Table 6: Line opacity ratios derived from our PdBI observations.
Lines Velocity Line opacity

interval ratios

HCO⁺/H¹³CO⁺ ( $2\leftarrow 1$ ) SW wings 28 (3)

HCN/H¹³CN ( $2\leftarrow 1$ ) SW wings 40 (-5+7)

HNC/HN¹³C ( $2\leftarrow 1$ ) SW 27 (3)

H¹³CN/HC¹⁵N ( $2\leftarrow 1$ ) SW 4.2 (0.3)

HN¹³C/H¹⁵NC ( $2\leftarrow 1$ ) SW 6 (-2+4)

HNC/H¹⁵NC ( $2\leftarrow 1$ ) SW 166 (-58+194)

HCO⁺/HC¹⁸O⁺ ( $2\leftarrow 1$ ) SW wings 53 (-10+16)

H¹³CO⁺/HC¹⁸O⁺ ( $2\leftarrow 1$ ) SW 2.01 (0.07)

HC¹⁸O⁺/HC¹⁷O⁺ ( $2\leftarrow 1$ ) SW 12 (-2+3)

CS/C³⁴S ( $4\leftarrow 3$ )) SW 10.4 (-0.7+0.8)

H₂S/H₂³⁴S ( $1_{10} \leftarrow 1_{01}$ ) SW 8 (1.5)

**Table 6:** Line opacity ratios derived from our PdBI observations.
Lines	Velocity	Line opacity
	interval	ratios
HCO⁺/H¹³CO⁺ ( $2\leftarrow 1$ )	SW wings	28 (3)
HCN/H¹³CN ( $2\leftarrow 1$ )	SW wings	40 (-5+7)
HNC/HN¹³C ( $2\leftarrow 1$ )	SW	27 (3)
H¹³CN/HC¹⁵N ( $2\leftarrow 1$ )	SW	4.2 (0.3)
HN¹³C/H¹⁵NC ( $2\leftarrow 1$ )	SW	6 (-2+4)
HNC/H¹⁵NC ( $2\leftarrow 1$ )	SW	166 (-58+194)
HCO⁺/HC¹⁸O⁺ ( $2\leftarrow 1$ )	SW wings	53 (-10+16)
H¹³CO⁺/HC¹⁸O⁺ ( $2\leftarrow 1$ )	SW	2.01 (0.07)
HC¹⁸O⁺/HC¹⁷O⁺ ( $2\leftarrow 1$ )	SW	12 (-2+3)
CS/C³⁴S ( $4\leftarrow 3$ ))	SW	10.4 (-0.7+0.8)
H₂S/H₂³⁴S ( $1_{10} \leftarrow 1_{01}$ )	SW	8 (1.5)

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