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Forbidden line diagnostics of photoevaporative disc winds

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journal contribution
posted on 14.06.2021, 10:50 by G Ballabio, RD Alexander, CJ Clarke
Photoevaporation driven by high-energy radiation from the central star plays an important role in the evolution of protoplanetary discs. Photoevaporative winds have been unambiguously detected through blue-shifted emission lines, but their detailed properties remain uncertain. Here we present a new empirical approach to make observational predictions of these thermal winds, seeking to fill the gap between theory and observations. We use a self-similar model of an isothermal wind to compute line profiles of several characteristic emission lines (in particular the [Ne II] line at 12.81 μm, and optical forbidden lines such as [O I] 6300 Å and [S II] 4068/4076 Å), studying how the lines are affected by parameters such as the gas temperature, disc inclinations, and density profile. Our model successfully reproduces blue-shifted lines with vpeak≲10 km s−1, which decrease with increasing disc inclination. The line widths increase with increasing disc inclinations and range from Δv∼15 to 30 km s−1. The predicted blue-shifts are mostly sensitive to the gas sound speed (and therefore the temperature). The observed [Ne II] line profiles are consistent with a thermal wind and point towards a relatively high sound speed, as expected for extreme-UV photoevaporation. However, the observed [O I] line profiles require lower temperatures, as expected in X-ray photoevaporation, and show a wider scatter that is difficult to reconcile with a single wind model; it seems likely that these lines trace different components of a multiphase wind. We also note that the spectral resolution of current observations remains an important limiting factor in these studies, and that higher resolution spectra are required if emission lines are to further our understanding of protoplanetary disc winds.

History

Citation

Monthly Notices of the Royal Astronomical Society, Volume 496, Issue 3, August 2020, Pages 2932–2945, https://doi.org/10.1093/mnras/staa1767

Author affiliation

School of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society

Volume

496

Issue

3

Pagination

2932 - 2945

Publisher

Oxford University Press (OUP) for Royal Astronomical Society

issn

0035-8711

eissn

1365-2966

Acceptance date

17/06/2020

Copyright date

2020

Available date

14/06/2021

Language

English