Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Quantitative spectroscopy of BA-type supergiants
Luminous BA-type supergiants have enormous potential for modernastrophysics. They allow topics ranging from non-LTE physics and theevolution of massive stars to the chemical evolution of galaxies andcosmology to be addressed. A hybrid non-LTE technique for thequantitative spectroscopy of these stars is discussed. Thorough testsand first applications of the spectrum synthesis method are presentedfor the bright Galactic objects η Leo (A0 Ib), HD 111613 (A2 Iabe),HD 92207 (A0 Iae) and β Ori (B8 Iae), based on high-resolution andhigh-S/N Echelle spectra. Stellar parameters are derived fromspectroscopic indicators, consistently from multiple non-LTE ionizationequilibria and Stark-broadened hydrogen line profiles, and they areverified by spectrophotometry. The internal accuracy of the methodallows the 1σ-uncertainties to be reduced to 1-2% in T_effand to 0.05-0.10 dex in log g. Elemental abundances are determined forover 20 chemical species, with many of the astrophysically mostinteresting in non-LTE (H, He, C, N, O, Mg, S, Ti, Fe). The non-LTEcomputations reduce random errors and remove systematic trends in theanalysis. Inappropriate LTE analyses tend to systematicallyunderestimate iron group abundances and overestimate the light andα-process element abundances by up to factors of two to three onthe mean. This is because of the different responses of these species toradiative and collisional processes in the microscopic picture, which isexplained by fundamental differences of their detailed atomic structure,and not taken into account in LTE. Contrary to common assumptions,significant non-LTE abundance corrections of ~0.3 dex can be found evenfor the weakest lines (Wλ 10 mÅ). Non-LTEabundance uncertainties amount to typically 0.05-0.10 dex (random) and~0.10 dex (systematic 1σ-errors). Near-solar abundances arederived for the heavier elements in the sample stars, and patternsindicative of mixing with nuclear-processed matter for the lightelements. These imply a blue-loop scenario for η Leo because offirst dredge-up abundance ratios, while the other three objects appearto have evolved directly from the main sequence. In the most ambitiouscomputations several ten-thousand spectral lines are accounted for inthe spectrum synthesis, permitting the accurate reproduction of theentire observed spectra from the visual to near-IR. This prerequisitefor the quantitative interpretation of intermediate-resolution spectraopens up BA-type supergiants as versatile tools for extragalacticstellar astronomy beyond the Local Group. The technique presented hereis also well suited to improve quantitative analyses of less extremestars of similar spectral types.
| Spectral Analysis of 4 Lacertae and ν Cephei
Elemental abundance analysis based on high S/N and high resolutionDominion Astrophysical Observatory spectrograms have been performed fortwo early type supergiants: 4 Lac (B9 Iab) and {\msf ν } Cep (A2 Ia).Lines as weak as of order 5 mÅ are employed in this study. Theprojected rotational velocities of these stars are 14 and 26 kms-1, respectively. Both stars show similar radial velocityamplitudes, macroturbulent velocities and the same general elementalabundance trends. Their He, CNO and light element abundances are solaror overabundant while the iron peak and heavy element abundances aresolar or underabundant. Detailed LTE model atmosphere abundance analysisshows that 4 Lac has nuclearly processed matter in its photosphere while{\msf ν } Cep does not.
| Chemical composition of evolved stars of high galactic latitude
We have carried out abundance analysis for a sample of high galacticlatitude supergiants in search of evolved stars.We find that HD 27381 has atmospheric parameters and an abundancepatternvery similar to that of the post-AGB star HD 107369.HD 10285 and HD 25291 are moderately metal-poor andshow the influence of mixing that has brought the productsof NeNa cycle to the surface.The high galactic latitude B supergiant HD 137569 shows selectivedepletion of refractory elements normally seen in post-AGB stars.We find that the high velocity B typestar HD 172324 shows moderate deficiency of Fe group elements butthe CNO abundances are verysimilar to that of disk B supergiants. The observed variations inthe radial velocities, transient appearance of emission componentsin hydrogen line profiles and doublingof O I lines at 7774 Å support the possibility of this star beinga pulsating variable or a binary star.
| Asphericity and clumpiness in the winds of Luminous Blue Variables
We present the first systematic spectropolarimetric study of LuminousBlue Variables (LBVs) in the Galaxy and the Magellanic Clouds, in orderto investigate the geometries of their winds. We find that at least halfof our sample show changes in polarization across the strong Hαemission line, indicating that the light from the stars is intrinsicallypolarized and therefore that asphericity already exists at the base ofthe wind. Multi-epoch spectropolarimetry on four targets revealsvariability in their intrinsic polarization. Three of these, AG Car, HRCar and P Cyg, show a position angle (PA) of polarization which appearsrandom with time. Such behaviour can be explained by the presence ofstrong wind-inhomogeneities, or “clumps” within the wind.Only one star, R 127, shows variability at a constant PA, and henceevidence for axi-symmetry as well as clumpiness. However, if viewed atlow inclination, and at limited temporal sampling, such a wind wouldproduce a seemingly random polarization of the type observed in theother three stars. Time-resolved spectropolarimetric monitoring of LBVsis therefore required to determine if LBV winds are axi-symmetric ingeneral. The high fraction of LBVs (>50%) showing intrinsicpolarization is to be compared with the lower ~20-25% for similarstudies of their evolutionary neighbours, O supergiants and Wolf-Rayetstars. We anticipate that this higher incidence is due to the lowereffective gravities of the LBVs, coupled with their variabletemperatures within the bi-stability jump regime. This is alsoconsistent with the higher incidence of wind asphericity that we find inLBVs with strong Hα emission and recent (last ~10 years) strongvariability.
| INTEGRAL observations of SS433: Results of a coordinated campaign
Results of simultaneous INTEGRAL and optical observations of thegalactic microquasar SS433 in May 2003 and INTEGRAL /RXTE observationsin March 2004 are presented. Persistent precessional variability with amaximum to minimum uneclipsed hard X-ray flux ratio of 4 isdiscovered. The 18 60 keV X-ray eclipse is found to be in phase withoptical and near infrared eclipses. The orbital eclipse observed byINTEGRAL in May 2003 is at least two times deeper and apparently widerthan in the soft X-ray band. The broadband 2 100 keV X-ray spectrumsimultaneously detected by RXTE/INTEGRAL in March 2004 can be explainedby bremsstrahlung emission from optically thin thermal plasma withkT 30 keV. Optical spectroscopy with the 6-m SAO BTA telescopeconfirmed the optical companion to be an A5 A7 supergiant. For the firsttime, spectorscopic indications of a strong heating effect in theoptical star atmosphere are found. The measurements of absorption lineswhich are presumably formed on the non-illuminated side of thesupergiant yield its radial velocity semi-amplitude K_v=132± 9 kms-1. The analysis of the observed hard X-ray light curve andthe eclipse duration, combined with the spectroscopically determinedoptical star radial velocity corrected for the strong heating effect,allows us to model SS433 as a massive X-ray binary. Assuming that thehard X-ray source in SS433 is eclipsed by the donor star that exactlyfills its Roche lobe, the masses of the optical and compact componentsin SS433 are suggested to be M_v≈ 30 M_ȯ and M_x≈ 9 M_ȯ,respectively. This provides further evidence that SS433 is a massivebinary system with supercritical accretion onto a black hole.
| On the massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| A near-infrared stellar spectral library: I. H-band spectra.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Non-LTE Line Formation for Hydrogen Revisited
We discuss aspects of non-LTE line formation for hydrogen in early-typestars. We evaluate the effect of variations in the electron-impactexcitation cross sections in model atoms of differing complexity bycomparison with observation. While the Balmer lines are basicallyunaffected by the choice of atomic data, the Paschen, Brackett, andPfund series members allow us to discriminate between the differentmodels. Non-LTE calculations based on the widely used approximationformulae of Mihalas, Heasley, & Auer and of Johnson fail tosimultaneously reproduce the optical and IR spectra over the entireparameter range. The use of data from ab initio calculations up toprincipal quantum number n<=7 largely solves the problem. Werecommend a reference model using the available data. This model is ofgeneral interest because of the ubiquity of the hydrogen spectrum.
| J - K DENIS photometry of a VLTI-selected sample of bright southern stars
We present a photometric survey of bright southern stars carried outusing the DENIS instrument equipped with attenuating filters. Theobservations were carried out not using the survey mode of DENIS, butwith individual target pointings. This project was stimulated by theneed to obtain near-infrared photometry of stars to be used in earlycommissioning observations of the ESO Very Large TelescopeInterferometer, and in particular to establish a network of brightcalibrator sources.We stress that near-infrared photometry is peculiarly lacking for manybright stars. These stars are saturated in 2MASS as well as in regularDENIS observations. The only other observations available for brightinfrared stars are those of the Two Micron Sky Survey dating from overthirty years ago. These were restricted to declinations above≈-30°, and thus cover only about half of the sky accessible fromthe VLTI site.We note that the final 2MASS data release includes photometry of brightstars, obtained by means of point-spread function fitting. However, thismethod only achieves about 30% accuracy, which is not sufficient formost applications.In this work, we present photometry for over 600 stars, each with atleast one and up to eight measurements, in the J and K filters. Typicalaccuracy is at the level of 0\fm05 and 0\fm04 in the J and K_s bands,respectively.Based on observations collected at the European Southern Observatory, LaSilla.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/413/1037
| Quantitative Spectroscopy of Supergiants
Blue supergiants of spectral types B and A are the visually brighteststars in spiral and irregular galaxies, with their most luminous members(at M_V=-10) outshining entire dwarf galaxies. This characteristicallows us to use them as probes to study the Local Universe in greatdetail. In principle, already the existing large telescopes andinstrumentation facilitate quantitative spectroscopy of these objects asfar as the Virgo and Fornax clusters of galaxies. Beyond theirchallenging stellar atmospheres and opportunities for testingsophisticated non-LTE physics they offer numerous applications to modernastrophysics. Quantitative spectroscopy of supergiants can contribute toimprove our understanding of massive star evolution. Galactic abundancegradients and abundance patterns, as can be obtained from studies oflarge ensembles of supergiants in our own and other galaxies, willfoster the understanding of galactochemical evolution. Finally, they arepromising independent indicators for calibrating the extragalacticdistance scale, by application of the wind momentum-luminosity and theflux-weighted gravity-luminosity relationships. In view of this largepotential, the objective of this thesis is to improve the status ofquantitative spectroscopy of BA-type supergiants and to provide firstapplications on a sample of Galactic and extragalactic targets, withinthe Local Group and beyond.
| On the Spectrum and Spectropolarimetry of Type Ic Hypernova SN 2003dh/GRB 030329
Spectroscopic and spectropolarimetric observations of SN 2003dh/GRB030329 obtained in 2003 May using the Subaru 8.2 m Telescope arepresented. The properties of the supernova (SN) are investigated througha comparison with spectra of the Type Ic hypernovae SN 1997ef and SN1998bw (hypernovae being a tentatively defined class of SNe with verybroad absorption features: these features suggest a large velocity ofthe ejected material and possibly a large explosion kinetic energy).Comparison with spectra of other hypernovae shows that the spectrum ofSN 2003dh obtained on 2003 May 8 and 9, i.e., 34-35 rest-frame daysafter the gamma-ray burst (GRB; for z=0.1685), are similar to those ofSN 1997ef obtained ~34-42 days after the fiducial time of explosion ofthat SN. The match with SN 1998bw spectra is not as good (at rest7300-8000 Å), but again spectra obtained ~33-43 days after GRB980425 are preferred. This indicates that the SN may have intermediateproperties between SNe 1997ef and 1998bw. On the basis of the analogywith the other hypernovae, the time of explosion of SN 2003dh is thenconstrained to be between -8 and +2 days of the GRB. The Si and O PCygni lines of SN 2003dh seem comparable to those of SN 1997ef, whichsuggests that the ejected mass in SN 2003dh may match that in SN 1997ef.Polarization was marginally detected at optical wavelengths. This isconsistent with measurements of the late afterglow, implying that itoriginated mostly in the interstellar medium of the host galaxy.Based on data obtained at the Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan (NAOJ).
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe 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/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| A highly sensitive search for magnetic fields in B, A and F stars
Circular spectropolarimetric observations of 74 stars were obtained inan attempt to detect magnetic fields via the longitudinal Zeeman effectin their spectral lines. The sample observed includes 22 normal B, A andF stars, four emission-line B and A stars, 25 Am stars, 10 HgMn stars,two lambda Boo stars and 11 magnetic Ap stars. Using the Least-SquaresDeconvolution multi-line analysis approach (Donati et al.\cite{donati97etal}), high precision Stokes I and V mean signatures wereextracted from each spectrum. We find absolutely no evidence formagnetic fields in the normal, Am and HgMn stars, with upper limits onlongitudinal field measurements usually considerably smaller than anypreviously obtained for these objects. We conclude that if any magneticfields exist in the photospheres of these stars, these fields are notordered as in the magnetic Ap stars, nor do they resemble the fields ofactive late-type stars. We also detect for the first time a field in theA2pSr star HD 108945 and make new precise measurements of longitudinalfields in five previously known magnetic Ap stars, but do not detectfields in five other stars classified as Ap SrCrEu. We also report newresults for several binary systems, including a new vsin i for therapidly rotating secondary of the Am-delta Del SB2 HD 110951. Based onobservations obtained using the MuSiCoS spectropolarimeter on theBernard Lyot telescope, l'Observatoire du Pic du Midi, France.
| Bispectrum speckle interferometry of IRC +10216: The dynamic evolution of the innermost circumstellar environment from 1995 to 2001
We present new near-infrared (JHK) bispectrum speckle-interferometrymonitoring of the carbon star IRC+10216 obtainedbetween 1999 and 2001 with the SAO 6 m telescope. The J-, H-, and K-bandresolutions are 50 mas, 56 mas, and 73 mas, respectively. The totalsequence of K-band observations covers now 8 epochs from 1995 to 2001and shows the dynamic evolution of the inner dust shell. The presentobservations show that the appearance of the dust shell has considerablychanged compared to the epochs of 1995 to 1998. Four main componentswithin a 0\farcs2 radius can be identified in the K-band images. Theapparent separation of the two initially brightest components A and Bincreased from ~ 191 mas in 1995 to ~ 351 mas in 2001. Simultaneously,component B has been fading and almost disappeared in 2000 whereas theinitially faint components C and D became brighter (relative to peakintensity). The changes of the images can be related to changes of theoptical depth caused, for instance, by mass-loss variations or new dustcondensation in the wind. Our recent two-dimensional radiative transfermodel of IRC +10216 suggests that the observedrelative motion of components A and B is not consistent with the outflowof gas and dust at the well-known terminal wind velocity of 15 kms-1. The apparent motion with a deprojected velocity of 19 kms-1 on average and of recently 27 km s-1 appearsto be caused by a displacement of the dust density peak due to dustevaporation in the optically thicker and hotter environment. The presentmonitoring, covering more than 3 pulsation periods, shows that thestructural variations are not related to the stellar pulsation cycle ina simple way. This is consistent with the predictions of hydrodynamicalmodels that enhanced dust formation takes place on a timescale ofseveral pulsation periods. The timescale of the fading of component Bcan well be explained by the formation of new dust in the circumstellarenvelope.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe 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/386/492, and from theauthors on CD-Rom.
| Orbital and Stellar Parameters of Omicron Leonis from Spectroscopy and Interferometry
We present a three-dimensional solution for the orbit of the double starOmicron Leonis, based on new photoelectric radial velocity data mainlyfrom the Observatoire de Haute-Provence and on interferometric dataobtained with the Navy Prototype Optical Interferometer, the Mark IIIStellar Interferometer, and the Palomar Testbed Interferometer. OmicronLeo's primary is a giant of type F9 and the secondary is an A5m dwarf,for which we derive masses of 2.12+/-0.01 Msolar and1.87+/-0.01 Msolar, respectively. The distance to the binaryis determined to be 41.4+/-0.1 pc. Combining the distance with themeasured apparent magnitudes and color differences between thecomponents yields luminosities of 39.4+/-2.4 Lsolar and15.4+/-1.0 Lsolar for primary and secondary, respectively.Data from the Palomar Testbed Interferometer taken at 2.2 μm are usedto constrain the photometry in the infrared.
| Non-LTE line formation for N: Abundances and stellar parameters. Model atom and first results on BA-type stars
A comprehensive model atom for non-LTE line formation calculations forneutral and singly-ionized nitrogen is presented. Highly accurateradiative and collisional atomic data are incorporated, recentlydetermined for astrophysical and fusion research using the R-matrixmethod in the close-coupling approximation. As a test and firstapplication of the model, nitrogen abundances are determined on thebasis of line-blanketed LTE model atmospheres for five stars, the mainsequence object Vega (A0 V) and the supergiantseta Leo (A0 Ib), HD 111613 (A2Iabe), HD 92207 (A0 Iae) and betaOri (B8 Iae), using high S/N and high-resolution spectra atvisual and near-IR wavelengths. The computed non-LTE line profiles fitthe observations excellently for a given nitrogen abundance in eachobject. Moreover, the ionization equilibrium of \ion{N}{i/ii} proves tobe a sensitive temperature indicator for late B-type and early A-typesupergiants - even at low metallicities - due to the apparent nitrogenoverabundance in these objects. All supergiants within our sample showan enrichment of nitrogen on the order of ~ 0.3-0.6 dex, indicating themixing of CN-cycled material into atmospheric layers, with the sum ofthe CNO abundances staying close to solar. This finding is in accordancewith recent stellar evolution models accounting for mass-loss androtation. For Vega, an underabundance of nitrogen by 0.25 dex is found,in good agreement with the similar underabundance of other lightelements. The dependence of the non-LTE effects on the atmosphericparameters is discussed with special emphasis on the supergiants where astrong radiation field at low particle densities favours deviations fromLTE. Non-LTE effects systematically strengthen the \ion{N}{i/ii} lines.For some N I lines in supergiants non-LTE abundance corrections inexcess of 1 dex are found and they react sensitively to modifications ofthe collisional excitation data. The influence of microturbulence on thestatistical-equilibrium calculations is also investigated: theline-strengths of the strong N I features show some sensitivity due tomodifications of the line-formation depths and the departurecoefficients, while the - in this parameter range - weak N Ii linesremain unaffected. Based on observations collected at the EuropeanSouthern Observatory, Chile (ESO Ndeg 62.H-0176).
| Non-LTE line-formation for neutral and singly-ionized carbon. Model atom and first results on BA-type stars
A comprehensive model atom for non-LTE line-formation calculations forneutral and singly-ionized carbon is presented. Highly accurateradiative and collisional atomic data are incorporated, recentlydetermined for astrophysical and fusion research using the R-matrixmethod in the close-coupling approximation. As a test and firstapplication of the model, carbon abundances are determined on the basisof line-blanketed LTE model atmospheres for five stars, the mainsequence object Vega (A0 V) and the supergiantseta Leo (A0 Ib), HD 111613 (A2Iabe), HD 92207 (A0 Iae) and betaOri (B8 Iae), using high S/N and high-resolution spectra atvisual and near-IR wavelengths. The computed non-LTE line profiles fitthe observations well for a single carbon abundance in each object. Fortwo supergiants, eta Leo and HD 111613, lines of both species aresimultaneously present in the spectra, giving consistent C I and C Iiabundances (within the error bars). However, the uncertainties of theabundances are large, on the order of ~ 0.3 dex(statistical+systematical error), thus the ionization equilibrium of\ion{C}{i/ii} is of restricted use for the determination of stellarparameters. All supergiants within our sample show a depletion of carbonon the order of 0.2-0.5 dex, indicating the mixing of CN-cycled materialinto the atmospheric layers, with the sum of the CNO abundancesremaining close to solar. This finding is in accordance with recentstellar evolution models accounting for mass-loss and rotation. ForVega, an underabundance of carbon by 0.3 dex is found, in excellentagreement with the similar underabundance of other light elements. Thedependence of the non-LTE effects on the atmospheric parameters isdiscussed and the influence of systematic errors is estimated. Specialemphasis is given to the supergiants where a strong radiation field atlow particle densities favours deviations from LTE. Non-LTE effectssystematically strengthen the \ion{C}{i/ii} lines. For the C I lines inthe infrared, a strong sensitivity to modifications in thephotoionization and collisional excitation data is found. An increasingdiscrepancy between our model predictions and the observations for the CIi doublet lambda lambda 6578-82 is perceived with rising luminosity,while the other C Ii doublet and quartet lines remain consistent.Furthermore, the influence of microturbulence on thestatistical-equilibrium calculations is investigated. Based onobservations collected at the European Southern Observatory, Chile (ESONdeg 62.H-0176).
| Non-LTE line formation for \ion{Mg}{I/II}: Abundances and stellar parameters. Model atom and first results on A-type stars
An extensive model atom for non-LTE line-formation calculations forneutral and singly-ionized magnesium is presented, taking into accountrecent improvements in the atomic data. As a test and first applicationof the model, stellar parameters and magnesium abundances are determinedon the basis of line-blanketed LTE model atmospheres for three stars:Vega (A0 V), eta Leo (A0 Ib) and HD 92207 (A0 Iae) using high S/N andhigh resolution spectra at visual and near-IR wavelengths. Theionization equilibrium of \ion{Mg}{i/ii} proves to be a sensibletemperature indicator for early A-type stars at all luminosities.Evidence is given that in late A and early F-type supergiants (T_effla \8000 K) the determination of accurate stellar parameters is hampered bythe presence of a pressure inversion region in the model atmospheres atline-formation depths. The \ion{Mg}{i/ii} lines in the observations arereproduced simultaneously by the calculated line profiles with highccuracy. For Vega spectral synthesis in the UV region of the\ion{Mg}{i/ii} resonance lines also proves excellent consistency withthe results from the visual. The dependence of the non-LTE effects onthe atmospheric parameters is discussed with special emphasis on thesupergiants where a strong radiation field at low particle densitiesfavours deviations from LTE, especially in the minor ionic species ofneutral magnesium. Non-LTE corrections turn out to be small in Mg I -typically la 0.3 dex - even in supergiants, but are essential for anaccurate effective temperature determination. From the Mg Ii spectrum,only the features at lambda lambda 4481 and 7877-96 Å reactsensitively to non-LTE effects. Furthermore, the influence ofmicroturbulence on the statistical-equilibrium calculations isinvestigated. The line strengths are found to be systematicallyaffected. Based on observations collected at the European SouthernObservatory, La Silla, Chile (ESO No 62.H-0176).
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| Helium and Carbon Abundances in Late-B and Early-A Supergiants
The abundances of carbon and helium were determined for representativelate-B and early-A supergiants based on the C I lines (9078, 9089, 9095,9112) in the near-IR, C II lines (4267, 6151), and the He I 6678 line,in an aim to investigate the nature of the envelope-mixing in theseevolved stars based on the anomaly (if any) of these elements combinedwith that of N and O published before. It turned out that N tends toincrease with a decrease in C, showing a tendency of conserving the sumof C+N nuclei, which suggests that the anomaly of C and N may bereasonably interpreted as being due to mixing of the CN-processedmaterial. However, this increase/decrease in N/C, indicative ofdredge-up of the H-burning product, is not accompanied by anyHe-enrichment. Even surprisingly, the observed tendency is just theopposite, i.e., [He/H] appears to decrease progressively in accordancewith a lowering of [C/H]. Instead of regarding this apparentcharacteristics as being real, we tentatively speculate that someactivity-related line-weakening mechanism (e.g., irradiance of X-rays)might act on the formation of He I lines, the extent of which isindirectly related to the efficiency of envelope mixing via stellarrotation.
| Near-Infrared Classification Spectroscopy: J-Band Spectra of Fundamental MK Standards
We present a catalog of J-band spectra for 88 fundamental MK standardstars observed at a resolving power of R~3000. This contribution servesas a companion atlas to the K-band spectra recently published by Wallace& Hinkle and the H-band atlas by Meyer and coworkers. We report datafrom 7400 to 9550 cm-1 (1.05-1.34 μm) for stars ofspectral types O7-M6 and luminosity classes I-V as defined in the MKsystem. In reducing these data, special care has been taken to removetime-variable telluric features of water vapor. We identify atomic andmolecular indexes that are both temperature and luminosity sensitivethat aid in the classification of stellar spectra in the J band. Inaddition to being useful in the classification of late-type stars, the Jband contains several features of interest in the study of early-typestellar photospheres. These data are available electronically foranonymous FTP in addition to being served on the World Wide Web.
| A spectral analysis of Deneb (A2 Iae)
This study presents a detailed model atmosphere abundance analysis ofDeneb which was performed using Kurucz LTE ATLAS9 model atmospheres. Theatmospheric parameters were determined from Mg I/II and Fe I/IIequilibrium, and by fitting the Hgamma profile and opticalregion spectrophotometry. The compromise values which best satisfy thesecriteria are T_eff = 9000 K and log g = 1.45. The Mg I, Mg II, Si II, TiII, Cr II, Fe I, and Fe II lines yield microturbulences of 3.60, 6.50,8.50, 8.00, 11.90, 3.60, and 10.40 km s-1, respectively. Anaverage microturbulence of 7 km s-1 was used for the otheratomic species. From a comparision of the synthetic spectrum with theobservations, the best value for the rotational velocity is v sin i = 25km s-1, and for the macroturbulent velocity is zeta = 14 kms-1, which are similar to those of some earlier derivedvalues. Also, the individual spectrograms have a range of radialvelocity variation of ~ 15 km s-1 which is approximatelyequal to macroturbulent velocity. These values of the macroturbulenceand the range of radial velocity variation are close to the sum of theamplitudes (10.44 km s-1) of all the pulsation periods foundby Lucy (\cite{lucy}), who also suggested that the surface motions ofthe atmosphere of Deneb can be identified with macroturbulence. Denebshowed a definite helium underabundance with a well determined He/Hvalue = 0.071. The CNO values (C being mildly deficient, N being inmoderate excess, and an O being slightly deficient) are consistent withthe mixing of the CNO-cycled products into the surface layers from theprocessed materials presumably dredged-up from the interior. Aluminiumis mildly deficient by ~ 0.24 dex with respect to solar value, whilesulfur is moderately underabundant by ~ 0.43 dex in Deneb. Mg and Sihave the solar abundances. The metal abundances (Ca to Ni) tend to begreater than solar except for Sc which is slightly deficient. The heavyelements abundances (Sr, Y, and Zr) are all greater than solar. Theseabundance patterns conform to the common tendency seen with other normalPopulation I A supergiants found by Venn (\cite{venna}). The rare-earthelements (Ba, La, and Eu) which have been unexplored in other Galacticearly A-type supergiants are significantly overabundant relative theSun. Based on data obtained at the Dominion Astrophysical Observatory,Herzberg Institute of Astrophysics, National Research Council of Canada.Table A.1 is only available electronically with the On-Line publicationat http://link.springer.de/link/service/00230/
| Non-LTE line formation for neutral oxygen. Model atom and first results on A-type stars
An extensive model atom for non-LTE line formation calculations for O Iis presented, taking into account recent improvements in the atomicdata. Based on line-blanketed LTE model atmospheres equivalent widthsare computed in LTE and non-LTE for the diagnostic O I lines of A- andlate B-type stars in the range Teff = 7500 K to 15000 K andluminosity classes V to Ia. Non-LTE abundance corrections are provided:they span a wide range in magnitude, from less than 0.1 dex for the weaklines in main sequence stars to more than 1.5 dex for the near-infraredlines in some supergiants. The dependence of the non-LTE effects on theatmospheric parameters is discussed with special emphasis onsupergiants. In particular, the near-infrared transitions are found toreact sensitively to the collisional excitation cross sections used inthe calculations. Further investigations concentrate on the influence ofmicroturbulence and on the rôle of wind outflow velocity fields onthe line formation. As a test and first application of the model, oxygenabundances for Vega (A0 V), etaLeo (A0 Ib) and HD 92207 (A0 Iae) arederived. The analysis of Vega confirms a slight oxygen underabundance (~ 0.3 dex) in this star while eta Leo and HD 92207 show a nearly solarvalue. For Vega the observed spectrum can be reproduced accurately bythe calculated line profiles. In supergiants consistent abundances canbe derived from the weak lines in the visible. At high luminosities theprominent near-infrared features - among other strong lines fromdifferent elemental species - are subject to additional broadening by anunidentified process which prevents an equally accurate theoreticalinterpretation. Based on observations collected at the European SouthernObservatory, Chile (ESO No 62.H-0176)
| The dynamical evolution of the fragmented, bipolar dust shell around the carbon star IRC +10 216 . Rapid changes of a PPN-like structure?
We present high-resolution J-, H-, and K-band observations and the firstH-K color image of the carbon star IRC +10 216. Theimages were reconstructed from 6 m telescope speckle interferogramsusing the bispectrum speckle interferometry method. The H and K imageswith resolutions between 70 mas and 92 mas consist of several compactcomponents within a 0\farcs2 radius and a fainter asymmetric nebula. Thebrightest four components are denoted with A to D in the order ofdecreasing brightness in the 1996 image. A comparison of our images from1995, 1996, 1997, and 1998 gives -- almost like a movie of five frames-- insight into the dynamical evolution of the inner nebula. Forinstance, the separation of the two brightest components A and Bincreased from 191 mas in 1995 to 265 mas in 1998. At the same time,component B is fading and the components C and D become brighter. TheX-shaped bipolar structure of the nebula, most prominently present inthe J-band image, implies an asymmetric mass-loss. Such asymmetries areoften present in protoplanetary nebulae but are unexpected for AGBstars. IRC +10 216 is thus likely to be very advancedin its AGB evolution, shortly before turning into a protoplanetarynebula. The cometary shapes of A in the H and J images and in the 0.79mu m and 1.06 mu m HST images suggest that the core of A is not thecentral star, but the southern lobe of a bipolar structure. The positionof the central star is probably at or near the position of component B,where the H-K color has a value of 4.2. If the star is at or near B,then the components A, C, and D are likely to be located at the innerboundary of the dust shell. Based on observations performed with the 6~mtelecope at the Special Astrophysical Observatory, Russia.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Leone |
| Ascensione retta: | 10h07m20.00s |
| Declinazione: | +16°45'46.0" |
| Magnitudine apparente: | 3.52 |
| Distanza: | 653.595 parsec |
| Moto proprio RA: | 0 |
| Moto proprio Dec: | 0 |
| B-T magnitude: | 3.462 |
| V-T magnitude: | 3.49 |
Cataloghi e designazioni:
|
INHABITED SKY
