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The Keele-Exeter young cluster survey - I. Low-mass pre-main-sequence stars in NGC 2169
We have used RCIC CCD photometry from the IsaacNewton telescope and intermediate-resolution spectroscopy from theGemini North telescope to identify and characterize low-mass (0.15

Empirical isochrones and relative ages for young stars, and the radiative-convective gap
We have selected pre-main-sequence (PMS) stars in 12 groups of notionalages ranging from 1 to 35 Myr, using heterogeneous membership criteria.Using these members we have constructed empirical isochrones in V, V - Icolour-magnitude diagrams. This allows us to identify clearly the gapbetween the radiative main sequence and the convective PMS (the R-Cgap). We follow the evolution of this gap with age and show that it canbe a useful age indicator for groups less than ~=15 Myr old. We alsoobserve a reduction in absolute spreads about the sequences with age.Finally, the empirical isochrones allow us to place the groups in orderof age, independently of theory. The youngest groups can be collatedinto three sets of similar ages. The youngest set is the ONC, NGC6530and IC5146 (nominally 1 Myr); next Cep OB3b, NGC2362, λ Ori andNGC2264 (nominally 3 Myr); and finally σ Ori and IC348 (nominally4-5 Myr). This suggests Cep OB3b is younger than previously thought, andIC348 older. For IC348 the stellar rotation rate distribution andfraction of stars with discs imply a younger age than we derive. Wesuggest this is because of the absence of O-stars in this cluster, whosewinds and/or ionizing radiation may be an important factor in theremoval of discs in other clusters.

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.

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.

A maximum-likelihood method for fitting colour-magnitude diagrams
We present a maximum-likelihood method for fitting two-dimensional modeldistributions to stellar data in colour-magnitude space. This allows oneto include (for example) binary stars in an isochronal population. Themethod also allows one to derive formal uncertainties for fittedparameters, and assess the likelihood that a good fit has been found. Weuse the method to derive an age of 38.5+3.5-6.5Myr and a true distance modulus of 7.79+0.11-0.05mag from the V versus V - I diagram of NGC2547 (the uncertainties are 67percent confidence limits, and the parameters are insensitive to theassumed binary fraction). These values are consistent with thosepreviously determined from low-mass isochronal fitting, and are thefirst measurements to have statistically meaningful uncertainties. Theage is also consistent with the lithium depletion age of NGC2547, andthe Hipparcos distance to the cluster is consistent with our value.The method appears to be quite general and could be applied to anyN-dimensional data set, with uncertainties in each dimension. However,it is particularly useful when the data are sparse, in the sense thatboth the typical uncertainties for a data point and the size ofstructure in the function being fitted are small compared with thetypical distance between data points. In this case binning the data willlose resolution, whilst the method presented here preserves it.Software implementing the methods described in this paper is availablefrom http://www.astro.ex.ac.uk/people/timn/tau-squared/.

The young star cluster NGC 2362: low-mass population and initial mass function from a Chandra X-ray observation
Context: . We study the stellar population of the very young cluster NGC2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only5 Myr old, has already cleared most of its inter- and circumstellardust, and with its small and uniform reddening offers a uniqueopportunity of studying its pre-main-sequence stellar population withminimal disturbance from a dense interstellar medium. Aims: .Ourmain purposes are to select cluster members down to low masses and tostudy their properties as a population (spatial properties, initial massfunction, and coronal properties). Methods: .We compare existingdeep optical photometry and Hα data with new X-ray data. We usecombined optical and X-ray criteria to select cluster members.Results: .We detect 387 X-ray sources down to log LX = 29.0(erg/s), and identify most of them (308) with star-like objects. Themajority (88%) of optically identified X-ray sources are found to bevery good candidate low-mass pre-main-sequence stars, with minimalfield-object contamination. This increases the known cluster census by asubstantial amount at low masses, with respect to previous optical/IRstudies. The fraction of stars with active accretion is found to be inthe range 5-9%. We find a significantly wider spatial distribution forlow-mass stars than for massive stars (mass segregation). We find only asmall spread around the low-mass cluster sequence in the HR diagram,indicating that star formation lasted only about 1-2 Myr. We havederived the cluster initial mass function, which appears to flatten (onthe low-mass side) at higher masses with respect to other very youngclusters. The quiescent X-ray emission of low-mass cluster stars isfound to be rather strictly correlated with the stellar bolometricluminosity: the small spread in this correlation puts an upper bound onthe amplitude of X-ray variability on time scales longer than one day(e.g., activity cycles) in such young coronal sources. We findsignificant X-ray spectral differences between low-mass stars brighterand fainter than log LX ˜ 30.3 (erg/s), respectively,with X-ray brighter stars showing hotter components (kT ˜ 2 keV),absent in fainter stars.

Spitzer Observations of the Orion OB1 Association: Second-Generation Dust Disks at 5-10 Myr
We report new Spitzer observations of intermediate-mass stars in tworegions of the Orion OB1 association located in the subassociations OB1a(~10 Myr) and OB1b (~5 Myr). In a representative sample of stars earlierthan F5 of both stellar groups, we find a population of stars surroundedof debris disks, without excess in the IRAC bands and without emissionlines in their optical spectra, but with a varying degree of 24 μmexcess. Comparing our samples with 24 μm observations ofintermediate-mass stars in other stellar groups, spanning a range ofages from 2.5 to 150 Myr, we find that debris disks are more frequentand have larger 24 μm excess at 10 Myr (OB1a). This trend agrees withpredictions of models of evolution of solids in the outer regions ofdisks (>30 AU), where large icy objects (~1000 km) begin to form at~10 Myr; the presence of these objects in the disk initiates acollisional cascade, producing enough dust particles to explain therelatively large 24 μm excess observed in OB1a. The dust luminosityobserved in the stellar groups older than 10 Myr declines roughly aspredicted by collisional cascade models. Combining Spitzer observations,optical spectra, and 2MASS data, we found a new Herbig Ae/Be star (HD290543) and a star (HD 36444) with a large 24 μm excess, both inOB1b. This last object could be explained as a intermediate stagebetween HAeBe and true debris systems or as a massive debris diskproduced by a collision between two large objects (>1000 km).

On the age of the TW Hydrae association and 2M1207334-393254
Aims.We have estimated the age of the young moving group TW HydraeAssociation, a cohort of a few dozen stars and brown dwarfs located nearthe Sun which share the same kinematic properties and, presumably, thesame origin and age. Methods: .The chronology has been determinedby analyzing different properties (magnitudes, colors, activity,lithium) of its members and comparing them with several well-known starforming regions and open clusters, as well as theoretical models. Inaddition, by using medium-resolution optical spectra of two M8 membersof the association (2M1139 and 2M1207 - an accreting brown dwarf with aplanetary mass companion), we have derived spectral types and measuredHα and lithium equivalent widths. We have also estimated theireffective temperature and gravity, which were used to produce anindependent age estimation for these two brown dwarfs. We have alsocollected spectra of 2M1315, a candidate member with a L5 spectral typeand measured its Hα equivalent width. Results: .Our ageestimate for the association, 10+10-7 Myr, agreeswith previous values cited in the literature. In the case of the twobrown dwarfs, we have derived an age of 15+15-10Myr, which also agree with our estimate for the whole group.Conclusions: .We compared our results with recent articles published onthe same subject using other techniques, and discuss the limits of theage-dating techniques.

Spitzer 24 μm Survey of Debris Disks in the Pleiades
We performed a 24 μm 2deg×1deg survey ofthe Pleiades cluster, using the MIPS instrument on Spitzer. Fifty-fourmembers ranging in spectral type from B8 to K6 show 24 μm fluxesconsistent with bare photospheres. All Be stars show excesses attributedto free-free emission in their gaseous envelopes. Five early-type starsand four solar-type stars show excesses indicative of debris disks. Wefind a debris disk fraction of 25% for B-A members and 10% for F-K3ones. These fractions appear intermediate between those for youngerclusters and for the older field stars. They indicate a decay with ageof the frequency of the dust production events inside the planetaryzone, with similar timescales for solar-mass stars as have been foundpreviously for A stars.

On the difference between nuclear and contraction ages
Context: .Ages derived from low mass stars still contracting onto themain sequence often differ from ages derived from the high mass onesthat have already evolved away from it. Aims: .We investigate thegeneral claim of disagreement between these two independent agedeterminations by presenting UBVRI photometry for the young galacticopen clusters NGC 2232, NGC 2516, NGC 2547 and NGC 4755, spanning theage range ~10-150 Myr Methods: .We derived reddenings, distances,and nuclear ages by fitting ZAMS and isochrones to color-magnitudes andcolor-color diagrams. To derive contraction ages, we used four differentpre-main sequence models, with an empirically calibratedcolor-temperature relation to match the Pleiades cluster sequence.Results: .When exclusively using the V vs. V-I color-magnitude diagramand empirically calibrated isochrones, there is consistency betweennuclear and contraction ages for the studied clusters. Although thecontraction ages seem systematically underestimated, in none of thecases do they deviate by more than one standard deviation from thenuclear ages.

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.

Precision Kinematics and Related Parameters of the α Persei Open Cluster
A kinematical study of the nearby open cluster α Persei ispresented based on the astrometric proper motions and positions in theTycho-2 catalog and Second USNO CCD Astrographic Catalog (UCAC2). Usingthe astrometric data and photometry from the Tycho-2 and ground-basedcatalogs, 139 probable members of the cluster are selected, 18 of themnew. By the classical convergent point method, systematic motions ofstars inside the cluster and velocity dispersions are estimated. Asdirectly observed, the upper limit on the internal velocity dispersionper coordinate is 1.1 km s-1. The actual velocity dispersionis much smaller than that value, since all of it appears to come fromthe expected errors of the astrometric proper motions. The relativeposition of the convergent point with respect to the cluster starsyields the ``astrometric'' radial velocity, which turns out larger by afew km s-1 than the mean observed spectroscopic radialvelocity. This implies an overall contraction of the cluster. Kinematicparallaxes are computed for each member, and an improved H-R diagram isconstructed. An age of 52 Myr is determined by isochrone fitting. Thestar α Per itself fits an isochrone of this age computed withovershooting from the boundary of the convective zone. The theoreticalmass of the star α Per is 6.65 Msolar. With respect tothe common center of mass, half of the higher mass members (earlier thanG) are located within a radius of 10.3 pc. The cluster appears to beroughly twice as large, or as sparse, as the Pleiades, retainingnonetheless a similar dynamical coherence. The low rate of binaries isanother feature of this cluster, where we find only about 20% of membersto be known or suspected spectroscopic, astrometric, or visual binariesor multiple systems. X-ray emitters in the cluster appear to have thesame dispersion of internal velocities as the rest of the membership.The cluster is surrounded by an extended, sparse halo of comovingdwarfs, which are found by combining the proper-motion data from UCAC2with Two Micron All-Sky Survey infrared photometry. Since many of theseexternal stars are outside the tidal radius, the cluster being in anactive stage of disintegration or evaporation could be considered. Thishypothesis is not supported by the weak compression and thenonmeasurable velocity dispersion found in the kinematic analysis. Asearch for stars ejected from the α Persei cluster is carried outby tracking a large number of nearby stars 70 Myr back in time andmatching their positions with the past location of the cluster. Only oneplausible ejection is found prior to 10 Myr ago. The nearby star GJ 82,an active M dwarf with a strong Hα emission, is likely a formermember ejected 47 Myr ago at 5 km s-1.

Spiral structure of the third galactic quadrant and the solution to the Canis Major debate
With the discovery of the Sagittarius dwarf spheroidal, a galaxy caughtin the process of merging with the Milky Way, the hunt for other suchaccretion events has become a very active field of astrophysicalresearch. The identification of a stellar ring-like structure inMonoceros, spanning more than 100°, and the detection of anoverdensity of stars in the direction of the constellation of CanisMajor (CMa), apparently associated to the ring, has led to thewidespread belief that a second galaxy being cannibalized by the MilkyWay had been found. In this scenario, the overdensity would be theremaining core of the disrupted galaxy and the ring would be the tidaldebris left behind. However, unlike the Sagittarius dwarf, which is wellbelow the Galactic plane and whose orbit, and thus tidal tail, is nearlyperpendicular to the plane of the Milky Way, the putative CMa galaxy andring are nearly co-planar with the Galactic disc. This severelycomplicates the interpretation of observations. In this Letter, we showthat our new description of the Milky Way leads to a completelydifferent picture. We argue that the Norma-Cygnus spiral arm defines adistant stellar ring crossing Monoceros and the overdensity is simply aprojection effect of looking along the nearby local arm. Our perspectivesheds new light on a very poorly known region, the third Galacticquadrant, where CMa is located.

An XMM-Newton observation of the young open cluster NGC 2547: coronal activity at 30 Myr
We report on XMM-Newton observations of the young open cluster NGC 2547which allow us to characterize coronal activity in solar-type stars, andstars of lower mass, at an age of 30Myr. X-ray emission is seen fromstars at all spectral types, peaking among G stars at luminosities(0.3-3keV) of Lx~= 1030.5ergs-1 anddeclining to Lx<= 1029.0ergs-1 amongM stars with masses >=0.2Msolar. Coronal spectra showevidence for multi-temperature differential emission measures and lowcoronal metal abundances of Z~= 0.3. The G- and K-type stars of NGC 2547follow the same relationship between X-ray activity and Rossby numberestablished in older clusters and field stars, although most of thesolar-type stars in NGC 2547 exhibit saturated or even supersaturatedX-ray activity levels. The median levels of Lx andLx/Lbol in the solar-type stars of NGC 2547 arevery similar to those in T-Tauri stars of the Orion Nebula cluster(ONC), but an order of magnitude higher than in the older Pleiades. Thespread in X-ray activity levels among solar-type stars in NGC 2547 ismuch smaller than in older or younger clusters.Coronal temperatures increase with Lx,Lx/Lbol and surface X-ray flux. The most activesolar-type stars in NGC 2547 have coronal temperatures intermediatebetween those in the ONC and the most active older zero-agemain-sequence (ZAMS) stars. We show that simple scaling argumentspredict higher coronal temperature in coronally saturated stars withlower gravities. A number of candidate flares were identified among thelow-mass members and a flaring rate [for total flare energies (0.3-3keV)> 1034erg] of one every350+350-120ks was found for solar-type stars,which is similar to rates found in the ONC and Pleiades. Comparison withROSAT High Resolution Imager (HRI) data taken 7yr earlier reveals thatonly 10-15 per cent of solar-type stars or stars with Lx >3 × 1029ergs-1 exhibit X-ray variability bymore than a factor of 2. This is comparable with clusters of similar agebut less than in both older and younger clusters. The similar medianlevels of X-ray activity and rate of occurrence for large flares in NGC2547 and the ONC demonstrate that the X-ray radiation environment aroundyoung solar-type stars remains relatively constant over their first30Myr.

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.

Formation and Evolution of Planetary Systems (FEPS): Primordial Warm Dust Evolution from 3 to 30 Myr around Sun-like Stars
We present data obtained with the Infrared Array Camera (IRAC) aboardthe Spitzer Space Telescope (Spitzer) for a sample of 74 young (t<30Myr old) Sun-like (0.7

51 Eridani and GJ 3305: A 10-15 Myr old Binary Star System at 30 Parsecs
Following the suggestion of Zuckerman and coworkers, we consider theevidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historicallyclassified as unrelated main-sequence stars in the solar neighborhood,are instead a wide physical binary system and members of the youngβ Pic moving group (BPMG). The BPMG is the nearest (d<~50 pc) ofseveral groups of young stars with ages around 10 Myr that arekinematically convergent with the Oph-Sco-Cen association (OSCA), thenearest OB star association. Combining South African AstronomicalObservatory optical photometry, Hobby-Eberly Telescope high-resolutionspectroscopy, Chandra X-Ray Observatory data, and Second US NavalObservatory CCD Astrograph Catalog kinematics, we confirm with highconfidence that the system is indeed extremely young. GJ 3305 itselfexhibits very strong magnetic activity but has rapidly depleted most ofits lithium. The 51 Eri/GJ 3305 system is the westernmost known memberof the OSCA, lying 110 pc from the main subgroups. The system is similarto the BPMG wide binary HD 172555/CD -64 1208 and the HD 104237 quintet,suggesting that dynamically fragile multiple systems can survive theturbulent environments of their natal giant molecular cloud complexes,while still having high dispersion velocities imparted. Nearby youngsystems such as these are excellent targets for evolved circumstellardisk and planetary studies, having stellar ages comparable to that ofthe late phases of planet formation.

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

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

Proper Motions of the HH 47 Jet Observed with the Hubble Space Telescope
We present a proper-motion study of the shock waves within the classicstellar jet HH 47 based on Hubble Space Telescope (HST) Hα and [SII] images of the region taken over two epochs. Individual knots withinthe jet and in the bow shock/Mach disk working surface of HH 47A movesignificantly in the 5 yr that separate the images, and the excellentspatial resolution of HST makes it possible to measure the propermotions with enough precision to easily observe differential motionsthroughout the flow. The bright portion of the jet emerges at37.5d+/-2.5d from the plane of the sky with an average velocity of 300km s-1. Dynamical ages of the shock waves in the jet rangefrom a few decades for knots recently ejected by the source to ~1300 yrfor the faint extended bow shock HH 47D. The jet curves, but motions ofknots in the jet are directed radially away from the exciting source,and velocity variability in the flow drives the shock waves that heatthe jet internally. The jet orientation angle varies with time by about15° and currently points to the northwestern portion of a cavityoutlined by a reflection nebula, where a quasi-stationary shock deflectsthe jet. The major working surface HH 47A is more complex than a simplebow shock/Mach disk and contains numerous clumps that move relative toone another with velocities of ~+/-40 km s-1. Small clumps orinstabilities affect the Mach disk, and dense clumps may move all theway through the working surface to cause the bumpy morphology seen atthe bow shock. A localized area between the bow shock and Mach diskvaries significantly between the two sets of images.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.

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.

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.

The lithium depletion boundary in NGC 2547 as a test of pre-main-sequence evolutionary models
Intermediate resolution spectroscopy from the European SouthernObservatory Very Large Telescope is analysed for 63 photometricallyselected low-mass (0.08-0.30 Msolar) candidates of the opencluster NGC 2547. We have confirmed membership for most of these starsusing radial velocities, and found that lithium remains undepleted forcluster stars with I > 17.54 +/- 0.14 and Ks > 14.86+/- 0.12. From these results, several pre-main-sequence evolutionarymodels give almost model independent ages of 34-36 Myr, with a precisionof 10 per cent. These ages are only slightly larger than the ages of25-35(+/-5) Myr obtained using the same models to fit isochrones tohigher mass stars descending towards the zero-age main-sequence, both inempirically calibrated and theoretical colour-magnitude diagrams. Thisagreement between age determinations in different mass ranges is anexcellent test of the current generation of low-mass pre-main-sequencestellar models and lends confidence to ages determined with eithermethod between 30 and 120 Myr.

A Long-lived Accretion Disk around a Lithium-depleted Binary T Tauri Star
We present a high-dispersion optical spectrum of St 34 and identify thesystem as a spectroscopic binary with components of similar luminosityand temperature (both M3 +/- 0.5). Based on kinematics, signatures ofaccretion, and location on an H-R diagram, we conclude that St 34 is aclassical T Tauri star belonging to the Taurus-Auriga T association.Surprisingly, however, neither component of the binary shows Li Iλ6708 absorption, the most universally accepted criterion forestablishing stellar youth. In this uniquely known instance, theaccretion disk appears to have survived longer than thelithium-depletion timescale. We speculate that the long-lived accretiondisk is a consequence of the sub-AU separation companion tidallyinhibiting, although not preventing, circumstellar accretion.Comparisons with pre-main-sequence evolutionary models imply, for eachcomponent of St 34, a mass of 0.37+/-0.08 Msolar and anisochronal age of 8+/-3 Myr, which is much younger than the predictedlithium-depletion timescale of ~25 Myr. Although a distance 38% closerthan that of Taurus-Auriga or a hotter temperature scale could reconcilethis discrepancy at 21-25 Myr, similar discrepancies in other systemsand the implications of an extremely old accreting Taurus-Auriga membersuggest instead a possible problem with evolutionary models. Regardless,the older age implied by St 34's depleted lithium is the firstcompelling evidence of a substantial age spread in this region.Additionally, since St 34's coeval comembers with early M spectral typeswould likewise fail the lithium test for youth, current membership listsmay be incomplete.

Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.

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Observation and Astrometry data

Right ascension:08h10m25.70s
Apparent magnitude:4.7

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NGC 2000.0NGC 2547

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