תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
LP 261-75/2MASSW J09510549+3558021: A Young, Wide M4.5/L6 Binary
We present new observations of the LP 261-75/2MASSW J09510549+3558021M4.5/L6 wide, common-proper-motion binary system. Optical spectroscopyof LP 261-75 shows strong Hα emission, and the star may also beassociated with the ROSAT FSC source 1RXS J095102.7+355824. The derivedchromospheric and coronal activity levels are consistent with those ofPleiades stars of the same spectral type, and we infer an age of 100-200Myr for the system. In that case, theoretical models suggest that the L6dwarf 2MASSW J09510549+3558021 is a low-mass brown dwarf, withM~0.02+0.01-0.005 Msolar.Based partly on observations obtained with the Apache Point Observatory3.5 m telescope, which is owned and operated by the AstrophysicalResearch Consortium.
| 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.
| 2MASS J22521073-1730134: A Resolved L/T Binary at 14 Parsecs
NICMOS images of the nearby late-type L dwarf 2MASS J22521073-1730134(DENIS-P J225210.73-173013.4) show that it is a close double, separation0.13". The companion is 1.0 mag fainter than the primary in the F110Wpassband and 1.55 mag fainter in the F170M images. The latter passbandis centered on the 1.6 μm methane band. The small separation and theabsence of an optical counterpart suggest that the two sources areassociated; the relatively blue infrared color suggests that 2MASSJ22521073-1730134B is a T-type binary companion of the late-L primary.This hypothesis is supported by infrared spectroscopy, which shows weakmethane absorption in both the H and K passbands. We estimate a distanceof 13.6+3.8-2.6 pc to the system and a projectedlinear separation of 1.75 AU. We consider the potential for measuringdynamical masses of the two components.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS 5-26555.
| Multiplicity among Widely Separated Brown Dwarf Companions to Nearby Stars: Gliese 337CD
We present Lick Natural Guide Star Adaptive Optics observations of theL8 brown dwarf Gl 337C, which is resolved for the first time into twoclosely separated (0.53"+/-0.03"), nearly equal magnitude componentswith a Ks flux ratio of 0.93+/-0.10. Companionship isinferred from the absence of a 3.6" offset source in Two Micron All SkySurvey or photographic plate images, implying that the observedsecondary component is a comoving late-type dwarf. With a projectedseparation of 11 AU and nearly equal magnitude components, Gl 337CD hasproperties similar to those of other known companion and fieldsubstellar binaries. Its long orbital period (estimated to be ~140-180yr) inhibits short-term astrometric mass measurements, but the Gl 337CDsystem is ideal for studying the L-T transition at a fixed age andmetallicity. From a compilation of all known widely separated (>~100AU) stellar-brown dwarf multiple systems, we find evidence that thebinary fraction of brown dwarfs in these systems is notably higher thanthat of field brown dwarfs, 45+15-13% versus18+7-4% for analogous samples. We speculate onpossible reasons for this difference, including the possibility thatdynamic (ejection) interactions that may form such wide pairspreferentially retain binary secondaries because of their greatercombined mass and/or ability to absorb angular momentum.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Comparison of Nuclear Starburst Luminosities between Seyfert 1 and 2 Galaxies Based on Near-Infrared Spectroscopy
We report on infrared K- (2-2.5 μm) and L-band (2.8-4.1 μm) slitspectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 μm samples. Apolycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 μm inthe L band is primarily used to investigate nuclear star-formingactivity in these galaxies. The 3.3 μm PAH emission is detected in 10sources (=43%), demonstrating that detection of nuclear star formationin a significant fraction of Seyfert 1 galaxies is now feasible. For thePAH-detected nuclei, the surface brightness values of the PAH emissionare as high as those of typical starbursts, suggesting that the PAHemission probes the putative nuclear starbursts in the dusty tori aroundthe central active galactic nuclei (AGNs). The magnitudes of the nuclearstarbursts are quantitatively estimated from the observed 3.3 μm PAHemission luminosities. The estimated starburst luminosities relative tosome indicators of AGN powers in these Seyfert 1 galaxies are comparedwith 32 Seyfert 2 galaxies in the same samples that we have previouslyobserved. We find that there is no significant difference in nuclearstarburst to AGN luminosity ratios of Seyfert 1 and 2 galaxies and thatnuclear starburst luminosity positively correlates with AGN power inboth types. Our results favor a slightly modified AGN unification model,which predicts that nuclear starbursts occurring in the dusty tori ofSeyfert galaxies are physically connected to the central AGNs, ratherthan the classical unification paradigm, in which the dusty tori simplyhide the central AGNs of Seyfert 2 galaxies and reprocess AGN radiationas infrared dust emission in Seyfert galaxies. No significantdifferences in nuclear star formation properties are recognizablebetween Seyfert 1 galaxies in the CfA and 12 μm samples.
| An Assessment of Dynamical Mass Constraints on Pre-Main-Sequence Evolutionary Tracks
We have assembled a database of stars having both masses determined frommeasured orbital dynamics and sufficient spectral and photometricinformation for their placement on a theoretical H-R diagram. Our sampleconsists of 115 low-mass (M<2.0 Msolar) stars, 27pre-main-sequence and 88 main-sequence. We use a variety of availablepre-main-sequence evolutionary calculations to test the consistency ofpredicted stellar masses with dynamically determined masses. Despitesubstantial improvements in model physics over the past decade, largesystematic discrepancies still exist between empirical and theoreticallyderived masses. For main-sequence stars, all models considered predictmasses consistent with dynamical values above 1.2 Msolar andsome models predict consistent masses at solar or slightly lower masses,but no models predict consistent masses below 0.5 Msolar,with all models systematically underpredicting such low masses by5%-20%. The failure at low masses stems from the poor match of mostmodels to the empirical main sequence below temperatures of 3800 K, atwhich molecules become the dominant source of opacity and convection isthe dominant mode of energy transport. For the pre-main-sequence samplewe find similar trends. There is generally good agreement betweenpredicted and dynamical masses above 1.2 Msolar for allmodels. Below 1.2 Msolar and down to 0.3 Msolar(the lowest mass testable), most evolutionary models systematicallyunderpredict the dynamically determined masses by 10%-30%, on average,with the Lyon group models predicting marginally consistent masses inthe mean, although with large scatter. Over all mass ranges, theusefulness of dynamical mass constraints for pre-main-sequence stars isin many cases limited by the random errors caused by poorly determinedluminosities and especially temperatures of young stars. Adopting awarmer-than-dwarf temperature scale would help reconcile the systematicpre-main-sequence offset at the lowest masses, but the case for this isnot compelling, given the similar warm offset at older ages between mostsets of tracks and the empirical main sequence. Over all age ranges, thesystematic discrepancies between track-predicted and dynamicallydetermined masses appear to be dominated by inaccuracies in thetreatment of convection and in the adopted opacities.
| Speckle Observations of Binary Stars with the WIYN Telescope. IV. Differential Photometry
Five hundred seventy-six magnitude difference measures are presented for260 binary stars. These measures are derived from CCD-based speckleobservations taken at the WIYN 3.5 m telescope at Kitt Peak NationalObservatory during the period 1997-2000. Separations of the systemsrange from over 1" down to near the diffraction limit of the telescope.A study of multiple measures of the same targets indicates that themeasures have a typical uncertainty of better than 0.13 mag per 2 minuteobservation, and that multiple observations can be averaged to arrive atsmaller uncertainties. Results presented here are also compared, insofaras it is possible, with measures in the Hipparcos Catalogue and toprevious studies using adaptive optics. No major systematic errors wereidentified.The WIYN Observatory is a joint facility of the University ofWisconsin-Madison, Indiana University, Yale University, and the NationalOptical Astronomy Observatory.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| J - K DENIS photometry of a VLTI-selected sample of bright southern stars
We present a photometric survey of bright southern stars carried outusing the DENIS instrument equipped with attenuating filters. Theobservations were carried out not using the survey mode of DENIS, butwith individual target pointings. This project was stimulated by theneed to obtain near-infrared photometry of stars to be used in earlycommissioning observations of the ESO Very Large TelescopeInterferometer, and in particular to establish a network of brightcalibrator sources.We stress that near-infrared photometry is peculiarly lacking for manybright stars. These stars are saturated in 2MASS as well as in regularDENIS observations. The only other observations available for brightinfrared stars are those of the Two Micron Sky Survey dating from overthirty years ago. These were restricted to declinations above≈-30°, and thus cover only about half of the sky accessible fromthe VLTI site.We note that the final 2MASS data release includes photometry of brightstars, obtained by means of point-spread function fitting. However, thismethod only achieves about 30% accuracy, which is not sufficient formost applications.In this work, we present photometry for over 600 stars, each with atleast one and up to eight measurements, in the J and K filters. Typicalaccuracy is at the level of 0\fm05 and 0\fm04 in the J and K_s bands,respectively.Based on observations collected at the European Southern Observatory, LaSilla.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/413/1037
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Simultaneous Optical and Near-Infrared Spectropolarimetry of Type 2 Seyfert Galaxies
We present optical and near-infrared spectropolarimetry of the nuclei offour type 2 Seyfert galaxies, Mrk 463E, Mrk 1210, NGC 1068, and NGC4388. The data were obtained simultaneously, covering the wavelengthrange of 0.46-2.5 μm. We model the polarizations from twodust-scattering components: (1) scattering in dusty regions inionization cones and (2) scattering in a torus surrounding a type 1nucleus. The polarizations from electron scattering in the cones anddichroic absorption by aligned dust grains in the torus are alsocompared with the observations. We confirmed that a combination ofelectron and dust scattering in the ionization cones is the preferredmechanism for the optical continuum polarization. For the near-infrared,dichroic absorption by aligned grains can explain the continuumpolarization of Mrk 463E and Mrk 1210 as well as NGC 1068. Visualoptical depths of the order of 10-20 are estimated for dichroicabsorption in these nuclei. Dust scattering in the torus, whose grainsize distribution is assumed to be the same as in the Galactic diffuseinterstellar medium, cannot reproduce the observed spectral slope of thenear-infrared polarization and total nuclear flux simultaneously.However, this might only indicate that the grain size distribution inthe torus of active galactic nuclei (AGNs) is different, and dustscattering with moderate optical depth and dominated by large grainsmight provide a reasonable explanation for the near-infrared radiationfrom AGNs.
| 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.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| Multiplicity among solar-type stars. III. Statistical properties of the F7-K binaries with periods up to 10 years
Two CORAVEL radial velocity surveys - one among stars in the solarneighbourhood, the other in the Pleiades and in Praesepe - are merged toderive the statistical properties of main-sequence binaries withspectral types F7 to K and with periods up to 10 years. A sample of 89spectroscopic orbits was finally obtained. Among them, 52 relate to afree-of-bias selection of 405 stars (240 field stars and 165 clusterstars). The statistics corrected for selection effects yield thefollowing results: (1) No discrepancy is found between the binariesamong field stars and the binaries in open cluster. The distributions ofmass ratios, of periods, the period-eccentricity diagram and the binaryfrequencies are all within the same error intervals. (2) Thedistribution of mass ratios presents two maxima: a broad peak from q ~0.2 to q ~ 0.7, and a sharp peak for q > 0.8 (twins). Both arepresent among the early-type as well as among the late-type part of thesample, indicating a scale-free formation process. The peak for q >0.8 gradually decreases when long-period binaries are considered.Whatever their periods, the twins have eccentricities significantlylower than the other binaries, confirming a difference in the formationprocesses. Twins could be generated by in situ formation followed byaccretion from a gaseous envelope, whereas binaries with intermediatemass ratios could be formed at wide separations, but they are madecloser by migration led by interactions with a circumbinary disk. (3)The frequency of binaries with P<10 years is about 14%. (4) About0.3% of binaries are expected to appear as false positives in a planetsearch. Therefore, the frequency of planetary systems among stars ispresently 7+4-2%. The extension of thedistribution of mass ratios in the planetary range would result in avery sharp and very high peak, well separated from the binary stars withlow mass ratios. Based on photoelectric radial-velocity measurementscollected at Haute-Provence observatory and on observations made withthe ESA Hipparcos astrometry satellite.
| 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.
| Astrometry and Photometry for Cool Dwarfs and Brown Dwarfs
Trigonometric parallax determinations are presented for 28 late-typedwarfs and brown dwarfs, including eight M dwarfs with spectral typesbetween M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8,and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz*)and/or near-IR wavelengths (JHK) is presented for these objects and for24 additional late-type dwarfs. Supplemented with astrometry andphotometry from the literature, including 10 L and two T dwarfs withparallaxes established by association with bright, usually Hipparcosprimaries, this material forms the basis for studying variouscolor-color and color-absolute magnitude relations. The I-J color is agood predictor of absolute magnitude for late M and L dwarfs.MJ becomes monotonically fainter with I-J color and withspectral type through late L dwarfs, then brightens for early T dwarfs.The combination of z*JK colors alone can be used to classify late M,early L, and T dwarfs accurately, as well as to predict their absolutemagnitudes, but is less effective at untangling the scatter among mid-and late L dwarfs. The mean tangential velocity of these objects isfound to be slightly less than that for dM stars in the solarneighborhood, consistent with a sample with a mean age of several Gyr.Using colors to estimate bolometric corrections and models to estimatestellar radii, effective temperatures are derived. The latest L dwarfsare found to have Teff~1360 K.
| Speckle Observations of Binary Stars with the WIYN Telescope. II. Relative Astrometry Measures during 1998-2000
Five hundred twelve relative astrometry measures are presented for 253double stars, including 53 double stars discovered by Hipparcos. In 15cases, relative astrometry is reported for the first time for newlyconfirmed pairs. In addition, 20 high-quality nondetections ofcompanions are reported for stars suspected of being nonsingle byHipparcos. Observations were taken using a fast-readout CCD camerasystem at the WIYN 3.5 m telescope at Kitt Peak, Arizona. In comparingthese measures with ephemeris predictions for binary stars with verywell known orbits, we find that the measurement precision is better than3 mas in separation and 1° in position angle per individualobservation. Measurement precision and detection capabilities are fullydiscussed, and confirmed orbital motion is reported in four cases of theHipparcos double star discoveries. The WIYN Observatory is a jointfacility of the University of Wisconsin-Madison, Indiana University,Yale University, and the National Optical Astronomy Observatory.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| A revision of the solar neighbourhood metallicity distribution
We present a revised metallicity distribution of dwarfs in the solarneighbourhood. This distribution is centred on solar metallicity. Weshow that previous metallicity distributions, selected on the basis ofspectral type, are biased against stars with solar metallicity orhigher. A selection of G-dwarf stars is inherently biased againstmetal-rich stars and is not representative of the solar neighbourhoodmetallicity distribution. Using a sample selected on colour, we obtain adistribution where approximately half the stars in the solarneighbourhood have metallicities higher than [Fe/H]=0. The percentage ofmid-metal-poor stars ([Fe/H]<-0.5) is approximately 4 per cent, inagreement with present estimates of the thick disc. In order to have ametallicity distribution comparable to chemical evolution modelpredictions, we convert the star fraction to mass fraction, and showthat another bias against metal-rich stars affects dwarf metallicitydistributions, due to the colour (or spectral type) limits of thesamples. Reconsidering the corrections resulting from the increasingthickness of the stellar disc with age, we show that the simpleclosed-box model with no instantaneous recycling approximation gives areasonable fit to the observed distribution. Comparisons with theage-metallicity relation and abundance ratios suggest that the simpleclosed-box model may be a viable model of the chemical evolution of theGalaxy at solar radius.
| Three Wide-Separation L Dwarf Companions from the Two Micron All Sky Survey: Gliese 337C, Gliese 618.1B, and HD 89744B
We present two confirmed wide-separation L dwarf common proper-motioncompanions to nearby stars and one candidate identified from the TwoMicron All Sky Survey. Spectral types from optical spectroscopy are L0V, L2.5 V, and L8 V. Near-infrared, low-resolution spectra of thecompanions are provided, as well as a grid of known objects spanning M6V-T dwarfs to support spectral type assignment for these and future Ldwarfs in the z'JHK bands. Using published measurements, weestimate ages of the companions from physical properties of theprimaries. These crude ages allow us to estimate companion masses usingtheoretical low-mass star and brown dwarf evolutionary models. The new Ldwarfs in this paper bring the number of known wide-binary(Δ>=100 AU) L dwarf companions of nearby stars to nine. One ofthe L dwarfs is a wide-separation companion to the F7 IV-V+extrasolarplanet system HD 89744Ab. Portions of the data presented here wereobtained at the W. M. Keck Observatory, which is operated as ascientific partnership among the California Institute of Technology, theUniversity of California, and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W. M. Keck Foundation. Observations at thePalomar Observatory were made as part of a continuing collaborationbetween the California Institute of Technology and Cornell University.The 60 inch telescope at Palomar Mountain is jointly owned by theCalifornia Institute of Technology and the Carnegie Institution ofWashington.
| 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
| ICCD Speckle Observations of Binary Stars. XXIII. Measurements during 1982-1997 from Six Telescopes, with 14 New Orbits
We present 2017 observations of 1286 binary stars, observed by means ofspeckle interferometry using six telescopes over a 15 year period from1982 April to 1997 June. These measurements constitute the 23dinstallment in CHARA's speckle program at 2 to 4 m class telescopes andinclude the second major collection of measurements from the MountWilson 100 inch (2.5 m) Hooker Telescope. Orbital elements are alsopresented for 14 systems, seven of which have had no previouslypublished orbital analyses.
| Measurements of Binary Stars with the Starfire Optical Range Adaptive Optics Systems
To investigate the relative photometry produced by adaptive opticswithin the isoplanatic patch, we observed four binaries, 10 UMa, φUMa, 81 Cnc, and κ UMa, with adaptive optics using natural guidestars on the 3.5 m telescope, as well as one binary, β Del, withadaptive optics using a laser guide star on the 1.5 m telescope at theStarfire Optical Range. Iterative blind deconvolution (IBD) andparametric blind deconvolution (PBD) techniques were used to postprocessthe data, which produced consistent results for position angles,separations, and magnitude differences. We also conducted simulationsthat verify the agreement between IBD and PBD and compared theirmeasurements to truth data. From the results of both observations andsimulations, we conclude that adaptive optics is well suited forproviding not only position angles and separations for close binaries,but also good relative magnitudes without quadrant ambiguity. From theobservations, we find that the secondary of 81 Cnc (separation=0.12")appears to be 0.12 mag brighter than the primary at 0.85 μm and is,therefore, cooler. We also derive a new orbit for κ UMa(separation=0.067"). Our results for β Del (ADS 14073) havesignificantly improved precision compared with the 1998 analyses of thesame data by ten Brummelaar and colleagues and by Roberts, tenBrummelaar, and Mason.
| Resolved double-lined spectroscopic binaries: A neglected source of hypothesis-free parallaxes and stellar masses
Double-lined spectroscopic binaries, once visually resolved (VB-SB2),provide hypothesis-free orbital parallaxes and masses of bothcomponents. Unlike eclipsing-spectroscopic binaries for which manyaccurate masses are already known (Andersen 1991; Andersen 1997), thenumber of VB-SB2 remains rather small. This paper presents 40 suchsystems for which published visual observations and radial velocitiesallow a simultaneous adjustment of both data sets. The precision of theindividual masses as well as the evolution of that precision withrespect to the published precision is investigated.
| New binary stars discovered by lunar occultations. V
We present recent results from our ongoing lunar occultation program onbinary stars at the TIRGO and Calar Alto observatories. Observations arepresented here for a total of seventeen sources, the majority of whichare resolved for the first time. These include SAO94621, IRAS 18154-1900, SAO98345, SAO 98363, SAO93947, SAO 97319, SAO97437, SAO 94986, SAO94488, DO 11742, SAO78027 and SAO 79799. Two speckle binarieswere also observed, namely SAO 98427 and SAO110723; this latter is discovered to be a quadruple system.SAO 77810 is found to be a new triple system. In thecases of SAO 94621 and SAO 98363,we have recorded two occultation light curves each, at differentposition angles: this allowed us to derive actual position angles andseparations for these binary systems. We report an observation of theoccultation binary SAO 94961, for which we could notdetect the companion. Finally, we report also an observation of thewell-studied multiple system SAO 97645 (zetaCnc), which is discussed in detail in a separate paper. Basedon observations collected at TIRGO (Gornergrat, Switzerland), and atCalar Alto (Spain). TIRGO is operated by CNR-CAISMI Arcetri, Italy.Calar Alto is operated by the German-Spanish Astronomical Center.
| On the Nature of Low-Mass Companions to Solar-like Stars
Low-mass companions (mass <70 Jupiter masses) to solar-like stars arecompared statistically to stellar-mass secondaries in binaries ofsimilar primary spectral types and orbital scales, based largely on thesurvey of Duquennoy & Mayor. To within the limits imposed byobservational constraints, the orbital properties of these low-masscompanions (LMCs) are statistically indistinguishable from those ofbinary systems. In both populations, orbital periods (P), semimajor axes(a), angular momenta (L), and binding energies (U) are all distributedapproximately as f(x)~x-1 for x=P,a,L,U. In both populations,eccentricities are broadly distributed approximately asf(e)~e-0.5, with no significant correlation with otherorbital elements, apart from a marked circularization of close orbits.The distribution of LMC masses is approximately a power law with indexbetween -1 and -2 there is ambiguous evidence in the data for a massspectrum bimodality about approximately 10 Jupiter masses. In bothpopulations the joint distributions of mass with all orbital propertiesare largely scattergrams, with no statistically significantcorrelations. The overall statistical properties of LMCs are suggestiveof a common formation mechanism with binary star systems. The similarform of the distributions of all orbital dynamic quantities in bothpopulations may result from postformation dissipative orbital decay.
| 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.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Evolution of the ``efficiency'' when different data sets are combined into a unified adjustment
Using a sample of binaries which were observed visually (VB) as well asspectroscopically for both components (SB2), we show that the efficiencyalmost always increases when a combined VB-SB2 solution replaces onebased only on VB.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | סרטן |
| התרוממות ימנית: | 09h12m17.60s |
| סירוב: | +14°59'46.0" |
| גודל גלוי: | 6.51 |
| מרחק: | 20.479 פארסק |
| תנועה נכונה: | -522.8 |
| תנועה נכונה: | 245.8 |
| B-T magnitude: | 7.403 |
| V-T magnitude: | 6.565 |
קטלוגים וכינוים:
|
INHABITED SKY
