The Changing Nebula Around the Hot R Coronae Borealis Star DY Centauri
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Mon. Not. R. Astron. Soc. 000, 1–?? () Printed 18 October 2018 (MN LATEX style file v2.2) The changing nebula around the hot R Coronae Borealis star DY Centauri N. Kameswara Rao1,2, David L. Lambert2, D. A. Garc´ıa-Hern´andez3,4, & Arturo Manchado3,4,5 1 543, 17th Main, IV Sector, HSR Layout, Bangalore 560102, India 2The W.J. McDonald Observatory, University of Texas, Austin, TX 78712-1083, USA 3Instituto De Astrof´ısica De Canarias, V´ıa L´actea s/n, E-38200 La Laguna, Tenerife, Spain 4Departmento de Astrof´ısica, Universidad de La Laguna(ULL), E-38206 La Laguna, Tenerife, Spain 5Consejo Superior de Investigaciones Cient´ıficas (CSIC), Spain Accepted Received ; in original form ABSTRACT Among the distinguishing characteristics of the remarkable hot R Coronae Borealis star DY Cen, which was recently found to be a spectroscopic binary, is the presence of nebular forbidden lines in its optical spectrum. A compilation of photometry from 1970 to the present suggests that the star has evolved to higher effective temperatures. Comparison of spectra from 2010 with earlier spectra show that between 2003 and 2010, the 6717 and 6730 A˚ emission lines of [S ii] underwent a dramatic change in their fluxes suggesting an increase in the nebula’s electron density of 290 cm−3 to 3140 cm−3 from 1989 to 2010 while the stellar temperature increased from 19500 K to 25000 K. The nebular radius is about 0.02 pc, 60000 times bigger than the semimajor axis of DY Cen binary system. Rapid changes of stellar temperature and its response by the nebula demonstrate stellar evolution in action. Key words: Star: individual: DY Cen: variables:RCB type: nebula, stellar evolution 1 INTRODUCTION lined spectroscopic binary (Rao et al. 2012), the only known binary among RCBs. The origins of the hydrogen-deficient RCB and EHe DY Centauri is known as a hot R Coronae Borealis (RCB) stars are not presently fully understood. Two proposals are star. RCB stars are a rare class of peculiar variable stars arXiv:1301.5773v1 [astro-ph.SR] 24 Jan 2013 commonly advocated. In one, a final helium shell flash or a with two principal defining characteristics: (i) they exhibit very late thermal pulse occurs on a cooling white dwarf star a propensity to fade at unpredictable times by up to about (Iben et al. 1983; Renzini 1990; Herwig 2000) which swells eight magnitudes as a result of obscuration by clouds of soot, the envelope of the star to supergiant dimensions for a few and (ii) they have a supergiant-like atmosphere that is very thousand years and the remaining hydrogen is convected in H-deficient, He-rich and C-rich. The subject of this paper, and consumed while helium and carbon are convected out to the remarkable star DY Cen, one of the hottest RCBs, is the surface. Living examples are believed to be Sakurai’s ob- even a peculiar RCB member on several accounts. It is one ject (V4334 Sgr) and FG Sge (Asplund et al. 1997; Jeffery & of the most hydrogen-rich RCBs. Its circumstellar environ- Sch¨onberner 2006). The other proposal involves the merger ment may be home to the fullerene C60 or more likely proto- of two white dwarfs: a carbon-oxygen white dwarf accretes fullerenes (Garc´ıa-Hern´andez et al. 2011a; 2012). In terms a helium white dwarf as a close binary orbit shrinks under of its chemical composition, DY Cen may have a composi- the influence of energy loss by gravitational radiation (Web- tion that sets it apart from most RCB and extreme helium bink 1984; Iben & Tutukov 1986). The merger leads to a (EHe) stars – see Jeffery & Heber’s (1993) abundance anal- swollen envelope around the C-O white dwarf which lasts ysis: DY Cen is not only uncharacteristically H-rich but is a few thousand years. Evidence from the chemical composi- very Fe-poor with a very high S/Fe ratio, thus has been clas- tions of RCB and EHe stars suggests that most are products sified as a minority RCB star (Lambert & Rao 1994); i.e., a of a merger (Garc´ıa-Hern´andez et al. 2009, 2010; Pandey & RCB with extraordinarily high Si/Fe and S/Fe ratios. How- Lambert 2011; Jeffery et al. 2011). ever, Jeffery et al. (2011) suggest that the Fe abundance was greatly underestimated in 1993. Finally, DY Cen is a single- DY Cen with an orbital period of about 39 days is ex- c RAS 2 N. Kameswara Rao et al. periencing mass loss and presumably mass transfer to a low In addition to these spectra from 2010, we draw on a mass companion.1 Moreover, it is likely to have experienced spectrum obtained in 2003 from the 3.9m AAT with the mass loss and transfer previously in arriving at its H-poor University College London Echelle Spectrograph (UCLES). condition. Rao et al. (2012) suggest DY Cen may evolve to The UCLES spectrograph was used with the 1.4” slit (slit a He-rich sdB star. Many such sdB stars are binaries. In this length of 6”)2 and three individual exposures of 1200 s each sense, DY Cen is a representative of a third way to form a were obtained. Wavelength coverage is from 4780 to 8800 A.˚ RCB star. The fact that its supergiant phase is short-lived The final S/N in the summed spectrum is 38 to 40 in the relative to life as a sdB accounts for the rarity of such hot continuum at 6300 A˚ to 6750 A˚ spectral region. Resolving RCBs relative to the sdB population. power is about 40000 as estimated from telluric [O i] lines. The presence of forbidden emission lines in DY Cen’s Also, considered here is the 1989 spectrum used by Rao et optical spectrum, was noticed in 1989 (Rao et al. 1993). al. (1993). This was acquired at CTIO with the Blanco 4- These lines were attributed to a nebula around DY Cen. In meter telescope and a Cassegrain echelle spectrograph. The this paper, we analyse the nebular lines and their evolution slit width was 2”3, which gives a resolving power of about over two decades. Nebular lines were especially prominent in 18000 with the wavelength coverage from 5480 to 6830 A˚ in a 2010 high-resolution spectrum with broad wavelength cov- 1989. A 1992 spectrum with Blanco telescope covered the erage. Inspection of earlier spectra show that physical con- interval 5480 to 7080 A˚ at a resolving power of 35000 (see ditions in the region emitting the nebular lines have changed Giridhar, Rao & Lambert 1996). since 1989. In particular, the electron density inferred from DY Cen was identified as a RCB variable by Hoffleit the ratio of the [S ii] 6717 and 6731 A˚ lines increased about (1930) from well-determined minima in 1897, 1901, 1924, eight-fold from 1989 to 2010. and 1929. No RCB type minima have been recorded since 1958 (Bateson 1978, A. A. Henden 2010, private communi- cation). Thus, DY Cen is not an active RCB star and all 2 SPECTROSCOPIC OBSERVATIONS the spectra discussed here were taken when the star was at maximum light. The general properties of DY Cen’s spectrum have been de- Quantitative interpretation of nebular emission lines re- scribed by Pollacco & Hill (1991), Jeffery & Heber (1993), quires knowledge of the line fluxes and, hence, of the con- Rao, Giridhar & Lambert (1993), Giridhar, Rao & Lam- tinuum fluxes across a spectrum. To obtain such fluxes, we bert (1996) and De Marco et al. (2002). Optical spectra use UBVRI photometry of the star reported for the period are presently a combination of photospheric absorption lines 1989 - 2010. DY Cen has been slowly fading for the last 40 ii ii iii (e.g. O , N , Si lines), emission lines due to a stellar to 50 years (Bateson 1978; Rao et al. 1993). This fading is ii i wind (mainly C , He often superposed on underlying ab- independent of RCB-like events. We have collected UBVRI ii ii sorption lines), and nebular emission lines of [S ], [N ], photometry reported in the literature. Figure 1 shows the i ii [O ], [Fe ], etc. In addition, circumstellar and interstellar variation of B, V, and R from 1970 to 2010. ii i i lines from Ca , Na , K and other species as well as the The earliest published UBV magnitudes are from mid- enigmatic diffuse interstellar bands (DIBs) are present in 1970 by Marino & Walker (1971): V, B-V and U-B are absorption (Garc´ıa-Hern´andez et al. 2012). 12.23, 0.39 and -0.62, respectively. U band measurements This paper considers new spectra from 2010 acquired are few and were obtained only in 1970 (see above) and dur- as a result of an ESO Director’s Discretionary Proposal. ing the 1982-83 period at SAAO by Kilkenny et al. (1985). The spectra were obtained on four nights in February-March The 1972 measurements are from V.E. Sherwood (quoted 2010 with the cross-dispersed echelle spectrograph UVES at by Rao, Giridhar & Lambert 1993). Pollacco & Hill (1991) the VLT at ESO’s Paranal Observatory. The UVES spec- obtained BV mesurements during 1987 May and June. The trograph was used with the 1.2” slit (slit length of 8” and 2007 BVRI measurements are from the AAVSO. It is clear P.A.=0 degrees) and the standard setting DIC2 (390+760). from the figure that there is a gradual increase in magnitude The seeing was always better than 1.5” and we obtained 11 over 40 years. The total variation is about 0.90 magnitude individual exposures of 1800 s each (total exposure time of in V and 0.83 magnitude in B in 37 years: the (B-V) colour 5.5 hours).