Transcript High-mass star forming regions: An ALMA view

MASER
Microwave Amplification by
Stimulated Emission of Radiation
• Statistical equilibrium equations
–
–
–
–
general expressions: coupling with radiation field
the excitation temperature: emission, absorption, and masers
the 2-level system: thermalization
the 3-level system: population inversion  MASER
• Astronomical masers
– observational characteristics: identification of a maser line
– masing species and their environments
– the outcome of maser observations: examples
…
Aij Bij Bji Cij Cji
i
j
…
Radiative transfer equation: the line case
2
A21 B21 B12 C21 C12
1
3-level system
3
A32
B32
B23
C32
C23
A31
A21
B21
B12
C21
B31
B13
C31
C13
2
C12
1
J=2
A21 ≈ 10 A10
A21
J=1
A10
J=0
nH2 ~ ncr
Tex(1-0) > TK
nH2 ~ ncr
Tex(1-0) < 0
MASER!!!
Astronomical masers
By definition a line is masing when Tex<0 i.e. τ<0
Identification of OH maser (Rieu et al. 1976)
towards cloud with background quasar:
off-source: TBoff=Tex(1-e-τ)
on-source: TBon=(Tex-Tquasar)(1-e-τ)
 Tex=-10 K, τ=-0.1 for 1720 MHz line of OH
TBon
TBoff
quasar
How to recognise maser lines
• Large TB: TB = |Tex| e|τ| e.g. TB=1010 K for H2O masers!
• Point-like source: maser emission is very beamed
 “spot” with angular size < 1 mas !
• Narrow line width: amplification favours line center
 line width ΔV < 1 km/s (cf. ΔVthermal > a few km/s)
• Multiple lines: velocity coherence needed along l.o.s.
• High time variability: exponential dependence on τ
 very sensitive to small changes of pumping,
amplification path, velocity coherence, orientation
 changes over < 1 day !
How to recognise maser lines
• Large TB: TB = |Tex| e|τ| e.g. TB=1010 K for H2O masers!
• Point-like source: maser emission is very beamed
 “spot” with angular size < 1 mas !
• Narrow line width: amplification favours line center
 line width ΔV < 1 km/s (cf. ΔVthermal > a few km/s)
• Multiple lines: velocity coherence needed along l.o.s.
• High time variability: exponential dependence on τ
 very sensitive to small changes of pumping,
amplification path, velocity coherence, orientation
 changes over < 1 day !
Cesaroni et al. (1991)
maser line
thermal line
maser emission
Hofner et al. (1994)
thermal emission
How to recognise maser lines
• Large TB: TB = |Tex| e|τ| e.g. TB=1010 K for H2O masers!
• Point-like source: maser emission is very beamed
 “spot” with angular size < 1 mas !
• Narrow line width: amplification favours line center
 line width ΔV < 1 km/s (cf. ΔVthermal > a few km/s)
• Multiple lines: velocity coherence needed along l.o.s.
• High time variability: exponential dependence on τ
 very sensitive to small changes of pumping,
amplification path, velocity coherence, orientation
 changes over < 1 day !
Variability of H2O maser
in the star forming region
L1204-G
2 months
1 year
Maser species and environments
• Many lines of many molecules are masing
• Maser lines from different environments:
– Galactic:
• SFR: H2O, OH, CH3OH, H2CO, NH3, SiO, Hnα, …
• Late Type Stars: H2O, OH, SiO, HCN, …
– Extragalactic:
• Star-burst galaxies: H2O, OH, H2CO
• AGN: H2O, OH
Water masers are strongest: in Orion, up to 1010 K
brightness or 1 LO isotropic luminosity in 1 line!!!
H2O molecule
22 GHz maser
61,652,3
Results from maser line observations
• Intensity useless to derive physical parameters of gas!
In fact, Tex< 0 cannot be Tk and τ<0 cannot be Ncol…
• Strong and pointlike: excellent targets for VLBI (mas!)
– Proper motion measurements
– Reconstruction of the 3D velocity field of the gas
– Distance estimates (statistical parallax, phase lag,
kinematical models)
– Magnetic field measurements from polarization (Zeeman)
– Clues on acceleration/deceleration from variability
– Different maser species trace different phenomena (e.g. disk
& outflow)
Proper motions of
H2O maser spots
in star forming region
Torrelles et al. (2001)
H2O masers in galaxy NGC4258
Miyoshi et al. (1995)
red
blue
0.13 pc
red
blue
central mass from Keplerian law:
M = V2R/G = 3.6 107 MO
density > 4 109 MO pc-3
black
hole!
OH & CH3OH
massive
protostar
masers
IRAS 20126+4104
Keplerian
rotation:
M*=7 MO
OH & CH3OH
Hypercompact
HII region
7mm free-free & H2O masers
Moscadelli et al. (2007)
Beltran et al. (2007)
500 AU
Hypercompact
HII region
7mm free-free & H2O masers
expands!
30 km/s
Bibliography
• Genzel 1991, in The Physics of Star Formation and
Early Stellar Evolution, p. 155
• M. Elitzur 1992, Astronomical Masers
• Cohen 1989, Rep. Prog. Phys. 52, 881
• Cosmic Masers: From Proto-Stars to Black Holes,
2002, proc. IAUS 206
• Stahler & Palla 2004, The Formation of Stars
• Astrophysical Masers and their Environment, 2008,
proc. IAUS 242