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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 FY08 and beyond will continue to lean 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 both a Fourier Transform MicroWave (FTMW) spectrometer, a THz laser difference frequency photomixer spectrometer, and is building a new THz time domain spectrometer (THz TDS) with funds from the NSF Chemistry Research Infrastructure and Facilities (CRIF) programs. 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 an ongoing GO-5 program, J.S. Carr, 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, 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/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 FY07/FY08, we continued our collaboration with the Mumma group, with studies of the spectacular outburst of Comet 17P/Homes forming our highest priority. Observations were carried out both with Keck NIRSPEC and with the newly commissioned Combined Array for Research in Millimeter Astronomy, or CARMA. Data reduction is now complete for the infrared echelle work, and an initial paper is in preparation by the Mumma group. For the CARMA observations, unanticipated data acquisition issues have made the reduction of the (u,v)-plane visibility data on this rapidly moving target a challenge, but appropriate new modules for the MIRIAD software suite used by the CARMA consortium has enabled a successful detection of the comet in just the past few weeks. We are presently analyzing the data cubes and hope to have a manuscript ready for submission by the fall of 2008. The greatly improved (u,v)-plane coverage should enable much more robust detections of features such as cometary jets (see Blake et al. 1999 for such mm-wave jets in Comet Hale-Bopp), while the improved sensitivity will enable a new generation of cometary research such as the measurement of the D/H ratio in Jupiter-family comets once the next generation 230 GHz receivers based on mixers developed for the Atacama Large Millimeter Array (ALMA) are installed in calendar year 2009.

The recent apparition of Comet C/2007 W1/Boattini offered one of the first realistic chances to attempt detections of the infrared emission from HDO. Over the nights of 9-10 July 2008 we observed Boattini in three NIRSPEC echelle settings for a total of ~2 hours clock time. With overhead, this resulted in somewhat less than an hour of actual integration time — insufficient to detect HDO, but a wealth of molecules are seen, as is outlined in Figure 1. This comet shows highly unusual organic grain signatures and very large ethane/water ratios. The Mumma group reduced the data in record time while we were at the telescope to optimize the second night’s observing, and two publications on this unique object are in preparation.

Our highest priority with Keck and the VLT has been to expand upon our 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). 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 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 regions of the solar nebula wherein the planetesimals that formed the asteroid belt and terrestrial planets originated.

We do not believe that IRS 46 is particularly special in any other respect than its favorable edge-on geometry, which facilitates absorption spectroscopy of the inner disk. Thus we have been working to improve our data reduction pipeline for the far more numerous inclined disks in the c2d IRS survey to search for molecular emission, and as Figure 2 shows we have been spectacularly successful. The spectra shown for AS 205N and DR Tau (Salyk et al. 2008) are among the more spectacular in terms of line-to-continuum excess, but at least 30-35% of the c2d disk targets reveal detectable gas emission features at R=600. From the c2d and our extensive Keck and VLT M-band CO survey we have selected a number of new targets for a successful Spitzer Cycle 5 Medium GO proposal with John Carr and Joan Najita whose goal is to search for molecular emission in a broader evolutionary range of disks. Our initial data on only six of the 27 sources reveals a large range in water/organic content in the inner few AU of the disks. Complementing these data are very deep Spitzer Cycle 4 observations of an additional 16 ~edge-on disk candidates to compare with systems such as IRS 46 and GV Tau (Gibb et al. 2007).

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We have used Keck NIRSPEC spectroscopy to make sure that the IRS-detected water emission arises from the inner disk, recent Keck results are shown in Figure 2. These beautiful data follow hot on the heels of our work on the L-band observations of the OH radical in collaboration with Avi Mandel and Mike Mumma at NASA GSFC. For inclined disks, the higher dust opacity at λ±3.5 μm and the smaller absolute abundances of most molecules compared to CO means that very high SNR spectra are required. In Mandell et al. (2008) we report the first detection of OH in the disks around Herbig Ae stars, while in Salyk et al. (2008) the high-resolution spectroscopic observations of water and OH in the disks around classical T Tauri stars (cTTs) are described. These results demonstrate that searches for molecular emission are possible even from ground-based telescopes if sufficient care is taken in achieving the requisite photon statistics and careful calibration. The unusual combination of collisional and radiative excitation of OH and water should provide access to both the physical conditions (through non-LTE excitation of such high dipole moment species) in and the chemical composition of the disk surface layers. The latter is pivotal for the question of whether the small organics seen in disks such as IRS46 go on to create the much more complex organics seen in meteorites; this depends critically on the oxygen fugacity in the inner disk. A Spitzer Space telescope press release describing this work may be found at http://www.spitzer.caltech.edu/Media/releases/ssc2008-06/release.shtml

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To access sources in the southern sky, we use CRIRES – a high-resolution near infrared 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 λ/Δλ = 100,000 (3 km/s) in the 3-5 µm spectral region. The detector is a 4 1k μ 1k 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, H₂O 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) are in the midst of a large (48 half-nights) program to obtain the high resolution 3-5 μm spectra of a sample of ~50 proto-planetary disks with CRIRES that began in spring 2007.

In comparison with other instruments on 8m-class telescopes capable of high-resolution 3-5 micron 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 have taken advantage of this, not only to carry out “standard’” spectroscopy, but also a dedicated program for spectro-astrometry of CO and other rovibrational 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 accuracy much higher than the FWHM of the primary beam. Figure 3 shows our initial CRIRES results, which demonstrates that (for signal-to-noise ratios of ≥ a few hundred) the relative spatial position of gas emission can routinely be determined with an accuracy of better than 0.1 AU for typical T Tauri disks in nearby star forming clouds! The high spectral resolution of CRIRES is critical for this application, which combines the spatial resolving power previously accessible only via infrared interferometry with the ability to directly probe the gas kinematics. Specifically, we can extract the position-velocity diagram (first moment map) along the slit on <0.1 AU scales. By rotating the slit to a few selected positions, we can "image" the line emission to yield parameters such as disk inclination, position angle, and, with an assumption of Keplerian rotation, the stellar mass. In our first applications (Pontoppidan et al. 2008), we have demonstrated that the nearly face-on transitional disk is warped, and directly measured the inner gas holes in the transitional disks SR 21 and HD 135344B. Our large Keck NIRSPEC M-band survey has revealed a large population of objects suitable for CRIRES follow up, and so this effort will be a major focus of the group in CY2008, with extended observing runs in April, August, and December 2008. An ESO press release describing this work may be found at http://www.eso.org/public/outreach/press-rel/pr-2008/pr-27-08.html

Our other major group effort will involve further laboratory study and astronomical 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/ 2002 T7 (LINEAR) 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). We are also expanding our search for species such as esters, and have recently used the MOPRA telescope in Australia to tentatively detect methyl acetate (Kelley et al. 2008, in prep; for the lab spectrum of this molecule see Figure 4).

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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 theoretically the role of esterification reactions in grain mantle chemistry with Prof. Eric Herbst at Ohio State and former group member Dr. Susanna Widicus Weaver, who in June 2008 began her appointment as Assistant Professor of Physical Chemistry at Emory University.

The methyl acetate lab spectrum alone shows the tremendous complexity of rotational spectra, and the challenge of molecular line searches in astronomical sources. We believe that a more “statistical” approach to detection of complex molecules is now warranted, and new capabilities recently deployed at the CSO are making such approaches possible by 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 carried out complete spectral line surveys in the 220-270 GHz region of many hot cores for the first time, and these observations and associated laboratory searches 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. Thanks to support from the NSF Chemistry Research Instrumentation and Facilities (CRIF) program we are installing a matrix isolation system that will use our existing photomixer spectrometer as a ‘THz sweeper’ to acquire the spectra of molecules in Ne, Ar, and hydrogen matrices. Significant (several percent) absorption features are expected even at 1% target/matrix mixtures only a few microns thick. Nevertheless, the scanning speeds will remain slow and new methods are urgently needed.

Thus, with the CRIF funds we are constructing a time domain spectrometer 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 generates 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 are now being acquired with the system outlined in Figure 5. The data shown here were acquired with a recently installed femtosecond Ti:Sapphire laser loaned to us by Caltech Prof. Scott Fraser in Biology. New, state-of-the-art fs Ti:Sapphire lasers are now operational in the laboratory, and will extend the frequency coverage to at least 10 THz through the use of electro-optic transmitters and receivers based on (110) ZnTe wafers.

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The matrix isolation unit has been integrated into the new system, and is undergoing initial tests with organic species such as L,L-Cystine whose THz optical constants are well known (see Figure 6). In fall 2008 we will be installing a Fourier Transform-InfraRed (FT-IR) spectrometer for the measurement of ice film and substrate thicknesses with support from the Herschel Laboratory Astrophysics program. We would like to stress here that the success of these substantial proposals was in large measure driven by our work supported by the Astrobiology program, and as such these funds have now been highly leveraged by additional Federal support.