Lack of thermal energy in superbubbles: Hint of cosmic rays?

Using analytic methods and 1D two-fluid simulations, we study the effect of cosmic rays (CRs) on the dynamics of interstellar superbubbles (ISBs) driven by multiple supernovae (SNe)/stellar winds in OB associations. In addition to CR advection and diffusion, our models include thermal conduction and radiative cooling. We find that CR injection at the reverse shock or within a central wind-driving region can affect the thermal profiles of ISBs and hence their X-ray properties. Even if a small fraction (10-20 per cent) of the total mechanical power is injected into CRs, a significant fraction of the ram pressure at the reverse shock can be transferred to CRs. The energy transfer becomes efficient if (1) the reverse shock gas Mach number exceeds a critical value (Mth ≳ 12) and (2) the CR acceleration time-scale τacc ∼κcr/v2 is shorter than the dynamical time, where κcr is a CR diffusion coefficient and v is the upstream velocity. We show that CR affected bubbles can exhibit a volume-averaged hot gas temperature 1-5 × 106 K, lower by a factor of 2 - 10 than without CRs. Thus, CRs can potentially solve the long-standing problem of the observed low ISB temperatures.