Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
The Reddening of Red Supergiants: When Smoke Gets in Your Eyes
Deriving the physical properties of red supergiants (RSGs) depends onaccurate corrections for reddening by dust. We use our recent modelingof the optical spectra of RSGs to address this topic. First, we findthat previous broadband studies have underestimated the correction forextinction in the visible, and hence the luminosities (if derived fromV); the shift in the effective wavelengths of the standard B and Vbandpasses necessitates using an effective value of the ratioR'V=4.2 to correct broadband photometry of RSGs ifRV=3.1 for early-type stars viewed through the same dust,where we have assumed the standard reddening law of Cardelli andcoauthors. Use of the Fitzpatrick reddening law would lead toR'V=3.8, as well as slightly lower values ofextinction derived from spectrophotometry, but results in slightlypoorer fits. Second, we find that a significant fraction of RSGs inGalactic OB associations and clusters show up to several magnitudes ofexcess visual extinction compared to OB stars in the same regions; weargue that this is likely due to circumstellar dust around the RSGs. Wealso show that the RSG dust production rate (as indicated by the 12μm excess) is well correlated with bolometric luminosity, contrary towhat has been found by earlier studies. The stars with the highestamount of extra visual extinction also show significant near-UV (NUV)excesses compared to the stellar models reddened by the standardreddening law. This NUV excess is likely due to scattering of the star'slight by the dust and/or a larger average grain size than that typicalof grains found in the diffuse interstellar medium. Similar excesseshave been attributed to circumstellar dust around R Coronae Borealisstars. Finally, we estimate that the RSGs contribute dust grains at therate of 3×10-8Msolar yr-1kpc-2 in the solar neighborhood, comparable to what weestimate for late-type WCs, 1×10-7Msolaryr-1 kpc-2. In the solar neighborhood thisrepresents only a few percent of the dust production (which is dominatedby low-mass AGBs), but we note that in low-metallicity starbursts, dustproduction by RSGs would likely dominate over other sources.
| The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCSstellar atmosphere models to determine the effective temperatures of 74Galactic red supergiants (RSGs). The stars are mostly members of OBassociations or clusters with known distances, allowing a criticalcomparison with modern stellar evolutionary tracks. We find we canachieve excellent matches between the observations and the reddenedmodel fluxes and molecular transitions, although the atomic lines Ca Iλ4226 and Ca II H and K are found to be unrealistically strong inthe models. Our new effective temperature scale is significantly warmerthan those in the literature, with the differences amounting to 400 Kfor the latest type M supergiants (i.e., M5 I). We show that the newlyderived temperatures and bolometric corrections give much betteragreement with stellar evolutionary tracks. This agreement provides acompletely independent verification of our new temperature scale. Thecombination of effective temperature and bolometric luminosities allowsus to calculate stellar radii; the coolest and most luminous stars (KWSgr, Case 75, KY Cyg, HD 206936=μ Cep) have radii of roughly 1500Rsolar (7 AU), in excellent accordance with the largeststellar radii predicted from current evolutionary theory, althoughsmaller than that found by others for the binary VV Cep and for thepeculiar star VY CMa. We find that similar results are obtained for theeffective temperatures and bolometric luminosities using only thedereddened V-K colors, providing a powerful demonstration of theself-consistency of the MARCS models.
| 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}
| Really Cool Stars and the Star Formation History at the Galactic Center
We present λ/Δλ=550-1200 near-infrared H and Kspectra for a magnitude-limited sample of 79 asymptotic giant branch andcool supergiant stars in the central ~5 pc (diameter) of the Galaxy. Weuse a set of similar spectra obtained for solar neighborhood stars withknown Teff and Mbol that is in the same range asthe Galactic center (GC) sample to derive Teff andMbol for the GC sample. We then construct the H-R diagram forthe GC sample. Using an automated maximum likelihood routine, we derivea coarse star formation history of the GC. We find that (1) roughly 75%of the stars formed in the central few parsecs are older than 5 Gyr; (2)the star formation rate (SFR) is variable over time, with a roughly 4times higher SFR in the last 100 Myr compared to the average SFR; (3)our model can match dynamical limits on the total mass of stars formedonly by limiting the initial mass function to masses above 0.7Msolar (this could be a signature of mass segregation or ofthe bias toward massive star formation from the unique star formationconditions in the GC); (4) blue supergiants account for 12% of the totalsample observed, and the ratio of red to blue supergiants is roughly1.5; and (5) models with isochrones with [Fe/H]=0.0 over all ages fitthe stars in our H-R diagram better than models with lower [Fe/H] in theoldest age bins, consistent with the finding of Ramírez et al.that stars with ages between 10 Myr and 1 Gyr have solar [Fe/H].
| The Evolutionary State of Stars in the NGC 1333S Star Formation Region
We present 2 μm near-IR spectroscopic observations of a sample of 33objects in the NGC 1333S active star-forming cluster centered on thepre-main-sequence star SSV 13. We have previously studied this regionphotometrically in the optical and near-IR, and with the addition ofthese near-IR spectra, we further probe the pre-main-sequence clustermembership and evolutionary state. From the atomic and molecularabsorption features observed, together with the earlier photometry, wederive spectral types, effective temperatures, masses, and ages of thestars and conclude that almost all (90%) the stars observed in thissample are pre-main-sequence objects. This result significantly refinesthe evolutionary information obtained from photometric evidence alone.Comparison with theoretical evolutionary tracks and isochrones suggeststhat our survey has sampled sources with masses in the range 0.2-2Msolar and stellar ages between 7×104 and1×108 yr with a preponderance of sources around3×106 yr. This implies the presence of low- tointermediate-mass T Tauri stars of evolutionary designation Class I toClass III. We conclude that star formation seems to have occurred inlikely several bursts rather than occurring coevally. Star formation insuch a region as NGC 1333S is likely significantly affected by the largenumber of active molecular outflows in the region, which could provide amechanism for cloud turbulence and the onset of subsequent starformation.
| The Star Clusters in the Irregular Galaxy NGC 4449
We examine the star clusters in the luminous irregular galaxy NGC 4449.We use a near-infrared spectrum and broadband images taken with theHubble Space Telescope to place a limit of 8-15 Myr on the age of thebright central object in NGC 4449. Its luminosity and size suggest thatit is comparable to young super star clusters. However, there is apeculiar nucleated-bar structure at the center of this star cluster, andwe suggest that this structure is debris from the interaction that hasproduced the counterrotating gas systems and extended gas streamers inthe galaxy. From the images we identify 60 other candidate compact starclusters in NGC 4449. Fourteen of these could be background ellipticalgalaxies or old globular star clusters. Of the star clusters, three, inaddition to the central object, are potentially super star clusters, andmany others are comparable to the populous clusters found in the LargeMagellanic Cloud. The star clusters span a large range in ages with noobvious peak in cluster formation that might be attributed to theinteraction that the galaxy has experienced. Based in part onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by AURA, Inc.,under NASA contract NAS 5-26555.
| Structure and Mass of a Young Globular Cluster in NGC 6946
Using the Wide Field Planetary Camera 2 on board the Hubble SpaceTelescope, we have imaged a luminous young star cluster in the nearbyspiral galaxy NGC 6946. Within a radius of 65 pc, the cluster has anabsolute visual magnitude, MV=-13.2, comparable to the mostluminous young ``super star clusters'' in the Antennae merger galaxy.UBV colors indicate an age of about 15 Myr. The cluster has a compactcore (radius ~1.3 pc) surrounded by an extended envelope with apower-law luminosity profile. The outer parts of the cluster profilegradually merge with the general field, making it difficult to measure aprecise half-light radius Re, but we estimateRe~13 pc. Combined with population synthesis models, theluminosity and age of the cluster imply a mass of8.2×105 Msolar for a Salpeter initial massfunction (IMF) extending down to 0.1 Msolar. If the IMF islognormal below 0.4 Msolar, then the mass decreases to5.5×105 Msolar. Depending on modelassumptions, the central density of the cluster is between5.3×103 and 1.7×104 Msolarpc-3, comparable to other high-density star-forming regions.We also estimate a dynamical mass for the cluster using high-dispersionspectra from the HIRES spectrograph on the Keck I telescope. The HIRESdata indicate a velocity dispersion of 10.0+/-2.7 km s-1 andimply a total cluster mass within 65 pc of(1.7+/-0.9)×106 Msolar. Comparing thedynamical mass with the mass estimates based on the photometry andpopulation synthesis models, we find that the mass-to-light ratio is atleast as high as for a Salpeter IMF extending down to 0.1Msolar, although a turnover in the IMF at 0.4Msolar is still possible within the ~1 σ errors. Thecluster will presumably remain bound, evolving into a globularcluster-like object. Based on observations with the NASA/ESA HubbleSpace Telescope and with the W. M. Keck Telescope.
| Near-Infrared Integral Field Spectroscopy and Mid-Infrared Spectroscopy of the Starburst Galaxy M82
We present new infrared observations of the central regions of thestarburst galaxy M82. The observations consist of near-infrared integralfield spectroscopy in the H and K bands obtained with the MPE 3Dinstrument and of λ=2.4-45 μm spectroscopy from the ShortWavelength Spectrometer (SWS) onboard the Infrared Space Observatory.These measurements are used, together with data from the literature, to(1) reexamine the controversial issue of extinction, (2) determine thephysical conditions of the interstellar medium (ISM) within thestar-forming regions, and (3) characterize the composition of thestellar populations. Our results provide a set of constraints fordetailed starburst modeling, which we present in a companion paper. Wefind that purely foreground extinction cannot reproduce the globalrelative intensities of H recombination lines from optical to radiowavelengths. A good fit is provided by a homogeneous mixture of dust andsources, and with a visual extinction of AV=52 mag. The SWSdata provide evidence for deviations from commonly assumed extinctionlaws between 3 and 10 μm. The fine-structure lines of Ne, Ar, and Sdetected with SWS imply an electron density of ~300 cm-3, andabundance ratios Ne/H and Ar/H nearly solar and S/H about one-fourthsolar. The excitation of the ionized gas indicates an average effectivetemperature for the OB stars of 37,400 K, with little spatial variationacross the starburst regions. We find that a random distribution ofclosely packed gas clouds and ionizing clusters and an ionizationparameter of ~10-2.3 represent well the star-forming regionson spatial scales ranging from a few tens to a few hundreds of parsecs.From detailed population synthesis and the mass-to-K-light ratio, weconclude that the near-infrared continuum emission across the starburstregions is dominated by red supergiants with average effectivetemperatures ranging from 3600 to 4500 K and roughly solar metallicity.Our data rule out significant contributions from older, metal-richgiants in the central few tens of parsecs of M82. Based on observationswith ISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, Netherlands, and theUnited Kingdom) and with the participation of ISAS and NASA. The SWS isa joint project of SRON and MPE.
| 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
| Stellar Iron Abundances at the Galactic Center
We present measurements of [Fe/H] for six M supergiant stars and threegiant stars within 2.5 pc of the Galactic center (GC) and one Msupergiant star within 30 pc of the GC. The results are based onhigh-resolution (λ/Δλ=40,000) K-band spectra, takenwith CSHELL at the NASA Infrared Telescope Facility. We determine theiron abundance by detailed abundance analysis, performed with thespectral synthesis program MOOG. The mean [Fe/H] of the GC stars isdetermined to be near solar, [Fe/H]=+0.12+/-0.22. Our analysis is adifferential analysis, as we have observed and applied the same analysistechnique to 11 cool, luminous stars in the solar neighborhood withsimilar temperatures and luminosities as the GC stars. The mean [Fe/H]of the solar neighborhood comparison stars, [Fe/H]=+0.03+/-0.16, issimilar to that of the GC stars. The width of the GC [Fe/H] distributionis found to be narrower than the width of the [Fe/H] distribution ofBaade's window in the bulge but consistent with the width of the [Fe/H]distribution of giant and supergiant stars in the solar neighborhood.
| The Star Clusters in the Starburst Irregular Galaxy NGC 1569
We examine star clusters in the irregular starburst galaxy NGC 1569 fromHubble Space Telescope images taken with filters F336W, F555W, andF814W. In addition to the two super-star clusters that are well known,we identify 45 other clusters that are compact but resolved. IntegratedUVI colors of the clusters span a large range, and comparison withcoeval evolutionary models suggests that the ages range from 2-3 Myr to1 Gyr. Most of the clusters have colors consistent with ages of <=30Myr, placing them at the end of the recent burst of star formation. Weexamine the radial surface brightness profiles of four of the clustersand fit King models to three of them. The colors of the clusters areapproximately constant with radius. The four clusters have half-lightradii and core radii that are in the range observed in present-dayglobular clusters in our Galaxy. However, they are somewhat lessconcentrated than the average globular cluster. The two well-knownsuper-star clusters have luminosities (and one has a known mass) thatare comparable to those of typical globular clusters. The other twoclusters and likely numerous others in the sample are similar to a smallglobular cluster and to R136 in the LMC. The conditions that producedthe recent starburst, therefore, have also been those necessary forproducing compact bright star clusters. We examine resolved stars in theouter parts of the super-star clusters. We find that cluster A containsmany bright blue stars. Some of the blue stars are bright enough to beevolved massive stars. There is also a small population of redsupergiants. Components A1 and A2 within cluster A have similar colors,and a two-dimensional color map does not offer evidence that onecomponent is dominated by red supergiants and the other is not. Theapparent contradiction of the presence of red supergiants withWolf-Rayet stars is allowed by the evolution of massive stars or mayinstead be due to an age spread within cluster A. The stars that weresolve around cluster B, on the other hand, contain a small populationof more normal blue massive stars and a large population of redsupergiants. The presence of the red supergiants is consistent with theview that cluster B is in its red supergiant phase. The presence of thered supergiant stars in clusters A and B is also verified innear-infrared spectra, where we find strong stellar CO absorptionfeatures. The various age indicators are consistent with a picture inwhich cluster B is of order 10-20 Myr old, the older stars in cluster Aare >=7 Myr old. The timescale to form the holes seen in Hα andH I is comparable to the age of cluster B. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS 5-26555.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| Spectral classification of the cool components of symbiotic stars
We have made near infrared spectroscopic observations of 10 symbioticand 27 K-M comparison stars. For stars later than M3, we found the banddepth of the triple-headed TiO absorption band to be sensitive totemperature and insensitive to gravity. We fitted the spectral type tothe band depth with a standard error of 0.22 of a subtype and derivedthe spectral types for 6 symbiotic stars. We measured the EW of a largenumber of lines including the CaII lines (very sensitive to luminosity),the FeI and TiO lines (moderately sensitive) and the NaI lines (notsensitive). The EW of these lines vary with the spectral type,particularly for stars later than M3, both spectral type and luminosityeffects must be considered. We give the luminosity classes for 10 of thebrighter symbiotic stars, they are all giant stars, showing no featuresof bright giants or supergiants.
| Library of Medium-Resolution Fiber Optic Echelle Spectra of F, G, K, and M Field Dwarfs to Giant Stars
We present a library of Penn State Fiber Optic Echelle (FOE)observations of a sample of field stars with spectral types F to M andluminosity classes V to I. The spectral coverage is from 3800 to 10000Å with a nominal resolving power of 12,000. These spectra includemany of the spectral lines most widely used as optical and near-infraredindicators of chromospheric activity such as the Balmer lines (Hαto Hepsilon), Ca II H & K, the Mg I b triplet, Na I D_1, D_2, He ID_3, and Ca II IRT lines. There are also a large number of photosphericlines, which can also be affected by chromospheric activity, andtemperature-sensitive photospheric features such as TiO bands. Thespectra have been compiled with the goal of providing a set of standardsobserved at medium resolution. We have extensively used such data forthe study of active chromosphere stars by applying a spectralsubtraction technique. However, the data set presented here can also beutilized in a wide variety of ways ranging from radial velocitytemplates to study of variable stars and stellar population synthesis.This library can also be used for spectral classification purposes anddetermination of atmospheric parameters (T_eff, logg, [Fe/H]). A digitalversion of all the fully reduced spectra is available via ftp and theWorld Wide Web (WWW) in FITS format.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| CCD spectra of MK standards and a preliminary extension of the MK classification to the yellow-red region.
Not Available
| Spectral and luminosity classification for the cool components in symbiotic stars
The near infrared spectra of 12 S-type symbiotic stars and 78 comparisonstars have been observed with moderate dispersion in five runs from 1992to 1997, the resolving power being R= (lambda )/(Delta lambda )>2000,with a signal to noise ratio S/N>100. The triple-headed absorptionband of TiO (lambda lambda 8432, 8422 and 8452 Ä) emerges when astar is later than M2, and the depth of the TiO absorption band is verysensitive to the spectral type (ST) and insensitive to the luminosityclass of the star. We fit a curve of spectral type against the index ofthe absorption depth of this band with a standard deviation sigma =0.37of a subdivision of one spectral type. The IR CaII triplet (lambdalambda 8498, 8542, 8662 Ä ), Fe I 8689 Ä, and Fe I 8675 Äare good luminosity indicators although the equivalent widths (EWs) ofthese lines clearly decrease for a star later than M3. When the star isa supergiant, the lines have a smaller central residual intensity andbroader wings than in the case of a normal giant. The Ca II 8662 Ä/Fe I 8675 Ä and Fe I 8689 Ä /Fe I 8675 Ä ratios are alsogood luminosity indicators for K-type giants. The latter is particularlyuseful when there are abundance anomalies. The metal-poor symbiotic starAG Dra is classified as a Ib or II giant, as is TX CVn, on the basis ofFe I 8689 Ä /Fe I 8675 Ä. 9 other symbiotic stars containingM-type cool components are classified as giants by direct comparison andquantitative analysis. Due to there being no known good ratio indicatorof luminosity for M-type stars in the band studied and because there isno metal abundance data for the symbiotic stars studied by us except forAG Dra, the results for these 9 symbiotic stars are only preliminary.The infrared Ca II triplet of most symbiotic stars clearly variesbetween the different observing runs. The different luminosity classesgiven to the same symbiotic star are probably caused by the variabilityof the lines of ionized elements, while in some cases they are affectedby a low metal abundance.
| Silicate and hydrocarbon emission from Galactic M supergiants
Following our discovery of unidentified infrared (UIR) band emission ina number of M supergiants in h and chi Per, we have obtained 10-μmspectra of a sample of 60 galactic M supergiants. Only three newsources, V1749 Cyg, UW Aql and IRC+40 427, appear to show the UIR bands;the others show the expected silicate emission or a featurelesscontinuum. The occurrence of UIR-band emission in M supergiants istherefore much higher in the h and chi Per cluster than in the Galaxy asa whole. Possible explanations for the origin and distribution of UIRbands in oxygen-rich supergiants are discussed. We use our spectra toderive mass-loss rates ranging from 10^-8 to 10^-4 M_solar yr^-1 for thenew sample, based on the power emitted in the silicate feature. Therelationship between mass-loss rate and luminosity for M supergiants isdiscussed, and correlations are explored between their mid-infraredemission properties.
| The ortho- to para- ratio of H_2 in the starburst of NGC253
We present measurements of several near-infrared emission lines from thenearby galaxy NGC253. We have been able to measure four H_2 lines acrossthe circumnuclear starburst, from which we estimate the ortho- to para-ratio of excited H_2 to be ~2. This indicates that the bulk of the H_2emission arises from photodissociation regions (PDRs), rather than fromshocks. This is the case across the entire region of active starformation. As the H_2 emission arises from PDRs, it is likely that theratio of H_2 to Brgamma (the bright hydrogen recombination line) is ameasure of the relative geometry of O and B stars and PDRs. Towards thenucleus of NGC253 the geometry is deduced to be tightly clustered O andB stars in a few giant HII regions that are encompassed by PDRs. Awayfrom the nuclear region, the geometry becomes that of PDRs bathed in arelatively diffuse ultraviolet radiation field. The rotation curves of1-0 S(1) and Brgamma suggest that the ionized gas is tracing a kineticsystem different from that of the molecular gas in NGC253, particularlyaway from the nucleus.
| 1--4 MU M Spectroscopy of Very Red Stars Found in an I-Band Objective Prism Survey
We present the 1.2--4.2 mu m spectroscopy of stars found in an objectiveprism survey of the galactic plane by Stephenson (1992, AAA 55.002.010).These stars were thought by Stephenson to be heavily reddened byinterstellar absorption. However, almost all of them have turned out tobe late-type stars with clear 2.3 mu m CO absorption, which means thatthey are intrinsically red as well. A few stars have AV ~ 9, but mostof them are only moderately reddened (AV = 1--5).
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| Medium-Resolution Spectra of Normal Stars in the K Band
An Atlas of 115 medium-resolution K-band (2.0--2.4 mu m) stellarspectra, spanning spectral types O--M and luminosity types I--V, ispresented. K-band spectra are also presented for one N- and one J-typecarbon star. A time series of spectra is presented for an S-type Miravariable. All the spectra are at a resolution of ~3000 (1.4 cm-1) andhave had the terrestrial absorption removed by dividing by a featurelessspectrum. The spectra are plotted with the major spectral featuresidentified and are available digitally.
| Accurate Two-dimensional Classification of Stellar Spectra with Artificial Neural Networks
We present a solution to the long-standing problem of automaticallyclassifying stellar spectra of all temperature and luminosity classeswith the accuracy shown by expert human classifiers. We use the 15Angstroms resolution near-infrared spectral classification systemdescribed by Torres-Dodgen & Weaver in 1993. Using the spectrum withno manual intervention except wavelength registration, artificial neuralnetworks (ANNs) can classify these spectra with Morgan-Keenan types withan accuracy comparable to that obtained by human experts using 2Angstroms resolution blue spectra, which is about 0.5 types (subclasses)in temperature and about 0.25 classes in luminosity. Accuratetemperature classification requires a hierarchy of ANNs, whileluminosity classification is most successful with a single ANN. Wepropose an architecture for a fully automatic classification system.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| An Atlas of the infrared spectral region. II. The late-type stars (G - M)
This Atlas illustrates the behavior of late type stars (F, G, K and M)in the near infrared 8400-8800 Angstrom region with a resolution ofabout 2 degrees. Seventeen figures illustrate the spectral sequence andluminosity classes V, III, Ib and Ia. Four figures illustrate peculiarspectra, namely those of Am stars, composites, weak metal stars and Sand C type objects. The complete Atlas is also available as FITS filesfrom the CDS de Strasbourg and other data centers.
| Really Cool Stars at the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.1988B&db_key=AST
| Hybrid stars and the reality of "dividing lines" among G to K bright giants and supergiants.
We present results of pointed ROSAT PSPC observations of 15 hybridstars/candidates, which have been analyzed in a homogenous way. 7 ofthese stars were observed in X-rays for the first time. 12 out of 15hybrid stars have been detected as X-ray sources, some of them close tothe detection limit. We conclude that essentially all hybrid stars asdefined by the simultaneous presence of transition region line emissionand cool stellar winds are X-ray sources if exposed sufficiently deep.The X-ray luminosities of hybrid stars cover a range between 2x10^27^and ~10^30^erg/s. Their X-ray surface fluxes can be as low as =~20erg/cm^2^/s and thus considerably lower than those of normal luminosityclass (LC) III giants. X-ray spectra of hybrid stars tend to be harderthan that of normal LC III giants, moreover, the X-ray brightest starshave the hardest spectra. We find that for K II giants the normalizedX-ray flux versus C IV flux obeys a power law with an exponent a=2.9,steeper than among normal giants (1.5). Hybrid K II stars are X-rayunderluminous by a factor of 5 to 20 compared to LC III giants at thesame level of normalized CIV flux f_CIV_/f_bol_; hybrid G supergiantsare even more X-ray deficient. We reanalyze the CaII wind dividing lineand find it vertical at B-V=1.45 for LC III giants. It is nearlyhorizontal between B-V=1.45 and 1.0 (at M_bol_=~-2...-3), and not welldefined for supergiants with B-V<1.0. We therefore suggest thatpossibly all LC II and Ib G and K giants are hybrid stars and that the"dividing line" concept in its simplest form is not valid for G/K giantsbrighter than M_bol_=~-2. Hybrid stars are supposed to be evolvedintermediate mass stars and their coronal activity may in principle bedetermined by the individual history of each star.
| The luminous starburst galaxy UGC 8387
We present broad-band J, H, and K images and K-band spectroscopy of theluminous starburst galaxy UGC 8387. The images show a disturbedmorphology, tidal tails, and a single elongated nucleus. Near infraredcolor maps constructed from the images reveal that the nucleus region ishighly reddened. Strong emission from the central 3 arcseconds in the2.166 micrometer Brackett gamma, 2.122 micrometer H2 v = 1-0 S(1), and2.058 micrometer He I lines is present in the K-band spectrum. From theBrackett gamma and published radio fluxes, we find an optical depthtoward the nucleus of tauV approximately 24. The CO bandheads produce strong absorption in the spectral region long-ward of 2.3micrometers. We measure a 'raw' CO index of 0.17 +/- 0.02 mag,consistent with a population of K2 supergiants of K4 giants. The nuclearcolors, however, are not consistent with an obscured population ofevolved stars. Instead, the red colors are best explained by an obscuredmixture of stellar and warm dust emission. The amount of dust emissionpredicted by the near-infrared colors exceeds that expected fromcomparisons to galactic H II regions. After correcting the spectrum ofUGC 8387 for dust emission and extinction, we obtain a CO index ofgreater than or equal to 0.25 mag. This value suggests the stellarcomponent of the 2.2 micrometer light is dominated by young supergiants.The infrared excess, LIR/LLy alpha derived for UGC8387 is lower than that observed in galactic H II regions and M82. Thisimplies that either the lower or upper mass cutoff of the initial massfunction must be higher than those of local star-forming regions andM82. The intense nuclear starburst in this galaxy is presumably theresult of merger activity; and we estimate the starburst age to be atleast a few times 107 yr.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Kertenkele |
| Sað Açýklýk: | 22h56m26.00s |
| Yükselim: | +49°44'01.0" |
| Görünürdeki Parlaklýk: | 4.95 |
| Uzaklýk: | 574.713 parsek |
| özdevim Sað Açýklýk: | -0.2 |
| özdevim Yükselim: | -2.2 |
| B-T magnitude: | 7.304 |
| V-T magnitude: | 5.178 |
Kataloglar ve belirtme:
|
INHABITED SKY
