Cluster Main-sequence Turnoff Research Articles

Discovery of a Double Blue Straggler Sequence in M15: New Insight into the Core-collapse Process

Abstract In this paper we report the discovery of a double blue straggler star (BSS) sequence in the core of the core-collapsed cluster M15 (NGC 7078). We performed a detailed photometric analysis of the extremely dense core of the cluster using a set of images secured with the Advanced Camera for Survey in the High Resolution Channel mode on board the Hubble Space Telescope. The proper combination of the large number of single frames in the near-UV (F220W), and blue (F435W) filters allowed us to perform a superb modeling of the point-spread function and an accurate deblending procedure. The color–magnitude diagram revealed the presence of two distinct parallel sequences of blue stragglers. In particular, the blue BSS sequence is characterized by the intriguing presence of two different branches. The first branch appears extremely narrow, it extends up to 2.5 mag brighter than the cluster main-sequence turnoff (MS-TO) point, and it is nicely reproduced by a 2 Gyr old collisional isochrone. The second branch extends up to 1.5 mag from the MS-TO and it is reproduced by a 5.5 Gyr old collisional isochrone. Our observations suggest that each of these branches is mainly constituted by a population of nearly coeval collisional BSSs of different masses generated during two episodes of high collisional activity. We discuss the possibility that the oldest episode corresponds to the core-collapse (CC) event (occurred about 5.5 Gyr ago), while the most recent one (occurred about 2 Gyr ago) is associated with a core oscillation in the post-CC evolution. The discovery of these features provides further strong evidence in support of the connection between the BSS properties and globular cluster dynamical evolution, and it opens new perspectives on the study of CC and post-CC evolution.

THE OVERLOOKED ROLE OF STELLAR VARIABILITY IN THE EXTENDED MAIN SEQUENCE OF LMC INTERMEDIATE-AGE CLUSTERS

ABSTRACT Intermediate-age star clusters in the Large Magellanic Cloud show extended main sequence turnoffs (MSTOs) that are not consistent with a canonical single stellar population. These broad turnoffs have been interpreted as evidence for extended star formation and/or stellar rotation. Since most of these studies use single frames per filter to do the photometry, the presence of variable stars near the MSTO in these clusters has remained unnoticed and their impact has been totally ignored. We model the influence of Delta Scuti using synthetic CMDs, adding variable stars following different levels of incidence and amplitude distributions. We show that Delta Scuti observed at a single phase will produce a broadening of the MSTO without affecting other areas of a CMD such as the upper MS or the red clump; furthermore, the amount of spread introduced correlates with cluster age, as observed. This broadening is constrained to ages ∼1–3 Gyr when the MSTO area crosses the instability strip, which is also consistent with observations. Variable stars cannot explain bifurcarted MSTOs or the extended MSTOs seen in some young clusters, but they can make an important contribution to the extended MSTOs in intermediate-age clusters.

THE PUTATIVE OLD, NEARBY CLUSTER LODÉN 1 DOES NOT EXIST*

ABSTRACT Astronomers have access to precious few nearby, middle-aged benchmark star clusters. Within 500 pc, there are only NGC 752 and Ruprecht 147 (R147) at 1.5 and 3 Gyr, respectively. The Database for Galactic Open Clusters (WEBDA) also lists Lodén 1 as a 2 Gyr cluster at a distance of 360 pc. If this is true, Lodén 1 could become a useful benchmark cluster. This work details our investigation of Lodén 1. We assembled archival astrometry (PPMXL) and photometry (2MASS, Tycho-2, APASS), and acquired medium resolution spectra for radial velocity measurements with the Robert Stobie Spectrograph at the Southern African Large Telescope. We observed no sign of a cluster main-sequence turnoff or red giant branch among all stars in the field brighter than . Considering the 29 stars identified by L.O. Lodén and listed on SIMBAD as the members of Lodén 1, we found no compelling evidence of kinematic clustering in proper motion or radial velocity. Most of these candidates are A stars and red giants, and their observed properties are consistent with distant field stars in the direction of Lodén 1 in the Galactic plane. We conclude that the old nearby cluster Lodén 1 is neither old, nor nearby, nor a cluster.

PHOTOMETRIC AND STRUCTURAL PROPERTIES OF NGC 6544: A COMBINED VVV-HUBBLE SPACE TELESCOPESTUDY

We combine archival Hubble Space Telescope imaging with wide-field near-infrared photometry to study the neglected metal-poor Galactic globular cluster NGC 6544. A high spatial resolution map of differential reddening over the inner portion of the cluster is constructed, revealing variations of up to half of the total reddening, and the resulting corrected color-magnitude diagrams reveal a sparse blue horizontal branch and centrally concentrated blue straggler population, verified via relative proper motions. Using the corrected photometry to investigate the cluster distance, reddening, and age via direct comparison to well-calibrated photometry of clusters with similar metallicities, we estimate (m – M)0 = 11.96, E(B – V) = 0.79, and an age coeval with M13 to within the relevant uncertainties. Although our data are insufficient to place tight constraints on the reddening law toward NGC 6544, we find no strong evidence that it is non-standard at optical or near-infrared wavelengths. We also provide near-infrared fiducial sequences extending nearly 2 mag below the cluster main sequence turnoff, generated from a statistically decontaminated sample of cluster stars. Lastly, we redetermine the cluster center and construct a radial number density profile which is well fit by an atypically flat power law with a slope of about 1.7. We discuss this result, together with a flattened main sequence luminosity function and inverted mass function, in the context of mass segregation and tidal stripping via interactions with Milky Way potential.

BLUE STRAGGLER MASSES FROM PULSATION PROPERTIES. I. THE CASE OF NGC 6541

We used high-spatial resolution images acquired with the WFC3 on board HST to probe the population of variable blue straggler stars in the central region of the poorly studied Galactic globular cluster NGC 6541. The time sampling of the acquired multi wavelength (F390W, F555W and F814W) data allowed us to discover three WUma stars and nine SX Phoenicis. Periods, mean magnitudes and pulsation modes have been derived for the nine SX Phoenicis and their masses have been estimated by using pulsation equations obtained from linear non adiabatic models. We found masses in the range 1.0-1.1Mo, with an average value of 1.06+-0.09 Mo (sigma = 0.04), significantly in excess of the cluster Main Sequence Turn Off mass (~ 0.75Mo). A mild trend between mass and luminosity seems also to be present. The computed pulsation masses turn out to be in very good agreement with the predictions of evolutionary tracks for single stars, indicating values in the range ~ 1.0-1.2 Mo for most of the BSS population, in agreement with what discussed in a number of previous studies.

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.

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.

Multi-Conjugate Adaptive Optics VLT imaging of the distant old open cluster FSR 1415

We employ the recently installed near-infrared Multi-Conjugate Adaptive Optics demonstrator (MAD) to determine the basic properties of a newly identified, old and distant, Galactic open cluster (FSR 1415). The MAD facility remarkably approaches the diffraction limit, reaching a resolution of 0.07 arcsec (in K), that is also uniform in a field of ∼1.8 arcmin in diameter. The MAD facility provides photometry that is 50 per cent complete at K ∼ 19. This corresponds to about 2.5 mag below the cluster main-sequence turn-off. This high-quality data set allows us to derive an accurate heliocentric distance of 8.6 kpc, a metallicity close to solar and an age of ∼2.5 Gyr. On the other hand, the deepness of the data allows us to reconstruct (completeness-corrected) mass functions (MFs) indicating a relatively massive cluster, with a flat core MF. The Very Large Telescope/MAD capabilities will therefore provide fundamental data for identifying/analysing other faint and distant open clusters in the Galaxy III and IV quadrants.

Mass Segregation and Tidal Tails of the Globular Cluster NGC 7492

We present a wide-field CCD photometric study of the Galactic globular cluster NGC 7492. The derived VR color-magnitude diagram (CMD) extends down to about 3.5 mag below the cluster main-sequence turnoff. The field covers 42' × 42', about 3 times larger than the known tidal radius of this cluster. The sample of cluster member candidates obtained by the CMD-mask process has been used to construct luminosity (LFs) and mass functions (MFs) and a surface density map. NGC 7492 has a very flat MF with very little variation in the slope with distance from the cluster center. However, there is a clear evidence for an increase of the MF slope from inner to outer regions, indicating mass segregation of the cluster. The surface density map of NGC 7492 shows extensions toward the Galactic anticenter (northeast) and northwest from the cluster center. A comparison of the LF for stars in the tails with that for stars within the tidal radius suggests that the extensions shown in the surface density map could be a real feature. The overall shape of NGC 7492 is significantly flattened. If the flattened shape of the NGC 7492 is caused by its rotation, the Galactic tidal field must have been an important influence, since the initial rotation would have been almost completely removed by dynamical relaxation.

Wide-Field CCD Photometry of the Globular Cluster M92

We present wide field CCD photometry of a galactic globular cluster M92 obtained in the V and I bands with the CFH12K mosaic CCD at the Canada-France-Hawaii Telescope. A well-defined color-magnitude diagram is derived down to 5 magnitudes fainter than the cluster main sequence turn-off. After removing the background contribution, we obtain luminosity and mass functions, surface density profiles, and the surface number density maps of the stars belonging to the cluster. The surface density profile of all stars shows that the cluster's halo extends at least out to ~30' from the cluster center in agreement with previous study, but the profile of faint stars at the very outer region of the cluster shows a different gradient compared with that of bright stars. For a mass function of the cluster, we find that the inner region of the cluster has x = 1.2+-0.2, whereas the outer region has x = 1.8 +-0.3, clearly indicating a mass segregation of the cluster. An estimate of the photometric mass of the cluster implies that the remnant populations (white dwarfs and neutron stars) contribute at least 25% of the total cluster mass. The surface density map of M92 shows some evidence that the tidal tail of M92 may be oriented perpendicular to the direction toward the Galactic center.

The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy

We present a deep color-magnitude diagram in the VI passbands of the globular cluster M54, a member of the Sagittarius dwarf galaxy. The data extend below the cluster's main sequence turn-off, allowing us to estimate the cluster's age. We find that M54 is 0.5--1.5 gigayears older than the Galactic globulars M68 and M5. In absolute terms, the age is comparable to the published age estimates of the other member clusters Arp 2 and Terzan 8, but is significantly older than the member cluster Terzan 7. An age estimate of the Sagittarius field population relative to M54 suggests that M54 is \gtrsim 3 Gyr older than the field. We discuss briefly the star formation history of the Sagittarius dwarf galaxy.

NTT Photometry in UBV of the Distant Globular Cluster NGC 1841

We present UBV CCD photometry of 1347 stars in the populous globular cluster NGC 1841 located approximately 15^deg^ from each of the Magellanic Clouds and probably dynamically associated with the LMC. The photometric limit was close to the cluster's main-sequence turnoff. The structure of the blue horizontal branch and possible radial dependence of the horizontal branch luminosity function are discussed. We estimate the cluster metallicity as [Fe/H]= -2.0+/-0.15, the reddening as E(B-V) = 0.17+/-0.03 and the true distance modulus as (m - M)_0_ = 18.26^m^+/- 0.11, somewhat lower than the majority of recent estimates of the LMC distance modulus.

Discovery of six short-period eclipsing binaries in the globular cluster M5

We report the identification of six short-period variables in the dense globular cluster M5. Our observations consist of multiple-epoch VI CCD images centred on the cluster and covering a 6.3 × 6.3 arcmin2 field. Of the six short-period variables, one is a possible blue straggler, four lie on the cluster main-sequence turn-off, and one is about 1 mag fainter than the main-sequence turn-off. The light curves of these six variables show that five of them are probably W UMa type contact eclipsing binaries and one of them is a semicontact Algol system. Owing to incomplete phase coverage, the type identification is fairly uncertain. The periods of these six interacting binaries are between 0.4 and 0.7 d. All six variables lie in the region beyond the 4rc radius, where rc is the cluster core radius of 24 arcsec. Using detailed artificial eclipsing binary star simulations, we have applied incompleteness corrections and estimate a total number of 11 short-period binaries in the region beyond 4rc. Adopting a flat period distribution and a contact binary lifetime of 3 Gyr, we estimate an overall primordial binary frequency of 28+11−5.8 per cent for the period range of 2.5 d to 550 yr. To make comparative studies of short-period main-sequence eclipsing binaries (SPMSEB) in different globular clusters, we estimate that the specific frequency of SPMSEB is 0.5 per cent in M5, excluding the possible blue straggler, similar to the fraction of 0.6 per cent derived in M71 from the data published by Yan & Mateo.

The age of the Large Magellanic Cloud cluster NGC 2193

The age of the LMC star cluster NGC 2193 is estimated as 2.2 + or - 0.6 Gyr from CCD photometry that reaches below the cluster main-sequence turnoff. The cluster is thus another member of the large set of LMC red clusters that have ages in the range of 1 to 3 Gyr. The principal cause of uncertainty in the estimated age is uncertainty in the LMC distance modulus; at (m - M)0 = 18.2, the age is estimated as 2.5 + or - 0.4 Gyr, while at (m - M)0 = 18.7, the age is 1.8 + or - 0.4 Gyr. A metal abundance of (Fe/H) = -0.5 + or - 0.3 dex is deduced for NGC 2193 from the color of the cluster giant branch. The C-M diagram for the LMC field near NGC 2193 shows the commonly found population of intermediate age stars but, in this case, the diagram is clearly dominated by a population of older stars. Isochrone fits indicate an age for this older population in excess of 7 Gyr, with some indication of ages perhaps as old as 13 Gyr. This latter population is the oldest LMC field population yet dated. 52 references.

SPECTROSCOPIC ORBITAL ELEMENTS FOR A DOUBLE-LINED BINARY IN NGC 752.

ABSTRACT Orbital elements for a double-lined spectroscopic binary in the intermediate-age galactic cluster NGC 752 are reported. The binary is located just above the cluster main-sequence turnoff, and is a confirmed proper-motion and radial-velocity member. The equal amplitudes of the primary and the secondary velocity curves indicate that the stars have equal masses.

Main-sequence photometry of the SMC globular cluster Kron 3

view Abstract Citations (60) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Main-sequence photometry of the SMC globular cluster Kron 3. Rich, R. M. ; Da Costa, G. S. ; Mould, J. R. Abstract A color-magnitude (C-M) diagram for the SMC cluster Kron 3 is presented and discussed. The cluster main-sequence turnoff is revealed for the first time, and comparison with theoretical isochrones indicates that the age of Kron 3 is in the range of 5-8 billion years. Current uncertainty in the SMC modulus prevents a more precise determination. A similar C-M diagram for a SMC background field was also determined. It indicates that, unlike the galactic halo, the SMC halo contains stars with a large range of ages, from 1 billion years upwards. An old population (with an age of more than 10 Gyr) is also apparent in the field. Publication: The Astrophysical Journal Pub Date: November 1984 DOI: 10.1086/162627 Bibcode: 1984ApJ...286..517R Keywords: Astronomical Photometry; Globular Clusters; Magellanic Clouds; Main Sequence Stars; Astronomical Catalogs; Horizontal Branch Stars; Stellar Color; Stellar Evolution; Stellar Magnitude; Astrophysics full text sources ADS | data products SIMBAD (1) NED (1)