V532 Oph Is a New R Coronae Borealis Star

The R Coronae Borealis (RCB) stars are a rare class of extremely interesting transition objects that have the potential to reveal critical details of the late stages of stellar evolution. These stars form a small group of carbon-rich supergiants that are defined by extreme hydrogen deficiency and unusual variability. RCB stars undergo large declines of up to 8 mag due to the formation of carbon dust at irregular intervals. Two scenarios have been proposed for the origin of an RCB star: the double degenerate (DD) and the final helium-shell flash (FF) models (Iben et al. 1996; Saio & Jeffery 2002). The former involves the merger of a CO WD and a He WD (Webbink 1984). In the latter, a star evolving into a planetary nebula (PN) central star expands to supergiant size by a FF (Fujimoto 1977; Renzini 1979). Clayton et al. (2005, 2007) found that some RCB stars have ¹⁸O/¹⁶O ratios that are orders of magnitude higher than those seen for any other known class of stars, which favors the WD merger scenario, while four RCB stars, including R CrB itself, show enhanced Li abundances, which favors the FF scenario. The obvious conclusion is that there are (at least) two evolutionary channels leading to the RCB stars, perhaps with the DD being the dominant mechanism.

Establishing the population of RCB stars in the Galaxy will place strong constraints on the lifetimes of these stars as well as their birthrate, which will, in turn, help to constrain the FF and DD scenarios. It has been predicted that there may be as many as 3000 RCB stars in the Galaxy as a whole based on the numbers found in the Magellanic Clouds, but only about 65 RCB stars have been discovered so far (Alcock et al. 2001; Tisserand et al. 2004; Zaniewski et al. 2005). The pace of discovery of RCB stars in the Galaxy has been accelerating with about 25 new RCB stars identified in the past 15 years (Benson et al. 1994; Clayton et al. 2002; Hesselbach et al. 2003; Zaniewski et al. 2005; Tisserand et al. 2008). A more accurate knowledge of the birthrates of RCB stars will constrain the frequency of low-mass WD mergers that produce them in the DD scenario. This will help us to establish the rates of more massive WD mergers that are thought to produce type Ia supernovae.

In the General Catalog of Variable Stars, V532 Oph is classified as an Algol-type eclipsing binary (Samus & Durlevich 2009). This identification was made by Swope (1942), who found that the brightness of this star (SVS 457) was constant from 1890 to 1939 except for one large drop in brightness that occurred in 1928 June (∼JD 2,425,413). At that time, V532 Oph dropped from its maximum, m_pg ∼ 12.6 mag, to >15.5 mag. No other significant variation in brightness was observed over 40 years. However, new ASAS-3 telescope photometry, covering the period from 2001 to 2009, shows that V532 Oph is not an eclipsing binary system. Its light curve shows large declines in brightness reminiscent of an RCB star. In this article, we present and interpret newly acquired photometry and spectroscopy of V532 Oph that show that this star is indeed an RCB star.

V532 Oph lies at α(2000) 17^h32^m42.61^s, δ(2000) −21°51'40.̋76 (UCAC2 23179321) (Zacharias et al. 2004). Figure 1 shows a chart with V532 Oph identified. Figure 2a shows the m_pg measurements presented in Swope (1942). The uncertainty in the m_pg measurements is on the order of ± 0.2 mag based on the scatter in the points. This photometry was measured on various plate/instrument/telescope combinations, so it is not necessarily very uniform across the 1890–1939 time frame. Most of the plate material was blue. It is unknown whether the fading of V532 Oph seen in the very early data is real. However, the decline in brightness seen ∼JD 2,425,362 (1928 April 25) is real. Figure 2b shows V-band data from the ASAS-3 telescope (Pojmanski 2002). The scale is 14^'' pixel^-1. A bright star, HD 15882 (V ∼ 8 mag), lies 1' from V532 Oph (see Fig. 1), so a small aperture must be used for the photometry. Therefore, we have plotted the ASAS-3 V-band photometry using the 2 pixel aperture. The uncertainties in the ASAS-3 points are typically ∼0.05 mag. The scatter in the photometry seems larger than this. A five-point boxcar smooth has been applied to the data plotted in Figure 2b. Also plotted in Figure 2b are two V-band data points downloaded from the AAVSO International Database (A. A. Henden 2008, private communication). BVRI photometry was obtained on JD 2,454,674 and 2,454,677. The two epochs are only three days apart, and the star did not appear to vary significantly in that time. Both observations give V = 12.8, B - V = 1.1, V - R = 0.4, V - I = 0.85.

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New spectroscopic observations of V532 Oph were obtained on 2008 September 3–8, when it was about 0.8 mag below maximum while recovering from a decline. The epoch of these spectra is marked on the light curve in Figure 2b. The spectra were obtained with the silicon intensified target (SIT)/CCD spectrograph on the South African Astronomical Observatory (SAAO) 1.9 m telescope at Sutherland, South Africa using grating 6, which has a resolution of and a useful range of about 3600–5400 Å at the angle setting used. The spectra have been extracted, flat-field corrected, sky subtracted, and wavelength calibrated. A flux calibration was done for the spectra using the spectrophotometric standard LTT7379 although due to the slit and variable seeing, the data are not photometric. Eight spectra were obtained of V532 Oph. These were combined by taking a median of all the spectra. This median spectrum is shown in Figure 3 along with spectra at a similar resolution of two RCB stars, W Men and HV 12842. The spectra of W Men and HV 12842 were also obtained with the SAAO 1.9 m telescope in 1997 November using the same setup described previously.

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Other than what is shown in Figure 2, there is little photometry of V532 Oph in the literature. The transformation from m_pg to Johnson V[V - m_pg = 0.17–1.09 × (B - V)] gives V - m_pg ∼ -0.9 mag, assuming (B - V) = 1.1 (Arp 1961; Pierce & Jacoby 1995). This delta is consistent with the difference in the maximum light brightness of V532 Oph as seen in Figure 2, V = 11.7 mag and m_pg = 12.6–12.7 mag. Therefore, there is good agreement between the data sets from ASAS-3 and Swope (1942). There is some indication that V532 Oph brightened by ∼0.1 mag between 1900 and 1930. The ASAS-3 light curve shows evidence for pulsations with a period of ∼50 days when the star is at maximum light, but there are not enough data to determine a period. This period is typical for RCB stars (Lawson et al. 1990).

There is also one epoch of IJK photometry taken with Denis (The Denis Consortium 2005) (JD 2,451,094.682577), I = 16.561 ± 0.10 mag, J = 13.448 ± 0.10 mag, and K = 9.333 ± 0.07 mag, and one epoch of JHK photometry taken with 2MASS (Cutri et al. 2003) (JD 2,450,963.6926), J = 9.050 ± 0.023 mag, H = 8.784 ± 0.051 mag, and K = 8.555 ± 0.021 mag. In various other catalogs, USNO-B1.0 (epoch 1971.5), USNO-A2.0 (epoch 1980.858), Hubble Space Telescope (HST) GSC 1.2 (epoch 1987.634), and HST GSC 2.2 (epoch 1997.244), V532 Oph appears to be at or near maximum light. V532 Oph was detected with IRAS (Helou & Walker 1988) only at 12 μm (F = 0.54 ± 0.06 Jy). Upper limits were obtained in the other bands. The IRAS flux in RCB stars is typically highest in the 12 μm band. The RCB dust shells have temperatures in the 600–900 K range and so their emission peaks are to the blue of 12 μm (Walker 1985).

V532 Oph was in a deep decline (ΔJ ≳ 4.4 mag) when the Denis data were obtained (JD 2,451,094; 1998 October 7). There is one major decline (JD 2,425,362; 1928 April 25) in the Swope (1942) data, shown in Figure 2a, and at least three major declines in the ASAS-3 data, shown in Figure 2b, all of which go below the faint limit (∼14 mag) of the telescope (ending near JD 2,452,600, 2002 November; JD 2,453,300–2,453,800, 2004 October–2006 March; and JD 2,454,400–2,454,600, 2007 October–2008 May). Nearly all known RCB stars spend a majority of their time at maximum light with a characteristic time between irregularly spaced declines of about 1100 days (Feast 1986). However, there is a wide variation in decline activity from star to star (Jurcsik 1996). Also, individual stars often vary from extremely active to extremely inactive on the timescale of decades (e.g., Mattei et al. 1991). V532 Oph seems to have been in an active phase over the past 10 years and was relatively inactive early in the twentieth century. However, it should be pointed out that the points plotted in Figure 2a are quite widely spaced, and some declines may have been missed. In a typical year, three or four plates were obtained over a 4 month period, and then nothing was obtained for 8 months.

A typical RCB star spectrum is characterized by weak or absent hydrogen lines and molecular bands (CH); strong carbon lines and molecular bands (CN, C₂); and little or no ¹³C (Clayton 1996; Alcock et al. 2001). The continuum seen in the spectrum of V532 Oph in Figure 3 is quite red, but its molecular features are quite weak, indicating that it is a significantly reddened, warm RCB star. The RCB stars show a range of effective temperatures, from the warm (6000–7000 K) RCB stars that show only weak molecular bands to the cool (∼5000 K) RCB stars that have much stronger molecular bands. The spectrum of V532 Oph is plotted along with two other warm RCB stars, W Men and HV 12842 (Feast 1972; Goldsmith et al. 1990). The characteristic Swan bands of C₂ are weak but present. CN bands are probably present as well. Hβ, if present, is extremely weak, and there is no sign of Hγ or Hδ. The CH band is also extremely weak. There is a large variation in the hydrogen deficiency of the RCB stars (Asplund et al. 2000). V854 Cen, for example, which is less hydrogen deficient, clearly shows the Balmer series (Kilkenny & Marang 1989). Also, there is no sign of the isotopic ¹²C¹³C Swan bandhead at 4744 Å in the spectrum of V532 Oph (Lloyd Evans et al. 1991).

V532 Oph lies at low Galactic latitude ∼6° and longitude ∼4°, so there is significant reddening along the line of sight. Using the reddening maps of Schlegel et al. (1998), the foreground extinction toward V532 Oph is estimated to be A_V ∼ 3 mag. There are only the two observations of B - V, mentioned previously, that exist for V532 Oph. They were obtained when the star was 1.1 mag below maximum as it recovered from a decline. The measured value of (B - V) = 1.1 will be the intrinsic stellar color reddened by a combination of circumstellar and interstellar dust. If V532 Oph is a warm RCB star, as indicated by its spectrum (T_eff ∼ 7000 K), then (B - V)_o ∼ 0.5 (Lawson et al. 1990). Then E(B - V) = 0.6, which implies A_V ∼ 2 mag (assuming R_V = 3.1). The absolute magnitude of V532 Oph should be M_V ∼ -5 mag (Clayton 1996; Alcock et al. 2001), so assuming V_max = 11.7 and a reddening of A_V = 2–3 mag, then it lies at a distance of 5.5–8.7 kpc. Therefore, this star may lie in the Galactic bulge similar to some other recently discovered RCB stars (Zaniewski et al. 2005; Tisserand et al. 2008).

The new data presented here clearly show that V532 Oph has all the characteristics of a typical RCB star including sudden, deep, irregularly spaced declines, an optical spectrum showing weak or absent hydrogen lines, the Swan bands of C₂, and no evidence for ¹³C. In addition, the star shows small pulsations typical of an RCB star and an infrared excess due to circumstellar dust. The combination of long-term photometric coverage, visible spectra, and IR photometry, used here for the definitive identification of V532 Oph as an RCB star, can be easily applied to other RCB candidates in the future now that all-sky survey telescopes such as ASAS-3 are available.

This paper is partly based on work supported financially by the National Research Foundation of South Africa. The research of DLW is supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC).