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.
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.
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.
Context. The second data release (DR2) from the European Space Agency mission Gaia took place on April 2018. DR2 included photometry for more than 1.3 × 109 sources in the three bands G, GBP, and ...GRP. Even though the Gaia DR2 photometry is very precise, there are currently three alternative definitions of the sensitivity curves that show significative differences. Aims. The aim of this paper is to improve the quality of the input calibration data to produce new compatible definitions of the G, GBP, and GRP bands and to identify the reasons for the discrepancies between previous definitions. Methods. We have searched the Hubble Space Telescope (HST) archive for Space Telescope Imaging Spectrograph (STIS) spectra with G430L+G750L data obtained with wide apertures and combined them with the CALSPEC library to produce a high quality spectral energy distribution (SED) library of 122 stars with a broad range of colors, including three very red stars. This library defines new sensitivity curves for G, GBP, and GRP using a functional analytical formalism. Results. The new sensitivity curves are significantly better than the two previous attempts we use as a reference, REV (Evans et al. 2018, A&A, 616, A4) and WEI (Weiler 2018, A&A, 616, A17). For G we confirm the existence of a systematic bias in magnitude and correct a color term present in REV. For GBP we confirm the need to define two magnitude ranges with different sensitivity curves and measure the cut between them at Gphot = 10.87 mag with a significant increase in precision. The new curves also fit the data better than either REV or WEI. For GRP, our new sensitivity curve fits the STIS spectra better and the differences with previous attempts reside in a systematic effect between ground-based and HST spectral libraries. Additional evidence from color–color diagrams indicate that the new sensitivity curve is more accurate. Nevertheless, there is still room for improvement in the accuracy of the sensitivity curves because of the current dearth of good-quality red calibrators: adding more to the sample should be a priority before Gaia data release 3 takes place.
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.
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.
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 introduce a Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution ...far-ultraviolet STIS spectroscopy of R136 using 17 contiguous 52 arcsec × 0.2 arcsec slits which together provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90 per cent of the 57 sources brighter than m
F555W = 16.0 mag within a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including R136b (O4 If/WN8). We measure wind velocities for 52 early-type stars from C ivλλ1548–51, including 16 O2–3 stars. For the first time, we spectroscopically classify all Weigelt and Baier members of R136a, which comprise three WN5 stars (a1–a3), two O supergiants (a5–a6) and three early O dwarfs (a4, a7, a8). A complete Hertzsprung–Russell diagram for the most massive O stars in R136 is provided, from which we obtain a cluster age of 1.5
$^{+0.3}_{-0.7}$
Myr. In addition, we discuss the integrated ultraviolet spectrum of R136, and highlight the central role played by the most luminous stars in producing the prominent He ii λ1640 emission line. This emission is totally dominated by very massive stars with initial masses above ∼100 M⊙. The presence of strong He ii λ1640 emission in the integrated light of very young star clusters (e.g. A1 in NGC 3125) favours an initial mass function extending well beyond a conventional upper limit of 100 M⊙. We include montages of ultraviolet spectroscopy for Large Magellanic Cloud O stars in the appendix. Future studies in this series will focus on optical STIS medium resolution observations.
Context.
The early-third
Gaia
data release (EDR3) parallaxes constitute the most detailed and accurate dataset that currently can be used to determine stellar distances in the solar neighborhood. ...Nevertheless, there is still room for improvement in their calibration and systematic effects can be further reduced in some circumstances.
Aims.
The aim of this paper is to determine an improved
Gaia
EDR3 parallax bias as a function of magnitude, color, and ecliptic latitude using a single method applied to stars in open clusters, globular clusters, the Large Magellanic Cloud, and the Small Magellanic Cloud.
Methods.
I study the behavior of the residuals or differences between the individual (stellar) parallaxes and the group parallaxes, which are assumed to be constant for the corresponding cluster or galaxy. This was done by first applying the Lindegren et al. (2021b, A&A, 649, A4) zero point and then calculating a new zero point from the residuals of the first analysis.
Results.
The Lindegren zero point shows very small residuals as a function of magnitude between individual and group parallaxes for
G
> 13 but significant ones for brighter stars, especially blue ones. The new zero point reduces those residuals, especially in the 9.2 <
G
< 13 range. The
k
factor that is used to convert from catalog parallax uncertainties to external uncertainties is small (1.1–1.7) for 9.2 <
G
< 11 and
G
> 13, intermediate (1.7–2.0) for 11 <
G
< 13, and large (>2.0) for
G
< 9.2. Therefore, significant corrections are needed to calculate distance uncertainties from
Gaia
EDR3 parallaxes for some stars. There is still room for improvement if future analyses add information from additional stellar clusters, especially for red stars with
G
< 11 and blue stars with
G
< 9.2. I also calculated
k
for stars with RUWE values between 1.4 and 8.0 and for stars with six-parameter solutions, allowing for a correct estiimation of their uncertainties.
We report magnetic and spectroscopic observations and modelling of the Of?p star CPD -28... 2561. Using more than 75 new spectra, we have measured the equivalent width variations and examined the ...dynamic spectra of photospheric and wind-sensitive spectral lines. A period search results in an unambiguous 73.41 d variability period. High-resolution spectropolarimetric data analysed using least-squares deconvolution yield a Zeeman signature detected in the mean Stokes V profile corresponding to phase 0.5 of the spectral ephemeris. Interpreting the 73.41 d period as the stellar rotational period, we have phased the equivalent widths and inferred longitudinal field measurements. The phased magnetic data exhibit a weak sinusoidal variation, with maximum of about 565 G at phase 0.5, and a minimum of about -335 G at phase 0.0, with extrema approximately in phase with the (double-wave) H... equivalent width variation. Modelling of the H... equivalent width variation assuming a quasi-3D magnetospheric model produces a unique solution for the ambiguous couplet of inclination and magnetic obliquity angles: (i, ...) or (..., i) = (35..., 90...). Adopting either geometry, the longitudinal field variation yields a dipole polar intensity Bd = 2.6 ± 0.9 kG, consistent with that obtained from direct modelling of the Stokes V profiles. We derive a wind magnetic confinement parameter ..., leading to an Alfven radius ..., and a Kepler radius ... This supports a physical scenario in which the H... emission and other line variability have their origin in an oblique, corotating 'dynamical magnetosphere' structure resulting from a magnetically channelled wind. Nevertheless, the details of the formation of spectral lines and their variability within this framework remain generally poorly understood. (ProQuest: ... denotes formulae/symbols omitted.)
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