Observations of the CH<sub>3</sub>OH 4<sub>2</sub>?5<sub>1</sub> E Line Toward the Sgr B2 Region
Observations of the CH3OH 42?51 E Line Toward the Sgr B2 Region
Journal of Astronomy and Space Sciences. 2011. Mar, 28(1): 13-16
Copyright ©2011, The Korean Space Science Society
This is an Open Access article distributed under the terms of theCreative Commons Attribution Non-Commercial License( permits unrestrictednon-commercial use, distribution, and reproduction in any medium,provided the original work is properly cited.
  • Received : January 20, 2011
  • Accepted : February 11, 2011
  • Published : March 15, 2011
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About the Authors
Young Chol Minh
Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea
Sang Joon Kim
School of Space Research, Kyunghee University, Yongin 446-701, Korea

The CH 3 OH 4 2 –5 1 E transition was observed toward the Sgr B2 region, including the Principal Cloud and its surroundings.This methanol transition shows an extended emission along the 2’N cloud, which is believed to be colliding with the Principal Cloud and may trigger the massive star formation in this cloud. This extended methanol emission may also suggest that the 2’N cloud is under shocks. We derive total methanol column density N(CH 3 OH) = 2.9 ±0.3 × 10 14 cm –2 toward the peak position of the extended emission. The fractional abundance of methanol is about 10 –9 , relative to the estimated total H 2 abundance, which is similar to the methanol abundances in quiet gas phase.
The Sgr B2 region in our Galactic center contains giant molecular clouds, which represent extreme cases of high luminosity star formation. The gas in the Sgr B2 region is mainly concentrated in a dense central component, “the Principal Cloud,” ~ 5–10 pc in diameter with a relatively high mean density, n(H2) ≈ 1 × 10 5 cm–3 (Irvine et al. 1987,Minh et al. 1998). Along the north-south direction there exist three prominent star-forming cores ‘inside’ the Principal Cloud, Sgr B2 (N), (M), and (S) which include clusters of compact HII regions having newly born massive stars. The density and temperature structures of this region are extremely complicated (Martìn-Pintado et al.1999, Rodrìguez-Fernàndez et al. 2002, and references therein). The massive star formations in the core of the Principal Cloud has been suggested to be triggered by cloud collisions (Hasegawa et al. 1994, Mehringer et al.1995, Oka et al. 1998, Sato et al. 2000) although there exist many difficulties in identifying different gas components by using the optically thick lines. The latest star formation may be occurring at the 2 arcmin north (2’N) position from Sgr B2 (M), where the isocyanic acid (HNCO) emission peaks probably resulting from the collision between the Principal Cloud and the 2’N cloud (Minh et al. 1998, Minh & Irvine 2006). We observed one of the methanol transitions in the 200 GHz band toward the Sgr B2 molecular cloud using the Swedish-European Southern Observatory (ESO) submillimeter telescope (SEST) in Chile, and discuss the morphology of its emission in this region.
Methanol is a slightly asymmetric rotor with hindered internal rotation and significant a- and b-axis dipole moments (μa = 0.885 D, μb = 1.44 D), and has a complex spectrum that is sensitive to interstellar conditions. The modeling of methanol excitation has been known to be extremely difficult. Even when the lines are not masing,they may be expected to be distinctly non-LTE excitation,except under the highest density conditions, where collisions could thermalize the level populations (Leurini et al. 2004, 2007). Methanol (CH 3 OH) is thought to form mainly on the grain surface in dense cores by the hydrogenation of CO, and observed to be one of the major solid components in dust ice mantles (Dartois et al. 1999, Gibb 2000). Methanol is the predominant O-bearing species in the gas phase after evaporation from dust grains, and the key species which initiates the early phase oxygen chemistry in hot cores (Rodgers & Charnley 2003). In addition to the fact that methanol masers are often found abundant very close to young stellar objects, its thermal emission is thought to unambiguously trace hot cores and/or outflows associated with massive star formation (Minh et al. 1993, Minier et al. 2000).
Various methanol lines have been detected toward the Sgr B2 molecular cloud, especially, toward the (M) and (N) positions of the Principal Cloud (Cummins et al. 1986, Mehringer & Menten 1997, Nummeline et al. 2000). In this paper, we report the observational results of the CH 3 OH 4 2 -5 1 E transition toward the Sgr B2 region, and discuss the emission properties in this cloud. Our study is mainly focused on the morphology of this emission associated with the 2’N Cloud, which is claimed to be colliding with the main gas concentration in Sgr B2, the Principal Cloud (Minh et al. 1998, Minh & Irvine 2006).
Observations were carried out using the 15 m SEST1 (Booth et al. 1989) on La Silla, Chile, in April 2000. The methanol (CH 3 OH) 4 2 -5 1 E transition at 216.95 GHz was observed using a dual channel SIS receiver during the survey of various molecular lines toward our Galactic center region (Minh et al. 2005, Minh & Irvine 2006). Two 1,500 channel acousto-optic spectrometers with channel spacing of 0.69 MHz were used but the frequency resolution of the system was approximately 1.2 MHz. The half-power beamwidth (HPBW) and main beam efficiency are 22” and 0.60, respectively, at 220 GHz. Spectra were taken with the position switching mode using the reference position (l, b) = (0.0º, 2.5º), and the map was made with a grid of 2 arcmin. The antenna temperatures T A * quoted in this paper have been corrected for antenna and atmospheric losses by means of the standard chopper wheel method, but not for possible beam dilution. The typical system temperatures were about 220–240 K (SSB) and typical rms (1 σ) of the spectra are ~120 mK.
Fig. 1 shows a spectrum observed toward the Sgr B2 (M) position. The line intensity appears to be strongly enhanced at this (M) position, relative to other observed points in this region, which is probably associated with massive star formation. This methanol line seems to have a broad wing component (Δv ≥ 30 km s –1 ), but higher spatial resolution observations are necessary for further studies. The velocity integrated intensity map of the observed methanol line is shown in Fig. 2 . The location of Sgr B1 is indicated with a filled square in the figure, which is an HII region complex (LaRosa et al. 2000). Sgr B1 is a part of the Ridge connecting the Galactic center radio Arc and the Sgr B2 cloud (Lis et al. 2001), which shows very weak molecular line emissions in our observations. In this paper we focus on the gas cloud associated with Sgr B2 (M) and its surroundings.
As shown in Fig. 2 the velocity integrated intensity of the 4 2 -5 1 line of methanol shows highly enhanced toward the (M) position, compared to other observed points in this region. The high enhancement of this methanol emission line, which appears to be unresolved in this ~22” beam, is thought to result from interactions between the hot cores and outflows from massive stars forming as a cluster in this region. The morphology of this extended emission shows a very close correlation with the 2’N cloud shown in Fig. 2 (Minh & Irvine 2006). The 2’N cloud, as a part of a large-scale “Ring” component (radius ~5 pc), may be experiencing the very first phase of collapse by colliding with the Principal Cloud, which could have resulted in the high activities of star formation in the Principal Cloud (Minh et al. 1998, Minh & Irvine 2006). Although the gas component in the Sgr B2 region is concentrated mainly in the Principal Cloud, we found no extra/extended emission toward the other part of the Principal Cloud at the observed sensitivity. It means that the dense
Lager Image
Spectrum of the CH3OH 42-51 line obtained toward (0.67º ?0.03º) in galactic coordinate which is roughly the Sgr B2 (M) position.
gas in the Principal Cloud is relatively quiet except the star-forming cores in this cloud (Irvine et al. 1987). On the other hand the relatively weak but extended emission of methanol toward the 2’N cloud may suggest that this cloud is under shocks, which makes the solid methanol in the grain mantles evaporate into the gas phase.
We estimate the total column density of methanol, by assuming optically thin emission and an LTE condition (Irvine et al. 1987), N(CH 3 OH) = 1.2 ±0.1 × 10 15 cm –2 toward (M), and 2.9 ±0.3 × 10 14 cm –2 toward the peak position of the extended emission. The errors come from the range of rotational temperature applied, 10–20 K, and the observed spectral noise (1 σ) gives about ≤ 5% and ~30% further uncertainty, for the values obtained toward (M) and the peak of the extended emission, respectively. Galactic methanol abundances are known to be about 10 –9 relative to H 2 for quiet dense clouds, but in smaller scales, methanol can be enhanced by factors of a few hundred over dark cloud values in shocks and outflows (Meier & Turner 2005). The total H 2 column density, N(H 2 ), is highly uncertain, but we assume that N(H 2 ) ≈ 1 × 10 24 cm –2 toward (M) and one-fourth of that toward the 2’N position (Irvine et al. 1987, Minh et al. 1998). Then we derive the “beam-averaged” fractional abundance of methanol, relative to the total H 2 abundance, to be about 10–9 for both Sgr B2 (M) and the peak of the extended emission. However, since the emission toward Sgr B2 (M) is unresolved
Lager Image
(Right) Velocity integrated intensity (∫ TA* dv) map of the observed CH3OH line in the velocity range of –50~150 km s–1. Contour lines start from 1 K km s–1 and increase by 1 K km s–1 for the extended emission except the Sgr B2 (M). The Sgr B1 position is indicated as a filled square and the small dots are observed points. The data obtained toward the filled circle position which is roughly the Sgr B2 (M) position has not been included in making this figure as explained in Section 3. (Left) Outlines of the Principal Cloud (grey scale with 10 K km s–1 steps between 200 and 300 K km s–1 for the 13CO 1–0 intensity) and the 2’N Cloud (solid line contour with a 20 K km s–1 step from 120 K km s–1 for the HNCO 404-303 intensity) reproduced from Minh & Irvine (2006). The Sgr B2 (M) and (N) positions are indicated.
in our beam, the fractional abundance of methanol will be much higher at the actual methanol core. And the estimated total H 2 column density toward the 2’N position includes the contribution from the Principal Cloud, which is probably significant. For better estimation of the methanol abundances, we need higher spatial resolution observations and multi-transition methanol observations, in addition to a better derivation of the total H 2 column density for separate cloud components in the Sgr B2 region.
1SEST is operated by Onsala Space Observatory, the Swedish National Facility for Radio Astronomy, with financial support from the Swedish Natural Science Research Council (NFR) and ESO.
We have observed the CH 3 OH 4 2 -5 1 transition toward the Sgr B2 region, including the Principal Cloud and its surroundings. This methanol emission shows a strong enhancement toward Sgr B2 (M), where massive stars are forming as a cluster. The low intensity emission shows to be extended mainly along the 2’N cloud, which was suggested to be colliding with the Principal Cloud (Minh et al. 1998, Minh & Irvine 2006). The methanol emission distribution confirms that the 2’N cloud is under shocks, assuming that shock makes the solid methanol in the grain mantles evaporate into gas phase. We derive total methanol column density N(CH 3 OH) = 2.9 ±0.3 × 10 14 cm –2 toward the peak position of the extended emission.
This work was supported by Mid-career Researcher Program through National Research Foundation (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) (No. 2009-0083999), and by KRF-2008-313-C00376. SJK acknowledges support from the World Class University (WCU) grant (R31-10016) of MEST.
Booth RS , Delgado G , Hagstrom M , Johansson LEB , Murphy DC 1989 The Swedish-ESO submillimetre telescope (SEST) A&A 216 315 - 324
Cummins SE , Linke RA , Thaddeus P 1986 A survey of the millimeter-wave spectrum of Sagittarius B2 ApJS 60 819 - 878    DOI : 10.1086/191102
Dartois E , et al. , Schutte W , et al. , Geballe TR , et al. , Demyk K , et al. , Ehrenfreund P , et al. 1999 Methanol: the second most abundant ice species towards the high-mass protostars RAFGL7009S and W 33A A&A et al. 342 L32 - L35
Gibb EL 2000 An inventory of interstellar ices toward the embedded protostar W33A ApJ 536 347 - 356    DOI : 10.1086/308940
Hasegawa T , Sato F , Whiteoak JB , Miyawaki R 1994 A large-scale cloud collision in the Galactic center molecular cloud near Sagittarius B21 ApJ 429 L77 - L80    DOI : 10.1086/187417
Irvine WM , Goldsmith PF , Hjalmarson Å , Hollenbach DJ , Thronson HA Jr 1987 Chemical abundances in molecular clouds in Interstellar processes Reidel Dordrecht 561 -
LaRosa TN , Kassim NE , Lazio TJW , Hyman SD 2000 A wide-field 90 centimeter VLA image of the Galactic center region ApJ 119 207 - 240    DOI : 10.1086/301168
Leurini S , et al. , Schilke P , et al. , Menten KM , et al. , Flower DR , et al. , Pottage JT , et al. 2004 Methanol as a diagnostic tool of interstellar clouds. I. Model calculations and application to molecular clouds ApJ et al. 422 573 - 585    DOI : 10.1051/0004-6361:20047046
Leurini S , Schilke P , Wyrowski F , Menten KM 2007 Methanol as a diagnostic tool of interstellar clouds. II. Modelling high-mass protostellar objects A&A 466 215 - 228    DOI : 10.1051/0004-6361:20054245
Lis DC , Serabyn E , Zylka R , Li Y 2001 Quiescent giant molecular cloud cores in the Galactic center ApJ 550 761 - 777    DOI : 10.1086/319815
Martìn-Pintado J , Gaume RA , Rodrìguez-Fernàndez N , de Vicente P , Wilson TL 1999 Hot expanding shells in the envelope of the Sagittarius B2 molecular cloud ApJ 519 667 - 686    DOI : 10.1086/307399
Mehringer DM , Menten KM 1997 44 GHz methanol masers and quasi-thermal emission in Sagittarius B2 ApJ 474 346 -    DOI : 10.1086/303454
Mehringer DM , Palmer P , Goss WM 1995 6 centimeter formaldehyde absorption toward the Sagittarius B star-forming complex ApJS 97 497 - 511    DOI : 10.1086/192148
Meier DS , Turner JL 2005 Spatially resolved chemistry in nearby galaxies. I. The center of IC 342 ApJ 618 259 - 280    DOI : 10.1086/426499
Minh YC , Haikala L , Hjalmarson A , Irvine WM 1998 Are clouds collapsing at the 2 north position of Sagittarius B2? ApJ 498 261 -    DOI : 10.1086/305558
Minh YC , Irvine WM 2006 The HNCO ring in the Sgr B2 region NewA 11 594 - 599    DOI : 10.1016/j.newast.2006.03.004
Minh YC , et al , Kim SJ , et al , Pak S , et al , Lee S , et al , Irvine WM , et al 2005 Molecular abundance variations near our Galactic center NewA et al 10 425 - 438    DOI : 10.1016/j.newast.2005.03.001
Minh YC , Ohishi M , Roh DG , Ishiguro M , Irvine WM 1993 Interferometric observations for oxygen-containing organic molecules toward Orion-KL ApJ 411 773 - 777    DOI : 10.1086/172880
Minier V , Booth RS , Conway JE 2000 VLBI observations of 6.7 and 12.2 GHz methanol masers toward high mass star-forming regions. I. Observational results: protostellar disks or outflows? A&A 362 1093 - 1108
Nummeline A , et al , Bergman P , et al , Hjalmarson Å , et al , Friberg P , et al , Irvine WM , et al 2000 A three-position spectral line survey of Sagittarius B2 between 218 and 263 GHZ. II. Data analysis ApJS et al 128 213 - 243    DOI : 10.1086/313376
Oka T , Hasegawa T , Sato F , Tsuboi M , Miyazaki A 1998 A large-scale CO survey of the Galactic center ApJS 118 455 - 515    DOI : 10.1086/313138
Rodgers SD , Charnley SB 2003 Chemical evolution in protostellar envelopes: cocoon chemistry ApJ 585 355 - 371    DOI : 10.1086/345497
Rodrìguez-Fernàndez NJ , Martìn-Pintado J , de Vicente P , Fuente A 2002 The warm molecular gas in the Galactic center Ap&SS 281 331 - 332    DOI : 10.1023/A:1019593524808
Sato F , Hasegawa T , Whiteoak JB , Miyawaki R 2000 Cloud collision-induced star formation in Sagittarius B2. I. Large-scale kinematics ApJ 535 857 - 868    DOI : 10.1086/308856