2023.6.28 highlights

Discovery of a new type of carbon-chain chemistry around massive young stellar objects – Hot Carbon-Chain Chemistry (HCCC) –

Carbon-chain molecules were classically known as “early-type specie”, because their abundances are high in starless cores, but become lower in evolved star-forming cores. However, it was proposed that carbon-chain molecules are formed in lukewarm gas (T ≈ 25 – 35 K) around low-mass protostars. In these regions, a reaction between CH4, which evaporates from dust grains, and C+ is a trigger of carbon-chain formation. This mechanism was named Warm Carbon-Chain Chemistry (WCCC). On the other hand, the carbon-chain chemistry around massive young stellar objects (MYSOs) has been unveiled so far.

An international team, including NAOJ, Max Planck Institute for Extraterrestrial Physics (Germany), University of Virginia (USA), has investigated chemical differentiation using molecular lines from carbon- chain species (HC3N, HC5N, CCH), oxygen (O)-bearing complex organic molecules (COMs), such as CH3OH and CH3OCH3, and nitrogen (N)-bearing COMs (CH3CN, CH2CHCN, CH3CH2CN) around five MYSO using ALMA Band 3 data. The HC5N (J = 35-34) line has been detected from three MYSOs, where two complex N- bearing COMs (CH2CHCN and CH3CH2CN) have been detected. Spatial distributions are compared among different types of molecules, and examples are shown in Figure 1. Emission peaks of the HC5N are associated with the continuum cores and consistent with peaks of the CH3OH line (Eup/k = 302.9 K). These results indicate that HC5N emission comes from hot (T ≥ 100 K) regions in which COMs evaporate into the gas phase from ice mantles.

Physical parameters, such as bolometric luminosity and temperature, were derived with the modified blackbody model using infrared data obtained by Spitzer and Herschel. There are no significant differences in luminosity and temperature between the three MYSOs associated with HC5N and the other two MYSOs without it. On the other hand, it has been found that the solid angle ratios between warm and cold components are higher in the three MYSOs associated with HC5N compared to the other two MYSOs. These results mean that HC5N has been detected from the more evolved hot cores with extended hot regions.

We have compared the results of observations and chemical simulations. The observed abundances of HC5N and O-/N-bearing COMs can be reproduced by the chemical simulations at the hot-core stage (T≈ 160-200 K). Hence, all of the results obtained by observations and simulations indicate that HC5N exists in hot regions in the case of MYSOs, and WCCC cannot explain the results around MYSOs.

Based on these results, we propose a new carbon-chain chemistry and name it Hot Carbon-Chain Chemistry (HCCC). The HCCC mechanism is essential for relatively stable species such as cyanopolyynes (HC2n+1N, n = 1,2,3,…). Figure 2 shows the carbon-chain chemistry occurring around MYSOs. HCCC refers to the following processes starting from the lukewarm region to the hot-core regions: cyanopolyynes are formed in the gas phase by neutral-neutral reactions between C2nH2 and CN, adsorbed onto dust grains and accumulated in the ice mantles during the warm-up stage (25 K < T < 100 K). When the temperature reaches above 100 K, cyanopolyynes evaporate into the gas phase and show their peak abundances.

The paper information

Title: Chemical Differentiation around Five Massive Protostars Revealed by ALMA -Carbon-Chain Species, Oxygen-/Nitrogen-Bearing Complex Organic Molecules-

Kotomi Taniguchi et al 2023 ApJS 267 4

Link for the paper; https://iopscience.iop.org/article/10.3847/1538-4365/acd110

arXiv: https://arxiv.org/abs/2304.13873

Figure 1. Comparisons of spatial distributions among CH3CN vs. CH3OH (left) and HC5N vs. CH3OH (right) toward the three MYSOs (G9.62+0.19, G10.47+0.03, and G12.89+0.49). The color scale shows the continuum emission and the white contours indicate the moment 0 maps of CH3OH. The black contours indicate the moment 0 maps of CH3CN (left) and HC5N (right). The ellipses indicate the angular resolutions: an open one corresponds to the continuum image, and white and black ones correspond to the moment 0 maps of CH3OH and the other two species, respectively.
Figure 2. A schematic view of carbon-chain chemistry around MYSOs, which is taken from Taniguchi et al. (2023) (https://arxiv.org/abs/2303.15769). WCCC refers to the carbon-chain formation in the lukewarm regions (25-35 K), while HCCC includes successive gas-phase formation, adsorption onto dust grains, accumulation in ice mantles (T < 100 K), and evaporation into the gas phase in hot regions (T > 100 K).