“Iron Star” AS 325: An Unusual Emission Line Eclipsing BinaryJill Gerke (NOAO, University of Arizona), Steve B. Howell (WIYN, NOAO), Sebastian Otero (CEA), Arne Henden (AAVSO)

AS 325 is a complex eclipsing binary system consisting of a BVe primary and a variable (K2III-M4III) red giant companion. The optical spectrum contains many emission lines, especially those of Fe II. This forest of Fe II emission lead to AS 325’s identification as an analog for XX Ophiuchi, the original and only other “iron star”. The mean brightness of AS 325 has been steadily increasing over the past 5 years, coinciding with an increase in the depth of the eclipse. New spectroscopic and photometric observations of AS 325 were obtained over the 2005 eclipse. The spectra show a generally flat continuum during the eclipse and a variable, slightly red, continuum outside of eclipse. The strength of the H emission lines (from the Be star) increase slightly during the eclipse, suggesting a decreased continuum level from the red star. This fact, along with the increased variability in the red portion of the continuum out of eclipse, suggests that the red star is eclipsed. Cool et al. (2005) used UV, optical, and IR spectra to propose that AS 325 consisted of a Be star and a K2.5 III star. Our new optical spectroscopy indicates that the red star now has a spectral type of M4III, suggesting that it is a variable giant and may account for the general system brightening. One interesting spectral feature is the Ca II H and K absorption lines. These lines are observed to disappear during eclipse and reappear afterwards. A model combination of a B4V and M4III reveals that the observed Ca II H and K absorption does not originate in either star and its presence and variability are unexplained. Using the established orbital period (513 days) and estimated masses for a B4V and an M4III star, we find the binary separation is ~4.8 au and the binary inclination must be at least 88 degrees. Due to the relative sizes of the two stars, the M4III will not be fully eclipsed by the 8 times smaller B4Ve star. Multi-color photometric observations of the eclipse, obtained by the AAVSO, help confirm our assumptions that the B star passes in front during the eclipse.

II. The figure at left shows the long term (ASAS-3) V light curve of AS 325. The 513 day periodic eclipses are clearly visible, confirming AS 325 is indeed a binary. The light curve shows variability in both the mean magnitude of the system and the depth of the eclipse. The increasing mean magnitude (by 0.6 mag) and depth of eclipse imply that one of the stellar components has increased in brightness. We find observational evidence that this brightness increase is due to variability of the red giant secondary star: changing from a K2.5III to a ~0.5-0.8 mag brighter M4III over the past few years.

III. The most recent eclipse of AS 325 occurred from 5 June 2005 to 30 August 2005. Three representative optical spectra are plotted at the right: near mid-eclipse (top) and 13 and 15 days after the eclipse ended. They are shown offset by a constant flux amount. Spectra taken during eclipse showed little variability and are relatively flat over the optical range. Spectra obtained out of eclipse show variability in the appearance of the continuum changing from flat to an increasing red slope. Note the absence of Ca H&K absorption during eclipse. The emission lines are always present but variable and P-Cyg wind features are seen at times (Cool et al. 2005).

I. The complex optical spectra of AS 325 has lead to much ambiguity in its understanding and classification. The confusion results from the many emission lines and essentially no stellar absorption features found in the optical spectrum. Optical spectra reveal strong hydrogen emission lines as well as emission from ionized metals such as Fe II, Ti II, and Cr II. Bopp and Howell (1989) cited these unusual emission lines to denote its similarity to XX Ophiuchi, Merrill’s iron star (1924). Neither star is really overly rich in Fe, and the low ionization metal emission lines seen in the optical are a result of wind material fluorescence. Bopp and Howell (1989) suggested the possibility that AS 325 was a binary system and Cool et al. (2005) proposed that AS 325 consists of a Be star and a K2.5 III star.

References: Cool, R.J. et al., 2005 (PASP, 117, 462): Bopp, B.W., & Howell, S.B. 1989 (PASP, 101, 981); Merrill 1924 (PASP, 36, 225)

V. In addition to being photometrically variable, we also find AS 325 to be spectroscopically variable. For example, spectra presented in Cool et al. (2005) differ from those obtained during our 2005 observations. Cool et al. proposed that AS 325 consists of a Be star and a K2.5 III star. The identification of a K2.5 III star was based on a K-band spectrum and showed a consistent match to AS 325 in the optical. On the left, we repeat the Cool et al. plot comparing the red spectrum of AS 325 to that of the single K2 III star HD 140573. However, our new optical spectroscopy (see VI) shows features in the (red) optical not present in the Cool et. al (2005) data. These features include TiO bands from the continuum of a red giant star. We conclude that the red star is now an M4III.

Our model (right) provides the correct “low-level bumps” seen in AS 325. These features include the Balmer jump of the B star and the TiO band heads of the M giant. While this model supports AS 325 as a binary containing a BV & MIII star, it can not explain the Ca II H and K absorption lines. These lines disappeared during eclipse (see III) and, when out of eclipse, they decrease in strength as the continuum becomes red. Our model composite spectrum shows that the observed Ca II H&K absorption lines are not from the photosphere of either star and the fact that the absorption goes away during eclipse seems to imply a formation site between the two stars.

IV. The cartoon to the left shows the relative sizes of the components in AS 325. We see that the Be star will occult only a small fraction of the secondary during eclipse. The red star now appears to be an M4 giant, larger and brighter than the K2 giant star found by Cool et al. (2005). This large change in spectral type over a period of a few years is not unheard of for a Mira or SR post-AGB type variable. Using typical masses and the orbital period, the two stars are ~4.8 au apart. These orbital parameters lead to a lower limit for the binary inclination of 88 degrees.

V. If the red star gets transited by the blue star, we would expect the system to become slightly less red during eclipse. Using AAVSO eclipse observations (left), we see that the B-V color of AS 325 is composite at all times, being “F8” at mid-eclipse and “G5” out of eclipse, i.e., slightly less red in eclipse. The R-I color, however, is nearly equal in/out of eclipse at 0.6. This suggests, as we have seen, that the M giant dominates the red optical continuum at all times while the blue optical changes during eclipse.

VI. To see if the relatively flat, featureless optical spectrum of AS 325 could be produced by a combination of the two proposed stellar components, we ignored the non-photospheric emission lines and co-added a normal B4V and an M4 III star. Normalizing each star at 5000A, we scaled the two stellar spectra to their correct relative brightness as viewed at the same distance form Earth.

We wish to thank