The general applicability of self-similar solutions for thermal disc winds. (arXiv:2106.05362v1 [astro-ph.EP])

<a href="http://arxiv.org/find/astro-ph/1/au:+Sellek_A/0/1/0/all/0/1">Andrew D. Sellek</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Clarke_C/0/1/0/all/0/1">Cathie J. Clarke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Booth_R/0/1/0/all/0/1">Richard A. Booth</a>

Thermal disc winds occur in many contexts and may be particularly important

to the secular evolution and dispersal of protoplanetary discs heated by high

energy radiation from their central star. In this paper we generalise previous

models of self-similar thermal winds – which have self-consistent morphology

and variation of flow variables – to the case of launch from an elevated base

and to non-isothermal conditions. These solutions are well-reproduced by

hydrodynamic simulations, in which, as in the case of isothermal winds launched

from the mid-plane, we find winds launch at the maximum Mach number for which

the streamline solutions extend to infinity without encountering a singularity.

We explain this behaviour based on the fact that lower Mach number solutions do

not fill the spatial domain. We also show that hydrodynamic simulations reflect

the corresponding self-similar models across a range of conditions appropriate

to photoevaporating protoplanetary discs, even when gravity, centrifugal

forces, or changes in the density gradient mean the problem is not inherently

scale free. Of all the parameters varied, the elevation of the wind base

affected the launch velocity and flow morphology most strongly, with

temperature gradients causing only minor differences. We explore how launching

from an elevated base affects Ne II line profiles from winds, finding it

increases (reduces) the full width at half maximum (FWHM) of the line at low

(high) inclination to the line of sight compared with models launched from the

disc mid-plane and thus weakens the dependence of the FWHM on inclination.

Thermal disc winds occur in many contexts and may be particularly important

to the secular evolution and dispersal of protoplanetary discs heated by high

energy radiation from their central star. In this paper we generalise previous

models of self-similar thermal winds – which have self-consistent morphology

and variation of flow variables – to the case of launch from an elevated base

and to non-isothermal conditions. These solutions are well-reproduced by

hydrodynamic simulations, in which, as in the case of isothermal winds launched

from the mid-plane, we find winds launch at the maximum Mach number for which

the streamline solutions extend to infinity without encountering a singularity.

We explain this behaviour based on the fact that lower Mach number solutions do

not fill the spatial domain. We also show that hydrodynamic simulations reflect

the corresponding self-similar models across a range of conditions appropriate

to photoevaporating protoplanetary discs, even when gravity, centrifugal

forces, or changes in the density gradient mean the problem is not inherently

scale free. Of all the parameters varied, the elevation of the wind base

affected the launch velocity and flow morphology most strongly, with

temperature gradients causing only minor differences. We explore how launching

from an elevated base affects Ne II line profiles from winds, finding it

increases (reduces) the full width at half maximum (FWHM) of the line at low

(high) inclination to the line of sight compared with models launched from the

disc mid-plane and thus weakens the dependence of the FWHM on inclination.

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