Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Finding benchmark brown dwarfs to probe the substellar initial mass function as a function of time
Using a simulated disc brown dwarf (BD) population, we find that newlarge area infrared surveys are expected to identify enough BDs coveringwide enough mass-age ranges to potentially measure the present day massfunction down to ~0.03Msolar, and the BD formation historyout to 10Gyr, at a level that will be capable of establishing if BDformation follows star formation. We suggest these capabilities are bestrealized by spectroscopic calibration of BD properties (Teff,g and [M/H]) which when combined with a measured luminosity and anevolutionary model can give BD mass and age relatively independent of BDatmosphere models. Such calibration requires an empirical understandingof how BD spectra are affected by variations in these properties, andthus the identification and study of `benchmark BDs' whose age andcomposition can be established independently.We identify the best sources of benchmark BDs as young open clustermembers, moving group members, and wide (>1000au) BD companions toboth subgiant stars and high-mass white dwarfs (WDs). To accuratelyasses the likely number of wide companion BDs available, we haveconstrained the wide L dwarf companion fraction using the 2-Micron AllSky Survey (2MASS), and find a companion fraction of2.7+0.7-0.5percent for separations of~1000-5000au. This equates to a BD companion fraction of34+9-6percent if one assumes an α~ 1companion mass function. Using this BD companion fraction, we simulatepopulations of wide BD binaries, and estimate that80+21-14 subgiant-BD binaries, and50+13-10 benchmark WD-BD binaries could beidentified using current and new facilities. The WD-BD binaries shouldall be identifiable using the Large Area Survey component of the UnitedKingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey, combinedwith the Sloan Digital Sky Survey. Discovery of the subgiant-BD binarieswill require a near-infrared imaging campaign around a large (~900)sample of Hipparcos subgiants. If identified, spectral studies of thesebenchmark BD populations could reveal the spectral sensitivities acrossthe Teff, g and [M/H] space probed by new surveys.
| 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.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| 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.
| Rotation and Chemical Abundances of Magnetic Ap/Bp Members of the Open Cluster NGC 6475
Membership in the open cluster NGC 6475 (age = 220 Myr) has beenconfirmed for the four magnetic Ap/Bp stars HD 162305, HD 162576, HD162725, and HD 320764. High-resolution spectra of the Ap stars, obtainedwithin the context of the POP project with the UVES spectrograph at theESO-VLT, were modelled in detail using LTE spectrum synthesis. Based ondirect comparison of the observed and calculated line profiles,projected rotational velocities and photospheric abundances of 11elements have been derived.
| 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.
| Lithium abundances of the local thin disc stars
Lithium abundances are presented for a sample of 181 nearby F and Gdwarfs with accurate Hipparcos parallaxes. The stars are on circularorbits about the Galactic centre and, hence, are identified as belongingto the thin disc. This sample is combined with two published surveys toprovide a catalogue of lithium abundances, metallicities ([Fe/H]),masses, and ages for 451 F-G dwarfs, almost all belonging to the thindisc. The lithium abundances are compared and contrasted with publishedlithium abundances for F and G stars in local open clusters. The fieldstars span a larger range in [Fe/H] than the clusters for which [Fe/H]~=0.0 +/- 0.2. The initial (i.e. interstellar) lithium abundance of thesolar neighbourhood, as derived from stars for which astration oflithium is believed to be unimportant, is traced from logɛ(Li) =2.2 at [Fe/H]=-1 to logɛ(Li) = 3.2 at +0.1. This form for theevolution is dependent on the assumption that astration of lithium isnegligible for the stars defining the relation. An argument is advancedthat this latter assumption may not be entirely correct, and, theevolution of lithium with [Fe/H] may be flatter than previouslysupposed. A sharp Hyades-like Li dip is not seen among the field starsand appears to be replaced by a large spread among lithium abundances ofstars more massive than the lower mass limit of the dip. Astration oflithium by stars of masses too low to participate in the Li dip isdiscussed. These stars show little to no spread in lithium abundance ata given [Fe/H] and mass.
| Eclipsing binaries in open clusters - I. V615 Per and V618 Per in h Persei
We derive absolute dimensions for two early-type main-sequence detachedeclipsing binaries in the young open cluster h Persei (NGC 869). V615Persei has a spectral type of B7 V and a period of 13.7 d. V618 Perseiis A2 V and has a period of 6.4 d. New ephemerides are calculated forboth systems. The masses of the component stars have been derived usinghigh-resolution spectroscopy and are 4.08 +/- 0.06 and 3.18 +/- 0.05Msolar for V615 Per and 2.33 +/- 0.03 and 1.56 +/- 0.02Msolar for V618 Per. The radii have been measured by fittingthe available light curves using EBOP and are 2.29 +/- 0.14 and 1.90 +/-0.09 Rsolar for V615 Per and 1.64 +/- 0.07 and 1.32 +/- 0.07Rsolar for V618 Per. By comparing the observed spectra ofV615 Per with synthetic spectra from model atmospheres we find that theeffective temperatures of the stars are 15000 +/- 500 K for the primaryand 11000 +/- 500 K for the secondary. The equatorial rotationalvelocities of the primary and secondary components of V615 Per are 28+/- 5 and 8 +/- 5 km s-1, respectively. Both components ofV618 Per rotate at 10 +/- 5 km s-1. The equatorial rotationalvelocities for synchronous rotation are about 10 km s-1 forall four stars. The time-scales for orbital circularization for bothsystems, and the time-scale for rotational synchronization of V615 Per,are much greater than the age of h Per. Their negligible eccentricitiesand equatorial rotational velocities therefore support the hypothesisthat they were formed by delayed break-up. We have compared the radii ofthese stars with models by the Granada and the Padova groups for starsof the same masses but different compositions. We conclude that themetallicity of the stars is Z~ 0.01. This appears to be the firstestimate of the bulk metallicity of h Per. Recent photometric studieshave assumed a solar metallicity so their results should be reviewed.
| XMM-Newton EPIC observations of stellar clusters and star forming regions
We report on observations of open clusters (OCs) and star formingregions (SFRs) obtained with the EPIC camera as part of the MissionScientist Guaranteed Time on XMM-Newton. These observations provide apowerful tool to investigate the evolution of coronal activity inlate-type convective stars and its dependence on magnetic fieldgeneration by dynamo processes. We discuss the motivations for thisprogram and present some results for the SFRs sigma Orionis (2-5Myr) and Taurus-Auriga (1-10 Myr) as well as for the OCs IC 2602(30 Myr), alpha Persei (50 Myr), Praesepe (600 Myr) andthe Hyades (600 Myr). We discuss imaging and spectral data providedby the EPIC MOS and PN detectors focussing on the determination of thecluster X-ray luminosity function and of the temperature structure,chemical abundances and time variability of cluster stars.Based on observations collected with the ESA mission XMM-Newton as partof the Mission Scientist (R. Pallavicini) Guaranteed Time
| Alkali-Activity Correlations in Open Clusters
We present a census of correlations between activity measures andneutral resonance lines of the alkali elements Li I and K I in openclusters and star-forming regions. The majority of very youngassociations and star formation regions show no evidence of Li-activitycorrelations, perhaps because their chromospheric activity indicatorshave a dominant origin in accretion processes with implied disk-clearingtimescales in the range of a few times 106 to~4×107 yr. Alkali-alkali and/or alkali-activitycorrelations are newly noted within IC 2391, M34, and perhaps Blanco 1and NGC 6475. Global X-ray luminosities are not as robust indicators astraditional optical indicators of alkali-activity correlations, nor areLi I-K I relations. Intracluster alkali-activity correlations are notglobal but are seen only within different intracluster subsamples,evincing rich behavior. Li- and K-activity correlations appear to gohand in hand, likely suggesting that at least some part of intraclusterLi variance is not due to real differential Li depletion. Although up to~90% of the star-to-star variance in Li I and K I within such asubsample can be related to that in optical chromospheric emission,significant Li dispersion above observational scatter may remain evenafter accounting for this. We suggest, for example, that at least threeindependent mechanisms (including a possible intracluster age spread)influence the distribution in the M34 Li-Teff plane. We arguethat Li-activity correlations are not illusory manifestations of aphysical Li-rotation connection. Although an unexpected correlationbetween Li, chromospheric emission, and the λ6455 Ca I feature incool M34 dwarfs indicates that the role of ``activity'' is played byspots/plages, we note that the alkali-activity correlations arequalitatively opposite in sign to other abundance anomalies beingrapidly delineated in active, young, cool stars.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| XMM-Newton observations of the young open cluster Blanco 1. I. X-ray spectroscopy and photometry
We present an X-ray study of the high metallicity young open clusterBlanco 1 based on XMM-Newton data. X-ray spectroscopy of cluster membersis presented for the first time as well as new X-ray distributionfunctions of late-type stars. We detected all known dF and dG stars inthe EPIC field and 80% and 90% of dK and dM stars, respectively. TheX-ray spectral analysis of the X-ray brightest cluster stars and X-raycolor analysis of a larger sample show that a model with twotemperatures (at about 0.3 and 1 keV) explains the quiescent activityphase spectra.We discuss also the nature of unidentified X-ray sources in the observedregion and their X-ray spectral properties.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.Appendix B is only available in electronic form athttp://www.edpsciences.org
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| Recent developments on studies of open clusters
Studies on open clusters are of great significance for astronomy andastrophysics, for which distinct developments have been made recently inboth observational and theoretical respects. In this paper, currentresearch of open cluster is reviewed, and membership determination,basic parameters determination, evolution of cluster and clustermembers, structure and dynamics of open cluster are also described indetail.
| The UVES Paranal Observatory Project: A Library of High- Resolution Spectra of Stars across the Hertzsprung-Russell Diagram
We present the UVES Paranal Observatory Project, consisting ofacquisition, reduction, and public release of high-resolution, largewavelength coverage, and high signal-to-noise ratio stellar spectraobtained with the UVES instrument of the ESO Very Large Telescope. Datafor about 400 stars across the Hertzsprung-Russell diagram have beentaken and reduced by members of the Paranal Science Operations Team andmade publicly available through a WEB interface at www.eso.org/uvespop.
| Brown Dwarfs in the Alpha Persei Cluster
We have obtained a new, deep, wide-field optical imaging survey of theyoung Alpha Persei cluster which reveals a well-populated lower mainsequence extending into the substellar mass regime. Subsequent infraredphotometry confirms that most of the candidate brown dwarfs are indeedlikely to be cluster members, with a predicted minimum mass of order0.035 solar masses. We have combined the new candidate list withprevious member catalogs to derive an IMF for Alpha Per; the slope ofthe IMF at the low mass end is α 0.6. The Alpha Per IMFslope is thus very similar to that found in the Pleiades.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| On the Coupling between Helium Settling and Rotation-induced Mixing in Stellar Radiative Zones. III. Applications to Light Elements in Population I Main-Sequence Stars
In the two previous papers of this series, we have discussed theimportance of the μ-gradients due to helium settling inrotation-induced mixing, first in an approximate analytical way, secondin a two-dimensional numerical simulation. We have found that, forslowly rotating low-mass stars, a process of ``creeping paralysis,'' inwhich the circulation and the diffusion are nearly frozen, may takeplace below the convective zone. Here we apply this theory to the caseof lithium and beryllium in Galactic clusters, especially the Hyades. Wetake into account the rotational braking with rotation velocitiesadjusted to the present observations. We find that two different cellsof meridional circulation appear on the hot side of the ``lithium dip''and that the creeping paralysis process occurs, not directly below theconvective zone, but deeper inside the radiative zone, at the top of thesecond cell. As a consequence, the two cells are disconnected, which maybe the basic reason for the lithium increase with effective temperatureon this side of the dip. On the cool side, there is just one cell ofcirculation, and the paralysis has not yet set in at the age of theHyades; the same modeling accounts nicely for the beryllium observationsas well as for the lithium ones.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Intrinsic spectral blueshifts in rapidly rotating stars?
Spectroscopic radial velocities for several nearby open clusters suggestthat spectra of (especially earlier-type) rapidly rotating stars aresystematically blueshifted by 3 km s-1 or more, relative tothe spectra of slowly rotating ones. Comparisons with astrometricallydetermined radial motions in the Hyades suggests this to be an absoluteblueshift, relative to wavelengths naively expected from stellar radialmotion and gravitational redshift. Analogous trends are seen also inmost other clusters studied (Pleiades, Coma Berenices, Praesepe, alphaPersei, IC 2391, NGC 6475, IC 4665, NGC 1976 and NGC 2516). Possiblemechanisms are discussed, including photospheric convection, stellarpulsation, meridional circulation, and shock-wave propagation, as wellas effects caused by template mismatch in determining wavelengthdisplacements. For early-type stars, a plausible mechanism is shock-wavepropagation upward through the photospheric line-forming regions. Suchwavelength shifts thus permit studies of certain types of stellaratmospheric dynamics and - irrespective of their cause - may influencededuced open-cluster membership (when selected from common velocity) anddeduced cluster dynamics (some types of stars might show fortuitousvelocity patterns).
| The evolution of lithium depletion in young open clusters: NGC 6475.
We have carried out a high resolution spectroscopic survey of the220-250 Myr old cluster NGC 6475: our main purpose isto investigate Li evolution during the early stages of the MainSequence. We have determined Li abundances for 33 late F to K-type X-rayselected cluster candidates, extending the samples already available inthe literature; for part of the stars we obtained radial and rotationalvelocities, allowing us to confirm the membership and to check forbinarity. We also estimated the cluster metallicity which turned out tobe over-solar ([Fe/H]=+0.14 +/- 0.06). Our Li analysis evidenced that(i) late F-type stars (Teff >≈ 6000 K) undergo a verysmall amount of Li depletion during the early phases on the ZAMS; (ii)G-type stars (6000 >≈ Teff >≈ 5500 K) instead dodeplete lithium soon after arrival on the ZAMS. Whereas this result isnot new, we show that the time scale for Li depletion in these stars isalmost constant between 100 and 600 Myr; (iii) we confirm that thespread observed in early K-type stars in younger clusters has convergedby 220 Myr. No constraints can be put on later-type stars. (iv) Finally,we investigate the effect of metallicity on Li depletion by comparingNGC 6475 with the similar age cluster M 34, but we show that the issueremains open, given the uncertain metallicity of the latter cluster. Byusing the combined NGC 6475+M 34 sample together with the Hyades and thePleiades, we compare quantitatively Li evolution from the ZAMS to 600Myr with theoretical predictions of standard models.Based on observations collected at the European Southern Observatory.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution
We compile a new sample of 89 open clusters with ages, distances andmetallicities available. We derive a radial iron gradient of about-0.099±0.008 dexkpc (unweighted) for the whole sample, which issomewhat greater than the most recent determination of oxygen gradientfrom nebulae and young stars. By dividing the clusters into age groups,we show that the iron gradient was steeper in the past and has evolvedslowly in time. Current data show a substantial scatter of the clustermetallicities indicating that the Galactic disk has undergone a veryrapid, inhomogeneous enrichment.Also, based on a simple, but quitesuccessful model of chemical evolution of the Milky Way disk, we make adetailed calculation of the iron abundance gradient and its timeevolution. The predicted current iron gradient is about -0.072 dexkpc.The model also predicts a steady flattening of the iron gradient withtime, which agrees with the result from our open cluster sample.
| Rotation and Activity in the Solar-Metallicity Open Cluster NGC 2516
We report new measures of radial velocities and rotation rates (vsini)for 51 F and early G stars in the open cluster NGC 2516 and combinethese with previously published data. From high signal-to-noise ratiospectra of two stars, we show that NGC 2516 has a relative ironabundance with respect to the Pleiades of Δ[Fe/H]=+0.04+/-0.07 atthe canonical reddening of E(B-V)=0.12, in contrast to previousphotometric studies that placed the cluster 0.2-0.4 dex below solar. Weconstruct a color-magnitude diagram based on radial velocity members andexplore the sensitivity of photometric determinations of the metallicityand distance to assumed values of the reddening. For a metal abundancenear solar, the Hipparcos distance to NGC 2516 is probablyunderestimated. Finally, we show that the distribution of rotation ratesand X-ray emission does not differ greatly from that of the Pleiades,when allowance is made for the somewhat older age of NGC 2516. Based onobservations obtained at the Anglo-Australian Telescope and onobservations obtained at the Cerro Tololo Inter-American Observatory,NOAO, which is operated by the Associated Universities for Research inAstronomy (AURA), Inc., under cooperative agreement with the NationalScience Foundation.
| Detailed Analysis of Nearby Bulgelike Dwarf Stars. II. Lithium Abundances
Li abundances are derived for a sample of bulgelike stars withisochronal ages of 10-11 Gyr. These stars have orbits with pericentricdistances, Rp, as small as 2-3 kpc and Zmax<1kpc. The sample comprises G and K dwarf stars in the metallicity range-0.80<=[Fe/H]<=+0.40. Few data on Li abundances in old turnoffstars (>=4.5 Gyr) within the present metallicity range are available.M67 (4.7 Gyr) and NGC 188 (6 Gyr) are the oldest studied metal-rich openclusters with late-type stars. Li abundances have also been studied fora few samples of old metal-rich field stars. In the present work, a highdispersion in Li abundances is found for bulgelike stars with allmetallicity ranges, comparable with values in M67. The role ofmetallicity and age on a Li depletion pattern is discussed. The possibleconnection between Li depletion and oxygen abundance due to atmosphericopacity effects is investigated.
| New Planetary Nebulae towards the galactic bulge. II. Objects surrounding the central area
In the second part of the objective-prism survey of PNe towards thegalactic centre we present the discovery of further 44 new planetarynebulae found in six ESO fields surrounding the central area and give arough description of these objects. Besides, we give coordinates of manyknown objects in the area of the survey. The discussion of the angulardiameters leads to the belief that the majority of our PNe is veryprobably located in or close to the galactic bulge.
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| ROSAT PSPC/HRI observations of the open cluster NGC 2422
We present the results of a ROSAT study of NGC 2422, a southern opencluster at a distance of about 470 pc, with an age close to thePleiades. Source detection was performed on two observations, a 10-ksPSPC and a 40-ks HRI pointing, with a detection algorithm based onwavelet transforms, particularly suited to detecting faint sources incrowded fields. We have detected 78 sources, 13 of which were detectedonly with the HRI, and 37 detected only with the PSPC. For each source,we have computed the 0.2-2.0 keV X-ray flux. Using optical data from theliterature and our own low-dispersion spectroscopic observations, wefind candidate optical counterparts for 62 X-ray sources, with more than80% of these counterparts being late type stars. We have assigned to theoptical sources an astrometric and/or a photometric membership flag,depending on which data are available to us. The number of sources (38of 62) with high membership probability counterparts is consistent withthat expected for Galactic plane observations at our sensitivity. Wehave computed maximum likelihood X-ray luminosity functions (XLF) for Fand early-G type stars with high membership probability. Heavy datacensoring due to our limited sensitivity permits determination of onlythe high-luminosity tails of the XLFs; the distributions areindistinguishable from those of the nearly coeval Pleiades cluster.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
NEWS@SKY (Science&Space News)
