Large distance ofε Aurigae inferred from interstellar absorption and reddening

Abstract

The long-period (P = 27.1 years) peculiar eclipsing binary ε Aur, which has recently completed its two year-long primary eclipse, has perplexed astronomers for over a century. The eclipse arises from the transit of a huge, cool and opaque, disk across the face of the F0 Iab star. One of the principal problems with understanding this binary is that the very small parallax of p = (1.53 ± 1.29) mas, implying a distance range of d ∼ (0.4−4.0) kpc, returned by a revised reduction of the Hipparcos satellite observations, is so uncertain that it precludes a trustworthy estimate of the luminosities and masses of the binary components. A reliable distance determination would help solve the nature of this binary and distinguish between competing models. A new approach is discussed here: we estimate the distance to ε Aur from the calibration of reddening and interstellar-medium gas absorption in the direction of the system. The distance to ε Aur is estimated from its measured E(B − V) and the strength of the diffuse interstellar band 6613.56 Å. Spectroscopy and UBV photometry of several B-and A-type stars (<1 • of ε Aur) were carried out. The distances of the reference stars were estimated from either measured or spectroscopic parallaxes. The range in distances of the reference stars is from 0.2 to 3.0 kpc. We find reasonably tight relations among E(B − V), EW, and I c (6613 Å feature) with distance. From these calibrations, a distance of d = (1.5 ± 0.5) kpc is indicated for ε Aur. If ε Aur is indeed at (or near) this distance, its inferred absolute visual magnitude of M V (−9.1 ± 1.1) mag for the F-supergiant indicates that it is a very young, luminous and massive star. Noteworthy, the high luminosity inferred here is well above the maximum value of M V −6. m 2 expected for (less-massive) post asymptotic giant branch supergiant stars. Thus, based on the circumstantial evidence, the higher-mass model appears to best explain the properties of this mysterious binary system. As a by-product of this study, our spectroscopy led to the finding that two of the stars used in the distance calibrations, HD 31617 and HD 31894, are newly discovered spectroscopic binaries, and HD 32328 is a new radial-velocity variable.

Time‐series high‐resolution spectroscopy and photometry of ε Aurigae from 2006–2013: Another brick in the wall

“…15) (Aitken & Dolittle 1932), Aurigae is a possible quintuple star system. As already mentioned by Guinan et al (2012), all of these putative components could be foreground or background field stars. We have therefore obtained high-resolution spectra for all of them.…”

“…16) and the EWs are accordingly uncertain. Our best value for EW(λ6613) is 110±10 mÅ which would place the star at a distance of 1.6±0.1 kpc according to the calibration in Guinan et al (2012). We note that the error of 10 mÅ for the EW is just a rms from repeated measurements and could be much larger, in particular when the expected calibration error would be added.…”

“…It converts into an abundance of log n(Li) = 1.35±0.10 (log n(H) = 12) from the NLTE tables in Pavlenko & Magazzu (1996). Differential UBV observations of this star secured at Hvar Observatory (12 individual observations in 5 nights) gave V = 10. m 93±0.09, B − V = 0. m 71±0.02, and U − B = 0. m 32±0.01 (Guinan et al 2012). However, B − V = 1.…”

“…16, Table 6). The λ6613 EW would suggest a distance of 2 kpc if the calibration by Guinan et al (2012) were valid.…”

“…Note that the errors of the van de Kamp value from Sproul-refractor plates were most likely underestimated because their positional and photometric elements did not agree with each other. However, a recent redetermination (Guinan et al 2012) of the interstellar absorption towards Aur places the system out to 1.5±0.5 kpc, which would favor the high-mass scenario.…”