The MACHO Project LMC Variable Star Inventory: X. The R Coronae Borealis Stars

The Astrophysical Journal (Impact Factor: 6.73). 02/2001; 554(1):298. DOI: 10.1086/321369
Source: arXiv

ABSTRACT We report the discovery of eight new R Coronae Borealis (RCB) stars in the Large Magellanic Cloud (LMC) using the MACHO project photometry database. The discovery of these new stars increases the number of known RCB stars in the LMC to thirteen. We have also discovered four stars similar to the Galactic variable DY Per. These stars decline much more slowly and are cooler than the RCB stars. The absolute luminosities of the Galactic RCB stars are unknown since there is no direct measurement of the distance to any Galactic RCB star. Hence, the importance of the LMC RCB stars. We find a much larger range of absolute magnitudes (M(V) = -2.5 to -5 mag) than inferred from the small pre-MACHO sample of LMC RCB stars. It is likely that there is a temperature - M(V)relationship with the cooler stars being intrinsically fainter. Cool (~5000 K) RCB stars are much more common than previously thought based on the Galactic RCB star sample. Using the fairly complete sample of RCB stars discovered in the MACHO fields, we have estimated the likely number of RCB stars in the Galaxy to be ~3,200. The SMC MACHO fields were also searched for RCB stars but none were found. Comment: 36 pages, Latex plus 16 additional tables. ApJ, in press

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    ABSTRACT: New optical spectra have been obtained with VLT/FORS2 of the final helium shell flash (FF) star, V605 Aql, which peaked in brightness in 1919. New models suggest that this star is experiencing a very late thermal pulse. The evolution to a cool luminous giant and then back to a compact hot star takes place in only a few years. V605 Aql, the central star of the planetary nebula (PN) A58, has evolved from Teff ~ 5000 K in 1921 to ~95,000 K today. There are indications that the new FF star, Sakurai's object (V4334 Sgr), which appeared in 1996, is evolving along a similar path. The abundances of Sakurai's object today and V605 Aql 80 years ago mimic the hydrogen-deficient R Coronae Borealis (RCB) stars, with 98% He and 1% C. The new spectra show that V605 Aql has stellar abundances similar to those seen in Wolf-Rayet [WC] central stars of PNe, with ~55% He, and ~40% C. The stellar spectrum of V605 Aql can be seen even though the star is not directly detected. Therefore, we may be seeing the spectrum in light scattered around the edge of a thick torus of dust seen edge-on. In the present state of evolution of V605 Aql, we may be seeing the not too distant future of Sakurai's object.
    The Astrophysical Journal 12/2008; 646(1):L69. · 6.73 Impact Factor
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    ABSTRACT: UBVR c I c photometric measurements are made of three cool stars with R Coronae Borealis type variability, ES Aql, SV Sge, and Z UMi. During their visual light minima ES Aql and Z UMi manifested brightness and color behavior typical of this type of variability: a decrease or increase in the color indices as the brightness decreases and an increase as the normal state is recovered. The molecular spectrum of Z UMi during the second half of the minimum with an weakness by about 4m was already normal, but the Na I D doublet lines were still filled with emission. High resolution profiles of the Na I D lines for SV Sge reveal a systematic shift to -10 km/s and an extended blue wing, which can be regarded as a consequence of a constant outflow of matter from the star's atmosphere that has not led to minima in the visual brightness even over a time of 9 years. Identification of the interstellar Na I D lines in the high resolution spectra has made it possible to verify the known color excess E(B-V) for Z UMi at a level 0m.02 ± 0m.11 and substantially reduce its value to 0m.03 for SV Sge. For the latter star this also opens up the possibility of a substantially lower absolute magnitude, M V ≤ 2m, than assumed previously.
    Astrophysics 12/2013; 56(4):501-517. · 0.37 Impact Factor
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    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.
    Astronomy and Astrophysics 01/2007; 472:247-256. · 5.08 Impact Factor

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