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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.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| Evidence for Spectropolarimetric Diversity in Type Ia Supernovae
We present single-epoch, postmaximum spectropolarimetry of four Type Iasupernovae (SNe Ia) that span a range of spectral and photometricproperties: SN 2002bf and SN 2004dt exhibit unusually high-velocity (HV)absorption lines. SN 1997dt is probably somewhat subluminous, and SN2003du is slightly overluminous. We detect polarization modulationsacross strong lines in all four objects, demonstrating that all areintrinsically polarized. However, the nature and degree of thepolarization varies considerably. Including all SNe Ia studied thus far,the following order emerges in terms of increasing strength ofline-polarization features: ordinary/overluminous
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| Interstellar 12C/13C ratios through CH^+λλ 3957,4232 absorption in local clouds: incomplete mixing in the ISM
The 12C/13C isotope ratio is a tracer of stellaryields and the efficiency of mixing in the ISM.12CH+/13CH+ is not affectedby interstellar chemistry, and is the most secure way of measuring12C/13C in the diffuse ISM.R=12C/13C is 90 in the solar system. Previousmeasurements of 12CH+λλ3957.7,4232.3and 13CH+λλ3958.2,4232.0 absorptiontoward nearby stars indicate some variations in12C/13C, with values ranging from 40 to 90suggesting inefficient mixing. Except for the cloud toward ζOph,these R values are strongly affected by noise. With UVES on the VLT wehave improved on the previous interstellar 12C/13Cmeasurements. The weighted 12C/13C ratio in thelocal ISM is 78.27 ± 1.83, while the weighted dispersion of ourmeasurements is 12.7, giving a 6.9σ scatter. Thus we report on a6.9σ detection of 16.2% root-mean-square variations in the carbonisotopic ratio on scales of ~100 pc: R= 74.7 ± 2.3 in theζOph cloud, while R = 88.6 ± 3.0 toward HD 152235 in theLupus clouds, R = 62.2 ± 5.3 towards HD 110432 in the Coalsack,and R = 98.9 ± 10.1 toward HD 170740. The observed variations in13C/12C are the first significant detection ofchemical heterogeneity in the local ISM.
| PSR B1929+10 revisited in X-rays
We performed timing and spectral analyses for PSR B1929+10, one of theoldest (~107 years) of the ordinary pulsars detected inX-rays, using archival ROSAT, ASCA and RXTE data. Pulsed emission wasdetected at a more than five sigma level for the combined ROSAT PSPC-Band previously unpublished HRI data. Our pulse profile is in agreementwith that obtained by Yancopoulos et al. (1994, ApJ, 429, 832, ROSATPSPC-B) but now with better statistics. The pulsed fraction in the ROSATX-ray band is 0.25 ± 0.04. The pulsed signal has also beendetected in the ASCA GIS data (0.5-5 keV) with a similar pulsed fractionof 0.36 ± 0.11. No significant timing signal is found in the RXTEPCA data (>2~keV). We found that the combined ROSAT PSPC-B and ASCAGIS spectrum can satisfactorily be described by a power-law as well asby a double black-body model but not by a single black-body model orblack-body plus power law model. Fitting the combined ROSAT/ASCA 0.1-10keV spectrum by a power-law model we obtain a photon index α of2.54 ± 0.12 and a neutral hydrogen column density NHtowards the source of 9.8-1.0+1.4 ×1020 cm-2. For a double black-body fit our resultsare T1 = 2.0-0.05+0.05 ×106~K, T2 = 6.9-0.35+0.23× 106 K and NH =4.4-1.1+2.1 × 1020cm-2. In both cases the derived value of NH ishigher than that adopted in earlier works, but our result is fullyconsistent with the larger distance estimate of 331 ± 10 pc fromparallax measurements combined with the hydrogen distributionmeasurements in the direction to the pulsar.
| New optical polarization measurements of quasi-stellar objects. The data
New linear polarization measurements (mainly in the V band) arepresented for 203 quasi-stellar objects (QSOs). The sample is made up of94 QSOs located in the North Galactic Pole (NGP) region and of 109 QSOsin the South Galactic Pole (SGP) region. First time measurements havebeen obtained for 184 QSOs. Among them, 109 known radio-emitters, 42known Broad Absorption Line (BAL) QSOs, and 1 gravitationally lensedquasi-stellar object. We found high polarization levels (p > 3%) for12 QSOs, including the BAL QSO SDSS J1409+0048. For10 objects, measurements obtained at different epochs do exist. Two ofthem show evidence for variability: the highly polarized BL Laccandidate PKS 1216-010 and the radio sourcePKS 1222+037.Based on observations collected at the European Southern Observatory(ESO, La Silla and Paranal). Table 4 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/757
| 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.
| UBVRI photopolarimetry of the long-period eclipsing AM Herculis binary V1309 Ori
We report simultaneous UBVRI photopolarimetric observations of thelong-period (7.98 h) AM Her binary V1309 Ori. The length and shape ofthe eclipse ingress and egress vary from night to night. We suggest thatthis is a result of the variation in the brightness of the accretionstream. By comparing the phases of circular polarization zero-crossoverswith previous observations, we confirm that V1309 Ori is wellsynchronized, and find an upper limit of 0.002 per cent for thedifference between the spin and orbital periods. We model thepolarimetry data using a model consisting of two cyclotron emissionregions at almost diametrically opposite locations, and centred atcolatitude β= 35° and 145° on the surface of the whitedwarf. We also present archive X-ray observations which show that thenegatively polarized accretion region is X-ray bright.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| 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.
| 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?
| 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.
| Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi
We present single-epoch spectropolarimetry of the Type II supernovae(SNe II) 1997ds, 1998A, and 1999gi. SN 1997ds and SN 1998A were bothobserved during the early photospheric phase, less than 50 days afterexplosion, while spectropolarimetry of SN 1999gi was obtained near thestart of the transition to the nebular phase, about 110 days afterexplosion. Uncorrected for interstellar polarization (ISP), SN 1997ds ischaracterized by pV=0.85%+/-0.02%, SN 1998A haspV=0.24%+/-0.05%, and SN 1999gi is polarized atpV=5.72%+/-0.01%. SN 1997ds and SN 1999gi exhibit distinctpolarization modulations (up to Δptot=1.6% in SN 1997dsand Δptot=1.0% in SN 1999gi) at the wavelengths of thestrongest spectral line features. While no spectral polarizationfeatures were observed in SN 1998A, the data are insensitive topolarization features at the levels confirmed in the other two objects.The low continuum polarization inferred for SN 1997ds and SN 1998A andthe amplitude of (or limits on) the polarization modulations areconsistent with those measured at similar epochs for SN 1987A and theType II plateau SN 1999em and supports the growing consensus thatcore-collapse events with hydrogen envelopes substantially intact at thetime of explosion are not significantly aspherical during the earlyphotospheric phase. The spectral shape of the high continuumpolarization of SN 1999gi closely resembles a ``Serkowski'' ISP curve(characterized by pmax=5.8%, θ=154deg, andλmax=5300 Å) and is inconsistent with thewavelength-independent nature of electron scattering expected for anaspherical SN atmosphere. Since Galactic reddening is minimal along thisline of sight, the majority of the observed polarization in SN 1999gi isbelieved to be due to ISP of the host galaxy, although significant (upto p~2%) intrinsic polarization cannot be ruled out. The potential powerof SN spectropolarimetry to study the properties of interstellar dust inexternal galaxies is described and applied to the SN 1999gi data, whereit is shown that if the polarization is indeed predominantlyinterstellar in origin, then RV=3.0+/-0.2 for the dust alongthis line of sight in NGC 3184.
| Profiles of blue and infrared diffuse interstellar bands
The paper presents a survey of profiles of reasonably strong diffuseinterstellar bands (DIBs) based on the extensive set of high-resolutionspectra acquired with the aid of echelle spectrographs installed at the2-m Terskol, 2-m Pic du Midi and 1-m SAO telescopes. The surveyeddiffuse interstellar bands cover the spectral ranges of blue andnear-infrared, i.e the DIBs not surveyed by Krełowski &Schmidt. The possible modifications caused by stellar and telluric linesare discussed. The very broad features such as 4430 are not discussedbecause the shapes of their profiles, extracted from echelle spectra,are very uncertain. The signal-to-noise (S/N) ratios of the spectra arenot high enough to enable discussion of the profiles of numerous weakinterstellar features discovered recently.
| The Metamorphosis of SN 1998bw
We present and discuss the photometric and spectroscopic evolution ofthe peculiar SN 1998bw, associated with GRB 980425, through an analysisof optical and near-IR data collected at ESO-La Silla. The spectroscopicdata, spanning the period from day -9 to day +376 (relative to Bmaximum), have shown that this supernova (SN) was unprecedented,although somewhat similar to SN 1997ef. Maximum expansion velocities ashigh as 3×104 km s-1 to some extent mask itsresemblance to other Type Ic SNe. At intermediate phases, betweenphotospheric and fully nebular, the expansion velocities(~104 km s-1) remained exceptionally high comparedto those of other recorded core-collapse SNe at a similar phase. Themild linear polarization detected at early epochs suggests the presenceof asymmetry in the emitting material. The degree of asymmetry, however,cannot be decoded from these measurements alone. The He I 1.083 and2.058 μm lines are identified, and He is suggested to lie in an outerregion of the envelope. The temporal behavior of the fluxes and profilesof emission lines of Mg I] λ4571, [O I] λλ6300,6364, and a feature ascribed to Fe are traced to stimulate futuremodeling work. The uniqueness of SN 1998bw became less obvious once itentered the fully nebular phase (after 1 yr), when it was very similarto other Type Ib/c-IIb objects, such as the Type Ib SN 1996N and theType IIb SN 1993J, even though SN 1998bw was 1.4 mag brighter than SN1993J and 3 mag brighter than SN 1996N at a comparable phase. Thelate-phase optical photometry, which extends up to 403 days after Bmaximum, shows that the SN luminosity declined exponentially butsubstantially faster than the decay rate of 56Co. Theultraviolet-optical-infrared bolometric light curve, constructed usingall available optical data and the early JHK photometry presented inthis work, shows a slight flattening starting on about day +300. Sinceno clear evidence of ejecta-wind interaction was found in the late-timespectroscopy (see also the work of Sollerman and coworkers), this may bedue to the contribution of the positrons since most γ-rays escapethermalization at this phase. A contribution from the superposed H IIregion cannot, however, be excluded. Based on observations collected atESO-La Silla.
| 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
| On the origin of the O and B-type stars with high velocities. II. Runaway stars and pulsars ejected from the nearby young stellar groups
We use milli-arcsecond accuracy astrometry (proper motions andparallaxes) from Hipparcos and from radio observations to retrace theorbits of 56 runaway stars and nine compact objects with distances lessthan 700 pc, to identify the parent stellar group. It is possible todeduce the specific formation scenario with near certainty for twocases. (i) We find that the runaway star zeta Ophiuchi and the pulsarPSR J1932+1059 originated about 1 Myr ago in a supernova explosion in abinary in the Upper Scorpius subgroup of the Sco OB2 association. Thepulsar received a kick velocity of ~ 350 km s-1 in thisevent, which dissociated the binary, and gave zeta Oph its large spacevelocity. (ii) Blaauw & Morgan and Gies & Bolton alreadypostulated a common origin for the runaway-pair AE Aur and mu Col,possibly involving the massive highly-eccentric binary iota Ori, basedon their equal and opposite velocities. We demonstrate that these threeobjects indeed occupied a very small volume ~ 2.5 Myr ago, and show thatthey were ejected from the nascent Trapezium cluster. We identify theparent group for two more pulsars: both likely originate in the ~ 50 Myrold association Per OB3, which contains the open cluster alpha Persei.At least 21 of the 56 runaway stars in our sample can be linked to thenearby associations and young open clusters. These include the classicalrunaways 53 Arietis (Ori OB1), xi Persei (Per OB2), and lambda Cephei(Cep OB3), and fifteen new identifications, amongst which a pair ofstars running away in opposite directions from the region containing thelambda Ori cluster. Other currently nearby runaways and pulsarsoriginated beyond 700 pc, where our knowledge of the parent groups isvery incomplete.
| Spectropolarimetry of broad Hα lines in radio galaxies: constraints on the geometry of the broad-line and scattering regions
We present optical spectropolarimetric observations of three radio-loudbroad-line active galactic nuclei: 3C332, 3C351 and 4C73.18, togetherwith new observations of 3C390.3, which we observed previously.Intrinsically polarized broad Hα emission was detected in 3C332and 3C390.3 at levels of ~3 per cent and ~1 per cent, respectively.Significant polarization was not detected in 3C351, while thepolarization measured in 4C73.18 (~1 per cent) is consistent withforeground contamination by the galactic interstellar medium. Although3C332 and 3C390.3 both exhibit `double-peaked' broad Hα lineprofiles, their polarization properties differ significantly. Thepolarized Hα emission in 3C332 has a similar profile to that seenin total flux and the continuum and line emission is polarized at70° to the axis of the radio source. In 3C390.3, however, the doublepeaks seen in total flux are suppressed in polarized flux and theHα line and continuum are polarized at ~5° to the axis of theradio emission. Comparison of these new observations of 3C390.3 withdata obtained 2 years previously reveals significant variations in bothtotal and polarized flux. The implications of these observations for therelationship between the geometry of the broad-line region and that ofthe scattering medium are discussed. We argue that models in which theHα emission arises in a relativistically rotating disc cannoteasily explain the observed polarization properties of 3C332 and, inparticular, 3C390.3. We consider instead a model in which the lineemission originates in a bipolar outflow and present calculationsshowing that the observed polarization properties of 3C390.3 can bebroadly explained by scattering from the inner wall of an obscuringtorus, while those of 3C332 are consistent with scattering by particlessituated above the poles of the torus. We outline a scheme in whicheither toroidal or polar scattering dominates in a given object,depending on the orientation of its radio jet axis to our line of sight.
| 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.
| Hot Stars and Cool Clouds: The Photodissociation Region M16
We present high-resolution spectroscopy and images of aphotodissociation region (PDR) in M16 obtained during commissioning ofthe near-infrared spectrometer (NIRSPEC) on the Keck II telescope. PDRsplay a significant role in regulating star formation, and M16 offers theopportunity to examine the physical processes of a PDR in detail. Wesimultaneously observe both the molecular and ionized phases of the PDRand resolve the spatial and kinematic differences between them. The mostprominent regions of the PDR are viewed edge-on. Fluorescent emissionfrom nearby stars is the primary excitation source, although collisionsalso preferentially populate the lowest vibrational levels ofH2. Variations in density-sensitive emission-line ratiosdemonstrate that the molecular cloud is clumpy, with an average densityn=3x105 cm-3. We measure the kinetic temperatureof the molecular region directly and find thatTH2=930 K. The observed density, temperature, andUV flux imply a photoelectric heating efficiency of 4%. In the ionizedregion, ni=5x103 cm-3 andTHII=9500 K. In the brightest regions of the PDR, therecombination line widths include a nonthermal component, which weattribute to viewing geometry. Data presented herein were obtained atthe W. M. Keck Observatory, which is operated as a scientificpartnership among the California Institute of Technology, the Universityof California, and the National Aeronautics and Space Administration.The Observatory was made possible by the generous financial support ofthe W. M. Keck Foundation.
| HR 4049: temporal variations in the structure of the circumstellar material
We present ultraviolet (UV) and optical spectropolarimetry of thepost-asymptotic giant branch star HR 4049 (= HD 89353). The polarizationis time-variable in both magnitude and position angle. The variations inthe polarization are tied to the orbital period and the circumstellarreddening and, in the optical, appear to arise from Rayleigh scatteringin a thin disc. However, the ultraviolet (λ <~ 3000 Å)polarization shows the signature of scattering from material in abipolar distribution. The near-UV (~ 2000-3000 Å) polarization hasapproximately the same position angle as the optical polarization andhence also arises from scattering in a disc. However, the componentperpendicular to the disc is of unknown origin. We discuss possiblesources for this component.
| Discovery of a high-speed outflow in 4C 74.26
We present intermediate-resolution optical spectropolarimetry of theradio-loud quasar 4C 74.26. Large variations in both the degree andposition angle of polarization are observed across the broad H alphaline. After continuum subtraction, we find that in polarized flux thisfeature is redshifted by ~ 2000 km s^-1 relative to its medianwavelength in total flux. We show that the H alpha polarization can beexplained by scattering in a high-speed outflow which, if it is directedalong the axis of the radio jet, must have a velocity >= 5000 km s^-1in order to account for the observed redshift. The polarization positionangle is not precisely orthogonal to that of the radio source,suggesting that the scattering outflow is either inhomogeneous, orsomewhat misaligned with the jet axis.
| Ultraviolet Interstellar Linear Polarization. V. Analysis of the Final Data Set
Using recent measurements of ultraviolet interstellar polarization, wehave examined its relationship to ultraviolet extinction and topolarization and extinction measurements in the visible and infrared.The relationship between the relative amount of ultraviolet polarizationand the parameter lambda_max, determined using only visible data, isconfirmed and strengthened, for example, by a tight correlation betweenp(6 mum^-1)/p_max and lambda^-1_max. A good fit to the wavelengthdependence of the polarization from the infrared to the ultraviolet canbe achieved with a five-parameter function combining a power law in theinfrared and a Serkowski-like function in the ultraviolet. Thepolarization efficiency (ratio of polarization to extinction) is less inthe ultraviolet than in the visual, and the ratio of these efficienciesincreases systematically with lambda^-1_max. We relate these effects tosystematic changes in the underlying aligned grain size distribution.The polarization efficiency of the grains causing the 2175 Åextinction bump along most sight lines is so (unusually) small that nostatistically significant polarization feature is detectable in thatwavelength region. Only two of 28 sight lines show a definitepolarization feature. The environments of the two bump sight lines aresimilar but not unique, and the mechanism for producing the polarizationfeature along only these two sight lines is still not known.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Ultraviolet interstellar linear polarization. IV. Cross-calibration between the Wisconsin ultraviolet photo-polarimeter experiment and the faint object spectrograph.
The Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) and theFaint Object Spectrograph (FOS) started gathering UV spectropolarimetryin 1990. Each observed a wide range of polarimetrically interestingobjects before being retired. This paper examines the polarimetriccalibration of each instrument and compares the absolute calibrationsthrough the use of the limited sample of ``standard stars'' observed byboth instruments. In general, there is good agreement between the FOSand WUPPE results at the 2sigma statistical level. Data from eachinstrument can be compared directly on a straightforward basis.
| Interstellar polarization at high galactic latitudes from distant stars. II. Evidence for depolarization starting at Z=~600pc.
We have measured linear polarization for 20 distant high latitude A andF stars, as a first step to extend our previous study (Berdyugin et al.1995A&A...294..568B: Paper I) of the North Galactic polar cap todistances larger than 600pc. The results give tentative evidence fordepolarization starting to be seen around Z=~500pc, possibly indicatingthat there is a significant change in the magnetic field direction.Depolarization requires that there is still light-polarizing dust in theZ range 500-1000pc and the lower limit to extinction inferred in Paper I(A_B_>0.11mag) must be increased by a yet undetermined amount.
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| Сазвежђа: | Змијоносац |
| Ректацензија: | 17h43m47.00s |
| Deклинација: | -07°04'46.0" |
| Apparent магнитуда: | 6.3 |
| Даљина: | 427.35 parsecs |
| Proper motion RA: | -4.9 |
| Proper motion Dec: | -12.7 |
| B-T magnitude: | 6.664 |
| V-T magnitude: | 6.331 |
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