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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. The experimental work is being carried out in Prof. Blake’s laboratories in the Caltech Beckman Institute, and the observational research in FY06 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), the Caltech Submillimeter Observatory (CSO), and the Keck telescope(s). In the laboratory, the Blake group operates 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 soon offer superb imaging performance in the 100/230 GHz windows now that the telescopes are fully operational at the new site in the Inyo Mountains (the full suite of correlator capabilities for CARMA is expected in summer 2006, first light was achieved in fall 2005). Our infrared program combines spectral surveys of young stellar objects (YSOs) with the IRS instrument aboard the Spitzer Space Telescope (taken as part of the “Cores to Disks” Legacy Science program, N.J. Evans, P.I.) and detailed Keck NIRSPEC follow up observations of key YSOs and comets. Over the coming years, we plan to utilize the THz heterodyne receivers under construction at Caltech/JPL for SOFIA and HERSCHEL as these platforms become operational (CY07/CY08, respectively, NASA budget permitting).

Two specific avenues are being pursued. The infrared spectra provide great insight into small-to-moderate sized molecules present in both the gas and icy grains surrounding protostars and in comets, as described below. We use these observations to establish the initial conditions for high temperature chemistry close to the star, and then couple these results with models and meteorite chemistry to determine which more complex organics should be the focus of laboratory study. The laboratory results then provide the necessary data for observational searches at microwave through THz frequencies using a variety of telescopes. Our latest results and some plans for the future are outlined next.

With Spitzer+Keck, our most exciting recent results center on the YSO IRS 46 in the Ophiuchus molecular cloud (Lahuis et al. 2006). As Figure 1 shows, this nearly edge-on disk shows high temperature absorption bands of the organic molecules HCN and acetylene with the Spitzer IRS. Follow up JCMT and Keck NIRSPEC (Fig. 1) observations demonstrate that these molecules are present in the inner disk or inner disk wind, and are thus tracing the high temperature organic chemistry long predicted to occur in the zones of the solar nebula that formed the planetesimals that led to the asteroid belt and terrestrial planets. We do not believe that IRS 46 is particularly special in any other respect than its very favorable geometry, and will be carrying out detailed observations this year to further characterize the chemical composition of this intriguing YSO.

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In cometary studies, over the past year we have completed our observational analyses centered on the OVRO and BIMA Millimeter 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 (Friedel et al. 2005, Remijan et al. 2006). 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). The Caltech team was also fortunate enough to participate with the Mumma group in observations of Comet Tempel 1 for the Deep Impact mission (Mumma et al. 2005), and an initial report on our joint observations of comet 73P has recently been submitted to Science (Villanueva et al. 2006). I presume Dr. Mumma and his colleagues will describe these in greater detail. Supporting these IR observations are additional aperture synthesis observations with the SubMillimeter Array (SMA) and CARMA, for which observations are ongoing at the time of this report.

Our other major focus of further laboratory study and data analysis will be searches for complex organics, especially esters, 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 comets should thus provide excellent targets 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 — will be of great interest to Astrobiology. We are also investigating the role of esterification reactions in grain mantle chemistry, and have recently completely assigned the rotational spectrum of methyl acetate (see Fig. 2), the next simplest ester after the only ester detected in hot cores — methyl formate. The spectrum alone shows the tremendous complexity of rotational spectra, and the challenge of molecular line searches in astronomical sources.

Importantly, new capabilities deployed at the CSO 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 expect to be able to acquire line confusion limited spectra of many hot cores for the first time, and upcoming runs are scheduled for the Galactic Center in May and Orion/Taurus in October 2006. These observations and associated laboratory searches will form the thesis research of two students in the Blake group partially supported by the NAI program at GSFC. The results of these theses will set the stage for even more ambitious work to be carried out with Herschel as part of the U.S. guaranteed time program.

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References

Blake, G.A., Qi, C., Hogerheijde, M.R., Gurwell, M.A. & Muhleman, D.O. 1999, Nature, 398, 213

Cooper, G., Kimmich, N., Belisle, W., Sarinana, J., Brabham, K., & Garrel, L. 2001, Nature, 414, 879

Friedl, D.N., Remijan, A., Snyder, L.E., A’Hearn, M.F., Blake, G.A., de Pater, I., Dickel, H.R., Forster, J.R., Hogerheijde, M.R., Kraybill, C., Looney, L.W., Palmer, P., and Wright, M.C.H. 2005, ApJ(Letters), 630, 623.

Lahuis, F., Boogert, A..C.A., van Dishoeck, E.F., Pontoppidan, K.M., Blake, G.A., Dullemond, C.P., Evans, N.J., Hogerheijde, M.R., Jörgensen, J.K., Kessler-Silacci, J.E., & Knez, C. 2006, Ap. J.(Letters), 636, L145.

Mumma, M.J., M.A. DiSanti, K. Magee-Sauer, B.P. Bonev, G.L. Villanueva, H. Kawakita, N. Dello Russo, E.L. Gibb, G.A. Blake, J.E. Lyke, R.D. Campbell, J. Aycock, A. Conrad, & G.M. Hill 2005, Science, 310, 270.

Remijan, A., Friedl, D.N., Snyder, L.E., A’Hearn, M.F., Blake, G.A., de Pater, I., Dickel, H.R., Forster, J.R., Hogerheijde, M.R., Kraybill, C., Looney, L.W., Palmer, P., & Wright, M.C.H. 2006, ApJ, in press.

Villanueva, G.L., Bone, B.P., Magee-Sauer, K., DiSanti, M.A., Salyk, C., Blake, G.A., & Mumma, M.J. 2006, Science, submitted.

Widicus, S.L., Drouin, B.J., Dyl, K.A., & Blake, G.A. 2003, J. Mol. Spec., 217, 278.

Widicus, S.L., Braakman, R., Kent, D.R., & Blake, G.A. 2004, J. Mol. Spec., 224, 101.

Widicus-Weaver, S.L., & Blake, G.A. 2005, ApJ(Letters), 624, L33.