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The Search for Super-saturation in Chromospheric Emission
We investigate if the super-saturation phenomenon observed at X-raywavelengths for the corona exists in the chromosphere for rapidlyrotating late-type stars. Moderate resolution optical spectra offast-rotating EUV- and X-ray-selected late-type stars were obtained.Stars in ? Per were observed in the northern hemisphere with theIsaac Newton 2.5 m telescope and Intermediate Dispersion Spectrograph.Selected objects from IC 2391 and IC 2602 were observed in the southernhemisphere with the Blanco 4 m telescope and R-C spectrograph at CTIO.Ca II H and K fluxes were measured for all stars in our sample. We findthe saturation level for Ca II K at log (L CaK/Lbol) = -4.08. The Ca II K flux does not show a decrease as afunction of increased rotational velocity or smaller Rossby number asobserved in the X-ray. This lack of "super-saturation" supports the ideaof coronal stripping as the cause of saturation and super-saturation instellar chromospheres and coronae, but the detailed underlying mechanismis still under investigation.
| Orbital and physical parameters of eclipsing binaries from the All-Sky Automated Survey catalogue. III. Two new low-mass systems with rapidly evolving spots
Aims: We present the results of our spectroscopic and photometricanalysis of two newly discovered low-mass detached eclipsing binariesfound in the All-Sky Automated Survey (ASAS) catalogue: ASASJ093814-0104.4 and ASAS J212954-5620.1. Methods: Using the Grating Instrument for Radiation Analysis with aFibre-Fed Echelle (GIRAFFE) on the 1.9-m Radcliffe telescope at theSouth African Astronomical Observatory (SAAO) and the University CollegeLondon Echelle Spectrograph (UCLES) on the 3.9-m Anglo-AustralianTelescope, we obtained high-resolution spectra of both objects andderived their radial velocities (RVs) at various orbital phases. The RVsof both objects were measured with the two-dimensional cross-correlationtechnique (TODCOR) using synthetic template spectra as references. Wealso obtained V and I band photometry using the 1.0-m Elizabethtelescope at SAAO and the 0.4-m Panchromatic Robotic Optical Monitoringand Polarimetry Telescopes (PROMPT) located at the Cerro TololoInter-American Observatory (CTIO). The orbital and physical parametersof the systems were derived with PHOEBE and JKTEBOP codes. We comparedour results with several sets of widely-used isochrones. Results:Our multi-epoch photometric observations demonstrate that both objectsshow significant out-of-eclipse modulations, which vary in time. Webelieve that this effect is caused by stellar spots, which evolve ontime scales of tens of days. For this reason, we constructed our modelson the basis of photometric observations spanning short time scales(less than a month). Our modeling indicates that (1) ASASJ093814-0104.04 is a main sequence active system withnearly-twin components with masses of M1 = 0.771 ±0.033 M&sun;, M2 = 0.768 ± 0.021M&sun; and radii of R1 = 0.772 ± 0.012R&sun; and R2 = 0.769 ± 0.013R&sun;. (2) ASAS J212954-5620.1 is a mainsequence active binary with component masses of M1 = 0.833± 0.017 M&sun;, M2 = 0.703 ± 0.013M&sun; and radii of R1 = 0.845 ± 0.012R&sun; and R2 = 0.718 ± 0.017R&sun;. Conclusions: Both systems seem to confirm thewell-known characteristic of active low-mass stars, for which theobserved radii are larger and the temperatures lower than predicted byevolutionary models. Other parameters agree within errors with themodels of main sequence stars. The time-varying spot configuration mayimply a variable level of activity, which may manifest itself in smallchanges of the measured radii.
| Orbital and physical parameters of eclipsing binaries from the All-Sky Automated Survey catalogue. II. Two spotted M < 1 M_? systems at different evolutionary stages
Aims: We present the results of our detailed spectroscopic andphotometric analysis of two previously unknown <1 M_? detachedeclipsing binaries: ASAS J045304-0700.4 and ASAS J082552-1622.8. Methods: With the HIgh Resolution Echelle Spectrometer (HIRES) on theKeck-I telescope, we obtained spectra of both objects covering largefractions of orbits of the systems. We also obtained V and I bandphotometry with the 1.0-m Elizabeth telescope of the South AfricanAstronomical Observatory (SAAO). The orbital and physical parameters ofthe systems were derived with the PHOEBE and JKTEBOP codes. Weinvestigated the evolutionary status of both binaries with several setsof widely-used isochrones. Results: Our modelling indicates that(1) ASAS J045304-0700.4 is an old, metal-poor, active system withcomponent masses of M1 = 0.8338 ± 0.0036M?, M2 = 0.8280 ± 0.0040M? and radii of R1 = 0.848 ± 0.005R? and R2 = 0.833 ± 0.005R?, which places it at the end of the Main Sequenceevolution - a stage rarely observed for this type of stars. (2) ASASJ082552-1622.8 is a metal-rich, active binary with component masses ofM1 = 0.7029 ± 0.0045 M?, M2= 0.6872 ± 0.0049 M? and radii of R1 =0.694+0.007-0.011 R? andR2 = 0.699+0.011-0.014R?. Both systems show significant out-of-eclipsevariations, probably owing to large, cold spots. We also investigatedthe influence of a third light in the second system.Light curves are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/526/A29
| 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.
| Spectroscopic survey of ASAS eclipsing variables: search for chromospherically active eclipsing binary stars - I
We have started a spectroscopic survey to identify new chromosphericallyactive components and low-mass pre-main sequence (PMS) stars in recentlydiscovered All Sky Automated Survey (ASAS) eclipsing binaries. In thispaper, we briefly describe our scientific motivation, the observingtools and the results obtained from the first phase of this survey.Using the available observing facilities in India, the spectroscopicobservations of a sample of 180 candidate eclipsing binary starsselected from ASAS-I&II releases were carried out during 2004-2006.The strength of H? emission was used to characterize the level ofchromospheric activity. Our spectroscopic survey reveals that out of 180stars about 36 binary systems show excess H? emission. One of theobjects in our sample, ASAS 081700-4243.8, displays very strong H?emission. Follow-up high-resolution spectroscopic observations revealthat this object is indeed very interesting and most likely a classicalBe-type system with K0III companion.
| Coronal Activity from the ASAS Eclipsing Binaries
We combine the catalog of eclipsing binaries from the All Sky AutomatedSurvey (ASAS) with the ROSAT All Sky Survey (RASS). The combinationresults in 836 eclipsing binaries that display coronal activity and isthe largest sample of active binary stars assembled to date. By usingthe (V-I) colors of the ASAS eclipsing binary catalog, we are able todetermine the distances and thus bolometric luminosities for themajority of eclipsing binaries that display significant stellaractivity. A typical value for the ratio of soft X-ray to bolometricluminosity is LX/Lbol? a few×10-4, similar to the ratio of soft X-ray to bolometricflux FX/Fbol in the most active regions of theSun. Unlike rapidly rotating isolated late-type dwarfs - stars withsignificant outer convection zones - a tight correlation between Rossbynumber and activity of eclipsing binaries is absent. We find evidencefor the saturation effect and marginal evidence for the so-called``super-saturation'' phenomena. Our work shows that wide-field stellarvariability searches can produce a high yield of binary stars withstrong coronal activity.The combined ASAS and RASS catalog, as well as the results of this workare available for download.
| EUVE J0825-16.3 and EUVE J1501-43.6: two dMe^ double-lined spectroscopic binaries.
High resolution echelle spectroscopic observations taken with the FEROSspectrograph at the ESO 2.2m telescope confirm the binary nature of thedMe stars EUVE J0825-16.3 and EUVE J1501-43.6, previously reported byChristian & Mathioudakis (2002). In these binary systems, emissionof similar intensity from both components is detected in the Na I D[1]& D[2], He I D[3], Ca II H & K, Ca II IRT and Balmer lines. Wehave determined precise radial velocities by cross-correlation withradial velocity standard stars, which have allowed us to obtain for thefirst time the orbital solution of these systems. Both binaries consistof two near-equal M0V components with an orbital period shorter than 3days. We have analyzed the behaviour of the chromospheric activityindicators (variability and possible flares). In addition, we havedetermined its rotational velocity and kinematics.
| Identification of New M Dwarfs in the Solar Neighborhood
We present the results from a spectroscopic study of 1080 nearby activeM dwarfs, selected by correlating the Two Micron All Sky Survey andROSAT catalogs. We have derived the spectral types and estimateddistances for all of our stars. The spectral types range between K5 andM6. Nearly half of our stars lie within 50 pc. We have measured theequivalent width of the Hα emission line. Our targets show anincrease in chromospheric activity from early to midspectral types, witha peak in activity around M5. Using the count rate and hardness ratiosobtained from the ROSAT catalog, we have derived the X-ray luminosities.Our stars display a ``saturation-type'' relation between thechromospheric and coronal activity. The relation is such thatlogLX/Lbol remains ``saturated'' at a value ofapproximately -3 for varying Hα equivalent width. We have found568 matches in the USNO-B catalog and have derived the tangentialvelocities for these stars. There is a slight trend of decreasingchromospheric activity with age, such that the stars with highervtan have lower Hα equivalent widths. The coronalemission, however, remains saturated at a value oflogLX/Lbol~-3 for varying tangential velocities,suggesting that the coronal activity remains saturated with age. We donot find any break in the saturation-type relation at the spectral typeat which stars become fully convective (~M3.5). Most of the stars in oursample show more coronal emission than the dMe stars in the Hyades andPraesepe and have vtan<40 km s-1, suggesting ayoung population.
| The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun
Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSATsurveys, a sample of 100 stars most luminous in X-rays within or arounda distance of 50 pc is culled. The smallest X-ray luminosity in thesample, in units of 1029 ergs s-1, isLX=9.8 the strongest source in the solar neighborhood is IIPeg, a RS CVn star, at LX=175.8. With respect to the originof X-ray emission, the sample is divided into partly overlapping classesof pre-main-sequence, post-T Tauri, and very young ZAMS objects (typeXY), RS CVn-type binary stars (type RS), other active short-periodbinaries, including binary BY Dra-type objects (type XO), apparentlysingle or long-period binary active evolved stars (type XG), contactbinaries of WU UMa kind (type WU), apparently single or long-periodbinary variable stars of BY Dra kind (type BY), and objects of unknownnature (type X?). Chromospherically active, short-period binaries (RSand XO) make up 40% of the brightest X-ray emitters, followed by youngstars (XY) at 30% and unknown sources (X?) at 15%. The fraction ofspectroscopically single evolved X-ray emitters of spectral classes IVand III is quite large (10%). The sources identified as RS CVn-typestars (RS, 23 objects) are considerably stronger in X-ray than theXY-objects and the other active binaries (XO and WU, 20 objects). Sevenobjects have LX>100, all RS except one XY, viz., BO Mic. Onlyfive (22%) RS objects have LX<25, while only three (10%)XY stars have LX>25. Formally, the limit of LX=25could serve as a statistical criterion to differentiate RS and XY stars.However, the other short-period binaries (including eclipsing stars ofAlgol and β Lyr type) have a distribution of LX verysimilar to the XY objects. The contact binaries (WU) appear to be muchweaker in X-rays than their detached counterparts of RS type, but thesample of the former is too small (three objects) to reach a firmconclusion. Sources matched with giants (either single or in binaries)are found to be significantly harder, with only 7% of hardness ratiosbelow 0, than subgiants (66% of HR1<0) and dwarfs (59% of HR1<0).Almost all objects in the sample are binary or multiple stars; thefraction of components (FC), defined as the total number of componentsin all binary and multiple systems divided by the sum of the totalnumber of components and single stars, is at least 0.90. The FC for theXY objects reaches 0.81, and for the unknown type 0.89. About 70% of RSobjects have also visual or astrometric companions, which makes themhierarchical multiple systems. The RS objects (mostly old, evolvedstars) and the XY stars have quite different kinematics. While the RSobjects move at considerable velocities in apparently random directionswith respect to the local standard of rest, the young stars have smallerand orderly velocities and tend to comprise expanding mini-associationssuch as the β Pic and the Tucana groups. The majority of the youngX-ray active stars belong to the Pleiades stream with the meanheliocentric velocity (U,V,W)=(-9.6,-21.8,-7.7) km s-1.
| 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
| High-Resolution Optical Observations of Extreme-Ultraviolet-selected Active Late-type Stars
We present high-resolution optical spectra of EUV-selected late-typestars. We have detected seven new spectroscopic binaries, including apair of F stars and an RS Canum Venaticorum candidate. Many of the starsin our sample show remarkable Hα emission with equivalent widthsup to 8 Å. Several sources show strong He I 5876 Å emission.We use cross-correlation techniques to estimate line-of-sight rotationalvelocities and find that they range from less than 5 to over 80 kms-1. Several of the stars show a strong Li 6707 Å line,which we use to determine their Li abundances. The EUV-to-bolometricluminosity, a measure of the coronal heating efficiency, is as high as10-3. The present sample extends the EUV-selected late-typestars to lower Rossby numbers and makes the coronal saturation boundarybetter pronounced. Our results indicate that there is a drop in the EUVemission from the corona in late M dwarfs possibly because of a changein the nature of the magnetic dynamo in fully convective stars.
| Faint Sources in the EUVE Survey: Identification of White Dwarfs, Active Late-Type Stars, and Galactic Nuclei
We report the classification of 21 new extreme-ultraviolet sources fromthe recent catalog of Lampton et al. The optical spectra presentedidentify the objects as 14 active late-type stars (including two doubleactive stars and a possible T Tauri star), three white dwarfs, and sixactive galactic nuclei (a Seyfert galaxy, the BL Lac object 1ES 1028+511[=EUVE J1031+508], and four quasi-stellar objects). We have detected CaII absorption lines in the BL Lac object and measured its redshift. Twoof the white dwarfs are unusually massive (M > 1.1 M?). Oursample of late-type stars includes five previously known high propermotion objects (EUVE J1004+503, J2244-332A,B, J1802+642, and J1131-346),of which one is the well-known flare star TX PsA (EUVE J2244-332B). Wereport an unusually high level of activity for the primary component ofthe TX PsA system (EUVE J2244-332A), which may indicate flare activity.The group of late-type stars is on average almost 3 mag fainter( ~ 13) than the typical member of the Extreme UltravioletExplorer (EUVE) all-sky survey catalog. All Galactic and extragalacticobjects were also detected in the ROSAT Position Sensitive ProportionalCounter survey, and most are at the faint limit of the EUVE detectors.These new identifications substantially increase the total number ofEUV-selected extragalactic sources.
| Extreme Ultraviolet Explorer Optical Identification Campaign.III. A Southern Hemisphere Sample of Active Late-Type Stars and White Dwarfs
We present optical identifications of 30 previously unidentified extremeultraviolet (EUV) and soft x-ray sources coinciding with the position offaint sources detected during the all-sky surveys of the ExtremeUltraviolet Explorer ({\euve}) in the 58-174 Angstroms (0.071-0.214 keV)band and of the \rosat\ Position Sensitive Proportional Counter (PSPC)in the 5-120 Angstroms (0.1-2.4 keV x-ray) band. We presentlow-resolution spectra of the possible optical counterparts of the x-rayand \euve\ sources obtained with 1.5 m telescope at CTIO in 1995 May andDecember and 1996 February. The optical counterparts for 23 sources areidentified as late-type (dKe and dMe) stars, and 4 are identified as newDA white dwarfs. All spectra of sources identified as late-type starsshow the Balmer series and Ca H and K lines in emission, indicating thesignature of chromospheric activity. For the white dwarfs, the jointdetection by the \rosat\ PSPC and the \euve\ Lexan band indicates thatthey have a low-abundance of metals and that their EUV emission ismostly attenuated by the local interstellar medium (ISM).
| An All-Sky Catalog of Faint Extreme Ultraviolet Sources
We present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects.
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