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Submillijansky Transients in Archival Radio Observations
We report the results of a 944 epoch survey for transient sources witharchival data from the Very Large Array spanning 22 years with a typicalepoch separation of 7 days. Observations were obtained at 5 or 8.4 GHzfor a single field of view and achieved a typical point-source detectionthreshold at the beam center of 300 μJy per epoch. Ten transientsources were detected with a significance threshold such that only onefalse positive would be expected. Of these transients, eight weredetected in only a single epoch. Two transients were too faint to bedetected in individual epochs but were detected in 2 month averages.None of the 10 transients was detected in longer term averages orassociated with persistent emission in the deep image produced from thecombination of all epochs. The cumulative rate for the short-timescaleradio transients above 370 μJy at 5 and 8.4 GHz is 0.07deg-2 yr-1<~R<~40 deg-2yr-1, where the uncertainty is due to the unknown duration ofthe transients, 20 minutes < tchar < 7 days. Atwo-epoch survey for transients will detect 1.5+/-0.4 transients persquare degree above a flux density of 370 μJy. Two transients areassociated with galaxies at z=0.040 and z=0.249. These may be similar tothe peculiar Type Ib/c radio supernova SN 1998bw associated with GRB980428. We detect no counterparts for six transients in deep optical andinfrared imaging. The hosts and progenitors of these transients areunknown.
| X-ray flares on the UV Ceti-type star CC Eridani: a ``peculiar'' time-evolution of spectral parameters
Context: Weak flares are supposed to be an important heating agent ofthe outer layers of stellar atmospheres. However, due to instrumentallimitations, only large X-ray flares have been studied in detail untilnow. Aims: We used an XMM-Newton observation of the very active BY-Dratype binary star CC Eri in order to investigate the properties of twoflares that are weaker than those typically studied in the literature. Methods: We performed time-resolved spectroscopy of the data taken withthe EPIC-PN CCD camera. A multi-temperature model was used to fit thespectra. We inferred the size of the flaring loops using thedensity-temperature diagram. The loop scaling laws were applied forderiving physical parameters of the flaring plasma. We also estimatedthe number of loops involved in the observed flares. Results: A largeX-ray variability was found. Spectral analysis showed that all theregions in the light curve, including the flare segments, arewell-described by a 3-T model with variable emission measures but,surprisingly, with constant temperatures (values of 3, 10 and 22 MK).The analysed flares lasted ~3.4 and 7.1 ks, with flux increases offactors 1.5-1.9. They occurred in arcades made of a few tens of similarcoronal loops. The size of the flaring loops is much smaller than thedistance between the stellar surfaces in the binary system, and evensmaller than the radius of each of the stars. The obtained results areconsistent with the following ideas: (i) the whole X-ray light curve ofCC Eri could be the result of a superposition of multiple low-energyflares; and (ii) stellar flares can be scaled-up versions of solarflares.Basedon observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA MemberStates and NASA.
| On X-Ray Optical Depth in the Coronae of Active Stars
We have investigated the optical thickness of the coronal plasma throughthe analysis of high-resolution X-ray spectra of a large sample ofactive stars observed with the High Energy Transmission GratingSpectrometer on Chandra. In particular, we probed for the presence ofsignificant resonant scattering in the strong Lyman series lines arisingfrom hydrogen-like oxygen and neon ions. The active RS CVn-type binariesII Peg and IM Peg and the single M dwarf EV Lac show significant opticaldepth. For these active coronae, the Lyα/Lyβ ratios aresignificantly depleted as compared with theoretical predictions and withthe same ratios observed in similar active stars. Interpreting thesedecrements in terms of resonance scattering of line photons out of theline of sight, we are able to derive an estimate for the typical size ofcoronal structures, and from these we also derive estimates of coronalfilling factors. For all three sources we find that both the photon pathlength as a fraction of the stellar radius and the implied surfacefilling factors are very small and amount to a few percent at most. Themeasured Lyα/Lyβ ratios are in good agreement with APEDtheoretical predictions, thus indicating negligible optical depth, forthe other sources in our sample. We discuss the implications for coronalstructuring and heating flux requirements. For the stellar sample as awhole, the data suggest increasing quenching of Lyα relative toLyβ as a function of both LX/Lbol and thedensity-sensitive Mg XI forbidden-to-intercombination line ratio, asmight generally be expected.
| X-ray emission from T Tauri stars and the role of accretion: inferences from the XMM-Newton extended survey of the Taurus molecular cloud
Context: T Tau stars display different X-ray properties depending onwhether they are accreting (classical T Tau stars; CTTS) or not(weak-line T Tau stars; WTTS). X-ray properties may provide insight intothe accretion process between disk and stellar surface. Aims: We usedata from the XMM-Newton Extended Survey of the Taurus molecular cloud(XEST) to study differences in X-ray properties between CTTS and WTTS. Methods: XEST data are used to perform correlation and regressionanalysis between X-ray parameters and stellar properties. Results: Weconfirm the existence of a X-ray luminosity (L_X) vs. mass (M) relation,L_X∝ M1.69~± 0.11, but this relation is aconsequence of X-ray saturation and a mass vs. bolometric luminosity(L_*) relation for the TTS with an average age of 2.4 Myr. X-raysaturation indicates LX = const.L_*, although the constant isdifferent for the two subsamples: const. = 10-3.73~±0.05 for CTTS and const. = 10-3.39~± 0.06 forWTTS. Given a similar L* distribution of both samples, theX-ray luminosity function also reflects a real X-ray deficiency in CTTS,by a factor of ≈2 compared to WTTS. The average electron temperaturesT_av are correlated with LX in WTTS but not in CTTS; CTTSsources are on average hotter than WTTS sources. At best marginaldependencies are found between X-ray properties and mass accretion ratesor age. Conclusions: The most fundamental properties are the twosaturation laws, indicating suppressed LX for CTTS. Wespeculate that some of the accreting material in CTTS is cooling activeregions to temperatures that may not significantly emit in the X-rayband, and if they do, high-resolution spectroscopy may be required toidentify lines formed in such plasma, while CCD cameras do not detectthese components. The similarity of the LX vs. T_avdependencies in WTTS and main-sequence stars as well as their similarX-ray saturation laws suggests similar physical processes for the hotplasma, i.e., heating and radiation of a magnetic corona.
| Rapid magnetic flux variability on the flare star CN Leonis
We present UVES/VLT observations of the nearby flare star CN Leocovering the Wing-Ford FeH band near 1 μm with high spectralresolution. Some of the FeH absorption lines in this band aremagnetically sensitive and allow a measurement of the mean magnetic fluxon CN Leo. Our observations, covering three nights separated by 48 hourseach, allow a clear detection of a mean magnetic field of Bf ≈ 2.2kG. The differential flux measurements show a night-to-night variabilitywith extremely high significance. Finally, our data strongly suggestmagnetic flux variability on time scales as low as 6 hours in line withchromospheric variability.Based on observationscollected at the European Southern Observatory, Paranal, Chile,077.D-0011.
| Fast colorimetry of the flare star EV Lacertae from UBVRI observations in 2004
Aims.We report results of a quantitative colorimetric UBVRI analysis oftwo flare events on the red dwarf EV Lac. The photometric data wereobtained in September 2004, during the multi-site synchronous monitoringfrom the four observatories in Ukraine, Russia, Greece, and Bulgaria.These observations confirmed the presence of small-scale high-frequencyoscillations (HFO) initially detected by Rodonó (1974, A&A,32, 337) and recently reconfirmed by the authors. Here we discuss thecolor characteristics of flares and HFO. Methods: Colorimetric analysishad been performed with the help of the time tracks in the UBVRIcolor-color diagrams from the earliest phase of flare development.Digital filtering technique was used to evaluate the time-dependentcolor indices. Results: As can be clearly seen in the diagrams, colorindices oscillate on a time scale of seconds, far exceeding instrumentalerrors. Regarding the HFO, we conclude that the bulk of a flareoscillates during a major part of its lifetime between the states ofhydrogen plasma opaque and transparent in the Balmer continuum. We findthat at the peaks of oscillations the color tracks drift into theregions of color-color diagrams corresponding to a blackbody radiation,which provides an estimate of color temperatures from 17 000 to 22 000K. We also find that flares cover ~1% of the stellar disc.
| The First Direct Measurements of Surface Magnetic Fields on Very Low Mass Stars
We present the first direct magnetic field measurements on M dwarfscooler than spectral class M4.5. Utilizing a new method based on the FeHband near 1 μm, we categorize the integrated surface magnetic flux aslow (well under 1 kG), intermediate (between 1 and about 2.5 kG), orstrong (greater than about 3 kG) for a set of more than 20 stars rangingfrom M2 down to M9. Along with the field, we also measure the rotationalbroadening (vsini) and Hα emission strength. Our goal is toadvance the understanding of how dynamo field production varies withstellar parameters for very low mass stars, how the field and emissionactivity are related, and whether there is a connection between therotation and magnetic flux. We find that fields are produced throughoutthe M dwarfs. In the mid-M stars, there is a clear connection betweenslow rotation and weak fields. In the late-M stars, rotation is alwaysmeasurable, and the strongest fields are associated with with the mostrapid rotators. Interestingly, these very cool rapid rotators appear tohave the largest magnetic flux in the whole sample (greater than in theclassical dMe stars). Hα emission is found to be a good proxy formagnetic fields, although the relation between the fractional emissionand the magnetic flux varies with effective temperature. The drop-off inthis fractional emission near the bottom of the main sequence is notaccompanied by a drop-off in magnetic flux. It is clear that themethodology we have developed can be further applied to discover moreabout the behavior of magnetic dynamos and activity in cool and fullyconvective objects.
| Theoretical mass loss rates of cool main-sequence stars
Context: The stellar mass loss rate is important for the rotationalevolution of a star and for its interaction with the circumstellarenvironment. The analysis of astrospheric absorption features enables anempirical determination of mass loss rates of cool stars other than theSun. Aims: We develop a model for the wind properties of coolmain-sequence stars, which comprises their wind ram pressures, massfluxes, and terminal wind velocities. Methods: The wind properties aredetermined through a polytropic magnetised wind model, assuming powerlaws for the dependence of the thermal and magnetic wind parameters onthe stellar rotation rate. We use the empirical data to constraintheoretical wind scenarios, which are characterised by different ratesof increase of the wind temperature, wind density, and magnetic fieldstrength. Results: Scenarios based on moderate rates of increase yieldwind ram pressures in agreement with most empirical constraints, butcannot account for some moderately rotating targets, whose high apparentmass loss rates are inconsistent with observed coronal X-ray andmagnetic properties. For fast magnetic rotators, the magneto-centrifugaldriving of the outflow can produce terminal wind velocities far inexcess of the surface escape velocity. Disregarding this aspect in theanalyses of wind ram pressures leads to overestimations of stellar massloss rates. The predicted mass loss rates of cool main-sequence stars donot exceed about ten times the solar value. Conclusions: .Our resultsare in contrast with previous investigations, which found a strongincrease of the stellar mass loss rates with the coronal X-ray flux.Owing to the weaker dependence, we expect the impact of stellar winds onplanetary atmospheres to be less severe and the detectability ofmagnetospheric radio emission to be lower then previously suggested.Considering the rotational evolution of a 1 {Mȯ} star,the mass loss rates and the wind ram pressures are highest during thepre-main sequence phase.
| Discovery of Nine New Companions to Nearby Young M Stars with the Altair AO System
We present results of a high-resolution, near-infrared survey of 41nearby, young (<~300 Myr) M0-M5.0 dwarfs using the Altair naturalguide star adaptive optics system at the Gemini North telescope. Twelveof the objects appear to be binaries, seven of which are reported herefor the first time. One triple system was discovered. Statisticalproperties are studied and compared with earlier (F to K) and later(>=M6 very low mass [VLM]) populations. We find that the separationdistribution of the binaries in this sample peaks at13+14-9 AU, which is consistent with previousmeasurements of early M binaries. Hence, early M binaries seem to occurin-on average-tighter systems than G binaries. At the same time they aresignificantly wider than field VLM binary stars. The distribution ofmass ratios q of primary and secondary stars was found to show anintermediate distribution between the strongly q-->1 peakeddistribution of field VLM systems and the almost flat distribution ofearlier type stars. Consequently, we show evidence for relatively young,early M binaries representing a transition between the well-knownearlier star distributions and the recently examined field VLMpopulation characteristics. Despite the fact that this survey wasdedicated to the search for faint brown dwarf and planetary masscompanions, all planetary mass candidates were background objects. Weexclude the existence of physical companions with masses greater than 10Jupiter masses (MJ) at separations of >~40 AU and massesgreater than 24MJ for separations >~10 AU around 37 of the41 observed objects.
| Search for cold debris disks around M-dwarfs
Debris disks are believed to be related to planetesimals left overaround stars after planet formation has ceased. The frequency of debrisdisks around M-dwarfs which account for 70% of the stars in the Galaxyis unknown while constrains have already been found for A- to K-typestars. We have searched for cold debris disks around 32 field M-dwarfsby conducting observations at λ = 850~μm with the SCUBAbolometer array camera at the JCMT and at λ = 1.2 mm with theMAMBO array at the IRAM 30-m telescopes. This is the first survey of alarge sample of M-dwarfs conducted to provide statistical constraints ondebris disks around this type of stars. We have detected a new debrisdisk around the M0.5 dwarf GJ 842.2 at λ = 850~μm, providingevidence for cold dust at large distance from this star (~300 AU). Bycombining the results of our survey with the ones of Liu et al. (2004),we estimate for the first time the detection rate of cold debris disksaround field M-dwarfs with ages between 20 and 200 Myr. This detectionrate is 13+6-8% and is consistent with thedetection rate of cold debris disks (9-23%) around A- to K-type mainsequence stars of the same age. This is an indication that cold disksmay be equally prevalent across stellar spectral types.
| A Simultaneous Optical and X-Ray Variability Study of the Orion Nebula Cluster. I. Incidence of Time-correlated X-Ray/Optical Variations
We present a database of BVRI time-series photometry of the Orion NebulaCluster obtained with two ground-based telescopes at differentlongitudes to provide simultaneous coverage with the 13 day Chandraobservation of the cluster. The resulting database of simultaneousoptical and X-ray light curves for some 800 pre-main-sequence (PMS)stars represents, by a factor of hundreds, the largest synoptic,multi-wavelength-regime, time-series study of young stars to date. Thisdatabase will permit detailed analyses of the relationship betweenoptical and X-ray variability among a statistically significant ensembleof PMS stars, with the goal of elucidating the origins of PMS X-rayproduction. In this first paper, we present the optical observations,describe the combined X-ray/optical database, and perform an analysis oftime-correlated variability in the optical and X-ray light curves. Weidentify 40 stars (representing 5% of our study sample) with possibletime-correlated optical and X-ray variability. Examples of both positiveand negative time correlations are found, possibly representing X-rayflares and persistent coronal features associated with both cool and hotsurface spots (i.e., magnetically active regions and accretion shocks).We also find two possible examples of ``white-light'' flares coincidentwith X-ray flares; these may correspond to the impulsive heating phasein solar-analog flares. However, although interesting, these representunusual cases. More generally, we find very little evidence to suggest adirect causal link between the sources of optical and X-ray variabilityin PMS stars. The conclusion that accretion is a primary driver of X-rayproduction in PMS stars is not supported by our findings.This study is part of the Chandra Orion Ultradeep Project (COUP).
| Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths
I present radio observations of 90 dwarf stars and brown dwarfs ofspectral type M5-T8. Three sources exhibit radio activity, in additionto the six objects previously detected in quiescence and outburst,leading to an overall detection rate of ~10% for objects later than M7.The inferred magnetic field strengths are ~102 G inquiescence and nearly 1 kG during flares, while the majority of thenondetected objects have B<~50 G. Depending on the configuration andsize of the magnetic loops, the surface fields may approach 1 kG even inquiescence, at most a factor of a few smaller than in early M dwarfs.With the larger sample of sources I find continued evidence for (1) asharp transition around spectral type of M7 from a ratio of radio toX-ray luminosity of log(LR/LX)~-15.5 to >~-12,(2) increased radio activity (LR/Lbol) with laterspectral type, in contrast to Hα and X-ray observations, and (3)an overall drop in the fraction of active sources from ~30% for M dwarfsto ~5% for L dwarfs, consistent with Hα and X-ray observations.Taken together, these trends suggest that some late M and L dwarfs arecapable of generating 0.1-1 kG magnetic fields, but the overall drop inthe fraction of such objects likely reflects changes in the structure ofthe chromospheres and coronae, possibly due to increasingly neutralatmospheres and/or a transition to a turbulent dynamo. Thesepossibilities can best be tested through simultaneous observations,which can trace the effect of magnetic dissipation in a direct, ratherthan a statistical, manner. Still, a more extended radio surveycurrently holds the best promise for measuring the magnetic fieldproperties of a large number of dwarf stars.
| From Radio to X-Ray: The Quiescent Atmosphere of the dMe Flare Star EV Lacertae
We report on multiwavelength observations spanning radio to X-raywavelengths of the M dwarf flare star EV Lacertae and probing thecharacteristics of the outer atmospheric plasma from the upperchromosphere to the corona. We detect the star at a wavelength of 2 cm(15 GHz) for the first time. UV and FUV line profiles show evidence ofnonthermal broadening, and the velocity width appears to peak at lowertemperatures than in the Sun; this trend is confirmed in another activeM dwarf flare star. Electron density measurements indicate nearlyconstant electron pressures between logT=5.2 and 6.4. At higher coronaltemperatures, there is a sharp increase of 2 orders of magnitude indensity (ne~1013 cm-3 at logT=6.9).X-ray, EUV, FUV, and NUV spectra constrain the differential emissionmeasure (DEM) from the upper chromosphere through the corona. Thecoronal pressures are inconsistent with the assumption of hydrostaticequilibrium, either through emission measure (EM) modeling orapplication of scaling laws, and imply large conductive loss rates and alarge energy input at the highest temperatures. The timescales forradiative and conductive losses in EV Lac's upper atmosphere imply thatsignificant continued heating must occur for the corona to maintain itsquiescent properties. The high-frequency radio detection requires thehigh-temperature X-ray-emitting coronal plasma to be spatially distinctfrom the radio emission source. Length scales in the low-temperaturecorona are markedly larger than those in the high-temperature corona,further suggestions of an inhomogeneous mixture of thermal andnonthermal coronal plasma.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| Magnetic Fields in M Dwarfs: Rapid Magnetic Field Variability in EV Lacertae
We report here our spectropolarimetric observations obtained using theCFHT Espadons high-resolution spectrograph of two M dwarf stars thatstandard models suggest are fully convective: EV Lac (M3.5) and HH And(M5.5). The difference in their rotational velocity makes them goodtargets for studying the dependence of the magnetic field topology in Mdwarfs on rotation. Our results reveal some aspects of the fieldtopology in EV Lac and HH And. We measured mean longitudinal magneticfield strengths (Bz) in EV Lac ranging from 18+/-3 to -40+/-3G. The Bz variations are seen to occur on a timescale of lessthan 50 minutes, which is significantly shorter than the rotationperiod, and are not due to a flaring event. We discuss some formationscenarios of the Zeeman signatures found in EV Lac. For HH And we couldnot detect circular polarization and thus we place an upper limit onBz of 5 G.
| Radio Constraints on Activity in Young Brown Dwarfs
We report on searches for radio emission from three of the nearest knownyoung brown dwarfs using the Very Large Array. We have obtainedsensitive upper limits on 3.6 cm emission from 2MASSW J1207334-393254,TWA 5B, and SSSPM J1102-3431, all of which are likely members of the ~8Myr old TW Hydrae association. We derive constraints on the magneticfield strength and the number density of accelerated electrons, underthe assumption that young brown dwarf atmospheres are able to producegyrosynchrotron emission, as seems to be indicated in older browndwarfs. For the young brown dwarf TWA 5B, the ratio of its detectedX-ray luminosity to the upper limit on radio luminosity places it withinthe expected range for young stars and older, active stars. Thus, itsbehavior is anomalous compared to older brown dwarfs, in which radioluminosity is substantially enhanced over the expected relationship. Ourobservations deepen the conundrum of magnetic activity in brown dwarfsand suggest that a factor other than age is more important fordetermining radio emission in cool substellar objects.
| Measuring Magnetic Fields in Ultracool Stars and Brown Dwarfs
We present a new method for direct measurement of magnetic fields onultracool stars and brown dwarfs. It takes advantage of the Wing-Fordbands of FeH, which are seen throughout the M and L spectral types.These molecular features are not as blended as other optical molecularbands, are reasonably strong through most of the spectral range, andexhibit a response to magnetic fields that is easier to detect thanother magnetic diagnostics, including the usual optical andnear-infrared atomic spectral lines that have heretofore been employed.The FeH bands show a systematic growth as the star gets cooler. We donot find any contamination by CrH in the relevant spectral region. Weare able to model cool and rapidly rotating spectra from warmer, slowlyrotating spectra utilizing an interpolation scheme based oncurve-of-growth analysis. We show that the FeH features can distinguishbetween negligible, moderate, and high magnetic fluxes on low-massdwarfs, with a current accuracy of about 1 kG. Two different approachesto extracting the information from the spectra are developed andcompared. Which one is superior depends on a number of factors. Wedemonstrate the validity of our new procedures by comparing the spectraof three M stars whose magnetic fluxes are already known from atomicline analysis. The low- and high-field stars are used to produceinterpolated moderate-strength spectra that closely resemble themoderate-field star. The assumption of linear behavior for the magneticeffects appears to be reasonable, but until the molecular constants arebetter understood, the method is subject to that assumption and ratherapproximate. Nonetheless, it opens a new regime of very low mass objectsto direct confirmation and testing of their magnetic dynamos.
| Dust in the solar system and in extra-solar planetary systems
Among the observed circumstellar dust envelopes a certain population,planetary debris disks, is ascribed to systems with optically thin dustdisks and low gas content. These systems contain planetesimals andpossibly planets and are believed to be systems that are most similar toour solar system in an early evolutionary stage. Planetary debris diskshave been identified in large numbers by a brightness excess in thenear-infrared, mid-infrared and/or submillimetre range of their stellarspectral energy distributions. In some cases, spatially resolvedobservations are possible and reveal complex spatial structures. Actingforces and physical processes are similar to those in the solar systemdust cloud, but the observational approach is obviously quite different:overall spatial distributions for systems of different ages for theplanetary debris disks, as opposed to detailed local information in thecase of the solar system. Comparison with the processes of dustformation and evolution observed in the solar system therefore helpsunderstand the planetary debris disks. In this paper, we review ourpresent knowledge of observations, acting forces, and major physicalinteractions of the dust in the solar system and in similar extra-solarplanetary systems.
| 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.
| Ca II H and K Chromospheric Emission Lines in Late-K and M Dwarfs
We have measured the profiles of the Ca II H and K chromosphericemission lines in 147 main-sequence stars of spectral type M5-K7 (masses0.30-0.55 Msolar) using multiple high-resolution spectraobtained during 6 years with the HIRES spectrometer on the Keck Itelescope. Remarkably, the average FWHM, equivalent widths, and lineluminosities of Ca II H and K increase by a factor of 3 with increasingstellar mass over this small range of stellar masses. We fit the Ca II Hand K lines with a double-Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of thesample stars display a central absorption that is typically redshiftedby ~0.1 km s-1 relative to the emission. This implies thatthe higher level, lower density chromospheric material has a smalleroutward velocity (or higher inward velocity) by 0.1 km s-1than the lower level material in the chromosphere, but the nature ofthis velocity gradient remains unknown. The FWHM of the Ca II H and Kemission lines increase with stellar luminosity, reminiscent of theWilson-Bappu effect in FGK-type stars. Both the equivalent widths andFWHM exhibit modest temporal variability in individual stars. At a givenvalue of MV, stars exhibit a spread in both the equivalentwidth and FWHM of Ca II H and K, due both to a spread in fundamentalstellar parameters, including rotation rate, age, and possiblymetallicity, and to the spread in stellar mass at a given MV.The K line is consistently wider than the H line, as expected, and itscentral absorption is more redshifted, indicating that the H and K linesform at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of Hαcorrelates with Ca II H and K only for stars having Ca II equivalentwidths above ~2 Å, suggesting the existence of a magneticthreshold above which the lower and upper chromospheres become thermallycoupled.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| Speckle interferometry of nearby multiple stars. III.
Not Available
| Metallicity of M dwarfs. I. A photometric calibration and the impact on the mass-luminosity relation at the bottom of the main sequence
We obtained high resolution ELODIE and CORALIE spectra for bothcomponents of 20 wide visual binaries composed of an F-, G- or K-dwarfprimary and an M-dwarf secondary. We analyse the well-understood spectraof the primaries to determine metallicities ([Fe/H]) for these 20systems, and hence for their M dwarf components. We pool thesemetallicities with determinations from the literature to obtain aprecise (±0.2 dex) photometric calibration of M dwarfmetallicities. This calibration represents a breakthrough in a fieldwhere discussions have had to remain largely qualitative, and it helpsus demonstrate that metallicity explains most of the large dispersion inthe empirical V-band mass-luminosity relation. We examine themetallicity of the two known M-dwarf planet-host stars, Gl876 (+0.02 dex) and Gl 436 (-0.03 dex), inthe context of preferential planet formation around metal-rich stars. Wefinally determine the metallicity of the 47 brightest single M dwarfs ina volume-limited sample, and compare the metallicity distributions ofsolar-type and M-dwarf stars in the solar neighbourhood.
| Oscillations of Optical Emission from Flare Stars and Coronal Loop Diagnostics
Based on an analogy between stellar and solar flares, we investigate theten-second oscillations detected in the U and B bands on the star EVLac. The emission pulsations are associated with fast magnetoacousticoscillations in coronal loops. We have estimated the magnetic field, B≈ 320 G; the temperature, T ≈ 3.7 × 107 K; and the plasmadensity, n ≈ 1.6 × 1011 cm-3, in the region of energy release.We provide evidence suggesting that the optical emission source islocalized at the loop footpoints.
| New Mass-Loss Measurements from Astrospheric Lyα Absorption
Measurements of stellar mass-loss rates are used to assess how windstrength varies with coronal activity and age for solar-like stars. Massloss generally increases with activity, but we find evidence that windssuddenly weaken at a certain activity threshold. Very active stars areoften observed to have polar starspots, and we speculate that themagnetic field geometry associated with these spots may be inhibitingthe winds. Our inferred mass-loss/age relation represents an empiricalestimate of the history of the solar wind. This result is important forplanetary studies as well as solar/stellar astronomy, since solar winderosion may have played an important role in the evolution of planetaryatmospheres.
| The `solar model problem' solved by the abundance of neon in nearby stars
The interior structure of the Sun can be studied with great accuracyusing observations of its oscillations, similar to seismology of theEarth. Precise agreement between helioseismological measurements andpredictions of theoretical solar models has been a triumph of modernastrophysics. A recent downward revision by 25-35 per cent of the solarabundances of light elements such as C, N, O and Ne (ref. 2) has,however, broken this accordance: models adopting the new abundancesincorrectly predict the depth of the convection zone, the depth profilesof sound speed and density, and the helium abundance. The discrepanciesare far beyond the uncertainties in either the data or the modelpredictions. Here we report neon-to-oxygen ratios measured in a sampleof nearby solar-like stars, using their X-ray spectra. The abundanceratios are all very similar and substantially larger than the recentlyrevised solar value. The neon abundance in the Sun is quite poorlydetermined. If the Ne/O abundance in these stars is adopted for the Sun,the models are brought back into agreement with helioseismologymeasurements.
| Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| X-ray properties of active M dwarfs as observed by XMM-Newton
We present a comparative study of X-ray emission from a sample of activeM dwarfs with spectral types using XMM-Newton observations of twosingle stars, AD Leonis and EV Lacertae, and two unresolved binarysystems, AT Microscopii and EQ Pegasi. The light curves reveal frequentflaring during all four observations. We perform a uniform spectralanalysis and determine plasma temperatures, abundances and emissionmeasures in different states of activity. Applying multi-temperaturemodels with variable abundances separately to data obtained with theEPIC and RGS detectors we are able to investigate the consistency of theresults obtained by the different instruments onboard XMM-Newton. Wefind that the X-ray properties of the sample M dwarfs are very similar,with the coronal abundances of all sample stars following a trend ofincreasing abundance with increasing first ionization potential, theinverse FIP effect. The overall metallicities are below solarphotospheric ones but appear consistent with the measured photosphericabundances of M dwarfs like these. A significant increase in theprominence of the hotter plasma components is observed during flareswhile the cool plasma component is only marginally affected by flaring,pointing to different coronal structures. AT Mic, probably a youngpre-main-sequence system, has the highest X-ray luminosity and exhibitsalso the hottest corona. While results of EQ Peg and EV Lac arepresented here for the first time, AT Mic and AD Leo have beeninvestigated before with different analysis approaches, allowing acomparison of the results.
| From Radio to X-Ray: Flares on the dMe Flare Star EV Lacertae
We present the results of a campaign to observe flares on the M dwarfflare star EV Lacertae over the course of two days in 2001 September,utilizing a combination of radio continuum, optical photometric andspectroscopic, ultraviolet spectroscopic, and X-ray spectroscopicobservations to characterize the multiwavelength nature of flares fromthis active, single, late-type star. We find flares in every wavelengthregion in which we observed. A large radio flare from the star wasobserved at both 3.6 and 6 cm and is the most luminous example of agyrosynchrotron flare yet observed on a dMe flare star. The radio flarecan be explained as encompassing a large magnetic volume, comparable tothe stellar disk, and involving trapped electrons that decay overtimescales of hours. Flux enhancements at 6 cm accompanied by highlynegatively circularly polarized emission (πc-->-100%)imply that a coherent emission mechanism is operating in the corona ofEV Lac. There are numerous optical white-light flares, and yet nosignature of emission-line response from the chromosphere appears. Twosmall ultraviolet enhancements differ in the amount of nonthermalbroadening present. There are numerous X-ray flares occurring throughoutthe observation, and an analysis of undispersed photons and gratingevents reveals no evidence for abundance variations. Higher temperaturesare present during some flares; however, the maximum temperatureachieved varies from flare to flare. There is no evidence for densityvariations during any flare intervals. In the multiwavelength context,the start of the intense radio flare is coincident with an impulsiveoptical U-band flare, to within 1 minute, and yet there is no signatureof an X-ray response. There are other intervals of time when opticalflaring and UV flaring is occurring, but these cannot be related to thecontemporaneous X-ray flaring: the time-integrated luminosities do notmatch the instantaneous X-ray flare luminosity, as one would expect forthe Neupert effect. We investigate the probability of chance occurrencesof flares from disparate wavelength regions producing temporalcoincidences but find that not all the flare associations can beexplained by a superposition of flares due to a high flaring rate. Wecaution against making causal associations of multiwavelength flaresbased solely on temporal correlations for high flaring rate stars suchas EV Lac.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
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| Сазвежђа: | Гуштер |
| Ректацензија: | 22h46m49.73s |
| Deклинација: | +44°20'02.4" |
| Apparent магнитуда: | 10.334 |
| Даљина: | 5.049 parsecs |
| Proper motion RA: | -705.6 |
| Proper motion Dec: | -460.3 |
| B-T magnitude: | 11.974 |
| V-T magnitude: | 10.47 |
Каталог и designations:
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