Bright Blue Stragglers Research Articles

Evolution and origin of Blue Stragglers in 47 Tucanae

AbstractUsing data from the core of 47 Tuc we have identified stars in different evolutionary stages in the colour-magnitude diagram, and used the effects of mass segregation on their radial distribution to study the evolution and origin of blue stragglers (BSS). We separate the BSS into 2 samples by their magnitude and find considerable differences in their distribution. Bright BSS are more centrally concentrated with mass estimates over twice the turn-off mass suggesting an origin involving a triple or multiple star system. The distribution of the faint BSS is close to that of the main-sequence (MS) binaries pointing to these stars as their likely progenitors. Using MESA models, we calculate the expected number of stars in each evolutionary stage and compare it with the observed number of stars. Results indicate that BSS have a post-MS evolution comparable to that of a normal star of the same mass and a MS-BSS lifetime of about 200 – 300 Myr.

Tracing the stellar component of low surface brightness Milky Way dwarf galaxies to their outskirts

Aims. We present results from deep and very spatially extended CTIO/DECam g and r photometry (reaching out to ~2 mag below the oldest main-sequence turn-off and covering ~20 deg2) around the Sextans dwarf spheroidal galaxy. We aim to use this dataset to study the structural properties of Sextans overall stellar population and its member stars in different evolutionary phases, as well as to search for possible signs of tidal disturbance from the Milky Way, which would indicate departure from dynamical equilibrium. Methods. We performed the most accurate and quantitative structural analysis to-date of Sextans’ stellar components by applying Bayesian Monte Carlo Markov chain methods to the individual stars’ positions. Surface density maps are built by statistically decontaminating the sample through a matched filter analysis of the colour-magnitude diagram, and then analysed for departures from axisymmetry. Results. Sextans is found to be significantly less spatially extended and more centrally concentrated than early studies suggested. No statistically significant distortions or signs of tidal disturbances were found down to a surface brightness limit of ~31.8 mag/arcsec2 in V-band. We identify an overdensity in the central regions that may correspond to previously reported kinematic substructure(s). In agreement with previous findings, old and metal-poor stars such as Blue Horizontal Branch stars cover a much larger area than stars in other evolutionary phases, and bright Blue Stragglers (BSs) are less spatially extended than faint ones. However, the different spatial distribution of bright and faint BSs appears consistent with the general age and metallicity gradients found in Sextans’ stellar component. This is compatible with Sextans BSs having formed by evolution of binaries and not necessarily due to the presence of a central disrupted globular cluster, as suggested in the literature. We provide structural parameters for the various populations analysed and make publicly available the photometric catalogue of point-sources as well as a catalogue of literature spectroscopic measurements with updated membership probabilities.

THE WFPC2 ULTRAVIOLET SURVEY: THE BLUE STRAGGLER POPULATION IN NGC 5824

We have used a combination of high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and wide-field ground-based observations, in ultraviolet and optical bands, to study the blue straggler star population of the massive outer halo globular cluster NGC 5824 over its entire radial extent. We have computed the center of the cluster and constructed the radial density profile from detailed star counts. The profile is well reproduced by a Wilson model with a small core (rc 4.''4) and a concentration parameter c 2.74. We also present the first age determination for this cluster. From a comparison with isochrones, we find t = 13 ± 0.5 Gyr. We discuss this result in the context of the observed age-metallicity relation of Galactic globular clusters. A total of 60 bright blue stragglers has been identified. Their radial distribution is found to be bimodal, with a central peak, a well-defined minimum at r ~ 20'', and an upturn at large radii. In the framework of the dynamical clock recently defined by Ferraro et al., this feature suggests that NGC 5824 is a cluster of intermediate dynamical age.

A FAR-ULTRAVIOLET SURVEY OF M80: X-RAY SOURCE COUNTERPARTS, STRANGE BLUE STRAGGLERS, AND THE RECOVERY OF NOVA T SCO

Using the ACS on HST, we have surveyed the FUV and NUV populations in the core region of M80. The CMD reveals large numbers of blue and extreme horizontal branch stars and blue stragglers, as well as approx. 60 objects lying in the region of the CMD where accreting and detached white dwarf binaries are expected. Overall, the blue straggler stars are the most centrally concentrated population, with their radial distribution suggesting a typical blue straggler mass of about 1.2 Msun. However, counterintuitively, the faint blue stragglers are significantly more centrally concentrated than the bright ones and a Kolmogorov-Smirnov test suggest only a 3.5% probability that both faint and bright blue stragglers are drawn from the same distribution. This may suggest that (some) blue stragglers get a kick during their formation. We have also been able to identify the majority of the known X-ray sources in the core with FUV bright stars. One of these FUV sources is a likely dwarf nova that was in eruption at the time of the FUV observations. This object is located at a position consistent with Nova 1860 AD, or T Scorpii. Based on its position, X-ray and UV characteristics, this system is almost certainly the source of the nova explosion. The radial distribution of the X-ray sources and of the cataclysmic variable candidates in our sample suggest masses > 1 Msun.

The Blue Straggler Population of the Globular Cluster M5: Comparison with M3

We have surveyed the blue straggler star population of the Galactic globular cluster M5 using high-resolution images of the core along with wide-field ground-based images reaching to more than 19 core radii. To gauge M5's relative efficiency of producing stragglers, we compared our sample to five studies of other globular clusters (mainly Ferraro et al. 1997b; Ferraro et al 2003; and Piotto et al. 2004). Using a "bright" sample selected in the same way as Ferraro et al. 1997b, we found a bimodal radial distribution similar to those found in three other luminous clusters. When the radial distributions for different clusters are scaled using the core radius, there is good cluster-to-cluster agreement in the size of the core straggler sample and the center of the "zone of avoidance". However, M5 has the smallest fraction of stragglers in the zone of avoidance of any of the clusters measured to date, and its zone of avoidance appears to be wider (in r / r_c) than that of M3, which has a very similar surface brightness profile. Both of these facts indicate that M5's straggler population has dynamically evolved to a larger extent than M3. Using an ultraviolet sample from Hubble Space Telescope selected in the same way as Ferraro et al. 2003 and Ferraro et al. 2004, we find that the frequency of blue stragglers in M5 is lower than all but two of the clusters examined. We also identified seven bright blue stragglers that were previously misidentified as HB stars by Sandquist et al. (2004). These bright stragglers are most likely the result of stellar collisions involving binary stars.

Deep Wide-FieldBVICCD Photometry of the Sextans Dwarf Spheroidal Galaxy

We present deep wide-field VI CCD photometry of the Sextans dwarf spheroidal galaxy (dSph) in the Local Group, covering a field of 42' × 28' located at the center of the galaxy (supplemented by short B photometry). The limiting magnitudes with 50% completeness are V = 24.4 mag and I = 23.6 mag. Color-magnitude diagrams of the Sextans dSph show a well-defined red giant branch (RGB), blue horizontal branch (BHB), prominent red horizontal branch (RHB), and asymptotic giant branch (AGB), as well as ~120 variable star candidates including RR Lyrae stars and anomalous Cepheids, ~230 blue stragglers (BSs), and main-sequence (MS) stars. The main-sequence turnoff (MSTO) of the old population is found to be located at V ≈ 23.7 mag and V-I ≈ 0.56. The distance to the galaxy is derived using the I-band magnitude of the tip of the RGB at ITRGB = 15.95 ± 0.04: (m - M)0 = 19.90 ± 0.06, for an adopted reddening of E(B-V) = 0.01. This estimate agrees well with the distance estimate based on the mean V-band magnitude of the HB at V(HB) = 20.37 ± 0.04. The mean metallicity of the RGB is estimated from the V-I color: [Fe/H] = -2.1 ± 0.1 (statistical error) ±0.2 (standard calibration error) dex, with a dispersion of σ[Fe/H] = 0.2 dex. The age of the MSTO of the main old population is estimated to be similar to that of the metal-poor Galactic globular cluster M92, and some stellar populations with younger age are seen. There is found to be one RGB bump at V = 19.95 ± 0.05 mag (MV = 0.03 mag) and a weak brighter bump at V = 19.35 ± 0.05 mag (MV = -0.58 mag), which is probably an AGB bump. The V-band luminosity function of the RGB and MS stars is in general similar to that of the globular cluster M92, with a slight excess of stars in the magnitude range brighter than the MSTO with respect to that of M92. The bright BSs are more centrally concentrated than the faint BSs. The V-band luminosity function of the BSs in the inner region is found to extend to a brighter magnitude and to have a flatter slope compared with that of the BSs in the outer region. Significant radial gradients are seen for several kinds of populations: the RHB, the red RGB, the red subgiant branch (SGB), and the bright BSs are more centrally concentrated toward the center of the galaxy compared with the BHB, the blue RGB, the blue SGB, and the faint BSs, respectively.

The Distribution of Collisionally Induced Blue Stragglers in the Color‐Magnitude Diagram

A primary production mechanism for blue stragglers in globular clusters is thought to be collisionally induced mergers, perhaps mediated by dynamical encounters involving binary stars. We model the formation and evolution of such blue stragglers and produce theoretical distributions of them in the color-magnitude diagram. We use a crude representation of cluster dynamics and detailed binary-single star encounter simulations to produce cross sections and rates for a variety of collisions. The results of the collisions are determined on the basis of smoothed-particle hydrodynamics simulations of realistic star models. The evolution of the collision products are then followed in detail using the Yale stellar evolution code. We present our results in the form of distributions in the observed color-magnitude diagram. We use our results to explore the effects of a variety of input assumptions on the number and kind of blue stragglers created by collisions. In particular, we describe the changes in the blue straggler distribution that result from using realistic collision products rather than the fully mixed assumption, and from changes in assumptions about the number and orbital-period distribution of the primordial binary population. We then apply our models to existing data from the core of M3, where the large blue straggler population is thought to be dominated by collision products. We find that we have difficulty successfully modeling the observed blue stragglers under a single consistent set of assumptions. However, if three particularly bright blue stragglers are considered to be part of a different observed population (as has been suggested previously for other clusters) and left out of the population, the remainder can be successfully modeled using realistic encounter products and assuming a 20% binary fraction with a plausible period distribution. Finally, we suggest a variety of routes toward a more comprehensive understanding of the blue straggler phenomenon.

Are Blue Stragglers Mixed during Collisions?

The smoothed particle hydrodynamics (SPH) simulations of Benz & Hills led to the assumption that blue stragglers created by collisions are initially fully mixed. The theoretical models of stars created under this assumption fit the observed blue straggler sequences well. Recently, however, Lombardi, Rasio, & Shapiro performed SPH calculations of stellar collisions using a more realistic polytropic index for the structure of stars near the main-sequence turnoff. Their results show that the collision products are not mixed and, in fact, have composition profiles similar to those of the original stars. We have calculated evolutionary tracks for both the products and fully mixed products and have compared the results to the very bright blue stragglers in the center of NGC 6397. The unmixed stars are not as blue and not as bright as the observed stars, and they only live on the main sequence for a very short time. To account for the observed blue stragglers in NGC 6397, the collision products must either be more than twice the turnoff mass or mixed by some process subsequent to the initial collision and merger.

Are all blue stragglers close binaries ?

view Abstract Citations (30) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Are all blue stragglers close binaries? Hintzen, P. ; Scott, J. ; Whelan, J. Abstract Radial velocity studies for five bright blue stragglers are reported, and cluster membership is confirmed for four of them. Two of the stars show no significant radial velocity variations on a time scale of about 75 days, and two others show no significant variation on time scales of 15 and 30 days, respectively. The star for which cluster membership is uncertain may be a velocity variable. The data do not support the theory of blue straggler production involving mass transfer in a close binary. A spread in formation time of the stars in a cluster is suggested to be a plausible explanation of the blue stragglers studied. Subject headings: binaries - open clusters - stellar evolution Publication: The Astrophysical Journal Pub Date: December 1974 DOI: 10.1086/153287 Bibcode: 1974ApJ...194..657H full text sources ADS | data products SIMBAD (8)