The Small Blue Straggler Star Population in the Dense Galactic Globular Cluster NGC 6752 (original) (raw)
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The Unimodal Distribution of Blue Straggler Stars in M75 (NGC 6864)
The Astrophysical Journal, 2012
We have used a combination of multiband high-resolution and wide-field ground-based observations to image the Galactic globular cluster M75 (NGC 6864). The extensive photometric sample covers the entire cluster extension, from the very central regions out to the tidal radius, allowing us to determine the center of gravity and to construct the most extended star density profile ever published for this cluster. We also present the first detailed star counts in the very inner regions. The star density profile is well reproduced by a standard King model with core radius r c ∼ 5.4 ′′ and intermediatehigh concentration c ∼ 1.75. The present paper presents a detailed study of the BSS population and its radial distribution. A total number of 62 bright BSSs (with m F255W 21, corresponding to m F555W 20) has been identified, and they have been found to be highly segregated in the cluster core. No significant upturn in the BSS frequency has been observed in the outskirts of M75, in contrast to several other clusters studied with the same technique. This observational fact is quite similar to what has been found in M79 (NGC 1904) by Lanzoni et al. (2007a). Indeed the BSS radial distributions in the two clusters is qualitatively very similar, even if in M75 the relative BSS frequency seems to decrease significantly faster than in M79: indeed it decreases by a factor of 5 (from 3.4 to 0.7) within 1 r c. Such evidence indicate that the vast majority of the cluster heavy stars (binaries) have already sunk to the core.
Blue Straggler Stars in the Unusual Globular Cluster NGC 6388
Astrophysical Journal, 2008
We have used multi-band high resolution HST WFPC2 and ACS observations combined with wide field ground-based observations to study the blue straggler star (BSS) population in the galactic globular cluster NGC 6388. As in several other clusters we have studied, the BSS distribution is found to be bimodal: highly peaked in the cluster center, rapidly decreasing at intermediate radii, and rising again at larger radii. In other clusters the sparsely populated intermediate-radius region (or ``zone of avoidance'') corresponds well to that part of the cluster where dynamical friction would have caused the more massive BSS or their binary progenitors to settle to the cluster center. Instead, in NGC 6388, BSS still populate a region that should have been cleaned out by dynamical friction effects, thus suggesting that dynamical friction is somehow less efficient than expected. As by-product of these observations, the peculiar morphology of the horizontal branch (HB) is also confirmed. In particular, within the (very extended) blue portion of the HB we are able to clearly characterize three sub-populations: ordinary blue HB stars, extreme HB stars, and blue hook stars. Each of these populations has a radial distribution which is indistinguishable from normal cluster stars.
2013
We present the first dynamical study of Blue Straggler Stars (BSSs) in three Galactic globular clusters, NGC 3201, NGC 5139 (ωCen), and NGC 6218, based on medium-resolution spectroscopy (R 10000) obtained with IMACS. Our BSS candidate selection technique uses HST/ACS and ESO/WFI photometric data out to >4.5 r_c. We use radial velocity measurements to discard non-members and achieve a success rate of ∼93%, which yields a sample of 116 confirmed BSSs. Using the penalized pixel fitting method (pPXF) we measure the vsin(i) values of the sample BSSs and find their distribution functions peaked at slow velocities with a long tail towards fast velocities in each globular cluster. We find that the BSSs in NGC 3201 and NGC 6218 which show vsin(i)>50 km s^-1 are all found in the central cluster regions, inside a projected 2 r_c, of their parent clusters. We find a similar result in ωCen for BSSs with vsin(i)>70 km s^-1 which are all, except for two, concentrated inside 2 r_c. In all ...
Another Nonsegregated Blue Straggler Population in a Globular Cluster: the Case of NGC 2419
The Astrophysical Journal, 2008
We have used a combination of ACS-HST high-resolution and wide-field SUB-ARU data in order to study the Blue Straggler Star (BSS) population over the entire extension of the remote Galactic globular cluster NGC 2419. The BSS population presented here is among the largest ever observed in any stellar system, with more than 230 BSS in the brightest portion of the sequence. The radial distribution of the selected BSS is essentially the same as that of the other cluster stars. In this sense the BSS radial distribution is similar to that of ω Centauri and unlike that of all Galactic globular clusters studied to date, which have highly centrally segregated distributions and, in most cases, a pronounced upturn in the external regions. As in the case of ω Centauri, this evidence indicates that NGC 2419 is not yet relaxed even in the central regions. This observational fact is in agreement with estimated half-mass relaxation time, which is of the order of the cluster age.
The Dynamical State and Blue Straggler Population of the Globular Cluster NGC 6266 (M62
Astronomical Journal, 2006
The millisecond pulsar population (whose members are all in binary systems) and the X-ray emitting population (more than 50 sources within the cluster half mass radius) suggest that NGC 6266 is in a dynamical phase particularly active in generating binaries through dynamical encounters. UV observations of the central region have been used to probe the population of blue straggler stars, whose origin might be also affected by dynamical interactions. The comparison with other globular clusters observed with a similar strategy shows that the blue straggler content in NGC 6266 is relatively low, suggesting that the formation channel that produces binary systems hosting neutron stars or white dwarfs is not effective in significantly increasing the blue straggler population. Moreover, an anticorrelation between millisecond pulsar content and blue straggler specific frequency in globular cluster seems emerging with increasing evidence.
HST observations of blue straggler stars in the core of the globular cluster M3
The core of the Galactic Globular Cluster M 3 (NGC 5272) has been observed with the WFPC2 through the filters F 255W , F 336W , F 555W , and F 814W. Using these observations along with a thorough reanalysis of earlier catalogs, we have produced a catalog of blue straggler stars (BSS) spanning the cluster. Earlier studies and the fainter part of our sample suffer severe selection biases. Our analysis is based on a more reliable bright global sample of 122 BSS. We confirm earlier suggestions that the radial BSS distribution in M 3 is bimodal. It is strongly peaked in the center, has a clear dip 100-200 from the center, and rises again at larger radii. The observed distribution agrees with the dynamical model of Sigurdsson et al. (1994) which takes into account both star collisions and merging of primordial binaries for the origin of BSS. The observed luminosity functions of BSS in the inner and outer parts of the cluster are different. Interpreting these using the models of Bailyn & Pinsonneault (1995), we suggest that the BSS in the inner cluster are formed by stellar collisions and those in the outer cluster from merging primordial binaries.
The Astrophysical Journal, 2008
We used a proper combination of multiband high-resolution and wide field multi-wavelength observations collected at three different telescopes (HST, LBT and CFHT) to probe Blue Straggler Star (BSS) populations in the globular cluster M53. Almost 200 BSS have been identified over the entire cluster extension. The radial distribution of these stars has been found to be bimodal (similarly to that of several other clusters) with a prominent dip at ∼ 60 ′′ (∼ 2r c ) from the cluster center. This value turns out to be a factor of two smaller than the radius of avoidance (r avoid , the radius within which all the stars of ∼ 1.2 M ⊙ have sunk to the core because of dynamical friction effects in an Hubble time). While in most of the clusters with a bimodal BSS radial distribution, r avoid has been found to be located in the region of the observed minimum, this is the second case (after NGC6388) where this discrepancy is noted. This evidence suggests that in a few clusters the dynamical friction seems to be somehow less efficient than expected.
UOCS – VII. Blue Straggler Populations of Open Cluster NGC 7789 with UVIT/AstroSat
Monthly Notices of the Royal Astronomical Society, 2022
NGC 7789 is a ∼1.6 Gyr old, populous open cluster located at ∼2000 pc. We characterize the blue straggler stars (BSS) of this cluster using the Ultraviolet (UV) data from the UVIT/AstroSat. We present spectral energy distributions (SED) of 15 BSS candidates constructed using multi-wavelength data ranging from UV to IR wavelengths. In 8 BSS candidates, a single temperature SED is found to be satisfactory. We discover hot companions in 5 BSS candidates. The hot companions with Teff ∼11750–15500 K, R ∼ 0.069–0.242 R⊙, and L ∼0.25–1.55 L⊙, are most likely extremely low mass (ELM) white dwarfs (WDs) with masses smaller than ∼0.18 M⊙, and thereby confirmed post mass transfer systems. We discuss the implication of this finding in the context of BSS formation mechanisms. Two additional BSS show excess in one or more UV filters, and may have a hot companion, however we are unable to characterize them. We suggest that at least 5 of the 15 BSS candidates (33 per cent) studied in this cluster h...
Variable Blue Straggler Stars in Open Cluster NGC 6819 Observed in the Kepler 'Superstamp' Field
arXiv (Cornell University), 2023
NGC 6819 is an open cluster of age 2.4 Gyr that was in the NASA Kepler spacecraft field of view from 2009 to 2013. The central part of the cluster was observed in a 200 x 200 pixel 'superstamp' during these four years in 30-minute cadence photometry, providing a unique long time-series high-precision data set. The cluster contains 'blue straggler' stars, i.e., stars on the main sequence above the cluster turnoff that should have left the main sequence to become red giants. We present light curves and pulsation frequency analyses derived from custom photometric reductions for five confirmed cluster members-four blue stragglers and one star near the main-sequence turnoff. Two of these stars show a rich spectrum of δ Scuti pulsation modes, with 236 and 124 significant frequencies identified, respectively, while two stars show mainly low-frequency modes, characteristic of γ Doradus variable stars. The fifth star, a known active x-ray binary, shows only several harmonics of two main frequencies. For the two δ Scuti stars, we use a frequency separation-mean-density relation to estimate mean density, and then use this value along with effective temperature to derive stellar mass and radius. For the two stars showing low frequencies, we searched for period-spacing sequences that may be representative of gravity-mode or Rossby-mode sequences, but found no clear sequences. The common age for the cluster members, considered along with the frequencies, will provide valuable constraints for asteroseismic analyses, and may shed light on the origin of the blue stragglers.
The Astrophysical Journal, 2005
We present a spectroscopic analysis of HST/STIS and FOS low-and intermediate-resolution spectroscopy of 55 stars in four globular clusters (47 Tucanae, M 3, NGC 6752, and NGC 6397). Stars hotter than T eff = 5750 K and with a signal-to-noise ratio larger than 15 were analyzed with non-Local Thermodynamic Equilibrium model atmospheres, and values for their effective temperatures and gravities were obtained. Using photometric fluxes, we also obtained radii, luminosities and spectroscopic masses.