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Observation and modelling of main-sequence star chromospheres - XV. New constraints on the dynamo mechanisms for dM1 stars
With the help of measures of rotation, radius and metallicity for aselected sample of dM1 stars (with Teff= 3460 ± 60 K),we aim to set new constraints on the dynamo mechanisms.We recover 913 high-resolution spectra for 97 different M1 dwarfs fromthe European Southern Observatory and Observatoire de Haute Provencedata bases. We present 660 new measurements of the Ca II resonance linesand 913 new measurements of the H? line for dM1 stars. We alsocompile other measurements available in the literature. In total, weobtain 2216 measures of the Ca II lines for 113 different dM1 stars.This represents the largest compilation of chromospheric linemeasurements at a single spectral type.We cross-correlate these magnetic activity indicators with variousstellar parameters to set new constraints on the dynamo mechanisms andon the properties of the outer atmosphere.We find a correlation of the Ca II line mean equivalent width with theabsolute magnitude and the metallicity. We correct the Ca II linemeasures from the metallicity effect and find that the surface flux inthe Ca II lines grows roughly as the power of 3.6 of the stellar radius.This corrected flux is a direct measure of magnetic activity at thechromospheric level. We find that the total magnetic activity levelgrows roughly as the power of 5.6 of the stellar radius. This trend isconfirmed by the correlation between the H? line and absolutemagnitude and the H? line luminosity and stellar radius: theH? luminosity grows roughly as the volume of the star for lowactivity dM1 stars and as the power of roughly 5/2 of the stellar radiusfor dM1e stars. The advantage of the H? line is that its formationin not dependent on metallicity.In contrast to the Ca II line, we find no correlation betweenLX and the absolute magnitude. We find that LXroughly correlates with the Ca II luminosity although the correlation isnot very good. This correlation shows that LX grows as thepower of 3/2 of the Ca II luminosity, i.e. the coronal emission growsfaster than the chromospheric emission.We find a correlation between the corrected Ca II line equivalent widthand P/sin i, i.e. the Ca II surface flux grows as the power of -1.5 ofthe rotation period. We also find a correlation between FX,the X-ray surface flux, and P/sin i: FX? (P/sini)-3.7. In other words, the coronal emission is much moredependent on the rotation period than the chromospheric emission.We find that the level of magnetic activity in dM1 stars is moredependent on the stellar radius than on rotation at the chromosphericlevel. We discuss the implications of these results on the models ofstellar dynamos. Based on observations available at Observatoire deHaute Provence and the European Southern Observatory data bases and onHipparcos parallax measurements.
| Dense gas and the nature of the outflows
We present the results of the observations of the (J,K) = (1,1) and the(J,K) = (2,2) inversion transitions of the NH3 moleculetoward a large sample of 40 regions with molecular or optical outflows,using the 37 m radio telescope of the Haystack Observatory. We detectedNH3 emission in 27 of the observed regions, which we mappedin 25 of them. Additionally, we searched for the616-523 H2O maser line toward sixregions, detecting H2O maser emission in two of them, HH265and AFGL 5173. We estimate the physical parameters of the regions mappedin NH3 and analyze for each particular region thedistribution of high density gas and its relationship with the presenceof young stellar objects. In particular, we identify the deflectinghigh-density clump of the HH270/110 jet. We were able to separate theNH3 emission from the L1641-S3 region into two overlappingclouds, one with signs of strong perturbation, probably associated withthe driving source of the CO outflow, and a second, unperturbed clump,which is probably not associated with star formation. We systematicallyfound that the position of the best candidate for the exciting source ofthe molecular outflow in each region is very close to an NH3emission peak. From the global analysis of our data we find that ingeneral the highest values of the line width are obtained for theregions with the highest values of mass and kinetic temperature. We alsofound a correlation between the nonthermal line width and the bolometricluminosity of the sources, and between the mass of the core and thebolometric luminosity. We confirm with a larger sample of regions theconclusion of Anglada et al. (1997) that the NH3 lineemission is more intense toward molecular outflow sources than towardsources with optical outflow, suggesting a possible evolutionary schemein which young stellar objects associated with molecular outflowsprogressively lose their neighboring high-density gas, weakening boththe NH3 emission and the molecular outflow in the process,and making optical jets more easily detectable as the total amount ofgas decreases.Appendix A is only available in electronic form at http://www.aanda.orgHaystack Observatorydata as FITS files are available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/527/A41
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| Observation and modelling of main-sequence star chromospheres - XIV. Rotation of dM1 stars
We have measured v sin i for a selected sample of dM1-typestars. We give 114 measurements of v sin i for 88 different stars, andsix upper detection limits. These are the first measurements of v sin ifor most of the stars studied here. This represents the largest sampleof v sin i measurements for M dwarfs at a given spectral type. For thesemeasurements, we used four different spectrographs: HARPS (ESO), SOPHIE(OHP), ÉLODIE (OHP) and UVES (ESO). Two of these spectrographs(HARPS and SOPHIE) are particularly stable in wavelength since they weredesigned for exoplanet searches.We measured v sin i down to an accuracy of 0.3kms-1 for thehighest resolution spectrographs and a detection limit of about1kms-1. We show that this unprecedented accuracy for M dwarfsin our data set is possible because all the targets have the samespectral type. This is an advantage and it facilitates the determinationof the narrowest line profiles for v sin i ~ 0. Although it is possibleto derive the zero-point profiles using several spectral types at atime. These values were combined with other measurements taken from theliterature. The total sample represents detected rotation for 100 stars(10 dM1e and 90 dM1 stars). We confirm our finding of Paper VII that thedistribution of the projected rotation period is bimodal for dM1 starswith a much larger sample, i.e. there are two groups of stars: the fastrotators with P/sin i ~ 4.5d and the slow rotators with P/sin i ~ 14.4d.There is a gap between these two groups. We find that the distributionof stars as a function of P/sin i has two very abrupt cuts, below 10dand above 18d. There are very few stars observed out of this range10-18d. We also observe that the distribution increases slightly from 18to 10d.We find that the M1 subdwarfs (very low metallicity dwarfs) rotate withan average period of P/sin i ~ 7.2d, which is about twice faster as themain group of normal M1 dwarfs. We also find a correlation for P/sin ito decrease with stellar radius among dM1e stars. Such a trend is alsoobserved in dM1 stars.We also derive metallicity and radius for all our target stars using thesame method as in Paper VII. We notably found that 11 of our targetstars are subdwarfs with metallicities below -0.5dex.Based on observations available at Observatoire de Haute Provence andthe European Southern Observatory data bases and on Hipparcos parallaxmeasurements.E-mail: eric_houdebine@yahoo.fr
| Potential Members of Stellar Kinematical Groups within 20 pc of the Sun
We analyze the kinematical histories of stars within 20 pc of the Sun,for which three-dimensional spatial coordinates and three-dimensionalvelocity vectors are available. From this sample, we extract members ofstellar kinematical groups (SKGs) in the following manner. First, in thethree-dimensional velocity space centered on the local standard of rest,we consider a sphere with a radius of 8 km s-1 centeredon the mean velocity vector of a particular SKG. Around each SKGvelocity center, we have found a significant excess of stars compared tobackground stars. For each candidate, in the three-dimensional spatialcoordinate space, its trajectory is traced back in time by the age ofthe relevant SKG, to estimate the distance from the SKG center at thetime of the SKG's birth by the epicycle approximation and verticalharmonic motion. It often happens that a star is a candidate member ofmultiple SKGs. Then we rank the candidacy to multiple SKGs based on thesmallness of distance separations. In this manner, we have kinematicallyselected 74 candidate members of SKGs out of a sample of 383 stars. Ifwe take into account stellar properties, the number is reduced to 68.
| Observation and modelling of main sequence star chromospheres - XIII. The NaI D1 and D2, and HeI D3 lines in dM1 stars
We investigate spectral lines of interest in dM1 stars, namely the NaID1 and D2 and HeI 5876 lines. We study in detail the line shapes of theNaI D1 and D2 lines. We find that these lines are strong and broad innormal dM1 stars and become weaker and narrower when metallicity is low,although our sample is insufficient in order to find out an empiricalcorrelation between these parameters.We find correlations between the CaII resonance line-mean equivalentwidth (EW) and vsini as well as between the NaI mean line core relativeflux and vsini. These correlations include low activity dM1 stars andshow that the NaI mean line core flux is a good chromosphericdiagnostic.We find a good correlation between the NaI D1 line core flux and the NaID2 line core flux. This correlation shows that the line core opticaldepths decrease with an increasing activity level, that is the oppositeof what was found for the CaII lines. The NaI D1 and D2 mean line coreflux also correlates well with the CaII mean EW and with the H?EW. We also compare these correlations to the available modelcomputations. We investigate in detail the shapes of the NaI D1 and D2lines through the full line widths at 85 per cent, 62 per cent and 35per cent of the continuum. The significant differences from one star toanother cannot be explained at this stage. Detailed modelling of thestellar photospheres will be necessary to interpret the observeddifferences.The HeI 5876 line is detected in only one dM1 star in our sample. Weobtain activity correlations between the HeI 5876 line EW and the CaIImean EW, and the H? EW.
| Magnetic Field Topology in Low-Mass Stars: Spectropolarimetric Observations of M Dwarfs
The magnetic field topology plays an important role in the understandingof stellar magnetic activity. While it is widely accepted that thedynamo action present in low-mass partially convective stars (e.g., theSun) results in predominantly toroidal magnetic flux, the field topologyin fully convective stars (masses below ~0.35 M sun) is stillunder debate. We report here our mapping of the magnetic field topologyof the M4 dwarf G 164-31 (or Gl 490B), which is expected to be fullyconvective, based on time series data collected from 20 hr ofobservations spread over three successive nights with the ESPaDOnSspectropolarimeter. Our tomographic imaging technique applied to timeseries of rotationally modulated circularly polarized profiles revealsan axisymmetric large-scale poloidal magnetic field on the M4 dwarf. Wethen apply a synthetic spectrum fitting technique for measuring theaverage magnetic flux on the star. The flux measured in G 164-31 is |Bf|= 3.2 ± 0.4 kG, which is significantly greater than the averagevalue of 0.68 kG determined from the imaging technique. The differenceindicates that a significant fraction of the stellar magnetic energy isstored in small-scale structures at the surface of G 164-31. OurH? emission light curve shows evidence for rotational modulationsuggesting the presence of localized structure in the chromosphere ofthis M dwarf. The radius of the M4 dwarf derived from the rotationalperiod and the projected equatorial velocity is at least 30% larger thanthat predicted from theoretical models. We argue that this discrepancyis likely primarily due to the young nature of G 164-31 rather thanprimarily due to magnetic field effects, indicating that age is animportant factor which should be considered in the interpretation ofthis observational result. We also report here our polarimetricobservations of five other M dwarfs with spectral types from M0 to M4.5,three of them showing strong Zeeman signatures.Based on observations made at the Canada-France-Hawaii Telescope,operated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique de France and the University of Hawaii.
| 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.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations
We have completed a high-resolution (R ≈ 60,000) opticalspectroscopic survey of 185 nearby M dwarfs identified using ROSAT datato select active, young objects with fractional X-ray luminositiescomparable to or greater than Pleiades members. Our targets are drawnfrom the NStars 20 pc census and the Moving-M sample with distancesdetermined from parallaxes or spectrophotometric relations. We limitedour sample to 25 pc from the Sun, prior to correcting forpre-main-sequence overluminosity or binarity. Nearly half of theresulting M dwarfs are not present in the Gliese catalog and have nopreviously published spectral types. We identified 30 spectroscopicbinaries (SBs) from the sample, which have strong X-ray emission due totidal spin-up rather than youth. This is equivalent to a 16% SBfraction, with at most a handful of undiscovered SBs. We estimate upperlimits on the age of the remaining M dwarfs using spectroscopic youthindicators such as surface gravity-sensitive indices (CaH and K I). Wefind that for a sample of field stars with no metallicity measurements,a single CaH gravity index may not be sufficient, as highermetallicities mimic lower gravity. This is demonstrated in a subsampleof metal-rich radial velocity (RV) standards, which appear to have lowsurface gravity as measured by the CaH index, yet show no other evidenceof youth. We also use additional youth diagnostics such as lithiumabsorption and strong Hα emission to set more stringent agelimits. Eleven M dwarfs with no Hα emission or absorption arelikely old (>400 Myr) and were caught during an X-ray flare. Weestimate that our final sample of the 144 youngest and nearest low-massobjects in the field is less than 300 Myr old, with 30% of them beingyounger than 150 Myr and four very young (lap10 Myr), representing agenerally untapped and well-characterized resource of M dwarfs forintensive planet and disk searches.Based on observations collected at the W. M. Keck Observatory and theCanada-France-Hawaii Telescope. The Keck Observatory is operated as ascientific partnership between the California Institute of Technology,the University of California, and NASA, and was made possible by thegenerous financial support of the W. M. Keck Foundation. The CFHT isoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii.
| χ Values for Blue Emission Lines in M Dwarfs
We compute values for blue emission lines in active M dwarfs. Usingflux-calibrated spectra from nearby M dwarfs and spectral M dwarftemplates from SDSS, we derive analytical relations that describe howthe values for the Ca II H and K as well as the Hβ, Hγ,Hδ, Hɛ, and H8 Balmer emission lines vary as a function ofspectral type and color. These derived values are important for numerousM dwarf studies where the intrinsic luminosity of emission lines cannotbe estimated due to uncertain distances and/or non-flux-calibratedspectra. We use these results to estimate the mean properties of blueemission lines in active-field M dwarfs from SDSS.
| Observation and modelling of main-sequence stellar chromospheres - VII. Rotation and metallicity of dM1 stars
We have measured v sini and metallicity from high-resolutionspectroscopic observations of a selected sample of dM1-type stars.To measure v sini, we first selected three template stars known fortheir slow rotation or their very low activity levels and thencross-correlated their spectra with those of our target stars. Theexcess broadening of the cross-correlation peaks gives v sini. Formetallicity, we compiled all available measurements from the literatureand correlated them with the stellar radius. Provided the parallax isknown, this new method allows us to derive metallicities for all ourtarget stars.We measured v sini to an accuracy of 2 kms-1. These valueswere combined with other measurements taken from the literature. We havedetected rotation in seven dM1e stars and 11 dM1 stars and upper limitsfor 20 other dM1 stars. Our results show that the distribution of therotation period may be bimodal for dM1 stars, i.e. there are two groupsof stars: the fast rotators with Prot ~ 6 d and the slowrotators with Prot ~ 24 d. There is a gap between these twogroups.We obtained a correlation between metallicity and stellar radius whichallows us to derive metallicities for all stars in our sample and moregenerally for all dM1 stars with [M/H] in the range -1.5 to 0.5 dex,with a reasonable accuracy. We compare this correlation to models andfind a significant disagreement in radii. However, the observed shape ofthe correlation is globally reproduced by the models. We derive themetallicity for 87 M1 dwarfs and subdwarfs.Based on observations collected at Observatoire de Haute Provence andthe European Southern Observatory and on Hipparcos parallaxmeasurements.E-mail: eric_houdebine@yahoo.fr
| 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.
| SHARC-II 350 ?m Observations of Thermal Emission from Warm Dust in z >= 5 Quasars
We present observations of four z >= 5 SDSS quasars at 350 ?m withthe SHARC-II bolometer camera on the Caltech Submillimeter Observatory.These are among the deepest observations that have been made by SHARC-IIat 350 ?m, and three quasars are detected at >=3?significance, greatly increasing the sample of 350 ?m (corresponds torest frame wavelengths of <60 ?m at z >= 5), detectedhigh-redshift quasars. The derived rest frame far-infrared (FIR)emission in the three detected sources is about five to ten timesstronger than that expected from the average spectral energydistribution (SED) of the local quasars given the same 1450 Åluminosity. Combining the previous submillimeter and millimeterobservations at longer wavelengths, the temperatures of the FIR-emittingwarm dust from the three quasar detections are estimated to be in therange of 39-52 K. Additionally, the FIR-to-radio SEDs of the three 350?m detections are consistent with the emission from typicalstar-forming galaxies. The FIR luminosities are ~1013 Lsun and the dust masses are >=108 Msun. These results confirm that huge amounts of warm dust canexist in the host galaxies of optically bright quasars as early as z ~6. The universe is so young at these epochs (~1 Gyr) that a rapiddust-formation mechanism is required. We estimate the size of the FIRdust-emission region to be about a few kpc, and further provide acomparison of the SEDs among different kinds of dust-emitting sources toinvestigate the dominant dust-heating mechanism.
| 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
| New periodic variable stars coincident with ROSAT sources discovered using SuperWASP
We present optical lightcurves of 428 periodic variable stars coincidentwith ROSAT X-ray sources, detected using the first run of the SuperWASPphotometric survey. Only 68 of these were previously recognised asperiodic variables. A further 30 of these objects are previously knownpre-main sequence stars, for which we detect a modulation period for thefirst time. Amongst the newly identified periodic variables, many appearto be close eclipsing binaries, their X-ray emission is presumably theresult of RS CVn type behaviour. Others are probably BY Dra stars,pre-main sequence stars and other rapid rotators displaying enhancedcoronal activity. A number of previously catalogued pulsating variables(RR Lyr stars and Cepheids) coincident with X-ray sources are also seen,but we show that these are likely to be misclassifications. We identifyfour objects which are probable low mass eclipsing binary stars, basedon their very red colour and light curve morphology.Tables 1 and 2 are also 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/467/785
| Spotting in stars with a low level of activity, close to solar activity
Data on the variability of the continuum optical emission are used forthe first time to estimate the degree of spotting in stars with activitylevels similar to that of the sun. It is shown that the amount ofspotting increases gradually from the sun to the highly spotted starsfor which Alekseev and Gershberg constructed the zonal model for thedistribution of spots. A close relationship is found between spottingand the power of the x-ray emission from stars with widely varyinglevels of activity.
| New Distant Companions to Known Nearby Stars. II. Faint Companions of Hipparcos Stars and the Frequency of Wide Binary Systems
We perform a search for faint, common proper motion companions ofHipparcos stars using the recently published Lépine-Shara ProperMotion-North catalog of stars with proper motionμ>0.15'' yr-1. Our survey uncovers a totalof 521 systems with angular separations3''<Δθ<1500'', with 15 triplesand 1 quadruple. Our new list of wide systems with Hipparcos primariesincludes 130 systems identified here for the first time, including 44 inwhich the secondary star has V>15.0. Our census is statisticallycomplete for secondaries with angular separations20''<Δθ<300'' and apparentmagnitudes V<19.0. Overall, we find that at least 9.5% of nearby(d<100 pc) Hipparcos stars have distant stellar companions withprojected orbital separations s>1000 AU. We observe that thedistribution in orbital separations is consistent with Öpik's law,f(s)ds~s-1ds, only up to a separation s~4000 AU, beyond whichit follows a more steeply decreasing power law f(s)ds~s-ldswith l=1.6+/-0.1. We also find that the luminosity function of thesecondaries is significantly different from that of the single stars'field population, showing a relative deficiency in low-luminosity(8
| Young Stars in the Camelopardalis Dust and Molecular Clouds. I. The Cam OB1 Association
The distribution of dust and molecular clouds in the direction ofGalactic longitudes 132--158° and latitudes ± 12\degr\ isinvestigated. The maps of dust distribution in the area were plottedfrom the following surveys: the star counts in the DSS I database byDobashi et al. (2005), the survey of the average infrared color excessesby Froebrich et al. (2007) and the thermal dust emission survey at 100μ m by Schlegel et al. (1998). The distribution of molecular cloudswas taken from the whole sky CO survey by Dame et al. (2001). All thesesurveys show very similar cloud patterns in the area. Using the radialvelocities of CO, the distances to separate clouds are estimated. Arevised list of the Cam OB1 association members contains 43 stars andthe open cluster NGC 1502. 18 young irregular variable and Hαemission stars are identified in the area. All this proves that the starforming process in the Camelopardalis clouds is still in progress.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| Multicolour CCD measurements of nearby visual double stars. II
We present accurate CCD astrometric and photometric data for 31 nearbyvisual double stars in the standard filters BVRI. The observations werecollected with a 1.3-m telescope in 2001-2002 The results consist ofrelative astrometric positions (epoch, angular separation and positionangle) and differential BVRI photometry of the components. Mean errorsare: 0.01 arcsec for the separation; 0.06 ° for the position angle;and 0.015m for the photometric data. Comparing the relative positions atdifferent epochs, we evaluate the physical association of the systems.We additionally derive fractional masses and true separations for themost probable binary systems and, whenever orbits are available, alsototal and component masses.Table \ref{t2} is only available in the electronic form athttp:www.edpsciences.org. Tables \ref{t4} and \ref{t5} are onlyavailable 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/qcat?J/A+A/422/1023Based on data obtained at the Skinakas Observatory and by the Hipparcosastrometry satellite. The Skinakas Observatory is a collaborativeproject of the University of Crete, the Foundation for Research andTechnology - Hellas, and the Max-Planck-Institut fürExtraterrestrische Physik.
| Meeting the Cool Neighbors. VIII. A Preliminary 20 Parsec Census from the NLTT Catalogue
Continuing our census of late-type dwarfs in the solar neighborhood, wepresent BVRI photometry and optical spectroscopy of 800 mid-type Mdwarfs drawn from the NLTT proper-motion catalog. The targets are takenboth from our own cross-referencing of the NLTT Catalogue and the 2MASSSecond Incremental Data Release, and from the revised NLTT compiledrecently by Salim & Gould. All are identified as nearby-starcandidates based on their location in the(mr,mr-Ks) diagram. Three hundred starsdiscussed here have previous astrometric, photometric, or spectroscopicobservations. We present new BVRI photometry for 101 stars, togetherwith low-resolution spectroscopy of a further 400 dwarfs. In total, wefind that 241 stars are within 20 pc of the Sun, while a further 70 liewithin 1 σ of our distance limit. Combining the present resultswith previous analyses, we have quantitative observations for 1910 ofthe 1913 candidates in our NLTT nearby-star samples. Eight hundredfifteen of those stars have distance estimates of 20 pc or less,including 312 additions to the local census. With our NLTT follow-upobservations essentially complete, we have searched the literature for Kand early-type M dwarfs within the sampling volume covered by the 2MASSsecond release. Comparing the resultant 20 pc census against predictednumbers, derived from the 8 pc luminosity function, shows an overalldeficit of ~20% for stellar systems and ~35% for individual stars.Almost all are likely to be fainter than MJ=7, and at leasthalf are probably as yet undiscovered companions of known nearby stars.Our results suggest that there are relatively few missing systems at thelowest luminosities, MJ>8.5. We discuss possible means ofidentifying the missing stars.
| 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.
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable 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/qcat?J/A+A/397/997
| 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
| Variability of the Photospheric Radiation of Active K-M Dwarfs and Their X-ray Luminosities
Variability of the photospheric radiation of 40 (dKe-dMe) dwarfs in thesolar neighborhood due to variations in the spottedness of theirsurfaces is analyzed based on the behavior of their mean annualbrightnesses over long time intervals. The amplitudes and characteristictime scales of the variations of the mean annual brightness are taken tobe indicators of photospheric activity and were used to infer the levelsof photospheric activity in the stars studied. The influence of axialrotation on the development of cyclic activity in young red dwarfs andF-M main-sequence stars is analyzed. The durations and amplitudes of thephotospheric variability of rapidly rotating (dK0e-dK5e) stars testifiesto a higher level of photospheric activity among red dwarfs andsolar-type stars. The X-ray luminosities of these stars grow with theamplitude of the variations of the mean annual brightness. However, thisis not typical of rapidly rotating M dwarfs, for which the X-rayemission varies by more than two orders of magnitude, although theirdegrees of spottedness are all virtually the same. A linear relationshipbetween the X-ray and bolometric luminosities is observed for young(dKe-dMe) stars, with their ratios log(L x/L bol) being about -3. Theseproperties can be used to determine whether a red dwarf is a young staror is already on the main sequence.
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