Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
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.
| Exploring the Frequency of Close-in Jovian Planets around M Dwarfs
We discuss our high-precision radial velocity results of a sample of 90M dwarfs observed with the Hobby-Eberly Telescope and the Harlan J.Smith 2.7 m Telescope at McDonald Observatory, as well as the ESO VLTand the Keck I telescopes, within the context of the overall frequencyof Jupiter-mass planetary companions to main-sequence stars. None of thestars in our sample show variability indicative of a giant planet in ashort-period orbit, with a<=1 AU. We estimate an upper limit of thefrequency f of close-in Jovian planets around M dwarfs as <1.27% (atthe 1 σ confidence level). Furthermore, we determine that theefficiency of our survey in noticing planets in circular orbits is 98%for companions with msini>3.8MJ and a<=0.7 AU. Foreccentric orbits (e=0.6) the survey completeness is 95% for all planetswith msini>3.5MJ and a<=0.7 AU. Our results pointtoward a generally lower frequency of close-in Jovian planets for Mdwarfs as compared to FGK-type stars. This is an important piece ofinformation for our understanding of the process of planet formation asa function of stellar mass.Based on data collected with the Hobby-Eberly Telescope, which isoperated by McDonald Observatory on behalf of the University of Texas atAustin, Pennsylvania State University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen. Also based on observationscollected at the European Southern Observatory, Chile (ESO programs65.L-0428, 66.C-0446, 267.C-5700, 68.C-0415, 69.C-0722, 70.C-0044,71.C-0498, 072.C-0495, 173.C-0606). Additional data were obtained at theW. M. Keck Observatory, which is operated as a scientific partnershipamong the California Institute of Technology, the University ofCalifornia, and the National Aeronautics and Space Administration(NASA), and with the McDonald Observatory Harlan J. Smith 2.7 mtelescope.
| 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.
| 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.
| Spatially resolving the accretion shocks on the rapidly-rotating M0 T-Tauri star MN Lupi
We obtained high-resolution, high-quality VLT/UVES spectra toreconstruct the two-dimensional surface structure of therapidly-rotating classical T-Tauri star MN Lupi on two separate nights.Both surface maps show a structured warm (5000 K) band centered aroundthe pole at a latitude of ≈65°. Located within the band are twohot spots with temperatures of approximately or possibly even in excessof 5800 K, i.e. 2000 K above the effective photospheric temperature.Both maps appear with an adjacent equatorial band of temperature 3400 K,some 400-500 K below the effective photospheric temperature. While weinterpret the two hot spots and the warm high-latitude band to be theheating points from two accretion impacts at the time of ourobservations and their redistributed energy trailed due to the faststellar rotation, respectively, the cool equatorial band may not be coolafter all but due to obscuration of the stellar surface by the innermostregion of the disk. The fact that the hot spots appear at high stellarlatitude is in agreement with the magnetospheric accretion model thatproposes material funnelling onto the star along a predominantly dipolarmagnetic field at roughly 50° latitude. The evidence of ongoing diskaccretion, together with the very fast rotation of MN Lupi of just 3-4times below its break-up velocity, suggests that the accretion mechanismis the cause of its rapid surface rotation. We present a model ofmagnetic star-disk coupling for MN Lupi that predicts a polar surfacemagnetic field of ≈3 kG.
| CHORIZOS: A χ2 Code for Parameterized Modeling and Characterization of Photometry and Spectrophotometry
We have developed CHi-square cOde for parameterRized modeling andcharacterIZation of phOtometry and Spectrophotmetry (CHORIZOS). CHORIZOScan use up to two intrinsic free parameters (e.g., temperature andgravity for stars, type and redshift for galaxies, or age andmetallicity for stellar clusters) and two extrinsic parameters (amountand type of extinction). The code uses χ2 minimization tofind all models compatible with the observed data in the modelN-dimensional (N=1, 2, 3, 4) parameter space. CHORIZOS can use eithercorrelated or uncorrelated colors as input and is specially designed toidentify possible parameter degeneracies and multiple solutions. Thecode is written in IDL and is available to the astronomical community.Here we present the techniques used, test the code, apply it to a fewwell-known astronomical problems, and suggest possible applications. Asa first scientific result from CHORIZOS, we confirm from photometry theneed for a revised temperature-spectral type scale for OB starspreviously derived from spectroscopy.
| High-resolution Doppler images of the spotted contact binary AE Phe
We present Doppler images of the short period (P= 0.362 d) W UMa binaryAE Phe. In order to obtain the necessary S/N ratio and time resolutionrequired to see individual star-spot features in highly rotationallybroadened profiles, we use least-squares deconvolution, which makes useof the information content of the several thousand lines in a typicalechelle spectrum. This yields a single rotation profile (free ofsidelobes due to blending) per spectrum with a typical S/N ratio ofseveral thousand.We use radial velocity curves, generated from standard profile fittingtechniques, to measure velocity amplitudes and the mass ratio. Failureto model star-spots with this method leads to a biased set of values,and we show that an imaging code is essential if accurate systemparameters are to be derived.Images are reconstructed from four nights of data which revealstar-spots at most latitudes on both components of the common envelopesystem. Our model requires that the primary component be several hundredK cooler than the secondary in order to reproduce the profile depthchanges with phase. In a two-temperature imaging model, we interpretthis as being due to 27 per cent greater - but unresolved - spot fillingon the primary relative to the secondary component. The images revealthat dark spots are present on both stars at various latitudes andlongitudes. Star-spots are also found in the neck region of bothcomponents, which appear to be darker on the side of each star leadingin rotation phase - particularly on the secondary component. Weinvestigate the reproducibility of the images from night to night andconclude that the star-spots evolve significantly on very shorttime-scales, of the order of 1 d. This is significantly faster than theweek time-scales found on active single stars and the Sun.
| 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Orbital Parameters for the Black Hole Binary XTE J1650-500
We present R-band photometry of the X-ray transient and candidate blackhole binary XTE J1650-500 obtained between 2003 May and August with the6.5 m Clay telescope. A timing analysis of these data reveals aphotometric period of 0.3205+/-0.0007 days (i.e., 7.63 hr) with apossible alias at 0.3785 days (9.12 hr). Our photometry completely rulesout the previously published spectroscopic period of 0.212 days (5.09hr). Consequently, we reanalyzed the 15 archival ESO VLT spectra(obtained 2002 June by Sanchez-Fernandez et al.) that were the basis ofthe previously published spectroscopic period. We used a ``rest-framesearch'' technique that is well suited for cases in which thesignal-to-noise ratio of individual spectra is low. For each of roughly1.1 million binary ephemerides, we summed all of the spectra in a trialrest frame of the secondary star, and each rest-frame spectrum wascross-correlated against a template spectrum. We then searched for theset of orbital parameters that produced the strongest cross-correlationvalue. The results confirmed the photometric period of 0.3205 days andruled out the alias period near 0.38 days. The best value for thevelocity semiamplitude of the companion star is K2=435+/-30km s-1, and the corresponding optical mass function isf(M)=2.73+/-0.56 Msolar. The spectral type of the companionstar is not well constrained because we only have six template spectraavailable to us. The K4 V template provides the best match; next-bestmatches are provided by the G5 V and K2 III templates. We also find thatthe accretion disk dominates the light in the R band where the diskfraction is 80% or higher, although this value should be treated withcaution owing to the poor signal-to-noise ratio and the limited numberof templates. The amplitude of the phased R-band light curve is 0.2 mag,which gives a lower limit to the inclination of50deg+/-3deg in the limiting case of nocontribution to the R-band light curve from the accretion disk. If themass ratio of XTE J1650-500 is similar to the mass ratios of other blackhole binaries, such as A0620-00 or GRS 1124-683 (e.g., Q>~10), thenour lower limit to the inclination gives an upper limit to the mass ofthe black hole in XTE J1650-500 of M1<~7.3Msolar. However, the mass can be considerably lower if theR-band flux is dominated by the accretion disk. For example, if theaccretion disk does contribute 80% of the flux, as our preliminaryresults suggest, then the black hole mass would be only about 4Msolar.Based on observations made with the Magellan 6.5 m Clay telescope at LasCampanas Observatory of the Carnegie Institution and with EuropeanSouthern Observatory (ESO) telescopes at the Paranal Observatory underprogram ID 69.D-0644(A).
| 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.
| 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
| 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.
| UBV(RI)C photometry of Hipparcos red stars
We present homogeneous and standardized UBV(RI)C photometryfor nearly 550 M stars selected from the Hipparcos satellite data baseusing the following selection criteria: lack of obvious variability (noHipparcos variability flag); δ<+10°(V-I)>1.7 and Vmagnitude fainter than about 7.6. Comparisons are made between thecurrent photometry, other ground-based data sets and Hipparcosphotometry. We use linear discriminant analysis to determine aluminosity segregation criterion for late-type stars, and principalcomponent analysis to study the statistical structure of the colourindices and to calibrate absolute magnitude in terms of (V-I) for thedwarf stars. Various methods are used to determine the mean absolutemagnitude of the giant stars. We find 10 dwarf stars, apparentlypreviously unrecognized (prior to Hipparcos) as being within 25pc,including five within 20pc.
| 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.
| The Solar Neighborhood. VI. New Southern Nearby Stars Identified by Optical Spectroscopy
Broadband optical spectra are presented for 34 known and candidatenearby stars in the southern sky. Spectral types are determined using anew method that compares the entire spectrum with spectra of more than100 standard stars. We estimate distances to 13 candidate nearby starsusing our spectra and new or published photometry. Six of these starsare probably within 25 pc, and two are likely to be within the ResearchConsortium on Nearby Stars (RECONS) horizon of 10 pc.
| 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
| Probing the {Na} BT I D and {K} BT I lambda 7699 resonance lines sensitivity to background opacity in late-type stars
We have measured the equivalent width WK of the K i resonanceline at 7699 Angstroms for a large sample of low activity late-typestars observed with high spectral resolution and we have verified thatthe relation WK vs. Teff is monotonicallydecreasing, for both dwarf and giant stars. This behaviour is differentfrom that of the Na I D lines for stars of the same type, which showedthat the relation WNa vs. Teff has a maximum forTeff ~ 4000 K, which is better defined for giants than fordwarfs (Tripicchio et al. 1997). The fit of the observed K I equivalentwidths by means of a NLTE spectral line synthesis using conventionalbackground opacity shows that, for dwarf stars, the adopted modelsoverestimate the observed WK for temperatures <~ 4000 K.This result is similar to that discussed for the Na I D lines in ourprevious paper. On the other hand, for giant stars with Teff<~ 3800 K these models in general underestimate WK. Thediscrepancies between observed and computed WK andWNa for cool stars are much stronger than the variations dueto uncertainties in either atmospheric model or line parameters, likeeffective temperature and surface gravity, or Van der Waals broadening.For M dwarf stars, the most convincing explanation for the disagreementis the lack of atomic and molecular line opacity in the adopted models.In fact, a NLTE spectral synthesis including an additional backgroundopacity reproduces with a good level of accuracy the equivalent widths,as well as the general shape of the profiles for both the Na I D and K Ilines, in a subsample of early-M dwarfs. Based on observations collectedat the European Southern Observatory (ESO), La Silla, Chile, and at theMcDonald Observatory, Mt. Locke, Texas, USA
| The NaI resonance lines as a spectroscopic test of late-type stellar atmospheres.
We have tested current models for the atmospheres (including photosphereand low chromosphere) of late-type stars using the D resonance lines ofneutral sodium as a diagnostic. To this end, we have measured theequivalent widths of the D lines for a sample of 39 dwarf and 45 giantlate-type stars observed with high spectral resolution. We constructedphotospheric models over a grid in effective temperature and surfacegravity spanning the spectral types F to M, and luminosity classes V andIII of the sample stars. The model photospheres were extended into thechromosphere by assuming a suitable scaling from the Sun, andtheoretical Nai D equivalent widths were computed over the grid ofmodels including the deviations from local thermodynamic equilibrium. Bytaking into account both the experimental errors and the possiblevariations of stellar parameters (effective temperature, surfacegravity, sodium abundance and microturbulence), the comparison betweenobserved and computed equivalent widths allows us to state that themodel atmospheres we have used can reproduce the observations for thetwo luminosity classes and for all the spectral types except for theM-type stars. We have discussed the importance of line blanketing in thespectral analysis of these stars, but at present we cannot conclude thatthis effect would reduce the discrepancy.
| 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
| 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 B jRI Photometric System
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..281G&db_key=AST
| Disk Accretion and Mass Loss from Young Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...452..736H&db_key=AST
| Medium-Resolution Near-Infrared (2.15-2.35 micron) Spectroscopy of Late-Type Main-Sequence Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2415A&db_key=AST
| 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
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Водолей |
| Прямое восхождение: | 22h02m10.27s |
| Склонение: | +01°24'00.9" |
| Видимая звёздная величина: | 9.256 |
| Расстояние: | 10.311 парсек |
| Собственное движение RA: | -452.3 |
| Собственное движение Dec: | -276.3 |
| B-T magnitude: | 11.015 |
| V-T magnitude: | 9.402 |
Каталоги и обозначения:
|
