1. Ay126: Fine Structure Line Emission from the Galaxy
S. R. Kulkarni

2. Cooling of CNM and WNM In both components H is largely neutral
Cooling is primarily by lines which are easily excited (relative to kT)
After H, He the most abundant elements are O, N, C
The ionization potentials (IP) are: 11.26(C), (N), (O) eV
Species
[CI], [CII], [OI], [SiII], SII] and [FeII] are present in the neutral medium. They are also present in the ionised medium, but generally in smaller amounts than more ionised species.
Ground state of NI is 4S (so no FSL)

3. Fine structure lines are forbidden
Arise in the same electronic configuration
Magnetic or Quadrupole transitions
Small A coeffecients
Need to compute critical density
Sometimes self absorption can be important

4. Ion / spectrum Transition Collider ΔΕ/k [K] λ [μm]
C+ / [CII] O0 / [OI]
Si+ / [SII]
2P3/2 -
2P1/2
H2, H, e- 92
157.7
3P1 -
3P2
H, e-
228
63.2
3P0 -
3P1 99
146
(H), e-
413
34.8
Of the most important forbidden lines in the ISM, only [CI], [CII], [OI], [SiII], SII] and [FeII] are present in the neutral medium. They are also present in the ionised medium, but generally in smaller amounts than more ionised species. .

5. The most important forbidden lines in the interstellar medium
The most important forbidden lines in the interstellar medium. Only CI, CII, OI, SiII, SII, and FeII are present in the neutral medium. They are also present in the ionised medium, but generally in smaller amounts than more ionised species. The collision strengths Ωul are for
collisions with electrons at a temperature of 104 K. The
critical densities
ncrit
= Aul/⟨σulv⟩
correspond to
collisions either with electrons (for Te ≃ 104 K), or with H2 molecules when between round brackets (for Tk
≃100K).

6. OII OI

7. CII 157 micron line

8. Observations of CII line
HII regions studied with KAO
First sensitive all-sky survey was carried out by FIRAS (Far-infrared Spectrometer) on COBE
Wide spread [CII] and [NII] 205 micron was detected
Correlation between [CII], [NII} and FIR was found
Herschel provided both high angular resolution (12 arcsec) and high velocity (0.1 km/s) observations of [CII].
Five hundred lines of sight in the Galaxy were observed

9. Contribution to the C+ line
HII regions
Photo-dissociation regions (PDR)
Interface between HII regions and molecular clouds
CNM
WNM
WIM

10. References Herschel Survey in [CII] line
J. L. Pineda t al. A&A 554, A103 (2013)
J. L. Pineda et al. A&A 570 A121 (2014)
Physics & Inference of [CII] line
P. F. Goldsmith et al. ApJS, 203, 13(2012)
Survey in the [NII] line
P. F. Goldsmith et al., ApJ 814, 133 (2015)
From ratios of fine structure lines concludes that this line does not arise in the WIM

11. Inferences from Analysis of Galactic Plane Observations of [CII]
Inferences from Analysis of Galactic Plane Observations of [CII] .. Pineda et al. 2013
[CII] emission arises from PDR (47%), ”CO-dark” H2 gas (28%), CNM (21%) and ionized gas (4%).
Global Inference: