UV Fluorescence Traces Gas and Lyα Evolution in Protoplanetary Disks

Nicole Arulanantham,Catherine C. Espaillat,H. M. Günther,Keri Hoadley,Alexander Brown,P. C. Schneider,Kevin France,Gregory J. Herczeg

doi:10.3847/1538-3881/ac1593

Abstract

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.

Highlights

The advent of the Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionized studies of circumstellar disks, providing images of planet-forming material at unprecedented spatial resolution and sensitivity
Consistent with previous studies (Herczeg et al 2004; France et al 2012), we find that the empirical ultraviolet emission from hot H2 (UV–H2) emitting radius is inside r < 2 au for all targets
The UV–CO appears to trace a different population of gas than the IR–CO emission (Salyk et al 2011; Brown et al 2013), which is produced by material that is roughly radially co-located with the UV–H2 (France et al 2012)

Summary

Introduction

The advent of the Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionized studies of circumstellar disks, providing images of planet-forming material at unprecedented spatial resolution and sensitivity. By mapping the dust distributions down to ∼5 au from the central star, submillimeter observations have revealed both small- and large-scale dust substructures in most disks (see, e.g., Andrews et al 2018; Huang et al 2018a; Long et al 2018). This includes all observed disks in the Taurus star-forming region with effective emission radii larger than 50 au, implying that disks without detected substructures are too compact to resolve the features without even higher resolution observations (Long et al 2019). The features are spectrally resolved in data from the Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST), allowing observers to extract both the empirical radial locations of emitting gas (France et al 2012) and the radial distributions of flux from the spectra (Hoadley et al 2015; Arulanantham et al 2018)

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Results

Discussion

Conclusion