Modeling Time Dependent Water Chemistry Due to Powerful X-ray Flares from T-Tauri Stars. (arXiv:1908.08048v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Waggoner_A/0/1/0/all/0/1">Abygail R. Waggoner</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Cleeves_L/0/1/0/all/0/1">L. Ilsedore Cleeves</a> (1), ((1) University of Virginia)
Young stars emit strong flares of X-ray radiation that penetrate the surface
layers of their associated protoplanetary disks. It is still an open question
as to whether flares create significant changes in disk chemical composition.
We present models of the time-evolving chemistry of gas-phase water during
X-ray flaring events. The chemistry is modeled at point locations in the disk
between 1 and 50 au at vertical heights ranging from the mid-plane to the
surface. We find that strong, rare flares, i.e., those that increase the
unattenuated X-ray ionization rate by a factor of 100 every few years, can
temporarily increase the gas-phase water abundance relative to H can by more
than a factor of $sim3-5$ along the disk surface (Z/R $ge$ 0.3). We report
that a “typical” flare, i.e., those that increase the unattenuated X-ray
ionization rate by a factor of a few every few weeks, will not lead to
significant, observable changes. Dissociative recombination of H$_3$O$^+$,
water adsorption and desorption onto dust grains, and ultraviolet photolysis of
water and related species are found to be the three dominant processes
regulating the gas-phase water abundance. While the changes are found to be
significant, we find that the effect on gas phase water abundances throughout
the disk is short-lived (days). Even though we do not see a substantial
increase in long term water (gas and ice) production, the flares’ large effects
may be detectable as time varying inner disk water ‘bursts’ at radii between 5
and 30 au with future far infrared observations.
Young stars emit strong flares of X-ray radiation that penetrate the surface
layers of their associated protoplanetary disks. It is still an open question
as to whether flares create significant changes in disk chemical composition.
We present models of the time-evolving chemistry of gas-phase water during
X-ray flaring events. The chemistry is modeled at point locations in the disk
between 1 and 50 au at vertical heights ranging from the mid-plane to the
surface. We find that strong, rare flares, i.e., those that increase the
unattenuated X-ray ionization rate by a factor of 100 every few years, can
temporarily increase the gas-phase water abundance relative to H can by more
than a factor of $sim3-5$ along the disk surface (Z/R $ge$ 0.3). We report
that a “typical” flare, i.e., those that increase the unattenuated X-ray
ionization rate by a factor of a few every few weeks, will not lead to
significant, observable changes. Dissociative recombination of H$_3$O$^+$,
water adsorption and desorption onto dust grains, and ultraviolet photolysis of
water and related species are found to be the three dominant processes
regulating the gas-phase water abundance. While the changes are found to be
significant, we find that the effect on gas phase water abundances throughout
the disk is short-lived (days). Even though we do not see a substantial
increase in long term water (gas and ice) production, the flares’ large effects
may be detectable as time varying inner disk water ‘bursts’ at radii between 5
and 30 au with future far infrared observations.
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