A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

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Table 2

MALT90 spectral-line transitions.

Transition^a	ν^b	E_u/k_B^c	n_crit^d	Comments^e
	[MHz]	[K]	[cm^-3]

H¹³CO⁺(J = 1−0)	86 754.330	4.16	1.6 × 10⁵	high-density and ionisation tracer;
				J = 1−0 is split into six hyperfine (hf) components^f
SiO(J = 2−1)	86 847.010	6.25	2 × 10⁶	shocked-gas/outflow tracer
HN¹³C(J = 1−0)	87 090.859	4.18	2 × 10⁵^g	high-density tracer;
				J = 1−0 is split into 11 hf components
				with four having a different frequency^h
C₂H(N_J, F = 1_3/2, 2 − 0_1/2, 1)	87 316.925	4.19	2 × 10⁵ⁱ	a tracer of photodissociation regions (PDRs);
				N_J = 1_3/2 − 0_1/2 is split into three hf components^j
HNCO(J_{K_a, K_b} = 4_0, 4 − 3_0, 3)	87 925.238	10.55	4.5 × 10⁶^k	hot core and shock-chemistry tracer;
				six hf components^l; a-type transition (ΔK_a = 0)
HNCO(J_{K_a, K_b} = 4_1, 3 − 3_1, 2)	88 239.027	53.86	2.3 × 10⁶^k	six hf components^l; a-type transition (ΔK_a = 0)
HCN(J = 1 − 0)	88 631.847	4.25	2.4 × 10⁶	high-density and infall tracer;
				J = 1 − 0 is split into three hf components^m
HCO⁺(J = 1 − 0)	89 188.526	4.28	1.7 × 10⁵	high-density, infall, and ionisation tracer;
				enhanced in outflows due to shock-induced UV radiationⁿ
HC¹³CCN(J = 10 − 9, F = 9 − 8)	90 593.059	23.91	1.7 × 10⁵^o	hot-core tracer; five hf components
HNC(J = 1 − 0)	90 663.572	4.35	2.9 × 10⁵	high-density tracer; three hf components
¹³C³⁴S(J = 2 − 1)	90 926.036	6.54	4.3 × 10⁵^p	high-density tracer
HC₃N(J = 10 − 9)	91 199.796	24.01	5.3 × 10⁵	high-density/hot-core tracer;
				six hf components
CH₃CN(J_K = 5₁ − 4₁)	91 985.316	20.39	4 × 10⁵^q	hot-core tracer; seven hf components
H41α	92 034.475	89.5^r	~ 5^s	ionised gas tracer; the principal quantum number
				changes from n = 42 to 41 → α-type radio recombination line
¹³CS(J = 2 − 1)	92 494.303	6.66	5 × 10⁵^k	high-density tracer; three hf components
N₂H⁺(J = 1 − 0)	93 173.480	4.47	1.5 × 10⁵	high-density/CO-depleted gas tracer;
				J = 1 − 0 line has 15 hf components out of which
				seven have a different frequency^t

Notes.

^(a)

The rotational transitions here occur in the vibrational ground state (v = 0).

^(b)

Rest frequencies adopted from the MALT90 webpage (http://malt90.bu.edu/parameters.html).

^(c)

Upper-state energy divided by the Boltzmann constant.

^(d)

Critical density at 15 K unless otherwise stated. Unless otherwise stated, the Einstein A coefficients and collision rates were adopted from the Leiden Atomic and Molecular Database (LAMDA (Schöier et al. 2005); http://home.strw.leidenuniv.nl/~moldata/).

^(e)

Comments on the species and transition in question.

^(f)

See e.g. Schmid-Burgk et al. (2004).

^(g)

Collision rate for HNC from LAMDA was used to estimate n_crit.

^(h)

van der Tak et al. (2009); Padovani et al. (2011).

⁽ⁱ⁾

From Lo et al. (2009).

^(j)

Reitblat (1980); Padovani et al. (2009); Spielfiedel et al. (2012).

^(k)

n_crit at 20 K.

^(l)

Lapinov et al. (2007).

^(m)

See e.g. Cao et al. (1993).

⁽ⁿ⁾

Rawlings et al. (2000), 2004.

^(o)

n_crit was estimated using the Einstein A coefficient from the Cologne Database for Molecular Spectroscopy (CDMS; Müller et al. 2005; http://www.astro.uni-koeln.de/cdms) and the HC₃N collision rate at 15 K from LAMDA.

^(p)

n_crit was estimated using the Einstein A coefficient from CDMS and the ¹²C³⁴S collision rate at 20 K from LAMDA.

^(q)

From SJF12.

^(r)

The energy of the n = 42 level (E = −E₀/n², where E₀ = 13.6 eV).

^(s)

Critical electron density at 10⁴ K (see Appendix D.1 in Gordon & Sorochenko 2009).

^(t)

See Table 2 in Pagani et al. (2009) and Table 1 in Keto & Rybicki (2010).

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