The Swings Effect

For astrophysicists, the "Swings band" refers to a set of particular lines in the spectrum of comets  and the "Swings effect" is that of fluorescence in the emission bands of these same cometary spectra. But for the University of Liege, the Swings band is undoubtedly the one formed by the many researchers that Polidore Swings  supported, encouraged and whose careers he promoted. And the Swings effect is the incredible dynamic that he instilled in 'his' Cointe Institute of Astrophysics.

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A good student during his humanities, the young Polidore -Pol- Swings received the First General Prize each year, materialized by various books that the lucky winner took home to read during the holidays. When he finished his second year at the Charleroi Athénée, it was love at first sight: among the books he received was the famous "Astronomie populaire" by Camille Flammarion. In the "A few stages in my life" that he wrote in 1970, Pol Swings saw the birth of his vocation: "How and why did I become an astrophysicist? My vocation is due to Camille Flammarion's admirable "Astronomie Populaire", which I obtained as a prize for excellence in 1922. I devoured this book from cover to cover during the summer of 1922 and from then on I was determined to do the impossible to become an astrophysicist.

"Doing the impossible' is the right expression because at the end of the First World War, astrophysics was in its infancy and was not taught as such in any Belgian university. Quite naturally, he enrolled for a doctorate (a title that at the time sanctioned a four-year university course) in physical sciences and mathematics at the University of Liege. But he did his final work in mathematical astronomy under the direction of Marcel Dehalu, astronomer and director of the Cointe observatory.  And astrophysics? Pol Swings discovered it abroad: Paris, Meudon and especially Warsaw. At least that is where he learned what was to become the core business of his career: molecular spectroscopy, even though at that time there was little question of applying it to the study of stars. For this, he had to wait until 1931 and his first stay at the Yerkes Observatory at the University of Chicago. There he made a decisive encounter with Otto Struve, an astronomer of Russian origin who had emigrated to the United States after the First World War. The two men became friends and pursued numerous research projects together.

In the meantime, in 1932, on his return to the University of Liège, Pol Swings was appointed lecturer and developed a spectroscopy laboratory which soon attracted more and more students. His publications are numerous, on various subjects, to the point that it is difficult to pinpoint one.

The interstellar medium

In 1937, however, one of them was to be highlighted. That year (1), with his friend Léon Rosenfeld, Pol Swings identified the first interstellar line due to a molecule (in this case CH). A major discovery. At that time, we didn't really care about the interstellar medium," confirms Eric Gosset, astrophysicist and F.R.S-FNRS research fellow at the University of Liège. We knew that isolated atoms were present but we did not imagine that these atoms could meet and form molecules. Swings studied the spectra of many distant stars in which he noticed the presence of a fine absorption line. He was soon convinced that this was a residual line caused by the CH radical spread among the tenuous matter in interstellar space. The discovery went relatively unnoticed, although Chandrasekhar, the future Nobel Prize winner, and Eddington, for example, noted its importance. The importance of the discovery is obvious, because the young researcher from Liège suggests that this first line and the following ones he will discover should be attributed to low-temperature dust. In the interstellar 'vacuum', atoms could therefore assemble to form molecules even if their density is less than one atom per cubic centimetre! And Pol Swings went further: he correctly interpreted this fact as the consequence of the very low temperature of the medium, which he estimated at about -270°C! It was not until 1965 and the work of Penzias and Wilson on the cosmological background radiation at 3K that his intuition was confirmed in a striking manner. And much later, the very precise measurements made by the Planck satellite (tested in Liège!) would further confirm the accuracy of Swings' very first measurement of the cosmological background!

Swings band and effect

Much of Pol Swings' work focuses on the spectra of a wide variety of celestial objects, including the sun and particularly its sunspots or the aurora borealis. But there is one category of these objects to which Pol Swings' name will always be attached: comets.

He devoted himself to deciphering the molecular bands of their spectra, thus revealing their composition and that of their atmosphere. But he is best known for explaining the bizarre nature of these spectra. The intensities of these molecular lines are indeed very variable," says Eric Gosset. At the time, there was no satisfactory explanation for this phenomenon. In 1941, Pol Swings was invited to the Lick Observatory in California. It was there that he formulated his main ideas on comets. In fact, several problems arose. First of all, for the same molecule, the intensity distribution is not the same when observed in the laboratory or in the comet; secondly, it varies from one comet to another; and thirdly, for the same comet, it varies according to the distance of the comet from the sun. Pol Swings will show that the anomalies observed in the intensities are due to a phenomenon of fluorescence (2) under the action of solar radiation: this is the 'Swings effect'. As for the variation according to the distance from the Sun, he showed that this is due to a Doppler effect (3) which manifests itself because of the speed of the comet in relation to the Sun (4).

Pol Swings is also tackling another enigma: that of the '4050 group', i.e. the presence, in cometary spectra, of emission lines around the wavelength (λ ) of 4050 angstroms (also found in absorption in the spectra of cold carbon stars). For a long time, the identification of the molecule responsible for this emission gave rise to several hypotheses. He demonstrated, in particular with Boris Rosen, also a researcher at the Institut d'Astrophysique de Liège, that the phenomenon is due to the presence of a polyatomic carbon compound, in this case the molecule C₃ (5). In an article entitled Flaming Stars (6), Otto Struve himself, in view of the importance of the work carried out by Swings over many years on the subject, suggested that the band close to 4050 angstroms should be called the "Swings band": "Because of the enormous amount of work which Professor Swings has done to advance our knowledge of the λ 4050 feature, and also because it is certain that this particular band will play an important role in astrophysics during the coming years, I think it appropriate to designate it in this article as the Swings band.

Cometary Atlas

The harvest of data thus collected and his exceptional knowledge of comets enabled Pol Swings to publish an 'Atlas of Representative Cometary Spectra' (7) with Leo Hazer in 1956. It was enthusiastically received by astrophysicists," emphasises Eric Gosset, "because it was a first on the subject. And it remained a reference on the subject for years, especially as Pol Swings' students then continued his work by updating it. The publication of such an atlas had in fact been decided at the Rome Congress of the International Astronomical Union in 1952. Such a book was of interest not only to comet scientists but also to researchers in solar physics or, for example, those interested in the physics of the upper atmosphere. But Pol Swings was the most qualified to fulfil this desire, which he did by publishing his book four years later. To achieve this, the Liège-based astrophysicist first collected 350 spectra of 36 different comets, scattered all over the world; he then studied them to highlight in his atlas various characteristics of the comets: intensity of the lines of molecular emissions, profiles of the emission bands, distribution of the emissions in different parts of the comet (tail, hair, nucleus), variation of the molecular emissions as a function of heliocentric distance, etc. In short, a true cometary identity card that astrophysicists were inspired by for a long time.

But Pol Swings was perhaps above all one of those dreamers who never doubted anything and never hesitated to ... make plans for a comet: In 1962, he proposed to ESRO (the European Space Research Organisation, the forerunner of today's ESA) to send a probe to the vicinity of a comet... In 2014, ESA's Rosetta probe orbited the comet Churumov-Guerassimenko and then released a small lander that would land there....

Learn more

his biography

his scientific publications

References

1. Consideration regarding Interstellar Molecules, P. Swings, L. Rosenfeld, The Astrophysical Journal, vol. 86, 1937.
2. Fluorescence is the emission of light caused by the excitation of electrons in molecules: they absorb a photon from the sun and then immediately give off this energy in the form of light.
3. The Doppler (or Doppler-Fizeau) effect is the shift in frequency of a wave as the distance between the transmitter and receiver varies over time. In astrophysics, its measurement provides information on the movement of stars and also on the movement of matter within them.
4. Complex Structure of Cometary Bands Tentatively Ascribed to the Contour of the Solar Spectrum, P. Swings, 1941, Lick Observatory Bulletin, 19, pp.131-136.
5. Rosen and P.Swings, Carbon stars, comets and combustion phenomena, Annals of Astrophysics, 16, 1953.