title: Discovery of Objects Richest in CH3NH2, Candidates for Future Glycine Surveys

authors: Masatoshi Ohishi (NAOJ)

abstract: It is widely accepted that prebiotic chemical evolution from small to large and complex molecules would have resulted in the Origin of Life. On the other hand there are two conflicting views where inorganic formation of complex organic molecules (hereafter COMs) occurred in the early Earth, on the Earth or out of the Earth. Ehrenfreund et al.[1] indicated that exogenous delivery of COMs by comets and/or asteroids to the early Earth could be larger than their terrestrial formation by three orders of magnitude. If amino acids are formed in interstellar clouds, significant amount of them may be delivered to planets. Detection of amino acids would accelerate the discussion concerning the universality of 'life'.

So far, many trials to detect the simplest amino acid, glycine (NH2CH2COOH), were made towards Sgr B2 and other high-mass forming regions, but none of them were successful. One idea to overcome this situation would be to search for precursors to glycine. Although the chemical evolution of interstellar N-bearing COMs is poorly known, methylamine (CH3NH2) is proposed as one precursor to glycine. CH3NH2 can be formed from abundant species, CH4 and NH3, on icy dust surface. Further methyleneimine (CH2NH) would be related to CH3NH2 [2][3]. Another possible route to form CH3NH2 is hydrogenation to HCN on dust surface [4][5]: HCN -> CH2NH -> CH3NH2. 

In the past CH2NH was reported only in Sgr B2, W51, Orion KL, and G34.3+0.15 [2]. In April 2013, we extended this survey by using the Nobeyama 45 m radio telescope towards CH3OH-rich sources. We succeeded to detect four new CH2NH sources. The derived fractional abundances of CH2NH relative to H2 are as high as 6x10^-8, implying that CH2NH may exist widely in the ISM [6].

If this is the case, further hydrogenation would efficiently produce CH3NH2. Based on this idea we conducted a survey of CH3NH2 towards CH2NH-rich sources in spring of 2014, and succeeded to detect CH3NH2 towards two sources. The estimated fractional abundance of CH3NH2 to H2 was ~ 10^-8, about 10 times higher than that reported towards SgrB2(N) [7].

Since it is well known that CO2 exists in most of molecular clouds, CH3NH2 could be a direct precursor candidate to glycine -- the simplest amino acid- , CH3NH2-rich sources would turn into promising glycine targets by ALMA. Such studies would also accelerate discussion regarding the exogenous delivery of prebiotic species to planets and connection between the Universe and life.

References: 
[1] Ehrenfreund et al. Rep. Prog. Phys., 65, 1427, 2002; 	[2] Holtom et al., ApJ, 626, 940, 2005; 	[3] Kim & Kaiser et al. ApJ, 729:68, 2011; 	[4] Dickens et al., ApJ, 479, 307, 1997;		[5] Theule et al., 2011, A& A, 534, A64;	[6] Suzuki et al., October 17, 2014, ISM workshop, Sapporo, Japan; 	[7] Halfen et al., ApJ, 767, id.66