Shielding functions for atoms and molecules in the ISRF

The shielding functions given here are explained in the various overview papers. These describe the reduction of photodissociation or ionisation rates when an interstellar radiation field is shielded by gas or dust, so that

k = θ×k0

where k < k0 are the shielded and unshielded rates and θ is the shielding function. We make independent calculations assuming shielding by interstellar dust, and line shielding by atomic and molecular gases (H, H2, CO, N2). There are some special calculations for the self-shielding of CO and N2.

[Download all dust and line shielding functions in a zip file.]

Click on a link below to get dust continuum and line shielding functions for that atom or molecule.

Al AlH C C2 C2H C2H2 C2H4 C2H5OH
C2H6 C3 C3H7OH CH CH+ CH2 CH2+ CH3
CH3CHO CH3CN CH3NH2 CH3OH CH3SH CH4 CH4+ CN
CO CO+ CO2 CS CS2 Ca Ca+ Cl
Co Cr Fe H H- H2 H2+ H2CO
H2O H2O2 H2S HC3H HC3N HCN HCO HCO+
HCl HCl+ HF HO2 K Li LiH Mg
MgH Mn N N2 N2O NH NH+ NH2
NH2CHO NH3 NO NO2 Na NaCl NaH Ni
O O2 O2+ O3 OCS OH OH+ P
PH PH+ Rb S S2 SH SH+ SO
SO2 Si SiH SiH+ SiO Ti Zn c-C3H
c-C3H2 l-C3H l-C3H2 l-C4 l-C4H l-C5H

Tabulated dust-shielding parameters

The dust shielding functions for isotropic interstellar radiation penetrating an infinite-slab constant-density dust mass decay approximately exponentially with dust column density (proportional to visual extinction AV). Then, the shielding function may be conveniently parameterised as:

θ = exp(−γexp×AV)

The accuracy of this parameterisation can be improved over the intermediate range of AV=0.1 to 3 by including an additional prefactor:

θ = (k0fit/k0)exp(−γfit×AV)

A more accurate parameterisation can be made with second-order integral exponential functions

θ = E2E2×AV).

To determine a photodestruction rate as a function of extinction:

k = k0×θ

Where k0 is the rate at the edge of the interstellar cloud. This will be slightly greater than 0.5× the unattenuated rates listed in this database, due to the restriction to 2π sr illumination, plus some backscattering from the cloud.

Fitted values for the various parameterisation schemes are given below for each atom and molecule.

[Download all γ factors in a csv file.]
Species Products γE2 γexp k0fit/k0 γfit
Al photoionisation 1.62 2.36 0.62 2.12
AlH photodissociation 1.67 2.42 0.63 2.19
AlH photoionisation 1.86 2.67 0.58 2.39
C photoionisation 2.77 3.76 0.50 3.42
C2 photodissociation 2.15 3.04 0.44 2.63
C2 photoionisation 3.12 4.19 0.52 3.86
C2H photodissociation 1.85 2.67 0.57 2.38
C2H2 photodissociation 1.83 2.64 0.56 2.35
C2H2 photoionisation 2.91 3.92 0.50 3.58
C2H4 photodissociation 1.70 2.49 0.54 2.19
C2H4 photoionisation 2.67 3.63 0.47 3.25
C2H5OH photodissociation 1.93 2.77 0.51 2.43
C2H5OH photoionisation 2.75 3.72 0.47 3.34
C2H6 photodissociation 2.07 2.94 0.50 2.59
C2H6 photoionisation 3.10 4.17 0.51 3.83
C3 photodissociation 1.64 2.39 0.61 2.14
C3 photoionisation 3.00 4.03 0.50 3.68
C3H7OH photodissociation 1.92 2.76 0.52 2.43
C3H7OH photoionisation 2.74 3.71 0.47 3.33
CH photodissociation 1.36 2.12 0.57 1.84
CH photoionisation 2.70 3.67 0.50 3.33
CH+ photodissociation 2.63 3.54 0.34 3.00
CH2 photodissociation 1.61 2.35 0.62 2.11
CH2+ photodissociation 1.86 2.73 0.40 2.27
CH3 photodissociation 1.73 2.50 0.61 2.25
CH3 photoionisation 2.36 3.26 0.50 2.92
CH3CHO photodissociation 1.69 2.46 0.59 2.19
CH3CHO photoionisation 2.49 3.42 0.50 3.08
CH3CN photodissociation 2.18 3.06 0.48 2.70
CH3CN photoionisation 3.12 4.19 0.50 3.84
CH3NH2 photodissociation 1.62 2.37 0.61 2.13
CH3NH2 photoionisation 2.31 3.21 0.49 2.86
CH3OH photodissociation 1.92 2.76 0.50 2.41
CH3OH photoionisation 2.80 3.78 0.48 3.42
CH3SH photodissociation 1.72 2.50 0.58 2.23
CH3SH photoionisation 2.29 3.18 0.51 2.85
CH4 photodissociation 2.19 3.08 0.48 2.71
CH4 photoionisation 3.21 4.31 0.51 3.97
CH4+ photodissociation 2.22 3.11 0.51 2.77
CN photodissociation 2.55 3.50 0.51 3.16
CO photodissociation 2.88 3.88 0.48 3.51
CO+ photodissociation 2.04 2.89 0.54 2.59
CO2 photodissociation 2.52 3.44 0.44 3.03
CS photodissociation 1.95 2.77 0.57 2.49
CS photoionisation 2.63 3.59 0.51 3.25
CS2 photodissociation 1.72 2.49 0.57 2.21
CS2 photoionisation 2.62 3.57 0.49 3.21
Ca photoionisation 1.60 2.34 0.61 2.10
Ca+ photoionisation 3.04 4.09 0.51 3.76
Cl photoionisation 3.21 4.30 0.51 3.97
Co photoionisation 1.70 2.47 0.59 2.21
Cr photoionisation 1.63 2.39 0.57 2.12
Fe photoionisation 1.81 2.62 0.56 2.33
H photoionisation 3.25 4.35 0.51 4.01
H- photoionisation 0.74 1.24 0.72 1.08
H2 photodissociation 3.11 4.18 0.51 3.85
H2+ photodissociation 1.94 2.78 0.51 2.44
H2CO photodissociation 1.74 2.54 0.55 2.24
H2CO photoionisation 2.69 3.66 0.49 3.30
H2O photodissociation 1.80 2.63 0.50 2.28
H2O photoionisation 3.18 4.27 0.51 3.94
H2O2 photodissociation 1.80 2.60 0.56 2.31
H2O2 photoionisation 2.88 3.88 0.48 3.52
H2S photodissociation 1.83 2.64 0.55 2.34
H2S photoionisation 2.58 3.53 0.49 3.18
HC3H photodissociation 1.43 2.15 0.62 1.91
HC3N photodissociation 1.79 2.59 0.57 2.31
HC3N photoionisation 3.02 4.07 0.51 3.73
HCN photodissociation 2.23 3.12 0.49 2.77
HCN photoionisation 3.24 4.34 0.51 4.01
HCO photodissociation 1.67 2.43 0.61 2.18
HCO+ photodissociation 2.68 3.67 0.55 3.37
HCl photodissociation 2.02 2.88 0.47 2.50
HCl photoionisation 3.18 4.27 0.51 3.94
HCl+ photodissociation 2.12 3.01 0.42 2.58
HF photodissociation 2.13 3.00 0.51 2.66
HO2 photodissociation 1.69 2.46 0.59 2.20
K photoionisation 1.70 2.48 0.56 2.19
Li photoionisation 1.68 2.45 0.59 2.19
LiH photodissociation 1.21 1.81 0.71 1.64
Mg photoionisation 1.67 2.43 0.58 2.16
MgH photodissociation 1.54 2.30 0.59 2.04
Mn photoionisation 1.61 2.35 0.61 2.10
N2 photodissociation 3.16 4.25 0.51 3.92
N2O photodissociation 1.98 2.81 0.54 2.50
N2O photoionisation 3.20 4.30 0.51 3.96
NH photodissociation 1.83 2.63 0.54 2.33
NH photoionisation 3.24 4.34 0.51 4.00
NH+ photodissociation 1.34 2.07 0.52 1.74
NH2 photodissociation 1.57 2.31 0.60 2.05
NH2 photoionisation 2.94 3.97 0.49 3.62
NH2CHO photodissociation 1.64 2.40 0.58 2.13
NH2CHO photoionisation 2.62 3.57 0.48 3.21
NH3 photodissociation 1.80 2.61 0.53 2.30
NH3 photoionisation 2.54 3.49 0.50 3.14
NO photodissociation 1.75 2.56 0.50 2.21
NO photoionisation 2.46 3.38 0.45 2.99
NO2 photodissociation 1.71 2.50 0.55 2.20
NO2 photoionisation 2.77 3.75 0.48 3.38
Na photoionisation 1.81 2.62 0.53 2.31
NaCl photodissociation 1.50 2.20 0.65 1.98
NaH photodissociation 1.17 1.77 0.71 1.59
Ni photoionisation 1.67 2.43 0.59 2.17
O2 photodissociation 1.69 2.45 0.59 2.19
O2 photoionisation 3.14 4.22 0.51 3.89
O2+ photodissociation 1.62 2.38 0.58 2.11
O3 photodissociation 1.49 2.25 0.56 1.96
O3 photoionisation 3.19 4.28 0.51 3.95
OCS photodissociation 1.68 2.46 0.57 2.18
OCS photoionisation 2.86 3.85 0.48 3.49
OH photodissociation 1.83 2.66 0.50 2.31
OH+ photodissociation 2.96 3.97 0.48 3.60
P photoionisation 2.51 3.45 0.51 3.11
PH photodissociation 1.71 2.48 0.58 2.21
PH+ photodissociation 1.16 1.93 0.49 1.57
Rb photoionisation 1.54 2.33 0.53 2.02
S photoionisation 2.58 3.53 0.48 3.17
S2 photodissociation 1.28 1.90 0.70 1.73
S2 photoionisation 2.36 3.27 0.48 2.90
SH photodissociation 1.64 2.40 0.60 2.15
SH photoionisation 2.90 3.92 0.50 3.57
SH+ photodissociation 1.79 2.83 0.27 2.19
SO photodissociation 1.93 2.76 0.57 2.48
SO photoionisation 2.52 3.46 0.53 3.15
SO2 photodissociation 1.94 2.77 0.52 2.45
SO2 photoionisation 3.16 4.25 0.51 3.92
Si photoionisation 1.81 2.61 0.56 2.33
SiH photodissociation 1.24 1.95 0.62 1.71
SiH+ photodissociation 0.94 1.55 0.61 1.30
SiO photodissociation 1.85 2.66 0.54 2.35
Ti photoionisation 1.96 2.81 0.51 2.47
Zn photoionisation 2.69 3.66 0.50 3.32
c-C3H photodissociation 1.45 2.15 0.64 1.93
c-C3H2 photodissociation 1.54 2.26 0.63 2.03
l-C3H photodissociation 1.32 2.08 0.59 1.82
l-C3H2 photodissociation 1.74 2.51 0.60 2.26
l-C4 photodissociation 1.52 2.22 0.64 2.00
l-C4H photodissociation 1.62 2.36 0.62 2.12
l-C5H photodissociation 1.14 1.76 0.66 1.56