UV Fluorescence Traces Gas and Lyα Evolution in Protoplanetary Disks

Abstract

Ultraviolet spectra of protoplanetary disks trace distributions of warm gas at radii where rocky planets form. We combine Hubble Space Telescope Cosmic Origins Spectrograph observations of H₂ and CO emission from 12 classical T Tauri stars to more extensively map inner disk surface layers, where gas temperature distributions allow radially stratified fluorescence from the two species. We calculate empirical emitting radii for each species under the assumption that the line widths are entirely set by Keplerian broadening, demonstrating that the CO fluorescence originates further from the stars r ~ 20 au) than the H₂ (r ~ 0.8 au). This is supported by 2D radiative transfer models, which show that the peak and outer radii of the CO flux distributions generally extend further into the outer disk than the H₂. These results also indicate that additional sources of Lyα photons remain unaccounted for, requiring more complex models to fully reproduce the molecular gas emission. As a first step, we confirm that the morphologies of the UV–CO bands and Lyα radiation fields are significantly correlated and discover that both trace the degree of dust disk evolution. The UV tracers appear to follow the same sequence of disk evolution as forbidden line emission from jets and winds, as the observed Lyα profiles transition between dominant red wing and dominant blue wing shapes when the high-velocity optical emission disappears. Our results suggest a scenario where UV radiation fields, disk winds and jets, and molecular gas evolve in harmony with the dust disks throughout their lifetimes.

Additional Information

© 2021. The American Astronomical Society. Received 2021 March 31; revised 2021 July 2; accepted 2021 July 13; published 2021 October 8. We are grateful to Zachary Taylor and Klaus Pontoppidan for enjoyable and insightful discussions in various stages of this work. We also thank the anonymous referee for their very detailed report, which greatly improved the clarity of this manuscript. NA was supported in part by NASA Earth and Space Science Fellowship grant 80NSSC17K0531, and KH is supported by the David & Ellen Lee Prize Postdoctoral Fellowship in Experimental Physics at Caltech. This paper also made use of data and financial support from HST program GO-15070, and AB acknowledges support from HST grant HST-GO-15128.001-A to the University of Colorado at Boulder. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work utilized the RMACC Summit supercomputer, which is supported by the National Science Foundation (awards ACI-1532235 and ACI-1532236), the University of Colorado Boulder, and Colorado State University. The Summit supercomputer is a joint effort of the University of Colorado Boulder and Colorado State University. This research made use of Astropy, 11 a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013; The Astropy Collaboration et al. 2018).

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Published - Arulanantham_2021_AJ_162_185.pdf

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