CAB: Towards the RNA-world in the interstellar medium -- detection of
  urea, and search of 2-amino-oxazole and simple sugars
  In the past decade, Astrochemistry has witnessed an impressive increase in
the number of detections of complex organic molecules. Some of these species
are of prebiotic interest such as glycolaldehyde, the simplest sugar, or amino
acetonitrile, a possible precursor of glycine. Recently, we have reported the
detection of two new nitrogen-bearing complex organics, glycolonitrile and
Z-cyanomethanimine, known to be intermediate species in the formation process
of ribonucleotides within theories of a primordial ribonucleic acid (RNA)-world
for the origin of life. In this paper, we present deep and high-sensitivity
observations toward two of the most chemically rich sources in the Galaxy: a
Giant Molecular Cloud in the center of the Milky Way (G+0.693-0.027) and a
proto-Sun (IRAS16293-2422 B). Our aim is to explore whether the key precursors
considered to drive the primordial RNA-world chemistry, are also found in
space. Our high-sensitivity observations reveal that urea is present in
G+0.693-0.027 with an abundance of about 5x10-11. This is the first detection
of this prebiotic species outside a star-forming region. Urea remains
undetected toward the proto-Sun IRAS16293-2422 B (upper limit to its abundance
of less than 2x10-11). Other precursors of the RNA-world chemical scheme such
as glycolaldehyde or cyanamide are abundant in space, but key prebiotic species
such as 2- amino-oxazole, glyceraldehyde or dihydroxyacetone are not detected
in either source. Future more sensitive observations targeting the brightest
transitions of these species will be needed to disentangle whether these large
prebiotic organics are certainly present in space.
