Seven-color CCD photometry in the Vilnius photometric system of 279 stars down to V=15mag in the open cluster M 67 area is obtained. 13 standard stars in the cluster are measured photoelectrically. Photometric spectral types have been determined for all the stars. The reddening of the cluster is found to be E(B-V)=0.045, the true distance modulus is 9.38mag and the age is 4x10^9^ years. For a description of the Vilnius photometric system, see e.g. <GCPD/21>
Seven color CCD photometry in the Vilnius system, supplemented by the Cousins I passband, has been obtained for 412 stars down to V=16mag in the M67 open cluster area. A special method of flat-fielding, giving high accuracy photometry, was used. Photometric spectral types and interstellar reddenings of all stars were determined. The mean mean reddening of the cluster stars is E(B-V)=0.04mag and the distance is 770pc.
We have used the 2nd version of the Catalogue of Radial Velocities with Astrometric Data (CRVAD-2, Cat. <III/254>) for a new determination of mean RVs of 363 open clusters and stellar associations considering their defined members from proper motions and photometry in the All-Sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5, Cat. <I/280>). For 330 clusters and associations we compiled previously published RVs from the literature, critically reviewed and partly revised them. The resulting Catalogue of Radial Velocities of Open Clusters and Associations (CRVOCA) provides RVs for 516 open clusters and stellar associations in the Solar neighbourhood. Objects in the catalogue are sorted in the order of increasing right ascension J2000.
We have used medium-resolution spectra to search for evidence that proto-stellar objects accrete at high rates during their early 'assembly phase'. Models predict that depleted lithium and reduced luminosity in T-Tauri stars are key signatures of 'cold' high-rate accretion occurring early in a star's evolution. We found no evidence in 168 stars in NGC 2264 and the Orion nebula cluster for strong lithium depletion through analysis of veiling-corrected 6708{AA} lithium spectral line strengths. This suggests that 'cold' accretion at high rates (dM/dt>=5x10^-4^M_{sun}_/yr) occurs in the assembly phase of fewer than 0.5 percent of 0.3<=M*<=1.9M_{sun}_ stars. We also find that the dispersion in the strength of the 6708{AA} lithium line might imply an age spread that is similar in magnitude to the apparent age spread implied by the luminosity dispersion seen in colour-magnitude diagrams. Evidence for weak lithium depletion (<10% in equivalent width) that is correlated with luminosity is also apparent, but we are unable to determine whether age spreads or accretion at rates less than 5x10^-4^M_{sun}_/yr are responsible.
A CCD study of the peculiar open cluster, Melotte 66, on the vbyHbeta system is presented. The V,(b-y) color-magnitude diagram of this metal-deficient, old disk cluster confirms the anomalous features found in photographic BV studies and extends the main sequence to V=20. The main sequence exhibits a (b-y) color range significantly larger than expected from photometric errors alone; the (b-y) colors are correlated with Hbeta implying that the dispersion is real and is not the result of variable reddening across the cluster. The m1 indices for the turnoff stars imply a spread in [Fe/H] too small to explain the color range. Though the subgiant region is poorly defined, the region above the turnoff in the Hertzsprung gap is richly populated by a group of "yellow stragglers" whose membership is confirmed by radial star counts and radial velocities. The giant branch is richly populated but shows a smaller spread in color at a given V than one would expect from the turnoff region. More important, the giants exhibit a large scatter in m1, in contradiction with the turnoff observations. Various explanations for these observations are discussed, including CN variations among the giants triggered by either binary evolution or rapid rotation among the main-sequence stars.
Photoelectric and photographic photometry of the open cluster Melotte 66 is presented. The CM diagram shows most of the characteristics of an old cluster. The giant branch is broad with its blue edge populated preferentially by stars from the outer parts of the cluster. There is no detectable horizontal subgiant sequence. The main sequence turn-off colour, two-colour diagram and the colour difference between the turn-off and the subgiants are used to estimate the age and composition. Melotte 66 appears to have reddening E(B-V)=0.17mag and ultraviolet excess d(U-B)~0.1mag corresponding to [Fe/H]=-0.3. The cluster is probably between 6 and 7x10^9^yr old. A distance modulus (m-M)o=12.4mag is derived, which implies that the cluster lies about 750pc from the galactic plane.
Previous studies of open clusters have shown that lithium depletion is not only strongly age dependent but also shows a complex pattern with other parameters that is not yet understood. For pre- and main-sequence late-type stars, these parameters include metallicity, mixing mechanisms, convection structure, rotation, and magnetic activity. We perform a thorough membership analysis for a large number of stars observed within the Gaia-ESO survey (GES) in the field of 20 open clusters, ranging in age from young clusters and associations, to intermediate-age and old open clusters. Based on the parameters derived from the GES spectroscopic observations, we obtained lists of candidate members for each of the clusters in the sample by deriving RV distributions and studying the position of the kinematic selections in the EW(Li) versus Teff plane to obtain lithium members. We used gravity indicators to discard field contaminants and studied [Fe/H] metallicity to further confirm the membership of the candidates. We also made use of studies using recent data from the Gaia DR1 and DR2 releases to assess our member selections. We identified likely member candidates for the sample of 20 clusters observed in GES (iDR4) with UVES and GIRAFFE, and conducted a comparative study that allowed us to characterize the properties of these members, as well as identify field contaminant stars, both lithium-rich giants and non-giant outliers. This work is the first step towards the calibration of the lithium-age relation and its dependence on other GES parameters. During this project we aim to use this relation to infer the ages of GES field stars, and identify their potential membership to young associations and stellar kinematic groups of different ages.
It is now well known that pre-main sequence models with inflated radii should be taken into account to simultaneously reproduce the colour-magnitude diagram and the lithium depletion pattern observed in young open star clusters. We test a new set of pre-main sequence models including radius inflation due to the presence of starspots or to magnetic inhibition of convection, using five clusters observed by the Gaia-ESO Survey, spanning the age range ~10-100Myr where such effects could be important. The Gaia-ESO Survey radial velocities are combined with astrometry from Gaia EDR3 to obtain clean lists of high-probability members for the five clusters. A Bayesian maximum likelihood method is adopted to fit the observed cluster sequences to theoretical predictions to derive the best model parameters and the cluster reddening and age. Models are calculated with different values of the mixing length parameter ({alpha}_ML_=2.0, 1.5 and 1.0), for the cases with no spots or with effective spot coverage {beta}_spot_=0.2 and 0.4. The models are also compared with the observed lithium depletion patterns. To reproduce the colour-magnitude diagram and the observed lithium depletion pattern in Gamma Vel A and B and in 25 Ori one needs to assume both a reduced convection efficiency, with {alpha}_ML_=1.0, and an effective surface spot coverage of about 20%. We obtained ages of 18^+1.5^_-4.0_ Myr and 21^+3.5^_-3.0_ Myr for Gamma Vel A and B, respectively, and 19^+1.5^_-7.0_ Myr for 25 Ori. However, a single isochrone is not sufficient to account for the lithium dispersion, and an increasing level of spot coverage as mass decreases seems to be required. On the other hand, the older clusters (NGC 2451B at 30^+3.0^_-5.0_Myr, NGC 2547 at 35^+4.0^_-4.0_Myr, and NGC 2516 at 138^+48^_-42_Myr) are consistent with standard models, with {alpha}_ML_=2.0 and no spots, except at low masses: a 20% spot coverage appears to better reproduce the sequence of M-type stars and might explain the observed spread in lithium abundances. The quality of Gaia-ESO data combined with Gaia allows us to gain important insights on pre-main sequence evolution. Models including starspots can provide a consistent explanation of the cluster sequences and lithium abundances observed in young clusters, although a range of starspot coverage is required to fully reproduce the data.
Young open clusters provide important clues to the interface between the main sequence and pre-main-sequence phases of stellar evolution. The young and nearby open cluster IC 2391 is well-suited to studies of these two evolutionary phases. We establish a bona fide set of cluster members and then analyze this set in terms of binary frequency, projected rotational velocities, [Fe/H], and lithium abundance. In the wake of the Hipparcos distance controversy for the Pleiades, we compare the main-sequence fitting distance modulus to the Hipparcos mean parallax for IC 2391.
Membership of Praesepe and Coma Berenices clusters
Short Name:
J/AJ/134/2340
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the results of a stellar membership survey of the nearby open clusters Praesepe and Coma Berenices. We have combined archival survey data from the SDSS, 2MASS, USNOB1.0, and UCAC-2.0 surveys to compile proper motions and photometry for ~5 million sources over 300{deg}^2^. Of these sources, 1010 stars in Praesepe and 98 stars in Coma Ber are identified as candidate members with probability >80%; 442 and 61 are identified as high-probability candidates for the first time. We estimate that this survey is >90% complete across a wide range of spectral types (F0-M5 in Praesepe, F5-M6 in Coma Ber). We have also investigated the stellar mass dependence of each cluster's mass and radius in order to quantify the role of mass segregation and tidal stripping in shaping the present-day mass function and spatial distribution of stars.
