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|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.
|Variable stars in the open cluster NGC 6791 and its surrounding field|
Aims.This work presents a high-precision variability survey in the fieldof the old, super metal-rich open cluster NGC 6791. Methods: The datasample consists of more than 75 000 high-precision CCD time seriesmeasurements in the V band obtained mainly at the Canada-France-HawaiiTelescope, with additional data from S. Pedro Mártir and Loianoobservatories, over a time span of ten nights. The field covers an areaof 42 × 28 arcmin^2. Results: We have discovered 260 newvariables and re-determined periods and amplitudes of 70 known variablestars. By means of a photometric evaluation of the membership in NGC6791, and a preliminary membership based on the proper motions, we givea full description of the variable content of the cluster andsurrounding field in the range 16⪉ V< 23.5. Accurate periods canbe given for the variables with P⪉ 4.0 d, while for ones with longerperiods the limited time-baseline hampered precise determinations. Wecategorized the entire sample as follows: 6 pulsating, 3 irregular, 3cataclysmic, 89 rotational variables and 61 eclipsing systems; moreover,we detected 168 candidate variables for which we cannot give avariability class since their periods are much longer than our timebaseline. Conclusions: On the basis of photometric considerations, andof the positions of the stars with respect to the center of the cluster,we inferred that 11 new variable stars are likely members of thecluster, for 22 stars the membership is doubtful and 137 are likelynon-members. We also detected an outburst of about 3 mag in the lightcurve of a very faint blue star belonging to the cluster and we suggestthat this star could be a new U Gem (dwarf nova) cataclysmic variable.Appendix A is only available in electronic form at http://www.aanda.org
|A new search for planet transits in NGC 6791|
Context: Searching for planets in open clusters allows us to study theeffects of dynamical environment on planet formation and evolution. Aims: Considering the strong dependence of planet frequency on stellarmetallicity, we studied the metal rich old open cluster NGC6791 and searched for close-in planets using the transittechnique. Methods: A ten-night observational campaign was performedusing the Canada-France-Hawaii Telescope (3.6 m), the San PedroMártir telescope (2.1 m), and the Loiano telescope (1.5 m). Toincrease the transit detection probability we also made use of theBruntt et al. (2003, A&A, 410, 323) eight-nights observationalcampaign. Adequate photometric precision for the detection of planetarytransits was achieved. Results: Should the frequency and properties ofclose-in planets in NGC 6791 be similar to thoseorbiting field stars of similar metallicity, then detailed simulationsforesee the presence of 2-3 transiting planets. Instead, we do notconfirm the transit candidates proposed by Bruntt et al. (2003, A&A,410, 323). The probability that the null detection is simply due tochance coincidence is estimated to be 3%-10%, depending on themetallicity assumed for the cluster. Conclusions: Possible explanationsof the null-detection of transits include: (i) a lower frequency ofclose-in planets in star clusters; (ii) a smaller planetary radius forplanets orbiting super metal rich stars; or (iii) limitations in thebasic assumptions. More extensive photometry with 3-4 m class telescopesis required to allow conclusive inferences about the frequency ofplanets in NGC 6791.Based on observation obtained at theCanada-France-Hawaii Telescope (CFHT) which isoperated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the CentreNational de la Recherche Scientifique of France, and theUniversity of Hawaii and on observations obtained at San PedroMártir 2.1 m telescope (Mexico),and Loiano 1.5 m telescope (Italy).
|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.
|Testing the universal stellar IMF on the metallicity distribution in the bulges of the Milky Way and M 31|
Aims.We test whether the universal initial mass function (UIMF) or theintegrated galaxial IMF (IGIMF) can be employed to explain themetallicity distribution (MD) of giants in the Galactic bulge. Methods:We make use of a single-zone chemical evolution model developed for theMilky Way bulge in the context of an inside-out model for the formationof the Galaxy. We checked whether it is possible to constrain the yieldsabove 80 Mȯ by forcing the UIMF and required that theresulting MD matches the observed ones. We also extended the analysis tothe bulge of M 31 to investigate a possible variation of the IMF amonggalactic bulges. Several parameters that have an impact on stellarevolution (star-formation efficiency, gas infall timescale) are varied. Results: We show that it is not possible to satisfactorily reproduce theobserved metallicity distribution in the two galactic bulges unlessassuming a flatter IMF (x ≤ 1.1) than the universal one. Conclusions: We conlude that it is necessary to assume a variation inthe IMF among the various environments.
|Whiting 1: the youngest globular cluster associated with the Sagittarius dwarf spheroidal galaxy|
Context: Recently, Carraro (2005) drew attention to the remarkable starcluster Whiting 1 by showing that it lies about 40 kpc from the Sun andis therefore unquestionably a member of the Galactic halo (b=-60.6 deg).Its Color Magnitude Diagram (CMD) indicated that Whiting 1 is very young(~5 Gyr) for a globular cluster. It is very likely that Whiting 1originated in a dwarf galaxy that has since been disrupted by the MilkyWay. Aims: The main goals of this investigation were to constrainbetter the age, metallicity, and distance of Whiting 1 and to assesswhether it belongs to a stellar stream from the Sagittarius dwarfspheroidal galaxy (Sgr dSph). Methods: Deep CCD photometry in the BVIpass-bands obtained with the VLT is used to improve the quality of theCMD and to determine the cluster's luminosity function and surfacedensity profile. High-resolution spectrograms obtained with Magellan areused to measure the cluster's radial velocity and to place limits on itspossible metallicity. The measurements of distance and radial velocityare used to test the cluster's membership in the stellar streams fromthe Sgr dSph. Results: From our CMD of Whiting 1, we derive newestimates for the cluster's age (6.5+1.0-0.5 Gyr),metallicity (Z=0.004±0.001, [Fe/H] = -0.65), and distance(29.4+1.8-2.0 kpc). From echelle spectrograms ofthree stars, we obtain -130.6±1.8 km s-1 for thecluster's radial velocity and show from measurements of two infra-redCaII lines that the [Fe/H] of the cluster probably lies in the range-1.1 to -0.4. Both the luminosity function and the surface densityprofile suggest that the cluster has undergone tidal stripping by theMilky Way. We demonstrate that the position of Whiting 1 on the sky, itsdistance from the Sun, and its radial velocity are identical to withinthe errors of both the theoretical predictions of the trailing stream ofstars from the Sgr dSph galaxy and the previous observations of the Mgiant stars that delineate the streams. Conclusions: With the additionof Whiting 1, there is now strong evidence that 6 globular clustersformed within the Sgr dSph. Whiting 1 is particularly interestingbecause it is the youngest and among the most metal rich. The relativelyyoung age of Whiting 1 demonstrates that this dwarf galaxy was able toform star clusters for a period of at least 6 Gyr, and the age andmetallicity of Whiting 1 are consistent with the age-metallicityrelationship in the main body of the Sgr dSph. The presence now ofWhiting 1 in the Galactic halo provides additional support for the viewthat the young halo clusters originated in dwarf galaxies that have beenaccreted by the Milky Way.Based on observations with the ESO VLT at the Paranal Observatory, underthe program 76.D-0128. This paper includes data gathered with the 6.5 mMagellan Telescopes located at Las Campanas Observatory, Chile. Tablesof the individual photometric measurements are available at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/181
|Element abundances in the metal-rich open cluster NGC 6253|
Context: We have carried out a big FLAMES survey of 10 Galactic openclusters aiming at different goals. One of them is the determination ofchemical abundances, to put constraints on the radial metallicitygradient in the disk and its evolution. One of the sample clusters isthe very metal-rich NGC 6253. Aims: We have obtained UVES highresolution spectra of seven candidate cluster members (from the turn offup to the red clump) with the goal of determining the chemicalcomposition of NGC 6253 and of investigating its origin and role in theinterpretation of the radial metallicity gradient in the disk. Methods:Equivalent width analysis and spectral synthesis were performed usingMOOG and Kurucz model atmospheres. Results: We derived abundances ofFe, α-, and Fe-peak elements, the light element Na, and thes-process element Ba. Excluding two likely non-members and the clumpgiant, whose metallicity from equivalent widths is overestimated, wefind an average [Fe/H] = +0.36±0.07 (rms) for the cluster. Formost of the other elements we derive solar abundance ratios.Based on observations collected at ESO telescopes under program072.D-0550(A). Table 3 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/185
|Integral Field Spectroscopy of a Candidate Disk Galaxy at z ~ 1.5 Using Laser Guide Star Adaptive Optics|
We present 0.1" resolution near-infrared integral field spectroscopy ofHα in a z=1.4781 star-forming galaxy, Q2343-BM133. Theseobservations were obtained with the OH Suppressing Infra-Red ImagingSpectrograph (OSIRIS) using the W. M. Keck Observatory laser guide staradaptive optics (LGS AO) system. Hα emission is resolved over a0.8''(6.8 kpc)×0.5''(4.3 kpc) region with a0.1" spatial resolution. We find a global flux of4.2+/-0.6×10-16 ergs s-1 cm-2,and detect a spatially resolved velocity gradient of ~134 kms-1 across the galaxy and a global velocity dispersion of73+/-9 km s-1. An upper limit of [N II]/Hα<~0.12 isinferred, which implies that this galaxy is not dominated by an activegalactic nucleus and has a metallicity at or below 1/2 solarmetallicity. We derive a star formation rate (SFR) of 47+/-6Msolar yr-1 and a dereddened SFR of 66+/-9Msolar yr-1. Two-dimensional kinematics forQ2343-BM133 fit well with an inclined disk model, with which we estimatean enclosed mass of 4.3×109 Msolar within5.5 kpc. A possible merger scenario is also presented, and cannot befully ruled out. We derive a virial mass of 1.1×1010Msolar for a disk geometry, using the observed velocitydispersion. We propose that Q2343-BM133 is currently at an early stageof disk formation at a look-back time of 9.3 Gyr.
|The Benchmark Cluster Reddening Project. II. A Reddening Value for M67|
Since 1954, 70 reddening determinations have been published for M67.Those results are of very diverse quality, and they imply only that thevalue of E(B-V) for the cluster lies between -0.02 and +0.14 mag. Inaddition, this uncertainty is habitually concealed by long-standingcitation practices. In response, a reformed reddening protocol called``alternative scholarship'' is applied, with tenets including (1)rigorous published foundations for reddening values, (2) accountabilityfor all extant reddening results for M67, (3) explicit quality controlof reddening techniques, (4) use of techniques with limited metallicitysensitivity, (5) explicit zero-point control of input data (whenpossible), (6) a reddening zero point based ultimately on the dust-freesolar neighborhood, and (7) adherence to statistical norms. Results fromreddening maps are then set aside pending further examination of theirzero points and accidental errors. In addition, reddening values derivedfrom color-magnitude analysis and from averages of published reddeningresults are likewise set aside. Five techniques that satisfy the adoptedprotocol are then selected and applied. By averaging results fromcluster A and F stars and K giants, one finds that the mean value ofE(B-V) for M67 is 41+/-4 mmag. It is also found that extant results fromadditional techniques are consistent with this result or else can be setaside for plausible reasons. The analysis yields a collateral value of[Fe/H], and this is consistent with the derived reddening value. Usingpublished high-dispersion data, it is found that the mean cluster valueof [Fe/H] is -0.009+/-0.009 dex. For M67 and the Hyades combined, nomean metallicity difference between giants and stars near the mainsequence is found that is >=0.036 dex at 95% confidence.
|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 (220.127.116.11) 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
|Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters|
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
|Properties of planets in binary systems. The role of binary separation|
Aims.The statistical properties of planets in binaries wereinvestigated. Any difference to planets orbiting single stars can shedlight on the formation and evolution of planetary systems. As planetswere found around components of binaries with very different separationand mass ratio, it is particularly important to study thecharacteristics of planets as a function of the effective gravitationalinfluence of the companion. Methods: .A compilation of planets inbinary systems was made; a search for companions orbiting stars recentlyshown to host planets was performed, resulting in the addition of twofurther binary planet hosts (HD 20782 and HD 109749). The probableoriginal properties of the three binary planet hosts with white dwarfscompanions were also investigated. Using this updated sample of planetsin binaries we performed a statistical analysis of the distributions ofplanet mass, period, and eccentricity, fraction of multiplanet systems,and stellar metallicity for planets orbiting components of tight andwide binaries and single stars. Results: .The only highlysignificant difference revealed by our analysis concerns the massdistribution of short-period planets. Massive planets in short periodorbits are found in most cases around the components of rather tightbinaries. The properties of exoplanets orbiting the components of widebinaries are compatible with those of planets orbiting single stars,except for a possible greater abundance of high-eccentricity planets.The previously suggested lack of massive planets with P>100 days inbinaries is not confirmed. Conclusions: .We conclude that thepresence of a stellar companion with separation smaller than 100-300 AUis able to modify the formation and/or migration and/or the dynamicalevolution history of giant planets while wide companions play a morelimited role.Table 1 and Appendices A-C are only available in electronic form athttp://www.aanda.org
|The absolute motion of the peculiar cluster NGC 6791|
We present improved values of the three components of the absolute spacevelocity of the open cluster NGC 6791. One HST ACS/WFC field withtwo-epoch observations provides astrometric measurements of objects in afield containing the cluster center. Identification of 60 backgroundgalaxies with sharp nuclei allows us to determine an absolute referencepoint, and measure the absolute proper motion of the cluster. We find(μ_α cosδ, μ_δ)J2000.0 = (-0.57 ± 0.13,-2.45 ± 0.12) mas yr-1, and adopt V_rad = -47.1± 0.7 km s-1 from the average of the published values.Assuming a Galactic potential, we calculate the Galactic orbit of thecluster for various assumed distances, and briefly discuss theimplications on the nature and the origin of this peculiar cluster.
|The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy|
Using HST ACS, we have obtained deep optical images reaching stars wellbelow the oldest main-sequence turnoff in the spheroid, tidal stream,and outer disk of Andromeda. We have reconstructed the star formationhistory in these fields by comparing their color-magnitude diagrams to agrid of isochrones calibrated to Galactic globular clusters observed inthe same bands. Each field exhibits an extended star formation history,with many stars younger than 10 Gyr but few younger than 4 Gyr.Considered together, the star counts, kinematics, and populationcharacteristics of the spheroid argue against some explanations for itsintermediate-age, metal-rich population, such as a significantcontribution from stars residing in the disk or a chance intersectionwith the stream's orbit. Instead, it is likely that this population isintrinsic to the inner spheroid, whose highly disturbed structure isclearly distinct from the pressure-supported metal-poor halo thatdominates farther from the galaxy's center. The stream and spheroidpopulations are similar, but not identical, with the stream's mean agebeing ~1 Gyr younger; this similarity suggests that the inner spheroidis largely polluted by material stripped from either the stream'sprogenitor or similar objects. The disk population is considerablyyounger and more metal-rich than the stream and spheroid populations,but not as young as the thin-disk population of the solar neighborhood;instead, the outer disk of Andromeda is dominated by stars of age 4-8Gyr, resembling the Milky Way's thick disk. The disk data areinconsistent with a population dominated by ages older than 10 Gyr andin fact do not require any stars older than 10 Gyr.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS 5-26555. These observations areassociated with proposals 9453 and 10265. Some of the data presentedherein were obtained at the W. M. Keck Observatory, which is operated asa scientific partnership among the California Institute of Technology,the University of California, and NASA. The Observatory was madepossible by the generous financial support of the W. M. Keck Foundation.
|The Outskirts of Spiral Galaxies: Evidence for Multiple Stellar Populations|
We present an analysis of the metallicity distribution functions offields projected along the minor axis for a sample of inclined spiralgalaxies in order to search for evidence of the presence of multiplestellar populations. In all cases, with very high confidence levels, thestellar populations appear to have asymmetric metallicity distributions.The mean metallicities of both stellar subpopulations, determined frommixture modeling of the metallicity distribution functions, correlatewith the parent galaxy luminosity. This suggests that the vast majorityof field stars probably formed in galactic fragments that were alreadyembedded in the dark matter halo of the final galaxy. The steepercorrelation between the mean stellar metallicity and parent galaxyluminosity is driven by an increasing fraction of metal-rich stars withincreasing galaxy luminosity. Metal-poor components show a largerdispersion in metallicity than metal-rich components. These propertiesare strikingly similar to those of globular cluster subpopulationsaround early-type galaxies. The properties of field stars along theminor axis are consistent with a formation scenario in which metal-poorstars formed in all galaxies, possibly as a result of the tidaldisruption of dwarflike objects. An additional metal-rich componentmight be related to the formation of the bulge and/or the disk.
|Chemical Evolution of the Galactic Bulge as Derived from High-Resolution Infrared Spectroscopy of K and M Red Giants|
We present chemical abundances in K and M red giant members of theGalactic bulge derived from high-resolution infrared spectra obtainedwith the Phoenix spectrograph on Gemini-South. The elements studied arecarbon, nitrogen, oxygen, sodium, titanium, and iron. The evolution of Cand N abundances in the studied red giants shows that their oxygenabundances represent the original values with which the stars were born.Oxygen is a superior element for probing the timescale of bulge chemicalenrichment via [O/Fe] versus [Fe/H]. The [O/Fe]-[Fe/H] relation in thebulge does not follow the disk relation, with [O/Fe] values fallingabove those of the disk. Titanium also behaves similarly to oxygen withrespect to iron. Based on these elevated values of [O/Fe] and [Ti/Fe]extending to large Fe abundances, it is suggested that the bulgeunderwent a more rapid chemical enrichment than the halo. In addition,there are declines in both [O/Fe] and [Ti/Fe] in those bulge targetswith the largest Fe abundances, signifying another source affectingchemical evolution: perhaps supernovae of Type Ia. Sodium abundancesincrease dramatically in the bulge with increasing metallicity, possiblyreflecting the metallicity-dependent yields from supernovae of Type II,although Na contamination from H-burning in intermediate-mass starscannot be ruled out.
|On the current status of open-cluster parameters|
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
|The Dwarf Satellites of M31 and the Galaxy|
The satellite systems of M31 and the Galaxy are compared. It is notedthat all five of the suspected stripped dwarf spheroidal (dSph) cores ofM31's companions are located within a projected distance of 40 kpc fromthe nucleus of this galaxy, whereas the normal dSph companions to thisobject have distances >40 kpc from the center of M31. All companionswithin 200 kpc25 kpc) satellites.
|Old open clusters as key tracers of Galactic chemical evolution. I. Fe abundances in NGC 2660, NGC 3960, and Berkeley 32|
Aims.We obtained high-resolution UVES/FLAMES observations of a sample ofnine old open clusters spanning a wide range of ages and Galactocentricradii. The goal of the project is to investigate the radial metallicitygradient in the disk, as well as the abundance of key elements (αand Fe-peak elements). In this paper we present the results for themetallicity of three clusters: NGC 2660 (age ~1 Gyr, Galactocentricdistance of 8.68 kpc), NGC 3960 (~1 Gyr, 7.80 kpc), and Be 32 (~6-7 Gyr,11.30 kpc). For Be 32 and NGC 2660, our study provides the firstmetallicity determination based on high-resolution spectra.Methods: .We performed equivalent width analysis with the spectral codeMOOG, which allows us to define a metallicity scale and build ahomogeneous sample. Results: .We find that NGC 3960 and NGC 2660have a metallicity that is very close to solar ([Fe/H] = +0.02 and+0.04, respectively), while the older Be 32 turns out to have [Fe/H] =-0.29.
|An Empirical Tool to Derive Metallicity, Reddening, and Distance for Old Stellar Populations from Near-Infrared Color-Magnitude Diagrams|
We present an empirical method to derive photometric metallicity,reddening, and distance to old stellar populations by using a few majorfeatures of the red giant branch (RGB) in near-IR color-magnitudediagrams. We combine the observed RGB features with a set of equationslinking the global metallicity [M/H] to suitable RGB parameters (colors,magnitudes, and slope), as calibrated from a homogeneous sample ofGalactic globular clusters with different metallicities. This techniquecan be applied to efficiently derive the main population parameters ofold stellar systems, with the goal of using ground-based adaptive opticsand space facilities to probe the stellar content of remote galaxies.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG), operated on the island La Palma bythe Fundacion Galileo Galilei of INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
|Survey for Transiting Extrasolar Planets in Stellar Systems. IV. Variables in the Field of NGC 1245|
The Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS)project is a search for planetary transits in open clusters. In thispaper we analyze the STEPSS observations of the open cluster NGC 1245 todetermine the variable-star content of the cluster. Out of 6787 starsobserved with V<22, of which ~870 are cluster members, we find 14stars with clear intrinsic variability that are potential clustermembers and 29 clear variables that are not cluster members. None ofthese variables have been previously identified. We present lightcurves, finder charts, and stellar/photometric data on these variableobjects. Several of the interacting binaries have estimated distancesconsistent with the cluster distance determined from isochrone fits tothe color-magnitude diagram. Four stars at the main-sequence turnoff ofthe cluster have light curves consistent with γ Doradusvariability. If these γ Doradus candidates are confirmed, theyrepresent the oldest and coolest members of this class of variablediscovered to date.
|Probing the Faintest Stars in a Globular Star Cluster|
NGC 6397 is the second closest globular star cluster to the Sun. Using 5days of time on the Hubble Space Telescope, we have constructed anultradeep color-magnitude diagram for this cluster. We see a cleartruncation in each of its two major stellar sequences. Faint redmain-sequence stars run out well above our observational limit and nearto the theoretical prediction for the lowest mass stars capable ofstable hydrogen burning in their cores. We also see a truncation in thenumber counts of faint blue stars, namely white dwarfs. This reflectsthe limit to which the bulk of the white dwarfs can cool over thelifetime of the cluster. There is also a turn toward bluer colors in theleast luminous of these objects. This was predicted for the very coolestwhite dwarfs with hydrogen-rich atmospheres as the formation ofH2 and the resultant collision-induced absorption cause theiratmospheres to become largely opaque to infrared radiation.
|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.
|High-Resolution Infrared Spectroscopy of the Old Open Cluster NGC 6791|
We report abundance analysis for six M giant members of the old opencluster NGC 6791, based on infrared spectroscopy (1.5-1.8 μm) atR=25,000, using the NIRSPEC spectrograph at the Keck II telescope. Wefind the iron abundance <[Fe/H]>=+0.35+/-0.02, confirming thesupersolar metallicity of this cluster as derived from opticalmedium-high resolution spectroscopy. We also measure C, O, and otherα-element abundances, finding a roughly solar value of[α/Fe] and <[C/Fe]>=-0.35. Our approach constrains [O/Fe]especially well, on the basis of the measurement of a number of OH linesnear 1.6 μm we find [O/Fe]=-0.07+/-0.03. The solar value of[α/Fe] is in contrast to the composition of similar stars in theGalactic bulge. We also find a low value of12C/13C~10, confirming the presence of extramixingprocesses during the red giant phase of evolution, up to supersolarmetallicities.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
|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.
|NGC 6791: An Exotic Open Cluster or the Nucleus of a Tidally Disrupted Galaxy?|
We report on high-resolution echelle spectroscopy of 20 giant stars inthe Galactic old open cluster NGC 6791, obtained with Hydra at the WIYNtelescope. High-precision radial velocity allows us to isolate 15 bonafide cluster members. From 10 of them we derive a global[M/H]=+0.39+/-0.05. We therefore confirm that NGC 6791 is extremelymetal-rich, exhibits a few marginally subsolar abundance ratios, andwithin the resolution of our spectra does not show evidence of spread inmetal abundance. With these new data we rederive the fundamental clusterparameters, suggesting that it is about 8 Gyr old and 4.3 kpc from theSun. The combination of its chemical properties, age, position, andGalactic orbit hardly makes NGC 6791 a genuine Population I opencluster. We discuss possible interpretations of the clusterpeculiarities, suggesting that the cluster might be what remains of amuch larger system whose initial potential well could have beensufficient to produce high-metallicity stars and which has beendepopulated by the tidal field of the Galaxy. Alternatively, its currentproperties may be explained by the perturbation of the Galactic bar onan object that originated well inside the solar ring, where the metalenrichment was very fast.
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