Evidence for a delay in the first contact of the 2009 eclipse of Epsilon AUR
ATel #2224; M. M.M. Santangelo, Osservatorio Astronomico di Capannori (OAC) Italy
on 2 Oct 2009; 21:23 UT
Credential Certification: Filippo Mannucci (email@example.com)
Subjects: Optical, Variables
We performed UBVR photoelectric photometry of epsilon AUR using an SSP-5A photometer attached to the 0.30-m f/10 telescope of OAC, in 41 nights from July 17th to September 28th 2009. The star lambda AUR was used as comparison, and the atmospheric extinction was also measured.
This OAC photometry of epsilon AUR shows that the 2009 eclipse of epsilon AUR did not still take place until at least August 17th. In fact, until August 17th 2009, the OAC's photometry shows only the normal outside-eclipse flickering of epsilon AUR. Most of these V data shows flickering between V mag 2.95 and 3.05, in perfect agreement with the V band value outside eclipse given in the literature. The predicted time of the first contact was August 6th 2009, but our measurements do not support this forecast. Until August 19th there is no evidence for the beginning of the first contact.
A linear fit of a subset of V band data from August 13th to September 28th gave a statistically significant (at 99% level) slope for a brightness decrease. This decrease should be due to the first contact; anyway there is clear evidence for a delay of about a dozen of days in the first contact. This delay supports the theory of a shrinking of the F supergiant (Saito & Kitamura 1986, Ap&SS 122, 387).
A time series analysis of the V band data was performed, from 0.016 to 2 c/d, using various algorithms. A marginal detection of a periodicity at the 5% significance level, but not at the 1% level, could be suspected.
The average B-V colour index is +0.574 in fair agreement with previous results for past eclipses. The average U-B colour is +0.195: a value bluer by about 0.11 magnitudes than that listed in the "Photoelectric catalogue" (Blanco et al., 1970, USNO Pub. 21, 178). The average V-R is +0.519 in good agreement with previous results. No significant linear trends, and no significant periodicities (from 0.016 to 2 c/d) at the 5% significance level are present in the time series of the colour indexes.
We also obtained low resolution long-slit spectra of epsilon AUR in the wavelenght range 380-900 nm. We used a Sbig DSS-7 spectrometer + ST9-XE CCD camera attached to the 0.30-m f/10 telescope of OAC, in 10 nights from July 10th to September 7th 2009.
Each spectrum of epsilon AUR was compared with the spectrum of the F0 Ia standard star phi CAS obtained in the same night. The main non-telluric features in OAC's spectra of epsilon AUR are the absorption lines of Na I at 589.3 nm and H-beta. Other non-telluric absorption features can be seen: the O I line at 777.4 nm, the H-gamma, the H Paschen series, a weak H-alpha, a weak Ca I at 612.2 and 616.2 nm, a weak Si II at 634.7 nm (blended with Mg II) and Si II at 637.1 nm. No emission lines are present in OAC's spectra of epsilon AUR. The O I absorption line at 777.4 nm is fairly strong, clearly indicating high luminosity. OAC's spectra of epsilon AUR can be classified as belonging to the F0 Ia or A9 Ia class.
The tracings of OAC's spectra of epsilon AUR are similar to that performed with the MIRA 0.91-m telescope some years ago (Torres-Dodgen & Weaver 1993, PASP 105, 693). The main difference between MIRA's and OAC's spectrum is that in MIRA's spectrum the H-alpha is completely absent, while in OAC's spectrum is present in absorption although quite weak. The weakness of the H-alpha absorption line in epsilon AUR is also the main difference between OAC's spectra of epsilon AUR and those of phi CAS taken in the same nights. In all spectra of phi CAS the strenght of H-alpha relative to other lines of the same spectrum is always stronger than the same relative strength in the spectra of epsilon AUR. From the comparison of OAC's and MIRA's spectra, it seems evident that this star shows strong H-alpha line variability on time scales of years.