- Email: [email protected]
Observational approach to molecular cloud evolution
Advances in Space Research 34 (2004) 519–523 www.elsevier.com/locate/asr
Observational approach to molecular cloud evolution Tomoharu Oka *, Satoshi Yamamoto, Mt. Fuji Submillimeter-wave Telescope Group Research Center for the Early Universe, Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received 3 January 2003; received in revised form 8 April 2003; accepted 14 April 2003
Abstract We have constructed a 1.2-m submillimeter-wave telescope at the summit of Mt. Fuji to observe molecular clouds in two CI lines, P1 –3 P0 (492 GHz) and 3 P2 –3 P1 (809 GHz). The telescope has been operated successfully for four observing seasons since the installation on 1998. We have obtained large-scale CI 492 GHz distributions of many molecular clouds, including Orion MC, Taurus MC, DR15, q-Oph, DR21, NGC2264, M17, W3, W44, W51, Rosette MC, covering more than 40 square degrees of the sky. The distribution of CI 492 GHz emission is found to be different from those of the 13 CO or C18 O emission in some clouds. We found C0 rich areas (C/CO 1) in several dark clouds without strong UV sources. Away from UV sources the spatial sequence appears to be Cþ /CO/C0 . This seems to be inconsistent with the standard photodissociation region picture. These results are discussed in relation to formation processes of molecular clouds and dense cloud cores. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. 3
Keywords: Submillimeter wave telescope; Molecular cloud evolution; Submillimeter CI lines
1. Introduction Diffuse pressure-bound clouds in the Galactic disk increase in density with decreasing temperature, and then evolve into self-gravitating molecular clouds: interstellar matter changes its form from ionized to atomic, and to molecular phase. Carbon, the 4th abundant element in the universe, also changes its major form from ionized carbon (Cþ ) to atomic carbon (C0 ), and to carbon monoxide (CO). Since the conversion timescale of C0 into CO is comparable to the dynamical timescale of molecular clouds (106 years; Suzuki et al., 1992), we might expect a C0 -rich age in the early stage of molecular cloud formation. The electronic ground state of a carbon atom is 3 P . Two magnetic dipole transitions, the 3 P1 –3 P0 (hereafter CI 1–0) line at 492 GHz and the 3 P2 –3 P1 (thereafter CI 2–1) line at 809 GHz, exist between the fine-structure levels. Since the 3 P1 level is 23.6 K above the ground level and the critical density is as low as 103 cm3 , the 1–0 line is expected to be a good tracer of atomic carbon *
Corresponding author. Tel.: +81-3-5841-4217; fax: +81-3-58414178. E-mail address: [email protected] (T. Oka).
in molecular clouds or cloud-forming regions. The CI 2– 1 line is rather difficult to detect, because it requires high temperature to excite the 3 P2 level, and because receiver performance drops steeply toward higher frequencies. If we could detect both CI lines, we can estimate excitation temperature, line optical depths, and column density under the LTE assumption. The detailed distribution of CI 1–0 emission around representative objects including photodissociation regions has been studied at high angular resolution (Minchin et al., 1994; Tauber et al., 1995; White and Sandell, 1995). Pioneering studies using a focal reducer installed on the CSO 10 m telescope have been made to survey the distribution of CI emission in several molecular clouds (Plume et al., 1994, 1999). In spite of these efforts, observed areas are small compared to the available maps of CO and its isopomers in molecular clouds.
2. Mt. Fuji submillimeter-wave telescope Exclusively to perform CI survey observations, we have constructed a submillimeter-wave telescope (Fig. 1; Sekimoto et al., 2000) at the summit of Mt. Fuji. Mt.
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.04.038
520
T. Oka et al. / Advances in Space Research 34 (2004) 519–523
of the telescope are controlled by a MicroSPARC-IIbased CPU board with a VME bus via GP-IB and serial interfaces. The control system is connected to the Nobeyama Radio Observatory via a commercial satellite communication system at a speed of 64 kbps. The telescope is operated remotely from the Hongo campus of the University of Tokyo. The telescope has been successfully operated for four observing seasons. In the CI 1–0 line more than 40 square degrees of sky have been surveyed. Several representative sources have been mapped in the CI 2–1 line. The excellent observing condition at the summit of Mt. Fuji in winter made it possible to perform an efficient survey in the submillimeter CI lines.
Fig. 1. The Mt. Fuji submillimeter-wave telescope.
Fuji is a dormant volcano with a 600-m diameter crater and is the highest mountain in Japan. In winter, temperatures are as low as )20 °C and there is a high percentage of fine days. The 220-GHz opacity at the summit is lower than 0.06 for 45% of time from November to March, thus the summit in winter is a site suitable for submillimeter-wave observations. The telescope has been installed August 1998, on Nishi-yasugawara (alt. 3725 m), about 200 m north of the highest peak, Kengamine (alt. 3776 m). The telescope has been transported to the site with a helicopter. Electric power is supplied from a weather station of the Meteorological Agency at Kengamine. The main reflector of the telescope has a 1.2-m diameter and a 0.48-m focal length. The surface accuracy is better than 10 lm in rms. The half-power beamwidth of the telescope at 492 GHz is 2:20 . We developed a triple-band receiver, in which SIS mixers for 492, 809, and 345 GHz bands are installed. Nb-based, parallelconnected twin junction (PCTJ) type SIS mixers fabricated at Nobeyama Radio Observatory were employed for all bands. All mixers are operated in double sideband mode. The receiver temperature is 120 K (DSB) at 492 GHz and 580 K (DSB) at 809 GHz (Noguchi et al., 2001). We use an acousto-optical radio spectrometer as a receiver backend. The total bandwidth of the spectrometer is 700 MHz, and the frequency resolution is 1.6 MHz. In summer, the telescope site is accessible by a bulldozer which can transport a maximum load of 1 ton. It takes about 3 h to the summit from the bulldozer base at Tarobo (alt. 1400 m). In winter, however, a bulldozer cannot reach the summit because of frozen snow. It takes about 9 h (depending on weather and physical condition) to walk to the summit with a professional mountain-guide, and this may be highly dangerous. Thus remote operation is indispensable. All instruments
3. C0 in dark clouds Dark clouds, which are free from intense far-ultraviolet (FUV) radiation, provide ideal sites for tracing chemical evolution of molecular clouds. However, CI emission from dark clouds is generally weak, about 2 K in antenna temperature. To date, we have obtained CI distributions in Heiles cloud 2, Lynds 1495, and Lynds 134 complex. 3.1. Heiles’ cloud 2 HeilesÕ cloud 2 (HCL2) is a dark cloud in the Taurus molecular cloud complex that is well-studied because of its proximity to the Sun (D ¼ 140 pc). Maezawa et al. (1999) observed the entire region of HCL2 in the CI 1–0 line (see Fig. 2). The CI intensity is generally weak in the dense core region in the north, and is brightest in the south end of HCL2, where C18 O emission is fairly weak. Toward the
Fig. 2. Maps of CI velocity-integrated intensity (gray scale) and C18 O integrated intensity (contours) in HCL2.
T. Oka et al. / Advances in Space Research 34 (2004) 519–523
CI emission peak, the C0 /CO column density ratio is evaluated to be greater than 0.8. This ratio is comparable to those found in diffuse clouds >1, although the visual extinction toward the C0 -rich area is higher than 6 mag. The C0 -rich area lies between the dense cores and a diffuse cloud which extends to the southeast. The IRAS 60 lm/100 lm color map shows a lack of enhancement in warm dust fraction toward the C0 -rich area. This may indicate that the origin of atomic carbon is unrelated to photodissociation processes. It is most likely that the cloud is in an early stage of chemical evolution. Then, dense core formation is taking place from northwest to southeast in this region. This direction is consistent with the result inferred for the TMC-1 ridge from the molecular abundance gradient (Hirahara et al., 1992).
521
Fig. 3. Maps of CI velocity-integrated intensity in Orion A and B molecular clouds.
3.2. Lynds 134 complex The Lynds 134 dark cloud complex consists of three dark clouds (L134, L169, L183) with similar mass. It is spatially isolated at high Galactic latitude (b 35°), and exhibits no sign of star formation. Ito et al. (2003) delineated the CI distribution in the entire region of the L134 cloud complex. Two clouds in the complex (L169, L183) show CI distributions similar to those of 13 CO emission, while one cloud (L134) exhibits a slight difference. A relatively C0 -rich area (C0 /CO 0.2) was found in the southeast edge of the L134 cloud. This indicates that the L134 cloud is in an earlier stage of chemical evolution.
associated with a very active star-forming region (Orion nebula), and has been extensively studied at various wavelengths. CI emission extends over the whole Orion molecular clouds irrespective of FUV field strength. The large-scale CI distributions in the Orion clouds are nearly identical to those of 13 CO obtained by Bally et al. (1987). C0 /CO column density ratios lie in a narrow range, 0.1–0.3. The similarity in CI/13 CO distribution is a general property of GMCÕs. This suggests that C0 in GMCs is not confined to a thin layer near the surface, but possibly coexists with CO.
5. C0 in the vicinity of FUV sources: Omc-1 region 4. C0 in giant molecular clouds Giant molecular clouds (GMCs) are formation sites of massive stars, which are sources of intense FUV photons. In the vicinity of intense FUV sources, C0 is thought to be formed by photodissociation of CO in the cloud surface exposed to FUV radiation. Thus, CI distributions in GMCs have been discussed in terms of photodissociation region (PDR) models (Hollenbach and Tielens, 1999, and references therein). In a planeparallel cloud illuminated by FUV radiation from one side, the major form of gas-phase carbon changes spatially from Cþ to C0 , and then to CO with distance to the UV source. According to this picture, atomic carbon would be confined to a thin layer near the surface. Nevertheless, CI observations to date have not detected such a thin C0 layer, nor have revealed the predicted phase configuration. We have obtained CI distributions of many GMCs including Orion A+B, DR15, DR21, NGC2264, M17, W3, W44, W51, and Rosette MC. Ikeda et al. (1999) observed the entire region of the Orion A+B giant molecular clouds in the CI 1–0 line (Fig. 3). The Orion A molecular cloud is the nearest GMC (D ¼ 450 pc)
In order to delineate distributions of C0 column density and excitation temperature in a cloud adjacent to intense FUV sources, Kuboi et al. (2003) mapped the OMC-1 region in the Orion A molecular cloud with the CI 2–1 line. The CI 2–1 emission is concentrated in the vicinity of the OB stars (h1 Ori and h2 Ori), while that of CI 1–0 emission extends over the whole molecular cloud ridge, from north to south. Column density and excitation temperature were determined under LTE assumption. The column density distribution was found to be very similiar to that of the CI 1–0 emission. Excitation temperature, on the other hand, is enhanced in the small region surrounding the OB stars, likely because of heating by the intense FUV field. Column density distributions of C0 and CO were compared (Fig. 4). Cþ 158 lm (CII) emission is known to be distributed in a small area around the OB stars. The CO column density has a peak in the Orion KL region. The C0 column density, however, has peaks further away from the OB stars than the CO column density peak. Thus, the phase configuration is Cþ /CO/ C0 from the OB stars. This is inconsistent with the standard PDR picture, which expects a spatial sequence of Cþ /C0 /CO.
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Fig. 4. Distributions of C0 (gray scale) and CO (contours) column densities.
6. Origin of the Cþ /CO/C0 configuration Cþ /CO/C0 is observed in various regions associated with OB stars, such as DR15 (Oka et al., 2001a), q-Oph (Kamegai et al., 2003), M17, and NGC1333 (Oka et al., in press). Thus, this configuration represents a general property of clouds irradiated by intense FUV radiation. The hypothesis in terms of unresolved clumps (e.g., Burton et al., 1990) with steady-state PDR surfaces cannot explain the spatial inversion of C0 and CO. To assess this problem, we constructed a simple 3-dimensional, time-dependent PDR model for the NGC 1333 cloud. After starting chemical reactions, Cþ inside of the cloud rapidly recombines into C0 within a thousand years, and then C0 is slowly converted into CO. These reactions proceed more quickly in high-density cores than in the low-density envelope, thereby leaving a C0 peak behind the CO cores at an age of 2 106 years. The C0 peak behind CO cores disappears after 107 years to reach a chemical equilibrium. Thus, the observed spatial inversion of C0 and CO could be understood in terms of chemical evolution.
7. Correlation with dynamical evolution Support for the chemical evolution comes from a correlation with dynamical evolution. Using the CI 1–0 data sets, Ikeda et al. (2002) identified 38 cores in the Orion A molecular cloud, and 42 cores in the Orion B molecular cloud. They derived virial theorem mass (MVT ) and LTE mass (MLTE ), as well as C0 and CO column densities for each core. Fig. 5 is a plot of MVT =MLTE mass ratio versus C0 /CO column density ratio. The ratios are well correlated. Since virial mass/LTE mass ratio is a measure of gravitational stability (e.g., Oka et al., 2001b), this relationship indicates that C0 /CO ratio decreases with increasing gravitational instability. Therefore, this result demonstrates that C0 /CO ratio could be a good tracer of molecular cloud evolution.
Fig. 5. A plot of MVT =MLTE mass ratio versus C0 /CO column density ratio for cloud cores in Orion A+B molecular clouds.
8. Summary We have performed large-scale CI surveys of nearby molecular clouds with the Mt. Fuji submillimeter-wave telescope. In these data set, we have discovered molecular clouds in the early stage of chemical evolution. The Cþ /CO/C0 configuration found in the vicinity of FUV sources can be understood in terms of chemical evolution as well. Virial analyses of cloud cores in Orion have demonstrated that C0 /CO ratio could be a good tracer of molecular cloud evolution.
Acknowledgements The authors are grateful to Professor Katsuhiko Sato and Professor Kazuo Makishima for their continuous encouragement and support throughout this work. This paper is based on the results of the Mt. Fuji submillimeter-wave telescope project in collaboration with Masafumi Ikeda, Hiroyuki Maezawa, Tetsuya Ito, Kazuhisa Kamegai, Takeshi Sakai, Nobuyuki Kuboi, Kunihiko Tanaka, Ken Shimbo, Masaaki Hayashida, KenÕichi Tatematsu, Yutaro Sekimoto, Takashi Noguchi, Keisuke Miyazawa, Hiroyuki Ozeki, Junji Inatani, Masatoshi Ohishi, Hideo Fujiwara, and Shuji Saito. References Bally, J., Stark, A., Wilson, R.W., et al. Filamentary structure in the Orion molecular cloud. Astrophys. J. 312, L45–L49, 1987. Burton, M.G., Hollenbach, D.J., Tielens, A.G.G.M. Line emission from clumpy photodissociation regions. Astrophys. J. 365, 620, 1990. Hirahara, Y., Suzuki, H., Yamamoto, S., et al. Mapping observations of sulfur-containing carbon-chain molecules in Taurus Molecular Cloud 1 (TMC-1). Astrophys. J. 394, 539–551, 1992.
T. Oka et al. / Advances in Space Research 34 (2004) 519–523 Hollenbach, D.J., Tielens, A.G.G.M. Photodissociation regions in the interstellar medium of galaxies. Rev. Mod. Phys. 71, 173, 1999. Ikeda, M., Maezawa, H., Ito, T., et al. Large-scale mapping observations of the CI (3 P1 –3 P0 ) and CO (J ¼ 3 to 2) lines toward the orion a molecular cloud. Astrophys. J. 527, L59–L62, 1999. Ikeda, M., Oka, T., Tatematsu, K., et al. The distribution of atomic carbon in the orion giant molecular cloud 1. Astrophys. J. Suppl. Ser. 139, 467–485, 2002. Ito, T., et al. Private communication, 2003. Kamegai, K., et al. Private communication. 2003. Kuboi, N., et al. Private communication, 2003. Maezawa, H., Ikeda, M., Ito, T., et al. Large-scale mapping observations of the CI 3 P1 –3 P0 line toward Heiles cloud 2 in the taurus dark cloud. Astrophys. J. 524, L129–L132, 1999. Minchin, N.R., White, G.J., Stutzki, J., et al. CI emission from the outflow and PDR in S140. Astron. Astrophys. 291, 250–260, 1994. Noguchi, T., Maezawa, H., Shi, S.-C., et al. Nb-based SIS mixer in the 800 GHz band. IEEE Trans. Appl. Supercond. 71, 167–170, 2001. Oka, T., Yamamoto, S., Iwata, M., et al. Atomic carbon and CO isotope emission in the vicinity of DR 15. Astrophys. J. 558, 176– 184, 2001a.
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Oka, T., Hasegawa, T., Sato, F., et al. Statistical properties of molecular clouds in the galactic center. Astrophys. J. 562, 348–362, 2001b. Oka, T., Iwata, M., Maczawa, H., et al. Submillimeter-wave CI spectral lines from the NGC 1333 region. Ap J, in press. Plume, R., Jaffe, D.T., Keene, J., et al. Observations of large-scale (CI) emission from S140. Astrophys. J. 425, L49–L52, 1994. Plume, R., Jaffe, D.T., Tatematsu, K., et al. Large-scale C I emission from molecular clouds with associated ultraviolet sources. Astrophys. J. 512, 768–783, 1999. Sekimoto, Y., Yamamoto, S., Oka, T., et al. Mount Fuji submillimeter-wave telescope. Rev. Sci. Instr. 71, 2895–2907, 2000. Suzuki, H., Yamamoto, S., Ohishi, M., et al. A survey of CCS, HCs N, HCs N, and NHs toward dark cloud cores and their production chemistry. Astrophys. J. 392, 551–570, 1992. Tauber, J.A., Lis, D.C., Keene, J., et al. Atomic carbon and CO isotope emission in the vicinity of the Orion Bar. Astron. Astrophys. 297, 567–573, 1995. White, G.J., Sandell, G. CI, CO and 790 lm continuum observations of the Orion molecular cloud and ionisation bar. Astron. Astrophys. 299, 179, 1995.
Observational approach to molecular cloud evolution Tomoharu Oka *, Satoshi Yamamoto, Mt. Fuji Submillimeter-wave Telescope Group Research Center for the Early Universe, Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received 3 January 2003; received in revised form 8 April 2003; accepted 14 April 2003
Abstract We have constructed a 1.2-m submillimeter-wave telescope at the summit of Mt. Fuji to observe molecular clouds in two CI lines, P1 –3 P0 (492 GHz) and 3 P2 –3 P1 (809 GHz). The telescope has been operated successfully for four observing seasons since the installation on 1998. We have obtained large-scale CI 492 GHz distributions of many molecular clouds, including Orion MC, Taurus MC, DR15, q-Oph, DR21, NGC2264, M17, W3, W44, W51, Rosette MC, covering more than 40 square degrees of the sky. The distribution of CI 492 GHz emission is found to be different from those of the 13 CO or C18 O emission in some clouds. We found C0 rich areas (C/CO 1) in several dark clouds without strong UV sources. Away from UV sources the spatial sequence appears to be Cþ /CO/C0 . This seems to be inconsistent with the standard photodissociation region picture. These results are discussed in relation to formation processes of molecular clouds and dense cloud cores. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. 3
Keywords: Submillimeter wave telescope; Molecular cloud evolution; Submillimeter CI lines
1. Introduction Diffuse pressure-bound clouds in the Galactic disk increase in density with decreasing temperature, and then evolve into self-gravitating molecular clouds: interstellar matter changes its form from ionized to atomic, and to molecular phase. Carbon, the 4th abundant element in the universe, also changes its major form from ionized carbon (Cþ ) to atomic carbon (C0 ), and to carbon monoxide (CO). Since the conversion timescale of C0 into CO is comparable to the dynamical timescale of molecular clouds (106 years; Suzuki et al., 1992), we might expect a C0 -rich age in the early stage of molecular cloud formation. The electronic ground state of a carbon atom is 3 P . Two magnetic dipole transitions, the 3 P1 –3 P0 (hereafter CI 1–0) line at 492 GHz and the 3 P2 –3 P1 (thereafter CI 2–1) line at 809 GHz, exist between the fine-structure levels. Since the 3 P1 level is 23.6 K above the ground level and the critical density is as low as 103 cm3 , the 1–0 line is expected to be a good tracer of atomic carbon *
Corresponding author. Tel.: +81-3-5841-4217; fax: +81-3-58414178. E-mail address: [email protected] (T. Oka).
in molecular clouds or cloud-forming regions. The CI 2– 1 line is rather difficult to detect, because it requires high temperature to excite the 3 P2 level, and because receiver performance drops steeply toward higher frequencies. If we could detect both CI lines, we can estimate excitation temperature, line optical depths, and column density under the LTE assumption. The detailed distribution of CI 1–0 emission around representative objects including photodissociation regions has been studied at high angular resolution (Minchin et al., 1994; Tauber et al., 1995; White and Sandell, 1995). Pioneering studies using a focal reducer installed on the CSO 10 m telescope have been made to survey the distribution of CI emission in several molecular clouds (Plume et al., 1994, 1999). In spite of these efforts, observed areas are small compared to the available maps of CO and its isopomers in molecular clouds.
2. Mt. Fuji submillimeter-wave telescope Exclusively to perform CI survey observations, we have constructed a submillimeter-wave telescope (Fig. 1; Sekimoto et al., 2000) at the summit of Mt. Fuji. Mt.
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.04.038
520
T. Oka et al. / Advances in Space Research 34 (2004) 519–523
of the telescope are controlled by a MicroSPARC-IIbased CPU board with a VME bus via GP-IB and serial interfaces. The control system is connected to the Nobeyama Radio Observatory via a commercial satellite communication system at a speed of 64 kbps. The telescope is operated remotely from the Hongo campus of the University of Tokyo. The telescope has been successfully operated for four observing seasons. In the CI 1–0 line more than 40 square degrees of sky have been surveyed. Several representative sources have been mapped in the CI 2–1 line. The excellent observing condition at the summit of Mt. Fuji in winter made it possible to perform an efficient survey in the submillimeter CI lines.
Fig. 1. The Mt. Fuji submillimeter-wave telescope.
Fuji is a dormant volcano with a 600-m diameter crater and is the highest mountain in Japan. In winter, temperatures are as low as )20 °C and there is a high percentage of fine days. The 220-GHz opacity at the summit is lower than 0.06 for 45% of time from November to March, thus the summit in winter is a site suitable for submillimeter-wave observations. The telescope has been installed August 1998, on Nishi-yasugawara (alt. 3725 m), about 200 m north of the highest peak, Kengamine (alt. 3776 m). The telescope has been transported to the site with a helicopter. Electric power is supplied from a weather station of the Meteorological Agency at Kengamine. The main reflector of the telescope has a 1.2-m diameter and a 0.48-m focal length. The surface accuracy is better than 10 lm in rms. The half-power beamwidth of the telescope at 492 GHz is 2:20 . We developed a triple-band receiver, in which SIS mixers for 492, 809, and 345 GHz bands are installed. Nb-based, parallelconnected twin junction (PCTJ) type SIS mixers fabricated at Nobeyama Radio Observatory were employed for all bands. All mixers are operated in double sideband mode. The receiver temperature is 120 K (DSB) at 492 GHz and 580 K (DSB) at 809 GHz (Noguchi et al., 2001). We use an acousto-optical radio spectrometer as a receiver backend. The total bandwidth of the spectrometer is 700 MHz, and the frequency resolution is 1.6 MHz. In summer, the telescope site is accessible by a bulldozer which can transport a maximum load of 1 ton. It takes about 3 h to the summit from the bulldozer base at Tarobo (alt. 1400 m). In winter, however, a bulldozer cannot reach the summit because of frozen snow. It takes about 9 h (depending on weather and physical condition) to walk to the summit with a professional mountain-guide, and this may be highly dangerous. Thus remote operation is indispensable. All instruments
3. C0 in dark clouds Dark clouds, which are free from intense far-ultraviolet (FUV) radiation, provide ideal sites for tracing chemical evolution of molecular clouds. However, CI emission from dark clouds is generally weak, about 2 K in antenna temperature. To date, we have obtained CI distributions in Heiles cloud 2, Lynds 1495, and Lynds 134 complex. 3.1. Heiles’ cloud 2 HeilesÕ cloud 2 (HCL2) is a dark cloud in the Taurus molecular cloud complex that is well-studied because of its proximity to the Sun (D ¼ 140 pc). Maezawa et al. (1999) observed the entire region of HCL2 in the CI 1–0 line (see Fig. 2). The CI intensity is generally weak in the dense core region in the north, and is brightest in the south end of HCL2, where C18 O emission is fairly weak. Toward the
Fig. 2. Maps of CI velocity-integrated intensity (gray scale) and C18 O integrated intensity (contours) in HCL2.
T. Oka et al. / Advances in Space Research 34 (2004) 519–523
CI emission peak, the C0 /CO column density ratio is evaluated to be greater than 0.8. This ratio is comparable to those found in diffuse clouds >1, although the visual extinction toward the C0 -rich area is higher than 6 mag. The C0 -rich area lies between the dense cores and a diffuse cloud which extends to the southeast. The IRAS 60 lm/100 lm color map shows a lack of enhancement in warm dust fraction toward the C0 -rich area. This may indicate that the origin of atomic carbon is unrelated to photodissociation processes. It is most likely that the cloud is in an early stage of chemical evolution. Then, dense core formation is taking place from northwest to southeast in this region. This direction is consistent with the result inferred for the TMC-1 ridge from the molecular abundance gradient (Hirahara et al., 1992).
521
Fig. 3. Maps of CI velocity-integrated intensity in Orion A and B molecular clouds.
3.2. Lynds 134 complex The Lynds 134 dark cloud complex consists of three dark clouds (L134, L169, L183) with similar mass. It is spatially isolated at high Galactic latitude (b 35°), and exhibits no sign of star formation. Ito et al. (2003) delineated the CI distribution in the entire region of the L134 cloud complex. Two clouds in the complex (L169, L183) show CI distributions similar to those of 13 CO emission, while one cloud (L134) exhibits a slight difference. A relatively C0 -rich area (C0 /CO 0.2) was found in the southeast edge of the L134 cloud. This indicates that the L134 cloud is in an earlier stage of chemical evolution.
associated with a very active star-forming region (Orion nebula), and has been extensively studied at various wavelengths. CI emission extends over the whole Orion molecular clouds irrespective of FUV field strength. The large-scale CI distributions in the Orion clouds are nearly identical to those of 13 CO obtained by Bally et al. (1987). C0 /CO column density ratios lie in a narrow range, 0.1–0.3. The similarity in CI/13 CO distribution is a general property of GMCÕs. This suggests that C0 in GMCs is not confined to a thin layer near the surface, but possibly coexists with CO.
5. C0 in the vicinity of FUV sources: Omc-1 region 4. C0 in giant molecular clouds Giant molecular clouds (GMCs) are formation sites of massive stars, which are sources of intense FUV photons. In the vicinity of intense FUV sources, C0 is thought to be formed by photodissociation of CO in the cloud surface exposed to FUV radiation. Thus, CI distributions in GMCs have been discussed in terms of photodissociation region (PDR) models (Hollenbach and Tielens, 1999, and references therein). In a planeparallel cloud illuminated by FUV radiation from one side, the major form of gas-phase carbon changes spatially from Cþ to C0 , and then to CO with distance to the UV source. According to this picture, atomic carbon would be confined to a thin layer near the surface. Nevertheless, CI observations to date have not detected such a thin C0 layer, nor have revealed the predicted phase configuration. We have obtained CI distributions of many GMCs including Orion A+B, DR15, DR21, NGC2264, M17, W3, W44, W51, and Rosette MC. Ikeda et al. (1999) observed the entire region of the Orion A+B giant molecular clouds in the CI 1–0 line (Fig. 3). The Orion A molecular cloud is the nearest GMC (D ¼ 450 pc)
In order to delineate distributions of C0 column density and excitation temperature in a cloud adjacent to intense FUV sources, Kuboi et al. (2003) mapped the OMC-1 region in the Orion A molecular cloud with the CI 2–1 line. The CI 2–1 emission is concentrated in the vicinity of the OB stars (h1 Ori and h2 Ori), while that of CI 1–0 emission extends over the whole molecular cloud ridge, from north to south. Column density and excitation temperature were determined under LTE assumption. The column density distribution was found to be very similiar to that of the CI 1–0 emission. Excitation temperature, on the other hand, is enhanced in the small region surrounding the OB stars, likely because of heating by the intense FUV field. Column density distributions of C0 and CO were compared (Fig. 4). Cþ 158 lm (CII) emission is known to be distributed in a small area around the OB stars. The CO column density has a peak in the Orion KL region. The C0 column density, however, has peaks further away from the OB stars than the CO column density peak. Thus, the phase configuration is Cþ /CO/ C0 from the OB stars. This is inconsistent with the standard PDR picture, which expects a spatial sequence of Cþ /C0 /CO.
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Fig. 4. Distributions of C0 (gray scale) and CO (contours) column densities.
6. Origin of the Cþ /CO/C0 configuration Cþ /CO/C0 is observed in various regions associated with OB stars, such as DR15 (Oka et al., 2001a), q-Oph (Kamegai et al., 2003), M17, and NGC1333 (Oka et al., in press). Thus, this configuration represents a general property of clouds irradiated by intense FUV radiation. The hypothesis in terms of unresolved clumps (e.g., Burton et al., 1990) with steady-state PDR surfaces cannot explain the spatial inversion of C0 and CO. To assess this problem, we constructed a simple 3-dimensional, time-dependent PDR model for the NGC 1333 cloud. After starting chemical reactions, Cþ inside of the cloud rapidly recombines into C0 within a thousand years, and then C0 is slowly converted into CO. These reactions proceed more quickly in high-density cores than in the low-density envelope, thereby leaving a C0 peak behind the CO cores at an age of 2 106 years. The C0 peak behind CO cores disappears after 107 years to reach a chemical equilibrium. Thus, the observed spatial inversion of C0 and CO could be understood in terms of chemical evolution.
7. Correlation with dynamical evolution Support for the chemical evolution comes from a correlation with dynamical evolution. Using the CI 1–0 data sets, Ikeda et al. (2002) identified 38 cores in the Orion A molecular cloud, and 42 cores in the Orion B molecular cloud. They derived virial theorem mass (MVT ) and LTE mass (MLTE ), as well as C0 and CO column densities for each core. Fig. 5 is a plot of MVT =MLTE mass ratio versus C0 /CO column density ratio. The ratios are well correlated. Since virial mass/LTE mass ratio is a measure of gravitational stability (e.g., Oka et al., 2001b), this relationship indicates that C0 /CO ratio decreases with increasing gravitational instability. Therefore, this result demonstrates that C0 /CO ratio could be a good tracer of molecular cloud evolution.
Fig. 5. A plot of MVT =MLTE mass ratio versus C0 /CO column density ratio for cloud cores in Orion A+B molecular clouds.
8. Summary We have performed large-scale CI surveys of nearby molecular clouds with the Mt. Fuji submillimeter-wave telescope. In these data set, we have discovered molecular clouds in the early stage of chemical evolution. The Cþ /CO/C0 configuration found in the vicinity of FUV sources can be understood in terms of chemical evolution as well. Virial analyses of cloud cores in Orion have demonstrated that C0 /CO ratio could be a good tracer of molecular cloud evolution.
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