Charles Ramsay Cowley (1934-2024)

Cowley was a major contributor to our understanding of the abundance distributions of the heavy and rare-earth elements in stars whose chemistry differs markedly from that of the Sun, as well as a persistent advocate for state-of-the-art spectroscopic investigations designed to improve the range and quality of the atomic parameters required for precision abundance work.

On 25 August 2024, Dr. Charles R. Cowley passed away nineteen days shy of his 90^th birthday. Perhaps best remembered for his work on stellar abundances in chemically peculiar (CP) stars, most notably HD 101065^[1] (“Przybylski’s star”), Chuck contributed broadly to the field of astronomical spectroscopy, where he was a tireless supporter of efforts to produce more and better fundamental atomic parameters like energy levels, oscillator strengths, and partition functions.

Chuck Cowley was born on 13 September 1934 in the territory of Guam, where his father, a US Marine, was deployed. He grew up in rural Virginia and attended Eagle Rock High School, where he graduated in 1951. Better known during this time (and throughout much of his early adult life) as “Bush”, he was a member of the football and basketball teams and President (and founding member) of the Science Club.

From 1951 to 1955, he attended the University of Virginia where he received a Bachelor’s degree in Chemistry. He remained at UVA to earn a Master’s degree in astronomy in 1958 under the guidance of A. N. Vyssotsky, classifying low resolution spectra and looking for stars with unusual properties. His first paper was published in 1956^[2] reporting brightness and spectral changes in BD+37^°2318.

In 1958, Chuck moved to the University of Michigan to work on his Ph.D., which he completed in 1963. His thesis was entitled “A Fine Analysis of CN in the Solar Atmosphere.” During this period, Chuck met and married Anne Pyne, and began a significant collaboration with her centering on the identification and analysis of peculiar and metallic line A stars.^[3] In 1963, the couple relocated to the University of Chicago where Chuck had accepted a position as an instructor in the Astronomy Department. While at Chicago, Chuck continued his studies of the solar atmosphere and collaborated with Brian Warner on what would become the first of numerous papers devoted to generating and applying new or improved gf-values in the analyses of stellar atmospheres.^[4]

In 1967, Chuck joined the faculty of the University of Michigan Department of Astronomy as an Assistant Professor. He would continue at UM for the remainder of his career, rising through the ranks to Associate Professor in 1970, Full Professor in 1977, and retiring as Professor Emeritus in 2007. His retirement memorandum from the University at that time noted that he had published more than 220 articles, reports and meeting abstracts. In addition, he completed two books, The Theory of Stellar Spectra^[5] and An Introduction to Cosmochemistry^[6], and co-edited (with M. M. Dworetsky and C. Mégessier) an IAU Colloquium Proceeding entitled “Upper Main Sequence Stars with Anomalous Abundances.”^[7] He also contributed a chapter to the 4^th edition of Allen’s Astrophysical Quantities entitled “Spectra” with W. L. Wiese, J. Fuhr, and L. A. Kuznetsova. During his 17 years of “retirement,” Chuck would add more than 80 new entries to his publication list to bring his total output in 2024 to just over 300 items.^[8] On two-thirds of these works, he was either the sole or the first author.

During his 57-year career at Michigan, Chuck became one of the world’s experts on elemental and isotopic abundance anomalies in the spectra of main sequence B and A type stars and their implications for nuclear and non-nuclear processes in these objects. Of particular interest to him were the abundances of the heavy and rare-earth species like barium, mercury, and platinum, and the 15 lanthanide elements.

To facilitate the identification of spectral lines belonging to a given atom or ion, Chuck and graduate student Mark Hartoog developed the method of wavelength coincidence statistics (WCS)^[9]. Here the probability that the observed number coincidences (“hits”) between the features in a star’s spectrum and those of a laboratory spectrum might arise by chance was computed using a large number (usually 10,000) of “trials” in which the laboratory wavelengths were randomly shifted by a small (positive or negative) amount. Instances in which the number of “hits” on the true lab list exceeded the mean number of “hits” on the fictitious laboratory line lists by more than three standard deviations were considered “significant” detections. Although not universally accepted, the technique gave good results in test cases against more traditional line identification methods, showing that WCS is at its best in finding line-rich atomic species that are only weakly present in a complicated stellar spectrum.^[10] Among the most interesting and controversial applications of the WCS methodology were the reported detections of short-lived isotopes of technitium and promethium in HD 101065 and HR 965^[11], and of uranium and thorium in HR 465.^[12,13]

To address the need and importance of precision atomic data for deriving accurate stellar abundances, Chuck sought out and maintained active collaborations with atomic physicists and laboratory spectroscopists both in the US and abroad throughout his career. Beginning in the late 1970’s, as result of his membership on the National Research Council’s Committee on the Line Spectra of the Elements (1978-81), Chuck established connections with researchers at the Johns Hopkins University and Argonne National Laboratory that provided him access to unpublished material containing wavelengths of the third spectrum of the lanthanides which facilitated the identification of Dy III^[14] in some CP stars. Later interactions with Robert Cowan at Los Alamos led to the use of his codes to calculate atomic structure parameters, particularly oscillator strengths, for the second and third spectra of cerium^[15] and neodymium^[16] for use in CP star abundance work. Additional collaborations with S. Johansson at Lund, J.-F. Wyart’s group in Orsay, É. Biémont’s team in Mons, and J. Lawler and associates at Wisconsin, as well as colleagues at NIST, spawned further gf output used to compute abundances for elements like rhenium, terbium^[17], and erbium^[18].

Beginning in about 2001, Chuck entered into a productive collaboration with S. Hubrig and F. Castelli, which yielded more than 25 papers and meeting presentations in the course of the next 17 years. The work continued the focus on abundance determinations in stars with unusual chemistries, but was extended to include the pre-main sequence Herbig Ae/Be stars with magnetic fields. Among the noteworthy contributions that emerged during this period was (i) the observation of wavelength shifts in the infrared triplet of singly-ionized calcium in magnetic Ap stars that are consistent with the presence (and sometimes dominance) of ⁴⁸Ca, the heaviest stable isotope, a result which posed some challenges for explanations invoking chemical fractionation^[19]; and (ii) the detection of a significant correlation of extreme abundance enhancements with second ionization energies for elements with Z > 30 in the late B star HD 65949, the explanation for which was posited to be the accretion of exotic r-processed material from a supernova followed by the in situ diffusion of the enriched accreted gas to the top of the atmosphere. The ability to precisely quantify the abundances of Ru, Re, and Os in this study was facilitated by the determination of new atomic parameters by co-authors^[20] at Mons.

In addition to his encyclopedic knowledge of stellar spectroscopy, Chuck Cowley possessed other traits that distinguished him as a quintessential scientist. He was a meticulous record keeper: virtually everything he did in connection with his researches was captured in his “scientist’s notebooks,” hardbound ledgers running to more than a score of volumes, all indexed and annotated in his careful and clear hand. Chuck also possessed a boundless curiosity, and was never afraid to admit when he didn’t understand something. Always a faithful attendee (even if only remotely in later years) at Departmental seminars and colloquia, Chuck was typically the first one to ask questions and often the best ones. At the same time, he was a humble man, rarely seeking the spotlight, while encouraging others forward and being quick to publicly acknowledge the contributions of his colleagues to the advancement of the work. In the words of Swetlana Hubrig: “His care about citing others’ work was amazing - he tried to mention in his papers every article, even only little related to the subject, just not to hurt anybody. I feel proud to have known him and have worked with him on a number of publications.” As long-time collaborator, Gautier Mathys recently wrote: “He was a model and a source of inspiration. Talking with him was always enlightening and thought-provoking.”

Chuck also took seriously his responsibility to provide leadership and service to his profession. In addition to his commitment to the NRC’s Committee on Line Spectra, he served as chair of the IAU’s Joint Working Group on Ap Stars from 1979 to 1988, as a member of the Scientific Organizing Committee for IAU Commission 29 on Stellar Spectra from 1979 to 1985, as a member of the IAU’s Commission B5 on Laboratory Astrophysics and Division G WG on Ap stars & Related Stars, and as a reviewer for the NRC Research Associateships Program from 1989 onward. To acknowledge his many contributions to stellar spectroscopy and in celebration of his 70^th birthday, Chuck was specially honored at the 8^th International Colloquium on Atomic Spectra and Oscillator Strengths in August 2004^[21].

As in his research, Chuck was passionate about and dedicated to his non-astronomy pursuits, principal among them, musical performance. Chuck had a special fondness for classical music and opera, and was a great fan of Swedish tenor, Jussi Bjorling. Chuck himself was a tenor and sang for many years with the University Choral Union in its annual performances of Handel’s “Messiah” and in the University’s May Festival events featuring world renown conductors and orchestras. He was well known locally as the “unofficial mascot” for the Michigan Marching Band in recognition for his regular attendance, rain or shine, at their fall weekday afternoon practices spanning a period of more than a decade.

Chuck’s love of music traveled with him when visiting colleagues abroad. Tanya Ryabchikova remembers well the 1985 IAU Colloquium 90 in Crimea: “I met Chuck in the Moscow aeroport and his first question was: Do I know the song 'Gori, gori, moya zvezda....'*, which he considered to be suitable for our conference. And then, the driver was terrorized by two crazy persons singing this song the whole way to the hotel.” Chuck visited Vienna at least four times during his career. On one extended stay, he took lodgings near the Volksoper so that he could easily and frequently attend musical performances there. During this time, he also managed to connect with an amateur vocal group in the city to be able to exercise his “pipes” as he liked to say and at the same time to practice his German. (Chuck gained proficiency in German by meticulously translating Albrecht Unsӧld’s classic textbook Physik der Sternatmosphӓren and through regular conversations with two staff ‘Feinmekanikers’ and native speakers during his grad school years at Michigan.)

Kutluay Yüce offers a fitting tribute: “I lost my great perfect friend, teacher, great universal scientist. I [will] always miss and remember Dr. Cowley. I hope that he rests in peace.”

Chuck’s request was to be cremated and that there be no memorial services. He is survived by his former spouse Anne, his two sons, David and James, and a granddaughter, Shelby.

*'Gori, gori, moya zvezda....' translates roughly to ‘Shine, shine, my star’ and is a well-known Russian romance, composed c.1847 by Peter Bulakhov and set to a poem written by Vladimir Chuevskiy.

Adapted and reproduced from https://lyricstranslate.com/en/gori-gori-moya-zvezla-shine-shine-my-star.html

Gould’s AstroGen entry

C. R. Cowley Obituary: Acknowledgements

Following Chuck’s practice of fully and properly acknowledging all those who had contributed to the successful prosecution of his research projects, we take this opportunity to thank the following individuals whose remembrances of and anecdotes about Chuck Cowley have enlivened this encomium: Anne and David Cowley, Peter Boyce, Swetlana Hubrig, Donald Kurtz, Lyudmila Mashonkina, Gautier Mathys, Tanya Ryabchikova, Glenn Wahlgren, Werner Weiss, and Kutluay Yüce.  In Chuck’s memory, we also express gratitude to his many other colleagues who, for lack of space, have not been explicitly acknowledged here but whose friendship and support immeasurably enriched Chuck’s personal and professional life.

[1] Cowley, C., Ryabchikova, T., Kupka, F., Bord, D. J., Mathys, G., & Bidelman, W. P. “Abundances in Przybylski’s Star”, 2000 MNRAS, 317, 299 DOI: 10.1046/j.1365-8711.2000.03578.x

[2] Cowley, C. R. “A Star with an Unusual Variable Spectrum”, 1956 PASP, 65, 537 DOI: 10.1086/126994

[3] Cowley, A., Cowley, C., Jaschek, M. & Jaschek, C. “A Study of the Bright A Stars: A Catalogue of Spectral Classifications”, 1969 AJ, 74, 375 DOI: 10.1086/110819

[4] Cowley, Charles & Warner, Brian “On the Determination and Astrophysical Application of Oscillator Strengths from Free-Burning Arcs”, 1967 AJ, 72, 791 DOI: 10.1086/110534

[5] Cowley, Charles R. The Theory of Stellar Spectra, 1970 (New York: Gordon & Breach), 260 pp.

[6] Cowley, Charles R. An Introduction to Cosmochemistry, 1995 (Cambridge: Cambridge University Press), 490 pp.

[7] Upper Main Sequence Stars with Anomalous Abundances, 1986 Proceedings of the 90^th IAU Colloquium, C.R. Cowley, M. M. Dworetsky, & C. Mégessier, eds. (Dordrecht: D. Reidel Publishing), 489 pp.

[8] NASA Astrophysics Data System - author: Cowley, Charles R. years: 1956-2024, downloaded 10 Sept 2024.

[9] Hartoog, M. R., Cowley, C. R. & Cowley, A. P. “The Application of Wavelength Coincidence Statistics to Line Identification: HR 465 and HR 7575”, 1973 ApJ, 182, 847 DOI: 10.1086/152188

[10] Cowley, Charles R. & Adelman, Saul J. “Line Identification Studies Using Traditional Techniques and Wavelength Coincidence Statistics”, 1990 PASP, 102, 1077 DOI: 10.1086/132736

[11] Cowley, C. R., Bidelman, W. P., Hubrig, S., Mathys, G. & Bord, D. J. “On the Possible Presence of Promethium in the Spectra of HD 101065 (“Przybylski’s Star”) and HR 965”, 2004 A&A, 419, 1087 DOI: 10.1051/0004-6361:20035726

[12] Cowley, C. R., & Hartoog, M. R. “Uranium in the Spectrum of HR 465”, 1972 ApJ Lett. 178, L9 DOI: 10.1086/181072

[13] Cowley, C. R., Allen, M. S. & Aikman, G. C. L. “Thorium in the Spectrum of HR 465”, 1975 Nature, 258, 311 DOI: 10.1038/258311a0

[14] Aikman, G. C. L., Cowley, C. R. & Crosswhite, Hannah M. “Dysprosium in Lines in the Spectra of Peculiar A and B Stars”, 1979 ApJ, 232, 812 DOI: 10.1086/157342

[15] Donald J. Bord, Charles R. Cowley, & Peggy L. Norquist “Oscillator Strength Calculations in Ce Ill: Application to HD 200311”, 1997 MNRAS 284, 869 DOI: 10.1093/mnras/284.4.869

[14] Charles R. Cowley & Donald J. Bord “The Third Spectrum of Neodymium: Applications to HR 6870 and γ Equulei”, 1998 ASP Conf. Series, J. C. Brandt, T. B. Ake, III, & C. C. Petersen, eds., Vol. 143, 346

[17] Lawler, J. E., Wickliffe, M. E., Cowley, C. R. & Sneden, C. “Atomic Transition Probabilities in Tb II with Applications to Solar and Stellar Spectra”, 2001 ApJ Suppl., 137, 341 DOI: 10.1086/323001

[18] Wyart, J.-F., Blaise, J., Bidelman, W. P., & Cowley, C. R “Energy Levels and Transition Probabilities in Doubly-Ionized Erbium (Er III)”, 1997 Phys. Scr., 56, 446 DOI: 10.1088/0031-8949/56/5/008

[19] Cowley, C. R., Hubrig, S., Castelli, F., González, J. F. & Wolff, B. “Heavy Calcium in CP Stars”, 2007 MNRAS, 377, 1579 DOI: 10.1111/j.1365-2966.2007.11719.x

[20] Cowley, C. R., Hubrig, S., Palmeri, P., Quinet, P., Biémont, É., Wahlgren, G. M., Schütz, O., & González, J. F. “HD 65949: Rosetta Stone or Red Herring”, 2010 MNRAS, 405, 1271 DOI: 10.1111/j.1365-2966.2010.16529.x

[21] Cowley, C. R. “Reminiscences: Exotic Stars and Exotic Elements—An Entirely Self-Serving Document”, 2005 Phys. Scr., T119, 26 DOI: 10.1088/0031-8949/2005/T119/004