As part of my Ph.D. dissertation, I have been conducting a comprehensive review of literature on Epsilon Aurigae. Each month I will select one paper to review or discuss, adding relevant information from more recent literature when necessary.
Even before the announcement at Don Hoard's press conference at the recent American Astronomical Society about the eps Aur system being in the so-called “low-mass” state, several other authors had proposed such an idea. In this issue of the CS newsletter, I will briefly discuss two papers that consider the low-mass scenario.
Before we continue, we first need to discuss what the “high” and “low” mass models mean, both in terms of the physical parameters and the evolutionary state of the system. The high-mass model would describe the (visible) F-star as a massive Population I F-supergiant with a mass of ~15 solar masses. Spectroscopic observations imply that the two components of the system are of almost equal mass (which is heaver is not known a priori). In this scenario, the disk (plus whatever is inside) have come in at about 13 solar masses. In general, it is assumed that disk itself has relatively little mass, therefore a majority of the 13 M0 resides inside of the disk as some object. It was proposed by Lissauer et. al. (1984) that the system itself might be a triple, in which the massive F-supergiant was accompanied by a pair of B-stars shrouded inside of the disk. This situation would allow for the mass ratio to be maintained, without creating excess light in the system, thereby agreeing with observations. In the low mass model, the (visible) F-star is described as a lightweight 4-5 solar mass AGB or Post-AGB star accompanied by a B-star enshrouded inside of a disk.
The first of the two papers I wish to discuss is not often cited in the literature (three times to-date to be exact) written by Takeuti (1986). His work is mostly theoretical, but he concludes that the F-star does not fit models of high-mass supergiants. Instead, he indicates that the best fitting model is one in which there is a “degenerate core and double shell sources with hydrogen-rich envelopes” which “describes a star just subsequent to the stage of mass ejection.” In this case, the degeneracy being discussed means that the core is packed as tightly as allowed by quantum mechanics, being supported by electron degeneracy pressure. In this case, the core would essentially be a white dwarf star that is still surrounded by material (the outer shells). Takeuti's paper continues by discussing possible masses for the F-star by using pulsation theory, indicating the mass of the F-star may be around 6 M0.
The second paper I wish to discuss presents some interesting observational evidence for something in the system having evolved through the AGB-phase. The second paper by Hinkle and Simon (1987) discusses the appearance of CO in the (infrared) spectrum of eps Aur after mid-eclipse. CO is a fairly strong molecule, but it is dissociated before 7000 K, therefore it cannot be in the F-supergiant (T ~ 7500+ K). The timing of its appearance and habitable zone of temperatures implies this CO cloud is associated with the disk itself. The authors also used a high-resolution spectrograph to observe the ratio of 12C to 13C (with typical exposure times around 300 minutes!). What they found was that the 12C/13C ratio is oddly low at 10. Massive F-supergiants typically have a 12C/13C ratio around 15-30. They conclude that this ratio is similar “to low mass stars along the AGB branch.”
As one can see, the low mass model does have supporting evidence, but historically there has been plenty of opposition to this scenario (see Guinan 2002). With the new information provided by Hoard, the high mass vs. low mass debate may finally be settled. The perplexing question that remains is how one can explain an AGB star, disk, and B star in one consistent evolutionary history story.
For further reference, please refer to:
Lissauer et. al. (1984):
The Epsilon Aurigae secondary - A binary embedded within a disk?
A low-mass model of epsilon Aurigae
Hinkle and Simon (1987)
Two micron CO absorption lines in the spectrum of Epsilon Aurigae during eclipse
Toward Solving the Mysteries of the Exotic Eclipsing Binary ε Aurigae: Two Thousand years of Observations and Future Possibilities