Turnoff Age Research Articles

CCD UBV(RI)KC photometry and dynamics of the open cluster NGC 1513

ABSTRACT We derive astrophysical parameters of the open cluster NGC 1513 by means of colour indices built with new CCD UBV(RI)KC photometry. Based on early-type members, the mean foreground reddening and total to selective extinction ratio are E(B − V) = 0.79 ± 0.09 mag and RV = 2.85 ± 0.05. Through the differential grid method, we derive the metal abundance [Fe/H] = −0.06 dex (Z = +0.013), which is consistent with the value [Fe/H] = −0.088 of the bright giant member – LAMOST 695710060. Z = +0.013 isochrone fit to the V × (B − V) colour–magnitude diagram leads to a turn-off age of 224 ± 27 Myr (thus an intermediate-age cluster), and a distance modulus of (V0 − MV) = 10.90 ± 0.15 mag, thus implying a distance from the Sun d = 1514 ± 105 pc. Within the uncertainties, our photometric distance is consistent with the value d = 1435 ± 14 pc from the Gaia DR3 parallax. We find signs of small mass segregation through a minimum spanning tree analysis for the 190 most massive stars, together with the rather steep mass function (χ = +2.39) slope. The high core to half-light radius ratio Rcore/Rh = 0.82, together with the compact half-light to tidal radius ratio Rh/Rt = 0.22, suggest that it is probably related to cluster-formation effects, due to little dynamical evolution, instead of driving its dynamical evolution by internal relaxation. Indeed, NGC 1513 is located in the second quadrant (ℓ = 152${_{.}^{\circ}}$59 and Galactocentric distance RGC = 9.57 kpc), which tends to minimize tidal effects by external processes and tidal disruption. Therefore, internal mass segregation effects in NGC 1513 seem to be more efficient than cluster evaporation processes. We find that NGC 1513 migrated about 0.50 kpc from its birth place.

The HST large programme on NGC 6752 – IV. The White Dwarf sequence

ABSTRACT We present our final study of the white dwarf cooling sequence (WD CS) in the globular cluster NGC 6752. The investigation is the main goal of a dedicated Hubble Space Telescope large program, for which all the observations are now collected. The WD CS luminosity function (LF) is confirmed to peak at mF606W ≃ 29.3 ± 0.1, consistent within uncertainties with what has been previously reported, and is now complete down to mF606W ≃ 29.7. We have performed robust and conclusive comparisons with model predictions that show how the theoretical LF for hydrogen envelope WD models closely follow the shape of the empirical LF. The magnitude of the peak of the observed LF is matched with ages between 12.7 and 13.5 Gyr, consistent with the cluster age derived from the main sequence turn off and subgiant branch. We also find that the impact of multiple populations within the cluster on the WD LF for mF606W below 27.3 is negligible, and that the presence of a small fraction of helium envelope objects is consistent with the data. Our analysis reveals a possible hint of an underestimate of the cooling time-scales of models in the magnitude range 28.1 < mF606W < 28.9. Finally, we find that hydrogen envelope models calculated with a new tabulation of electron conduction opacities in the transition between moderate and strong degeneracy provide WD ages that are too small in comparison to the main sequence turnoff age.

$$\varvec{U\!BV\!I}$$ CCD photometry of Berkeley 55 open cluster

Fundamental astrophysical parameters have been derived for Berkeley 55 (Be 55) open cluster based on the UBVI charge-coupled device (CCD) photometric data observed using a AZT-22 1.5 m telescope at Maidanak Astronomical Observatory (MAO) in Uzbekistan. The mean reddening is obtained as \(E(B-V)=1.77\pm 0.10\,\mathrm {mag}\) from early-type members. The zero-age main sequence fitting in the \(Q_{V\lambda }\) – \(Q'\) diagrams indicates the distance modulus, \((V_{0}-M_{V})=12.4\pm 0.20\,\mathrm {mag}\,(d=3.02\pm 0.28\,\mathrm {kpc}\)). This photometric distance is consistent with the distances of Gaia EDR3 (\(d = 3.09 \pm 0.16\,\mathrm {kpc}\)) and period–luminosity relation (\(d =\) 2.78 ± 0.32 kpc) of its Cepheid S5 within the uncertainties. This distance also locates the cluster near the Perseus spiral arm. The Geneva isochrone fittings to the Hertzsprung–Russell diagram and observational colour–magnitude diagrams derive turn-off age, 85 ± 13 Myr, by taking care of five red supergiants/bright giants. The possible inconsistencies in the locations of the bright giants with the rotating/non-rotating isochrones may be due to both the age spread of stars in young open clusters and the diversity in rotational velocities.

Estimating stellar ages and metallicities from parallaxes and broadband photometry: successes and shortcomings

A deep understanding of the Milky Way galaxy, its formation and evolution requires observations of huge numbers of stars. Stellar photometry, therefore, provides an economical method to obtain intrinsic stellar parameters. With the addition of distance information – a prospect made real for more than a billion stars with the second Gaia data release – deriving reliable ages from photometry is a possibility. We have developed a Bayesian method that generates 2D probability maps of a star’s age and metallicity from photometry and parallax using isochrones. Our synthetic tests show that including a near-UV passband enables us to break the degeneracy between a star’s age and metallicity for certain evolutionary stages. It is possible to find well-constrained ages and metallicities for turn-off and sub-giant stars with colours including a U band and a parallax with uncertainty less than ∼20%. Metallicities alone are possible for the main sequence and giant branch. We find good agreement with the literature when we apply our method to the Gaia benchmark stars, particularly for turn-off and young stars. Further tests on the old open cluster NGC 188, however, reveal significant limitations in the stellar isochrones. The ages derived for the cluster stars vary with evolutionary stage, such that turn-off ages disagree with those on the sub-giant branch, and metallicities vary significantly throughout. Furthermore, the parameters vary appreciably depending on which colour combinations are used in the derivation. We identify the causes of these mismatches and show that improvements are needed in the modelling of giant branch stars and in the creation and calibration of synthetic near-UV photometry. Our results warn against applying isochrone fitting indiscriminately. In particular, the uncertainty on the stellar models should be quantitatively taken into account. Further efforts to improve the models will result in significant advancements in our ability to study the Galaxy.

Improved Main-sequence Turnoff Ages of Young Open Clusters: Multicolor UBV Techniques and the Challenges of Rotation

Abstract Main-sequence turnoff ages in young open clusters are complicated by turnoffs that are sparse, have high binarity fractions, can be affected by differential reddening, and typically include a number of peculiar stars. Furthermore, stellar rotation can have a significant effect on a star’s photometry and evolutionary timescale. In this paper we analyze in 12 nearby open clusters, ranging in age from 50 to 350 Myr, how broadband UBV color–color relations can be used to identify turnoff stars that are Be stars, blue stragglers, certain types of binaries, or those affected by differential reddening. This UBV color–color analysis also directly measures a cluster’s E(B − V) and estimates its [Fe/H]. The turnoff stars unaffected by these peculiarities create a narrower and more clearly defined cluster turnoff. Using four common isochronal models, two of which consider rotation, we fit cluster parameters using these selected turnoff stars and the main sequence. Comparisons of the photometrically fit cluster distances to those based on parallaxes from Gaia data release 2 find that they are consistent for all clusters. For older (>100 Myr) clusters, such as the Pleiades and the Hyades, comparisons to ages based on the lithium depletion boundary method find that these cleaned turnoff ages agree to within ∼10% for all four isochronal models. For younger clusters, however, only the Geneva models that consider rotation fit turnoff ages consistent with lithium-based ages, while the ages based on non-rotating isochrones quickly diverge to become 30%–80% younger. This illustrates the importance of rotation in deriving ages in the youngest (<100 Myr) clusters.

A BAYESIAN ANALYSIS OF THE AGES OF FOUR OPEN CLUSTERS

ABSTRACT In this paper we apply a Bayesian technique to determine the best fit of stellar evolution models to find the main sequence turn-off age and other cluster parameters of four intermediate-age open clusters: NGC 2360, NGC 2477, NGC 2660, and NGC 3960. Our algorithm utilizes a Markov chain Monte Carlo technique to fit these various parameters, objectively finding the best-fit isochrone for each cluster. The result is a high-precision isochrone fit. We compare these results with the those of traditional “by-eye” isochrone fitting methods. By applying this Bayesian technique to NGC 2360, NGC 2477, NGC 2660, and NGC 3960, we determine the ages of these clusters to be 1.35 ± 0.05, 1.02 ± 0.02, 1.64 ± 0.04, and 0.860 ± 0.04 Gyr, respectively. The results of this paper continue our effort to determine cluster ages to a higher precision than that offered by these traditional methods of isochrone fitting.

POPULATION SYNTHESIS OF CATACLYSMIC VARIABLES. I. INCLUSION OF DETAILED NUCLEAR EVOLUTION

We have carried out an extensive population synthesis study of the ensemble properties of the present-day population of cataclysmic variables (PDCVs) that takes into account the nuclear evolution of high-mass donors close to the bifurcation and dynamical instability limits. Assuming the interrupted magnetic braking paradigm, we confirm many of the general features associated with the observed CV population and find enormous diversity in their secular properties. We predict that nearly half of the non-magnetic CVs with Porb > 6 hours are at least mildly evolved (i.e., greater than one-half of their MS turn-off age). Some of these systems contribute to the observed population of PDCVs in the period gap. We also see an enhancement by up to a factor of two in the probability of detecting CVs at the `minimum period'. This spike is quite narrow (approximately 5 minutes) and is attenuated because of the spectrum of WD masses and partly by the evolution of the donors. Our syntheses imply that there should be a very rapid decline in the number of ultracompact CVs (such as AM CVns). We find that between 0.05 to 1 percent of PDCVs could be UCs, and thus it is likely that the CV channel is probably not the primary contributor to the intrinsic population of UCs (especially for Porb < 30 minutes). Finally, a preliminary analysis of our results suggests that WDs in PDCVs experience a net gain in mass of less than about 0.1 Msun as a result of high mass-transfer rates early in their evolution.

Optimum crossbreeding systems for goats in low-input livestock production system in Kenya

The aim of the current simulation study was to maximize the amount (kg) of high value meat arising from surplus males and females per kilogram of the goat population and, maximize weight gains to turnoff age. Subsequently, the study aimed to decrease turnoff age, maximization of reproduction rates and minimization of the mature weight of does in the low-input livestock production system in Kenya. Three crossbreeding systems and a synthetic breed development were evaluated, namely; (a) straight breeding system that utilized pure Small East African goat (SEAG) where parental lines were maintained to generate the desired terminal crosses, (b) three-breed crossbreeding system that utilized SEAG as pure breed, Galla goat (GG) as the first cross sire breed, and Improved Boer goat (IBG) as the terminal sire, (c) two-breed rotational crossbreeding system where the SEAG and IBG were purebreds, and (d) synthetic breed development system utilizing SEAG×IBG. The HotCross crossbreeding simulation software was used to assess the predicted performance of the different goat breed crosses under conditions of agro ecological zones (AEZ) V and VI. A model was developed to compare these crossbreeding systems, and showed the optimum numbers required in each stage of a cross to maximize production. In the AEZ V, it was found that the three-breed terminal system gave 18.2kg (76%) improvement over straightbreds per doe mated per year. The multi-breed composite realized 20.4kg (86%) improvement over pure SEAG. In the AEZ VI, it was found that the three-breed terminal system gave 10kg (64%) improvement over straightbreds per doe mated per year. The multi-breed composite realized 14.6kg (94%) improvement straightbreds per doe mated per year. The two-breed rotation was worse than the multi-breed composite in both environments. This implies that in low-input livestock production system, a multi-breed composite may be the crossbreeding system of choice, so long as supportive infrastructure is put in place.

Sejong Open Cluster Survey (SOS) – III. The young open cluster NGC 1893 in the H ii region W8

We present a UBV I and H alpha photometric study of the young open cluster NGC 1893 in the H II region W8 (IC 410 or Sh 2-236). A total of 65 early-type members are selected from photometric diagrams. A mean reddening of the stars is <E(B-V)> = 0.563 +/- 0.083 mag. The published photometric data in the near- and mid-infrared passbands are used to test the reddening law toward the cluster, and we confirm that the reddening law is normal (R_V = 3.1). Zero-age main sequence fitting gives a distance modulus of V_0 - M_V = 12.7 +/- 0.2 mag, equivalent to 3.5 +/- 0.3 kpc. From H alpha photometry 125 H alpha emission stars and candidates are identified as pre-main sequence (PMS). The lists of young stellar objects and X-ray sources published by previous studies allow us to select a large number of PMS members down to 1 M_sun. Isochrone fitting in the Hertzsprung-Russell diagram gives a turn-off age of 1.5 Myr and the median age of 1.9 Myr from the PMS members with a spread of 5 Myr. We derive the initial mass function (IMF) for stars with mass larger than 1 M_sun. The slope of the IMF (Gamma = -1.3 +/- 0.1) is well consistent with the Salpeter/Kroupa IMF. A total mass of the cluster appears to be in excess of 1,300 M_sun. Finally, we estimate the mass accretion rate of 82 PMS members in the mass range of 0.6 M_sun to 5 M_sun.

A consistency test of white dwarf and main sequence ages: NGC 6791

NGC 6791 is an open cluster that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (∼8 Gyr) and the age derived from the cut-off of the white dwarf luminosity function (∼6 Gyr) were found to be significantly different. Here we demonstrate that the origin of this age discrepancy lies in an incorrect evaluation of the white dwarf cooling ages, and we show that when the relevant physical separation processes are included in the calculation of white dwarf sequences both ages are coincident.

THE WHITE DWARF AGE OF NGC 2477

We present deep photometric observations of the open cluster NGC 2477 using HST/WFPC2. By identifying seven cluster white dwarf candidates, we present an analysis of the white dwarf age of this cluster, using both the traditional method of fitting isochrones to the white dwarf cooling sequence, and by employing a new Bayesian statistical technique that has been developed by our group. This new method performs an objective, simultaneous model fit of the cluster and stellar parameters (namely age, metallicity, distance, reddening, as well as individual stellar masses, mass ratios, and cluster membership) to the photometry. Based on this analysis, we measure a white dwarf age of 1.035 +/- 0.054 +/- 0.087 Gyr (uncertainties represent the goodness of model fits and discrepancy among models, respectively), in good agreement with the cluster's main sequence turnoff age. This work is part of our ongoing work to calibrate main sequence turnoff and white dwarf ages using open clusters, and to improve the precision of cluster ages to the ~5% level.

EMPLOYING A NEW,BVIcPHOTOMETRIC SURVEY OF IC 4665 TO INVESTIGATE THE AGE OF THIS YOUNG OPEN CLUSTER

We present a new, BVIc photometric survey of the young open cluster IC4665, which improves on previous studies of this young cluster by incorporating a rigorous standardization procedure, thus providing high-fidelity colors and magnitudes for cluster members. We use this new photometric dataset to reevaluate the properties (age and distance) of IC4665. Namely, using a statistical approach incorporating Tau^2 CMD modeling, we measure a pre-main-sequence isochrone age and distance of 36+-9 Myr and 360+-12 pc, as well as a upper-main-sequence turn-off age and distance of 42+-12 Myr and 357+-12 pc. These ages and distances are highly dependent on the isochrone model and color used for the fitting procedure, with a possible range of ~10-20 Myr in age and ~20 pc in distance. This spread in calculated ages and distances seen between colors and models is likely due to limitations in the individual membership catalogs and/or systematic differences in the predicted stellar parameters from the different sets of models. Interestingly, when we compare the isochrone ages for IC4665 to the published lithium depletion boundary age, 28+-5 Myr, we observe that this cluster does not appear to follow the trend of isochrone ages being 1.5 times smaller than lithium depletion boundary ages. In addition, comparing the overall magnetic activity (X-ray and H-alpha emission) in IC4665 with other well studied open clusters, we find the observed activity distributions for this young cluster are best characterized by assuming an age of 30-40 Myr, thus in agreement with our pre-main-sequence and turn-off isochrone ages for IC4665. Overall, although some age discrepancies do exist, particularly in the ages measured from pre-main-sequence isochrones, the range of possible IC4665 ages derived from the various dating techniques employed here is relatively small compared to that found for other well studied open clusters.

EVOLUTION OF WHITE DWARF STARS WITH HIGH-METALLICITY PROGENITORS: THE ROLE OF22Ne DIFFUSION

Motivated by the strong discrepancy between the main sequence turn-off age and the white dwarf cooling age in the metal-rich open cluster NGC 6791, we compute a grid of white dwarf evolutionary sequences that incorporates for the first time the energy released by the processes of 22Ne sedimentation and of carbon/oxygen phase separation upon crystallization. The grid covers the mass range from 0.52 to 1.0 Msun, and it is appropriate for the study of white dwarfs in metal-rich clusters. The evolutionary calculations are based on a detailed and self-consistent treatment of the energy released from these two processes, as well as on the employment of realistic carbon/oxygen profiles, of relevance for an accurate evaluation of the energy released by carbon/oxygen phase separation. We find that 22Ne sedimentation strongly delays the cooling rate of white dwarfs stemming from progenitors with high metallicities at moderate luminosities, whilst carbon/oxygen phase separation adds considerable delays at low luminosities. Cooling times are sensitive to possible uncertainties in the actual value of the diffusion coefficient of 22Ne. Changing the diffusion coefficient by a factor of 2, leads to maximum age differences of approx. 8-20% depending on the stellar mass. We find that the magnitude of the delays resulting from chemical changes in the core is consistent with the slow down in the white dwarf cooling rate that is required to solve the age discrepancy in NGC 6791.

A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

NGC 6791 is a well studied open cluster that it is so close to us that can be imaged down to very faint luminosities. The main-sequence turn-off age ( approximately 8 Gyr) and the age derived from the termination of the white dwarf cooling sequence ( approximately 6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, (22)Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf ((12)C and (16)O) is expected to occur. This sequence of events is expected to introduce long delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791.

GALACTIC GLOBULAR AND OPEN CLUSTERS IN THE SLOAN DIGITAL SKY SURVEY. II. TEST OF THEORETICAL STELLAR ISOCHRONES

We perform an extensive test of theoretical stellar models for main-sequence (MS) stars in ugriz, using cluster fiducial sequences obtained in the previous paper of this series. We generate a set of isochrones using the Yale Rotating Evolutionary Code with updated input physics, and derive magnitudes and colors in ugriz from MARCS model atmospheres. These models match cluster MSs over a wide range of metallicity within the errors of the adopted cluster parameters. However, we find a large discrepancy of model colors at the lower MS (T {sub eff} {approx}< 4500 K) for clusters at and above solar metallicity. We also reach similar conclusions using the theoretical isochrones of Girardi et al. and Dotter et al., but our new models are generally in better agreement with the data. Using our theoretical isochrones, we also derive MS-fitting distances and turnoff ages for five key globular clusters, and demonstrate the ability to derive these quantities from photometric data in the Sloan Digital Sky Survey. In particular, we exploit multiple color indices (g - r, g - i, and g - z) in the parameter estimation, which allows us to evaluate internal systematic errors. Our distance estimates, with an error of {sigma}{sub (m-M)}more » = 0.03-0.11 mag for individual clusters, are consistent with Hipparcos-based subdwarf-fitting distances derived in the Johnson-Cousins or Stroemgren photometric systems.« less

INVERTING COLOR-MAGNITUDE DIAGRAMS TO ACCESS PRECISE STAR CLUSTER PARAMETERS: A NEW WHITE DWARF AGE FOR THE HYADES

We have extended our Bayesian modeling of stellar clusters—which uses main-sequence stellar evolution models, a mapping between initial masses and white dwarf (WD) masses, WD cooling models, and WD atmospheres—to include binary stars, field stars, and two additional main-sequence stellar evolution models. As a critical test of our Bayesian modeling technique, we apply it to Hyades UBV photometry, with membership priors based on proper motions and radial velocities, where available. Under the assumption of a particular set of WD cooling models and atmosphere models, we estimate the age of the Hyades based on cooling WDs to be 648 ± 45 Myr, consistent with the best prior analysis of the cluster main-sequence turnoff (MSTO) age by Perryman et al. Since the faintest WDs have most likely evaporated from the Hyades, prior work provided only a lower limit to the cluster’s WD age. Our result demonstrates the power of the bright WD technique for deriving ages and further demonstrates complete age consistency between WD cooling and MSTO ages for seven out of seven clusters analyzed to date, ranging from 150 Myr to 4 Gyr.

Towards a precise white dwarf cooling age of a globular cluster

AbstractThe white dwarf cooling age of a globular star cluster provides a potentially precise method of determining the ages of these ancient systems. This age-dating technique should be viewed as one distinct from that of turn-off ages, with a largely different set of input physics and problems. As such the ages produced by these two methods are complimentary and we seek convergent to the same value. In addition to deep photometry and astrometry of cluster stars, precise distances to the clusters and their reddenings are required. Theoretical models of both main sequence stars and cooling white dwarfs are also needed as well as the masses of the white dwarfs and an initial-final mass relationship. In this contribution I discuss a potentially precise approach to cluster distances via a geometric technique (comparing the internal proper motion dispersion of cluster stars with their radial velocity dispersion) and spectroscopically determined masses of M4 white dwarfs at the top of the cooling sequence. These latter data extend the initial-final mass relationship down to the lowest mass stars that are currently forming white dwarfs.

A Suprime-Cam study of the stellar population of the Ursa Major I dwarf spheroidal galaxy

We present deep and wide V , I CCD photometry of the Ursa Major I (UMa I) dwarf spheroidal galaxy (dSph) in the Local Group. The images of the galaxy were taken with the Subaru/Suprime-Cam wide field camera, covering a field of 34´ $\times$ 27´ located at the centre of the galaxy. The colour-magnitude diagram (CMD) of the UMa I dSph shows a steep and narrow red giant branch (RGB), blue and red horizontal branch (HB), and main sequence (MS) stars. A well-defined main sequence turn-off (MSTO) is found to be located at $V_{0,\rm MSTO}$ ~ 23.5 mag. The distance modulus is derived as $(m-M)_0$ = 19.93 ± 0.1 (corresponding to a distance D = 96.8 ± 4 kpc) from the V -band magnitude of the horizontal branch ($V_{0,\rm HB}$ = 20.45 ± 0.02). The mean metallicity of the RGB stars is estimated by the $V-I$ colour as [Fe/H] ~ -2.0. The turn-off age estimated by overlaying the theoretical isochrones reveals that most of stars in the UMa I dSph are formed at a very early epoch (~12 Gyr ago). The isopleth map of stellar number density of the UMa I dSph, based upon the resolved star counts of MS, RGB, HB stars as well as blue stragglers (BS), shows that the morphology of the UMa I dSph is quite irregular and distorted, suggesting that the galaxy is in a process of disruption. The very old and metal-poor nature of the stellar population implies that the star formation history of this newly discoverd faint dSph may have been different from other well-known “classical” dSphs, which show significant stellar populations of intermediate age. The stellar population of the UMa I dSph closely resembles that of Galactic old metal-poor globular clusters, but its size is typical of Galactic dSphs ( r e = 188 [pc], $r_{1/2}$ = 300 [pc]), and the shape of its spatial density contours suggests that it is undergoing tidal disruption. These characteristics of stellar population and spatial distribution of the faint galaxies help us to understand how they formed and evolved, and give a hint to the nature of the building blocks of hierarchical galaxy formation.

Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low‐Mass White Dwarfs

We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ 0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS spectra, suggests that most of these stars are undermassive, <M> = 0.43 +/- 0.06 Msun, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791's evolved stars must have lost enough mass on the red giant branch to avoid the flash, and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen core stars and therefore the corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.

NGC 7419: a young open cluster with a number of very young intermediate mass pre-MS stars

We present a photometric and spectroscopic study of the young open cluster NGC 7419, which is known to host a large number of classical Be stars for reasons not well understood. Based on CCD photometric observations of 327 stars in UBV passbands, we estimated the cluster parameters as, reddening (E(B V)) = 1.65±0.15 mag and distance = 2900 ± 400 pc. The turn-off age of the cluster was estimated as 25± 5 Myr using isochrone fits. UBV data of the stars were combined with the JHK data from 2MASS and were used to create the near infrared (NIR) (J H) vs (H K) colourcolour diagram. A large fraction of stars (42%) was found to have NIR excess and their location in the diagram was used to identify them as intermediate mass pre-MS stars. The isochrone fits to pre-MS stars in the optical colour-magnitude diagram showed that the turn-on age of the cluster is 0.3 –3 Myr. This indicates that there has been a recent episode of star formation in the vicinity of the cluster.