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Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction
We relate the equivalent widths of 11 diffuse interstellar bands,measured in the spectra of 49 stars, to different colour excesses in theultraviolet. We find that most of the observed bands correlatepositively with the extinction in the neighbourhood of the2175-Åbump. Correlation with colour excesses in other parts of theextinction curve is more variable from one diffuse interstellar band toanother; we find that some diffuse bands (5797, 5850 and 6376 Å)correlate positively with the overall slope of the extinction curve,while others (5780 and 6284 Å) exhibit negative correlation. Wediscuss the implications of these results on the links between thediffuse interstellar band carriers and the properties of theinterstellar grains.
| Spectroscopic and Photometric Analysis of the O-Type Binary V1182 Aquilae: A Close Eclipsing System with a Luminous Third Body
We obtained high-resolution spectroscopy and UBV photometry of theO-type eclipsing binary V1182 Aql. In the spectra lines of a thirdcomponent were found; the presence of third light is also supported bythe solution of the light curves. New masses for both components of thebinary were derived: M1=31.0 and M2=16.6Msolar. These values differ considerably from those given byBell et al., which were obtained by neglecting the presence of a thirdbody. With Teff~43,000 K the primary component is much hotterthan expected for the previously assumed spectral type O8. Itsclassification has to be revised to O5.5, which makes V1182 Aql probablythe earliest eclipsing binary in the Galaxy. The mass of the primary issmaller than suggested by evolutionary models, while the secondaryparameters agree with a position close to the zero-age main sequence(ZAMS). The third body, which manifests itself by strong lines in theoptical spectrum and by a third light contribution of ~17% as deducedfrom the light curve solution might be detectable by interferometricmeasurements.Based on observations collected at the European Southern Observatory, LaSilla, Chile, and at the German-Spanish Astronomical Centre, Calar Alto,operated by the Max-Planck-Institut für Astronomie, Heidelberg,jointly with the Spanish National Commission for Astronomy; also basedon spectral data retrieved from the ELODIE archive at Observatoire deHaute Provence (OHP).
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| Is G84.0+0.8 a high mass star formation site near the edge of the Pelican nebula?
We present visible and near-infrared observations of the G84.0+0.8 HIIregion, a bright compact knot projected within the boundaries of the W80complex dominated by the North America and Pelican nebulae. The spectrumof the nebula indicates a temperature of the ionizing stellar spectrumT* ≃ 40 000{-}45 000 K (corresponding to a O7-O5 star)and a density of the HII region n ≃ 460 cm-3, with aforeground extinction of AV ≃ 5.9 mag. A comparison ofnarrow-band near-infrared images through the Brγ and the H2 S(1)v=1 → 0 filters shows that G84.0+0.8 consists of a fan-shapedcavity in a molecular cloud at least partly bounded by aphotodissociation region, filled with Brγ-emitting ionized gas,and with a compact cluster at the tip of the fan. The brightest star atthe position of the cluster is found to be a late G-type interloper.While membership of G84.0+0.8 in the local arm is well established fromexisting radial velocity measurements of the ionized gas, we find thatthe ionizing flux estimated from the size and density of the nebula onthe one hand, and the radio continuum properties of the nebula on theother hand, are well below the expected ionizing flux of a mid, or evenlate, O-type star. We consider the possibility that G84.0+0.8 might beexternally ionized by a nearby mid-O star. Currently availableobservations do not definitely confirm or reject the membership ofG84.0+0.8 in the W80 complex, although a larger distance seems favoredby the available data. Nevertheless, we can firmly rule out thepossibility that it represents a massive star forming site in thatcomplex, as its appearance as a compact HII region containing anembedded cluster may lead one to think.
| To see or not to see a bow shock. Identifying bow shocks with Hα allsky surveys
OB-stars have the highest luminosities and strongest stellar winds ofall stars, which enables them to interact strongly with theirsurrounding ISM, thus creating bow shocks. These offer us an idealopportunity to learn more about the ISM. They were first detected andanalysed around runaway OB-stars using the IRAS allsky survey by vanBuren et al. (1995, AJ, 110, 2614). Using the geometry of such bowshocks information concerning the ISM density and its fluctuations canbe gained from such infrared observations. As to help to improve the bowshock models, additional observations at other wavelengths, e.g.Hα, are most welcome. However due to their low velocity these bowshocks have a size of 1°, and could only be observed as awhole with great difficulties. In the light of the new Hα allskysurveys (SHASSA/VTSS) this is no problem any more. We developeddifferent methods to detect bow shocks, e.g. the improved determinationof their symmetry axis with radial distance profiles. Using twoHα-allsky surveys (SHASSA/VTSS), we searched for bow shocks andcompared the different methods. From our sample we conclude, that thecorrelation between the direction of both proper motion and the symmetryaxis determined with radial distance profile is the most promisingdetection method. We found eight bow shocks around HD17505, HD 24430, HD48099, HD 57061, HD92206, HD 135240, HD149757, and HD 158186 from 37 candidatestaken from van Buren et al. (1995, AJ, 110, 2614). Additionally to thetraditional determination of ISM parameters using the standoff distanceof the bow shock, another approach was chosen, using the thickness ofthe bow-shock layer. Both methods lead to the same results, yieldingdensities ( 1 cm-3) and the maximal temperatures (104 K), that fit well to the up-to-date picture of the WarmIonised Medium.
| The ionizing star of the North America and Pelican nebulae
We present the results of a search for the ionizing star of the NorthAmerica (NGC 7000) and the Pelican (IC 5070) nebulae complex. Theapplication of adequate selection criteria to the 2MASS JH KSbroad-band photometry allows us to narrow the search down to 19preliminary candidates in a circle of 0o 5 radius containingmost of the L935 dark cloud that separates both nebulae. Follow-upnear-infrared spectroscopy shows that most of these objects are carbonstars and mid-to-late-type giants, including some AGB stars. Two of thethree remaining objects turn out to be later than spectral type B andthus cannot account for the ionization of the nebula, but a thirdobject, 2MASS J205551.25+435224.6, has infrared properties consistentwith it being a mid O-type star at the distance of the nebulae complexand reddened by AV ≃ 9.6. We confirm its O5V spectraltype by means of visible spectroscopy in the blue. This star has thespectral type required by the ionization conditions of the nebulae andphotometric properties consistent with the most recent estimates oftheir distance. Moreover, it lies close to the geometric center of thecomplex that other studies have proposed as the most likely location forthe ionizing star, and is also very close to the position inferred fromthe morphology of cloud rims detected in radio continuum. Given thefulfillment of all the conditions and the existence of only one star inthe whole search area that satisfies them, we thus propose 2MASSJ205551.25+435224.6 as the ionizing star of the North America/Pelicancomplex.Based on observations collected at the Centro AstronómicoHispano-Alemán (CAHA) at Calar Alto, operated jointly by theMax-Planck Institut für Astronomie and the Instituto deAstrofísica de Andalucía (CSIC).Figure 1 is only available in electronic form athttp://www.edpsciences.org
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| Optical photometry and spectral classification in the field of the open cluster NGC 6996 in the North America Nebula
We present and discuss broad band CCD UBV(I)_C photometry and lowresolution spectroscopy for stars in the region of the open cluster NGC6996, located in the North America Nebula. The new data allow us totightly constrain the basic properties of this object. We revise thecluster size, which in the past has been significantly underestimated.The width of the Main Sequence is mainly interpreted in terms ofdifferential reddening, and indeed the stars' color excessEB-V ranges from 0.43 to 0.65, implying the presence of asignificant and evenly distributed dust component. We cross-correlateour optical photometry with near infrared photometry from 2MASS, and bymeans of spectral classification we are able to build extinction curvesfor an handful of bright members. We find that the reddening slope andthe total to selective absorption ratio R_V toward NGC 6996 areanomalous. Moreover the reddening-corrected colors and magnitudes allowus to derive estimates for the cluster distance and age, which turn outto be 760 ± 70 pc (V0-MV = 9.4 ±0.2) and 350 Myr, respectively. Based on our results, we suggestthat NGC 6996 is located in front of the North America Nebula, and doesnot seem to have any apparent relationship with it.Based on observations carried out at Asiago and Teramo Observatories,Italy.Photometry is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/419/149
| On the relation between diffuse bands and column densities of H2, CH and CO molecules
Mutual relations between column densities of H2, CH and COmolecules as well as between the latter and strengths of the major 5780and 5797 diffuse bands are presented and discussed. The CH radical seemsto be a good H2 tracer, possibly better than CO. It is alsodemonstrated that the molecular fraction of the H2 moleculeis correlated with an intensity ratio of 5797 and 5780 DIBs, suggestingthe possible formation of narrow DIB carriers in denser clouds,dominated by molecular hydrogen and reasonably shielded from ionizing UVradiation by small dust grains.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/949
| Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| Some Diffuse Interstellar Bands Related to Interstellar C2 Molecules
We have investigated the correlations between the equivalent widths of21 selected diffuse interstellar bands (DIBs) and the correspondinginterstellar column densities N(C2), N(CN), and N(CH), toward53 stars with color excesses 0.11<=E(B-V)<=1.99. The observationaldata were derived primarily from echelle spectra acquired at R=38,000 aspart of our extensive, continuing survey of the bands. All but six ofthe 53 final spectra show signal-to-noise ratios >=800 at 5780Å. The principal result presented here is that seven of the 21bands prove to be examples of ``the C2 DIBs,'' a class ofweak, narrow bands whose normalized equivalent widthsWλ(X)/Wλ (λ6196) are wellcorrelated specifically with N(C2)/E(B-V) via power laws. Incontrast, the similarly normalized equivalent widths of the 14 other,well-known DIBs analyzed here are uncorrelated, or weaklyanticorrelated, with N(C2)/E(B-V), to within theobservational uncertainties. Thus, the polyatomic molecule(s) presumedto cause these seven C2 DIBs may bear a direct chemicalrelation to C2 that is not shared by the polyatomic moleculesputatively responsible for the other 14 bands. The C2 DIBsalso show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) inour particular sample of light paths, although generally with shallowerslopes in the case of N(CN) and with greater scatter in the case ofN(CH). Eleven additional C2 DIBs are also identified but arenot analyzed here. Among the 18 C2 DIBs identified, fourapparently have not been previously detected. The λ4963 band isgenerally the strongest of the 18 C2 DIBs, while theλ4734 band shows the most sensitive correlation withN(C2).Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| Gas-Phase Iron Abundances and Depletions in Translucent Interstellar Lines of Sight from Far Ultraviolet Spectroscopic Explorer Observations of Fe II Lines
Far Ultraviolet Spectroscopic Explorer (FUSE) wavelength coverageincludes several weak- to moderate-strength lines of Fe II, allowing thedetermination, through curve-of-growth analysis, of accurate Fe IIabundances and hence iron depletions. We have analyzed Fe II absorptionlines toward 18 of the reddened stars included in the FUSE survey ofmolecular hydrogen abundances in translucent clouds. Our analysis isbased on equivalent width measurements and curves of growth, aided bythe fact that some of the observed lines are weak enough to be on thelinear part of the curve of growth. In interpreting our abundance anddepletion results, we have combined our data with those of an earliersurvey of interstellar iron abundances and depletions in diffuse clouds,based on Copernicus data. The principal result of our survey is thatiron depletions, known from earlier work to increase with averageline-of-sight density for diffuse clouds, do not continue to increasewith either density or extinction in translucent clouds; i.e., there isno significant trend of increasing depletion with increasing extinctionor molecular fraction. This may be due to the fact that our data setdoes not probe lines of sight with greater average volume densities thanthose that were covered by the previous Copernicus-based survey of irondepletions. We conclude by reevaluating the definition of translucentclouds, based on the lack of enhanced iron depletions in our sample.
| The Spectral Energy Distribution and Mass-Loss Rate of the A-Type Supergiant Deneb
A stellar wind module has been developed for the PHOENIX stellaratmosphere code for the purpose of computing non-LTE, line-blanketed,expanding atmospheric structures and detailed synthetic spectra of hotluminous stars with winds. We apply the code to observations of Deneb,for which we report the first positive detections of millimeter andcentimeter emission (obtained using the Submillimeter Common-UserBolometric Array and the Very Large Array) as well a strong upper limiton the 870 μm flux (using the Heinrich Hertz Telescope). The slope ofthe radio spectrum shows that the stellar wind is partially ionized. Wereport a uniform-disk angular diameter measurementθUD=2.40+/-0.06 mas from the Navy PrototypeOptical Interferometer (NPOI). The measured bolometric flux andcorrected NPOI angular diameter yield an effective temperature of8600+/-500 K. Least-squares comparisons of synthetic spectral energydistributions from 1220 Å to 3.6 cm with the observations provideestimates for the effective temperature and the mass-loss rate of~=8400+/-100 K and (8+/-3)×10-7 Msolaryr-1, respectively. This range of mass-loss rates isconsistent with that derived from high-dispersion UV spectra whennon-LTE metal-line blanketing is considered. We are unable achieve areasonable fit to a typical Hα P Cygni profile with any modelparameters over a reasonable range. This is troubling because theHα profile is the observational basis for the windmomentum-luminosity relationship.
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| An atlas of 2.4 to 4.1 mu m ISO/SWS spectra of early-type stars
We present an atlas of spectra of O- and B-type stars, obtained with theShort Wavelength Spectrometer (SWS) during the Post-Helium program ofthe Infrared Space Observatory (ISO). This program is aimed at extendingthe Morgan & Keenan classification scheme into the near-infrared.Later type stars will be discussed in a separate publication. Theobservations consist of 57 SWS Post-Helium spectra from 2.4 to 4.1 μm, supplemented with 10 spectra acquired during the nominal mission witha similar observational setting. For B-type stars, this sample providesample spectral coverage in terms of subtype and luminosity class. ForO-type stars, the ISO sample is coarse and therefore is complementedwith 8 UKIRT Larcmin -band observations. In terms of the presence ofdiagnostic lines, the Larcmin -band is likely the most promising of thenear-infrared atmospheric windows for the study of the physicalproperties of B stars. Specifically, this wavelength interval containsthe Bralpha , Pfgamma , and other Pfund lines which are probes ofspectral type, luminosity class and mass loss. Here, we present simpleempirical methods based on the lines present in the 2.4 to 4.1 mu minterval that allow the determination of i) the spectral type of Bdwarfs and giants to within two subtypes; ii) the luminosity class of Bstars to within two classes; iii) the mass-loss rate of O stars and Bsupergiants to within 0.25 dex. Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA. The appendix is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin.qcat?J/A+A/384/473
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| Chemical Abundances of OB Stars in Five OB Associations
We present LTE abundances of magnesium, aluminum, sulfur, and iron andnon-LTE abundances of carbon, nitrogen, oxygen, and silicon for a sampleof 15 slowly rotating B stars belonging to five OB associations: CygOB3, Cyg OB7, Lac OB1, Vul OB1, and Cep OB3. These OB associations lieon the Galactic plane and are situated within 3 kpc of the Sun. Of theeight elements sampled, non-LTE abundances for C, N, O, and Si, as wellas LTE abundances for Al and Fe, generally show subsolar abundances,with typical underabundances of ~0.2-0.4 dex. The LTE abundances for Mgand S tend to fall closer to solar values in the five associations.Whether the somewhat larger abundances derived for Mg and S, relative tothe other six elements studied, are significantly different will requirefurther work, while the modest, but persistent, underabundances(relative to solar) found for the other elements confirm a number ofprevious studies of young disk OB stars lying relatively near to theSun. The five associations studied here do not span a significant rangeof Galactocentric distances; however, their derived abundances agreewith what would be expected based upon previous studies that have mappedabundance versus Galactocentric distance and measured abundancegradients in the Milky Way disk.
| Detection of the Faint Companion in the Massive Binary HD 199579
We present new radial velocity data for the massive binary HD 199579 O6V((f)) based upon spectra obtained from IUE, the Canada-France-HawaiiTelescope, KPNO, the David Dunlap Observatory, and the GSUMultiple-Telescope Telescope. We derive a revised period,P=48.5216+/-0.0015 days, and improved orbital elements that agree withthe earlier elements within their errors. We applied a Dopplertomography algorithm to the KPNO spectra to reconstruct the individualprimary and secondary spectra in the red, yielding the first detectionof the secondary's spectrum. The spectral features observed, impliedmass ratio (M2/M1=4+/-1), and magnitude difference(▵V=2.5+/-0.3) are all consistent with a secondary of type B1 V-B2V. The maximum angular separation of the components is predicted to be~1.2 mas, and thus the binary is an important target for opticalinterferometry.
| A possible sets of diffuse bands originating at the same carrier
This paper discusses measurements of eight selected diffuse interstellarbands (DIBs): lambda lambda 5793, 5809, 5819, 5828, 6196, 6397, 6614 and6660 performed in high resolution, high S/N spectra of 41 reddenedstars. Central depths, considered less error-prone than equivalentwidths, are measured and mutual correlations between the selected DIBsare analyzed. Tight correlations between the DIBs: 5809, 6196, 6614 and6660 may suggest their common origin despite their widths differing by afactor of up to 2. The performed simulations prove that this fact doesnot preclude a common, molecular carrier of such features.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| In-Flight Calibration of the ROSAT HRI Ultraviolet Sensitivity
Comparing measured and estimated count rates of a few selected samplestars, we confirm the validity and provide the in-flight calibration ofthe ROSAT HRI UV/visible effective area model in Zombeck et al. Thecount rate estimates for Betelgeuse derived with this model are inagreement with the measured HRI upper limit. This result is alsoconfirmed in an erratum by Berghöfer et al. aimed at revising theirprevious calculation, which was overestimated by more than 2 orders ofmagnitude. Adopting this ROSAT HRI UV/visible effective area model andmeasured UV/visible spectra of a set of sample stars covering the rangeof Teff 3000-40,000 K, we have built the calibration curvesto estimate UV/visible contamination count rates for any star of knownTeff, mv, and NH.
| The Search for Interstellar C60
The optical region of a number of reddened O-type stars has beenexamined on Keck I HIRES spectrograms (R=45,000) for evidence ofinterstellar C60. No absorption features were detected nearthe laboratory C60 wavelengths 3857 and 3980 Å. Aninterstellar feature is present at 6220.8 Å, but it isunacceptably far from the laboratory gas-phase wavelength of 6217.5Å. It is probably just another of the weak diffuse interstellarbands (DIBs), which are numerous in that spectral region. The mostastronomically promising C60 feature was measured in thelaboratory at 3284 Å in liquid or solid matrices. Its gas-phasewavelength can be inferred either from matrix shifts of C60features at longer wavelengths or from high-temperature gas-phasemeasurements. On that evidence, the interstellar feature could fallanywhere between about 3244 and 3306 Å. Its width is uncertain buthere is taken to be about 1 Å . No interstellar absorption fittingthese specifications and as strong as 16 mÅ has been detected inthe stars observed, including Cyg OB2/8A of E(B-V)=1.60. It follows thatin that particular line of sight and for the assumed FWHM of 1 Å ,N(C60)<4.5×1011 cm-2. However,some recent laboratory spectroscopy suggests that its width may be verymuch larger, in which case this limit would be invalid. At this upperlimit, the corresponding number of carbon atoms contained in neutralC60 indicates that that molecule would be only a minorcontributor to the total amount of C in that direction, and would beless than 1% of the amount that may be tied up in the DIBs. StrongerC60 bands are known in the laboratory at approximately 2110and 2566 Å, but the upper limit on 3284 Å suggests that theywill not be easy to detect without high resolution, high signal-to-noiseratio (S/N) satellite spectroscopy and better laboratory gas-phasewavelengths. An estimate of the column density ofC+60, under the assumption that the 9577, 9632Å bands are indeed due to C+60 and that thelaboratory f-value is correct, indicates that theC+60/C60 abundance in that line ofsight is greater than 100.
| On the Correlation between CO Absorption and Far-Ultraviolet Nonlinear Extinction toward Galactic OB Stars
A sample of 59 sight lines to reddened Galactic OB stars was examinedfor correlations of the strength of the CO Fourth Positive(A1Π-X1Σ+) absorption bandsystem with the ultraviolet interstellar extinction curve parameters. Weused archival high-dispersion NEWSIPS IUE spectra to measure the COabsorption for comparison with parametric fits of the extinction curvesfrom the literature. A strong correlation with the nonlinear far-UVcurvature term was found with greater absorption, normalized to E(B-V),being associated with more curvature. A weaker trend with the linearextinction term was also found. Mechanisms for enhancing CO in dustenvironments exhibiting high nonlinear curvature are discussed.
| Far infrared spectroscopy of FU Ori objects. ISO-LWS observations
We present the results of the first spectrophotometric observations of asample of FU Ori objects obtained with the Long Wavelength Spectrometer(LWS) on board the Infrared Space Observatory (ISO). The [OI] (63 mu m)and the [CII] (158 mu m) lines are commonly observed in all spectra(both ON and OFF source). The observational novelty is the presence inmost of the sources of the transition of ionised nitrogen [NII] (122 mum), which is not detected in other objects in a similar evolutionaryphase. This line probes low ionisation and low density material noteasily traced by other lines. Line intensities and intensity ratios areused along with model predictions to infer the prevailing mechanisms forline excitation. To reconcile our far-infrared spectroscopy withprevious knowledge of these objects, the simultaneous presence of twocomponents is required: well localised J-shocks, responsible for the[OI] emission, and an extended low density ionised medium produced by UVphotons from the disc boundary layer, responsible for the [NII] and[CII] emission. A few molecular lines (CO, OH, H2O)associated with relatively cold and dense peaks are revealed and theirintensities are in good agreement with the proposed scenario. Otherionic lines ([OIII] and [NIII]) are detected in two sources in the CygOB7 region and likely trace the presence of nearby HII regions. ISO isan ESA project with instruments funded by ESA Member States and with theparticipation of ISAS and NASA
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Osservazione e dati astrometrici
| Costellazione: | Cigno |
| Ascensione retta: | 20h56m34.70s |
| Declinazione: | +44°55'30.0" |
| Magnitudine apparente: | 5.96 |
| Distanza: | 1204.819 parsec |
| Moto proprio RA: | 0.4 |
| Moto proprio Dec: | -1 |
| B-T magnitude: | 5.984 |
| V-T magnitude: | 5.962 |
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