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51. Hipparcos southern B8-F2 stars radial velocities
ID:
ivo://CDS.VizieR/J/A+AS/135/503
Title:
Hipparcos southern B8-F2 stars radial velocities
Short Name:
J/A+AS/135/503
Date:
21 Oct 2021
Publisher:
CDS
Description:
Radial velocities have been determined for a sample of B8-F2 type stars observed by the Hipparcos satellite. Observations were obtained within the framework of an ESO key-program. Radial velocities have been measured using a cross-correlation method, the templates being a grid of synthetic spectra. The obtained precision depends on effective temperature and projected rotational velocity of the star as well as on a possible asymmetry of the correlation peak generally due to secondary components. New spectroscopic binaries have been detected from these a symmetries and the variability of the measured radial velocity. Simulations of binary and triple systems have been performed. For binaries our results have been compared with Hipparcos binary data. Adding the variable radial velocities, the minimum binary fraction has been found 60% for physical systems. Radial velocities have been determined for 581 B8-F2 stars, 159 being new. Taking into account published radial velocities, 39% south A-type stars with V magnitude lower than 7.5 have a radial velocity.
52. Hot stars emitting in X-ray. I.
ID:
ivo://CDS.VizieR/J/A+A/506/1055
Title:
Hot stars emitting in X-ray. I.
Short Name:
J/A+A/506/1055
Date:
21 Oct 2021
Publisher:
CDS
Description:
Following the advent of increasingly sensitive X-ray observatories, deep observations of early-type stars became possible. However, the results for only a few objects or clusters have until now been reported and there has been no large survey comparable to that based upon the ROSAT All-Sky Survey (RASS). A limited survey of X-ray sources, consisting of all public XMM observations (2XMMi) and slew survey data (XMMSL1), is now available. The X-ray counterparts to hot, massive stars have been searched for in these catalogs. About 300 OB stars were detected with XMM. Half of them were bright enough for a spectral analysis to be possible, and we make available the detailed spectral properties that were derived. The X-ray spectra of O stars are represented well by low (<1keV) temperature components and seem to indicate that an absorption column is present in addition to the interstellar contribution. The X-ray fluxes are well correlated with the bolometric fluxes, with a scatter comparable to that of the RASS studies and thus larger than found previously with XMM for some individual clusters. These results contrast with those of B stars that exhibit a large scatter in the L_X-L_BOL relation, no additional absorption being found, and the fits indicate a plasma at higher temperatures. Variability (either within one exposure or between multiple exposures) was also investigated whenever possible: short-term variations are far more rare than long-term ones (the former affects a few percent of the sample, while the latter concerns between one third and two thirds of the sources).
53. HST Observations of SMC N88A
ID:
ivo://CDS.VizieR/J/A+A/347/841
Title:
HST Observations of SMC N88A
Short Name:
J/A+A/347/841
Date:
21 Oct 2021
Publisher:
CDS
Description:
High-resolution Hubble Space Telescope images have allowed us for the first time to resolve the compact SMC ionized ``blob'' N88A (diameter ~3.5arcsec or 1pc). This very young HII, region, which is hatching from its natal molecular cloud, is heavily affected by absorbing dust associated with the cloud. The interstellar reddening towards N88A is on average A_v_~1.5mag and strikingly rises to more than 3.5mag in a narrow dust band crossing the core of the HII region. Such a high extinction is unprecedented for an HII region in the metal-poor SMC. We present the photometry of some 70 stars lying towards the OB association at the center of which lies N88A. The exciting star(s) of N88A is not detected, due to the heavy extinction. The chronology of star formation is discussed for the whole region.
54. HST photometry in 6 dwarf galaxies
ID:
ivo://CDS.VizieR/J/AJ/143/74
Title:
HST photometry in 6 dwarf galaxies
Short Name:
J/AJ/143/74
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a comprehensive study of young stellar populations in six dwarf galaxies in or near the Local Group: Phoenix, Pegasus, Sextans A, Sextans B, WLM, and NGC 6822. Their star-forming regions, selected from GALEX wide-field far-UV imaging, were imaged (at sub-pc resolution) with the WFPC2 camera on board the Hubble Space Telescope (HST) in six bandpasses from far-UV to IR to detect and characterize their hot massive star content. This study is part of HST treasury survey program HST-GO-11079; the general data characteristics and reduction procedures are detailed in this paper and results are presented for the first six galaxies. From a total of 180 HST images, we provide catalogs of the multi-band stellar photometry and derive the physical parameters of massive stars by analyzing it with model-atmosphere colors. We use the results to infer ages, number of massive stars, extinction, and spatial characteristics of the young stellar populations. The hot massive star content varies largely across our galaxy sample, from an inconspicuous presence in Phoenix and Pegasus to the highest relative abundance of young massive stars in Sextans A and WLM. Albeit to a largely varying extent, most galaxies show a very young population (a few Myrs, except for Phoenix), and older ones (a few 10^7^ years in Sextans A, Sextans B, NGC 6822, and WLM, ~10^8^yr in Phoenix and Pegasus), suggesting discrete bursts of recent star formation in the mapped regions. The hot massive star content (indicative of the young populations) broadly correlates with the total galaxy stellar mass represented by the integrated optical magnitude, although it varies by a factor of ~3 between Sextans A, WLM, and Sextans B, which have similar M_V_. Extinction properties are also derived.
55. HST photometry of young stellar populations in M31
ID:
ivo://CDS.VizieR/J/AJ/144/142
Title:
HST photometry of young stellar populations in M31
Short Name:
J/AJ/144/142
Date:
21 Oct 2021
Publisher:
CDS
Description:
We studied the young stellar populations of 22 star-forming regions in the Andromeda galaxy (M31), with Hubble Space Telescope (HST) multi-band imaging from far-UV to I. The regions were selected from Galaxy Evolution Explorer (GALEX) wide-field far-UV imaging; they sample different environments and galactocentric distances from 6 to 22kpc. They were imaged with 30 HST fields (360 distinct images, in six bandpasses), with a pixel scale of 0.38pc projected on the sky, at the distance of M31. This study is part of HST treasury survey program HST-GO-11079, which includes star-forming regions in eight Local Group galaxies. We provide a merged catalog of six-band stellar photometry in the 30 M31 fields, containing 118036 sources brighter than V and B~23mag. Each HST field covers about 0.3kpc^2^ in M31, and contains up to ~7000 stars, of which the number varies by a factor of >7 among the target regions; a large fraction of the sample are hot massive stars, due to our choice of filters and exposures. We derived stellar physical parameters and interstellar extinction for individual sources by spectral energy distribution analysis with model-atmosphere colors, and used the results to infer ages, massive stars content, and extinction of the star-forming regions. Reddening is up to E(B-V)<~0.6mag in some OB associations, and lowest in the outermost regions (average of <~0.12mag in OB184 at 21.9kpc). We examined the spatial distribution (clustering) of the hot massive stars, and defined OB associations on various spatial scales from compact to wider, more spread out ones. A hierarchical structuring is observed, with small compact groups arranged within large complexes. Their areas vary from less than 10 to 10^5^pc^2^, and masses are up to {approx}10^5^M_{sun}_, in the scales sampled by our analysis. Their cumulative mass distribution follows a power law, at least in part of the sampled regime. Hot-star counts in the young regions compare very well with integrated measurements of UV flux from GALEX.
56. Hydrogen-Line Absorption in Early-Type Stars
ID:
ivo://CDS.VizieR/II/40
Title:
Hydrogen-Line Absorption in Early-Type Stars
Short Name:
II/40
Date:
21 Oct 2021
Publisher:
CDS
Description:
Photoelectric determinations of H-{gamma} absorption line intensity have been made of stars in selected clusters and associations as well as in the general galactic field. Interference filters having a width at half intensity of 45{AA} were used to isolate spectral regions centered on 4280{AA}, H-{gamma} and 4410{AA}. The {Gamma} indices derived, which represent a measure of H-{gamma} absorption, can be used in conjunction with unreddened values of either (U-B) or (B-V) for spectral and luminosity class determinations of stars in the spectral range O6 to A0.
57. Infrared emission of young HII regions
ID:
ivo://CDS.VizieR/J/A+A/579/A71
Title:
Infrared emission of young HII regions
Short Name:
J/A+A/579/A71
Date:
21 Oct 2021
Publisher:
CDS
Description:
Investigating the relationship between radio and infrared emission of HII regions may help shed light on the nature of the ionizing stars and the formation mechanism of early-type stars in general. We have taken advantage of recent unbiased surveys of the Galactic plane such as Herschel/Hi-GAL and VLA/CORNISH to study a bona fide sample of young HII regions located in the Galactic longitude range 10{deg}-65{deg} by comparing the mid- and far-IR continuum emission to the radio free-free emission at 5GHz. We have identified the Hi-GAL counterparts of 230 CORNISH HII regions and reconstructed the spectral energy distributions of 204 of these by complementing the Hi-GAL fluxes with ancillary data at longer and shorter wavelengths. Using literature data, we obtained a kinematical distance estimate for 200 HII regions with Hi-GAL counterparts and determined their luminosities by integrating the emission of the corresponding spectral energy distributions. We have also estimated the mass of the associated molecular clumps from the (sub)millimeter flux densities. Our main finding is that for ~1/3 of the HII regions the Lyman continuum luminosity appears to be greater than the value expected for a zero-age main-sequence star with the same bolometric luminosity. This result indicates that a considerable fraction of young, embedded early-type stars presents a "Lyman excess" possibly due to UV photons emitted from shocked material infalling onto the star itself and/or a circumstellar disk. Finally, by comparing the bolometric and Lyman continuum luminosities with the mass of the associated clump, we derive a star formation efficiency of 5%. The results obtained suggest that accretion may still be present during the early stages of the evolution of HII regions, with important effects on the production of ionizing photons and thus on the circumstellar environment. More reliable numerical models describing the accretion process onto massive stars are required to shed light on the origin of the observed Lyman excess.
58. Intermediate high-velocity clouds distances
ID:
ivo://CDS.VizieR/J/MNRAS/415/1105
Title:
Intermediate high-velocity clouds distances
Short Name:
J/MNRAS/415/1105
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present optical spectra of 403 stars and quasi-stellar objects in order to obtain distance limits towards intermediate- and high-velocity clouds (IHVCs), including new Fibre-fed Extended Range Optical Spectrograph (FEROS) observations plus archival ELODIE, FEROS, High Resolution Echelle Spectrometer (HIRES) and Ultraviolet and Visual Echelle Spectrograph (UVES) data.
59. Interstellar Ca II line equivalent widths
ID:
ivo://CDS.VizieR/J/ApJ/634/451
Title:
Interstellar Ca II line equivalent widths
Short Name:
J/ApJ/634/451
Date:
21 Oct 2021
Publisher:
CDS
Description:
We show that the equivalent widths of the well-known interstellar CaII H and K lines can be used to determine the distances to OB stars in our Galaxy. The equivalent widths, measured in the spectra of 147 early-type stars, are strongly related to the Hipparcos parallaxes of those objects. The lines fitted to the parallax-equivalent width data are given by the formulae {pi}=1/[2.78EW(K)+95] and {pi}=1/[4.58EW(H)+102], where {pi} is in arcseconds and EW is in milliangstroms.
60. Interstellar CaII lines for 290 stars
ID:
ivo://CDS.VizieR/J/A+A/507/833
Title:
Interstellar CaII lines for 290 stars
Short Name:
J/A+A/507/833
Date:
21 Oct 2021
Publisher:
CDS
Description:
We attempt to extend the relation between the strengths of the interstellar CaII lines and the distances to early-type stars to objects beyond 1 kiloparsec, with the line saturation taken into account. We measure the CaII K and CaII H equivalent widths, and compute Ca II column densities for 262 lines of sight towards early-type stars with available Hipparcos parallaxes (pi). The targets are located within a few hundred parsecs of the Galactic plane, and span all the range of Galactic longitudes. We fit the N_CaII_ - parallax relation with a function of the form pi=1/(a.N_CaII_+b), using a maximum-likelihood approach to take account of errors in both variables. We use the resultant formula to estimate distances to stars in OB associations and clusters, and compare them to those found in the literature, usually estimated by spectrophotometric methods.

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