科研之友
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摘要
Absorption spectra toward Herschel 36 (Her 36) for the (A) over tilde (1)Pi <- (X) over tilde (1)Sigma transitions of CH+ in the J = 1 excited rotational level and for the (A) over tilde (2)Delta <- (X) over tilde (2)Pi transitions of CH in the J = 3/2 excited fine structure level have been analyzed. These excited levels are above their ground levels by 40.1 K and similar to 25.7 K and indicate high radiative temperatures of the environment of 14.6 K and 6.7 K, respectively. The effect of the high radiative temperature is more spectacular in some diffuse interstellar bands (DIBs) observed toward Her 36; remarkable extended tails toward red (ETRs) were observed. We interpret these ETRs as being due to a small decrease of the rotational constants upon excitation of the excited electronic states. Along with radiative pumping of a great many high-J rotational levels, this causes the ETRs. In order to study this effect quantitatively, we have developed a model calculation in which the effects of collisions and radiation are treated simultaneously. The simplest case of linear molecules is considered. It has been found that the ETR is reproduced if the fraction of the variation of the rotational constant, beta = (B' - B)/B, is sufficiently high (3%-5%) and the radiative temperature is high (T-r > 50 K). Although modeling for general molecules is beyond the scope of this paper, the results indicate that the prototypical DIBs lambda 5780.5, lambda 5797.1, and lambda 6613.6 which show the pronounced ETRs are due to polar molecules that are sensitive to the radiative excitation. The requirement of high beta favors relatively small molecules with three to six heavy atoms. DIBs lambda 5849.8, lambda 6196.0, and lambda 6379.3 that do not show the pronounced ETRs are likely due to non-polar molecules or large polar molecules with small beta.
- 出版日期2013-8-10
