The general applicability of self-similar solutions for thermal disc winds

Andrew D Sellek,Richard A Booth,Cathie J Clarke

doi:10.1093/mnras/stab1693

Andrew D Sellek, Richard A Booth + Show 1 more

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https://doi.org/10.1093/mnras/stab1693

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Abstract

ABSTRACT 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 generalize 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 midplane, 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 midplane and thus weakens the dependence of the FWHM on inclination.

Highlights

In this paper we explore the large scale kinematics and morphology of thermally driven winds in cases where the wind temperature structure and the density profile at the flow base are prescribed to investigate to what extent we can separate these kinematics and morphology from the microphysics
In order to isolate the effects of the temperature gradients from the resultant slower launch, we show an isothermal streamline with the Mach number reduced to match the non-isothermal case
We have demonstrated that the effects of gravity and centrifugal force make only a small impact on the launch velocity and streamline morphology of thermal winds and that radial temperature gradients cannot decrease Mb by much more than ∼ 10 per cent compared to the isothermal case

Summary

Introduction

Winds originating from accretion discs are thought to account for blueshifted features in a number of astrophysical spectra, such as the Low Velocity Component (LVC) of forbidden emission lines such as [Ne ] and [O ] in protoplanetary disc line spectra (Hartigan et al 1995; Pascucci & Sterzik 2009; Rigliaco et al 2013; Simon et al 2016; Banzatti et al 2019; Pascucci et al 2020), warm absorbers in AGN spectra (e.g. Mizumoto et al 2019; Laha et al 2021; Ganguly et al 2021) and Fe lines from X-ray binaries (e.g. Begelman et al 1983; Higginbottom et al 2020). Various effects may drive or assist the acceleration of the wind: from magnetic tension, to centrifugal forces, to thermal pressure gradients and radiation pressure (Alexander et al 2014; King & Pounds 2015; Ercolano & Pascucci 2017). Understanding the kinematics of these winds is of importance both for interpreting these observations and for assessing their effects both on the accretion disc and their surroundings. A thermal wind may be launched from a disc so long as the thermal energy of heated gas in its upper layers (which is converted to kinetic energy by pressure gradients) is sufficient to overcome the gravitational potential of the star, leading to unbound material.

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