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Register a new userModeling Time Dependent Water Chemistry Due to Powerful X-ray Flares from T-Tauri Stars
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Abygail R. Waggoner
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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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Context: T Tauri stars have X-ray luminosities ranging from L_X = 10^28-10^32 erg/s. These luminosities are similar to UV luminosities (L_UV 10^30-10^31 erg/s) and therefore X-rays are expected to affect the physics and chemistry of the upper layers of their surrounding protoplanetary disks. Aim: The effects and importance of X-rays on the chemical and hydrostatic structure of protoplanetary disks are investigated, species tracing X-ray irradiation (for L_X >= 10^29 erg/s) are identified and predictions for [OI], [CII] and [NII] fine structure line fluxes are provided. Methods: We have implemented X-ray physics and chemistry into the chemo-physical disk code ProDiMo. We include Coulomb heating and H2 ionization as heating processes and primary and secondary ionization due to X-rays in the chemistry. Results: X-rays heat up the gas causing it to expand in the optically thin surface layers. Neutral molecular species are not much affected in their abundance and spatial distribution, but charged species such as N+, OH+, H2O+ and H3O+ show enhanced abundances in the disk surface. Conclusions: Coulomb heating by X-rays changes the vertical structure of the disk, yielding temperatures of ~ 8000 K out to distances of 50 AU. The chemical structure is altered by the high electron abundance in the gas in the disk surface, causing an efficient ion-molecule chemistry. The products of this, OH+, H2O+ and H3O+, are of great interest for observations of low-mass young stellar objects with the Herschel Space Observatory. [OI] (at 63 and 145 mic) and [CII] (at 158 mic) fine structure emission are only affected for L_X > 10^30 erg/s.
Getman et al. (2021) reports the discovery, energetics, frequencies, and effects on environs of $>1000$ X-ray super-flares with X-ray energies $E_X sim 10^{34}-10^{38}$~erg from pre-main sequence (PMS) stars identified in the $Chandra$ MYStIX and SFiNCs surveys. Here we perform detailed plasma evolution modeling of $55$ bright MYStIX/SFiNCs super-flares from these events. They constitute a large sample of the most powerful stellar flares analyzed in a uniform fashion. They are compared with published X-ray super-flares from young stars in the Orion Nebula Cluster, older active stars, and the Sun. Several results emerge. First, the properties of PMS X-ray super-flares are independent of the presence or absence of protoplanetary disks inferred from infrared photometry, supporting the solar-type model of PMS flaring magnetic loops with both footpoints anchored in the stellar surface. Second, most PMS super-flares resemble solar long duration events (LDEs) that are associated with coronal mass ejections. Slow rise PMS super-flares are an interesting exception. Third, strong correlations of super-flare peak emission measure and plasma temperature with the stellar mass are similar to established correlations for the PMS X-ray emission composed of numerous smaller flares. Fourth, a new correlation of loop geometry is linked to stellar mass; more massive stars appear to have thicker flaring loops. Finally, the slope of a long-standing relationship between the X-ray luminosity and magnetic flux of various solar-stellar magnetic elements appears steeper in PMS super-flares than for solar events.
We present evidence for rotational modulation of X-ray flares by an analysis of four outbursts on late-type stars. Three of these flares have been observed on T Tauri Stars and one on Algol. The structure of the X-ray lightcurves observed in this selection of flare events is untypical in that the maximum emission extends over several hours producing a round hump in the lightcurve instead of a sharp peak. We explain this deviation from the standard shape of a flare lightcurve as the result of a flare erupting on the back side of the star and gradually moving into the line of sight due to the stars rotation. An estimate for the decay timescale and the size of the X-ray emitting volume is derived. Spectral information supports our proposition that changes of the visible volume are responsible for the observed time development of these flares.
We present 3.6 to 70 {mu}m Spitzer photometry of 154 weak-line T Tauri stars (WTTS) in the Chamaeleon, Lupus, Ophiuchus and Taurus star formation regions, all of which are within 200 pc of the Sun. For a comparative study, we also include 33 classical T Tauri stars (CTTS) which are located in the same star forming regions. Spitzer sensitivities allow us to robustly detect the photosphere in the IRAC bands (3.6 to 8 {mu}m) and the 24 {mu}m MIPS band. In the 70 {mu}m MIPS band, we are able to detect dust emission brighter than roughly 40 times the photosphere. These observations represent the most sensitive WTTS survey in the mid to far infrared to date, and reveal the frequency of outer disks (r = 3-50 AU) around WTTS. The 70 {mu}m photometry for half the c2d WTTS sample (the on-cloud objects), which were not included in the earlier papers in this series, Padgett et al. (2006) and Cieza et al. (2007), are presented here for the first time. We find a disk frequency of 19% for on-cloud WTTS, but just 5% for off- cloud WTTS, similar to the value reported in the earlier works. WTTS exhibit spectral energy distributions (SEDs) that are quite diverse, spanning the range from optically thick to optically thin disks. Most disks become more tenuous than Ldisk/L* = 2 x 10^-3 in 2 Myr, and more tenuous than Ldisk/L* = 5 x 10^-4 in 4 Myr.
Residual gas in disks around young stars can spin down stars, circularize the orbits of terrestrial planets, and whisk away the dusty debris that is expected to serve as a signpost of terrestrial planet formation. We have carried out a sensitive search for residual gas and dust in the terrestrial planet region surrounding young stars ranging in age from a few Myr to ~10 Myr in age. Using high resolution 4.7 micron spectra of transition objects and weak T Tauri stars, we searched for weak continuum excesses and CO fundamental emission, after making a careful correction for the stellar contribution to the observed spectrum. We find that the CO emission from transition objects is weaker and located further from the star than CO emission from non-transition T Tauri stars with similar stellar accretion rates. The difference is possibly the result of chemical and/or dynamical effects (i.e., a low CO abundance or close-in low-mass planets). The weak T Tauri stars show no CO fundamental emission down to low flux levels (5 x 10^(-20) - 10^{-18} W/m^2). We illustrate how our results can be used to constrain the residual disk gas content in these systems and discuss their potential implications for star and planet formation.
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