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Spectral synthesis analysis and radial velocity study of the northern F-, G- and K-type flare stars
In this paper, we present a study of the general physical and chemicalproperties and radial velocity monitoring of young active stars. Wederive temperatures, logg, [Fe/H], v sini and Rspec valuesfor eight stars. The detailed analysis reveals that the stars are nothomogeneous in their principal physical parameters or in the agedistribution. In 4/5, we found a periodic radial velocity signal whichoriginates in surface features; the fifth is surprisingly inactive andshows little variation.
| Frequency of Debris Disks around Solar-Type Stars: First Results from a Spitzer MIPS Survey
We have searched for infrared excesses around a well-defined sample of69 FGK main-sequence field stars. These stars were selected withoutregard to their age, metallicity, or any previous detection of IRexcess; they have a median age of ~4 Gyr. We have detected 70 μmexcesses around seven stars at the 3 σ confidence level. Thisextra emission is produced by cool material (<100 K) located beyond10 AU, well outside the ``habitable zones'' of these systems andconsistent with the presence of Kuiper Belt analogs with ~100 times moreemitting surface area than in our own planetary system. Only one star,HD 69830, shows excess emission at 24 μm, corresponding to dust withtemperatures >~300 K located inside of 1 AU. While debris disks withLdust/L*>=10-3 are rare around oldFGK stars, we find that the disk frequency increases from 2%+/-2% forLdust/L*>=10-4 to 12%+/-5% forLdust/L*>=10-5. This trend in thedisk luminosity distribution is consistent with the estimated dust inour solar system being within an order of magnitude greater or less thanthe typical level around similar nearby stars. Although there is nocorrelation of IR excess with metallicity or spectral type, there is aweak correlation with stellar age, with stars younger than a gigayearmore likely to have excess emission.
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| Stars within 15 Parsecs: Abundances for a Northern Sample
We present an abundance analysis for stars within 15 pc of the Sunlocated north of -30° declination. We have limited our abundancesample to absolute magnitudes brighter than +7.5 and have eliminatedseveral A stars in the local vicinity. Our final analysis list numbers114 stars. Unlike Allende Prieto et al. in their consideration of a verysimilar sample, we have enforced strict spectroscopic criteria in thedetermination of atmospheric parameters. Nevertheless, our results arevery similar to theirs. We determine the mean metallicity of the localregion to be <[Fe/H]>=-0.07 using all stars and -0.04 when interlopersfrom the thick disk are eliminated.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| The Brown Dwarf Desert at 75-1200 AU
We present results of a comprehensive infrared coronagraphic search forsubstellar companions to nearby stars. The research consisted of (1) a178-star survey at Steward and Lick observatories, with opticalfollow-up from Keck Observatory, capable of detecting companions withmasses greater than 30 MJ, and semimajor axes between about140 to 1200 AU; (2) a 102-star survey using the Keck Telescope, capableof detecting extrasolar brown dwarfs and planets typically more massivethan 10 MJ, with semimajor axes between about 75 and 300 AU.Only one brown dwarf companion was detected, and no planets. Thefrequency of brown dwarf companions to G, K, and M stars orbitingbetween 75 and 300 AU is measured to be 1%+/-1%, the most precisemeasurement of this quantity to date. The frequency of massive (greaterthan 30 MJ) brown dwarf companions at 120-1200 AU is found tobe f=0.7%+/-0.7%. The frequency of giant planet companions with massesbetween 5 and 10 MJ orbiting between 75 and 300 AU ismeasured here for the first time to be no more than ~3%. Together withother surveys that encompass a wide range of orbital separations, theseresults imply that substellar objects with masses between 12 and 75MJ form only rarely as companions to stars. Theories of starformation that could explain these data are only now beginning toemerge.
| The shortest period M dwarf eclipsing system BW3 V38. II. Determination of absolute elements
The spectroscopic data for the short-period (0.1984 deg) eclipsingbinary V38, discovered by the OGLE micro-lensing team in Baade's Windowfield BW3, are analyzed. Radial velocity curves are derived frommid-resolution spectra obtained with EMMI-NTT at ESO - La Silla, and asimultaneous solution of the existing light curve by OGLE and of the newradial velocity curves is obtained. The system is formed by almost twinM3e dwarf components that are very close, but not yet in contact. Thespectra of both dwarfs show signatures of the presence of strongchromospheres. Spectroscopy definitely confirms, therefore, what wassuggested on the basis of photometry: BW3 V38 is indeed a unique system,as no other similar binary with M components and in such a tight orbitis known. Within the limits posed by the relatively large errors, dueto the combined effect of system faintness and of the constraints onexposure time, the derived physical parameters seem to agree with therelations obtained from the other few known eclipsing binaries with latetype components (which indicate a discrepancy between the availableevolutionary models and the data at 10% level). A possibleexplanation is the presence of strong magnetic fields and fast rotation(that applies to the BW3 V38 case as well). A simple computation of thesystem secular evolution by angular momentum loss and spin-orbitsynchronization shows that the evolution of a system with M dwarfcomponents is rather slow, and indicates as well a possible reason whysystems similar to BW3 V38 are so rare.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| The 77th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1152 variable stars recently designated in the system ofthe General Catalogue ofVariable Stars. New designations are alsointroduced for two ``old'' variables whose corrected coordinates bringthem to other constellations.
| 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.
| The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs
We present the results of a new study on the relationship betweencoronal X-ray emission and stellar rotation in late-type main-sequencestars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0,including 110 field stars and 149 members of the Pleiades, Hyades, alphaPersei, IC 2602 and IC 2391 open clusters. All the stars have beenobserved with ROSAT, and most of them have photometrically-measuredrotation periods available. Our results confirm that two emissionregimes exist, one in which the rotation period is a good predictor ofthe total X-ray luminosity, and the other in which a constant saturatedX-ray to bolometric luminosity ratio is attained; we present aquantitative estimate of the critical rotation periods below which starsof different masses (or spectral types) enter the saturated regime. Inthis work we have also empirically derived a characteristic time scale,taue , which we have used to investigate the relationshipbetween the X-ray emission level and an X-ray-based Rossby numberRe = Prot/taue: we show that ourempirical time scale taue resembles the theoreticalconvective turnover time for 0.4 <~ M/Msun <~ 1.2, butit also has the same functional dependence on B-V asLbol-1/2 in the color range 0.5 <~ B-V <~1.5. Our results imply that - for non-saturated coronae - theLx - Prot relation is equivalent to theLx/Lbol vs. Re relation. Tables 1 and 2are only available in electronic form at \ http://www.edpsciences.org
| K dwarfs and the chemical evolution of the solar cylinder
K dwarfs have lifetimes older than the present age of the Galactic disc,and are thus ideal stars for investigating the chemical evolution of thedisc. We have developed several photometric metallicity indicators for Kdwarfs, based on a sample of accurate spectroscopic metallicities for 34disc and halo G and K dwarfs. The photometric metallicities lead us todevelop a metallicity index for K dwarfs based only on their position inthe colour-absolute-magnitude diagram. Metallicities have beendetermined for 431 single K dwarfs drawn from the Hipparcos catalogue,selecting the stars by absolute magnitude and removing multiple systems.The sample is essentially a complete reckoning of the metal content innearby K dwarfs. We use stellar isochrones to mark the stars by mass,and select a subset of 220 of the stars, which is complete within anarrow mass interval. We fit the data with a model of the chemicalevolution of the solar cylinder. We find that only a modest cosmicscatter is required to fit our age-metallicity relation. The modelassumes two main infall episodes for the formation of the halo-thickdisc and thin disc, respectively. The new data confirm that the solarneighbourhood formed on a long time-scale of the order of 7 Gyr.
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| A new look at dynamo cycle amplitudes
We explore the dependence of the amplitude of stellar dynamo cyclevariability (as seen in the Mount Wilson Ca II HK timeseries data) onother stellar parameters. We find that the fractional cycle amplitudeA_cyc (i.e. the ratio of the peak-to-peak variation to the average)decreases somewhat with mean activity, increases with decreasingeffective temperature, but is not correlated with inverse Rossby numberRo-1. We find that A_cyc increases with the ratio of cycleand rotational frequencies omega_cyc /Omega along two, nearly parallelbranches.
| K-band spectroscopy of the intermediate polar XY Ari
We present the K-band infrared spectrum of the intermediate polar XYAri. The spectrum confirms the cataclysmic binary nature of XY Ari,showing emission lines of Hei (λ2.0587μm) and the Brackett andPaschen series of Hi. The broad nature of these lines suggests an originin an accretion disc. The spectrum is strongly reddened by absorptionwithin the molecular cloud Lynds 1457 and shows prominent absorptionfeatures from the secondary star, from which we determine a spectraltype for the secondary of M0V. The secondary contributes 80+/-6 per centof the K-band light. We derive a visual extinction to XY Ari ofAv=11.5+/-0.3 and a distance of d=270+/-100pc, placing XY Aribehind the molecular cloud.
| A new look at the relationship between activity, dynamo number and Rossby number in late-type stars
The correlation between stellar activity, as measured by the indicatorΔRHK, and the Rossby number Ro in late-type stars isrevisited in light of recent developments in solar dynamo theory.Different stellar interior models, based on both mixing-length theoryand the full spectrum of turbulence, are used in order to see to whatextent the correlation of activity with Rossby number is modeldependent, or otherwise can be considered universal. Although we findsome modest model dependence, we find that the correlation of activitywith Rossby number is significantly better than with rotation periodalone for all the models we consider. Dynamo theory suggests thatactivity should scale with the dynamo number. A current model of thesolar dynamo, the so-called interface dynamo, proposes that theamplification of the toroidal magnetic field by differential rotation(the ω-effect) and the production of the poloidal magnetic fieldfrom toroidal by helical turbulence (the α-effect) take place indifferent, adjacent layers near the base of the convection zone. A newscale analysis based on the interface dynamo shows that the appropriatedynamo number does not depend on the Rossby number alone, but alsodepends on an additional dimensionless factor related to thedifferential rotation. This leads to a new interpretation of thecorrelation between activity and Rossby number, which in turn leads tosome conclusions about the magnitude of differential rotation in thedynamo layers of late-type main-sequence stars.
| A search for previously unrecognized metal-poor subdwarfs in the Hipparcos astrometric catalogue
We have identified 317 stars included in the Hipparcos astrometriccatalogue that have parallaxes measured to a precision of better than 15per cent, and the location of which in the(MV,(B-V)T) diagram implies a metallicitycomparable to or less than that of the intermediate-abundance globularcluster M5. We have undertaken an extensive literature search to locateStrömgren, Johnson/Cousins and Walraven photometry for over 120stars. In addition, we present new UBV(RI)C photometry of 201of these candidate halo stars, together with similar data for a further14 known metal-poor subdwarfs. These observations provide the firstextensive data set of RCIC photometry ofmetal-poor, main-sequence stars with well-determined trigonometricparallaxes. Finally, we have obtained intermediate-resolution opticalspectroscopy of 175 stars. 47 stars still lack sufficient supplementaryobservations for population classification; however, we are able toestimate abundances for 270 stars, or over 80 per cent of the sample.The overwhelming majority have near-solar abundance, with theirinclusion in the present sample stemming from errors in the colourslisted in the Hipparcos catalogue. Only 44 stars show consistentevidence of abundances below [Fe/H]=-1.0. Nine are additions to thesmall sample of metal-poor subdwarfs with accurate photometry. Weconsider briefly the implication of these results for clustermain-sequence fitting.
| Levels of coronal and chromospheric activity in late-type stars and various types of dynamo waves
We analyze the X-ray emission and chromospheric activity of late-type F,G, and K stars studied in the framework of the HK project. More powerfulcoronas are possessed by stars displaying irregular variations of theirchromospheric emission, while stars with cyclic activity arecharacterized by comparatively modest X-ray luminosities and ratios ofthe X-ray to bolometric luminosity L X/L bol. This indicates that thenature of processes associated with magnetic-field amplification in theconvective envelope changes appreciably in the transition from small tolarge dynamo numbers, directly affecting the character of the(α-Ω) dynamo. Due to the strong dependence of both thedynamo number and the Rossby number on the speed of axial rotation,earlier correlations found between various activity parameters and theRossby number are consistent with our conclusions. Our analysis makes itpossible to draw the first firm conclusions about the place of solaractivity among analogous processes developing in active late-type stars.
| 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.
| 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
| Infrared spectroscopy of cataclysmic variables - III. Dwarf novae below the period gap and nova-like variables
We present K-band spectra of the short-period dwarf novae YZ Cnc, LYHya, BK Lyn, T Leo, SW UMa and WZ Sge, the nova-like variables DW UMa,V1315 Aql, RW Tri, VY Scl, UU Aqr and GP Com, and a series of fielddwarf stars with spectral types ranging from K2 to M6. The spectra ofthe dwarf novae are dominated by emission lines of Hi and Hei. The largevelocity and equivalent widths of these lines, in conjunction with thefact that the lines are double-peaked in the highest inclinationsystems, indicate an accretion disc origin. In the case of YZ Cnc and TLeo, for which we obtained time-resolved data covering a completeorbital cycle, the emission lines show modulations in their equivalentwidths that are most probably associated with the bright spot (theregion where the gas stream collides with the accretion disc). There areno clear detections of the secondary star in any of the dwarf novaebelow the period gap, yielding upper limits of 10-30per cent for thecontribution of the secondary star to the observed K-band flux. Inconjunction with the K-band magnitudes of the dwarf novae, we use thederived secondary star contributions to calculate lower limits to thedistances to these systems. The spectra of the nova-like variables aredominated by broad, single-peaked emission lines of Hi and Hei - eventhe eclipsing systems we observed do not show the double-peaked profilespredicted by standard accretion disc theory. With the exception of RWTri, which exhibits Nai, Cai and 12CO absorption featuresconsistent with a M0V secondary contributing 65per cent of the observedK-band flux, we find no evidence for the secondary star in any of thenova-like variables. The implications of this result are discussed.
| An IUE Atlas of Pre-Main-Sequence Stars. II. Far-Ultraviolet Accretion Diagnostics in T Tauri Stars
We use our ultraviolet (UV) atlas of pre-main-sequence stars constructedfrom all useful, short-wavelength, low-resolution spectra in theInternational Ultraviolet Explorer (IUE) satellite Final Archive toanalyze the short-wavelength UV properties of 49 T Tauri stars (TTSs).We compare the line and continuum fluxes in these TTSs with each otherand with previously published parameters of these systems, includingrotation rate, infrared excess, and mass accretion rate. Theshort-wavelength continuum in the classical TTSs (CTTSs) appears tooriginate in a ~10,000 K optically thick plasma, while in the naked TTSs(NTTSs-stars without dusty disks) the continuum appears to originate inthe stellar atmosphere. We show that all of the TTSs in our sample liein the regime of ``saturated'' magnetic activity due to their smallRossby numbers. However, while some of the TTSs show emission linesurface fluxes consistent with this saturation level, many CTTSs showsignificantly stronger emission than predicted by saturation. In thesestars, the emission line luminosity in the high ionization lines presentin the spectrum between 1200 and 2000 Å correlates well with themass accretion rate. Therefore, we conclude that the bulk of theshort-wavelength emission seen in CTTSs results from accretion relatedprocesses and not from dynamo-driven magnetic activity. Using CTTSs withknown mass accretion rates, we calibrate the relationship between M andLC IV to derive the mass accretion rate for some CTTSs whichfor various reasons have never had their mass accretion rates measured.Finally, several of the CTTSs show strong emission from molecularhydrogen. While emission from H2 cannot form in gas at atemperature of ~105 K, the strength of the molecular hydrogenemission is nevertheless well correlated with all the other emissionsdisplayed in the IUE short-wavelength bandpass. This suggests that theH2 emission is in fact fluorescent emission pumped by theemission (likely Lyα) from hotter gas.
| Time Evolution of the Magnetic Activity Cycle Period. II. Results for an Expanded Stellar Sample
We further explore nondimensional relationships between the magneticdynamo cycle period P_cyc, the rotational period P_rot, the activitylevel (as observed in Ca II HK), and other stellar properties byexpanding the stellar sample studied in the first paper in this series.We do this by adding photometric and other cycles seen in active starsand the secondaries of CV systems and by selectively adding less certaincycles from the Mount Wilson HK survey; evolved stars, long-term HKtrends and secondary P_cyc are also considered. We confirm that moststars with age t>~0.1 Gyr occupy two roughly parallel branches,separated by a factor of ~6 in P_cyc, with the ratio of cycle androtational frequencies ω_cyc/Ω~Ro^-0.5, where Ro is theRossby number. Using the model of the first paper in this series, thisresult implies that the α effect increases with mean magneticfield (contrary to the traditional α-quenching concept) and thatα and ω_cyc decrease with t. Stars are not strictlysegregated onto one or the other branch by activity level, though thehigh-ω_cyc/Ω branch is primarily composed of inactive stars.The expanded data set suggests that for t>~1 Gyr, stars can havecycles on one or both branches, though among older stars, those withhigher (lower) mass tend to have their primary P_cyc on the lower(upper) ω_cyc/Ω branch. The Sun's ~80 yr Gleissberg cycleagrees with this scenario, suggesting that long-term activity ``trends''in many stars may be segments of long (P_cyc~50-100 yr) cycles not yetresolved by the data. Most very active stars (P_rot<3 days) appear tooccupy a new, third branch with ω_cyc/Ω~Ro^0.4. Many RS CVnvariables lie in a transition region between the two most activebranches. We compare our results with various models, discuss theirimplications for dynamo theory and evolution, and use them to predictP_cyc for three groups: stars with long-term HK trends, stars in youngopen clusters, and stars that may be in Maunder-like magnetic minima.
| Lifetime of Surface Features and Stellar Rotation: A Wavelet Time-Frequency Approach
We explore subtle variations in disk-integrated measurements spanning<~18 yr of stellar surface magnetism by using a newly developedtime-frequency gapped wavelet algorithm. We present results based onanalysis of the Mount Wilson Ca II H and K emission fluxes in four,magnetically active stars (HD 1835 [G2 V], HD 82885 [G8 IV-V], HD 149661[K0 V], and HD 190007 [K4 V]) and sensitivity tests using artificialdata. When the wavelet basis is appropriately modified (i.e., when thetime-frequency resolution is optimized), the results are consistent withthe existence of spatially localized and long-lived Ca II features(assumed here as activity regions that tend to recur in narrowlyconfined latitude bands), especially in HD 1835 and HD 82885. Thisinterpretation is based on the observed persistence of relativelylocalized Ca II wavelet power at a narrow range of rotationaltimescales, enduring as long as >~10 yr.
| 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.
| Stellar Encounters with the Oort Cloud Based on HIPPARCOS Data
We have combined Hipparcos proper-motion and parallax data for nearbystars with ground-based radial velocity measurements to find stars thatmay have passed (or will pass) close enough to the Sun to perturb theOort cloud. Close stellar encounters could deflect large numbers ofcomets into the inner solar system, which would increase the impacthazard at Earth. We find that the rate of close approaches by starsystems (single or multiple stars) within a distance D (in parsecs) fromthe Sun is given by N= 3.5D^2.12 Myr^-1, less than the number predictedby a simple stellar dynamics model. However, this value is clearly alower limit because of observational incompleteness in the Hipparcosdata set. One star, Gliese 710, is estimated to have a closest approachof less than 0.4 pc 1.4 Myr in the future, and several stars come within1 pc during a +/-10 Myr interval. We have performed dynamicalsimulations that show that none of the passing stars perturb the Oortcloud sufficiently to create a substantial increase in the long-periodcomet flux at Earth's orbit.
| Spectroscopy of Brown Dwarf Candidates in the rho Ophiuchi Molecular Core
We present an analysis of low-resolution infrared spectra for 20 browndwarf candidates in the core of the rho Ophiuchi molecular cloud.Fifteen of the sources display absorption-line spectra characteristic oflate-type stars. By comparing the depths of water vapor absorption bandsin our candidate objects with a grid of M dwarf standards, we derivespectral types that are independent of reddening. Optical spectroscopyof one brown dwarf candidate confirms the spectral type derived from thewater bands. Combining their spectral types with published near-infraredphotometry, effective temperatures and bolometric stellar luminositiesare derived, enabling us to place our sample on the Hertzsprung-Russelldiagram. We compare the positions of the brown dwarf candidates in thisdiagram with two sets of theoretical models in order to estimate theirmasses and ages. Considering uncertainties in placing the candidates inthe H-R diagram, six objects consistently lie in the brown dwarf regimeand another five objects lie in the transition region between stellarand substellar objects. The ages inferred for the sample are consistentwith those derived for higher mass association members. Three of thenewly identified brown dwarfs display infrared excesses at lambda=2.2mum, suggesting that young brown dwarfs can have active accretion disks.Comparing our mass estimates of the brown dwarf candidates with thosederived from photometric data alone suggests that spectroscopy is anessential component of investigations of the mass functions of youngclusters.
| Catalogue and bibliography of the UV Cet-type flare stars and related objects in the solar vicinity
This new catalogue of flare stars includes 463 objects. It containsastrometric, spectral and photometric data as well as information on theinfrared, radio and X-ray properties and general stellar parameters.From the total reference list of about 3400 articles, partial listsselected by objects, authors, key words and by any pairs of thesecriteria can be obtained Tables 1, 2 and 3 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.
| 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
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새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
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관측 및 측정 데이터
천체목록:
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INHABITED SKY
