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Subarcsecond Mid-Infrared Observations of NGC 6240: Limitations of Active Galactic Nucleus-Starburst Power Diagnostics
In order to examine the relative importance of powerful starbursts andCompton-thick active galactic nuclei (AGNs) in NGC 6240, we haveobtained mid-infrared images and low-resolution spectra of the galaxywith subarcsecond spatial resolution using the Keck telescopes. Despitethe high spatial resolution (~200 pc) of our data, no signature of thehidden AGNs has been detected in the mid-infrared. The southern nucleus,which we show provides 80%-90% of the total 8-25 μm luminosity of thesystem, has a mid-infrared spectrum and a mid-/far-infrared spectralenergy distribution consistent with starbursts. At the same time,however, it is also possible to attribute up to 60% of the bolometricluminosity to an AGN, consistent with X-ray observations, if the AGN isheavily obscured and emits mostly in the far-infrared. This ambiguityarises because the intrinsic variation of properties among a givengalaxy population (e.g., starbursts) introduces at least a factor of afew uncertainty even into the most robust AGN-starburst diagnostics. Weconclude that with present observations it is not possible to determinethe dominant power source in galaxies when AGN and starburstluminosities are within a factor of a few of each other.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Lifting the Iron Curtain: Toward an Understanding of the Iron Stars XX Oph and AS 325
We present new optical, near-infrared, and archival ultravioletobservations of XX Ophiuchi and AS 325, two proposed ``iron'' stars.These unusual stars have optical spectra dominated by emission linesarising from hydrogen, as well as ionized metals such as iron, chromium,and titanium. Both stars have been classified as ``iron'' stars, and anumber of exotic models have been presented for their origin. Using 2years of moderately high resolution optical spectroscopy, the first highsignal-to-noise ratio K-band spectroscopy of these sources (whichreveals stellar photospheric absorption lines), and new near-infraredinterferometric observations, we confirm that both systems are composedof two stars, likely binaries, containing a hot Be star with an evolvedlate-type secondary. The hydrogen emission features arise in the hotwind from the Be star, while the corresponding P-Cygni absorption linesare produced from dense material in the expanding, radiation-driven windaround each system. The optical Fe II emission lines are pumped byultraviolet Fe II absorption lines through fluorescence. Contrary tosome claims in the literature, the spectral features of XX Oph and AS325 are quite similar, evidence that they are comparable systems. Weexamine the variability of the spectral morphology and radial velocitymotions of both sources. We also study the variability of XX Oph duringa major photometric event and find that the spectral nature of thesystem varies during the event. A comparison of the velocity of theabsorption-line components in our new spectra with those in theliterature show that the structure of the stellar wind from XX Oph haschanged since the system was observed in 1951.
| Visual Star Colours from Instrumental Photometry
In order to display graphically the visual colours of stars and otherastronomical objects, photometric broadband R, V, B colours are used toproxy for the r, g, b colours of the three visual sensors of the eye.From photometric Johnson B-V and V-R colour indices, R, V, and Bmagnitudes (V = 0) are calculated, and from these the respectivebrightnesses (r, v = 1 = g, and b) are calculated. After suitablenormalization these are then placed in a ternary diagram having r, g,and b as the vertices. All B-V and V-R are adjusted so that the Sunfalls in the same place as a blackbody at 5800 K. The resulting ternaryplot shows all of its objects (stars, planets) in their visual coloursat their relative positions in the ternary diagram. The star coloursdisplayed on a computer monitor screen or as a print with a colourprinter are more vivid than the usual visual impressions of isolatedstars, undoubtedly because of properties of the dark-adapted eye, butdouble-star pairs with contrasting colours correspond nicely totelescopic visual impressions.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165}
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| Seasonal variation of Titan's stratospheric ethylene (C2H4) observed
All previous observations of seasonal change on Titan have been ofphysical phenomena such as clouds and haze. We present here the firstobservational evidence of chemical change in Titan's atmosphere. Imagestaken during 1999-2002 (late southern spring on Titan) with the W.M.Keck I 10-meter telescope at 8-13 μm show a significant accumulationof ethylene (C2H4) in the south polar stratosphereas well as north-south stratospheric temperature variation (colder atpoles). Our observations restrict this newly discovered south polarethylene accumulation to latitudes south of 60° S. The only otherobservations of the spatial distribution of C2H4were those of Voyager I, which found a significant north polaraccumulation in early northern spring. We see no build-up in the north,although the highest northern latitudes are obstructed from view in thecurrent season. Our observations constrain any unobserved north polaraccumulation of C2H4 to north of 50° Nlatitude. Comparison of the Voyager I results with our new results showseasonal chemical change has occurred in Titan's atmosphere.
| Angular Diameters of Stars from the Mark III Optical Interferometer
Observations of 85 stars were obtained at wavelengths between 451 and800 nm with the Mark III Stellar Interferometer on Mount Wilson, nearPasadena, California. Angular diameters were determined by fitting auniform-disk model to the visibility amplitude versus projected baselinelength. Half the angular diameters determined at 800 nm have formalerrors smaller than 1%. Limb-darkened angular diameters, effectivetemperatures, and surface brightnesses were determined for these stars,and relationships between these parameters are presented. Scatter inthese relationships is larger than would be expected from themeasurement uncertainties. We argue that this scatter is not due to anunderestimate of the angular diameter errors; whether it is due tophotometric errors or is intrinsic to the relationship is unresolved.The agreement with other observations of the same stars at the samewavelengths is good; the width of the difference distribution iscomparable to that estimated from the error bars, but the wings of thedistribution are larger than Gaussian. Comparison with infraredmeasurements is more problematic; in disagreement with models, coolerstars appear systematically smaller in the near-infrared than expected,warmer stars larger.
| Infrared Colors and Variability of Evolved Stars from COBE DIRBE Data
For a complete 12 μm flux-limited sample of 207 IRAS sources(F12>=150 Jy, |b|>=5deg), the majority ofwhich are AGB stars (~87%), we have extracted light curves in seveninfrared bands between 1.25 and 60 μm using the database of theDiffuse Infrared Background Experiment (DIRBE) instrument on the CosmicBackground Explorer (COBE) satellite. Using previous infrared surveys,we filtered these light curves to remove data points affected by nearbycompanions and obtained time-averaged flux densities and infraredcolors, as well as estimates of their variability at each wavelength. Inthe time-averaged DIRBE color-color plots, we find clear segregation ofsemiregulars, Mira variables, carbon stars, OH/IR stars, and red giantswithout circumstellar dust (i.e., V-[12]<5) and with little or novisual variation (ΔV<0.1 mag). The DIRBE 1.25-25 μm colorsbecome progressively redder and the variability in the DIRBE databaseincreases along the oxygen-rich sequence nondusty slightly varying redgiants-->SRb/Lb-->SRa-->Mira-->OH/IR and the carbon-richSRb/Lb-->Mira sequence. This supports previous assertions that theseare evolutionary sequences involving the continued production andejection of dust. The carbon stars are redder than their oxygen-richcounterparts for the same variability type, except in theF12/F25 ratio, where they are bluer. Of the 28sources in the sample not previous noted to be variable, 18 are clearlyvariable in the DIRBE data, with amplitudes of variation of ~0.9 mag at4.9 μm and ~0.6 mag at 12 μm, consistent with them being verydusty Mira-like variables. We also present individual DIRBE light curvesof a few selected stars. The DIRBE light curves of the semiregularvariable L2 Pup are particularly remarkable. The maxima at1.25, 2.2, and 3.5 μm occur 10-20 days before those at 4.9 and 12μm, and, at 4.9 and 12 μm, another maximum is seen between the twonear-infrared maxima.
| JHK Standards for Small Telescopes
The AAVSO Futures meeting, held in Madison, WI, in May 2001, proposedthat the AAVSO support near-infrared research with small telescopes. Aphotometer, the SSP-4, has been developed to provide J- and H-bandcapability for a reasonable cost. However, proper calibrated photometryrequires a set of standard stars. This paper describes such a set ofstars, suitable for small telescopes, and with accurate coordinates,proper motions, and high-quality magnitudes.
| First light observations with TIFR near-infrared camera.
The TIFR near-infrared camera (TIRCAM) is based on the SBRC InSb focalplane array (58×62 pixels) sensitive between 1-5 μm. TIRCAM hadits first engineering run at Gurusikhar 1.2 meter PRL telescope at MountAbu during March-April 2001. The first light observations with TIRCAMwere quite successful. Several infrared standard stars and the TrapeziumCluster in Orion region were observed in the J, H and K bands. In anarrow band at 3.9 μm (nbL), some bright stars could be detected fromthe Gurusikhar site. The performance of TIRCAM is discussed in the lightof preliminary observations in nbL band.
| Kinematics of Hipparcos Visual Binaries. II. Stars with Ground-Based Orbital Solutions
This paper continues kinematical investigations of the Hipparcos visualbinaries with known orbits. A sample, consisting of 804 binary systemswith orbital elements determined from ground-based observations, isselected. The mean relative error of their parallaxes is about 12% andthe mean relative error of proper motions is about 4%. However, even 41%of the sample stars lack radial velocity measurements. The computedGalactic velocity components and other kinematical parameters are usedto divide the stars with known radial velocities into kinematical agegroups. The majority (92%) of binaries from the sample are thin diskstars, 7.6% have thick disk kinematics and only two binaries have halokinematics. Among them, the long-period variable Mira Ceti has a verydiscordant {Hipparcos} and ground-based parallax values. From the wholesample, 60 stars are ascribed to the thick disk and halo population.There is an urgent need to increase the number of the identified halobinaries with known orbits and substantially improve the situation withradial velocity data for stars with known orbits.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| Long-Term VRI Photometry of Small-Amplitude Red Variables. I. Light Curves and Periods
We report up to 5000 days of VRI photometry, from a robotic photometrictelescope, of 34 pulsating red giants, namely, TV Psc, EG And, Z Psc, RZAnd, 4 Ori, RX Lep, UW Lyn, η Gem, μ Gem, ψ1 Aur,V523 Mon, V614 Mon, HD 52690, Y Lyn, BC CMi, X Cnc, UX Lyn, RS Cnc, VYUMa, ST UMa, TU CVn, FS Com, SW Vir, 30 Her, α1 Her,V642 Her, R Lyr, V450 Aql, V1293 Aql, δ Sge, EU Del, V1070 Cyg, WCyg, and μ Cep, as well as a few variable comparison stars. V, R, andI variations are generally in phase. The length and density of the dataenable us to look for variations on timescales ranging from days toyears. We use both power-spectrum (Fourier) analysis and autocorrelationanalysis, as well as light-curve analysis; these three approaches arecomplementary. The variations range from regular to irregular, but inmost of the stars, we find a period in the range of 20-200 days, whichis probably due to low-order radial pulsation. In many of the stars, wealso find a period which is an order of magnitude longer. It may be dueto rotation, or it may be due to a new kind of convectively inducedoscillatory thermal mode, recently proposed by P. Wood.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| On the Wilson-Bappu relationship in the Mg II k line
An investigation is carried out on the Wilson-Bappu effect in the Mg Iik line at 2796.34 Å. The work is based on a selection of 230 starsobserved by both the IUE and HIPPARCOS satellites, covering a wide rangeof spectral types (F to M) and absolute visual magnitudes (-5.4<=MV <=9.0). A semi-automatic procedure is used to measurethe line widths, which applies also in the presence of strong centralabsorption reversal. The Wilson-Bappu relationship here provided isconsidered to represent an improvement over previous recent results forthe considerably larger data sample used, as well as for a properconsideration of the measurement errors. No evidence has been found fora possible dependence of the WB effect on stellar metallicity andeffective temperature.
| 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 Variability of K5-M Stars
I investigate the Hipparcos Satellite photometry of K5-M stars to seethe pattern of activity of these stars. A few stars for which furtherstudy is desirable are identified.
| The First 50 Years at Palomar, 1949-1999 Another View: Instruments, Spectroscopy and Spectrophotometry and the Infrared
We review the research on a wide variety of topics using data obtainedwith the 200-inch Hale telescope. Using state-of-the-art spectrographs,photometers, spectrometers and infrared detectors, the Palomarastronomers investigated the spectra of stars, interstellar matter, AGNsand quasars in great detail. Spectral resolutions ranged from 1000 A forbroad-band photometry to 0.04 A using interferometric techniques.
| Near-Infrared Classification Spectroscopy: J-Band Spectra of Fundamental MK Standards
We present a catalog of J-band spectra for 88 fundamental MK standardstars observed at a resolving power of R~3000. This contribution servesas a companion atlas to the K-band spectra recently published by Wallace& Hinkle and the H-band atlas by Meyer and coworkers. We report datafrom 7400 to 9550 cm-1 (1.05-1.34 μm) for stars ofspectral types O7-M6 and luminosity classes I-V as defined in the MKsystem. In reducing these data, special care has been taken to removetime-variable telluric features of water vapor. We identify atomic andmolecular indexes that are both temperature and luminosity sensitivethat aid in the classification of stellar spectra in the J band. Inaddition to being useful in the classification of late-type stars, the Jband contains several features of interest in the study of early-typestellar photospheres. These data are available electronically foranonymous FTP in addition to being served on the World Wide Web.
| Variability of Quasars at 10 Microns
Twenty-five low-redshift quasars have been monitored for several decadesat five near- and midinfrared wavelengths to detect rapid variationsthat would indicate that a nonthermal component was present in the ``10μm bump.'' Such variability has apparently been detected in severalradio-loud quasars and in the radio-quiet quasar PG 1535+547. Inaddition, the structure function of PG 1226+023 shows that an apparentlyperiodic component is present in its near-infrared emission.
| High resolution spectroscopy over lambda lambda 8500-8750 Å for GAIA. I. Mapping the MKK classification system
We present an Echelle+CCD high resolution spectroscopic atlas (0.25Ä/pix dispersion, 0.43 Ä FWHM resolution and 20 000 resolvingpower) mapping the MKK classification system over the interval lambdalambda 8500-8750 Ä. The wavelength interval is remarkably free fromtelluric lines and it is centered on the near-IR triplet of Ca II, thehead of hydrogen Paschen series and several strong metallic lines. Thespectra of 131 stars of types between O4 and M8 and luminosity classes Ithrough V are included in the atlas. Special care was put in maintainingthe highest instrumental homogeneity over the whole set of data. Thecapability to derive accurate MKK spectral types from high resolutionobservations over the interval lambda lambda 8500-8750 Ä isdiscussed. The observations have been performed as part of an evaluationstudy of possible spectroscopic performances for the astrometric missionGAIA planned by ESA. Tables~3 and 4 are 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/ Abstract.html}\fnmsep\thanks{ Thespectra of the stars listed in Table~2 are also available in electronicform at the CDS or via the personal HomePagehttp://ulisse.pd.astro.it/Astro/Atlases/}\fnmsep\thanks{ Figures 3--28are only available in electronic form at http://www.edpsciences.com
| The Infrared Spectral Classification of Oxygen-rich Dust Shells
This paper presents infrared spectral classifications for a flux-limitedsample of 635 optically identified oxygen-rich variables includingsupergiants and sources on the asymptotic giant branch (AGB). Severalclasses of spectra from oxygen-rich dust exist, and these can bearranged in a smoothly varying sequence of spectral shapes known as thesilicate dust sequence. Classification based on this sequence revealsseveral dependencies of the dust emission on the properties of thecentral star. Nearly all S stars show broad emission features fromalumina dust, while most of the supergiants exhibit classic featuresfrom amorphous silicate dust. Mira variables with symmetric light curvesgenerally show broad alumina emission, while those with more asymmetriclight curves show classic silicate emission. These differences may arisefrom differences in the photospheric C/O ratio.
| Near-Infrared Classification Spectroscopy: H-Band Spectra of Fundamental MK Standards
We present a catalog of H-band spectra for 85 stars of approximatelysolar abundance observed at a resolving power of 3000 with the KPNOMayall 4 m Fourier Transform Spectrometer. The atlas covers spectraltypes O7-M5 and luminosity classes I-V as defined in the MK system. Weidentify both atomic and molecular indices and line ratios that aretemperature and luminosity sensitive, allowing spectral classificationto be carried out in the H-band. The line ratios permit spectralclassification in the presence of continuum excess emission, which iscommonly found in pre-main-sequence or evolved stars. We demonstratethat with spectra of R = 1000 obtained at signal-to-noise ratio >50,it is possible to derive spectral types within +/-2 subclasses forlate-type stars. These data are available electronically through theAstronomical Data Center in addition to being served on the World WideWeb.
| The luminosity index for M stars and the distance to the LMC.
Not Available
| Radii and effective temperatures for K and M giants and supergiants. II.
Not Available
| The ROSAT all-sky survey catalogue of optically bright late-type giants and supergiants
We present X-ray data for all late-type (A, F, G, K, M) giants andsupergiants (luminosity classes I to III-IV) listed in the Bright StarCatalogue that have been detected in the ROSAT all-sky survey.Altogether, our catalogue contains 450 entries of X-ray emitting evolvedlate-type stars, which corresponds to an average detection rate of about11.7 percent. The selection of the sample stars, the data analysis, thecriteria for an accepted match between star and X-ray source, and thedetermination of X-ray fluxes are described. Catalogue only available atCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| On the X-ray emission from M-type giants
We have searched for X-ray emission from M-type giants and supergiantslisted in the Bright Star Catalogue (BSC) in the data of the ROSATall-sky survey (RASS). These stars lie to the far right of the X-raydividing line and are thus not expected to be X-ray sources. Any X-raydetection would therefore violate the common paradigm of X-ray darkM-type giants beyond the X-ray dividing line. We found 11 BSC M-typegiants and supergiants to coincide with RASS X-ray sources. While for 4stars the X-ray emission is very likely related to their cataclysmic orsymbiotic nature or can be attributed to a visual G-type companion, theother 7 stars are candidates for intrinsic X-ray emission. Of theseobjects, 3 have a rather large offset between optical and X-rayposition, so their proper identification with late-type giants is atleast questionable. For the remaining four stars, we obtained opticallow-dispersion spectra in order to search for emission lines indicativeof a possible symbiotic nature. None of these stars shows any brightemission lines, so they are probably quite normal M-type giants. Wediscuss possible origins of X-ray emission in these stars and theimportance of evolutionary aspects. Based on observations by the ROSATand Calar Alto Observatories
| Quantitative spectral classification of galactic disc K-M stars from spectrophotometric measurements
New spectral observations for 47 southern galactic red supergiantsobtained with the new RUBIKON spectrophotometer (developed at theAstronomisches Institut der Ruhr-Universitat Bochum) at the Bochum 61-cmtelescope on La Silla are presented. The spectra range from 4800 to 7700A and their resolution is 10 A. The mean error of absolute fluxes is0.028 mag and that of relative fluxes 0.021 mag. The spectra will beavailable at the Strasbourg Stellar Database (CDS). Together with datataken from recently published spectral catalogues, the new observationshave been used to define spectral indices as measures of the strengthsof the following features: Fe i+TiOalpha_1, Mgb+TiOalpha_0,NaD+TiOgamma'_1, TiOgamma'_0 and TiOgamma_1 systems. The indices havebeen checked against errors introduced by reductions, interstellarreddening and different resolutions of different spectral catalogues,and have been found to be very insensitive to all these effects.Therefore, different catalogues may be combined without any loss ofaccuracy and homogeneity. The mean error of a single index has beenfound to be 0.011 mag. For stars from K4 to M7, a strong temperaturedependence is found for all indices. For the Fe i+TiO and especially theMgb+TiO features, a strong dependence on luminosity has also beenobserved. These indices therefore have been combined to form aluminosity index, while the others together form a spectral index. Thecombined indices have been calibrated in terms of MK data using thestepwise linear regression technique, and may be used for quantitativetwo- dimensional spectral classification of late K- and M-type stars.The mean error of the classification is 0.6 of spectral subtype and 0.8of luminosity class, which is much higher than would be expected fromthe uncertainty of the indices alone (which, e.g., for an M4 giantcorrespond to an uncertainty of 0.1 of spectral subtype and 0.3 ofluminosity class). This may be explained by the uncertainty of theoriginal MK classifications and the variability of some programme stars.
| Hotspots on late-type supergiants
We present the results of a diffraction-limited imaging survey of threenearby M-supergiants using interferometric techniques at the 4.2-mWilliam Herschel Telescope. Between 1992 January and 1993 December allthree stars (alpha Ori, alpha Sco and alpha Her) showed unambiguousevidence at optical wavelengths for asymmetric structures close to, oron, the stellar surface, implying that such features must be a commonphenomenon on M-supergiants. Typically these unresolved features, or`hotspots', numbered between one and three, contributed between 5 and 20per cent of the total flux from the source, and had similar colours tothe emission from the stellar disc. The observed time-scales forvariations of the hotspots were in the range 3 to 9 months, and are thusconsistent with a convective origin, as first suggested bySchwarzschild, and with their contributing towards the irregularvariability seen in this class of star. The apparent ubiquity of suchsurface asymmetries implies that the circularly symmetric models oftenused to interpret high spatial resolution observations of these starswill need to be modified if systematic errors are not to be introduced.
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