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The Solar Neighborhood. XXVIII. The Multiplicity Fraction of Nearby Stars from 5 to 70 AU and the Brown Dwarf Desert around M Dwarfs
We report on our analysis of Hubble Space Telescope/NICMOS snapshothigh-resolution images of 255 stars in 201 systems within ~10 pc of theSun. Photometry was obtained through filters F110W, F180M, F207M, andF222M using NICMOS Camera 2. These filters were selected to permit clearidentification of cool brown dwarfs through methane contrast imaging.With a plate scale of 76 mas pixel-1, NICMOS can easilyresolve binaries with subarcsecond separations in the19farcs5×19farcs5 field of view. We previously reported fivecompanions to nearby M and L dwarfs from this search. No new companionswere discovered during the second phase of data analysis presented here,confirming that stellar/substellar binaries are rare. We establishmagnitude and separation limits for which companions can be ruled outfor each star in the sample, and then perform a comprehensivesensitivity and completeness analysis for the subsample of 138 M dwarfsin 126 systems. We calculate a multiplicity fraction of0.0+3.5 -0.0% for L companions to M dwarfsin the separation range of 5-70 AU, and 2.3+5.0-0.7% for L and T companions to M dwarfs in theseparation range of 10-70 AU. We also discuss trends in thecolor-magnitude diagrams using various color combinations and presentastrometry for 19 multiple systems in our sample. Considering theseresults and results from several other studies, we argue that theso-called brown dwarf desert extends to binary systems with low-massprimaries and is largely independent of primary mass, mass ratio, andseparations. While focusing on companion properties, we discuss how thequalitative agreement between observed companion mass functions andinitial mass functions suggests that the paucity of brown dwarfs ineither population may be due to a common cause and not due to binaryformation mechanisms.
| Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program . II. Activity and radial velocity
Owing to their low mass and luminosity, M dwarfs are ideal targets ifone hopes to find low-mass planets similar to Earth using the radialvelocity (RV) method. However, stellar magnetic cycles could add noiseor even mimic the RV signal of a long-period companion. We extend ourprevious study of the correlation between activity cycles and long-termRV variations for K dwarfs to the lower-end of the main sequence. Ourobjective is to detect any correlations between long-term activityvariations and the observed RV of a sample of M dwarfs. We use a sampleof 27 M-dwarfs with a median observational timespan of 5.9 years. Thecross-correlation function (CCF) with its parameters RV, bisectorinverse slope (BIS), full width at half maximum (FWHM), and contrast arecomputed from the HARPS spectrum. The activity index is derived usingthe Na i D doublet. These parameters are compared with the activitylevel of the stars to search for correlations. We detect RV variationsup to ~5 m s-1 that we can attribute to activity cycleeffects. However, only 36% of the stars with long-term activityvariability appear to have had their RV affected by magnetic cycles, onthe typical timescale of ~6 years. Therefore, we suggest a carefulanalysis of activity data when searching for extrasolar planets usinglong-timespan RV data.Based on observations made with the HARPS instrument on the ESO 3.6-mtelescope at La Silla Observatory under programme ID072.C-0488(E).Tables with the data used for Figs. A.1-A.27 are onlyavailable at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/541/A9AppendixA is available in electronic form at http://www.aanda.org
| Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program. I. Comparison of activity indices
Context. The search for extra-solar planets similar to Earth is becominga reality, but as the level of the measured radial-velocity reaches thesub-m s-1, stellar intrinsic sources of noise capable ofhiding the signal of these planets from scrutiny become more important. Aims: Other stars are known to have magnetic cycles similar tothat of the Sun. The relationship between these activity variations andthe observed radial-velocity is still not satisfactorily understood.Following our previous work, which studied the correlation betweenactivity cycles and long-term velocity variations for K dwarfs, we nowexpand it to the lower end of the main sequence. In this first paper ouraim is to assess the long-term activity variations in the low end of themain sequence, having in mind a planetary search perspective. Methods: We used a sample of 30 M0-M5.5 stars from the HARPS M-dwarfplanet search program with a median timespan of observations of 5.2years. We computed chromospheric activity indicators based on the Ca iiH and K, H?, He i D3, and Na i D1 and D2 lines. All data werebinned to average out undesired effects such as rotationally modulatedatmospheric inhomogeneities. We searched for long-term variability ofeach index and determined the correlations between them. Results:While the SCa II, H?, and Na i indices showedsignificant variability for a fraction of our stellar sample (39%, 33%,and 37%, respectively), only 10% of our stars presented significantvariability in the He i index. We therefore conclude that this index isa poor activity indicator at least for this type of stars. Although theH? shows good correlation with SCa II for the mostactive stars, the correlation is lost when the activity level decreases.This result appears to indicate that the Ca ii - H? correlation isdependent on the activity level of the star. The Na i lines correlatevery well with the SCa II index for the stars with lowactivity levels we used, and are thus a good chromospheric activityproxy for early-M dwarfs. We therefore strongly recommend the use of theNa i activity index because the signal-to-noise ratio in the sodiumlines spectral region is always higher than for the calcium lines.Based on observations made with the HARPS instrument on the ESO 3.6-mtelescope at La Silla Observatory under programme ID 072.C-0488(E).
| Chromospheric Activity of Southern Stars from the Magellan Planet Search Program
I present chromospheric-activity measurements of ~670 F, G, K, and Mmain-sequence stars in the Southern Hemisphere, from ~8000 archivalhigh-resolution echelle spectra taken at Las Campanas Observatory since2004. These stars were targets from the Old Magellan Planet Search, andare now potential targets for the New Magellan Planet Search that willlook for rocky and habitable planets. Activity indices (S values) arederived from Ca II H and K line cores and then converted to the MountWilson system. From these measurements, chromospheric (logR'HK) indices are derived, which are then used as indicatorsof the level of radial-velocity jitter, age, and rotation periods thesestars present.Based on observations obtained with the Magellan Telescopes, operated bythe Carnegie Institution, Harvard University, University of Michigan,University of Arizona, and the Massachusetts Institute of Technology.
| UBV(RI)C JHK observations of Hipparcos-selected nearby stars
We present homogeneous, standardized UBV(RI)C photometry forover 700 nearby stars selected on the basis of Hipparcos parallaxes.Additionally, we list JHK photometry for about half of these stars, aswell as L photometry for 86 of the brightest. A number of stars withpeculiar colours or anomalous locations in various colour-magnitudediagrams are discussed.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| The effect of activity on stellar temperatures and radii
Context: Recent analyses of low-mass eclipsing binary stars haveunveiled a significant disagreement between the observations andpredictions of stellar structure models. Results show that theoreticalmodels underestimate the radii and overestimate the effectivetemperatures of low-mass stars but yield luminosities that accord withobservations. A hypothesis based upon the effects of stellar activitywas put forward to explain the discrepancies. Aims: In this paper westudy the existence of the same trend in single active stars and providea consistent scenario to explain systematic differences between activeand inactive stars in the H-R diagram reported earlier. Methods: Theanalysis is done using single field stars of spectral types late-K and Mand computing their bolometric magnitudes and temperatures throughinfrared colours and spectral indices. The properties of the stars insamples of active and inactive stars are compared statistically toreveal systematic differences. Results: After accounting for a numberof possible bias effects, active stars are shown to be cooler thaninactive stars of similar luminosity therefore implying a larger radiusas well, in proportions that are in excellent agreement with those foundfrom eclipsing binaries. Conclusions: The present results generalisethe existence of strong radius and temperature dependences on stellaractivity to the entire population of low-mass stars, regardless of theirmembership in close binary systems.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/478/507
| Further observations of Hipparcos red stars and standards for UBV(RI)C photometry
We present homogeneous and standardized UBV(RI)C JHKphotometry for over 100 M stars selected from an earlier paper on thebasis of apparent photometric constancy. L photometry has been obtainedfor stars brighter than about L = 6. Most of the stars have asubstantial number of UBV(RI)C observations and, it is hoped,will prove useful as red supplementary standards. Additionally, we listJHK photometry for nearly 300 Hipparcos red stars not selected asstandards, as well as L photometry for the brightest stars.
| The NaI D resonance lines in main-sequence late-type stars
We study the sodium D lines (D1: 5895.92Å D2: 5889.95Å) inlate-type dwarf stars. The stars have spectral types between F6 and M5.5(B - V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82and 0.6. We obtained medium-resolution echelle spectra using the 2.15-mtelescope at the Argentinian observatory Complejo Astronómico ElLeoncito (CASLEO). The observations have been performed periodicallysince 1999. The spectra were calibrated in wavelength and in flux. Adefinition of the pseudo-continuum level is found for all ourobservations. We also define a continuum level for calibration purposes.The equivalent width of the D lines is computed in detail for all ourspectra and related to the colour index (B - V) of the stars. Whenpossible, we perform a careful comparison with previous studies.Finally, we construct a spectral index (R'D) as the ratiobetween the flux in the D lines and the bolometric flux. We find that,once corrected for the photospheric contribution, this index can be usedas a chromospheric activity indicator in stars with a high level ofactivity. Additionally, we find that combining some of our results, weobtain a method to calibrate in flux stars of unknown colour.
| Hα and the Ca II H and K lines as activity proxies for late-type stars
Context: The main chromospheric activity indicator is the S index, whichis the ratio of the flux in the core of the Ca II H and K lines to thecontinuum nearby, and is well studied for stars from F to K. Anotherchromospheric proxy is the Hα line, which is believed to betightly correlated with the Ca II index. Aims: In this work wecharacterize both chromospheric activity indicators, the one associatedwith the H and K Ca II lines and the other with Hα, for the wholerange of late type stars, from F to M. Methods: We present periodicmedium-resolution echelle observations covering the complete visualrange, taken at the CASLEO Argentinean Observatory over 7 years. We usea total of 917 flux-calibrated spectra for 109 stars that range from F6to M5. We statistically study these two indicators for stars ofdifferent activity levels and spectral types. Results: We directlyderive the conversion factor that translates the known S index to fluxin the Ca II cores, and extend its calibration to a wider spectralrange. We investigate the relation between the activity measurements inthe calcium and hydrogen lines, and found that the usual correlationobserved is the product of the dependence of each flux on stellarcolour, and not the product of similar activity phenomena.Tables 1 and 2 and full Figs. 1 and 6 are only available in electronicform at http://www.aanda.org
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Library of flux-calibrated echelle spectra of southern late-type dwarfs with different activity levels
We present Echelle spectra of 91 late-type dwarfs, of spectral typesfrom F to M and of different levels of chromospheric activity, obtainedwith the 2.15 m telescope of the CASLEO Observatory located in theArgentinean Andes. Our observations range from 3890 to 6690 Å, ata spectral resolution from 0.141 to 0.249 Å per pixel(R=λ/δ λ ≈ 26 400). The observations were fluxcalibrated with the aid of long slit spectra. A version of thecalibrated spectra is available via the World Wide Web.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/699The spectra are available as FITS and ascii-files at the URL:http://www.iafe.uba.ar/cincunegui/spectra/Table2.html. They are alsoavailable 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/414/699. When convertingthe fits to ascii, the spectra were oversampled to a constant δλ ≈ 0.15 Å.Table 2 is also available in electronic form at the CDS via anonymous
| 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.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| 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
| Nearby Microlensing Events: Identification of the Candidates for theSpace Interferometry Mission
The Space Interferometry Mission (SIM) is the instrument of choice whenit comes to observing astrometric microlensing events where nearby,usually high proper motion, stars (``lenses'') pass in front of moredistant stars (``sources''). Each such encounter produces a deflectionin the source's apparent position that, when observed by SIM, can leadto a precise mass determination of the nearby lens star. We search forlens-source encounters during the 2005-2015 period using Hipparcos, ACT,and NLTT to select lenses, and USNO-A2.0 to search for the correspondingsources, and rank these by the SIM time required for a 1% massmeasurement. For Hipparcos and ACT lenses, the lens distance andlens-source impact parameter are precisely determined so that the eventsare well characterized. We present 32 candidates beginning with a 61 CygA event in 2012 that requires only a few minutes of SIM time. ProximaCentauri and Barnard's star each generate several events. For NLTTlenses, the distance is known only to a factor of 3, and the impactparameter only to 1''. Together, these produce uncertaintiesof a factor ~10 in the amount of SIM time required. We present a list of146 NLTT candidates and show how single-epoch CCD photometry of thecandidates could reduce the uncertainty in SIM time to a factor of ~1.5.
| The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Supplementary southern standards for UBV(RI)c photometry
We present UBV(RI)c photometry for 80 southern red and blue stars foruse as additional standards. The data are tied to the Johnson UBV andCousins (RI)c systems and extend the range of the available stars forcolor equation determination, especially in (U-B) for blue stars and(V-R) and (V-I) for red stars. Comparisons with published data are madeand particularly good agreement is found with Bessell for the red(Gliese) stars.
| Astrometric positions of stars with high proper motions in the Southern Hemisphere
Several stars with large proper motions, cited by W.J. Luyten, wereincluded in the preliminary programme for the HIPPARCOS mission. Whenperforming preparatory measurements of plates, difficulties wereencountered in identifying certain of these stars when relying only onpublished coordinates. We have taken advantage of this work whichrelates to the southern sky in order to determine the astrometricposition of the greatest possible number of these objects, even forthose which were not included in the programme. Catalogue 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/Abstract.html
| 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
| The Stellar Content of Star Stream I
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....111.1615E&db_key=AST
| The general catalogue of trigonometric [stellar] paralaxes
Not Available
| A volume-limited ROSAT survey of extreme ultraviolet emission from all nondegenerate stars within 10 parsecs
We report the results of a volume-limited ROSAT Wide Field Camera (WFC)survey of all nondegenerate stars within 10 pc. Of the 220 known starsystems within 10 pc, we find that 41 are positive detections in atleast one of the two WFC filter bandpasses (S1 and S2), while weconsider another 14 to be marginal detections. We compute X-rayluminosities for the WFC detections using Einstein Imaging ProportionalCounter (IPC) data, and these IPC luminosities are discussed along withthe WFC luminosities throughout the paper for purposes of comparison.Extreme ultraviolet (EUV) luminosity functions are computed for singlestars of different spectral types using both S1 and S2 luminosities, andthese luminosity functions are compared with X-ray luminosity functionsderived by previous authors using IPC data. We also analyze the S1 andS2 luminosity functions of the binary stars within 10 pc. We find thatmost stars in binary systems do not emit EUV radiation at levelsdifferent from those of single stars, but there may be a fewEUV-luminous multiple-star systems which emit excess EUV radiation dueto some effect of binarity. In general, the ratio of X-ray luminosity toEUV luminosity increases with increasing coronal emission, suggestingthat coronally active stars have higher coronal temperatures. We findthat our S1, S2, and IPC luminosities are well correlated withrotational velocity, and we compare activity-rotation relationsdetermined using these different luminosities. Late M stars are found tobe significantly less luminous in the EUV than other late-type stars.The most natural explanation for this results is the concept of coronalsaturation -- the idea that late-type stars can emit only a limitedfraction of their total luminosity in X-ray and EUV radiation, whichmeans stars with very low bolometric luminosities must have relativelylow X-ray and EUV luminosities as well. The maximum level of coronalemission from stars with earlier spectral types is studied also. Tounderstand the saturation levels for these stars, we have compiled alarge number of IPC luminosities for stars with a wide variety ofspectral types and luminosity classes. We show quantitatively that ifthe Sun were completely covered with X-ray-emitting coronal loops, itwould be near the saturation limit implied by this compilation,supporting the idea that stars near upper limits in coronal activity arecompletely covered with active regions.
| 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 low mass Hyades and the evaporation of clusters
The 135 single stars and 85 binary systems, redder than R-I = +0.34 magand brighter than V = 17 mag, between alpha = 3.75 h and 5.0 h and delta= +5 deg and + 25 deg show a luminosity function that differsconsiderably from that of the general field stars within 20 pc of theSun. The ratio of double star components to single cluster membersincreases markedly with decreasing luminosity. Forty-three single starsand 16 binary systems that are members of the Hyades supercluster within20 pc of the Sun show the same luminosity function as the field stars inthat region. Fifty percent of the cluster members and 40 percent of thesupercluster members are components of binary stars. The equivalentwidths of H-alpha appear to support a range of ages (approximately 8 to16 x 108 yr) for the cluster stars and demonstrate that theoldest objects are in the supercluster. A list of cluster members, whichmay include the end of the stable main sequence, but for which accurate(R-I) photometry is not available, is included. The half-dozen knownparallax stars of the faintest luminosity contain at least onesupercluster member, TVLM 868-110639, which is probably beyond thestable, nuclear burning main sequence as a 'transitional' or 'brown'dwarf.
| 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.
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Datos observacionales y astrométricos
| Constelación: | Centauro |
| Ascensión Recta: | 12h37m52.23s |
| Declinación: | -52°00'05.3" |
| Magnitud Aparente: | 10.7 |
| Distancia: | 9.658 parsecs |
| Movimiento Propio en Ascensión Recta: | -1023.6 |
| Movimiento Propio en Declinación: | 31.3 |
| B-T magnitude: | 13.57 |
| V-T magnitude: | 10.937 |
Catálogos y designaciones:
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