title: Some Major Questions Concerning Ice Chemistry on Grains

authors: Eric Herbst

abstract: Astrochemistry is undergoing a period of growth spurred on by a generation of new and more powerful telescopes.  Analysis of both current and future data requires, in my view, better and more  detailed chemical simulations for diverse sources in both our galaxy and others.  A few examples of the need for better simulations will be given.  I will then talk about our research program in developing improved gas-grain chemical simulations.   The grain surface chemistry will be emphasized because this field of chemistry is still much less certain than gas-phase chemistry for a variety of reasons [1].  Topics to be discussed include the relative advantages of rate equation and stochastic approaches, the difficulty in converting laboratory results into results useful for the interstellar medium, mechanisms for chemistry occurring in bulk ice, non-thermal desorption mechanisms, and isomeric abundances.   But not only must the chemistry be treated better, the physics must also be improved, and I will discuss our preliminary efforts to develop models including both gas-grain chemistry and 3D magnetohydrodynamics.  Initial calculations show that there is a severe problem in the treatment of molecular hydrogen in interstellar ices.  A possible resolution of this problem will be discussed [2]. 



References
[1]	Q. Chang, & E. Herbst, 2014, ApJ 787, 135.
[2]	U. Hincelin, Q. Chang, & E. Herbst, 2015, A&A, 574, A24