- 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.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/466/989
- Title:
- Hot ammonia from NGC 6334I and NGC 6334I(N)
- Short Name:
- J/A+A/466/989
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The massive twin cores NGC 6334I and I(N) are in different evolutionary stages and hence ideal targets to study evolutionary variations within the same larger-scale environment. Here, we study the warm, compact gas components. We imaged the two regions with the Australia Telescope Compact Array (ATCA) at high angular resolution in the NH3(3, 3) to (6, 6) inversion lines.
- 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).
- 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.
- 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.
- ID:
- ivo://CDS.VizieR/J/ApJ/452/238
- Title:
- HST photometry of I Zw 18 massive stars
- Short Name:
- J/ApJ/452/238
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Blue compact dwarf galaxies are tiny galaxies that are dominated by intense star-forming regions. Thus, they have been thought to represent a different and extreme environment for star formation compared to the Milky Way and many other nearby galaxies. In this paper we use images obtained with the Hubble Space Telescope to resolve the main body of one of these galaxies, I Zw 18, into stars for the first time. This galaxy is also one of the most metal-poor galaxies known and is sometimes argued to be evolutionarily young. Broadband colors are used to determine the bulk characteristics of the resolved stellar population to an F555W magnitude of about 26, or M_(V,0)~-4 (O9.5 V star). Narrow-band images are used to look for emission characteristic of Wolf-Rayet stars. Color-magnitude diagrams reveal a broad main sequence of massive stars as well as blue and red supergiants, although there is a surprising lack of Wolf-Rayet stars. About half the massive stars are located in two groups, corresponding to the two knots of emission identified in ground-based images, and the rest are distributed between these regions and in the outer parts of the galaxy. We find that the northwestern of these two regions resolves in Halpha images into a small H II region plus a complicated shell structure that encircles the northern stellar association. There are additional, larger loops and filaments of ionized gas up to 450pc from the center of the galaxy. While the shell could be as young as the stars it encircles, the larger ionized gas structures must be older for reasonable models, and their presence implies that there has been a previous generation of massive stars at least several tens of millions of years ago. However, we find no evidence in these data for stars that must be older than this. The larger sites of most recent star formation are located northwest and south of the central part of the galaxy. Thus, we find that I Zw 18 is somewhat more complicated than had previously been thought, both in terms of its star formation history and the state of its interstellar medium. At the same time, however, the stellar populations look relatively normal, and the spatial concentration of massive stars is closer to that of large OB associations in nearby galaxies rather than to that exemplified by the compact cluster R136 in the Large Magellanic Cloud.
- 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.
- 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.
- 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.
- 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.
