Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| The symmetric dust shell and the central star of the bipolar planetary nebula NGC6537*†
We present high-resolution images of the strongly bipolar planetarynebula NGC6537, obtained with the Hubble Space Telescope (HST) and withthe infrared adaptive optics system on the Very Large Telescope. Thecentral star is detected for the first time. Using the multibandphotometry and constraints from the dynamical age of the nebula, wederive a temperature in the range 1.5-2.5 × 105 K, aluminosity ~103Lsolar and a core massMc~ 0.7-0.9Msolar. The progenitor mass is probablyin the range Mi= 3-7Msolar. The extinction mapshows a largely symmetric, and compact dust structure, which is mostlikely a shell, located at the neck of the bipolar flow, only 2-4arcsecfrom the star. The dust shell traces a short-lived phase of very highmass loss at the end of the asymptotic giant branch (AGB). The dynamicalage of the shell and bipolar lobes are very similar but the morphologiesare very different. The data suggests that the mass loss during theejection of the compact shell was largely spherically symmetric, and thepronounced bipolarity formed afterwards. The dynamical ages of thebipolar lobes and dust shell are similar, which is consistent withsuggestions that bipolar structures form in a run-away event at the verylast stages of the AGB mass loss. The inner edge of the dust shell isionized, and polycyclic aromatic hydrocarbon (PAH) emission is seen justoutside the ionized gas. We associate the PAH emission with thephotodissociation region of the molecular shell.
| Surface abundances of light elements for a large sample of early B-type stars - IV. The magnesium abundance in 52 stars - a test of metallicity
From high-resolution spectra a non-local thermodynamic equilibriumanalysis of the MgII 4481.2-Åfeature is implemented for 52 earlyand medium local B stars on the main sequence (MS). The influence of theneighbouring line AlIII 4479.9-Åis considered. The magnesiumabundance is determined; it is found that logɛ(Mg) = 7.67 +/-0.21 on average. It is shown that uncertainties in the microturbulentparameter Vt are the main source of errors inlogɛ(Mg). When using 36 stars with the most reliableVt values derived from OII and NII lines, we obtain the meanabundance logɛ(Mg) = 7.59 +/- 0.15. The latter value isprecisely confirmed for several hot B stars from an analysis of the MgII7877-Åweak line. The derived abundance logɛ(Mg) = 7.59 +/-0.15 is in excellent agreement with the solar magnesium abundancelogɛsolar (Mg) = 7.55 +/- 0.02, as well as with theproto-Sun abundance logɛps(Mg) = 7.62 +/- 0.02. Thus,it is confirmed that the Sun and the B-type MS stars in ourneighbourhood have the same metallicity.
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| Interstellar Ca II Line Intensities and the Distances of the OB stars
We show that the equivalent widths of the well-known interstellar Ca IIH and K lines can be used to determine the distances to OB stars in ourGalaxy. The equivalent widths, measured in the spectra of 147 early-typestars, are strongly related to the Hipparcos parallaxes of thoseobjects. The lines fitted to the parallax-equivalent width data aregiven by the formulae π=1/[2.78EW(K)+95] and π=1/[4.58EW(H)+102],where π is in arcseconds and EW is in milliangstroms. The form of theformulae, yielding a finite parallax even for zero absorption, showsthat space within ~100 pc of the Sun contains very little Ca II, whichis in agreement with the known dimensions of the Local Bubble. Using CaII lines for distance determination does not require the knowledge ofthe absolute magnitude of the object; it is thus well suited for targetsfor which the absolute calibration is either not precise (OBsupergiants) or not available at all (peculiar objects). We alsodemonstrate that neither the reddening E(B-V) nor the equivalent widthsof interstellar K I and CH lines are suitable candidates for distanceestimation, their relation with parallaxes being far less tight than forCa II.
| VLT L-band mapping of the Galactic center IRS 3-IRS 13 region. Evidence for new Wolf-Rayet type stars
This paper presents L-band ISAAC and NAOS/CONICA (VLT) spectroscopicobservations of the IRS 3-IRS 13 Galactic Center region. The ISAAC dataallowed us to build the first spectroscopic data cube of the region inthe L-band domain. Using the L-band spectrum of the extinction along theline of sight towards the GC derived in a previous paper (Moultaka etal. 2004, A&A, 425, 529), it was also possible to correct the cubefor the foreground extinction. Maps of the water ice and hydrocarbonabsorption line strength were then derived. These maps are importantdiagnostics of the interstellar and circumstellar medium because waterices are observed in molecular clouds while hydrocarbons are usuallygood tracers of the diffuse ISM. These maps support our previous resultsthat the absorption features most probably occur in the local Galacticcenter medium and can be associated with the individual sources.Moreover, turbulence seems to affect the studied region of theminispiral, which appears like a mixture of a dense and diffuse medium.Comparison of the concentrations of ice and hydrocarbon absorptionsaround IRS 13E, IRS 6E, and IRS 2, with similar concentrations at thelocation of the extended continuum emission around IRS 3, suggests thatthese sources might present outflows interacting with the surroundingISM. It was also possible to derive Brα and Pfγ emissionline maps. The results suggest that the physical conditions of the ISMare not uniform in the observed region of the minispiral especially atthe edges of the minicavity. The emission line maps allowed us to findthree sources with broad lines corresponding to an FWHM deconvolved linewidth of about 1100 km s-1 and moving towards us with aradial velocity of about -300 km s-1. These sources are mostprobably new Wolf-Rayet type stars located in projection to the northand west of IRS 3. Their derived radial velocities and proper motionsshow that only two of them might belong to the two rotating disks ofyoung stars reported by Genzel et al. (2003, ApJ, 594, 812) and Levin& Beloborodov (2003, ApJ, 590, L33). Previously, NAOS/CONICA (NACO)data allowed us to resolve the IRS13E3 region into two components E3N,and E3c (Eckart et al. 2004, ApJ, 602, 760). The new spectroscopic NACOdata show that E3c is a good candidate for a Wolf-Rayet type star. Inaddition, three sources (η, ζ, and γ) out of the eightvery red sources located in the IRS13N complex also presented in Eckartet al. (2004, ApJ, 602, 760) have been resolved spectroscopically withNACO. The spectra presented in this paper show that the red colors ofthe sources are probably due to extended dust emission.
| On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| Correlations between diffuse interstellar bands and atomic lines
We present and discuss correlations between strengths of the well-known,strong interstellar atomic lines of KI and CaII, and four selected,strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850and 6614. In order to analyse a homogeneous sample of echellehigh-resolution spectra it has been chosen to use measurements fromTerskol Observatory in Northern Caucasus plus a selected number ofhigher resolution observations performed using other instruments. Wedemonstrate that the strength of certain DIBs correlate well withneutral potassium lines and to a much lower degree with ionized calciumlines. This fact suggests that the degree of irradiation of a cloud withUV photons, capable to ionize interstellar atoms, plays a crucial rolein the formation/maintenance of certain molecular species: possiblecarriers of DIBs.
| Surface abundances of light elements for a large sample of early B-type stars - III. An analysis of helium lines in spectra of 102 stars
Non-local thermodynamic equilibrium analysis of He I lines in spectraof 102 B stars is implemented in order to derive the helium abundanceHe/H, the microturbulent parameter Vt and the projectedrotation velocity v sini. A simultaneous determination of He/H andVt for the stars is effected by analysing equivalent widthsof the 4471- and 4922-Å lines primarily as indicators of He/H andthe 4713-, 5016-, 5876- and 6678-Å lines primarily as indicatorsof Vt. The rotation velocities v sini are found from profilesof the same lines. It is shown that, when Vt > 7 kms-1, the Vt(He I) values determined from He Ilines are systematically overestimated as compared with theVt(OII, NII) values derived from OII and NII lines. Thisdiscrepancy is especially appreciable for hot evolved B giants withVt(He I) = 16-23 km s-1 and may indicate a failureof classical model atmospheres to represent the strong He I lines forthese stars.Two programme stars, HR 1512 and 7651, are found to be helium-weakstars. The remaining 100 stars are divided into three groups accordingto their masses M. The microturbulent parameter Vt(He I) islow for all stars of group A (M= 4.1-6.9 Msolar) and for allstars with the relative ages t/tMS < 0.8 of group B (M=7.0-11.2 Msolar). Their Vt(He I) values are withinthe 0 to 5 km s-1 range, as a rule; the mean value isVt= 1.7 km s-1. Only evolved giants of group B,which are close to the termination of the main-sequence (MS)evolutionary phase (t/tMS > 0.8), show Vt(He I)up to 11 km s-1. The helium abundance He/H is correlated withthe relative age t/tMS in both groups; the averaged He/Henhancement during the MS phase is 26 per cent. For group C, containingthe most massive stars (M= 12.4-18.8 Msolar), theVt(He I) values display a correlation with t/tMS,varying from 4 to 23 km s-1. The He/H determination for hotevolved B giants of the group with Vt(He I) > 15 kms-1 depends on a choice between the Vt(He I) andVt(OII, NII) scales. The mean He/H enrichment by 67 per centduring the MS phase is found, if the abundances He/H are based on theVt(OII, NII) scale; however, two evolved giants withespecially high v sini, HR 7446 and 7993, show the He/H enhancement byabout a factor of 2.5. When using the same Vt scale, we founda trend of He/H with projected rotational velocities v sini a largedispersion for v sini > 150 km s-1 can result fromdifferences in masses M.A comparison with the stellar model computations with rotationallyinduced mixing shows that the observed helium enrichment during the MSphase can be explained by rotation with initial velocities 250-400 kms-1. The He/H distribution on M and v sini based on theVt(OII, NII) scale seems to be in better agreement with thetheory than one based on the Vt(He I) scale. The mean valueHe/H = 0.10 derived for stars in the zero age main sequence (ZAMS)vicinity can be adopted as the typical initial helium abundance forearly B stars in the solar neighbourhood.
| 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.
| 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.
| 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.
| Grey extinction in the solar neighbourhood?
Some of the close O and B dwarfs appear to be fainter than indicated bytheir Hipparcos distances, intrinsic absolute magnitudes attributed totheir spectral types, and estimated selective interstellar extinction.This discrepancy is explained in the paper by the grey (neutral)interstellar extinction in the visual range of spectrum. The measure ofsuch an effect is related to discrete features of the interstellarmatter.
| Surface abundances of light elements for a large sample of early B-type stars - II. Basic parameters of 107 stars
Effective temperatures Teff, surface gravities logg andinterstellar extinctions AV are found for 107 B stars.Distances d of the stars, which are based on the derivedTeff, logg and AV values, show good agreement withthose obtained from the Hipparcos parallaxes. Comparing theTeff and logg values with evolutionary computations, we infermasses, radii, luminosities, ages and relative ages of the stars.Empirical relations between the Teff and logg parameters, onthe one hand, and the photometric indices Q, [c1] and β,on the other hand, are constructed; these relations give a fast methodfor the Teff and logg estimation of early and medium B stars.Inclusion of the infrared J, H and K colours into the Teff,logg and AV determination shows that (i) the Teffand logg parameters are altered only slightly; (ii) the AVvalue is rather sensitive to these colours, so an accuracy better than0.05mag in the JHK data is necessary for precise AVevaluation.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| A Search for Fine Structure within the 4430 Å Diffuse Interstellar Band
The dependence of the 4430 Å diffuse interstellar band (DIB) ondust extinction or total gas column density indicates that this DIB,like the narrower features seen in the yellow-red portion of thespectrum, does not continue to grow in strength beyond a certainthreshold. One possible interpretation is that the 4430 Å DIBsaturates at high column densities, suggesting that this band mightconsist of unresolved narrow features (such as molecular rotationallines), individually saturated, that are blended at the resolving powerof previous observations. In this study we use the Ultra High ResolutionFacility on the Anglo-Australian Telescope to assess that question byseeking evidence for fine structure within the 4430 Å DIB at thevery high spectral resolving power ofΔλ/λ~106. Our target stars include linesof sight where apparent rotational line structure has been observed inother DIBs. We find no evidence of fine structure within the 4430Å DIB, suggesting that the broadening of this band is intrinsic.If so, the lack of fine structure may indicate a molecular transitionfollowed by very rapid internal conversion or a molecular transitionconsisting of overlapping rotational lines. In either case, our resultssupport the hypothesis that the apparent saturation of the band as afunction of extinction is a result of the ``skin effect,'' in which thestrength of DIBs stops increasing with extinction because the DIBs formprimarily in the outer boundaries of interstellar clouds.
| Very high resolution profiles of 6196 Åand 6614 Ådiffuse interstellar bands
We present a careful analysis of very high resolution (R=220 000)profiles of two well correlated diffuse interstellar bands (DIBs): 6196and 6614 Å observed along 7 lines of sight free of the Dopplersplitting in interstellar atomic lines. The high signal-to-noise ratio(S/N) of the spectra ( ~ 600-1000) allows us to draw definite conclusionthat the ratio of equivalent widths of the two DIBs is not alwaysexactly the same, i.e. they do not seem to originate at the samecarrier. The DIB profiles vary from object to object but the variationsare different for every DIB. The width of 6196 DIB may change by up to1.5 times lowest value while that of 6614 remains almost constant. Thebroadening of 6196 is not caused by the Doppler splitting, absent inatomic interstellar lines. Possibly this fact reflects a strongdependence of the carrier excitation pattern on small variations ofphysical parameters. The variations of substructure strength ratiosinside the 6614 DIB profile, evident in our spectra, are apparentlycaused by a mechanism different from that which broadens DIB 6196. Basedon data collected at the ESO 3.6 m telescope operated on La SillaObservatory, Chile.
| On the detection of the linear C5 molecule in the interstellar medium
An upper limit of the column density of the C5 linearmolecule in translucent interstellar clouds is estimated fromhigh-resolution (R=80000) and very high signal-to-noise ratio (~1000)echelle spectra. It is 1012cm-2 per E(B-V)=1 (twoorders of magnitude lower than that of C2).
| Stellar populations in Seyfert 2 galaxies. I. Atlas of near-UV spectra
We have carried out a uniform spectroscopic survey of Seyfert 2 galaxiesto study the stellar populations of the host galaxies. New spectra havebeen obtained for 79 Southern galaxies classified as Seyfert 2 galaxies,7 normal galaxies, and 73 stars at a resolution of 2.2 Å over thewavelength region 3500-5300 Å. Cross-correlation between thestellar spectra is performed to group the individual observations into44 synthesis standard spectra. The standard groups include a solarabundance sequence of spectral types from O5 to M3 for dwarfs, giants,and supergiants. Metal-rich and metal-weak F-K giants and dwarfs arealso included. A comparison of the stellar data with previouslypublished spectra is performed both with the individual spectra and thestandard groups. For each galaxy, two distinct spatial regions areconsidered: the nucleus and the external bulge. Spectroscopic variationsfrom one galaxy to another and from the central to the external regionare briefly discussed. It is found that the central region of a Seyfert2 galaxy, after subtracting the bulge stellar population, always shows anear-UV spectrum similar to one of three representative categories: a)many strong emission lines and only two visible absorption lines (Ca IiK and G band) (Sey2e); b) few emission lines, many absorption lines, anda redder continuum than the previous category (Sey2a); c) an almost flatcontinuum and high-order Balmer lines seen in absorption (Sey2b). Theproportion of Seyfert 2 galaxies belonging to each class is found to be22%, 28%, and 50% respectively. We find no significative differencesbetween morphology distributions of Seyfert 2 galaxies with Balmer linesdetected in absorption and the rest of the sample. This quick lookthrough the atlas indicates that half of Seyfert 2 galaxies harbour ayoung stellar population (about or less than 100 Myr) in their centralregion, clearly unveiled by the high order Balmer series seen inabsorption. Based on observations collected at the European SouthernObservatory, Chile (ESO 65.P-0014(A)). Tables 1-3 and 8 and Fig. A.1(Appendix A) are only available in electronic form athttp://www.edpsciences.org
| 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.
| Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars
We present the results of statistical analyses of a sample of 627 Bestars. The parameters of intrinsic polarization (p*),projected rotational velocity (v sin i), and near IR excesses have beeninvestigated. The values of p* have been estimated for a muchlarger and more representative sample of Be stars (~490 objects) thanpreviously. We have confirmed that most Be stars of early spectral typehave statistically larger values of polarization and IR excesses incomparison with the late spectral type stars. It is found that thedistributions of p* diverge considerably for the differentspectral subgroups. In contrast to late spectral types (B5-B9.5), thedistribution of p* for B0-B2 stars does not peak at the valuep*=0%. Statistically significant differences in the meanprojected rotational velocities (/line{vsin i}) are found for differentspectral subgroups of Be stars in the sense that late spectral typestars (V luminosity class) generally rotate faster than early types, inagreement with previously published results. This behaviour is, however,not obvious for the III-IV luminosity class stars. Nevertheless, thecalculated values of the ratio vt/vc of the truerotational velocity, vt, to the critical velocity forbreak-up, vc, is larger for late spectral type stars of allluminosity classes. Thus, late spectral type stars appear to rotatecloser to their break-up rotational velocity. The distribution of nearIR excesses for early spectral subgroups is bi-modal, the position ofthe second peak displaying a maximum value E(V-L)~ 1 . m 3for O-B1.5 stars, decreasing to E(V-L)~0. m8 for intermediatespectral types (B3-B5). It is shown that bi-modality disappears for latespectral types (B6-B9.5). No correlations were found betweenp* and near IR excesses and between E(V-L) and vsin i for thedifferent subgroups of Be stars. In contrast to near IR excesses, arelation between p* and far IR excesses at 12 mu m is clearlyseen. A clear relation between p* and vsin i (as well asbetween p* and /line{vsin i}/vc) is found by thefact that plots of these parameters are bounded by a ``triangular"distribution of p*: vsin i, with a decrease of p*towards very small and very large vsin i (and /line{vsini}/vc) values. The latter behaviour can be understood in thecontext of a larger oblateness of circumstellar disks for the stars witha rapid rotation. From the analysis of correlations between differentobservational parameters we conclude that circumstellar envelopes forthe majority of Be stars are optically thin disks with the range of thehalf-opening angle of 10degr
| BCD spectrophotometry of stars with the B[e] phenomenon. I. Fundamental parameters
Low resolution spectra in the lambda lambda3500 -4600 Å wavelengthrange of 23 stars with the B[e] phenomenon are presented. Spectralclassification of 15 program stars was performed using the BCDspectrophotometric system, based on the study of the Balmerdiscontinuity, which is independent of interstellar and circumstellardust extinctions and of circumstellar gas emissions and/or absorptions.From calibrations of the (lambda1 ,D) BCD parameters wedetermined the (Teff,log g, Mbol) of the studiedstars. For stars where this method could not be applied, we tried toestimate the temperature of the central star by using the Balmer and Hei emission lines and/or their visible energy distribution. The colourtemperature and the temperatures obtained from the study of Balmer andHe i lines are consistent with each other. The new results are comparedwith those obtained previously by other authors and discussed for eachstar individually. For some stars, differences between the effectivetemperatures derived using the BCD classification system and thoseobtained elsewhere, based on photometric or spectroscopic analysis,imply spectral-type classification disagreements ranging from 2-3 up to6 B sub-spectral types. The fundamental parameters of AS 119,CD-24°5721, Hen2-91, HD 316375 and BD-11°4747 were determinedfor the first time. A simple method was introduced to calculate total(interstellar+circumstellar) dust extinction towards the studied stars.For HD 53179, which is a double stellar system, and for HD 45677 and HD50138, which are suspected to be binaries, we predicted thecharacteristics of the components that are consistent with the observed(lambda1 ,D) parameters. However, the possible binarity of HD45677 and HD 50138 still needs to be confirmed spectroscopically. Dataobtained in CASLEO operated under agreement between the CONICET and theUniversities of La Plata, Córdoba and San Juan, Argentina.Partially based on observations done at ESO La Silla, Chili.
| 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
| Surface abundances of light elements for a large sample of early B-type stars - I. Spectral observations of 123 stars; measurements of hydrogen and helium lines; infrared photometry
High-resolution spectral observations of 123 B0-B5 stars in the mainsequence evolutionary phase were obtained at two observatories, namelythe McDonald Observatory (McDO) and the Crimean AstrophysicalObservatory (CrAO). Accurate equivalent widths W of two Balmer lines,Hβ and Hγ, and ten Hei lines were obtained for all the stars,as well as of the Heiiλ4686 line for the hottest ones. A carefulanalysis of the measured equivalent widths was performed. It is shownthat there is a very good agreement between the W values derived fromthe McDO and CrAO spectra for 14 common stars. A comparison withpublished data leads to the conclusion that the W values measuredearlier by some authors for strong Hei lines are very likely to beunderestimated. Infrared photometric observations in the J, H, and Kbands were performed for 70 programme stars. All these data will be usedin other papers: in particular for the Teff and loggdetermination and for the He, C, N and O abundance analyses.
| Oxygen and helium abundances in Galactic Hii regions - I. Observations
Absolute integrated line fluxes of Hii regions have been measured usinga Fabry-Perot spectrophotometer. We describe the observations andcalibration procedures. Fluxes are given for 36 Hii regions withGalactocentric distances ranging from 6.6 to 17.7kpc. Several emissionlines have been measured, mainly [Oii] λλ3726 and 3629,Hβ, [Oiii] λ5007, Hei λ5876 and Hα. The veryfaint [Oiii] λ4363 line has been measured in six regions,allowing a direct determination of the electron temperature. Newphotometric distances have been derived based on data from theliterature. A discussion of these results in terms of extinction,electron density and temperature, and oxygen and helium abundances isgiven in Paper II.
| A new survey of diffuse interstellar bands (5650 - 6865 Å)
This paper presents a new systematic survey of diffuse interstellarbands in the optical wavelength range from 5650 to 6865 Ä based onechelle spectra of medium-reddened early-type stars acquired at McDonaldObservatory. Adding up interstellar spectra of the same type (as it wasdone by Krełowski et al. \cite{K97}) we reached a very highsignal-to-noise ratio. In this spectral range 89 new features have beendiscovered, 62 of them certain. In the whole spectral range of thissurvey we have found 240 features, 213 of them certain. Figure~4 is onlyavailable in electronic form at http://www.edpsciences.org
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Águila |
| Ascensión Recta: | 19h36m53.50s |
| Declinación: | -07°01'39.0" |
| Magnitud Aparente: | 4.95 |
| Distancia: | 446.429 parsecs |
| Movimiento Propio en Ascensión Recta: | 1 |
| Movimiento Propio en Declinación: | -2.8 |
| B-T magnitude: | 4.892 |
| V-T magnitude: | 4.942 |
Catálogos y designaciones:
|
