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A Classification of H II Regions Based on Oxygen and Helium Lines: The Cases of TOL 2146-391 and TOL 0357-3915
We present long-slit spectrophotometry of two H II galaxies: TOL2146-391 and TOL 0357-3915. We performed a detailed analysisthat involves abundance determinations relaxing the assumption ofhomogeneous temperature. The temperature inhomogeneity values, t2, were obtained through two methods: (1) comparingabundances from oxygen recombination lines to abundances fromcollisionally excited lines and (2) by using the line intensity ratiosof a set of He I lines together with the HELIO10 program. We find thatthe HELIO10 program is a good alternative to obtain a t 2value in photoionized regions where recombination lines of heavyelements are not available. We have plotted 27 high- and low-metallicityH II regions in an oxygen degree of ionization versus t 2diagram; we find areas populated by H II regions and areas void of them;the physical characteristics of each area are discussed. In addition, anaverage t 2 value can be determined for the objects in eacharea. We propose to use this langt 2rang value for the caseswhere a direct measurement of t 2 cannot be determined.Based on observations collected at the European Southern Observatory,Chile, proposal number ESO 69.C-0203(A).
| Analysis of Two Small Magellanic Cloud H II Regions Considering Thermal Inhomogeneities: Implications for the Determinations of Extragalactic Chemical Abundances
We present long-slit spectrophotometry considering the presence ofthermal inhomogeneities (t 2) of two H II regions in theSmall Magellanic Cloud (SMC): NGC 456 and NGC 460. Physical conditionsand chemical abundances were determined for three positions in NGC 456and one position in NGC 460, first under the assumption of uniformtemperature and then allowing for the possibility of thermalinhomogeneities. We determined t 2 values based on threedifferent methods: (1) by comparing the temperature derived using oxygenforbidden lines with the temperature derived using helium recombinationlines (RLs), (2) by comparing the abundances derived from oxygenforbidden lines with those derived from oxygen RLs, and (3) by comparingthe abundances derived from ultraviolet carbon forbidden lines withthose derived from optical carbon RLs. The first two methods averaged t2 = 0.067 ± 0.013 for NGC 456 and t 2 =0.036 ± 0.027 for NGC 460. These values of t 2 implythat when gaseous abundances are determined with collisionally excitedlines they are underestimated by a factor of nearly two. From theseobjects and others in the literature, we find that in order to accountfor thermal inhomogeneities and dust depletion, the O/H ratio inlow-metallicity H II regions should be corrected by 0.25-0.45 dexdepending on the thermal structure of the nebula or by 0.35 dex if suchinformation is not available.Based on observations collected at the European Southern Observatory,Chile, proposal number ESO 69.C-0203(A).
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Chemical Composition of Two H II Regions in NGC 6822 Based on VLT Spectroscopy
We present long-slit spectrophotometry of regions V and X of the LocalGroup irregular galaxy NGC 6822. The data consist of VLT FORSobservations in the 3450-7500 Å range. We have obtained electrontemperatures and densities using different line intensity ratios. Wehave derived the He, C, and O abundances relative to H based onrecombination lines; the abundance ratios among these elements arealmost independent of the temperature structure of the nebulae. We havealso determined the N, O, Ne, S, Cl, and Ar abundances based oncollisionally excited lines; the ratios of these abundances relative tothat of H depend strongly on the temperature structure of the nebulae.The chemical composition of NGC 6822 V is compared with those of theSun, the Orion Nebula, NGC 346 in the SMC, and 30 Doradus in the LMC.The value of O/H derived from recombination lines is in good agreementwith that derived by Venn and coworkers from two A-type supergiants inNGC 6822.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 69.C-0203(A).
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Dwarf K and M stars in the southern hemisphere.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972AJ.....77..486U&db_key=AST
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