Detections Of 1,3-Propanediol And Hydroxyacetone In IRAS 16293-2422

So far,209 molecular species are detected in interstellar space,which are used for solving different astrophysical problems.Complex organic molecules(COMs),which are defined in astrochemistry as organic molecules consisting of six or more atoms,are widely found in the interstellar medium(hereafter ISM).Among COMs,the polyhy-droxylated compounds(polyols)are components of nucleic acids(e.g.,RNA,DNA),cell membranes and also act as energy sources thus are vital to all known lifeforms.Examples of polyols are sugars,sugar alcohols and sugar acids.1,3–propanediol is a typically sugar alcohol.Hydroxyacetone is one of the smallest molecules that contain both hydroxyl and carbonyl group on neighboring carbon atoms.This sterical config-uration is characteristic for saccharides and determines their biochemical activity.The attempt to search for hydroxyacetone toward the massive star formation region Sagit-tarius B2(N)was unsuccessful.This has caused considerable interests as the existence of polyols/saccharides and the process to form these molecules in ISM may help with answering the question of the origin and evolution of life in the universe.The Class 0 protostar IRAS 16293–2422(hereafter I16293)contains at least two sources A and B with a separation of 5^??(?700 AU).Many line surveys have been carried out towards I16293 by using single dish telescopes and interferometers.The results show that I16293 exhibits a rich chemistry and chemical complexity of complex organic molecules(COMs).There are many oxygen–bearing COMs that have been detected in I16293.Examples are glycolaldehyde,ethanol,ethylene glycol,acetic acid,propanal and others.Since the 1,3–propanediol is a structural derivative of ethylene glycol,and hydroxyacetone closely resembles glycolaldehyde,I16293 is very likely to be a source to search for 1,3–propanediol and hydroxyacetone.We report the first detection of a polyol 1,3–propanediol(CH₂OHCH₂CH₂OH),in the solar–type protostar I16293,using the Atacama Large Millimeter Array(ALMA)science verification(SV)data at Band 4.We analyzed the data and found fifteen un-blended transitions of the g GG^?g conformer of CH₂OHCH₂CH₂OH.From our Local Thermodynamic Equilibrium(LTE)analysis,we derived that the excitation temperature is 200±18 K and the column density of 1,3–propanediol is(6.8±1.0)×10¹⁵cm^-2,which results in an abundance of 4×10^-10with respect to molecular hydrogen.Also,we present our method to identify the g GG^?g conformer of CH₂OHCH₂CH₂OH in the source and propose its formation mechanism.This may provide basis for future study of the origin and evolution of polyols in the interstellar medium.We also report the first detection of CH₃COCH₂OH in the I16293.In total 64transitions of CH₃COCH₂OH with energies ranging from 25 to 331 K are identified.From our LTE analysis,we derived that the rotational temperature of CH₃COCH₂OH is 160±21 K and the column density is(1.2±1.0)×10¹⁶cm^-2,which results in a fractional abundance of 7×10^-10with respect to molecular hydrogen.We present the identification of CH₃COCH₂OH in I16293 and propose a brief formation mechanism.The unambiguous identification of hydroxyacetone may provide the basis for future study of the origin and evolution of saccharides in the interstellar medium.Our understanding of the composition of the interstellar medium is mainly coming from the observation.At present,we identified two new molecules in addition to 209molecules that have been observed in the interstellar medium,which are presented by the CDMS database.This work will enrich our knowledge to understanding the composition of the interstellar medium.