Understanding the water emission in the mid- and far-IR from protoplanetary disks around T Tauri stars

Abstract

Aims. We investigate which properties of protoplanetary disks around T Tauri stars affect the physics and chemistry in the regions where mid-and far-IR water lines originate and their respective line fluxes. We search for diagnostics for future observations. Methods. With the code ProDiMo, we build a series of models exploring a large parameter space, computing rotational and rovibrational transitions of water in nonlocal thermodynamic equilibrium (non-LTE). We select a sample of transitions in the mid-IR regime and the fundamental ortho and para water transitions in the far-IR. We investigate the chemistry and the local physical conditions in the line emitting regions. We calculate Spitzer spectra for each model and compare far-IR and mid-IR lines. In addition, we use mid-IR colors to tie the water line predictions to the dust continuum. Results. Parameters affecting the water line fluxes in disks by more than a factor of three are : the disk gas mass, the dust-to-gas mass ratio, the dust maximum grain size, interstellar medium (ISM) UV radiation field, the mixing parameter of Dubrulle settling, the disk flaring parameter, and the dust size distribution. The first four parameters affect the mid-IR lines much more than the far-IR lines. Conclusions. A key driver behind water spectroscopy is the dust opacity, which sets the location of the water line emitting region. We identify three types of parameters, including those (1) affecting global disk opacity and opacity function (maximum dust size and dust size distribution); (2) affecting global disk opacity (dust-to-gas mass ratio, Dubrulle settling, disk gas mass); and (3) not affecting disk opacity (flaring parameter, ISM UV radiation field, fraction of PAHs). Parameters, such as dust-to-gas ratio, ISM radiation field, and dust size distribution, affect the mid-IR lines more, while the far-IR transitions are more affected by the flaring index. The gas mass greatly affects lines in both regimes. Higher spectral resolution and line sensitivities, like from the James Webb Space Telescope, are needed to detect a statistically relevant sample of individual water lines to distinguish further between these types of parameters.

The Depletion of Water During Dispersal of Planet-Forming Disk Regions

“…Table 1 summarizes the properties, samples, and detection rates for all major surveys of water vapor in protoplanetary disks, including this work. The general trends identified to date are shown in Figure 1, and are compared to expectations produced by recent models of water in disks (Antonellini et al 2015;Walsh et al 2015). 7 We investigate the measured line fluxes against the masses of the central stars,  M , as an alternative way to look at the spectral-type dependencies found by Pontoppidan et al (2010a).…”

“…Water emission at Spitzer wavelengths has been interpreted in most disks as coming from an optically thick surface layer within the snow line radius, at temperatures of 300-700 K Salyk et al 2011a). Modeling of the velocity unresolved Spitzer spectra (Δv ∼ 400 km s −1 ) provided estimates of the disk emitting region to the inner few au in disks (e.g., Najita et al 2011;Antonellini et al 2015;Walsh et al 2015). At 12.4 μm, within the Spitzer-IRS range, a few water lines have been resolved in velocity in four disks with VLT-VISIR and Gemini-TEXES (Δv ∼ 15 and ∼ 3.5 km s −1 , respectively; Lagage et al 2004;Lacy et al 2002), providing support to the emitting regions estimated by models for the Spitzer spectra (Pontoppidan et al 2010b;Banzatti et al 2014;Salyk et al 2015).…”

Consistent dust and gas models for protoplanetary disks

“…Here, n H is the total hydrogen number density. The CO ice line is reasonably well described by the T dust = 20 K line, but a rate equilibrium approach works even better (Antonellini 2016, white dashed line). The disk shows a ring of high CO 2 abundance inside 1 au.…”

“…Beyond the snow line, water freezes out onto the cold dust grains. The water ice reservoir is outlined well by a rate equilibrium approach (Antonellini 2016, yellow dashed line) or using the water vapor pressure together with the local radiation field (Min et al 2016a, white dashed line).…”

“…For CO, the black contour shows the PDR parameter log χ/ n H = −3.5 and the blue contour T dust = 20 K where CO starts to freeze out on dust grains. For CO#, the white dashed contours show dust temperatures of 15, 20 and 25 K and the blue dashed line shows the CO ice line estimate from rate equilibrium (Antonellini 2016). For water ice, two approximations of the snow line are indicated: (1) Estimate based on the local density, dust temperature and radiation field (Min et al 2016a, yellow dashed) and (2) Estimate from rate equilibrium (Antonellini 2016, blue dashed).…”