Observing in the infrared presents a number of advantages, in addition to being the only way to probe species and physical domains that are not observed at other wavelengths.
The most immediate advantage is the limited interstellar extinction, which allow the observer to diagnose physical conditions of jets embedded in dense environments, such as those of young protostars. Given AK^0.1Av, in the infrared one can ideally observe regions ten times more embedded than at visual wavelengths. In practice, this advantage is limited by technical problems, which make IR observations less sensitive than in the optical (see below). IR observations, in addition, make it possibile to diagnose gas at low excitation, not emitted at optical wavelength, and thus to reveal physical conditions that would be otherwise unproven. Indeed, gas excited at temperatures < 5000 K and densities >104 cm~3 cools down only through atomic and molecular IR lines.
Important lines from abundant atoms and ions such as [C i] , [S i] , [S ii] A6731, [N i] , [O i] A6300, [Fe ii] are found in a wide range of wavelength spanning near- to far-IR (see Sect 3.2 and Table 1), while H2 has a near-IR
Table 1. Bright forbidden lines in the near- and mid-IR spectral range
Species Transition Xa Eup nbcr
|im K cnf3
[S ii] 2Pi/2 -2 D5/2 [C i] 1D2 -3 Pi [C i] 1D2 -3 P2
1.040 4.12104 3.0106
1.041 4.12104 3.0106
1.029 3.52104 1.7106
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