Gravitational wave background from coalescence of black hole binaries population
This paper has been withdrawn by the authors because it had major revisions.
astro-ph.CO↗
arXiv subjects
Raffaella Schneider
Publications and source records attributed to Raffaella Schneider.
The first low-mass stars: critical metallicity or dust-to-gas ratio?
We explore the minimal conditions which enable the formation of metal-enriched solar and sub-solar mass stars. We find that in the absence of dust grains, gas fragmentation occurs at densities nH ~ [10^4-10^5]cm^{-3} when the metallicity exceeds Z ~ 10^{-4} Zsun. The resulting fragmentation masses are > 10 Msun. The inclusion of Fe and Si cooling does not affect the thermal evolution as this is dominated by molecular cooling even for metallicities as large as Z = 10^{-2} Zsun. The presence of dust is the key driver for the formation of low-mass stars. We focus on three representative core-collapse supernova (SN) progenitors, and consider the effects of reverse shocks of increasing strength: these reduce the depletion factors, fdep = Mdust/(Mdust+Mmet), alter the shape of the grain size distribution function and modify the relative abundances of grain species and of metal species in the gas phase. We find that the lowest metallicity at which fragmentation occurs is Z=10^{-6} Zsun for gas pre-enriched by the explosion of a 20 Msun primordial SN (fdep > 0.22) and/or by a 35 Msun, Z=10^{-4} Zsun SN (fdep > 0.26); it is ~ 1 dex larger, when the gas is pre-enriched by a Z = 10^{-4} Zsun, 20 Msun SN (fdep > 0.04). Cloud fragmentation depends on the depletion factor and it is suppressed when the reverse shock leads to a too large destruction of dust grains. These features are all consistent with the existence of a minimum dust-to-gas ratio, Dcr, above which fragmentation is activated. We derive a simple analytic expression for Dcr which, for grain composition and properties explored in the present study, reads Dcr = [2.6 - 6.3] x 10^{-9}. When the dust-to-gas ratio of star forming clouds exceeds this value, the fragmentation masses range between 0.01 Msun and 1 Msun, thus enabling the formation of the first low-mass stars.
astro-ph.CO↗
The origin of dust in high redshift QSOs: the case of SDSS J1148+5251
We present a semi-analytical model for the formation and evolution of a high redshift quasar (QSO). We reconstruct a set of hierarchical merger histories of a 10^13 Msun dark matter halo and model the evolution of the corresponding galaxy and of its central super massive black hole. The code GAMETE/QSODUST consistently follows: the black hole assembly via both coalescence with other black holes and gas accretion; the build up and star formation history of the quasar host galaxy, driven by binary mergers and mass accretion; the evolution of gas, stars, metals in the interstellar medium (ISM), accounting for mass exchanges with the external medium (infall and outflow processes); dust formation in Supernova (SN) ejecta and in the stellar atmosphere of Asymptotic Giant Branch (AGB) stars, dust destruction by interstellar shocks and grain growth in molecular clouds; the AGN feedback which powers a galactic-scale wind, self-regulating the black hole growth and eventually halting star formation.We use this model to study the case of SDSS J1148+5251 observed at redshift 6.4, exploring different star formation histories for the QSO host galaxy.
astro-ph.CO↗
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.
astro-ph.CO↗
A decline and fall in the future of Italian Astronomy?
On May 27th 2010, the Italian astronomical community learned with concern that the National Institute for Astrophysics (INAF) was going to be suppressed, and that its employees were going to be transferred to the National Research Council (CNR). It was not clear if this applied to all employees (i.e. also to researchers hired on short-term contracts), and how this was going to happen in practice. In this letter, we give a brief historical overview of INAF and present a short chronicle of the few eventful days that followed. Starting from this example, we then comment on the current situation and prospects of astronomical research in Italy.
astro-ph.IM↗
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.
astro-ph.CO↗
Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption
We discuss expectations for the absorption of high-energy gamma-rays by gamma-gamma pair production with intergalactic radiation fields (IRFs) at very high redshifts (z~5-20), and the prospects thereof for probing the cosmic reionization era. For the evolving IRF, a semi-analytical model incorporating both Population II and Population III stars is employed, which is consistent with a wide variety of existing high-z observations including QSO spectral measurements, WMAP Thomson depth constraints, near-IR source count limits, etc. We find that the UV IRF below the Lyman edge energy with intensities in the range of a few times 10^{-19} erg cm^{-2} s^{-1} Hz^{-1} sr^{-1} can cause appreciable attenuation above ~12 GeV at z~5, down to ~6-8 GeV at z>~8-10. This may be observable in the spectra of blazars or gamma-ray bursts by the Fermi Gamma-ray Space Telescope or next generation facilities such as the Cherenkov Telescope Array, Advanced Gamma-ray Imaging System or 5@5, providing invaluable insight into early star formation and cosmic reionization.
astro-ph.CO↗
Metals, dust and the cosmic microwave background: fragmentation of high-redshift star-forming clouds
We investigate the effects of the Cosmic Microwave Background (CMB) radiation field on the collapse of prestellar clouds. Using a semi-analytic model to follow the thermal evolution of clouds with varying initial metallicities and dust contents at different redshifts, we study self-consistently the response of the mean Jeans mass at cloud fragmentation to metal line-cooling, dust-cooling and the CMB. In the absence of dust grains, at redshifts z < 10 moderate characteristic masses (of 10s of Msun) are formed when the metallicity is 10^{-4} Zsun < Z < 10^{-2.5} Zsun; at higher metallicities, the CMB inhibits fragmentation and only very large masses (of ~ 100s of Msun) are formed. These effects become even more dramatic at z > 10 and the fragmentation mass scales are always > 100s of Msun, independent of the initial metallicity. When dust grains are present, sub-solar mass fragments are formed at any redshift for metallicities Z > 10^{-6} Zsun because dust-cooling remains relatively insensitive to the presence of the CMB. When Z > 10^{-3} Zsun, heating of dust grains by the CMB at z > 5 favors the formation of larger masses, which become super-solar when Z > 10^{-2} Zsun and z > 10. Finally, we discuss the implications of our result for the interpretation of the observed abundance patterns of very metal-poor stars in the galactic halo.
astro-ph.CO↗
The influence of magnetic fields on the thermodynamics of primordial star formation
We explore the effects of magnetic energy dissipation on the formation of the first stars. For this purpose, we follow the evolution of primordial chemistry in the presence of magnetic fields in the post-recombination universe until the formation of the first virialized halos. From the point of virialization, we follow the protostellar collapse up to densities of $\sim10^{12}$ cm$^{-3}$ in a one-zone model. In the intergalactic medium (IGM), comoving field strengths of $\gtrsim0.1$ nG lead to Jeans masses of $10^8 M_\odot$ or more and thus delay gravitational collapse in the first halos until they are sufficiently massive. During protostellar collapse, we find that the temperature minimum at densities of $\sim10^3$ cm$^{-3}$ does not change significantly, such that the characteristic mass scale for fragmentation is not affected. However, we find a significant temperature increase at higher densities for comoving field strengths of $\gtrsim0.1$ nG. This may delay gravitational collapse, in particular at densities of $\sim10^9$ cm$^{-3}$, where the proton abundance drops rapidly and the main contribution to the ambipolar diffusion resistivity is due to collisions with Li$^+$. After the formation of the protostar, the increased gas temperatures may enhance the protostellar accretion rate. Our model confirms that initial weak magnetic fields may be amplified considerably during gravitational collapse and become dynamically relevant. For instance, a comoving field strength above $10^{-5}$ nG will be amplified above the critical value for the onset of jets which can magnetize the IGM.
astro-ph.CO↗
Gravitational wave backgrounds and the cosmic transition from Population III to Population II stars
Using the results of a numerical simulation which follows the evolution, metal enrichment and energy deposition of both Population III and Population II stars, we predict the redshift dependence of the formation rate of black hole remnants of Population III stars with masses 100- 500Msun and of neutron stars(black holes) remnants of Population II stars with masses 8-20Msun (20-40Msun). We describe the gravitational wave spectrum produced by Population III and Population II sources adopting the most appropriate signals available in the literature and we compute the stochastic backgrounds resulting from the cumulative emission of these sources throughout the history of the Universe. With the aim of assessing whether these backgrounds might act as foregrounds for signals generated in the Inflationary epoch, we compare their amplitudes with the sensitivity of currently planned and future ground/space-based interferometers.
astro-ph.CO↗
Stellar sources of dust in the high redshift Universe
With the aim of investigating whether stellar sources can account for the >10^8 Msun dust masses inferred from mm/sub-mm observations of samples of 5<z<6.4 quasars,we develop a chemical evolution model which follows the evolution of metals and dust on the stellar characteristic lifetimes, taking into account dust destruction mechanisms.Using a grid of stellar dust yields as a function of the initial mass and metallicity over the range 1-40 Msun and 0-1 Zsun,we show that the role of AGB stars in cosmic dust evolution at high redshift might have been over-looked.We apply the chemical evolution model with dust to the host galaxy of the most distant quasar at z=6.4, SDSS J1148+5251.Given the current uncertainties on the star formation history of the host galaxy, we have considered two models: (i) a star formation history obtained in a numerical simulation by Li et al.(2007) which predicts that a large stellar bulge is already formed at z=6.4,and (ii) a constant star formation rate of 1000 Msun/yr, as suggested by the observations if most of the FIR luminosity is due to young stars.The total mass of dust predicted at z=6.4 by the first model is 2x10^8Msun,within the range of values inferred by observations,with a substantial contribution (80%) of AGB-dust.When a constant star formation rate is adopted,the contribution of AGB-dust decreases to 50% but the total mass of dust formed is a factor 2 smaller.Both models predict a rapid enrichment of the ISM with metals and a relatively mild evolution of the carbon abundance,in agreement with observational constraints. This supports the idea that stellar sources can account for the dust observed but show that the contribution of AGB stars to dust production cannot be neglected, even at the most extreme redshifts currently accessible to observations.
astro-ph.CO↗
The Spatial Distribution of the Galactic First Stars I: High-Resolution N-body Approach
We study the spatial distribution of Galactic metal-free stars by combining an extremely high-resolution (7.8 X 10^5 solar masses per particle) Cold Dark Matter N-body simulation of the Milky-Way with a semi-analytic model of metal enrichment. This approach allows us to resolve halos with virial temperatures down to the 10^4K atomic cooling limit, and it is sufficiently flexible to make a number of robust conclusions, despite the extremely uncertain properties of the first stars. Galactic metal-free stars are formed over a large redshift range, which peaks at z~10, but continues down to z~5, contributing stars at wide range of Galactocentric radii. Stars containing only metals generated by primordial stars are similarly widespread. Neither changing the efficiency of metal dispersal by two orders of magnitude, nor drastically changing the approximations in our semi-analytical model can affect these result. Thus, if they have sufficiently long lifetimes, a significant number of stars formed in initially primordial star clusters should be found in the nearby Galactic halo regardless of the specifics of metal-free star formation. Observations of metal abundances in Galactic halo stars should be taken as directly constraining the properties of primordial stars, and the lack of metal-free halo stars today should be taken as strongly suggesting a 0.8 solar mass lower limit on the primordial initial mass function.
astro-ph↗
Fragmentation of star-forming clouds enriched with the first dust
The thermal and fragmentation properties of star-forming clouds have important consequences on the corresponding characteristic stellar mass. The initial composition of the gas within these clouds is a record of the nucleosynthetic products of previous stellar generations. In this paper we present a model for the evolution of star-forming clouds enriched by metals and dust from the first supernovae, resulting from the explosions of metal-free progenitors with masses in the range 12 - 30 Msun and 140 - 260 Msun. Using a self-consistent approach, we show that: (i) metals depleted onto dust grains play a fundamental role, enabling fragmentation to solar or sub-solar mass scales already at metallicities Zcr = 10^{-6} Zsun; (ii) even at metallicities as high as 10^{-2} Zsun, metals diffused in the gas-phase lead to fragment mass scales which are ~ 100 Msun; (iii) C atoms are strongly depleted onto amorphous carbon grains and CO molecules so that CII plays a minor role in gas cooling, leaving OI as the main gas-phase cooling agent in low-metallicity clouds. These conclusions hold independently of the assumed supernova progenitors and suggest that the onset of low-mass star formation is conditioned to the presence of dust in the parent clouds.
astro-ph↗
Detecting Primordial Stars
We study the detectability of primordial metal-free stars. Cosmological enrichment is a local process that takes place over an extended redshift range. While the duration of this transition depends on several unknown factors, in all cases late-forming metal-free stars are found in ~ 10^8 solar mass objects, which are just large enough to cool, but small enough to not be clustered near areas of previous star formation. We discuss the observational properties of these objects, some of which may have already been detected in ongoing Lyman-alpha surveys. If metal-free stars have masses between 140 and 260 solar masses, they are expected to end their lives as pair-production supernovae (PPSNe). Using the KEPLER code, we calculate a set of light curves for these objects. The peak luminosities of typical PPSNe are only slightly greater than those of Type Ia supernovae, but they remain bright much longer (~ 1 year) and have hydrogen lines. Ongoing supernova searches are able to place stringent limits on the fraction of very massive stars out to redshift 2. The planned Joint Dark Energy Mission satellite will be able to extend these constraints out to redshift 6
astro-ph↗
Early Enrichment of Quasars by First Stars
Studies of the broad emission-line regions (BLRs) in quasars have revealed solar or higher enrichment levels up to the highest redshifts. In combination with the presence of large amounts of dust in QSOs at $z$ about 6, this implies that substantial amounts of star formation and nucleosynthesis took place at significantly earlier epochs. Here, we examine whether a top-heavy stellar initial mass function (IMF) is indicated by current data, by modelling the contributions from different regions of the IMF, including Type Ia/II and pair instability supernovae, to the metal synthesis in BLRs. We find that, in order to reproduce the observations of roughly solar values of N/C and Fe/Mg in these objects, (i) stars with a present-day IMF are sufficient, regardless of their metallicity, (ii) zero-metallicity stars with a top-heavy IMF severely underproduce N/C, and (iii) the contribution of Type Ia SNe is not strongly required by the data. Therefore, stars of mass about 1-40 $M_\odot$ must have existed at $z$ about 10-20, possibly coeval with any hypothesized stars of masses exceeding about 100 $M_\odot$ at these epochs. This is in agreement with the nucleosynthetic abundance pattern detected in extremely metal-poor stars in the galactic halo.
astro-ph↗
The Detectability of the First Stars and Their Cluster Enrichment Signatures
We conduct a comprehensive investigation of the detectability of the first stars and their enrichment signatures in galaxy clusters. We show that the mean metallicity of outflows from objects containing these Population III (PopIII) stars is well above the critical transition metallicity (Z_cr \sim 10^-4) that marks the formation of normal stars. Thus the fraction of PopIII objects formed as a function of redshift is heavily dependent on the distribution of metals and fairly independent of the precise value of Z_cr. Using an analytical model of inhomogenous structure formation, we study the evolution of PopIII objects as a function of the star formation efficiency, IMF, and efficiency of outflow generation. For all models, PopIII objects tend to be in the 10^6.5-10^7.0 solar mass range, just large enough to cool within a Hubble time, but small enough that they are not clustered near areas of previous star formation. Although the mean metallicity exceeds Z_cr at a redshift of 15 in all models, the peak of PopIII star formation occurs at z \sim 10, and such stars continue to form well into the observable range. We discuss the observational properties of these objects, some of which may have already been detected in ongoing surveys of high-redshift Lyman-alpha emitters. Finally, we combine our PopIII distributions with the yield models of Heger and Woosley (2002) to study their impact on the intracluster medium (ICM) in galaxy clusters. We find that PopIII stars can contribute no more than 20% of the iron observed in the ICM, although their peculiar elemental yields help to reconcile theoretical models with the observed Fe and Si/Fe abundances. However, these stars tend to overproduce S/Fe and their associated SN heating falls far short of the observed level of 1 keV per ICM gas particle.
astro-ph↗
Gamma Ray Bursts from the First Stars: Neutrino Signals
If the first (PopIII) stars were very massive, their final fate is to collapse into very massive black holes. Once a proto-black hole has formed into the stellar core, accretion continues through a disk. It is widely accepted, although not confirmed, that magnetic fields drive an energetic jet which produces a burst of TeV neutrinos by photon-meson interaction, and eventually breaks out of the stellar envelope appearing as a Gamma Ray Burst (GRB). Based on recent numerical simulations and neutrino emission models, we predict the expected neutrino diffuse flux from these PopIII GRBs and compare it with the capabilities of present and planned detectors as AMANDA and IceCube. If beamed into 1% of the sky, we find that the rate of PopIII GRBs is $\le 4 \times 10^6$ yr$^{-1}$. High energy neutrinos from PopIII GRBs could dominate the overall flux in two energy bands [$10^4 - 10^5$] GeV and [$10^5 - 10^6$] GeV of neutrino telescopes. The enhanced sensitivities of forthcoming detectors in the high-energy band (AMANDA-II, IceCube) will provide a fundamental insight on the characteristic explosion energies of PopIII GRBs and will constitute a unique probe of the the Initial Mass Function (IMF) of the first stars and of the redshift $z_f$ marking the metallicity-driven transition from a top-heavy to a normal IMF. The current upper limit set by AMANDA-B10 implies that such transition must have occurred not later than $z_f =9.2$ for the most plausible jet energies. Based on such results, we speculate that PopIII GRBs, if not chocked, could be associated with a new class of events detected by BeppoSax, the Fast X-ray Transient (FXTs), which are bright X-ray sources, with peak energies in the 2-10 keV band and durations between 10-200 s.
astro-ph↗
Gravitational waves from cosmological compact binaries
We consider gravitational waves emitted by various populations of compact binaries at cosmological distances. We use population synthesis models to characterize the properties of double neutron stars, double black holes and double white dwarf binaries as well as white dwarf-neutron star, white dwarf-black hole and black hole-neutron star systems. We use the observationally determined cosmic star formation history to reconstruct the redshift distribution of these sources and their merging rate evolution. The gravitational signals emitted by each source during its early-inspiral phase add randomly to produce a stochastic background in the low frequency band with spectral strain amplitude between 10^{-18} Hz^{-1/2} and 5 10^{-17} Hz^{-1/2} at frequencies in the interval [5 10^{-6}-5 10^{-5}] Hz. The overall signal which, at frequencies above 10^{-4}Hz, is largely dominated by double white dwarf systems, might be detectable with LISA in the frequency range [1-10] mHz and acts like a confusion limited noise component which might limit the LISA sensitivity at frequencies above 1 mHz.
astro-ph↗