Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
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.
| 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.
| 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
| 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.
| 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.
| 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.
| uvbyCaHβ CCD Photometry of Clusters. VII. The Intermediate-Age Anticenter Cluster Melotte 71
CCD photometry on the intermediate-band uvbyCaHβ system ispresented for the anticenter, intermediate-age open cluster Melotte 71.Restricting the data to probable single members of the cluster using thecolor-magnitude diagram and the photometric indices alone generates asample of 48 F dwarfs on the unevolved main sequence. The averageE(b-y)=0.148+/-0.003 (standard error of the mean [s.e.m.]) orE(B-V)=0.202+/-0.004 (s.e.m.), where the errors refer to internal errorsalone. With this reddening, [Fe/H] is derived from both m1and hk, using Hβ and b-y as the temperature index, with excellentagreement among the four approaches and a final weighted average of[Fe/H]=-0.17+/-0.02 (s.e.m.) for the cluster, on a scale in which theHyades has [Fe/H]=+0.12. When adjusted for the higher reddeningestimate, the previous metallicity estimates from Washington photometryand from spectroscopy are now in agreement with the intermediate-bandresult. From comparisons to isochrones of appropriate metallicity, thecluster age and distance are determined as 0.9+/-0.1 Gyr and(m-M)=12.2+/-0.1 or (m-M)0=11.6+/-0.1. At this distance fromthe Sun, Mel 71 has a galactocentric distance of 10.0 kpc on a scale inwhich the Sun is 8.5 kpc from the Galactic center. Based on its age,distance, and elemental abundances, Mel 71 appears to be a less populousanalog to NGC 3960.
| 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.
| 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.
| 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.
| Open cluster remnants: an observational overview
The state of the art in the current research on the final stages of OpenStar Clusters dynamical evolution is presented. The focus will be onobservational studies and their achievements. By combining togetherphotometric, spectroscopic and astrometric material, a new look at thisexciting topic is now possible, thus providing a solid observationalfoundation for a new generation of theoretical studies on the subject.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| vbyCaHβ CCD Photometry of Clusters. VI. The Metal-deficient Open Cluster NGC 2420
CCD photometry on the intermediate-band vbyCaHβ system is presentedfor the metal-deficient open cluster NGC 2420. Restricting the data toprobable single members of the cluster using the CMD and the photometricindices alone generates a sample of 106 stars at the cluster turnoff.The average E(b-y)=0.035+/-0.003 (s.e.m.) or E(B-V)=0.050+/-0.004(s.e.m.), where the errors refer to internal errors alone. With thisreddening, [Fe/H] is derived from both m1 and hk, using b-yand Hβ as the temperature index. The agreement among the fourapproaches is reasonable, leading to a final weighted average of[Fe/H]=-0.37+/-0.05 (s.e.m.) for the cluster, on a scale where theHyades has [Fe/H]=+0.12. When combined with the abundances from DDOphotometry and from recalibrated low-resolution spectroscopy, the meanmetallicity becomes [Fe/H]=-0.32+/-0.03. It is also demonstrated thatthe average cluster abundances based on either DDO data orlow-resolution spectroscopy are consistently reliable to 0.05 dex orbetter, contrary to published attempts to establish an open clustermetallicity scale using simplistic offset corrections among differentsurveys.
| Exploring Halo Substructure with Giant Stars. VIII. The Extended Structure of the Sculptor Dwarf Spheroidal Galaxy
We explore the spatial distribution of stars in the Sculptor dwarfspheroidal (dSph) galaxy over an area of 7.82 deg2, includingcoverage of the central region but extending mostly south and east ofthe dSph core. Two methods are used to identify stars that are mostlikely associated with the dSph, and these filtered samples of stars areused to map its spatial structure. First, following the method ofprevious contributions in this series, we utilize Washington M,T2+DDO51 photometry to identify red giant branch (RGB) starcandidates with approximately the same distance and metallicity as theSculptor dSph. Second, a prominent blue horizontal branch (BHB)population provides a fairly populous and pure sample of Sculptor starshaving broadband colors unlike the bulk of the Galactic field starpopulation. A spectroscopically observed subset of Sculptor candidatestars (147 total stars: ~5% of all Sculptor candidates and ~10% ofSculptor giant candidates) yields a systemic heliocentric velocity forthe system of vhel=110.43+/-0.79 km s-1, in goodagreement with previous studies. We also find a global velocitydispersion of σv=8.8+/-0.6 km s-1, withslight indications of a rise in the velocity dispersion past ~0.4rlim.These spectra also provide a check on the reliability of our candidateSculptor giant sample to M~19 94% of the photometrically selectedSculptor giant star candidates with follow-up spectroscopy are found tobe kinematically associated with Sculptor, while 4 out of 10 starsoutside of our Sculptor giant star selection criteria that we testedspectroscopically appear to be velocity members of Sculptor. Thesepercentages are in agreement with results for an additional 22 Sculptorfield stars with radial velocities in the literature. All availablevelocities show that our methodology for picking Sculptor giants is bothreliable and conservative. Thus, these giant star samples should providea reliable means to explore the structure of the Sculptor dSph.Nevertheless, considerable care has been taken to assess the level ofbackground contamination in our photometric sample to ensure anaccurately derived density profile of the Sculptor dSph to large radii.Multiple background assessments verify that we detect a considerablestellar density of Sculptor stars to the limits of our main survey areafor both the RGB and BHB candidate samples. While we find that a Kingprofile of limiting radius rlim=79.6 arcmin fits the densityprofile of Sculptor well to ~60', beyond this, we identify a ``break''in the profile and a clearly detected population of Sculptor starsfollowing a Σ~r-2 decline to more than 2rlim. Thisbreak population must signify either the presence of an extremely broaddistribution of bound ``halo stars'' around the Sculptor dSph or thepresence of unbound tidal debris. If the latter is true, we determine afractional mass-loss rate of approximately 0.042 Gyr-1 forthe Sculptor dSph. Additional support for the notion that there is tidaldisruption comes from the two-dimensional distribution of our Sculptorcandidate stars; both the RGB and BHB samples show increasinglyelongated isodensity contours with radius that point to an apparentstretching reminiscent of what is seen in models of disrupting satellitegalaxies. Finally, we find that RGB stars that are more likely to bemetal-poor (based on their color and magnitude) are significantly lesscentrally concentrated and therefore constitute the primary contributingstellar population to the likely tidally stripped parts of the dSph.
| Mass functions and structure of the young open cluster NGC 6611
We use J, H and KS 2MASS photometry to study colour-magnitude(CMDs) and colour-colour diagrams, structure and mass distribution inthe ionizing open cluster NGC 6611. Reddening variation throughout thecluster region is taken into account followed by field-stardecontamination of the CMDs. Decontamination is also applied to derivethe density profile and luminosity functions in the core, halo andoverall (whole cluster) regions. The field-star decontamination showedthat the lower limit of the main sequence (MS) occurs at ≈5 M_ȯ.Based on the fraction of KS excess stars in the colour-colourdiagram we estimate an age of 1.3±0.3 Myr which is consistentwith the presence of a large number of pre-main sequence (PMS) stars.The distance from the Sun was estimated from known O V stars in thecluster area and the turn-on stars connecting the PMS and MS, resultingin d_ȯ=1.8±0.5 kpc. The radial density distributionincluding MS and PMS stars is fitted by a King profile with a coreradius R_core=0.70±0.08 pc. The cluster density profile mergesinto the background at a limiting radius R_lim=6.5±0.5 pc. Fromthe field-star subtracted luminosity functions we derive the massfunctions (MFs) in the form φ(m)∝ m-(1+χ). Inthe halo and through the whole cluster the MFs have slopesχ=1.52±0.13 and χ=1.45±0.12, respectively, thusslightly steeper than Salpeter's IMF. In the core the MF is flat,χ=0.62±0.16, indicating some degree of mass segregation sincethe cluster age is a factor 2 larger than the relaxation time. Becauseof the very young age of NGC 6611, part of this effect appears to berelated to the molecular cloud-fragmentation process itself. We detect362±120 PMS stars. The total observed mass including detected MS(in the range 5{-}85 M_ȯ) and PMS stars amounts to 1600 M_ȯ,thus more massive than the Trapezium cluster. Compared to older openclusters of different masses, the overall NGC 6611 fits in the relationsinvolving structural and dynamical parameters. However, the core isatypical in the sense that it looks like an old/dynamically evolvedcore. Again, part of this effect must be linked to formation processes.
| On the ages of exoplanet host stars
We obtained spectra, covering the CaII H and K region, for 49 exoplanethost (EH) stars, observable from the southern hemisphere. We measuredthe chromospheric activity index, R'{_HK}. We compiled previouslypublished values of this index for the observed objects as well as theremaining EH stars in an effort to better smooth temporal variations andderive a more representative value of the average chromospheric activityfor each object. We used the average index to obtain ages for the groupof EH stars. In addition we applied other methods, such as: Isochrone,lithium abundance, metallicity and transverse velocity dispersions, tocompare with the chromospheric results. The kinematic method is a lessreliable age estimator because EH stars lie red-ward of Parenago'sdiscontinuity in the transverse velocity dispersion vs dereddened B-Vdiagram. The chromospheric and isochrone techniques give median ages of5.2 and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The medianage of F and G EH stars derived by the isochrone technique is 1-2 Gyrolder than that of identical spectral type nearby stars not known to beassociated with planets. However, the dispersion in both cases is large,about 2-4 Gyr. We searched for correlations between the chromosphericand isochrone ages and L_IR/L* (the excess over the stellarluminosity) and the metallicity of the EH stars. No clear tendency isfound in the first case, whereas the metallicy dispersion seems toslightly increase with age.
| Properties of five low-contrast open clusters in the third quadrant
We derive photometric, structural and dynamical evolution-relatedparameters of five as yet unstudied low-contrast open clusters locatedin the third quadrant using 2MASS data. The target clusters are Czernik31, Czernik 32, Haffner 9, Haffner 11 and Trumpler 13. We apply astatistical field-star decontamination procedure to infer on theintrinsic colour-magnitude diagram (CMD) morphology which is criticalfor such low-contrast objects. Consequently, it became possible toderive accurate reddening, age, distance from the Sun and Galactocentricdistance for the five clusters. In the structural andluminosity/mass-function analyses we apply a colour-magnitude filterwhich encompasses the cluster evolutionary CMD sequences and excludesstars with discrepant colours. Using this procedure we derive core andlimiting radii, mass function (MF) slope, total mass, mass density andrelaxation time. We derive ages in the range 140-1100 Myr,Galactocentric distances within 7.7-11.4 kpc, and total masses within360-2900 M_ȯ. Reflecting large-scale mass segregation, the MF slopein the core is significantly flatter than that in the halo of the fiveclusters. Although some of the present clusters are relatively youngerthan the Gyr-old clusters, they present evidence of advanced dynamicalevolution. This kind of study has become possible because of thephotometric uniformity and spatial coverage of 2MASS which allows aproper subtraction of the field-star contamination on the target CMDs.The present study indicates that low-contrast clusters can be studiedwith 2MASS, particularly after field-star subtraction, which isimportant since most of the unstudied open clusters belong to thisclass.
| 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.
| Structure and stellar content analysis of the open cluster M 11 with 2MASS photometry
An overall analysis of the structure and stellar content of M 11 ispresented, thanks to the wide-angle 2MASS spatial coverage. We derivephotometric and structural parameters and discuss the spatial dependanceof the luminosity and mass functions. Photometric parameters basicallyagree with previous ones mostly based on the optical. We obtained a coreradius of 1.23 pc and a tidal radius of 29 pc. In particular, thecluster is populous enough so that the tidal radius could be obtained byfitting the three-parameter King profile to the radial distribution ofstars. We analyzed the spatial distribution of mass functions, findingthat the slope changes from -0.73 in the core to +2.88 in the outerhalo. The spatial distribution of mass function slopes derived from2MASS agrees with that derived from optical CCD data, which furtherconfirms the reliability of 2MASS data for future analyses of this kindat comparable observational limits. We detect mass segregation up todistances from the center of ≈20 arcmin. We emphasize that the massfunction slope in the core is flatter than anywhere else as aconsequence of mass segregation. The derived total cluster mass is≈11 000 M_ȯ.
| 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.
| Detailed analysis of open clusters: A mass function break and evidence of a fundamental plane
We derive photometric, structural and dynamical evolution-relatedparameters of 11 nearby open clusters with ages in the range 70 Myr to 7Gyr and masses in the range ≈400 M_ȯ to ≈5300 M_ȯ. Theclusters are homogeneously analysed by means of J, H and KS2MASS photometry, which provides spatial coverage wide enough toproperly take into account the contamination of the cluster field byGalaxy stars. Structural parameters such as core and limiting radii arederived from the background-subtracted radial density profiles.Luminosity and mass functions (MFs) are built for stars later than theturnoff and brighter than the 2MASS PSC 99.9% completeness limit. Thetotal mass locked up in stars in the core and the whole cluster, as wellas the corresponding mass densities, are calculated by taking intoaccount the observed stars (evolved and main sequence) and extrapolatingthe MFs down to the H-burning mass limit, 0.08 M_ȯ. We illustratethe methods by analysing for the first time in the near-infrared thepopulous open clusters NGC 2477 and NGC 2516. For NGC 2477 we derive anage of 1.1 ± 0.1 Gyr, distance from the Sun d_ȯ=1.2 ±0.1 kpc, core radius Rcore=1.4 ± 0.1 pc, limitingradius Rlim=11.6 ± 0.7 pc and total massmtot≈(5.3±1.6) × 103 M_ȯ.Large-scale mass segregation in NGC 2477 is reflected in the significantvariation of the MF slopes in different spatial regions of the cluster,and in the large number-density of giant stars in the core with respectto the cluster as a whole. For NGC 2516 we derive an age of 160 ±10 Myr, d_ȯ=0.44 ± 0.02 kpc, Rcore=0.6 ±0.1 pc, Rlim=6.2 ± 0.2 pc andmtot≈(1.3±0.2) × 103 M_ȯ.Mass-segregation in NGC 2516 shows up in the MFs. Six of the 11 clusterspresent a slope break in the MF occurring at essentially the same massas that found for the field stars in Kroupa's universal IMF. The MFbreak is not associated to cluster mass, at least in the clusters inthis paper. In two clusters the low-mass end of the MF occurs above theMF break. However, in three clusters the MF break does not occur, atleast for the mass range m≥0.7 M_ȯ. One possibility isdynamical evolution affecting the MF slope distribution. We also searchfor relations of structural and evolutionary parameters with age andGalactocentric distance. The main results for the present sample are:(i) cluster size correlates both with age and Galactocentric distance;(ii) because of size and mass scaling, core and limiting radii, and coreand overall mass correlate; (iii) massive (m≥1000 M_ȯ) andless-massive clusters follow separate correlation paths on the planecore radius and overall mass; (iv) MF slopes of massive clusters arerestricted to a narrow range, while those of the less-massive onesdistribute over a wider range. Core and overall MF flattening is relatedto the ratio (τ) of age to relaxation time. For large values ofτ the effects of large-scale mass segregation and low-mass starsevaporation can be observed in the MFs. In this sense, τ appears tocharacterize the evolutionary state of the clusters. We conclude thatappreciable slope flattenings in the overall MFs of the less-massiveclusters take ~6 times longer to occur than in the core, while in themassive clusters they take a time ~13 times longer. We investigatecluster parameters equivalent to those determining the fundamental planeof ellipticals. These parameters are: overall mass, projected massdensity and core radius. We conclude that in the present sample there isevidence of a fundamental plane. Larger samples are necessary to pindown this issue.
| Spatial dependence of 2MASS luminosity and mass functions in the old open cluster NGC 188
Luminosity and mass functions in the old open cluster NGC 188 areanalysed by means of J and H 2MASS photometry, which provides uniformityand spatial coverage for a proper background subtraction. With an age ofabout 6-8 Gyr, NGC 188 is expected to be suffering the effects ofadvanced dynamical evolution. Indeed, previous works in optical bandshave suggested the presence of mass segregation. Within theuncertainties, the observed projected radial density profile of NGC 188departs from the two-parameter King model in two inner regions, whichreflects the non-virialized dynamical state and possibly, some degree ofnon-sphericity in the spatial shape of this old open cluster. Fits withtwo and three-parameter King models to the radial distribution of starsresulted in a core radius Rcore=1.3±0.1 pc and a tidalradius Rtidal=21±4 pc, about twice as large as thevisual limiting radius. The concentration parameter c=1.2±0.1 ofNGC 188 makes this open cluster structurally comparable to the looseglobular clusters. The present 2MASS analysis resulted in significantslope variations with distance in the mass function φ(m)∝m-(1+χ), being flat in the central parts(χ=0.6±0.7) and steep in the cluster outskirts(χ=7.2±0.6). The overall mass function has a slopeχ=1.9±0.7, slightly steeper than a standard Salpeter massfunction. In this context, NGC 188 is similar to the 3.2 Gyr,dynamically evolved open cluster M 67. Solar metallicity Padovaisochrone fits to the near-infrared colour-magnitude diagram of NGC 188resulted in an age of 7.0±1.0 Gyr. The best fit, obtained withthe 7.1 Gyr isochrone, produced a distance modulus(m-M)0=11.1±0.1, E(B-V)=0.0, and a distance to the Sundȯ=1.66±0.08 kpc. The observed stellar mass (inthe range 0.98 Mȯ- 1.08 Mȯ) in NGC 188is mobs=380±12 Mȯ. A simpleextrapolation of the observed overall mass function to stars with 0.08Mȯ resulted in a total present mass ofmtot(1.8±0.7)×104Mȯ. On the other hand, for a more realistic initial massfunction which flattens in the low-mass range, the total mass in NGC 188drops to mtot(3.8±1.6)×103Mȯ. Since mass-loss processes such as evaporation andtidal stripping have been occurring in this old open cluster for about 7Gyr, the primordial mass in NGC 188 must have been significantly largerthan 4×103 Mȯ. We also examinedthe consequences of the presence of unresolved binaries and concludedthat, even if dominant in the CMD, binaries alone are not responsiblefor the flat central mass function, which supports the mass-segregationscenario.
| Probing the nature of possible open cluster remnants with the Southern Proper Motion Program
We discuss the nature of eleven Possible Open Cluster Remnants (POCRs)by using absolute proper motions from the Southern Proper Motion (SPM)Program 3 (Girard et al. [CITE], AJ, 127, 3060) combined with nearinfrared photometry from 2MASS. The analysis is done by considering thedistribution of stars in the Color-Magnitude and the Vector Pointdiagrams. We successfully probed the capabilities of the SPM catalog todetect a physical group by looking at the open cluster Blanco 1.However, within the uncertainties of the SPM3 catalog and basing on2MASS photometry we conclude that only one - ESO 282SC26 - out of elevenobjects turns out to be a probable physical group. We suggest it is anopen cluster 1.3 Gyr old and located 1.4 kpc from the Sun.
| Proper motion measurements as indicators of binarity in open clusters
We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyrusing UCAC2 proper motion data and 2MASS photometry, which allows us toreach stellar masses down to ≈0.7 M_ȯ. We employ in this work anapproach in which the background proper motion contribution isstatistically subtracted in order to obtain the cluster's intrinsicproper motion distribution. For each cluster we consider the projectedvelocity distributions in the core and off-core regions separately. Inthe projected velocity distribution of all sample clusters we find awell-defined low-velocity peak, as well as an excess in the number ofstars at larger velocities. The low-velocity peak is accounted for bythe random motion of the single stars, while the high-velocity excesscan be attributed to the large velocity changes produced by asignificant fraction of unresolved binaries in a cluster. We derivekinematic parameters of the single-star distribution, in particular theprojected velocity dispersion. The relatively large velocity dispersionsderived in this work may reflect the non-virialized state of theclusters. Based on the relative number of high-velocity (binary) andsingle stars, we inferred for the sample clusters unresolved binaryfractions in the range 15%≤f_bin≤54%, for both core and off-coreregions. Stars with a projected velocity exceeding the maximum reachedby the single-star distribution are identified in 2MASS J×(J-H)colour magnitude diagrams. The asymmetry observed in the distribution ofthese stars around the main sequence is consistent with models ofmain-sequence widening resulting from unresolved binaries combined with2MASS photometric uncertainties. The present results suggest that caremust be taken when applying proper-motion filters to sort out members,especially binaries in a star cluster. This paper shows that propermotions turn out to be a useful tool for identifying high-velocity starsas unresolved binary cluster members, and as a consequence, map andquantify the binary component in colour magnitude diagrams.
| Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC
Open clusters potentially provide an ideal environment for the searchfor transiting extrasolar planets, since they feature a relatively largenumber of stars of the same known age and metallicity at the samedistance. With this motivation, over a dozen open clusters are now beingmonitored by four different groups. We review the motivations andchallenges for open cluster transit surveys for short-period giantplanets. Our photometric monitoring survey of Galactic southern openclusters, the Extrasolar Planet Occultation Research/Open Clusters(EXPLORE/OC) project, was designed with the goals of maximizing thechance of finding and characterizing planets and of providing astatistically valuable astrophysical result in the case of nodetections. We use the EXPLORE/OC data from two open clusters, NGC 2660and NGC 6208, to illustrate some of the largely unrecognized issuesfacing open cluster surveys, including severe contamination by Galacticfield stars (>80%) and the relatively low number of cluster membersfor which high-precision photometry can be obtained. We discuss how acareful selection of open cluster targets under a wide range of criteriasuch as cluster richness, observability, distance, and age can meet thechallenges, maximizing chances to detect planet transits. In addition,we present the EXPLORE/OC observing strategy to optimize planetdetection, which includes high-cadence observing and continuouslyobserving individual clusters rather than alternating between targets.
| Blue Stragglers in Galactic Open Clusters and Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic openclusters are studies in this work, in which 27 Galactic open clusters ofages >=1 Gyr are selected as the working sample. Based on thephotometric observations of these open clusters, a synthetic integratedspectrum has been made for the stellar population of each cluster. Theeffects of blue straggler (BS) stars on the conventional simple stellarpopulation (SSP) model are analyzed on an individual cluster basis. Itis shown that the BSs, whose positions in the color-magnitude diagramscannot be predicted by the current single-star evolution theory, requiresignificant modifications to the integrated properties of theoreticalSSP model. The synthesized integrated spectral energy distributions(ISEDs) of our sample clusters are dramatically different from those ofSSPs based on an isochrone only. The BS-corrected ISEDs of stellarpopulations show systematic enhancements toward shorter wavelengths inthe spectra. When measured with broadband colors in unresolvableconditions, the age of a stellar population can be seriouslyunderestimated by the conventional SSP model. Therefore, considering thecommon existence of BS components in real stellar populations, we shouldexpect considerable alterations of the conventional ISEDs when we applythe technique of evolutionary population synthesis to more complicatedstellar systems.
| uvbyCaHβ CCD Photometry of Clusters. V. The Metal-Deficient Open Cluster NGC 2243
CCD photometry on the intermediate-band uvbyCaHβ systemis presented for the metal-deficient open cluster NGC 2243. Restrictingthe data to probable single members of the cluster using thecolor-magnitude diagram (CMD) and the photometric indices alonegenerates a sample of 100 stars at the cluster turnoff. The averageE(b-y)=0.039+/-0.003 (s.e.m.) or E(B-V)=0.055+/-0.004 (s.e.m.), wherethe errors refer to internal errors alone. With this reddening, [Fe/H]is derived from both m1 and hk, using b-y and Hβ as thetemperature index. The agreement among the four approaches is excellent,leading to a final weighted average of [Fe/H]=-0.57+/-0.03 (s.e.m.) forthe cluster, on a scale where the Hyades has [Fe/H]=+0.12. Using acombination of photometric and spectroscopic data, 27 probable clustermembers are identified and used to delineate the red giant branch and awell-defined clump at V=13.70, while eliminating the so-called secondclump at V=14.1. Interpolation between isochrones of appropriate [Fe/H]leads to an apparent modulus of m-M=13.15+/-0.1 and an age of 3.8+/-0.2Gyr. A differential CMD comparison with Ber 29, a cluster with agalactocentric distance almost twice that of NGC 2243, constrains Ber 29to be at least as young and as metal-rich as NGC 2243.
| Y2 Isochrones with an Improved Core Overshoot Treatment
Convective core overshoot affects stellar evolution rates and the datingof stellar populations. In this paper, we provide a patch to theY2 isochrones with an improved treatment of convective coreovershoot. The new tracks cover the transition mass range from noconvective core to a fully developed convective core. We compare theimproved isochrones to CMDs of a few well-observed open star clusters inthe Galaxy and the Large Magellanic Cloud. Finally, we discuss futureprospects for improving the treatment of core overshoot with the help ofasteroseismology.
| NGC 2180: A disrupting open cluster?
The spatial dependence of luminosity and mass functions of evolved openclusters is discussed in this work using J and H 2MASS photometry, whichallows a wide spatial coverage and proper background determination. Thetarget objects are the intermediate-age open cluster NGC 3680(ℓ=286.76o and b=16.92o), which has beenreported as being in an advanced state of dissolution, and NGC 2180(ℓ=203.85o, b=-7.01o), a possible dynamicallyevolved open cluster. At the 1-σ level, NGC 2180 presents a radialdistribution of stars that can be represented by a King law. We concludethat, although in an advanced dynamical state (mass segregated), NGC3680 does not present strong signs of dissolution, having luminosity andmass functions very similar to those of the τage≈3.2Gyr open cluster M 67. On the other hand, NGC 2180 appears to have flat,eroded luminosity functions throughout its structure, suggesting that inaddition to mass segregation, Galactic tidal stripping has beeneffective in depleting this object. Accordingly, NGC 2180 may be themissing link between evolved open clusters and remnants. For NGC 2180 weinfer a distance to the Sun dȯ=0.91 ± 0.15 kpc,an age τage≈710 Myr and an observed stellar mass ofmobs˜47 M&odot. For NGC 3680 we derivean age τage≈1.6 Gyr, E(B-V)=0.0 anddȯ=1.00 ± 0.09 kpc, confirming previousestimates. The observed stellar mass mobs≈130Mȯ agrees with previous values. We study both clustersin the context of dynamical states estimated from diagnostic diagramsinvolving photometric and structural parameters. Both objects appear tobe dynamically evolved systems. In particular, NGC 2180 is closer toopen cluster remnants than NGC 3680.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
Katalógusok és elnevezések:
|
