Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
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}
| Ca II K Emission-Line Asymmetries Among Red Giants
Measurements of the asymmetry of the K2 emission line of CaII have been made for a sample of bright field giants with B-V>1.15observed with the Cassegrain echelle spectrometer on the McDonaldObservatory 2.1 m telescope. The asymmetry of the Ca II K2line is quantified through measurement of a parameter V/R, which isdefined as the ratio between the maximum counts recorded in the violetand red components of the double-peaked emission profile. Red-maximumasymmetry (V/R<1.0) is found in our sample of 35 giants only amongstars with B-V>1.35, a trend that is still maintained (with oneexception) with the inclusion of an additional sample of giantspreviously observed by us with the same spectrograph. Althoughexceptional stars can be found in the literature, the data support anearlier finding by R. Stencel that among luminosity class III fieldgiants the occurrence of V/R<1.0 is generally restricted to effectivetemperatures cooler than 4320 K. This limit may coincide with the onsetof pulsation.
| High-Precision Stellar Radial Velocities in the Galactic Center
We present radial velocities for 85 cool stars projected onto thecentral parsec of the Galaxy. The majority of these velocities haverelative errors of ~1 km s-1, or a factor of ~30-100 smallerthan those previously obtained with proper-motion or other radialvelocity measurements for a similar stellar sample. The error in atypical individual stellar velocity, including all sources ofuncertainty, is 1.7 km s-1. Two similar data sets wereobtained 1 month apart, and the total error in the relative velocitiesis 0.80 km s-1 in the case where an object is common to bothdata sets. The data are used to characterize the velocity distributionof the old population in the Galactic center. We find that the starshave a Gaussian velocity distribution with a mean heliocentric velocityof -10.1+/-11.0 km s-1 (blueshifted) and a standard deviationof 100.9+/-7.7 km s-1 the mean velocity of the sample isconsistent with no bulk line-of-sight motion with respect to the localstandard of rest. At the 1 σ level, the data are consistent with asymmetric velocity distribution about any arbitrary axis in the plane ofthe sky. We find evidence for a flattening in the distribution oflate-type stars within a radius of ~0.4 pc and infer a volume densitydistribution of r-1/4 in this region. Finally, we establish afirst epoch of radial velocity measurements that can be compared withsubsequent epochs to measure small accelerations (1 km s-1yr-1), corresponding to the magnitude expected over a timespan of several years for stars nearest to Sgr A*.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof 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.
| Wing Near-Infrared, TiO-Band, and V-Band Photometry of Chromospherically Active Star λ Andromedae
As a pilot program, Wing near-IR, TiO-band, and V-band photometry isbeing conducted of the RS Canum Venaticorum type, chromosphericallyactive, G8 IV-III star λ Andromedae. The objective is toinvestigate a possible relationship between variation of the ~54 dayrotationally starspot modulated visual light curve and TiO absorptionstrength. The TiO (γ,0,0) absorption band strength at λ=719nm is very sensitive to temperature for cool stars and manifests itselfin cooler starspot regions (T<=4000 K). TiO photometry has anadvantage over conventional photometry in that it provides unambiguousmeasures of the fractional cool starspot coverage. In addition, as thestars rotate, the variation in the TiO index yields information aboutthe longitudinal distribution of the starspots. Importantly, combiningthe TiO photometry with the V-band and near-IR light curves allows thediscrimination of white-light faculae (=hot spot) and cool starspotcontributions. Initial results of this study indicate that the observedV-band and near-IR continua light variations found for λ Andprimarily arise from bright spot (plage) features rather than darkstarspots as is usually assumed. This is in contrast to current theoriesthat the visual light variation is solely due to dark spots. Modelsusing both bright and dark spot features have been developed and arebeing used to fit the light and TiO-index curves. The models account forcool/hot spot characteristics such as projected filling factor andtemperature. The long-term variation of V light and TiO index have beeninvestigated to search for any activity cycles.
| 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.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable 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/qcat?J/A+A/397/997
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| 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.
| Zirconium to Titanium Ratios in a Large Sample of Galactic S Stars
The [Zr/Ti] ratio for a large sample of Galactic S stars has beendetermined using high-quality, high-resolution spectra. The pattern ofZr enhancements in intrinsic and extrinsic S stars is found to differ,and the [Zr/Ti] ratio in the extrinsic S stars clearly links them to thestrong barium stars. In addition, the pattern of [Zr/Ti] ratios seems toindicate that the progression of spectral type M to MS to S to SC is duelargely to an increase in the abundance of s-process elements and notsolely to a changing C/O ratio as claimed by some investigators (such asScalo and Ross).
| 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.
| 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
| 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.
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| The CaII-M_v Correlation (Wilson-Bappu Effect) Calibrated by HIPPARCOS Parallaxes
We have used Hipparcos parallaxes to derive absolute visual magnitudesof G, K, and M stars with Ca II emission line widths previously measuredby O. C. Wilson. A linear relationship similar to the one derivedoriginally by Wilson & Bappu and improved by Lutz & Kelker wasfound from M_v=+7 to -2. For stars brighter than M_v=-2 a substantialnumber of stars show Ca II emission lines that are broader than expectedfrom the linear fit. Most of those stars are bright giants andsupergiants of type G. In appendices we show some sample Ca II profilesand identify emission lines of Fe II as well as the Hepsilon line insome stars.
| An analysis of the Johnson et al. Catalina UBVRI photometry for second order extinction effects.
Not Available
| Photometric modelling of starspots - I. A Barnes-Evans-like surface brightness-colour relation using (Ic-K)
In the first part of this work, the empirical correlation of stellarsurface brightness FV with (Ic-K) broad-bandcolour is investigated by using a sample of stars cooler than the Sun. Abilinear correlation is found to represent well the brightness of G, Kand M giant stars. The change in slope occurs at (Ic-K)~2.1or at about the transition from K to M spectral types. The samerelationship is also investigated for dwarf stars and found to bedistinctly different from that of the giants. The dwarf star correlationdiffers by an average of -0.4 in (Ic-K) or by a maximum inFV of ~-0.1, positioning it below that of the giants, withboth trends tending towards convergence for the hotter stars in oursample. The flux distribution derived from theFV-(Ic-K) relationship for the giant stars,together with that derived from an FV-(V-K) relationship andthe blackbody flux distribution, is then utilized to compute syntheticlight V and colour (V-R)c, (V-I)c and (V-K) curvesof cool spotted stars. We investigate the effects on the amplitudes ofthe curves by using these FV-colour relations and by assumingthe effective gravity of the spots to be lower than the gravity of theunspotted photosphere. We find that the amplitudes produced by using theFV-(Ic-K) relationship are larger than thoseproduced by the other two brightness correlations, meaning smallerand/or warmer spots.
| Revision and Calibration of MK Luminosity Classes for Cool Giants by HIPPARCOS Parallaxes
The Hipparcos parallaxes of cool giants are utilized in two ways in thispaper. First, a plot of reduced parallaxes of stars brighter than 6.5,as a function of spectral type, for the first time separates members ofthe clump from stars in the main giant ridge. A slight modification ofthe MK luminosity standards has been made so that luminosity class IIIbdefines members of the clump, and nearly all of the class III stars fallwithin the main giant ridge. Second, a new calibration of MK luminosityclasses III and IIIb in terms of visual absolute magnitudes has beenmade.
| 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.
| Stellar radii of M giants
We determine the stellar radii of the M giant stars in the Hipparcoscatalogue that have a parallax measured to better than 20% accuracy.This is done with the help of a relation between a visual surfacebrightness parameter and the Cousins (V - I) colour index, which wecalibrate with M giants with published angular diameters.The radii of(non-Mira) M giants increase from a median value of 50 R_Sun at spectraltype M0 III to 170 R_Sun at M7/8 III. Typical intermediate giant radiiare 65 R_Sun for M1/M2, 90 R_Sun for M3, 100 R_Sun for M4, 120 R_Sun forM5 and 150 R_Sun for M6. There is a large intrinsic spread for a givenspectral type. This variance in stellar radius increases with latertypes but in relative terms, it remains constant.We determineluminosities and, from evolutionary tracks, stellar masses for oursample stars. The M giants in the solar neighbourhood have masses in therange 0.8-4 M_Sun. For a given spectral type, there is a close relationbetween stellar radius and stellar mass. We also find a linear relationbetween the mass and radius of non-variable M giants. With increasingamplitude of variability we have larger stellar radii for a given mass.
| 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.
| Averaged energy distributions in the stellar spectra.
Not Available
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| 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.
| Systematic Errors in the FK5 Catalog as Derived from CCD Observations in the Extragalactic Reference Frame.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114..850S&db_key=AST
| UBVRI Standard Stars at Northern Declinations
Accurate values of V, B--V and U--B are given 123 stars of magnitudesfrom 6 to 11 at high northern declinations. For 36 stars also V--R andR--I color indices are given.
| Transformations from Theoretical Hertzsprung-Russell Diagrams to Color-Magnitude Diagrams: Effective Temperatures, B-V Colors, and Bolometric Corrections
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...469..355F&db_key=AST
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Pegasus |
| Ascensió Recta: | 00h14m36.20s |
| Declinació: | +20°12'24.0" |
| Magnitud Aparent: | 4.8 |
| Distancia: | 99.9 parsecs |
| Moviment propi RA: | 90.2 |
| Moviment propi Dec: | 0.6 |
| B-T magnitude: | 6.84 |
| V-T magnitude: | 4.983 |
Catàlegs i designacions:
|
