תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Cool Subdwarf Investigations. II. Multiplicity
Cool subdwarfs of types K and M are the fainter counterparts of coolmain-sequence dwarfs that dominate the Galactic population. In thispaper, we present the results of an optical speckle survey of 62confirmed cool subdwarf systems within 60 pc. We have resolved twonew companions and confirmed two previously known companions withseparations 0farcs13-3farcs29. After including previously known widecompanions and all known spectroscopic binaries, we determine themultiplicity rate of cool subdwarfs to be 26% ± 6%, which issomewhat lower than comparable main-sequence stars that have amultiplicity rate of 37% ± 5%. We find that only 3% of the coolsubdwarfs surveyed have companions within 10 AU, 3% have companionsbetween 10 and 100 AU, and 14% have companions beyond 100 AU.The other 6% of cool subdwarfs are spectroscopic binaries. This is verydifferent from K/M dwarfs that have most companions (13%) at separationscloser than 10 AU. However, because a search for close binariesamong a large sample of nearby cool subdwarfs remains elusive, it is notyet settled whether or not the multiplicity rates are significantlydifferent. Nonetheless, several different observational results andtheories pointing to a possible dearth of subdwarf multiples arediscussed.
| The usage of Strömgren photometry in studies of local group dwarf spheroidal galaxies. Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients
Aims.In this paper we demonstrate how Strömgren uvby photometry canbe efficiently used to: 1. identify red giant branch stars that aremembers in a dwarf spheroidal galaxy; 2. derive age-independentmetallicities for the same stars and quantify the associated errors. Methods: Strömgren uvby photometry in a 11 × 22 arcmin fieldcentered on the Draco dwarf spheroidal galaxy was obtained using theIsaac Newton Telescope on La Palma. Members of the Draco dSph galaxywere identified using the surface gravity sensitive c1 indexwhich discriminates between red giant and dwarf stars. Thus enabling usto distinguish the (red giant branch) members of the dwarf spheroidalgalaxy from the foreground dwarf stars in our galaxy. The method isevaluated through a comparison of our membership list with membershipclassifications in the literature based on radial velocities and propermotions. The metallicity sensitive m1 index was used toderive individual and age-independent metallicities for the members ofthe Draco dSph galaxy. The derived metallicities are compared to studiesbased on high resolution spectroscopy and the agreement is found to bevery good. Results: We present metallicities for 169 members of the redgiant branch in the Draco dwarf spheroidal galaxy (the largest sample todate). The metallicity distribution function for the Draco dSph galaxyshows a mean [Fe/H] = -1.74 dex with a spread of 0.24 dex. Thecorrelation between metallicity and colour for the stars on the redgiant branch is consistent with a dominant old, and coeval population.There is a possible spatial population gradient over the field with themost metal-rich stars being more centrally concentrated than themetal-poor stars.Based on observations made with the Isaac Newton Telescope, operated onthe Island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias. Guest User, Canadian Astronomy Data Centre, which isoperated by the Herzberg Institute of Astrophysics, National ResearchCouncil of Canada. Full Tables 2 and 6 are only available athttp://www.aanda.org
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Calibrating M Dwarf Metallicities Using Molecular Indices
We report progress in the calibration of a method to determine cooldwarf star metallicities using molecular band strength indices. Themolecular band index to metallicity relation can be calibrated usingchemical abundances calculated from atomic-line equivalent widthmeasurements in high-resolution spectra. Building on previous work, wehave measured Fe and Ti abundances in 32 additional M and K dwarf starsto extend the range of temperature and metallicity covered. A test ofour analysis method using warm star-cool star binaries shows we cancalculate reliable abundances for stars warmer than 3500 K. We have usedabundance measurements for warmer binary or cluster companions toestimate abundances in six additional cool dwarfs. Adding stars measuredin our previous work and others from the literature provides 76 starswith Fe abundance and CaH2 and TiO5 index measurements. The CaH2molecular index is directly correlated with temperature. TiO5 depends ontemperature and metallicity. Metallicity can be estimated to within+/-0.3 dex within the bounds of our calibration, which extends fromroughly [Fe/H]=+0.05 to -1.0, with a limited extension to -1.5.
| Metallicity of M dwarfs. I. A photometric calibration and the impact on the mass-luminosity relation at the bottom of the main sequence
We obtained high resolution ELODIE and CORALIE spectra for bothcomponents of 20 wide visual binaries composed of an F-, G- or K-dwarfprimary and an M-dwarf secondary. We analyse the well-understood spectraof the primaries to determine metallicities ([Fe/H]) for these 20systems, and hence for their M dwarf components. We pool thesemetallicities with determinations from the literature to obtain aprecise (±0.2 dex) photometric calibration of M dwarfmetallicities. This calibration represents a breakthrough in a fieldwhere discussions have had to remain largely qualitative, and it helpsus demonstrate that metallicity explains most of the large dispersion inthe empirical V-band mass-luminosity relation. We examine themetallicity of the two known M-dwarf planet-host stars, Gl876 (+0.02 dex) and Gl 436 (-0.03 dex), inthe context of preferential planet formation around metal-rich stars. Wefinally determine the metallicity of the 47 brightest single M dwarfs ina volume-limited sample, and compare the metallicity distributions ofsolar-type and M-dwarf stars in the solar neighbourhood.
| Probing the LHS Catalog. II. Faint Proper-Motion Stars
We present low-resolution spectroscopic observations of faintproper-motion stars from the LHS Catalogue, concentrating on stars withmr>16.5 and μ>0.5" yr-1. The presentpaper includes observations and spectral classifications for 294 Mdwarfs, M subdwarfs (sdM), and extreme M subdwarfs (esdM). We alsoidentify white dwarfs among the faintest LHS stars. We havecross-referenced this sample against the Two Micron All Sky Survey(2MASS) sources, and list data for the detected objects. We discussstars of individual interest, as well as the characteristics of theoverall sample. As expected, a significant number of the stars in thisproper-motion-selected sample are halo subdwarfs, including an esdMdwarf, LHS 3481, that is likely to lie within 20 pc of the Sun. None ofthe subdwarfs show Hα emission.
| The Red-Sequence Cluster Survey. I. The Survey and Cluster Catalogs for Patches RCS 0926+37 and RCS 1327+29
The Red-Sequence Cluster Survey (RCS) is a ~100 deg2,two-filter imaging survey in the RC and z'filters, designed primarily to locate and characterize galaxy clustersto redshifts as high as z=1.4. This paper provides a detaileddescription of the survey strategy and execution, including a thoroughdiscussion of the photometric and astrometric calibration of the surveydata. The data are shown to be calibrated to a typical photometricuncertainty of 0.03-0.05 mag, with total astrometric uncertainties lessthan 0.25" for most objects. We also provide a detailed discussion ofthe adaptation of a previously described cluster search algorithm (thecluster red-sequence method) to the vagaries of real survey data, withparticular attention to techniques for accounting for subtle variationsin survey depths caused by changes in seeing and sky brightness andtransparency. A first catalog of RCS clusters is also presented, for thesurvey patches RCS 0926+37 and RCS 1327+29. These catalogs, representingabout 10% of the total survey and comprising a total of 429 candidateclusters and groups, contain a total of 67 cluster candidates at aphotometric redshift of 0.9
| Metallicity measurements using atomic lines in M and K dwarf stars
We report the first survey of chemical abundances in M and K dwarf starsusing atomic absorption lines in high-resolution spectra. We havemeasured Fe and Ti abundances in 35 M and K dwarf stars using equivalentwidths measured from λ/Δλ~ 33000 spectra. Ouranalysis takes advantage of recent improvements in model atmospheres oflow-temperature dwarf stars. The stars have temperatures between 3300and 4700 K, with most cooler than 4100 K. They cover an iron abundancerange of -2.44 < [Fe/H] < +0.16. Our measurements show [Ti/Fe]decreasing with increasing [Fe/H], a trend similar to that measured forwarmer stars, where abundance analysis techniques have been tested morethoroughly. This study is a step towards the observational calibrationof procedures to estimate the metallicity of low-mass dwarf stars usingphotometric and low-resolution spectral indices.
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| Magnesium Isotopic Abundance Ratios in Cool Stars
From high-resolution spectra of 61 cool dwarfs and giants, Mg isotopicabundance ratios 24Mg:25Mg:26Mg arederived from spectral synthesis of the MgH A-X lines near 5140 Å.Our sample spans the range -2.5<=[Fe/H]<=0.1, including the firstmeasurements of Mg isotope ratios in stars with metallicities below[Fe/H]=-2.0. We confirm the decrease in 25Mg/24Mgand 26Mg/24Mg with decreasing [Fe/H], as predictedby recent models of Galactic chemical evolution in which the Mg isotopesare produced in massive stars. A subset of kinematically identifiedthin-disk stars have Mg isotope ratios in excellent agreement with thepredictions. Within the measurement uncertainties, these thin-disk starsshow no scatter about the predictions. Several of our stars are likelymembers of the thick disk, and their high Mg isotopic ratios may reflectthe nucleosynthetic history of the thick disk, which is distinct fromthe predictions for, and observations of, the thin disk. For thick-diskand halo stars we find a scatter in 25Mg/24Mg and26Mg/24Mg exceeding our measurement uncertaintiesand increasing with increasing metallicity. Our data suggest that anadditional source of 25Mg and 26Mg is required.Intermediate-mass asymptotic giant branch stars are likely candidates.
| A Search for Cool Subdwarfs: Stellar Parameters for 134 Candidates
The results of a search for cool subdwarfs are presented. Kinematic (U,V, and W) and stellar parameters (Teff, logg, [Fe/H], andξt) are derived for 134 candidate subdwarfs based onhigh-resolution spectra. The observed stars span 4200K
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| The u'g'r'i'z' Standard-Star System
We present the 158 standard stars that define the u'g'r'i'z' photometricsystem. These stars form the basis for the photometric calibration ofthe Sloan Digital Sky Survey. The defining instrument system andfilters, the observing process, the reduction techniques, and thesoftware used to create the stellar network are all described. Webriefly discuss the history of the star selection process, thederivation of a set of transformation equations for theUBVRCIC system, and plans for future work.
| Nearby Microlensing Events: Identification of the Candidates for theSpace Interferometry Mission
The Space Interferometry Mission (SIM) is the instrument of choice whenit comes to observing astrometric microlensing events where nearby,usually high proper motion, stars (``lenses'') pass in front of moredistant stars (``sources''). Each such encounter produces a deflectionin the source's apparent position that, when observed by SIM, can leadto a precise mass determination of the nearby lens star. We search forlens-source encounters during the 2005-2015 period using Hipparcos, ACT,and NLTT to select lenses, and USNO-A2.0 to search for the correspondingsources, and rank these by the SIM time required for a 1% massmeasurement. For Hipparcos and ACT lenses, the lens distance andlens-source impact parameter are precisely determined so that the eventsare well characterized. We present 32 candidates beginning with a 61 CygA event in 2012 that requires only a few minutes of SIM time. ProximaCentauri and Barnard's star each generate several events. For NLTTlenses, the distance is known only to a factor of 3, and the impactparameter only to 1''. Together, these produce uncertaintiesof a factor ~10 in the amount of SIM time required. We present a list of146 NLTT candidates and show how single-epoch CCD photometry of thecandidates could reduce the uncertainty in SIM time to a factor of ~1.5.
| M-Subdwarfs: Spectroscopic Classification and the Metallicity Scale
We present a spectroscopic classification system for M-dwarfs andM-subdwarfs based on quantitative measures of TiO and CaH features inthe region lambda lambda 6200 - 7400 Angstroms. Our sample of cool starscovers the range from solar metallicity stars to the most extremesubdwarfs known. Using synthetic spectra computed by Allard andHauschildt (1995), we derive metallicities for the stars. Stars areclassified as dwarfs (M V), subdwarfs (sdM), or extreme subdwarfs(esdM). These classifications correspond to [m/H] ~ 0.0, -1.2, and -2.0respectively. Our metallicity scale agrees with theoretical HR diagramsand HST globular cluster measurements. We discuss some nearby subdwarfsof particular interest in light of our metallicity scale.
| Determination of effective temperatures for an extended sample of dwarfs and subdwarfs (F0-K5).
We have applied the InfraRed Flux Method (IRFM) to a sample of 475dwarfs and subdwarfs in order to derive their effective temperatureswith a mean accuracy of about 1.5%. We have used the new homogeneousgrid of theoretical model atmosphere flux distributions developed byKurucz (1991, 1993) for the application of the IRFM. The atmosphericparameters of the stars cover, roughly, the ranges:3500K<=T_eff_<=8000K -3.5<=[Fe/H]<=+0.53.5<=log(g)<=5. The monocromatic infrared fluxes at the continuum,and the bolometric fluxes are derived using recent results, whichsatisfy the accuracy requeriments of the work. Photometric calibrationshave been revised and applied to estimate metallicities, although directspectroscopic determinations were preferred when available. The adoptedinfrared absolute flux calibration, based on direct optical measurementsof angular stellar diameters, sets the effective temperatures determinedusing the IRFM on the same scale than those obtained by direct methods.We derive three temperatures, T_J_, T_H_ and T_K_, for each star usingthe monochromatic fluxes at different infrared wavelengths in thephotometric bands J, H, and K. They show good consistency over 4000 K,and no trend with wavelength may be appreciated. We provide a detaileddescription of the steps followed for the application of the IRFM, aswell as the sources of the errors associated to the different inputs ofthe method, and their transmission into the final temperatures. We alsoprovide comparison with previous works.
| The Resolved Stellar Population of Leo A
New observations of the resolved stellar population of the extremelymetal-poor Magellanic dwarf irregular galaxy Leo A in Thuan-Gunn r, g,i, and narrowband H? filters are presented. Using the recentCepheid variable star distance determination to Leo A by Hoessel et al.,we are able to create an accurate color-magnitude diagram (CMD). We haveused the Bavesian inference method described by Tolstoy & Saha tocalculate the likelihood of a Monte Carlo simulation of the stellarpopulation of Leo A being a good match to the data within the wellunderstood errors in the data. The magnitude limits on our data aresensitive enough to look back at ~1 Gyr of star formation history at thedistance of Leo A. To explain the observed ratio of red to blue stars inthe observed CMD, it is necessary to invoke either a steadily decreasingstar formation rate toward the present time or gaps in the starformation history. We also compare the properties of the observedstellar population with the known spatial distribution of the H I gasand H II regions to support the conclusions from CMD modeling. Weconsider the possibility that currently there is a period of diminishedstar formation in Leo A, as evidenced by the lack of very young stars inthe CMD and the faint H II regions. How the chaotic H I distribution,with no observable rotation, fits into our picture of the evolution ofLeo A is as yet unclear.
| 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.
| Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
| Determination of bolometric fluxes for F, G and K subdwarfs
Bolometric fluxes for 118 dwarfs and subdwarfs with 4000K less thanTeff less than 7000K and +0.5 less than (Fe/H) less than -3.5are derived by integrating UBVRIJHK photometry. For these stars (typicalof the galactic and globular cluster main sequences) at least 80% of theenergy emitted is measured by those bands. A grid of theoretical modelline-blanketed flux distributions ws used in order to estimate the fluxout-side these photometric bands. The method, based on the use ofsynthetic magnitudes, provides bolometric fluxes with an estimateduncertainty in the order of 2%. The result obtained have been used tocalibrate the bolometric flux versus K, (V-K) and Fe/H), in order toderive an expression of general application. A by-product of this studyis the bolometric correction for main sequence stars, which is tabulatedfor different metallicities. Comparisons of these results with thoseobtained by other authors are presented and discussed.
| The general catalogue of trigonometric [stellar] paralaxes
Not Available
| Broad band JHK infrared photometry of an extended sample of late type dwarfs and subdwarfs.
The results of a long term programme of broad band JHK photometry, for asample of 360 late type stars, made at the Observatorio del Teide(Tenerife, Spain) are presented. Transformations between thesemagnitudes and those of several currently used systems (CIT (Elias etal. 1982 and Carney 1983), Johnson (Johnson 1966, and Lee 1970), and ESO(Bouchet et al. 1991)) are proposed. A comparison to the narrow-bandsystem of Selby et al. (1988) has been made, in order to check theaccuracy of the photometric system. A mean internal accuracy better than0.02mag in the three bands can be inferred from the comparison to thelarge number of stars in common with Carney (1983), and from thedispersion of the multiple measured stars. The list of standards, thefilter passbands and effective wavelengths, together with correlationsbetween the extinction coefficients, ultimately characterize thephotometric IR system of the Observatorio del Teide (TCS). Data ofcomparable quality previously published have been added in order tocomplete the sample. This way the final sample consists of 550 stars.From the analysis of optical and IR colour:colour diagrams, we maydeduce that the range F0-K0 is properly sampled for0.1>[Fe/H]>-3.0. In the range K0-M4, no reliable photometricestimates of metallicity can be assigned, and only a small number ofstars have spectroscopic determination of the metallicity. Nevertheless,after kinematical considerations, the stars in this spectral range arealso expected to sample the galactic populations of dwarfs. Themetallicity effects on the IR and optical colour:colour diagrams arebriefly discussed.
| Secondary photometric standard stars for the Thuan-Gunn and Johnson-Kron-Cousins systems
CCD photometry is presented for 119 stars observed in the Thuan-Gunnphotometric system (the passbands vgr). The sample includes 82 stars forwhich magnitudes in this system have not previously been published.Photometry in the Johnson-Kron-Cousins photometric system (BV or UBV)has been obtained for a subsample of 85 of the stars. Of those starswhich are not primary standard stars for the Thuan-Gunn system 70observed in Gunn r, 48 observed in Gunn g, and 37 observed in Gunn vhave sufficient repeat measurements that they may be used as secondarystandard stars for CCD photometry. The typical uncertainties for themagnitudes of these stars are 0.006 mag.
| Low-mass stars in cooling-flow galaxies
Many cooling-flow models of X-ray data predict a very large populationof low-mass stars in otherwise normal elliptical galaxies. To date, noconclusive evidence either in favour of or against the formation of thissignificant population of low-mass stars has been found, largely becauseof the lack of appropriate techniques. The aim of this paper is toinvestigate the feasibility of near-infrared photometry and spectroscopyas a potential diagnostic for the existence of a substantial populationof low-mass stars in cooling-flow ellipticals. The technique discussedhere, based on the CO absorption band at 2.3 microns, appears to be verypromising for this purpose because the 2.3-micron CO absorption band isa sensitive luminosity class indicator. We assume that a cooling-flowgalaxy is a normal elliptical galaxy with a low-mass star accretionpopulation. By simulating the K-band spectrum of the cooling-flowgalaxy, we investigate the sensitivity of the broad-band V - K colourand the CO index on the mass function and accretion rate of theaccretion population. We find that the CO index is a much more sensitiveindicator of the accretion population in the CO-M_V_ plane than the V -K colour is in the (V - K)M_V_ plane. An accretion population with amass of approximately 3 X 10^12^ M_sun_ can be detected with highconfidence in the CO-M_V_ plane. If the upper mass cut-off of theaccretion population is less than 0.5 M_sun_ then its CO index isapproximately - 0.4 and is independent of the stellar mass function. TheCO index of any cooling- flow galaxy lies in the range -0.05 < CO< 0. 15. We also perform differential spectral analysis by simulatingthe K-band spectrum (2-2.5 microns) of a cooling-flow galaxy under ourabove assumption. By varying the mass accretion rate and thesignal-to-noise ratio, we study the sensitivity of spectral fitting, inorder to constrain the mass function parameters of the accretionpopulation. We find that a signal-to-noise ratio of about 100 allowsuseful constraints of mass function parameters, provided that the massof the accretion population is more than about 3 X 10^12^ M_sun_.Smaller accretion masses require larger signal-to-noise data for usefulspectral fitting. Accretion rates of low-mass stars on to the centralgalaxy that are larger than about 500 M_sun_ yr^-1^, with a duration ofa Hubble time, result in a cooling-flow galaxy that is brighter than theaverage brightest galaxy of a large cluster of galaxies. A cooling-flowgalaxy with such an accretion rate is entirely composed of the accretionpopulation and is physically inconsistent with the existence of a`parent' elliptical galaxy.
| The importance of surface inhomogeneities for K and M dwarf chromospheric fluxes
We present published and archived spectroscopic and spectrophotometricdata of H-alpha, Ca II, Mg II, and X-rays for a large sample of K and Mdwarfs. The data set points to the importance that surfaceinhomogeneities have in the flux luminosity diagrams in these late-typedwarfs, irrespective of whether the Balmer lines are in emission orabsorption. Although supporting the fact that cooler stars exhibitincreasing levels of surface activity, evident through an increasingincidence of Balmer emission, surface inhomogeneities, or variations inthe local temperature and density structure, at the chromospheric level,dominate the total Ca II and Mg II fluxes. We show that the flux-fluxand luminosity-luminosity relations indicate differing extents ofinhomogeneity from the chromosphere through to the corona. A goodcorrelation between Ca II and Mg II fluxes indicates that they areformed in overlapping regions of the chromosphere, so that thecontribution of surface inhomogeneities is not evident from thisparticular flux-flux diagram. In the region of the upper chromospherethrough to the transition and corona, the correlation between Ly-alphaand X-ray fluxes indicates regions with similar levels of arealinhomogeneity. This appears to be uncorrelated with that at thechromospheric level.
| CCD surface photometry for E and S0 galaxies in the Coma cluster
The paper presents CCD surface photometry in Johnson B and Gunn r for 33galaxies in the Coma cluster and for 13 galaxies in the field or inother clusters, obtained with the Danish 1.54-m telescope at the ESO, LaSilla, Chile in the period April 3-10, 1989. The data were calibratedwith standard stars. Data for all galaxies include derivations ofeffective radius, mean surface brightness, and total magnitude. Goodagreement was found between the photometry and the global parameterspresented with data of Dressler, Faber, Burstein and theircollaborators, indicating that the quality of the CCD data is fullyadequate for studies of the large-scale motions in the universe.
| Chromospheric diagnostics in late-type stars
Moderate and low resolution spectroscopic observations of late typedwarfs covering a broad range of activity are presented. The strength ofthe TiO band in 4762 A is used for spectral classification. Calibratedfluxes for the Balmer and the Ca II H and K lines are derived. H-alphaequivalent widths are given for those stars where the line is inabsorption as well as those in emission. It is confirmed that stars withno detectable or very weak H-alpha do exist. It is also shown that starswith very weak or no detectable emission in Ca II H and K also exist,this occurring preferentially in late M dwarfs. Although this can beexplained in terms of reduced nonradiative heating in the chromosphere,the results cannot be conclusive due to the limited resolution of thedata.
| Photometric calibration of the HST wide-field/planetary camera. I - Ground-based observations of standard stars
This paper describes the observation and analysis of field stars,undertaken as a step toward understanding the photometric properties ofthe filters and CCDs aboard the Wide-Field/Planetary Camera of theHubble Space Telescope. Ground-based CCD observations have been madesimulating 15 of the most important WF/PC passbands. Data are presentedhere for an equatorial network of stars including many UBVRI standardsand some spectrophotometric standards. They serve to establish aphotometric system that will be used in the calibration of in-flightdata and that is useful for the calibration of other ground-based data.Transformations between WF/PC and other photometric systems arediscussed, as well as the effects of interstellar absorption andatmospheric extinction on data in the WF/PC system. Synthetic photometryhas been used as an aid in the data analysis and is described.
| The circumstellar environment of the emission-line star LkH-alpha 101
The environs of the premain-sequence, emission-line star, LkH-alpha 101,have been observed by broad-band CCD imaging, high-resolution opticalspectroscopy, IRAS imaging, and single-dish, as well as interferometric,millimeter-line mapping. LkH-alpha 101 exhibits a high mass-loss rate(0.000011) low-velocity (350 km/s) ionized wind typical of early-typepremain-sequence stars (Barsony, 1989). The millimeter interferometermaps show that this wind has cleared out a cavity in the molecular cloudsurrounding LkH-alpha 101, allowing the rapid expansion of thepreviously observed VLA H II region (Becker and White, 1988).
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | בתולה |
| התרוממות ימנית: | 13h18m56.72s |
| סירוב: | -03°04'18.0" |
| גודל גלוי: | 10.649 |
| מרחק: | 35.881 פארסק |
| תנועה נכונה: | -643 |
| תנועה נכונה: | -130.2 |
| B-T magnitude: | 12.265 |
| V-T magnitude: | 10.783 |
קטלוגים וכינוים:
|
