arXiv subjects

Vo Hong Minh Phan

Publications and source records attributed to Vo Hong Minh Phan.

21 records · Page 2Linked to original sources

arXiv2021

Erosion of Icy Interstellar Objects by Cosmic Rays and Implications for `Oumuamua

We study the destruction and modification of icy interstellar objects by cosmic rays and gas collisions. Using the cosmic-ray flux measured in the local interstellar medium as well as inferred from gamma-ray observations at the different galactocentric radii, we find that cosmic-ray erosion is significant for interstellar objects made of common types of ices. Interestingly, cosmic-ray heating might destroy icy interstellar objects very efficiently such that the initial size of an N$_2$ fragment as suggested by \citet{jackson2021} to explain the composition of `Oumuamua should be at least 0.5 km in size in order to survive the journey of about 0.5 Gyr in the ISM and might be even larger if it originated from a region with an enhanced cosmic-ray flux. This implies an initial N$_2$ mass that is at least an order of magnitude larger than the final value, exacerbating the N$_2$ mass budget deficiency for explaining `Oumuamua. The erosion time due to cosmic-ray heating and gas collisions also allows us to set approximate limits on the initial size for other types of icy interstellar objects, e.g. composed of CO, CO$_2$, or CH$_4$. For a given initial size, we constrain the maximum distance to the birth site for interstellar objects with different speeds. We also find that cosmic-ray and gas heating could entirely modify the ice structure before destroying interstellar objects.

astro-ph.GA↗

arXiv2021

Stochastic Fluctuations of Low-Energy Cosmic Rays and the Interpretation of Voyager Data

Data from the Voyager probes have provided us with the first measurement of cosmic ray intensities at MeV energies, an energy range which had previously not been explored. Simple extrapolations of models that fit data at GeV energies, e.g. from AMS-02, however, fail to reproduce the Voyager data in that the predicted intensities are too high. Oftentimes, this discrepancy is addressed by adding a break to the source spectrum or the diffusion coefficient in an ad hoc fashion, with a convincing physical explanation yet to be provided. Here, we argue that the discrete nature of cosmic ray sources, which is usually ignored, is instead a more likely explanation. We model the distribution of intensities expected from a statistical model of discrete sources and show that its expectation value is not representative, but has a spectral shape different from that for a typical configuration of sources. The Voyager proton and electron data are however compatible with the median of the intensity distribution. We stress that this model can explain the Voyager data without requiring any unphysical breaks.

astro-ph.HE↗

arXiv2021

Cosmic Ray Small Scale Anisotropies in Slab Turbulence

In the standard picture of cosmic ray transport the propagation of charged cosmic rays through turbulent magnetic fields is described as a random walk with cosmic rays scattering on magnetic field turbulence. This is in good agreement with the highly isotropic cosmic ray arrival directions as this diffusion process effectively isotropizes the cosmic ray distribution. High-statistics observatories like IceCube and HAWC have however observed significant deviations from isotropy down to very small angular scales. This is in strong tension with this standard picture of cosmic ray propagation. While large scale multipoles arise naturally, for example due to the earth's motion relative to the isotropic cosmic ray distribution, there is no intuitive mechanism to account for the observed anisotropies at smaller angular scales. By relaxing one of the standard assumptions of quasi linear theory and treating correlations between fluxes of cosmic rays from different directions explicitly we show that higher multipoles also are to be expected from particle propagation through turbulent magnetic fields. We present a first analytical calculation of the angular power spectrum assuming a physically motivated model of the magnetic field turbulence and find good agreement with numerical simulations.

astro-ph.HE↗