The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analog of large star-formation events in the distant universe. We determined the recent formation history and the initial ...mass function (IMF) of massive stars in 30 Doradus on the basis of spectroscopic observations of 247 stars more massive than 15 solar masses (Formula: see text). The main episode of massive star formation began about 8 million years (My) ago, and the star-formation rate seems to have declined in the last 1 My. The IMF is densely sampled up to 200 Formula: see text and contains 32 ± 12% more stars above 30 Formula: see text than predicted by a standard Salpeter IMF. In the mass range of 15 to 200 Formula: see text, the IMF power-law exponent is Formula: see text, shallower than the Salpeter value of 2.35.
ABSTRACT
We cross-match the Alma catalogue of OB stars with Gaia DR2 astrometry and photometry as a first step towards producing a clean sample of massive stars in the solar neighbourhood with a high ...degree of completeness. We analyse the resulting colour–absolute magnitude diagram to divide our sample into categories and compare extinction estimates from two sources, finding problems with both of them. The distances obtained with three different priors are found to have few differences among them, indicating that Gaia DR2 distances are robust. An analysis of the 3D distribution of massive stars in the solar neighbourhood is presented. We show that a kinematically distinct structure we dub the Cepheus spur extends from the Orion–Cygnus spiral arm towards the Perseus arm and is located above the Galactic mid-plane, likely being related to the recently discovered Radcliffe wave. We propose that this corrugation pattern in the Galactic disc may be responsible for the recent enhanced star formation at its crests and troughs. We also discuss our plans to extend this work in the immediate future.
Context. On September 2016 the first data from Gaia were released (DR1). The first release included photometry for over 109 sources in the very broad G system. Aims. The aims here are to test the ...correspondence between G magnitudes in DR1 and the synthetic equivalents derived using spectral energy distributions from observed and model spectrophotometry; to correct the G passband curve; and to measure the zero point in the Vega system. Methods. I have computed the synthetic G and Tycho-2 BTVT photometry for a sample of stars using the Next Generation Spectral Library (NGSL) and the Hubble Space Telescope (HST) CALSPEC spectroscopic standards. Results. I have found that the nominal G passband curve is too blue for the DR1 photometry, as shown by the presence of a color term in the comparison between observed and synthetic magnitudes. A correction to the passband applying a power law in λ with an exponent of 0.783 eliminates the color term. The corrected passband has a Vega zero point of 0.070 ± 0.004 mag.
We present the second installment of GOSSS, a massive spectroscopic survey of Galactic O stars, based on new homogeneous, high signal-to-noise ratio, R ~ 2500 digital observations from both ...hemispheres selected from the Galactic O-Star Catalog (GOSC). In this paper we include bright stars and other objects drawn mostly from the first version of GOSC, all of them south of delta = -20degrees, for a total number of 258 O stars. We also revise the northern sample of Paper I to provide the full list of spectroscopically classified Galactic O stars complete to B = 8, bringing the total number of published GOSSS stars to 448. Extensive sequences of exceptional objects are given, including the early Of/WN, O Iafpe, Ofc, ON/OC, Onfp, Of?p, and Oe types, as well as double/triple-lined spectroscopic binaries. The new spectral subtype O9.2 is also discussed. The magnitude and spatial distributions of the observed sample are analyzed. We also present new results from OWN, a multi-epoch high-resolution spectroscopic survey coordinated with GOSSS that is assembling the largest sample of Galactic spectroscopic massive binaries ever attained. The OWN data combined with additional information on spectroscopic and visual binaries from the literature indicate that only a very small fraction (if any) of the stars with masses above 15-20 M sub(middot in circle) are born as single systems. In the future we will publish the rest of the GOSSS survey, which is expected to include over 1000 Galactic O stars.
Context.
The young massive cluster Westerlund 1 offers the promise of a grand laboratory for the study of high-mass star evolution, but its basic parameters are still poorly known.
Aims.
In this ...paper, we aim at a better characterisation of the cluster by determining some basic kinematic properties and analysing the area surrounding the cluster and the population in its foreground.
Methods.
We have used
Gaia
early data release 3 (EDR3) data, together with spectra of a large sample of luminous stars in the field surrounding Westerlund 1, to explore the extent of the cluster. We carried out a non-parametric analysis of proper motions and membership determination. We investigated the reddening and proper motions of several dozen OB stars and red supergiants less than one degree away from Westerlund 1.
Results.
We identify a population of kinematic members of Westerlund 1 that largely includes the known spectroscopic members. From their EDR3 parallaxes, we derive a distance to the cluster of $ 4.23^{+0.23}_{-0.21} $ kpc. We analyse the extinction in this direction, finding that it increases by a large amount around 2.8 kpc, which in all likelihood is due to dark clouds associated with the Scutum-Crux arm. As a consequence, we hardly see any stars at distances comparable (or higher) than that of the cluster. The proper motions of Westerlund 1, however, are very similar to those of stars in the field surrounding it which are – almost without exception – less distant, but distinct. We find a second, astrometrically well-defined population in the foreground (
d
≈ 2 kpc), centred ∼8′ away, which is likely connected to the possible open cluster BH 197. Westerlund 1 is very elongated, an effect that seems real and not driven by the very heavy extinction to the east and south. We find a low-density halo extending to distances up to 10′ from the cluster centre, mainly in the north-west quadrant. A few OB stars at larger distances from the cluster, most notably the luminous blue variable (LBV) MN48, share its proper motions, suggesting that Westerlund 1 has little or no peculiar motion with respect to the field population of the Norma arm. Despite this, we are unable to find any red supergiant that could belong to an extended population related to the cluster, although we observe several dozen such objects in the foreground, demonstrating the richness of the field population along this sightline. We find a substantial population of luminous OB members obscured by several more magnitudes of extinction than most known members. These objects, mostly located in the central region of the cluster, increase the population of OB supergiants by about 25%.
We present the first installment of a massive spectroscopic survey of Galactic O stars, based on new, high signal-to-noise ratio, R ~ 2500 digital observations from both hemispheres selected from the ...Galactic O-Star Catalog of Maiz Apellaniz et al. and Sota et al. The spectral classification system is rediscussed and a new atlas is presented, which supersedes previous versions. Extensive sequences of exceptional objects are given, including types Ofc, ON/OC, Onfp, Of?p, Oe, and double-lined spectroscopic binaries. The remaining normal spectra bring this first sample to 184 stars, which is close to complete to B = 8 and north of Delta *d = --20? and includes all of the northern objects in Maiz Apellaniz et al. that are still classified as O stars. The systematic and random accuracies of these classifications are substantially higher than previously attainable, because of the quality, quantity, and homogeneity of the data and analysis procedures. These results will enhance subsequent investigations in Galactic astronomy and stellar astrophysics. In the future, we will publish the rest of the survey, beginning with a second paper that will include most of the southern stars in Maiz Apellaniz et al.
In this paper, we present high-resolution HST/COS observations of the extreme magnetic O star NGC 1624-2. These represent the first ultraviolet spectra of this archetypal object. We examine the ...variability of its wind-sensitive resonance lines, comparing it to that of other known magnetic O stars. In particular, the observed variations in the profiles of the Civ and Siiv doublets between low state and high state are the largest observed in any magnetic O-type star, consistent with the expected properties of NGC 1624-2's magnetosphere. We also observe a redshifted absorption component in the low state, a feature not seen in most stars. We present preliminary modelling efforts based on the Analytic Dynamical Magnetosphere (ADM) formalism, demonstrating the necessity of using non-spherically symmetric models to determine wind/magnetospheric properties of magnetic O stars.
Context. On the one hand, the second data release of the Gaia mission (Gaia DR2) has opened a trove of astrometric and photometric data for Galactic clusters within a few kiloparsecs of the Sun. On ...the other hand, lucky imaging has been an operational technique to measure the relative positions of visual binary systems for a decade and a half. This time is sufficient to apply the results of the technique to the calculation of orbits of some massive multiple systems within ∼1 kpc of the Sun. Aims. As part of an ambitious research program to measure distances to Galactic stellar groups (including clusters) containing O stars, I start with two of the nearest examples: Collinder 419 in Cygnus and NGC 2264 in Monoceros. The main ionizing source for each cluster is a multiple system with an O-type primary: HD 193 322 and 15 Mon, respectively. For each of the two multiple systems, I aim to derive new astrometric orbits for the Aa,Ab components. Methods. First, I present a method that uses Gaia DR2 G + GBP + GRP photometry, positions, proper motions, and parallaxes to obtain the membership and distance of a stellar group and applied these values to Collinder 419 and NGC 2264. Second, I present a new code that calculates astrometric orbits by searching the whole seven-parameter orbit space and apply it to HD 193 322 Aa,Ab and 15 Mon Aa,Ab using, as input, literature data from the Washington Double Star Catalog and the AstraLux measurements recently presented by our group in another paper published this year. Results. I obtain Gaia DR2 distances of 1006+37−34 1006 − 34 + 37 $ 1006^{+37}_{-34} $ pc for Collinder 419 and 719 ± 16 pc for NGC 2264; the main contribution to the uncertainties comes from the spatial covariance of the parallaxes. The two NGC 2264 subclusters are at the same distance (within the uncertainties) and show a significant relative proper motion. The distances are shown to be robust. HD 193 322 Aa,Ab follows an eccentric ( e = 0.58+0.03−0.04 e = 0 . 58 − 0.04 + 0.03 $ e = 0.58^{+0.03}_{-0.04} $ ) orbit with a period of 44 ± 1 a and the three stars it contains have a total mass of 76.1+9.9−7.4 76 . 1 − 7.4 + 9.9 $ 76.1^{+9.9}_{-7.4} $ M⊙. The orbit of 15 Mon Aa,Ab is even more eccentric ( e = 0.770+0.023−0.030 e = 0 . 770 − 0.030 + 0.023 $ e = 0.770^{+0.023}_{-0.030} $ ); it has a period of 108 ± 12 a and a total mass of 45.1+3.6−3.3 45 . 1 − 3.3 + 3.6 $ 45.1^{+3.6}_{-3.3} $ M⊙ for its two stars.
Context.
Massive stars at low metallicity are among the main feedback agents in the early Universe and in present-day star forming galaxies. When in binaries, these stars are potential progenitors of ...gravitational-wave events. Knowledge of stellar masses is a prerequisite to understanding evolution and feedback of low-metallicity massive stars.
Aims.
Using abundant spectroscopic and photometric measurements of an outstandingly bright eclipsing binary, we compare its dynamic, spectroscopic, and evolutionary mass estimates and develop a binary evolution scenario.
Methods.
We comprehensively studied the eclipsing binary system, AzV 476, in the Small Magellanic Cloud (SMC). The light curve and radial velocities were analyzed to obtain the orbital parameters. The photometric and spectroscopic data in the UV and optical were analyzed using the Potsdam Wolf–Rayet (PoWR) model atmospheres. The obtained results are interpreted using detailed binary-evolution tracks including mass transfer.
Results.
AzV 476 consists of an O4 IV-III((f))p primary and an O9.5: Vn secondary. Both components have similar current masses (20
M
⊙
and 18
M
⊙
) obtained consistently from both the orbital and spectroscopic analysis. The effective temperatures are 42 kK and 32 kK, respectively. The wind mass-loss rate of log(
Ṁ
∕(
M
⊙
yr
−1
)) = −6.2 of the primary is a factor of ten higher than a recent empirical prescription for single O stars in the SMC. Only close-binary evolution with mass transfer can reproduce the current stellar and orbital parameters, including orbital separation, eccentricity, and the rapid rotation of the secondary. The binary evolutionary model reveals that the primary has lost about half of its initial mass and is already core helium burning.
Conclusions.
Our comprehensive analysis of AzV 476 yields a consistent set of parameters and suggests previous case B mass transfer. The derived stellar masses agree within their uncertainties. The moderate masses of AzV 476 underline the scarcity of bright massive stars in the SMC. The core helium burning nature of the primary indicates that stripped stars might be hidden among OB-type populations.
ABSTRACT
We have started an ambitious program to determine if the full diversity of extinction laws is real or if some of it is due to calibration or methodological issues. Here we start by analysing ...the information on near-infrared (NIR) extinction in a Two Micron All-Sky Survey (2MASS) stellar sample with good quality photometry and very red colours. We calculate the extinction at 1 μm, A1, and the power-law exponent, α (Aλ = A1λ−α), for the 2MASS stars located in the extinction trajectory in the H − K versus J − H plane expected for red giants with A1 > 5 mag. We test the validity of the assumption about the nature of those stars, whether a single or multiple values of α are needed, and the spatial variations of the results. Most (∼83 per cent) of those stars can indeed be explained by high-extinction red giants and the rest are composed of extinguished asymptotic giant branch (AGB) stars (mostly O-rich), blended sources, and smaller numbers of other objects, a contaminant fraction that can be reduced with the help of Gaia Data Release 2 (DR2) data. Galactic red giants experience a NIR extinction with α ∼ 2.27 and an uncertainty of a few hundredths of a magnitude. There is no significant spread in α even though our sample is widely distributed and has a broad range of extinctions. Differences with previous results are ascribed to the treatment of non-linear photometric effects and/or the contaminant correction. Future research should concentrate in finding the correct functional form for the NIR extinction law. In the appendix, we detail the treatment of non-linear photometric effects in the 2MASS bands.
PDF
