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Astrophysics Research and Analysis Program

Electron impact cross sections for excitation to vibrational levels of the H2 ground state: Elucidating the origin of warm H2 in protoplanetary disks.

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Electron impact cross sections for excitation to vibrational levels of the H2 ground state: Elucidating the origin of warm H2 in protoplanetary disks.

Our objective is to accurately determine the electron impact excitation cross sections for excitation of molecular hydrogen (H2) into vibrational levels of the ground state. Our motivation is to elucidate the role of electron collisions in creating the population of 'warm' H2 that gives rise to (much) of the Far-Ultraviolet (FUV) emission observed in protoplanetary disks of many classical T Tauri stars. Many observed astrophysical FUV emissions result from hydrogen gas being excited by H Lyman-alpha (121.6 nm). However, it is energetically impossible for H Lyman-alpha photons to excite H2 to any electronic state from the ground state unless initially ro-vibrationally excited to a (v, J ) level that is 0.985 eV or above the (0,0) level. This corresponds to molecules primarily in the v =2 and higher levels. Regardless, H Lyman-alpha pumping alone cannot be responsible for H2 FUV emission in regions of low ro-vibrational populations. However, if there is a population of ground state v>1, then H Lyman-alpha pumping can be significant. Recent observations strongly suggest that low energy electrons (< 6 eV), play an important role in the production of ro-vibrationally excited H2 in the warm regions of protoplanetary systems. Unfortunately, the electron impact excitation cross sections, required to quantitatively understand the role of electrons in this context, remain inadequate. To address this, we propose to: 1. Measure electron impact excitation cross sections of H2 for the excitations of the ground state v=0 level to the v=1,2,3,4,5,6, and 7 levels of the ground state via electron energy-loss spectroscopy (EELS) and a novel two-step electron-photon impact procedure. 2. Qualitatively reproduce the observed H2 FUV emissions from protoplanetary systems using our proposed two-step electron-impact + Lyman-alpha irradiation of H2 by recording FUV spectra at several electron impact energies and, therefore, a selection of ground state v level target population distributions. 3. Using our non-local thermodynamic equilibrium (non-LTE) model for H2, in conjunction with the new electron impact excitation cross sections and the known H Lyman-alpha profiles of the lamp and those of Linsky et al. (ApJ, 766(2), 69 2013), we will model the FUV emissions resulting from H Lyman-alpha pumping of electron excited warm H2 in our lab experiments and in protostellar disks, respectively.

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