The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime

Abstract

Context. Near-to mid-infrared observations of molecular emission from protoplanetary disks show that the inner regions are rich in small organic volatiles (e.g., C 2 H 2 and HCN). Trends in the data suggest that disks around cooler stars (T eff ≈ 3000 K) are potentially (i) more carbon-rich; and (ii) more molecule-rich than their hotter counterparts (T eff > ∼ 4000 K). Aims. We explore the chemical composition of the planet-forming region (<10 AU) of protoplanetary disks around stars over a range of spectral types (from M dwarf to Herbig Ae) and compare with the observed trends. Methods. Self-consistent models of the physical structure of a protoplanetary disk around stars of different spectral types are coupled with a comprehensive gas-grain chemical network to map the molecular abundances in the planet-forming zone. The effects of (i) N 2 self shielding; (ii) X-ray-induced chemistry; and (iii) initial abundances, are investigated. The chemical composition in the "observable" atmosphere is compared with that in the disk midplane where the bulk of the planet-building reservoir resides. Results. M dwarf disk atmospheres are relatively more molecule rich than those for T Tauri or Herbig Ae disks. The weak far-UV flux helps retain this complexity which is enhanced by X-ray-induced ion-molecule chemistry. N 2 self shielding has only a small effect in the disk molecular layer and does not explain the higher C 2 H 2 /HCN ratios observed towards cooler stars. The models underproduce the OH/H 2 O column density ratios constrained in Herbig Ae disks, despite reproducing (within an order of magnitude) the absolute value for OH: the inclusion of self shielding for H 2 O photodissociation only increases this discrepancy. One possible explanation is the adopted disk structure. Alternatively, the "hot" H 2 O (T > ∼ 300 K) chemistry may be more complex than assumed. The results for the atmosphere are independent of the assumed initial abundances; however, the composition of the disk midplane is sensitive to the initial main elemental reservoirs. The models show that the gas in the inner disk is generally more carbon rich than the midplane ices. This effect is most significant for disks around cooler stars. Furthermore, the atmospheric C/O ratio appears larger than it actually is when calculated using observable tracers only. This is because gas-phase O 2 is predicted to be a significant reservoir of atmospheric oxygen.Conclusions. The models suggest that the gas in the inner regions of disks around cooler stars is more carbon rich; however, calculations of the molecular emission are necessary to definitively confirm whether the chemical trends reproduce the observed trends.

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

“…This sample revealed two major trends: (1) water line fluxes increase with increasing stellar luminosity (and mass; see Figures 1 and 20, and Salyk et al 2011a), and in contrast with the first trend, (2) the water vapor frequency is much higher in disks around stars of < 0.2  < M 1.5  M (with a frequency of 60%-80%) than in disks around stars of  > M 1.5  M (with a frequency of <20%; see Table 1, Figure 10, and Pontoppidan et al 2010a). Recent chemical modeling of the molecular content of inner disks by Walsh et al (2015), while able to reproduce the luminosity trend, was still unable to reproduce the absence of Table 1 are shown as black dots.…”

“…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).…”

“…Three of these disks were previously published in Mandell et al (2012). Three main-sequence stars were observed to be used as photospheric standards, sampling stellar spectral types from K to M (with spectral types taken from Gray et al 2006;Koen et al 2010): NLTT29176 (K3), Walsh et al 2015 Spitzer fluxes Spitzer limits Antonellini et al 2016 S p it z e r s e n s it iv it y S p it z e r s e n s it iv it y HD111631 (K7), and HIP49986 (M1.5). These spectra are used to correct the 2.9 μm H 2 O and OH emission spectra, as explained in Section 3.1.…”

“…Walsh et al (2015) suggested that disks around low-mass stars may be too cold to produce strong water emissions. The luminosity dependence shown in Figure 1 may provide an explanation for the non-detection of water in these disks with Spitzer (only a tentative detection was reported by Pascucci et al 2013): models produce fluxes at or below the measured sensitivity limits, due to a radial extent of the warm water-emitting layer that is ∼10 times smaller than in a disk around a solar-mass star (∼0.1 au; see Walsh et al 2015). However, according to models, these disks would still show prominent organic emission lines from a disk atmosphere at 0.1-10 au, only mildly UV-irradiated as compared to higher-mass stars.…”