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The" Dynamical Clock": Dating the Internal Dynamical Evolution of Star

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The Rendiconti Lincei. Scienze Fisiche e Naturali. LINCEI PRIZEWINNERS manuscript No. (will be inserted by the editor) The “dynamical clock”: dating the internal dynamical evolution of star clusters with Blue Straggler Stars Francesco R. Ferraro · Barbara Lanzoni · Emanuele Dalessandro Received: November 12, 2019 / Accepted: January 12, 2020 Abstract We discuss the observational properties of PACS PACS 97.20.Rp · 97.10.Zr · 98.20.Gm · a special class of objects (the so-called “Blue Straggler 98.10.+z Stars”, BSSs) in the framework of using this stellar pop- ulation as probe of the dynamical processes occurring in high-density stellar systems. Indeed, the shape of the 1 Introduction BSS radial distribution and their level of central concen- tration are powerful tracers of the stage of dynamical Globular Clusters (GCs) are compact aggregates of up evolution reached by the host cluster since formation. to a million stars held together by their mutual gravi- Hence, they can be used as empirical chronometers able tational attraction in a nearly spherical configuration. to measure the dynamical age of stellar systems. In ad- They are sub-galactic structures, with typical masses 4 6 dition, the presence of a double BSS sequence in the of 10 − 10 M⊙, ages as old as the Hubble time (∼ 13 color-magnitude diagram is likely the signature of the Gyr), and cores that present the most extreme stel- most extreme dynamical process occurring in globular lar densities in the Universe, reaching up to a few mil- cluster life: the core collapse event. Such a feature can lions of stars per cubic parsec. GCs are the most pop- therefore be used to reveal the occurrence of this pro- ulous and oldest systems where stars can be individu- cess and, for the first time, even date it. ally observed. At odds with what happens in galaxies, where the orbital motion of stars primarily depends Keywords Faint blue stars (blue stragglers) · on the average gravitational potential, GCs are “col- Hertzsprung-Russell, color-magnitude, and color-color lisional” systems, where two-body interactions cause diagrams · Globular clusters in the Milky Way · Stellar kinetic-energy exchanges among stars and gravitational dynamics and kinematics perturbations to their orbits, bringing the cluster to- ward a thermodynamically relaxed state in a timescale (relaxation time) that can be significantly shorter that The author F. R. Ferraro received the international prize its age. For this reason they represent unique cosmic “L. Tartufari” for Astronomy in 2018, attributed by the Ac- arXiv:2001.07435v1 [astro-ph.GA] 21 Jan 2020 laboratories to study the fundamental physical processes cademia Nazionale dei Lincei, Rome. characterizing multi-body dynamics. Because of such Francesco R. Ferraro interactions, heavy stars tend to progressively sink to- Dipartimento di Fisica e Astronomia, Universit`adegli Studi ward the central region of the cluster (dynamical fric- di Bologna, Via Gobetti 93/2, I–40129 Bologna, Italy Tel.: +39-051-2095774 tion), while low-mass stars can escape from the sys- E-mail: [email protected] tem (evaporation). This yields a progressive contrac- Barbara Lanzoni tion of the core, producing an impetuous increase of Dipartimento di Fisica e Astronomia, Universit`adegli Studi its density virtually toward infinity: the so-called “core di Bologna, Via Gobetti 93/2, I–40129 Bologna, Italy collapse”. The runaway contraction is thought to be Emanuele Dalessandro halted by the formation and hardening of binary sys- INAF-Osservatorio di Astrofisica & Scienza dello Spazio, via tems, and the post-core collapse phase is characterized Gobetti 93/3, I–40129 Bologna, Italy by core oscillations, with several episodes of high cen- 2 Francesco R. Ferraro et al. M3 Fig. 1 Left: indicative illustration of the location of BSSs (large blue circles) in a Temperature - Luminosity diagram. Luminosities are expressed in Solar units, temperatures in Kelvin. Right: artistic illustration of the two main BSS formation channels, namely stellar collisions (upper panel), and vampirism phenomena between two companion stars in a binary system (lower panel). tral density followed by stages during which the clus- holds for star clusters. Thus, a proper characterization ter rebounds toward a structure with lower density and of any GC requires the knowledge, not only of its inter- more extended core. The recurrent gravitational inter- nal structure and kinematics, but also of its dynamical actions among stars thus modify the structure of the age. system over the time (the so-called ”dynamical evo- lution”), with a time-scale (the relaxation time) that depends in a very complex way on the initial and the 1.1 Blue Stragglers as gravitational test particles of local conditions, thus differing from cluster to cluster GC dynamics and, within the same system, from high- to low-density regions (e.g., Meylan & Heggie 1997). The internal dynamical activity of GCs is thought to also generate a variety of stellar exotica, as blue strag- As a consequence of the internal dynamical evolu- gler stars (BSSs) and interacting binaries containing tion, clusters born with a given size progressively de- heavily degenerate objects, like black holes and neutron velop more and more compact cores, the timescale of stars (see Bailyn, 1995; Ransom et al., 2005; Cool et al., these changes being hard to determine. Indeed, esti- 1998; Strader et al., 2012; Cadelano et al., 2017, 2018; mating the formation epoch of a cluster (corresponding Dalessandro et al., 2014; Pallanca et al., 2013, 2017; Ferraro et al., to the chronological age of its stars) is relatively sim- 2015). Among these, BSSs are certainly the most abun- ple from the measure of the luminosity of the Main dant and they are the easiest to distinguish from nor- Sequence Turn-Off (MS-TO) level, while measuring its mal stars in a color-magnitude diagram (CMD), since “dynamical age” (corresponding to the level of dynam- they define a sort of sequence extending brighter and ical evolution it reached since formation) is much more bluer than the cluster MS-TO point, mimicking a sub- challenging. Following an analogy to human experience, population of young stars (see Fig. 1; Sandage, 1953; as people with the same biological age can be in very Ferraro et al., 1992, 1993, 1997a, 2003; Leigh et al., 2007; different physical shapes, so stellar aggregates with the Moretti et al., 2008; Simunovic & Puzia, 2016; Parada et al., same chronological age can have reached quite differ- 2016). ent levels of internal dynamical evolution. While the Their history dates back to 1953, when the Ameri- age of people is easily readable in the identity card, de- can astronomer Allan Sandage first discovered this puz- termining their physical shape is not straightforward zling population of stars that seemed to go against the (and it depends on the capacity of correctly reading a rules of the stellar evolution theory, in the Galactic few characteristics impressed on their body). The same GC Messier 3 (M3). He dubbed them “stragglers” be- The stellar dynamical clock 3 Fig. 2 Ultraviolet CMDs of two massive globular clusters (namely, M3 and NGC 2808). In both cases, BSSs are plotted as large blue circles. In the right panel, the position of the main evolutionary sequences are also marked. cause they are located outside the main evolutionary Leigh et al., 2013; Xin et al., 2015; Beccari et al., 2019; sequences in the CMD and they seem to be trailing in Portegies Zwart , 2019). the evolution with respect to the vast majority of stars Irrespective of their formation mechanism, BSSs rep- in the cluster. The presence of these (apparently) young resent a population of heavy objects (MBSS = 1.0 − stars in a GC was completely unexpected, since star for- 1.4M⊙; e.g., Fiorentino et al., 2014; Raso et al., 2019; mation essentially stopped 13 billion years ago in these see also Ferraro et al., 2016) orbiting in an “ocean” of systems. Indeed, BSSs are not thought to be a young lighter stars (the average stellar mass in an old GC is stellar population, and it is widely accepted that they hMi =0.3M⊙; e.g., Djorgovski, 1993). For this reason, are hydrogen-burning stars more massive than the MS- BSSs can be used as powerful gravitational probes to TO objects (Shara et al., 1997; Gilliland et al., 1998). investigate key physical processes (such as mass segre- However, the details of their formation mechanism are gation and dynamical friction) characterizing the dy- not completely understood yet, and two main mech- namical evolution of star clusters (e.g., Ferraro et al., anisms are commonly advocated to explain their ori- 2009, 2012; Dalessandro et al., 2013a; Simunovic et al., gin (see Fig. 1): (1) direct collisions (COLL), in which 2014). But, how difficult is to collect complete samples the stars might actually merge, mix their nuclear fuel of BSSs in GCs? and “re-stoke” the fires of nuclear fusion (Hills & Day, 1976), and (2) mass-transfer (MT) in tight binary sys- tems, where the less massive object acts as a “vam- 1.2 The UV route to search for BSSs in GCs pire”, siphoning fresh hydrogen from its more massive Because of their high surface temperatures (Teff ∼ 6500- companion, possibly up to the complete coalescence 9000 K), BSSs are among the brightest objects at ultra- of the two stars (McCrea, 1964). Both processes are violet (UV) wavelengths in Galactic GCs (see Fig. 2). suggested to add fresh fuel (hydrogen) into the stel- Thus, their systematic search strongly benefitted from lar core, thus prolonging the star lifetime and making the advent of UV space facilities. More than 20 years it look more youthful (blueness and brightness being ago we promoted the so-called UV route to BSS study the attributes of stellar youth).
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