내용
사진
사진 업로드
DSS Images Other Images
관련 글
Chemical Composition and Differential Time-Series CCD Photometry of V2314 Ophiuchi: A New λ Bootis-Type Star
We investigated V2314 Oph using high-resolution (R=45,000) spectralobservations. The atmospheric parameters were determined using iron-lineabundance analysis. We derived a mean effective temperature ofTeff=7510 K, mean surface gravity of logg=3.30, metallicityof [Fe/H]=-0.59, and rotation velocity of vsini=38 km s-1.The abundances of 19 chemical elements were determined. The abundancepattern of V2314 Oph was found to be similar to that of λBootis-type stars. V2314 Oph is also a variable star, as are a majorityof λ Bootis stars. A frequency analysis of 10 selected nights ofprecise differential photometry has preliminarily revealed fourpulsational modes. The multiperiodic solution of the light curve isprobably more similar to a low-amplitude Population I δ Scutivariable pattern, as is apparent for many λ Bootis stars. Forasteroseismic investigations of the star it is necessary to obtain moreprolonged, continuous time-series observations. These features, as wellas the small proper motion and relatively large rotational velocity ofV2314 Oph, specify the star as not likely to be a field SX PhoenicisPopulation II variable, as was suggested previously.
| Oxygen Abundance of Open Cluster Dwarfs
We present oxygen abundances of dwarfs in the young open cluster IC 4665deduced from the O I λ7774 triplet lines and of dwarfs in theopen cluster Pleiades derived from the [O I] λ6300 forbiddenline. Stellar parameters and oxygen abundances were derived using thespectroscopic synthesis tool SME (Spectroscopy Made Easy). We find adramatic increase in the upper boundary of the O I triplet abundanceswith decreasing temperature in the dwarfs of IC 4665, consistent withthe trend found by Schuler et al. in the open clusters Pleiades and M34and to a less extent in the cool dwarfs of the Hyades by Schuler et al.and UMa by King & Schuler. By contrast, oxygen abundances derivedfrom the [O I] λ6300 forbidden line for stars in the Pleiades andthe Hyades from Schuler et al. are constant within the errors. Possiblemechanisms that may lead a varying oxygen triplet line abundance areexamined, including systematic errors in the stellar parameterdeterminations, the NLTE effects, surface activities, and granulation.The age-related stellar surface activities (especially the chromosphericactivities) are suggested by our analysis to be responsible for thelarge spreads of oxygen triplet line abundances.
| Hyades Morphology and Star Formation
Perryman and collaborators found that for the Hyades cluster thefraction of multiple stars increases from G stars to early-A stars. Wediscuss here whether this may be a general property of star formation orwhether collisions in the cluster environment change the fractions ofbinary stars. A star ring of mainly F and G stars is seen around theHyades cluster core, supposedly created by a shock wave due to asupernova explosion. These ring stars show the same fraction of binarystars as observed for the F and G stars in the core of the cluster. Thissuggests that collisions in the high stellar density cluster center didnot measurably change the multiple-star fractions. There is so far onlyevidence of this one supernova explosion in the Hyades. If this isindeed the only one, then only one massive star was born in the cluster.There are also fewer than 10 white dwarf descendants of B stars found inthe cluster. Unless many white dwarfs and neutron stars evaporated fromthe cluster, the initial mass function for stellar masses >2Msolar must have been very steep, corresponding to a Salpeterfunction with an exponent of less than -3.3.
| 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.
| The Monitor project: searching for occultations in young open clusters
The Monitor project is a photometric monitoring survey of nine young(1-200Myr) clusters in the solar neighbourhood to search for eclipses byvery low mass stars and brown dwarfs and for planetary transits in thelight curves of cluster members. It began in the autumn of 2004 and usesseveral 2- to 4-m telescopes worldwide. We aim to calibrate the relationbetween age, mass, radius and where possible luminosity, from the Kdwarf to the planet regime, in an age range where constraints onevolutionary models are currently very scarce. Any detection of anexoplanet in one of our youngest targets (<~10Myr) would also provideimportant constraints on planet formation and migration time-scales andtheir relation to protoplanetary disc lifetimes. Finally, we will usethe light curves of cluster members to study rotation and flaring inlow-mass pre-main-sequence stars.The present paper details the motivation, science goals and observingstrategy of the survey. We present a method to estimate the sensitivityand number of detections expected in each cluster, using a simplesemi-analytic approach which takes into account the characteristics ofthe cluster and photometric observations, using (tunable) best-guessassumptions for the incidence and parameter distribution of putativecompanions, and we incorporate the limits imposed by radial velocityfollow-up from medium and large telescopes. We use these calculations toshow that the survey as a whole can be expected to detect over 100 younglow and very low mass eclipsing binaries, and ~3 transiting planets withradial velocity signatures detectable with currently availablefacilities.
| New brown dwarfs in Upper Sco using UKIDSS Galactic Cluster Survey science verification data
We present first results from a deep (J = 18.7), wide-field(6.5deg2) infrared (ZY JHK) survey in the Upper Scoassociation conducted within the science verification phase of theUnited Kingdom Infrared Telescope Infrared Deep Sky Survey GalacticCluster Survey (GCS). Cluster members define a sequence well separatedfrom field stars in the (Z - J, Z) colour-magnitude diagram. We haveselected a total of 164 candidates with J = 10.5-18.7 mag from the (Z -J, Z) and (Y - J, Y) diagrams. We further investigated the location ofthose candidates in the other colour-magnitude and colour-colourdiagrams to weed out contaminants. The cross-correlation of the GCScatalogue with the Two-Micron All-Sky Survey data base confirms themembership of 116 photometric candidates down to 20 Jupiter masses asthey lie within a 2σ circle centred on the association meanmotion. The final list of cluster members contains 129 sources withmasses between 0.3 and 0.007 Msolar. We extracted a dozen newlow-mass brown dwarfs below 20 MJup, the limit of previoussurveys in the region. Finally, we have derived the mass function inUpper Sco over the 0.3-0.01 Msolar mass range, best fit by asingle segment with a slope of index α = 0.6 +/- 0.1, in agreementwith previous determination in open clusters.Based on observations made with the United Kingdom Infrared Telescope,operated by the Joint Astronomy Centre on behalf of the UK ParticlePhysics and Astronomy Research Council.E-mail: nl41@star.le.ac.uk
| The Chemical Compositions of Stars with Planets: A Review
A number of trends among the properties of exoplanets have becomeevident in the years since the first one was announced in 1995. Oneparticularly interesting trend began to emerge in 1997: the incidence ofgiant planets correlates with the metallicity of the host star. This hassince been established with a high degree of statistical significance byseveral research groups. Other, more subtle trends are beginning toappear as the sample size continues to grow and the statistics improve.I review the state of our knowledge concerning the observedcompositional trends and their possible causes and suggest severalresearch directions.
| The UKIRT Infrared Deep Sky Survey Early Data Release
This paper defines the UKIRT Infrared Deep Sky Survey (UKIDSS) EarlyData Release (EDR). UKIDSS is a set of five large near-infrared surveysbeing undertaken with the United Kingdom Infrared Telescope Wide FieldCamera (WFCAM). The programme began in 2005 May and has an expectedduration of 7yr. Each survey uses some or all of the broad-band filtercomplement ZYJHK. The EDR is the first public release of data to theEuropean Southern Observatory (ESO) community. All worldwide releasesoccur after a delay of 18 months from the ESO release. The EDR providesa small sample data set, ~50deg2 (about 1 per cent of thewhole of UKIDSS), that is a lower limit to the expected quality offuture survey data releases. In addition, an EDR+ data set contains allEDR data plus extra data of similar quality, but for areas not observedin all of the required filters (amounting to ~220deg2). Thefirst large data release, DR1, will occur in mid-2006. We providedetails of the observational implementation, the data reduction, theastrometric and photometric calibration and the quality controlprocedures. We summarize the data coverage and quality (seeing,ellipticity, photometricity, depth) for each survey and give a briefguide to accessing the images and catalogues from the WFCAM ScienceArchive.
| Pleiades low-mass brown dwarfs: the cluster L dwarf sequence
Aims.We present a search for low-mass brown dwarfs in the Pleiades opencluster. The identification of Pleiades members fainter and cooler thanthose currently known allows us to constrain evolutionary models for Ldwarfs and to extend the study of the cluster mass function to lowermasses. Methods: .We conducted a 1.8 deg2 near-infraredJ-band survey at the 3.5 m Calar Alto Telescope, with completenessJ_cpl˜ 19.0. The detected sources were correlated with those ofpreviously available optical I-band images (I_cpl˜ 22). Using a Jversus I-J colour-magnitude diagram, we identified 18 faint red L-typecandidates, with magnitudes 17.4 3.2.If Pleiades members, their masses would span~0.040-0.020~Mȯ. We performed follow-up HK_s-bandimaging to further confirm their cluster membership by photometry andproper motion. Results: .Out of 11 IJ candidates with propermotion measurements, we find six cluster members, two non-members andthree whose membership is uncertain and depends on the intrinsicvelocity dispersion of Pleiades brown dwarfs. This dispersion (>4 masyr-1) is at least four times that of cluster stars withmasses ⪆1 Mȯ. Five of the seven other IJ candidatesare discarded because their J-Ks colours are bluer than thoseof confirmed members. Our least massive proper motion members areBRB 28 and 29 (~25 M_Jup). The J versus I-J sequenceof the L-type candidates at J>18 is not as red as theoretical modelspredict; it rather follows the field L-dwarf sequence translated to thecluster distance. This sequence overlapping, also observed in the Jversus J-H and J-K diagrams, suggests that Pleiades and field L dwarfsmay have similar spectral energy distributions and luminosities, andthus possibly similar radii. Also, we find α= 0.5±0.2 for apower-law approximation dN/dM ∝ M-α of the surveymass spectrum in the mass range 0.5-0.026~Mȯ. This valueis similar to that of much younger clusters, indicating no significantdifferential evaporation of low-mass Pleiades members relative to moremassive ones.
| On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| Caroline Herschel's catalogue of nebulae
Not Available
| The effect of heavy element opacity on pre-main sequence Li depletion
Context: .Recent 3-D analysis of the solar spectrum data suggests asignificant change of the solar chemical composition. This may affectthe temporal evolution of the surface abundance of light elements sincethe extension of the convective envelope is largely affected by theinternal opacity value. Aims: .We analyse the influence of theadopted solar mixture on the opacity in the convective envelope ofpre-main sequence (PMS) stars and thus on PMS lithium depletion. Thesurface Li abundance depends on the relative efficiency of severalprocesses, some of them still not known with the required precision;this paper thus analyses one of the aspects of this "puzzle".Methods: .Focusing on PMS evolution, where the largest amount of Liburning occurs, we computed stellar models for three selected masses(0.8, 1.0 and 1.2 Mȯ, with Z=0.013, Y=0.27, α=1.9)by varying the chemical mixture, that is the internal elementdistribution in Z. We analysed the contribution of the single elementsto the opacity at the temperatures and densities of interest for Lidepletion. Several mixtures were obtained by varying the abundance ofthe most important elements one at a time; we then calculated thecorresponding PMS Li abundance evolution. Results: .We found thata mixture variation does change the Li abundance: at fixed totalmetallicity, the Li depletion increases when increasing the fraction ofelements heavier than O.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Finding benchmark brown dwarfs to probe the substellar initial mass function as a function of time
Using a simulated disc brown dwarf (BD) population, we find that newlarge area infrared surveys are expected to identify enough BDs coveringwide enough mass-age ranges to potentially measure the present day massfunction down to ~0.03Msolar, and the BD formation historyout to 10Gyr, at a level that will be capable of establishing if BDformation follows star formation. We suggest these capabilities are bestrealized by spectroscopic calibration of BD properties (Teff,g and [M/H]) which when combined with a measured luminosity and anevolutionary model can give BD mass and age relatively independent of BDatmosphere models. Such calibration requires an empirical understandingof how BD spectra are affected by variations in these properties, andthus the identification and study of `benchmark BDs' whose age andcomposition can be established independently.We identify the best sources of benchmark BDs as young open clustermembers, moving group members, and wide (>1000au) BD companions toboth subgiant stars and high-mass white dwarfs (WDs). To accuratelyasses the likely number of wide companion BDs available, we haveconstrained the wide L dwarf companion fraction using the 2-Micron AllSky Survey (2MASS), and find a companion fraction of2.7+0.7-0.5percent for separations of~1000-5000au. This equates to a BD companion fraction of34+9-6percent if one assumes an α~ 1companion mass function. Using this BD companion fraction, we simulatepopulations of wide BD binaries, and estimate that80+21-14 subgiant-BD binaries, and50+13-10 benchmark WD-BD binaries could beidentified using current and new facilities. The WD-BD binaries shouldall be identifiable using the Large Area Survey component of the UnitedKingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey, combinedwith the Sloan Digital Sky Survey. Discovery of the subgiant-BD binarieswill require a near-infrared imaging campaign around a large (~900)sample of Hipparcos subgiants. If identified, spectral studies of thesebenchmark BD populations could reveal the spectral sensitivities acrossthe Teff, g and [M/H] space probed by new surveys.
| A deep wide-field optical survey in the young open cluster Collinder 359
We present the first deep, optical, wide-field imaging survey of theyoung open cluster Collinder 359, complemented by near-infraredfollow-up observations. This study is part of a large programme aimed atexamining the dependence of the mass function on environment and time.We have surveyed 1.6 square degrees in the cluster, in the I and zfilters, with the CFH12K camera on the Canada-France-Hawaii 3.6-mtelescope down to completeness and detection limits in both filters of22.0 and 24.0 mag, respectively. Based on their location in the optical(I-z, I) colour-magnitude diagram, we have extracted new cluster membercandidates in Collinder 359 spanning 1.3-0.03 Mȯ,assuming an age of 60 Myr and a distance of 450 pc for the cluster. Wehave used the 2MASS database as well as our own near-infrared photometryto examine the membership status of the optically-selected clustercandidates. Comparison of the location of the most massive members inCollinder 359 in a (B-V, V) diagram with theoretical isochrones suggeststhat Collinder 359 is older than α Per but younger than thePleiades. We discuss the possible relationship between Collinder 359 andIC 4665 as both clusters harbour similar parameters, including propermotion, distance, and age.
| Photometric Analysis of the Contact Binary V513 Herculis
Not Available
| Exploring the lower mass function in the young open cluster IC 4665
We present a study of the young (30-100 Myr) open cluster IC 4665 withthe aim to determine the shape of the mass function well into the browndwarf regime. We photometrically select 691 low-mass stellar and 94brown dwarf candidate members over an area of 3.82 square degreescentred on the cluster. K-band follow-up photometry and Two-MicronAll-Sky Survey data allow a first filtering of contaminant objects fromour catalogues. A second filtering is performed for the brightest starsusing proper motion data provided by the Tycho-2 and UCAC2 publiccatalogues. Contamination by the field population for the lowest massobjects is estimated using same latitude control fields. We fit observedsurface densities of various cluster populations with King profiles andfind a consistent tidal radius of 1.0°. The presence of possiblemass segregation is discussed. In most respects investigated, IC 4665 issimilar to other young open clusters at this age: (1) a power law fit tothe mass function between 1 and 0.04 Mȯ results in bestfit for a slope of -0.6; (2) a cusp in the mass function is noticed atabout the substellar boundary with respect to the power law description,the interpretation of which is discussed; (3) a fraction between 10-19%for BDs with M 0.03 Mȯ to total members; (4) abest-fit lognormal function to the full mass distribution shows anaverage member mass of 0.32 Mȯ, if IC 4665 has an age of50 Myr.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| Monitor: transiting planets and brown dwarfs in star forming regions and young open clusters
The Monitor project is a large scale photometric monitoring survey often star forming regions and open clusters aged between 1 and 200 Myrusing wide-field optical cameras on 2-4 m telescopes worldwide. Theprimary goal of the project is to search for close-in planets and browndwarfs at young ages through the detection of transit events. Suchdetections would provide unprecedented constraints on planet formationand migration time-scales, as well as on evolutionary models of planetsand brown dwarfs in an age range where such constraints are very scarce.Additional science goals include rotation period measurements and theanalysis of flares and accretion-related variability.Workshop ``Ultralow-mass star formation and evolution'', see AN 326, No. 10www.ast.cam.ac.uk/suz/monitor/monitor.php
| Rotation- and temperature-dependence of stellar latitudinal differential rotation
More than 600 high resolution spectra of stars with spectral type F andlater were obtained in order to search for signatures of differentialrotation in line profiles. In 147 stars the rotation law could bemeasured, with 28 of them found to be differentially rotating.Comparison to rotation laws in stars of spectral type A reveals thatdifferential rotation sets in at the convection boundary in theHR-diagram; no star that is significantly hotter than the convectionboundary exhibits the signatures of differential rotation. Four lateA-/early F-type stars close to the convection boundary and at v sin{i}≈ 100 km s-1 show extraordinarily strong absolute shear atshort rotation periods around one day. It is suggested that this is dueto their small convection zone depth and that it is connected to anarrow range in surface velocity; the four stars are very similar inTeff and v sin{i}. Detection frequencies of differentialrotation α = ΔΩ/Ω > 0 were analyzed in starswith varying temperature and rotation velocity. Measurable differentialrotation is more frequent in late-type stars and slow rotators. Thestrength of absolute shear, ΔΩ, and differential rotationα are examined as functions of the stellar effective temperatureand rotation period. The highest values of ΔΩ are found atrotation periods between two and three days. In slower rotators, thestrongest absolute shear at a given rotation rateΔΩmax is given approximately byΔΩmax ∝ P-1, i.e.,αmax ≈ const. In faster rotators, bothαmax and ΔΩmax diminish lessrapidly. A comparison with differential rotation measurements in starsof later spectral type shows that F-stars exhibit stronger shear thancooler stars do and the upper boundary in absolute shear ΔΩwith temperature is consistent with the temperature-scaling law found inDoppler Imaging measurements.
| Revisiting the population of Galactic open clusters
We present results of a study of the galactic open cluster populationbased on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2,Hipparcos and other catalogues. The sample of optical clusters fromASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry planeof the clusters' distribution is determined to be at Z_0=-22±4pc, and the scale height of open clusters is only 56±3 pc. Thetotal surface density and volume density in the symmetry plane areΣ= 114 kpc-2 and D(Z_0)=1015 kpc-3,respectively. We find the total number of open clusters in the Galacticdisk to be of order of 105 at present. Fluctuations in thespatial and velocity distributions are attributed to the existence offour open cluster complexes (OCCs) of different ages containing up to afew tens of clusters. Members in an OCC show the same kinematicbehaviour, and a narrow age spread. We find, that the youngest clustercomplex, OCC 1 (log t<7.9), with 19 deg inclination to the Galacticplane, is apparently a signature of Gould's Belt. The most abundant OCC2 complex has moderate age (log t≈8.45). The clusters of thePerseus-Auriga group, having the same age as OCC 2, but differentkinematics are seen in breaks between Perseus-Auriga clouds. The oldest(log t≈8.85) and sparsest group was identified due to a large motionin the Galactic anticentre direction. Formation rate and lifetime ofopen clusters are found to be 0.23±0.03 kpc-2Myr-1 and 322±31 Myr, respectively. This implies atotal number of cluster generations in the history of the Galaxy between30 to 40. We estimate that less than about 10% of the total Galacticstellar disk population has ever passed an open cluster membership.
| Kinematics of the Gould belt based on open clusters.
Not Available
| Spectroscopic Abundance Analysis of Dwarfs in the Young Open Cluster IC 4665
We report a detailed spectroscopic abundance analysis for a sample of 18F-K dwarfs of the young open cluster IC 4665. Stellar parameters andelement abundances of Li, O, Mg, Si, Ca, Ti, Cr, Fe, and Ni have beenderived using the spectroscopic synthesis tool SME (Spectroscopy MadeEasy). Within the measurement uncertainties the iron abundance isuniform, with a standard deviation of 0.04 dex. No correlation is foundbetween the iron abundance and the mass of the stellar convective zoneor between the Li abundance and the Fe abundance. In other words, ourresults do not reveal any signature of accretion and therefore do notsupport the scenario that stars with planets (SWPs) acquire theiron-average higher metallicity compared to field stars via accretion ofmetal-rich planetary material. Instead, the higher metallicity of SWPsmay simply reflect the fact that planetary formation is more efficientin high-metallicity environs. However, since so many details of theplanetary system formation processes remain poorly understood, furtherstudies are needed for a final settlement of the problem of the highmetallicity of SWPs. The standard deviation of [Fe/H] deduced from ourobservations, taken as an upper limit on the metallicity dispersionamong the IC 4665 member stars, has been used to constrainprotoplanetary disk evolution, terrestrial and giant planets formation,and evolution processes. The total reservoir of heavy elements retainedby the nascent disks is limited, and high retention efficiency ofplanet-building material is supported. Under modest surface density, gasgiant planets are expected to form in locally enhanced regions or startefficient gas accretion when they only have a small core of a few Earthmasses. Our results do not support the possibility that the migration ofgas giants and the circularization of terrestrial planets' orbits areregulated by their interaction with a residual population ofplanetesimals and dust particles.
| Rotation and variability of young very low-mass objects
Variability studies are an important tool to investigate key propertiesof stars and brown dwarfs. From photometric monitoring we are able toobtain information about rotation and magnetic activity, which areexpected to change in the mass range below 0.3 solar masses, since thesefully convective objects cannot host a solar-type dynamo. On the otherhand, spectroscopic variability information can be used to obtain adetailed view on the accretion process in very young objects. In thispaper, we report about our observational efforts to analyse thevariability and rotational evolution of young brown dwarfs and verylow-mass stars.
| ULMSF conference summary
Ten years on from the discovery of the first brown dwarf and the firstexoplanet, how well have we progressed in our understanding of theselow-mass objects? In particular how well do we understand theirformation? The strong impression from this conference was that theformation of brown dwarfs was just a continuation of the star formationprocess, no special additional mechanism is indicated.
| Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS
We have performed a new and homogeneous analysis of all the Li dataavailable in the literature for main sequence stars (spectral-types fromlate F to K) in open clusters. In the present paper we focus on adetailed investigation of MS Li depletion and its time scales for starsin the 6350-5500 K effective temperature range. For the first time, wewere able to constrain the age at which non-standard mixing processes,driving MS Li depletion, appear. We have also shown that MS Li depletionis not a continuous process and cannot be simply described by at-α law. We confirm that depletion becomes ineffectivebeyond an age of 1-2 Gyr for the majority of the stars, leading to a Liplateau at old ages. We compared the empirical scenario of Li as afunction of age with the predictions of three non-standard models. Wefound that models including only gravity waves as main mixing processare not able to fit the Li vs. age pattern and thus this kind of mixingcan be excluded as the predominant mechanism responsible for Lidepletion. On the other hand, models including slow mixing induced byrotation and angular momentum loss, and in particular those includingalso diffusive processes not related to rotation, can explain to someextent the empirical evidence. However, none of the currently proposedmodels can fit the plateau at old ages.
| An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards
In this paper we present a new method for deriving UV through IRextinction curves, based on the use of stellar atmosphere models toprovide estimates of the intrinsic (i.e., unreddened) stellar spectralenergy distributions (SEDs), rather than unreddened (or lightlyreddened) standard stars. We show that this ``extinction withoutstandards'' technique greatly increases the accuracy of the derivedextinction curves and allows realistic estimations of the uncertainties.An additional benefit of the technique is that it simultaneouslydetermines the fundamental properties of the reddened stars themselves,making the procedure valuable for both stellar and interstellar studies.Given the physical limitations of the models we currently employ, thetechnique is limited to main-sequence and mildly evolved B stars.However, in principle, it can be adapted to any class of star for whichaccurate model SEDs are available and for which the signatures ofinterstellar reddening can be distinguished from those of the stellarparameters. We demonstrate how the extinction without standards curvesmake it possible to (1) study the uniformity of curves in localizedspatial regions with unprecedented precision, (2) determine therelationships between different aspects of curve morphology, (3) producehigh-quality extinction curves from low color excess sight lines, and(4) derive reliable extinction curves for mid to late B stars, therebyincreasing spatial coverage and allowing the study of extinction in openclusters and associations dominated by such stars. The application ofthis technique to the available database of UV through IR SEDs, and tofuture observations, will provide valuable constraints on the nature ofinterstellar grains and on the processes that modify them, and it willenhance our ability to remove the multiwavelength effects of extinctionfrom astronomical energy distributions.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| A near-infrared survey for new low-mass members in α Per
We present a near-infrared (K'-band) survey of 0.7 square degree area inthe α Persei open cluster (age = 90 Myr, distance = 182 pc)carried out with the Omega-Prime camera on the Calar Alto 3.5-mtelescope. Combining optical data (Rc and I_c) obtained withthe KPNO/MOSA detector and presented in Stauffer et al. (1999) with theK' observations, a sample of new candidate members has been extractedfrom the optical-infrared colour-magnitude diagram. The location ofthese candidates in the colour-colour diagram suggests that two-thirdsof them are actually reddened background giants. About 20 new candidatemembers with masses between 0.3 and 0.04 Mȯ are added tothe ~400 known α Per cluster members. If they are indeed αPer members, four of the new candidates would be brown dwarfs. Wediscuss the advantages and drawbacks of the near-infrared survey ascompared to the optical selection method. We also describe the outcomeof optical spectroscopy obtained with the Twin spectrograph on the CalarAlto 3.5-m telescope for about 30 candidates, including selected membersfrom the optical sample presented in Barrado y Navascués et al.(2002) and from our joint optical/infrared catalogue. These resultsargue in favour of the optical selection method for this particularcluster.
| Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-star
We have derived the mean broadening profile of the star V 102 in theregion of the open cluster IC 4665 from high resolution spectroscopy. Ata projected equatorial rotation velocity of v sin i = (105 ± 12)km s-1 we find strong deviation from classical rotation. Wediscuss several scenarios, the most plausible being strong differentialrotation in latitudinal direction. For this scenario we find adifference in angular velocity of ΔΩ = 3.6 ± 0.8 radd-1 (ΔΩ/Ω = 0.42 ± 0.09). From theHα line we derive a spectral type of A9 and support photometricmeasurements classifying IC 4665 V 102 as a non-member of IC 4665. Atsuch early spectral type this is the strongest case of differentialrotation observed so far. Together with three similar stars, IC 4665 V102 seems to form a new class of objects that exhibit extremelatitudinal shear in a very shallow convective envelope.Based on observations carried out at the European Southern Observatory,Paranal, 71.D-0127(A).
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
