Using Graphics Processing Units to solve the classical N-body problem in physics and astrophysics

Graphics Processing Units (GPUs) can speed up the numerical solution of various problems in astrophysics including the dynamical evolution of stellar systems; the performance gain can be more than a factor 100 compared to using a Central Processing Unit only. In this work I describe some strategies to speed up the classical $N$-body problem using GPUs. I show some features of the $N$-body code HiGPUs as template code. In this context, I also give some hints on the parallel implementation of a regularization method and I introduce the code HiGPUs-R. Although the main application of this work concerns astrophysics, some of the presented techniques are of general validity and can be applied to other branches of physics such as electrodynamics and QCD.Comment: 6 pages, 3 figures, to be published in the proccedings "GPU Computing in High Energy Physics", September 10-12, 2014, Pisa, Ital

Do open star clusters evolve toward energy equipartition?

We investigate whether open clusters (OCs) tend to energy equipartition, by means of direct N-body simulations with a broken power-law mass function. We find that the simulated OCs become strongly mass segregated, but the local velocity dispersion does not depend on the stellar mass for most of the mass range: the curve of the velocity dispersion as a function of mass is nearly flat even after several half-mass relaxation times, regardless of the adopted stellar evolution recipes and Galactic tidal field model. This result holds both if we start from virialized King models and if we use clumpy sub-virial initial conditions. The velocity dispersion of the most massive stars and stellar remnants tends to be higher than the velocity dispersion of the lighter stars. This trend is particularly evident in simulations without stellar evolution. We interpret this result as a consequence of the strong mass segregation, which leads to Spitzer's instability. Stellar winds delay the onset of the instability. Our simulations strongly support the result that OCs do not attain equipartition, for a wide range of initial conditions

High Precision, High Performance Simulations of Astrophysical Stellar Systems

The main target of this work is the discussion of the modern techniques (software and hardware) apt to solve numerically the $N$-body problem in order to develop a numerical code with highest as possible speed and accuracy performance. In particular, we will introduce a new high precision, high performance, code (called \code) which solves the $N$-body problem exploiting both a high order time integration algorithm (the Hermite's 6th order integrator) and the modern hardware represented by Graphics Processing Units (GPUs), which work as powerful computing accelerators. I will describe in details \code showing how GPUs can be efficiently exploited for gravitational $N$-body simulations up to a large number of particles ($N\simeq 10^7$) with a degree of precision and speed impossible to reach until 5 years ago. Being quite new technologies, the GPUs have not been fully exploited so far; this is why, in this Thesis, I will discuss modern numerical techniques associated with the $N$-body problem, starting from the set up of initial conditions up to the computation of the dynamical evolution of dense and populous stellar systems using GPUs and the two main languages (OpenCL and CUDA) apt to program them. I will present also results of the application of \code to study the emerging state, and rapid mass segregation, of intermediate-$N$, young, stellar systems after their violent relaxation process. These objects have been investigated simulating systems composed by stars of different masses, including a central star-mass black hole as well as a model of gas residual of the mother cloud, starting from \lq cold\rq to \lq warm\rq initial conditions. Moreover, thanks to the high adaptability of the developed software, our group is investigating the formation and the evolution of the innermost region of galaxies (Nuclear Star Clusters). This is, surely, a modern topic, which has not yet received an adequate self-consistent explanation neither from theoretical nor a numerical point of view

High Precision, High Performance Simulations of Astrophysical Stellar Systems

The main target of this work is the discussion of the modern techniques (software and hardware) apt to solve numerically the $N$-body problem in order to develop a numerical code with highest as possible speed and accuracy performance. In particular, we will introduce a new high precision, high performance, code (called \code) which solves the $N$-body problem exploiting both a high order time integration algorithm (the Hermite's 6th order integrator) and the modern hardware represented by Graphics Processing Units (GPUs), which work as powerful computing accelerators. I will describe in details \code showing how GPUs can be efficiently exploited for gravitational $N$-body simulations up to a large number of particles ($N\simeq 10^7$) with a degree of precision and speed impossible to reach until 5 years ago. Being quite new technologies, the GPUs have not been fully exploited so far; this is why, in this Thesis, I will discuss modern numerical techniques associated with the $N$-body problem, starting from the set up of initial conditions up to the computation of the dynamical evolution of dense and populous stellar systems using GPUs and the two main languages (OpenCL and CUDA) apt to program them. I will present also results of the application of \code to study the emerging state, and rapid mass segregation, of intermediate-$N$, young, stellar systems after their violent relaxation process. These objects have been investigated simulating systems composed by stars of different masses, including a central star-mass black hole as well as a model of gas residual of the mother cloud, starting from \lq cold\rq to \lq warm\rq initial conditions. Moreover, thanks to the high adaptability of the developed software, our group is investigating the formation and the evolution of the innermost region of galaxies (Nuclear Star Clusters). This is, surely, a modern topic, which has not yet received an adequate self-consistent explanation neither from theoretical nor a numerical point of view

Preventable proportion of intubation-associated pneumonia: role of adherence to a care bundle

Objective: The aim of the present study was to estimate the preventable proportion of Intubation-Associated Pneumonia (IAP) in the Intensive Care Units (ICUs) participating in the Italian Nosocomial Infections Surveillance in ICUs (SPIN-UTI) network, taking into account differences in intrinsic patients’ risk factors, and additionally considering the compliance with the European bundle for IAP prevention. Methods: A prospective patient-based survey was conducted and all patients staying in ICU for more than 2 days were enrolled in the surveillance. Compliance with the bundle was assessed using a questionnaire for each intubated patient. A twofold analysis by the parametric g-formula was used to compute the number of infections to be expected if the infection incidence in all ICUs could be reduced to that one of the top-tenth-percentile-ranked ICUs and to that one of the ICU with the highest compliance to all five bundle components. Results: A total of 1,840 patients and of 17 ICUs were included in the first analysis showing a preventable proportion of 44% of IAP. In a second analysis on a subset of data, considering compliance with the European bundle, a preventable proportion of 40% of IAP was shown. A significant negative trend of IAP incidences was observed with increasing number of bundle components performed (p<0.001) and a strong negative correlation between these two factors was shown (r = -0.882; p = 0.048). Conclusions: The g-formula controlled for time-varying factors is a valuable approach for estimating the preventable proportion of IAP and the impact of interventions, based entirely on an observed population in a real-world setting. However, both the study design that cannot definitively prove a causative relationship between bundle compliance and IAP risk, and the small number of patients included in the care bundle compliance analysis, may represent limits of the study and further and larger studies should be conducted

Intermediate mass black holes in globular clusters: effects on jerks and jounces of millisecond pulsars

Globular clusters may host intermediate mass black holes (IMBHs) at their centres. Here, we propose a new method for their identification using millisecond pulsars (MSPs) as probes. We show that measuring the first (jerk) and second (jounce) derivatives of the accelerations of an ensemble of MSPs will let us infer the presence of an IMBH in a globular cluster better than measuring the sole accelerations. We test this concept by simulating a set of star clusters with and without a central IMBH to extract the distributions of the stellar jerks and jounces. We then apply this technique to the ensemble ofMSPs in the Galactic globular cluster 47 Tucanae. Current timing observations are insufficient to constrain the presence of an IMBH and can only be used to pose upper limits on its mass. But, with few more years of observations it will be possible to test for the presence of a central IMBH with mass smaller than similar to 1000 M-circle dot. We conclude that jerks and jounces help significantly in reducing the upper limit of the mass of IMBHs in Galactic globular clusters

Neoadjuvant Trastuzumab Emtansine and Pertuzumab in Human Epidermal Growth Factor Receptor 2-Positive Breast Cancer: Three-Year Outcomes From the Phase III KRISTINE Study.

PurposeThe KRISTINE study compared neoadjuvant trastuzumab emtansine plus pertuzumab (T-DM1+P) with docetaxel, carboplatin, trastuzumab plus P (TCH+P) for the treatment human epidermal growth factor receptor 2-positive stage II to III breast cancer. T-DM1+P led to a lower pathologic complete response rate (44.4% v 55.7%; P = .016), but fewer grade 3 or greater and serious adverse events (AEs). Here, we present 3-year outcomes from KRISTINE.MethodsPatients were randomly assigned to neoadjuvant T-DM1+P or TCH+P every 3 weeks for six cycles. Patients who received T-DM1+P continued adjuvant T-DM1+P, and patients who received TCH+P received adjuvant trastuzumab plus pertuzumab. Secondary end points included event-free survival (EFS), overall survival, patient-reported outcomes (measured from random assignment), and invasive disease-free survival (IDFS; measured from surgery).ResultsOf patients, 444 were randomly assigned (T-DM1+P, n = 223; TCH+P, n = 221). Median follow-up was 37 months. Risk of an EFS event was higher with TDM-1+P (hazard ratio [HR], 2.61 [95% CI, 1.36 to 4.98]) with more locoregional progression events before surgery (15 [6.7%] v 0). Risk of an IDFS event after surgery was similar between arms (HR, 1.11 [95% CI, 0.52 to 2.40]). Pathologic complete response was associated with a reduced risk of an IDFS event (HR, 0.24 [95% CI, 0.09 to 0.60]) regardless of treatment arm. Overall, grade 3 or greater AEs (31.8% v 67.7%) were less common with T-DM1+P. During adjuvant treatment, grade 3 or greater AEs (24.5% v 9.9%) and AEs leading to treatment discontinuation (18.4% v 3.8%) were more common with T-DM1+P. Patient-reported outcomes favored T-DM1+P during neoadjuvant treatment and were similar to trastuzumab plus pertuzumab during adjuvant treatment.ConclusionCompared with TCH+P, T-DM1+P resulted in a higher risk of an EFS event owing to locoregional progression events before surgery, a similar risk of an IDFS event, fewer grade 3 or greater AEs during neoadjuvant treatment, and more AEs leading to treatment discontinuation during adjuvant treatment

Astrophysical and Cosmological Relevance of the High-Frequency Features in the Stochastic Gravitational-Wave Background

The stochastic gravitational-wave background (SGWB) produced by merging neutron stars features a peak in the kHz frequency band. In this paper, we develop a theoretical framework to exploit such a distinguishing feature through a Markov Chain Monte Carlo analysis using a simulated data-set of SGWB measurements within this frequency band. The aim is to use the peak of the SGWB as an observable to constrain a selection of astrophysical and cosmological parameters that accurately describe the SGWB. We examine how the variation of these parameters impacts the morphology of the SGWB. Given our priors on astrophysical and cosmological parameters, we show that the values of the chirp mass and common envelope efficiency of the binary systems are retrieved with percent accuracy, as well as the cosmological expansion history populated by these binaries, represented by the Hubble constant, the matter abundance and the effective equation of state of the dark energy.Comment: 10 pages, 6 figure

Isolated and dynamical black hole mergers with B-POP: the role of star formation and dynamics, star cluster evolution, natal kicks, mass and spins, and hierarchical mergers

The current interpretation of LIGO–Virgo–KAGRA data suggests that the primary mass function of merging binary black holes (BBHs) at redshift z ≲ 1 contains multiple structures, while spins are relatively low. Theoretical models of BBH formation in different environments can provide a key to interpreting the population of observed mergers, but they require the simultaneous treatment of stellar evolution and dynamics, galaxy evolution, and general relativity. We present B-POP, a population synthesis tool to model BBH mergers formed in the field or via dynamical interactions in young, globular, and nuclear clusters. Using B-POP, we explore how black hole (BH) formation channels, star cluster evolution, hierarchical mergers, and natal BH properties affect the population of BBH mergers. We find that the primary mass distribution of BBH mergers extends beyond M1≃200 M⊙, and the effective spin parameter distribution hints at different natal spins for single and binary BHs. Observed BBHs can be interpreted as members of a mixed population comprised of ∼34 per cent(66 per cent) isolated (dynamical) BBHs, with the latter likely dominating at redshift z \u3e 1. Hierarchical mergers constitute the 4.6−7.9 per cent of all mergers in the reference model, dominating the primary mass distribution beyond M1\u3e65 M⊙. The inclusion of cluster mass-loss and expansion causes an abrupt decrease in the probability for mergers beyond the third generation to occur. Considering observational biases, we find that 2.7−7.5 per cent of mock mergers involve intermediate-mass black hole (IMBH) seeds formed via stellar collisions. Comparing this percentage to observed values will possibly help us to constrain IMBH formation mechanisms