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X-Ray Flaring on the dMe Star, Ross 154
We present results from two Chandra imaging observations of Ross 154, anearby flaring M dwarf star. During a 61 ks ACIS-S exposure, a verylarge flare occurred (the equivalent of a solar X3400 event, withLX=1.8×1030 ergs s-1) in whichthe count rate increased by a factor of over 100. The early phase of theflare shows evidence for the Neupert effect, followed by a further riseand then a two-component exponential decay. A large flare was alsoobserved at the end of a later 48 ks HRC-I observation. Emission fromthe nonflaring phases of both observations was analyzed for evidence oflow-level flaring. From these temporal studies we find that microflaringprobably accounts for most of the ``quiescent'' emission and that,unlike for the Sun and the handful of other stars that have beenstudied, the distribution of flare intensities does not appear to followa power law with a single index. Analysis of the ACIS spectra, which wascomplicated by exclusion of the heavily piled-up source core, suggeststhat the quiescent Ne/O abundance ratio is enhanced by a factor of ~2.5compared to the commonly adopted solar abundance ratio and that the Ne/Oratio and overall coronal metallicity during the flare appear to beenhanced relative to quiescent abundances. Based on the temperatures andemission measures derived from the spectral fits, we estimate the lengthscales and plasma densities in the flaring volume and also track theevolution of the flare in color-intensity space. Lastly, we searched fora stellar wind charge exchange X-ray halo around the star but withoutsuccess; because of the relationship between mass-loss rate and the halosurface brightness, not even an upper limit on the stellar mass-lossrate can be determined.
| Far-Infrared Properties of M Dwarfs
We report the mid- and far-infrared properties of nearby M dwarfs.Spitzer MIPS measurements were obtained for a sample of 62 stars at 24μm, with subsamples of 41 and 20 stars observed at 70 and 160 μm,respectively. We compare the results with current models of M starphotospheres and look for indications of circumstellar dust in the formof significant deviations of K-[24 μm] colors and 70 μm/24 μmflux ratios from the average M star values. At 24 μm, all 62 of thetargets were detected; 70 μm detections were achieved for 20 targetsin the subsample observed, and no detections were seen in the 160 μmsubsample. No clear far-infrared excesses were detected in our sample.The average far-infrared excess relative to the photospheric emission ofthe M stars is at least 4 times smaller than the similar average for asample of solar-type stars. However, this limit allows the averagefractional infrared luminosity in the M-star sample to be similar tothat for more massive stars. We have also set low limits(10-4 to 10-9 M⊕ depending onlocation) for the maximum mass of dust possible around our 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.
| The substellar mass function in σ Orionis. II. Optical, near-infrared and IRAC/Spitzer photometry of young cluster brown dwarfs and planetary-mass objects
Aims.We investigate the mass function in the substellar domain down to afew Jupiter masses in the young σ Orionis open cluster (3±2Ma, d = 360^+70_-60 pc). Methods: We have performed a deep IJ-bandsearch, covering an area of 790 arcmin2 close to the clustercentre. This survey was complemented with an infrared follow-up in theHK_s- and Spitzer 3.6-8.0 μm-bands. Using colour-magnitude diagrams,we have selected 49 candidate cluster members in the magnitude interval16.1 mag < I < 23.0 mag. Results: Accounting for flux excesses at8.0 μm and previously known spectral features of youth, we identify30 objects as bona fide cluster members. Four are first identified fromour optical-near infrared data. Eleven have most probable masses belowthe deuterium burning limit which we therefore classify as candidateplanetary-mass objects. The slope of the substellar mass spectrum(Δ N / Δ {M} ≈ a { M}-α) in the massinterval 0.11 M_ȯ < {M} < 0.006 M_ȯ is α = +0.6± 0.2. Any mass limit to formation via opacity-limitedfragmentation must lie below 0.006 M_ȯ. The frequency of σOrionis brown dwarfs with circumsubstellar discs is 47±9 %. Conclusions: The continuity in the mass function and in the frequency ofdiscs suggests that very low-mass stars and substellar objects, evenbelow the deuterium-burning mass limit, share the same formationmechanism.Alexander von Humboldt Fellow at the Max-Planck-Institut fürAstronomie Appendices are only available in electronic form athttp://www.aanda.org
| Low-mass visual binaries in the solar neighbourhood: The case of HD 141272
We search for stellar and substellar companions of young nearby stars toinvestigate stellar multiplicity and formation of stellar and substellarcompanions. We detect common proper-motion companions of stars viamulti-epoch imaging. Their companionship is finally confirmed withphotometry and spectroscopy. Here we report the discovery of a newco-moving (13 σ) stellar companion ˜ 17.8 arcsec (350 AU inprojected separation) north of the nearby star HD 141272 (21 pc). WithEMMI/NTT optical spectroscopy we determined the spectral type of thecompanion to be M3±0.5V. The derived spectral type as well as thenear infrared photometry of the companion are both fully consistent witha 0.26+0.07-0.06 Mȯ dwarf locatedat the distance of HD 141272 (21 pc). Furthermore the photometry datarules out the pre-main sequence status, since the system is consistentwith the ZAMS of the Pleiades.Based on observations obtained on La Silla in ESO programs 77.C-0572(A)and Calar Alto project number F06-3.5-016.
| Analysis and modeling of high temporal resolution spectroscopic observations of flares on AD Leonis
We report the results of a high temporal resolution spectroscopicmonitoring of the flare star AD Leo. During 4 nights,more than 600 spectra were taken in the optical range using the IsaacNewton Telescope (INT) and the Intermediate Dispersion Spectrograph(IDS). We observed a large number of short and weak flares occurringvery frequently (flare activity > 0.71 h-1). This isconsistent with the very important role that flares can play in stellarcoronal heating. The detected flares are non white-light flares and,although most solar flares are of this kind, very few such events havebeen observed previously in stars. The behaviour of differentchromospheric lines (Balmer series from Hα to H{11}, Ca II H &K, Na I D1 and D2, He I 4026 Å and He I D3) was studied in detailfor a total of 14 flares. We estimated the physical parameters of theflaring plasma by using a procedure that assumes a simplified slab modelof flares. All the obtained physical parameters are consistent withpreviously derived values for stellar flares, and the areas - less than2.3% of the stellar surface - are comparable with the size inferred forother solar and stellar flares. We studied the relationships between thephysical parameters and the area, duration, maximum flux and energyreleased during the detected flares.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| 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.
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| High Temporal Resolution Spectroscopic Observations of the Flare Star V1054 Oph
We present the results of a high temporal resolution spectroscopicmonitoring of the flare (UV Cet type) star V1054 Oph (Wolf 630AB),classified as a dM3.5e visual binary system. Intermediate resolutionspectra have been taken during four nights (2 5 April 2001) using theIDS spectrograph of the 2.5 m Isaac Newton Telescope (INT) (La Palma,Canary Islands, Spain). The V1054 Oph spectra show very strong emissionlines even in its quiescent state. The analysis of the temporalevolution of the observed emission lines (from Hβ toH11 and the Ca II H and K lines) reveals four strong flaresand several weak flares. We have studied in detail the behaviour of thechromospheric lines during the different phases (pre-flare, impulsiveand gradual decay) of these flares. The observed flares last from˜25 to 95 min. The equivalent width of the Hβ linechanges by a factor up to ˜2.3. Broad wings and asymmetric(red-shifted) lines are observed as well.
| Near-Ultraviolet Spectra of Nine M Dwarf Stars, or a Second Effort to Find Optical Coronal Lines in M Dwarf Stars
We have searched for optical coronal lines in the 3100-3700 Åregion of eight M dwarf stars with rather low levels of activity. Thisbrief survey supplements a similar search in 15 active stars publishedin 1991. No coronal lines could be identified. However, the emissionspectra including lines of H I, He I, Ca II, Ca I, Si I, and Fe I aredescribed and illustrated. Radial velocities of the emission lines showno systematic differences from the stellar absorption lines. Coronaewith temperatures similar to those in the solar corona seem to be rareamong the M dwarfs, although at least one example has been found bySchmitt & Wichmann.
| Fe XIII coronal line emission in cool M dwarfs
We report on a search for the Fe XIII forbidden coronal line at 3388.1Å in a sample of 15 M-type dwarf stars covering the whole spectralclass as well as different levels of activity. A clear detection wasachieved for LHS 2076 during a major flare and for CN Leo, where theline had been discovered before. For some other stars the situation isnot quite clear. For CN Leo we investigated the timing behaviour of theFe XIII line and report a high level of variability on a timescale ofhours which we ascribe to microflare heating.Based on observations collected at the European Southern Observatory,Paranal, Chile, 68.D-0166A.
| 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.
| High Temporal Resolution Spectra of Flare Stars
Not Available
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| 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.
| Meeting the Cool Neighbors. IV. 2MASS 1835+32, a Newly Discovered M8.5 Dwarf within 6 Parsecs of the Sun
We present observations of 2MASSI J1835379+325954, a previouslyunrecognized late-type M dwarf within 6 pc of the Sun. Identified basedanalysis of the Two Micron All Sky Survey optical spectroscopy andphotometry indicate a spectral type of M8.5. The star has a propermotion of 0.759" yr-1 and is clearly visible on both POSS Iand POSS II photographic plate material, but it may have escapeddetection in previous surveys through its proximity to the Galacticplane. We discuss potential implications for the completeness of thelocal stellar census.
| The 0.4-Msun eclipsing binary CU Cancri. Absolute dimensions, comparison with evolutionary models and possible evidence for a circumstellar dust disk
Photometric observations in the R and I bands of the detached M-typedouble-lined eclipsing binary CU Cnc have beenacquired and analysed. The photometric elements obtained from theanalysis of the light curves have been combined with an existingspectroscopic solution to yield high-precision (errors la 2%) absolutedimensions: MA=0.4333+/-0.0017 Msun,MB= 0.3980+/-0.0014 Msun,RA=0.4317+/-0.0052 Rsun, andRB=0.3908+/-0.0094 Rsun. The mean effectivetemperature of the system has been estimated to be Teff=3140+/-150 K by comparing multi-band photometry (optical and infrared)with synthetic colors computed from state-of-the-art model atmospheres.Additionally, we have been able to obtain an estimate for the age ( ~320 Myr) and chemical composition ([Fe/H]~ 0.0) of the binary systemthrough its membership of the Castor moving group. With all theseobservational constraints, we have carried out a critical test of recentstellar models for low-mass stars. The comparison reveals that mostevolutionary models underestimate the radius of the stars by as much as10%, thus confirming the trend observed by Torres & Ribas(\cite{TR02}) for YY Gem and V818Tau. In the mass-absolute magnitude diagram, CU Cnc isobserved to be dimmer than other stars of the same mass and this makesthe comparison with stellar models not so compelling. After ruling out anumber of different scenarios, the apparent faintness of CU Cnc can beexplained if its components are some 10% cooler than similar-mass starsor if there is some source of circumstellar dust absorption. The lattercould be a tantalizing indirect evidence for a coplanar (Vega-like)dusty disk around this relatively young M-type binary.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/239}
| 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
| Lithium Depletion Boundary in a Pre-Main-Sequence Binary System
A lithium depletion boundary is detected in HIP 112312 (GJ 871.1 A andB), an ~12 Myr old pre-main-sequence binary system. A strong (EW~300mÅ) Li λ6708 absorption feature is seen in the secondary(~M4.5), while no Li λ6708 feature is detected in the primary(~M4). The physical companionship of the two stars is confirmed by theircommon proper motions. Current theoretical pre-main-sequenceevolutionary models cannot simultaneously match the observed colors,brightnesses, and Li depletion patterns of this binary system. At theage upper limit of 20 Myr, contemporary theoretical evolutionary modelspredict a too slow Li depletion. If true Li depletion is a fasterprocess than predicted by theoretical models, the ages of open clusters(Pleiades, α Persei, and IC 2391) estimated from the Li depletionboundary method are all overestimated. Because of the importance of theopen cluster age scale, development of self-consistent theoreticalmodels to match the HIP 112312 data is desirable.
| Flare Heating in Stellar Coronae
An open question in the field of solar and stellar astrophysics is thesource of heating that causes stellar coronae to reach temperatures ofmillions of degrees. One possibility is that the coronae are heated by alarge number of small flares. On the Sun, flares with energies as low asthose of microflares are distributed with energy as a power law of theform (dN/dE)~E-α, with α~1.8, and α appearsto increase to values of 2.2-2.7 for flares of lower energy. If theslope exceeds the critical value of 2, then in principle the entirecoronal energy input can be ascribed to flares that are increasinglyless energetic but are more numerous. Previous analyses of flares inlight curves of active stars have shown that this index generally isgreater than 2, although it may be as low as 1.6 when strong flaresalone are considered. Here we investigate the contribution of very weakflares, covering the milliflare energy range, to the coronal luminosityof low-mass active stars. We analyze Extreme Ultraviolet Explorer/DeepSurvey events data from FK Aqr, V1054 Oph, and AD Leo and conclude thatin all these cases, the coronal emission is dominated by flares to suchan extent that in some cases, the entire emission can be ascribed toflare heating. We have developed a new method to directly model for thefirst time stochastically produced flare emission, includingundetectable flares, and their effects on the observed photon arrivaltimes. We find that αFKAqr=2.60+/-0.34,αV1054Oph=2.74+/-0.35, andαADLeo=2.03-2.32, and the flare component accounts fora large fraction (generally greater than 50%) of the total flux.
| UBV(RI)C photometry of Hipparcos red stars
We present homogeneous and standardized UBV(RI)C photometryfor nearly 550 M stars selected from the Hipparcos satellite data baseusing the following selection criteria: lack of obvious variability (noHipparcos variability flag); δ<+10°(V-I)>1.7 and Vmagnitude fainter than about 7.6. Comparisons are made between thecurrent photometry, other ground-based data sets and Hipparcosphotometry. We use linear discriminant analysis to determine aluminosity segregation criterion for late-type stars, and principalcomponent analysis to study the statistical structure of the colourindices and to calibrate absolute magnitude in terms of (V-I) for thedwarf stars. Various methods are used to determine the mean absolutemagnitude of the giant stars. We find 10 dwarf stars, apparentlypreviously unrecognized (prior to Hipparcos) as being within 25pc,including five within 20pc.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| The WARPS Survey. VI. Galaxy Cluster and Source Identifications from Phase I
We present in catalog form the optical identifications for objects fromthe first phase of the Wide Angle ROSAT Pointed Survey (WARPS). WARPS isa serendipitous survey of relatively deep, pointed ROSAT observationsfor clusters of galaxies. The X-ray source detection algorithm used byWARPS is Voronoi Tessellation and Percolation (VTP), a technique whichis equally sensitive to point sources and extended sources of lowsurface brightness. WARPS-I is based on the central regions of 86 ROSATPSPC fields, covering an area of 16.2 square degrees. We describe herethe X-ray source screening and optical identification process forWARPS-I, which yielded 34 clusters at 0.06
| A Near-Infrared, Wide-Field, Proper-Motion Search for Brown Dwarfs
A common proper-motion survey of M dwarf stars within 8 pc of the Sunreveals no new stellar or brown dwarf companions at wide separations(~100-1400 AU). This survey tests whether the brown dwarf ``desert''extends to large separations around M dwarf stars and further exploresthe census of the solar neighborhood. The sample includes 66 stars northof -30° and within 8 pc of the Sun. Existing first-epoch images arecompared with new J-band images of the same fields an average of 7 yrlater to reveal proper-motion companions within a ~4' radius of theprimary star. No new companions are detected to a J-band limitingmagnitude of ~16.5, corresponding to a companion mass of ~40 Jupitermasses for an assumed age of 5 Gyr at the mean distance of the objectsin the survey, 5.8 pc.
| The Solar Neighborhood. VI. New Southern Nearby Stars Identified by Optical Spectroscopy
Broadband optical spectra are presented for 34 known and candidatenearby stars in the southern sky. Spectral types are determined using anew method that compares the entire spectrum with spectra of more than100 standard stars. We estimate distances to 13 candidate nearby starsusing our spectra and new or published photometry. Six of these starsare probably within 25 pc, and two are likely to be within the ResearchConsortium on Nearby Stars (RECONS) horizon of 10 pc.
| Hα flares from V404 Cyg in quiescence
We present a spectrophotometric study of short-term optical variabilityin the quiescent black hole X-ray transient V404 Cyg. This includes twonights of high-time-resolution Hα spectroscopy with which weresolve much of the time variability, and a further six nights ofarchival spectroscopy with lower time resolution but higher spectralresolution. We find significant variability in most of the dataconsidered, with both the Hα line and the continuum often varyingin a correlated way. This includes both dramatic flares lasting a fewhours in which the line flux nearly doubles and lower-level flickering.The strongest flares involve development of asymmetry in the lineprofile, with the red wing usually strongest independent of orbitalphase. It is unclear why this is the case, but we discuss severalpossible explanations. We consider the energetics of the flares andcompare with plausible models including chromospheric activity on thecompanion star, local magnetic reconnection events within the disc andvarying irradiation from close to the black hole. Based on the lineprofile changes during the flares, we conclude that the most likelyorigin for the variability is variable photoionization by the centralsource, although local flares within the disc cannot be ruled out.
| Relative orientation of orbits in triple stars
Statistical analysis of the relative alignment of inner and outer orbitsin triple systems resulting from a dynamical decay of small-N clusters(N le10 ) is presented and compared to the statistics of real multiplestars. The distribution of the relative angle Phi between the angularmomentum vectors of inner and outer orbits in triple stars formed bydecay is shown to depend on the initial cluster conditions likegeometry, mass function, rotational and thermal energy. For a realisticset of initial conditions, a modest alignment of orbital momentumvectors is found, in good agreement with the latest observational dataon visual multiple stars exhibiting an average < Phi > between67o and 79o. The relation between eccentricitiesof outer orbits and period ratios for both simulated and real triples isconsistent with a slightly adjusted formulation of the stabilitycriterium by Mardling & Aarseth (2001). Dynamical decay cantherefore explain the weak correlation of orbital orientations observedin multiple stars. Using modern high-resolution techniques, the observedstatistics of Phi should be extended as it will allow one to sensitivelyconstrain properties of initial clusters. Table 4 is only available inelectronic form at the 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/384/1030
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Observation and Astrometry data
| Constellation: | Ophiuchus |
| Right ascension: | 16h55m28.77s |
| Declination: | -08°20'11.0" |
| Apparent magnitude: | 9.048 |
| Distance: | 5.74 parsecs |
| Proper motion RA: | -809.9 |
| Proper motion Dec: | -894 |
| B-T magnitude: | 11.055 |
| V-T magnitude: | 9.214 |
Catalogs and designations:
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