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Excitation Collisionnelle du formaldéhyde interstellaire : Théorie et Observations

Abstract : Molecular hydrogen is the simplest and most abundant molecule in the Universe. Owing to the possible different nuclear spin alignments, it presents two forms, ortho- and para-H$_2$. The ortho-to-para ratio of H$_2$ is a fundamental parameter to understand the (inelastic and reactive) collisional processes in molecular astrophysical media. In this thesis, we focus on the determination of the ortho-to-para ratio of H$_2$ in dark clouds, where H$_2$ cannot be directly observed. To this aim, we use the formaldehyde molecule (H2CO) whose rotational excitation in these sources is dominated by H2 collisions. A peculiar transition has been selected in our study: the 6-cm transition (4.8 GHz) of ortho-H$_2$CO observed in absorption against the cosmic microwave background. Previous studies have shown that collisional effects can explain this (antimaser) absorption but the impact of the ortho- and para- form of H$_2$ on this absorption has been so far ignored. We first present high-precision computations of the (de)excitation rates of (ortho-, para-) H$_2$CO by (ortho-, para-)H$_2$. Significant differences are observed between ortho- and para-H$_2$ rates. We then use these collisional rates in radiative transfer calculations in order to model 6-cm observations carried out with the Green Bank Telescope towards 3 different dark clouds (B68, L134N and TMC-1). We show that the differences in para and ortho-H2 collisional rates have a significant impact on the modeling, allowing us to put interesting constraints on the ortho-to-para ratio of H$_2$ in dark clouds. We consider in detail the example of B68, prototype of pre-stellar cores.
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Contributor : Nicolas Troscompt <>
Submitted on : Monday, February 8, 2010 - 7:37:25 PM
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Nicolas Troscompt. Excitation Collisionnelle du formaldéhyde interstellaire : Théorie et Observations. Astrophysique [astro-ph]. Université Joseph-Fourier - Grenoble I, 2009. Français. ⟨tel-00454581⟩

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