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Project Progress

As part of the overall Astrobiology Node at the NASA Goddard Space Flight Center, whose goal is an understanding of the Origin and Evolution of Organics in Planetary Systems (Mike Mumma, P.I.), Co-Investigator Blake is directing both laboratory and astronomical spectroscopy programs. The goal of these observations is to determine whether complex organics are detectable in the circumstellar accretion disks that encircle young stars and in the comae of comets. Targets of study are being selected in collaboration with Node scientists investigating the organic speciation in carbonaceous chondrites, and this past year has seen exceptional progress along many fronts. The experimental work is being carried out in Prof. Blake’s laboratories in the Caltech Beckman Institute, and the observational research in FY05 and beyond leans heavily on the extensive suite of Caltech telescopes, especially the Combined Array for Research in Millimeter Astronomy (or CARMA, the merging of the Caltech Owens Valley Millimeter Array and the Berkeley-Illinois-Maryland Array at Hat Creek, CA) and the Caltech Submillimeter Observatory (CSO). In the laboratory, the Blake group operates both a Fourier Transform MicroWave (FTMW) spectrometer, a THz laser difference frequency photomixer spectrometer, and high resolution mid- to near-IR diode laser spectrometers. This results in continuous coverage from the microwave to the optical with exceptional sensitivity. For observational work, the CSO has receivers that operate through all of the atmospheric windows between 180-980 GHz, and CARMA will offer superb imaging performance in the 100/230 GHz windows once the telescopes are fully operational at the new site in the Inyo Mountains (expected first light in winter 2005). In the far-infrared, we plan to utilize the heterodyne receivers under construction at Caltech/JPL for SOFIA and HERSCHEL as these platforms become operational (CY06/CY08, respectively), while our infrared observations are conducted primarily with the Keck telescope through time allocated to Caltech and with the IRS instrument aboard the Spitzer Space Telescope.

Over the past year our major millimeter-wave observational analyses centered on the OVRO and BIMA Array spectra and images of the comets C/NEAT (2001 Q4) & C/LINEAR (2002 T7) that reached perihelion on 2004 April 26 and 2004 May 17 respectively. Both comets passed within 0.3-0.4 AU of the Earth, and were well placed for observations from the northern hemisphere. With water production rates near perihelion in excess of ~10²⁹ mol/s, these apparitions provided us with a unique opportunity to test hypotheses about the physical and chemical processes in the inner regions of cometary comae developed from our highly successful observations of Comet Hale-Bopp in 1997 (Blake et al. 1999). Our first paper on the millimeter-wave observations of HCN has now appeared in the Astrophysical Journal (Friedel et al. 2005), and our next step is to compare the physical model developed to explain the HCN emission with those developed by the GSFC Astrobiology team to explain a suite of infrared observations of these comets. Furthermore, these results form an interesting counterpoint to the compelling Deep Impact spectra gathered at Keck by the Astrobiology team and recently accepted for publication in Science (Mumma et al. 2005).

In addition, a major focus of further data analysis will be searches for complex organics, especially sugars and polyalcohols, whose spectra we have recently assigned in the laboratory (Widicus et al. 2003, 2004). CSO observations undertaken near perihelion (D. Lis 2004, priv. commun.) have revealed that C/LINEAR (2002 T7) has the highest
CH₃OH/H₂O yet measured for a comet, and so should provide an excellent target for the more complex poly-hydroxylated compounds known to be present in carbonaceous chondrites (Cooper et al. 2001). For example, we have tentatively detected the simplest three carbon ketone sugar (or ketose), 1,3-dihydroxyacetone, at the CSO (Widicus-Weaver & Blake 2005), and the confirmation of this detection with other microwave and (sub)millimeter-wave telescopes – one of the most complex species yet discovered in star- and planet-forming environments – is our highest priority and will be of great interest to Astrobiology. Toward this end, we have begun initial Ka- and Q-band microwave observations with the GBT. The GBT may well serve as an exceptional and complementary platform for the detection of complex molecules, and our first observing sessions demonstrate very good capabilities over the 40-50 GHz window that optimize the conflicting constraints of line strengths (better at higher frequencies) and telescope performance (better at lower frequencies) — provided the weather cooperates!

The spectra obtained for comets C/NEAT (2001 Q4) & C/LINEAR (2002 T7) are also being compared to those we are currently collecting from a number of young protostars and circumstellar disks. New capabilities at the CSO in particular are drastically increasing the throughput of searches for complex organics. In particular, new receivers and spectrometers have been installed that increase the bandwidth of data collected in a single local oscillator setting by a factor of eight. These bandwidth improvements along with more sensitive detectors mean that what formerly required many nights of integration can now be achieved in only an hour or two. With this technology we have begun acquiring line confusion limited spectra of many hot cores for the first time, and upcoming runs for Orion/Taurus are scheduled in November-December 2005.

Finally, by combining the high resolution spectroscopic capabilities of the NIRSPEC instrument on Keck with the astonishing sensitivity of the Spitzer IRS, we are making the first assault on the organic chemistry in circumstellar disks. Edge-on disks offer particularly promising targets, with two recent examples providing interesting contrasts of the kinds of information available. Toward CRBR 2242, for example, we detect the signatures of organic molecules in the ices located in the outer regions of this potentially protoplanetary disk, and the abundances so obtained can be directly compared to those in comets (Pontoppidan et al. 2005). More excitingly, perhaps, we have recently detected the gas phase absorption bands of the organics HCN, acetylene, and CO₂ that we believe arise from the inner disk encircling IRS46 (another ~edge on disk in Ophiuchus, Lahuis et al. 2005). If substantiated by further observations, this source would provide the first opportunity to examine the hot (several hundred Kelvin) organic chemistry predicted to occur in the planetesimal formation region sampled by carbonaceous chondrites in our own solar system.