Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
The Infrared Telescope Facility (IRTF) Spectral Library: Cool Stars
We present a 0.8-5 μm spectral library of 210 cool stars observed ata resolving power of R ≡ λ/Δλ ~ 2000 with themedium-resolution infrared spectrograph, SpeX, at the 3.0 m NASAInfrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars havewell-established MK spectral classifications and are mostly restrictedto near-solar metallicities. The sample not only contains the F, G, K,and M spectral types with luminosity classes between I and V, but alsoincludes some AGB, carbon, and S stars. In contrast to some otherspectral libraries, the continuum shape of the spectra is measured andpreserved in the data reduction process. The spectra are absolutely fluxcalibrated using the Two Micron All Sky Survey photometry. Potentialuses of the library include studying the physics of cool stars,classifying and studying embedded young clusters and optically obscuredregions of the Galaxy, evolutionary population synthesis to studyunresolved stellar populations in optically obscured regions of galaxiesand synthetic photometry. The library is available in digital form fromthe IRTF Web site.
| Reaching the boundary between stellar kinematic groups and very wide binaries. The Washington double stars with the widest angular separations
Aims: I look for and characterise very wide binaries and multiplesystems with projected physical separations larger than s = 0.1 pc,which is generally believed to be a sharp upper limit to thedistribution of wide binary semimajor axes. Methods: Iinvestigated in detail 30 Washington double stars with angularseparations of ? > 1000 arcsec. I discarded 23 of them asprobably unbound systems based on discordant astrometry, photometry,spectral types, and radial velocities. The remaining seven systems weresubject to a comprehensive data compilation and derivation(multi-wavelength photometry, heliocentric distance, multiplicity, age,mass, metallicity, membership in a young kinematic group). Results: Of the seven very wide systems, six have projected physicalseparations greater than the hypothetical cutoff at s = 0.1 pc and fourhave separations s > 0.2 pc. Although there are two systems in youngkinematic groups (namely HD 136654 and BD+32 2572 in the HyadesSupercluster, and AU Mic and AT Mic AB in the ? Pictoris movinggroup), there is no clear prevalence of young systems (? < 1 Ga)among these very wide binaries. Finally, I compare the binding energiesof the seven systems with those of other weakly bound systems in thefield.Appendix A is only available in electronic form at http://www.aanda.org
| Radio Interferometric Planet Search. I. First Constraints On Planetary Companions For Nearby, Low-Mass Stars From Radio Astrometry
Radio astrometry of nearby, low-mass stars has the potential to be apowerful tool for the discovery and characterization of planetarycompanions. We present a Very Large Array survey of 172 active M dwarfsat distances of less than 10 pc. Twenty-nine stars were detectedwith flux densities greater than 100 μJy. We observed seven ofthese stars with the Very Long Baseline Array at milliarcsecondresolution in three separate epochs. With a detection threshold of500 μJy in images of sensitivity 1σ ~ 100 μJy, wedetected three stars three times (GJ 65B, GJ 896A, GJ 4247), one startwice (GJ 285), and one star once (GJ 803). Two stars were undetected(GJ 412B and GJ 1224). For the four stars detected in multiple epochs,residuals from the optically determined apparent motions have anroot-mean-square deviation of ~0.2 milliarcseconds, consistent withstatistical noise limits. Combined with previous optical astrometry,these residuals provide acceleration upper limits that allow us toexclude planetary companions more massive than 3-6 M Jup at adistance of ~1 AU with a 99% confidence level.
| Discovery of the Widest Very Low Mass Field Binary
We present the discovery of the widest (~6700 AU) very low mass (VLM)field binary to date, found in a proper motion cross-match of the SloanDigital Sky Survey and the Two Micron All Sky Survey. Our follow-upJ-band imaging provides a 10 year baseline for measuring proper motions.Consequently, we are able to confirm the common proper motion of thepair to within 10 mas, implying a 99.5% probability of their physicalassociation. Near-infrared spectra of the components indicate spectraltypes of M6 ± 1 and M7 ± 1. The system resides at aspectroscopic distance of 105 ± 13 pc and has an angularseparation of 63farcs38 ± 0farcs05. We have used evolutionarymodels to infer component masses of 0.105+0.029-0.017 M sun and 0.091+0.010-0.007 M sun. The large separation and lowbinding energy of this system can provide constraints for formationmodels of VLM stars.
| Cool Subdwarf Investigations. II. Multiplicity
Cool subdwarfs of types K and M are the fainter counterparts of coolmain-sequence dwarfs that dominate the Galactic population. In thispaper, we present the results of an optical speckle survey of 62confirmed cool subdwarf systems within 60 pc. We have resolved twonew companions and confirmed two previously known companions withseparations 0farcs13-3farcs29. After including previously known widecompanions and all known spectroscopic binaries, we determine themultiplicity rate of cool subdwarfs to be 26% ± 6%, which issomewhat lower than comparable main-sequence stars that have amultiplicity rate of 37% ± 5%. We find that only 3% of the coolsubdwarfs surveyed have companions within 10 AU, 3% have companionsbetween 10 and 100 AU, and 14% have companions beyond 100 AU.The other 6% of cool subdwarfs are spectroscopic binaries. This is verydifferent from K/M dwarfs that have most companions (13%) at separationscloser than 10 AU. However, because a search for close binariesamong a large sample of nearby cool subdwarfs remains elusive, it is notyet settled whether or not the multiplicity rates are significantlydifferent. Nonetheless, several different observational results andtheories pointing to a possible dearth of subdwarf multiples arediscussed.
| χ Values for Blue Emission Lines in M Dwarfs
We compute values for blue emission lines in active M dwarfs. Usingflux-calibrated spectra from nearby M dwarfs and spectral M dwarftemplates from SDSS, we derive analytical relations that describe howthe values for the Ca II H and K as well as the Hβ, Hγ,Hδ, Hɛ, and H8 Balmer emission lines vary as a function ofspectral type and color. These derived values are important for numerousM dwarf studies where the intrinsic luminosity of emission lines cannotbe estimated due to uncertain distances and/or non-flux-calibratedspectra. We use these results to estimate the mean properties of blueemission lines in active-field M dwarfs from SDSS.
| A catalogue of chromospherically active binary stars (third edition)
The catalogue of chromospherically active binaries (CABs) has beenrevised and updated. With 203 new identifications, the number of CABstars is increased to 409. The catalogue is available in electronicformat where each system has a number of lines (suborders) with a uniqueorder number. The columns contain data of limited numbers of selectedcross references, comments to explain peculiarities and the position ofthe binarity in case it belongs to a multiple system, classicalidentifications (RS Canum Venaticorum, BY Draconis), brightness andcolours, photometric and spectroscopic data, a description of emissionfeatures (CaII H and K, Hα, ultraviolet, infrared),X-ray luminosity, radio flux, physical quantities and orbitalinformation, where each basic entry is referenced so users can go to theoriginal sources.
| An in-depth study of the pre-polar candidate WX Leonis Minoris
Optical photometry, spectroscopy, and XMM-Newton ultraviolet and X-rayobservations with full phase coverage are used for an in-depth study ofWX LMi, a system formerly termed a low-accretion rate polar. We find aconstant low-mass accretion rate, dot{M} ˜ 1.5 ×10-13 M_ȯ yr-1, a peculiar accretion geometrywith one spot not accessible via Roche-lobe overflow, a low temperatureof the white dwarf, T_eff< 8000 K, and the secondary very likelyRoche-lobe underfilling. All this lends further support to the changedview on WX LMi and related systems as detached binaries, i.e. magneticpost-common envelope binaries without significant Roche-lobe overflow inthe past. The transfer rate determined here is compatible with accretionfrom a stellar wind. We use cyclotron spectroscopy to determine theaccretion geometry and to constrain the plasma temperatures. Bothcyclotron spectroscopy and X-ray plasma diagnostics reveal low plasmatemperatures below 3 keV on both accretion spots. For the low-dot{m},high-B plasma at the accretion spots in WX LMi, cyclotron coolingdominates thermal plasma radiation in the optical. Optical spectroscopyand X-ray timing reveal atmospheric, chromospheric, and coronal activityat the saturation level on the dM4.5 secondary star.
| Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables
Context: . Barnes-Evans type relations provide an empirical relationshipbetween the surface brightness of stars and their color. They are widelyused for measuring the distances to stars of known radii, as theRoche-lobe filling secondaries in cataclysmic variables (CVs).Aims: . The calibration of the surface brightness of field dwarfs ofnear-solar metalicity with spectral types A0 to L8 covers all secondaryspectral types detectable in CVs and related objects and will aid in themeasurement of their distances. Methods: . The calibrations arebased on the radii of field dwarfs measured by the Infrared Flux Methodand by interferometry. Published photometry is used and homogenized tothe Cousins Rc and Ic and the CIT JHK photometricsystems. The narrow band surface brightness at 7500 Å is based onour own and published spectrophotometry. Care is taken to select thedwarfs for near-solar metalicity, appropriate to CVs, and to avoiderrors caused by unrecognized binarity. Results: . Relations areprovided for the surface brightness in V, R_c, I_c, J, H, K and in anarrow band at 7500 Å as functions of V-K and of spectral type.The method is tested with selected CVs for which independent informationon their distances is available. The observed spread in the radii ofearly M-dwarfs of given mass or luminosity and its influence on thedistance measurements of CVs is discussed. Conclusions: . As longas accurate trigonometric parallaxes are not routinely available for alarge number of CVs, the surface brightness method remains a reliablemeans of determining distances to CVs in which a spectral signature ofthe secondary star can be discerned.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| A 110 MG cyclotron harmonic in the optical spectrum of RX J1554.2+2721
We report the detection of a 110 MG cyclotron harmonic in theSDSS-spectrum of the magnetic cataclysmic variable (MCV) RXJ1554.2+2721. This feature was noted earlier by others but remainedunexplained. The wide spectral coverage of the new spectrum togetherwith the earlier detection of a Zeeman split Ly? line in a fieldof 144 MG makes the identification almost unambiguous. We propose toexplain the non-conforming UV-optical photospheric temperature of thewhite dwarf by an as yet unobserved cyclotron component in theultraviolet which also could significantly contribute to the overallenergy balance of the accretion process.
| The case and fate of HD 75767 - neutron star or supernova?
We report the discovery of the nearby (d= 24 pc) HD 75767 as an eightbillion year old quadruple system consisting of a distant M dwarf pair,HD 75767 C-D, in orbit around the known short-period P= 10.25 dsingle-lined binary HD 75767 A-B, the primary of which is a solar-like Gstar. On the reasonable assumption of synchronous orbital rotation aswell as rotational and orbital coplanarity for the inner pair, we getMB= 0.96Msolar for the unseen HD 75767 B, that is,the case of a massive white dwarf. Upon future evolution, mass transfertowards HD 75767 B will render the MA= 0.96MsolarG-type primary, now a turnoff star, to become a helium white dwarf ofMA~ 0.33Msolar. Depending on the mass accretionrate, accretion efficiency and composition of the massive white dwarf,this in turn may result in a collapse of HD 75767 B with the formationof a millisecond pulsar, i.e. the creation of a low-mass binary pulsar(LMBP), or, instead, a Type Ia supernova explosion and the completedisruption of HD 75767 B. Irrespective of which scenario applies, wepoint to the importance of the distant M dwarfs as the likely agents forthe formation of the inner, short-period HD 75767 A-B pair, and hence apath that particularly avoids preceding phases of common envelopeevolution.
| 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.
| 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.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| Meeting the Cool Neighbors. VIII. A Preliminary 20 Parsec Census from the NLTT Catalogue
Continuing our census of late-type dwarfs in the solar neighborhood, wepresent BVRI photometry and optical spectroscopy of 800 mid-type Mdwarfs drawn from the NLTT proper-motion catalog. The targets are takenboth from our own cross-referencing of the NLTT Catalogue and the 2MASSSecond Incremental Data Release, and from the revised NLTT compiledrecently by Salim & Gould. All are identified as nearby-starcandidates based on their location in the(mr,mr-Ks) diagram. Three hundred starsdiscussed here have previous astrometric, photometric, or spectroscopicobservations. We present new BVRI photometry for 101 stars, togetherwith low-resolution spectroscopy of a further 400 dwarfs. In total, wefind that 241 stars are within 20 pc of the Sun, while a further 70 liewithin 1 σ of our distance limit. Combining the present resultswith previous analyses, we have quantitative observations for 1910 ofthe 1913 candidates in our NLTT nearby-star samples. Eight hundredfifteen of those stars have distance estimates of 20 pc or less,including 312 additions to the local census. With our NLTT follow-upobservations essentially complete, we have searched the literature for Kand early-type M dwarfs within the sampling volume covered by the 2MASSsecond release. Comparing the resultant 20 pc census against predictednumbers, derived from the 8 pc luminosity function, shows an overalldeficit of ~20% for stellar systems and ~35% for individual stars.Almost all are likely to be fainter than MJ=7, and at leasthalf are probably as yet undiscovered companions of known nearby stars.Our results suggest that there are relatively few missing systems at thelowest luminosities, MJ>8.5. We discuss possible means ofidentifying the missing stars.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| Target Selection for SETI. II. Tycho-2 Dwarfs, Old Open Clusters, and the Nearest 100 Stars
We present the full target list and prioritization algorithm developedfor use by the microwave search for technological signals at the SETIInstitute. We have included the Catalog of Nearby Habitable StellarSystems (HabCat, described in Paper I), all of the nearest 100 stars and14 old open clusters. This is further augmented by a subset of theTycho-2 catalog based on reduced proper motions, and this larger catalogshould routinely provide at least three target stars within the largeprimary field of view of the Allen Telescope Array. The algorithm forprioritizing objects in the full target list includes scoring based onthe subset category of each target (i.e., HabCat, cluster, Tycho-2, ornearest 100), its distance (if known), and its proximity to the Sun onthe color-magnitude diagram.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| 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.
| A Near-Infrared, Wide-Field, Proper-Motion Search for Brown Dwarfs
A common proper-motion survey of M dwarf stars within 8 pc of the Sunreveals no new stellar or brown dwarf companions at wide separations(~100-1400 AU). This survey tests whether the brown dwarf ``desert''extends to large separations around M dwarf stars and further exploresthe census of the solar neighborhood. The sample includes 66 stars northof -30° and within 8 pc of the Sun. Existing first-epoch images arecompared with new J-band images of the same fields an average of 7 yrlater to reveal proper-motion companions within a ~4' radius of theprimary star. No new companions are detected to a J-band limitingmagnitude of ~16.5, corresponding to a companion mass of ~40 Jupitermasses for an assumed age of 5 Gyr at the mean distance of the objectsin the survey, 5.8 pc.
| 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.
| Stellar encounters with the solar system
We continue our search, based on Hipparcos data, for stars which haveencountered or will encounter the solar system(García-Sánchez et al. \cite{Garcia}). Hipparcos parallaxand proper motion data are combined with ground-based radial velocitymeasurements to obtain the trajectories of stars relative to the solarsystem. We have integrated all trajectories using three different modelsof the galactic potential: a local potential model, a global potentialmodel, and a perturbative potential model. The agreement between themodels is generally very good. The time period over which our search forclose passages is valid is about +/-10 Myr. Based on the Hipparcos data,we find a frequency of stellar encounters within one parsec of the Sunof 2.3 +/- 0.2 per Myr. However, we also find that the Hipparcos data isobservationally incomplete. By comparing the Hipparcos observations withthe stellar luminosity function for star systems within 50 pc of theSun, we estimate that only about one-fifth of the stars or star systemswere detected by Hipparcos. Correcting for this incompleteness, weobtain a value of 11.7 +/- 1.3 stellar encounters per Myr within one pcof the Sun. We examine the ability of two future missions, FAME andGAIA, to extend the search for past and future stellar encounters withthe Sun.
| A Coronagraphic Survey for Companions of Stars within 8 Parsecs
We present the technique and results of a survey of stars within 8 pc ofthe Sun with declinations δ>-35° (J2000.00). The survey,designed to find without color bias faint companions, consists ofoptical coronagraphic images of the 1' field of view centered on eachstar and infrared direct images with a 32" field of view. The imageswere obtained through the optical Gunn r and z filters and the infraredJ and K filters. The survey achieves sensitivities up to 4 absolutemagnitudes fainter than the prototype brown dwarf, Gliese 229B. However,this sensitivity varies with the seeing conditions, the intrinsicbrightness of the star observed, and the angular distance from the star.As a result, we tabulate sensitivity limits for each star in the survey.We used the criterion of common proper motion to distinguish companionsand to determine their luminosities. In addition to the brown dwarf Gl229B, we have identified six new stellar companions of the sample stars.Since the survey began, accurate trigonometric parallax measurements formost of the stars have become available. As a result, some of the starswe originally included should no longer be included in the 8 pc sample.In addition, the 8 pc sample is incomplete at the faint end of the mainsequence, complicating our calculation of the binary fraction of browndwarfs. We assess the sensitivity of the survey to stellar companionsand to brown dwarf companions of different masses and ages.
| X-ray emission from nearby M-dwarfs: the super-saturation phenomenon
A rotation rate and X-ray luminosity analysis is presented for rapidlyrotating single and binary M-dwarf systems. X-ray luminosities for themajority of both single and binary M-dwarf systems with periods below~=5-6d (equatorial velocities, Veq>~6kms-1) areconsistent with the current rotation-activity paradigm, and appear tosaturate at about 10-3 of the stellar bolometric luminosity.The single M-dwarf data show tentative evidence for the super-saturationphenomenon observed in some ultra-fast rotating(>~100kms-1) G- and K-dwarfs in the IC 2391, IC 2602 andAlpha Persei clusters. The IC 2391M star VXR60b is the least X-rayactive and most rapidly rotating of the short period(Prot<~2d) stars considered herein, with a period of0.212d and an X-ray activity level of about 1.5 sigma below the meanX-ray emission level for most of the single M-dwarf sample. For thisstar, and possibly one other, we cautiously believe that we haveidentified the first evidence of super-saturation in M-dwarfs. If we arewrong, we demonstrate that only M-dwarfs rotating close to theirbreak-up velocities are likely to exhibit the super-saturation effect atX-ray wavelengths. The M-dwarf X-ray data also show that there is noevidence for any difference in the X-ray behaviour between the singleand binary systems, because for the single stars, the meanlogLxLbol=-3.21+/-0.04(0.2<~Prot<~10.1d), whereas for the binary stars, themean logLxLbol=-3.19+/-0.10(0.8<~Prot<~10.4d). Furthermore, we show that extremelyX-ray active M-dwarfs exhibit a blue excess of about 0.1magnitudes inU-B compared with less active field M-dwarfs. Such an excess level iscomparable to that observed for extremely chromospherically activeM-dwarfs. Moreover, as is the case for M-dwarf Caii H and K activitylevels, there is an exclusion zone of X-ray activity between theextremely active M-dwarfs and the less active ones.
| A TiO study of the black hole binary GRO J0422+32 in a very low state
We present 53 simultaneous photometric (I band) and spectroscopic(6900-9500Å) observations of GRO J0422+32, taken during 1997December. From these we determine that J0422+32 was in its lowest stateyet observed, at I=20.44+/-0.08. Using relative spectrophotometry, weshow that it is possible to correct very accurately for telluricabsorption. Following this, we use the TiO bands at 7055 and 7589Åfor a radial velocity study and thereby obtain a semi-amplitude of378+/-16kms-1, which yieldsf(M)=1.191+/-0.021Msolar andq9.0+2.2-2.7, consistent with previousobservations. We further demonstrate that this little-explored method isvery powerful for such systems. We also determine a new orbitalephemeris of HJD=2450274.4156+/-0.0009+0.2121600+/-0.0000002E. We seesome evidence for an ellipsoidal modulation, from which we determine theorbital inclination of J0422+32 to be less than 45°. We thereforecalculate a minimum mass for the primary of 2.22Msolar,consistent with a black hole, but not necessarily the supermassive oneproposed recently (1997) by Beekman et al. We obtain an M4-5 spectraltype for the secondary star, and determine that the secondarycontributes 38+/-2per cent of the flux that we observe from J0422+32over the range 6950-8400Å. From this we calculate the distance tothe system to be 1.39+/-0.15kpc.
| Photometric modelling of starspots - I. A Barnes-Evans-like surface brightness-colour relation using (Ic-K)
In the first part of this work, the empirical correlation of stellarsurface brightness FV with (Ic-K) broad-bandcolour is investigated by using a sample of stars cooler than the Sun. Abilinear correlation is found to represent well the brightness of G, Kand M giant stars. The change in slope occurs at (Ic-K)~2.1or at about the transition from K to M spectral types. The samerelationship is also investigated for dwarf stars and found to bedistinctly different from that of the giants. The dwarf star correlationdiffers by an average of -0.4 in (Ic-K) or by a maximum inFV of ~-0.1, positioning it below that of the giants, withboth trends tending towards convergence for the hotter stars in oursample. The flux distribution derived from theFV-(Ic-K) relationship for the giant stars,together with that derived from an FV-(V-K) relationship andthe blackbody flux distribution, is then utilized to compute syntheticlight V and colour (V-R)c, (V-I)c and (V-K) curvesof cool spotted stars. We investigate the effects on the amplitudes ofthe curves by using these FV-colour relations and by assumingthe effective gravity of the spots to be lower than the gravity of theunspotted photosphere. We find that the amplitudes produced by using theFV-(Ic-K) relationship are larger than thoseproduced by the other two brightness correlations, meaning smallerand/or warmer spots.
| Young Low-Mass Stars and Brown Dwarfs in IC 348
I present new results from a continuing program to identify andcharacterize the low-mass stellar and substellar populations in theyoung cluster IC 348 (0.5-10 Myr). Optical spectroscopy has revealedyoung objects with spectral types as late as M8.25. The intrinsic J-Hand H-K colors of these sources are dwarflike, whereas the R-I and I-Jcolors appear intermediate between the colors of dwarfs and giants.Furthermore, the spectra from 6500 to 9500 Å are reproduced wellwith averages of standard dwarf and giant spectra, suggesting that suchaverages should be used in the classification of young late-typesources. An H-R diagram is constructed for the low-mass population in IC348 (K6-M8). The presumably coeval components of the young quadruplesystem GG Tau (White et al.) and the locus of stars in IC 348 are usedas empirical isochrones to test the theoretical evolutionary models. Thecalculations of Burrows et al. do not appear to be consistent with thedata at these earliest stages of stellar evolution. There is fairagreement between the data and the model isochrones of D'Antona &Mazzitelli, except near the hydrogen-burning limit. The agreement cannotbe improved by changing the conversion between spectral types andeffective temperatures. On the other hand, for the models of Baraffe etal., an adjustment of the temperature scale to progressively warmertemperatures at later M types, intermediate between dwarfs and giants,brings all components of GG Tau onto the same model isochrone and givesthe population of IC 348 a constant age and age spread as a function ofmass. When other observational constraints are considered, such as thedynamical masses of GM Aur, DM Tau, and GG Tau A, the models of Baraffeet al. are the most consistent with observations of young systems. Withcompatible temperature scales, the models of both D'Antona &Mazzitelli and Baraffe et al. suggest that the hydrogen-burning masslimit occurs near M6 at ages of <~10 Myr. Thus, several likely browndwarfs are discovered in this study of IC 348, with masses down to~20-30 MJ.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Arabaci |
| Sað Açýklýk: | 07h10m01.84s |
| Yükselim: | +38°31'46.1" |
| Görünürdeki Parlaklýk: | 11.68 |
| Uzaklýk: | 6.36 parsek |
| özdevim Sað Açýklýk: | -445.4 |
| özdevim Yükselim: | -943.3 |
| B-T magnitude: | 13.059 |
| V-T magnitude: | 11.794 |
Kataloglar ve belirtme:
|
