Mario Vietri

Bio: Mario Vietri is an academic researcher. The author has contributed to research in topics: Pulsar & Cosmic ray. The author has an hindex of 3, co-authored 5 publications receiving 90 citations.

The peculiar shape of the inner galactic rotation curve

Abstract: La courbe de rotation apparente de la galaxie, deduite d'observations HI et CO, presente un pic etroit (ν max ≃250-260 kms −1 ) a r=500 pc, puis un lent declin pour 600 pc≤r≤1,5 kpc. On montre que cette morphologie ne peut etre reconciliee avec les observations que si le bulbe de la galaxie n'est pas a symetrie axiale et le potentiel resultant triaxial

Coronal gamma-ray bursts as the sources of ultra-high-energy cosmic rays?

Abstract: I consider the possibility that Ultra High Energy Cosmic Rays are accelerated in Gamma Ray Bursts located in the Galactic corona, thus circumventing the problem raised by Greisen--Zatsepin--Kuz'min cutoff. The acceleration of UHECRs could occur in the pulsars which, in the coronal GRB model, produce them: the same parameters that permit fitting GRBs' observations in the model of Podsiadlowski, Rees and Ruderman (1995) lead to an estimate of the highest achievable energies corresponding to that of the Bird et al (1994) event, and to very low luminosities in cosmic rays. I show that, if the observations of Milgrom and Usov (1995a) are confirmed, the extragalactic GRBs' model for the acceleration of UHECRs is untenable, but the same constraint does not apply to the coronal model. Also, I show that the efficiency of particle acceleration needs be much smaller (and less demanding) than in cosmological models of GRBs. Uncertainties remain about the ensuing cosmic ray spectral distribution. I also briefly discuss observational strategies to distinguish between the two possibilities.

The z-structure of the disc of NGC 3115

Abstract: Les observations photometriques du disque de la galaxie SO NGC 3115 presentees par Capaccioli M., Held E.V. et Nieto J.L. (1987, Astr. J., 94) sont analysees dans le but de donner une interpretation dynamique de la structure du disque

Non-radial instabilities in collapsing galaxies : an analytically solvable model

Abstract: La stabilite d'une famille de modeles sans collision de l'effondrement des galaxies est examinee vis-a-vis des perturbations de type fluide qui n'alterent pas le profil de la densite radiale mais distordent la forme des galaxies.

Some comments on the electrodynamics of binary pulsars

Abstract: We consider the electrodynamics of in-spiraling binary pulsars, showing that there are two distinct ways in which they may emit radiation. On the one hand, even if the pulsars do not rotate, we show that in vacuo orbital rotation generates magnetic quadrupole emission, which, in the late stages of the binary evolution becomes nearly as effective as magnetic dipole emission by a millisecond pulsar. On the other hand, we show that interactions of the two magnetic fields generate powerful induction electric fields, which cannot be screened by a suitable distribution of charges and currents like they are in isolated pulsars. We compute approximate electromotive forces for this case.

The sagittarius dwarf galaxy: a model for evolution in a triaxial milky way halo

Abstract: We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential that can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a)Φ = 0.72 and intermediate/major axis ratio (b/a)Φ = 0.99 at radii 20 kpc < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7°, 0°), (0°, 90°), and (97°, 0°) respectively, corresponding to a nearly oblate ellipsoid whose minor axis is contained within the Galactic disk plane. This particular disk/halo orientation is difficult to reconcile within the general context of galactic dynamics (and cold dark matter models in particular), suggesting either that the orientation may have evolved significantly with time or that inclusion of other non-axisymmetric components (such as the gravitational influence of the Magellanic Clouds) in the model may obviate the need for triaxiality in the dark matter halo. The apparent proper motion of Sgr in this model is estimated to be (μ l cos b, μ b ) = (–2.16, 1.73) mas yr–1, corresponding to a Galactocentric space velocity (U, V, W) = (230, –35, 195) km s–1 . Based on the velocity dispersion in the stellar tidal streams, we estimate that Sgr has a current bound mass M Sgr = 2.5+1.3 –1.0 × 108 M ☉. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies. These models predict a strong evolution in the MDF of the model Sgr dwarf with time, indicating that the chemical abundances of stars in Sgr at the present day may be significantly different than the abundances of those already contributed to the Galactic stellar halo. We conclude by using the new N-body model to re-evaluate previous claims of the association of miscellaneous halo substructure with the Sgr dwarf.

Dynamics of Barred Galaxies

Abstract: Some 30% of disc galaxies have a pronounced central bar features in the disc plane and many more have weaker features of a similar kind. Kinematic data indicate that the bar constitutes a major non-axisymmetric component of the mass distribution and that the bar pattern tumbles rapidly about the axis normal to the disc plane. The observed motions are consistent with material within the bar streaming along highly elongated orbits aligned with the rotating major axis. A barred galaxy may also contain a spheroidal bulge at its centre, spirals in the outer disc and, less commonly, other features such as a ring or lens. Mild asymmetries in both the light and kinematics are quite common. The authors review the main problems presented by these complicated dynamical systems and summarize the effort so far made towards their solution, emphasizing results which appear secure.

The Effect of the Outer Lindblad Resonance of the Galactic Bar on the Local Stellar Velocity Distribution

Abstract: Hydrodynamic modeling of the inner Galaxy suggests that the radius of the outer Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of the Sun. How does this resonance affect the distribution function in the outer parts of a barred disk, and can we identify any effect of the resonance in the velocity distribution actually observed in the solar neighborhood? To answer these questions, detailed simulations of the velocity distribution, f(v), in the outer parts of an exponential stellar disk with nearly flat rotation curve and a rotating central bar have been performed. For a model resembling the old stellar disk, the OLR causes a distinct feature in f(v) over a significant fraction of the outer disk. For positions up to 2 kpc outside the OLR radius and at bar angles of ~10°–70°, this feature takes the form of a bimodality between the dominant mode of low-velocity stars centered on the local standard of rest (LSR) and a secondary mode of stars predominantly moving outward and rotating more slowly than the LSR. Such a bimodality is indeed present in f(v) inferred from the Hipparcos data for late-type stars in the solar neighborhood. If one interprets this observed bimodality as induced by the OLR—and there are hardly any viable alternatives—then one is forced to deduce that the OLR radius is slightly smaller than R0. Moreover, by a quantitative comparison of the observed with the simulated distributions, one finds that the pattern speed of the bar is 1.85 ± 0.15 times the local circular frequency, where the error is dominated by the uncertainty in bar angle and local circular speed. Also, other, less prominent but still significant, features in the observed f(v) resemble properties of the simulated velocity distributions, in particular a ripple caused by orbits trapped in the outer 1:1 resonance.

The Sagittarius Dwarf Galaxy: a Model for Evolution in a Triaxial Milky Way Halo

Abstract: We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (< 3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential which can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a)_Phi = 0.72 and intermediate/major axis ratio (b/a)_Phi = 0.99 at radii 20 < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7, 0), (0, 90), and (97, 0) respectively, corresponding to a nearly-oblate ellipsoid whose minor axis is contained within the Galactic disk plane. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies. (Abridged).

A COBE Model of the Galactic Bar and Disk

Abstract: A model of the bar and old stellar disk of the Galaxy has been derived from the survey of the Diffuse Infrared Background Experiment (DIRBE) of the Cosmic Background Explorer at wavelengths of 1.25, 2.2, 3.5, and 4.9 μm. It agrees very well with the data, except in directions in which the near-infrared optical depth is high. Among the conclusions drawn from the model is that the Sun is located approximately 16.5 pc above the midpoint of the Galactic plane. The disk has an outer edge 4 kpc from the Sun and is warped like the H I layer. It has a central hole roughly the diameter of the inner edge of the 3 kpc molecular cloud ring, and within that hole lies a bright, strong, early-type bar, tilted approximately 14° from the Sun-Galactic center line. The model has 47 free parameters. The model is discussed in detail, and contour plots and images of the residuals are presented.