Model exploration of near-IR ro-vibrational CO emission as a tracer of inner cavities in protoplanetary disks

Abstract

Context. Near-IR observations of protoplanetary disks provide information about the properties of the inner disk. High resolution spectra of abundant molecules such as CO can be used to determine the disk structure in the warm inner parts. The v2/v1 ro-vibrational ratio of v 1−0 and v 2−1 transitions has been recently observed to follow distinct trends with the CO emitting radius, in a sample of TTauri and Herbig disks; these trends have been empirically interpreted as due to inner disk depletion from gas and dust. Aims. In this work we use existing thermo-chemical disk models to explore the interpretation of these observed trends in ro-vibrational CO emission. Methods. We use the radiation thermo-chemical code ProDiMo, exploring a set of previously published models with different disk properties and varying one parameter at a time: the inner radius, the dust-to-gas mass ratio, the gas mass. In addition, we use models where we change the surface density power law index, and employ a larger set of CO ro-vibrational levels, including also fluorescence from the first electronic state. We investigate these models for both TTauri and Herbig star disks. Finally, we include a set of DIANA models for individual TTauri and Herbig disks which were constructed to reproduce a large set of multi-wavelength observations. Results. This modeling exploration highlights promising parameters to explain the trends observed in ro-vibrational CO emission. Our models with an increasing inner radius can naturally match the trend observed for TTauris, where the vertical spread in the data could also be accounted for by different values for dust-to-gas mass ratio and disk gas mass in different disks. Our models match the CO vibrational ratio observed in Herbig disks only in the case of large inner holes, instead, and cannot produce the low ratios measured in many disks. The models do produce an inversion in the trend, where v 2−1 /v 1−0 increases instead of decreasing for CO radii larger than a few au. This is a consequence of the P(4) v 2−1 line becoming optically thin and super-thermally excited. In our models, this does not require invoking UV fluorescence pumping. Conclusions. Our modeling explorations suggest that the observed decrease of v 2−1 /v 1−0 with CO radius in TTauri disks can be a consequence of inside-out disk depletion. For the Herbig disks, instead, a more complex inner disk structure may be needed to explain the observed trends in the excitation of CO emission as a function of emitting radius: disk gaps emptied of dust, partially depleted in gas and/or possibly a disk structure with inverted surface density profile. These structures need to be further investigated in future work.

Hints for icy pebble migration feeding an oxygen-rich chemistry in the inner planet-forming region of disks

“…The only two millimeter cavities that are consistent with the model are those detected in TW Hya and HD 143006, which are among the few observed at the highest spatial resolution achievable with ALMA. The radius of infrared CO emission R co (Figure 9, middle) generally seems to probe closely the size of an inner region depleted in dust, as proposed in previous work Banzatti et al 2017Banzatti et al , 2018Bosman et al 2019;Antonellini et al 2020). The trend provided by the model is well matched by most datapoints, but some disks with inner cavities clearly deviate from the model (these are marked in the figure).…”

Scanning disk rings and winds in CO at 0.01-10 au: a high-resolution $M$-band spectroscopy survey with IRTF-iSHELL

“…In fact, models show that the excitation and kinematics of fundamental CO emission should be very sensitive to several properties, including the local gas-to-dust ratio and dust opacity, the radial size of an inner dust cavity, the UV flux, and the vertical disk scale height, suggesting a key role for CO spectra in revealing different inner disk structures (e.g. Woitke et al 2016;Hein Bertelsen et al 2016a;Bosman et al 2019;Antonellini et al 2020). Recently, observations revealed a tight correlation between the CO vibrational v2/v1 ratio (spanning almost a factor 100, between 0.02 and 1) and the IR excess emission from hot dust in inner disks (Banzatti et al 2018), further supporting a key role for dust mixing and suggesting that the observed CO spectra are excited in disk regions with a wide range in gas-to-dust ratios (from ISM levels of ∼ 100 up to > 10, 000; Bosman et al 2019).…”

“…Najita et al 2003;Salyk et al 2011a;. Moreover, the relatively high Einstein A-coefficients of fundamental CO v = 1 − 0 transitions have proven effective in tracing the depletion of gas in inner disks, providing a complementary tool to ALMA to study disk evolution and planet formation in the innermost disk region (Brittain et al 2003;Salyk et al 2009;Banzatti et al 2017Banzatti et al , 2018Bosman et al 2019;Antonellini et al 2020), in a few cases by directly spatially resolving gas in inner dust cavities through spectro-astrometry (Pontoppidan et al 2008). Modeling of the gas surface density drop in inner dust cavities from the CO emission observed in some disks has been found consistent with dynamical interactions with massive planets (Brittain et al 2007b;Carmona et al 2014Carmona et al , 2017Bosman et al 2019).…”

Water UV-shielding in the Terrestrial Planet-forming Zone: Implications from Water Emission

“…Observations show that CO 2 is not a dominant carrier of either carbon or oxygen (Pontoppidan & Blevins 2014;Bosman et al 2017). Detailed models of water get stuck on a degeneracy between gas-to-dust abundance and water abundance (Meijerink et al 2009;Blevins et al 2016), whereas models of the CO rovibrational lines are very sensitive to the assumed structures Antonellini et al 2020). As such, C/O ratios have only been inferred from infrared data in TW Hya, where H 2 lines are available to infer the total column and the inner disk structure can be resolved ).…”