The Radial Distribution and Excitation of H₂ around Young Stars in the HST-ULLYSES Survey

Abstract

The spatial distribution and evolution of gas in the inner 10 au of protoplanetary disks form the basis for estimating the initial conditions of planet formation. Among the most important constraints derived from spectroscopic observations of the inner disk are the radial distributions of the major gas phase constituents, how the properties of the gas change with inner disk dust evolution, and how the chemical abundances and excitation conditions are influenced by the high-energy radiation from the central star. We present a survey of the radial distribution, excitation, and evolution of inner disk molecular hydrogen (H2) obtained as part of the Hubble Space Telescope-ULLYSES program. We analyze far-UV spectroscopy of 71 (63 accreting) pre-main-sequence systems in ULLYSES DR5 to characterize the H2 emission lines, H2 dissociation continuum emission, and major photochemical/disk evolution driving the UV emissions (Lyα, UV continuum, and C iv). We use the widths of the H2 emission lines to show that most fluorescent H2 arises between 0.1 and 1.4 au from the parent star, and show positive correlations of the average emitting radius with the accretion luminosity and with the dust disk mass. We find a strong correlation between H2 dissociation emission and both the accretion-dominated Lyα luminosity and the inner disk dust clearing, painting a picture where water molecules in the inner 3 au are exposed to and dissociated by strong Lyα emission as the opacity of the inner disk declines with time.

Line Profiles of Forbidden Emission Lines and What They Can Tell Us About Protoplanetary Disk Winds

“…Usually, the accretion luminosity is not measured directly because the source is mostly in the UV and is efficiently absorbed before reaching the observer, but inferred from tight correlations with H lines (Fang et al 2018). France et al (2023) directly measure the UV H 2 , Lyα, C IV 1600 Å bump, and FUV continuum between 912 and 1760 Å (the major constituents of the accretion FUV continuum) using Hubble Space Telescope observations, and confirm the results of previous studies about the typical luminosity of this source. We approximate the shape of the FUV source with a blackbody with T eff = 12,000 K following the previous theoretical work (Gorti & Hollenbach 2009;EO16;Wang et al 2019).…”

A Multiwavelength, Multiepoch Monitoring Campaign of Accretion Variability in T Tauri Stars from the ODYSSEUS Survey. I. HST Far-UV and Near-UV Spectra

“…For the last two scenarios, there is an expected correlation between the H 2 bump luminosity and L acc , since accretion should produce Lyα photons (e.g., Alencar et al 2012). France et al (2023) find strong correlations between the H 2 bump and both Lyα and Wide-field Infrared Survey Explorer W3−W4 colors, which they suggest reinforces that the H 2 bump originates in the inner disk, where Lyα photons can reach and dissociate the H 2 O content of the inner disk. Espaillat et al (2019) suggest that the H 2 bump is driven by changes in the surface density in the inner disk that propagate through the accretion column and lead to a change in the accretion rate.…”

“…UV line luminosities have previously been found to correlate with mass accretion (Calvet et al 2004;Yang et al 2012;Ardila et al 2013;Ingleby et al 2013;). Additionally, a broad region near 1600 Å dubbed the "H 2 bump" (e.g., Bergin et al 2004;Herczeg et al 2004) has also been linked to accretion (see Ingleby et al 2009Ingleby et al , 2012Yang et al 2012;France et al 2017France et al , 2023Thanathibodee et al 2018;Espaillat et al 2019). Here we measure the luminosities of selected UV lines, the FUV and NUV continua, and the H 2 bump.…”

The Dynamic, Chimeric Inner Disk of PDS 70

“…For one thing, a wind would screen the disk itself from the Lyα photons that pump such emission, precluding it from being the source. Such emission is common among classical T Tauri stars and generally arises from the inner (1 au) disk region (Arulanantham et al 2021;France et al 2023), and a correlation with the narrow-line component of forbidden atomic emission, i.e., [O I] (Gangi et al 2023), suggests a connection with winds. Fluorescent H 2 emission is rare among weak-lined and lowaccreting T Tauri stars; another notable exception is EP Cha (RECX-11, France et al 2012), which is also a "dipper" star.…”