title: Detectability of Prolate Symmetric-Top Molecules in Diffuse Clouds  by a "Hot-Axis Effect"

authors: M. Araki(1), S. Takano(2), N. Kuze(3), T. Oyama(1), K. Tsukiyama(1)
(1) Faculty of Science Division I, Tokyo University of Science,  Japan
(2) Nobeyama Radio Observatory and the Graduate University for Advanced Studies (Sokendai), Japan
(3) Faculty of Science and Technology, Sophia University, Japan


abstract: The aim of this study is to detect prolate symmetric-top molecules in diffuse clouds by observations of absorption lines by a "hot-axis effect." Molecules in low excitation temperatures in diffuse clouds can be detected by only absorption. A prolate symmetric-top molecule, such as CH3CN and CH3CCH, has a special advantage to detect absorption lines. The K rotation (rotation around molecular axis) is excited by collisions in a high kinetic (collisional) temperate and is not cooled by radiation. On the other hand, the J rotation is cooled well by radiation due to a permanent dipole moment and the low radiative temperature. As a result, the special advantage is that the J = K rotational levels have population concentrations. We call the population concentrations a "hot-axis effect." Absorption lines of the J + 1 <- J (= K) and ĢK = 0 transitions indicated by the red arrows in Figure 2 can be strong. To calculate the population concentrations, both of radiation and collisions should be considered simultaneously. In the case of a linear molecule the equation for rotational distribution by radiation and collisions have been derived by Oka et al. Recently we have derived the equation in the case of a symmetric-top molecule having the hot-axis effect. Using the equation the population concentrations of CH3CN and CH3CCH were calculated and the intensities of absorption lines were estimated. Therefore it is suggested that the absorption lines enhanced by the hot-axis effect can be the powerful probe of the molecules in diffuse clouds.