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Публикации по объекту
A Uniform Analysis of 118 Stars with High-contrast Imaging: Long-period Extrasolar Giant Planets are Rare Around Sun-like Stars
We expand on the results of Nielsen et al., using the null result forgiant extrasolar planets around the 118 target stars from the Very LargeTelescope (VLT) NACO H- and Ks-band planet search (conducted byMasciadri and collaborators in 2003 and 2004), the VLT and MMTSimultaneous Differential Imager survey, and the Gemini Deep PlanetSurvey to set constraints on the population of giant extrasolar planets.Our analysis is extended to include the planet luminosity models ofFortney et al., as well as the correlation between stellar mass andfrequency of giant planets found by Johnson et al. Doubling the samplesize of FGKM stars strengthens our conclusions: a model for extrasolargiant planets with power laws for mass and semimajor axis as given byCumming et al. cannot, with 95% confidence, have planets beyond 65 AU,compared to the value of 94 AU reported by Nielsen et al., using themodels of Baraffe et al. When the Johnson et al. correction for stellarmass (which gives fewer Jupiter-mass companions to M stars with respectto solar-type stars) is applied, however, this limit moves out to 82 AU.For the relatively new Fortney et al. models, which predict fainterplanets across most of parameter space, these upper limits, with andwithout a correction for stellar mass, are 182 and 234 AU, respectively.
| Detectability of Exoplanets in the ? Pic Moving Group with the Gemini Planet Imager
We model the detectability of exoplanets around stars in the ? PicMoving Group (BPMG) using the Gemini Planet Imager (GPI), acoronagraphic instrument designed to detect companions by imaging.Members of the BPMG are considered promising targets for exoplanetsearches because of their youth (~12 Myr) and proximity (median distance~35 pc). We wrote a modeling procedure to generate hypotheticalcompanions of given mass, age, eccentricity, and semi-major axis, andplace them around BPMG members that fall within the V-band range of theGPI. We count companions lying within the GPI's field of view and H-bandfluxes that have a host-companion flux ratio placing them within itssensitivity as possible detections. The fraction of companions thatcould be detected depends on their brightness at 12 Myr, and henceformation mechanism, and on their distribution of semi-major axes. Weused brightness models for formation by disk instability andcore-accretion. We considered the two extreme cases of the semi-majoraxis distribution—the log-normal distribution of the nearby F- andG-type stars and a power-law distribution indicated by the exoplanetsdetected by the radial velocity technique. We find that the GPI coulddetect exoplanets of all the F and G spectral type stars in the BPMGsample with a probability that depends on the brightness model andsemi-major axis distribution. At spectral type K-M1, exoplanetdetectability depends on brightness and hence distance of the host star.GPI will be able to detect the companions of M stars later than M1 onlyif they are closer than 10 pc. Of the four A stars in the BPMG sample,only one has a V-band brightness in the range of GPI; the others are toobright.
| Deep imaging survey of young, nearby austral stars . VLT/NACO near-infrared Lyot-coronographic observations
Context. High contrast and high angular resolution imaging is theoptimal search technique for substellar companions to nearby stars atphysical separations larger than typically 10 AU. Two distinctpopulations of substellar companions, brown dwarfs and planets, can beprobed and characterized. As a result, fossile traces of processes offormation and evolution can be revealed by physical and orbitalproperties, both for individual systems and as an ensemble. Aims:Since November 2002, we have conducted a large, deep imaging, survey ofyoung, nearby associations of the southern hemisphere. Our goal isdetection and characterization of substellar companions with projectedseparations in the range 10-500 AU. We have observed a sample of 88stars, primarily G to M dwarfs, younger than 100 Myr, and within 100 pcof Earth. Methods: The VLT/NACO adaptive optics instrument of theESO Paranal Observatory was used to explore the faint circumstellarenvironment between typically 0.1 and 10''. Diffraction-limitedobservations in H and K_s-band combined with Lyot-coronagraphy enabledus to reach primary star-companion brightness ratios as small as10-6. The existence of planetary mass companions couldtherefore be probed. We used a standardized observing sequence toprecisely measure the position and flux of all detected sources relativeto their visual primary star. Repeated observations at several epochsenabled us to discriminate comoving companions from background objects. Results: We report the discovery of 17 new close (0.1-5.0'')multiple systems. HIP 108195 AB and C (F1 III-M6), HIP 84642 AB (a~14AU, K0-M5) and TWA22 AB (a~1.8 AU; M6-M6) are confirmed comovingsystems. TWA22 AB is likely to be a rare astrometric calibrator that canbe used to test evolutionary model predictions. Among our completesample, a total of 65 targets were observed with deep coronagraphicimaging. About 240 faint companion candidates were detected around 36stars. Follow-up observations with VLT or HST for 83% of these starsenabled us to identify a large fraction of background contaminants. Ourlatest results that pertain to the substellar companions to GSC08047-00232, AB Pic and 2M1207 (confirmed during this survey andpublished earlier), are reviewed. Finally, a statistical analysis of ourcomplete set of coronagraphic detection limits enables us to placeconstraints on the physical and orbital properties of giant planetsbetween typically 20 and 150 AU.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/509/A52
| Search for associations containing young stars (SACY). III. Ages and Li abundances
Context: Our study is a follow-up of the SACY project, an extended highspectral resolution survey of more than two thousand opticalcounterparts to X-ray sources in the southern hemisphere targeted tosearch for young nearby association. Nine associations have either beennewly identified, or have had their member list revised. Groupsbelonging to the Sco-Cen-Oph complex are not considered in the presentstudy. Aims: These nine associations, with ages of between about 6Myr and 70 Myr, form an excellent sample to study the Li depletion inthe pre-main sequence (PMS) evolution. In the present paper, weinvestigate the use of Li abundances as an independent clock toconstrain the PMS evolution. Methods: Using our measurements ofthe equivalent widths of the Li resonance line and assuming fixedmetallicities and microturbulence, we calculated the LTE Li abundancesfor 376 members of various young associations. In addition, weconsidered the effects of their projected stellar rotation.Results: We present the Li depletion as a function of age in the firsthundred million years for the first time for the most extended sample ofLi abundances in young stellar associations. Conclusions: A clearLi depletion can be measured in the temperature range from 5000 K to3500 K for the age span covered by the nine associations studied in thispaper. The age sequence based on the Li-clock agrees well with theisochronal ages, the ?Cha association being the only possibleexception. The lithium depletion patterns for the associations presentedhere resemble those of the young open clusters with similar ages,strengthening the notion that the members proposed for these loose youngassociations have indeed a common physical origin. The observed scatterin the Li abundances hampers the use of Li in determining reliable agesfor individual stars. For velocities above 20 km s-1,rotation seems to play an important role in inhibiting the Li depletion.Based on observations collected at the ESO - La Silla and at theLNA-OPD.Tables [see full textsee full text]-[see full textsee full text] areonly available in electronic form at http://www.aanda.org
| No Transition Disk? Infrared Excess, PAH, H2, and X-Rays from the Weak-Lined T Tauri Star DoAr 21
As part of a program to understand disk dispersal and the interplaybetween circumstellar disks and X-ray emission, we present newhigh-resolution mid-infrared (IR) imaging, high-resolution opticalspectroscopy, and Chandra grating X-ray spectroscopy of the weak-lined TTauri star DoAr 21. DoAr 21 (age <106 yr and mass ~2.2 Msun based on evolutionary tracks) is a strong X-ray emitter,with conflicting evidence in the literature about its disk properties.It shows weak but broad Hα emission (reported here for the firsttime since the 1950s); polarimetric variability; polycyclic aromatichydrocarbon (PAH) and H2 emission; and a strong, spatiallyresolved 24 μm excess in archival Spitzer photometry. Geminisub-arcsecond-resolution 9-18 μm images show that there is little orno excess mid-IR emission within 100 AU of the star; the excess emissionis extended over several arcseconds and is quite asymmetric. Theextended emission is bright in the ultraviolet (UV)-excited λ =11.3 μm PAH emission feature. A new high-resolution X-ray gratingspectrum from Chandra shows that the stellar X-ray emission is very hardand dominated by continuum emission; it is well fit by amulti-temperature thermal model, typical of hard coronal sources, andshows no evidence of unusually high densities. A flare during the X-rayobservation shows a temperature approaching 108 K. We arguethat the far-UV emission from the transition region is sufficient toexcite the observed extended PAH and continuum emission, and that theH2 emission may be similarly extended and excited. While thisextended emission may be a disk in the final stages of clearing, it alsocould be more akin to a small-scale photodissociation region than aprotoplanetary disk, highlighting both the very young ages(<106 yr) at which some stars are found without disks andthe extreme radiation environment around even late-typepre-main-sequence stars.
| Starspots
Starspots are created by local magnetic fields on the surfaces of stars,just as sunspots. Their fields are strong enough to suppress theoverturning convective motion and thus block or redirect the flow ofenergy from the stellar interior outwards to the surface andconsequently appear as locally cool and therefore dark regions againstan otherwise bright photosphere (Biermann in Astronomische Nachrichten264:361, 1938; Z Astrophysik 25:135, 1948). As such, starspots areobservable tracers of the yet unknown internal dynamo activity and allowa glimpse into the complex internal stellar magnetic field structure.Starspots also enable the precise measurement of stellar rotation whichis among the key ingredients for the expected internal magnetictopology. But whether starspots are just blown-up sunspot analogs, we donot know yet. This article is an attempt to review our current knowledgeof starspots. A comparison of a white-light image of the Sun (G2V, 5Gyr) with a Doppler image of a young solar-like star (EK Draconis;G1.5V, age 100 Myr, rotation 10 × ? Sun) and witha mean-field dynamo simulation suggests that starspots can be ofsignificantly different appearance and cannot be explained with ascaling of the solar model, even for a star of same mass and effectivetemperature. Starspots, their surface location and migration pattern,and their link with the stellar dynamo and its internal energytransport, may have far reaching impact also for our understanding oflow-mass stellar evolution and formation. Emphasis is given in thisreview to their importance as activity tracers in particular in thelight of more and more precise exoplanet detections around solar-like,and therefore likely spotted, host stars.
| 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
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is 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/501/941
| 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).
| Lithium Depletion of Nearby Young Stellar Associations
We estimate cluster ages from lithium depletion in fivepre-main-sequence groups found within 100 pc of the Sun: the TW Hydraeassociation, η Chamaeleontis cluster, β Pictoris moving group,Tucanae-Horologium association, and AB Doradus moving group. Wedetermine surface gravities, effective temperatures, and lithiumabundances for over 900 spectra through least-squares fitting tomodel-atmosphere spectra. For each group, we compare the dependence oflithium abundance on temperature with isochrones from pre-main-sequenceevolutionary tracks to obtain model-dependent ages. We find that theη Cha cluster and the TW Hydrae association are the youngest, withages of 12+/-6 Myr and 12+/-8 Myr, respectively, followed by the βPic moving group at 21+/-9 Myr, the Tucanae-Horologium association at27+/-11 Myr, and the AB Dor moving group at an age of at least 45 Myr(whereby we can only set a lower limit, since the models-unlike realstars-do not show much lithium depletion beyond this age). Here theordering is robust, but the precise ages depend on our choice of bothatmospheric and evolutionary models. As a result, while our ages areconsistent with estimates based on Hertzsprung-Russell isochrone fittingand dynamical expansion, they are not yet more precise. Our observationsdo show that with improved models, much stronger constraints should befeasible, as the intrinsic uncertainties, as measured from the scatterbetween measurements from different spectra of the same star, are verylow: around 10 K in effective temperature, 0.05 dex in surface gravity,and 0.03 dex in lithium abundance.
| High levels of surface differential rotation on the young G0 dwarf HD171488
We present high-resolution images of the young, rapidly rotating G0dwarf HD171488, using both Stokes I and Stokes V data. The observationswere secured with the MuSiCoS spectropolarimeter at Telescope BernardLyot from 2005 May 31 to June 10. The photospheric surface brightnessdistributions show a strong and slightly decentred polar cap thatdominates over weak high- and low-latitude spot features. Thelarge-scale magnetic field topology shows a strong ring of anticlockwiseazimuthal field with a latitudinal dependence on polarity and largeregions of radial field with negative polarity at all latitudes. Usingthe good phase coverage of our data, we measure the differentialrotation on HD171488. The results indicate that the equator laps thepole every 12 days for brightness data and 13 days for magnetic data,which is the highest measurement of differential rotation obtained usingZeeman-Doppler imaging techniques.Spectropolarimetric observations were obtained, from 2005 May 31 to June10, with the MuSiCoS echelle spectropolarimeter at the Telescope BernardLyot (Observatoire du Pic du Midi, France).E-mail: s.v.jeffers@uu.nl
| 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.
| Spitzer MIPS Observations of Stars in the β Pictoris Moving Group
We present Multiband Imaging Photometer for Spitzer (MIPS) observationsat 24 and 70 μm for 30 stars, and at 160 μm for a subset of 12stars, in the nearby (~30 pc), young (~12 Myr) β Pictoris movinggroup (BPMG). In several cases, the new MIPS measurements resolve sourceconfusion and background contamination issues in the IRAS data for thissample. We find that 7 members have 24 μm excesses, implying a debrisdisk fraction of 23%, and that at least 11 have 70 μm excesses (diskfraction of >=37%). Five disks are detected at 160 μm (out of abiased sample of 12 stars observed), with a range of 160/70 flux ratios.The disk fraction at 24 and 70 μm, and the size of the excessesmeasured at each wavelength, are both consistent with an ``inside-out''infrared excess decrease with time, wherein the shorter wavelengthexcesses disappear before longer wavelength excesses, and consistentwith the overall decrease of infrared excess frequency with stellar age,as seen in Spitzer studies of other young stellar groups. Assuming thatthe infrared excesses are entirely due to circumstellar disks, wecharacterize the disk properties using simple models and fractionalinfrared luminosities. Optically thick disks, seen in the younger TW Hyaand η Cha associations, are entirely absent in the BPMG. Additionalflux density measurements at 24 and 70 μm are reported for nineTucana-Horologium association member stars. Since this is <20% of theassociation membership, limited analysis on the complete disk fractionof this association is possible.
| Coronae of Young Fast Rotators
AB Dor, Speedy Mic, and Rst 137B are in their early post-T Taurievolutionary phase (<100 Myr), at the age of fastest rotation in thelife of late-type stars. They straddle the coronalsaturation-supersaturation boundary first defined by young stars in openclusters. High-resolution Chandra X-ray spectra have been analyzed tostudy their coronal properties as a function of coronal activityparameters Rossby number, LX/Lbol, and a coronaltemperature index. Differences between stars suggest that assupersaturation is reached the DEM slope below the temperature of peakDEM becomes shallower, while the DEM drop-off above this temperaturebecomes more pronounced. A larger sample comprising our three targetsand 22 active stars studied in the recent literature reveals a generalincrease of plasma at T>~107 K toward thesaturated-supersaturated boundary but a decline beyond this amongsupersaturated stars. The coronal Fe abundances of the stellar sampleare inversely correlated with LX/Lbol, decliningslowly with rising LX/Lbol, but with a much moresharp decline atLX/Lbol>~3×10-4. For dwarfsthe Fe abundance is also well correlated with Rossby number. The coronalO/Fe ratios for dwarfs show a clear increase with decreasing Rossbynumber, apparently reaching saturation at [O/Fe]=0.5 at the coronalsupersaturation boundary.
| X-Atlas: An Online Archive of Chandra's Stellar High-Energy Transmission Grating Observations
The high-resolution X-ray spectroscopy made possible by the 1999deployment of the Chandra X-ray Observatory has revolutionized ourunderstanding of stellar X-ray emission. Many puzzles remain, however,particularly regarding the mechanisms of X-ray emission from OB stars.Although numerous individual stars have been observed in highresolution, realizing the full scientific potential of theseobservations will necessitate studying the high-resolution Chandra dataset as a whole. To facilitate the rapid comparison and characterizationof stellar spectra, we have compiled a uniformly processed database ofall stars observed with the Chandra High-Energy Transmission Grating(HETG). This database, known as X-Atlas, is accessible through a Webinterface with searching, data retrieval, and interactive plottingcapabilities. For each target, X-Atlas also features predictions of thelow-resolution ACIS spectra convolved from the HETG data for comparisonwith stellar sources in archival ACIS images. Preliminary analyses ofthe hardness ratios, quantiles, and spectral fits derived from thepredicted ACIS spectra reveal systematic differences between the high-and low-mass stars in the atlas and offer evidence for at least twodistinct classes of high-mass stars. A high degree of X-ray variabilityis also seen in both high- and low-mass stars, including Capella, longthought to exhibit minimal variability. X-Atlas contains over 130observations of approximately 25 high-mass stars and 40 low-mass starsand will be updated as additional stellar HETG observations becomepublic. The atlas has recently expanded to nonstellar point sources, andLow-Energy Transmission Grating (LETG) observations are currently beingadded as well.
| On the kinematic evolution of young local associations and the Scorpius-Centaurus complex
Context: Over the last decade, several groups of young (mainly low-mass)stars have been discovered in the solar neighbourhood (closer than ~100pc), thanks to cross-correlation between X-ray, optical spectroscopy andkinematic data. These young local associations - including an importantfraction whose members are Hipparcos stars - offer insights into thestar formation process in low-density environments, shed light on thesubstellar domain, and could have played an important role in the recenthistory of the local interstellar medium. Aims: To study the kinematicevolution of young local associations and their relation to other youngstellar groups and structures in the local interstellar medium, thuscasting new light on recent star formation processes in the solarneighbourhood. Methods: We compiled the data published in theliterature for young local associations. Using a realistic Galacticpotential we integrated the orbits for these associations and theSco-Cen complex back in time. Results: Combining these data with thespatial structure of the Local Bubble and the spiral structure of theGalaxy, we propose a recent history of star formation in the solarneighbourhood. We suggest that both the Sco-Cen complex and young localassociations originated as a result of the impact of the inner spiralarm shock wave against a giant molecular cloud. The core of the giantmolecular cloud formed the Sco-Cen complex, and some small cloudlets ina halo around the giant molecular cloud formed young local associationsseveral million years later. We also propose a supernova in young localassociations a few million years ago as the most likely candidate tohave reheated the Local Bubble to its present temperature.
| 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 Nearby Young Visual Binary HIP 115147 and Its Common Proper Motion Companion LSPM J2322+7847
We report a late M-type, common proper motion companion to a nearbyyoung visual binary HIP 115147 (V368 Cep), separated by 963" from theprimary K0 dwarf. This optically dim star was identified as a candidatehigh proper motion, nearby dwarf LSPM J2322+7847 by Lépine in2005. The wide companion is one of the latest post-T Tauri low-massstars found within 20 pc. We obtain a trigonometric parallax of51.6+/-0.8 mas, in good agreement with the Hipparcos parallax of theprimary star (50.7+/-0.6 mas). Our BVRI photometric data andnear-infrared data from 2MASS are consistent with LSPM J2322+7847 beingbrighter by 1 mag in Ks than field M dwarfs atV-Ks=6.66, which indicates its pre-main-sequence status. Weconclude that the most likely age of the primary HIP 115147 and its 11"companion HIP 115147B is 20-50 Myr. The primary appears to be older thanits close analog PZ Tel (age 12-20 Myr) and members of the TWAassociation (7 Myr).
| 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.
| SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets around Young Stars
We present a review of precursor observing programs for the SIMPlanetQuest Key Project devoted to detecting Jupiter-mass planets aroundyoung stars. In order to ensure that the stars in the sample are free ofvarious sources of astrometric noise that might impede the detection ofplanets, we have initiated programs to collect photometry, high-contrastimages, interferometric data, and radial velocities for stars in boththe northern and southern hemispheres. We have completed a high-contrastimaging survey of target stars in Taurus and the Pleiades and found nodefinitive common proper motion companions within 1" (140 AU) of the SIMtargets. Our radial velocity surveys have shown that many of the targetstars in Sco-Cen are fast rotators, and a few stars in Taurus and thePleiades may have substellar companions. Interferometric data of a fewstars in Taurus show no signs of stellar or substellar companions withseparations of 5-50 mas. The photometric survey suggests thatapproximately half of the stars initially selected for this program arevariable to a degree (1 σ > 0.1 mag) that would degrade theastrometric accuracy achievable for that star. While the precursorprograms are still a work in progress, we provide a comprehensive listof all targets and rank them according to their viability as a result ofthe observations taken to date. The observable that removes by far themost targets from the SIM young stellar object (YSO) program isphotometric variability.
| Rotation and Activity of Pre-Main-Sequence Stars
We present a study of rotation (vsini) and chromospheric activity(Hα equivalent width) based on an extensive set of high-resolutionoptical spectra obtained with the MIKE instrument on the 6.5 m MagellanClay telescope. Our targets are 74 F-M dwarfs in four young stellarassociations, spanning ages from 6 to 30 Myr. By comparing Hα EWsin our sample to results in the literature, we see a clear evolutionarysequence: Chromospheric activity declines steadily from the T Tauriphase to the main sequence. Using activity as an age indicator, we finda plausible age range for the Tuc-Hor association of 10-40 Myr. Between5 and 30 Myr, we do not see evidence for rotational braking in the totalsample, and thus angular momentum is conserved, in contrast to youngerstars. This difference indicates a change in the rotational regulationat ~5-10 Myr, possibly because disk braking cannot operate longer thantypical disk lifetimes, allowing the objects to spin up. Therotation-activity relation is flat in our sample; in contrast tomain-sequence stars, there is no linear correlation for slow rotators.We argue that this is because young stars generate their magnetic fieldsin a fundamentally different way from main-sequence stars, and not justthe result of a saturated solar-type dynamo. By comparing our rotationalvelocities with published rotation periods for a subset of stars, wedetermine ages of 13+7-6 and9+8-2 Myr for the η Cha and TWA associations,respectively, consistent with previous estimates. Thus we conclude thatstellar radii from evolutionary models by Baraffe et al. (1998) are inagreement with the observed radii to within +/-15%.
| High-resolution X-ray spectroscopy of T Tauri stars in the Taurus-Auriga complex
Context: Differences have been reported between the X-ray emission ofaccreting and non-accreting stars. Some observations have suggested thataccretion shocks could be responsible for part of the X-ray emission inclassical T Tauri stars (CTTS). Aims: We present high-resolutionX-ray spectroscopy for nine pre-main sequence stars in order to test theproposed spectroscopic differences between accreting and non-accretingpre-main sequence stars. Methods: We used X-ray spectroscopy fromthe XMM-Newton Reflection Grating Spectrometers and the EPICinstruments. We interpret the spectra using optically thin thermalmodels with variable abundances, together with an absorption columndensity. For BP Tau and AB Aur we derive electron densities from the Ovii triplets. Results: Using the O vii/O viii count ratios as adiagnostic for cool plasma, we find that CTTS display a soft excess(with equivalent electron temperatures of ≈2.5-3 MK) when comparedwith WTTS or zero-age main-sequence stars. Although the O vii triplet inBP Tau is consistent with a high electron density (3.4 ×1011 cm-3), we find low density for the accretingHerbig star AB Aur (ne < 1010 cm-3).The element abundances of accreting and non-accreting stars are similar.The Ne abundance is found to be high (4-6 times the Fe abundance) in allK and M-type stars. In contrast, for the three G-type stars (SU Aur, HD283572, and HP Tau/G2), we find an enhanced Fe abundance (0.4-0.8 timessolar photospheric values) compared to later-type stars. Conclusions: Adding the results from our sample to formerhigh-resolution studies of T Tauri stars, we find a soft excess in allaccreting stars, but in none of the non-accretors. On the other hand,high electron density and high Ne/Fe abundance ratios do not seem to bepresent in all accreting pre-main sequence stars.
| On the common origin of the AB Doradus moving group and the Pleiades cluster
AB Doradus (AB Dor) is the nearest identified moving group. As withother such groups, the age is important for understanding of several keyquestions. It is important, for example, in establishing the origin ofthe group and also in comparative studies of the properties of planetarysystems, eventually surrounding some of the AB Dor group members, withthose existing in other groups. For AB Dor two rather differentestimates for its age have been proposed: the first one, of the order of50 Myr, by Zuckerman and coworkers from a comparison with theTucana/Horologium moving group and a second one of about 100-125 Myr byLuhman and coworkers from colour-magnitude diagrams. Using this lastvalue and the closeness in velocity space of AB Dor and the Pleiadesgalactic cluster, Luhman and coworkers suggested coevality for thesesystems. Because strictly speaking such a closeness does not stillguarantee coevality, here we address this problem by computing andcomparing the full 3D orbits of AB Dor, Pleiades, α Persei and IC2602. The latter two open clusters have estimated ages of about 85-90and 50 Myr. The resulting age 119 +/- 20 Myr is consistent with AB Dorand Pleiades being coeval. Our solution and the scenario of open clusterformation proposed by Kroupa and collaborators suggest that the AB Dormoving group may be identified with the expanding subpopulation (GroupI) present in this scenario. We also discuss other related aspects asiron and lithium abundances, eventual stellar mass segregation duringthe formation of the systems and possible fraction of debris discs inthe AB Dor group.
| Coronal Abundances in Orion Nebula Cluster Stars
Following the Chandra Orion Ultradeep Project (COUP) observation, wehave studied the chemical composition of the hot plasma in a sample of146 X-ray-bright pre-main-sequence stars in the Orion Nebula Cluster(ONC). We report measurements of individual element abundances for asubsample of 86 slightly absorbed and bright X-ray sources, usinglow-resolution X-ray spectra obtained from the Chandra ACIS instrument.The X-ray emission originates from a plasma with temperatures andelemental abundances very similar to those of active coronae in olderstars. A clear pattern of abundances versus first ionization potential(FIP) is evident if solar photospheric abundances are assumed asreference. The results are validated by extensive simulations. Theobserved abundance distributions are compatible with a single pattern ofabundances for all stars, although a weak dependence on flare loop sizemay be present. The abundance of calcium is the only one which appearsto vary substantially between stars, but this quantity is affected byrelatively large uncertainties. The ensemble properties of theX-ray-bright COUP sources confirm that the iron in the emitting plasmais underabundant with respect to both the solar composition and theaverage stellar photospheric values. Comparison of the present plasmaabundances with those of the stellar photospheres and those of thegaseous component of the nebula indicates a good agreement for all theother elements with available measurements, and in particular for thehigh-FIP elements (Ne, Ar, O, and S) and for the low-FIP element Si. Weconclude that there is evidence of a significant chemical fractionationeffect only for iron, which appears to be depleted by a factor 1.5-3with respect to the stellar composition.
| CCD B & V Photometry of the Young, Active, Late-type Star PZ Telescopii
We present new CCD B and V photometry, obtained at the BrightwaterObservatory in June and July 2006, of PZ Telescopii (HD 174429), ayoung, rapidly rotating late-type star with an active chromosphere. TheCCD data were collected with a short focal-length telescope, giving afield of view near 0.80 x 0.55 deg, so target and comparison stars couldbe observed simultaneously. A periodogram analysis of the PZ Tel datareturned a value of 0.94 d, consistent with earlier photometry. We foundthe amplitude of variation was ~0.06 mag in B and V, with evidence fora small colour variation of between 0.01 and 0.02 mag, the star beingredder when fainter. From a comparison with previously publishedphotometry we find that the amplitude of V-light variation is amongstthe smallest ever observed. There is an indication that there was amaximum in the long-term brightness level near V=8.28 around the year2000, with the current data being several hundredths of a magnitudefainter.
| 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.
| 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
| Accretion Disks around Young Stars: Lifetimes, Disk Locking, and Variability
We report the findings of a comprehensive study of disk accretion andrelated phenomena in four of the nearest young stellar associationsspanning 6-30 million years in age, an epoch that may coincide with thelate stages of planet formation. We have obtained ~650 multiepochhigh-resolution optical spectra of 100 low-mass stars that are likelymembers of the η Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), βPictoris (~12 Myr), and Tucanae-Horologium (~30 Myr) groups. Our datawere collected over 12 nights between 2004 December and 2005 July on theMagellan Clay 6.5 m telescope. Based on Hα line profiles, alongwith a variety of other emission lines, we find clear evidence ofongoing accretion in 3 out of 11 η Cha stars and 2 out of 32 TWHydrae members. None of the 57 β Pic or Tuc-Hor members showsmeasurable signs of accretion. Together, these results imply significantevolution of the disk-accretion process within the first several Myr ofa low-mass star's life. While a few disks can continue to accrete for upto ~10 Myr, our findings suggest that disks accreting for beyond thattimescale are rather rare. This result provides an indirect constrainton the timescale for gas dissipation in inner disks and, in turn, ongas-giant planet formation. All accretors in our sample are slowrotators, whereas nonaccretors cover a large range in rotationalvelocities. This may hint at rotational braking by disks at ages up to~8 Myr. Our multiepoch spectra confirm that emission-line variability iscommon even in somewhat older T Tauri stars, among which accretors tendto show particularly strong variations. Thus, our results indicate thataccretion and wind activity undergo significant and sustained variationsthroughout the lifetime of accretion disks.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Телескоп |
| Прямое восхождение: | 18h53m05.87s |
| Склонение: | -50°10'49.9" |
| Видимая звёздная величина: | 8.414 |
| Расстояние: | 49.652 парсек |
| Собственное движение RA: | 15.8 |
| Собственное движение Dec: | -84.1 |
| B-T magnitude: | 9.415 |
| V-T magnitude: | 8.497 |
Каталоги и обозначения:
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