Cyanopolyyne Chemistry around Massive Young Stellar Objects

Abstract

Recent radio astronomical observations have revealed that HC 5 N, the second shortest cyanopolyyne (HC 2n+1 N), is abundant around some massive young stellar objects (MYSOs), which is not predicted by classical carbon-chain chemistry. For example, the observed HC 5 N abundance toward the G28.28−0.36 MYSO is higher than that in L1527, which is one of the warm carbon chain chemistry (WCCC) sources, by more than one order of magnitude . In this paper, we present chemical simulations of hot-core models with a warm-up period using the astrochemical code Nautilus. We find that the cyanopolyynes are formed initially in the gas phase and accreted onto the bulk and surface of granular ice mantles during the lukewarm phase, which occurs at 25 < T < 100 K. In slow warm-up period models, the peak abundances occur as the cyanopolyynes desorb from dust grains after the temperature rises above 100 K. The lower limits of the abundances of HC 5 N, CH 3 CCH, and CH 3 OH observed in the G28.28−0.36 MYSO can be reproduced in our hot-core models, after their desorption from dust grains. Moreover, previous observations suggested chemical diversity in envelopes around different MYSOs. We discuss possible interpretations of relationships between stages of the star-formation process and such chemical diversity, such as the different warm-up timescales. This timescale depends not only on the mass of central stars but also on the relationship between the size of warm regions and their infall velocity.

A spectral survey of CH3CCH in the Hot Molecular Core G331.512-0.103

“…Grain-surface reactions are also proposed in order to explain the observed abundances of CH 3 CCH (Hickson et al 2016;Guzmán et al 2018). Regarding massive environments, Taniguchi et al (2019) have constructed hot-core models to investigate the formation pathways of cyanopolyynes and other carbonchain species, including CH 3 CCH and c-C 3 H 2 , around MYSOs. They found chemical similarities between methyl acetylene, methane and cyanopolyynes, being all triggered by CH 4 sublimation from dust grains.…”

Clump-scale chemistry in the NGC2264-D cluster-forming region

“…The HC 3 N/CH 3 OH ratios in the n3 field have larger values than those in the n5 field systematically. The HC 3 N abundance is likely less sensitive to protostellar evolution compared to CH 3 OH, because HC 3 N is continually formed from CH 4 and/or C 2 H 2 , which are sublimated from dust grains, in warm or hot regions (25 K < T < 100 K) (e.g., Taniguchi et al 2019a). Thus, these results imply that CH 3 OH is efficiently sublimated from dust grains in the n5 field.…”

“…1). The n3 position is located at the northern edge of NGC 2264-D and the integrated intensity ratio of HC 3 N and CH 3 OH, I(HC 3 N)/I(CH 3 OH), at this position shows the highest value among the other positions in NGC 2264-C and NGC 2264-D. A variety in the I(HC 3 N)/I(CH 3 OH) ratio implies a chemical differentiation in the clusterforming region, suggestive of different evolutionary histories and/or different environments (Spezzano et al 2016;Taniguchi et al 2019a;Spezzano et al 2020). The n3 position is located to the west of the IRS2 source (IRAS 06382+0939).…”

Chemical compositions in the vicinity of protostars in Ophiuchus

“…Recent observations have shown chemical differentiation not only around low-mass YSOs, but around high-mass YSOs as well (Taniguchi et al 2018(Taniguchi et al , 2019b(Taniguchi et al , 2021a. Although the origin of the chemical differentiation around YSOs is still controversial, three possible factors have been proposed: the different timescale of the prestellar collapse (Sakai et al 2008), the different ultraviolet (UV) radiation field (Spezzano et al 2016), and the different timescale of the warm-up stage (Taniguchi et al 2019a). In order to reveal the effects of the above factors, we need to investigate molecular spatial distributions on large scales (e.g., the clump scale).…”