A Search for Chemical Evolutional Indicators in High-Mass Star-Forming Regions
K. Taniguchi,1 M. Saito,2,3 T. K. Sridharan4 and T. Minamidani3,5
1Departments of Astronomy and Chemistry, University of Virginia, USA
2TMT-J Project, National Astronomical Observatory of Japan, Japan
3Department of Astronomical Science, SOKENDAI, Japan
4
Harvard-Smithsonian Center for Astrophysics, USA
5Nobeyama Radio Observatory, National Astronomical Observatory of Japan, Japan

Carbon-chain molecules have been known as good chemical evolutional indicators in lowmass star-forming regions (e.g., [1],[2]). They are abundant in young starless cores anddecrease in star-forming cores. On the other hand, chemical evolutional indicators have notbeen clearly established in high-mass star-forming regions, nevertheless there were many
attempts (e.g., [3]).
We have carried out survey observations of HC3N, HC5N, CCS, cyclic-C3H2, and N2H+
toward 17 high-mass starless cores (HMSCs)[4] and 28 high-mass protostellar objects(HMPOs)[5] using the Nobeyama 45-m radio telescope. The main purpose of this survey
project is to find good chemical evolutional indicators, which enable us to find the very earlystage of high-mass protostars containing the initial conditions of massive star formation. We
investigated several molecular combinations and found that the N(N2H+)/N(HC3N) ratio is agood candidate for chemical evolutional indicators in high-mass star-forming regions. Figure
1 (a) shows the relationship between the column density ratio of N(N2H+)/N(HC3N) and theHC3N column density in HMSCs and HMPOs. The ratio decreases from HMSC to HMPO.
Surprisingly, this tendency is opposite to that in low-mass star-forming regions as shown
Figure 1 (b). One possible explanation for the difference between high-mass and low-massstar-forming regions is the higher temperature in high-mass star-forming regions; CH4 and/or
C2H2 evaporated from grain mantles form HC3N, whereas N2H+ is destroyed by CO
molecules liberated from dust.
(a) High-Mass Star-Forming Regions

1e+0141014

N(HC3N) [cm-2]

HMSC associated with hotcore species and/or SiO
HMSC

1012
1e+012Evolu

11

N[CCS] (1012 cm-2)

HMPOOff-HMPO

1e+0131013

1011
1e+011(b) Low-Mass Star-Forming Regions
100Evolu

10tio
n1tion1010+N(N2H )/N(HC3N)

100
100

0.1
1101001000
10000
N[NH3]/N[CCS]
T. Hirota et al. (2009) [2]Figure 1: Chemical evolutional indicators in (a) high-mass star-forming regions and (b) low-mass starforming regions. (a) Off-HMPO means that the center positions of the telescope were off from the 1.2 mm
dust continuum emission peaks.

In addition, HMSCs, which were classified based on the mid-infrared (8.3 μm) observations,associated with CH3OH, CH3CN, and/or SiO are plotted between HMSCs and HMPOs. Thissuggests that we can find very-early-stage high-mass protostars embedded within dense cores,using the N(N2H+)/N(HC3N) ratio.
We compare the detection rates of carbon-chain molecules between HMPOs and low-mass
protostars[6]. The detection rates of cyanopolyynes in HMPOs (93% for HC3N and 50% forHC5N[7]) are higher than those in low-mass protostars (75% and 31%, respectively), whereas
CCS has been more frequently detected in low-mass protostars (88%) compared to HMPOs(46%). These results imply that carbon-chain chemistry around protostars is different between
high-mass and low-mass protostars.

References
[1]
[2]
[3]
[4]
[5]
[6]
[7]

T. Suzuki, S. Yamamoto, M. Ohishi et al., 1992, ApJ, 392, 551.
T. Hirota, M. Ohishi, and S. Yamamoto, 2009, ApJ, 699, 585.
T. Sakai, N. Sakai, K. Kamegai et al., 2008, ApJ, 678, 1049.
T. K. Sridharan, H. Beuther, M. Saito, F. Wyrowski, and P. Shilke, 2005, ApJ, 634, L57.
T. K. Sridharan, H. Beuther, P. Schilke, K. M. Menten, and F. Wyrowski, 2002, ApJ, 566, 931.
C. J. Law, K. I. Oberg, J. B. Bergner and D. Graninger, 2018, ApJ, 863, 88.
K. Taniguchi, M. Saito, T. K. Sridharan, and T. Minamidani, 2018, ApJ, 854, 133.