CH3OH Masers in Massive Star-forming Regions
Y. C. Minh(Korea Astronomy and Space Science Institute: KASI)‣ Methanol is an alcohol, which has nearly the same geometry as water,

HCHHO
Hwith one H being replaced by a methyl group (-CH3).
‣ Since the methyl group has three equivalent H atoms, the combinationsof the nuclear spins of these three H atoms create symmetry statescalled A and E. The A state corresponds to all spins being parallel for a
total of I = 3/2, while the E state has I=1/2.
‣ The internal rotation of the methyl group against the HO-C framework
is referred to as torsion.
‣ A slightly asymmetric rotor with hindered internal rotation andsignificant a- and b-axis dipole moments (µa=0.885D, µb=1.44D),
methanol has a complex spectrum that is sensitive to IS conditions.

1. Observations of Methanol
2. Methanol Chemistry
3. Thermal Emission in Astronomical Sources
4. SMA Observations
5. Summary11. Observations of Methanol
Rotational Energy Levels of Methanol

Thermal Emissions - Gas
Phase Observations

Although methanol is an asymmetric rotator, it is nearly a prolate
symmetric top. -> rotational level, K
Many of the rotational lines have been detected. Observable lines
cover a wide range of energies and line strengths, allowing for a
detailed analysis of excitation conditions and opacities.

Vatt’les et al. 1999

Excitation and radiative transfer modeling efforts: e.g., Leurini et al. 2004, 2007

CSO 350 µm Line Survey of Orion KL Comito et al. 2005

Even when not masing, methanol may be
expected to distinctly non-LTE excitation,
except under the highest density conditions,where collisions could thermalize the level
populations.
2Solid Methanol Observed• Methanol has six atoms - 12 vibrational modes:• They corresponds to O-H stretch (1), C-Hstretches (2, 3, 9), C-O stretch (8), CH3 rocks(7, 11), OH bend (6), CH3 deformations (4, 5, 10),and 12 torsion (1 - 11 : 3 - 10 µm)• 3.54 µm band (C-H stretch): W3A, NGC7538(IRS1),

GL2136• 8.9 and 9.7 µm band (CH3 rock and CO stretch):

GL2136• 6.85 µm (C-H bending): W3A, NGC7538(IRS9)Boogert et al. 2004: (Spitzer satellite)
The main ice constituents: H2O, CO2, CO, CH3OH (3.47, 6.85 μm)

Ice Mantle: H2O, CH3OH (1-30 %), NH3 (a few - 30 %), CO,CO2, OCS, ..
– Daughter molecules: CH3CN, HCOOCH3, HCN, CH3CH2CN,
CH3CH2OH,…
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Methanol Masers✤ First methanol masers in 1971 toward Orion nebula (Barrettet al. 1971)✤ Over 20 methanol rotational transitions have been found tobe masers: in star-forming regions, associated with IRsources, molecular outflows and compact HII regions.
✤ Probably a sign for star-birth of massive YSOs.
✤ They are divided into two classes, I and II.
✤ Class I: probably produced by collisional pumping(generally well separated from the center of activity)✤ Class II: probably by radiative excitation at IR (spatially wellcorrelated with YSOs OH masers

6.7 GHz methanol maser in G23.657-0.127(Bartkiewicz et al. 2005)

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2. Methanol Chemistry
Gas Phase Formation• Methanol Abundance: 0.4-1 x 10-8 in cold dark clouds> 10-5 in hot cores• Formation in the gas phase,

via the radiative association reaction:
CH3+ + H2O  CH3OH2+ + h
followed by the electron recombination to CH3OH (50%)
and H2CO (50%)
Miller et al. 1991, Lee et al. 1996, Turner 1998
• Destruction is dominated by reactions with C+ (mostimportant for 2.5 < Avo < 5.0 mag) , He+, H3+, and UV
dissociation (most important for Avo < 2.5 mag)
• Many uncertainties in methanol related reactions– Model expectations are less by 3-4 orders ofmagnitudes than the observed values– Whether surface CH3OH can be desorbed to produce

the observed gas-phase CH3OH is controversial in darkclouds.

Turner 1998
5Solid Methanol - Grain Chemistry• CH3OH is a “primary” ice that forms other products after nearby stars turn on.
– Laboratory work (Bernstein et al. 1995): the mixtures of H2O/CH3OH/CO/NH3 ices irradiatedby UV  complex organic species  at the expense of CH3OH as the primary carbon
source• Solid CH3OH: hydrogenation of CO (or H2CO) (Charnley et al. 1992, Caselli et al. 1994, Gibb etal. 2000).
– may be inefficient in quiescent sources: high CO/H2O ratio source show no solid CH3OH
feature (Elias 16)– laboratory works (Hiraoka et al. 1994)• CH3OH formation may need energetic processing (Chiar et al. 1996)-> Mainly we detect CH3OH toward massive star-forming region

Low-mass SF Cores
Massive Proto-stars

Yellow: Lab experiments
CO: apolar, polar
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Desorption and Sublimation• Heating by CRs or UV followed by thermal
evaporation of the surface molecules

Sublimation Temperature
CO ~ 20K, H2S ~ 40K, NH3 ~ 65K,
H2O, CH3OH ~ 100K

• UV photodesorption: direct interaction withsurface molecules

Eherenfreund 2000

Abundances vs. time of large O-bearing (dotted lines) and simple
N-bearing (solid lines) organic molecules in hot cores following
the evaporation of representative interstellar ices, T=300K(Rodgers & Charnley 2001)• More volatile species are evaporated before more tightly bound species.
• Small-scale spatial differentiation is observed in the chemical composition of many hot cores.
• N/O differences: Orion KL(Hot Core, Compact Ridge), W3(OH) (Wyrowski et al. 1999), G29.96-0.02 (Pratap etal. 1999), G5.89-0.39 (Thompson & Macdonald 1999), and Sgr B2 (Nummelin et al. 2000).
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3. Thermal Emission in Astronomical Sources

Methanol: traces outflowsor shocks

Minh et al. in preparation

Methanol in Orion-KL: NMA, CH3OH
153-144 A- (Minh et al. 1993)

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4. SMA Observations towards W75N and DR21(OH)(contour) 1.3 mm continuum, (triangles) UC HII, (+) H2O
masers, (dot) OH masers [Shepherd et al. 2001]

Cygnus X is one of the richest
molecular and HII complexes
located at less than 3 kpc fromthe Sun. The molecular complex
is massive (4x106 Mo) andextends over ~100 pc indiameter.

Detected Molecular Lines: ≥ 100 transitions

Sub-Millimeter Array (SMA, 8x 6m, 0.5 km, Hawaii)
2006, W75N (230 GHz; 345 GHz),  = 1” (230 GHz), 2” (345 GHz)

• C17O, H13CO+, H2CO, CH3OH (~ 50 lines), 13CH3OH• HCOOH, HCOOCH3, CH3OCH3, CH3CHO, CH3CH2OH• C34S, H2CS, H2S, SO, 33SO, 34SO, SO2, 34SO2• SiO• HC3N, DCN, HN13C• CH2CHCN, CH3CH2CN, NH2CHO, …

Continuum Emission

Dust T: 100 - 200 K

230 GHz

W75N
216.6 GHz ----------------------------------------------------------------------- (2 GHz) ----------------------------------------------------------------------- 218.6 GHz1385 µm -------------------------------------------------------------------------- (12.6 µm) ----------------------------------------------------------------------- 1372 µm226.5 GHz ------------------------------------------------------------------------ (2 GHz) ----------------------------------------------------------------------- 228.5 GHz
1324 µm ------------------------------------------------------------------------ (11.6 µm) ------------------------------------------------------------------------- 1313 µm

336.8 GHz ----------------------------------------------------------------------- (2 GHz) ------------------------------------------------------------------------ 338.8 GHz
891 µm --------------------------------------------------------------------------- (5.2 µm) ---------------------------------------------------------------------------- 885 µm

347.5 GHz ------------------------------------------------------------------------ (2 GHz) ------------------------------------------------------------------------ 349.5 GHz
863 µm ---------------------------------------------------------------------------- (4.9 µm) ---------------------------------------------------------------------------- 858 µm

Methanol Lines Observed
W75N and DR21(OH)W75N

Minh et al. 2010, ApJ, in press

H2COSiO0)
)PU$PSF



4J0

HC3N

SO2



7-"4PVSDFT)0.BTFST0).BTFSTCH3OHCS

Minh et al. 2010, ApJ, in press

DR21(OH)

10.0
05.0
42:23:00.0

Dec. (J2000)

55.0
50.0
45.0
40.0
35.0
22:30.0
25.0(contour) 3.1mm continuum, (triangle) 2.7mm continuum,(x) OH maser, (circle) H2O masers, (square) H2CO peaks,(star) CH3OH 2k-1k (96 GHz) [Liechti et al. 1997]

03.0

02.0

01.0

20:39:00.0

R.A. (J2000)

Minh et al., in preparation

38:59.0

34SO
2C17O

H13CO+C2H3CN

MM1
230 GHz continuum

MM1MM2HC3N

H13CO+H2S

Beam

H2O maser

H2COHC3N

H2CS
H2O maser

IRAC: 8 µm
CH3OH maser

SiO

CH3OH maser

HCOOCH3
Beam

1” = 0.015 pc at 3 kpc
0.015 pc = 3000 AU

SO2

H2O maserMethanol Masers and Thermal Emission
218440: 4(2,2)-3(1,2) E [45.5K]

Maser 1

Maser 3Maser 4Maser 2

SEDs
Spitzer IRAC 190

10.08042:23:00.07060

Dec. (J2000)

50.0

5040.04030.03020.0
20

22:10.01004.0

03.0

02.0

01.0

20:39:00.0R.A. (J2000)

59.0

38:58.0
0

CH3OH abundances at nearmaser positions:(10 K) 1.0 x 1012 cm-2

Summary• Methanol– Oxygen chemistry– Many transitions in radio and IR, and masers– Traces massive star-forming activities

HCHOHH• More observations– thermal emission: toward various objects, including our solar system objects
chemical variations, ..
– methanol masers: relation with other masers, kinematics, association withother features like outflows, or other SF features, ...
probable correlation with regional chemical gradients• More studies of its chemistry - especially surface chemistry on the icy dust grains– ice mixture, evaporation, dissociation,..
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