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Differential Radial Velocities and Stellar Parameters of Nearby Young Stars
Radial velocity searches for substellar-mass companions have focusedprimarily on stars older than 1 Gyr. Increased levels of stellaractivity in young stars hinders the detection of solar system analogs,and therefore until recently there has been a prejudice againstinclusion of young stars in radial velocity surveys. Adaptive opticssurveys of young stars have given us insight into the multiplicity ofyoung stars, but only for massive, distant companions. Understanding thelimit of the radial velocity technique, restricted to high-mass,close-orbiting planets and brown dwarfs, we began a survey of youngstars of various ages. While the number of stars needed to carry outfull analysis of the problems of planetary and brown dwarf populationand evolution is large, the beginning of such a sample is included here.We report on 61 young stars ranging in age from the β Pictorisassociation (~12 Myr) to the Ursa Major association (~300 Myr). Thisinitial search resulted in no stars showing evidence of companionslarger than ~1MJup-2MJup in short-period orbits atthe 3 σ level. We also present derived stellar parameters, as mosthave unpublished values. The chemical homogeneity of a cluster, andpresumably of an association, may help to constrain true membership, sowe present [Fe/H] abundances for the stars in our sample.

LP 261-75/2MASSW J09510549+3558021: A Young, Wide M4.5/L6 Binary
We present new observations of the LP 261-75/2MASSW J09510549+3558021M4.5/L6 wide, common-proper-motion binary system. Optical spectroscopyof LP 261-75 shows strong Hα emission, and the star may also beassociated with the ROSAT FSC source 1RXS J095102.7+355824. The derivedchromospheric and coronal activity levels are consistent with those ofPleiades stars of the same spectral type, and we infer an age of 100-200Myr for the system. In that case, theoretical models suggest that the L6dwarf 2MASSW J09510549+3558021 is a low-mass brown dwarf, withM~0.02+0.01-0.005 Msolar.Based partly on observations obtained with the Apache Point Observatory3.5 m telescope, which is owned and operated by the AstrophysicalResearch Consortium.

The lights of other days
The concept of first lights can be thought of as the most recentiteration in an extended interplay between the idea that the worldaround us can be explained in terms of processes that are still going onand amenable to examination (“uniformitarianism”) and theidea that many important things happened only long ago when the universewas very different (“catastrophism”). As is usually the casein such matters, there is a fair amount of truth on each side, but, justat the moment, the catastrophists are slightly in the lead.

New Praesepe white dwarfs and the initial mass-final mass relation
We report the spectroscopic confirmation of four further white dwarfmembers of Praesepe. This brings the total number of confirmed whitedwarf members to 11, making this the second largest collection of theseobjects in an open cluster identified to date. This number is consistentwith the high-mass end of the initial mass function of Praesepe beingSalpeter in form. Furthermore, it suggests that the bulk of Praesepewhite dwarfs did not gain a substantial recoil kick velocity frompossible asymmetries in their loss of mass during the asymptotic giantbranch phase of evolution. By comparing our estimates of the effectivetemperatures and the surface gravities of WD0833+194, WD0840+190,WD0840+205 and WD0843+184 to modern theoretical evolutionary tracks, wehave derived their masses to be in the range 0.72-0.76 Msolarand their cooling ages ~300 Myr. For an assumed cluster age of 625 +/-50 Myr, the inferred progenitor masses are between 3.3 and 3.5Msolar. Examining these new data in the context of theinitial mass-final mass relation, we find that it can be adequatelyrepresented by a linear function (a0 = 0.289 +/-0.051,a1 = 0.133 +/- 0.015) over the initial mass range 2.7-6Msolar. Assuming an extrapolation of this relation to largerinitial masses is valid and adopting a maximum white dwarf mass of 1.3Msolar, our results support a minimum mass for core-collapsesupernovae progenitors in the range ~6.8-8.6 Msolar.

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.

Automated analysis of eclipsing binary light curves - II. Statistical analysis of OGLE LMC eclipsing binaries
In the first paper of this series, we presented EBAS - Eclipsing BinaryAutomated Solver, a new fully automated algorithm to analyse the lightcurves of eclipsing binaries, based on the EBOP code. Here, we apply thenew algorithm to the whole sample of 2580 binaries found in the OpticalGravitational Lensing Experiment (OGLE) Large Magellanic Cloud (LMC)photometric survey and derive the orbital elements for 1931 systems. Toobtain the statistical properties of the short-period binaries of theLMC, we construct a well-defined subsample of 938 eclipsing binarieswith main-sequence B-type primaries. Correcting for observationalselection effects, we derive the distributions of the fractional radiiof the two components and their sum, the brightness ratios and theperiods of the short-period binaries. Somewhat surprisingly, the resultsare consistent with a flat distribution in log P between 2 and 10 d. Wealso estimate the total number of binaries in the LMC with the samecharacteristics, and not only the eclipsing binaries, to be about 5000.This figure leads us to suggest that (0.7 +/- 0.4) per cent of themain-sequence B-type stars in the LMC are found in binaries with periodsshorter than 10 d. This frequency is substantially smaller than thefraction of binaries found by small Galactic radial-velocity surveys ofB stars. On the other hand, the binary frequency found by Hubble SpaceTelescope (HST) photometric searches within the late main-sequence starsof 47 Tuc is only slightly higher and still consistent with thefrequency we deduced for the B stars in the LMC.

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.

Spectral synthesis analysis and radial velocity study of the northern F-, G- and K-type flare stars
In this paper, we present a study of the general physical and chemicalproperties and radial velocity monitoring of young active stars. Wederive temperatures, logg, [Fe/H], v sini and Rspec valuesfor eight stars. The detailed analysis reveals that the stars are nothomogeneous in their principal physical parameters or in the agedistribution. In 4/5, we found a periodic radial velocity signal whichoriginates in surface features; the fifth is surprisingly inactive andshows little variation.

Modelling space-based high-precision photometry for asteroseismic applications
We present a new formalism to model and simulate space-basedhigh-precision photometric time-series of stochastically oscillatingstars. The first component of the formalism models time-series of CCDimages with all important instrumental effects incorporated, while thesecond component models stellar light curves as if there was noinstrumental deterioration. The formalism can and is being used toanalyse ground-based asteroseismic time-series, as well as to predictthe performance of a space-based instrument during its design phase.Next to the description of the formalism, we demonstrate its usefulnessby including two short but real-life applications.

New stellar members of the Coma Berenices open star cluster
We present the results of a survey of the Coma Berenices open starcluster (Melotte 111), undertaken using proper motions from theUSNO-B1.0 (United States Naval Observatory) and photometry from theTwo-Micron All-Sky Survey (2MASS) Point Source catalogues. We haveidentified 60 new candidate members with masses in the range 1.007

Asteroseismic constraints on the Pleiades distance
The global parameters of the Pleiades cluster are derived from thecomparison of observed and computed frequencies of six multi-periodicdelta Scuti stars. The best frequency fits lead to a distance modulusof 5.70 in good agreement with the pre-Hipparcos MS fitting methods.

A re-examination of the dispersion of lithium abundance of Pleiades member stars
The activity and dispersion of lithium abundance at a given color in thePleiades member stars are re-examined in the present article. It isfound that most of researchers underestimated the activity of thePleiades stars. The activity of stars (including spot activities) andthe influence of the inhomogeneous reddening effects over the cluster onthe Li abundance dispersion are also restudied. The main conclusionobtained is that there is no solid evidence to suggest that the observedLi abundance dispersion represents the existence of the true differenceof the Li abundance in the atmosphere of the cluster members with thesame effective temperature. By contraries, if it is not the whole, mostof the apparent Li abundance dispersion is caused by the atmosphericeffect of active stars. The correlation between the Li abundance andstellar rotation very likely is only a reflection of the correlationbetween the Li abundance and stellar activity.

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.

The Nature of Interstellar Gas toward the Pleiades Revealed in Absorption Lines
We present high-resolution, high signal-to-noise ratio absorption-lineobservations of CN, Ca II, Ca I, CH+, and CH along 20 linesof sight toward members of the Pleiades. The acquired data enable themost detailed study to date of the interaction between cluster stars andthe surrounding interstellar gas. Total equivalent widths are consistentwith previous investigations, except where weaker features are detectedowing to our greater sensitivity. Mean b-values for the molecularspecies indicate that toward most of the Pleiades, CH is associated withthe production of CH+ rather than CN. An analysis of radialvelocities reveals a kinematic distinction between ionized atomic gasand molecular and neutral gas. Molecular components are found withvelocities in the local standard of rest of either ~+7 or ~+9.5 kms-1, with the higher velocity components being associatedwith the strongest absorption. Atomic gas traced by Ca II shows a strongcentral component at vLSR~+7 km s-1, exhibitingvelocity gradients indicative of cloud-cluster interactions. Gas densityestimates derived from measured CH/CH+ column density ratiosshow good agreement with those inferred from H2 rotationalpopulations, yielding typical values of n~50 cm-3. Our modelsdo not include the important time-dependent effects on CH+formation, which may ultimately be needed to extract physical conditionsin these clouds.

Mergers of Close Primordial Binaries
We study the production of main-sequence mergers of tidally synchronizedprimordial short-period binaries. The principal ingredients of ourcalculation are the angular momentum loss rates inferred from thespin-down of open cluster stars and the distribution of binaryproperties in young open clusters. We compare our results with theexpected number of systems that experience mass transfer in thepost-main-sequence phases of evolution and compute the uncertainties inthe theoretical predictions. We estimate that main-sequence mergers canaccount for the observed number of single blue stragglers in M67.Applied to the blue straggler population, this implies that such mergersare responsible for about one-quarter of the population of halo bluemetal-poor stars and at least one-third of the blue stragglers in openclusters for systems older than 1 Gyr. The observed trends as a functionof age are consistent with a saturated angular momentum loss rate forrapidly rotating tidally synchronized systems. The predicted number ofblue stragglers from main-sequence mergers alone is comparable to thenumber observed in globular clusters, indicating that the net effect ofdynamical interactions in dense stellar environments is to reduce ratherthan increase the blue straggler population. A population of subturnoffmergers of order 3%-4% of the upper main sequence population is alsopredicted for stars older than 4 Gyr, which is roughly comparable to thesmall population of highly Li-depleted halo dwarfs. Other observationaltests are discussed.

The SPITZER c2d Survey of Weak-Line T Tauri Stars. I. Initial Results
Using the Spitzer Space Telescope, we have observed 90 weak-line andclassical T Tauri stars in the vicinity of the Ophiuchus, Lupus,Chamaeleon, and Taurus star-forming regions as part of the Cores toDisks (c2d) Spitzer Legacy project. In addition to the Spitzer data, wehave obtained contemporaneous optical photometry to assist inconstructing spectral energy distributions. These objects werespecifically chosen as solar-type young stars with low levels ofHα emission, strong X-ray emission, and lithium absorption, i.e.,weak-line T Tauri stars, most of which were undetected in the mid- tofar-IR by the IRAS survey. Weak-line T Tauri stars are potentiallyextremely important objects in determining the timescale over which diskevolution may take place. Our objective is to determine whether theseyoung stars are diskless or have remnant disks that are below thedetection threshold of previous infrared missions. We find that only5/83 weak-line T Tauri stars have detectable excess emission between 3.6and 70 μm, which would indicate the presence of dust from the innerfew tenths of an AU out to the planet-forming regions a few tens of AUfrom the star. Of these sources, two have small excesses at 24 μmconsistent with optically thin disks; the others have optically thickdisks already detected by previous IR surveys. All of the sevenclassical T Tauri stars show excess emission at 24 and 70 μm althoughtheir properties vary at shorter wavelengths. Our initial results showthat disks are rare among young stars selected for their weak Hαemission.

AmFm Stars as a Test of Rotational Mixing Models
Stellar evolution models have been calculated for stars of 1.7-2.5Msolar with both the Geneva-Toulouse and Montreal codes. Inthe Geneva-Toulouse code, the internal evolution of angular momentum iscalculated self-consistently along with the transport of a few species.In the Montreal code, the transport of 24 species is treated in detail,taking into account radiative accelerations, thermal diffusion, andgravitational settling, along with the turbulent diffusion coefficientscalculated in the Geneva-Toulouse code. It is verified that the twocodes lead to very similar internal structure for a given mass. Thecalculated surface abundances are compared to abundance anomaliesobserved on AmFm stars. It is found that with approximately the sameparameters as used for other types of stars, the Geneva-Toulouse codeleads to turbulent transport coefficients that produce abundanceanomalies consistent with the observed ones for HD 73045, HD 23610, andSirius. Taking into account the effect of the anisotropy of turbulenceon vertical transport plays an important role, although the level ofanisotropy in stellar envelopes is very uncertain; this effect isusually neglected in calculations. The stabilizing effect of the meanmolecular weight gradient can also be important. The current level ofaccuracy of observed abundances only permits to choose within aone-parameter family of models. To distinguish between turbulent modelswith different interior profiles, an accuracy of 0.03 dex is required ofabundance determinations, a level of accuracy that is not currentlyachieved. It is also shown that taking into account thepre-main-sequence evolution of the rotation profile leads to animportant reduction in the Ω dependence of turbulent transport forslow rotators.

The Gl569 Multiple System
We report the results of high spectral and angular resolution infraredobservations of the multiple system Gl569A and B that were intended tomeasure the dynamical masses of the brown dwarf binary believed tocomprise Gl569B. Our analysis did not yield this result but, instead,revealed two surprises. First, at age ~100 Myr, the system is youngerthan had been reported earlier. Second, our spectroscopic andphotometric results provide support for earlier indications that Gl569Bis actually a hierarchical brown dwarf triple rather than a binary. Ourresults suggest that the three components of Gl569B have roughly equalmass, ~0.04 Msolar.

The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.

Equilibrium Star Cluster Formation
We argue that rich star clusters take at least several local dynamicaltimes to form and so are quasi-equilibrium structures during theirassembly. Observations supporting this conclusion include morphologiesof star-forming clumps, momentum flux of protostellar outflows fromforming clusters, age spreads of stars in the Orion Nebula cluster (ONC)and other clusters, and the age of a dynamical ejection event from theONC. We show that these long formation timescales are consistent withthe expected star formation rate in turbulent gas, as recently evaluatedby Krumholz & McKee. Finally, we discuss the implications of thesetimescales for star formation efficiencies, the disruption of gas bystellar feedback, mass segregation of stars, and the longevity ofturbulence in molecular clumps.

The Low-Mass Initial Mass Function of the Field Population in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations
We present V- and I-equivalent HST WFPC2 stellar photometry of an areain the Large Magellanic Cloud (LMC), located to the west of the bar ofthe galaxy, which accounts for the general background field of its innerdisk. The WFPC2 observations reach magnitudes as faint as V=25 mag, andthe large sample of more than 80,000 stars allows us to determine indetail the present-day mass function (PDMF) of the detectedmain-sequence stars, which is identical to the initial mass function(IMF) for masses M<~1 Msolar. The low-mass main-sequencemass function of the LMC field is found not to have a uniform slopethroughout the observed mass range; i.e., the slope does not follow asingle power law. This slope changes at about 1 Msolar tobecome more shallow for stars with smaller masses down to the lowestobserved mass of ~0.7 Msolar, giving clear indications offlattening for even smaller masses. We verified statistically that forstars with M<~1 Msolar the IMF has a slope Γ around-2, with an indicative slope Γ~=-1.4 for0.7<~M/Msolar<~0.9, while for more massive stars themain-sequence mass function becomes much steeper with Γ~=-5. Themain-sequence luminosity function (LF) of the observed field is in verygood agreement with the Galactic LF as it was previously found. Takinginto account several assumptions concerning evolutionary effects, whichshould have changed through time the stellar content of the observedfield, we reconstruct qualitatively its IMF for the whole observed massrange (0.7<~M/Msolar<~2.3), and we find that the numberof observed evolved stars is not large enough to have affectedsignificantly the form of the IMF, which thus is found almost identicalto the observed PDMF.

Binaries Like to Be Twins: Implications for Doubly Degenerate Binaries, the Type Ia Supernova Rate, and Other Interacting Binaries
A recently published sample of 21 detached eclipsing binaries in theSmall Magellanic Cloud provides a valuable test of the binary massfunction for massive stars. We show that 50% of detached binaries havecompanions with very similar masses,q=M2/M1>0.87, where M1 andM2 denote the masses of the two binary components,M1>=M2. A Salpeter relative mass function forthe secondary is very strongly excluded, and the data are consistentwith a flat mass function containing 55% of the systems and a ``twin''population with q>0.95 containing the remainder. We survey theexisting literature on binary mass ratios and conclude that asignificant twin population (of order 20%-25%) exists in binaries thatare likely to interact across a broad range of stellar masses andmetallicity. Interactions involving twins have distinctly differentproperties from those involving stars of unequal mass; the secondarieswill tend to be evolved, and common-envelope evolution is qualitativelydifferent. The implications of such a population for both binaryinteractions and star formation are substantial, and we present someexamples. We argue that twin systems may provide a natural stellarpopulation to explain the recently proposed prompt channel for Type Iasupernovae, and the presence of a twin population dramatically reducesthe maximum inferred merger rate between neutron stars (NSs) and blackholes relative to the NS-NS merger rate. Twins may also be important forunderstanding the tendency of white dwarf and NS binaries to be nearlyequal in mass, and inclusion of twins in population studies will boostthe blue straggler production rate.

Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574
While following up L dwarf candidates selected photometrically from theTwo Micron All Sky Survey, we uncovered an unusual object designated2MASS J01415823-4633574. Its optical spectrum exhibits very strong bandsof vanadium oxide but abnormally weak absorptions by titanium oxide,potassium, and sodium. Morphologically, such spectroscopiccharacteristics fall intermediate between old field early-L dwarfs[log(g)~5] and very late M giants [log(g)~0], leading us to favor lowgravity as the explanation for the unique spectral signatures of this Ldwarf. Such a low gravity can be explained only if this L dwarf is muchlower in mass than a typical old field L dwarf of similar temperatureand is still contracting to its final radius. These conditions imply avery young age. Further evidence of youth is found in the near-infraredspectrum, including a triangular-shaped H-band continuum, reminiscent ofyoung brown dwarf candidates discovered in the Orion Nebula Cluster.Using the above information along with comparisons to brown dwarfatmospheric and interior models, our current best estimate is that thisL dwarf has an age of 1-50 Myr and a mass of 6-25MJ. Althoughthe lack of a lithium detection (pseudo-equivalent width <1 Å)might appear to contradict other evidence of youth, we suggest thatlithium becomes weaker at lower gravity like all other alkali lines andthus needs to be carefully considered before being used as a diagnosticof age or mass for objects in this regime. The location of 2MASS0141-4633 on the sky coupled with a distance estimate of ~35 pc and theabove age estimate suggests that this object may be a brown dwarf memberof either the 30 Myr old Tucana/Horologium association or the ~12 Myrold β Pic moving group. Distance as determined throughtrigonometric parallax (underway) and a measure of the total spacemotion are needed to test this hypothesis.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.

The Formation and Evolution of Planetary Systems (FEPS): Discovery of an Unusual Debris System Associated with HD 12039
We report the discovery of a debris system associated with the ~30 Myrold G3/5V star HD 12039 using Spitzer Space Telescope observations from3.6-160 μm. An observed infrared excess(LIR/L*=1×10-4) above theexpected photosphere for λ>~14 μm is fit by thermallyemitting material with a color temperature of T~110 K, warmer than themajority of debris disks identified to date around Sun-like stars. Theobject is not detected at 70 μm with a 3 σ upper limit 6 timesthe expected photospheric flux. The spectrum of the infrared excess canbe explained by warm, optically thin material comprised ofblackbody-like grains of size >~7 μm that reside in a beltorbiting the star at 4-6 AU. An alternate model dominated by smallergrains, near the blowout size a~0.5 μm, located at 30-40 AU is alsopossible but requires the dust to have been produced recently, sincesuch small grains will be expelled from the system by radiation pressurein approximately a few times 102 yr.

Testing Theoretical Evolutionary Models with AB Doradus C and the Initial Mass Function
We assess the constraints on the evolutionary models of young low-massobjects that are provided by the measurements of the companion AB Dor Cby Close and coworkers and by a new comparison of model-derived IMFs ofstar-forming regions to the well-calibrated IMF of the solarneighborhood. After performing an independent analysis of all of theimaging and spectroscopic data for AB Dor C that were obtained by Close,we find that AB Dor C (which has no methane) is not detected at asignificant level (S/N~1.2) in the SDI data when one narrowband image issubtracted from another but that it does appear in the individual SDIframes, as well as the images at J, H, and Ks. Although ourbroadband photometry for AB Dor C is consistent with that of Close, theuncertainties that we measure are larger. Using the age of τ=75-150Myr recently estimated for AB Dor by Luhman and coworkers, theluminosity predicted by the models of Chabrier and Baraffe is consistentwith the value that we estimate from the photometry for AB Dor C. Wemeasure a spectral type of M6+/-1 from the K-band spectrum of AB Dor C,which is earlier than the value of M8+/-1 reported by Close and isconsistent with the model predictions when a dwarf temperature scale isadopted. In a test of these evolutionary models at much younger ages, weshow that the low-mass IMFs that they produce for star-forming regionsare similar to the IMF of the solar neighborhood. If the masses of thelow-mass stars and brown dwarfs in these IMFs of star-forming regionswere underestimated by a factor of 2 as suggested by Close, then the IMFcharacterizing the current generation of Galactic star formation wouldhave to be radically different from the IMF of the solar neighborhood.Based on observations made with ESO telescopes at the ParanalObservatories under program ID 60.A-9026. This publication makes use ofdata products from the Two Micron All Sky Survey, which is a jointproject of the University of Massachusetts and the Infrared Processingand Analysis Center/California Institute of Technology, funded by theNational Aeronautics and Space Administration and the National ScienceFoundation.

A Hubble Space Telescope Advanced Camera for Surveys Search for Brown Dwarf Binaries in the Pleiades Open Cluster
We present the results of a high-resolution imaging survey for browndwarf binaries in the Pleiades open cluster. The observations werecarried out with the Advanced Camera for Surveys (Pavlovsky andcoworkers) on board the Hubble Space Telescope. Our sample consists of15 bona fide brown dwarfs. We confirm two binaries and detect theirorbital motion, but we did not resolve any new binary candidates in theseparation range between 5.4 and 1700 AU and masses in the range0.035-0.065 Msolar. Together with the results of our previousstudy (Martín and coworkers), we can derive a visual binaryfrequency of 13.3+13.7-4.3% for separationsgreater than 7 AU, masses in the range 0.055-0.065 Msolar,and mass ratios in the range 0.45-0.9

Oxygen from the λ7774 High-Excitation Triplet in Open Cluster Dwarfs: Hyades
Oxygen abundances have been derived from the near-IR, high-excitation OI λ7774 triplet in high-resolution, high signal-to-noise ratiospectra of 45 Hyades dwarfs using standard one-dimensional,plane-parallel LTE models. Effective temperatures of the stellar samplerange from 4319 to 6301 K, and the derived relative O abundances as afunction of Teff evince a trichotomous morphology. AtTeff>6100 K, there is evidence of an increase in the Oabundances with increasing Teff, consistent with non-LTE(NLTE) predictions. At intermediate Teff (5450K<=Teff<=6100 K), the O abundances are flat, andstar-to-star values are in good agreement, having a mean value of[O/H]=+0.25+/-0.02 however, systematic errors at the <~0.10 dex levelmight exist. The O abundances for stars with Teff<=5450 Kshow a striking increase with decreasing Teff, in starkcontrast to expectations and canonical NLTE calculations. The coolHyades triplet results are compared to those recently reported fordwarfs in the Pleiades cluster and the UMa moving group; qualitativedifferences between the trends observed in these stellar aggregatespoint to a possible age-related diminution of triplet abundance trendsin cool open cluster dwarfs. Correlations with age-related phenomena,i.e., chromospheric activity and photospheric spots, faculae, and/orplages, are investigated. No correlation with Ca II H+K chromosphericactivity indicators is observed. Multicomponent LTE ``toy'' models havebeen constructed in order to simulate photospheric temperatureinhomogeneities that could arise from the presence of starspots, and wedemonstrate that photospheric spots are a plausible source of thetriplet trends among the cool dwarfs.Based on observations obtained with the Mayall 4 m telescope at KittPeak National Observatory, a division of the National Optical AstronomyObservatory, which is operated by the Association of Universities forResearch in Astronomy, Inc., under cooperative agreement with theNational Science Foundation.This paper includes data taken with the Harlan J. Smith 2.7 m telescopeat the McDonald Observatory of the University of Texas at Austin.

OB Stars in the Solar Neighborhood. II. Kinematics
Using the spatial classification method and the structural parametersestimated for the Gould Belt (GB) and the local Galactic disk (LGD) froma previous paper, we have evaluated spatial membership probabilities fora sample of O and B stars from the Hipparcos catalog (Perryman andcoworkers) with available proper motions and radial velocity data. Thus,being able to study the space velocity fields of both systems, weconclude that the GB and the LGD present different statisticaldistributions, both in velocity space and in phase space. In light oftheir possible existence as distinct systems, we analyze differentkinematic aspects in the vicinity of the Sun, and we find the GB to beresponsible for the highly negative vertex deviation found for the OBstars in the solar neighborhood. We also find that the GB noticeablyalters the estimation of the Oort constants in the Galactic disk; thus,in order to establish comparisons with other kinematic studies based onolder stellar populations, a careful pruning of the GB members must beperformed. Further analysis of the GB velocity field and the movinggroups that contribute to it suggest the possibility that the GB can bethe result of a chance superposition of moving groups. We propose forfuture investigations the dynamical analysis of these moving groups inorder to probe the origin of the GB.

A Large-Area Search for Low-Mass Objects in Upper Scorpius. I. The Photometric Campaign and New Brown Dwarfs
We present a wide-field photometric survey covering ~200 deg2toward the Upper Scorpius OB association. Data taken in the R and Ibands with the Quest-2 camera on the Palomar 48 inch (1.2 m) telescopewere combined with the Two Micron All Sky Survey JHKS surveyand used to select candidate pre-main-sequence stars. Follow-upspectroscopy with the Palomar 200 inch (5.1 m) telescope of 62 candidatelate-type members identified 43 stars that have surface gravitysignatures consistent with association membership. From theoptical/near-infrared photometry and derived spectral types we constructan H-R diagram for the new members and find 30 likely new brown dwarfs,nearly doubling the known substellar population of the Upper Scorpius OBassociation. Continuation of our spectroscopic campaign should revealhundreds of new stellar and substellar members.

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.

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Observation and Astrometry data

Right ascension:03h47m00.00s
Apparent magnitude:1.2

Catalogs and designations:
Proper NamesPleiades
Seven Sisters   (Edit)
MessierM 45

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