תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Ca II H and K Chromospheric Emission Lines in Late-K and M Dwarfs
We have measured the profiles of the Ca II H and K chromosphericemission lines in 147 main-sequence stars of spectral type M5-K7 (masses0.30-0.55 Msolar) using multiple high-resolution spectraobtained during 6 years with the HIRES spectrometer on the Keck Itelescope. Remarkably, the average FWHM, equivalent widths, and lineluminosities of Ca II H and K increase by a factor of 3 with increasingstellar mass over this small range of stellar masses. We fit the Ca II Hand K lines with a double-Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of thesample stars display a central absorption that is typically redshiftedby ~0.1 km s-1 relative to the emission. This implies thatthe higher level, lower density chromospheric material has a smalleroutward velocity (or higher inward velocity) by 0.1 km s-1than the lower level material in the chromosphere, but the nature ofthis velocity gradient remains unknown. The FWHM of the Ca II H and Kemission lines increase with stellar luminosity, reminiscent of theWilson-Bappu effect in FGK-type stars. Both the equivalent widths andFWHM exhibit modest temporal variability in individual stars. At a givenvalue of MV, stars exhibit a spread in both the equivalentwidth and FWHM of Ca II H and K, due both to a spread in fundamentalstellar parameters, including rotation rate, age, and possiblymetallicity, and to the spread in stellar mass at a given MV.The K line is consistently wider than the H line, as expected, and itscentral absorption is more redshifted, indicating that the H and K linesform at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of Hαcorrelates with Ca II H and K only for stars having Ca II equivalentwidths above ~2 Å, suggesting the existence of a magneticthreshold above which the lower and upper chromospheres become thermallycoupled.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| High Proper Motion Stars. IV. Radial Velocities of 166 Luyten Half-Second Stars
We present 178 radial velocity measurements for 166 late-type starsselected from the Luyten half-second (LHS) proper motion catalog. Spacevelocities are given for all but two of them. Most of the stars liewithin 25 pc of the Sun, but the list includes a handful ofhigh-velocity transients from the halo population. None of the derivedspace velocities is high enough, however, to provide any constraint onthe escape speed at the solar circle. Twenty-six stars are discussed insomewhat more detail, and evidence is adduced that several of them maybe velocity variables.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| Hubble Space Telescope WFPC2 Study of the Trapezium Cluster: The Influence of Circumstellar Disks on the Initial Mass Function
We have performed the first measures of mass accretion rates in the coreof the Orion Nebula Cluster. Four adjacent fields centered on theTrapezium stars have been imaged in the U and B bands using the WideField Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope. Weobtained photometry for 91 stars in the U band (F336W) and 71 stars inthe B band (F439W). The WFPC2 archive was also searched to obtaincomplementary V-band (F547M) and I-band (F791W) photometry. In thispaper we focus our attention on a group of 40 stars with known spectraltypes and complete UBVI WFPC2 photometry. We locate each star on the H-Rdiagram, considering both the standard ISM reddening law withRV=3.1 and the ``anomalous'' reddening law withRV=5.5 more appropriate for the Orion Nebula. Then we derivethe stellar masses and ages by comparing with the evolutionary tracksand isochrones calculated by D'Antona & Mazzitelli and Palla &Stahler. Approximately three-quarters of the sources show excessluminosity in the U band, which we attribute to mass accretion. Theknown correlation between the U-band excess and the total accretionluminosity, recalibrated for our photometric system, allows us toestimate the accretion rates, which are all found to be in the range10-8 to 10-12 Msolar yr-1.For stars older than 1 Myr, there is some evidence of a relation betweenmass accretion rates and stellar age. Overall, mass accretion ratesappear lower than those measured by other authors in the Orion flankingfields or in Taurus-Auriga. Mass accretion rates remain low even in thevicinity of the 10-5 Msolar yr-1 birthline of Palla & Stahler, suggesting that in the core of theTrapezium cluster, disk accretion has been recently depressed by anexternal mechanism. We suggest that the UV radiation generated by theTrapezium OB stars, responsible for the disk evaporation, may also causethe drop of the mass accretion rate. In this scenario, low-mass starsmay terminate their pre-main-sequence evolution with masses lower thanthose they would have reached if disk accretion could have proceededundisturbed until the final disk consumption. In OB associations thelow-mass end of the initial mass function (IMF) may therefore beaffected by the rapid evolution of the most massive cluster's stars,causing a surplus of ``accretion-aborted,'' very low mass stars andbrown dwarfs and a deficit of intermediate-mass stars. This trend is inagreement with recent observations of the IMF in the Trapezium cluster.Based on observations taken with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS5-26555.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are only available in electronic form at the CDS viaanonymous ftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1083
| 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.
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| 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 Palomar/MSU Nearby Star Spectroscopic Survey. III. Chromospheric Activity, M Dwarf Ages, and the Local Star Formation History
We present high-resolution echelle spectroscopy of 676 nearby M dwarfs.Our measurements include radial velocities, equivalent widths ofimportant chromospheric emission lines, and rotational velocities forrapidly rotating stars. We identify several distinct groups by theirHα properties and investigate variations in chromospheric activityamong early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limitedsample together with a relationship between age and chromosphericactivity, we show that the rate of star formation in the immediate solarneighborhood has been relatively constant over the last 4 Gyr. Inparticular, our results are inconsistent with recent large bursts ofstar formation. We use the correlation between Hα activity and ageas a function of color to set constraints on the properties of L and Tdwarf secondary components in binary systems. We also identify a numberof interesting stars, including rapid rotators, radial velocityvariables, and spectroscopic binaries. Observations were made at the 60inch telescope at Palomar Mountain, which is jointly owned by theCalifornia Institute of Technology and the Carnegie Institution ofWashington.
| Studies of multiple stellar systems - IV. The triple-lined spectroscopic system Gliese 644
We present a radial velocity study of the triple-lined system Gliese 644and derive spectroscopic elements for the inner and outer orbits withperiods of 2.9655 and 627d. We also utilize old visual data, as well asmodern speckle and adaptive optics observations, to derive a newastrometric solution for the outer orbit. These two orbits togetherallow us to derive masses for each of the three components in thesystem: MA=0.410+/-0.028 (6.9 per cent),MBa=0.336+/-0.016 (4.7 per cent), andMBb=0.304+/-0.014 (4.7 per cent)Msolar. We suggestthat the relative inclination of the two orbits is very small. Ourindividual masses and spectroscopic light ratios for the three M starsin the Gliese 644 system provide three points for the mass-luminosityrelation near the bottom of the main sequence, where the relation ispoorly determined. These three points agree well with theoretical modelsfor solar metallicity and an age of 5Gyr. Our radial velocities forGliese 643 and vB 8, two common proper motion companions of Gliese 644,support the interpretation that all five M stars are moving together ina physically bound group. We discuss possible scenarios for theformation and evolution of this configuration, such as the formation ofall five stars in a sequence of fragmentation events leading directly tothe hierarchical configuration now observed, versus formation in a smallN cluster with subsequent dynamical evolution into the presenthierarchical configuration.
| 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
| Photometric Measurements of the Fields of More than 700 Nearby Stars
In preparation for optical/IR interferometric searches for substellarcompanions of nearby stars, we undertook to characterize the fields ofall nearby stars visible from the Northern Hemisphere to determinesuitable companions for interferometric phase referencing. Because theKeck Interferometer in particular will be able to phase-reference oncompanions within the isoplanatic patch (30") to about 17th magnitude atK, we took images at V, r, and i that were deep enough to determine iffield stars were present to this magnitude around nearby stars using aspot-coated CCD. We report on 733 fields containing 10,629 measurementsin up to three filters (Gunn i, r and Johnson V) of nearby stars down toabout 13th magnitude at V.
| The peculiar M giant HD 154791. Optical counterpart of the X-ray source 4U1700+24
HD 154791 is the only optical counterpart of an hard X-ray source(4U1700+24) with an apparently normal M giant spectrum. We present herenew high S/N spectroscopic observations of this interesting and poorlystudied object, pointing out a number of peculiarities never noticedbefore; therefore we derive a possible scenario involving a wide binarysystem, where an old, slowly rotating, neutron star is weeklyinteracting with its late optical companion. We foresee that this systemcould evolve, in a short time, into a normal LMXB. Based on datacollected at the Loiano Observatory
| Hubble Space Telescope observations of the black hole X-ray transient GRO J0422+32 near quiescence
We present HST/FOS ultraviolet and optical spectroscopy of the blackhole X-ray transient GRO J0422+32 shortly before the system reachedquiescence. We find that the accretion spectrum from 2500 to 9000 A canbe very well fitted by a self-absorbed synchrotron model, withsuperposed H I and Mg II emission lines. The explanations that wesuggest for this spectrum are either that it is due to active coronalregions above a geometrically thin accretion disc, or that the disc isevaporated into an advective flow.
| Photometric Separation of Stellar Properties Using SDSS Filters
Using synthetic photometry of Kurucz model spectra, we explore thecolors of stars as a function of temperature, metallicity, and surfacegravity with Sloan Digital Sky Survey (SDSS) filters, u'g'r'i'z'. Thesynthetic colors show qualitative agreement with the few publishedobservations in these filters. We find that the locus of synthetic starsis basically two-dimensional for 4500 < T < 8000 K, whichprecludes simultaneous color separation of the three basic stellarcharacteristics we consider. Colors including u' contain the mostinformation about normal stellar properties; measurements in this filterare also important for selecting white dwarfs. We identify two differentsubsets of the locus in which the loci separate by either metallicity orsurface gravity. For 0.5 < g' - r' < 0.8 (corresponding roughly toG stars), the locus separates by metallicity; for photometric error of afew percent, we estimate metallicity to within ~0.5 dex in this range.In the range -0.15 < g' - r' < 0.00 (corresponding roughly to Astars), the locus shows separation by surface gravity. In both cases, weshow that it is advantageous to use more than two colors whendetermining stellar properties by color. Strategic observations in SDSSfilters are required to resolve the source of a ~5% discrepancy betweensynthetic colors of Gunn-Stryker stars, Kurucz models, and externaldeterminations of the metallicities and surface gravities. The syntheticstar colors can be used to investigate the properties of any normal starand to construct analytic expressions for the photometric prediction ofstellar properties in special cases.
| Rotation and chromospheric activity in field M dwarfs
We have obtained high resolution spectra for a volume-limited sample of118 field M dwarfs. From these observations we derive projectedrotational velocities and fluxes in the H_alpha and H_beta lines. 8stars are double-lined spectroscopic binaries with measured or probableperiods short enough for rotation to be tidally synchronized with theorbit, and another 11 are visual binaries where we cannot yet separatethe lines of the two stars. Of the remaining 99 stars, 24 haverotational velocities above our detection limit of ~ 2 km.s(-1) , andsome are quite fast rotators, including two with v sin i\ =~ 30 km.s(-1)and one with v sin i\ =~ 50 km.s(-1) . Given the small radii of Mdwarfs, these moderate rotational velocities correspond to rather shortmaximum rotational periods, of only 7-8 hours. These three stars aregood candidates for Doppler imaging. We find that rotation is stronglycorrelated with both spectral type and kinematic population: all starswith measurable rotation are later than M3.5, and all but one havekinematic properties typical of the young disk, or intermediate betweenthe young disk and the the old disk. We interpret this correlation asevidence for a spin-down timescale that increases with decreasing mass.At the age of the old disk or halo, all stars earlier than M5-M6(0.1-0.15Msun) have spun-down to below our detection limit,while at the age of the young disk this has only happened for starsearlier than M3.5. The one star with measurable rotation and akinematics intermediate between old disk and population II has spectraltype M6. It is probably an old star whose mass is low enough that it hasretained significant rotation up to present, still consistently withlonger spin-down times for lower mass stars. We observe, on the otherhand, no conspicuous change in the v sin i\ distribution or activitypattern at the mass (M ~ 0.35 Msun) below which stars remainfully convective down to the main sequence. These new data areconsistent with a saturated correlation between rotation and activity,similar to the one observed for younger or more massive stars:L_X/Lbol and L_{H_alpha }/Lbol both correlate withv sin i\ for v sin i\ -5km.s^{-1} and then saturate at respectively10^{-2.5} and 10^{-3.5}$. Based on observations made at the Observatoirede Haute-Provence (CNRS), France Tables 2 and 4 are also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.
| Observation and modelling of main sequence stellar chromospheres. VI. Halpha_ and CaII line observations of M1 dwarfs and comparison with models.
We compare hydrogen and calcium line calculations for dM1 (Teff=3500K)stellar chromospheres with high resolution observations of a selectedsample of stars with the same spectral type ((R-I)_K_=0.875+/-0.05). Webring evidence that grids of uniform model atmospheres in theplane-parallel and hydrostatic equilibrium approximations can reproducethe average spectral signatures throughout the entire activity range.Observations confirm that when magnetic activity level rises, theHα line is first weak, then increases in absorption strength,rapidly fills in and eventually goes into emission. We obtain acorrelation between the Hα line width and equivalent width that isin good agreement with our model calculations. Simultaneous Hα andCaII line observations allow to remove the degeneracy in Hαequivalent width for low activity (weak absorption) and intermediateactivity stars (filled in profiles). We show that the latter grouprepresents a significant proportion of the stellar population. Withinthe active stars group, we find an exclusion zone in the[0.25Å;-1Å] Hα equivalent width domain, that can besimply explained by the rapid change from the absorption to the emissionregimes when the chromospheric pressure increases. In our sample of 154stars, covering a large luminosity range, we found no ``zero-Hα''stars but instead a minimum (possibly ``basal'') Hα equivalentwidth of ~0.20Å which, with reference to our models, suggests atransition region column mass of log(M)~-5.5. This implies that for anoverwhelming majority of M1 type dwarfs the amount of non-thermal energyinput in the chromosphere is much higher than in the Sun, and byreference to acoustic heating calculations, that they are alsomagnetically much more active (per unit area). Our observations provideevidence for gradual and important changes in the integrated physicalproperties of the chromosphere throughout the activity range. Forexample, the equivalent widths of the H and K line cores are tightlycorrelated with their ratio, the later decreasing from low activity(~1.50 at -0.2Å) to high activity stars (~1.06 at -15Å). TheK lines are also typically 30% broader than the H lines and their widthsincrease with increasing activity level. This suggests that theiroptical depths in their region of formation also increase withincreasing activity level. Our calcium line calculations reproduce theobserved trends. We confirm a near UV and blue excess in active dMestars that increases with activity level; in average 0.12 magnitudes inU-B (and up to 0.26mag.) and 0.03 magnitudes in B-V. This excess isabout three times larger than expected from our calculations for a givenatmospheric pressure, and together with discrepancies between models andobservations for spectral lines, all converge to imply that highpressure plages with a filling factor of about 30% are present on thesestars. Low metallicity halo dwarfs in our sample also exhibit a U-Bexcess, but in the case of single dMe stars an effect of metallicity isexcluded. In our sample, single dMe stars are more luminous than theirless active absorption line counterparts. We present anactivity-luminosity relationship for the CaII lines; namely, the CaIIline fluxes rise as the power of 5.4 of the stellar radius. Hence, weexpect the stellar magnetic flux to rise approximately as the power of7.4 of the radius; an important constraint for the dynamo mechanism.Such a correlation is also found with Hα and L_X_, the X-rayluminosity. With our present understanding, these correlations are, atleast partly, activity-metallicity relationships. We emphasize theimportance of metallicity on stellar activity as a whole, i.e., metaldeficient stars are also activity deficient.
| The Palomar/MSU Nearby Star Spectroscopic Survey.II.The Southern M Dwarfs and Investigation of Magnetic Activity
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2799H&db_key=AST
| Photometry of Stars with Large Proper Motion
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2300W&db_key=AST
| The empirical scale of temperatures of the low main sequence (F0V-K5V).
We have calibrated the effective temperatures of the low main sequencestars ranging spectral types from F0 to K5 versus [Fe/H] and colours(B-V), (R-I), (V-R), (V-I), (V-K), (J-H), (J-K) and ubvy-β, using alarge sample of dwarfs and subdwarfs. The effective temperatures, scaledto direct T_eff_ determinations via reliable angular diametermeasurements, were derived applying the InfraRed Flux Method with thenew grid of atmosphere models developed by Kurucz (1993). We have fittedpolynomial functions of the form θ_eff_=P(colour,[Fe/H]) usingthe least squares method. The precision of the fits ranges from 30K for(V-K) to 154K for (J-H). The new relations have been compared toprevious calibrations. We also provide the empirical intrinsic colours(U-B), (B-V), (R-I), (V-R), (V-I), (V-K), (J-H), (J-K) and β, inthe ranges: 4000K[Fe/H]>-2.5.
| 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.
| Distribution and Corrlation of Age, Abundance, and Motion of Lower Main Sequence Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....111..466E&db_key=AST
| Observation and modelling of main sequence star chromospheres. V. Ultraviolet excess emission in active M dwarfs.
The variation in the continuum intensity (50-5000nm) of late-type Mdwarfs is investigated via two grids of model atmospheres with differenttemperature minima. It is shown that the (E)UV intensity is mostlydependent on the transition region pressure, although the temperatureminimum also plays an important role. We also observe a significantfrequency redistribution of the photospheric flux when changing theminimum temperature, and a black-body type of emission from the lowerchromosphere. We examine the formation of the continuum and point outthat, although some differences appear for very low or very highactivity levels, in general the global picture is much alike the Sun. Weshow that the UV continua are very good diagnostics of cool dwarfatmospheres, from the temperature minimum to the transition region. Ourcalculations give a good overview of the domains where physicalparameters and spectral signatures are most likely observed. We computethe UBV(RIJKL)_J_ broad band fluxes for our models and conclude that thechromospheric contribution should be detectable in the U band andpossibly also in the B band. We compare our calculations to recent highresolution observations for selected stars in a narrow spectral range((R-I)_K_=0.875+/-0.05); we show that Hα line profiles behave asexpected, with a tight correlation between the line width and equivalentwidth. Hα emission line stars show an excess in U-B color but notin B-V. They are also more luminous than their less active absorptionline counterparts, which indicates that active dwarfs have not yetreached the main sequence and are intermediate between T Tauri stars andmain sequence stars. The anomalously large proportion of active starstowards late spectral types is attributed to the very slow contractionphase for low mass stars. We calculate the fluxes in the ExtremeUltraviolet Explorer Sn/SiO band (500-740A) and show that they arecompatible with observed upper limits. We further compare our resultsfor the upper activity range to pre-main sequence models andobservations. They strongly support the case for a chromosphericcontribution to Hα and the blue/UV excess for those objects (TTauris, naked T Tauris, YSOs). An important conclusion is that at highpressures, corresponding to active dMe stellar atmospheres, thechromosphere becomes a very efficient radiator at continuum wavelengths.From log(M)~-5 (column mass), radiative losses in the continuum riseexponentially and faster than in Hydrogen spectral lines because oftheir larger optical depths. As a consequence, Hi spectral lines have asmall or negligible contribution to the total Hi (lines and continua)and white light radiative budget. We show that the hydrogen seriesdominate the radiative cooling in spectral lines (from 40% to 90%) forHα emission line stars. The cooling in the (E)UV continuumoverwhelms the total radiative budget and is much larger than that inoutstanding chromospheric and transition region lines (e.g. CaII, MgII,HI Lyman and Balmer). We propose this as a possible contribution for theapparent saturation observed in some spectral lines, thereforequestioning the suggestion of saturation in magnetic activity levels.
| 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.
| Ground-Based Detection of Terrestrial Extrasolar Planets by Photometry: The Case for CM Draconis
Not Available
| The Palomar/MSU Nearby-Star Spectroscopic Survey. I. The Northern M Dwarfs -Bandstrengths and Kinematics
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.1838R&db_key=AST
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | קאסיופיאה |
| התרוממות ימנית: | 01h02m38.87s |
| סירוב: | +62°20'42.2" |
| גודל גלוי: | 9.578 |
| מרחק: | 10.056 פארסק |
| תנועה נכונה: | 730.3 |
| תנועה נכונה: | 88.4 |
| B-T magnitude: | 11.474 |
| V-T magnitude: | 9.735 |
קטלוגים וכינוים:
|
INHABITED SKY
