Interplay between Escaping Cosmic Rays and Interstellar Medium: Driving of Galactic Winds and Shaping the Local Proton Spectrum

Abstract

We study the effects of escaping cosmic rays (CRs) on the interstellar medium (ISM) around their source with spherically symmetric CR-hydrodynamical simulations taking into account the evolution of the CR energy spectrum, radiative cooling, and thermal conduction. We show how the escaping CRs accelerate and heat the ISM fluid depending on the CR diffusion coefficient. The CR heating effects are potentially responsible for the recent observations of the unexpected H$α$ and [OIII]$λ$5007 lines in old supernova remnants. The implied gas outflow by CRs can be comparable to the Galactic star formation rate, compatible with the Galactic wind required for the metal-polluted halo gas and the production of eROSITA bubbles. Assuming a locally suppressed CR diffusion and a few nearby CR sources in the Local Bubble, we also propose alternative interpretations for the Galactic CR proton spectrum around the Earth measured with CALET, AMS02, and Voyager I.

Figures (12)

• Figure 1: The CR pressure profiles for the cases of ${\cal D}_0=10^{26}~{\rm cm^2~s^{-1}}$ (red), $10^{27}~{\rm cm^2~s^{-1}}$ (orange), and $10^{28}~{\rm cm^2~s^{-1}}$ (blue). The dashed, solid, and dotted lines are the profiles at $t=0.5$ kyr, $10$ kyr, and $1$ Myr, respectively.
• Figure 2: The fluid velocity profiles for the cases of ${\cal D}_0=10^{26}~{\rm cm^2~s^{-1}}$ (red), $10^{27}~{\rm cm^2~s^{-1}}$ (orange), and $10^{28}~{\rm cm^2~s^{-1}}$ (blue). The dashed, solid, and dotted lines are the profiles at $t=0.5$ kyr, $10$ kyr, and $1$ Myr, respectively.
• Figure 3: Temporal evolutions of the differential CR energy density, $\epsilon p{\cal N}_{\rm cr}$, for the case of ${\cal D}_0=10^{26}~{\rm cm^2~s^{-1}}$. The energy transfer by the mechanical work by CRs, $-tv_{\rm g}\partial_rP_{\rm cr}$, is also shown by the black curves. The positive $-tv_{\rm g}\partial_rP_{\rm cr}$ indicates the energy transfer from CRs to the fluid, while the negative value indicates the energy gain of CRs via the compression of the fluid.
• Figure 4: The solid lines are the volume averaged CR energy spectrum in the whole simulation box ($r\leq300$ pc) at $t=1$ Myr (upper) and $10$ Myr (lower) for the cases of ${\cal D}_0=10^{26}~{\rm cm^2~s^{-1}}$ (red), $10^{27}~{\rm cm^2~s^{-1}}$ (orange), and $10^{28}~{\rm cm^2~s^{-1}}$ (blue). The dotted lines are spectra of the CRs escaped from the simulation box. The dashed thick lines are the sum of the escaped and remaining components. The thin dashed line (black) shows the initial spectrum common to all the cases.
• Figure 5: Temporal evolutions of the pressure $P_{\rm g}$ (the solid black curve) and temperature $T_{\rm g}$ (the dashed black curve) for ${\cal D}_0=10^{27}~{\rm cm^2~s^{-1}}$. We also exhibit the CR heating term $t|{\cal V}_{\rm A}\partial_r\varepsilon_{\rm cr}|$ (red) and the thermal conduction term $-(t/r^2)\partial_r\left(r^2{\cal K}\partial_rT_{\rm g}\right)$ (lightblue).