Comprehensive Study of Thermal Desorption of Grain-surface Species by Accretion Shocks around Protostars

Abstract

We conducted numerical simulations of the dust heating in accretion shocks induced by the interaction between the infalling envelope and the Keplerian disk surrounding a protostar, in order to investigate the thermal desorption of molecules from the dust-grain surfaces. It is thought that the surfaces of the amorphous dust grains are inhomogeneous; various adsorption sites with different binding energies should therefore exist. We assumed that the desorption energy has a Gaussian distribution and investigated the effect of the desorption energy distribution on the desorption-efficiency evaluation. We calculated the desorption fractions of the grain-surface species for wide ranges of input parameters and summarized our results in a shock diagram. The resultingshock diagram suggests that the enhanced line emissions around protostars observed using the Atacama Large Millimeter Array cannot be explained by the thermal desorption in an accretion shockif typical interstellar dust-grain sizes ( 0.1 m m ) and a single desorption energy are considered. On the other hand, if significantly smaller dust grains are the main grain-surface species carriers and the desorption energy has a Gaussian distribution, the origin of the enhanced line emission can be explained by the accretion shock heating scenario for all of the three protostars examined in this study: IRAS 04368+2557, IRAS 04365+2535, and IRAS 16293-2422. The small-grain-carrier supposition is quite reasonable when the dust grains have a power-law size distribution because the smaller grains primarily contribute to the dust-grain surface area.

Questioning the spatial origin of complex organic molecules in young protostars with the CALYPSO survey

“…An enhanced desorption of COMs in accretion shocks at the centrifugal barrier has been proposed by Oya et al (2016) and Csengeri et al (2018) to explain the results of observations performed with ALMA toward IRAS16293A and the high-mass protostar G328.2551-0.5321, respectively. Theoretical calculations show that this is indeed a possible mechanism if the grain population carrying the adsorbed molecules is dominated by small grains of size ∼0.01 µm (Miura et al 2017). If we assume that each protostellar disk has an accretion shock at the radius of its centrifugal barrier, and that this shock should lead to the desorption of the COMs formed in the ice mantles of dust grains then we would expect a strong correlation between the detection of a disk and the detection of COM emission, as well as a good match between the disk radius and the radius of the COM emission.…”

“…The lack of COM emission in sources with detected and resolved disk-like structures (L1527, SerpM-SMM4, SerpS-MM22, GF9-2) questions the accretion shock scenario as a general mechanism for the release of COMs in the gas phase around protostars. Specific shock parameters may be required for this process to be efficient (see, e.g., Miura et al 2017). However, methanol emission was detected with a size (FWHM) of about 1 ′′ by Sakai et al (2014b) toward L1527 with ALMA, which roughly corresponds to the radius of the centrifugal barrier (∼100 au) determined by Sakai et al (2014a) in this source.…”

Physical and chemical fingerprint of protostellar disc formation

“…Material from the inner envelope falls onto the circumstellar disc and produces accretion shocks at the envelope-disc interface, releasing molecules from the dust grains and altering the chemistry. Miura et al (2017) investigated the thermal desorption of molecules from the dust-grain surface by accretion shocks and found that the enhancement of some species (such as SO) can be explained by the accretion shock scenario, where the main parameters are the grain size, the pre-shock gas number density, and the shock velocity. Taking a shock velocity of 10 km s −1 , Miura et al (2017) predicted that SO 2 can be released from the dust-grain surface for a pre-shock gas number density of ∼10 7 cm −3 .…”

Modeling C-shock Chemistry in Isolated Molecular Outflows

“…To accurately simulate the chemistry within shocked media, the nautilus code and KIDA-2014 network needed to be enhanced to include a number of additional shock-related processes and reactions. These include additional high-temperature reactions from Harada et al (2010) and Garrod (2013), the physical structure and sputtering processes as described by Jiménez-Serra et al (2008), and the evolution of the dust temperature as described by Miura et al (2017), of which all will be described in detail in the subsequent sections. The high degree of chemical complexity within the network used, especially for the ice chemistry, allows us to simulate the chemistry of much larger COMs than previous models (Lesaffre et al 2013;Dzyurkevich et al 2017;Codella et al 2017).…”

“…In order to accurately assess the significance of enhancement through desorption versus chemical processing in shocks, it is crucial to consider the effects of the dust temperature on the system. Therefore, we adopted the formalism from Miura et al (2017) and Aota et al (2015), which considers the effects from the adiabatic heating of high-velocity particle collisions and the conductive heating from the dust residing in hot gas.…”