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Rotation Modulations and Distributions of the Flare Occurrence Rates on the Surface of Five UV Ceti Type Stars
In this study, we considered stellar spots, stellar flares, and also therelation between these two magnetic proccesses that take place on UV Cetstars. In addition, the hypothesis about slow flares described byGurzadyan (1986 Ap&SS, 125, 127) was investigated. All of thesediscussions were based on the results of three years of observations ofUV Cet-type stars: AD Leo, EV Lac, V1005 Ori, EQ Peg, and V1054 Oph.First of all, the results show that stellar spot activity occurs on thestellar surface of EV Lac, V1005 Ori, and EQ Peg, while AD Leo does notshow any short-term variability and V1054 Oph does not exhibit anyvariability. We report on new ephemerides for EV Lac, V1005 Ori, and EQPeg, obtained from time-series analyses. The phases, computed atintervals of 0.10 phase length, where the mean flare occurence rates toobtain maximum amplitude; also, the phases of rotational modulation werecompared in order to investigate whether there is any longitudinalrelation between stellar flares and spots. Although the results showthat flare events are related with spotted areas on stellar surfacesduring some of the observing seasons, we did not find any clearcorrelation among them. Finally, it was tested whether slow flares arefast flares occurring on the opposite side of the stars according to thedirection of the observers, as mentioned in a hypothesis developed byGurzadyan (1986). The flare occurence rates revealthat both slow and fast flares can occur in any rotational phases. Theflare occurence rates of both fast and slow flares vary in the same wayalong the longitudes for all program stars. These results are notexpected based on the case mentioned in the hypothesis.
| 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.
| Infrared Spectroscopic Observations of the Secondary Stars of Short-period Sub-gap Cataclysmic Variables
We present K-band spectroscopy of short-period, "sub-gap" cataclysmicvariable (CV) systems obtained using ISAAC on the Very Large Telescope.We show the infrared (IR) spectra for nine systems below the 2-3 hrperiod gap: V2051 Oph, V436 Cen, EX Hya, VW Hyi, Z Cha, WX Hyi, V893Sco, RZ Leo, and TY PsA. We are able to clearly detect the secondarystar in all but WX Hyi, V893 Sco, and TY PsA. We present the firstdirect detection of the secondary stars of V2051 Oph, V436 Cen, anddetermine new spectral classifications for EX Hya, VW Hyi, Z Cha, and RZLeo. We find that the CO band strengths of all but Z Cha appear normalfor their spectral types, in contrast to their longer period cousinsabove the period gap. This brings the total number of CVs and pre-CVswith moderate resolution (R >~ 1500) IR spectroscopy to 61 systems:19 pre-CVs, 31 non-magnetic systems, and 11 magnetic or partiallymagnetic systems. We discuss the trends seen in the IR abundancepatterns thus far and highlight a potential link between anomalousabundances seen in the IR with the C IV/N V anomaly seen in theultraviolet. We present a compilation of all systems with sufficientresolution IR observations to assess the CO band strengths and, byproxy, obtain an estimate on the C abundance on the secondary star.
| Saturation Levels for White-light Flares of Flare Stars: Variation of Minimum Flare Duration for Saturation
Taking into account results obtained from models and from statisticalanalyses of obtained parameters, we discuss flare activity levels andflare characteristics of five UV Ceti stars. We present the parametersof unpublished flares detected over two years of observations of V1005Ori. We compare parameters of the U-band flares detected over severalseasons of observations of AD Leo, EV Lac, EQ Peg, V1054 Oph, and V1005Ori. Flare frequencies calculated for all program stars and maximumenergy levels of the flares are compared, and we consider which is themost correct parameter as an indicator of flare activity levels. Usingthe One Phase Exponential Association function, the distributions offlare equivalent duration versus flare total duration are modeled foreach program star. We use the Independent Samples t-Test in thestatistical analyses of the parameters obtained from the models. Theresults reveal some properties of flare processes occurring on thesurfaces of UV Ceti type stars. (1) Flare energies cannot be higher thana specific value regardless of the length of the flare total duration.This must be a saturation level for white-light flares occurring inflare processes observed in the U band. Thus, for the first time it isshown that white-light flares have a saturation in a specific energyrange. (2) The span values, which are the difference between theequivalent durations of flares with the shortest and longest totaldurations, are almost equal for each star. (3) The half-life values,minimum flare durations for saturation, increase toward the laterspectral types. (4) Both maximum total durations and maximum rise timescomputed from the observed flares decrease toward the later spectraltypes among the UV Ceti stars. According to the maximum energy levelsobtained from the models, both EV Lac and EQ Peg are more active thanthe other three program stars, while AD Leo is the most active flarestar according to the flare frequencies.
| A New Method for Classifying Flares of UV Ceti Type Stars: Differences Between Slow and Fast Flares
In this study, a new method is presented to classify flares derived fromthe photoelectric photometry of UV Ceti type stars. This method is basedon statistical analyses using an independent samples t-test. The dataused in analyses were obtained from four flare stars observed between2004 and 2007. The total number of flares obtained in the observationsof AD Leo, EV Lac, EQ Peg, and V1054 Oph is 321 in the standard JohnsonU band. As a result flares can be separated into two types, slow andfast, depending on the ratio of flare decay time to flare rise time. Theratio is below 3.5 for all slow flares, while it is above 3.5 for allfast flares. Also, according to the independent samples t-test, there isa difference of about 157 s between equivalent durations of slow andfast flares. In addition, there are significant differences betweenamplitudes and rise times of slow and fast flares.
| UBV(RI)C JHK observations of Hipparcos-selected nearby stars
We present homogeneous, standardized UBV(RI)C photometry forover 700 nearby stars selected on the basis of Hipparcos parallaxes.Additionally, we list JHK photometry for about half of these stars, aswell as L photometry for 86 of the brightest. A number of stars withpeculiar colours or anomalous locations in various colour-magnitudediagrams are discussed.
| Imaging Young Giant Planets From Ground and Space
High-contrast imaging can find and characterize gas giant planets aroundnearby young stars and the closest M stars, complementing radialvelocity and astrometric searches by exploring orbital separationsinaccessible to indirect methods. Ground-based coronagraphs are alreadyprobing within 25 AU of nearby young stars to find objects as small as .This paper contrasts near-term and future ground-based capabilities withhigh-contrast imaging modes of the James Webb Space Telescope (JWST).Monte Carlo modeling reveals that JWST can detect planets with masses assmall as across a broad range of orbital separations. We present newcalculations for planet brightness as a function of mass and age forspecific JWST filters and extending to .
| Reaching the boundary between stellar kinematic groups and very wide binaries. The Washington double stars with the widest angular separations
Aims: I look for and characterise very wide binaries and multiplesystems with projected physical separations larger than s = 0.1 pc,which is generally believed to be a sharp upper limit to thedistribution of wide binary semimajor axes. Methods: Iinvestigated in detail 30 Washington double stars with angularseparations of ? > 1000 arcsec. I discarded 23 of them asprobably unbound systems based on discordant astrometry, photometry,spectral types, and radial velocities. The remaining seven systems weresubject to a comprehensive data compilation and derivation(multi-wavelength photometry, heliocentric distance, multiplicity, age,mass, metallicity, membership in a young kinematic group). Results: Of the seven very wide systems, six have projected physicalseparations greater than the hypothetical cutoff at s = 0.1 pc and fourhave separations s > 0.2 pc. Although there are two systems in youngkinematic groups (namely HD 136654 and BD+32 2572 in the HyadesSupercluster, and AU Mic and AT Mic AB in the ? Pictoris movinggroup), there is no clear prevalence of young systems (? < 1 Ga)among these very wide binaries. Finally, I compare the binding energiesof the seven systems with those of other weakly bound systems in thefield.Appendix A is only available in electronic form at http://www.aanda.org
| 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.
| A new population of cool stars and brown dwarfs in the Lupus clouds
Context: Most studies of the stellar and substellar populations ofstar-forming regions rely on using the signatures of accretion,outflows, disks, or activity characterizing the early stages of stellarevolution. However, these signatures rapidly decay with time. Aims: We present the results of a wide-area study of the stellarpopulation of clouds in the Lupus star-forming region. When combinedwith 2MASS photometry, our data allow us to fit the spectral energydistributions of over 150 000 sources and identify possible new membersbased on their photospheric fluxes, independent of any display of thesignposts of youth. Methods: We used the Wide Field Imager (WFI)at the La Silla 2.2 m telescope to image an area of more than 6 squaredegrees in the Lupus 1, 3 and 4 clouds in the R_C, I_C, and z_WFI bands,selected so as to overlap with the areas observed in the Spitzer LegacyProgram “From molecular cores to planet-forming disks”. Wecomplement our data with 2MASS photometry to sample the spectral energydistribution from 0.6 ?m to 2.2 ?m. We validate our method on thecensus of known members of the Lupus clouds, for which spectroscopicclassification is available. The temperatures derived for cool objectsare generally accurate, with most of the exceptions attributed toveiling, strong emission lines at short wavelengths, near-infraredexcess, variability, or the presence of close companions. Results: Considering that the dereddened fluxes of most cool (T_eff< 3500 K) young stellar objects at the distance of Lupus occupy a gapbetween those typical both of field cool dwarfs and of backgroundgiants, we identify a new population of cool members of Lupus 1 and 3.The approximately 130 new members are only moderately concentratedtoward the densest clouds, they appear to have ages in the same range asthe known members, and very few show the infrared excess caused by warmdisks. This population is absent in Lupus 4. Conclusions: Thisnew population of Lupus members seems to be composed of stars and browndwarfs that have lost their inner disks on a timescale of a few Myr orless. Almost all these objects are in low extinction regions. Wespeculate that dissipation of unshielded disks caused by nearby O starsor fast collapse of the pre-(sub)stellar cores triggered by the passageof old supernova shocks may have led to disk properties and evolutionarypaths very different from those resulting from the more quiescentenvironment provided by dense molecular clouds.Based on observations carried out at the ESO/La Silla, Chile underproposals numbers 69.C-0260(A) and 71.C-0259(A).Tables 4 and 9 are only available in electronic form athttp://www.aanda.org
| Tracers of Chromospheric Structure. I. Observations of Ca II K and Hα in M Dwarfs
We report on our observing program4This paper is based onobservations obtained with the Apache Point Observatory 3.5 m telescope,which is owned and operated by the Astrophysical Research Consortium.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California and theNational Aeronautics and Space Administration. The Observatory was madepossible by the generous financial support of the W. M. Keck Foundation.to capture simultaneous spectra of Ca II and Balmer lines in a sample ofnearby M3 dwarfs. Our goal is to investigate the chromospherictemperature structure required to produce these lines at the observedlevels. We find a strong positive correlation between instantaneousmeasurements of Ca II K and the Balmer lines in active stars, althoughthese lines may not be positively correlated in time-resolvedmeasurements. The relationship between Hα and Ca II K remainsambiguous for weak and intermediate activity stars, with Hαabsorption corresponding to a range of Ca II K emission. A similarrelationship is also observed between Ca II K and the higher-orderBalmer lines. As our sample consists of a single spectral type,correlations between these important chromospheric tracers cannot beascribed to continuum effects, as suggested by other authors. These dataconfirm prior nonsimultaneous observations of the Hα line behaviorwith increasing activity, showing an initial increase in the Hαabsorption with increasing Ca II K emission, prior to Hα fillingin and eventually becoming a pure emission line in the most activestars. We also compare our optical measurements with archival UV andX-ray measurements, finding a positive correlation between thechromospheric and coronal emission for both high and intermediateactivity stars. We compare our results with previous determinations ofthe active fraction of low-mass stars, and discuss them in the contextof surface inhomogeneity. Lastly, we discuss the application of thesedata as empirical constraints on new static models of quiescent M dwarfatmospheres.
| Common Proper Motion Companions to Nearby Stars: Ages and Evolution
A set of 41 nearby stars (closer than 25 pc) is investigated which havevery wide binary and common proper motion (CPM) companions at projectedseparations between 1000 and 200,000 AU. These companions are identifiedby astrometric positions and proper motions from the NOMAD catalog.Based mainly on measures of chromospheric and X-ray activity, ageestimation is obtained for most of 85 identified companions.Color-absolute magnitude diagrams are constructed to test whether CPMcompanions are physically related to the primary nearby stars and havethe same age. Our carefully selected sample includes three remote whitedwarf companions to main-sequence stars and two systems (55 Cnc and GJ777A) of multiple planets and distant stellar companions. Ten new CPMcompanions, including three of extreme separations, are found. Multiplehierarchical systems are abundant; more than 25% of CPM components arespectroscopic or astrometric binaries or multiples themselves. Two newastrometric binaries are discovered among nearby CPM companions, GJ 264and HIP 59000, and preliminary orbital solutions are presented. TheHyades kinematic group (or stream) is presented broadly in the sample,but we find few possible thick-disk objects and no halo stars. Itfollows from our investigation that moderately young (age<~1 Gyr)thin-disk dwarfs are the dominating species in the near CPM systems, ingeneral agreement with the premises of the dynamical survival paradigm.
| Comparative statistics and origin of triple and quadruple stars
The statistics of catalogued quadruple stars consisting of two binaries(hierarchy 2 + 2), is studied in comparison with triple stars, withrespective sample sizes of 81 and 724. Seven representative quadruplesystems are discussed in greater detail. The main conclusions are asfollows. (i) Quadruple systems of ? Lyr type with similar massesand inner periods are common, in 42 per cent of the sample the outermass ratio is above 0.5 and the inner periods differ by less than 10times. (ii) The distributions of the inner periods in triple andquadruple stars are similar and bimodal. The inner mass ratios do notcorrelate with the inner periods. (iii) The statistics of outer periodsand mass ratios in triples and quadruples are different. The medianouter mass ratio in triples is 0.39 independently of the outer period,which has a smooth distribution. In contrast, the outer periods of 25per cent quadruples concentrate in the narrow range from 10 to 100yr,the outer mass ratios of these tight quadruples are above 0.6 and theirtwo inner periods are similar to each other. (iv) The outer and innermass ratios in triple and quadruple stars are not mutually correlated.In 13 per cent of quadruples both inner mass ratios are above 0.85(double twins). (v) The inner and outer orbital angular momenta andperiods in triple and quadruple systems with inner periods above 30dshow some correlation, the ratio of outer-to-inner periods is mostlycomprised between 5 and 104. In the systems with small periodratios the directions of the orbital spins are correlated, while in thesystems with large ratios they are not. The properties of multiple starsdo not correspond to the products of dynamical decay of small clusters,hence the N-body dynamics is not the dominant process of theirformation. On the other hand, rotationally driven (cascade)fragmentation possibly followed by migration of inner and/or outerorbits to shorter periods is a promising scenario to explain the originof triple and quadruple stars.
| The effect of activity on stellar temperatures and radii
Context: Recent analyses of low-mass eclipsing binary stars haveunveiled a significant disagreement between the observations andpredictions of stellar structure models. Results show that theoreticalmodels underestimate the radii and overestimate the effectivetemperatures of low-mass stars but yield luminosities that accord withobservations. A hypothesis based upon the effects of stellar activitywas put forward to explain the discrepancies. Aims: In this paper westudy the existence of the same trend in single active stars and providea consistent scenario to explain systematic differences between activeand inactive stars in the H-R diagram reported earlier. Methods: Theanalysis is done using single field stars of spectral types late-K and Mand computing their bolometric magnitudes and temperatures throughinfrared colours and spectral indices. The properties of the stars insamples of active and inactive stars are compared statistically toreveal systematic differences. Results: After accounting for a numberof possible bias effects, active stars are shown to be cooler thaninactive stars of similar luminosity therefore implying a larger radiusas well, in proportions that are in excellent agreement with those foundfrom eclipsing binaries. Conclusions: The present results generalisethe existence of strong radius and temperature dependences on stellaractivity to the entire population of low-mass stars, regardless of theirmembership in close binary systems.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/478/507
| Further observations of Hipparcos red stars and standards for UBV(RI)C photometry
We present homogeneous and standardized UBV(RI)C JHKphotometry for over 100 M stars selected from an earlier paper on thebasis of apparent photometric constancy. L photometry has been obtainedfor stars brighter than about L = 6. Most of the stars have asubstantial number of UBV(RI)C observations and, it is hoped,will prove useful as red supplementary standards. Additionally, we listJHK photometry for nearly 300 Hipparcos red stars not selected asstandards, as well as L photometry for the brightest stars.
| Southern Very Low Mass Stars and Brown Dwarfs in Wide Binary and Multiple Systems
The results of the Königstuhl survey in the Southern Hemisphere arepresented. I have searched for common proper motion companions to 173field very low mass stars and brown dwarfs with spectral types >M5.0V and magnitudes J<~14.5 mag. I have measured for the first time thecommon proper motion of two new wide systems containing very low masscomponents, Königstuhl 2 AB and 3 A-BC. Together withKönigstuhl 1 AB and 2M 0126-50 AB, they are among the widestsystems in their respective classes (r=450-11,900 AU). I have determinedthe minimum frequency of field wide multiples (r>100 AU) withlate-type components at 5.0%+/-1.8% and the frequency of field widelate-type binaries with mass ratios q>0.5 at 1.2%+/-0.9%. Thesevalues represent a key diagnostic of evolution history and low-mass starand brown dwarf formation scenarios. In addition, the proper motions of62 field very low mass dwarfs are measured here for the first time.
| Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables
Context: . Barnes-Evans type relations provide an empirical relationshipbetween the surface brightness of stars and their color. They are widelyused for measuring the distances to stars of known radii, as theRoche-lobe filling secondaries in cataclysmic variables (CVs).Aims: . The calibration of the surface brightness of field dwarfs ofnear-solar metalicity with spectral types A0 to L8 covers all secondaryspectral types detectable in CVs and related objects and will aid in themeasurement of their distances. Methods: . The calibrations arebased on the radii of field dwarfs measured by the Infrared Flux Methodand by interferometry. Published photometry is used and homogenized tothe Cousins Rc and Ic and the CIT JHK photometricsystems. The narrow band surface brightness at 7500 Å is based onour own and published spectrophotometry. Care is taken to select thedwarfs for near-solar metalicity, appropriate to CVs, and to avoiderrors caused by unrecognized binarity. Results: . Relations areprovided for the surface brightness in V, R_c, I_c, J, H, K and in anarrow band at 7500 Å as functions of V-K and of spectral type.The method is tested with selected CVs for which independent informationon their distances is available. The observed spread in the radii ofearly M-dwarfs of given mass or luminosity and its influence on thedistance measurements of CVs is discussed. Conclusions: . As longas accurate trigonometric parallaxes are not routinely available for alarge number of CVs, the surface brightness method remains a reliablemeans of determining distances to CVs in which a spectral signature ofthe secondary star can be discerned.
| New members in the Upper Scorpius association from the UKIRT Infrared Deep Sky Survey Early Data Release†
We present the results of a 9.3-deg2 infrared (ZYJHK) surveyin the Upper Scorpius association extracted from the UKIRT Infrared DeepSky Survey (UKIDSS) Galactic Cluster Survey Early Data Release. We haveselected a total of 112 candidates from the (Z - J,Z) colour-magnitudediagram over the Z = 12.5-20.5 magnitude range, corresponding to M =0.25-0.01 Msolar at an age of 5 Myr and a distance of 145 pc.Additional photometry in J and K filters revealed most of them asreddened stars, leaving 32 possible members. Among them, 15 have propermotion consistent with higher-mass members from Hipparcos and opticalspectra with strong Hα in emission and weak gravity features. Wehave also extracted two lower-mass candidate members for which nooptical spectra are in hand. Three members exhibit strong Hαequivalent widths (>20 Å), suggesting that they could stillundergo accretion, whereas two other dwarfs show signs of chromosphericactivity. The likelihood of the binarity of a couple of new stellar andsubstellar members is discussed as well.Based on observations made with the United Kingdom Infrared Telescope,operated by the Joint Astronomy Centre on behalf of the UK ParticlePhysics and Astronomy Research Council.Based on observations collected with the European Southern Observatory(ESO) 3.6-m/EFOSC2 at ESO, La Silla, Chile (ESO programme 076C-0237).‡E-mail: nl41@star.le.ac.uk
| Improved u'g'r'i'z' to UBVRCIC Transformation Equations for Main-Sequence Stars
We report improved transformation equations between the u'g'r'i'z' andUBVRCIC photometric systems. Although the detailsof the transformations depend on luminosity class, we find a typical rmsscatter on the order of 0.001 mag if the sample is limited tomain-sequence stars. Furthermore, we find that an accuratetransformation requires complex, multicolor dependencies for the bluerbandpasses. Results for giant stars will be reported in a subsequentpaper.
| Keck HIRES Spectroscopy of Four Candidate Solar Twins
We use high signal-to-noise ratio, high-resolution Keck HIRESspectroscopy of four solar twin candidates (HIP 71813, 76114, 77718, and78399) pulled from our Hipparcos-based Ca II H and K survey to carry outparameter and abundance analyses of these objects. Our spectroscopicTeff estimates are ~100 K hotter than the photometric scale of therecent Geneva-Copenhagen survey; several lines of evidence suggest thephotometric temperatures are too cool at solar Teff. At the same time,our abundances for the three solar twin candidates included in theGeneva-Copenhagen survey are in outstanding agreement with thephotometric metallicities; there is no sign of the anomalously lowphotometric metallicities derived for some late-G UMa group and Hyadesdwarfs. A first radial velocity determination is made for HIP 78399 andUVW kinematics derived for all stars. HIP 71813 appears to be akinematic member of the Wolf 630 moving group (a structure apparentlyreidentified in a recent analysis of late-type Hipparcos stars), but itsmetallicity is 0.1 dex higher than the most recent estimate of thisgroup's metallicity. While certainly solar-type stars, HIP 76114 and77718 are a few percent less massive, significantly older, andmetal-poor compared to the Sun; they are neither good solar twincandidates nor solar analogs providing a look at the Sun at some otherpoint in its evolution. HIP 71813 appears to be an excellent solaranalog of ~8 Gyr age. Our results for HIP 78399 suggest the promise ofthis star as a solar twin may be equivalent to the ``closest ever solartwin,'' HR 6060; follow-up study of this star is encouraged.
| New neighbours. VI. Spectroscopy of DENIS nearby stars candidates
We present spectra of 36 nearby star candidates and 3 red giantcandidates, identified in the DENIS database. 32 of the dwarf candidatesare nearby red dwarfs, with spectral types from M 5.5 to M 8.5. Out of11 targets with low proper motion (? < 0.1 arcsec yr^-1) but aReduced Proper Motion above an inclusive threshold, 9 are red dwarfs.The 4 contaminants are all reddened F-K main sequence stars, and couldhave been eliminated by checking for small well-known high latitudemolecular clouds. These stars might be of interest as probes ofinterstellar absorption. For the red dwarfs we derive spectral types andspectroscopic distances, using a new calibration of the PC3 spectralindex to absolute magnitudes in the I, J, H and K photometric bands. Weconfirm 2 new members of the 12 pc volume (2 new M 8.5), and one M 7.5NLTT object closer than 10 pc; and show that one quarter of the starswith photometric distances under 30 pc have too small a proper motionfor inclusion in the NLTT catalog.
| The Central Region of Barred Galaxies: Molecular Environment, Starbursts, and Secular Evolution
Stellar bars drive gas into the circumnuclear (CN) region of galaxies.To investigate the fate of the CN gas and star formation (SF), we studya sample of barred nonstarbursts and starbursts with high-resolution CO,optical, Hα, radio continuum, Brγ, and HST data, and findthe following. (1) The inner kiloparsec of bars differs markedly fromthe outer disk. It hosts molecular gas surface densitiesΣgas-m of 500-3500 Msolar pc-2,gas mass fractions of 10%-30%, and epicyclic frequencies of several100-1000 km s-1 kpc-1. Consequently, in the CNregion gravitational instabilities can only grow at high gas densitiesand on short timescales, explaining in part why powerful starburstsreside there. (2) Across the sample, we find bar pattern speeds withupper limits of 43-115 km s-1 pc-1 and outer innerLindblad resonance radii of >500 pc. (3) Barred starbursts andnonstarbursts have CN SF rates of 3-11 and 0.1-2 Msolaryr-1, despite similar CN gas masses. TheΣgas-m value in the starbursts is larger (1000-3500Msolar pc-2) and close to the Toomre criticaldensity over a large region. (4) Molecular gas makes up 10%-30% of theCN dynamical mass and fuels large CN SF rates in the starbursts,building young, massive, high-V/σ components. Implications forsecular evolution along the Hubble sequence are discussed.
| Probing the LHS Catalog. II. Faint Proper-Motion Stars
We present low-resolution spectroscopic observations of faintproper-motion stars from the LHS Catalogue, concentrating on stars withmr>16.5 and μ>0.5" yr-1. The presentpaper includes observations and spectral classifications for 294 Mdwarfs, M subdwarfs (sdM), and extreme M subdwarfs (esdM). We alsoidentify white dwarfs among the faintest LHS stars. We havecross-referenced this sample against the Two Micron All Sky Survey(2MASS) sources, and list data for the detected objects. We discussstars of individual interest, as well as the characteristics of theoverall sample. As expected, a significant number of the stars in thisproper-motion-selected sample are halo subdwarfs, including an esdMdwarf, LHS 3481, that is likely to lie within 20 pc of the Sun. None ofthe subdwarfs show Hα emission.
| Absolute magnitudes for late-type dwarf stars for Sloan photometry
We present a new formula for absolute magnitude determination forlate-type dwarf stars as a function of (g-r) and (r-i) for Sloanphotometry. The absolute magnitudes estimated by this approach arebrighter than those estimated by colour-magnitude diagrams, and theyreduce the luminosity function rather close to the luminosity functionof Hipparcos.
| 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.
| The Solar Neighborhood. IX. Hubble Space Telescope Detections of Companions to Five M and L Dwarfs within 10 Parsecs of the Sun
We report the detections of low-mass companions to five M and L dwarfswithin 10 pc of the Sun using the Hubble Space Telescope (HST)Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Follow-upobservations using the HST Advanced Camera for Surveys (ACS) and FineGuidance Sensor 1r (FGS1r) confirm our NICMOS discoveries of companionsto the L4.5 dwarf GJ 1001B (LHS 102B) and the M5 dwarf LHS 224,respectively. Images obtained with the Astrophysical Research Consortium3.5 m telescope at Apache Point Observatory verify our discovery of acompanion to the M3 dwarf G239-25. Our NICMOS images confirm thepreviously suspected duplicity of the M3 dwarfs GJ 54 and GJ 84. Thecomponents of GJ 1001BC and LHS 224AB have nearly equal luminosities inall the ACS and/or NICMOS bandpasses in which they were observed. Themagnitudes of GJ 54A and B in one FGS1r bandpass and four NICMOSbandpasses differ by <~1. GJ 84B and G239-25B are ~4 mag fainter thantheir M3 companions in the NICMOS bandpasses. We estimate from ourNICMOS photometry that GJ 84B and G239-25B have spectral types M7 andM8, respectively, and masses near the lower limit for sustained hydrogenburning. The apparent separations of GJ 1001BC, GJ 54AB, and LHS 224ABsuggest orbital periods less than 5 yr. These binary dwarfs are idealcandidates for continued astrometric monitoring that will quickly yieldaccurate dynamic masses needed to constrain the mass-luminosity relationfor low-mass stars and brown dwarfs.
| L' and M' Photometry of Ultracool Dwarfs
We have compiled L' (3.4-4.1 μm) and M' (4.6-4.8 μm) photometry of63 single and binary M, L, and T dwarfs obtained at the United KingdomInfrared Telescope using the Mauna Kea Observatory filter set. Thiscompilation includes new L' measurements of eight L dwarfs and 13 Tdwarfs and new M' measurements of seven L dwarfs, five T dwarfs, and theM1 dwarf Gl 229A. These new data increase by factors of 0.6 and 1.6,respectively, the numbers of ultracool dwarfs (Teff<~2400K) for which L' and M' measurements have been reported. We computeLbol, BCK, and Teff for 42 dwarfs whoseflux-calibrated JHK spectra, L' photometry, and trigonometric parallaxesare available, and we estimate these quantities for nine other dwarfswhose parallaxes and flux-calibrated spectra have been obtained.BCK is a well-behaved function of near-infrared spectral typewith a dispersion of ~0.1 mag for types M6-T5 it is significantly morescattered for types T5-T9. Teff declines steeply andmonotonically for types M6-L7 and T4-T9, but it is nearly constant at~1450 K for types L7-T4 with assumed ages of ~3 Gyr. This constantTeff is evidenced by nearly unchanging values of L'-M'between types L6 and T3. It also supports recent models that attributethe changing near-infrared luminosities and spectral features across theL-T transition to the rapid migration, disruption, and/or thinning ofcondensate clouds over a narrow range of Teff. The L' and M'luminosities of early-T dwarfs do not exhibit the pronounced humps orinflections previously noted in the I through K bands, but insufficientdata exist for types L6-T5 to assert that ML' andMM' are strictly monotonic within this range of types. Wecompare the observed K, L', and M' luminosities of L and T dwarfs in oursample with those predicted by precipitating-cloud and cloud-free modelsfor varying surface gravities and sedimentation efficiencies. The modelsindicate that the L3-T4.5 dwarfs generally have higher gravities(logg=5.0-5.5) than the T6-T9 dwarfs (logg=4.5-5.0). The predicted M'luminosities of late-T dwarfs are 1.5-2.5 times larger than thosederived empirically for the late-T dwarfs in our sample. Thisdiscrepancy is attributed to absorption at 4.5-4.9 μm by CO, which isnot expected under the condition of thermochemical equilibrium assumedin the models. Our photometry and bolometric calculations indicate thatthe L3 dwarf Kelu-1 and the T0 dwarf SDSS J042348.57-041403.5 areprobable binary systems. We computelog(Lbol/Lsolar)=-5.73+/-0.05 andTeff=600-750 K for the T9 dwarf 2MASSI J0415195-093506, whichsupplants Gl 570D as the least luminous and coolest brown dwarfpresently known.
| New Hipparcos-based Parallaxes for 424 Faint Stars
We present a catalog of 424 common proper-motion companions to Hipparcosstars with good (>3 σ) parallaxes, thereby effectively providingnew parallaxes for these companions. Compared with typical stars in theHipparcos catalog, these stars are substantially dimmer. The catalogincludes 20 white dwarfs and an additional 29 stars withMV>14, the great majority of the latter being M dwarfs.
| Star Clusters in the Galactic Anticenter Stellar Structure and the Origin of Outer Old Open Clusters
The Galactic anticenter stellar structure (GASS) has been identifiedwith excess surface densities of field stars in several large-area skysurveys and with an unusual, stringlike grouping of five globularclusters. At least two of these are diffuse, young ``transitional''clusters between open and globular types. Here we call attention to thefact that four younger open or transitional clusters extend thepreviously identified, stringlike cluster grouping, with at least onehaving a radial velocity consistent with the previously found GASSvelocity-longitude trend. All nine clusters lie close to a plane tipped17° to the Galactic plane. This planar orientation is used to foragefor additional potential cluster members in the inner Galaxy, and anumber are found along the same plane and stringlike sequence, includingalmost all 15 known outer, old open clusters. Tidal accretion of a dwarfsatellite galaxy on a low-inclination orbit-perhaps the GASSsystem-appears to be a plausible explanation for the origin of theouter, old open and transitional clusters of the Milky Way. We use theseclusters to explore the age-metallicity relation of the putativeaccreted GASS progenitor. Finally, we provide the first radial velocityof a star in the cluster BH 176 and discuss its implications.
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