Table 2: I Zw 18 NW: model parameters and properties.
Parameters Run^a Model^b

of model N0-N1 M1 M2 M3 M4

2-3 4 5 6 7

Central source parameters

T1/10⁴ K 10. 10. 8. 8. 12.

L1/10⁴¹ erg s^-1 3.5-1.6 3.5 2.0 1.6 1.25

$\delta _4$ .12-.67 0.73 0.83 0.93 0.24

T2/10⁴ K 4. 4. 4. 5. 4.

L2/10⁴¹ erg s^-1 3.5-1.6 3.5 2.0 1.6 2.5

log(Q) - 51. 1.04-.71 1.054 0.829 0.779 0.734

-log( $Q_{\rm He}$ /Q) .462-.447 0.445 0.523 0.493 0.539

Ionized shell parameters

$\epsilon$ .0042-.31 1.00 1.00 1.00 1.00

$P_{\rm in}/k$ /10⁵ CGS 38.-8.3 5.01 3.40 2.96 2.71

$P_{\rm out}/k$ /10⁵ CGS - 23.2 21.7 25.4 26.8

$\tau_{\rm c}$ - 5.7 4.9 4.0 4.3

$f^{\rm cov}_1$ 1.-0.43 0.20 0.26 0.23 0.29

$f^{\rm cov}_2$ - - 0.30 0.50 0.60

$\tau_{\rm m}$ or $\tau_{\rm m}$ (2) .96-300. 270. 1.21 1.46 1.00

Elemental abundances by number (H = 10⁷)

C 60-52 45.8 38.8 35.3 47.4

N 8.1-5.3 3.9 4.1 4.0 3.8

O 198-192 172. 168. 162. 173.

Ne 33-30 26.4 25.7 24.8 26.9

S (Table 1) 3.4-3.9 5.0 4.3 4.3 5.0

Ar .77-.90 1.14 0.99 0.98 1.13

Fe 4.8-5.8 5.8 6.1 6.0 6.5

Mean shell properties weighted by $N_{\rm e}$ $\times$ $N_{\rm ion}$ , except for $N_{\rm e}$

$Q_{\rm abs}$ /Q .20-.43 .200 .343 .380 .426

$M_{\rm gas}$ /10⁶ $M_{\odot}$ .15-.92 1.02 1.52 1.74 1.79

H⁺/H .998-.98 .957 .961 .963 .948

O⁰/O .00-.017 .041 .038 .037 .052

O⁺/O .076-.10 .122 .125 .129 .131

O²⁺/O .910-.85 .784 .793 .790 .773

O³⁺/O .014-.03 .049 .043 .043 .042

$T_{\rm e}$ (H⁺)/10⁴ K 1.65-1.73 1.873 1.859 1.896 1.839

$T_{\rm e}$ (O⁰)/10⁴ K 1.61-1.14 1.040 1.013 1.012 1.003

$T_{\rm e}$ (O⁺)/10⁴ K 1.63-1.43 1.320 1.315 1.309 1.272

$T_{\rm e}$ (O²⁺)/10⁴ K 1.66-1.75 1.911 1.915 1.961 1.898

$T_{\rm e}$ (O³⁺)/10⁴ K 1.76-2.1 2.576 2.402 2.449 2.479

$N_{\rm e}$ (H⁺)/ cm^-3 99-18 17.3 11.1 9.8 9.4

$N_{\rm e}$ (O⁰)/ cm^-3 99-8.5 46.7 34.0 37.8 41.1

$N_{\rm e}$ (O⁺)/ cm^-3 99-16 62.7 48.0 54.0 54.8

$N_{\rm e}$ (O²⁺)/ cm^-3 99-18 15.4 10.2 8.9 8.5

$N_{\rm e}$ (O³⁺)/ cm^-3 100-19 10.1 7.4 6.3 5.7

t²(H⁺) .002-.014 .032 .026 .028 .030

t²(O⁰) .001-.018 .022 .024 .023 .020

t²(O⁺) .0013-.02 .021 .024 .024 .023

t²(O²⁺) .002-.008 .013 .010 .010 .010

t²(O³⁺) .0010-05 .0008 .0006 .0007 .0016

^a Constant $N_{\rm H}$ . N0: $N_{\rm H}$ = 92 cm^-3; N1: $N_{\rm H}$ = 17.0 cm^-3.
^b $N_{\rm H}$ from thermal pressure of Eq. (1).

**Table 2:** I Zw 18 NW: model parameters and properties.
Parameters	Run^a	Model^b
of model	N0-N1	M1	M2	M3	M4
	2-3	4	5	6	7
Central source parameters
T1/10⁴ K	10.	10.	8.	8.	12.
L1/10⁴¹ erg s^-1	3.5-1.6	3.5	2.0	1.6	1.25
$\delta _4$	.12-.67	0.73	0.83	0.93	0.24
T2/10⁴ K	4.	4.	4.	5.	4.
L2/10⁴¹ erg s^-1	3.5-1.6	3.5	2.0	1.6	2.5
log(Q) - 51.	1.04-.71	1.054	0.829	0.779	0.734
-log( $Q_{\rm He}$ /Q)	.462-.447	0.445	0.523	0.493	0.539
Ionized shell parameters
$\epsilon$	.0042-.31	1.00	1.00	1.00	1.00
$P_{\rm in}/k$ /10⁵ CGS	38.-8.3	5.01	3.40	2.96	2.71
$P_{\rm out}/k$ /10⁵ CGS	-	23.2	21.7	25.4	26.8
$\tau_{\rm c}$	-	5.7	4.9	4.0	4.3
$f^{\rm cov}_1$	1.-0.43	0.20	0.26	0.23	0.29
$f^{\rm cov}_2$	-	-	0.30	0.50	0.60
$\tau_{\rm m}$ or $\tau_{\rm m}$ (2)	.96-300.	270.	1.21	1.46	1.00
Elemental abundances by number (H = 10⁷)
C	60-52	45.8	38.8	35.3	47.4
N	8.1-5.3	3.9	4.1	4.0	3.8
O	198-192	172.	168.	162.	173.
Ne	33-30	26.4	25.7	24.8	26.9
S (Table 1)	3.4-3.9	5.0	4.3	4.3	5.0
Ar	.77-.90	1.14	0.99	0.98	1.13
Fe	4.8-5.8	5.8	6.1	6.0	6.5
Mean shell properties weighted by $N_{\rm e}$ $\times$ $N_{\rm ion}$ , except for $N_{\rm e}$
$Q_{\rm abs}$ /Q	.20-.43	.200	.343	.380	.426
$M_{\rm gas}$ /10⁶ $M_{\odot}$	.15-.92	1.02	1.52	1.74	1.79
H⁺/H	.998-.98	.957	.961	.963	.948
O⁰/O	.00-.017	.041	.038	.037	.052
O⁺/O	.076-.10	.122	.125	.129	.131
O²⁺/O	.910-.85	.784	.793	.790	.773
O³⁺/O	.014-.03	.049	.043	.043	.042
$T_{\rm e}$ (H⁺)/10⁴ K	1.65-1.73	1.873	1.859	1.896	1.839
$T_{\rm e}$ (O⁰)/10⁴ K	1.61-1.14	1.040	1.013	1.012	1.003
$T_{\rm e}$ (O⁺)/10⁴ K	1.63-1.43	1.320	1.315	1.309	1.272
$T_{\rm e}$ (O²⁺)/10⁴ K	1.66-1.75	1.911	1.915	1.961	1.898
$T_{\rm e}$ (O³⁺)/10⁴ K	1.76-2.1	2.576	2.402	2.449	2.479
$N_{\rm e}$ (H⁺)/ cm^-3	99-18	17.3	11.1	9.8	9.4
$N_{\rm e}$ (O⁰)/ cm^-3	99-8.5	46.7	34.0	37.8	41.1
$N_{\rm e}$ (O⁺)/ cm^-3	99-16	62.7	48.0	54.0	54.8
$N_{\rm e}$ (O²⁺)/ cm^-3	99-18	15.4	10.2	8.9	8.5
$N_{\rm e}$ (O³⁺)/ cm^-3	100-19	10.1	7.4	6.3	5.7
t²(H⁺)	.002-.014	.032	.026	.028	.030
t²(O⁰)	.001-.018	.022	.024	.023	.020
t²(O⁺)	.0013-.02	.021	.024	.024	.023
t²(O²⁺)	.002-.008	.013	.010	.010	.010
t²(O³⁺)	.0010-05	.0008	.0006	.0007	.0016

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