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Table 3

Radio continuum spectral index, free-free optical depth at 74 MHz, and ISM properties (emission measures (EM) and electronic densities (ne)) computed from our fit to the spectra presented in Fig. 3.

Absorption from extended envelopes of normal H II regions (EHEs)

Source (alias) New results Literature


α τ74 EM (a) ne (a) α (b) τ74 (c) Refs.
[cm−6 pc] [cm−3]
G18.8 + 0.3 (Kes 67) − 0.373 ± 0.010 0.043 ± 0.030 150 ± 130 1.2 ± 0.6 − 0.46 to − 0.42 0.06 ± 0.03 1, 2; 1
G21.8 − 0.6 (Kes 69) − 0.505 ± 0.018 0.088 ± 0.026 440 ± 140 2.1 ± 0.3 − 0.56 to − 0.50 0.11 ± 0.04 1, 2; 1
G29.7 − 0.3 (Kes 75) − 0.659 ± 0.014 0.176 ± 0.035 870 ± 190 2.9 ± 0.3 − 0.73 to − 0.59 0.11 ± 0.04 1, 3; 1
G41.1 − 0.3 (3C 397) − 0.356 ± 0.013 0.141 ± 0.013 720 ± 70 2.7 ± 0.2 − 0.59 to − 0.46 0.13 ± 0.01 1, 2, 4; 1
Absorption: Special cases(d)

Source (alias) New results Literature


α τ74 EM ne α (b) τ74 (c) Refs.
[cm−6 pc] [cm−3]

G23.3 − 0.3 (W41) − 0.628 ± 0.040 1.214 ± 0.155 10 × 103 60 − 0.63 to − 0.48 1.04 ± 0.11 1, 5; 1
G27.4 + 0.0 (Kes 73) − 0.690 ± 0.035 0.878 ± 0.220 8–15 × 103 70–100 − 0.71 ± 0.11 0.72 ± 0.32 1; 1
G31.9 + 0.0 (3C 391) − 0.521 ± 0.004 1.210 ± 0.051 600–2500(e) 10–40(e) − 0.54 to − 0.49 1.1 1, 2, 6; 6
G39.2 − 0.3 (3C 396) − 0.351 ± 0.010 0.063 ± 0.020 − 0.53 to − 0.34 0.12 ± 0.04 1, 2, 4, 7; 1
G43.3 − 0.2 (W49B) − 0.461 ± 0.009 0.580 ± 0.043 6−19 × 103 500 (f) ≃ − 0.47 0.14 ± 0.05 1, 2; 8
No absorption

Source (alias) New results Literature


α α (b) Refs.

G4.5 + 6.8 (Kepler) − 0.655 ± 0.010 − 0.65 to − 0.62 1, 9
G21.5 − 0.9 { − 0.044 ± 0.013 ν ≲ 38 GHz − 0.09 to +0.08 1, 2, 10, 11, 12
− 0.546 ± 0.273 ν > 38 GHz − 0.57 to −0.37 4, 12
G28.6 − 0.1 − 0.690 ± 0.057 − 0.6 to − 0.5 13
G120.1 + 1.4 (Tycho) − 0.624 ± 0.004 − 0.65 to − 0.58 1, 2, 9, 14, 15
G130.7 + 3.1 (3C 58) { − 0.076 ± 0.008 ν ≲ 12 GHz − 0.10 to   − 0.04 1, 2, 11, 14, 16, 17
− 0.383 ± 0.022 ν > 12 GHz − 0.58 to   − 0.45 17, 18

Notes. (a) For EHEs, EM and ne calculations were done using a typical electron temperature of 5000 K and an average path length of 100 pc (Kassim 1989a). (b) Numbers without errors indicate that uncertainties are not given in the references. (c) Free-free continuum optical depth reported in the literature at a frequency ν were extrapolated to 74 MHz according to . (d) The properties of the absorbing medium are analysed in Sect. 6.3. (e) The listed EM and ne values were extracted from the analysis in Brogan et al. (2005). (f)The estimate ne = 500 cm−3 comes from Zhu et al. (2014) for the postshock near-IR line emitting gas.

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