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Observation and modelling of main sequence star chromospheres. VIII. High resolution observations of M and K dwarf chromospheric lines
Aims: We report on high resolution observations of chromospheric linesfor M and K dwarfs. Methods: We observed up to 13 spectral linesper star: the Hα, Hβ, Hɛ, Paɛ, Pa{8}, CaII H,CaII IR triplet, CaI 6572Å, NaI D{1}/D{2} and HeI D{3} lines. Weobserved two dMe stars, one dM(e) star, 5 dM stars and 3 dK stars. Results: We observed a self-reversal in the emission core of the CaIIH line for the brightest stars only, indicating a rather optically thickregion of formation. We present original spectra of the NaI doublet andthe CaII IR triplet for active dMe stars and less active dM stars. Coreemission is detected in the Sodium lines and the CaII IR triplet linesfor the most active M dwarf AU Mic. We investigate the differencespectra between active dMe stars and dM stars and show that theseprovide interesting new constraints for the NLTE-radiation transfermodelling of the chromospheres. In our sample, emission Hαprofiles have a rather homogeneous FWHM of about 1.5 Å. This,according to our previous modelling, can be interpreted as the signatureof a rather constant temperature break in the chromosphere. We foundthat one of our targets (MCC 332) is a binary with a faint but activeHα emission component. For the first time we detect thePaɛ line for six dwarfs. It appears as weak absorption withpossible weak wing emission in AU Mic. The region of the Pa{8} line wasobserved but the line was not detected.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| Kinematic analysis and membership status of TWA22 AB
Context: TWA22 was initially regarded as a member of the TW Hydraeassociation (TWA). In addition to being one of the youngest (≈8 Myr)and nearest (≈20 pc) stars to Earth, TWA22 has proven to be veryinteresting after being resolved as a tight, very low-mass binary. Thisbinary can serve as a very useful dynamical calibrator for pre-mainsequence evolutionary models. However, its membership in the TWA hasbeen recently questioned despite due to the lack of accurate kinematicmeasurements. Aims: Based on proper motion, radial velocity, andtrigonometric parallax measurements, we aim here to re-analyze themembership of TWA22 to young, nearby associations. Methods: Usingthe ESO NTT/SUSI2 telescope, we observed TWA22 AB during 5 differentobserving runs over 1.2 years to measure its trigonometric parallax andproper motion. This is a part of a larger project measuringtrigonometric parallaxes and proper motions of most known TWA members ata sub-milliarcsec level. HARPS at the ESO 3.6 m telescope was also usedto measure the system's radial velocity over 2 years. Results: Wereport an absolute trigonometric parallax of TWA22 AB, π =57.0±0.7 mas, corresponding to a distance 17.5±0.2 pc fromEarth. Measured proper motions of TWA 22AB areμαcos(δ) = -175.8±0.8 mas/yr andμδ = -21.3±0.8 mas/yr. Finally, from HARPSmeasurements, we obtain a radial velocity V_rad = 14.8±2.1 kms-1. Conclusions: A kinematic analysis of TWA22 ABspace motion and position implies that a membership of TWA22 AB to knownyoung, nearby associations can be excluded except for the βPictoris and TW Hydrae associations. Membership probabilities based onthe system's Galactic space motion and/or the trace-back techniquesupport a higher chance of being a member to the β Pictorisassociation. Membership of TWA22 in the TWA cannot be fully excludedbecause of large uncertainties in parallax measurements and radialvelocities and to the uncertain internal velocity dispersion of itsmembers.Based on observations performed at the European Southern Observatory,Chile (76.C-0543, 077.C-0112, 078.C-0158, 079.C-0229). Table 4 is onlyavailable in electronic form at http://www.aanda.org
| Infrared Spectrograph Characterization of a Debris Disk Around an M-Type Star in NGC 2547
We present 5 to 15 μm Spitzer Infrared Spectrograph (IRS)low-resolution spectral data of a candidate debris disk around an M4.5star identified as a likely member of the ~40 Myr old clusterNGC 2547. The IRS spectrum shows a silicate emission feature,indicating the presence of warm, small, (sub)micron-sized dust grains inthe disk. Of the 15 previously known candidate debris disks aroundM-type stars, the one we discuss in this paper is the first to have anobserved mid-infrared spectrum and is also the first to have measuredsilicate emission. We combined the IRS data with ancillary data(optical, JHKs , and Spitzer InfraRed Array Camera and 24μm data) to build the spectral energy distribution (SED) of thesource. Monte Carlo radiation transfer modeling of the SED characterizedthe dust disk as being very flat (h 100 = 2 AU) and extendinginward within at least 0.13 AU of the central star. Our analysisshows that the disk is collisionally dominated and is likely a debrisdisk.
| Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations
We have completed a high-resolution (R ≈ 60,000) opticalspectroscopic survey of 185 nearby M dwarfs identified using ROSAT datato select active, young objects with fractional X-ray luminositiescomparable to or greater than Pleiades members. Our targets are drawnfrom the NStars 20 pc census and the Moving-M sample with distancesdetermined from parallaxes or spectrophotometric relations. We limitedour sample to 25 pc from the Sun, prior to correcting forpre-main-sequence overluminosity or binarity. Nearly half of theresulting M dwarfs are not present in the Gliese catalog and have nopreviously published spectral types. We identified 30 spectroscopicbinaries (SBs) from the sample, which have strong X-ray emission due totidal spin-up rather than youth. This is equivalent to a 16% SBfraction, with at most a handful of undiscovered SBs. We estimate upperlimits on the age of the remaining M dwarfs using spectroscopic youthindicators such as surface gravity-sensitive indices (CaH and K I). Wefind that for a sample of field stars with no metallicity measurements,a single CaH gravity index may not be sufficient, as highermetallicities mimic lower gravity. This is demonstrated in a subsampleof metal-rich radial velocity (RV) standards, which appear to have lowsurface gravity as measured by the CaH index, yet show no other evidenceof youth. We also use additional youth diagnostics such as lithiumabsorption and strong Hα emission to set more stringent agelimits. Eleven M dwarfs with no Hα emission or absorption arelikely old (>400 Myr) and were caught during an X-ray flare. Weestimate that our final sample of the 144 youngest and nearest low-massobjects in the field is less than 300 Myr old, with 30% of them beingyounger than 150 Myr and four very young (lap10 Myr), representing agenerally untapped and well-characterized resource of M dwarfs forintensive planet and disk searches.Based on observations collected at the W. M. Keck Observatory and theCanada-France-Hawaii Telescope. The Keck Observatory is operated as ascientific partnership between the California Institute of Technology,the University of California, and NASA, and was made possible by thegenerous financial support of the W. M. Keck Foundation. The CFHT isoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii.
| New Debris Disks Around Young, Low-Mass Stars Discovered with the Spitzer Space Telescope
We present 24 μm and 70 μm Multiband Imaging Photometer forSpitzer (MIPS) observations of 70 A through M-type dwarfs with estimatedages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed timeprogram, including a re-analysis of some previously published sourcephotometry. Our sample is selected from stars with common youthindicators such as lithium abundance, X-ray activity, chromosphericactivity, and rapid rotation. We compare our MIPS observations toempirically derived Ks -[24] colors as a function of thestellar effective temperature to identify 24 μm and 70 μmexcesses. We place constraints or upper limits on dust temperatures andfractional infrared luminosities with a simple blackbody dust model. Weconfirm the previously published 70 μm excesses for HD 92945, HD112429, and AU Mic, and provide updated flux density measurements forthese sources. We present the discovery of 70 μm excesses for fivestars: HD 7590, HD 10008, HD 59967, HD 73350, and HD 135599. HD 135599is also a known Spitzer IRS (InfraRed Spectrograph) excess source, andwe confirm the excess at 24 μm. We also present the detection of 24μm excesses for 10 stars: HD 10008, GJ 3400A, HD 73350, HD 112429, HD123998, HD 175742, AT Mic, BO Mic, HD 358623 and Gl 907.1. We find thatlarge 70 μm excesses are less common around stars with effectivetemperatures of less than 5000 K (3.7+7.6–1.1%) than around stars with effective temperaturesbetween 5000 K and 6000 K (21.4+9.5 –5.7%),despite the cooler stars having a younger median age in our sample (12Myr vs. 340 Myr). We find that the previously reported excess for TWA13A at 70 μm is due to a nearby background galaxy, and the previouslyreported excess for HD 177724 is due to saturation of the near-infraredphotometry used to predict the mid-infrared stellar flux contribution.In the Appendix, we present an updated analysis of dust grain removaltimescales due to grain-grain collisions and radiation pressure,Poynting-Robertson (P-R) drag, stellar wind drag, and planet-dustdynamical interaction. We find that drag forces can be important fordisk dynamics relative to grain-grain collisions for L IR/L* < 10–4, and that stellar wind drag ismore important than P-R drag for K and M dwarfs, and possibly for young(<1 Gyr) G dwarfs as well.
| A Formation Scenario of Young Stellar Groups in the Region of the Scorpio Centaurus OB Association
The main objective of this work is to investigate the role played byLower Centaurus Crux (LCC) and Upper Centaurus Lupus (UCL), bothsubcomponents of the Scorpio Centaurus OB association (Sco-Cen), in theformation of the groups β Pictoris, TW Hydrae, and the ηChamaeleontis cluster. The dynamical evolution of all the stellar groupsinvolved and of the bubbles and shells blown by LCC and UCL arecalculated, and followed from the past to the present. This leads to aformation scenario in which (1) the groups β Pictoris, TW Hydraewere formed in the wake of the shells created by LCC and UCL, (2) theyoung cluster η Chamaeleontis was born as a consequence of thecollision of the shells of LCC and UCL, and (3) the formation of UpperScorpius (US), the other main subcomponent of the Sco-Cen association,may have been started by the same process that created ηChamaeleontis.
| Nearby Young Stars Selected by Proper Motion. I. Four New Members of the β Pictoris Moving Group From The Tycho-2 Catalog
We describe a procedure to identify stars from nearby moving groups andassociations out of catalogs of stars with large proper motions. We showthat from the mean motion vector of a known or suspected moving group,one can identify additional members of the group based on proper motiondata and photometry in the optical and infrared, with minimalcontamination from background field stars. We demonstrate this techniqueby conducting a search for low-mass members of the β Pictorismoving group in the Tycho-2 catalog. All known members of the movinggroup are easily recovered, and a list of 51 possible candidates isgenerated. Moving group membership is evaluated for 33 candidates basedon X-ray flux from ROSAT, Hα line emission, and radial velocitymeasurement from high-resolution infrared spectra obtained at InfraredTelescope Facility. We confirm three of the candidates to be new membersof the group: TYC 1186-706-1, TYC 7443-1102-1, and TYC 2211-1309-1 whichare late-K and early-M dwarfs 45-60 pc from the Sun. We also identify acommon proper motion companion to the known β Pictoris Moving Groupmember TYC 7443-1102-1, at a 26farcs3 separation; the new companion isassociated with the X-ray source 1RXS J195602.8 – 320720. We arguethat the present technique could be applied to other large proper motioncatalogs to identify most of the elusive, low-mass members of knownnearby moving groups and associations.Based on data obtained in part with the 2.4 m Hiltner telescope of theMDM observatory. Based on data obtained in part with the CTIO 1.5 mtelescope, operated by SMARTS, the Small and Medium Aperture TelescopeSystem consortium, under contract with the Associated Universities forResearch in Astronomy (AURA).
| 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.
| 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.
| T-Lyr1-17236: A Long-Period Low-Mass Eclipsing Binary
We describe the discovery of a 0.68+0.52 Msolar eclipsingbinary (EB) with an 8.4 day orbital period, found through a systematicsearch of 10 fields of the Trans-atlantic Exoplanet Survey (TrES). Suchlong-period low-mass EBs constitute critical test cases for resolvingthe long-standing discrepancy between the theoretical and observationalmass-radius relations at the bottom of the main sequence. It has beensuggested that this discrepancy may be related to strong stellarmagnetic fields, which are not properly accounted for in currenttheoretical models. All previously well-characterized low-massmain-sequence EBs have periods of a few days or less, and theircomponents are therefore expected to be rotating rapidly as a result oftidal synchronization, thus generating strong magnetic fields. Incontrast, the binary system described here has a period that is morethan 3 times longer than previously characterized low-mass main-sequenceEBs, and its components rotate relatively slowly. It is thereforeexpected to have a weaker magnetic field and to better match theassumptions of theoretical stellar models. Our follow-up observations ofthis EB yield preliminary stellar properties that suggest it is indeedconsistent with current models. If further observations confirm a lowlevel of activity in this system, these determinations would providesupport for the hypothesis that the mass-radius discrepancy is at leastpartly due to magnetic activity.
| New M Dwarf Debris Disk Candidates in NGC 2547
With only six known examples, M dwarf debris disks are rare, even thoughM dwarfs constitute the majority of stars in the Galaxy. After finding anew M dwarf debris disk in a shallow mid-infrared observation of NGC2547, we present a considerably deeper Spitzer MIPS image of the region,with a maximum exposure time of 15 minutes pixel-1. Amongsources selected from a previously published membership list, weidentify nine new M dwarfs with excess emission at 24 μm tracing warmmaterial close to the snow line of these stars, at orbital radii of lessthan 1 AU. We argue that these are likely debris disks, suggesting thatplanet formation is under way in these systems. Interestingly, theestimated excess fraction of M stars appears to be higher than that of Gand K stars in our sample.
| Evolution of Debris Disks
Circumstellar dust exists around several hundred main sequence stars.For the youngest stars, that dust could be a remnant of theprotoplanetary disk. Mostly it is inferred to be continuouslyreplenished through collisions between planetesimals in belts analogousto the Solar System's asteroid and Kuiper belts, or in collisionsbetween growing protoplanets. The evolution of a star's debris disk isindicative of the evolution of its planetesimal belts and may beinfluenced by planet formation processes, which can continue throughoutthe first gigayear as the planetary system settles to a stableconfiguration and planets form at large radii. Evidence for thatevolution comes from infrared photometry of large numbers of debrisdisks, providing snapshots of the dust present at different evolutionaryphases, as well as from images of debris disk structure. This reviewdescribes the theoretical framework within which debris disk evolutiontakes place and shows how that framework has been constrained byobservations.
| Further Constraints on the Presence of a Debris Disk in the Multiplanet System Gliese 876
Using both the Very Large Array (VLA) at 7 mm wavelength, and theAustralia Telescope Compact Array (ATCA) at 3 mm, we have searched formicrowave emission from cool dust in the extrasolar planetary systemGliese 876 (Gl 876). Having detected no emission above our 3σdetection threshold of 135 μJy, we rule out any dust disk with eithera mass greater than 0.0006 M ⊕ or less than ~250 AUacross. This result improves on previous detection aperture thresholdsby an order of magnitude, and it has some implications for the dynamicalmodeling of the system. It also is consistent with the Greaves et al.hypothesis that relates the presence of a debris disk to close-inplanets. Due to the dust-planetesimal relationship, our null result mayalso provide a constraint on the population or composition of the dustand small bodies around this nearby M dwarf.
| Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations
We examine the implications for the distribution of extrasolar planetsbased on the null results from two of the largest direct imaging surveyspublished to date. Combining the measured contrast curves from 22 of thestars observed with the VLT NACO adaptive optics system by Masciadri andcoworkers and 48 of the stars observed with the VLT NACO SDI and MMT SDIdevices by Biller and coworkers (for a total of 60 unique stars), weconsider what distributions of planet masses and semimajor axes can beruled out by these data, based on Monte Carlo simulations of planetpopulations. We can set the following upper limit with 95% confidence:the fraction of stars with planets with semimajor axis between 20 and100 AU, and mass above 4 MJup, is 20% or less. Also, with adistribution of planet mass of dN/dM~M-1.16 in the range of0.5-13 MJup, we can rule out a power-law distribution forsemimajor axis (dN/da~aα) with index 0 and upper cutoffof 18 AU, and index -0.5 with an upper cutoff of 48 AU. For thedistribution suggested by Cumming et al., a power-law of index -0.61, wecan place an upper limit of 75 AU on the semimajor axis distribution. Ingeneral, we find that even null results from direct imaging surveys arevery powerful in constraining the distributions of giant planets (0.5-13MJup) at large separations, but more work needs to be done toclose the gap between planets that can be detected by direct imaging,and those to which the radial velocity method is sensitive.
| 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 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).
| Signatures of Dynamical Star Formation in the Ophiuchus Association of Pre-Main-Sequence Stars
A sample of 58 probable members of the association of pre-main-sequencestars around the filamentary ρ Ophiuchi cloud is investigated. Usingastrometric proper motions from the UCAC-2 catalog and the convergentpoint method, the total heliocentric systemic velocity is estimated at19 km s-1, and the mean distance at 145 pc. A small,statistically insignificant difference between the geometric convergentpoint and the actual direction of velocity defined by the observedradial velocity implies a small, if any, rate of expansion of theassociation. The Ophiuchus association appears to have a considerabledepth, with half of the members lying within 15 pc of the center. Whenthe moving cluster distances are taken into account, the near-infraredMKs versus (J-Ks) H-R diagram reveals an apparentlarge spread of ages between 14 Myr and younger than 1 Myr. Most of theyoungest stars are located along a slightly curved strip just south ofthe densest cloud, extending in the east-west direction roughly alignedwith the central streamer of warm dust. The intersection of this stripwith a thin segment of shocked dust visible in the IRAS 12 μm map at(α,δ)=(16h31m,-24.5deg) ismarked with a small cluster of probably very young stars around ROXs43A. The large extent and depth of the association, the moderate rate ofexpansion, the spread in ages of about 14 Myr, and the alignment of veryyoung stars with the dusty streamer point at a dynamical mode of starformation in this region, scattered far and wide around the main core.
| An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT
We present the results of a survey of 45 young (<~250 Myr), close(<~50 pc) stars with the Simultaneous Differential Imager (SDI)implemented at the VLT and the MMT for the direct detection ofextrasolar planets. As part of the survey, we observed 54 objects,consisting of 45 close, young stars; two more distant (<150 pc),extremely young (<=10 Myr) stars; three stars with known radialvelocity planets; and four older, very nearby (<=20 pc) solaranalogs. Our SDI devices use a double Wollaston prism and a quad filterto take images simultaneously at three wavelengths surrounding the 1.62μm methane absorption bandhead found in the spectrum of cool browndwarfs and gas giant planets. By differencing adaptive optics-correctedimages in these filters, speckle noise from the primary star issignificantly attenuated, resulting in photon (andflat-field)-noise-limited data. In our VLT data, we achieved H-bandcontrasts>~10 mag (5 σ) at a separation of 0.5" from theprimary star on 45% of our targets and H-band contrasts>~9 mag at aseparation of 0.5" on 80% of our targets. With these contrasts, we canimage (5 σ detection) a 7 MJ planet 15 AU from a 70 MyrK1 star at 15 pc or a 7.8 MJ planet at 2 AU from a 12 Myr Mstar at 10 pc. We detected no candidates with S/N>2 σ whichbehaved consistently like a real object. From our survey null result, wecan rule out (with 93% confidence) a model planet population whereN(a)~constant out to a distance of 45 AU.Steward Observatory, University of Arizona, Tucson, AZ 85721.
| Kinematics of the Scorpius-Centaurus OB association
A fine structure related to the kinematic peculiarities of threecomponents of the Scorpius-Centaurus association (LCC, UCL, and US) hasbeen revealed in the UV-velocity distribution of Gould Belt stars. Wehave been able to identify the most likely members of these groups byapplying the method of analyzing the two-dimensional probability densityfunction of stellar UV velocities that we developed. A kinematicanalysis of the identified structural components has shown that, ingeneral, the center-of-mass motion of the LCC, UCL, and US groupsfollows the motion characteristic of the Gould Belt, notably itsexpansion. The entire Scorpius-Centaurus complex is shown to possess aproper expansion with an angular velocity parameter of 46 ± 8 kms‑1 kpc‑1 for the kinematic centerwith l 0 = ‑40° and R 0 = 110 pc found.Based on this velocity, we have estimated the characteristic expansiontime of the complex to be 21 ± 4 Myr. The proper rotationvelocity of the Scorpius-Centaurus complex is lower in magnitude, isdetermined less reliably, and depends markedly on the data quality.
| Mass and Temperature of the TWA 7 Debris Disk
We present photometric detections of dust emission at 850 and 450 μmaround the pre-main-sequence M1 dwarf TWA 7 using the SCUBA camera onthe James Clerk Maxwell Telescope. These data confirm the presence of acold dust disk around TWA 7, a member of the TW Hydrae Association(TWA). Based on the 850 μm flux, we estimate the mass of the disk tobe 18 Mlunar (0.2 M⊕) assuming a massopacity of 1.7 cm2 g-1 with a temperature of 45 K.This makes the TWA 7 disk (d=55 pc) an order of magnitude more massivethan the disk reported around AU Microscopii (GL 803), the closest (9.9pc) debris disk detected around an M dwarf. This is consistent with TWA7 being slightly younger than AU Mic. We find that the mid-IR andsubmillimeter data require the disk to be comprised of dust at a rangeof temperatures. A model in which the dust is at a single radius fromthe star, with a range of temperatures according to grain size, is aseffective at fitting the emission spectrum as a model in which the dustis of uniform size, but has a range of temperatures according todistance. We discuss this disk in the context of known disks in the TWAand around low-mass stars; a comparison of masses of disks in the TWAreveals no trend in mass or evolutionary state (gas-rich vs. debris) asa function of spectral type.
| 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.
| Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| Spitzer 24 μm Observations of Open Cluster IC 2391 and Debris Disk Evolution of FGK Stars
We present 24 μm Spitzer MIPS photometric observations of the ~50 Myropen cluster IC 2391. Thirty-four cluster members ranging in spectraltype from B3 to M5 were observed in the central square degree of thecluster. Excesses indicative of debris disks were discovered around oneA star, six FGK stars, and possibly one M dwarf. For the cluster membersobserved to their photospheric limit, we find a debris disk frequency of10+17-3% for B-A stars and31+13-9% for FGK stars using a 15% relative excessthreshold. Relative to a model of decaying excess frequency, thefrequency of debris disks around A-type stars appears marginally low forthe cluster's age while that of FGK stars appears consistent. Scenariosthat may qualitatively explain this result are examined. We concludethat planetesimal activity in the terrestrial region of FGK stars iscommon in the first ~50 Myr and decays on timescales of ~100 Myr.Despite luminosity differences, debris disk evolution does not appear todepend strongly on stellar mass.
| Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables
Context: . Barnes-Evans type relations provide an empirical relationshipbetween the surface brightness of stars and their color. They are widelyused for measuring the distances to stars of known radii, as theRoche-lobe filling secondaries in cataclysmic variables (CVs).Aims: . The calibration of the surface brightness of field dwarfs ofnear-solar metalicity with spectral types A0 to L8 covers all secondaryspectral types detectable in CVs and related objects and will aid in themeasurement of their distances. Methods: . The calibrations arebased on the radii of field dwarfs measured by the Infrared Flux Methodand by interferometry. Published photometry is used and homogenized tothe Cousins Rc and Ic and the CIT JHK photometricsystems. The narrow band surface brightness at 7500 Å is based onour own and published spectrophotometry. Care is taken to select thedwarfs for near-solar metalicity, appropriate to CVs, and to avoiderrors caused by unrecognized binarity. Results: . Relations areprovided for the surface brightness in V, R_c, I_c, J, H, K and in anarrow band at 7500 Å as functions of V-K and of spectral type.The method is tested with selected CVs for which independent informationon their distances is available. The observed spread in the radii ofearly M-dwarfs of given mass or luminosity and its influence on thedistance measurements of CVs is discussed. Conclusions: . As longas accurate trigonometric parallaxes are not routinely available for alarge number of CVs, the surface brightness method remains a reliablemeans of determining distances to CVs in which a spectral signature ofthe secondary star can be discerned.
| 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.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): sample selection and the cluster logN-logS
In this paper we serendipitously identify X-ray cluster candidates usingXMM-Newton archival observations complemented by five-band opticalphotometric follow-up observations (r~ 23 mag) as part of the X-rayIdentification (XID) programme. Our sample covers an area of ~2.1deg2 (15 XMM-Newton fields) and comprises a total of 21 (19serendipitous + two target) extended X-ray sources to the limitfx (0.5-2 keV) ~ 6 × 10-15 ergs-1 cm-2, with a high probability (>99.9 percent) of being extended on the XMM-Newton images. Of the 21 X-rayclusters, 14 are detected for the first time while seven arespectroscopically confirmed in the literature. Exploiting the opticaldata available for these fields we discover that >~68 per cent of theX-ray cluster candidates are associated with optical galaxyoverdensities. We also attempt to constrain the redshifts of our clustercandidates using photometric methods. We thus construct the photometricredshift distribution of galaxies in the vicinity of each X-ray selectedcluster candidate and search for statistically significant redshiftpeaks against that of the background distribution of field galaxies.Most of our clusters have photometric or spectroscopic redshifts in therange 0.4 < z < 0.6. Comparison of photometric with spectroscopicredshift estimates for the confirmed clusters suggests that our simplemethod is robust out to z~ 0.5. For clusters at higher z, deeper opticaldata are required to estimate reliable photometric redshifts. Using thesample of the 19 serendipitous X-ray selected cluster candidates, weestimate their surface density down to fx (0.5-2 keV) ~ 6× 10-15 erg s-1 cm-2 and find itto be in fair agreement with previous and recent studies.
| A Spitzer Study of Dusty Disks around Nearby, Young Stars
We have obtained Spitzer Space Telescope MIPS (Multiband ImagingPhotometer for Spitzer) observations of 39 A- through M-type dwarfs,with estimated ages between 12 and 600 Myr; IRAC observations for asubset of 11 stars; and follow-up CSO SHARC II 350 μm observationsfor a subset of two stars. None of the objects observed with IRACpossess infrared excesses at 3.6-8.0 μm however, seven objectsobserved with MIPS possess 24 and/or 70 μm excesses. Four objects(κ Phe, HD 92945, HD 119124, and AU Mic), with estimated ages12-200 Myr, possess strong 70 μm excesses, >=100% larger thantheir predicted photospheres, and no 24 μm excesses, suggesting thatthe dust grains in these systems are cold. One object (HD 112429)possesses moderate 24 and 70 μm excesses with a color temperature,Tgr=100 K. Two objects (α1 Lib and HD177724) possess such strong 24 μm excesses that their 12, 24, and 70μm fluxes cannot be self-consistently modeled using a modifiedblackbody despite a 70 μm excess >2 times greater than thephotosphere around α1 Lib. The strong 24 μm excessesmay be the result of emission in spectral features, as observed towardthe Hale-Bopp star HD 69830.
| Where Are the M Dwarf Disks Older Than 10 Million Years?
We present 11.7 μm observations of nine late-type dwarfs obtained atthe Keck I 10 m telescope in 2002 December and 2003 April. Our targetswere selected for their youth or apparent IRAS 12 μm excess. For allnine sources, excess infrared emission is not detected. We find thatstellar wind drag can dominate the circumstellar grain removal andplausibly explain the dearth of M dwarf systems older than 10 Myr withcurrently detected infrared excesses. We predict that M dwarfs possessfractional infrared excesses on the order ofLIR/L*~10-6 and that this may bedetectable with future efforts.
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