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Satoshi Ohashi, Ken'ichi Tatematsu, Kosuke Fujii, Patricio Sanhueza, Quang Nguyen Luong, Minho Choi, Tomoya Hirota, Norikazu Mizuno, Chemical evolution of the HC3N and N2H+ molecules in dense cores of the Vela C giant molecular cloud complex, Publications of the Astronomical Society of Japan, Volume 68, Issue 1, February 2016, 3, https://doi.org/10.1093/pasj/psv104
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Abstract
We have observed the HC3N(J = 10–9) and N2H+ (J = 1–0) lines toward the Vela C molecular clouds with the Mopra 22 m telescope to study the chemical characteristics of dense cores. The intensity distributions of these molecules are similar to each other at an angular resolution of 53″, corresponding to 0.19 pc, suggesting that these molecules trace the same dense cores. We identified 25 local peaks in the velocity-integrated intensity maps of the HC3N and/or N2H+ emission. Assuming local thermodynamic equilibrium conditions, we calculated the column densities of these molecules and found a tendency for the N2H+/HC3N abundance ratio to be low in starless regions while it seems to be high in star-forming regions, similar to the tendencies in the NH3/CCS, NH3/HC3N, and N2H+/CCS abundance ratios found in previous studies of dark clouds and the Orion A giant molecular cloud (GMC). We suggest that carbon chain molecules, including HC3N, may trace chemically young molecular gas, and that N-bearing molecules, such as N2H+, may trace later stages of chemical evolution in the Vela C molecular clouds. It may be possible that the N2H+/HC3N abundance ratio of ∼1.4 divides the star-forming and starless peaks in Vela C, although it is not as clear as those in NH3/CCS, NH3/HC3N, and N2H+/CCS for the Orion A GMC. This less clear separation may be caused by our lower spatial resolution or the misclassification of star-forming and starless peaks due to the larger distance of Vela C. It might also be possible that the HC3N (J = 10–9) transition is not a good chemical evolution tracer compared with CCS (J = 4–3 and 7–6) transitions.
1 Introduction
In nearby cold dark clouds, carbon chain molecules such as HC3N and CCS, and N-bearing molecules such as NH3 and N2H+, are thought to be good tracers of chemical evolution (e.g., Hirahara et al. 1992; Suzuki et al. 1992; Benson et al. 1998; Maezawa et al. 1999; Hirota et al. 2002, 2009). Carbon chain molecules are produced in chemically young evolutionary stages by ion–molecular reactions and depleted through both gas-phase reactions and adsorption onto dust grains (Aikawa et al. 2001; Hirota et al. 2010). On the other hand, N2H+ is produced in later evolutionary stages and is hardly depleted compared with other species because the nitrogen molecule N2 (the precursor of N2H+) is produced in late stages of the gas phase chemistry (Aikawa et al. 2001). Therefore, it is suggested that carbon chain molecules, CCS and HC3N, trace the early chemical evolutionary stage, whereas N-bearing molecules, NH3 and N2H+, trace later stages of the chemical evolution. Suzuki et al. (1992) observed CCS (JN = 43–32, JN = 21–10), HC3N (J = 5–4), and NH3 (J, K = 1, 1) emission toward nearby cold dark cores and found that CCS and HC3N are abundant in starless cores while NH3 is abundant in star-forming cores. Benson, Caselli, and Myers (1998) observed CCS (JN = 43–32) and N2H+ (J = 1–0) emission toward dense cores in nearby cold dark clouds, and showed that the column density of CCS is anticorrelated with that of N2H+ among the cores. Therefore, it is established that the column density ratios of N-bearing molecules to carbon chain molecules can be indicators of chemical evolution in cold dark clouds.
On the other hand, the chemical evolution of molecular dense cores located in giant molecular clouds (GMCs) has not been extensively explored in comparison with that of nearby cold dark clouds. The dense cores in nearby dark clouds are known to be cold (∼10 K), less turbulent, and show isolated low mass star-forming regions, while those in GMCs are warmer, turbulent, and often show the formation of star clusters. Because most stars in the Galaxy are formed in GMCs, it is of great interest to know whether or not these molecules are indicators of chemical evolution in GMCs. It has been shown that the column density ratios of NH3/HC3N, NH3/CCS, and N2H+/CCS are low in starless regions, while they are high in the star-forming regions in the Orion A GMC (Ohashi et al. 2014; Tatematsu et al. 2014a). This suggests that these molecules may be indicators of the chemical evolution in the Orion A GMC. Tatematsu et al. (2014b) observed CCS and NH3 emission toward a starless core in the Orion A GMC with the Very Large Array, and revealed that the CCS emission surrounds the NH3 core. This configuration resembles that of the N2H+ and CCS distribution in the Taurus L1544 prestellar core showing a collapsing motion. It remains to be established whether these molecules can generally be indicators of chemical evolution even in GMCs by investigating dense cores located in other GMCs.
We focus on one of the near GMCs located in the southern hemisphere, the Vela C molecular cloud complex. The Vela molecular clouds, or “Vela Molecular Ridge” (Murphy & May 1991), is one of closest GMCs within the Galactic plane, and consists of four components labelled A through D. The Vela C giant molecular cloud complex is the most massive component of the Vela region, and accompanies a bright H ii region, RCW 36, associated with an early type star (spectral type O5-B0: Massi et al. 2003). The distance is 700 pc (Liseau et al. 1992). Yamaguchi et al. (1999) mapped 12CO, 13CO, and C18O (J = 1–0) lines in Vela C, and detected molecular outflows. By Herschel PACS and SPIRE observations of 70, 160, 250, 350, and 500 μm dust continuum emissions, Hill et al. (2011) identified high column density filaments over the whole region, and showed differences in the column density and temperature probability distribution function (PDF) among regions. They revealed that the column density PDF of the “Centre-Ridge region,” which contains a high-mass star, is flatter than other regions, and shows a high column density tail.
In this study, we investigate the chemical characteristics of molecular cloud cores in the Vela C giant molecular cloud complex through mapping observations of HC3N and N2H+. The purpose of this study is to investigate whether HC3N (a carbon chain molecule) and N2H+ (an N-bearing molecule) are indicators of chemical evolution in a GMC.
2 Observations
Observations were carried out with the Mopra 22 m telescope located in Australia from 2014 May 10 to 25. We used the 3 mm MMIC receiver that can simultaneously record dual polarization data. We simultaneously observed HC3N (J = 10–9) at 90.978989 GHz and N2H+ (J = 1–0) at 93.1737767 GHz. The spectrometer employed was the Mopra Spectrometer (MOPS) digital filter bank in the zoom-band mode. The spectral resolution was 33.57 kHz (∼0.11 km s−1). At 90 GHz, the half-power beam width (HPBW) and “extended beam efficiency” of the telescope are 42″ and 50%, respectively (Ladd et al. 2005). We observed source 81 of the Vela-D molecular cloud identified by Morales Ortiz et al. (2012) once a day to check the stability of the intensity calibration. The absolute intensity accuracy was estimated to be less than 10%. We mapped 5′ × 5′ box areas in the on-the-fly (OTF) mode. We observed a total of seven regions, which were selected from bright C18O emission peaks. The C18O (J = 1–0) observations were previously carried out by Yamaguchi et al. (1999) with 2′ resolution. Figure 1 shows the locations of the observed areas superimposed on a Herschel SPIRE 500 μm map. The black boxes indicate the OTF mapping area and white contours represent the N2H+J = 1–0 velocity-integrated intensity map. The obtained data were smoothed to a HPBW of 53″ with a 2D Gaussian function in order to improve the signal-to-noise ratio. The angular resolution is equivalent to 0.19 pc at a distance of 700 pc. It is worth noting that this resolution will identify objects that will form groups of stars. The typical rms noise level is about 0.08 K per channel. The telescope pointing was checked every 1.0 hr by observing the SiO maser source L2 Pup [RA, Dec(J2000.0) = 7h13m32|${^{\rm s}_{.}}$|31, −44°38′24|${^{\prime\prime}_{.}}$|1]. The pointing accuracy was better than 10″. The upper energy levels (Eu/k) for HC3N (J = 10–9) and N2H+ (J = 1–0) transitions are 24.01 and 4.47 K, respectively. The critical densities for these transitions are 3 × 105 and 5 × 105 cm−3 at 20 K, respectively (Sanhueza et al. 2012). However, Shirley (2015) suggested that N2H+ is excited at lower densities than the critical density, and the effective excitation densities may be lower than 5 × 105 cm−3.
The observed data were reduced using “Livedata” and “Gridzilla,” which are the most typical data reduction packages for the Mopra telescope (Barnes et al. 2001).1 “Livedata” fits and subtracts a linear baseline. The output of “Livedata” is recorded in single-dish fits format (sdfits). “Gridzilla” uses this output to regrid data onto a data cube using Gaussian smoothing. The final data cubes were smoothed to 15″ grid size.
3 Results and discussion
3.1 Integrated intensity maps
Figures 2 through 8 show the integrated intensity maps in |$T_{\rm A}^*$| scale, with the HC3N J = 10–9 contour maps superimposed on the N2H+J = 1–0 grayscale images. We plot the location of protostars identified by thermal emission at five wavebands from 70 to 500 μm using PACS and SPIRE cameras on board the Herschel Space Observatory (Giannini et al. (2012); T. Giannini 2015 private communication for the protostar coordinates). We also plot protostar candidates identified by the near-infrared (J, H, Ks) survey carried out by Baba et al. (2006) and the AKARI-FIS Point Source Catalogue, which are Class 0 protostellar candidates (Sunada et al. 2009). We identified 25 local peaks in the velocity-integrated intensity maps of the HC3N J = 10–9 and/or N2H+J = 1–0 emission in these seven regions. The criteria for identification are more than five sigma above the noise level for N2H+ and/or three sigma above the noise level for HC3N. If the HC3N peak is within the HPBW of the N2H+ peak, we use the HC3N peak position. The identification of N2H+ emission peaks is more strict so that we can perform hyperfine fitting to satellite components. In table 1, we list the positions of the detected HC3N J = 10–9 and/or N2H+J = 1–0 local peaks in the velocity-integrated intensity map and the line parameters of the HC3N spectra measured toward these positions. The peak intensities and linewidths are obtained through Gaussian fitting. If local intensity peaks coincide with the protostars within 53″, we define the peaks as star-forming cores. If local intensity peaks do not coincide with the protostars, we define the peaks as starless cores. A total of 12 peak positions were identified as star-forming. We assume that both lines trace sufficiently dense gas, and the starless cores identified in these lines will eventually form protostars. We found that the distributions of the HC3N and N2H+ velocity-integrated intensities are similar to each other, suggesting that these molecular lines trace the same dense cores. Taking into account the fact that the scale of different distributions between HC3N and N2H+ occurs at 0.1 pc scales in the OMC-3 region (Tatematsu et al. 2008), less prominent differences of intensity distributions would be due to our larger spatial resolution of 0.19 pc.
Peak position . | l . | b . | |$T_{\rm A}^*$| . | v LSR(HC3N) . | dv(HC3N) . | Protostar . |
---|---|---|---|---|---|---|
name . | (°) . | (°) . | (K) . | (km s−1) . | (km s−1) . | . |
1 | 264.161 | 1.986 | ||||
2 | 264.140 | 1.960 | 0.41 ± 0.06 | 6.6 ± 0.1 | 1.00 ± 0.18 | P1 |
3 | 264.113 | 1.969 | 0.20 ± 0.07 | 6.8 ± 0.1 | 0.75 ± 0.31 | P2 |
4 | 264.086 | 1.944 | 0.22 ± 0.05 | 6.5 ± 0.3 | 1.10 ± 0.59 | |
5 | 264.148 | 1.926 | 0.26 ± 0.06 | 6.9 ± 0.1 | 0.66 ± 0.15 | P3 |
6 | 264.31|$\phantom{0}$| | 1.733 | 0.30 ± 0.07 | 7.1 ± 0.1 | 0.83 ± 0.21 | |
7 | 264.292 | 2.006 | 0.16 ± 0.07 | 7.2 ± 0.2 | 0.70 ± 0.35 | P4 |
8 | 264.304 | 1.725 | 0.23 ± 0.06 | 6.6 ± 0.1 | 0.99 ± 0.31 | P5 |
9 | 264.299 | 1.746 | P6 | |||
10 | 264.279 | 1.733 | 0.24 ± 0.09 | 6.8 ± 0.1 | 0.37 ± 0.21 | |
11 | 264.265 | 1.67|$\phantom{0}$| | 0.21 ± 0.07 | 6.9 ± 0.1 | 0.77 ± 0.30 | |
12 | 264.975 | 1.633 | P8 | |||
13 | 265.015 | 1.618 | 0.07 ± 0.06 | 7.0 ± 0.4 | 1.02 ± 1.00 | |
14 | 264.965 | 1.587 | 0.29 ± 0.06 | 6.4 ± 0.1 | 0.99 ± 0.24 | P11 |
15 | 264.957 | 1.601 | 0.26 ± 0.06 | 6.4 ± 0.1 | 1.00 ± 0.27 | |
16 | 265.138 | 1.439 | 0.35 ± 0.05 | 8.3 ± 0.1 | 1.37 ± 0.71 | |
265.138 | 1.439 | 0.24 ± 0.03 | 6.6 ± 0.2 | 2.29 ± 0.34 | ||
17 | 265.152 | 1.437 | 0.32 ± 0.04 | 7.1 ± 0.1 | 2.05 ± 0.32 | P12 |
18 | 265.167 | 1.433 | 0.29 ± 0.04 | 7.2 ± 0.2 | 2.14 ± 0.34 | P13 |
19 | 265.336 | 1.392 | 0.33 ± 0.06 | 6.5 ± 0.1 | 1.18 ± 0.23 | P14 |
20 | 265.292 | 1.432 | 0.13 ± 0.09 | 6.8 ± 0.1 | 0.85 ± 0.24 | P15 |
21 | 266.212 | 0.888 | 0.36 ± 0.08 | 4.9 ± 0.1 | 0.60 ± 0.14 | |
22 | 266.245 | 0.871 | 0.23 ± 0.08 | 5.1 ± 0.1 | 0.67 ± 0.26 | |
23 | 266.285 | 0.918 | ||||
24 | 266.279 | 0.928 | 0.29 ± 0.06 | 4.5 ± 0.1 | 1.24 ± 0.27 | |
25 | 266.278 | 0.941 | 0.22 ± 0.05 | 4.8 ± 0.2 | 1.40 ± 0.37 |
Peak position . | l . | b . | |$T_{\rm A}^*$| . | v LSR(HC3N) . | dv(HC3N) . | Protostar . |
---|---|---|---|---|---|---|
name . | (°) . | (°) . | (K) . | (km s−1) . | (km s−1) . | . |
1 | 264.161 | 1.986 | ||||
2 | 264.140 | 1.960 | 0.41 ± 0.06 | 6.6 ± 0.1 | 1.00 ± 0.18 | P1 |
3 | 264.113 | 1.969 | 0.20 ± 0.07 | 6.8 ± 0.1 | 0.75 ± 0.31 | P2 |
4 | 264.086 | 1.944 | 0.22 ± 0.05 | 6.5 ± 0.3 | 1.10 ± 0.59 | |
5 | 264.148 | 1.926 | 0.26 ± 0.06 | 6.9 ± 0.1 | 0.66 ± 0.15 | P3 |
6 | 264.31|$\phantom{0}$| | 1.733 | 0.30 ± 0.07 | 7.1 ± 0.1 | 0.83 ± 0.21 | |
7 | 264.292 | 2.006 | 0.16 ± 0.07 | 7.2 ± 0.2 | 0.70 ± 0.35 | P4 |
8 | 264.304 | 1.725 | 0.23 ± 0.06 | 6.6 ± 0.1 | 0.99 ± 0.31 | P5 |
9 | 264.299 | 1.746 | P6 | |||
10 | 264.279 | 1.733 | 0.24 ± 0.09 | 6.8 ± 0.1 | 0.37 ± 0.21 | |
11 | 264.265 | 1.67|$\phantom{0}$| | 0.21 ± 0.07 | 6.9 ± 0.1 | 0.77 ± 0.30 | |
12 | 264.975 | 1.633 | P8 | |||
13 | 265.015 | 1.618 | 0.07 ± 0.06 | 7.0 ± 0.4 | 1.02 ± 1.00 | |
14 | 264.965 | 1.587 | 0.29 ± 0.06 | 6.4 ± 0.1 | 0.99 ± 0.24 | P11 |
15 | 264.957 | 1.601 | 0.26 ± 0.06 | 6.4 ± 0.1 | 1.00 ± 0.27 | |
16 | 265.138 | 1.439 | 0.35 ± 0.05 | 8.3 ± 0.1 | 1.37 ± 0.71 | |
265.138 | 1.439 | 0.24 ± 0.03 | 6.6 ± 0.2 | 2.29 ± 0.34 | ||
17 | 265.152 | 1.437 | 0.32 ± 0.04 | 7.1 ± 0.1 | 2.05 ± 0.32 | P12 |
18 | 265.167 | 1.433 | 0.29 ± 0.04 | 7.2 ± 0.2 | 2.14 ± 0.34 | P13 |
19 | 265.336 | 1.392 | 0.33 ± 0.06 | 6.5 ± 0.1 | 1.18 ± 0.23 | P14 |
20 | 265.292 | 1.432 | 0.13 ± 0.09 | 6.8 ± 0.1 | 0.85 ± 0.24 | P15 |
21 | 266.212 | 0.888 | 0.36 ± 0.08 | 4.9 ± 0.1 | 0.60 ± 0.14 | |
22 | 266.245 | 0.871 | 0.23 ± 0.08 | 5.1 ± 0.1 | 0.67 ± 0.26 | |
23 | 266.285 | 0.918 | ||||
24 | 266.279 | 0.928 | 0.29 ± 0.06 | 4.5 ± 0.1 | 1.24 ± 0.27 | |
25 | 266.278 | 0.941 | 0.22 ± 0.05 | 4.8 ± 0.2 | 1.40 ± 0.37 |
Peak position . | l . | b . | |$T_{\rm A}^*$| . | v LSR(HC3N) . | dv(HC3N) . | Protostar . |
---|---|---|---|---|---|---|
name . | (°) . | (°) . | (K) . | (km s−1) . | (km s−1) . | . |
1 | 264.161 | 1.986 | ||||
2 | 264.140 | 1.960 | 0.41 ± 0.06 | 6.6 ± 0.1 | 1.00 ± 0.18 | P1 |
3 | 264.113 | 1.969 | 0.20 ± 0.07 | 6.8 ± 0.1 | 0.75 ± 0.31 | P2 |
4 | 264.086 | 1.944 | 0.22 ± 0.05 | 6.5 ± 0.3 | 1.10 ± 0.59 | |
5 | 264.148 | 1.926 | 0.26 ± 0.06 | 6.9 ± 0.1 | 0.66 ± 0.15 | P3 |
6 | 264.31|$\phantom{0}$| | 1.733 | 0.30 ± 0.07 | 7.1 ± 0.1 | 0.83 ± 0.21 | |
7 | 264.292 | 2.006 | 0.16 ± 0.07 | 7.2 ± 0.2 | 0.70 ± 0.35 | P4 |
8 | 264.304 | 1.725 | 0.23 ± 0.06 | 6.6 ± 0.1 | 0.99 ± 0.31 | P5 |
9 | 264.299 | 1.746 | P6 | |||
10 | 264.279 | 1.733 | 0.24 ± 0.09 | 6.8 ± 0.1 | 0.37 ± 0.21 | |
11 | 264.265 | 1.67|$\phantom{0}$| | 0.21 ± 0.07 | 6.9 ± 0.1 | 0.77 ± 0.30 | |
12 | 264.975 | 1.633 | P8 | |||
13 | 265.015 | 1.618 | 0.07 ± 0.06 | 7.0 ± 0.4 | 1.02 ± 1.00 | |
14 | 264.965 | 1.587 | 0.29 ± 0.06 | 6.4 ± 0.1 | 0.99 ± 0.24 | P11 |
15 | 264.957 | 1.601 | 0.26 ± 0.06 | 6.4 ± 0.1 | 1.00 ± 0.27 | |
16 | 265.138 | 1.439 | 0.35 ± 0.05 | 8.3 ± 0.1 | 1.37 ± 0.71 | |
265.138 | 1.439 | 0.24 ± 0.03 | 6.6 ± 0.2 | 2.29 ± 0.34 | ||
17 | 265.152 | 1.437 | 0.32 ± 0.04 | 7.1 ± 0.1 | 2.05 ± 0.32 | P12 |
18 | 265.167 | 1.433 | 0.29 ± 0.04 | 7.2 ± 0.2 | 2.14 ± 0.34 | P13 |
19 | 265.336 | 1.392 | 0.33 ± 0.06 | 6.5 ± 0.1 | 1.18 ± 0.23 | P14 |
20 | 265.292 | 1.432 | 0.13 ± 0.09 | 6.8 ± 0.1 | 0.85 ± 0.24 | P15 |
21 | 266.212 | 0.888 | 0.36 ± 0.08 | 4.9 ± 0.1 | 0.60 ± 0.14 | |
22 | 266.245 | 0.871 | 0.23 ± 0.08 | 5.1 ± 0.1 | 0.67 ± 0.26 | |
23 | 266.285 | 0.918 | ||||
24 | 266.279 | 0.928 | 0.29 ± 0.06 | 4.5 ± 0.1 | 1.24 ± 0.27 | |
25 | 266.278 | 0.941 | 0.22 ± 0.05 | 4.8 ± 0.2 | 1.40 ± 0.37 |
Peak position . | l . | b . | |$T_{\rm A}^*$| . | v LSR(HC3N) . | dv(HC3N) . | Protostar . |
---|---|---|---|---|---|---|
name . | (°) . | (°) . | (K) . | (km s−1) . | (km s−1) . | . |
1 | 264.161 | 1.986 | ||||
2 | 264.140 | 1.960 | 0.41 ± 0.06 | 6.6 ± 0.1 | 1.00 ± 0.18 | P1 |
3 | 264.113 | 1.969 | 0.20 ± 0.07 | 6.8 ± 0.1 | 0.75 ± 0.31 | P2 |
4 | 264.086 | 1.944 | 0.22 ± 0.05 | 6.5 ± 0.3 | 1.10 ± 0.59 | |
5 | 264.148 | 1.926 | 0.26 ± 0.06 | 6.9 ± 0.1 | 0.66 ± 0.15 | P3 |
6 | 264.31|$\phantom{0}$| | 1.733 | 0.30 ± 0.07 | 7.1 ± 0.1 | 0.83 ± 0.21 | |
7 | 264.292 | 2.006 | 0.16 ± 0.07 | 7.2 ± 0.2 | 0.70 ± 0.35 | P4 |
8 | 264.304 | 1.725 | 0.23 ± 0.06 | 6.6 ± 0.1 | 0.99 ± 0.31 | P5 |
9 | 264.299 | 1.746 | P6 | |||
10 | 264.279 | 1.733 | 0.24 ± 0.09 | 6.8 ± 0.1 | 0.37 ± 0.21 | |
11 | 264.265 | 1.67|$\phantom{0}$| | 0.21 ± 0.07 | 6.9 ± 0.1 | 0.77 ± 0.30 | |
12 | 264.975 | 1.633 | P8 | |||
13 | 265.015 | 1.618 | 0.07 ± 0.06 | 7.0 ± 0.4 | 1.02 ± 1.00 | |
14 | 264.965 | 1.587 | 0.29 ± 0.06 | 6.4 ± 0.1 | 0.99 ± 0.24 | P11 |
15 | 264.957 | 1.601 | 0.26 ± 0.06 | 6.4 ± 0.1 | 1.00 ± 0.27 | |
16 | 265.138 | 1.439 | 0.35 ± 0.05 | 8.3 ± 0.1 | 1.37 ± 0.71 | |
265.138 | 1.439 | 0.24 ± 0.03 | 6.6 ± 0.2 | 2.29 ± 0.34 | ||
17 | 265.152 | 1.437 | 0.32 ± 0.04 | 7.1 ± 0.1 | 2.05 ± 0.32 | P12 |
18 | 265.167 | 1.433 | 0.29 ± 0.04 | 7.2 ± 0.2 | 2.14 ± 0.34 | P13 |
19 | 265.336 | 1.392 | 0.33 ± 0.06 | 6.5 ± 0.1 | 1.18 ± 0.23 | P14 |
20 | 265.292 | 1.432 | 0.13 ± 0.09 | 6.8 ± 0.1 | 0.85 ± 0.24 | P15 |
21 | 266.212 | 0.888 | 0.36 ± 0.08 | 4.9 ± 0.1 | 0.60 ± 0.14 | |
22 | 266.245 | 0.871 | 0.23 ± 0.08 | 5.1 ± 0.1 | 0.67 ± 0.26 | |
23 | 266.285 | 0.918 | ||||
24 | 266.279 | 0.928 | 0.29 ± 0.06 | 4.5 ± 0.1 | 1.24 ± 0.27 | |
25 | 266.278 | 0.941 | 0.22 ± 0.05 | 4.8 ± 0.2 | 1.40 ± 0.37 |
3.2 Hyperfine fitting of N2H+ and column density
We fitted a hyperfine component model to the N2H+ spectra, and derived the optical depth, LSR velocity, linewidth, and excitation temperature assuming a uniform excitation temperature in the N2H+ hyperfine components. We also assumed that the N2H+ emission fills the whole beam, that is, the beam filling factor is equal to unity. Figure 9 plots the linewidths of HC3N against those of N2H+. The dashed line delineates the case that they are equal. We found a good correlation between them, suggesting that the molecular emission likely comes from the same volume. This is supported by the fact that the centroid velocities of the N2H+ and HC3N lines are consistent with each other (see also tables 1 and 2). The intrinsic line strengths of the hyperfine components are adopted from Tiné et al. (2000). The optical depth τtot is the sum of the optical depths of all the hyperfine components. At peak 16, we identified two velocity components and we fit a two-velocity-component hyperfine model. Figure 10 shows examples of hyperfine fit results at the peak positions of the cores. The column density is calculated by assuming local thermodynamic equilibrium (LTE). The formulation can be found, for example, in equations (96) and (97) of Mangum and Shirley (2015). The range of the optical depths and the excitation temperatures are: τtot = 0.9–9.6 and Tex = 3.3–10.4 K (see also table 2). Even in the most prominent hyperfine line (F1 = 2–1, F = 3–2), the optical thickness is 0.259 τtot(N2H+) (Tiné et al. 2000). Therefore, each N2H+ hyperfine emission line is optically thin in general because the median value of τtot(N2H+) was estimated to be ∼2.4. The 1 σ error of the N2H+ column density was estimated by Δτtot of the hyperfine fit results. However, if we use the lower limit of the τtot − Δτtot, Tex(N2H+) can be too high to accept, that is, Tex(N2H+) is higher than Tex(CO) ∼ 10–20 K from Yamaguchi et al. (1999). In these cases, we use the symmetric error bars in the log scale by adopting the upper limit τtot + Δτtot.
Peak position . | τtot(N2H+) . | v LSR (N2H+) . | dv (N2H+) . | T ex . | N(N2H+) . | N(HC3N) . |
---|---|---|---|---|---|---|
name . | . | (km s−1) . | (km s−1) . | (K) . | (cm−2) . | (cm−2) . |
1 | 1.2 ± 1.1 | 7.40 ± 0.06 | 1.70 ± 0.21 | 4.7 ± 1.5 | (5.7|$^{+5.3}_{-2.7}$|)E+12 | <7.50E+12 |
2 | 6.2 ± 2.2 | 6.68 ± 0.04 | 1.00 ± 0.11 | 3.8 ± 0.2 | (1.3|$^{+4.8}_{-3.5}$|)E+13 | (1.2 ± 0.3)E+13 |
3 | 3.5 ± 2.9 | 6.78 ± 0.03 | 0.60 ± 0.07 | 4.2 ± 0.8 | (5.0|$^{+4.2}_{-2.3}$|)E+12 | (3.4 ± 1.8)E+12 |
4 | 5.2 ± 4.2 | 7.04 ± 0.06 | 0.82 ± 0.16 | 3.9 ± 0.5 | (9.1|$^{+7.3}_{-4.1}$|)E+12 | (6.6 ± 3.8)E+12 |
5 | 5.6 ± 2.6 | 6.96 ± 0.03 | 0.65 ± 0.06 | 3.6 ± 0.2 | (7.0|$^{+3.3}_{-2.2}$|)E+12 | (7.3 ± 1.9)E+12 |
6 | 3.9 ± 3.6 | 7.22 ± 0.04 | 0.63 ± 0.09 | 3.9 ± 0.7 | (5.2|$^{+5.0}_{-2.6}$|)E+12 | (6.7 ± 2.3)E+12 |
7 | 5 | <2.2E+12 | (1.9 ± 1.3)E+12 | |||
8 | 3.6 ± 2.4 | 6.88 ± 0.06 | 1.20 ± 0.19 | 3.7 ± 0.4 | (8.6|$^{+5.7}_{-3.4}$|)E+12 | (7.4 ± 3.1)E+12 |
9 | 6.0 ± 2.6 | 7.54 ± 0.03 | 0.70 ± 0.07 | 3.8 ± 0.2 | (8.6|$^{+3.5}_{-2.5}$|)E+12 | <5.0E+12 |
10 | 3.0 ± 2.6 | 6.84 ± 0.05 | 0.84 ± 0.12 | 4.1 ± 0.8 | (5.8|$^{+5.2}_{-2.7}$|)E+12 | (2.1 ± 1.4)E+12 |
11 | 5 | <2.4E+12 | (2.7 ± 1.4)E+12 | |||
12 | 1.7 ± 1.6 | 7.44 ± 0.03 | 0.80 ± 0.09 | 5.4 ± 1.7 | (4.8|$^{+4.7}_{-2.4}$|)E+12 | <2.9E+12 |
13 | 5 | <3.1E+12 | (1.2 ± 1.2)E+12 | |||
14 | 3.8 ± 1.7 | 6.41 ± 0.03 | 0.99 ± 0.10 | 4.5 ± 0.5 | (1.0|$^{+4.4}_{-3.1}$|)E+13 | (5.6 ± 1.8)E+12 |
15 | 9.6 ± 7.5 | 6.63 ± 0.05 | 0.56 ± 0.11 | 3.4 ± 0.2 | (9.7|$^{+7.6}_{-4.3}$|)E+12 | (1.1 ± 0.4)E+13 |
16 | 1.0 ± 0.8 | 8.28 ± 0.05 | 1.23 ± 0.12 | 9.2 ± 4.5 | (1.0|$^{+0.8}_{-0.5}$|)E+13 | (5.9 ± 3.2)E+12 |
1.9 ± 1.7 | 6.61 ± 0.10 | 1.14 ± 0.20 | 4.8 ± 1.5 | (6.2|$^{+5.8}_{-3.0}$|)E+12 | (9.8 ± 1.9)E+12 | |
17 | 0.9 ± 0.4 | 7.12 ± 0.02 | 1.81 ± 0.07 | 10.4 ± 3.1 | (1.7|$^{+8.2}_{-5.5}$|)E+13 | (8.1 ± 1.7)E+12 |
18 | 2.1 ± 0.6 | 7.23 ± 0.03 | 1.58 ± 0.08 | 5.9 ± 0.7 | (1.3|$^{+0.4}_{-0.3}$|)E+13 | (8.9 ± 1.9)E+12 |
19 | 1.7 ± 0.9 | 6.46 ± 0.03 | 1.27 ± 0.09 | 7.0 ± 1.8 | (1.1|$^{+0.6}_{-0.4}$|)E+13 | (5.1 ± 1.4)E+12 |
20 | 3.0 ± 2.0 | 6.85 ± 0.03 | 0.69 ± 0.06 | 4.9 ± 1.0 | (6.3|$^{+4.2}_{-2.5}$|)E+12 | (1.9 ± 1.4)E+12 |
21 | 9.1 ± 6.4 | 4.88 ± 0.04 | 0.49 ± 0.08 | 3.3 ± 0.2 | (7.7|$^{+5.4}_{-3.2}$|)E+12 | (1.1 ± 0.4)E+13 |
22 | 1.4 ± 1.4 | 4.98 ± 0.04 | 0.99 ± 0.12 | 5.3 ± 1.4 | (4.7|$^{+4.6}_{-2.3}$|)E+12 | (5.6 ± 1.2)E+12 |
23 | 3.7 ± 2.5 | 4.60 ± 0.04 | 0.87 ± 0.12 | 5.2 ± 1.1 | (1.0|$^{+0.7}_{-0.4}$|)E+13 | <3.4E+12 |
24 | 2.8 ± 2.3 | 4.59 ± 0.04 | 0.82 ± 0.10 | 4.6 ± 1.1 | (6.2|$^{+5.2}_{-2.8}$|)E+12 | (6.9 ± 1.9)E+12 |
25 | 5 | <4.3E+12 | (5.2 ± 1.8)E+12 |
Peak position . | τtot(N2H+) . | v LSR (N2H+) . | dv (N2H+) . | T ex . | N(N2H+) . | N(HC3N) . |
---|---|---|---|---|---|---|
name . | . | (km s−1) . | (km s−1) . | (K) . | (cm−2) . | (cm−2) . |
1 | 1.2 ± 1.1 | 7.40 ± 0.06 | 1.70 ± 0.21 | 4.7 ± 1.5 | (5.7|$^{+5.3}_{-2.7}$|)E+12 | <7.50E+12 |
2 | 6.2 ± 2.2 | 6.68 ± 0.04 | 1.00 ± 0.11 | 3.8 ± 0.2 | (1.3|$^{+4.8}_{-3.5}$|)E+13 | (1.2 ± 0.3)E+13 |
3 | 3.5 ± 2.9 | 6.78 ± 0.03 | 0.60 ± 0.07 | 4.2 ± 0.8 | (5.0|$^{+4.2}_{-2.3}$|)E+12 | (3.4 ± 1.8)E+12 |
4 | 5.2 ± 4.2 | 7.04 ± 0.06 | 0.82 ± 0.16 | 3.9 ± 0.5 | (9.1|$^{+7.3}_{-4.1}$|)E+12 | (6.6 ± 3.8)E+12 |
5 | 5.6 ± 2.6 | 6.96 ± 0.03 | 0.65 ± 0.06 | 3.6 ± 0.2 | (7.0|$^{+3.3}_{-2.2}$|)E+12 | (7.3 ± 1.9)E+12 |
6 | 3.9 ± 3.6 | 7.22 ± 0.04 | 0.63 ± 0.09 | 3.9 ± 0.7 | (5.2|$^{+5.0}_{-2.6}$|)E+12 | (6.7 ± 2.3)E+12 |
7 | 5 | <2.2E+12 | (1.9 ± 1.3)E+12 | |||
8 | 3.6 ± 2.4 | 6.88 ± 0.06 | 1.20 ± 0.19 | 3.7 ± 0.4 | (8.6|$^{+5.7}_{-3.4}$|)E+12 | (7.4 ± 3.1)E+12 |
9 | 6.0 ± 2.6 | 7.54 ± 0.03 | 0.70 ± 0.07 | 3.8 ± 0.2 | (8.6|$^{+3.5}_{-2.5}$|)E+12 | <5.0E+12 |
10 | 3.0 ± 2.6 | 6.84 ± 0.05 | 0.84 ± 0.12 | 4.1 ± 0.8 | (5.8|$^{+5.2}_{-2.7}$|)E+12 | (2.1 ± 1.4)E+12 |
11 | 5 | <2.4E+12 | (2.7 ± 1.4)E+12 | |||
12 | 1.7 ± 1.6 | 7.44 ± 0.03 | 0.80 ± 0.09 | 5.4 ± 1.7 | (4.8|$^{+4.7}_{-2.4}$|)E+12 | <2.9E+12 |
13 | 5 | <3.1E+12 | (1.2 ± 1.2)E+12 | |||
14 | 3.8 ± 1.7 | 6.41 ± 0.03 | 0.99 ± 0.10 | 4.5 ± 0.5 | (1.0|$^{+4.4}_{-3.1}$|)E+13 | (5.6 ± 1.8)E+12 |
15 | 9.6 ± 7.5 | 6.63 ± 0.05 | 0.56 ± 0.11 | 3.4 ± 0.2 | (9.7|$^{+7.6}_{-4.3}$|)E+12 | (1.1 ± 0.4)E+13 |
16 | 1.0 ± 0.8 | 8.28 ± 0.05 | 1.23 ± 0.12 | 9.2 ± 4.5 | (1.0|$^{+0.8}_{-0.5}$|)E+13 | (5.9 ± 3.2)E+12 |
1.9 ± 1.7 | 6.61 ± 0.10 | 1.14 ± 0.20 | 4.8 ± 1.5 | (6.2|$^{+5.8}_{-3.0}$|)E+12 | (9.8 ± 1.9)E+12 | |
17 | 0.9 ± 0.4 | 7.12 ± 0.02 | 1.81 ± 0.07 | 10.4 ± 3.1 | (1.7|$^{+8.2}_{-5.5}$|)E+13 | (8.1 ± 1.7)E+12 |
18 | 2.1 ± 0.6 | 7.23 ± 0.03 | 1.58 ± 0.08 | 5.9 ± 0.7 | (1.3|$^{+0.4}_{-0.3}$|)E+13 | (8.9 ± 1.9)E+12 |
19 | 1.7 ± 0.9 | 6.46 ± 0.03 | 1.27 ± 0.09 | 7.0 ± 1.8 | (1.1|$^{+0.6}_{-0.4}$|)E+13 | (5.1 ± 1.4)E+12 |
20 | 3.0 ± 2.0 | 6.85 ± 0.03 | 0.69 ± 0.06 | 4.9 ± 1.0 | (6.3|$^{+4.2}_{-2.5}$|)E+12 | (1.9 ± 1.4)E+12 |
21 | 9.1 ± 6.4 | 4.88 ± 0.04 | 0.49 ± 0.08 | 3.3 ± 0.2 | (7.7|$^{+5.4}_{-3.2}$|)E+12 | (1.1 ± 0.4)E+13 |
22 | 1.4 ± 1.4 | 4.98 ± 0.04 | 0.99 ± 0.12 | 5.3 ± 1.4 | (4.7|$^{+4.6}_{-2.3}$|)E+12 | (5.6 ± 1.2)E+12 |
23 | 3.7 ± 2.5 | 4.60 ± 0.04 | 0.87 ± 0.12 | 5.2 ± 1.1 | (1.0|$^{+0.7}_{-0.4}$|)E+13 | <3.4E+12 |
24 | 2.8 ± 2.3 | 4.59 ± 0.04 | 0.82 ± 0.10 | 4.6 ± 1.1 | (6.2|$^{+5.2}_{-2.8}$|)E+12 | (6.9 ± 1.9)E+12 |
25 | 5 | <4.3E+12 | (5.2 ± 1.8)E+12 |
Peak position . | τtot(N2H+) . | v LSR (N2H+) . | dv (N2H+) . | T ex . | N(N2H+) . | N(HC3N) . |
---|---|---|---|---|---|---|
name . | . | (km s−1) . | (km s−1) . | (K) . | (cm−2) . | (cm−2) . |
1 | 1.2 ± 1.1 | 7.40 ± 0.06 | 1.70 ± 0.21 | 4.7 ± 1.5 | (5.7|$^{+5.3}_{-2.7}$|)E+12 | <7.50E+12 |
2 | 6.2 ± 2.2 | 6.68 ± 0.04 | 1.00 ± 0.11 | 3.8 ± 0.2 | (1.3|$^{+4.8}_{-3.5}$|)E+13 | (1.2 ± 0.3)E+13 |
3 | 3.5 ± 2.9 | 6.78 ± 0.03 | 0.60 ± 0.07 | 4.2 ± 0.8 | (5.0|$^{+4.2}_{-2.3}$|)E+12 | (3.4 ± 1.8)E+12 |
4 | 5.2 ± 4.2 | 7.04 ± 0.06 | 0.82 ± 0.16 | 3.9 ± 0.5 | (9.1|$^{+7.3}_{-4.1}$|)E+12 | (6.6 ± 3.8)E+12 |
5 | 5.6 ± 2.6 | 6.96 ± 0.03 | 0.65 ± 0.06 | 3.6 ± 0.2 | (7.0|$^{+3.3}_{-2.2}$|)E+12 | (7.3 ± 1.9)E+12 |
6 | 3.9 ± 3.6 | 7.22 ± 0.04 | 0.63 ± 0.09 | 3.9 ± 0.7 | (5.2|$^{+5.0}_{-2.6}$|)E+12 | (6.7 ± 2.3)E+12 |
7 | 5 | <2.2E+12 | (1.9 ± 1.3)E+12 | |||
8 | 3.6 ± 2.4 | 6.88 ± 0.06 | 1.20 ± 0.19 | 3.7 ± 0.4 | (8.6|$^{+5.7}_{-3.4}$|)E+12 | (7.4 ± 3.1)E+12 |
9 | 6.0 ± 2.6 | 7.54 ± 0.03 | 0.70 ± 0.07 | 3.8 ± 0.2 | (8.6|$^{+3.5}_{-2.5}$|)E+12 | <5.0E+12 |
10 | 3.0 ± 2.6 | 6.84 ± 0.05 | 0.84 ± 0.12 | 4.1 ± 0.8 | (5.8|$^{+5.2}_{-2.7}$|)E+12 | (2.1 ± 1.4)E+12 |
11 | 5 | <2.4E+12 | (2.7 ± 1.4)E+12 | |||
12 | 1.7 ± 1.6 | 7.44 ± 0.03 | 0.80 ± 0.09 | 5.4 ± 1.7 | (4.8|$^{+4.7}_{-2.4}$|)E+12 | <2.9E+12 |
13 | 5 | <3.1E+12 | (1.2 ± 1.2)E+12 | |||
14 | 3.8 ± 1.7 | 6.41 ± 0.03 | 0.99 ± 0.10 | 4.5 ± 0.5 | (1.0|$^{+4.4}_{-3.1}$|)E+13 | (5.6 ± 1.8)E+12 |
15 | 9.6 ± 7.5 | 6.63 ± 0.05 | 0.56 ± 0.11 | 3.4 ± 0.2 | (9.7|$^{+7.6}_{-4.3}$|)E+12 | (1.1 ± 0.4)E+13 |
16 | 1.0 ± 0.8 | 8.28 ± 0.05 | 1.23 ± 0.12 | 9.2 ± 4.5 | (1.0|$^{+0.8}_{-0.5}$|)E+13 | (5.9 ± 3.2)E+12 |
1.9 ± 1.7 | 6.61 ± 0.10 | 1.14 ± 0.20 | 4.8 ± 1.5 | (6.2|$^{+5.8}_{-3.0}$|)E+12 | (9.8 ± 1.9)E+12 | |
17 | 0.9 ± 0.4 | 7.12 ± 0.02 | 1.81 ± 0.07 | 10.4 ± 3.1 | (1.7|$^{+8.2}_{-5.5}$|)E+13 | (8.1 ± 1.7)E+12 |
18 | 2.1 ± 0.6 | 7.23 ± 0.03 | 1.58 ± 0.08 | 5.9 ± 0.7 | (1.3|$^{+0.4}_{-0.3}$|)E+13 | (8.9 ± 1.9)E+12 |
19 | 1.7 ± 0.9 | 6.46 ± 0.03 | 1.27 ± 0.09 | 7.0 ± 1.8 | (1.1|$^{+0.6}_{-0.4}$|)E+13 | (5.1 ± 1.4)E+12 |
20 | 3.0 ± 2.0 | 6.85 ± 0.03 | 0.69 ± 0.06 | 4.9 ± 1.0 | (6.3|$^{+4.2}_{-2.5}$|)E+12 | (1.9 ± 1.4)E+12 |
21 | 9.1 ± 6.4 | 4.88 ± 0.04 | 0.49 ± 0.08 | 3.3 ± 0.2 | (7.7|$^{+5.4}_{-3.2}$|)E+12 | (1.1 ± 0.4)E+13 |
22 | 1.4 ± 1.4 | 4.98 ± 0.04 | 0.99 ± 0.12 | 5.3 ± 1.4 | (4.7|$^{+4.6}_{-2.3}$|)E+12 | (5.6 ± 1.2)E+12 |
23 | 3.7 ± 2.5 | 4.60 ± 0.04 | 0.87 ± 0.12 | 5.2 ± 1.1 | (1.0|$^{+0.7}_{-0.4}$|)E+13 | <3.4E+12 |
24 | 2.8 ± 2.3 | 4.59 ± 0.04 | 0.82 ± 0.10 | 4.6 ± 1.1 | (6.2|$^{+5.2}_{-2.8}$|)E+12 | (6.9 ± 1.9)E+12 |
25 | 5 | <4.3E+12 | (5.2 ± 1.8)E+12 |
Peak position . | τtot(N2H+) . | v LSR (N2H+) . | dv (N2H+) . | T ex . | N(N2H+) . | N(HC3N) . |
---|---|---|---|---|---|---|
name . | . | (km s−1) . | (km s−1) . | (K) . | (cm−2) . | (cm−2) . |
1 | 1.2 ± 1.1 | 7.40 ± 0.06 | 1.70 ± 0.21 | 4.7 ± 1.5 | (5.7|$^{+5.3}_{-2.7}$|)E+12 | <7.50E+12 |
2 | 6.2 ± 2.2 | 6.68 ± 0.04 | 1.00 ± 0.11 | 3.8 ± 0.2 | (1.3|$^{+4.8}_{-3.5}$|)E+13 | (1.2 ± 0.3)E+13 |
3 | 3.5 ± 2.9 | 6.78 ± 0.03 | 0.60 ± 0.07 | 4.2 ± 0.8 | (5.0|$^{+4.2}_{-2.3}$|)E+12 | (3.4 ± 1.8)E+12 |
4 | 5.2 ± 4.2 | 7.04 ± 0.06 | 0.82 ± 0.16 | 3.9 ± 0.5 | (9.1|$^{+7.3}_{-4.1}$|)E+12 | (6.6 ± 3.8)E+12 |
5 | 5.6 ± 2.6 | 6.96 ± 0.03 | 0.65 ± 0.06 | 3.6 ± 0.2 | (7.0|$^{+3.3}_{-2.2}$|)E+12 | (7.3 ± 1.9)E+12 |
6 | 3.9 ± 3.6 | 7.22 ± 0.04 | 0.63 ± 0.09 | 3.9 ± 0.7 | (5.2|$^{+5.0}_{-2.6}$|)E+12 | (6.7 ± 2.3)E+12 |
7 | 5 | <2.2E+12 | (1.9 ± 1.3)E+12 | |||
8 | 3.6 ± 2.4 | 6.88 ± 0.06 | 1.20 ± 0.19 | 3.7 ± 0.4 | (8.6|$^{+5.7}_{-3.4}$|)E+12 | (7.4 ± 3.1)E+12 |
9 | 6.0 ± 2.6 | 7.54 ± 0.03 | 0.70 ± 0.07 | 3.8 ± 0.2 | (8.6|$^{+3.5}_{-2.5}$|)E+12 | <5.0E+12 |
10 | 3.0 ± 2.6 | 6.84 ± 0.05 | 0.84 ± 0.12 | 4.1 ± 0.8 | (5.8|$^{+5.2}_{-2.7}$|)E+12 | (2.1 ± 1.4)E+12 |
11 | 5 | <2.4E+12 | (2.7 ± 1.4)E+12 | |||
12 | 1.7 ± 1.6 | 7.44 ± 0.03 | 0.80 ± 0.09 | 5.4 ± 1.7 | (4.8|$^{+4.7}_{-2.4}$|)E+12 | <2.9E+12 |
13 | 5 | <3.1E+12 | (1.2 ± 1.2)E+12 | |||
14 | 3.8 ± 1.7 | 6.41 ± 0.03 | 0.99 ± 0.10 | 4.5 ± 0.5 | (1.0|$^{+4.4}_{-3.1}$|)E+13 | (5.6 ± 1.8)E+12 |
15 | 9.6 ± 7.5 | 6.63 ± 0.05 | 0.56 ± 0.11 | 3.4 ± 0.2 | (9.7|$^{+7.6}_{-4.3}$|)E+12 | (1.1 ± 0.4)E+13 |
16 | 1.0 ± 0.8 | 8.28 ± 0.05 | 1.23 ± 0.12 | 9.2 ± 4.5 | (1.0|$^{+0.8}_{-0.5}$|)E+13 | (5.9 ± 3.2)E+12 |
1.9 ± 1.7 | 6.61 ± 0.10 | 1.14 ± 0.20 | 4.8 ± 1.5 | (6.2|$^{+5.8}_{-3.0}$|)E+12 | (9.8 ± 1.9)E+12 | |
17 | 0.9 ± 0.4 | 7.12 ± 0.02 | 1.81 ± 0.07 | 10.4 ± 3.1 | (1.7|$^{+8.2}_{-5.5}$|)E+13 | (8.1 ± 1.7)E+12 |
18 | 2.1 ± 0.6 | 7.23 ± 0.03 | 1.58 ± 0.08 | 5.9 ± 0.7 | (1.3|$^{+0.4}_{-0.3}$|)E+13 | (8.9 ± 1.9)E+12 |
19 | 1.7 ± 0.9 | 6.46 ± 0.03 | 1.27 ± 0.09 | 7.0 ± 1.8 | (1.1|$^{+0.6}_{-0.4}$|)E+13 | (5.1 ± 1.4)E+12 |
20 | 3.0 ± 2.0 | 6.85 ± 0.03 | 0.69 ± 0.06 | 4.9 ± 1.0 | (6.3|$^{+4.2}_{-2.5}$|)E+12 | (1.9 ± 1.4)E+12 |
21 | 9.1 ± 6.4 | 4.88 ± 0.04 | 0.49 ± 0.08 | 3.3 ± 0.2 | (7.7|$^{+5.4}_{-3.2}$|)E+12 | (1.1 ± 0.4)E+13 |
22 | 1.4 ± 1.4 | 4.98 ± 0.04 | 0.99 ± 0.12 | 5.3 ± 1.4 | (4.7|$^{+4.6}_{-2.3}$|)E+12 | (5.6 ± 1.2)E+12 |
23 | 3.7 ± 2.5 | 4.60 ± 0.04 | 0.87 ± 0.12 | 5.2 ± 1.1 | (1.0|$^{+0.7}_{-0.4}$|)E+13 | <3.4E+12 |
24 | 2.8 ± 2.3 | 4.59 ± 0.04 | 0.82 ± 0.10 | 4.6 ± 1.1 | (6.2|$^{+5.2}_{-2.8}$|)E+12 | (6.9 ± 1.9)E+12 |
25 | 5 | <4.3E+12 | (5.2 ± 1.8)E+12 |
For HC3N, the o ptical depth has been derived by using multitransitional observations (J = 4–3, J = 10–9, J =12–11, and J =16–15) toward several GMCs, including Orion A GMC, by Bergin, Snell, and Goldsmith (1996), and the opacity was found to be <1 at all positions. The HC3N column density is calculated by assuming LTE and optically thin emission. The formulation can be found in equation (13) of Sanhueza et al. (2012), for example. The excitation temperature Tex(HC3N) is assumed to be equal to that for N2H+. This assumption has some uncertainties, but the same excitation will be a best guess because both lines are thought to be optically thin and would be only subthermally excited. We confirmed that the excitation temperature of N2H+ does not change between the star-forming and starless peaks, suggesting that the excitation condition of the dense cores would not be changed. When hyperfine fitting is not successful due to low signal-to-noise ratios, we adopt Tex(HC3N) = 5 K, which is the average value of Tex(N2H+). In the case of non-detection of these lines, we estimate the upper limit of the column densities from the 3 σ noise level. In table 2 we summarize the derived column densities.
3.3 Integrated intensity ratio and abundance ratio
We show the ratio of the velocity-integrated intensity of the N2H+ main hyperfine group J = 1–0, F1 = 2–1 to that of the HC3N J = 10–9 emission against the N2H+ column density in figure 11. The integrated intensity ratios of N2H+/HC3N are always greater than unity. It is also found that the star-forming peaks have high ratios compared with those of starless peaks, suggesting that the N2H+/HC3N integrated intensity ratio increases with time as star formation evolves. However, the substantial overlap between the star-forming and starless peaks can be seen. This unclear boundary will be discussed in the following section.
In molecular dense cores located in the Orion A GMC, it has been shown that the column density ratios of N2H+/CCS, NH3/CCS, and NH3/HC3N are high in star-forming regions, while low in starless regions (Ohashi et al. 2014; Tatematsu et al. 2014a). These results suggest that the carbon chain molecules (HC3N and CCS) trace chemically young gas, while N-bearing molecules (N2H+ and NH3) trace later stages of chemical evolution. We investigate whether the N2H+/HC3N abundance ratio can be an indicator of the chemical evolution in Vela C. Figure 12 shows the column density of N2H+ against that of HC3N. We found a positive correlation between these column densities. We also found a systematic difference in the column densities between the star-forming and starless peaks. Figure 13 shows the column density ratio of N2H+/HC3N against the linewidth of HC3N. We find, on average, that the column density ratio of N2H+/HC3N seems to be low toward starless peaks, while it seems to be high toward star-forming peaks. The average value of the N2H+/HC3N abundance ratio, excluding the data with upper limits, is 1.6|$^{+0.5}_{-0.4}$| for star-forming (median value is 1.5) and 1.2|$^{+0.4}_{-0.3}$| for starless peaks (median value is 0.9). This is similar to the tendency found in the Orion A GMC and nearby cold dark clouds. Ohashi et al. (2014) suggested that column density ratio of NH3/HC3N decreases with increasing linewidth of HC3N in the Orion A cloud. However, we found no correlation between column density ratio of N2H+/HC3N and linewidths. The criterion between star-forming and starless in Vela C is found around N2H+/HC3N ∼ 1.4. That is, N2H+/HC3N may be ≲1.4 in starless peaks, and ≳1.4 in star-forming peaks. Differences in the filling factors of these two molecules may affect the column density ratio. If the filling factor is equal to 0.5, the column density is 1.3–1.4 times higher than that in the case of unity. Without having precise measurements of the filling factor, we assume the same filling factor, that is, unity for both of them. Even if the filling factor of HC3N is systematically larger or smaller than that of N2H+, we will obtain the same tendency in comparison between starless and star-forming peaks, although the absolute value of the ratio may change. It should be noted that the uncertainties of the ratios are still large and we have to confirm these trends with high sensitivity observations in future. Tatematsu et al. (2014a) observed CCS J = 7–6 and N2H+J = 1–0 emission toward dense cores in the Orion A cloud, and showed a clearer boundary between star-forming and starless at ∼2–3 (see their figure 20). However, in the present study, the boundary between star-forming and starless is less evident. Our large beam observations may have failed to resolve the core. If so, the column density of HC3N may be underestimated with our observations, and the column density ratio of N2H+/HC3N will become smaller than our estimate in such cold dense regions. Furthermore, identifications of protostars may not be complete in Vela C. Giannini et al. (2012) identified protostars if 70 μm flux obtained by the Herschel Space Observatory is more than 3 σ from the best modified black body fit. Embedded protostars may not be identified by these criteria and we may have misidentified star-forming cores as starless cores. Finally, it may also be possible that the HC3N J = 10–9 transition is not as appropriate a chemical evolution tracer as the CCS J = 7–6 transition. This is because the upper state energy for the HC3N J = 10–9 transition is ∼24 K, which is higher than that for the CCS J = 7–6 transition of ∼15 K. The HC3N J = 10–9 transition may be emitted in warm gas in star-forming regions rather than in young cold gas. It is also known that HC3N emissions show a wing emission toward Orion KL (Ungerechts et al. 1997) and strong emission in the circumnuclear disk of NGC 1068 (Takano et al. 2014). The HC3N molecule will not always trace chemically young gas.
Finally, figure 14 shows the column density ratio of N2H+/HC3N against Galactic longitude. The lower longitude corresponds to the northern part of Vela C and the higher longitude corresponds to the southern part. The boundary between star-forming and starless cores may remain unchanged along the different locations of the cores. We find no evidence for global chemical variation in Vela C, in contrast to what has been observed in the Orion A GMC by Tatematsu et al. (2010).
4 Summary
We have observed the HC3N (J = 10–9) and N2H+ (J = 1–0) lines toward the Vela C molecular cloud. We found, on average, that the N2H+/HC3N abundance ratio tends to be low in starless peaks while it tends to be high in star-forming peaks. This tendency is consistent with those found in the Orion A cloud and dark clouds. The criterion between star-forming and starless found in Vela C may be at N(N2H+)/N(HC3N) ∼ 1.5. However, this separation is not clear compared with that of N(N2H+)/N(CCS) in the Orion A cloud, suggesting that our spatial resolution of 0.19 pc is insufficient to resolve spatial distribution differences between HC3N and N2H+, that the identification of protostars may be incomplete, or that the HC3N J = 10–9 transition is not an appropriate chemical indicator in comparison with the CCS J = 7–6 transition.
We thank the anonymous referee for helpful comments. S. O. thanks Teresa Giannini for providing unpublished protostar data. Data analysis was carried out on the common use data analysis computer system at the Astronomy Data Center, ADC, of the National Astronomical Observatory of Japan.
References