Dust dynamics and evolution in expanding H ii regions. I. Radiative drift of neutral and charged grains

Abstract

We consider dust drift under the influence of stellar radiation pressure during the pressure-driven expansion of an H ii region using the chemo-dynamical model MAR-ION. Dust size distribution is represented by four dust types: conventional polycyclic aromatic hydrocarbons (PAHs), very small grains (VSGs), big grains (BGs) and also intermediate-sized grains (ISGs), which are larger than VSGs and smaller than BGs. The dust is assumed to move at terminal velocity determined locally from the balance between the radiation pressure and gas drag. As Coulomb drag is an important contribution to the overall gas drag, we evaluate a grain charge evolution within the H ii region for each dust type. BGs are effectively swept out of the H ii region. The spatial distribution of ISGs within the H ii region has a double peak structure, with a smaller inner peak and a higher outer peak. PAHs and VSGs are mostly coupled to the gas. The mean charge of PAHs is close to zero, so they can become neutral from time to time because of charge fluctuations. These periods of neutrality occur often enough to cause the removal of PAHs from the very interior of the H ii region. For VSGs, the effect of charge fluctuations is less pronounced but still significant. We conclude that accounting for charge dispersion is necessary to describe the dynamics of small grains.

Molecular envelope around the HII region RCW 120

“…Thus, the dust is well mixed with the gas in the molecular envelope of RCW 120. Simulations of Akimkin et al (2015Akimkin et al ( , 2017 show how charged dust can be expelled from an H ii region by radiation pressure, and estimate that the dust-to-gas mass ratio for the interior of an H ii region like RCW 120 can be less than 50% of the canonical value. However, the drifting dust is stopped inside the collected dense molecular envelope of the H ii region due to high gas density, and dust-to-gas mass ratio there remains near the canonical of 1:100.…”

Restructuring and destruction of hydrocarbon dust in the interstellar medium

“…Note that, when considering interactions of an ion with a dust grain, the charge of the grain must also be taken into account in the Coulomb factor, but we have not concerned ourselves with this issue, since we assume that all the dust grains are neutral. In fact, the grain charge can substantially influence some microprocesses; for example, it can play an important role in the dynamics of dust in HII regions and shocks [65,66], by determining the relative velocity of the dust grains and gas.…”

Non-monotonic spatial distribution of the interstellar dust in astrospheres: finite gyroradius effect

“…The first one is the neglecting of the drag force due to interaction of dust grains with protons and electrons through direct and Coulomb collisions (Draine & Salpeter 1979). Several authors take this force into account in simulations of the interstellar dust motion in astrospheres and H ii regions (see, e.g., Ochsendorf et al 2014;Akimkin et al 2015). The drag force depends on the dust radius, relative velocity between plasma and dust (v rel ), plasma number density (np = ρ/mp) and temperature (T).…”

“…The potential is usually positive due to an influence of accretion of protons, photoelectric emission of stellar and interstellar radiation and secondary electron emission (see, e.g. Kimura & Mann 1998;Akimkin et al 2015). Table 3 gives radii of dust grains for a d = 0.18 − 5.4 Figure 8.…”