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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.
| The young open cluster NGC 2129
The first charge-coupled device UBV(RI)C photometric study inthe area of the doubtful open cluster NGC 2129 is presented. Photometryof a field offset 15 arcmin northwards is also provided, to probe theGalactic disc population towards the cluster. Using star counts, propermotions from the UCAC2 catalogue, colour-magnitude and colour-colourdiagrams, we demonstrate that NGC 2129 is a young open cluster. Thecluster radius is 2.5 arcmin, and across this region we find evidence ofsignificant differential reddening, although the reddening law seems tobe normal towards its direction. Updated estimates of the clusterfundamental parameters are provided. The mean reddening is found to beE(B-V) = 0.80 +/- 0.08 and the distance modulus is (m-M)0=11.70 +/- 0.30. Hence, NGC 2129 is located at 2.2 +/- 0.2 kpc from theSun inside the Local spiral arm. The age derived from 37 photometricallyselected members is estimated to be approximately 10 Myr. These starsare used to provide new estimates of the cluster absolute proper-motioncomponents.
| 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.
| A Hot DQ White Dwarf in the Open Star Cluster M35
We report the discovery of a hot DQ white dwarf, NGC 2168:LAWDS 28, thatis a likely member of the 150 Myr old cluster NGC 2168 (M35). Thespectrum of the white dwarf (WD) is dominated by C II features. Theeffective temperature is difficult to estimate but likely >~20,000 Kbased on the temperatures of hot DQ WDs with similar spectra. NGC2168:LAWDS 28 provides further evidence that hot DQ WDs may be the``missing'' high-mass helium-atmosphere white dwarfs. Based on publishedstudies, we find that the DBA WD LP 475-242 is likely a member of theHyades open cluster, as often assumed. These two white dwarfs are theentire sample of known He-atmosphere WDs in open clusters with turnoffmasses >=2 Msolar. Based on the number of known cluster DAwhite dwarfs and a redetermination of the H-atmosphere to He-atmosphereratio, commonly known as the DA:DB ratio, we reexamine the hypothesisthat the H- to He-atmosphere ratio in open clusters is the same as theratio in the field. Under this hypothesis, we calculate that fiveHe-atmosphere WDs are expected to have been discovered, with aprobability of finding fewer than three He-atmosphere white dwarfs of0.08, or at the ~2 σ level.
| 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.
| The Bologna Open Cluster Chemical Evolution Project: Midterm Results from the Photometric Sample
We describe a long-term project aimed at deriving information on thechemical evolution of the Galactic disk from a large sample of openclusters. The main property of this project is that all clusters areanalyzed in a homogeneous way to guarantee the robustness of the rankingin age, distance, and metallicity. Special emphasis is devoted to theevolution of the earliest phases of the Galactic disk evolution, forwhich clusters have superior reliability with respect to other types ofevolution indicators. The project is twofold: on one hand we derive theage, distance, and reddening (and indicative metallicity) byinterpreting deep and accurate photometric data with stellar evolutionmodels, and on the other hand, we derive the chemical abundances fromhigh-resolution spectroscopy. Here we describe our overall goals andapproaches and report on the midterm project status of the photometricpart, with 16 clusters already studied, covering an age interval from0.1 to 6 Gyr and galactocentric distances from 6.6 to 21 kpc. Theimportance of quantifying the theoretical uncertainties by deriving thecluster parameters with various sets of stellar models is emphasized.Stellar evolution models assuming overshooting from convective regionsappear to better reproduce the photometric properties of the clusterstars. The examined clusters show a clear metallicity dependence on thegalactocentric distance and no dependence on age. The tight relationbetween cluster age and magnitude difference between the main-sequenceturnoff and the red clump is confirmed.
| Toward the Detection of Transiting Hot Earths and Hot Neptunes in Open Clusters
Radial velocity searches for extrasolar planets have recently detectedseveral very low mass (7-20 M_oplus) planets in close orbits withperiods less than 10 days. We consider the prospects for detecting theanalogs of these planets in Galactic open clusters via transits. Weoutline the requirements for constructing a transit survey that wouldallow one to probe such ``Hot Earths'' and ``Hot Neptunes.''Specifically, we present a simple criterion for detection that definesthe minimum aperture required to detect planets of a given radius in acluster at a given distance. Adopting photometric precisions that havebeen demonstrated in state-of-the-art variability surveys, we thenpredict the number of planets one could potentially detect withambitious transit surveys toward several open clusters. Dedicatedsurveys lasting more than 20 nights with Pan-STARRS toward the Hyadesand Praesepe could detect a handful of Hot Earths, if the majority ofstars host such planets. Similar surveys with larger aperture telescopes(eg CFHT, MMT), toward M67, M35, M50, and M37 could detect Hot Neptunes,provided that their frequency is greater than 1%. The majority ofplanets will be detected around M dwarfs; detecting Hot Neptunes aroundsuch primaries requires photometric precisions of approx 1%, whereas HotEarths require approx 0.1 %. We discuss potential hurdles in detectingand confirming small planets in ground-based surveys, includingcorrelated noise, false positives, and intrinsic stellar variability.
| 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.
| Washington photometry of open cluster giants: two moderately metal-poor anticentre clusters
New photometric data in the Washington system are presented for redgiant candidates in NGC 1817 and 2251, two open clusters located towardsthe Galactic anticentre direction. In the case of NGC 2251, theWashington data are supplemented with new UBV and David DunlapObservatory (DDO) photoelectric photometry. Published radial velocitiesare used to separate field stars from cluster giants. The photometricdata yield an effective temperature and metal abundance for each clustermember. Five independent Washington abundance indices yield meanmetallicities of [Fe/H]= 0.25 +/- 0.04 for NGC 1817 and 2251,respectively. From combined BV and DDO data, we also derive E(B-V) =0.21 +/- 0.03 and [Fe/H]DDO=-0.14 +/- 0.05 for NGC 2251. Bothobjects are then found to be on the metal-poor side of the distributionof open clusters, their metallicities being compatible with theexistence of a radial abundance gradient in the disc. Using the WEBDAOpen Cluster data base and the available literature, we re-examined theoverall properties of a sample of 30 clusters located towards theGalactic anticentre with the distances, ages and metallicitiesavailable. This cluster sample presents no evidence of an abundancegradient perpendicular to the Galactic plane, nor is an age-metallicityrelation found. However, a radial abundance gradient of -0.093 dexkpc-1 is derived over a Galactocentric distance of 14 kpc, agradient which is in keeping with most recent determinations. This valuepractically does not change when all clusters with basic parametersknown up to this date are considered.
| Astrometry Test of MSCRED IRAF Software Package
Astrometric properties of images obtained using MSCRED IRAF softwarepackage are investigated. CCD observations of M35 open cluster weretaken in an observation run between October 10 and 17, 2004 atBelogradchick Observatory (Bulgaria) using the 60cm Cassegrain telescopeendowed with a 1kX1k Apogee 47P CCD camera, ensuring to a field size of6.16 square minutes, with a scale of 0.722arcsec/pixel in 2X2 binnedmode. Dithered images are resampled and reansambled into a single largerimage for which the World Coordinate System (WCS) solution is recomputedand investigated.
| A dynamical calibration of the mass-luminosity relation at very low stellar masses and young ages
Mass is the most fundamental parameter of a star, yet it is also one ofthe most difficult to measure directly. In general, astronomers estimatestellar masses by determining the luminosity and using the`mass-luminosity' relationship, but this relationship has never beenaccurately calibrated for young, low-mass stars and brown dwarfs. Massesfor these low-mass objects are therefore constrained only by theoreticalmodels. A new high-contrast adaptive optics camera enabled the discoveryof a young (50million years) companion only 0.156arcseconds (2.3AU) fromthe more luminous (> 120 times brighter) star AB Doradus A. Here wereport a dynamical determination of the mass of the newly resolvedlow-mass companion AB Dor C, whose mass is 0.090 +/- 0.005 solar masses.Given its measured 1-2-micrometre luminosity, we have found that thestandard mass-luminosity relations overestimate the near-infraredluminosity of such objects by about a factor of ~2.5 at young ages. Theyoung, cool objects hitherto thought to be substellar in mass aretherefore about twice as massive, which means that the frequency ofbrown dwarfs and planetary mass objects in young stellar clusters hasbeen overestimated.
| The open-cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution
Recent studies of white dwarfs in open clusters have provided newconstraints on the initial-final mass relationship (IFMR) formain-sequence stars with masses in the range 2.5-6.5Msolar.We re-evaluate the ensemble of data that determines the IFMR and arguethat the IFMR can be characterized by a mean IFMR about which there isan intrinsic scatter. We investigate the consequences of the IFMR forthe observed mass distribution of field white dwarfs using populationsynthesis calculations. We show that while a linear IFMR predicts a massdistribution that is in reasonable agreement with the recent resultsfrom the Palomar-Green survey, the data are better fitted by an IFMRwith some curvature. Our calculations indicate that a significant (~28)percentage of white dwarfs originating from a single star evolution hasmasses in excess of ~0.8Msolar, obviating the necessity forpostulating the existence of a dominant population of high-mass whitedwarfs that arise from binary star mergers.
| Discovery of 13 New Variable Stars in the Field of the Open Cluster NGC 2168 (M35)
A wide-field time-series CCD photometric survey of variable stars in thefield of the open cluster NGC 2168 was carried out using the BATCSchmidt telescope. In total 13 new variable stars are discovered withthree W UMa systems, one EA type and two EB type eclipsing binaries (oneof them could be a W UMa system), and seven pulsating stars includingthree candidates of δ Scuti stars.
| Detection of a Young Stellar Population in the Background of Open Clusters in the Third Galactic Quadrant
We report the detection of a young stellar population (<=100 Myr) inthe background of nine young open clusters belonging to a homogenoussample of 30 star clusters in the third Galactic quadrant (at217deg<=l<=260deg). Deep and accurate UBVRIphotometry allows us to measure model-independent age and distance forthe clusters and the background population with high confidence. Thispopulation is exactly the same population (the blue plume) recentlydetected in three intermediate-age open clusters and suggested to be a<=1-2 Gyr old population belonging to the Canis Major (CMa)overdensity (Bellazzini et al.; Martínez-Delgado et al.).However, we find that the young population in those three clusters andin six clusters of our sample follows the pattern of the Norma-Cygnusspiral arm as defined by CO clouds remarkably well, while in the otherthree program clusters it lies in the Perseus arm. We finally provideone example (out of 21) of a cluster that does not show any backgroundpopulation, demonstrating that this population is not ubiquitous towardCMa.
| The Age and Progenitor Mass of Sirius B
The Sirius AB binary system has masses that are well determined frommany decades of astrometric measurements. Because of the well-measuredradius and luminosity of Sirius A, we employed the TYCHO stellarevolution code to determine the age of the Sirius AB binary systemaccurately, at 225-250 Myr. Note that this fit requires the assumptionof solar abundance and the use of the new Asplund et al. primordialsolar metallicity. No fit to Sirius A's position is possible using theold Grevesse & Sauval scale. Because the Sirius B white dwarfparameters have also been determined accurately from space observations,the cooling age could be determined from recent calculations by Fontaineet al. or Wood to be 124+/-10 Myr. The difference in the two ages yieldsthe nuclear lifetime and mass of the original primary star,5.056+0.374-0.276 Msolar. This resultyields, in principle, the most accurate data point at relatively highmasses for the initial-to-final mass relation. However, the analysisrelies on the assumption that the primordial abundance of the Siriusstars was solar, based on membership in the Sirius supercluster. Arecent study suggests that its membership in the group is by no meanscertain.
| The Substellar Mass Function: A Bayesian Approach
We report our efforts to constrain the form of the low-mass star andbrown dwarf mass function via Bayesian inference. Recent surveys of M,L, and T dwarfs in the local solar neighborhood are an essentialcomponent of our study. Uncertainties in the age distribution of localfield stars make reliable inference complicated. We adopt a wide rangeof plausible assumptions about the rate of Galactic star formation andshow that their deviations from a uniform rate produce little effect onthe resulting luminosity function for a given mass function. As anancillary result, we calculate the age distribution for M, L, and Tspectral types. We demonstrate that late L dwarfs, in particular, aresystematically younger than objects with earlier or later spectraltypes, with a mean age of 3 Gyr. Finally, we use a Bayesian statisticalformalism to evaluate the probability of commonly used mass functions inthe light of recent discoveries. We consider three functional forms ofthe mass function, including a two-segment power law, a single power lawwith a low-mass cutoff, and a lognormal distribution. Our results showthat at a 60% confidence level the power-law index α for thelow-mass arm of a two-segment power law has a value between -0.6 and 0.6for objects with masses between 0.04 and 0.10 Msolar. Thebest-fit index is α=0.3+/-0.6 at the 60% confidence level for asingle-segment mass function. Current data require this function toextend to at least 0.05 Msolar with no restrictions placed ona lower mass cutoff. Inferences of the parameter values for a lognormalmass function are virtually unaffected by recent estimates of the localspace density of L and T dwarfs. We find that we have no preferenceamong these three forms using this method. We discuss current and futurecapabilities that may eventually discriminate between mass functionmodels and refine estimates of their associated parameter values.
| From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main-Sequence Stellar Evolution in Open Clusters
I explore the current ability of both white dwarf cooling theory andmain-sequence stellar evolution theory to accurately determine stellarpopulation ages by comparing ages derived using both techniques for openclusters ranging from 0.1 to 4 Gyr. I find good agreement between whitedwarf and main-sequence evolutionary ages over the entire age rangecurrently available for study. I also find that directly comparingmain-sequence turnoff ages to white dwarf ages is only weakly sensitiveto realistic levels of errors in cluster distance, metallicity, andreddening. Additional detailed comparisons between white dwarf andmain-sequence ages have tremendous potential to refine and calibrateboth of these important clocks, and I present new simulations ofpromising open cluster targets. The most demanding requirements forthese white dwarf studies are very deep (V>=25-28) clusterobservations made necessary by the faintness of the oldest white dwarfs.
| A Robust Measure of Tidal Circularization in Coeval Binary Populations: The Solar-Type Spectroscopic Binary Population in the Open Cluster M35
We present a new homogeneous sample of 32 spectroscopic binary orbits inthe young (~150 Myr) main-sequence open cluster M35. The distribution oforbital eccentricity versus orbital period (e-logP) displays a distincttransition from eccentric to circular orbits at an orbital period of ~10days. The transition is due to tidal circularization of the closestbinaries. The population of binary orbits in M35 provide a significantlyimproved constraint on the rate of tidal circularization at an age of150 Myr. We propose a new and more robust diagnostic of the degree oftidal circularization in a binary population based on a functional fitto the e-logP distribution. We call this new measure the ``tidalcircularization period.'' The tidal circularization period of a binarypopulation represents the orbital period at which a binary orbit withthe most frequent initial orbital eccentricity circularizes (defined ase=0.01) at the age of the population. We determine the tidalcircularization period for M35, as well as for seven additional binarypopulations spanning ages from the pre-main sequence (~3 Myr) to thelate main sequence (~10 Gyr), and use Monte Carlo error analysis todetermine the uncertainties on the derived circularization periods. Weconclude that current theories of tidal circularization cannot accountfor the distribution of tidal circularization periods with populationage.WIYN Open Cluster Study XXII.
| The Dearth of Massive, Helium-rich White Dwarfs in Young Open Star Clusters
Spectra have been obtained of 21 white dwarfs (WDs) in the direction ofthe young, rich open star cluster NGC 2099. This represents anappreciable fraction (>30%) of the cluster's total WD population. Themean derived mass of the sample is 0.8 Msolar-about 0.2Msolar larger than the mean seen among field WDs. Asurprising result is that all of the NGC 2099 WDs have hydrogen-richatmospheres (DAs); none exhibit helium-rich ones (DBs) or any otherspectral class. The number ratio in the field at the temperatures of theNGC 2099 WDs is DA/DB ~ 3.5. While the probability of seeing no DB WDsin NGC 2099 solely by chance is ~2%, if we include WDs in other openclusters of similar age it then becomes highly unlikely that the dearthof DB WDs in young open clusters is just a statistical fluctuation. Weexplore possible reasons for the lack of DBs in these clusters andconclude that the most promising scenario for the DA/DB number ratiodiscrepancy in young clusters is that hot, high-mass WDs do not developlarge enough helium convection zones to allow helium to be brought tothe surface and turn a hydrogen-rich WD into a helium-rich one.Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck.Gemini is an international partnership managed by the Association ofUniversities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. The W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA, was made possible by the generous financial support of the W. M.Keck Foundation.
| The Initial-Final Mass Relationship: Spectroscopy of White Dwarfs in NGC 2099 (M37)
We present new observations of very faint white dwarfs (WDs) in the richopen star cluster NGC 2099 (M37). Following deep, wide-field imaging ofthe cluster using the Canada-France-Hawaii Telescope, we have nowobtained spectroscopic observations of candidate WDs using both theGemini Multi-Object Spectrograph on Gemini North and the Low-ResolutionImaging Spectrometer on Keck. Of our 24 WD candidates (all fainter thanV=22.4), 21 are spectroscopically confirmed to be bona fide WDs, four orfive of which are most likely field objects. Fitting 18 of the 21 WDspectra with model atmospheres, we find that most WDs in this clusterare quite massive (0.7-0.9 Msolar), as expected given thecluster's young age (650 Myr) and, hence, high turnoff mass (~2.4Msolar). We determine a new initial-final mass relationshipand almost double the number of existing data points from previousstudies. The results indicate that stars with initial masses between 2.8and 3.4 Msolar lose 70%-75% of their mass through stellarevolution. For the first time, we find some evidence of a metallicitydependence on the initial-final mass relationship.Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck.Gemini is an international partnership managed by the Association ofUniversities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. The W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA, was made possible by the generous financial support of the W. M.Keck Foundation.
| 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.
| Outbursts on normal stars. FH Leo misclassified as a novalike variable
We present high resolution spectroscopy of the common proper motionsystem FH Leo (components HD 96273 andBD+07 2411B), which has been classified as a novalike variabledue to an outburst observed by Hipparcos, and we present and review theavailable photometry. We show from our spectra that neither star canpossibly be a cataclysmic variable, instead they are perfectly normallate-F and early-G stars. We measured their radial velocities andderived the atmospheric fundamental parameters, abundances of severalelements including Fe, Ni, Cr, Co, V, Sc, Ti, Ca and Mg, and we derivethe age of the system. From our analysis we conclude that the stars doindeed constitute a physical binary. However, the observed outburstcannot be readily explained. We examine several explanations, includingpollution with scattered light from Jupiter, binarity, microlensing,background supernovae, interaction with unseen companions and planetaryengulfment. While no explanation is fully satisfactory, the scatteredlight and star-planet interaction scenarios emerge as the least unlikelyones, and we give suggestions for further study.
| 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.
| Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much ofmodern astrophysics. However, a quantitative prediction of the starformation rate and the initial distribution of stellar masses remainselusive. For several decades it has been thought that the star formationprocess is primarily controlled by the interplay between gravity andmagnetostatic support, modulated by neutral-ion drift (known asambipolar diffusion in astrophysics). Recently, however, bothobservational and numerical work has begun to suggest that supersonicturbulent flows rather than static magnetic fields control starformation. To some extent, this represents a return to ideas popularbefore the importance of magnetic fields to the interstellar gas wasfully appreciated. This review gives a historical overview of thesuccesses and problems of both the classical dynamical theory and thestandard theory of magnetostatic support, from both observational andtheoretical perspectives. The outline of a new theory relying on controlby driven supersonic turbulence is then presented. Numerical modelsdemonstrate that, although supersonic turbulence can provide globalsupport, it nevertheless produces density enhancements that allow localcollapse. Inefficient, isolated star formation is a hallmark ofturbulent support, while efficient, clustered star formation occurs inits absence. The consequences of this theory are then explored for bothlocal star formation and galactic-scale star formation. It suggests thatindividual star-forming cores are likely not quasistatic objects, butdynamically collapsing. Accretion onto these objects varies depending onthe properties of the surrounding turbulent flow; numerical models agreewith observations showing decreasing rates. The initial massdistribution of stars may also be determined by the turbulent flow.Molecular clouds appear to be transient objects forming and dissolvingin the larger-scale turbulent flow, or else quickly collapsing intoregions of violent star formation. Global star formation in galaxiesappears to be controlled by the same balance between gravity andturbulence as small-scale star formation, although modulated by coolingand differential rotation. The dominant driving mechanism instar-forming regions of galaxies appears to be supernovae, whileelsewhere coupling of rotation to the gas through magnetic fields orgravity may be important.
| The astrophysics of cool white dwarfs
Electronic Article Available from Elsevier Science.
| Variability in the stellar initial mass function at low and high mass: three-component IMF models
Three-component models of the initial mass function (IMF) are made toconsider possible origins for the observed relative variations in thenumbers of brown dwarfs, solar-to-intermediate-mass stars and high-massstars. The differences between the IMFs observed for clusters, field andremote field are also discussed. Three distinct physical processes thatshould dominate the three stellar mass regimes are noted. Thecharacteristic mass for most star formation is identified with thethermal Jeans mass in the molecular cloud core, and this presumablyleads to the middle mass range by the usual collapse and accretionprocesses. Pre-stellar condensations (PSCs) observed in millimetre-wavecontinuum studies presumably form at this mass. Significantly smallerself-gravitating masses require much larger pressures and may arisefollowing dynamical processes inside these PSCs, including discformation, tight-cluster ejection, and photoevaporation as studiedelsewhere, but also gravitational collapse of shocked gas in collidingPSCs. Significantly larger stellar masses form in relatively lowabundance by normal cloud processes, possibly leading to steep IMFs inlow-pressure field regions, but this mass range can be significantlyextended in high-pressure cloud cores by gravitationally focused gasaccretion on to PSCs and by the coalescence of PSCs. These modelssuggest that the observed variations in brown dwarf,solar-to-intermediate-mass and high-mass populations are the result ofdynamical effects that depend on environmental density and velocitydispersion. They accommodate observations ranging from shallow IMFs incluster cores to Salpeter IMFs in average clusters and whole galaxies tosteep and even steeper IMFs in field and remote field regions. They alsosuggest how the top-heavy IMFs in some starburst clusters may originateand they explain bottom-heavy IMFs in low surface brightness galaxies.
| Low mass stars, brown dwarf candidates and the mass function of the young open cluster NGC 2547
We present a catalogue of RcIcZ photometry over anarea of 0.855 square degrees, centred on the young open cluster NGC2547. The survey is substantially complete to limits of Rc=21.5, Ic= 19.5, Z= 19.5. We use the catalogue to define asample of NGC 2547 candidates with model-dependent masses of about 0.05-1.0 Msolar. After correcting for incompleteness andestimating contamination by foreground field dwarfs, we investigate themass function of the cluster, its binary content, and search forevidence of mass segregation among the lower mass stars. There is ampleevidence for mass segregation between high (>3 Msolar) andlower mass stars, but over the range 0.1 < M < 0.7Msolar, the data are consistent with no further masssegregation. By fitting King profiles we conclude that at least 60 percent of the low-mass stellar population are contained within our survey.The cluster mass function is remarkably similar to the Pleiades for0.075 < M < 0.7 Msolar. Because of its age (~= 30 Myr),we demonstrate that this mass function is robust to a number ofsystematic uncertainties likely to affect older and younger clusters andis therefore one of the best available estimates for the initial massfunction in young disc populations. For 0.05 < M < 0.075Msolar there is some evidence for a deficit of brown dwarfsin NGC 2547 compared with other clusters. This deficit may extend tolower masses or may only be a dip, perhaps caused by an imperfectunderstanding of the mass-magnitude relationship at temperatures ofaround 2800 K. Incompleteness in both our survey and the luminosityfunctions from which we estimate contamination by foreground objectsleave this question open. The binary fraction for systems with massratios greater than about 0.5 is 20-35 per cent for M dwarfs in NGC2547, quite consistent with that found in the field and other youngclusters. The full photometric catalogue and our lists of candidatecluster members are made available in electronic format.
| Interpreting the colour-magnitude diagrams of open star clusters through numerical simulations
We present detailed comparisons between high quality observationalcolour-magnitude diagrams (CMDs) of open star clusters and syntheticCMDs based on Monte Carlo numerical simulations. The comparisons accountfor all of the main parameters which determine the shape of the CMD fora stellar population. For the four clusters studied, NGC 6819, 2099(M37), 2168 (M35) and 2323 (M50), we derive reddening, distance, age,binary fraction, star formation rate and indicative metallicity bycomparing the locations and density of points in the observed CMDs tothe simulated CMDs. We estimate the uncertainties related to stellarevolution theories by adopting various sets of stellar models for all ofthe synthetic CMDs and discuss which stellar models provide thetheoretical CMDs that best reproduce the observations.
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