Abstract
Interstellar emission and absorption lines are discussed, including optical recombination lines, the 21cm hydrogen line, and broad lines such as the interstellar Lyman-alpha line. This chapter ends with a discussion of the curve of growth.
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Burton, W.B.: In: Pfenniger, D., Bartholdi, P. (eds.) The Galactic Interstellar Medium, p. 1. Berlin, Springer (1992). Discussion on the H 21 cm line and its application to the study of the interstellar medium and the Galaxy’s structure. See also the article by W.B. Burton in Verschuur, G.L. & Kellermann, K.I. (eds.). Galactic and Extragalactic Radio Astronomy, referred to in Chapter 2, page 61
Fuhr, J.R., Wiese, W.L.: In: Lide, D. (ed.) Atomic Transition Probabilities. Handbook of Chemistry and Physics, p. 10. CRC, Boca Raton (1991). Basic reference for numerical values of oscillator strengths and other parameters. See also the previous work by W. L. Wiese et al. Atomic Transition Probabilities. Washington, NBS, 1966
Grevesse, N., Noels, A., Sauval, A.J.: In: Holt, S.S., Sonne-Born, G.G. (eds.) Cosmic Abundances. ASP Conference Series, p. 117. American Society of Pacific, San Francisco (1996). Contains an updated discussion on “cosmic” abundance
Jenkins, E.B.: In: Houziaux, L., Butler, H.E. (eds.) IAU Symposium 36, p. 281. Reidel, Dordrecht (1970). Includes an updated discussion on the interstellar Lyman-α line
Mihalas, D., Binney, J.: Galactic Astronomy. Freeman, San Francisco (1981). Excellent discussion on kinematic aspects of the Galaxy’s structure and the LSR. It also includes a summary of the main properties of stars and interstellar gas
Morton, D.C.: Atomic data for resonance absorption lines. I – Wavelengths longward of the Lyman limit. Astrophys J Suppl 77, 119 (1991). Interstellar line f-values compilation. For some examples of ultraviolet interstellar line analysis see also Astrophys. J. vol. 147, p.1017, 1967 and Astrophys. J. vol. 197, p.85, 1975. Figure 4.7 is based on data taken from this reference
Osterbrock, D.: Astrophysics of Gaseous Nebulae and Active Galactic Nuclei. University Science Books, Mill Valley (1989). Referred to in Chapter 1. Excellent discussion on emission lines observed in interstellar photoionized nebulae
Scheffler, H., Elsässer, H.: Physics of the Galaxy and Interstellar Matter. Springer, Berlin (1988). Referred to in Chapter 1. Includes a good discussion on the principal emission and absorption interstellar lines and examples. Figure 4.6 is based on data taken from this reference
Spitzer, L.: Physical Processes in the Interstellar Medium. Wiley, New York (1978). Referred to in Chapter 1. Excellent discussion on the main characteristics of interstellar emission and absorption lines and of curves of growth. Figure 4.8 is based on data taken from this reference
Weaver, H., Williams, D.R.W.: The Berkeley low-latitude survey of neutral hydrogen Part I. Profiles. Astron. Astrophys. 8, 1 (1973). Example of a large survey (Berkeley) of neutral H emission in the interstellar gas. See also Astron. Astrophys. vol. 17, p.1, 1974. Figure 4.1 is based on data taken from this reference
Wilson, T.L., Matteucci, F.: Abundances in the interstellar medium. Astron. Astrophys. Rev. 4, 1 (1992). Review article on interstellar abundances. See also B.D. Savage e K.R. Sembach. Annual Rev. Astron. Astrophys. vol. 34, p.279, 1996
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Exercises
Exercises
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4.1
Make a Gaussian fit to the brightness temperature profile of Fig. 4.1, assuming four interstellar clouds in the considered direction. What would be the column density for each of these clouds?
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4.2
The H Lyman-α line involves a transition between two levels j and k, whose parameters are λ jk = 1,215.67 Å, g j = 2, g k = 6, and A kj = 6.265 × 108 s−1. (a) Calculate the oscillator strength f jk for this line. (b) Calculate the dissipation constant Γ k . (c) Calculate the Doppler width Δν D, assuming a kinetic temperature T = 80 K for the H cloud. (d) Consider a region in the line radiative wings where Δν ≃ 10 Δν D. Show that in this case (Δν)2 ≫ (Γ k /4π)2. (e) Estimate the optical depth in the line wings, if the H column density is N H = 3 × 1020 cm−2. Which fraction of the original intensity is absorbed in this region? Suppose that all H is in the ground state. (f) Calculate the optical depth in the line center. (g) Calculate the FWHM. (h) In which region the line becomes so weak that τ ≃ 1?
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4.4
A spectral line with central wavelength λ is formed in a region characterized by kinetic temperature T and microturbulence velocity v t . (a) Assuming that the broadening of the line is due to the Doppler process, how could the Doppler width of this line be written? (b) Considering the SI III line with λ = 1,206 Å in a cloud with T = 80 K, what must be the value of the turbulent velocity so that the Doppler width increases by a factor of two?
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4.5
Measurements of the equivalent width of the absorption NaI D lines at λ = 5,890 Å in the direction of star HD 190066 (type B1I) give the result W ~ 400 mÅ. (a) Assume this is a weak line and calculate the column density of neutral Na atoms in the direction of the star. Show that in this case, the following relation is valid:
$$ N\simeq \frac{{11.3W}}{{{\lambda^2}f}}, $$where N is in cm−2, W is in mÅ, and λ is in cm. Use f = 0.65. (b) Analysis of the line saturation suggests a correction factor of the order of 6 for the column density. Apply this factor to result (a) and estimate the Na total column density, assuming that 99 % of the sodium atoms are ionized.
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Maciel, W.J. (2013). Interstellar Emission and Absorption Lines. In: Astrophysics of the Interstellar Medium. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-3767-3_4
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