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Identification of NSV 11661 with MisV0134: A possible post-AGB variableFebruary 10, 2000
Yoshida, S.; Kadota, K.; Kato, T.
This report discusses on the identification of NSV 11661 with MisV0134 and the nature of the object as a possible post-AGB variable.
NSV 11661 is recorded as a suspected variable star discovered by Hoffmeister. It was classified as an irregular type variable star with a photographic brightness variation between 17 - 20 mag in the New Catalogue of Suspected Variable Stars (Kukarkin et al. 1982). The cataloged position is R.A. 18h59m3, Decl. +15o40' (1950.0).
In the course of the MISAO Project variable star survey, the variability of a star at R.A. 19h01m38s.64, Decl. +15o43'08".0 (2000.0), was discovered from unfiltered CCD images taken by Kadota, with a range of 13.5 - 14.3 mag between 1999 April and July. There are no known variable stars within 5 arcmin, except for NSV 11661 whose cataloged position is 103 arcsec from the object. Because of the large angular distance and large difference of brightness, we judged it as a new variable star and named as MisV0134 (Yoshida et al. 1999).
However, Kato researched the Hoffmeister's original chart (Hoffmeister 1965) and discovered that the cataloged position of NSV 11661 was erroneous and it was the same object as MisV0134. In the original chart, the star at the position of MisV0134 was marked as NSV 11661, originally named as S9039. But the measured position described in the paper, R.A. 18h55m.0, Decl. +14o43' (1855.0), had a large error.
As a result, our discovery of MisV0134 confirmed the variability of NSV 11661. Table 1 shows the photometry, measured automatically by the PIXY system from unfiltered CCD images taken by Kadota. The magnitudes were measured using the USNO-A1.0 catalog based on a preliminary V magnitude calculated from R and B magnitude in the catalog based on Kato's (1998) equation:
V = R + 0.375 (B - R).
Table 1: Photometry
The large difference between Hoffmeister's photometry and ours implies that the star is a red variable. But the color of NSV 11661 inferred from Hoffmeister's paper is not noticeably red, since no remark on the color is assigned. It is unusual as a red variable star because many other Mira type variable stars were remarked.
NSV 11661 is also identified with IRAS 18593+1538 with fluxes 0.67 Jy for 12 micron, 0.59 Jy for 25 micron, and the upper limits of 0.40 Jy, 3.83 Jy for 60 micron, 100 micron respectively. The 12-micron flux to 25 micron flux ratio is small as 1.14, while the ratio is larger than 1.5 in case of most red variable stars. The 25-micron flux to 60 micron flux ratio is larger than 1.48. Therefore, this object is probably an OH/IR star, a post-AGB star in the transition between a red variable star and a planetary nebula (Zijlstra et al. 1990).
The difference between 12-micron flux and 25 micron flux is 0.08. The difference between 25-micron flux and 60 micron flux is larger than 0.19. According to another diagram, the possible classification becomes a variable star with very thick O-rich circumstellar shell or a variable star with relatively hot dust close to the star and relatively cold dust at large distance (Van der Veen and Habing 1988). The large difference between the magnitude from the Palomar blue plate by Hoffmeister and magnitude from the unfiltered CCD images taken by Kadota could be explained by the existence of dust shell.
We would like to thank Dr. Brian Skiff for discussion on the analysis of the IRAS flux.