The dynamical state of the globular clusters Rup 106 and IC 4499

The dynamical evolution of globular clusters is theoretically described by a series of well known events typical of N-body systems. Still, the identification of observational signatures able to empirically describe the stage of dynamical evolution of a stellar system of the density typical of a globular cluster, represents a challenge. In this paper we study the dynamical age of the globular clusters Rup 106 and IC 4499. To this aim, we study the radial distribution of the Blue Straggler Stars via the A+ parameter and of the slope of the Main Sequence Mass Function. Both tracers show that Rup 106 and IC 4499 are dynamically young clusters where dynamical friction has just started to segregate massive stars towards the clusters' centre. In fact, we observe that the Blue Straggler stars are more centrally concentrated in both clusters than the reference population. On the same line, we find that in both cases the slope of the mass function significantly decreases as a function of the cluster-centric distances. This result provides additional support for the use of the the radial distribution of the blue stragglers as a powerful observationally convenient indicator of the cluster dynamical age.Comment: Accepted for publication on A&

The Exotic Zoo of Millisecond Pulsars in Globular Clusters: a multi-wavelength study

This work is focused on the study of millisecond pulsars in globular cluster by using multi-wavelength observations. Radio observations have been used to search for and timing the pulsars. An alternative method to search for very faint pulsars is first presented and then successfully applied to observations of stellar system Terzan 5, leading to the discovery of three new pulsars. The update of the timing solutions of 9 pulsars in M28 is then presented. The timing solutions now cover a data span of 10 years and provide spin, astrometric and orbital properties for all the systems. For the case of the eccentric binaries M28C and M28D, post-Keplerian corrections to the orbit have been measured in order to derive the masses of the binary components. The measurement of the pulsar proper motions allowed to constrain the cluster motion as a whole and its orbit around the Galaxy. Finally, the pulsar spin and orbital period derivatives have been used to measure their accelerations induced by the cluster potential field. Optical observations have been used to search for millisecond pulsar optical counterparts. Six new companion stars have been discovered. In particular, four companions turned out to be He white dwarfs. One companion turned out to be a faint and non-degenerate object, strongly affected by heating of the stellar side exposed to the pulsar flux. Finally, one companion is a main-sequence star which shows Hα emission likely due to a low-level mass transfer. Furthermore, we identified the companion star to a transient low-mass X-ray binary. This companion turned out to be a sub-giant branch star and therefore this system is likely in a very early phase of the mass accretion stage. Conclusion are drawn in the final chapter, where the evolution of millisecond pulsars is discussed and possible future developments are suggested

Using long-term millisecond pulsar timing to obtain physical characteristics of the bulge globular cluster Terzan 5

Over the past decade the discovery of three unique stellar populations and a large number of confirmed pulsars within the globular cluster Terzan 5 has raised questions over its classification. Using the long-term radio pulsar timing of 36 millisecond pulsars in the cluster core, we provide new measurements of key physical properties of the system. As Terzan 5 is located within the galactic bulge, stellar crowding and reddening make optical and near infrared observations difficult. Pulsar accelerations, however, allow us to study the intrinsic characteristics of the cluster independent of reddening and stellar crowding and probe the mass density profile without needing to quantify the mass to light ratio. Relating the spin and orbital periods of each pulsar to the acceleration predicted by a King model, we find a core density of $1.58\times$10$^6$ M$_\odot$ pc$^{-3}$, a core radius of 0.16 pc, a pulsar density profile $n\propto r^{-3.14}$, and a total mass of M$_{\rm T}$($R_\perp<$1.0 pc)$\simeq3.0\times$10$^5$ M$_\odot$ assuming a cluster distance of 5.9 kpc. Using this information we argue against Terzan 5 being a disrupted dwarf galaxy and discuss the possibility of Terzan 5 being a fragment of the Milky Way's proto-bulge. We also discuss whether low-mass pulsars were formed via electron capture supernovae or exist in a core full of heavy white dwarfs and hard binaries. Finally we provide an upper limit for the mass of a possible black hole at the core of the cluster of 3.0$\times$10$^4$ M$_\odot$.Comment: 27 pages, 20 figures, 5 tables, thesis research, accepte

The structural properties of multiple populations in the dynamically young globular cluster NGC 2419

NGC 2419 is likely the globular cluster (GC) with the lowest dynamical age in the Galaxy. This makes it an extremely interesting target for studying the properties of its multiple populations (MPs), as they have been likely affected only modestly by long-term dynamical evolution effects. Here we present for the first time a detailed analysis of the structural and morphological properties of the MPs along the whole extension of this remote and massive GC by combining high-resolution HST and wide-field ground-based data. In agreement with formation models predicting that second population (SP) stars form in the inner regions of the first population (FP) system, we find that the SP is more centrally concentrated than the FP. This may provide constraints on the relative concentrations of MPs in the cluster early stages of the evolutionary phase driven by two-body relaxation. In addition, we find that the fraction of FP stars is larger than expected from the general trend drawn by Galactic GCs. If, as suggested by a number of studies, NGC 2419 formed in the Sagittarius dwarf galaxy and was later accreted by the Milky Way, we show that the observed FP fraction may be explained as due to the transition of NGC 2419 to a weaker tidal field (its current Galactocentric distance is d_gc~95 kpc) and consequently to a reduced loss rate of FP stars.Comment: Accepted for publication in A&

Discovery of a Double Sequence of Blue Straggler Stars in the Core-collapsed Globular Cluster NGC 6256

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 alpha(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 A(rh)(+) 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

Light element variations within the different age-metallicity populations in the nucleus of the Sagittarius dwarf

The cluster M54 lies at the centre of the Sagittarius dwarf spheroidal galaxy, and therefore may be the closest example of a nuclear star cluster. Either in-situ star formation, inspiralling globular clusters, or a combination have been invoked to explain the wide variety of stellar sub-populations in nuclear star clusters. Globular clusters are known to exhibit light element variations, which can be identified using the photometric construct called a chromosome map. In this letter, we create chromosome maps for three distinct age-metallicity sub-populations in the vicinity of M54. We find that the old, metal-poor population shows the signature of light element variations, while the young and intermediate-age metal rich populations do not. We conclude that the nucleus of Sagittarius formed through a combination of in-situ star formation and globular cluster accretion. This letter demonstrates that properly constructed chromosome maps of iron-complex globular clusters can provide insight into the formation locations of the different stellar populations.Comment: Accepted by MNRAS Letter

Empirical measurement of the dynamical ages of three globular clusters and some considerations on the use of the dynamical clock

We have used the ``dynamical clock'' to measure the level of dynamical evolution reached by three Galactic globular clusters (namely, NGC 3201, NGC 6316 and NGC 6440). This is an empirical method that quantifies the level of central segregation of blue stragglers stars (BSSs) within the cluster half-mass radius by means of the $A^+_{rh}$ parameter, defined as the area enclosed between the cumulative radial distribution of BSSs and that of a lighter population. The total sample with homogeneous determinations of $A^+_{rh}$ now counts a gran-total of 59 clusters: 52 old GCs in the Milky Way (including the three investigated here), 5 old clusters in the Large Magellanic Cloud, and 2 young systems in the Small Magellanic Cloud. The three objects studied here nicely nest into the correlation between $A^+_{rh}$ and the central relaxation time defined by the previous sample, thus proving and consolidating the use of the dynamical clock as an excellent tracer of the stage of star cluster dynamical evolution in different galactic environments. Finally, we discuss the advantages of using the dynamical clock as an indicator of star cluster dynamical ages, compared to the present-day central relaxation time.Comment: 16 pages and 8 figures, in press in the Ap

Monitoring Moisture Diffusion after Contact Sponge Application

The contact sponge method is applied on a piece of clay brick. According to the standard, the sponge is moistened with water, applied on the surface of the material by means of a cup, and weighted before and after the application. It allows us to determine the amount of water absorbed by the porous material by unit area and unit time. After the application, the moistened area begins to evaporate and cool down. The IR camera is used to monitor the temperature variation of the imprint of the sponge. Meanwhile, moisture diffuses on the material as well. The IR camera is used to monitor the in-plane diffusion of moisture by following the imprint of the sponge that enlarges with time. A suitable model is used to evaluate the shape of the imprint that varies with time

Fast rotating Blue Straggler Stars in the Globular Cluster NGC3201

We used high resolution spectra acquired at the Magellan Telescope to measure radial and rotational velocities of approximately 200 stars in the Galactic globular cluster NGC 3201. The surveyed sample includes Blue Stragglers Stars (BSSs) and reference stars in different evolutionary stages (main sequence turn-off, sub-giant, red giant and asymptotic giant branches). The average radial velocity value ($\langle V_r\rangle = 494.5 \pm 0.5$ km s$^{-1}$) confirms a large systemic velocity for this cluster and was used to distinguish 33 residual field interlopers. The final sample of member stars counts 67 BSSs and 114 reference stars. Similarly to what is found in other clusters, the totality of the reference stars has negligible rotation ($<20$ km s$^{-1}$), while the BSS rotational velocity distribution shows a long tail extending up to $\sim 200$ km s$^{-1}$, with 19 BSSs (out of 67) spinning faster than 40 km s$^{-1}$. This sets the percentage of fast rotating BSSs to $\sim 28\%$. Such a percentage is roughly comparable to that measured in other loose systems ($\omega$ Centauri, M4 and M55) and significantly larger than that measured in high-density clusters (as 47 Tucanae, NGC 6397, NGC 6752 and M30). This evidence supports a scenario where recent BSS formation (mainly from the evolution of binary systems) is occurring in low-density environments. We also find that the BSS rotational velocity tends to decrease for decreasing luminosity and surface temperature, similarly to what is observed in main sequence stars. Hence, further investigations are needed to understand the impact of BSS internal structure on the observed rotational velocities.Comment: Accepted for pubblication in ApJ: 14 pages, 10 figure

PSR J1641+3627F: a low-mass He white dwarf orbiting a possible high-mass neutron star in the globular cluster M13

We report on the discovery of the companion star to the millisecond pulsar J1631+3627F in the globular cluster M13. By means of a combination of optical and near-UV high-resolution observations obtained with the Hubble Space Telescope, we identified the counterpart at the radio source position. Its location in the color-magnitude diagrams reveals that the companion star is a faint (V \sim 24.3) He-core white dwarf. We compared the observed companion magnitudes with those predicted by state-of-the-art binary evolution models and found out that it has a mass of 0.23 \pm 0.03 Msun, a radius of 0.033^+0.004_-0.005 Rsun and a surface temperature of 11500^+1900_-1300 K. Combining the companion mass with the pulsar mass function is not enough to determine the orbital inclination and the neutron star mass; however, the last two quantities become correlated: we found that either the system is observed at a low inclination angle, or the neutron star is massive. In fact, assuming that binaries are randomly aligned with respect to the observer line of sight, there is a \sim 70% of probability that this system hosts a neutron star more massive than 1.6 Msun. In fact, the maximum and median mass of the neutron star, corresponding to orbital inclination angles of 90 deg and 60 deg, are M_NS,max = 3.1 \pm 0.6 Msun and M_NS,med = 2.4 \pm 0.5 Msun, respectively. On the other hand, assuming also an empirical neutron star mass probability distribution, we found that this system could host a neutron star with a mass of 1.5 \pm 0.1 Msun if orbiting with a low-inclination angle around 40 deg.Comment: Accepted for publication by Ap