Philip Solomon, one of the pioneers and leading researchers in molecular astrophysics, died on 30 April 2008 at his apartment on the upper west side of Manhattan after a battle with cancer. His pioneering research included both theoretical and very extensive observational studies of stellar atmospheres, interstellar molecules, high redshift galaxies, and the Earth's stratosphere. Phil was Distinguished Professor at The State University of New York [SUNY], Stony Brook, where he had been since 1974.
Phil was born on 29 March 1939 in Manhattan, New York City, to Nat and Betty Solomon. Nat Solomon was a labor organizer and a printer. Phil attended the University of Wisconsin, where he received his BS in 1959 and where he met his future wife Sheila who was studying art. His Ph.D., "On the Role of Light Molecules in Astrophysics," was also from the University of Wisconsin under the guidance of Art Code and Bob Bless. After postdoctoral positions at Princeton and lectureships at Columbia and the University of California, San Diego, Phil spent two years as a Professor at the University of Minnesota. After two years at the Institute for Advanced Study in Princeton, he came to SUNY, Stony Brook, as Professor of Astronomy in the Department of Earth and Space Sciences. In 1988 Phil was selected as a Humboldt Senior Distinguished Scientist, and, in 1999, he was honored with the rank of Distinguished Professor at SUNY. Phil took sabbatical and other leaves at Churchill College and the Institute of Astronomy, Cambridge; the Institute for Advanced Study; l'Ecole Normale Superieure, Paris; Institut d'Astrophysique, Paris; and the Institut de Radioastronomie Millimetrique [IRAM], France. Phil published more than 160 papers and supervised seven Ph.D. students. He served on numerous review, visiting, and advisory panels.
Phil's first theoretical research focused on opacity and abundance of light molecules such as H2, CO, and CN in stellar atmospheres, but then shifted quickly to the formation, excitation, and astrophysics of interstellar molecules, which had just been discovered in the late 1960s. In 1969, Phil and Chandra Wickramasinghe were among the first to suggest that the denser interstellar clouds, which were deficient in atomic hydrogen, were principally molecular hydrogen with the H2 formed on the surface of cold dust grains and protected from dissociating UV by a self-shielding H2 layer at the cloud surface. With L. Lucy, Phil then developed the radiative transfer and mass-loss mechanism operative in hot OB star winds and QSOs--where the radiative momentum is absorbed in resonance lines of ions.
In the late 1960s and early 1970s, the detection and mapping of interstellar molecules moved rapidly from the early discoveries of maser emission in H2O and OH to the thermal emission lines of simple molecules like CO, CN, CS, and HCN, to more complex species containing up to thirteen atoms. Phil was a major force in pushing these new detections and in using the thermal emission as astrophysical probes. This explosive growth of spectroscopic detections occurred primarily as a result of Phil's collaborations with A. Penzias, K. Jefferts, R. Wilson, and P. Thaddeus, along with other competing groups using the NRAO 36-foot telescope at Kitt Peak. This was a most exciting period with the mm-wave window finally accessible to spectroscopy and each observing run on the telescope typically yielded one or two new detections. Phil was probably the one most responsible for providing the astrophysical motivation to push the technology towards mm-wavelengths. He clearly elucidated the fact that high densities were required for the thermal excitation of the higher dipole moment molecules such CS and HCN--at the same time pointing out, for the first time, the critical role of line photon trapping in the optically thick lines.
In collaboration with N. Scoville and D. Sanders, Phil initiated the early surveys of CO emission from the Milky Way molecular gas. They first pointed out that the molecular gas resides largely in self-gravitating clouds (not in pressure equilibrium with the diffuse atomic and ionized gas) and coined the term Giant Molecular Clouds. They also discovered that the molecular gas had a Galactic distribution very different from the atomic gas--a massive central concentration within 300 pc of the Galactic center and the molecular cloud "ring" at 4 to 8 kpc radius. Throughout this work, he clearly elucidated the astrophysical importance of the dense molecular clouds as the sites of virtually all star formation. During this period, Phil, in collaboration with W. Klemperer, provided important theoretical insight into the formation of simple diatomic molecules via gas-phase charge exchange reactions.
As the sensitivity of mm-wave receivers improved, Phil extended the observations of CO and HCN emission first to nearby galaxies in collaboration with J. Barrett and L. Sage and then to higher luminosity, more-distant objects with Y. Gao, D. Downes, and P. Vanden Bout. Solomon and Gao very significantly showed that for the most luminous infrared galaxies, the infrared luminosities were linearly proportional to the amount of dense, star-forming gas (as traced by HCN) and hence, they argued, that the source of luminosity could be entirely star formation rather than quasar activity. With Downes, Phil was introduced to mm-wave interferometry (at last, I should say, since he was basically a single-dish astronomer at heart), and they were among the first to produce high-resolution images of the most luminous infrared galaxies. Phil's most recent work, in collaboration with Vanden Bout, Carilli, and Maddalena to detect CO and HCN from the highest redshift galaxies was on-going, but resulted in a recent Annual Reviews paper that provides a forward look to the future of this field.
Phil also realized that mm-wave spectroscopy could be used for remote sensing of the terrestrial stratosphere to trace the abundance of ClO and hence monitor the ozone layer depletion. This work, in collaboration with R. deZafra, J. Barrett, and A. Parrish, led to their setting up remote, automated observing systems in Antarctica and in Hawaii; these efforts continued over twenty years up to the present.
For those of us who collaborated with Phil, he will be greatly missed. Phil had a keen sense for interesting and significant science; he had a real enthusiasm for discovery; and he enjoyed the competition of forefront scientific research where recognition of significance was vital but where discussion of interpretation was rational, albeit with strong argument. The pleasure of an observing run with Phil was supreme due to his enthusiasm and focus on the astrophysics; these runs also were creative since if the original plans did not quite pan out, he was always ready to modify the observations to take advantage of what one learned from the data coming into the telescope. Often this resulted in much better science--in contrast to the current paradigm with fixed observing programs and queue observing. Phil was also a major presence at scientific meetings giving stimulating talks and provoking challenging discussions.
Phil is survived by his wife Sheila, daughter Nina, son-in-law John, granddaughter Sarah, and brother Mark