A comprehensive analysis of the cool RCB star DY Persei

Abstract

Context. Theoretically, the number of cool Galactic R Coronae Borealis stars (RCBs) should be greater than the warm RCBs, however to date, only a few candidates have been detected. Aims. Observations of the extremely cool RCB candidate, DY Per, and the anonymous nearby star (the "Taipei star") are presented to specify its fundamental parameters and evolutionary status. Methods. CCD BVRI photometry and low-resolution spectroscopy at deep light decline was carried out in 2004. A high-resolution spectrum was gathered near the maximum light in 2002, and a qualitative analysis was made relative to the typical carbon stars of various types. Results. Near the light maximums, the nearby anonymous star with a separation of ∼2. 5 reported by Začs et al. (2005a, A&A, 438, L13) was outshined by DY Per. However, in the B and V band the components are resolved at deep light decline and the nearby star is somewhat brighter in B than DY Per itself. BV(R) individual magnitudes of the components were estimated using PSF photometry for the first time. The light decline in 2004 was the deepest ever observed for DY Per, B = 18.16 mag. The individual colour indices of DY Per at light decline, (B − V) = 2.35 and (V − R) 1.9, are not typical for carbon stars. In the spectrum, absorption features of C 2 and CN molecules prevail, however, prominent C 2 absorption bands are significantly veiled at deep-light decline and a broad (FWHM ∼ 600 km s −1 ) emission feature of Na i D12 is visible. Emissions are suspected in the C 2 (1,0) bandhead and in Ca i line at 4227 Å. Analysis of DY Per relative to the sample of carbon stars, verifies the significant hydrogen deficiency, high carbon abundance, relatively high 12 C/ 13 C ratio and solar metallicity. Thus DY Per could be a prototype of cool extreme Galactic RCBs. The nearby star does not vary appreciably in the B and V bands, B = 17.8 mag. With the observed colour indices (B − V) = 1.00 and (V − R) 0.6, it may be a G0 dwarf not physically related to DY Per, although the distances to both stars are similar, d ∼ 1.5 kpc.

Carbon stars

“…DY Per, the best studied of the group, has fairly regular minima of depth about 2 mag in a period of 792 d and minima of up to 5 mag which are more erratic but which keep approximately to the 792 d cycle (Alksnis et al 2002). The type star is at most moderately H-deficient (Keenan & Barnbaum 1997;Začs et al 2007), while s-process elements are not enhanced. 12 C/ 13 C is similar to the values found in typical N stars (Začs et al 2005(Začs et al , 2007, in contrast to the absence of 13 C in the spectra of most RCB stars.…”

“…The type star is at most moderately H-deficient (Keenan & Barnbaum 1997;Začs et al 2007), while s-process elements are not enhanced. 12 C/ 13 C is similar to the values found in typical N stars (Začs et al 2005(Začs et al , 2007, in contrast to the absence of 13 C in the spectra of most RCB stars. Absorption components of Na I D are attributed to two separate clouds of ejected material with velocities up to 170 km s −1 (Keenan & Barnbaum 1997;Začs et al 2005).…”

Signatures of pulsations and mass loss in the spectra of post-AGB stars

“…DY Per seems to be a prototype of Galactic cool R CrB star with confirmed hydrogen deficiency, high carbon abundance, high 12 C/ 13 C ratio, solar metallicity, and confirmed episodes of mass ejections (Začs et al 2007).…”

A new search for R Coronae Borealis stars in the SMC

“…A subclass of RCB stars called DY Persei also exists. The DY Per stars tend to be cooler and have more symmetrical drops in comparison to RCBs (e.g., [1,12], and references therein). Our goal was to search for new RCB stars in the Small Magellanic Cloud (SMC), using VI data from the OGLE project ( [10]).…”