Abstract
Abstract We report the first detection of a substantial brightening event in an isotopologue of a key molecular ion, HCO+, within a protoplanetary disk of a T Tauri star. The H13CO+ rotational transition was observed three times toward IM Lup between 2014 July and 2015 May with the Atacama Large Millimeter/submillimeter Array. The first two observations show similar spectrally integrated line and continuum fluxes, while the third observation shows a doubling in the disk-integrated line flux compared to the continuum, which does not change between the three epochs. We explore models of an X-ray active star irradiating the disk via stellar flares, and find that the optically thin H13CO+ emission variation can potentially be explained via X-ray-driven chemistry temporarily enhancing the HCO+ abundance in the upper layers of the disk atmosphere during large or prolonged flaring events. If the HCO+ enhancement is indeed caused by an X-ray flare, future observations should be able to spatially resolve these events and potentially enable us to watch the chemical aftermath of the high-energy stellar radiation propagating across the face of protoplanetary disks, providing a new pathway to explore ionization physics and chemistry, including electron density, in disks.
Highlights
The gas-phase abundances of key molecular ions, including HCO+, N2H+, DCO+, and H2D+, are sensitive to high energy processes in the dense interstellar medium (e.g., Caselli et al 1998; Williams et al 1998; Caselli et al 2002; Dalgarno 2006)
We report the first detection of a substantial brightening event in an isotopologue of a key molecular ion, HCO+, within a protoplanetary disk of a T Tauri star
The first two observations show similar spectrally integrated line and continuum fluxes, while the third observation shows a doubling in the disk integrated J = 3−2 line flux compared to the continuum, which does not change between the three epochs
Summary
The gas-phase abundances of key molecular ions, including HCO+, N2H+, DCO+, and H2D+, are sensitive to high energy (keV – GeV) processes in the dense interstellar medium (e.g., Caselli et al 1998; Williams et al 1998; Caselli et al 2002; Dalgarno 2006). Abundances of such molecular ions, especially HCO+ (Cleeves et al 2014), are strongly influenced by X-rays from their host star (Glassgold et al 2012). Shorter term variability is associated with magnetic reconnection events in the corona, similar to solar X-ray flares but with greater magnitude and frequency (e.g., Feigelson et al 2007). Such events can result in luminosity changes up to two orders of magnitude within a matter of hours. Given the sensitivity of molecular ions to energetic processes, it has been an open question whether the bulk chemistry of the disk responds to stellar X-ray variabil-
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