Table 4: Fractional abundances relative to C₂H. The assumed column densities of C₂H are recalled in the first row. CP stands for Cyanopolyyne Peak. References are: ^(a) Pratap et al. 1997, ^(b) Fossé et al. 2001, ^(c) Guélin et al. 1982, ^(d) Fossé 2003, ^(e) Dickens et al. 2000, ^(f) Takano et al. 1998. Since no C₆H and HC₃N measurements were available at the Horsehead IR peak, the corresponding abundances relative to C₂H are given for the hydrocarbon peak.
Molecule Horsehead IC 63 L1688-W TMC-1 L134N

IR peak area (0 $^{\prime \prime }$ , -0 $^{\prime \prime }$ ) ( $\Delta \alpha=-21$ $^{\prime \prime }$ ) (CP)

C₂H (10¹³ cm^-2) 16 5.3 3.0 7.2 ^(a) 15 ^(d)

c-C₃H 0.024 <0.023 - 0.140 ^(b) 0.050 ^(d)

l-C₃H 0.013 <0.110 - 0.012 ^(b) 0.040 ^(d)

c-C₃H₂ 0.058 0.085 0.100 0.800 ^(b) 0.330 ^(d)

l-C₃H₂ <0.002 - - 0.030 ^(b) 0.004 ^(d)

CH₃C₂H <0.069 - - 1.4 ^(a) 0.070 ^(d)

C₄H 0.069 0.062 0.500 2.8 ^(c) 0.735 ^(d)

C₆H 0.004 - - 0.115 ^(b) 0.008 ^(d)

HN¹³C 0.002 <0.002 - 0.056 ^(a) 0.054 ^(e)

HC₃N 0.003 - - 2.2 ^(f) 0.127 ^(e)

**Table 4:** Fractional abundances relative to C₂H. The assumed column densities of C₂H are recalled in the first row. CP stands for Cyanopolyyne Peak. References are: ^(a) Pratap et al. 1997, ^(b) Fossé et al. 2001, ^(c) Guélin et al. 1982, ^(d) Fossé 2003, ^(e) Dickens et al. 2000, ^(f) Takano et al. 1998. Since no C₆H and HC₃N measurements were available at the Horsehead IR peak, the corresponding abundances relative to C₂H are given for the hydrocarbon peak.
Molecule	Horsehead	IC 63	L1688-W	TMC-1	L134N
	IR peak area	(0 $^{\prime \prime }$ , -0 $^{\prime \prime }$ )	( $\Delta \alpha=-21$ $^{\prime \prime }$ )	(CP)
C₂H (10¹³ cm^-2)	16	5.3	3.0	7.2 ^(a)	15 ^(d)
c-C₃H	0.024	<0.023	-	0.140 ^(b)	0.050 ^(d)
l-C₃H	0.013	<0.110	-	0.012 ^(b)	0.040 ^(d)
c-C₃H₂	0.058	0.085	0.100	0.800 ^(b)	0.330 ^(d)
l-C₃H₂	<0.002	-	-	0.030 ^(b)	0.004 ^(d)
CH₃C₂H	<0.069	-	-	1.4 ^(a)	0.070 ^(d)
C₄H	0.069	0.062	0.500	2.8 ^(c)	0.735 ^(d)
C₆H	0.004	-	-	0.115 ^(b)	0.008 ^(d)
HN¹³C	0.002	<0.002	-	0.056 ^(a)	0.054 ^(e)
HC₃N	0.003	-	-	2.2 ^(f)	0.127 ^(e)

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