Primordial binary evolution and blue stragglers

In this talk, we will simply show the consequences of binary population synthesis for blue stragglers, such as the integrated spectral energy distribution (ISED), the color-magnitude diagram, the specific frequency, and the influences on colors. Blue stragglers have been found in all stellar populations and they are an important population component in both stellar evolution and star clusters. Much evidence shows that blue stragglers are relevant to primordial binaries. The binary population synthesis study shows that primordial binary evolution may produce blue stragglers at any given times. The specific frequency in this way decreases with time first, then increases when the age is larger than 10 Gyr, while that from angular momentum loss induced by magnetic braking in low-mass binaries increases with time and exceeds that of primordial binary evolution in a population older than 3 Gyr. Meanwhile, blue stragglers resulting from primordial binary evolution are dominant contributors to the ISEDs in ultraviolet and blue bands in a population between 0.3 and 2.0 Gyr. The mass fraction of the lost matter from the primary accreted by the secondary, beta, significantly affects on the final results, e.g. the specific frequency of blue stragglers decreasing with beta, blue stragglers produced from a high value of beta being more massive, then contributing more to the ISEDs of the host clusters. For old open clusters, it is appropriate to adopt a higher value of beta when the primary is in HG at the onset of mass transfer. Our study also shows that, for most Galactic open clusters, the specific frequency of blue stragglers obtained from our simulations is much lower than that of observations, which has been discussed in this talk.

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.

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.

Most stars are members of binaries, and the evolution of a star in a close binary system differs from that of an ioslated star due to the proximity of its companion star. The components in a binary system interact in many ways and binary evolution leads to the formation of many peculiar stars, including blue stragglers and hot subdwarfs. We will discuss binary evolution and the formation of blue stragglers and hot subdwarfs, and show that those hot objects are important in the study of evolutionary population synthesis (EPS), and conclude that binary interactions should be included in the study of EPS. Indeed, binary interactions make a stellar population younger (hotter), and the far-ultraviolet (UV) excess in elliptical galaxies is shown to be most likely resulted from binary interactions. This has major implications for understanding the evolution of the far-UV excess and elliptical galaxies in general. In particular, it implies that the far-UV excess is not a sign of age, as had been postulated prviously and predicts that it should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals.

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.

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

Variable stars have historically served as tools and laboratories in our understanding of stellar formation and evolution, the formation of star clusters, the calibration of distance determination methods, and a variety of other areas. In particular, the study of eclipsing binaries (EBs) in globular clusters (GCs) can provide direct distance estimates to clusters as well as constraints for the turnoff masses of GC stars. Knowing precise distances to GCs would constitute an independent check of widely used distance determination methods in astronomy. Obtaining masses of GC turnoff stars provides a fundamental test of low-metallicity stellar models that calculate ages for these stars—and thus the GCs themselves—and thereby allow a lower limit estimate of the age of the universe. My dissertation research with M. Mateo at the University of Michigan consists of a monitoring survey of 10 Galactic GCs with the aim of identifying photometrically variable EBs around or below the main-sequence turnoff (MSTO). My thesis work comprises the results of our research on the GCs NGC 3201, M10, and M12. Our observing strategy is aimed at detecting variables with periods between 0.2 and around 5 days and , thereby optimizing our chance to find the 16.5 ! V ! 20 valuable detached EBs in the target clusters. In addition, we correct for differential reddening variations across the cluster fields of up to several tenths of magnitudes in V by internally creating a differential extinction map with an arcminute resolution from our cluster photometry data and calculate the additional reddening zero point using isochrone fitting. The resulting improvement in the appearance of the color-magnitude diagrams (CMDs) of the clusters is considerable (see K. von Braun & M. Mateo 2001, AJ, 121, 1522). We obtained approximately 200 VI epochs with about 20,000 stars per image for each of the three clusters. Analysis of these data revealed the existence of 14 variable stars (11 EBs) in the field of NGC 3201, three variables (one EB) in the field of M10, and two EBs in the field of M12. Spectroscopic followup work showed that only one variable (a blue straggler W Ursa Majoris contact EB) in the field of NGC 3201 is associated with the cluster. Another W UMa EB is most likely a member of M12, based on its location in M12’s CMD and its empirically calculated absolute magnitude. The rest of the variable stars we detected are members of the Galactic disk (see K. von Braun & M. Mateo 2002, AJ, 123, 279; and K. von Braun et al. 2002, AJ, 124, 2067 for details). We thus calculate a ratio of observable, short-period, main-sequence binaries to main-sequence stars of around 1/500 for the Galactic disk (consistent with literature estimates) and 1/9000 for GC members. The latter value is considerably smaller than the commonly quoted value of 1/1000. While the discrepancy may be due to small number statistics, we attribute it to the fact that we lose about 20% of our fields to crowding toward the cluster center where the binary fraction should be highest after a few cluster relaxation times. In my dissertation, I show the cluster fields and CMDs (before and after dereddening) with the locations of the variable stars and our differential extinction maps, as well as the phased light curves and spectra (wherever applicable) of the variable stars. For additional information or to obtain a copy of the dissertation, please contact me or visit http://www.ciw.edu/vonbraun.

Yellow straggler stars (YSSs) fall above the subgiant branch in optical color–magnitude diagrams (CMDs), between the blue stragglers and the red giants. YSSs may represent a population of evolved blue stragglers, but none have the direct and precise mass and radius measurements needed to determine their evolutionary states and formation histories. Here we report the first asteroseismic mass and radius measurements of such a star, the yellow straggler S1237 in the open cluster M67. We apply asteroseismic scaling relations to a frequency analysis of the Kepler K2 light curve and find a mass of 2.9 ± 0.2 M ⊙ and a radius of 9.2 ± 0.2 R ⊙. This is more than twice the mass of the main-sequence turnoff in M67, suggesting that S1237 is indeed an evolved blue straggler. S1237 is the primary in a spectroscopic binary. We update the binary orbital solution and use spectral energy distribution fitting to constrain the CMD location of the secondary star. We find that the secondary is likely an upper main-sequence star near the turnoff, but a slightly hotter blue straggler companion is also possible. We then compare the asteroseismic mass of the primary to its mass from CMD fitting, finding that the photometry implies a mass and radius more than 2σ below the asteroseismic measurement. Finally, we consider formation mechanisms for this star and suggest that S1237 may have formed from dynamical encounters resulting in stellar collisions or a binary merger.

The nearby, metal-rich, halo globular cluster NGC 6366 has been observed in order to obtain its color-magnitude diagram and to search for variable stars. The color-magnitude diagram indicates that the cluster has a reddening E (B − V) of 0.80 (higher than most previous determinations) and a distance of 3.0 kpc. The cluster has numerous blue straggler stars, and a list of 27 candidate blue stragglers is given. However, no blue stragglers are found to be variable, and the implications of this finding are discussed. The one RR Lyrae star in the cluster is studied, and its period and light curve are determined. The structural parameters of the cluster are determined

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.

Forty-eight blue straggler stars have been discovered in NGC 5466, the only Galactic globular cluster known to contain an anomalous Cepheid of the sort found in dwarf galaxies. The stars were identified in color-magnitude diagrams constructed from photometry of deep photographic plates taken with the Canada-France-Hawaii 3.6 m telescope (calibrated with new UBV photoelectric photometry), and from point spread function photometry of CCD frames taken with the Palomar 5 m telescope. The stars typically have magnitudes ∼ 19 m .1 and colors ∼ 0 m .2. Forty-two of the 48 stars are situated inside of R=2.5 arcmin (see Fig.1), the projected radius containing half the cluster luminosity, and only six stars are found between 2.5 and 9 arcmin. A one-sided, two-sample Kolmogorov-Smirnov test (using the CCD data) establishes at the 98% significance level that the blue stragglers are more centrally concentrated than the subgiant stars of the same magnitude. By fitting multi-component King models to the projected radial distributions (Fig.2), the mean mass of the blue stragglers is shown to be ∼1.5 to two times larger than the masses of the stars that contributed the light from which the core and tidal radii were derived (i.e. M(Blue Str.)=1.3±0.3 M⊙). Because the central relaxation time for NGC 5466 is much less than the cluster age, the different radial distributions are attributed to mass segregation. A similar mass segregation is also observed in the globular cluster NGC 5053, where Nemec and Cohen (1986, in preparation) have recently identified ∼30 blue stragglers. The low stellar aensity and small escape velocity of NGC 5466 make a recent epoch of star formation (during which the blue stragglers might have formed as massive single stars) seem unlikely. Instead, the blue stragglers probably are either close binary systems that have transferred mass, or are coalesced stars. The very low frequency of stellar collisions expected in the center of NGC 5466 suggests that the blue stragglers are primordial binary systems. The simultaneous presence in NGC 5466 of the blue stragglers and the anomalous Cepheid V19, and their relative numbers, supports the hypothesis that there is an evolutionary connection between the two types of stars. By fitting theoretical isochrones to the photographic c-m diagram, NGC 5466 is estimated to have an age of 18±3 Gyr.

We report the discovery of a hot white dwarf (WD) companion to a blue straggler star (BSS) in the globular cluster (GC) NGC 5466, based on observations from the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat. The Spectral Energy Distribution (SED) of the Far-UV detected BSS NH 84 was constructed by combining the flux measurements from four filters of UVIT, with GALEX, GAIA, and other ground-based observations. The SED of NH 84 reveals the presence of a hot companion to the BSS. The temperature and radius of the BSS (T K, R/R ) derived from Gemini spectra and SED fitting using Kurucz atmospheric models are consistent with each other. The temperature and radius of the hotter companion of NH 84 (T K, R/R ) derived by fitting Koester WD models to the SED suggest that it is likely to be a hot WD. The radial velocity derived from the spectra along with the proper motion from GAIA DR2 confirms NH 84 to be a kinematic member of the cluster. This is the second detection of a BSS-WD candidate in a GC, and the first in the outskirts of a low-density GC. The location of this BSS in NGC 5466 along with its dynamical age supports the mass-transfer pathway for BSS formation in low-density environments.

For the past 9 years we have been monitoring the radial velocities of 13 blue stragglers in the old open cluster M67. For the 9 blue stragglers with rotational velocities no larger than about 100 km s −1 we have used the CfA digital speedometers to measure more than 500 radial velocities. To get reliable velocity correlations we use synthetic rotating templates computed from a grid of Kurucz model atmospheres. Four of the blue stragglers rotate too rapidly to allow successful velocity correlations with the CfA instruments. For three of these we have used a CCD spectrograph at Kitt Peak and similar reduction procedures (Morse et al. 1991.

We present the first far-UV (FUV) imaging results of the intermediate-age Galactic open cluster NGC 2818 that has a planetary nebula (PN) within the field using images taken from the Ultra Violet Imaging Telescope (UVIT) aboard AstroSat. We identify cluster members by combining UVIT-detected sources with Gaia EDR3 data. We detect four bright and hot blue straggler stars (BSSs) and two yellow straggler stars (YSSs) based on their location in optical and FUV–optical color–magnitude diagrams. Based on the parameters estimated using spectral energy distributions, we infer that BSSs are either collisional products or might have undetectable white dwarf (WD) companions. Our photometric analysis of YSSs confirms their binarity, consistent with the spectroscopic results. We find YSSs to be formed through a mass-transfer scenario and the hot components are likely to be A-type subdwarfs. A comparison of the radial velocity, Gaia EDR3 proper motion of the PN with the cluster, and reddening toward the PN and the cluster does not rule out the membership of the PN. Comparing the central star’s position with theoretical post‐AGB (pAGB) models suggest that it has already entered the WD cooling phase, and its mass is deduced to be ∼0.66 M ⊙. The corresponding progenitor mass turns out to be ∼2.1 M ⊙, comparable to the turn-off mass of the cluster, implying that the progenitor could have formed in the cluster. We suggest that the NGC 2818 might be one of the few known clusters to host a PN, providing a unique opportunity to test stellar evolution models.

This chapter explores how we might use the observed statistics of blue stragglers in globular clusters to shed light on their formation. This means we will touch on topics also discussed elsewhere in this book, such as the discovery and implications of bimodal radial distributions and the “double sequences” of blue stragglers that have recently been found in some clusters. However, we will focus particularly on the search for a “smoking gun” correlation between the number of blue stragglers in a given globular cluster and a physical cluster parameter that would point towards a particular formation channel. As we shall see, there is little evidence for an intrinsic correlation between blue straggler numbers and stellar collision rates, even in dense cluster cores. On the other hand, there is a clear correlation between blue straggler numbers and the total (core) mass of the cluster. This would seem to point towards a formation channel involving binaries, rather than dynamical encounters. However, the correlation between blue straggler numbers and actual binary numbers—which relies on recently determined empirical binary fractions—is actually weaker than that with core mass. We explain how this surprising result may be reconciled with a binary formation channel if binary fractions depend almost uniquely on core mass. If this is actually the case, it would have significant implications for globular cluster dynamics more generally.KeywordsBlue StragglersGlobular ClustersDynamic EncounterDense Cluster CoresStellar CollisionsThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.