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Revealing the chemical composition and its evolution in high-mass star-forming regions

[Outline]
A research group led by Kotomi Taniguchi (research associate at the University of Virginia), including scientists from the National Astronomical Observatory of Japan (NAOJ) and Harvard-Smithsonian Center for Astrophysics, has investigated the initial chemical composition and chemical evolution in high-mass star-forming regions, where stars with more than 8 solar masses are born. The group found a useful combination of molecules to investigate these regions. In particular, the column density ratio decreases from starless cores to star-forming cores. In addition, this N(N2H+)/N(HC3N) ratio can be used to find newborn massive protostars, which are still surrounded by thick gas and dust. The decrease in the ratio is opposite to that found in low-mass star-forming regions. This can be explained by the presence of molecules which are sublimated from dust grains. Our results suggest that such molecules sublimated from dust grains play essential roles in the gas-phase reactions even in the early stage of massive star formation.
These results have been published in the February 20 issue of the American astrophysics journal, The Astrophysical Journal.

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Figure 1.Artist’ image showing that molecules sublimated from dust grains affect chemical reactions in the gas phase around high-mass protostellar object. (enlarged view)

[Scientific Background]
High-mass stars are defined as stars with more than 8 solar masses. High-mass star-forming regions are farther from our Sun than low-mass star-forming regions and high-mass stars evolve faster than low-mass ones. Thus, it is difficult to investigate high-mass star formation mechanisms by observations and there still remain many questions regarding their formation. These stars are usually born in cluster regions. Recent research has shown that our Sun was born in such a cluster region. Therefore, studies investigating the evolution of high-mass star-forming regions will lead to better understanding of the formation of the solar system.
Radio telescopes can detect molecular emission lines, which can be used to derive the chemical composition of the gas in a given source. The chemical composition evolves with the formation of stars and includes information about past conditions. Hence, revealing the initial chemical composition and chemical evolution in high-mass star-forming regions can be related to studies on formation mechanisms of organic molecules, including amino acids, which have been detected in meteorites and comets in the solar system.

[Contents of Research and Results]
Using the Nobeyama 45 m radio telescope, we have carried out survey observations toward high-mass starless cores (HMSCs) and high-mass protostellar objects (HMPOs), which were previously identified by infrared observations. Carbon-chain molecules, which are unique species in the interstellar medium, have been known as early type species in low-mass star-forming regions, because they are abundant in young starless cores and decrease in abundance in star-forming cores. We decided to observe carbon-chain molecules because we assumed that they are suitable species for investigating the initial chemical composition even in high-mass star-forming regions. We have observed HC3N, CCS, and cyclic-C3H2. Unlike these species, N2H+ increases from starless cores to star-forming cores in low-mass star-forming regions. We then have carried out observations of N2H+, because of the possibility that we might find good chemical evolutionary indicators in high-mass star-forming regions combining N2H+ with carbon-chain species. Our results are the deepest survey observations of carbon-chain molecules in high-mass star-forming regions. We found that the column density ratio of N(N2H+)/N(HC3N) is a good chemical evolutionary indicator in high-mass star-forming regions (Figure 2). The N(N2H+)/N(HC3N) ratio decreases from HMSCs to HMPOs. Moreover, we can probably find newborn massive protostars, which cannot be recognized by infrared observations due to thick gas and dust surrounding the protostars. The evolutionary decrease in the N(N2H+)/N(HC3N) ratio is opposite to that found in low-mass star-forming regions. Our research group can explain the difference between high-mass and low-mass star-forming regions, taking the different physical conditions between these regions into consideration. The decrease in the N(N2H+)/N(HC3N) ratio indicates that HC3N is formed and N2H+ is destroyed. Molecules subliming from dust grains are considered to lead to HC3N formation and N2H+ destruction. Methane (CH4) and acetylene (C2H2), which are sublimated from dust grains, can efficiently form HC3N in the gas phase. CO molecules sublimate from dust surfaces with dust temperatures above 20 K and react with N2H+ to destroy it. Hence, molecules evaporated from dust grains play essential roles in the gas-phase reactions just after the massive protostars are born, which leads to the opposite tendency in high-mass star-forming regions compared to low-mass star-forming regions.

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Figure 2. Chemical evolutionary indicator in high-mass star-forming regions. The vertical and horizontal axes show the HC3N column density, N(HC3N), and the column density ratio of N(N2H+)/N(HC3N), respectively. The plot of red open circle means that IRAS positions were not at exact continuum peak positions, but the beam covered the continuum core in the beam edge. The plot of blue diamond indicates that the infrared observations were recognized as coming from an HMSC but molecules frequently detected in star-forming cores (CH3OH and CH3CN) or SiO, a molecular outflow tracer, have been detected. These sources appear to contain newborn massive stars in the dense cores but cannot be found by infrared observations due to surrounding thick gas and dust.

[Future Work]
In the group of Professor Eric Herbst at the University of Virginia, we are currently investigating the relationship between massive star formation and abundances of carbon-chain molecules with chemical simulations involving large networks of chemical reactions. In addition, we will carry out survey observations to study the chemical composition toward Herbig Ae/Be stars, which are intermediate-mass protostars, with the Nobeyama 45-m telescope. From these studies, we will reveal the effects not only of the mass of central stars but also of surrounding physical conditions on the chemical composition. Such studies will help to reveal the initial condition of molecular clouds where our Sun was born and how the solar system developed.

[Members]
Kotomi Taniguchi (Departments of Astronomy and Chemistry, University of Virginia / Research associate, Virginia Initiative on Cosmic Origins Fellow)
Masao Saito (National Astronomical Observatory of Japan, TMT project / Professor)
T. K. Sridharan (Harvard-Smithsonian Center for Astrophysics)
Tetsuhiro Minamidani (Nobeyama Radio Observatory / Assistant Professor)

These results have been published in the February 20 issue of The Astrophysical Journal.
Taniguchi et al., "Survey Observations to Study Chemical Evolution from High-Mass Starless Cores to High-Mass Protostellar Objects II. HC3N and N2H+"
The Astrophysical Journal, 872:154 (23pp), 2019 February 20
doi: 10.3847/1538-4357/ab001e

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