We know what a nova is, but how?

Question

I work with astrophysicists and require some basic knowledge of many astronomical sources, however research priorities often demand that most of human knowledge on a subject be taken for granted.

I am currently researching galactic novae, and I find it relevant to summarize their history briefly when presenting my research to certain audiences. Unfortunately, I am unable to find any source material which describes how we know one key aspect of the events: that they are an accreting white dwarf in a stellar binary. This fact appears to be so well founded that no scientific paper feels obligated to cite it when stated, but basic resources like astronomical encyclopedia also make no reference that I've seen.

How do we know that novae are binary systems?

E.g., have follow-up observations clearly identified the white dwarf and its companion? Or do other astronomical measurements strongly confirm this binary hypothesis (and make it all but obviously true)? I apologize if it's as simple as "someone looked through a telescope, and it was pretty obvious" -- in my experience no revelation in astrophysics is nearly so simple, but certainly this could be the case.

Answer 1

Following a reference to Darley et al., ApJ 746, 61 (2012) from your Wikipedia link gives a (very technical) discussion of nova progenitors, including distinctions between nova systems where the secondary stars are main sequence or supergiant stars, and distinctions among white dwarfs with different chemistries. The first sentence of that paper is

A classical nova (CN) outburst occurs in an interacting binary system comprising a white dwarf (WD, the primary) and typically a late-type main-sequence (MS) star (the secondary) that fills its Roche lobe (Crawford & Kraft, 1956).

That suggests the 1956 paper is the original proposal for the Roche overflow model of the classical nova. Like many original-idea papers, it's a pretty clear read. But for your question, Crawford and Kraft seem to hedge about whether the "blue star" in their particular pair needs to be a white dwarf:

[T]he observed luminosity of the blue star is essentially due to the energy released by the accreted material. This view is strengthened also by the fact that the blue star occupies a peculiar position in the H-R diagram. It lies 10.5 vis. mag. below the main sequence but about 4 mag. above the most luminous white dwarfs, whose effective temperature it exceeds by about 8000° K. Unless the blue star is essentially degenerate, it can readily be shown that the small radius implies such a high internal temperature that electron scattering is the principal source of opacity. A simple calculation based on the standard model then yields a luminosity 8 mag. brighter than is observed.

In other words, Crawford and Kraft don't come out and say "definitely a WD," but if it's a non-degenerate star, it's a very strange one. More modern observations of novae are compared to detailed models of the dynamics of the surface dynamics, models which have been debated vigorously for decades; the current generation of comparisons to data are sensitive to details like the amount of helium accumulating on the white dwarf's surface during the nova event. It seems unlikely that such details could even come close if the underlying assumptions about the basic physics of the erupting star were wrong.

Note that a classical nova system can be thought of as a type of contact binary star. For any reasonable estimate of the size of the giant star, a distance of 10 AU between the two members of the pair seems like an overlarge estimate. Ten astronomical units of separation viewed from a distance of 50 parsecs is already a gap of 0.1 seconds of arc. I wouldn't expect to see visible-light photographs showing both the giant star and the white dwarf, but rather that all of the information about the binary systems comes from spectroscopy.

Answer 2

The real key, I suspect, was that observations of "postnovae" -- classical novae after the nova outburst, when the light from the outburst itself no long obscured light from the underlying system -- often showed clear characteristics of binary stars. This took the form of periodic dips in the light curve, suggestive of eclipses, or direct spectroscopic evidence for binary motion, or both.

This is discussed, with references (including the Crawford & Kraft 1956 reference that rob mentions in his answer), in Section 2.2 of the 1978 review article by Gallagher & Starrfield in Annual Reviews of Astronomy & Astrophysics. Section 2.4 discusses some of the evidence then available for the primaries being accreting white dwarfs.

(If you're not already aware of it, review articles in Ann.Rev.A&A are often a good place to look for answers to questions like this. Sometimes earlier articles are better for certain questions, because they're closer in time to when people were still figuring things out, and so they go over the early evidence in more detail than a later article would.)