster formation, along with a better understanding of Galactic structures. Aims. In the past decade, studies of OC groupings have either been based solely on spatial criteria or have also included tangential velocities for identifications. In contrast to previous approaches, we assumed that real OC groupings occupy a well defined area in the sky and show similar velocity vectors. For the first time, we have used 6D phase-space information, including radial velocities from the RAdial Velocity Experiment (RAVE) and other catalogues, for the detection of OC groupings. We also checked the age spread of potential candidates to distinguish between genuine groupings and chance alignments. Methods. We explored the Catalogue of Open Cluster Data (COCD) and determined 6D phase-space information for 432 out of 650 listed OCs and compact associations. The group identification was performed using an adapted version of the Friends-of-Friends algorithm, as used in cosmology, with linking lengths of 100 pc and 10-20 km s super(-1). For the verification of the identified structures, we applied Monte Carlo simulations with randomised samples. Results. For the linking lengths 100 pc and 10 km s super(-1), we detected 19 groupings, including 14 pairs, 4 groups with 3-5 members, and 1 complex with 15 members. The Monte Carlo simulations revealed that, in particular, the complex is most likely genuine, whereas pairs are more likely chance alignments. A closer look at the age spread of the complex and the comparison between spatial distributions of young and old cluster populations suggested that OC groupings likely originated from a common molecular cloud.
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The very massive binary NGC 3603-A1 Schnurr, O.; Casoli, J.; Chené, A.-N. ...
Monthly Notices of the Royal Astronomical Society Letters,
September 2008, Volume:
389, Issue:
1
Journal Article
Peer reviewed
Open access
Using Very Large Telescope/Spectrograph for INtegral Field Observation in the Near-Infrared (VLT/SINFONI), we have obtained repeated adaptive-optics assisted, near-infrared spectroscopy of the three ...central WN6ha stars in the core of the very young (∼1 Myr), massive and dense Galactic cluster NGC 3603. One of these stars, NGC 3603-A1, is a known 3.77 d, double-eclipsing binary, while another one, NGC 3603-C, is one of the brightest X-ray sources among all known Galactic WR stars, which usually is a strong indication for binarity. Our study reveals that star C is indeed an 8.9-d binary, although only the WN6ha component is visible in our spectra; therefore, we temporarily classify star C as an SB1 system. A1, on the other hand, is found to consist of two emission-line stars of similar, but not necessarily of identical spectral type, which can be followed over most the orbit. Using radial velocities for both components and the previously known inclination angle of the system, we are able to derive absolute masses for both stars in A1. We find M1 = (116 ± 31) M⊙ for the primary and M2 = (89 ± 16) M⊙ for the secondary component of A1. While uncertainties are large, A1 is intrinsically half a magnitude brighter than WR20a, the current record holder with 83 and 82 M⊙, respectively; therefore, it is likely that the primary in A1 is indeed the most massive star weighed so far.
We report the discovery of a bow-shock-producing star in the vicinity of the young massive star cluster NGC 3603 using archival data of the Spitzer Space Telescope. Follow-up optical spectroscopy of ...this star with Gemini-South led to its classification as O6 V. The orientation of the bow shock and the distance to the star (based on its spectral type) suggest that the star was expelled from the cluster, while the young age of the cluster (∼2 Myr) implies that the ejection was caused by a dynamical few-body encounter in the cluster's core. The relative position on the sky of the O6 V star and a recently discovered O2 If*/WN6 star (located on the opposite side of NGC 3603) allows us to propose that both objects were ejected from the cluster via the same dynamical event - a three-body encounter between a single (O6 V) star and a massive binary (now the O2 If*/WN6 star). If our proposal is correct, then one can 'weigh' the O2 If*/WN6 star using the conservation of the linear momentum. Given a mass of the O6 V star of 30 M, we found that at the moment of ejection the mass of the O2 If*/WN6 star was 175 M. Moreover, the observed X-ray luminosity of the O2 If*/WN6 star (typical of a single star) suggests that the components of this originally binary system have merged (e.g., because of encounter hardening).
Aims. WISE provides an infrared all-sky survey that aims at completing our knowledge on the possibly dramatically increasing number of brown dwarfs with lower temperatures. We search for the nearest ...representatives of the coolest brown dwarfs, which will be very interesting for detailed follow-up observations, once they have been discovered. Methods. We used the preliminary data release from WISE, then selected bright candidates with colours typical of late-T dwarfs, and tried to match them with faint 2MASS and SDSS objects, to determine their proper motions, and to follow them up spectroscopically. Results. We have identified two new ultracool brown dwarfs with large proper motions, WISE J0254+0223 (2.5 arcsec/yr) and WISE J1741+2553 (1.5 arcsec/yr). With their w1 − w2 ~ 3.0 and J − w2 ~ 4.0 colour indices, we expect both to have spectral types of ~T8-T10 and absolute magnitude of Mw2 ~ 14. We confirm WISE J1741+2553 as a T9-T10 dwarf from near-infrared spectroscopy with LBT/LUCIFER1. From their bright WISE w2 magnitudes of 12.7 and 12.3, we estimate distances of 5.5\hbox{$^{+2.3}_{-1.6}$}+2.3-1.6 pc and 4.6\hbox{$^{+1.2}_{-1.0}$}+1.2-1.0 pc and tangential velocities of ~65 km s-1 and ~34 km s-1 that indicate Galactic thick and thin disk membership, respectively.
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ABSTRACT
The hot WN star WR 2 (HD 6327) has been claimed to have many singular characteristics. To explain its unusually rounded and relatively weak emission line profiles, it has been proposed that ...WR 2 is rotating close to break-up with a magnetically confined wind. Alternatively, the line profiles could be explained by the dilution of WR 2’s spectrum by that of a companion. In this paper, we present a study of WR 2 using near-infrared AO imaging and optical spectroscopy and polarimetry. Our spectra reveal the presence of weak photospheric absorption lines from a B 2.5–4V companion, which however contributes only 5–10 per cent to the total light, suggesting that the companion is a background object. Therefore, its flux cannot be causing any significant dilution of the WR star’s emission lines. The absence of intrinsic linear continuum polarization from WR 2 does not support the proposed fast rotation. Our Stokes V spectrum was not of sufficient quality to test the presence of a moderately strong organized magnetic field but our new modelling indicates that to confine the wind the putative magnetic field must be significantly stronger than was previously suggested sufficiently strong as to make its presence implausible.
We present a mini-survey of Galactic Be stars mainly undertaken with the Large Binocular Telescope (LBT). Be stars show morphological features with hydrogen emission lines and an infrared excess, ...attributed to warm circumstellar dust. In general, these features are assumed to arise from dense, non-spherical, disc-forming circumstellar material in which molecules and dust can condensate. Due to the lack of reliable luminosities, the class of Galactic Be stars contains stars at very different stellar evolutionary phases like Herbig AeBe, supergiants or planetary nebulae. We took near-infrared long-slit K-band spectra for a sample of Galactic Be stars with the LBT-LUCI 1. Prominent spectral features, such as the Brackett γ line and CO band heads are identified in the spectra. The analysis shows that the stars can be characterized as evolved objects. Among others we find one luminous blue variable candidate (MWC 314), one supergiant Becandidate with 13CO (MWC 137), and in two cases (MWC 623 and AS 381) indications for the existence of a late-type binary companion, complementary to previous studies. For MWC 84, IR spectra were taken at different epochs with LBT-LUCI 1 and the GNIRS spectrograph at the Gemini North telescope. The new data show the disappearance of the circumstellar CO emission around this star, previously detectable over decades. Also no signs of a recent prominent eruption leading to the formation of new CO disc emission are found during 2010 and 2013.
Context. Although many new brown dwarf (BD) neighbours have recently been discovered thanks to new sky surveys in the mid- and near-infrared (MIR, NIR), their numbers are still more than five times ...lower than those of stars in the same volume. Aims. Our aim is to detect and classify new BDs to eventually complete their census in the immediate solar neighbourhood. Methods. We combined multi-epoch data from sky surveys at different wavelengths to detect BD neighbours of the Sun by their high proper motion (HPM). We concentrated on relatively bright MIR (w2 < 13.5) BD candidates from the Wide-field Infrared Survey Explorer (WISE) expected to be so close to the Sun that they may also be seen in older NIR (Two Micron All Sky Survey – 2MASS –; DEep Near-Infrared Survey – DENIS) or even red optical (Sloan Digital Sky Survey (SDSS) i- and z-band, SuperCOSMOS Sky Surveys (SSS) I-band) surveys. With low-resolution NIR spectroscopy we classified the new BDs and estimated their distances and velocities. Results. We have discovered the HPM (μ ~ 470 mas/yr) T7.5 dwarf, WISE J0521+1025, which is at d = 5.0 ± 1.3 pc from the Sun the nearest known T dwarf in the northern sky, and two early T dwarfs, WISE J0457−0207 (T2) and WISE J2030+0749 (T1.5), with proper motions of ~120 and ~670 mas/yr and distances of 12.5 ± 3.1 pc and 10.5 ± 2.6 pc, respectively. The last one was independently discovered and also classified as a T1.5 dwarf by Mace and coworkers. All three show thin disc kinematics. They may have been overlooked in the past owing to overlapping images and because of problems with matching objects between different surveys and measuring their proper motions.
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The past decade and a half has seen the design and execution of several ground-based spectroscopic surveys, both Galactic and Extragalactic. Additionally, new surveys are being designed that extend ...the boundaries of current surveys. In this context, many important considerations must be done when designing a spectrograph for the future. Among these is the determination of the optimum wavelength coverage. In this work, we present a new code for determining the wavelength ranges that provide the optimal amount of information to achieve the required science goals for a given survey. In its first mode, it utilizes a user-defined list of spectral features to compute a figure-of-merit for different spectral configurations. The second mode utilizes a set of flux-calibrated spectra, determining the spectral regions that show the largest differences among the spectra. Our algorithm is easily adaptable for any set of science requirements and any spectrograph design. We apply the algorithm to several examples, including 4MOST, showing the method yields important design constraints to the wavelength regions.
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