Scientists release one-of-a-kind collection of infrared spectra produced at NASA Ames Research Center onto the World Wide Web (www.astrochemistry.org/pahdb). Duplicating the harsh conditions of cold interstellar space in their laboratories and computers, NASA scientists created a unique collection of polycyclic aromatic hydrocarbon (PAH) spectra primarily to interpret mysterious infrared emission detected by ground, air, and space-based observatories. Besides being of great interest to astronomers, the value of the PAH spectral database extends far beyond the immediate needs of NASA and astronomy. A PAH spectral database has a large and very diverse set of important applications. PAHs are a major product of combustion, they remain in the environment, and they are carcinogenic. Consequently they are important, for example, to scientists, educators, policy makers, and consultants working in the fields of medicine, health, chemistry, fuel composition, engine design, environmental assessment, environmental monitoring, and environmental protection. This PAH database is a new tool for people working in all of these fields.
The web site contains over 800 spectra of polycyclic aromatic hydrocarbons (PAHs) in their neutral and electrically charged states and tools to download PAH spectra ranging in temperature from minus 470 to 2000 degrees Fahrenheit. PAHs are flat, chicken-wire shaped, nano-sized molecules. Thanks to these spectra, PAHs are now known to be abundant throughout the Universe but often in exotic forms not readily available on Earth. They are thought to be produced in the outflows from carbon-rich stars by processes similar to combustion in oxygen poor flames that produces PAH-rich soots on Earth.
Astronomical observations in the 1970's and 1980's discovered mysterious infrared radiation from space. While the infrared signature hinted that PAHs might be responsible, laboratory spectra of only a handful of small, individual PAHs were available to test this idea. To make matters worse, these were only for neutral PAHs, not for PAHs as they were expected in interstellar space: electrically charged, free, very cold, individual molecules in the gas. The only spectra available at that time were of tiny crystals containing many PAH molecules stuck together suspended in oils and salt pellets-these PAHs were not isolated, not cold, and not charged. By the mid-1990's, observations from space showed this infrared emission was surprisingly common and widespread across the Universe implying that the unknown carrier was abundant and important. The need for PAH spectra measured under astronomical conditions to understand this was critical to make progress.
To provide these spectra a team of scientists led by Louis Allamandola at NASA Ames Research Center developed a program in the late 1980's to measure PAH spectra under simulated astronomical conditions experimentally and with computer software. This 7 member team is made up of experts in many different fields. "This group has put in a tremendous effort over the past 5 years to create this web accessable database," said Allamandola. He continued, "There are now about 800 spectra in the database. Six hundred of these have been theoretically computed and two hundred have been measured in the laboratory. The theoretical spectra span the range from 2 to 2000 microns, the experimental spectra cover 2 to 25 microns."
Christiaan Boersma, formerly at the University of Groningen in the Netherlands, is now a NASA Postdoctoral Fellow at Ames. He is one of the three astronomers on the team who routinely use the PAH spectra in the database to interpret astronomical observations. He designed and developed many parts of the current website and its user tools. Boersma said, ``The spectra in the database have given insights into the composition of the PAHs in space that were impossible to obtain any other way. In the near future these spectra will be especially valuable for interpreting observations made with NASA's new airborne observatory, the 'Stratospheric Observatory for Infrared Astronomy' (SOFIA) and the European Space Agency's (ESA) Herschel Telescope," He continued, "These telescopes are pioneering the far-infrared radiation region, opening a new window on the sky. Since the database shows PAHs have many far-infrared transitions, they will add to the pioneering discoveries made by these observatories." Turning to the website and database, Boersma explained, "We tried to make the website as easy to use and as useful as possible. Instead of presenting long tables of numbers and letting users sort through it all on their own, website users can graphically interact with the data. The database can be interrogated using many criteria, such as PAH charge, composition and spectral signatures. Several tools even allow users to do some initial analysis online. For example, spectra can be combined to create a `composite' spectrum that can be directly compared to an unknown. Furthermore, the all data can be downloaded." "We intend to expand the database with new sets of data periodically and plan to add more useful tools along the way '' Boersma concluded.
Charles Bauschlicher, Jr., also at NASA Ames, is one of the first computational chemists to calculate the IR spectra of PAHs and their ions. Using quantum chemical calculations he and Alessandra Ricca of the SETI Institute computed all of the spectra in the computational portion of the database. Bauschlicher described the history of the computational effort saying “When we started this project, our hope was to help interpret the experimental spectra, but over time, our computational capabilities grew to a point that we are able to study molecules much larger than can be studied in the laboratory. In addition, we were soon using theory to study species that are very difficult to create and measure in a laboratory, but could be common in space.” Ricca added “I am very excited about the future of this research, because we have only scratched the surface of what theory can contribute to our understanding of PAHs. For example, we are now studying clusters of large PAHs and extremely large individual PAHs containing more than 100 carbon atoms.”
Andrew Mattioda, also at NASA Ames, and Douglas Hudgins, now at NASA Headquarters in Washington DC, both experimental physical chemists, measured all of the spectra in the experimental portion of the database under astrophysically relevant conditions. Mattioda explained, "We accomplished this feat by first isolating the molecules in an inert matrix at very low temperatures, nearly -450 F, much like the molecules are isolated in space. We then measure the spectra of both the neutral and ionized PAHs." Their experimental work includes PAHs containing a nitrogen atom in the molecule's framework, molecules affectionately known as PANHs. Mattioda adds, "Although the PANHs are more dangerous, biologically speaking, then the PAH counterparts, they are very important in biochemistry as well as the search for life elsewhere in the universe. Our laboratory research has revealed previously unknown spectroscopic and electronic properties of PAHs and PANHs. We are rewriting the organic chemistry textbooks where polycyclic compounds are concerned." In Mattioda’s view “Given the biological processes that rely on PAH type molecules, understanding the distribution of PAHs in the universe could provide insight into the distribution of life in the universe.”
Els Peeters, formerly a postdoctoral fellow at NASA Ames, is now a Professor of Astronomy at the University of Western Ontario and connected to the SETI Insitute. Peeters pioneered the application of the lab and theoretical spectral database to astronomical spectra and guided the direction of its expansion from the astronomer's perspective. Analyzing observational data obtained with the Infrared Space Observatory and the Spitzer Space Telescope, she identified several classes of astronomical PAH spectra which are related to different types of astronomical objects. "Just as with handwriting and language analysis," Peeters explained, "by comparing the spectral signature from each of these different astronomical objects with the different PAH signatures in the database, we were able to pinpoint the types of PAHs in these objects. When you realize that this works not only in our Galaxy, the Milky Way, but across the Universe, it's pretty amazing." Peeters continued, "We are now able to use each 'letter' in the signature to tell us specifics about the PAHs in space, their electrical charge, size, shape and so on. In addition, we found small but real mismatches with the spectral database, indicating that something was missing. Bringing all this information to bear on this cosmic emission has revealed the types and amounts of different PAHs present in space and how they evolve from their birth site in red giant stars, to the interstellar medium between the stars, and ultimately into star-forming regions and proto-planetary disks." Peeters concluded, "Thanks to this synthesis of lab data with astronomical observations, just as a weatherwoman uses satellite pictures to forecast the weather, these emission bands are being developed into a diagnostic tool to probe the local environmental conditions in our galaxy, The Milky Way, out to nearby galaxies and all across the distant Universe."
Jan Cami, a former postdoctoral fellow at NASA Ames and now a Professor of Physics & Astronomy at the University of Western Ontario and connected to the SETI Insitute, laid the conceptual foundation for the database and website. He developed algorithms to match astronomical observations with spectra from the database, and that comparison tells us precisely how many PAHs of which kind are present in various astronomical objects. He explains: “Being able to almost perfectly reproduce astronomical observations with earth-based laboratory experiments and theoretical calculations is very cool and rewarding. That alone makes the NASA Ames PAH database unique – it is the only database in the world with enough PAH information to be relevant to astrophysical environments. Not only that, the database tells us what kind of PAHs like to hang out together and in what corner of the Universe they do so. And that has led to some unexpected scientific results – for instance, we can now say that a significant fraction of these PAHs in space are negatively charged, which was deemed very unlikely up to now. We can also show that emission at certain wavelengths originates from smaller molecules, while larger molecules dominate other wavelength ranges, .... There’s a humbling aspect to the database too – when it dawns on you that there are about as many PAH molecules in some of these huge astronomical objects as there are in our earthly laboratory experiments.”
Douglas Hudgins joined the team in the early 1990's. He pioneered the experimental techniques that are now routinely used in many laboratories to measure PAH spectra under extraterrestrial conditions. Hudgins, now a Program Scientist at NASA Headquarters in Washington DC, is thrilled to see the Ames database coming to fruition. "The whole reason that NASA's Astrophysics Division supports laboratory research is because the resultant data is essential for analyzing and interpreting the observations of NASA's space observatories. Actually doing the experiments or the calculations is only part of the job, though; just as important is getting those data into the hands of scientists in a convenient and useful format so that they can put them to work. This new database will do exactly that. It is the culmination of a vision that Lou and I had 20 years ago when we started out on this adventure."
"We started this work motivated to test the PAH hypothesis and, if born out, provide the spectra needed to exploit the PAH model and develop it into a new probe of the wide variety and vast number of astronomical objects that show the PAH emission spectrum. This field has exploded far beyond my wildest dreams and, in my opinion, this spectral collection is what broke the spell that astrochemistry was limited to simple species and marginal for astrophysics." Allamandola said. "Thanks to the incredible sensitivity of the Spitzer Space Telescope, the PAH signature is seen across the Universe, removing any doubt of the importance of these species. There is even evidence for PAH emission from very distant galaxies at redshifts of 3, indicating these complex organic molecules were produced only a few billion years after the Big Bang. This means that enough carbon was available to drive a rich organic chemistry far earlier in the history of the Universe than people thought only a few years ago. When you consider that the discovery of simple, garden variety molecules like ammonia (NH3), formaldehyde (H2CO) and carbon monoxide (CO) in space made headlines in the 60's and 70's, this is incredible. Up till then, space was thought to be chemically barren. If this isn't enough, as shown in the figures of the galaxy, M82, Spitzer has shown there are even PAHs in the space between galaxies! Beyond a doubt, PAHs are an important part of modern astrophysics."
Pioneering observations made by the European Infrared Space Observatory (ISO) and the unprecedented sensitivity of NASA's Spitzer Space Telescope (SST) have seen the PAH infrared signature from many astronomical objects within our Galaxy, The Milky Way (Figure of Orion) and from most other galaxies across the Universe. This database and user tools will see immediate use by astronomers throughout the world as they probe PAH emission to the edge of the universe with increasingly sensitive telescopes.
The database and tools will become available on the web at www.astrochemistry.org/pahdb on August 2nd, 2010. A paper describing the website and details about the computational spectra in the database will be published in the Astrophysical Journal Supplement Series on August 2nd as well. Details about the experimental spectra in the database will be described in a forthcoming publication.
This research is supported by the Space Science and Astrobiology Division at Ames Research Center and the Science Mission Directorate at NASA Headquarters, Washington, D.C.