摘要: EX Lup is a low-mass pre-main sequence star that occasionally shows
accretion-related outbursts. Here, we present JWST/MIRI medium resolution
spectroscopy obtained for EX Lup fourteen years after its powerful outburst. EX
Lup is now in quiescence and displays a Class II spectrum. We detect a forest
of emission lines from molecules previously identified in infrared spectra of
classical T Tauri disks: H2O, OH, H2, HCN, C2H2, and CO2. The detection of
organic molecules demonstrates that they are back after disappearing during the
large outburst. Spectral lines from water and OH are for the first time
de-blended and will provide a much improved characterization of their
distribution and density in the inner disk. The spectrum also shows broad
emission bands from warm, sub-micron size amorphous silicate grains at 10 and
18 um. During the outburst, in 2008, crystalline forsterite grains were
annealed in the inner disk within 1 au, but their spectral signatures in the 10
um silicate band later disappeared. With JWST we re-discovered these crystals
via their 19.0, 20.0, and 23.5 um emission, whose strength implies that the
particles are at ~3 au from the star. This suggests that crystalline grains
formed in 2008 were transported outwards and now approach the water snowline,
where they may be incorporated into planetesimals. Containing several key
tracers of planetesimal and planet formation, EX Lup is an ideal laboratory to
study the effects of variable luminosity on the planet-forming material and may
provide explanation for the observed high crystalline fraction in solar system
comets.
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