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X-ray optical depth diagnostics of T Tauri accretion shocks
Context: In classical T Tauri stars, X-rays are produced by two plasmacomponents: a hot low-density plasma, with frequent flaring activity,and a high-density lower temperature plasma. The former is coronalplasma related to the stellar magnetic activity. The latter component,never observed in non-accreting stars, could be plasma heated by theshock formed by the accretion process. However its nature is still beingdebated. Aims: Our aim is to probe the soft X-ray emission fromthe high-density plasma component in classical T Tauri stars to checkwhether this plasma is heated in the accretion shock or whether it iscoronal plasma. Methods: High-resolution X-ray spectroscopyallows us to measure individual line fluxes. We analyze X-ray spectra ofthe classical T Tauri stars MP Muscae and TWHydrae. Our aim is to evaluate line ratios to search foroptical depth effects, which are expected in the accretion-drivenscenario. We also derive the plasma emission measure distributions EMD,to investigate whether and how the EMD of accreting and non accretingyoung stars differ. The results are compared to those obtained for thenon-accreting weak-line T Tauri star TWA 5. Results: We find evidence of resonance scattering in the strongestlines of MP Mus, supporting the idea that soft X-rays are produced byplasma heated in the accretion shock. We also find that the EMD of MPMus has two peaks: a cool peak at temperatures expected for plasmaheated in the accretion shock, and a hot peak typical of coronal plasma.The shape of the EMD of MP Mus appears to be the superposition of theEMD of a pure coronal source, like TWA 5, and an EMD alike that of TWHya, which is instead dominated by shock-heated plasma.
| Rotational Velocities for M Dwarfs
We present spectroscopic rotation velocities (v sin i) for 56 M dwarfstars using high-resolution Hobby-Eberly Telescope High ResolutionSpectrograph red spectroscopy. In addition, we have also determinedphotometric effective temperatures, masses, and metallicities ([Fe/H])for some stars observed here and in the literature where we couldacquire accurate parallax measurements and relevant photometry. We haveincreased the number of known v sin i values for mid M stars by around80% and can confirm a weakly increasing rotation velocity withdecreasing effective temperature. Our sample of v sin is peak at lowvelocities (~3 km s-1). We find a change in therotational velocity distribution between early M and late M stars, whichis likely due to the changing field topology between partially and fullyconvective stars. There is also a possible further change in therotational distribution toward the late M dwarfs where dust begins toplay a role in the stellar atmospheres. We also link v sin i to age andshow how it can be used to provide mid-M star age limits. When allliterature velocities for M dwarfs are added to our sample, there are198 with v sin i <= 10 km s-1 and 124 in themid-to-late M star regime (M3.0-M9.5) where measuring precision opticalradial velocities is difficult. In addition, we also search the spectrafor any significant Hα emission or absorption. Forty three percentwere found to exhibit such emission and could represent young, activeobjects with high levels of radial-velocity noise. We acquired twoepochs of spectra for the star GJ1253 spread by almost one month and theHα profile changed from showing no clear signs of emission, toexhibiting a clear emission peak. Four stars in our sample appear to below-mass binaries (GJ1080, GJ3129, Gl802, and LHS3080), with both GJ3129and Gl802 exhibiting double Hα emission features. The tablespresented here will aid any future M star planet search target selectionto extract stars with low v sin i.Based on observations obtained with the Hobby-Eberly Telescope, which isa joint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| Flares on a Bp Star
Two large X-ray flares have been reported from the direction of amagnetic B2p star (σ Ori E). Sanz-Forcada et al. havesuggested that the flares did not occur on the B2p star but on acompanion of late spectral type. A star which is a candidate for alate-type flare star near σ Ori E has recently been identified byBouy et al. However, based on the properties of the flares, andbased on a recent model of rotating magnetospheres, we argue that,rather than attributing the two flares to a late-type dwarf, it is aviable hypothesis that the flares were magnetic phenomena associatedwith the rotating magnetosphere of the B2p star itself.
| The Closely Positioned Three Radio Transients in the Nasu 1.4 GHz Wide-Field Survey
In 2004, we started the Nasu 1.4 GHz wide-field survey to study thedynamic universe by simultaneously performing the surveillance of thewide-field sky. We have detected six radio transients with fluxdensities larger than 1 Jy. In this paper, we report the detection ofthree new radio transients in the drift-scanning observations at δ~ +41fdg7. Based on their positions in the sky and flux densities, weconsidered the candidate types of astronomical objects for transients.As a result, it is suggested that the three radio transients could bemassive radio flares associated with known extragalactic sources, suchas active galactic nuclei, which are usually very faint in X-ray andradio wavelengths. The three transients were found in the sky of 2°× 1°, while nine transients, including the three transientsreported in this paper, are distributed at both high and low Galacticlatitude. According to the observational results, we suggest that thewide-field survey simultaneously performed would be useful in searchingfor radio transients to study the dynamic radio sky.
| Radio Interferometric Planet Search. I. First Constraints On Planetary Companions For Nearby, Low-Mass Stars From Radio Astrometry
Radio astrometry of nearby, low-mass stars has the potential to be apowerful tool for the discovery and characterization of planetarycompanions. We present a Very Large Array survey of 172 active M dwarfsat distances of less than 10 pc. Twenty-nine stars were detectedwith flux densities greater than 100 μJy. We observed seven ofthese stars with the Very Long Baseline Array at milliarcsecondresolution in three separate epochs. With a detection threshold of500 μJy in images of sensitivity 1σ ~ 100 μJy, wedetected three stars three times (GJ 65B, GJ 896A, GJ 4247), one startwice (GJ 285), and one star once (GJ 803). Two stars were undetected(GJ 412B and GJ 1224). For the four stars detected in multiple epochs,residuals from the optically determined apparent motions have anroot-mean-square deviation of ~0.2 milliarcseconds, consistent withstatistical noise limits. Combined with previous optical astrometry,these residuals provide acceleration upper limits that allow us toexclude planetary companions more massive than 3-6 M Jup at adistance of ~1 AU with a 99% confidence level.
| M Dwarfs in Sloan Digital Sky Survey Stripe 82: Photometric Light Curves and Flare Rate Analysis
We present a flare rate analysis of 50,130 M dwarf light curves in SloanDigital Sky Survey Stripe 82. We identified 271 flares using acustomized variability index to search ~2.5 million photometricobservations for flux increases in the u and g bands. Every image of aflaring observation was examined by eye and with a point-spreadfunction-matching and image subtraction tool to guard against falsepositives. Flaring is found to be strongly correlated with theappearance of Hα in emission in the quiet spectrum. Of the 99flare stars that have spectra, we classify eight as relatively inactive.The flaring fraction is found to increase strongly in stars with reddercolors during quiescence, which can be attributed to the increasingflare visibility and increasing active fraction for redder stars. Theflaring fraction is strongly correlated with |Z| distance such that moststars that flare are within 300 pc of the Galactic plane. We deriveflare u-band luminosities and find that the most luminous flares occuron the earlier-type m dwarfs. Our best estimate of the lower limit onthe flaring rate (averaged over Stripe 82) for flares with Δu>= 0.7 mag on stars with u < 22 is 1.3 flareshr-1 deg-2 but can vary significantlywith the line of sight.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| GCRT J1742-3001: A New Radio Transient Toward the Galactic Center
We report the detection of a new transient radio source, GCRTJ1742-3001, located ~1° from the Galactic center. The source wasdetected 10 times from late 2006 to 2007 May in our 235 MHz transientmonitoring program with the Giant Metrewave Radio Telescope (GMRT). Theradio emission brightened in about one month, reaching a peak observedflux density of ~100 mJy on 2007 January 28, and decaying to ~50 mJy by2007 May when our last monitoring observation was made. Two additionalfaint, isolated 235 MHz detections were made in mid-2006, also with theGMRT. GCRT J1742-3001 is unresolved at each epoch, with typicalresolutions of ~20'' × 10''. No polarization information isavailable from the observations. Based on nondetections in observationsobtained simultaneously at 610 MHz, we deduce that the spectrum of GCRTJ1742-3001 is very steep, with a spectral index less than about-2. Follow-up radio observations in 2007 September at 330 MHz and1.4 GHz, and in 2008 February at 235 MHz yielded no detections. No X-raycounterpart is detected in a serendipitous observation obtained with theX-ray telescope aboard the Swift satellite during the peak of the radioemission in early 2007. We consider the possibilities that GCRTJ1742-3001 is either a new member of an existing class of radiotransients, or is representative of a new class, possibly having noassociated X-ray emission.
| On the magnetic topology of partially and fully convective stars
We compare the amount of magnetic flux measured in Stokes V and Stokes Iin a sample of early- and mid-M stars around the boundary to fullconvection (~M 3.5). Early-M stars possess a radiative core, mid-M starsare fully convective. While Stokes V is sensitive to the net polarity ofmagnetic flux arising mainly from large-scale configurations, Stokes Imeasurements can see the total mean flux. We find that in early-Mdwarfs, only ~6% of the total magnetic flux is detected in Stokes V.This ratio is more than twice as large, ~14%, in fully convective mid-Mdwarfs. The bulk of the magnetic flux on M-dwarfs is not seen in StokesV. This is presumably because magnetic flux is mainly stored in smallscale components. There is also more to learn about the effect of theweak-field approximation on the accuracy of strong field detections. Inour limited sample, we see evidence for a change in magnetic topology atthe boundary to full convection. Fully convective stars store a 2-3times higher fraction of their flux in fields visible to Stokes V. Weestimate the total magnetic energy detected in Stokes I and compare itto results from Stokes V. We find that in early-M dwarfs only ~0.5% ofthe total magnetic energy is detected in Stokes V while this fraction is~2.5% in mid-M dwarfs.
| Evidence for Magnetic Flux Saturation in Rapidly Rotating M Stars
We present magnetic flux measurements in seven rapidly rotating Mdwarfs. Our sample stars have X-ray and Hα emission indicative ofsaturated emission, i.e., emission at a high level, independent ofrotation rate. Our measurements are made using near-infrared FeHmolecular spectra observed with the High Resolution Echelle Spectrometerat Keck. Because of their large convective overturn times, the rotationvelocity of M stars with small Rossby numbers is relatively slow anddoes not hamper the measurement of Zeeman splitting. The Rossby numbersof our sample stars are as small as 0.01. All our sample stars exhibitmagnetic flux of kG strength. We find that the magnetic flux saturatesin the same regime as saturation of coronal and chromospheric emission,at a critical Rossby number of around 0.1. The filling factors of bothfield and emission are near unity by then. We conclude that the strengthof surface magnetic fields remains independent of rotation rate belowthat; making the Rossby number yet smaller by a factor of 10 has littleeffect. These saturated M-star dynamos generate an integrated magneticflux of roughly 3 kG, with a scatter of about 1 kG. The relation betweenemission and flux also has substantial scatter.
| Energy flux determines magnetic field strength of planets and stars
The magnetic fields of Earth and Jupiter, along with those of rapidlyrotating, low-mass stars, are generated by convection-driven dynamosthat may operate similarly (the slowly rotating Sun generates its fieldthrough a different dynamo mechanism). The field strengths of planetsand stars vary over three orders of magnitude, but the critical factorcausing that variation has hitherto been unclear. Here we report anextension of a scaling law derived from geodynamo models to rapidlyrotating stars that have strong density stratification. The unifyingprinciple in the scaling law is that the energy flux available forgenerating the magnetic field sets the field strength. Our scaling lawfits the observed field strengths of Earth, Jupiter, young contractingstars and rapidly rotating low-mass stars, despite vast differences inthe physical conditions of the objects. We predict that the fieldstrengths of rapidly rotating brown dwarfs and massive extrasolarplanets are high enough to make them observable.
| χ Values for Blue Emission Lines in M Dwarfs
We compute values for blue emission lines in active M dwarfs. Usingflux-calibrated spectra from nearby M dwarfs and spectral M dwarftemplates from SDSS, we derive analytical relations that describe howthe values for the Ca II H and K as well as the Hβ, Hγ,Hδ, Hɛ, and H8 Balmer emission lines vary as a function ofspectral type and color. These derived values are important for numerousM dwarf studies where the intrinsic luminosity of emission lines cannotbe estimated due to uncertain distances and/or non-flux-calibratedspectra. We use these results to estimate the mean properties of blueemission lines in active-field M dwarfs from SDSS.
| Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs
We report on radio observations of the M8.5 dwarf LSR J1835+3259 and theL3.5 dwarf 2MASS J00361617+1821104, which provide the strongest evidenceto date that the electron cyclotron maser instability is the dominantmechanism producing radio emission in the magnetospheres of ultracooldwarfs. As has previously been reported for the M9 dwarf TVLM 513-46546,periodic pulses of 100% circularly polarized, coherent radio emissionare detected from both dwarfs with periods of 2.84+/-0.01 and3.08+/-0.05 hr, respectively, for LSR J1835+3259 and 2MASSJ00361617+1821104. Importantly, periodic unpolarized radio emission isalso detected from 2MASS J00361617+1821104, and brightness temperaturelimitations rule out gyrosynchrotron radiation as a source of this radioemission. The unpolarized emission from this and other ultracool dwarfsis also attributed to electron cyclotron maser emission, which hasbecome depolarized on traversing the ultracool dwarf magnetosphere,possibly due to propagations effects such as scattering. Based onavailable vsini data in the literature and rotation periods derived fromthe periodic radio data for the three confirmed sources of electroncyclotron maser emission, TVLM 513-46546, LSR J1835+3259, and 2MASSJ00361617+1821104, we determine that the rotation axes of all threedwarfs are close to perpendicular to our line of sight. This suggests apossible geometrical selection effect due to the inherent directivity ofelectron cyclotron maser emission, that may account for the previouslyreported relationship between radio activity and vsini observed forultracool dwarfs. We also determine the radius of the dwarf LSRJ1835+3259 to be >=0.117+/-0.012 Rsolar. The implied sizeof the radius, together with the bolometric luminosity of the dwarf,suggests that either LSR J1835 is a young- or intermediate-age browndwarf, or that current theoretical models underestimate the radii ofultracool dwarfs.
| Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM 513-46546
We present the first simultaneous radio, X-ray, ultraviolet, and opticalspectroscopic observations of the M8.5 dwarf TVLM 513-46546, with aduration of 9 hr. These observations are part of a program to study theorigin of magnetic activity in ultracool dwarfs, and its impact onchromospheric and coronal emission. Here we detect steady quiescentradio emission superposed with multiple short-duration, highly polarizedflares; there is no evidence for periodic bursts previously reported forthis object, indicating their transient nature. We also detect softX-ray emission, with LX/Lbol~10-5.1,the faintest to date for any object later than M5, and a possible X-rayflare. TVLM 513-46546 continues the trend of severe violation of theradio/X-ray correlation in ultracool dwarfs, by nearly 4 orders ofmagnitude. From the optical spectroscopy we find that the Balmer lineluminosity exceeds the X-ray luminosity by a factor of a few, ruling outchromospheric heating by coronal X-ray emission. More importantly, wedetect sinusoidal Hα and Hβ equivalent width light curveswith a period of 2 hr, matching the rotation period of TVLM 513-46546.This behavior points to a corotating chromospheric hot spot or anextended magnetic structure, with a covering fraction of about 50%. Thisfeature may be transitory based on the apparent decline in light-curvepeak during the four observed maxima. From the radio data we infer alarge-scale and steady magnetic field of ~102 G. Alarge-scale field is also required by the sinusoidal Balmer lineemission. The radio flares, on the other hand, are produced in acomponent of the field with a strength of ~3 kG and a likely multipolarconfiguration. The overall lack of correlation between the variousactivity indicators suggests that the short-duration radio flares do nothave a strong influence on the chromosphere and corona, and that thechromospheric emission is not the result of coronal heating.
| The effect of activity on stellar temperatures and radii
Context: Recent analyses of low-mass eclipsing binary stars haveunveiled a significant disagreement between the observations andpredictions of stellar structure models. Results show that theoreticalmodels underestimate the radii and overestimate the effectivetemperatures of low-mass stars but yield luminosities that accord withobservations. A hypothesis based upon the effects of stellar activitywas put forward to explain the discrepancies. Aims: In this paper westudy the existence of the same trend in single active stars and providea consistent scenario to explain systematic differences between activeand inactive stars in the H-R diagram reported earlier. Methods: Theanalysis is done using single field stars of spectral types late-K and Mand computing their bolometric magnitudes and temperatures throughinfrared colours and spectral indices. The properties of the stars insamples of active and inactive stars are compared statistically toreveal systematic differences. Results: After accounting for a numberof possible bias effects, active stars are shown to be cooler thaninactive stars of similar luminosity therefore implying a larger radiusas well, in proportions that are in excellent agreement with those foundfrom eclipsing binaries. Conclusions: The present results generalisethe existence of strong radius and temperature dependences on stellaractivity to the entire population of low-mass stars, regardless of theirmembership in close binary systems.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/478/507
| The Coherent Radio Emission from the RS CVn Binary HR 1099
The Australia Telescope was used in March-April 2005 to observe the1.384 and 2.368-GHz emissions from the RS CVn binary HR 1099 in twosessions, each of 9-h duration and 11 days apart. Two intervals ofhighly polarised emission, each lasting 2-3 h, were recorded. Duringthis coherent emission we employed a recently installed facility tosample the data at 78-ms intervals to measure the fine temporalstructure and, in addition, all the data were used to search for finespectral structure. We present the following observational results: (1)~100% left-hand circularly polarised emission was seen at both 1.384 and2.368 GHz during separate epochs; (2) the intervals of highly polarisedemission lasted for 2-3 h on each occasion; (3) three 22-minintegrations made at 78-ms time resolution showed that the modulationindex of the Stokes V parameter increased monotonically as theintegration time was decreased and was still increasing at ourresolution limit; (4) the extremely fine temporal structure stronglyindicates that the highly polarised emission is due to anelectron-cyclotron maser operating in the corona of one of the binarycomponents; (5) the first episode of what we believe is ECME(electron-cyclotron maser emission) at 1.384 GHz contained a regularfrequency structure of bursts with FWHM ~48 MHz, which drifted acrossthe spectrum at ~0.7 MHz min-1. Our second episode of ECME at2.368 GHz contained wider-band frequency structure, which did not permitus to estimate an accurate bandwidth or direction of drift; (6) the twoECME events reported in this paper agree with six others reported in theliterature in occurring in the binary orbital phase range 0.5-0.7 (7) inone event of 8-h duration, two independent maser sources were operatingsimultaneously at 1.384 and 2.368 GHz. We discuss two kinds of masersources that may be responsible for driving the observed events that webelieve are powered by ECME. One is based on the widely reported`loss-cone anisotropy', the second on an auroral analogue, which isdriven by an unstable `horseshoe' distribution of fast-electronvelocities with respect to the magnetic field direction. Generally, wefavour the latter, because of its higher growth rate and the possibilityof the escape of radiation which has been emitted at the fundamentalelectron cyclotron frequency. If the auroral analogue is operating, themagnetic field in the source cavity is ~500 G at 1.384 GHz and ~850 G at2.368 GHz; the source brightness temperatures are of the orderTB ~ 1015 K. We suggest that the ECME source maybe an aurora-like phenomenon due to the transfer of plasma from the K2subgiant to the G5 dwarf in a strong stellar wind, an idea that is basedon VLBA maps showing the establishment of an 8.4 GHz source near the G5dwarf at times of enhanced radio activity in HR 1099.
| The Gemini Deep Planet Survey
We present the results of the Gemini Deep Planet Survey, a near-infraredadaptive optics search for giant planets and brown dwarfs around 85nearby young stars. The observations were obtained with the Altairadaptive optics system at the Gemini North telescope, and angulardifferential imaging was used to suppress the speckle noise of thecentral star. Typically, the observations are sensitive to angularseparations beyond 0.5" with 5 σ contrast sensitivities inmagnitude difference at 1.6 μm of 9.5 at 0.5", 12.9 at 1", 15.0 at2", and 16.5 at 5". These sensitivities are sufficient to detect planetsmore massive than 2 MJ with a projected separation in therange 40-200 AU around a typical target. Second-epoch observations of 48stars with candidates (out of 54) have confirmed that all candidates areunrelated background stars. A detailed statistical analysis of thesurvey results is presented. Assuming a planet mass distributiondn/dm~m-1.2 and a semimajor-axis distributiondn/da~a-1, the 95% credible upper limits on the fraction ofstars with at least one planet of mass 0.5-13 MJ are 0.28 forthe range 10-25 AU, 0.13 for 25-50 AU, and 0.093 for 50-250 AU; thisresult is weakly dependent on the semimajor-axis distribution power-lawindex. The 95% credible interval for the fraction of stars with at leastone brown dwarf companion having a semimajor axis in the range 25-250 AUis 0.019+0.083-0.015, irrespective of anyassumption on the mass and semimajor-axis distributions. Theobservations made as part of this survey have resolved the stars HD14802, HD 166181, and HD 213845 into binaries for the first time.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).
| The X-ray spectra of the flaring and quiescent states of YZ CMi observed by XMM-Newton
We analyse the X-ray spectrum of the active late-type star YZ CMi(M4.5V); for quiescent as well as active stages, we derive emissionmeasure (EM) distributions, elemental abundances, and electrontemperatures and densities, which are in turn used to estimate flareloop lengths as well as coronal magnetic field strengths.YZ CMi was observed in the wavelength range 1-40 Å by the X-raydetectors RGS, EPIC-MOS and EPIC-pn onboard XMM-Newton. Some flaresoccurred during the observation. We perform a multi-temperature fit andmodel the differential EM of both the flaring and the quiescent parts ofthe spectrum and derive the coronal temperature distribution, EMs, andelemental abundances of the flaring and quiescent states.The observed temperature covers a range from about 1.3 to 42 MK. Thetotal volume EM in this temperature interval is13.7+/-.8×1050cm-3 for the quiescent stateand 21.7+/-1.4×1050cm-3 for the activestate. The abundance pattern in the quiescent state shows some depletionof low first ionization potential (FIP) elements relative to high-FIPelements, indicating the presence of an I(nverse)FIP effect in thisactive star. No abundance differences between the quiescent and theactive states are established.Based on the X-ray light curves in combination with the temperature,density and EM, the coronal magnetic field strength at flare-site isfound to be between 50 and 100 G and the flaring loop lengths areestimated to be in the range of 5 -13 × 109 cm.
| The narrowest M-dwarf line profiles and the rotation-activity connection at very slow rotation
Context: The rotation-activity connection explains stellar activity interms of rotation and convective overturn time. It is well establishedin stars of spectral types F-K as well as in M-type stars of youngclusters, in which rotation is still very rapid even among M-dwarfs. Therotation-activity connection is not established in field M-dwarfs,because they rotate very slowly, and detecting rotation periods orrotational line broadening is a challenge. In field M-dwarfs, saturationsets in below v_rot = 5 km s-1, hence they are expected topopulate the non-saturated part of the rotation-activity connection. Aims: This work for the first time shows intrinsically resolved spectrallines of slowly rotating M-dwarfs and makes a first comparison toestimates of convective velocities. By measuring rotation velocities ina sample of mostly inactive M-dwarfs, the unsaturated part of therotation-activity connection is followed into the regime of very lowactivity. Methods: Spectra of ten M-dwarfs are taken at a resolvingpower of R = 200000 at the CES in the near infrared region wheremolecular FeH has strong absorption bands. The intrinsically very narrowlines are compared to model calculations of convective flows, androtational broadening is measured. Results: In one star, an upper limitof v sin i = 1 km s-1 was found, significant rotation wasdetected in the other nine objects. All inactive stars show rotationbelow or equal to 2 km s-1. In the two active stars AD Leoand YZ CMi, rotation velocities are found to be 40-50% below the resultsfrom earlier studies. Conclusions: The rotation activity connectionholds in field early-M stars, too. Activity and rotation velocities ofthe sample stars are well in agreement with the relation found inearlier and younger stars. The intrinsic absorption profiles ofmolecular FeH lines are consistent with calculations from atomic Felines. Investigation of FeH line profiles is a very promising tool tomeasure convection patterns at the surfaces of M-stars.Based on observationscarried out at the European Southern Observatory, La Silla,PID 076.D-0092.
| Spotting in stars with a low level of activity, close to solar activity
Data on the variability of the continuum optical emission are used forthe first time to estimate the degree of spotting in stars with activitylevels similar to that of the sun. It is shown that the amount ofspotting increases gradually from the sun to the highly spotted starsfor which Alekseev and Gershberg constructed the zonal model for thedistribution of spots. A close relationship is found between spottingand the power of the x-ray emission from stars with widely varyinglevels of activity.
| 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.
| Near-Ultraviolet Spectra of Flares on YZ CMi
Near-ultraviolet spectroscopic data obtained with the Hubble SpaceTelescope STIS (Space Telescope Imaging Spectrograph) instrument on thedMe flare star YZ Canis Minoris (YZ CMi) were analyzed. Flare and quietintervals were identified from the broadband near-UV light curve, andthe spectrum of each flare was separately extracted. Mg II and Fe IIline profiles show similar behavior during the flares. Two large flaresallowed time-resolved spectra to be analyzed, revealing a very broadcomponent to the Mg II k line profile in at least one flare spectrum(F9b). If interpreted as a velocity, this component requireschromospheric material to be moving with FWHM ~ 250 km s-1,implying kinetic energy far in excess of the radiative energy. The Mg IIk flare line profiles were compared to recent radiative hydrodynamicmodels of flare atmospheres undergoing electron beam heating. The modelssuccessfully predict red enhancements in the line profile, with atypical velocity of a few km s-1, but do not reproduce theflares showing blue enhancements, or the strongly broadened lineobserved in flare F9b. A more complete calculation of redistributioninto the line wings, including the effect of collisions with theelectron beam, may resolve the origin of the excess line broadening.
| 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.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
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| A Survey for Spectroscopic Binaries among Very Low Mass Stars
We report on the results of a survey for radial velocity variability ina heterogeneous sample of very low mass stars and brown dwarfs. Onedistinguishing characteristic of the survey is its time span, whichallows an overlap between spectroscopic binaries and those that can befound by high angular resolution imaging. Despite our relatively lowvelocity precision, we are able to place a new constraint on the totalbinary fraction in these objects, which suggests that they are morelikely the result of extending the same processes at work at highermasses into this mass range, rather than a distinct mode of formation.Our basic result is that there are 6+/-2 out of 53, or11%+0.07-0.04, spectroscopic binaries in theseparation range 0-6 AU, nearly as many as resolved binaries. This leadsto an estimate of an upper limit of 26%+/-10% for the binary fraction ofvery low mass objects (it is an upper limit because of the possibleoverlap between the spectroscopic and resolved populations). Areasonable estimate for the very low mass binary fraction is 20%-25%. Weconsider several possible separation and frequency distributions,including the one found for GK stars, a compressed version of that, aversion of the compressed distribution truncated at 15 AU, and atheoretical distribution that considers the evaporation of small-Nclusters. We conclude that the latter two bracket the observations,which may mean that these systems form with intrinsically smallerseparations due to their smaller mass and then are truncated due totheir smaller binding energy. We do not find support for the ``ejectionhypothesis'' as their dominant mode of formation, particularly in viewof the similarity in the total binary fraction compared with slightlymore massive stars and the difficulty this mechanism has in producingnumerous binary systems. Our conclusions must be viewed as tentativeuntil studies with larger and better-posed samples and higher velocityprecision are conducted.
| Opacity in the upper atmospheres of active stars. II. AD Leonis
We present FUV and UV spectroscopic observations of AD Leonis, with theaim of investigating opacity effects in the transition regions oflate-type stars. The C III lines in FUSE spectra show significantopacity during both the quiescent and flaring states of AD Leonis, withup to 30% of the expected flux being lost during the latter. Other FUSEemission lines tested for opacity include those of O VI, while C IV, SiIV and N V transitions observed with stis are also investigated. Theselines only reveal modest amounts of opacity with losses during flaringof up to 20%. Optical depths have been calculated for homogeneous andinhomogeneous geometries, giving path lengths of ≈20-60 km and≈10-30 km, respectively, under quiescent conditions. However pathlengths derived during flaring are ≈2-3 times larger. These valuesare in excellent agreement with both estimates of the small-scalestructure observed in the solar transition region, and path lengthsderived previously for several other active late-type stars.
| Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares
We report on radiative hydrodynamic simulations of M dwarf stellarflares and compare the model predictions to observations of severalflares. The flares were simulated by calculating the hydrodynamicresponse of a model M dwarf atmosphere to a beam of nonthermalelectrons. Radiative back-warming through numerous soft X-ray,extreme-ultraviolet, and ultraviolet transitions are also included. Theequations of radiative transfer and statistical equilibrium are treatedin non-LTE for many transitions of hydrogen, helium, and the Ca II ion,allowing the calculation of detailed line profiles and continuumradiation. Two simulations were carried out, with electron beam fluxescorresponding to moderate and strong beam heating. In both cases we findthat the dynamics can be naturally divided into two phases: an initialgentle phase in which hydrogen and helium radiate away much of the beamenergy and an explosive phase characterized by large hydrodynamic waves.During the initial phase, lower chromospheric material is evaporatedinto higher regions of the atmosphere, causing many lines and continuato brighten dramatically. The He II 304 line is especially enhanced,becoming the brightest line in the flaring spectrum. The hydrogen Balmerlines also become much brighter and show very broad line widths, inagreement with observations. We compare our predicted Balmer decrementsto decrements calculated for several flare observations and find thepredictions to be in general agreement with the observations. During theexplosive phase both condensation and evaporation waves are produced.The moderate flare simulation predicts a peak evaporation wave of ~130km s-1 and a condensation wave of ~30 km s-1. Thevelocity of the condensation wave matches velocities observed in severaltransition region lines. The optical continuum also greatly intensifies,reaching a peak increase of 130% (at 6000 Å) for the strong flare,but does not match observed white-light spectra.
| 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.
| 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.
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