Context.
The recent early third data release (EDR3) from the
Gaia
mission has produced parallaxes for 1.468 × 10
9
sources with better quality than those reported in the previous data release. ...Nevertheless, there are calibration issues with the data that require corrections to the published values and uncertainties.
Aims.
We want to properly characterize the behavior of the random and systematic uncertainties of the
Gaia
EDR3 parallaxes in order to maximize the precision of the derived distances without compromising their accuracy. We also aim to provide a step-by-step procedure for the calculation of distances to stars and stellar clusters when using these parallaxes.
Methods.
We reanalyzed some of the data presented in the calibration papers for quasar and Large Magellanic Cloud (LMC) parallaxes and combine these results with measurements for six bright globular clusters. We calculated the angular covariance of EDR3 parallaxes at small separations (up to a few degrees) based on the LMC results and combined it with the results for larger angles using quasars to obtain an approximate analytical formula for the angular covariance over the whole sky. The results for the six globular clusters were used to validate the parallax bias correction as a function of magnitude, color, and ecliptic latitude and to determine the multiplicative constant
k
used to convert internal uncertainties to external ones.
Results.
The angular covariance at zero separation is estimated to be 106 μas
2
, yielding a minimum (systematic) uncertainty for EDR3 parallaxes of 10.3 μas for individual stars or compact stellar clusters. This value can be slightly reduced for globular clusters that span ≳30′ after considering the behavior of the angular covariance of the parallaxes for small separations. A recent parallax bias correction is found to work quite well, except perhaps for the brighter magnitudes, where improvements may be possible. The value of
k
is found to be 1.1–1.7 and to depend on
G
. We find that stars with moderately large values of the renormalized unit weight error can still provide useful parallaxes, albeit with larger values of
k
. We give accurate and precise
Gaia
EDR3 distances to the six globular clusters, and for the specific case of 47 Tuc we are able to beat the angular covariance limit through the use of the background Small Magellanic Cloud as a reference and derive a high-precision distance of 4.53 ± 0.06 kpc. Finally, a recipe for the derivation of distances to stars and stellar clusters using
Gaia
EDR3 parallaxes is given.
Context. O stars are excellent tracers of the intervening ISM because of their high luminosity, blue intrinsic SED, and relatively featureless spectra. We are currently conducting the Galactic O-Star ...Spectroscopic Survey (GOSSS), which is generating a large sample of O stars with accurate spectral types within several kpc of the Sun. Aims. We aim to obtain a global picture of the properties of dust extinction in the solar neighborhood based on optical-NIR photometry of O stars with accurate spectral types. Methods. We have processed a carefully selected photometric set with the CHORIZOS code to measure the amount E(4405 − 5495) and type R5495 of extinction towards 562 O-type stellar systems. We have tested three different families of extinction laws and analyzed our results with the help of additional archival data. Results. The Maíz Apellániz et al. (2014, A&A, 564, A63) family of extinction laws provides a better description of Galactic dust that either the Cardelli et al. (1989, ApJ, 345, 245) or Fitzpatrick (1999, PASP, 111, 63) families, so it should be preferentially used when analysing samples similar to the one in this paper. In many cases O stars and late-type stars experience similar amounts of extinction at similar distances but some O stars are located close to the molecular clouds left over from their births and have larger extinctions than the average for nearby late-type populations. In qualitative terms, O stars experience a more diverse extinction than late-type stars, as some are affected by the small-grain-size, low-R5495 effect of molecular clouds and others by the large-grain-size, high-R5495 effect of H II regions. Late-type stars experience a narrower range of grain sizes or R5495, as their extinction is predominantly caused by the average, diffuse ISM. We propose that the reason for the existence of large-grain-size, high-R5495 regions in the ISM in the form of H II regions and hot-gas bubbles is the selective destruction of small dust grains by EUV photons and possibly by thermal sputtering by atoms or ions.
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 lack of high-precision long-term continuous photometric data for large samples of stars has impeded the large-scale exploration of pulsational variability in the OB star regime. As a ...result, the candidates for in-depth asteroseismic modelling have remained limited to a few dozen dwarfs. The TESS nominal space mission has surveyed the southern sky, including parts of the galactic plane, yielding continuous data across at least 27 d for hundreds of OB stars.
Aims.
We aim to couple TESS data in the southern sky with ground-based spectroscopy to study the variability in two dimensions, mass and evolution. We focus mainly on the presence of coherent pulsation modes that may or may not be present in the predicted theoretical instability domains and unravel all frequency behaviour in the amplitude spectra of the TESS data.
Methods.
We compose a sample of 98 OB-type stars observed by TESS in Sectors 1–13 and with available multi-epoch, high-resolution spectroscopy gathered by the IACOB and OWN surveys. We present the short-cadence 2 min light curves of dozens of OB-type stars, which have one or more spectra in the IACOB or OWN database. Based on these light curves and their Lomb–Scargle periodograms, we performed variability classification and frequency analysis. We placed the stars in the spectroscopic Hertzsprung–Russell diagram to interpret the variability in an evolutionary context.
Results.
We deduce the diverse origins of the mmag-level variability found in all of the 98 OB stars in the TESS data. We find among the sample several new variable stars, including three hybrid pulsators, three eclipsing binaries, high frequency modes in a Be star, and potential heat-driven pulsations in two Oe stars.
Conclusions.
We identify stars for which future asteroseismic modelling is possible, provided mode identification is achieved. By comparing the position of the variables to theoretical instability strips, we discuss the current shortcomings in non-adiabatic pulsation theory and the distribution of pulsators in the upper Hertzsprung–Russell diagram.
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.)
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.
ABSTRACT
We have detected a broad interstellar absorption band centred close to 7700 Å and with a full width at half-maximum (FWHM) of 176.6 ± 3.9 Å. This is the first such absorption band detected ...in the optical range and is significantly wider than the numerous diffuse interstellar bands (DIBs). It remained undiscovered until now because it is partially hidden behind the A telluric band produced by O2. The band was discovered using STIS@HST spectra and later detected in a large sample of stars of diverse type (OB stars, BA supergiants, red giants), using further STIS and ground-based spectroscopy. The EW of the band is measured and compared with our extinction and K i λλ7667.021, 7701.093 measurements for the same sample. The carrier is ubiquitous in the diffuse and translucent Galactic interstellar medium (ISM) but is depleted in the environment around OB stars. In particular, it appears to be absent or nearly so in sightlines rich in molecular carbon. This behaviour is similar to that of the σ-type DIBs, which originate in the low/intermediate-density UV-exposed ISM but are depleted in the high-density UV-shielded molecular clouds. We also present an update on our previous work on the relationship between E(4405–5495) and R5495 and incorporate our results into a general model of the ISM.
Context.
Dynamical interactions in young stellar clusters can eject massive stars early in their lives and significantly alter their mass functions. If all of the most massive stars are lost, we are ...left with an orphan cluster.
Aims.
We study the Bermuda cluster (Villafranca O-014 NW), the most significant young stellar group in the North America and Pelican nebulae, and the massive stars that may have been ejected from it to test if it has been orphaned.
Methods.
We use
Gaia
EDR3 parallaxes and proper motions to search for walkaway and runaway stars in the vicinity of the North America and Pelican nebulae. The candidates are analyzed with a combination of spectroscopy and photometry to assess their nature and their trajectories are traced back in time to determine at what time they left the Bermuda cluster.
Results.
We detect three ejection events, dubbed the Bajamar, Toronto, and HD 201 795 events, which expelled (a minimum of) five, two, and two systems, respectively, or six, three, and three stars if we count the individual components in spectroscopic and eclipsing binaries. The events took place 1.611 ± 0.011 Ma, 1.496 ± 0.044 Ma, and 1.905 ± 0.037 Ma ago, respectively, but our analysis is marginally consistent with the first two being simultaneous. We detect bow shocks in WISE images associated with four of the ejected systems; their orientation agrees with that of their relative proper motions with respect to the cluster. Combining the three events, the Bermuda cluster has lost over 200
M
⊙
, including its three most massive stars, so it can be rightfully considered an orphan cluster. One consequence is that the present-day mass function of the cluster has been radically altered from its top-heavy initial value to one compatible with a Kroupa-like function. Another is that the cluster is currently expanding with a dynamical timescale consistent with the cause being the ejection events. A scenario in which the Bermuda cluster was formed in a conveyor-belt fashion over several hundreds of ka or even 1 Ma is consistent with all the observables.
A close encounter of the massive kind Apellaniz, J Maiz; Sana, H; Barba, R H ...
Monthly notices of the Royal Astronomical Society,
01/2017, Letnik:
464, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We have used (i) Hubble Space Telescope Advanced Camera for Surveys imaging and Space Telescope Imaging Spectrograph spectroscopy, (ii) ground-based Precision Integrated-Optics Near-infrared Imaging ...ExpeRiment/Very Large Telescope long-baseline interferometry, and (iii) ground-based spectroscopy from different instruments to study the orbit of the extreme multiple system HD 93...129 Aa,Ab, which is composed of (at least) two very massive stars in a long-period orbit with e > 0.92, which will pass through periastron in 2017/2018. In several ways, the system is an ... Car precursor. Around the time of periastron passage, the two very strong winds will collide and generate an outburst of non-thermal hard X-ray emission without precedent in an O+O binary since astronomers have been able to observe above Earth's atmosphere. A coordinated multiwavelength monitoring in the next two years will enable a breakthrough understanding of the wind interactions in such extreme close encounters. Furthermore, we have found evidence that HD 93 129 Aa may be a binary system itself. In that case, we could witness a three-body interaction which may yield a runaway star or a stellar collision close to or shortly after the periastron passage. Either of those outcomes would be unprecedented, as they are predicted to be low-frequency events in the Milky Way. (ProQuest: ... denotes formulae/symbols omitted.)
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