Molecule formation in dust-poor irradiated jets

Abstract

Context. Recent ALMA observations suggest that the highest velocity part of molecular protostellar jets ( 80 km s −1 ) are launched from the dust-sublimation regions of the accretion disks ( 0.3 au). However, formation and survival of molecules in inner protostellar disk winds, in the presence of a harsh far-ultraviolet (FUV) radiation field and the absence of dust, remain unexplored. Aims. We aim at determining if simple molecules such as H 2 , CO, SiO, and H 2 O can be synthesized and spared in fast and collimated dust-free disk winds or if a fraction of dust is necessary to explain the observed molecular abundances. Methods. This work is based on a recent version of the Paris-Durham shock code designed to model irradiated environments. Fundamental properties of the dust-free chemistry are investigated from single point models. A laminar 1D disk wind model is then built using a parametric flow geometry. This model includes time-dependent chemistry and the attenuation of the radiation field by gas-phase photoprocesses. The influence of the mass-loss rate of the wind and of the fraction of dust on the synthesis of the molecules and on the attenuation of the radiation field is studied in detail. Results. We show that a small fraction of H 2 (≤ 10 −2 ), primarily formed through the H − route, can efficiently initiate molecule synthesis such as CO and SiO above T K ∼ 800 K. We also propose new gas-phase formation routes of H 2 that can operate in strong visible radiation fields, involving for instance CH + . The attenuation of the radiation field by atomic species (eg. C, Si, S) proceeds through continuum self-shielding. This process ensures efficient formation of CO, OH, SiO, H 2 O through neutral-neutral reactions, and the survival of these molecules. Class 0 dust-free winds with high mass-loss rates (Ṁ w ≥ 2 × 10 −6 M yr −1 ) are predicted to be rich in molecules if warm (T K ≥ 800 K). Interestingly, we also predict a steep decrease in the SiO-to-CO abundance ratio with the decline of mass-loss rate, from Class 0 to Class I protostars. The molecular content of disk winds is very sensitive to the presence of dust and a mass-fraction of surviving dust as small as 10 −5 significantly increases the H 2 O and SiO abundances. Conclusions. Chemistry of high velocity jets is a powerful tool to probe their content in dust and uncover their launching point. Models of internal shocks are required to fully exploit the current (sub-)millimeter observations and prepare future JWST observations.

Self-generated ultraviolet radiation in molecular shock waves

“…where h is Planck's constant and σ ν is the frequency dependent cross-section. We use the cross-sections of Heays et al (2017) rescaled onto a smaller wavelength grid by Tabone et al (2020) for the most important photoreactions, the photo-ionisation of C, S, and Si, and both photo-ionisation and -dissociation of CH, OH, H 2 O, O 2 , and C 2 . For all other photoreactions, we use the form…”

Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy

“…One explanation for L1448-MM is that the jet gas is rich in atomic Si originating from its dust-free launch position in the inner disk. This then leads to SiO formation in the jet by gas-phase processes, a mechanism proposed originally by Glassgold et al (1991) and revisited theoretically by Tabone et al (2020), not because of sputtering of ambient dust grains. Whether SiO is also seen in the broad component then depends on the details of the wind-cloud interaction.…”

Which molecule traces what: chemical diagnostics of protostellar sources

“…The fact that the EHV jet tracers are dominated by O-bearing molecules has been associated with a low C/O ratio in the jet material (Tafalla et al 2010). For high mass-loss rates, molecules are produced efficiently in the jet from the launched atomic material (Glassgold et al 1991;Raga et al 2005;Tabone et al 2020). Additionally the ratio of SiO-to-CO can indicate the presence of dust in the launched material, which can in turn inform about the jet launching radius, i.e., whether it is inside or outside the dust sublimation radius (Tabone et al 2020).…”

“…For high mass-loss rates, molecules are produced efficiently in the jet from the launched atomic material (Glassgold et al 1991;Raga et al 2005;Tabone et al 2020). Additionally the ratio of SiO-to-CO can indicate the presence of dust in the launched material, which can in turn inform about the jet launching radius, i.e., whether it is inside or outside the dust sublimation radius (Tabone et al 2020). The new detections of high-velocity jets suggest that this process may be occurring in every young Class 0 object.…”