Notes and References. Observations of N_H are taken from the compilation of Bellomi et al. (2020) (Table A.1) or computed from measurements of the reddening E(B − V), as N_H = 5.8 × 10²¹ E(B − V) cm⁻², performed by ⁽¹⁾Chaffee & White (1982), ⁽²⁾van Dishoeck & Black (1989), ⁽³⁾Federman et al. (1994), ⁽⁴⁾Fruscione et al. (1994), ⁽⁵⁾Diplas & Savage (1994), ⁽⁶⁾Crane et al. (1995), ⁽⁷⁾Welsh et al. (1997), ⁽⁸⁾Savage et al. (2001), ⁽⁹⁾Cartledge et al. (2004), ⁽¹⁰⁾Ritchey et al. (2006), ⁽¹¹⁾Bowen et al. (2008), ⁽¹²⁾Sheffer et al. (2008), and ⁽¹³⁾Welty & Crowther (2010). If no measurement of the reddening is found, N_H is derived from the Galactic dust extinction A_V, as N_H = 1.87 × 10²¹ A_V cm⁻², estimated by ^(IR)Schlafly & Finkbeiner (2011) and available on the NASA/IPAC infrared science archive. Observations of N(CH⁺) are taken from ^(a)Federman (1982) (Table 1), ^(b)Lambert & Danks (1986) (Table 1), ^(c)Crane et al. (1995) (Table 2), ^(d)Rachford et al. (2002) (Table 2), ^(e)Gredel et al. (2002) (Table 11) ^(f)Pan et al. (2004) (Table 6), ^(g)Ritchey et al. (2006) (Table 4), and ^(h)Sheffer et al. (2008) (Table 4).