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Ýlgili Makaleler
Multi-periodic oscillations of HD 32887 and HD 81797.
In this paper we present two evolved stars, HD 32887 and HD 81797, whichshow multi-periodic oscillations. We observed the oscillations by meansof the precise radial velocity technique with the simultaneouscalibration method. The high-resolution spectra of each star have beenobtained with FEROS at the 2.2 m-MPG/ESO telescope in La SillaObservatory, Chile. We found variation in the stellar radial velocitiesand spectral line profiles. The periods of the oscillations are fromseveral hours up to few days. The sources of the short-term oscillationsof HD 32887 and HD 81797 are obviously due to stellar pulsations, whichare similar to solar-like oscillations. In particular, in HD 81797 wefound a clear correlation between the variation in the asymmetry of thespectral line profile, measured in the bisector velocity spans, and theradial velocity. Both stars have bisector velocity spans which also showoscillations. The periods of the bisector oscillations are similar tothose of the radial velocity variation. The detection of themulti-periodic oscillations in HD 32887 and HD 81797 makes these star tobe amenable targets for asteroseismology, in particular, of stars in thered giant branch.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| Dust Morphology and Composition in FU Orionis Systems
FU Orionis stars are a small group of pre-main-sequence stars known forlarge-amplitude optical variability. These objects also exhibitmultiwavelength phenomena suggestive of active accretion from acircumstellar disk. We present high spatial resolution mid-IR imagingand spectroscopy, submillimeter photometry, and 3-4 μm photometry offour FU Ori-class objects, RNO 1B and C, Z CMa, and Par 21, and oneobject classified as a pre-FU Ori star, V380 Ori. We resolve multiple IRsources and extended emission in the RNO 1B/C system, and we discuss indetail their association with disk activity and the source of theInfrared Astronomical Satellite far-IR and radio maser emission in thisfield. We derive dust temperatures and masses for all sources anddiscuss how dust composition and morphology is related to theevolutionary stage of these objects.
| VLA Observations of ζ Aurigae: Confirmation of the Slow Acceleration Wind Density Structure
Studies of the winds from single K and early M evolved stars indicatethat these flows typically reach a significant fraction of theirterminal velocity within the first couple of stellar radii. The mostdetailed spatially resolved information of the extended atmospheres ofthese spectral types comes from the ζ Aur eclipsing binaries.However, the wind acceleration inferred for the evolved primaries inthese systems appears significantly slower than for stars of similarspectral type. Since there are no successful theories for mass loss fromK and early M evolved stars, it is important to place strong empiricalconstraints on potential models and determine whether this difference inacceleration is real or an artifact of the analyses. We have undertakena radio continuum monitoring study of ζ Aurigae (K4 Ib + B5 V)using the Very Large Array to test the wind density model of Baade etal. that is based on Hubble Space Telescope (HST) Goddard HighResolution Spectrograph ultraviolet spectra. ζ Aur was monitored atcentimeter wavelengths over a complete orbital cycle, and fluxvariations during the orbit are found to be of similar magnitude tovariations at similar orbital phases in the adjacent orbit. Duringeclipse, the flux does not decrease, showing that the radio emissionoriginates from a volume substantially larger thanR3K~(150Rsolar)3 surroundingthe B star. Using the one-dimensional density model of the K4 Ibprimary's wind derived from HST spectral line profile modeling andelectron temperature estimates from previous optical and new HSTstudies, we find that the predicted radio fluxes are consistent withthose observed. Three-dimensional hydrodynamic simulations indicate thatthe accretion flow perturbations near the B star do not contributesignificantly to the total radio flux from the system, consistent withthe radio eclipse observations. Our radio observations confirm the slowwind acceleration for the evolved K4 Ib component. ζ Aur's velocitystructure does not appear to be typical of single stars with similarspectral types. This highlights the need for more comprehensivemultiwavelength studies for both single stars, which have been sadlyneglected, and other ζ Aur systems to determine if its windproperties are typical.
| A giant planet around the massive giant star HD 13189
Most extrasolar planet discoveries using radial velocity measurementshave been for solar-like G-stars. In order to understand better the rolestellar mass for the formation of planets we must learn more about thefrequency of planetary companions around both high- and low-mass stars.Radial velocity searches for planets around high mass main-sequencestars are difficult due to the paucity of lines and often rapid rotationof these early-type stars. On the other hand, evolved stars that havemoved off the main sequence offer us the possibility of searching forplanets around higher mass stars by means of precise radial velocitymeasurements. Here we present radial velocity measurements for the starHD 13189 using measurements taken at the Thüringer LandessternwarteTautenburg, the Harlan J. Smith Telescope at McDonald Observatory, andthe Hobby-Eberly Telescope. We classify the spectral type of this staras K2 with luminosity class II. The radial velocity measurements showlong-period variations with a period of 472 days and an amplitude of 173m s-1. The Ca II S-index is consistent with an inactive starand this shows no variations with the radial velocity period. We alsoinvestigated possible changes in the line shapes by measuring spectralline bisectors. These show no variations with the radial velocityperiod. We interpret the 472-day period as being caused by a sub-stellarcompanion. Based on the estimated absolute magnitude and a comparison toevolutionary tracks we estimate the mass of the progenitor star between2 and 7 M_ȯ which results in a projected mass of the companion of msin i = 8 20 M_J. HD 13189 may be the most massive star known to possessan extrasolar planet. This suggests that the formation of giant planetscan also occur around early-type stars. HD 13189 also shows significantshort term radial velocity variability on time scales of days that ismost likely due to stellar oscillations. This behavior is typical for Kgiant stars.
| Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch
We present the results of our radial velocity (RV) measurements of G andK giants, concentrating on the presence of multiple systems in oursample. Eighty-three giants have been observed for 2.5 years with thefiber-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in LaSilla, Chile. Seventy-seven stars (93%) of the targets have beenanalyzed for RV variability using simultaneous Th-Ar calibration and across-correlation technique. We estimate the long-term precision of ourmeasurement as better than 25 m s-1. Projected rotationalvelocities have been measured for most stars of the sample. Within ourtime-base only 21 stars (or 27%) show variability below 2\sigma, whilethe others show RV variability with amplitudes up to several kms-1. The large amplitude (several km s-1) andshape (high eccentricity) of the RV variations for 11 of the programstars are consistent with stellar companions, and possibly brown dwarfcompanions for two of the program stars. In those systems for which afull orbit could be derived, the companions have minimum masses from0.6 M\sun down to 0.1 M\sun. To thesemultiple systems we add the two candidates of giant planets alreadydiscovered in the sample. This analysis shows that multiple systemscontribute substantially to the long-term RV variability of giant stars,with about 20% of the sample being composed of multiple systems despitescreening our sample for known binary stars. After removing binaries,the range of RV variability in the whole sample clearly decreases, butthe remaining stars retain a statistical trend of RV variability withluminosity: luminous cool giants with B-V≥1.2 show RV variationswith \sigma_{/lineRV} > 60 m s-1, while giants with B-V< 1.2 including those in the clump region exhibit less variability orthey are constant within our accuracy. The same trend is observed withrespect to absolute visual magnitudes: brighter stars show a largerdegree of variability and, when plotted in the RV variability vs.magnitude diagram a trend of increasing RV scatter with luminosity isseen. The amplitude of RV variability does not increase dramatically, aspredicted, for instance, by simple scaling laws. At least two luminousand cooler stars of the sample show a correlation between RV andchromospheric activity and bisector asymmetry, indicating that in thesetwo objects RV variability is likely induced by the presence of(chromospheric) surface structures.Based on observations collected at the 1.52 m-ESO telescope at the LaSilla Observatory from Oct 1999 to Feb. 2002 under ESO programs and theESO-Observatório Nacional, Brazil, agreement and in part onobservations collected on the Alfred Jensch 2 m telescope of theThüringer Landessternwarte Tautenburg.
| Stellar Dynamics in the Galactic Center: 1000 Stars in 100 Nights
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| A Stochastic Model of Large Scale Motions in the Wind of Evolved Late-type Stars and its First Application to l Velorum
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| NLTE Radiative Transfer in the Extended Atmospheres and Winds of Cool Stars
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| The Wilson-Bappu effect: A tool to determine stellar distances
Wilson & Bappu (\cite{orig}) have shown the existence of aremarkable correlation between the width of the emission in the core ofthe K line of Ca II and the absolute visual magnitude of late-typestars.Here we present a new calibration of the Wilson-Bappu effect based on asample of 119 nearby stars. We use, for the first time, widthmeasurements based on high resolution and high signal to noise ratio CCDspectra and absolute visual magnitudes from the Hipparcos database.Our primary goal is to investigate the possibility of using theWilson-Bappu effect to determine accurate distances to single stars andgroups.The result of our calibration fitting of the Wilson-Bappu relationshipis MV=33.2-18.0 log W0, and the determinationseems free of systematic effects. The root mean square error of thefitting is 0.6 mag. This error is mostly accounted for by measurementerrors and intrinsic variability of W0, but in addition apossible dependence on the metallicity is found, which becomes clearlynoticeable for metallicities below [Fe/H] ~ -0.4. This detection ispossible because in our sample [Fe/H] ranges from -1.5 to 0.4.The Wilson-Bappu effect can be used confidently for all metallicitiesnot lower than ~ -0.4, including the LMC. While it does not provideaccurate distances to single stars, it is a useful tool to determineaccurate distances to clusters and aggregates, where a sufficient numberof stars can be observed.We apply the Wilson-Bappu effect to published data of the open cluster M67; the retrieved distance modulus is of 9.65 mag, in very goodagreement with the best distance estimations for this cluster, based onmain sequence fitting.Observations collected at ESO, La Silla.
| A stochastic model of large scale motions in the wind of evolved late-type stars and its first application to lamda Velorum.
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| A catalog of rotational and radial velocities for evolved stars. II. Ib supergiant stars
Rotational velocity vsin i and mean radial velocity are presented for asample of 231 Ib supergiant stars covering the spectral region F, G andK. This work is the second part of the large survey carried out with theCORAVEL spectrometer to establish the behavior of the rotation for starsevolving off the main sequence (De Medeiros & Mayor 1999). Thesedata will add constraints to the study of the rotational behavior inevolved stars, as well as solid information concerning tidalinteractions in binary systems and on the link between rotation,chemical abundance and activity in stars of intermediate masses. Basedon observations collected at the Haute-Provence Observatory,Saint-Michel, France and at the European Southern Observatory, La Silla,Chile Table 1 is 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/395/97
| A multi-epoch spectrophotometric atlas of symbiotic stars
A multi-epoch, absolute-fluxed spectral atlas extending from about 3200to 9000 Å is presented for 130 symbiotic stars, including membersof the LMC, SMC and Draco dwarf galaxies. The fluxes are accurate tobetter than 5% as shown by comparison with Tycho and ground-basedphotometric data. The spectra of 40 reference objects (MKK cool giantstandards, Mira and Carbon stars, planetary nebulae, white dwarfs, hotsub-dwarfs, Wolf-Rayet stars, classical novae, VV Cep and Herbig Ae/Beobjects) are provided to assist the interpretation of symbiotic starspectra. Astrometric positions and counterparts in astrometriccatalogues are derived for all program symbiotic stars. The spectra areavailable in electronic form from the authors. Based on observationscollected with the telescopes of the European Southern Observatory (ESO,Chile) and of the Padova & Asiago Astronomical Observatories(Italy). Tables 2 and 3 are only available in electronic form (a) at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/383/188, and (b) from thepersonal home page http://ulisse.pd.astro.it/symbio_atlas/ Figures 4-256are only available in electronic form (a) at http://www.edpsciences.organd (b) from the personal home pagehttp://ulisse.pd.astro.it/symbio_atlas/
| Mid-infrared observations of methanol maser sites and ultracompact H ii regions: signposts of high-mass star formation
N-band (10.5μm) and/or Q-band (20.0μm) images taken with MANIAC onthe ESO/MPI 2.2-m telescope are presented for 31 methanol maser sitesand 19 ultracompact (UC) Hii regions. Most of the maser sites and UC Hiiregions are coincident with mid-infrared (MIR) sources to within thepositional uncertainties of ~3arcsec, consistent with the maser emissionbeing powered by the MIR source. The IRAS source positions, however, donot always coincide with the MIR sources. Based on an average infraredspectral energy distribution, we deduce that the MIR objects areluminous enough that they should also produce a strong ionizingradiation. Some sources are consistent with stars of later spectraltype, but not all can be. A number of maser sites show no detectableradio continuum emission associated with MIR emission, despite apowering source luminous enough potentially to produce an UC Hii region.Since no signs of an UC Hii region are detected here, these maser sitesmight be produced during a very early stage of stellar evolution. Wepresent objects that show evidence of outflow activity stemming from amaser site, exhibiting CO and/or CS line profiles indicative of outflowscoincident with the MIR source. These cases are promising examples ofmaser sites signposting the earliest stages of high-mass star formation.
| N-Band Imaging of Seyfert Nuclei and the Mid-Infrared-X-Ray Correlation
We present new mid-infrared (N-band) images of a sample of eight nearbySeyfert galaxies. In all of our targets, we detect a central unresolvedsource, which in some cases has been identified for the first time. Inparticular, we have detected the mid-infrared emission from the activenucleus of NGC 4945, which previously remained undetected at anywavelength but hard X-rays. We also detect circumnuclear extendedemission in the Circinus galaxy along its major axis and find marginalevidence for extended circumnuclear emission in NGC 3281. The highspatial resolution (1.7") of our data allows us to separate the flux ofthe nuclear point sources from the extended circumnuclear starburst (ifpresent). We complement our sample with literature data for a number ofnonactive starburst galaxies and relate the nuclear N-band flux topublished hard (2-10 keV) X-ray fluxes. We find tight and well-separatedcorrelations between the nuclear N-band flux and X-ray flux for bothSeyfert and starburst nuclei that span over 3 orders of magnitude inluminosity. We demonstrate that these correlations can be used as apowerful classification tool for galactic nuclei. For example, we findstrong evidence against NGC 1808 currently harboring an active Seyfertnucleus based on its position in the mid-infrared-X-ray diagram. On theother hand, we confirm that NGC 4945 is in fact a Seyfert 2 galaxy.
| 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
| Moderate-Resolution Near-Infrared Spectroscopy of Cool Stars: A New K-Band Library
I present an atlas of near-infrared K-band spectra of 31 late-typegiants and supergiants and two carbon stars. The spectra were obtainedat resolving powers of 830 and 2000, and have a signal-to-noise ratio>~100. These data are complemented with results from similar existinglibraries in both K and H band, and they are used to identify varioustools useful for stellar population studies at moderate resolution. Ifocus on several of the most prominent absorption features and (1)investigate the effects of spectral resolution on measurements of theirequivalent width (EW), (2) examine the variations with stellarparameters of the EWs, and (3) construct composite indices as indicatorsof stellar parameters and of the contribution from excess continuumsources commonly found in star-forming and AGN galaxies. Among thefeatures considered, the 12CO (2,0) and 12CO (6,3)bandheads together with the Si I 1.59 μm feature, first proposed byOliva, Origlia, and coworkers, constitute the best diagnostic set forstellar spectral classification and for constraining the excesscontinuum emission. The Ca I 2.26 μm and Mg I 2.28 μm featuresoffer alternatives in the K band to the 12CO (6,3) bandheadand Si I feature.
| Ca II activity and rotation in F-K evolved stars
Ca II H and K high resolution observations for 60 evolved stars in thefield and in 5 open clusters are presented. From these spectrachromospheric fluxes are derived, and a homogeneous sample of more than100 giants is built adding data from the literature. In addition, formost stars, rotational velocities were derived from CORAVELobservations. By comparing chromospheric emission in the cluster starswe confirm the results of Pasquini & Brocato (1992): chromosphericactivity depends on the stellar effective temperature, and mass, whenintermediate mass stars (M ~ 4 Msun) are considered. TheHyades and the Praesepe clump giants show the same level of activity, asexpected from stars with similar masses and effective temperatures. Adifference of up to 0.4 dex in the chromospheric fluxes among the Hyadesgiants is recorded and this sets a clear limit to the intrinsic spreadof stellar activity in evolved giants. These differences in otherwisevery similar stars are likely due to stellar cycles and/or differencesin the stellar initial angular momentum. Among the field stars none ofthe giants with (V-R)o < 0.4 and Ia supergiants observedshows a signature of Ca II activity; this can be due either to the realabsence of a chromosphere, but also to other causes which preclude theappearance of Ca II reversal. By analyzing the whole sample we find thatchromospheric activity scales linearly with stellar rotational velocityand a high power of stellar effective temperature: F'k ~Teff7.7 (Vsini)0.9. This result can beinterpreted as the effect of two chromospheric components of differentnature: one mechanical and one magnetic. Alternatively, by using theHipparcos parallaxes and evolutionary tracks, we divide the sampleaccording to the stellar masses, and we follow the objects along anevolutionary track. For each range of masses activity can simply beexpressed as a function of only one parameter: either theTeff or the angular rotation Omega , with laws F'k~ Omega alpha , because angular velocity decreases witheffective temperature along an evolutionary track. By using theevolutionary tracks and the observed Vsini we investigate the evolutionof the angular momentum for evolved stars in the range 1-5Msun. For the 1.6-3 solar mass stars the data are consistentwith the IOmega =const law while lower and higher masses follow a lawsimilar to IOmega 2=const, where I is the computed stellarmomentum of inertia. We find it intriguing that Vsini remains almostconstant for 1Msun stars along their evolution; if a similarbehavior is shared by Pop II stars, this could explain the relativelyhigh degree of activity observed in Pop II giants. Finally, through theuse of models, we have verified the consistency of the F'k ~Omega alpha and the IOmega beta = Const lawsderived, finding an excellent agreement. This representation, albeitcrude (the models do not consider, for instance, mass losses) representsthe evolution of Ca II activity and of the angular momentum in asatisfactory way in most of the portion of HR diagram analyzed.Different predictions could be tested with observations in selectedclusters. Based on observations collected at ESO, La Silla. Tables 1-3are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Winds and mass-loss from evolved, low-gravity cool stars.
Not Available
| GHRS Observations of Cool, Low-Gravity Stars. V. The Outer Atmosphere and Wind of the Nearby K Supergiant lambda Velorum
UV spectra of lambda Velorum taken with the Goddard High ResolutionSpectrograph (GHRS) on the Hubble Space Telescope are used to probe thestructure of the outer atmospheric layers and wind and to estimate themass-loss rate from this K5 Ib-II supergiant. VLA radio observations atlambda=3.6 cm are used to obtain an independent check on the windvelocity and mass-loss rate inferred from the UV observations.Parameters of the chromospheric structure are estimated frommeasurements of UV line widths, positions, and fluxes and from the UVcontinuum flux distribution. The ratios of optically thin C II] emissionlines indicate a mean chromospheric electron density of logN_e~8.9+/-0.2cm^-3. The profiles of these lines indicate a chromospheric turbulence(v_0~25-36 km s^-1), which greatly exceeds that seen in either thephotosphere or wind. The centroids of optically thin emission lines ofFe II and of the emission wings of self-reversed Fe II lines indicatethat they are formed in plasma approximately at rest with respect to thephotosphere of the star. This suggests that the acceleration of the windoccurs above the chromospheric regions in which these emission linephotons are created. The UV continuum detected by the GHRS clearlytraces the mean flux-formation temperature as it increases with heightin the chromosphere from a well-defined temperature minimum of 3200 K upto about 4600 K. Emission seen in lines of C III] and Si III] providesevidence of material at higher than chromospheric temperatures in theouter atmosphere of this noncoronal star. The photon-scattering windproduces self-reversals in the strong chromospheric emission lines,which allow us to probe the velocity field of the wind. The velocitiesto which these self-absorptions extend increase with intrinsic linestrength, and thus height in the wind, and therefore directly map thewind acceleration. The width and shape of these self-absorptions reflecta wind turbulence of ~9-21 km s^-1. We further characterize the wind bycomparing the observations with synthetic profiles generated with theLamers et al. Sobolev with Exact Integration (SEI) radiative transfercode, assuming simple models of the outer atmospheric structure. Thesecomparisons indicate that the wind in 1994 can be described by a modelwith a wind acceleration parameter beta~0.9, a terminal velocity of29-33 km s^-1, and a mass-loss rate~3x10^-9 M_solar yr^-1. Modeling ofthe 3.6 cm radio flux observed in 1997 suggests a more slowlyaccelerating wind (higher beta) and/or a higher mass-loss rate thaninferred from the UV line profiles. These differences may be due totemporal variations in the wind or from limitations in one or both ofthe models. The discrepancy is currently under investigation.
| Mid-Infrared Imaging of the Young Binary Star HEN 3-600: Evidence for a Dust Disk around the Primary
We present high-resolution mid-infrared observations of the nearbylate-type young binary system Hen 3-600. The binary, at a distance of~50 pc, could be a member of the TW Hydrae association, the nearestknown group of young stars, with an age of a few million years. Ourimages make it possible for the first time to determine which star inthe pair, separated by 1.4", harbors the mid-infrared excess detected byIRAS. In the near-infrared, where the radiation is primarilyphotospheric, Hen 3-600A (M3) and Hen 3-600B (M3.5) have a flux ratio of1.6. At 4.8, 10.8, and 18.2 mum, the primary becomes increasinglydominant over the secondary, suggesting that most of the circumstellardust in the system resides around Hen 3-600A. Comparison of the spectralenergy distribution (SED) of Hen 3-600A to the median SED of classical TTauri stars suggests that its disk may be truncated by the secondary andprovides tentative evidence for a central disk hole. The distribution ofdust in the Hen 3-600 system may provide important clues to theformation and evolution of protoplanetary disks in close binaries.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Constraints of age, distance and progenitor of the supernova remnant RX J0852.0-4622/GRO J0852-4642
The discovery of the nearest young supernova remnant RX J0852.0-4622 /GRO J0852-4642 in the Galaxy by ROSAT and COMPTEL has been reportedrecently. Age and distance are determined to ~ 680 years and ~ 200 pc bythe X-ray diameter and the gamma -ray line flux of radioactive44Ti. Here we discuss the implications of the X-ray spectraand of the fact that 1.8 MeV gamma -ray line emission from the decay of26Al has been measured from the Vela region with a certainfraction possibly associated with the new SNR. We estimate anuncertainty of the age of +/- 100 yrs for a fixed yield of44Ti. The highest values of 44Ti yield provided bycurrent supernova explosion models give worst case upper limits of 1100yrs for the age and of 500 pc for the distance. Also the unknownionization stage of 44Ti adds to the uncertainty of age anddistance which is at most another 35% on top. Both the energy balancecompiled for the remnant and yield predictions for 44Ti and26Al by supernova models favour a core-collapse event. Twopoint sources have been found in the vicinity of the explosion center,either one of these might be the neutron star left by the supernova. Ifthere is a neutron star the X-ray count rates of the two point sourcesprovide an upper limit of the blackbody surface temperature, which isvery unlikely to exceed 3x105 K. The supernova might havebeen observed some 700 +/- 150 yrs ago, but based on the data of SN1181, e.g., there is a realistic chance that it has been missed if thesupernova was sub-luminous.
| The central depth of the Ca II triplet lines as a discriminant of chromospheric activity in late type stars
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| Large Variations in the Winds of Single Cool Giants: lambda Velorum and gamma Crucis
UV spectra of the "noncoronal" single K supergiant lambda Vel and of thesingle M giant gamma Cru obtained with IUE and the Hubble SpaceTelescope at various epochs indicate that the profiles of many linesformed in the wind exhibit striking alterations in shape with time. Weparameterize the wind profiles in terms of an empirical optical depthtau emp by reflecting the red wing about line center and comparing thereflected intensity with that of the blue wing. In the lambda Vel windthe terminal velocity v&infy; was found to be close to 40 km s-1 in1978, 1982, and 1994, but was at least 20 km s-1 greater in 1990. Thefaster wind in 1990 also had a total optical depth that was a factor of2-6 times greater than at the other epochs.
| On Chromospheric Heating Mechanisms of ``Basal Flux'' Stars
Several pieces of evidence have been pieced together over recent yearsto support the notion that the chromospheric emission measured fromstars with convection zones results in part from the upward propagationand dissipation of acoustic waves. One argument, based on a statisticalanalysis of available UV data of such stars across the H-R diagram,suggests the presence of an omnipresent "basal" level of chromosphericheating, which has been postulated as resulting from nonlinear acousticwave heating. However, with few exceptions, no studies have been madethat test more directly the intrinsically dynamic nature of thisshock-heating mechanism. Therefore, in order to search for more directsignatures of such upward-propagating shock waves in lines of C II, weexamined Goddard High-Resolution Spectrograph spectra of several evolvedstars that have "basal" levels of activity. No evidence is found tosupport the presence of such waves as a dominant component of theheating mechanism. Instead, behavior reminiscent of the solar transitionregion is seen, suggesting a magnetic heating mechanism for these stars.We conclude that upward-propagating shock waves do not dominate theobserved radiative losses from chromospheres of stars exhibiting typical"basal" behavior, and we suggest that the nonmagnetic origin of thebasal components of all convective stars must be called into question.New solar data from the SUMER instrument on SOHO also suggest problemswith the acoustic-wave interpretation, although further work iswarranted. In the course of this work, we also found a simpleexplanation for previously noted discrepancies between calculated andobserved ratios of C II lines in the spectrum of alpha Ori.
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| Takýmyýldýz: | Yelken |
| Sað Açýklýk: | 09h07m59.80s |
| Yükselim: | -43°25'57.0" |
| Görünürdeki Parlaklýk: | 2.21 |
| Uzaklýk: | 175.747 parsek |
| özdevim Sað Açýklýk: | -23.4 |
| özdevim Yükselim: | 14.4 |
| B-T magnitude: | 4.412 |
| V-T magnitude: | 2.379 |
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