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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 FY07 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, a THz Time Domain Spectrometer (THS TDS), 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 offers superb imaging performance in the 100/230 GHz windows now that the telescopes are fully operational at the new site in the Inyo Mountains. 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 now completed “Cores to Disks” Legacy Science program, N.J. Evans, P. I.) and detailed Keck-NIRSPEC / VLT-CRIRES 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’s 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/comet 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.

In the area of cometary studies in FY06, we collaborated extensively with the Mumma group in observations of Comet Tempel 1 for the Deep Impact mission. Initial results from the NIRSPEC monitoring of the pre- and post-impact outgassing are presented in Mumma et al. (2005), more extensive modeling is presently underway. We also carried out joint Keck observations of comet 73P in the spring of 2006, as described in Villanueva et al. (2006).

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For the brightest apparitions, the IR data are combined with mm-wave aperture synthesis studies (Friedel et al. 2005). In the past, this involved separate observations with the OVRO and BIMA Millimeter Arrays, as outlined in our work on the comets C/NEAT (2001 Q4) & C/LINEAR (2002 T7) that reached perihelion on 26 April 2004 and 17 May 2004, 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). Future work will utilize CARMA, whose vastly improved imaging performance will enable a new generation of cometary research such as the measurement of the D/H ratio in Jupiter-family comets.

Our highest priority with Keck has been to follow up the exciting discovery of gas phase absorption bands from organics in the inner disk of 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. Subsequent JCMT and Keck NIRSPEC observations demonstrated 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 regions of the solar nebula wherein the planetesimals that formed the asteroid belt and terrestrial planets originated. A press release on this work may be found at
http://www.spitzer.caltech.edu/Media/releases/ssc2005-26/index.shtml.
We do not believe that IRS 46 is particularly special in any other respect than its very favorable geometry, which facilitates absorption spectroscopy of the inner disk. We have thus been searching the Spitzer archive for additional candidate sources. Gibb et al. (2007) have recently announced the 3 μm detection of such features toward the GV Tau primary, what does Spitzer reveal?

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A non-optimal reduction of the Spitzer IRS ShortHi GV Tau archival data indeed reveals the same bands as seen in IRS 46 (with approximately half the optical depth seen toward IRS 46). From our extensive Keck and VLT M-band CO survey we have selected an additional sixteen ~edge-on disk candidates for further study with the Spitzer IRS in Cycle 4, including much deeper integrations on GV Tau. The successful results for this proposal were announced in June 2007, data are forthcoming. Keck observations are also planned for the fall of 2007 and winter 2007-2008.

Whether or not such small organic species go on to create the much more complex organics seen in meteorites depends critically on the oxygen fugacity in the inner disk. We have therefore expended considerable effort to measure this with Keck-NIRSPEC via L-band observations of the OH radical in collaboration with Avi Mandel and Mike Mumma at NASA’s GSFC. For inclined disks, the higher dust opacity at ~3.5 μm wavelength, and the smaller absolute abundances of most molecules compared with CO means that very high SNR spectra are required. Figure 2 demonstrates that such results are possible even from ground-based telescopes if sufficient care is taken to achieve the requisite photon statistics and careful calibration. This first detection of OH in the disks around Herbig Ae stars should provide access to both the physical conditions (through non-LTE excitation of high dipole moment species such as OH) in and the chemical composition of the disk surface layers.

To access sources in the southern sky, we will use CRIRES (a high-resolution near-IR spectrometer recently commissioned for the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile). The instrument offers a combination of AO-assisted spatial resolution (0.1”) and a spectral resolution of 100,000 (3 km/s) in the 3-5 μm spectral region. The detector is a 4 1k x1k mosaic, combining the advantages of long-slit spectroscopy with the wide spectral range otherwise only available in cross-dispersed spectrometers. This makes CRIRES uniquely suited for observations of ro-vibrational transitions of CO, OH, and other molecules in proto-planetary disks. We (Pontoppidan and Blake) are part of the science verification program, and in collaboration with Prof. Ewine van Dishoeck (Leiden Observatory) and Dr. Alain Smette (ESO, CRIRES instrument scientist) will be starting a large (48 half-nights) program to obtain the high resolution 3-5 micron spectra of a sample of ~50 proto-planetary disks with CRIRES in spring 2007. Our initial results on sources in Chameleon and Ophiuchus are spectacular.

In comparison with other instruments on 8m-class telescopes capable of high-resolution 3-5 μm spectroscopy, CRIRES is the only generally available instrument designed for use with an AO system that can acquire M-band spectra with the dispersion needed to sample the emission lines beyond 5-10 AU. We intend to take advantage of this, not only to carry out “standard” spectroscopy, but also a dedicated program for spectro-astrometry of CO ro-vibrational lines in proto-planetary disks. Spectro-astrometry, a technique well-known in optical spectroscopy, takes advantage of the ability to determine the centroid of the point-spread function with an accuracy much higher than the FWHM of the primary beam. Initial tests with CRIRES and similar instruments show that, for signal-to-noise ratios greater than a few hundred, the relative spatial position of a line can routinely be determined with an accuracy of 1/50th of a pixel, corresponding to 1-2 milli-arcsec for typical T Tauri disks in nearby star forming clouds. Central to the applicability of the spectro-astrometry technique to CO ro-vibrational lines is the high spectral resolution of CRIRES. Typical CO ro-vibrational lines from protoplanetary disks will have FWHM of 5-30 km/s. Since a disk in Keplerian rotation will produce symmetric lines, any astrometric signal will be smeared out and disappear if the lines are not fully spectrally resolved. With the high resolution of CRIRES, one can extract a position-velocity diagram along the slit on 0.1 AU scales. Rotating the slit in steps of 45° will “image” the line emission to yield parameters such as disk inclination, position angle, and, with an assumption of Keplerian rotation, the stellar mass.
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. comm.) reveal 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 chrondrites (Cooper et al. 2001). For example, we have tentatively detected the simplest three carbon ketone sugar (or ketose), 1,3-dihydroxyacetone, at the CSO (Figure 1, Widicus-Weaver & Blake 2005), and the confirmation of this detection of one of the most complex species yet discovered in star- and planet-forming environments with other microwave and (sub)millimeter-wave telescopes 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. 3), the next simplest ester after the only ester detected in hot cores (methyl formate); and 1,3-propanediol, the second simplest diol (ethylene glycol, a known interstellar compound, being the simplest). Figure 3 shows the tremendous complexity of rotational spectra of even small organics, and the challenge of molecular line searches in astronomical sources.

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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 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 2007. 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.
Herschel and SOFIA may also offer a novel means of overcoming the spectral line confusion problem that exists at microwave frequencies by examining the THz torsional modes of complex species. Even for moderately complex species such as glycine, the gas phase torsional bands are quite specific, yet little is known about them. In addition, much of the relevant chemistry occurs in the ices that are present on the surfaces of dust grains, yet our understanding of the THz properties of either the ice or the grains themselves is poor. We are therefore installing a matrix isolation system to use our existing photomixer spectrometer as a ‘THz sweeper’ to acquire the spectra of molecules in Ne, Ar, and H2 matrices. Significant (several ) absorption features are expected even at 1 target/matrix mixtures only a few mm thick. Nevertheless, the scanning speeds will remain slow and new methods are urgently needed.

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Thus, we have begun to investigate the use of time domain techniques in which THz antennae are placed onto optoelectronic substrates with short recombination times, such as low temperature grown (LTG) GaAs (trec<250 fs). Illumination of such devices with ultrafast laser optical pulses can be used to generate sub-ps THz radiation with bandwidths of > 2 – 4 THz. Moreover, coherent sub-ps detection of the electric field is possible, which enables both the real and imaginary refractive indices of materials to be measured. The overall sensitivity can be >105, and a variety of solid state and gas phase THz spectra have been acquired with such systems. Two examples are shown in Figure 4, which presents the THz spectra of all-trans retinal, an important chromophore in photosynthesis, and L,L-cystine. The retinal low frequency modes which mediate the photoisomerization process near in vivo are nicely resolved as the sample is cooled to cryogenic temperatures, while L,L-cystine retains sharp features even at room temperature.
We have recently installed a femtosecond Ti:Sapphire laser loaned to us by Caltech Prof. Scott Fraser in Biology, and are building up the associated optics and electronics needed to generate THz pulses. The matrix isolation instrument arrived in April 2007, and we expect to begin acquiring condensed phase THz spectra by the late summer of 2007. Follow up gas phase studies with our CW photomixer spectrometer will then determine the degree of matrix perturbation and provide the necessary data for astronomical searches. Both thermal and laser vaporization sources have already been optimized with the Flygare-Balle spectrometer available to the group.

References:
Blake, G., Qi, C., Hogerheijde, M., Gurwell, M. & Muhleman, D. 1999, Nature 398, 213.
Cooper, G., Kimmich, N., Belisle, W., Sarinana, J., Brabham, K., and 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, Ap. J. 630, 623.
Gibb, E. et al. 2007, Ap. J. 660, 1572.
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., Jorgensen, J. K., Kessler-Silacci, J. E., and Knez, C. 2006, Ap. J. 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, and 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., and Wright, M. C. H. 2006, Ap. J. 643, 567.
Villanueva, G. L. et al. 2006, Ap. J. 650, L87.
Walther, M., Fischer, B., Schall, M., Helm, H., & Uhd Jepsen, P. 2000, C. P. L. 332, 389.
Widicus, S.L., Drouin, B.J., Dyl, K.A., and Blake, G.A. 2003, J. Mol. Spec. 217, 278.
Widicus, S.L., Braakman, R., Kent, D.R., and Blake, G.A. 2004, J. Mol. Spec. 224, 101.
Widicus-Weaver, S. L., and Blake, G. A. 2005, Ap. J. 624, L33.
Yamamoto, K., Kabir, M. H., & Tominaga, K. 2005, J. Opt. Soc. Am. B 22, 2417.