Binary population synthesis for blue stragglers

The puzzling existence of Blue Straggler Stars (BSSs) implies that they must form in relatively recent events, after the majority of the constituent globular cluster (GC) stellar population was formed. In this thesis we compile a large set of independent work to help understand the formation of BSSs. In Chapter 2 we present new proper-motion cleaned BSS catalogs in 38 Milky Way GCs based on multi-passband and multi-epoch treasury survey data from the Hubble Space Telescope. We study the accuracy of our proper motion measurements using estimates of central velocity dispersions and find very good agreement with previous studies in the literature. Finally, we present a homogeneous BSS selection method, that expands the classic BSS selection parameter space to more evolved BSS evolutionary stages. In Chapter 3 we present the largest ever data set of velocity dispersion profiles of BSSs, which confirm that BSSs are populations that have been greatly affected by two-body relaxation, as it has been previously expected based on basic equipartition principles. In addition, we show from photometric BSS mass estimates that there is a striking correlation between the average BSS mass and the cluster dynamical age. We relate the measured proper-motion velocity dispersions with the photometric BSS mass estimates and use them to propose a novel interpretation of BSS CMDs and its relationship with the binary-evolution and dynamical formation channels. In Chapter 4 we present the first dynamical study of BSSs in three galactic GCs: NGC 3201, NGC 5139 (ωCen), and NGC 6218. We measure the v sin(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 each of the GC sample which show vsin(i)>70 km s−1 are all found in the central cluster regions. In all globular clusters we find rapidly rotating BSSs that have relatively high differential radial velocities which likely put them on hyperbolic orbits, suggestive of strong dynamical interactions in the past. In Chapter 5 we present a multi-passband photometric study of the BSS population in NGC 1261. The inner BSS population is found to have two distinct sequences in the color-magnitude diagram, similar to double BSS sequences detected in other GCs. These well defined sequences are presumably linked to single short-lived events such as core collapse, which are expected to boost the formation of BSSs. In agreement with this, we find a BSS sequence in NGC 1261 which can be well reproduced individually by a theoretical model prediction of a 2 Gyr old population of stellar collision products, which are expected to form in the denser inner regions during short-lived core contraction phases. We argue that NGC 1261 can be considered a candidate for the post-core-collapse bounce state seen in dynamical simulations of old GCs.

We identified blue straggler stars (BSSs) in 53 open clusters utilizing data from Gaia DR3. Most of these clusters are situated in the outer regions of the Galactic disk, encompassing structures such as the warp and the Outer arm. We analyzed their astrometric parameters and determined that 48 of them demonstrate high reliability in the radial density profile. Furthermore, through manual isochrone fitting and visual inspection, we confirmed 119 BSS candidates and identified 328 additional possible candidates within these clusters. Our results contribute to a 46% increase in the sample size of BSSs in open clusters for regions of the Galactic disk where R gc > 12 kpc. We observed that the new samples are fainter compared to those identified in the past. Additionally, we investigated the maximum fractional mass excess (M e ) of the BSSs in open clusters, including previously published BSS samples. Our findings indicate a strong correlation between the capability to produce highest-M e BSSs and the mass of their host clusters. This observation appears to reinforce a fundamental principle whereby an increase in the mass of a star cluster correlates with a higher likelihood of stellar mergers. In contrast, we observe minimal correlation between maximum-M e and the cluster age. Among clusters containing BSSs, younger clusters (0.5–1 Gyr) display a scarcity of high-M e BSSs. This scarcity may be attributed to the absence of more massive clusters within this age range.

Blue stragglers have been found in all populations. They are important in both stellar evolution and stellar population synthesis. Much evidence shows that blue stragglers are relevant to primordial binaries. Here, we summarize the links between binary evolution and blue stragglers, describe the characteristics of blue stragglers originating from different binary evolutionary channels and show their consequences for binary population synthesis, such as for the integrated spectral-energy distribution, the colour–magnitude diagram, their specific frequency, and their influence on colours, etc.

We present a kinematic and dynamical analysis of six Galactic open clusters – NGC 2204, NGC 2660, NGC 2262, Czernik 32, Pismis 18, and NGC 2437, using Gaia DR3. We used Bayesian and Gaussian Mixture Model (GMM) methods to identify cluster members, but chose GMM because it is more appropriate for low-mass stars. Estimated distances range from 1.76 to 4.20 kpc and ages from 0.199 to 1.95 Gyr, confirming their intermediate-age nature. King model fits indicate compact morphologies, with core radii of 1–10 arcmin and cluster radii of 5–24 arcmin. We identify 13 blue straggler stars and 3 yellow straggler stars members, whose central concentrations suggest origins via mass transfer or stellar collisions. The mass function slopes (0.96–1.19) are flatter than the Salpeter value, which indicates that these clusters have undergone dynamical mass segregation. Orbit integration within a Galactic potential indicates nearly circular orbits (eccentricities 0.02–0.10), vertical excursions within $\pm$132 pc, and guiding radii near the solar circle, suggesting disc confinement. These clusters likely formed in the thin disc and are shaped by Galactic tidal perturbations, facilitating the rapid loss of low-mass members. Additionally, 12 variable stars were found across 4 clusters using Transiting Exoplanet Survey Satellite (TESS) light curves, including $\gamma$ Doradus and SPB pulsators, eclipsing binaries, and a yellow straggler candidate. Periods were derived via Lomb–Scargle analysis. Two eclipsing binaries (TIC 94229743 and TIC 318170024) were modelled using PHOEBE, yielding mass ratios of 1.37 and 2.16, respectively. Our findings demonstrate that integrating orbital dynamics and variable star studies presents valuable insights into the evolutionary pathways of open clusters.

view Abstract Citations (146) References (144) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Starbursts, Blue Stragglers, and Binary Stars in Local Superclusters and Groups. II. The Old Disk and Halo Populations Eggen, Olin J. ; Iben, Icko, Jr. Abstract A study of the distribution in the HR diagram of stars in several old-disk population aggregates and in several halo-population aggregates indicates that the blue straggler phenomenon is ubiquitous. In all the cases considered, several stars are found to lie far from the locus defined by a semiempirical isochrone that best fits the distribution of the bulk of the stars in the aggregate. The results support the starburst interpretation of blue stragglers in young-disk aggregates. Publication: The Astronomical Journal Pub Date: February 1989 DOI: 10.1086/114993 Bibcode: 1989AJ.....97..431E Keywords: Blue Stars; Cepheid Variables; Eclipsing Binary Stars; Star Formation; Starburst Galaxies; Stellar Spectrophotometry; Main Sequence Stars; Open Clusters; Star Clusters; Star Distribution; Stellar Luminosity; Stellar Winds; Astrophysics; STARS: BINARIES; STARS: PROPER-MOTION full text sources ADS | data products SIMBAD (182) Related Materials (1) Part 1: 1988AJ.....96..635E

Stellar collisions are an important formation channel for blue straggler stars in globular and old open clusters. Hydrodynamical simulations have shown that the remnants of such collisions are out of thermal equilibrium, are not strongly mixed and can rotate very rapidly. Detailed evolution models of collision products are needed to interpret observed blue straggler populations and to use them to probe the dynamical history of a star cluster. We expand on previous studies by presenting an efficient procedure to import the results of detailed collision simulations into a fully implicit stellar evolution code. Our code is able to evolve stellar collision products in a fairly robust manner and allows for a systematic study of their evolution. Using our code we have constructed detailed models of the collisional blue stragglers produced in the $N$-body simulation of M67 performed by Hurley \emph{et al.} in 2005. We assume the collisions are head-on and thus ignore the effects of rotation in this paper. Our detailed models are more luminous than normal stars of the same mass and in the same stage of evolution, but cooler than homogeneously mixed versions of the collision products. The increased luminosity and inefficient mixing decrease the remaining main-sequence lifetimes of the collision products, which are much shorter than predicted by the simple prescription commonly used in $N$-body simulations.

We present a statistical approach to investigating the dynamical evolution of the old open cluster Trumpler 19. We identified 810 cluster members using an ensemble-based unsupervised machine learning method applied to Gaia Data Release 3 astrometric data. From the color–magnitude diagram, we identified 18 blue straggler stars (BSSs) in Trumpler 19. The mass function of the cluster shows a flatter slope, indicating strong mass segregation and advanced dynamical evolution. We fitted the radial surface density profile and found that the concentration parameter c > 1, suggesting that the cluster has formed a clear core–halo structure as a result of dynamical evolution. We characterized the mass segregation among the cluster members as well as BSSs using the Minimum Spanning Trees method, indicating a significant central concentration. Additionally, the sedimentation level of the BSSs is measured as A r h + = 0.28 ± 0.05 , further supporting radial segregation. To probe the BSS formation mechanisms, we estimated their fractional mass excess (M e ), supporting binary mass transfer and mergers as the dominant channels. This is further supported by the presence of six variable BSSs. The dynamical evolution of the cluster is further assessed through its tidal interaction with the Galaxy. Trumpler 19 appears to be within the tidal regime, where strong Galactic tidal forces have a significant influence on the dynamical evolution. This indicates that the cluster may have undergone significant mass-loss processes, potentially leading to its eventual disruption, which is further supported by the orbit analysis of the cluster. We found that Trumpler 19 may have lost more than 95% of its initial mass due to dynamical evolution.

Many population synthesis and stellar evolution studies have addressed the evolution of close binary systems in which the primary is a compact remnant and the secondary is filling its Roche lobe, thus triggering mass transfer. Although tidal locking is expected in such systems, most studies have neglected the rotationally-induced mixing that may occur. Here we study the possible effects of mixing in the mass-losing stars for a range in secondary star masses and metallicities. We find that tidal locking can induce rotational mixing prior to contact and thus affect the evolution of the secondary star if the effects of the Spruit-Tayler dynamo are included both for angular momentum and chemical transport. Once contact is made, the effect of mass transfer tends to be more rapid than the evolutionary time scale, so the effects of mixing are no longer directly important, but the mass transfer strips matter to inner layers that may have been affected by the mixing. These effects are enhanced for secondaries of 1-1.2 Msun and for lower metallicities. We discuss the possible implications for the paucity of carbon in the secondaries of the cataclysmic variable SS Cyg and the black hole candidate XTE J1118+480 and for the progenitor evolution of Type Ia supernovae. We also address the issue of the origin of blue straggler stars in globular and open clusters. We find that for models that include rotation consistent with that observed for some blue straggler stars, evolution is chemically homogeneous. This leads to tracks in the HR diagram that are brighter and bluer than the non-rotating main-sequence turn-off point. Rotational mixing could thus be one of the factors that contribute to the formation of blue stragglers.

The nature of the pseudohorizontal branch in the galactic cluster M67 is investigated. A differential technique is used to determine the masses of these stars relative both to the turnoff-point stars and to the ZAMS. The pseudohorizontal-branch stars are found to have masses in the range 1.3 < 93'tP..)'0 < 2.5; a mass lower than or equal to the tumoffpoint mass seems to be ruled out. This result suggests that these stars are analogs of the blue stragglers found in other galactic clusters rather than Population I analogs of Population II blue horizontal-branch stars. As by-products of this investigation, the zero-age main sequence is calibrated in terms of effective temperature and log g for the fi-(h-y) and fl-c1 planes; a value of reddening of = 0.04 or = 0.06 is derived for M 67 and the metal content of the M 67 stars is estimated to be solar or slightly lower. Key words: galactic clusters - stellar evolution - blue stragglers

THE nearby globular cluster NGC6397, 2.2 kpc from us, is an evolved cluster which has passed through core collapse1–3; its core region is expanding after an earlier dynamical phase of contraction. Its proximity means that observations of it with CCD (charge-coupled device) detectors in arcsecond-seeing conditions simulate Hubble Space Telescope observations of more-distant objects, such as the archetypal post-core-collapse globular cluster M15, and are capable of testing the predictions of some recent models4,5 of post-collapse cluster dynamics. We have discovered in the central region of NGC6397 a population of 'blue stragglers'6, stars that are too hot for their luminosity in relation to subgiant stars. Five of these blue stragglers are within 0.07 pc of the centre of the cluster, and are brighter than the rest by one magnitude. We suggest that the blue stragglers are either coalesced stars resulting from stellar collisions expected to occur especially during core-collapse7,8, or belong to a population of primordial close binaries that could provide the energy for the expansion of the core4.

We present a homogeneous study of blue straggler stars across ten outer halo globular clusters, three classical dwarf spheroidal and nine ultra-faint galaxies based on deep and wide-field photometric data taken with MegaCam on the Canada-France-Hawaii Telescope. We find blue straggler stars to be ubiquitous among these Milky Way satellites. Based on these data, we can test the importance of primordial binaries or multiple systems on blue straggler star formation in low density environments. For the outer halo globular clusters we find an anti-correlation between the specific frequency of blue straggler and absolute magnitude, similar to that previously observed for inner halo clusters. When plotted against density and encounter rate, the frequency of blue stragglers are well fitted by single trends with smooth transitions between dwarf galaxies and globular clusters, which points to a common origin for their blue stragglers. The fraction of blue stragglers stays constant and high in the low encounter rate regime spanned by our dwarf galaxies, and decreases with density and encounter rate in the range spanned by our globular clusters. We find that young stars can mimic blue stragglers in dwarf galaxies only if their ages are 2.5+/-0.5 Gyr and they represent ~1-7% of the total number of stars, which we deem highly unlikely. These results point to mass-transfer or mergers of primordial binaries or multiple systems as the dominant blue straggler formation mechanism in low density systems.

This paper investigates the spatial structure and tidal tails of the nearby ($\sim$1100 pc) old open cluster NGC 2112 ($\sim$1.2 Gyr) based on reliable cluster members from Gaia-DR3. A robust clustering method is used to calculate membership probabilities for sample stars within $2.^{\!\!\!\circ }5$ ($\sim$48 pc) of the cluster center. More than 1200 cluster members are identified based on the membership probabilities. We find that the cluster members can be naturally segregated into two distinct components (core and halo) using the Gaussian mixture model. The major and minor axes of the core and halo are determined using principal component analysis (PCA). The lengths of the semi-major axes of the core and halo are estimated to be $\sim\! 22^{\prime }$ ($\sim$7 pc) and $\sim 86^{\prime }$ ($\sim$28 pc), respectively. We find that the core and halo have minor to major axis ratios of $e\sim 0.92$ and 0.75 respectively, indicating that the halo is strongly influenced by the external tidal field. Moreover, we find two $\sim$36 pc long tidal tails around the cluster, nearly aligned with the orbit of the cluster. The tidal tails contain various types of stars, including main-sequence stars, main-sequence turn-off (MSTO) stars, blue straggler (BS) stars, and red giant branch stars. The high-mass tail stars (e.g., MSTO stars, BS stars) could not have formed only via star evaporation driven by two-body relaxation. We estimate that the observed tidal tails could have formed within the last $\sim$30 Myr.

We used a combination of high-resolution optical images acquired with the Hubble Space Telescope and near-IR wide-field data to investigate the stellar density profile and the population of blue straggler stars (BSSs) in the Galactic globular cluster NGC 6256, with the aim of probing its current stage of internal dynamical evolution. We found that the inner stellar density profile significantly deviates from a King model, while it is well reproduced by a steep cusp with a power-law slope α cusp = −0.89, thus implying that the cluster is currently in the post-core-collapse (PCC) phase. This is also confirmed by the very high segregation level of the BSS population measured through the parameter. We also found that the distribution of BSSs in the color–magnitude diagram is characterized by a collimated blue sequence and a red more sparse component, as already observed in three other PCC clusters. A comparison with appropriate collisional models demonstrates that the vast majority of the BSSs lying along the collimated blue sequence is consistent with a generation of coeval (1 Gyr old) stars with different masses originated by an event that highly enhanced the collisional rate of the system (i.e., the core collapse). This study confirms that the segregation level of BSSs is a powerful dynamical diagnostic also of star cluster in a very advanced stage of dynamical evolution. Moreover, it pushes forward the possibility of using the morphology of BSSs in a color–magnitude diagram as a tracer of the core-collapse and subsequent dynamical evolutionary phases.

The sedimentation level of blue straggler stars (BSS) has been shown to be a great tool to investigate the dynamical states of globular clusters (GCs). The area enclosed between the cumulative radial distributions of BSS and a reference population up to the half-mass radius of the clusters, $A^+_{\mathrm{rh}}$, is known to be a measure of the sedimentation of BSS in GCs. In this work, we calculate $A^+_{\mathrm{rh}}$ for 11 open clusters (OCs) using a combined list of main-sequence turn-off stars, sub-giant branch stars, and red giant branch stars as reference population. The BSS, the reference populations, and the cluster members are identified using the proper motions and parallaxes from the Gaia DR2 data. In a subset of clusters, the BSS are confirmed cluster members on the basis of radial velocity information available in the literature. Using the Pearson and Spearman rank correlation coefficients, we find weak correlations between the estimated values of $A^+_{\mathrm{rh}}$ and other markers of dynamical ages of the clusters, i.e. the number of central relaxations a cluster has experienced since its formation, and the structural parameters of the clusters. Based on statistical tests, we find that these correlations are similar to the corresponding correlations among the less evolved GCs, albeit within large errors.

(abridged) We present an in-depth study of the hard-binary population of the old (7 Gyr) open cluster NGC 188. The main-sequence solar-type hard binaries in NGC 188 are nearly indistinguishable from similar binaries in the Galactic field. We find a global solar-type main-sequence hard-binary frequency in NGC 188 of 29 +/- 3 % for binaries with periods less than 10^4 days. For main-sequence hard binaries in the cluster we observe a log-period distribution that rises towards our detection limit, a roughly Gaussian eccentricity distribution centered on e = 0.35 (for binaries with periods longer than the circularization period), and a secondary-mass distribution that rises towards lower-mass companions. Importantly, the NGC 188 blue straggler binaries show significantly different characteristics than the solar-type main sequence binaries in NGC 188. We observe a blue straggler hard-binary frequency of 76 +/- 19 %, three times that of the main sequence. The blue straggler binary eccentricity - log period distribution is distinct from that of the main sequence at the 99% confidence level, with the majority of the blue straggler binaries having periods of order 1000 days and lower eccentricities. The secondary-mass distribution for these long-period blue straggler binaries is narrow and peaked with a mean value of about 0.5 Msun. Predictions for mass-transfer products are most closely consistent with the binary properties of these NGC 188 blue stragglers, which comprise two-thirds of the blue straggler population. Additionally we compare the NGC 188 binaries to those evolved within the sophisticated Hurley et al. (2005) N-body open cluster simulation. We find that additional simulations with initial conditions that are better motivated by observations are necessary to properly investigate the dynamical evolution of a rich binary population in open clusters like NGC 188.