N-body models of globular clusters: metallicity, half-light radii and mass-to-light ratios

Size differences of approx. 20% between red (metal-rich) and blue (metal-poor) sub-populations of globular clusters have been observed, generating an ongoing debate as to weather these originate from projection effects or the difference in metallicity. We present direct N-body simulations of metal-rich and metal-poor stellar populations evolved to study the effects of metallicity on cluster evolution. The models start with N = 100000 stars and include primordial binaries. We also take metallicity dependent stellar evolution and an external tidal field into account. We find no significant difference for the half-mass radii of those models, indicating that the clusters are structurally similar. However, utilizing observational tools to fit half-light (or effective) radii confirms that metallicity effects related to stellar evolution combined with dynamical effects such as mass segregation produce an apparent size difference of 17% on average. The metallicity effect on the overall cluster luminosity also leads to higher mass-to-light ratios for metal-rich clusters.Comment: 14 pages, 10 figures, accepted for publication in MNRA

Star formation at the Galactic Centre: coevolution of multiple young stellar discs

Studies of the Galactic Centre suggest that in-situ star formation may have given rise to the observed stellar population near the central supermassive black hole (SMBH). Direct evidence for a recent starburst is provided by the currently observed young stellar disc (2-7Myr) in the central 0:5 pc of the Galaxy. This result suggests that star formation in galactic nuclei may occur close to the SMBH and produce initially attened stellar discs. Here we explore the possible build-up and evolution of nuclear stellar clusters near SMBHs through in-situ star formation producing stellar discs similar to those observed in the Galactic Centre and other nuclei. We make use of N-body simulations to model the evolution of multiple young stellar discs, and explore the potential observable signatures imprinted by such processes. Each of the five simulated discs is evolved for 100Myr before the next one is introduced in the system. We find that populations born at different epochs show different morphologies and kinematics. Older and presumably more metal poor populations are more relaxed and extended, while younger populations show a larger amount of rotation and attening. We conclude that star formation in central discs can reproduce the observed properties of multiple stellar populations in galactic nuclei differing in age, metallicity and kinematic properties

Using binaries in globular clusters to catch sight of intermediate-mass black holes

The dynamical evolution of globular clusters (GCs) is tied to their binary population, as binaries segregate to the cluster centre, leading to an increased binary fraction in the core. This central overabundance of mainly hard binaries can serve as a source of energy for the cluster and might significantly affect the observed kinematics of the GC. We analyse the binary fractions and distributions of 95 simulated GCs, with and without an intermediate-mass black hole (IMBH) in their centre. We show that an IMBH will not only halt the segregation of binaries towards the cluster centre but also, directly and indirectly, disrupt the binaries that segregate, thus depleting binaries in the cluster core. We illustrate this by showing that clusters with an IMBH have fewer binaries and flatter radial binary distributions than their counterparts without one. These binary fraction and distribution differences provide an additional indicator of a central IMBH in GCs.Full article: Aros et al. 2021, MNRAS, 508, 4385 (https://doi.org/10.1093/mnras/stab2872

Evidence-based posttraumatic stress disorder treatment in a community sample : military-affiliated versus civilian patient outcomes

Posttraumatic stress disorder (PTSD) is a significant mental health issue among military service members and veterans. Although the U.S. Department of Veterans Affairs (VA) provides crucial resources for behavioral health care, many veterans seek mental health services through community clinics. Previous research illustrates that military and veteran patients benefit less from evidence-based treatments (EBTs) for PTSD than civilians. However, most PTSD treatment outcome research on military and veteran populations is conducted in VA or military settings. Little is known about outcomes among military-affiliated patients in community settings. The primary aim of this study was to directly compare civilian versus military-affiliated patient outcomes on PTSD and depression symptoms using the PTSD Checklist for DSM-5 (PCL-5) and the nine-item Patient Health Questionnaire (PHQ-9) in a community setting. Participants (N = 502) included military-affiliated (veteran, Guard/Reservist, active duty) and civilian patients who engaged in cognitive processing therapy (CPT) or prolonged exposure (PE) for PTSD in community clinics. Both groups demonstrated significant reductions on the PCL-5, military-affiliated: d = −0.91, civilian: d = -1.18; and PHQ-9, military-affiliated: d = -0.65, civilian: d = -0.88, following treatment. However, military-affiliated patients demonstrated smaller posttreatment reductions on the PCL-5, Mdiff = 5.75, p =.003, and PHQ-9, Mdiff = 1.71, p =.011, compared to civilians. Results demonstrate that military-affiliated patients benefit from EBTs for PTSD, albeit to a lesser degree than civilians, even in community settings. These findings also highlight the importance of future research on improving EBTs for military personnel with PTSD

Globular cluster ejection, infall, and the host dark matter halo of the Pegasus dwarf galaxy

International audienceRecent photometric observations revealed a massive, extended (M_GC ≳ 10^5 M_⊙; R_h ∼ 14 pc) globular cluster (GC) in the central region (D_3D ≲ 100 pc) of the low-mass (M_* ∼ 5 × 10^6 M_⊙) dwarf irregular galaxy Pegasus. This massive GC offers a unique opportunity to study star cluster inspiral as a mechanism for building up nuclear star clusters, and the dark matter (DM) density profile of the host galaxy. Here, we present spectroscopic observations indicating that the GC has a systemic velocity of ΔV = 3 ± 8 km s^−1 relative to the host galaxy, and an old, metal-poor stellar population. We run a suite of orbital evolution models for a variety of host potentials (cored to cusped) and find that the GC’s observed tidal radius (which is ∼3 times larger than the local Jacobi radius), relaxation time, and relative velocity are consistent with it surviving inspiral from a distance of D_gal ≳ 700 pc (up to the maximum tested value of D_gal = 2000 pc). In successful trials, the GC arrives to the galaxy centre only within the last ∼1.4 ± 1 Gyr. Orbits that arrive in the centre and survive are possible in DM haloes of nearly all shapes, however to satisfy the GC’s structural constraints a galaxy DM halo with mass M_DM ≃ 6 ± 2 × 10^9 M_⊙, concentration c ≃ 13.7 ± 0.6, and an inner slope to the DM density profile of −0.9 ≤ γ ≤ −0.5 is preferred. The gas densities necessary for its creation and survival suggest the GC could have formed initially near the dwarf’s centre, but then was quickly relocated to the outskirts where the weaker tidal field permitted an increased size and relaxation time – with the latter preserving the former during subsequent orbital decay