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Constraints on Long-period Planets from an L'- and M-band Survey of Nearby Sun-like Stars: Modeling Results
We have carried out an L'- and M-band adaptive optics (AO) extrasolarplanet imaging survey of 54 nearby, Sun-like stars using the Clio cameraat the MMT. Our survey concentrates more strongly than all others todate on very nearby F, G, and K stars, in that we have prioritizedproximity higher than youth. Our survey is also the first to includeextensive observations in the M band, which supplemented the primary L'observations. These longer-wavelength bands are most useful for verynearby systems in which low-temperature planets with red IR colors(i.e., H - L', H - M) could be detected. The survey detectedno planets, but set interesting limits on planets and brown dwarfs inthe star systems we investigated. We have interpreted our null result bymeans of extensive Monte Carlo simulations and constrained thedistributions of extrasolar planets in mass M and semimajor axis a. Ifplanets are distributed according to a power law with dN vprop M? a ? dMda, normalized to be consistentwith radial velocity (RV) statistics, we find that a distribution with? = -1.1 and ? = -0.46, truncated at 110 AU, isruled out at the 90% confidence level. These particular values of? and ? are significant because they represent the mostplanet-rich case consistent with current statistics from RVobservations. With 90% confidence no more than 8.1% of stars like thosein our survey have systems with three widely spaced, massive planetslike the A star HR 8799. Our observations show that giant planets inlong-period orbits around Sun-like stars are rare, confirming theresults of shorter-wavelength surveys and increasing the robustness ofthe conclusion.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| Constraints on Long-period Planets from an L'- and M-band Survey of Nearby Sun-like Stars: Observations
We present the observational results of an L'- and M-band adaptiveoptics imaging survey of 54 nearby, Sun-like stars for extrasolarplanets, carried out using the Clio camera on the MMT. We haveconcentrated more strongly than all other planet-imaging surveys to dateon very nearby F, G, and K stars, prioritizing stellar proximity higherthan youth. Ours is also the first survey to include extensiveobservations in the M band, which supplement the primary L'observations. Models predict much better planet/star flux ratios at theL' and M bands than at more commonly used shorter wavelengths (i.e., theH band). We have carried out extensive blind simulations with fakeplanets inserted into the raw data to verify our sensitivity, and toestablish a definitive relationship between source significance in? and survey completeness. We find 97% confident-detectioncompleteness for 10? sources, but only 46% for 7?sources—raising concerns about the standard procedure of assuminghigh completeness at 5?, and demonstrating that blind sensitivitytests to establish the significance-completeness relation are animportant analysis step for all planet-imaging surveys. We discovered apreviously unknown ~0.15 M sun stellar companion to the F9star GJ 3876, at a projected separation of about 80 AU. Twelveadditional candidate faint companions are detected around other stars.Of these, 11 are confirmed to be background stars and one is apreviously known brown dwarf. We obtained sensitivity to planetary-massobjects around almost all of our target stars, with sensitivity toobjects below 3 M Jup in the best cases. Constraints onplanet populations based on this null result are presented in ourModeling Results paper.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| Reaching the boundary between stellar kinematic groups and very wide binaries . II. ? Librae + KU Librae: a common proper motion system in Castor separated by 1.0 pc
Aims: I investigate the gravitational binding of a nearby commonproper motion system in the young Castor moving group (? ~ 200 Ma),which is formed by the bright quadruple star ? Lib (Zubenelgenubi)and the young solar analogue KU Lib. The system has an exceptionallywide angular separation of about 2.6 deg, which corresponds to aprojected physical separation of about 1.0 pc. Methods: Icompiled basic information on the system, compared its binding energywith those of other weakly bound systems in the field, and studied thephysical separations of resolved multiple systems in Castor. Results: KU Lib has roughly the same proper motion, parallacticdistance, radial velocity, and metallicity as the young hierarchicalquadruple system ? Lib. It also displays youth features. Theresemblance between these basic parameters and the relatively highestimated binding energy indicate that the five stars aregravitationally bound. KU Lib and ? Lib constitute the widestknown multiple system in all mass domains, and probably represent themost extreme example of young wide binaries on the point of beingdisrupted. Besides this, I make a comprehensive compilation of starcandidates in Castor, including new ones.
| Multiwavelength observations of a giant flare on CN Leonis. III. Temporal evolution of coronal properties
Context. Stellar flares affect all atmospheric layers from thephotosphere over chromosphere and transition region up into the corona.Simultaneous observations in different spectral bands allow to obtain acomprehensive picture of the environmental conditions and the physicalprocesses going on during different phases of the flare. Aims: Weinvestigate the properties of the coronal plasma during a giant flare onthe active M dwarf CN Leo observed simultaneously with the UVESspectrograph at the VLT and XMM-Newton. Methods: From the X-raydata, we analyze the temporal evolution of the coronal temperature andemission measure, and investigate variations in electron density andcoronal abundances during the flare. Optical Fe XIII line emissiontraces the cooler quiescent corona. Results: Although of rathershort duration (exponential decay time ?_LC < 5 min), the X-rayflux at flare peak exceeds the quiescent level by a factor of ?100.The electron density averaged over the whole flare is greater than 5× 1011 cm-3. The flare plasma shows anenhancement of iron by a factor of ?2 during the rise and peak phaseof the flare. We derive a size of <9000 km for the flaring structurefrom the evolution of the the emitting plasma during flare rise, peak,and decay. Conclusions: The characteristics of the flare plasmasuggest that the flare originates from a compact arcade instead of asingle loop. The combined results from X-ray and optical data furtherconfine the plasma properties and the geometry of the flaring structurein different atmospheric layers.Based on observations collected at the European Southern Observatory,Paranal, Chile, 077.D-0011(A) and on observations obtained withXMM-Newton, an ESA science mission with instruments and contributionsdirectly funded by ESA Member States and NASA.
| A Detailed Study of the Rise Phase of a Long Duration X-Ray Flare in the Young Star TWA 11B
We analyzed a long duration flare observed in a serendipitous XMM-Newtondetection of the M star CD-39 7717B (TWA 11B), member of the youngstellar association TW Hya (~8 Myr). Only the rise phase (with aduration of ~35 ks) and possibly the flare peak were observed. We tookadvantage of the high count rate of the X-ray source to carry out adetailed analysis of its spectrum during the whole exposure. After acareful analysis, we interpreted the rise phase as resulting from theignition of a first group of loops (event A) which triggered asubsequent two-ribbon flare (event B). Event A was analyzed using asingle-loop model, while a two-ribbon model was applied for event B.Loop semi-lengths of ~4 R * were obtained. Such largestructures had been previously observed in very young stellar objects(~1-4 Myr). This is the first time that they have been inferred in aslightly more evolved star. The fluorescent iron emission line at 6.4keV was detected during event B. Since TWA 11B seems to have no disk,the most plausible explanation found for its presence in the X-rayspectrum of this star is collisional- or photo-ionization. As far as weare concerned, this is only the third clear detection of Fe photosphericfluorescence in stars other than the Sun.
| Imaging Young Giant Planets From Ground and Space
High-contrast imaging can find and characterize gas giant planets aroundnearby young stars and the closest M stars, complementing radialvelocity and astrometric searches by exploring orbital separationsinaccessible to indirect methods. Ground-based coronagraphs are alreadyprobing within 25 AU of nearby young stars to find objects as small as .This paper contrasts near-term and future ground-based capabilities withhigh-contrast imaging modes of the James Webb Space Telescope (JWST).Monte Carlo modeling reveals that JWST can detect planets with masses assmall as across a broad range of orbital separations. We present newcalculations for planet brightness as a function of mass and age forspecific JWST filters and extending to .
| Reaching the boundary between stellar kinematic groups and very wide binaries. The Washington double stars with the widest angular separations
Aims: I look for and characterise very wide binaries and multiplesystems with projected physical separations larger than s = 0.1 pc,which is generally believed to be a sharp upper limit to thedistribution of wide binary semimajor axes. Methods: Iinvestigated in detail 30 Washington double stars with angularseparations of ? > 1000 arcsec. I discarded 23 of them asprobably unbound systems based on discordant astrometry, photometry,spectral types, and radial velocities. The remaining seven systems weresubject to a comprehensive data compilation and derivation(multi-wavelength photometry, heliocentric distance, multiplicity, age,mass, metallicity, membership in a young kinematic group). Results: Of the seven very wide systems, six have projected physicalseparations greater than the hypothetical cutoff at s = 0.1 pc and fourhave separations s > 0.2 pc. Although there are two systems in youngkinematic groups (namely HD 136654 and BD+32 2572 in the HyadesSupercluster, and AU Mic and AT Mic AB in the ? Pictoris movinggroup), there is no clear prevalence of young systems (? < 1 Ga)among these very wide binaries. Finally, I compare the binding energiesof the seven systems with those of other weakly bound systems in thefield.Appendix A is only available in electronic form at http://www.aanda.org
| 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.
| Magnetic Fields of Nondegenerate Stars
Magnetic fields are present in a wide variety of stars throughout the HRdiagram and play a role at basically all evolutionary stages, fromvery-low-mass dwarfs to very massive stars, and from young star-formingmolecular clouds and protostellar accretion discs to evolvedgiants/supergiants and magnetic white dwarfs/neutron stars. These fieldsrange from a few ?G (e.g., in molecular clouds) to TG and more (e.g.,in magnetic neutron stars); in nondegenerate stars in particular, theyfeature large-scale topologies varying from simple nearly axisymmetricdipoles to complex nonaxsymmetric structures, and from mainly poloidalto mainly toroidal topologies. After recalling the main techniques ofdetecting and modeling stellar magnetic fields, we review the existingproperties of magnetic fields reported in cool, hot, and youngnondegenerate stars and protostars, and discuss our understanding of theorigin of these fields and their impact on the birth and life of stars.
| 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.
| 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.
| Coronal properties of the EQ Pegasi binary system
Context: The activity indicators of M dwarfs are distinctly differentfor early and late types. The coronae of early M dwarfs display highX-ray luminosities and temperatures, a pronounced inverse FIP effect,and frequent flaring to the extent that no quiescent level can bedefined in many cases. For late M dwarfs, fewer but more violent flareshave been observed, and the quiescent X-ray luminosity is much lower. Aims: To probe the relationship between coronal properties with spectraltype of active M dwarfs, we analyze the M3.5 and M4.5 components of theEQ Peg binary system in comparison with other active M dwarfs ofspectral types M0.5 to M5.5. Methods: We investigate the timingbehavior of both components of the EQ Peg system, reconstruct theirdifferential emission measure, and investigate the coronal abundanceratios based on emission-measure independent line ratios from theirChandra HETGS spectra. Finally we test for density variations indifferent states of activity. Results: The X-ray luminosity of EQ Peg A(M3.5) is by a factor of 6-10 brighter than that of EQ Peg B (M4.5).Like most other active M dwarfs, the EQ Peg system shows an inverse FIPeffect. The abundances of both components are consistent within theerrors; however, there seems to be a tendency toward the inverse FIPeffect being less pronounced in the less active EQ Peg B when comparingthe quiescent state of the two stars. This trend is supported by ourcomparison with other M dwarfs. Conclusions: As the X-ray luminositydecreases with later spectral type, so do coronal temperatures and flarerate. The amplitude of the observed abundance anomalies, i.e. theinverse FIP effect, declines; however, clear deviations from solarabundances remain.
| Large-scale magnetic topologies of mid M dwarfs
We present in this paper, the first results of a spectropolarimetricanalysis of a small sample (~20) of active stars ranging from spectraltype M0 to M8, which are either fully convective or possess a very smallradiative core. This study aims at providing new constraints on dynamoprocesses in fully convective stars.This paper focuses on five stars of spectral type ~M4, i.e. with massesclose to the full convection threshold (~=0.35Msolar), andwith short rotational periods. Tomographic imaging techniques allow usto reconstruct the surface magnetic topologies from the rotationallymodulated time-series of circularly polarized profiles. We find that allstars host mainly axisymmetric large-scale poloidal fields. Three starswere observed at two different epochs separated by ~1 yr; we find themagnetic topologies to be globally stable on this time-scale.We also provide an accurate estimation of the rotational period of allstars, thus allowing us to start studying how rotation impacts thelarge-scale magnetic field.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) and the Télescope Bernard Lyot (TBL). CFHT is operated bythe National Research Council of Canada, the Institut National desScience de l'Univers of the Centre National de la Recherche Scientifiqueof France (INSU/CNRS) and the University of Hawaii, while the TBL isoperated by CNRS/INSU.E-mail: jmorin@ast.obs-mip.fr (JM); donati@ast.obs-mip.fr (J-FD);petit@ast.obs-mip.fr (PP); xavier.delfosse@obs.ujf-grenoble.fr (XD);thierry.forveille@obs.ujf-grenoble.fr (TF); albert@cfht.hawaii.edu (LA);auriere@ast.obs-mip.fr (MA); remi.cabanac@ast.obs-mip.fr (RC);dintrans@ast.obs-mip.fr (BD); rfares@ast.obs-mip.fr (RF);tgastine@ast.obs-mip.fr (TG); mmj@st-andrews.ac.uk (MMJ);ligniere@ast.obs-mip.fr (FL); fpaletou@ast.obs-mip.fr (FP);jramirez@mesiog.obspm.fr (JR); sylvie.theado@ast.obs-mip.fr (ST)
| Large-scale magnetic topologies of early M dwarfs
We present here additional results of a spectropolarimetric survey of asmall sample of stars ranging from spectral type M0 to M8 aimed atinvestigating observationally how dynamo processes operate in stars onboth sides of the full convection threshold (spectral type M4).The present paper focuses on early M stars (M0-M3), that is above thefull convection threshold. Applying tomographic imaging techniques totime series of rotationally modulated circularly polarized profilescollected with the NARVAL spectropolarimeter, we determine the rotationperiod and reconstruct the large-scale magnetic topologies of six earlyM dwarfs. We find that early-M stars preferentially host large-scalefields with dominantly toroidal and non-axisymmetric poloidalconfigurations, along with significant differential rotation (andlong-term variability); only the lowest-mass star of our subsample isfound to host an almost fully poloidal, mainly axisymmetric large-scalefield resembling those found in mid-M dwarfs.This abrupt change in the large-scale magnetic topologies of M dwarfs(occurring at spectral type M3) has no related signature on X-rayluminosities (measuring the total amount of magnetic flux); it thussuggests that underlying dynamo processes become more efficient atproducing large-scale fields (despite producing the same flux) atspectral types later than M3. We suspect that this change relates to therapid decrease in the radiative cores of low-mass stars and to thesimultaneous sharp increase of the convective turnover times (withdecreasing stellar mass) that models predict to occur at M3; it may alsobe (at least partly) responsible for the reduced magnetic brakingreported for fully convective stars.Based on observations obtained at the Télescope Bernard Lyot(TBL), operated by the Institut National des Science de l'Univers of theCentre National de la Recherche Scientifique of France.E-mail: donati@ast.obs-mip.fr (J-FD); jmorin@ast.obs-mip.fr (JM);petit@ast.obs-mip.fr (PP); xavier.delfosse@obs.ujf-grenoble.fr (XD);thierry.forveille@obs.ujf-grenoble.fr (TF); auriere@ast.obs-mip.fr (MA);remi.cabanac@ast.obs-mip.fr (RC); dintrans@ast.obs-mip.fr (BD);rfares@ast.obs-mip.fr (RF); tgastine@ast.obs-mip.fr (TG);mmj@st-and.ac.uk (MMJ); lignieres@ast.obs-mip.fr (FL);fpaletou@ast.obs-mip.fr (FP); julio.ramirez@obspm.fr (JCRV);sylvie.theado@ast.obs-mip.fr (ST)
| A catalogue of chromospherically active binary stars (third edition)
The catalogue of chromospherically active binaries (CABs) has beenrevised and updated. With 203 new identifications, the number of CABstars is increased to 409. The catalogue is available in electronicformat where each system has a number of lines (suborders) with a uniqueorder number. The columns contain data of limited numbers of selectedcross references, comments to explain peculiarities and the position ofthe binarity in case it belongs to a multiple system, classicalidentifications (RS Canum Venaticorum, BY Draconis), brightness andcolours, photometric and spectroscopic data, a description of emissionfeatures (CaII H and K, Hα, ultraviolet, infrared),X-ray luminosity, radio flux, physical quantities and orbitalinformation, where each basic entry is referenced so users can go to theoriginal sources.
| The moderate magnetic field of the flare star Proxima Centauri
We report moderate magnetic flux of 450 G < Bf < 750 G (3?)on the nearby M 5.5 flare star Proxima Centauri. A high resolution UVESspectrum was used to measure magnetic flux from Zeeman broadening inabsorption lines of molecular FeH around 1 ?m. The magnetic flux wefind is relatively weak compared with classical strong flare stars, butso are Proxima's flaring rates and actual emission levels. We comparewhat is known about the rotation rate, Rossby number, and activitylevels in this star to relations between these quantities that are beingdeveloped more generally for M dwarfs. We conclude that the magneticflux is higher than the best estimates of the Rossby number from periodmeasurements. On the other hand, the activity levels of Proxima Centauriare at the high end of what could be expected based on the measuredfield, but not so high as to exceed the natural scatter in theserelations (other stars lie along this high envelope as well).
| 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.
| X-Ray Flaring on the dMe Star, Ross 154
We present results from two Chandra imaging observations of Ross 154, anearby flaring M dwarf star. During a 61 ks ACIS-S exposure, a verylarge flare occurred (the equivalent of a solar X3400 event, withLX=1.8×1030 ergs s-1) in whichthe count rate increased by a factor of over 100. The early phase of theflare shows evidence for the Neupert effect, followed by a further riseand then a two-component exponential decay. A large flare was alsoobserved at the end of a later 48 ks HRC-I observation. Emission fromthe nonflaring phases of both observations was analyzed for evidence oflow-level flaring. From these temporal studies we find that microflaringprobably accounts for most of the ``quiescent'' emission and that,unlike for the Sun and the handful of other stars that have beenstudied, the distribution of flare intensities does not appear to followa power law with a single index. Analysis of the ACIS spectra, which wascomplicated by exclusion of the heavily piled-up source core, suggeststhat the quiescent Ne/O abundance ratio is enhanced by a factor of ~2.5compared to the commonly adopted solar abundance ratio and that the Ne/Oratio and overall coronal metallicity during the flare appear to beenhanced relative to quiescent abundances. Based on the temperatures andemission measures derived from the spectral fits, we estimate the lengthscales and plasma densities in the flaring volume and also track theevolution of the flare in color-intensity space. Lastly, we searched fora stellar wind charge exchange X-ray halo around the star but withoutsuccess; because of the relationship between mass-loss rate and the halosurface brightness, not even an upper limit on the stellar mass-lossrate can be determined.
| 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 XMM-Newton serendipitous survey. IV. Optical identification of the XMM-Newton medium sensitivity survey (XMS)
Aims:X-ray sources at intermediate fluxes (a few ×10-14 erg cm-2 s-1) with a sky densityof ~100 deg-2 are responsible for a significant fraction ofthe cosmic X-ray background at various energies below 10 keV. The aim ofthis paper is to provide an unbiased and quantitative description of theX-ray source population at these fluxes and in various X-ray energybands. Methods: We present the XMM-Newton Medium sensitivitySurvey (XMS), including a total of 318 X-ray sources found among theserendipitous content of 25 XMM-Newton target fields. The XMS comprisesfour largely overlapping source samples selected at soft (0.5-2 keV),intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV)bands, the first three of them being flux-limited. Results: Wereport on the optical identification of the XMS samples, complete to85-95%. At the flux levels sampled by the XMS we find that the X-ray skyis largely dominated by Active Galactic Nuclei. The fraction of stars insoft X-ray selected samples is below 10%, and only a few per cent forhard selected samples. We find that the fraction of optically obscuredobjects in the AGN population stays constant at around 15-20% for softand intermediate band selected X-ray sources, over 2 decades of flux.The fraction of obscured objects amongst the AGN population is larger(~35-45%) in the hard or ultra-hard selected samples, and constantacross a similarly wide flux range. The distribution in X-ray-to-opticalflux ratio is a strong function of the selection band, with a largerfraction of sources with high values in hard selected samples. Sourceswith X-ray-to-optical flux ratios in excess of 10 are dominated byobscured AGN, but with a significant contribution from unobscured AGN.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA). Based on observations made with the INT/WHT, TNG andNOT operated on the island of La Palma by the Isaac Newton Group, theCentro Galileo Galilei and the Nordic Optical Telescope ScienceAssociation respectively, in the Spanish Observatorio del Roque de losMuchachos. Based on observations collected at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto, operatedjointly by the Max-Planck Institut für Astronomie and the Institutode Astrofísica de Andalucía (CSIC). Based on observationscollected at the European Southern Observatory, Paranal, Chile, as partof programme 75.A-0336.
| Speckle Interferometry at the US Naval Observatory. XIII.
The results of 1424 speckle interferometric observations of doublestars, made with the 26 inch (66 cm) refractor of the US NavalObservatory, are presented. Each speckle interferometric observation ofa system represents a combination of over 2000 short-exposure images.These observations are averaged into 1053 mean relative positions andrange in separation from 0.36″ to 61.92″, with a medianseparation of 10.31″. This is the 13th in a series of paperspresenting measurements obtained with this system and covers the period2006 January 12-December 29. Included in these data are nine oldermeasurements whose positions were previously deemed possibly aberrantbut are no longer classified this way following a confirmingobservation. This paper also includes the first data obtained using anew "secondary" camera, designed and built at USNO.
| The XMM-Newton serendipitous survey. III. The AXIS X-ray source counts and angular clustering
Context: Recent results have revised upwards the total X-ray background(XRB) intensity below ~10 keV, therefore an accurate determination ofthe source counts is needed. There are also contradictory results on theclustering of X-ray selected sources. Aims: We have studied theX-ray source counts in four energy bands: soft (0.5-2 keV), hard (2-10keV), XID (0.5-4.5 keV) and ultra-hard (4.5-7.5 keV) in order toevaluate the contribution of sources at different fluxes to the X-raybackground. We have also studied the angular clustering of X-ray sourcesin those bands. Methods: AXIS (An XMM-NewtonInternational Survey) is a survey of 36 high Galactic latitudeXMM-Newton observations covering 4.8deg2 in the Northern sky and containing 1433 serendipitousX-ray sources detected with 5-? significance. This survey hassimilar depth to the XMM-Newton catalogues andtherefore serves as a pathfinder to explore their possibilities. We havecombined this survey with shallower and deeper surveys, and fitted thesource counts with a Maximum Likelihood technique. Using only AXISsources we have studied the angular correlation using a novel robusttechnique. Results: Our source counts results are compatible withmost previous samples in the soft, XID, ultra-hard and hard bands. Wehave improved on previous results in the hard band. The fractions of theXRB resolved in the surveys used in this work are 87%, 85%, 60% and 25%in the soft, hard, XID and ultra-hard bands, respectively. Extrapolationof our source counts to zero flux is not sufficient to saturate the XRBintensity. Only galaxies and/or absorbed AGN could contribute theremaining unresolved XRB intensity. Our results are compatible, withinthe errors, with recent revisions of the XRB intensity in the soft andhard bands. The maximum fractional contribution to the XRB comes fromfluxes within about a decade of the break in the source counts(~10-14 cgs), reaching ~50% of the total in the soft and hardbands. Angular clustering (widely distributed over the sky and notconfined to a few deep fields) is detected at 99-99.9% significance inthe soft and XID bands, with no detection in the hard and ultra-hardband (probably due to the smaller number of sources). We cannot confirmthe detection of significantly stronger clustering in the hard-spectrumhard sources. Conclusions: Medium depth surveys such as AXIS areessential to determine the evolution of the X-ray emission in theUniverse below 10 keV.Table 1 and Appendices are only available in electronic form athttp://www.aanda.org Based on observations obtained with XMM-newton, anESA science mission with instruments and contributions funded by ESAMember States and the USA (NASA). Tables 2 and 3 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/27
| 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.
| New periodic variable stars coincident with ROSAT sources discovered using SuperWASP
We present optical lightcurves of 428 periodic variable stars coincidentwith ROSAT X-ray sources, detected using the first run of the SuperWASPphotometric survey. Only 68 of these were previously recognised asperiodic variables. A further 30 of these objects are previously knownpre-main sequence stars, for which we detect a modulation period for thefirst time. Amongst the newly identified periodic variables, many appearto be close eclipsing binaries, their X-ray emission is presumably theresult of RS CVn type behaviour. Others are probably BY Dra stars,pre-main sequence stars and other rapid rotators displaying enhancedcoronal activity. A number of previously catalogued pulsating variables(RR Lyr stars and Cepheids) coincident with X-ray sources are also seen,but we show that these are likely to be misclassifications. We identifyfour objects which are probable low mass eclipsing binary stars, basedon their very red colour and light curve morphology.Tables 1 and 2 are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/J/A+A/467/785
| Hubble Space Telescope NICMOS Observations of NGC 1333: The Ratio of Stars to Substellar Objects
We present an analysis of NICMOS photometry and low-resolution grismspectroscopy of low-mass stars and substellar objects in the youngstar-forming region NGC 1333. Our goal is to constrain the ratio oflow-mass stars to substellar objects down to 20MJup in thecluster, as well as constrain the cluster initial mass function (IMF)down to 30MJup, in combination with a previous survey of NGC1333 by Wilking et al. Our survey covers four fields of 51.2" ×51.2", centered on brown dwarf candidates previously identified inWilking et al. We extend previous work based on the use of a water vaporindex for spectral typing to wavelengths accessible with NICMOS on theHubble Space Telescope. Spectral types were derived for the 14 brightestobjects in our fields, ranging from <=M0 to M8, which, at the age ofthe cluster (0.3 Myr), correspond to a range in mass of >=0.25-0.02Msolar. In addition to the spectra, we present an analysis ofthe color-magnitude diagram using pre-main-sequence evolutionary modelsof D'Antona & Mazzitelli. Using an extinction-limited sample, wederive the ratio of low-mass stars to brown dwarfs. Comparisons of theobserved ratio to that expected from the field IMF of Chabrier indicatethat the two results are consistent. We combine our data with those ofWilking et al. to compute the ratio of intermediate-mass stars (0.1-1.0Msolar) to low-mass objects (0.03-0.1 Msolar) inthe cluster. We also report the discovery of a faint companion to thepreviously confirmed brown dwarf ASR 28, as well as a possible outflowsurrounding ASR 16. If the faint companion is confirmed as a clustermember, it would have a mass of ~5MJup (mass ratio 0.15) at aprojected distance of 350 AU, similar to that of 2MASS 1207-3923B.
| New Distant Companions to Known Nearby Stars. II. Faint Companions of Hipparcos Stars and the Frequency of Wide Binary Systems
We perform a search for faint, common proper motion companions ofHipparcos stars using the recently published Lépine-Shara ProperMotion-North catalog of stars with proper motionμ>0.15'' yr-1. Our survey uncovers a totalof 521 systems with angular separations3''<Δθ<1500'', with 15 triplesand 1 quadruple. Our new list of wide systems with Hipparcos primariesincludes 130 systems identified here for the first time, including 44 inwhich the secondary star has V>15.0. Our census is statisticallycomplete for secondaries with angular separations20''<Δθ<300'' and apparentmagnitudes V<19.0. Overall, we find that at least 9.5% of nearby(d<100 pc) Hipparcos stars have distant stellar companions withprojected orbital separations s>1000 AU. We observe that thedistribution in orbital separations is consistent with Öpik's law,f(s)ds~s-1ds, only up to a separation s~4000 AU, beyond whichit follows a more steeply decreasing power law f(s)ds~s-ldswith l=1.6+/-0.1. We also find that the luminosity function of thesecondaries is significantly different from that of the single stars'field population, showing a relative deficiency in low-luminosity(8
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The periodic variations of a white-light flare observed with ULTRACAM
High time resolution observations of a white-light flare on the activestar EQ PegB show evidence of intensity variations with a period of≈10 s. The period drifts to longer values during the decay phase ofthe flare. If the oscillation is interpreted as an impulsively-excited,standing-acoustic wave in a flare loop, the period implies a loop lengthof ≈3.4 Mm and ≈6.8 Mm for the case of the fundamental mode andthe second harmonic, respectively. However, the small loop lengths implya very high modulation depth making the acoustic interpretationunlikely. A more realistic interpretation may be that of a fast-MHDwave, with the modulation of the emission being due to the magneticfield. Alternatively, the variations could be due to a series ofreconnection events. The periodic signature may then arise as a resultof the lateral separation of individual flare loops or current sheetswith oscillatory dynamics (i.e., periodic reconnection).
| MK Classification and Dynamical Masses for Late-Type Visual Binaries
On the basis of slit spectra obtained with the SCORPIO spectral cameraattached to the 2.6 m telescope of the V. Ambartsumian ByurakanAstrophysical Observatory (Armenia), MK classifications for 30 visualbinaries comprising mostly late K and M type stars are presented.Comparison with other determinations shows that this configurationprovides a reliable MK classification. Dynamical masses for 25 systemsare computed. Using standard mass-luminosity calibrations, individualmass sums for 11 pairs consisting of virtually single, nonvariabledwarfs are calculated, showing a good agreement with correspondingdynamical masses. The dynamical parallax of HIP 112354 is closer to thetrigonometric parallax given in the Yale General Catalogue ofTrigonometric Stellar Parallaxes (van Altena et al.) than to theHipparcos parallax.
| 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.
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