The first magnetic field in a star other than the Sun was detected in 1947 in the star 78 Vir. Today, we know that about 10% of these intermediate-mass and high-mass stars have strong, large-scale surface magnetic fields whose origin has remained a mystery till today. It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields. The massive star τ Sco is a well-known member of the group of magnetic stars and is a blue straggler given its apparently young age compared to that of other members of the Upper Scorpius association. Here, we present 3D magnetohydrodynamic simulations of the coalescence of two massive main-sequence stars and 1D stellar evolution computations of the subsequent evolution of the merger product that can explain τ Sco’s magnetic field, apparent youth and other observed characteristics. We argue that field amplification in stellar mergers is a general mechanism to form strongly-magnetised massive stars. Such stars are promising progenitors of magnetars, which may give rise to some of the enigmatic fast radio bursts, and their supernova explosions may be affected by the strong magnetic fields.

In Kawahara et al. (2018) and Masuda et al. (2019), we reported the discovery of four self-lensing binaries consisting of F/G-type stars and (most likely) white dwarfs whose masses range from 0.2 to 0.6 solar masses. Here we present their updated system parameters based on new radial velocity data from the Tillinghast Reflector Echelle Spectrograph at the Fred Lawrence Whipple Observatory, and the Gaia parallaxes and spectroscopic parameters of the primary stars. We also briefly discuss the astrophysical implications of these findings.

The observational properties of a special class of stars (the so-called Blue Straggler stars - BSSs) in Globular Clusters are discussed in the framework of using this stellar population as probe of the dynamical processes occurring in high-density stellar systems. In particular, the shape of the BSS radial distribution and their level of central segregation have been found to be powerful tracers of the level of the dynamical evolution of the hosting cluster, thus allowing the definition of an empirical chronometer able to measure the dynamical age of star clusters.

Using data from the core of 47 Tuc we have identified stars in different evolutionary stages in the colour-magnitude diagram, and used the effects of mass segregation on their radial distribution to study the evolution and origin of blue stragglers (BSS). We separate the BSS into 2 samples by their magnitude and find considerable differences in their distribution. Bright BSS are more centrally concentrated with mass estimates over twice the turn-off mass suggesting an origin involving a triple or multiple star system. The distribution of the faint BSS is close to that of the main-sequence (MS) binaries pointing to these stars as their likely progenitors. Using MESA models, we calculate the expected number of stars in each evolutionary stage and compare it with the observed number of stars. Results indicate that BSS have a post-MS evolution comparable to that of a normal star of the same mass and a MS-BSS lifetime of about 200 – 300 Myr.

Binary evolution can produce different blue-straggler binaries, for example, blue stragglers with a bright, red component, or with a faint, blue component. In globular clusters, these blue-straggler binaries are generally observed as a single star, because two components can not be distinguished. Therefore, these blue-straggler binaries can be located in different regions of the color-magnitude diagram of globular clusters, e.g. blue sequence and red sequence observed in M30. We suggest that binary evolution can contribute to the blue stragglers in both of the sequences. Some blue stragglers in the blue sequence may have a faint white dwarf companion, while the red sequence includes some binaries experiencing mass transfer. It should be noted that the red sequence may also have other binaries, for example, the binaries just finished the mass transfer, and the binaries including a blue straggler (the accretors) that have evolved away from the blue sequence.

Explosive stellar transients arise from diverse situations, including deaths of massive stars, a variety of thermonuclear outbursts, and compact-object mergers. Stellar interactions are heavily implicated in explaining the observed populations of events, and not only those where binarity is obviously involved. Relationships between these classes probably help to elucidate our understanding; for example; the production of double neutron-star mergers from field binaries is thought to be heavily biased towards routes involving stripped core-collapse supernovæ. As we gain an ever more synoptic view of the changing sky, theorists should be mindful of developing an ability to take robust quantitative advantage of the available population information to help constrain the physics. This is complementary to aiming for deep understanding of individual events.

Gamma Cassiopeiae is an enigmatic Be star with unusually hard, strong X-ray emission compared with normal main-sequence B stars. The origin has been debated for decades between two theories: mass accretion onto a hidden compact companion and a magnetic dynamo driven by the star-Be disk differential rotation. There has been no decisive signature found that supports either theory, such as a pulse in X-ray emission or the presence of large-scale magnetic field. In a ~100 ksec duration observation of the star with the Suzaku X-ray observatory in 2011, we detected six rapid X-ray spectral hardening events called “softness dips”. All the softness dip events show symmetric softness ratio variations, and some of them have flat bottoms apparently due to saturation. The softness dip spectra are best described by either ~40% or ~70% partial covering absorption to kT ~12 keV plasma emission by matter with a neutral hydrogen column density of ~2 − 8 × 1021cm−2, while the spectrum outside of these dips is almost free of absorption. This result suggests that two distinct X-ray emitting spots in the γ Cas system, perhaps on a white dwarf companion with dipole mass accretion, are occulted by blobs in the Be stellar wind, the Be disk, or rotating around the white dwarf companion. The formation of a Be star and white dwarf binary system requires mass transfer between two stars; γ Cas may have experienced such activity in the past.

In this paper I present an overview of the main observational properties of a special class of exotic objects (the so-called Blue Straggler Stars, BSSs) in Galactic Globular Clusters (GCs). The BSS specific frequency and their radial distribution are discussed in the framework of using this stellar population as probe of GC internal dynamics. In particular, the shape of the BSS radial distribution has been found to be a powerful tracer of the dynamical evolution of stellar systems, thus allowing the definition of an empirical “clock”able to measure the dynamical age of stellar aggregates from pure observational properties.

We present a direct N-body simulation modeling the evolution of the old (7 Gyr) open cluster NGC 188. This is the first N-body open cluster simulation whose initial binary population is directly defined by observations of a specific open cluster: M35 (150 Myr). We compare the simulated color–magnitude diagram at 7 Gyr to that of NGC 188, and discuss the blue stragglers produced in the simulation. We compare the solar-type main-sequence binary period and eccentricity distributions of the simulation to detailed observations of similar binaries in NGC 188. These results demonstrate the importance of detailed observations in guiding N-body open cluster simulations. Finally, we discuss the implications of a few discrepancies between the NGC 188 model and observations and suggest a few methods for bringing N-body open cluster simulations into better agreement with observations.

Star clusters are ideal laboratories to test the theory of stellar evolution and provide very tight constraints on the concept of single stellar poputions (SSPs). Observations show that some stars fail to conform to the theoretical evolutionary scenario applicable to single stars. These special objects, particularly blue stragglers, present a challenge to our current theory of stellar evolution. They may be very important in the context of the integrated spectral properties of clusters. Here, we review the construction of SSP models, both empirically using star clusters and theoretically based on binary-interaction theory.

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.

The presence of blue straggler stars (BSs) as secure members of star clusters poses a major challenge to the conventional picture of simple stellar population (SSP) models. The models are based on the stellar evolution theory of single stars, while the major formation mechanisms of BSs are all correlated with stellar interactions. Based on a sufficient working sample including 100 Galactic open clusters, one Galactic globular cluster, and seven Magellanic Cloud star clusters, we discuss the modifications of the properties of broad-band colors and Lick indices of the standard SSP models due to BS populations.

During the past decade, the number known shallow-amplitude, yet totally eclipsing, extreme-mass-ratio binaries has increased from one (AW UMa, which is now in doubt) to about a dozen. Statistics are accumulating that will tell us the nature of these once rare systems. These individual interacting field binaries are believed to be the progenitors of FK Comae-type fast-rotating subgiants, which are similar to the cluster stars called ‘variable blue stragglers.’ We review our recent observations and new analyses of the individual systems, V409 Hya, GSC 1283 0053 Ori, GSC 2537 520 CVn, using the 2007 Wilson Code and summarize results from our previous observations (V802 Aql, V902 Sgr) and those from the literature (e.g., CK Boo, GR Vir, HV Aqr, UY UMa, EM Pis, CU Tau, TV Mus) to disclose a preliminary picture of the character of this interesting class of binary stars.

In this contribution we review the main observational properties of Blue Stragglers Stars (BSS) in galactic GCs. A flower of results on the BSS frequency, radial distribution, and chemical composition are presented and discussed.

We present a detailed dynamical study of the old (7 Gyr) open cluster NGC 188. Our combined radial-velocity data set spans a baseline of 35 years, a magnitude range of 12 ≤ V ≤ 16.5, and a 1° diameter region on the sky. Our magnitude limits include solar-mass main-sequence stars, subgiants, giants, and blue stragglers, and our spatial coverage extends radially to 11.5 core radii. We have measured radial velocities for 1014 stars in the direction of NGC 188 with a precision of 0.4 km s−1, and have calculated radial-velocity membership probabilities for stars with ≥ 3 measurements. We find 420 stars to be high-probability cluster members, including 137 spectroscopic binaries. These detectable binaries all have orbital periods of less than 104 days, and thus are hard. We have derived orbit solutions for 67 member binary stars, and use our 35 main-sequence binaries with orbit solutions to compare the eccentricity and period distributions with simulated observations of the Hurley et al. (2005) model of M67 (4.5 Gyr). We also compare the spatial distributions of cluster member populations.

We present an analysis of our deep far- (FUV) and near-ultraviolet (NUV) photometry of the core region of the dense globular cluster M 15. Our FUV-NUV colour-magnitude diagram (CMD) is the deepest one presented for a globular cluster so far, and shows all hot stellar populations expected in a globular cluster, such as horizontal branch stars, blue stragglers, white dwarfs, cataclysmic variables and even main sequence stars. The main sequence turn-off is clearly visible and the main sequence stars form a prominent track that extends at least two magnitudes below the main sequence turn-off. We compare and discuss the radial distribution of the various stellar populations that show up in the FUV. We search for variability amongst our FUV sources and tentatively classify our variable candidates based on an analysis of the UV colours and variability properties. We find that RR Lyraes, Cepheids, and SX Phoenicis exhibit massive variability amplitudes in this waveband (several mags).

By means of high-resolution and wide-field observations in the UV and optical bands we have derived the radial distribution of the Blue Stragglers Star (BSS) population in a number of galactic globular clusters. Monte-Carlo dynamical simulations have then been used to interpret the observed radial distributions in terms of percentage of collisional and mass-transfer BSS populating each cluster. I will present the main results thus obtained and an overall cluster–to–cluster comparison for the whole sample collected so far, mainly focusing on the clues that such an approach provides about the BSS formation mechanisms.

The extensive stellar radial-velocity surveys of the WIYN Open Cluster Study now allow comprehensive studies of the solar-type hard-binary populations in open clusters as a function of age. We first describe an empirical “initial” hard-binary population as derived from the young open cluster NGC 2168 (M35). Given the limited analyses so far, the cluster binary population is indistinguishable from that of the field. We then compare the hard-binary population in the old open cluster NGC 188 to the binary population in the sophisticated N-body simulations of the old cluster M67 by Hurley et al. The binary populations in the cluster and the simulation show significant differences in binary frequency and fraction of circularized binaries, while otherwise showing similar orbital eccentricity distributions. Since the simulations were designed to match the encounter products in M67, such as blue stragglers, the large reduction in binary fraction indicated by the empirical results likely will also require changes in the simulation physics producing blue stragglers and other anomalous stars arising from stellar dynamics. We present three case studies of stars in open clusters which very likely are products of dynamical encounters between binaries and either single stars or other binaries: the M67 blue straggler S1082, the M67 sub-subgiant S1113, and the horizontal branch star 6819-3002 in the intermediate-age open cluster NGC 6819. Finally, we remind the reader of recent empirical results on the rates of tidal interactions, using tidal circularization periods in open clusters. Every indication is that current theories underestimate the effectiveness of tidal circularization, a result that need to be incorporated into dynamical simulations of dense stellar systems.

We present preliminary work on the formation scenario of blue straggler stars by mass transfer in binary systems. More precisely, using Smoothed Particle Hydrodynamics (SPH), we want to model only the outer parts of the stars in order to get a much greater spatial resolution of the mass transfer flow itself. The inner boundary conditions are achieved using the so-called ghost particles and by replacing the inner mass by a central point mass. Stability of this central point mass is crucial, and it is shown that we get reasonable results. These simulations should give us indications on which layers of the donor star are actually transferred to the other star as well as how mass is transferred and how it settles on the accretor. This work is aimed at getting distinct observational signatures which would help identifying the dominant formation mechanism of blue straggler stars.

The old open cluster M 67 is an ideal test case for current star cluster evolution models because of its dynamically evolved structure and rich stellar populations that show clear signs of interaction between stellar, binary and cluster evolution. Here we discuss a direct N-body model of M 67. This model of 12,000 single stars and 12,000 binaries is evolved from zero-age and takes full account of cluster dynamics as well as stellar and binary evolution. At an age of 4 Gyr the model cluster matches the mass and structure of M 67 as constrained by observations. We discuss the role of the primordial binary population and the cluster environment in shaping the nature of the stellar populations of M 67, with a focus on X-ray binaries and blue stragglers.