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Asteroseismology of the β Cephei star 12 (DD) Lacertae: photometric observations, pulsational frequency analysis and mode identification
We report a multisite photometric campaign for the β Cephei star 12Lacertae. 750 h of high-quality differential photoelectricStrömgren, Johnson and Geneva time-series photometry were obtainedwith nine telescopes during 190 nights. Our frequency analysis resultsin the detection of 23 sinusoidal signals in the light curves. Ten ofthose correspond to independent pulsation modes, and the remainder arecombination frequencies. We find some slow aperiodic variability such asthat seemingly present in several β Cephei stars. We perform modeidentification from our colour photometry, derive the spherical degree lfor the five strongest modes unambiguously and provide constraints on lfor the weaker modes. We find a mixture of modes of 0 <=l<= 4. Inparticular, we prove that the previously suspected rotationally splittriplet within the modes of 12 Lac consists of modes of different ltheir equal frequency splitting must thus be accidental.One of the periodic signals we detected in the light curves is argued tobe a linearly stable mode excited to visible amplitude by non-linearmode coupling via a 2:1 resonance. We also find a low-frequency signalin the light variations whose physical nature is unclear; it could be aparent or daughter mode resonantly coupled. The remaining combinationfrequencies are consistent with simple light-curve distortions.The range of excited pulsation frequencies of 12 Lac may be sufficientlylarge that it cannot be reproduced by standard models. We suspect thatthe star has a larger metal abundance in the pulsational driving zone, ahypothesis also capable of explaining the presence of β Cepheistars in the Large Magellanic Cloud.
| Detailed spectroscopic analysis of the Trapezium cluster stars inside the Orion nebula. Rotational velocities, stellar parameters, and oxygen abundances
We present the results of a spectroscopic analysis of the Trapeziumcluster stars inside the Orion nebula. The rotational velocities wereobtained using the Fourier analysis method, and we found agreement withvalues derived by the usual method based on linewidth measurements. Therotational velocity derived for θ1 Ori C by thismethod is consistent with the variability of some of its spectralfeatures that have a period of 15.42 days. By means of the fit of H, HeI, and He II observed profiles with Fastwind synthetic profiles, stellarparameters and wind characteristics were derived. This methodology letus estimate the errors associated with these parameters, and we foundthat macroturbulence effects have to be included for a good fit to theHe I-II lines in the spectrum of θ1 Ori C. By means ofa very accurate study, oxygen abundances were derived for the threeB0.5V stars θ1 Ori A, D, and θ2 OriB. Final abundances are consistent with the nebular gas-phase resultspresented in Esteban et al. (2004) and are lower than those given byCunha & Lambert (1994). Our results suggest a lower dust depletionfactor of oxygen than previous estimations for the Orion nebula.
| Multiwavelength monitoring of BD +53°2790, the optical counterpart to 4U 2206+54
We present the results of our long-term monitoring of BD+53°2790, the optical counterpart to the X-ray source4U 2206+54. Unlike previous studies that classify thesource as a Be/X-ray binary, we find that its optical and infraredproperties differ from those of typical Be stars: the variability of theV/R ratio is not cyclical; there are variations in the shape andstrength of the Hα emission line on timescales less than 1 day;and no correlation between the EW and the IR magnitudes or colors isseen. Our observations suggest that BD +53°2790is very likely a peculiar O9.5V star. In spite of exhaustive searches wecannot find any significant modulation in any emission line parameter oroptical/infrared magnitudes. Spectroscopy of the source extending fromthe optical to the K-band confirms the peculiarity of the spectrum: notonly are the He lines stronger than expected for an O9.5V star but alsothere is no clear pattern of variability. The possibility thatBD +53°2790 is an early-type analogue toHe-strong stars (like θ1 Ori C) is discussed.
| Astrophysical parameters for the eclipsing binary u Herculis
Not Available
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Chemical composition of Galactic OB stars. II. The fast rotator ζ Ophiuchi
ζ Oph, HD 149757, is an O9.5Vnn star with a very high projected rotational velocity (v sin i ≥340 km s-1). It is also a classical runaway star due to itshigh proper motion. We perform a quantitative analysis of its opticalspectrum in order to measure important observables of the star such asits mass, effective temperature, luminosity and He, C, N, and Oabundances. Comparing these observed values to those predicted by therotating evolutionary models of the Geneva group we find that none ofthe two sets of models (those of 2000 and 2003) is capable ofreproducing the characteristics of the star. Nevertheless, due to itsrunaway nature, the reason for this discrepancy may be that the star isnot the result of the evolution of a single object, but the product ofthe evolution of a close binary system.
| O stars with weak winds: the Galactic case
We study the stellar and wind properties of a sample of Galactic Odwarfs to track the conditions under which weak winds (i.e. mass lossrates lower than 10-8 Mȯ yr-1)appear. The sample is composed of low and high luminosity dwarfsincluding Vz stars and stars known to display qualitatively weak winds.Atmosphere models including non-LTE treatment, spherical expansion andline blanketing are computed with the code CMFGEN (Hillier & Miller1998, ApJ, 496, 407). Both UV and Hα lines are used to derive windproperties while optical H and He lines give the stellar parameters. Wefind that the stars of our sample are usually 1 to 4 Myr old. Mass lossrates of all stars are found to be lower than expected from thehydrodynamical predictions of Vink et al. (2001, A&A, 369, 574). Forstars with log {L}/{Lȯ} 5.2, the reduction is byless than a factor 5 and is mainly due to the inclusion of clumping inthe models. For stars with log {L}/{Lȯ} 5.2 thereduction can be as high as a factor 100. The inclusion of X-rayemission (possibly due to magnetic mechanisms) in models with lowdensity is crucial to derive accurate mass loss rates from UV lines,while it is found to be unimportant for high density winds. The modifiedwind momentum - luminosity relation shows a significant change of slopearound this transition luminosity. Terminal velocities of low luminositystars are also found to be low. Both mass loss rates and terminalvelocities of low L stars are consistent with a reduced line forceparameter α. However, the physical reason for such a reduction isstill not clear although the finding of weak winds in Galactic starsexcludes the role of a reduced metallicity. There may be a link betweenan early evolutionary state and a weak wind, but this has to beconfirmed by further studies of Vz stars. X-rays, through the change inthe ionisation structure they imply, may be at the origin of a reductionof the radiative acceleration, leading to lower mass loss rates. Abetter understanding of the origin of X-rays is of crucial importancefor the study of the physics of weak winds.
| Spectral analysis of early-type stars using a genetic algorithm based fitting method
We present the first automated fitting method for the quantitativespectroscopy of O- and early B-type stars with stellar winds. The methodcombines the non-LTE stellar atmosphere code fastwind from Puls et al.(2005, A&A, 435, 669) with the genetic algorithm based optimizationroutine pikaia from Charbonneau (1995, ApJS, 101, 309), allowing for ahomogeneous analysis of upcoming large samples of early-type stars (e.g.Evans et al. 2005, A&A, 437, 467). In this first implementation weuse continuum normalized optical hydrogen and helium lines to determinephotospheric and wind parameters. We have assigned weights to theselines accounting for line blends with species not taken into account,lacking physics, and/or possible or potential problems in the modelatmosphere code. We find the method to be robust, fast, and accurate.Using our method we analysed seven O-type stars in the young cluster CygOB2 and five other Galactic stars with high rotational velocities and/orlow mass loss rates (including 10 Lac, ζ Oph, and τ Sco) thathave been studied in detail with a previous version of fastwind. Thefits are found to have a quality that is comparable or even better thanproduced by the classical “by eye” method. We defineerrorbars on the model parameters based on the maximum variations ofthese parameters in the models that cluster around the global optimum.Using this concept, for the investigated dataset we are able to recovermass-loss rates down to ~6 × 10-8~Mȯyr-1 to within an error of a factor of two, ignoringpossible systematic errors due to uncertainties in the continuumnormalization. Comparison of our derived spectroscopic masses with thosederived from stellar evolutionary models are in very good agreement,i.e. based on the limited sample that we have studied we do not findindications for a mass discrepancy. For three stars we findsignificantly higher surface gravities than previously reported. Weidentify this to be due to differences in the weighting of Balmer linewings between our automated method and “by eye” fittingand/or an improved multidimensional optimization of the parameters. Theempirical modified wind momentum relation constructed on the basis ofthe stars analysed here agrees to within the error bars with thetheoretical relation predicted by Vink et al. (2000, A&A, 362, 295),including those cases for which the winds are weak (i.e. less than a fewtimes 10-7 Mȯ yr-1).
| 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.
| Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
We present column density measurements of the HD molecule in theinterstellar gas toward 17 Galactic stars. The values for the seven mostheavily reddened sightlines, with E(B-V) = 0.38-0.72, are derived fromobservations with the Far Ultraviolet Spectroscopic Explorer (FUSE). Theother ten values are from a reanalysis of spectra obtained withCopernicus. In all cases, high-resolution ground-based observations of KI and/or the CH molecule were used to constrain the gas velocitystructure and to correct for saturation effects. Comparisons of thecolumn densities HD, CH, CN, and K I in these 17 sightlines indicatethat HD is most tightly correlated with CH. Stringent lower limits tothe interstellar D/H ratio, derived from the HD/2H2 ratio,range from 3.7 × 10-7 to 4.3 × 10-6.Our results also suggest that the HD/H2 ratio increases withthe molecular fraction f(H2) and that the interstellar D/Hratio might be obtained from HD by probing clouds with f(H2) 1. Finally, we note an apparent relationship between the molecularfractions of hydrogen and deuterium.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| 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.
| Quantitative Studies of the Far-Ultraviolet, Ultraviolet, and Optical Spectra of Late O- and Early B-Type Supergiants in the Magellanic Clouds
We present quantitative studies of eight late O- and early B-typesupergiants in the Magellanic Clouds using far-ultraviolet FarUltraviolet Spectroscopic Explorer, ultraviolet InternationalUltraviolet Explorer/Hubble Space Telescope, and optical VLT-UVESspectroscopy. Temperatures, mass-loss rates, and CNO abundances areobtained using the non-LTE, spherical, line-blanketed model atmospherecode of Hillier & Miller. We support recent results for lowertemperatures of OB-type supergiants as a result of stellar winds andblanketing, which amounts to ~2000 K at B0 Ia. In general,Hα-derived mass-loss rates are consistent with UV and far-UVspectroscopy, although from consideration of the S IVλλ1063, 1073 doublet, clumped winds are preferred overhomogenous models. AV 235 (B0 Iaw) is a notable exception, which has anunusually strong Hα profile that is inconsistent with the otherBalmer lines and UV wind diagnostics. We also derive CNO abundances forour sample, revealing substantial nitrogen enrichment, with carbon andoxygen depletion. Our results are supported by comparison with theGalactic supergiant HD 2905 (BC0.7 Ia) for which near-solar CNOabundances are obtained. This bolsters previous suggestions that``normal'' OB-type supergiants exhibit atmospheric compositionsindicative of partial CNO processing.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated by Johns Hopkins Universityunder NASA contract NAS5-32985. Also based in part on observationscollected at the European Southern Observatory Very Large Telescope inprogram 67.D-0238, plus archival data obtained with the NASA-ESA HubbleSpace Telescope and the NASA-ESA-PPARC International UltravioletExplorer.
| 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.
| 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.
| Fast Line-Profile Variability in the Spectra of O Stars
A program of the search for and analysis of profile variability in thespectra of bright O supergiants with a time resolution of 5-30 min isdescribed. Preliminary results of the spectroscopic observations of thestars lambda Ori, alpha Cam, and 19 Cep with the 1-m SpecialAstrophysical Observatory telescope in 2001 are presented. Line-profilevariability was detected in the spectra of all the stars studied;variability in the H_alpha and C III lambda5696 lines in the spectrum oflambda Ori has been found for the first time. The variability amplitudeis 4-5% for 19 Cep and 1-2% for alpha Cam and lambda Ori on time scalesfrom several hours to 3 or 4 days, and the width of the variablefeatures reaches 2 Angstroms (100 km/s). We detected cyclical variationsin the He II lambda4686 and C III lambda5696 line profiles in thespectrum of lambda Ori on time scales of 1.3-1.6 days. Rapid profilevariations on time scales of 3.5-7 h were found in the violet parts ofthe H_alpha and He I lambda4715 line profiles in the spectrum of lambdaOri A.
| Kinematical Structure of the Local Interstellar Medium: The Galactic Anticenter Hemisphere
A survey of interstellar Na I D1 and D2 absorption features in thespectra of 104 early-type stars in the second and third Galacticquadrants reveals the large-scale kinematics of the interstellar gaswithin the Galactic anticenter hemisphere at distances from the Sunbetween ~70 and ~250 pc. Employing a technique that uses both the radialvelocities and column densities of the Na I absorptions produced by theintervening gas we have identified the velocity vectors and determinedthe spatial distribution of eight interstellar clouds in the volumeexplored. The average internal H I+H2 densities of the cloudsrange between 0.03 and 1.7 cm-3, and their masses between 80and 104 Msolar, although uncertainties in thesizes of the clouds, their possible extension beyond the regionexplored, and the presence of denser gas embedded in the larger cloudsimply that these will tend to be lower limits. We have clearlyidentified clumps of denser gas immersed in the low-density gas in oneof the clouds; these clumps show internal H I+H2 densities oforder 50 cm-3. Although we are not able to detect anyinterstellar Na I within 70 pc, the sizes of some of the clouds implythat their near edges are within that range of distances from the Sun.With respect to the local standard of rest the clouds move withvelocities between 19 and 54 km s-1. Their velocity vectorsdo not support the view of a local interstellar medium uniquelydominated by expansion from centers in the Scorpio-Centaurus OBassociation; our results suggest that this expansion is present in theGalactic center hemisphere but in the Galactic anticenter hemisphere isrestricted to the immediate neighborhood of the Sun.
| Quantitative Spectral Analysis of Early B-Type Supergiants in the Sculptor Galaxy NGC 300
The spectra of two early B-type supergiant stars in the Sculptor spiralgalaxy NGC 300 are analyzed by means of non-LTE line-blanketed unifiedmodel atmospheres, aimed at determining their chemical composition andthe fundamental stellar and wind parameters. For the first time, adetailed chemical abundance pattern (He, C, N, O, Mg, and Si) isobtained for a B-type supergiant beyond the Local Group. The derivedstellar properties are consistent with those of other Local Group B-typesupergiants of similar types and metallicities. One of the stars shows anear solar metallicity, while the other one resembles more a SMC Bsupergiant. The effects of the lower metallicity can be detected in thederived wind momentum.Based on observations obtained at the ESO Very Large Telescope.
| 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.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.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/411/447
| Non-LTE abundances of magnesium, aluminum and sulfur in OB stars near the solar circle
Non-LTE abundances of magnesium, aluminum and sulfur are derived for asample of 23 low-v sin i stars belonging to six northern OBassociations of the Galactic disk within 1 kpc of the Sun. Theabundances are obtained from the fitting of synthetic line profiles tohigh resolution spectra. A comparison of our results with HII regionabundances indicates good agreement for sulfur while the cepheidabundances are higher. The derived abundances of Mg show good overlapwith the cepheid results. The aluminum abundances for OB stars aresignificantly below the cepheid values. But, the OB star results show adependence with effective temperature and need further investigation.The high Al abundances in the cepheids could be the result of mixing. Adiscussion of the oxygen abundance in objects near the solar circlesuggests that the current mean galactic oxygen abundance in this regionis 8.6-8.7 and in agreement with the recently revised oxygen abundancein the solar photosphere. Meaningful comparisons of the absolute S, Aland Mg abundances in OB stars with the Sun must await a reinvestigationof these elements with 3D hydrodynamical model atmospheres for the Sun.No abundance gradients are found within the limited range ingalactocentric distances in the present study. Such variations would beexpected only if there were large metallicity gradients in the disk.
| Gas-phase recombination, grain neutralization and cosmic-ray ionization in diffuse gas
Atomic ions are mostly neutralized by small grains (or PAH molecules) incurrent theories of heating and cooling in cool diffuse clouds; in themain they do not recombine with free electrons. This alters theionization balance by depressing n(H+) and n(He+)while carbon generally remains nearly fully once-ionized: chargeexchange with atomic oxygen and formation of H2 and OH alsodepress n(H+) in partly molecular gas. Seemingly restrictiveempirical limits on {zetaH are relaxed and higher values forzetaH are favored in a wide range of circumstances, whengrain neutralization is recognized. Maintenance of the proton density atlevels needed to reproduce observations of HD requires zeta,SUB>H>~ 2x 10-16 s-1, but such models naturallyexplain the presence of both HD and H3+ inrelatively tenuous H I clouds. In dense gas, a higher ionization ratecan account for high observed fractions of atomic hydrogen, andrecognition of the effects of grain neutralization can resolve a majorparadox in the formation of sulfur-bearing compounds.
| The relation between far-UV and visible extinctions
For directions of sufficient reddening (/E(B-V)>~0.25), there is asimple relation between the slope of the extinction curve in the far-UVand /E(B-V). Regardless of direction, the far-UV extinction curve isproportional to 1/λn e-2E(B-V)/λ(/λ in μm, /n=4), in accordance with the idea that reddenedstars spectra are contaminated by scattered light (Zagury, 2001b). Thisrelation is not compatible with the standard theory of extinction whichstates that far-UV and visible extinctions are due to different classesof particle. In that model the two (far-UV and visible) extinctions varythus independently according to the proportion of each type of particle.In preceding papers I have shown that the standard theory cannot explainUV observations of nebulae, and is contradicted by the UV spectra ofstars with very low reddening: for how long shall the standard theory beconsidered as the interpretation of the extinction curve?
| Fundamental parameters of Galactic luminous OB stars VI. Temperatures, masses and WLR of Cyg OB2 supergiants
We have analyzed six OB supergiants and one giant covering spectraltypes from O3 to B1 in the Galactic OB association Cyg OB2 by means ofan updated version of FASTWIND (Santolaya-Rey et al. \cite{sph97}) thatincludes an approximate treatment of metal line blocking and blanketing.This large coverage in spectral type allows us to derive a newtemperature scale for Galactic O supergiants that is lower than the oneobtained by using pure H-He models, either plane-parallel andhydrostatic or spherical with mass-loss. The lower temperatures are thusa combined effect of line blanketing and the large mass-loss rates. Insome cases, the newly derived effective temperature is reduced by up to8000 K. Changes are larger for earlier stars with large mass-loss rates.As a consequence, luminosities are modified as well, which results in alower number of emerging ionizing photons and reduces the massdiscrepancy. Although there are still significant differences betweenspectroscopic and evolutionary masses, we do not find any obvioussystematic pattern of those differences. We derive mass-loss rates andthe corresponding wind momentum-luminosity relation for the analyzedstars. Although consistent with previous results by Puls et al.(\cite{puls96}) for Galactic stars, our relation is better defined dueto a reduction of errors related to stellar distances and points to apossible separation between extreme Of stars (Of+,Of*) and stars with more moderate morphologies. However thisfinding is only tentative, as the statistics are still scarce. The INTis operated in the island of La Palma by the ING in the SpanishObservatorio de El Roque de los Muchachos of the Instituto deAstrofísica de Canarias.
| The present-day chemical composition of the LMC
High-resolution observations of five OB-type main-sequence stars in theLarge Magellanic Cloud (LMC) have been obtained with the UCLéchelle spectrograph on the 3.9-m Anglo-Australian Telescope.These spectra have been analysed using LTE model-atmosphere techniques,to derive stellar atmospheric parameters and chemical compositions. Asthese stars are located within the hydrogen burning main-sequence band,their surface abundances should reflect those of the present-dayinterstellar medium. Detailed line-by-line differential analyses havebeen undertaken relative to Galactic comparison stars. We conclude thatthere exists a general metal deficiency of -0.31+/-0.04 dex within theLMC, and find no significant abundance variations between cluster andfield stars. There is also tentative evidence to suggest a lower oxygento iron abundance ratio, and an over-deficiency of magnesium relative tothe other alpha -elements. These are discussed in terms of previousabundance analyses and models of discontinuous (or bursting) starformation within the LMC. Finally, there is some evidence to suggest agreater chemical enrichment of material within the H Ii region LH 104.
| On the effective temperatures and surface gravities of superficially normal main sequence band B and A stars
Effective temperatures and surface gravities for 48 main sequence band Band A stars were found by matching optical region spectrophotometry andHγ profiles with the predictions of ATLAS9 solar composition modelatmospheres. When these values were compared with those found usingStrömgren uvbybeta photometry based on ATLAS6 model atmospheres, wefound a difference (photometry-spectrophotometry) of 25+/- 118 K for 29stars with 8000 K le Teff <= 10 050 K compared to 76 +/-105 K for 14 stars with 10 050 K <= Teff <= 17 000 K.The surface gravity scales are in agreement. These stars aresufficiently hot that their effective temperatures and surface gravitydeterminations are unaffected by discrepancies due to the choice ofMixing-Length or Canuto-Mazzitelli convection theories.
| Chemical composition of Galactic OB stars. I. CNO abundances in O9 stars
We present NLTE abundances of CNO for a sample of four O9 stars in theGalaxy, together with new determinations of their stellar parameters,T_eff, log g, epsilon (He) and microturbulence. These new analyses takeinto account the effect of line-blocking in the spectral synthesis withour classical NLTE, plane-parallel and hydrostatic model atmospheres.The sample includes three O9 He normal stars: two dwarfs, HD 214680 andHD 34078, and one supergiant, HD 209975, and one fast rotating giantwith a preliminary high He overabundance, HD 191423 with epsilon(He)=0.20. We find first that the consideration of microturbulence inthe spectral synthesis for the fast rotator leads to a considerablylower He abundance, epsilon (He)=0.12. The CNO abundances of the threeHe normal stars are in good agreement with the values in the literaturefor Galactic B dwarfs with no evidence of mixing, and show that they allhave the same chemical composition. We also discuss however the possibleCNO contamination of the supergiant HD 209975. For the fast rotator wefind that the abundances show the trend of the CNO contamination: a Noverabundance together with C and O depletion. The N/C and N/O ratios ofour stars as a function of their projected rotational velocities areconsistent with the predictions of the recent evolutionary models ofMeynet & Maeder (\cite{Mey&Mae00}). The INT is operated on theisland of La Palma by the RGO in the Spanish Obervatorio de El Roque delos Muchachos of the Instituto de Astrofísica de Canarias.
| A Study of the Wavelength Calibration of NEWSIPS High-Dispersion Spectra
In this study we cross-correlate many IUE echellograms of a variety ofwell-observed stars to evaluate systematic error sources in thewavelength zero points (velocities) of all three cameras. We firstevaluate differences between the final archived (``NEWSIPS'') and theoriginally processed (``IUESIPS'') echellograms. These show a markedtime dependence in zero point for the SWP camera resulting from severalrevisions of wavelength calibration coefficients used for IUESIPS.Smaller offsets are present for the LWR camera between the twoprocessings. We also evaluated small-amplitude fluctuations in the zeropoints of the NEWSIPS wavelength calibration spectra themselves. In thecase of the SWP camera, these variations are too complicated to havebeen completely removed in the NEWSIPS wavelength calibration. We alsoexamine wavelength zero-point disparities between data obtained boththrough the small and large entrance apertures as well as forobservations made by different target acquisitions of faint and brightstars. We also find that statistical differences between thesealternative observing modes are virtually nil. For large-apertureobservations, the dominant error source is the placement of the targetin the aperture. These give rise to non-Gaussian, extended ``tails'' inapparent velocity. We also searched for spurious trends. Except for apossible trend for faint objects with SWP camera data, we cannot detectsignificant dependences with time. Additionally, we discovered a trendwith telescope focus for data sets derived from intensive monitoringcampaigns of bright stars. These exhibit a repeatable, 1 day ``radialvelocity variation'' with a semiamplitude of nearly 3 km s-1.This pattern appears to be a by-product of fluctuations in telescopefocus caused by operational procedures to maintain the ambientinstrument temperature. In the second part of the paper, we measure themean zero-point errors of NEWSIPS echellogram data with respect tolaboratory results by using the Goddard High Resolution Spectrograph(GHRS) spectral atlas of the O9 V spectral standard 10 Lacertae as anintermediary reference. We find that the derived apparent velocitydifference for this star is essentially zero: -1+/-3.5 kms-1. Several less precise comparisons lead to similarresults, including cross-correlations of (1) spectra of 10 Lac and twostars with similar spectra, HD 93521 and HD 60753; (2) lines in commonwith the SWP camera and GHRS and STIS atlases of Arcturus and Procyon;and (3) interstellar lines in the GHRS spectrum of the white dwarfG191-B2B. The zero points of the NEWSIPS-processed long-wavelengthcameras are evaluated and are also found to be nearly zero (+/-5 kms-1) relative to the Arcturus and Procyon atlas calibrationsand relative to one another. In general, these results do not supportthe suggestion by González-Riestra and coworkers that correctionsshould be introduced to the wavelength scales of various NEWSIPShigh-dispersion data products. Despite our optimistic results, it isobvious that using small IUE data sets from large-aperture observationsof arbitrarily chosen stars can contain velocity errors of at least afew km s-1.
| Photometric Monitoring of Bright Be Stars. IV. 1996-1999
We report long-term UBV observations of 15 bright, active Be stars,namely: X Persei, EW Canis Majoris, θ Coronae Borealis, 4 (V839)Herculis, 88 (V744) Herculis, 66 (V2048) Ophiuchi, NW Serpentis, CXDraconis, 12 (V395) Vulpeculae, 28 (V1624) Cygni, QR Vulpeculae, 59(V832) Cygni, EW Lacertae, ο Andromedae, and KX Andromedae. Theobservations were made in 1996-1999 through the Automatic PhotometricTelescope Service in Arizona and through the American Association ofVariable Star Observers (AAVSO) photoelectric photometry program andhave been added to a database extending back 20 years. We describe thestars' recent behavior and also comment on the long-term behavior ofsome of them. They vary photometrically on timescales ranging from abouta day to many years.
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