On the triple origin of blue stragglers (original) (raw)
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The Evolution of Blue Stragglers Formed via Stellar Collisions
The Astronomical Journal, 1998
We have used the results of recent smoothed particle hydrodynamic simulations of colliding stars to create models appropriate for input into a stellar evolution code. In evolving these models, we find that little or no surface convection occurs, precluding angular momentum loss via a magnetically-driven stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers which have been formed by collisions. Angular momentum transfer to either a circumstellar disk (possibly collisional ejecta) or a nearby companion are plausible mechanisms for explaining the observed low rotation velocities of blue stragglers Under the assumption that the blue stragglers seen in NGC 6397 and 47 Tuc have been created solely by collisions, we find that the majority of blue stragglers cannot have been highly mixed by convection or meridional circulation currents at anytime during their evolution. Also, on the basis of the agreement between the predictions of our non-rotating models and the observed blue straggler distribution, the evolution of blue stragglers is apparently not dominated by the effects of rotation.
Multiple stellar populations and their influence on blue stragglers
Monthly Notices of the Royal Astronomical Society, 2010
It has become clear in recent years that globular clusters are not simple stellar populations, but may host chemically distinct sub-populations, typically with an enhanced helium abundance. These helium-rich populations can make up a substantial fraction of all cluster stars.
Chapter 11 The Multiple Origin of Blue Straggler Stars: Theory vs. Observations
2016
Blue straggler stars (BSSs) are stars that appear to be anomalously young com-pared to other stars of their population. In particular, BSSs lie along an extension of the main sequence (MS) in the colour-magnitude diagram, a region from which most of the stars of equal mass and age have already evolved. Such stars appear
First Evidence of Circumstellar Disks around Blue Straggler Stars
The Astrophysical Journal, 2004
We present an analysis of optical HST/STIS and HST/FOS spectroscopy of 6 blue stragglers found in the globular clusters M 3, NGC 6752 and NGC 6397. These stars are a subsample of a set of ∼50 blue stragglers and stars above the main sequence turn-off in four globular clusters which will be presented in an forthcoming paper. All but the 6 stars presented here can be well fitted with non-LTE model atmospheres. The 6 misfits, on the other hand, possess Balmer jumps which are too large for the effective temperatures implied by their Paschen continua. We find that our data for these stars are consistent with models only if we account for extra absorption of stellar Balmer photons by an ionized circumstellar disk. Column densities of H i and Ca ii are derived as are the the disks' thicknesses. This is the first time that a circumstellar disk is detected around blue stragglers. The presence of magnetically-locked disks attached to the stars has been suggested as a mechanism to lose the large angular momentum imparted by the collision event at the birth of these stars. The disks implied by our study might not be massive enough to constitute such an angular momentum sink, but they could be the leftovers of once larger disks.
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.
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 ...
Collisions of Main-Sequence Stars and the Formation of Blue Stragglers in Globular Clusters
The Astrophysical Journal, 1996
We report the results of new SPH calculations of parabolic collisions between two main-sequence stars in a globular cluster. Such collisions are directly relevant to the formation of blue stragglers. In particular, we consider parent stars of mass M/M T O = 0.2, 0.5, 0.75, and 1, where M T O is the cluster turnoff mass (typically about 0.8 M ⊙). Low-mass stars (with M = 0.2M T O or 0.5M T O) are assumed to be fully convective and are therefore modeled as n = 1.5 polytropes. Stars at the turnoff (with M = M T O) are assumed to be mostly radiative and are modeled as n = 3 polytropes. Intermediate-mass stars (with M = 0.75M T O) are modeled as composite polytropes consisting of a radiative core with polytropic index n = 3 and a convective envelope with n = 1.5. We focus our study on the question of hydrodynamic mixing of helium and hydrogen, which plays a crucial role in determining the observable characteristics of blue stragglers. In all cases we find that there is negligible hydrodynamic mixing of helium into the outer envelope of the merger remnant. The amount of hydrogen mixed into the core of the merger depends strongly on the entropy profiles of the two colliding stars. For two stars with nearly equal masses (and hence entropy profiles) very little hydrodynamic mixing occurs at all, especially if they are close to the turnoff point. This is because the hydrogen-rich material from both stars maintains, on average, a higher specific entropy than the helium-rich material. If the two parent stars are close to turnoff, very little hydrogen is present at the center of the merger remnant and the main-sequence lifetime of the blue straggler could be very short. In contrast, during a collision between two stars of sufficiently different masses (mass ratio q ∼ < 0.5), the hydrogen-rich material