Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~rminchin/arp220pub/2007AAS...21114107G.pdf Дата изменения: Thu Jan 3 21:51:22 2008 Дата индексирования: Sun Apr 13 00:40:44 2008 Кодировка: Поисковые слова: molecules

The Detection of Prebiotic Molecules in the ULIRG Arp 220
Ghosh, T., Catinella B., Lebron M., Lerner M.S., Minchin R., Momjian E., Salter C.J. (NAIC - Arecibo Observatory)
ABSTRACT
Organic molecules in space are thought to form mostly on the surface of dust grains. Heating events, such as the formation of a protostar, release the icy grain mantles into the gas phase. Once released, these molecules may form amino acids by the combination of organic species known as "prebiotic" molecules. Methanimine (CH2NH) is one such pre-biotic molecule. This can either combine with HCN to form aminoacetonitrile (H2CNNH2) and then glycine, the simplest amino acid, via hydrolysis (Strecker Synthesis), or combine with Formic Acid (HCOOH) directly to form glycine (NH2CH2COOH). Methanimine has been detected towards Sgr B2, and tentatively in the nearby galaxy, NGC 253, but has never been seen beyond the neighborhood of our Galaxy (i.e. beyond ~5 Mpc). We are conducting a cm-wave molecular line census in Arp 220, the nearest Ultraluminous Infrared Galaxy (ULIRG) and the prototype OHmegamaser source, using the Arecibo 305-m telescope. Among our results to date is the first detection of the prebiotic molecule methanimine in a distant galaxy. Methanimine is seen in emission, and is likely to be a weak maser. Also detected are three "direct l-type transitions with J = 0" of Hydrogen Cyanide (HCN) from the J= 4, 5 and 6 levels. These HCN lines, previously undetected in any source, appear in absorption against the continuum radiation of this galaxy. The HCN J=2 line should lie near 1370 MHz, but is not detected. This is likely to be due to foreground free-free absorption within Arp 220.

Arp 220 (HST)

Observations
Using the Arecibo 305-m telescope, a complete spectral scan of Arp 220 between 1.1 &10 GHz with 24 kHz resolution is in progress. The WAPP spectrometer is used in its recently-commissioned "dual-board" mode with simultaneous 8 в 100-MHz coverage in both polarizations. A modified Double Position Switching technique was used: a 5-min ON/OFF scan on Arp 220 being followed by a 1-min ON/OFF on a calibrator, J1531+2402.

HCN Absorption

Methanimine Emission

The spectra show the first astronomical detection of the v2=1 direct l-type absorption lines of HCN with vibrational levels J = 4, 5 and 6. The relative line integrals suggest an excitation temperature of ~120-150 K. The non-detection of the J=2 vibrational level is likely to be due to free-free absorption in the foreground ionized ISM.

Reactions
The emission spectrum of the blended 110­111 methanimine (CH2NH) multiplet, whose 6 transitions are marked by vertical lines. Using an angular size upper limit of 0.27в0.21 (from a published H2CO map), we derive a lower limit on the brightness temperature of ~2800 K. Since this is more than the decomposition temperature of methanimine (1300 K), this detection is likely to represent weak maser emission. Methanimine is a pre-biotic molecule which can1form the simplest amino acid, glycine (NH2CH2COOH), either by (1) first combining with HCN to form aminoacetonitrile (NH2CH2CN), with subsequent hydrolysis:

or (2) directly combining with HCOOH: