내용
사진
사진 업로드
DSS Images Other Images
관련 글
Optical spectroscopy of high proper motion stars: new M dwarfs within 10 pc and the closest pair of subdwarfs
We present spectra of 59 nearby star candidates, M dwarfs and whitedwarfs, previously identified using high proper motion catalogues andthe DENIS database. We review the existing spectral classificationschemes and spectroscopic parallax calibrations in the near-infrared Jband and derive spectral types and distances of the nearby candidates.Forty-two stars have spectroscopic distances smaller than 25 pc, threeof them being white dwarfs. Two targets lie within 10 pc, one M8 star at10.0 pc (APMPM J0103-3738), and one M4 star at 8.3 pc (L 225-57). Onestar, LHS 73, is found to be among the few subdwarfs lying within 20 pc.Furthermore, together with LHS 72, it probably belongs to the closestpair of subdwarfs we know.
| 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.
| Finding benchmark brown dwarfs to probe the substellar initial mass function as a function of time
Using a simulated disc brown dwarf (BD) population, we find that newlarge area infrared surveys are expected to identify enough BDs coveringwide enough mass-age ranges to potentially measure the present day massfunction down to ~0.03Msolar, and the BD formation historyout to 10Gyr, at a level that will be capable of establishing if BDformation follows star formation. We suggest these capabilities are bestrealized by spectroscopic calibration of BD properties (Teff,g and [M/H]) which when combined with a measured luminosity and anevolutionary model can give BD mass and age relatively independent of BDatmosphere models. Such calibration requires an empirical understandingof how BD spectra are affected by variations in these properties, andthus the identification and study of `benchmark BDs' whose age andcomposition can be established independently.We identify the best sources of benchmark BDs as young open clustermembers, moving group members, and wide (>1000au) BD companions toboth subgiant stars and high-mass white dwarfs (WDs). To accuratelyasses the likely number of wide companion BDs available, we haveconstrained the wide L dwarf companion fraction using the 2-Micron AllSky Survey (2MASS), and find a companion fraction of2.7+0.7-0.5percent for separations of~1000-5000au. This equates to a BD companion fraction of34+9-6percent if one assumes an α~ 1companion mass function. Using this BD companion fraction, we simulatepopulations of wide BD binaries, and estimate that80+21-14 subgiant-BD binaries, and50+13-10 benchmark WD-BD binaries could beidentified using current and new facilities. The WD-BD binaries shouldall be identifiable using the Large Area Survey component of the UnitedKingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey, combinedwith the Sloan Digital Sky Survey. Discovery of the subgiant-BD binarieswill require a near-infrared imaging campaign around a large (~900)sample of Hipparcos subgiants. If identified, spectral studies of thesebenchmark BD populations could reveal the spectral sensitivities acrossthe Teff, g and [M/H] space probed by new surveys.
| 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.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| 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.
| A Dedicated M Dwarf Planet Search Using The Hobby-Eberly Telescope
We present the first results from our planet-search program using the9.2 m Hobby-Eberly Telescope (HET) at McDonald Observatory to detectplanets around M-type dwarf stars by means of high-precision radialvelocity (RV) measurements. Although more than 100 extrasolar planetshave been found around solar-type stars of spectral type F-K, there isonly a single M dwarf (GJ 876) known to harbor a planetary system. Withthe current incompleteness of Doppler surveys with respect to M dwarfs,it is not yet possible to decide whether this is due to a fundamentaldifference in the formation history and overall frequency of planetarysystems in the low-mass regime of the Hertzsprung-Russell diagram, orsimply an observational bias. Our HET M dwarf survey plans to survey 100M dwarfs in the next 3 to 4 years, with the primary goal being to answerthis question. Here we present the results from the first year of thesurvey, which show that our routine RV precision for M dwarfs is 6 ms-1. We found that GJ 864 and GJ 913 are binary systems withas yet undetermined periods, while five out of 39 M dwarfs reveal a highRV scatter and represent candidates for having short-period planetarycompanions. For one of them, GJ 436 (rms=20.6 m s-1), we havealready obtained follow-up observations, but no periodic signal ispresent in the RV data.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.
| 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.
| 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 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.
| 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.
| Infrared Colors at the Stellar/Substellar Boundary
We present new infrared JHK photometry for 61 halo and disk stars aroundthe stellar/substellar boundary. We also present new L' photometry for21 of these stars and for 40 low-mass stars taken from the Leggettphotometry compilation. These data are combined with available opticalphotometry and astrometric data to produce color-color and absolutemagnitude-color diagrams; the current sample extends the similar workpresented in the 1992 paper into more metal-poor and lower mass regimes.The disk and halo sequences are compared to the predictions of thelatest model atmospheres and structural models. We find good agreementbetween observation and theory except for known problems in the V and Hpassbands, probably due to incomplete molecular data for TiO, metalhydrides, and H_2O. The metal-poor M subdwarfs are well matched by themodels since oxide opacity sources are less important in this case. Theknown extreme M subdwarfs have metallicities about one-hundredth solar,and the coolest subdwarfs have T_eff ~ 3000 K with masses~0.09M/M_ȯ. The grainless models are not able to reproduce the fluxdistributions of disk objects with T_eff < 2500 K; however, apreliminary version of the NextGen-Dusty models that includeshomogeneous formation and extinction by dust grains is able to match thecolors of these very cool objects. The least luminous objects in thissample are GD 165B, three DENIS objects-DBD 0205, DBD 1058, and DBD1228-and Kelu-1. These have T_eff ~ 2000 K and are at or below thestellar limit with masses <=0.075M/M_ȯ. Photometry alone cannotconstrain these parameters further since the age is unknown, butpublished lithium detections for two of these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr) and substellar (mass<= 0.06M/M_ȯ).
| 3C 254: the alignment effect and unification schemes
3C 254 is a radio-loud quasar at z=0.734. Optical line and continuumemission from the underlying galaxy is clearly extended and aligned withthe radio axis; the object shows the so-called `alignment effect' whichis often seen in powerful radio galaxies. This is the clearest case yetof the continuum alignment effect in a radio-loud quasar. The object isone of the most lobe-dominated 3C quasars; the significance of thealigned emission in this source is discussed in terms oforientation-based unification schemes for radio-loud quasars and radiogalaxies. 3C 254 is a very asymmetric radio source and it is shown thatthe radio structure on the side with the shortest nucleus-hotspotdistance is interacting with the emission-line gas surrounding thequasar. It is also shown that the quasar is surrounded by an overdensityof faint objects, consistent with a cluster or group of galaxies aroundthe object.
| 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
| 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.
| 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
| Photographic astrometry of binar and proper-motion stars: 8.
300 trigonometric parallaxes, 15 revised binary-star orbits, and 24 massratios are listed and annotated.
| Long term variability in dwarf M stars
Broadband VRI observations for 43 dwarf M stars have been made over theperiod 1980-1991, and 21 of them, including eight UBV standard stars,show long term variability at the 95% confidence level. While itundoubtedly exists, periodic behavior cannot be demonstrated from thesedata except perhaps in the cases of GL 213 and GL 876 which appear toshow periods of 2.7 and 2.9 yr, respectively. Substantial short termvariations in the light of GL 277A (= VV Lyn) are also noted.
| Far infrared properties of late type dwarfs. Infrared fluxes of K & M dwarfs
IRAS fluxes/upper limits are presented for a large sample of K and Mdwarfs. Good agreement is found between the 12 micrometer fluxes andthose derived from the photospheric models of Mould (1976).Relationships between the optical and infrared colors are derived. Theactive dMe/dKe stars appear systematically brighter in the infraredcompared with the less active dM/dK stars, which could be attributed tomore efficient nonradiative heating in their atmosphere. Any systematicdifferences found in our results when compared with those obtained fromprevious studies are attributed to the different analysis packages used.
| 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.
| Photometry of dwarf K and M stars
An observational program using UBVRI photometry is presented for 688stars from among the dwarf K and M stars already found spectroscopicallyby Vyssotsky (1958). Of these, 211 have not been observedphotometrically. These observations were obtained over a period ofseveral years at the Kitt Peak National Observatory using a GaAsphotomultiplier with an 0.9 m reflector. Based on night-to-nightvariations in the measures of individual stars, the internal errors maybe estimated to be roughly 0.01 mag for the colors and 0.015 for the Vmagnitudes. The photometric parallaxes reported for each star werecomputed in the manner discussed by Weis (1986).
| Analysis of Neckel and Chini standard stars in the UBVRI photometric system
This paper studies the list of 60 faint standard stars of Neckel andChini (1980) in the UBVRI Johnson photometric system. We have re-reducedthe standard stars using our own method of reduction from a large numberof selected observations made at the Observatorio Astronomico Nacionaland at the Centro Astronomico Hispano-Aleman, both at Calar Alto, Spain,and at the Observatorio del Roque de los Muchachos, La Palma Island,Spain. The reduction method used is explained and the results are givenfor the four colors used: (B-V), (U-B), (V-R), and (V-I), together withthe V magnitude, that is, we have corrected many standard stars for eachcolor and magnitude. These results are supported with graphs whereresidual color is plotted against corresponding color (or magnitude).Finally, our (V-R) and (V-I) results are compared to those of Taylor etal. (1989), with a good correspondence.
| Infrared colors of low-mass stars
A total of 322 red dwarf stars are studied in a review of IR IJHKphotometry to discern chromospheric activity and kinematic dataregarding metallicity effects in the IR color:color diagrams. Themetallicity variations are employed to assess changes in the H(-)continuum opacity and water-vapor characterizations. The stars areclassified in terms of metal-richness with five categories includingyoung disk, old disk, and halo types with attention given to the inverserelationship between metallicity and water-band absorption strength. Theresults include IR photometric parallax relations for each metallicitygroup and absolute magnitudes for single stars as well as temperatures,intrinsic colors, and spectral types. The body of data is useful forconstraining models of the interiors and atmospheres of this class ofstars.
| Multiplicity among M dwarfs
Surveys of M dwarfs within 20 pc are examined to determine the incidenceof stellar companions. Observational data are drawn from high-qualitysurveys, including IR-array imaging, precise velocities, IR speckleinterferometry, and visual imaging, and their respective incompletenessis determined. The number of companions per AU of semimajor axis iscomputed and found to decline monotonically toward larger semimajoraxes. The period distribution exhibits a unimodal broad maximum with apeak in the range 9-220 yr, corresponding to separations of 3-30 AU,similar to that for G dwarf binaries. Fifty-eight percent of nearby Mdwarf primaries are found to be single, and 42 +/- 9 percent havecompanions. The M-dwarf binary frequency is lower than that for G dwarfs(about 57 percent), owing partly to the smaller range of companionmasses included, i.e., companions less massive than the M- or G-typeprimary.
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
