Constraining modified theories of gravity with gravitational wave stochastic background

The direct discovery of gravitational waves has finally opened a new observational window on our Universe, suggesting that the population of coalescing binary black holes is larger than previously expected. These sources produce an unresolved background of gravitational waves, potentially observables by ground-based interferometers. In this paper we investigate how modified theories of gravity, modeled using the ppE formalism, affect the expected signal, and analyze the detectability of the resulting stochastic background by current and future ground-based interferometers. We find the constraints that AdLIGO would be able to set on modified theories, showing that they may significantly improve the current bounds obtained from astrophysical observations of binary pulsars.Comment: Results updated to match the version accepted on Phys. Rev. Let

Where does galactic dust come from?

Here we investigate the origin of the dust mass (Mdust) observed in the Milky Way (MW) and of dust scaling relations found in a sample of local galaxies from the DGS and KINGFISH surveys. To this aim, we model dust production from Asymptotic Giant Branch (AGB) stars and supernovae (SNe) in simulated galaxies forming along the assembly of a Milky Way-like halo in a well resolved cosmic volume of 4cMpc using the GAMESH pipeline. We explore the impact of different sets of metallicity and mass-dependent AGB and SN dust yields on the predicted Mdust. Our results show that models accounting for grain destruction by the SN reverse shock predict a total dust mass in the MW that is a factor of ~4 lower than observed, and can not reproduce the observed galaxy-scale relations between dust and stellar masses, and dust-to-gas ratios and metallicity, with a smaller discrepancy in galaxies with low metallicity (12 + log(O/H) < 7.5) and low stellar masses (Mstar < 10^7 Msun). In agreement with previous studies, we suggest that competing processes in the interstellar medium must be at play to explain the observed trends. Our result reinforces this conclusion by showing that it holds independently of the adopted AGB and SN dust yields.Comment: 6 pages, 4 figures. Accepted version for publication in MNRA

The origin of the most iron-poor star

We investigate the origin of carbon-enhanced metal-poor (CEMP) stars starting from the recently discovered $\rm [Fe/H]<-7.1$ star SMSS J031300 (Keller et al. 2014). We show that the elemental abundances observed on the surface of SMSS J031300 can be well fit by the yields of faint, metal free, supernovae. Using properly calibrated faint supernova explosion models, we study, for the first time, the formation of dust grains in such carbon-rich, iron-poor supernova ejecta. Calculations are performed assuming both unmixed and uniformly mixed ejecta and taking into account the partial destruction by the supernova reverse shock. We find that, due to the paucity of refractory elements beside carbon, amorphous carbon is the only grain species to form, with carbon condensation efficiencies that range between (0.15-0.84), resulting in dust yields in the range (0.025-2.25)M$_{\odot}$. We follow the collapse and fragmentation of a star forming cloud enriched by the products of these faint supernova explosions and we explore the role played by fine structure line cooling and dust cooling. We show that even if grain growth during the collapse has a minor effect of the dust-to-gas ratio, due to C depletion into CO molecules at an early stage of the collapse, the formation of CEMP low-mass stars, such as SMSS J031300, could be triggered by dust cooling and fragmentation. A comparison between model predictions and observations of a sample of C-normal and C-rich metal-poor stars supports the idea that a single common pathway may be responsible for the formation of the first low-mass stars.Comment: 14 pages, 8 figures, accepted for publication in ApJ. Rephrased sentence in section 5 to avoid text overlap with arXiv:1307.2239 in their model descriptio

Perturbative approach to the structure of rapidly rotating neutron stars

We construct models of rotating stars using the perturbative approach introduced by J. Hartle in 1967, and a set of equations of state proposed to model hadronic interactions in the inner core of neutron stars. We integrate the equations of stellar structure to third order in the angular velocity and show, comparing our results to those obtained with fully non linear codes, to what extent third order corrections are needed to accurately reproduce the moment of inertia of a star which rotates at rates comparable to that of the fastest isolated pulsars.Comment: 17 pages, 5 figures, minor changes to match version accepted by Phys. Rev.

Nanostructured tin-carbon/ LiNi0.5Mn1.5O4 lithium-ion battery operating at low temperature

An advanced lithium ion battery using nanostructured tinecarbon lithium alloying anode and a high voltage LiNi0.5Mn1.5O4 spinel-type cathode is studied, with particular focus to the low temperature range. The stable behavior of the battery is assured by the use of an electrolyte media based on a LiPF6 salt dissolved in EC-DEC-DMC, i.e. a mixture particularly suitable for the low temperature application. Cycling tests, both in half cells and in full lithium ion battery using the SneC anode and the LiNi0.5Mn1.5O4 cathode, performed in a temperature range extending from room temperature to "30 C, indicate that the electrode/electrolyte configuration here adopted may be suitable for effective application in the lithium ion battery field. The full cell, cycled at -5 °C, shows stable capacity of about 105 mAh g-1 over more than 200 chargee-discharge cycles that is considered a relevant performance considering the low temperature region

Stochastic background of gravitational waves emitted by magnetars

Two classes of high energy sources in our galaxy are believed to host magnetars, neutron stars whose emission results from the dissipation of their magnetic field. The extremely high magnetic field of magnetars distorts their shape, and causes the emission of a conspicuous gravitational waves signal if rotation is fast and takes place around a different axis than the symmetry axis of the magnetic distortion. Based on a numerical model of the cosmic star formation history, we derive the cosmological background of gravitational waves produced by magnetars, when they are very young and fast spinning. We adopt different models for the configuration and strength of the internal magnetic field (which determines the distortion) as well as different values of the external dipole field strength (which governs the spin evolution of magnetars over a wide range of parameters). We find that the expected gravitational wave background differs considerably from one model to another. The strongest signals are generated for magnetars with very intense toroidal internal fields ($\sim 10^{16}$ G range) and external dipole fields of $\sim 10^{14}$, as envisaged in models aimed at explaining the properties of the Dec 2004 giant flare from SGR 1806-20. Such signals should be easily detectable with third generation ground based interferometers such as the Einstein Telescope.Comment: 9 pages, 5 figures, accepted for publication in MNRA

Stochastic backgrounds of gravitational waves from extragalactic sources

Astrophysical sources emit gravitational waves in a large variety of processes occurred since the beginning of star and galaxy formation. These waves permeate our high redshift Universe, and form a background which is the result of the superposition of different components, each associated to a specific astrophysical process. Each component has different spectral properties and features that it is important to investigate in view of a possible, future detection. In this contribution, we will review recent theoretical predictions for backgrounds produced by extragalactic sources and discuss their detectability with current and future gravitational wave observatories.Comment: 10 pages, 9 figures, proceedings of the GWDAW 10 Conference, submitted to Class. & Quantum Gra