A Search for Eclipsing Binaries in Galactic Globular Clusters

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.

Blue Straggler Stars (BSSs) are one of the keys to understand in a better way how stars evolve and interact with each other in different star clusters. They appear to be hotter and brighter than the Main Sequence Turn Off Point (MSTO) and, therefore, they should have evolved off the Main-Sequence branch. We properly select, for Open Clusters (OCs) and Globular Clusters (GCs), star cluster members based on proper motions and parallaxes provided by Gaia Data Release 3 (DR3). Using isochrones models and selection criteria from previous studies, we select and classify stars as BSS, Yellow Straggler Stars and Red Stragglers Stars (evolved BSSs). We identify BSSs to be present in all our GCs and in 42/129 (∼ 33 %) of studied OCs. We counted a total of 4399 BSSs; 434 (∼ 10 %) located in OCs and 3965 (90 %) located in GCs. Clusters younger than ∼ 500 Myr do not show the presence of BSSs in our sample. We obtain astrophysical parameters from 3 different methods (color–temperature relations, isochrone–fitting models and parameters from Gaia DR3 spectra) such as the effective temperature Teff, star mass M, and surface gravity log(g). We find values for BSS Teff to be ∼ (6800 ± 585) K in GCs and ∼ (7570 ± 1400) K in OCs; and an average mass of ⟨MBSS⟩ = (1.75 ± 0.45) M⊙ in OCs and ⟨MBSS⟩ = (1.02 ± 0.1) M⊙ in GCs. For every BSS, we compute the difference of the BSS mass and the MSTO mass of its parent cluster, normalized by the MSTO mass, and called it Me based on previous studies. This parameter is classified as low–Me (Me < 0.5, BSS likely formed through mass-transfer) and high–Me (0.5 < Me < 1.0, BSS likely formed through mergers). For OCs we find a percentage 81.34 % high–Me and 18.66 % low–Me. For GCs, we find 94.25 % low–Me and 5.25 % high–Me. Comparing Me against the BSS age obtained with isochrone modes, we were able to detect: i) GC BSSs that are most likely to be formed through collisions show a “boost” in their percentage/fraction for stars with an age ∼ 1 − 2 Gyr, in agreement with reported age for core-collapse events in GCs found in previous studies; ii) a double sequence for GC BSSs, where we conclude that these sequences correspond to a pre merger/closer-binary interaction and post merger/closer-binary interaction of BSS formation

We have resolved stars in the core of the old SMC globular cluster NGC 121 with images from the refurbished Hubble Space Telescope and Wide Field Planetary Camera 2. Our photometry reaches 1.5 mag below the main-sequence turnoff, with negligible field contamination. A blue straggler star (BSS) sequence of 42 candidates is clearly seen—the first such detection in an extragalactic globular cluster. The BSSs are more centrally concentrated than the subgiant, red giant, and horizontal branch stars. Various blending and completeness tests demonstrate that at least 23 of our candidates are genuine BSSs. The BSSs extend about 1.8 mag brighter than the main-sequence turnoff, in accord with similar sequences in Galactic globular clusters. One BSS candidate is 2.4 mag brighter than the turnoff, as luminous as the brightest BSS seen in Galactic globulars. The cluster red horizontal branch is clearly detected and well populated, with a modest blueward extension. The relaxation times of the cluster stars and the BSSs are such that most BSSs are in energy equipartition with the other stars. We find ΔV= 3.3 ± 0.1 mag, corresponding to a cluster age that is 2 Gyr younger than that of most Galactic globular clusters and of one of the oldest LMC globulars.

We have studied ultraviolet (UV) bright sources in the Galactic globular cluster (GGC) NGC 4590 using Ultraviolet Imaging Telescope onboard the AstroSat satellite. Using UV-optical colour–magnitude diagrams (CMDs), we have identified and characterized the sources of different evolutionary stages i.e. blue horizontal branch stars (BHBs), extremely blue horizontal branch stars (EHBs), blue straggler stars (BSs), variable stars, etc. We estimated effective temperature (Teff), gravity (log (g)), luminosity (Lbol), and hence the radius (R) of these hot stars by fitting spectral energy distribution with the help of stellar atmosphere models. Two new far-UV bright cluster member stars situated near the core of the cluster have been detected; one of them is an EHB star and the other one is either in its post-blue hook evolutionary phase or in white dwarf phase. The evolutionary status of all the hot stars, identified in the cluster, has been investigated by using various evolutionary models. We find the massive and younger BSs are concentrated at the centre of the cluster whereas the older and less massive BSs are distributed throughout the cluster. The BSs normalized radial distribution seems to be bi-modal with a minimum located at rmin = 4.3 rc. We calculated A+ parameter of the cluster which is obtained using cumulative normalized radial distribution of horizontal branch stars (HBs) and BSs. We measured this value up to half-mass radius of the cluster to be +0.13, which indicates that NGC 4590 is one of the youngest clusters among dynamically intermediate age GGCs with a dynamical age of 0.423 ± 0.096 Gyr.

We present a study of the detection and recovery efficiency of the Rubin Observatory for detached eclipsing binaries (EBs) in the galactic field, globular clusters (GCs), and open clusters (OCs), with a focus on comparing two proposed observing strategies: a standard cadence (baseline) and a cadence that samples the galactic plane more evenly (colossus). We generate realistic input binary populations in all observing fields of the Rubin Observatory, simulate the expected observations in each filter, and attempt to characterize the EBs using these simulated observations. In our models, we predict that the baseline cadence will enable the Rubin Observatory to observe about three million EBs; our technique could recover and characterize nearly one million of these in the field and thousands within star clusters. If the colossus cadence is used, the number of recovered EBs would increase by an overall factor of about 1.7 in the field and in globular clusters, and a factor of about 3 in open clusters. Including semidetached and contact systems could increase the number of recovered EBs by an additional factor of about 2.5 to 3. Regardless of the cadence, observations from the Rubin Observatory could reveal statistically significant physical distinctions between the distributions of EB orbital elements between the field, GCs, and OCs. Simulations such as these can be used to bias correct the sample of Rubin Observatory EBs to study the intrinsic properties of the full binary populations in the field and star clusters.

There is now strong evidence that the close binary fraction (P < 104 days; a < 10 au) of solar-type stars (M 1 ≈ 0.6–1.5 ) decreases significantly with metallicity. Although early surveys showed that the observed spectroscopic binary (SB) fractions in the galactic disk and halo are similar (e.g., Carney–Latham sample), these studies did not correct for incompleteness. In this study, we examine five different surveys and thoroughly account for their underlying selection biases to measure the intrinsic occurrence rate of close solar-type binaries. We reanalyze (1) a volume-limited sample of solar-type stars, (2) the Carney-Latham SB survey of high proper motion stars, (3) various SB samples of metal-poor giants, (4) the APOGEE survey of radial velocity (RV) variables, and (5) eclipsing binaries (EBs) discovered by Kepler. The observed APOGEE RV variability fraction and Kepler EB fraction both decrease by a factor of ≈4 across −1.0 < [Fe/H] < 0.5 at the 22σ and 9σ confidence levels, respectively. After correcting for incompleteness, all five samples/methods exhibit a quantitatively consistent anticorrelation between the intrinsic close binary fraction (a < 10 au) and metallicity: F close = 53% ± 12%, 40% ± 6%, 24% ± 4%, and 10% ± 3% at [Fe/H] = −3.0, −1.0, −0.2 (mean field metallicity), and +0.5, respectively. We present simple fragmentation models that explain why the close binary fraction of solar-type stars strongly decreases with metallicity while the wide binary fraction, close binary fraction of OB stars, and initial mass function are all relatively constant across −1.5 ≲ [Fe/H] < 0.5. The majority of solar-type stars with [Fe/H] ≲ −1.0 will interact with a stellar companion, which has profound implications for binary evolution in old and metal-poor environments such as the galactic halo, bulge, thick disk, globular clusters, dwarf galaxies, and high-redshift universe.

The 222~000 I-band light curves of variable stars detected by the OGLE-II survey in the direction of the Galactic Bulge have been searched for eclipsing binaries (EBs). A previously developed code to analyze lightcurve shapes and identify long period variables (LPVs) has been adapted to identify EBs. The parameters in the modified code have been optimised to recover a list of about 140 detached EBs in the Small Magellanic Cloud previously identified in the literature as particularly well suited for distance estimates (and wich have periods \more 0.85 days). The power of the code is demonstrated by identifying 16 and 178 previously uncatalogued EBs in the SMC and LMC, respectively. Among the 222~000 variable stars in the direction of the Galactic Bulge 3053 EBs have been identified. Periods and phased lightcurves are presented.

Blue straggler stars (BSSs) are brighter and bluer (hotter) than the main-sequence (MS) turnoff and they are known to be more massive than MS stars.Two main scenarios for their formation have been proposed:collision-induced stellar mergers (COL-BSSs),or mass-transfer in binary systems (MT-BSSs).Depleted surface abundances of C and O are expected for MT-BSSs,whereas no chemical anomalies are predicted for COL-BSSs.Both MT- and COL-BSSs should rotate fast, but braking mechanisms may intervene with efficiencies and time-scales not well known yet,thus preventing a clear prediction of the expected rotational velocities.Within this context,an extensive survey is ongoing by using the multi-object spectrograph FLAMES@VLT,with the aim to obtain abundance patterns and rotational velocities for representative samples of BSSs in several Galactic GCs.A sub-population of CO-depleted BSSs has been identified in 47 Tuc,with only one fast rotating star detected.For this PhD Thesis work I analyzed FLAMES spectra of more than 130 BSSs in four GCs:M4,NGC 6397,M30 and ω Centauri.This is the largest sample of BSSs spectroscopically investigated so far.Hints of CO depletion have been observed in only 4-5 cases (in M30 and ω Centauri),suggesting either that the majority of BSSs have a collisional origin,or that the CO-depletion is a transient phenomenon.Unfortunately,no conclusions in terms of formation mechanism could be drawn in a large number of cases,because of the effects of radiative levitation. Remarkably,however,this is the first time that evidence of radiative levitation is found in BSSs hotter than 8200 K.Finally, we also discovered the largest fractions of fast rotating BSSs ever observed in any GCs:40% in M4 and 30% in ω Centauri.While not solving the problem of BSS formation,these results provide invaluable information about the BSS physical properties,which is crucial to build realistic models of their evolution.

UBV CCD photometry for 7217 stars in the loose Galactic globular cluster IC 4499 is presented. The new colour-magnitude diagram (CMD) reaches 2mag below the main-sequence turn-off (TO), yielding the first view of the subgiant branch and the upper main sequence. Beyond confirming the already known overall morphology of the horizontal and giant branches, these new data lead to the following results. (i) The cluster interstellar reddening, independently obtained from the two-colour diagram, is |$E(B-V)=0.25\pm0.05.$| (ii) The mean metallicity derived via different photometric observables measured in the CMD turns out to be [Fe/H]=-1.75±-0.20, about 0.2-0.3 dex lower than that obtained via spectroscopy. (iii) 64 candidate blue stragglers (BSS) have been identified. They are more concentrated than the subgiant stars of similar brightness (at >3σ). (iv) Coupling the apparent luminosity of the horizontal branch, |$V_\text {HB}=17.65\pm0.04$|⁠, with |$V_\text {TO}=20.90\pm0.10$| [and |$(B-V)_\text {TO}=0.657\pm 0.003$|], one gets |$\Delta V^\text {TO}_\text {HB}=3.25\pm0.12$|⁠, a value much lower than 3.55±0.09 found by Buonanno, Corsi & Fusi Pecci for 18 well-studied clusters. With the adopted metallicity, this suggests that IC 4499 is younger (by 3-4 Gyr) than most globular clusters of similar metallicity. This conclusion is further supported by differential comparisons of the main branches in the CMDs of a wide set of reference clusters. (v) Together with Pal 12, Ruprecht 106, Arp 2 and Terzan 7, this is the fifth Galactic globular cluster showing significant evidence for large age differences in the medium-far Galactic halo.

Context. Blue stragglers stars (BSSs) are thought to form in globular clusters by two main formation channels: i) mergers induced by stellar collisions; and ii) coalescence or mass transfer between companions in binary systems. The detailed study of the BSS properties is therefore crucial for understanding the binary evolution mechanisms and the complex interplay between dynamics and stellar evolution in dense stellar systems. Aims. We present the first comparison between the BSS specific frequency and the binary fraction in the core of a sample of Galactic globular clusters, with the aim of investigating the relative efficiency of the two proposed formation mechanisms. Methods. We derived the frequency of BSSs in the core of thirteen low-density Galactic globular clusters by using deep ACS@HST observations and investigated its correlation with the binary fraction and various other cluster parameters. Results. We observed a correlation between the BSS specific frequency and the binary fraction. The significance of the correlation increases by including a further dependence on the cluster’s central velocity dispersion. Conclusions. We conclude that the unperturbed evolution of primordial binaries could be the dominant BSS formation process, at least in low-density environments.

view Abstract Citations (31) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CCD Photometry for the Galactic Globular Cluster NGC 6584 Sarajedini, Ata ; Forrester, Wendy L. Abstract We present CCD photometry in the B and V passbands for the Galactic globular cluster NGC 6584. The color-,agmotide diagram reveals a horizontal branch (HB) populated on both the red and blue sides of the RR Llyrae instability strip. The cluster contains 42 confirmed RR Lyrae variables, and we find an additional 56 stars which are possible RR Lyraes. The mean magnitude of the RR Lyraes on our CCD frames is combined with that of the nonvariable stars to yield (V(HB)) = +/- 0.03. Using a polynomial fit to the cluster giant stars, we derive the color of the giant branch at the level of the HB to be (B-V)g = 0.89 +/- 0.02. Our photometry has also been used to isolate the red giant branch clump which forms when stars pause and sometimes reverse their course as they evolve up the giant branch. Its luminosity is primarily dependent upon the cluster metallicity, and we have used this fact to derive a metallicity of (Fe/H = -1.44 +/- 0.16 for NGC 6584. Coupled with the color of the giant branch given above, this yields a cluster reddening of E)B-V) = 0.07 +/- 0.04. To the extent that our photometry can be used to study the age of NGC 6584, we find it to be comparable to M3, which is not surprising given their similar HB morphologies. The color-magnitude diagram also exhibits a significant population of blue straggler stars. From a comparison of the blue straggler radial distribution to that of the subgiant stars with similar V magnitudes, we find that the blue stragglers are more centrally concentrated than the subgiants, indicative of a higher mass for the blue stragglers. Publication: The Astronomical Journal Pub Date: March 1995 DOI: 10.1086/117345 Bibcode: 1995AJ....109.1112S Keywords: Globular Clusters; Horizontal Branch Stars; Metallicity; Red Giant Stars; Stellar Color; Stellar Magnitude; Subgiant Stars; Variable Stars; Charge Coupled Devices; Chronology; Color-Magnitude Diagram; Radial Distribution; Star Distribution; Stellar Spectrophotometry; Astronomy; GLOBULAR CLUSTERS: INDIVIDUAL: NGC 6584; GALAXY: HALO; GALAXY: ABUNDANCES full text sources ADS | data products SIMBAD (6) CDS (1)

Using CCD color-magnitude diagrams of Galactic globular clusters in the B and V filter bands, we focus on the main sequence turnoff region and its usefulness in studying blue straggler stars, relative cluster ages, and the luminosity of the horizontal branch. In addition, we have measured several basic photometric parameters for the clusters in our program; these include NGC 288, NGC 3201, NGC 4372, NGC 4590 (M68), IC 4499, NGC 5897, NGC 6101, NGC 6171 (M107), NGC 6496, NGC 6584, NGC 6723, NGC 6809 (M55), NGC 6934, and NGC 6981 (M72). Of the 14 globular clusters in our sample, 11 appear to have definite blue straggler sequences. In the majority of cases, the blue stragglers are found to be more centrally concentrated and therefore more massive than the subgiant branch stars. Furthermore, the characteristics of the clusters which contain blue stragglers span practically the entire observed range for Galactic globular clusters. It therefore seems that all such systems must contain blue stragglers. The HR Diagram of the blue stragglers reveals that all of these stars occupy the same general region of this diagram, but the mean location of the individual cluster sequences do scale with metal abundance in the expected fashion. Finally, the luminosity function indicates the possible identification of blue stragglers which have evolved away from their initial positions. There exists a range in age among the Galactic globular clusters of 3 to 5 gigayears. Such a range suggests a relatively slow formation timescale for the Galactic halo. In addition, from the variation of age as a function of horizontal branch type, we conclude that age is the 'second parameter' which, along with metallicity, governs the behavior of the horizontal branch. Finally, we derive a simple algebraic expression that describes the slope of the horizontal branch luminosity - metallicity relation in terms of three relative cluster parameters. Using available values of these quantities in 8 globular clusters, we compute a slope of 0.15 ± 0.05 (mag/dex) for the variation of horizontal branch luminosity with metallicity.

The current status of the search and study of Blue Stragglers (BSS) in Galactic globular clusters is reviewed leading to a catalog of 425 BSS- candidates in 21 clusters. A new interpretative scenario is presented, based on the claimed detection of a BSS-progeny located on the red Horizontal Branch (HB) in the various clusters, as first suggested by Renzini and Fusi Pecci (1988, ARA&A, 26,199) for M3. The cluster concentration and integrated luminosity are important driving parameters when considering the properties of the BSS population and HB morphology, now partially connected to each other. Implications and problems emerged within the framework of cluster stellar evolution are discussed and some speculations are presented. BSS luminosity functions (LF) are also presented and discussed. Though selection effects and poor statistics still dominate the available database, the total BSS LF shows a clear peak at M_V_~+3.2 and the brightest BSS are about 2.5 mag brighter than the corresponding turnoff stars. Concerning the BSS location with respect to the extension of the zero age main sequence (ZAMS) and its dependence on metallicity, the BSS sequence always has a much larger intrinsic width than expected from photometric errors and is systematically shifted towards the red with respect to the ZAMS. This redward shift appears to be slightly related with metallicity. Finally, a preliminary analysis of the population ratios between detected BSS in a cluster, N_BSS_, and their claimed progeny on the HB, N^HB^_BSP_, leads to N_BSS_/N^HB^_BSP_~6.

Binary stars should be present in relatively large numbers in globular clusters (GCs). While individual binary systems have been previously observed, until now, no large population had been detected in a dense GC. As part of this thesis, we developed a new technique to determine the fraction of main-sequence stars which are binaries. We analyzed the redward spread of the main-sequence ridge-line of the post-core-collapse GC NGC~6752. These data, obtained with post-refurbishment HST data, indicate that in the inner core-radius, the 3-σ lower and upper limits on the binary fraction are 15% and 38%, respectively. The region outside the core has a 3-σ upper limit on the binary fraction of 16%, and is consistent with no binaries. This rapid change in binary fraction is clear evidence of mass segregation operating within the inner 3 core-radii. We also find that the binary fraction increases at fainter magnitudes; mass segregation would also account for such behavior. We discuss the likely effects on the cluster's luminosity function (LF). In particular, we find that if the contribution of binaries to the LF was not removed, the apparent number of low mass stars would be over-estimated. I also examined the time-series data and discovered that one of the blue stragglers in NGC~6752 is an SX Phe pulsator. Its pulsation mass is 1-1.2 M⊙, consistent with the mass of the best fit stellar models from the Yale Isochrones, and also consistent with any of the blue straggler formation scenarios. No contact or eclipsing binaries were unambiguously detected. However, many low-amplitude variable candidates were observed at the limit of our ability to detect variability. The putative period of these candidate objects is consistent with these objects being produced via tidal capture events. In a time-series study of NGC~6397, we found a contact binary. This W~UMa system probably has a mass greater than that of main sequence turn-off stars implying that when this system coalesces due to angular momentum loss, it will become a blue straggler star. The frequency of W~UMas in this cluster is not statistically different from that of other GCs where ∼0.1% of the main-sequence stars are contact binarie.

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.