HST UV Spectroscopy of the Planet-Hosting T Tauri Star PDS 70

TL;DR

This study presents the first UV spectra of PDS 70, revealing chromospheric activity, circumstellar gas, and insights into the star's accretion and disk evolution, with implications for young planet-hosting stars nearing disk dispersal.

Contribution

First UV spectra of PDS 70 obtained with HST, providing new insights into its stellar atmosphere, circumstellar environment, and accretion processes.

Findings

Detection of chromospheric and transition region emission lines.

Presence of fluorescent H2 lines indicating irradiated circumstellar gas.

Estimated accretion rate consistent with previous measurements.

Abstract

We summarize Hubble Space Telescope (HST) UV observations of the weak-lined T Tauri star (wTTS) PDS 70 obtained with the Space Telescope Imaging Spectrograph (STIS). These observations provide the first far-UV (FUV) and near-UV (NUV) spectra of PDS 70. Ground-based observations have so far revealed two formative giant planets orbiting in a wide gap in its circumstellar disk. Both the star and young planets are thought to still be accreting. The HST spectra provide new insight into physical conditions in the star's outer atmosphere and circumstellar environment. The spectra are dominated by chromospheric and transition region emission lines with maximum formation temperatures log T = 4.5 - 5.2 K. Stellar continuum emission is present in the NUV but we find no significant FUV continuum, as could arise from accretion shocks. Several fluorescent FUV H2 emission lines are present, a surprising result since H2 lines are usually undetected in wTTS. The H2 lines likely originate in irradiated circumstellar gas that could serve as a reservoir for the star's waning accretion. A previously established correlation between C IV line luminosity and accretion rate yields $\dot{M}_{acc}$ $\sim$ 10$^{-10}$ $M_{\odot}$ yr$^{-1}$, consistent with previous estimates. ALMA disk gas models imply strong absorption of stellar X-ray and UV (XUV) radiation near the star, effectively shielding the planets. Inner disk gas is exposed to ongoing photoevaporation by XUV radiation and the disk is nearing the end of its expected lifetime, making PDS 70 an important example of a young planet-hosting star in the late stage of accretion.