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Ýlgili Makaleler
The nearby population of M-dwarfs with WISE: a search for warm circumstellar dust
Context. Circumstellar debris disks are important because of theirconnection and interaction with planetary systems. An efficient way toidentify these systems is through their infrared excess. Most studies sofar concentrated on early-type or solar-type stars, but less effort hasgone into investigating M-dwarfs, which pose the additional problem thattheir mid-infrared colors are poorly known. Aims: We characterizethe infrared photometric behavior of M-dwarfs and search for infraredexcess in nearby M-dwarfs taken from the volume-limited RECONS sample,concentrating on mid-infrared wavelengths corresponding to warm(≳100 K) dust. We then check whether the population of theselate-type stars has a significantly different fraction of infraredexcess compared to earlier-type stars. Methods: We used datarecently released from the WISE satellite, which provides the mostsensitive mid-infrared all-sky survey to date. Our sample consists of 85sources encompassing 103 M-dwarfs. We compared this sample to Spitzerdata and matched it to the 2MASS catalog. We derived empirical infraredcolors from these data and discuss the intrinsic spread within thesecolors as well as the errors from WISE and 2MASS. Based on this, wechecked the stars for infrared excess and discuss the minimum excess wewould be able to detect. Results: Other than the M8.5 dwarf SCR1845-6357 A, where the mid-infrared excess is produced by a known T6companion, we detect no excesses in any of our sample stars. The limitswe derive for the 22 μm excess are slightly higher than the usualdetection limit of ~10-15% for Spitzer studies, but including the 12μm band and the [12]-[22] color in our analysis allows us to derivetight constraints for the fractional dust luminosityLdust/L⋆. Comparing our findings toearlier-type stars, we show that this result is consistent with theassumption that M-dwarf excesses in the mid-inrared is as frequent asexcesses around earlier-type stars. The low detection rate of0-0.0+1.3% we derive for our sample in that caseis an age effect. We also present a tentative excess detection at 22μm around the known cold debris disk M-dwarf AU Mic, which is notpart of our statistical sample. Conclusions: There is still noclear detection of a mid-infrared excess around any old (≳30 Myr)main-sequence M-dwarf. It is unclear whether this is due to a differentdust evolution around M-dwarfs compared to earlier-type stars or whetherthis is an age effect combined with the difficulties involved insearching M-dwarfs for infrared excesses. A significantly larger sampleof well-studied M-dwarfs is required to solve this question. At the sametime, their behavior at longer wavelengths, which are sensitive tocolder dust, needs further investigation.Appendix A is available in electronic form at http://www.aanda.org
| The Gaia spectrophotometric standard stars survey - I. Preliminary results
We describe two ground-based observing campaigns aimed at building agrid of approximately 200 spectrophotometric standard stars (SPSS), withan internal ≃1 per cent precision and tied to Vega within ≃3per cent, for the absolute flux calibration of data gathered by Gaia,the European Space Agency (ESA) astrometric mission. The criteria forthe selection and a list of candidates are presented, together with adescription of the survey strategy and the adopted data analysismethods. We also discuss a short list of notable rejected SPSScandidates and difficult cases, based on identification problems,literature discordant data, visual companions and variability. In fact,all candidates are also monitored for constancy (within ±5 mmag,approximately). In particular, we report on a CALSPEC standard, 1740346,that we found to be a δ Scuti variable during our short-termmonitoring (1-2 h) campaign. Based on data obtained within the Gaia DataProcessing and Analysis Consortium (DPAC) - and coordinated by theGround-based Observations for Gaia (GBOG) working group - at varioustelescopes; see acknowledgments.
| The Solar Neighborhood. XXVIII. The Multiplicity Fraction of Nearby Stars from 5 to 70 AU and the Brown Dwarf Desert around M Dwarfs
We report on our analysis of Hubble Space Telescope/NICMOS snapshothigh-resolution images of 255 stars in 201 systems within ~10 pc of theSun. Photometry was obtained through filters F110W, F180M, F207M, andF222M using NICMOS Camera 2. These filters were selected to permit clearidentification of cool brown dwarfs through methane contrast imaging.With a plate scale of 76 mas pixel-1, NICMOS can easilyresolve binaries with subarcsecond separations in the19farcs5×19farcs5 field of view. We previously reported fivecompanions to nearby M and L dwarfs from this search. No new companionswere discovered during the second phase of data analysis presented here,confirming that stellar/substellar binaries are rare. We establishmagnitude and separation limits for which companions can be ruled outfor each star in the sample, and then perform a comprehensivesensitivity and completeness analysis for the subsample of 138 M dwarfsin 126 systems. We calculate a multiplicity fraction of0.0+3.5 -0.0% for L companions to M dwarfsin the separation range of 5-70 AU, and 2.3+5.0-0.7% for L and T companions to M dwarfs in theseparation range of 10-70 AU. We also discuss trends in thecolor-magnitude diagrams using various color combinations and presentastrometry for 19 multiple systems in our sample. Considering theseresults and results from several other studies, we argue that theso-called brown dwarf desert extends to binary systems with low-massprimaries and is largely independent of primary mass, mass ratio, andseparations. While focusing on companion properties, we discuss how thequalitative agreement between observed companion mass functions andinitial mass functions suggests that the paucity of brown dwarfs ineither population may be due to a common cause and not due to binaryformation mechanisms.
| Further Defining Spectral Type "Y" and Exploring the Low-mass End of the Field Brown Dwarf Mass Function
We present the discovery of another seven Y dwarfs from the Wide-fieldInfrared Survey Explorer (WISE). Using these objects, as well as thefirst six WISE Y dwarf discoveries from Cushing et al., we furtherexplore the transition between spectral types T and Y. We find that theT/Y boundary roughly coincides with the spot where the J - Hcolors of brown dwarfs, as predicted by models, turn back to the red.Moreover, we use preliminary trigonometric parallax measurements to showthat the T/Y boundary may also correspond to the point at which theabsolute H (1.6 ?m) and W2 (4.6 ?m) magnitudes plummet. We usethese discoveries and their preliminary distances to place them in thelarger context of the solar neighborhood. We present a table thatupdates the entire stellar and substellar constituency within 8 pc ofthe Sun, and we show that the current census has hydrogen-burning starsoutnumbering brown dwarfs by roughly a factor of six. This factor willdecrease with time as more brown dwarfs are identified within thisvolume, but unless there is a vast reservoir of cold brown dwarfsinvisible to WISE, the final space density of brown dwarfs is stillexpected to fall well below that of stars. We also use these new Y dwarfdiscoveries, along with newly discovered T dwarfs from WISE, toinvestigate the field substellar mass function. We find that the overallspace density of late-T and early-Y dwarfs matches that from simulationsdescribing the mass function as a power law with slope -0.5
| Metallicity and Temperature Indicators in M Dwarf K-band Spectra: Testing New and Updated Calibrations with Observations of 133 Solar Neighborhood M Dwarfs
We present K-band spectra for 133 nearby (d < 33 ps) M dwarfs,including 18 M dwarfs with reliable metallicity estimates (as inferredfrom an FGK type companion), 11 M dwarf planet hosts, more than 2/3 ofthe M dwarfs in the northern 8 pc sample, and several M dwarfs from theLSPM catalog. From these spectra, we measure equivalent widths of the Caand Na lines, and a spectral index quantifying the absorption due toH2O opacity (the H2O-K2 index). Using empiricalspectral type standards and synthetic models, we calibrate theH2O-K2 index as an indicator of an M dwarf's spectral typeand effective temperature. We also present a revised relationship thatestimates the [Fe/H] and [M/H] metallicities of M dwarfs from their NaI, Ca I, and H2O-K2 measurements. Comparisons to modelatmosphere provide a qualitative validation of our approach, but alsoreveal an overall offset between the atomic line strengths predicted bymodels as compared to actual observations. Our metallicity estimatesalso reproduce expected correlations with Galactic space motions andH? emission line strengths, and return statistically identicalmetallicities for M dwarfs within a common multiple system. Finally, wefind systematic residuals between our H2O-based spectraltypes and those derived from optical spectral features with previouslyknown sensitivity to stellar metallicity, such as TiO, and identify theCaH1 index as a promising optical index for diagnosing the metallicitiesof near-solar M dwarfs.
| Refined Metallicity Indices for M Dwarfs Using the SLoWPoKES Catalog of Wide, Low-mass Binaries
We report the results from spectroscopic observations of 113 ultra-wide,low-mass binary systems, largely composed of M0-M3 dwarfs, from theSLoWPoKES catalog of common proper motion pairs identified in the SloanDigital Sky Survey. Radial velocities of each binary member were used toconfirm that they are comoving and, consequently, to further validatethe high fidelity of the SLoWPoKES catalog. Ten stars appear to bespectroscopic binaries based on broad or split spectral features,supporting previous findings that wide binaries are likely to behierarchical systems. We measured the Hα equivalent width of thestars in our sample and found that components of 81% of the observedpairs have similar Hα levels. The difference in Hαequivalent width among components with similar masses was smaller thanthe range of Hα variability for individual objects. We confirmthat the Lépine et al. ζ-index traces iso-metallicity locifor most of our sample of M dwarfs. However, we find a small systematicbias in ζ, especially in the early-type M dwarfs. We use our sampleto recalibrate the definition of ζ. While representing a smallchange in the definition, the new ζ is a significantly betterpredictor of iso-metallicity for the higher-mass M dwarfs.
| Effect of different stellar galactic environments on planetary discs - I. The solar neighbourhood and the birth cloud of the Sun
We have computed trajectories, distances and times of closest approachesto the Sun by stars in the solar neighbourhood with known position,radial velocity and proper motions. For this purpose, we have used afull potential model of the Galaxy that reproduces the local z-force,the Oort constants, the local escape velocity and the rotation curve ofthe Galaxy. From our sample, we constructed initial conditions, withinobservational uncertainties, with a Monte Carlo scheme for the 12 mostsuspicious candidates because of their small tangential motion. We findthat the star Gliese 710 will have the closest approach to the Sun, witha distance of approximately 0.34 pc in 1.36 Myr in the future. We showthat the effect of a flyby with the characteristics of Gliese 710 on a100 au test particle disc representing the Solar system is negligible.However, since there is a lack of 6D data for a large percentage ofstars in the solar neighbourhood, closer approaches may exist. Wecalculate parameters of passing stars that would cause notable effectson the solar disc. Regarding the birth cloud of the Sun, we performedexperiments to reproduce roughly the observed orbital parameters such aseccentricities and inclinations of the Kuiper belt. It is now known thatin Galactic environments, such as stellar formation regions, the stellardensities of new born stars are high enough to produce close encounterswithin 200 au. Moreover, in these Galactic environments, the velocitydispersion is relatively low, typically ?˜ 1-3 kms-1. We find that with a velocity dispersion of ˜1 kms-1 and an approach distance of about 150 au, typical ofthese regions, we obtain approximately the eccentricities andinclinations seen in the current Solar system. Simple analyticalcalculations of stellar encounters effects on the Oort Cloud arepresented.
| The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog. II. Statistical Parallax Analysis
We present a statistical parallax analysis of low-mass dwarfs from theSloan Digital Sky Survey. We calculate absolute r-band magnitudes(Mr ) as a function of color and spectral type andinvestigate changes in Mr with location in the Milky Way. Wefind that magnetically active M dwarfs are intrinsically brighter inMr than their inactive counterparts at the same color orspectral type. Metallicity, as traced by the proxy ?, also affectsMr , with metal-poor stars having fainter absolute magnitudesthan higher metallicity M dwarfs at the same color or spectral type.Additionally, we measure the velocity ellipsoid and solar reflex motionfor each subsample of M dwarfs. We find good agreement between ourmeasured solar peculiar motion and previous results for similarpopulations, as well as some evidence for differing motions of early andlate M-type populations in U and W velocities that cannot be attributedto asymmetric drift. The reflex solar motion and the velocitydispersions both show that younger populations, as traced by magneticactivity and location near the Galactic plane, have experienced lessdynamical heating. We introduce a new parameter, the independentposition altitude (IPA), to investigate populations as a function ofvertical height from the Galactic plane. M dwarfs at all types exhibitan increase in velocity dispersion when analyzed in comparable IPAsubgroups.
| Mt. Suhora Survey - Searching for Pulsating M Dwarfs. I
We present the first results of our M dwarf survey in search for stellarpulsation in low mass main sequence stars. Theoretical calculationspredict that ? mechanism might drive a fundamental radial modein these stars and therefore pulsations could be observedphotometrically. Although M dwarfs are known for their flare and spotactivity they have not yet been the subject of dedicated time-seriessurveys for pulsation. In this work we include the light curves andamplitude spectra of 46 M dwarfs, which have been observed during thefirst two years of our survey. We did not detect any pulsations yet. Asa by-product of our search, we describe the light curves of some flare Mdwarfs. The survey will last for two more years and during that periodmore than a hundred of M0-M4 type main sequence stars will be observed.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| Chromospheric Activity and Jitter Measurements for 2630 Stars on the California Planet Search
We present time series measurements of chromospheric activity for morethan 2600 main-sequence and subgiant stars on the California PlanetSearch (CPS) program with spectral types ranging from about F5V to M4Vfor main-sequence stars and from G0IV to about K5IV for subgiants. Thelarge data set of more than 44,000 spectra allows us to identify anempirical baseline floor for chromospheric activity as a function ofcolor and height above the main sequence. We define ?S as anexcess in emission in the Ca II H and K lines above the baselineactivity floor and define radial velocity jitter as a function of?S and B - V for main-sequence and subgiant stars. Although thejitter for any individual star can always exceed the baseline level, wefind that K dwarfs have the lowest level of jitter. The lack ofcorrelation between observed jitter and chromospheric activity in Kdwarfs suggests that the observed jitter is dominated by instrumental oranalysis errors and not astrophysical noise sources. Thus, given thelong-term precision for the CPS program, radial velocities are notcorrelated with astrophysical noise for chromospherically quiet K dwarfstars, making these stars particularly well suited for the highestprecision Doppler surveys. Chromospherically quiet F and G dwarfs andsubgiants exhibit higher baseline levels of astrophysical jitter than Kdwarfs. Despite the fact that the rms in Doppler velocities iscorrelated with the mean chromospheric activity, it is rare to seeone-to-one correlations between the individual time series activity andDoppler measurements, diminishing the prospects for correctingactivity-induced velocity variations in F and G dwarfs.Based on observations obtained at the Keck Observatory and LickObservatory, which are operated by the University of California.
| Accurate Coordinates and 2MASS Cross Identifications for (Almost) All Gliese Catalog Star
We provide precise J2000, epoch 2000 coordinates, andcross-identifications to sources in the 2MASS Point Source Catalog fornearly all stars in the Gliese, Gliese-Jahreiss, and Woolley catalogs ofnearby stars. The only Gliese objects where we were not successful aretwo Gliese sources that are actually QSOs; two proposed companions tobrighter stars, which we believe do not exist; four stars included inone of the catalogs but identified there as only optical companions; oneprobable plate flaw; and two stars that simply remain unrecovered. Forthe 4251 recovered stars, 2693 have coordinates based on Hipparcospositions, 1549 have coordinates based on 2MASS data, and 9 havepositions from other astrometric sources. All positions have beencalculated at epoch 2000 using proper motions from the literature, whichare also given here.
| Constraints on Long-period Planets from an L'- and M-band Survey of Nearby Sun-like Stars: Modeling Results
We have carried out an L'- and M-band adaptive optics (AO) extrasolarplanet imaging survey of 54 nearby, Sun-like stars using the Clio cameraat the MMT. Our survey concentrates more strongly than all others todate on very nearby F, G, and K stars, in that we have prioritizedproximity higher than youth. Our survey is also the first to includeextensive observations in the M band, which supplemented the primary L'observations. These longer-wavelength bands are most useful for verynearby systems in which low-temperature planets with red IR colors(i.e., H - L', H - M) could be detected. The survey detectedno planets, but set interesting limits on planets and brown dwarfs inthe star systems we investigated. We have interpreted our null result bymeans of extensive Monte Carlo simulations and constrained thedistributions of extrasolar planets in mass M and semimajor axis a. Ifplanets are distributed according to a power law with dN vprop M? a ? dMda, normalized to be consistentwith radial velocity (RV) statistics, we find that a distribution with? = -1.1 and ? = -0.46, truncated at 110 AU, isruled out at the 90% confidence level. These particular values of? and ? are significant because they represent the mostplanet-rich case consistent with current statistics from RVobservations. With 90% confidence no more than 8.1% of stars like thosein our survey have systems with three widely spaced, massive planetslike the A star HR 8799. Our observations show that giant planets inlong-period orbits around Sun-like stars are rare, confirming theresults of shorter-wavelength surveys and increasing the robustness ofthe conclusion.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| Constraints on Long-period Planets from an L'- and M-band Survey of Nearby Sun-like Stars: Observations
We present the observational results of an L'- and M-band adaptiveoptics imaging survey of 54 nearby, Sun-like stars for extrasolarplanets, carried out using the Clio camera on the MMT. We haveconcentrated more strongly than all other planet-imaging surveys to dateon very nearby F, G, and K stars, prioritizing stellar proximity higherthan youth. Ours is also the first survey to include extensiveobservations in the M band, which supplement the primary L'observations. Models predict much better planet/star flux ratios at theL' and M bands than at more commonly used shorter wavelengths (i.e., theH band). We have carried out extensive blind simulations with fakeplanets inserted into the raw data to verify our sensitivity, and toestablish a definitive relationship between source significance in? and survey completeness. We find 97% confident-detectioncompleteness for 10? sources, but only 46% for 7?sources—raising concerns about the standard procedure of assuminghigh completeness at 5?, and demonstrating that blind sensitivitytests to establish the significance-completeness relation are animportant analysis step for all planet-imaging surveys. We discovered apreviously unknown ~0.15 M sun stellar companion to the F9star GJ 3876, at a projected separation of about 80 AU. Twelveadditional candidate faint companions are detected around other stars.Of these, 11 are confirmed to be background stars and one is apreviously known brown dwarf. We obtained sensitivity to planetary-massobjects around almost all of our target stars, with sensitivity toobjects below 3 M Jup in the best cases. Constraints onplanet populations based on this null result are presented in ourModeling Results paper.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood
Debris discs - analogous to the asteroid and Kuiper-Edgeworth belts inthe Solar system - have so far mostly been identified and studied inthermal emission shortward of 100?m. The Herschel space observatoryand the Submillimetre Common-User Bolometer Array-2 (SCUBA-2) camera onthe James Clerk Maxwell Telescope will allow efficient photometricsurveying at 70 to 850?m, which allows for the detection of coolerdiscs not yet discovered, and the measurement of disc masses andtemperatures when combined with shorter wavelength photometry. TheSCUBA-2 Unbiased Nearby Stars survey (SUNS) and the Disc Emission via aBias-free Reconnaissance in the Infrared/Submillimetre (DEBRIS) HerschelOpen Time Key Project are complementary legacy surveys observing samplesof ~500 nearby stellar systems. To maximize the legacy value of thesesurveys, great care has gone into the target selection process. Thispaper describes the target selection process and presents the targetlists of these two surveys.
| Rotation and Magnetic Activity in a Sample of M-Dwarfs
We have analyzed the rotational broadening and chromospheric activity ina sample of 123 M-dwarfs, using spectra taken at the W.M. KeckObservatory as part of the California Planet Search program. We findthat only seven of these stars are rotating more rapidly than ourdetection threshold of v sin i ? 2.5 km s-1.Rotation appears to be more common in stars later than M3 than in theM0-M2.5 mass range: we estimate that less than 10% of early-M stars aredetectably rotating, whereas roughly a third of those later than M4 showsigns of rotation. These findings lend support to the view thatrotational braking becomes less effective in fully convective stars. Bymeasuring the equivalent widths of the Ca II H and K lines for the starsin our sample, and converting these to approximate L Ca/Lbol measurements, we also provide constraints on theconnection between rotation and magnetic activity. Measurable rotationis a sufficient, but not necessary condition for activity in our sample:all the detectable rotators show strong Ca II emission, but so too do asmall number of non-rotating stars, which we presume may lie at highinclination angles relative to our line of sight. Our data areconsistent with a "saturation-type" rotation-activity relationship, withactivity roughly independent of rotation above a threshold velocity ofless than 6 km s-1. We also find weak evidence for a"gap" in L Ca/L bol between a highly activepopulation of stars, which typically are detected as rotators, andanother much less active group.
| Search for cold debris disks around M-dwarfs. II
Although 70% of the stars in the Galaxy are M-dwarfs, thermal emissionsearches for cold debris disks have been conducted mostly for A-type andsolar-type stars. We report on new ?=1.2 mm continuumobservations of thirty M-dwarfs, using the MAMBO-2 bolometer arraycamera at the IRAM 30 m telescope. For a statistical analysis, wecombine these data with our prior SCUBA and MAMBO-2 observations of 20other M-dwarfs. Our sample consists of M-dwarfs in moving groups, withrelatively young ages, and of nearby M-dwarfs with unknown ages. Onlyone cold debris disk (GJ842.2) was detected significantly. We comparethe implied disk abundance constraints with those found in twocomparable submillimeter surveys of 10 to 190 Myr old A- and FGK-typestars. For the 19 youngest (ages less than 200 Myr) M-dwarfs in oursample, we derive a cold disk fraction of5.3+10.5-5.0%, compared to15+11.5-11.5% for FGK-stars and22+33-20% for A-stars. Hence, for this age group,there is an apparent trend of fewer cold disks for later stellar types.Although its statistical significance is marginal, this trend isstrengthened by the deeper observations of our M-dwarf sample. We derivea cold disk fraction of <10% for the older (likely a few Gyr)M-dwarfs in our sample. Finally, although inconclusively related to adebris disk, we present the complex millimeter structure found aroundthe position of the M 1.5 dwarf GJ526 in our sample.
| Rotational Velocities for M Dwarfs
We present spectroscopic rotation velocities (v sin i) for 56 M dwarfstars using high-resolution Hobby-Eberly Telescope High ResolutionSpectrograph red spectroscopy. In addition, we have also determinedphotometric effective temperatures, masses, and metallicities ([Fe/H])for some stars observed here and in the literature where we couldacquire accurate parallax measurements and relevant photometry. We haveincreased the number of known v sin i values for mid M stars by around80% and can confirm a weakly increasing rotation velocity withdecreasing effective temperature. Our sample of v sin is peak at lowvelocities (~3 km s-1). We find a change in therotational velocity distribution between early M and late M stars, whichis likely due to the changing field topology between partially and fullyconvective stars. There is also a possible further change in therotational distribution toward the late M dwarfs where dust begins toplay a role in the stellar atmospheres. We also link v sin i to age andshow how it can be used to provide mid-M star age limits. When allliterature velocities for M dwarfs are added to our sample, there are198 with v sin i <= 10 km s-1 and 124 in themid-to-late M star regime (M3.0-M9.5) where measuring precision opticalradial velocities is difficult. In addition, we also search the spectrafor any significant Hα emission or absorption. Forty three percentwere found to exhibit such emission and could represent young, activeobjects with high levels of radial-velocity noise. We acquired twoepochs of spectra for the star GJ1253 spread by almost one month and theHα profile changed from showing no clear signs of emission, toexhibiting a clear emission peak. Four stars in our sample appear to below-mass binaries (GJ1080, GJ3129, Gl802, and LHS3080), with both GJ3129and Gl802 exhibiting double Hα emission features. The tablespresented here will aid any future M star planet search target selectionto extract stars with low v sin i.Based on observations obtained with the Hobby-Eberly Telescope, which isa joint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| 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.
| The effect of activity on stellar temperatures and radii
Context: Recent analyses of low-mass eclipsing binary stars haveunveiled a significant disagreement between the observations andpredictions of stellar structure models. Results show that theoreticalmodels underestimate the radii and overestimate the effectivetemperatures of low-mass stars but yield luminosities that accord withobservations. A hypothesis based upon the effects of stellar activitywas put forward to explain the discrepancies. Aims: In this paper westudy the existence of the same trend in single active stars and providea consistent scenario to explain systematic differences between activeand inactive stars in the H-R diagram reported earlier. Methods: Theanalysis is done using single field stars of spectral types late-K and Mand computing their bolometric magnitudes and temperatures throughinfrared colours and spectral indices. The properties of the stars insamples of active and inactive stars are compared statistically toreveal systematic differences. Results: After accounting for a numberof possible bias effects, active stars are shown to be cooler thaninactive stars of similar luminosity therefore implying a larger radiusas well, in proportions that are in excellent agreement with those foundfrom eclipsing binaries. Conclusions: The present results generalisethe existence of strong radius and temperature dependences on stellaractivity to the entire population of low-mass stars, regardless of theirmembership in close binary systems.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/478/507
| Search for cold debris disks around M-dwarfs
Debris disks are believed to be related to planetesimals left overaround stars after planet formation has ceased. The frequency of debrisdisks around M-dwarfs which account for 70% of the stars in the Galaxyis unknown while constrains have already been found for A- to K-typestars. We have searched for cold debris disks around 32 field M-dwarfsby conducting observations at λ = 850~μm with the SCUBAbolometer array camera at the JCMT and at λ = 1.2 mm with theMAMBO array at the IRAM 30-m telescopes. This is the first survey of alarge sample of M-dwarfs conducted to provide statistical constraints ondebris disks around this type of stars. We have detected a new debrisdisk around the M0.5 dwarf GJ 842.2 at λ = 850~μm, providingevidence for cold dust at large distance from this star (~300 AU). Bycombining the results of our survey with the ones of Liu et al. (2004),we estimate for the first time the detection rate of cold debris disksaround field M-dwarfs with ages between 20 and 200 Myr. This detectionrate is 13+6-8% and is consistent with thedetection rate of cold debris disks (9-23%) around A- to K-type mainsequence stars of the same age. This is an indication that cold disksmay be equally prevalent across stellar spectral types.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Kinematic structure of the corona of the Ursa Major flow found using proper motions and radial velocities of single stars
Aims.We study the kinematic structure of peripheral areas of the UrsaMajoris stream (Sirius supercluster). Methods.We use diagrams ofindividual stellar apexes developed by us and the classical technique ofproper motion diagrams generalized to a star sample distributed over thesky. Results.Out of 128 cluster members we have identified threecorona (sub)structures comprised of 13, 13 and 8 stars. Thesubstructures have a spatial extension comparable to the size of thecorona. Kinematically, these groups are distinguished by their propermotions, radial velocities and by the directions of their spatialmotion. Coordinates of their apexes significantly differ from those ofthe apexes of the stream and its nucleus. Our analysis shows that thesesubstructures do not belong to known kinematic groups, such as Hyades orCastor. We find kinematic inhomogeneity of the corona of the UMa stream.
| Ca II H and K Chromospheric Emission Lines in Late-K and M Dwarfs
We have measured the profiles of the Ca II H and K chromosphericemission lines in 147 main-sequence stars of spectral type M5-K7 (masses0.30-0.55 Msolar) using multiple high-resolution spectraobtained during 6 years with the HIRES spectrometer on the Keck Itelescope. Remarkably, the average FWHM, equivalent widths, and lineluminosities of Ca II H and K increase by a factor of 3 with increasingstellar mass over this small range of stellar masses. We fit the Ca II Hand K lines with a double-Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of thesample stars display a central absorption that is typically redshiftedby ~0.1 km s-1 relative to the emission. This implies thatthe higher level, lower density chromospheric material has a smalleroutward velocity (or higher inward velocity) by 0.1 km s-1than the lower level material in the chromosphere, but the nature ofthis velocity gradient remains unknown. The FWHM of the Ca II H and Kemission lines increase with stellar luminosity, reminiscent of theWilson-Bappu effect in FGK-type stars. Both the equivalent widths andFWHM exhibit modest temporal variability in individual stars. At a givenvalue of MV, stars exhibit a spread in both the equivalentwidth and FWHM of Ca II H and K, due both to a spread in fundamentalstellar parameters, including rotation rate, age, and possiblymetallicity, and to the spread in stellar mass at a given MV.The K line is consistently wider than the H line, as expected, and itscentral absorption is more redshifted, indicating that the H and K linesform at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of Hαcorrelates with Ca II H and K only for stars having Ca II equivalentwidths above ~2 Å, suggesting the existence of a magneticthreshold above which the lower and upper chromospheres become thermallycoupled.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| Metallicity of M dwarfs. I. A photometric calibration and the impact on the mass-luminosity relation at the bottom of the main sequence
We obtained high resolution ELODIE and CORALIE spectra for bothcomponents of 20 wide visual binaries composed of an F-, G- or K-dwarfprimary and an M-dwarf secondary. We analyse the well-understood spectraof the primaries to determine metallicities ([Fe/H]) for these 20systems, and hence for their M dwarf components. We pool thesemetallicities with determinations from the literature to obtain aprecise (±0.2 dex) photometric calibration of M dwarfmetallicities. This calibration represents a breakthrough in a fieldwhere discussions have had to remain largely qualitative, and it helpsus demonstrate that metallicity explains most of the large dispersion inthe empirical V-band mass-luminosity relation. We examine themetallicity of the two known M-dwarf planet-host stars, Gl876 (+0.02 dex) and Gl 436 (-0.03 dex), inthe context of preferential planet formation around metal-rich stars. Wefinally determine the metallicity of the 47 brightest single M dwarfs ina volume-limited sample, and compare the metallicity distributions ofsolar-type and M-dwarf stars in the solar neighbourhood.
| 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.
| The χ Factor: Determining the Strength of Activity in Low-Mass Dwarfs
We describe a new, distance-independent method for calculating themagnetic activity strength in low-mass dwarfs,LHα/Lbol. Using a well-observed sample ofnearby stars and cool standards spanning spectral type M0.5 to L0, wecompute χ, the ratio between the continuum flux near Hα andthe bolometric flux, fλ6560/fbol. Thisratio can be multiplied by the measured equivalent width of the Hαemission line to yield LHα/Lbol. We provideχ values for all objects in our sample, and also fits to χ as afunction of color and average values by spectral type. This method wasused by West et al. to examine trends in magnetic activity strength inlow-mass stars.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| A Medium-Resolution Near-Infrared Spectral Library of Late-Type Stars. I.
We present an empirical infrared spectral library of medium-resolution(R~2000-3000) H (1.6 μm) and K (2.2 μm) band spectra of 218 redstars, spanning a range of [Fe/H] from ~-2.2 to ~+0.3. The sampleincludes Galactic disk stars, bulge stars from Baade's window, and redgiants from Galactic globular clusters. We report the values of 19indices covering 12 spectral features measured from the spectra in thelibrary. Finally, we derive calibrations to estimate the effectivetemperature, and diagnostic relationships to determine the luminosityclasses of individual stars from near-infrared spectra.This paper is part of a larger effort aimed at building a near-IRspectral library to be incorporated in population synthesis models, aswell as at testing synthetic stellar spectra.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
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| Takýmyýldýz: | Ejderha |
| Sað Açýklýk: | 16h25m24.63s |
| Yükselim: | +54°18'14.8" |
| Görünürdeki Parlaklýk: | 10.194 |
| Uzaklýk: | 6.582 parsek |
| özdevim Sað Açýklýk: | 434.4 |
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| B-T magnitude: | 12.34 |
| V-T magnitude: | 10.372 |
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