2.2. MIT enthusiasm

Chia Ch'iao Lin was not an astronomer. Since the pre-war time, he had been studying fluid flows. By the 1960s, he had had over 60 publications, a monograph on hydrodynamic stability (Lin 1955), a world recognition of an applied science expert, and a solid reputation at the department of mathematics in the Massachusetts Institute of Technology (MIT) where he worked since 1947. But he did feel a continual interest in astronomy, being admired with strict analytical papers by Chandrasekhar, with M. Schwarzschild' work on stellar structure, with Zwicky's morphological method. In 1961 this side interest became Lin's life-long vitality. That spring, on visit in Princeton, ³⁸ he attended the aforementioned conference on interstellar matter and, having become familiar with the developments in galaxy research, he got captured by the problem of the persistent spiral structure. ³⁹

Back in MIT, Lin conveyed his galactic enthusiasm to his young colleagues Hunter and Toomre. ⁴⁰ For quick acquaintance with current periodicals, a `reading group' was formed; ⁴¹ a "friendly back-and-forth atmosphere" (Toomre) warmed open discussions and working visits of Woltjer and Lust, organized by Lin; ⁴² Lebovitz was hired in the department. ⁴³ In 1962, Shu arrived there for doing his undergraduate course work under Lin's guidance, ⁴⁴ and Hunter with Toomre, their instructorship finished, left MIT, one back for Cambridge, UK, the other for Princeton; their first papers appeared in 1963.

Hunter and Toomre made their debut in galaxy dynamics on a vital problem already posed but yet unanswered very basically (Kuzmin 1956; Burbidge et al 1959): How to connect the empirical rotation curves of galaxies with their equilibrium mass distribution? Toomre (1963) set forth a general mathematical method, and for a razor-thin disk model he derived a series of solutions well known nowadays as Toomre's models of n^th order (Binney & Tremaine 1987, p.44). ⁴⁵ Hunter (1963) used a distinct thin-disk approximation and found another series of exact solutions. The simplest there was the case of uniform rotation and surface density µ₀(r) (1 - r² / R²)^1/2. For it only was the analytical study of equilibrium stability possible, and Hunter did it "using only pencil, paper, and Legendre polynomials" (Toomre 1977, p.464). This cold disk proved unstable for a wide span of axisymmetric and non-axisymmetric oscillation modes. ⁴⁶ These papers by Toomre and Hunter had paved the way for further works on kinematical models and global dynamics of flat stellar systems.

³⁸ Stromgren invited him for discussions on stellar structure (Lin), largely in relation to his fresh interest in hydrodynamics of liquid helium (Lin 1959). Back.

³⁹ In his early spiral papers, Lin often quoted Oort's statement reproduced in Sect. 1.3. Back.

⁴⁰ At that time, the department of mathematics in MIT was vigorously enlarging its applied side. Hunter and Toomre were hired there in 1960, just after they had got their PhD degrees in fluid dynamics in England. Initially, they hoped to collaborate with Backus (Hunter; Toomre), a recognized leader in geomagnetic problems, but as he left MIT that year already, they two "soon caught some of Lin's fever for problems in the dynamics of galaxies". "Almost at the moment I first met him in fall 1960 I was struck with his breadth of scientific interests, his really excellent spoken English, [...] and his genuinely gracious manner of dealing with other people". (Toomre) Back.

⁴¹ "[We] were all becoming interested in astrophysical problems together. We read Martin Schwarzschild's book on stellar structure together". (Hunter) Back.

⁴² "It was a real pleasure to have such a thoughtful and articulate theoretical astrophysicist as Woltjer so close to chat with about this thing or that. [...] It was from his informal lectures that summer that I learned for the first time not only how Dutch and Australian radio astronomers working in parallel had more or less mapped the spiral arms of this Galaxy from the velocity maps, but also how astonishingly thin - and yet curiously bent - is our layer of 21-cm gas". (Toomre) Back.

⁴³ "I had just received my PhD [working with Chandrasekhar], I wished to pursue applied mathematics, and I had received an offer of an instructorship from one of the best applied-mathematics departments in the country. Lin's motive I can only speculate on. He was interested in moving in the direction of astronomy and of the spiral-structure problem and perhaps figured I would be a useful participant. If this is the case, I suppose my stay at MIT may have been somewhat disappointing to him because I spent all of it in close collaboration with Chandrasekhar on a quite different set of problems". (Lebovitz) Back.

⁴⁴ "I began work with C.C. Lin in summer 1962 as an undergraduate research assistant and continued through the fall and spring 1963, on the topic of spiral structure in galaxies as my undergraduate thesis project in physics at MIT [...] I knew Lin from even earlier because he is a close friend of my father". (Shu) Back.

⁴⁵ Toomre's model 1 reproduced the result by Kuzmin (1956) then unknown to Toomre (Binney & Tremaine 1987, p.43). Back.

⁴⁶ The stability of differentially rotating cold disks Hunter studied in his subsequent paper (Hunter 1965). Back.