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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.
| 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.
| 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.
| 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.
| 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.
| Spectroscopy of Active and Inactive M Dwarfs in Praesepe
Spectroscopy of 222 late-type dwarfs in the Praesepe open cluster isused to measure the strengths of molecular and atomic features and toexamine their sensitivity to chromospheric activity as measured by thestrength of Hα emission. For this cluster we confirm the resultsof Hawley and coworkers that some TiO features are selectively weakenedamong active field stars. However, a similar effect for CaOH is onlyweakly confirmed. Using differential measures of the strengths offeatures with respect to the median in narrow color bins, we also findthat absorption in the Ca II IR triplet is selectively weakened in moreactive stars, while absorption at K I and Na I is strengthened. Acomparison of the distribution of the Hα strengths with V-I colorin Praesepe and the similarly aged Hyades cluster indicates that theonset of M dwarf activity occurs at a significantly redder color inPraesepe than in the Hyades. Analysis of similar data on eight clustersfrom the literature is used to explore the relationship between age andactivity in open clusters. We quantify the dependence on cluster age ofthe location along the main sequence for the onset of activity,confirming that the critical color at which activity becomes prominentis a nearly linear function of the logarithm of the age.
| Identification of New M Dwarfs in the Solar Neighborhood
We present the results from a spectroscopic study of 1080 nearby activeM dwarfs, selected by correlating the Two Micron All Sky Survey andROSAT catalogs. We have derived the spectral types and estimateddistances for all of our stars. The spectral types range between K5 andM6. Nearly half of our stars lie within 50 pc. We have measured theequivalent width of the Hα emission line. Our targets show anincrease in chromospheric activity from early to midspectral types, witha peak in activity around M5. Using the count rate and hardness ratiosobtained from the ROSAT catalog, we have derived the X-ray luminosities.Our stars display a ``saturation-type'' relation between thechromospheric and coronal activity. The relation is such thatlogLX/Lbol remains ``saturated'' at a value ofapproximately -3 for varying Hα equivalent width. We have found568 matches in the USNO-B catalog and have derived the tangentialvelocities for these stars. There is a slight trend of decreasingchromospheric activity with age, such that the stars with highervtan have lower Hα equivalent widths. The coronalemission, however, remains saturated at a value oflogLX/Lbol~-3 for varying tangential velocities,suggesting that the coronal activity remains saturated with age. We donot find any break in the saturation-type relation at the spectral typeat which stars become fully convective (~M3.5). Most of the stars in oursample show more coronal emission than the dMe stars in the Hyades andPraesepe and have vtan<40 km s-1, suggesting ayoung population.
| Radial Velocity Studies of Close Binary Stars. XI.
Radial-velocity measurements and sine-curve fits to orbital radialvelocity variations are presented for 10 close binary systems: DU Boo,ET Boo, TX Cnc, V1073 Cyg, HL Dra, AK Her, VW LMi, V566 Oph, TV UMi, andAG Vir. With this contribution, the David Dunlap Observatory program hasreached the point of 100 published radial velocity orbits. The radialvelocities have been determined using an improved fitting technique thatuses rotational profiles to approximate individual peaks in broadeningfunctions. Three systems, ET Boo, VW LMi, and TV UMi, are found to bequadruple, while AG Vir appears to be a spectroscopic triple. ET Boo, amember of a close visual binary with Pvis=113 yr, waspreviously known to be a multiple system, but we show that the secondcomponent is actually a close, noneclipsing binary. The new observationshave enabled us to determine the spectroscopic orbits of the companion,noneclipsing pairs in ET Boo and VW LMi. A particularly interesting caseis VW LMi, for which the period of the mutual revolution of the twospectroscopic binaries is only 355 days. While most of the studiedeclipsing pairs are contact binaries, ET Boo is composed of twodouble-lined detached binaries, and HL Dra is a single-lined detached orsemidetached system. Five systems of this group have been observedspectroscopically before: TX Cnc, V1073 Cyg, AK Her (as a single-linedbinary), V566 Oph, and AG Vir, but our new data are of much higherquality than in the previous studies.Based on data obtained at the David Dunlap Observatory, University ofToronto, Canada.
| 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.
| 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.
| 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.
| Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
Aims.We report a deep X-ray survey of the young (~140 Myr), rich opencluster NGC 2516 obtained with the EPIC camera on board the XMM-Newtonsatellite. Methods: .By combining data from six observations, ahigh sensitivity, greater than a factor of 5 with respect to recentChandra observations, has been achieved. Kaplan-Meier estimators of thecumulative X-ray luminosity distribution are built, statisticallycorrected for non members contaminants and compared to those of thenearly coeval Pleiades. The EPIC spectra of the X-ray brightest starsare fitted using optically thin model plasma with one or two thermalcomponents. Results: .We detected 431 X-ray sources and 234 ofthem have as optical counterparts cluster stars spanning the entire NGC2516 Main Sequence. On the basis of X-ray emission and opticalphotometry, we indicate 20 new candidate members of the cluster; at thesame time we find 49 X-ray sources without known optical or infraredcounterpart. The X-ray luminosities of cluster stars span the range logLX (erg s-1) = 28.4-30.8. The representativetemperatures span the 0.3-0.6 keV (3.5-8 MK) range for the coolcomponent and 1.0-2.0 keV (12-23 MK) for the hot one; similar values arefound in other young open clusters like the Pleiades, IC 2391, andBlanco 1. While no significant differences are found in X-ray spectra,NGC 2516 solar type stars are definitely less luminous in X-rays thanthe nearly coeval Pleiades. The comparison with a previous ROSAT surveyreveals the lack of variability amplitudes larger than a factor of 2 insolar type stars in a 11 yr time scale of the cluster and thusactivity cycles like in the Sun are probably absent or have a differentperiod and amplitude in young stars.
| Rotation- and temperature-dependence of stellar latitudinal differential rotation
More than 600 high resolution spectra of stars with spectral type F andlater were obtained in order to search for signatures of differentialrotation in line profiles. In 147 stars the rotation law could bemeasured, with 28 of them found to be differentially rotating.Comparison to rotation laws in stars of spectral type A reveals thatdifferential rotation sets in at the convection boundary in theHR-diagram; no star that is significantly hotter than the convectionboundary exhibits the signatures of differential rotation. Four lateA-/early F-type stars close to the convection boundary and at v sin{i}≈ 100 km s-1 show extraordinarily strong absolute shear atshort rotation periods around one day. It is suggested that this is dueto their small convection zone depth and that it is connected to anarrow range in surface velocity; the four stars are very similar inTeff and v sin{i}. Detection frequencies of differentialrotation α = ΔΩ/Ω > 0 were analyzed in starswith varying temperature and rotation velocity. Measurable differentialrotation is more frequent in late-type stars and slow rotators. Thestrength of absolute shear, ΔΩ, and differential rotationα are examined as functions of the stellar effective temperatureand rotation period. The highest values of ΔΩ are found atrotation periods between two and three days. In slower rotators, thestrongest absolute shear at a given rotation rateΔΩmax is given approximately byΔΩmax ∝ P-1, i.e.,αmax ≈ const. In faster rotators, bothαmax and ΔΩmax diminish lessrapidly. A comparison with differential rotation measurements in starsof later spectral type shows that F-stars exhibit stronger shear thancooler stars do and the upper boundary in absolute shear ΔΩwith temperature is consistent with the temperature-scaling law found inDoppler Imaging measurements.
| Revisiting the population of Galactic open clusters
We present results of a study of the galactic open cluster populationbased on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2,Hipparcos and other catalogues. The sample of optical clusters fromASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry planeof the clusters' distribution is determined to be at Z_0=-22±4pc, and the scale height of open clusters is only 56±3 pc. Thetotal surface density and volume density in the symmetry plane areΣ= 114 kpc-2 and D(Z_0)=1015 kpc-3,respectively. We find the total number of open clusters in the Galacticdisk to be of order of 105 at present. Fluctuations in thespatial and velocity distributions are attributed to the existence offour open cluster complexes (OCCs) of different ages containing up to afew tens of clusters. Members in an OCC show the same kinematicbehaviour, and a narrow age spread. We find, that the youngest clustercomplex, OCC 1 (log t<7.9), with 19 deg inclination to the Galacticplane, is apparently a signature of Gould's Belt. The most abundant OCC2 complex has moderate age (log t≈8.45). The clusters of thePerseus-Auriga group, having the same age as OCC 2, but differentkinematics are seen in breaks between Perseus-Auriga clouds. The oldest(log t≈8.85) and sparsest group was identified due to a large motionin the Galactic anticentre direction. Formation rate and lifetime ofopen clusters are found to be 0.23±0.03 kpc-2Myr-1 and 322±31 Myr, respectively. This implies atotal number of cluster generations in the history of the Galaxy between30 to 40. We estimate that less than about 10% of the total Galacticstellar disk population has ever passed an open cluster membership.
| Influence of Gravity Waves on the Internal Rotation and Li Abundance of Solar-Type Stars
The Sun's rotation profile and lithium content have been difficult tounderstand in the context of conventional models of stellar evolution.Classical hydrodynamic models predict that the solar interior mustrotate highly differentially, in disagreement with observations. It hasrecently been shown that internal waves produced by convection insolar-type stars produce an asymmetric, shear layer oscillation, similarto Earth's quasi-biennial oscillation, that leads to efficient angularmomentum redistribution from the core to the envelope. We presentresults of a model that successfully reproduces both the rotationprofile and the surface abundance of lithium in solar-type stars ofvarious ages.
| A Dozen δ Scuti Stars in the Open Cluster NGC 1817
We obtained high-precision time-series observations of stars in the opencluster NGC 1817 in order to find δ Scuti stars among the clustermembers. The detection of 12 δ Scuti stars, of which our datasuggest that 11 are cluster members, makes NGC 1817 a key target forasteroseismology of 1.5-2.5 Msolar stars. One of the clustermember δ Scuti stars is also an eclipsing binary, potentiallyoffering very strong constraints for theoretical modeling. In addition,we find one δ Scuti star candidate, a γ Dor candidate, twovariables of currently unknown type, and two eclipsing binary systems.We also describe a method for combining the oscillation frequencies ofseveral δ Scuti stars in a single cluster to obtain informationabout the order n of the excited modes. This method will eventually beused to constrain the theoretical models to be compared with preciseoscillation frequencies determined from a future multisite campaign(asteroseismology).
| The Prospects for Finding Brown Dwarfs in Eclipsing Binary Systems and Measuring Brown Dwarf Properties
We present the results of a simulation to investigate the prospects ofmeasuring mass, age, radius, metallicity, and luminosity data for browndwarfs in fully eclipsing binary systems around dwarf spectral typesfrom late K to early M that could be identified by ultra-wide-fieldtransit surveys such as SuperWASP. These surveys will monitorapproximately a million K and M dwarfs with |b|>20deg(where blending of sources is not a significant problem) at a levelsufficient to detect transits of low-luminosity companions. We look atthe current observational evidence for such systems and suggest that ~1%of late K and early-to-mid M dwarfs could have a very close (~0.02 AU)brown dwarf companion. With this assumption, and using SuperWASP as anexample, our simulation predicts that ~400 brown dwarfs in fullyeclipsing binary systems could be discovered. All of these eclipsingbinaries could yield accurate brown dwarf mass and radius measurementsfrom radial velocity and follow-up light curve measurements. Byinferring the brown dwarf effective temperature distribution, assuming auniform age spread and an α=0.5 companion brown dwarf massfunction, the simulation estimates that brown dwarf brightness couldalso be measurable (at the 10% level) for ~60 of these binary systemsfrom near-infrared follow-up light curves of the secondary eclipse. Weconsider irradiation of these brown dwarfs by their primary stars andconclude that it would be below the 10% level for ~70% of them. Thismeans that in these cases, the measured brown dwarf brightnesses shouldessentially be the same as those of free-floating counterparts. Thepredicted age distribution of the primaries is dominated by youngsystems, and ~20 binaries could be younger than 1 Gyr. Irradiation willbe below the 10% level for ~80% of these. We suggest that many of theseyoung binary systems will be members of ``kinematic moving groups,''allowing their ages to be accurately constrained.
| The Low-Mass Limit for Total Mass of W UMa-type Binaries
The observations of W UMa type stars show a well-defined short-periodlimit of 0.22 d, which is equivalent to a lower mass limit ofapproximately 1 M_odot for the total binary mass. It is currentlybelieved that cool contact binaries are formed from detached binarieslosing angular momentum (AM) via a magnetized wind. Orbital evolution ofdetached binaries with various component masses was followed until theprimary component reached the critical Roche surface and the Roche lobeoverflow (RLOF) began. It was assumed that the minimum initial, i.e.,ZAMS, orbital period of such binaries is equal to 2 d and that thecomponents lose AM just as single stars. According to the mass-dependentformula for AM loss rate of single stars, derived in this paper, the AMloss time scale increases substantially with decreasing stellar mass.The formula was applied to binaries with the initial primary componentmasses between 1.0 M_odot and 0.6 M_odot and two values of mass ratioq=1 and 0.5.Detailed calculations show that the time needed to reach RLOF by a 1M_odot primary is of the order of 7.5 Gyr, but it increases to more than13 Gyr for a binary with an initial primary mass of 0.7 M_odot. Binarieswith less massive primaries have not yet had time to reach RLOF evenwithin the age of the Universe. This sets a lower mass limit for thepresently existing contact binaries at about 1.0 M_odot--1.2 M_odot, ina good agreement with observations.
| Crossing into the substellar regime in Praesepe
We present the results of a deep optical 2.6 deg2 survey withnear-infrared (NIR) follow-up measurements of the intermediate-agedPraesepe open cluster. The survey is complete to Ic= 21.3, Z=20.5, corresponding to M~ 0.06Msolar assuming a cluster ageof 0.5 Gyr. Using three to five passbands to constrain clustermembership, we identify 32 new low-mass cluster members, at least fourof which are likely to be substellar. We use the low-mass census totrace the region where the sequence moves away from the NEXTGEN towardsthe DUSTY regime at Teff= 2200 K. In doing so, we identifyfour unresolved binaries, yielding a substellar binary fraction (BF) of~30 per cent. The BFs appear to decrease below 0.1 Msolar, incontrast to the rising fractions found in the Pleiades. We also identifya paucity of late M dwarfs, thought to be due to a steepening in themass-luminosity relation at these spectral types, and compare theproperties of this gap in the sequence to those observed in youngerclusters. We note an overdensity of faint sources in the region of theso-called subcluster (possibly an older smaller cluster withinPraesepe), and subsequently derive the luminosity and mass functions(MFs) for the main Praesepe cluster, revealing a turnover near thesubstellar boundary. We conclude by presenting astrometric measurementsfor low-mass Praesepe candidates from the literature and rule out as alikely foreground dwarf RPr1, hitherto thought to be a substellarmember.
| 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.
| A search for substellar members in the Praesepe and σ Orionis clusters
We have conducted deep photometric searches for substellar members ofthe Praesepe (0.5-1 Gyr) and σ Orionis (3 Myr) star clusters usingthe Sloan i' and z' broad-band filters, with the 3.5-m and the 5-m Haletelescopes on the Calar Alto and Palomar Observatories. The total areasurveyed was 1177 arcmin2 and 1122 arcmin2 towardsthe central regions of Praesepe and σ Orionis, respectively. The5-σ detection limit of our survey is measured at i' = 24.5 and z'= 24 mag, which according to state-of-the-art evolutionary modelscorresponds to masses of 50-55 M_Jup (Praesepe) and 6 M_Jup (σOrionis), i.e., well within the substellar regime. Besides recoveringpreviously known cluster members reported in the literature, we haveidentified new photometric candidates in both clusters whose massesexpand the full range covered by our study. In σ Orionis,follow-up near-infrared photometry has allowed us to confirm the likelycluster membership of three newly discovered planetary-mass objects. Thesubstellar mass function of σ Orionis, which is complete from thestar-brown dwarf borderline down to 7 M_Jup, rises smoothly with a slopeof α = 0.6 +0.5-0.1 (dN/dM ˜M-α). One of the faintest Praesepe candidates for whichwe have obtained follow-up near-infrared JHKs photometryclosely fits the expected optical and infrared photometric sequence ofthe cluster. From its colors, we have estimated its spectral type to bebetween L4 and L6. If confirmed as a true Praesepe member, it wouldbecome the first L-type brown dwarf (50-60 M_Jup) identified in anintermediate-age star cluster. Our derivation of the Praesepe massfunction, which is based on state-of-the-art evolutionary models,depends strongly on the cluster age. For the youngest possible ages(500-700 Myr), our results suggest that there is a deficit of Praesepebrown dwarfs in the central regions of the cluster, while the similaritybetween the Praesepe and σ Orionis mass functions increasesqualitatively for models older than 800 Myr.
| Mass transfer in eccentric binary stars
The concept of Roche lobe overflow is fundamental to the theory ofinteracting binaries. Based on potential theory, it is dependent on allthe relevant material corotating in a single frame of reference.Therefore if the mass losing star is asynchronous with the orbitalmotion or the orbit is eccentric, the simple theory no longer appliesand no exact analytical treatment has been found. We use an analyticapproximation whose predictions are largely justified by smoothedparticle hydrodynamic simulations (SPH). We present SPH simulations ofbinary systems with the same semi-major axis a= 5.55 Rsolar,masses M1= 1 Msolar, M2= 2Msolar and radius R1= 0.89 Rsolar forthe primary star but with different eccentricities e= 0.4, 0.5, 0.6 and0.7. In each case the secondary star is treated as a point mass. When e=0.4 no mass is lost from the primary while at e= 0.7 catastrophic masstransfer, partly through the L2 point, takes place nearperiastron. This would probably lead to common-envelope evolution ifstar 1 were a giant or to coalescence for a main-sequence star. Inbetween, at e>= 0.5, some mass is lost through the L1point from the primary close to periastron. However, rather than beingall accreted by the secondary, some of the stream appears to leave thesystem. Our results indicate that the radius of the Roche lobe issimilar to circular binaries when calculated for the separation andangular velocity at periastron. Part of the mass loss occurs through theL2 point.
| Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation
We study the kinematics of 129 W UMa binaries and we discuss itsimplications on the contact binary evolution. The sample is found to beheterogeneous in the velocity space. That is, kinematically younger andolder contact binaries exist in the sample. A kinematically young (0.5Gyr) subsample (moving group) is formed by selecting the systems thatsatisfy the kinematical criteria of moving groups. After removing thepossible moving group members and the systems that are known to bemembers of open clusters, the rest of the sample is called the fieldcontact binary (FCB) group. The FCB group is further divided into fourgroups according to the orbital period ranges. Then, a correlation isfound in the sense that shorter-period less-massive systems have largervelocity dispersions than the longer-period more-massive systems.Dispersions in the velocity space indicate a 5.47-Gyr kinematical agefor the FCB group. Compared with the field chromospherically activebinaries (CABs), presumably detached binary progenitors of the contactsystems, the FCB group appears to be 1.61 Gyr older. Assuming anequilibrium in the formation and destruction of CAB and W UMa systems inthe Galaxy, this age difference is treated as an empirically deducedlifetime of the contact stage. Because the kinematical ages (3.21, 3.51,7.14 and 8.89 Gyr) of the four subgroups of the FCB group are muchlonger than the 1.61-Gyr lifetime of the contact stage, the pre-contactstages of the FCB group must dominantly be producing the largedispersions. The kinematically young (0.5 Gyr) moving group covers thesame total mass, period and spectral ranges as the FCB group. However,the very young age of this group does not leave enough room forpre-contact stages, and thus it is most likely that these systems wereformed in the beginning of the main sequence or during thepre-main-sequence contraction phase, either by a fission process or mostprobably by fast spiralling in of two components in a common envelope.
| Pleiades low-mass brown dwarfs: the cluster L dwarf sequence
Aims.We present a search for low-mass brown dwarfs in the Pleiades opencluster. The identification of Pleiades members fainter and cooler thanthose currently known allows us to constrain evolutionary models for Ldwarfs and to extend the study of the cluster mass function to lowermasses. Methods: .We conducted a 1.8 deg2 near-infraredJ-band survey at the 3.5 m Calar Alto Telescope, with completenessJ_cpl˜ 19.0. The detected sources were correlated with those ofpreviously available optical I-band images (I_cpl˜ 22). Using a Jversus I-J colour-magnitude diagram, we identified 18 faint red L-typecandidates, with magnitudes 17.4 3.2.If Pleiades members, their masses would span~0.040-0.020~Mȯ. We performed follow-up HK_s-bandimaging to further confirm their cluster membership by photometry andproper motion. Results: .Out of 11 IJ candidates with propermotion measurements, we find six cluster members, two non-members andthree whose membership is uncertain and depends on the intrinsicvelocity dispersion of Pleiades brown dwarfs. This dispersion (>4 masyr-1) is at least four times that of cluster stars withmasses ⪆1 Mȯ. Five of the seven other IJ candidatesare discarded because their J-Ks colours are bluer than thoseof confirmed members. Our least massive proper motion members areBRB 28 and 29 (~25 M_Jup). The J versus I-J sequenceof the L-type candidates at J>18 is not as red as theoretical modelspredict; it rather follows the field L-dwarf sequence translated to thecluster distance. This sequence overlapping, also observed in the Jversus J-H and J-K diagrams, suggests that Pleiades and field L dwarfsmay have similar spectral energy distributions and luminosities, andthus possibly similar radii. Also, we find α= 0.5±0.2 for apower-law approximation dN/dM ∝ M-α of the surveymass spectrum in the mass range 0.5-0.026~Mȯ. This valueis similar to that of much younger clusters, indicating no significantdifferential evaporation of low-mass Pleiades members relative to moremassive ones.
| The origin of the magnetic fields in white dwarfs
Magnetic white dwarfs with fields in excess of ~106 G (thehigh field magnetic white dwarfs; HFMWDs) constitute about ~10 per centof all white dwarfs and show a mass distribution with a mean mass of~0.93 Msolar compared to ~0.56 Msolar for allwhite dwarfs. We investigate two possible explanations for theseobservations. First, that the initial-final mass relationship (IFMR) isinfluenced by the presence of a magnetic field and that the observedHFMWDs originate from stars on the main sequence that are recognized asmagnetic (the chemically peculiar A and B stars). Secondly, that theIFMR is essentially unaffected by the presence of a magnetic field, andthat the observed HFMWDs have progenitors that are not restricted tothese groups of stars. Our calculations argue against the formerhypothesis and support the latter. The HFMWDs have a higher than averagemass because on the average they have more massive progenitors and notbecause the IFMR is significantly affected by the magnetic field. Arequirement of our model is that ~40 per cent of main-sequence starsmore massive than ~4.5 Msolar must either have magneticfields in the range of ~10-100 G, which is below the current level ofdetection, or generate fields during subsequent stellar evolutiontowards the white dwarf phase. In the former case, the magnetic fieldsof the HFMWDs could be fossil remnants from the main-sequence phaseconsistent with the approximate magnetic flux conservation.
| Basic physical parameters of a selected sample of evolved stars
We present the detailed spectroscopic analysis of 72 evolved stars,which were previously studied for accurate radial velocity variations.Using one Hyades giant and another well studied star as the referenceabundance, we determine the [Fe/H] for the whole sample. Thesemetallicities, together with the T_eff values and the absolute V-bandmagnitude derived from Hipparcos parallaxes, are used to estimate basicstellar parameters (ages, masses, radii, (B-V)0 and log g)using theoretical isochrones and a Bayesian estimation method. The(B-V)0 values so estimated turn out to be in excellentagreement (to within ~0.05 mag) with the observed (B-V), confirming thereliability of the T_eff-(B-V)0 relation used in theisochrones. On the other hand, the estimated log g values are typically0.2 dex lower than those derived from spectroscopy; this effect has anegligible impact on [Fe/H] determinations. The estimated diametersθ have been compared with limb darkening-corrected ones measuredwith independent methods, finding an agreement better than 0.3 maswithin the 1<θ<10 mas interval (or, alternatively, findingmean differences of just 6%). We derive the age-metallicity relation forthe solar neighborhood; for the first time to our knowledge, such arelation has been derived from observations of field giants rather thanfrom open clusters and field dwarfs and subdwarfs. The age-metallicityrelation is characterized by close-to-solar metallicities for starsyounger than ~4 Gyr, and by a large [Fe/H] spread with a trend towardslower metallicities for higher ages. In disagreement with other studies,we find that the [Fe/H] dispersion of young stars (less than 1 Gyr) iscomparable to the observational errors, indicating that stars in thesolar neighbourhood are formed from interstellar matter of quitehomogeneous chemical composition. The three giants of our sample whichhave been proposed to host planets are not metal rich; this result is atodds with those for main sequence stars. However, two of these starshave masses much larger than a solar mass so we may be sampling adifferent stellar population from most radial velocity searches forextrasolar planets. We also confirm the previous indication that theradial velocity variability tends to increase along the RGB, and inparticular with the stellar radius.
| Numerical Simulations of the Hyades Dynamics and the Nature of the Moving Hyades Cluster
We present our numerical simulations of the dynamical evolution of theHyades open cluster. The simulations were performed usinga modifiedNBODY6 algorithm that included tidal forces and a realistic orbit of thecluster in a gravitational field described by the Miyamoto-Nagaipotential. Our goal was to study the nature of movingclu sters. We showthat the stars that were earlier cluster members could be lateridentified within a sphere of 50 pc in diameter around the Sun. Thenumber of such stars for the chosen initial mass and virial radius ofthe cluster does not exceed ten. The maximum space velocity of thesestars relative to the core of the current cluster does not exceed 3 kms-1. Our numerical simulations confirm the assumption that some of themoving clusters near the Sun could consist of stars that have escapedfrom open clusters in the course of their dynamical evolution.
| 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.
| 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.
| Population analysis of open clusters: radii and mass segregation
Aims.Based on our well-determined sample of open clusters in the all-skycatalogue ASCC-2.5 we derive new linear sizes of some 600 clusters, andinvestigate the effect of mass segregation of stars in open clusters. Methods: .Using statistical methods, we study the distribution oflinear sizes as a function of spatial position and cluster age. We alsoexamine statistically the distribution of stars of different masseswithin clusters as a function of the cluster age. Results: .Nosignificant dependence of the cluster size on location in the Galaxy isdetected for younger clusters (<200 Myr), whereas older clustersinside the solar orbit turned out to be, on average, smaller thanoutside. Also, small old clusters are preferentially found close to theGalactic plane, whereas larger ones more frequently live farther awayfrom the plane and at larger Galactocentric distances. For clusters with(V - M_V) < 10.5, a clear dependence of the apparent radius on agehas been detected: the cluster radii decrease by a factor of about 2from an age of 10 Myr to an age of 1 Gyr. A detailed analysis shows thatthis observed effect can be explained by mass segregation and does notnecessarily reflect a real decrease of cluster radii. We found evidencefor the latter for the majority of clusters older than 30 Myr. Among theyoungest clusters (between 5 and 30 Myr), there are some clusters with asignificant grade of mass segregation, whereas some others show nosegregation at all. At a cluster age between 50 and 100 Myr, thedistribution of stars of different masses becomes more regular overcluster area. In older clusters the evolution of the massive stars isthe most prominent effect we observe.
| 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.
| Ultracool dwarf binaries
We review here the multiplicity properties of ultracool dwarfs (spectraltype later than M6) observed in three different environments and atthree different ages: in the field, where the objects are relatively old(1-5 Gyrs) and isolated, in the Pleiades young (120 Myr) opencluster, and in the young (5 Myr) Upper Scorpius OB association(USco). While the field and Pleiades populations seem to have verysimilar properties, the preliminary results obtained in USco might showsignificant differences. If confirmed, it would mean that the phenomenaresponsible for the ``final'' properties of ultracool dwarfsmultiplicity are still at work at the age of USco, but are already overat the age of the Pleiades.
| Rotation and variability of young very low-mass objects
Variability studies are an important tool to investigate key propertiesof stars and brown dwarfs. From photometric monitoring we are able toobtain information about rotation and magnetic activity, which areexpected to change in the mass range below 0.3 solar masses, since thesefully convective objects cannot host a solar-type dynamo. On the otherhand, spectroscopic variability information can be used to obtain adetailed view on the accretion process in very young objects. In thispaper, we report about our observational efforts to analyse thevariability and rotational evolution of young brown dwarfs and verylow-mass stars.
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