Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
The Infrared Ca II Triplet as Metallicity Indicator
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
| Searching for Earth Analogs Around the Nearest Stars: The Disk Age-Metallicity Relation and the Age Distribution in the Solar Neighborhood
The chemical composition of Earth's atmosphere has undergone substantialevolution over the course of its history. It is possible, even likely,that terrestrial planets in other planetary systems have undergonesimilar changes; consequently, the age distribution of nearby stars isan important consideration in designing surveys for Earth analogs.Valenti & Fischer provide age and metallicity estimates for 1039 FGKdwarfs in the solar neighborhood. Using the Hipparcos catalog as areference to calibrate potential biases, we have extractedvolume-limited samples of nearby stars from the Valenti-Fischer dataset. Unlike other recent investigations, our analysis shows clearevidence for an age-metallicity relation in the local disk, albeit withsubstantial dispersion at any epoch. The mean metallicity increases from~-0.3 dex at a look-back time of ~10 Gyr to ~+0.15 dex at the presentday. Supplementing the Valenti-Fischer measurements with literaturemetallicity data to give a complete volume-limited sample, the agedistribution of nearby FGK dwarfs is broadly consistent with a uniformstar-formation rate over the history of the Galactic disk. In strikingcontrast, most stars known to have (gas giant) planetary companions areyounger than 5 Gyr; however, stars with planetary companions within 0.4AU have a significantly flatter age distribution, indicating that thosesystems are stable on timescales of many gigayears. Several of theolder, lower metallicity host stars have enhanced [α/Fe] ratios,implying membership of the thick disk. If the frequency of terrestrialplanets is also correlated with stellar metallicity, then the median ageof such planetary system is likely to be ~3 Gyr. We discuss theimplications of this hypothesis in designing searches for Earth analogsamong the nearby stars.
| Cassini States with Dissipation: Why Obliquity Tides Cannot Inflate Hot Jupiters
Some short-period exoplanets (``hot Jupiters'') are observed by theirtransits to have anomalously large radii. It has been suggested thatthese planets are in a resonance involving persistent misalignment andsynchronous precession of their spin and orbital angular momenta (aCassini state) and that the attendant tidal heating inflates the planet.We argue against this. Using explicit tidal integrations, we show thatalthough an oblique Cassini state can dissipate many times the planet'srotational energy, the rate of dissipation must be much less thanhypothesized. Dissipation causes the planetary spin to lie at an angleto the plane containing the orbital and total angular momenta. Ifdissipation is too rapid, this angle becomes so large that Cassiniequilibrium is lost. A separate consideration limits the total energythat can be extracted from the orbit. The source of the torque on theorbit, either an oblique parent star or an inclined third body, alignswith the orbit as it absorbs the angular momentum shed by the planet.Alignment removes the orbital precession required by the Cassini state.In combination with observational bounds on the mass and semimajor axisof a possible second planet and with bounds on the stellar rotation andobliquity, these constraints make it very unlikely that obliquity tidescan be the explanation for inflated hot Jupiters, especially HD 209458b.
| Characterization of open cluster remnants
Context: Despite progress in the theoretical knowledge of open clusterremnants and the growing search for observational identifications inrecent years, open questions still remain. The methods used to analyzeopen cluster remnants and criteria to define them as physical systemsare not homogeneous. In this work we present a systematic method forstudying these objects that provides a view of their properties andallows their characterization. Aims: Eighteen remnant candidates areanalyzed by means of photometric and proper motion data. These dataprovide information on objects and their fields. We establish criteriafor characterizing open cluster remnants, taking observationaluncertainties into account. Methods: 2MASS J and H photometry isemployed (i) to study structural properties of the objects by means ofradial stellar density profiles, (ii) to test for any similarity betweenobjects and fields with a statistical comparison method applied to thedistributions of stars in the CMDs, and (iii) to obtain ages, reddeningvalues, and distances from the CMD, taking an index of isochrone fitinto account. The UCAC2 proper motions allowed an objective comparisonbetween objects and large solid angle offset fields. Results: Theobjective analysis based on the present methods indicates 13open-cluster remnants in the sample. Evidence of the presence of binarystars is found, as expected for dynamically evolved systems. Finally, weinfer possible evolutionary stages among remnants from the structure,proper motion, and CMD distributions. The low stellar statistics forindividual objects is overcome by means of the construction of compositeproper motion and CMD diagrams. The distributions of remnants in thecomposite diagrams resemble the single-star and unresolved binary stardistributions of open clusters.
| Blue Straggler Stars in Galactic Open Clusters and the Simple Stellar Population Model
The presence of blue straggler stars (BSs) as secure members of Galacticopen clusters (OCs) poses a major challenge to the conventional pictureof simple stellar population (SSP) models. These are based on thestellar evolution theory of single stars, whereas the major formationmechanisms of BSs are all correlated with stellar interactions. We haveillustrated this in a previous study based on a small sample of old (age>=1 Gyr) Galactic OCs. However, for the purpose of demonstrating thecontributions of BSs to the conventional SSP models statistically andsystematically, a large database with sufficient coverage of age andmetallicity is definitely needed. The working sample now includes 100Galactic OCs with ages ranging from 0.1 to 10 Gyr. The contributions ofBSs to the integrated light of their host clusters are calculated on anindividual cluster basis. The general existence of BSs in our starcluster sample dramatically alters the predictions of conventional SSPmodels in terms of their integrated properties. Neglecting theconsequences of nonstandard evolutionary products, such as BSs, instellar populations, very large uncertainties can be made in analyzingtheir integrated spectral energy distributions at unresolvableconditions. The current work strongly suggests that when evolutionarypopulation synthesis technique is used to study the properties ofunresolved stellar populations in galaxies, the contributions of BSsshould be taken into account.
| A survey of open clusters in the u'g'r'i'z' filter system. III. Results for the cluster NGC 188.
We continue our series of papers describing the results of a photometricsurvey of open star clusters, primarily in the southern hemisphere,taken in the u'g'r'i'z' filter system. The entire observed samplecovered more than 100 clusters, but here we present data only on NGC188, which is one of the oldest open clusters known in the Milky Way. Wefit the Padova theoretical isochrones to our data. Assuming a solarmetallicity for NGC 188, we find a distance of 1700+/-100 pc, an age of7.5+/-0.7 Gyr, and a reddening E(B-V) of 0.025+/-0.005. This yields adistance modulus of 11.23+/-0.14.
| Galactic Interstellar Gas Cloud Mass Functions: A Simple Quantitative Approach
We present here a simple approach to understanding the gas cloud massdistribution function by simulating formation and destruction of gasclouds and gas clumps in the ISM. We include as relevant processescoagulation to form bigger clouds, as well as disruption by collisionsand the removal of gas by collapse to form stars. We evolve initial setsof preexisting gas clumps with a range of initial distribution functions(flat, Gaussian, fractal) for their physical parameters and withdifferent geometrical forms (spherical or elongated) for the individualclouds, and constrain them within an imaginary box representinggravitational bounding, applying the kinematic laws of nonelasticcollisions. The results agree well with observations of the massdistribution function of Galactic giant gas clouds if we choose aGaussian for the initial distribution function, and initial gas cloudswhich are quasi-spherical.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| K-band magnitude of the red clump as a distance indicator
We have investigated how the K-band magnitude of the red clump [M_K(RC)]depends on age and metallicity, using 2MASS infrared data for a sampleof 24 open clusters with known distances. We show that a constant valueof M_K(RC)=-1.57 ± 0.05 is a reasonable assumption to use indistance determinations for clusters with metallicity between -0.5 and+0.4 dex and age between 108.5 and 109.9 years.Figures 8 and 9 are available in electronic form at http://www.aanda.org
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| Faint open clusters with 2MASS: BH 63, Lyngå 2, Lyngå 12 and King 20
Context: .Structural and dynamical parameters of faint open clusters areprobed with quality 2MASS-photometry and analytical procedures developedfor bright clusters. Aims: .We derive fundamental parameters ofthe faint open clusters Lyngå 2, BH 63, Lyngå 12 and King20, the last three of which have no prior determinations. We also focuson the structure and dynamical state of these clusters. Methods:.J, H and Ks 2MASS photometry with errors smaller than 0.2mag are used to build CMDs, radial density profiles, colour-colourdiagrams, luminosity and mass functions. Colour-magnitude filters areused to isolate probable member stars. Field-star decontamination isapplied to Lyngå 2, Lyngå 12 and King 20. Results:.Reddening values are in the range 0.22≤E(B-V)≤1.9, with BH 63 themost reddened object. Ages of Lyngå 2, King 20, Lyngå 12 andBH 63 are ≈90, ≈200, ≈560 and ≈700 Myr, respectively. Theradial density distributions of Lyngå 12 and King 20 arewell-represented by King profiles. Lyngå 2 and BH 63 are verysmall with core and limiting radii of ≈0.12 pc and ≈1.5 pc. Yet,they fit in the small-radii tail of the open cluster size distribution.Lyngå 12 and King 20 have R_core≈0.43 pc and R_lim≈3.9 pc.Lyngå 2 and Lyngå 12 are inside the Solar circle. Totalstellar masses (extrapolating the MFs to stars with 0.08 M_ȯ) rangefrom ≈340 M_ȯ (BH 63) to ≈2300 M_ȯ (Lyngå 12).Observed masses are ~1/4 of these values. In all clusters the core massfunction is flatter than the halo's. Conclusions: .Faint openclusters can be probed with 2MASS when associated with colour-magnitudefilters and field-star decontamination. BH 63 appears to be in anadvanced dynamical state, both in the core and halo. To a lesser degreethe same applies to King 20. Marginal evidence of dynamical evolution ispresent in the cores of Lyngå 2 and Lyngå 12.
| Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960
Aims.We derive accurate parameters related to the CMD, structure anddynamical state of M 52 and NGC 3960, whose fields are affected bydifferential reddening. Previous works estimated their ages in theranges 35-135 Myr and 0.5-1.0 Gyr, respectively. Methods: .J, Hand Ks 2MASS photometry with errors <0.2 mag is used tobuild CMDs, radial density profiles, luminosity and mass functions, andcorrect for differential reddening. Field-star decontamination isapplied to uncover the cluster's intrinsic CMD morphology, andcolour-magnitude filters are used to isolate stars with high probabilityof being cluster members. Results: .The differential-reddeningcorrected radial density profile of M 52 follows King's law with coreand limiting radii of R_core =0.91 ± 0.14 pc and R_lim =8.0± 1.0 pc. NGC 3960 presents an excess of the stellar density overKing's profile (R_core = 0.62 ± 0.11 pc and R_lim =6.0 ±0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 areR_tidal=13.1 ± 2.2 pc and R_tidal=10.7 ± 3.7 pc. Clusterages of M 52 and NGC 3960 derived with Padova isochrones are constrainedto 60 ± 10 Myr and 1.1 ± 0.1 Gyr. In M 52 the core MF(χ_core=0.89 ± 0.12) is flatter than the halo's(χ_halo=1.65 ± 0.12). In NGC 3960 they are χ_core=-0.74± 0.35 and χ_halo=1.26 ± 0.26. The mass locked up inMS/evolved stars in M 52 is ~1200 M_ȯ, and the total mass(extrapolated to 0.08M_ȯ) is ~3800 M_ȯ. The total mass in NGC3960 is ~1300 M_ȯ. Conclusions: .Compared to open clusters indifferent dynamical states studied with similar methods, the core andoverall parameters of M 52 are consistent with an open cluster moremassive than 1000 M_ȯ and ~60 Myr old, with some mass segregationin the inner region. The core of NGC 3960 is in an advanced dynamicalstate with strong mass segregation in the core/halo region, while thesomewhat flat overall MF (χ≈ 1.07) suggests low-mass starevaporation. The excess stellar density in the core may suggestpost-core collapse. The dynamical evolution of NGC 3960 may have beenaccelerated by the tidal Galactic field, since it lies ≈0.5 kpcinside the Solar circle.
| 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.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| Detection of K_s-excess stars in the 14 Myr open cluster NGC 4755
Aims.We derive the structure, distribution of MS and PMS stars anddynamical state of the young open cluster NGC 4755. We explore thepossibility that, at the cluster age, some MS and PMS stars stillpresent infrared excesses related to dust envelopes and proto-planetarydiscs. Methods: .J, H and Ks 2MASS photometry is usedto build CMD and colour-colour diagrams, radial density profiles,luminosity and mass functions. Field-star decontamination is applied touncover the cluster's intrinsic CMD morphology and detect candidate PMSstars. Proper motions from UCAC2 are used to determine clustermembership. Results: .The radial density profile follows King'slaw with a core radius Rcore=0.7 ± 0.1 pc and alimiting radius Rlim=6.9 ± 0.1 pc. The cluster agederived from Padova isochrones is 14 ± 2 Myr. Field-stardecontamination reveals a low-MS limit at ≈1.4 M_ȯ. The core MF(χ=0.94 ± 0.16) is flatter than the halo's (χ=1.58± 0.11). NGC 4755 contains 285 candidate PMS stars of age 1{-}15 Myr, and a few evolved stars. The mass locked up in PMS, MS andevolved stars amounts to 1150 M_ȯ. Proper motions show thatK_s-excess MS and PMS stars are cluster members. K_s-excess fractions inPMS and MS stars are 5.4 ± 2.1% and 3.9 ± 1.5%respectively, consistent with the cluster age. The core is deficient inPMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halobut they are scarce in the core. Conclusions: .Compared to openclusters in different dynamical states studied with similar methods, NGC4755 fits relations involving structural and dynamical parameters in theexpected locus for its age and mass. On the other hand, the flatter coreMF probably originates from primordial processes related to parentmolecular cloud fragmentation and mass segregation over 14 Myr. Starformation in NGC 4755 began ≈14 Myr ago and proceeded for about thesame length of time. Detection of K_s-excess emission in member MS starssuggests that some circumstellar dust discs survived for 10^7 yr,occurring both in some MS and PMS stars for the age and spread observedin NGC 4755.
| Astrometry with Optic at WIYN
The astrometric precision of Orthogonal Transfer CCDs (OTCCDs) isinvestigated, using the OPTIC camera at WIYN 3.5-m telescope. Theprecision of the positions, determined with the Yale image-centeringcode, was used to establish astrometric design requirements for theanticipated WIYN One Degree Imager camera that will use similartechnology.OTCCDs are able to electronically compensate for real-time image motion,providing tip/tilt corrections without additional optics or moving parts(Tonry et al. 1997, PASP, 109, 1154). The Orthogonal Parallel TransferImaging Camera (OPTIC) consists of two 2K by 4K OTCCDs arranged in asingle dewar mounted adjacent to each other with a small gap between thechips. Each chip is electronically divided into two regions, and each ofthese is further subdivided into two parts: a guide region and a scienceregion. When mounted on WIYN, the camera has a scale of 0.14arcsec/pixel, and a 10 × 10 arcmin field of view. OPTIC has a readnoise of < 4 electrons when read at 160 kpix/sec and a nominal gainof 1.45 e^-/ADU.To test the astrometric performance of OPTIC, we imaged the open clusterNGC 188, during 7 nights in october 2003, using Johnson-V and Gunn-ifilters ranged from 0.6 arcsec to 3 arcsec, but only frames with < 2arsec were considered for this study. After a careful internalastrometric reduction, we estimate that OPTIC achieves a precision of 2milliarseconds over the dynamical range of the CCDs. This precision isobviously affected by the stability of the atmosphere, so these resultsdemonstrate that the unique charge tranfer donde by OPTIC, which tracksthe motion of a guide star, is able to stabilize the field within theatmospheric isokinetic region to the level of a few milli-arcseconds.
| BVI photometry of the very old open cluster Berkeley 17*
We have obtained BVI CCD imaging of Berkeley 17, an anticentre opencluster that competes with NGC 6791 as the oldest known open cluster.Using the synthetic colour-magnitude diagrams (CMDs) technique withthree sets of evolutionary tracks, we have determined that its age is8.5-9.0 Gyr, it distance modulus is (m - M)0 = 12.2, with areddening of E(B - V) = 0.62-0.60. Differential reddening, if present,is at the 5 per cent level. All these values have been obtained usingmodels with metallicity about half of solar (Z = 0.008 or 0.01 dependingon the stellar evolution tracks), which allows us to reproduce thefeatures of the cluster CMD better than other metallicities. Finally,from the analysis of a nearby comparison field, we think to haveintercepted a portion of the disrupting Canis Major dwarf galaxy.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| δ Scuti-type nature of the high-amplitude variable star GSC4619-450
We present the results of spectroscopy and B, V CCD photometry carriedout from 2003 to 2005 of GSC4619-450, a high-amplitude pulsatingvariable discovered by Zhang et al., [Zhang, X.B., Deng, L., Zhou, X.,Xin, Y., 2004. MNRAS, 355, 1369]. From this study, the star isclassified as a high-amplitude δ Scuti star. A Fourier analysisbased on the B and V photometric data indicates that GSC 4619-450 is amono-periodic radial pulsator, with a period of 0.13341195 days. From ashort time line we find that the pulsating period of this star isincreasing rapidly with a rate of 9.1 × 10‑2s/yr, and is thus suggested to be highly evolved and undergoing rapidevolving at the present time. A spectral type of F0 is assigned to thevariable considering the results from both the spectroscopy andphotometry.
| The old anticentre open cluster Berkeley 32: membership and fundamental parameters*
We have obtained medium- and low-resolution spectroscopy and BVI CCDimaging of Berkeley 32, an old open cluster which lies in the anticentredirection. From the radial velocities of 48 stars in the clusterdirection, we found that 31 of them, in crucial evolutionary phases, areprobable cluster members, with an average radial velocity of +106.7(σ= 8.5) km s-1.From isochrone fitting to the colour-magnitude diagrams of Berkeley 32,we have obtained an age of 6.3 Gyr, (m-M)0= 12.48 and E(B-V)= 0.10. The best fit is obtained with Z= 0.008. A consistent distance,(m-M)0~= 12.6 +/- 0.1, has been derived from the meanmagnitude of red clump stars with confirmed membership; we may assume(m-M)0~= 12.55 +/- 0.1.The colour-magnitude diagram of the nearby field observed to check forfield stars contamination looks intriguingly similar to that of theCanis Major overdensity.
| The Metallicity of the Old Open Cluster NGC 6791
We have observed four red clump stars in the very old, metal-rich opencluster NGC 6791 to derive its metallicity using the high-resolutionspectrograph SARG mounted on the Galileo National Telescope (TNG). Usinga spectrum synthesis technique, we obtain an average value of[Fe/H]=+0.47 (+/-0.04, rms=0.08) dex. Our method was tested on μ Leo,a well-studied, metal-rich field giant. We also derive average oxygenand carbon abundances for NGC 6791 from synthesis of [O I] at 6300Å and C2 at 5086 Å, finding [O/Fe]~=-0.3 and[C/Fe]~=-0.2.Based on observations made with the Italian Telescopio Nazionale Galileo(TNG) operated on the island of La Palma by the Fundación GalileoGalilei of the INAF (Istituto Nazionale di Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
| 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.
| Element abundances of unevolved stars in the open cluster M 67
Context: .The star-to-star scatter in lithium abundances observed amongotherwise similar stars in the solar-age open cluster M 67 is one of themost puzzling results in the context of the so called "lithium problem".Among other explanations, the hypothesis has been proposed that thedispersion in Li is due to star-to-star differences in Fe or otherelement abundances which are predicted to affect Li depletion. Aims: .The primary goal of this study is the determination of themetallicity ([Fe/H]), α- and Fe-peak abundances in a sample ofLi-poor and Li-rich stars belonging to M 67, in order to test thishypothesis. By comparison with previous studies, the presentinvestigation also allows us to check for intrinsic differences in theabundances of evolved and unevolved cluster stars and to draw moresecure conclusions on the abundance pattern of this cluster.Methods: .We have carried out an analysis of high resolution UVES/VLTspectra of eight unevolved and two slightly evolved cluster membersusing MOOG and measured equivalent widths. For all the stars we havedetermined [Fe/H] and element abundances for O, Na, Mg, Al, Si, Ca, Ti,Cr and Ni. Results: .We find an average metallicity [Fe/H] =0.03±0.01, in very good agreement with previous determinations.All the [ X/Fe] abundance ratios are very close to solar. Thestar-to-star scatter in [Fe/H] and [ X/Fe] ratios for all elements,including oxygen, is lower than 0.05 dex, implying that the largedispersion in lithium among cluster stars is not due to differences inthese element abundances. We also find that, when using a homogeneousscale, the abundance pattern of unevolved stars in our sample is verysimilar to that of evolved stars, suggesting that, at least in thiscluster, RGB and clump stars have not undergone any chemical processing.Finally, our results show that M 67 has a chemical composition that isrepresentative of the solar neighborhood.
| Extended Strömgren Photoelectric Photometry in NGC 752
Photoelectric photometry on the extended Strömgren system (uvbyCa)is presented for 7 giants and 21 main-sequence stars in the old opencluster NGC 752. Analysis of the hk data for the turnoff stars yields anew determination of the cluster mean metallicity. From 10 single-starmembers, [Fe/H]=-0.06+/-0.03, where the error quoted is the standarderror of the mean and the Hyades abundance is set at [Fe/H]=+0.12. Thisresult is unchanged if all 20 stars within the limits of the hkmetallicity calibration are included. The derived [Fe/H] is in excellentagreement with past estimates, using properly zeroed m1 data,transformed moderate-dispersion spectroscopy, and recent high-dispersionspectroscopy.
| UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley17. Color-magnitude diagrams (CMDs) are constructed and reach well pastthe main-sequence turnoff for both clusters. Cluster ages are determinedby means of isochrone fitting to the cluster CMDs. These fits areconstrained to agree with spectroscopic metallicity and reddeningestimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188and 10.0+/-1.0 Gyr for Berkeley 17, where the errors refer touncertainties in the relative age determinations. These ages arecompared to the ages of relatively metal-rich inner halo/thick-diskglobular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyryounger than the thick-disk globular clusters. These results confirm thestatus of Berkeley 17 as one of the oldest known open clusters in theMilky Way, and its age provides a lower limit to the age of the Galacticdisk.
| The N2K Consortium. V. Identifying Very Metal-rich Stars with Low-Resolution Spectra: Finding Planet-Search Targets
We present empirical calibrations that provide estimates of stellarmetallicity, effective temperature, and surface gravity as a function ofLick IDS indices. These calibrations have been derived from a trainingset of 261 stars for which (1) high-precision measurements of [Fe/H],Teff, and logg have been made using spectral-synthesisanalysis of HIRES spectra, and (2) Lick indices have also been measured.Estimation of atmospheric parameters with low-resolution spectroscopyrather than photometry has the advantage of producing a highly accuratemetallicity calibration, and requires only one observation per star. Ourcalibrations have identified a number of bright (V<9) metal-richstars that are now being screened for hot-Jupiter-type planets. Usingthe Yonsei-Yale stellar models, we show that the calibrations providedistance estimates accurate to ~20% for nearby stars. We have alsoinvestigated the possibility of constructing a ``planeticity''calibration to predict the presence of planets based on stellarabundance ratios but find no evidence that a convincing relation of thistype can be established. High metallicity remains the best singleindicator that a given star is likely to harbor extrasolar planets.
| Hot Populations in M87 Globular Clusters
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.
| Astrometry with OPTIC at WIYN
The astrometric precision of Orthogonal Transfer CCDs (OTCCDs) isinvestigated by using OPTIC camera at the WIYN 3.5-m telescope. Theprecision of the positions, determined by the Yale image-centering code,will be used to establish astrometric design requirements for theanticipated WIYN One Degree Imager Camera that will use similartechnology.
| Galactic Orbits of Globular Clusters in a Barred Galaxy
We study the effect of a bar in the galactic orbits of forty-fiveglobular clusters whose absolute proper motions are known. The orbitalcharacteristics of the orbits are compared with those obtained for thecase of an axisymmetric galactic potential. Tidal radii are computed anddiscussed for both cases.
| Deep Astrometric Standards and Galactic Structure
The advent of next-generation imaging telescopes, such as the LargeSynoptic Survey Telescope (LSST) and the Panoramic Survey Telescope andRapid Response System (Pan-STARRS), has revitalized the need for deepand precise reference frames. The proposed weak-lensing observationswith these facilities put the highest demands on image quality over wideangles on the sky. It is particularly difficult to achieve asubarcsecond point-spread function on stacked images, where preciseastrometry plays a key role. Current astrometric standards areinsufficient to achieve the science goals of these facilities. We thuspropose the establishment of a few selected deep (V=25) astrometricstandards (DAS). These will enable a reliable geometric calibration ofsolid-state mosaic detectors in the focal plane of large ground-basedtelescopes, and will make a substantial contribution to ourunderstanding of stellar populations in the Milky Way. In this paper weexamine the need for such standards and discuss the strategy forselecting them and their acquisition and reduction techniques. Thefeasibility of DAS is demonstrated by a pilot study around the opencluster NGC 188, using the Kitt Peak National Observatory 4 m CCD Mosaiccamera, and by Subaru Suprime-Cam observations. The goal of reaching anaccuracy of 5-10 mas in positions and obtaining absolute proper motionsgood to 2 mas yr-1 over a several square-degree area ischallenging, but reachable with the NOAO 4 m telescopes and CCD mosaicimagers, or a similar setup. Our proposed DAS aims to establish fourfields near the Galactic plane, at widely separated coordinates. Inaddition to their utilitarian purpose for DAS, the data we will obtainin these fields will enable fundamental Galactic science in their ownright. The positions, proper motions, and VI photometry of faint starswill address outstanding questions of Galactic disk formation andevolution, stellar buildup, and mass assembly via merger events.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
Katalógusok és elnevezések:
|
