Photoionization calculations of the radiation force due to spectral lines in AGNs. (arXiv:1812.01773v1 [astro-ph.GA])

Photoionization calculations of the radiation force due to spectral lines in AGNs. (arXiv:1812.01773v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Dannen_R/0/1/0/all/0/1">Randall C. Dannen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Proga_D/0/1/0/all/0/1">Daniel Proga</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kallman_T/0/1/0/all/0/1">Timothy R. Kallman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Waters_T/0/1/0/all/0/1">Tim Waters</a>

One of the main physical mechanisms that could drive mass outflows in AGNs is
radiation pressure on spectral lines. Although this mechanism is conceptually
straightforward to understand, the actual magnitude of the radiation force is
challenging to compute because the force depends on the physical conditions in
the gas, as well as the strength, spectral energy distribution (SED), and
geometry of the radiation field. We present results from our photoionization
and radiation transfer calculations of the so-called force multiplier, $M$,
using the same radiation field to compute the gas photoionization and thermal
balance. Our method is general and can be used for an arbitrary SED. Here we
focus on describing results for two SEDs that correspond to a Type 1 and Type 2
AGN. We use the photoionization code XSTAR and take into account the most
up-to-date and complete atomic data and line list. Our main results are the
following: 1) for a fixed value of the optical depth parameter, $M$ is not a
monotonic function of the photoionization parameter $xi$. Although $M$ starts
to decrease with $xi$ for $xi> 1$ as shown by others, this decrease in our
calculations is relatively gradual and in fact $M$ can increase by a factor of
few at $xi approx 10-1000$ (details depend on the assumed SED). The main
dynamically relevant effect of this behavior is that the multiplier can stay
larger than 1 for $xi$ as high as $1000$; 2) at these same $xi$ for which the
multiplier is higher than in previous calculations, the gas is thermally
unstable to isobaric perturbations. We discuss implications of our results in
the context of AGN winds that are observed in the UV and X-ray bands, and we
confirm previous findings that the force multiplier depends on very many lines,
not a few very optically thick lines.

One of the main physical mechanisms that could drive mass outflows in AGNs is
radiation pressure on spectral lines. Although this mechanism is conceptually
straightforward to understand, the actual magnitude of the radiation force is
challenging to compute because the force depends on the physical conditions in
the gas, as well as the strength, spectral energy distribution (SED), and
geometry of the radiation field. We present results from our photoionization
and radiation transfer calculations of the so-called force multiplier, $M$,
using the same radiation field to compute the gas photoionization and thermal
balance. Our method is general and can be used for an arbitrary SED. Here we
focus on describing results for two SEDs that correspond to a Type 1 and Type 2
AGN. We use the photoionization code XSTAR and take into account the most
up-to-date and complete atomic data and line list. Our main results are the
following: 1) for a fixed value of the optical depth parameter, $M$ is not a
monotonic function of the photoionization parameter $xi$. Although $M$ starts
to decrease with $xi$ for $xi> 1$ as shown by others, this decrease in our
calculations is relatively gradual and in fact $M$ can increase by a factor of
few at $xi approx 10-1000$ (details depend on the assumed SED). The main
dynamically relevant effect of this behavior is that the multiplier can stay
larger than 1 for $xi$ as high as $1000$; 2) at these same $xi$ for which the
multiplier is higher than in previous calculations, the gas is thermally
unstable to isobaric perturbations. We discuss implications of our results in
the context of AGN winds that are observed in the UV and X-ray bands, and we
confirm previous findings that the force multiplier depends on very many lines,
not a few very optically thick lines.

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