NSV 11154 Is a New R Coronae Borealis Star
Nutsinee Kijbunchoo, Geoffrey C. Clayton, Timothy C. Vieux, N. Dickerman, T. C. Hillwig, D. L. Welch, Ashley Pagnotta, Sumin Tang, J. E. Grindlay, A. Henden
aa r X i v : . [ a s t r o - ph . S R ] A ug NSV 11154 Is a New R Coronae Borealis Star
Nutsinee Kijbunchoo , Geoffrey C. Clayton , Timothy C. Vieux , N. Dickerman , T. C.Hillwig , D. L. Welch , Ashley Pagnotta , Sumin Tang , J. E. Grindlay , and A. Henden ABSTRACT
NSV 11154 has been confirmed as a new member of the rare hydrogen-deficient R Coronae Borealis (RCB) stars based on new photometric and spectro-scopic data. Using new photometry, as well as archival plates from the Harvardarchive, we have constructed the historical lightcurve of NSV 11154 from 1896 tothe present. The lightcurve shows the sudden, deep, irregularly spaced declinescharacteristic of RCB stars. The visible spectrum is typical of a cool (T eff . C Swan bands, and noevidence of C . In addition, the star shows small pulsations typical of an RCBstar, and an infrared excess due to circumstellar dust with a temperature of ∼ ∼ Department of Physics & Astronomy, Louisiana State University, Baton Rouge, LA 70803 USA; [email protected], [email protected], [email protected],[email protected] Department of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383;[email protected] Department of Physics and Astronomy, McMaster University, 1280 Main Street West, Hamilton, ONL8S 4M1 Canada; [email protected] Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138;[email protected], [email protected] American Association of Variable Star Observers, 49 Bay State Rd., Cambridge, MA 02138;[email protected]
1. Introduction
The R Coronae Borealis (RCB) stars represent an extremely rare class of variable stars(Clayton 1996). They are cool supergiants, which are carbon-rich and hydrogen deficient.Their defining characteristic is large irregular declines in brightness of up to 8 mag caused bythe formation of carbon dust. Two scenarios have been suggested which attempt to clarifythe origins of the RCB stars, the double degenerate (DD), and the final helium-shell flash(FF). The DD model suggests that RCB stars are formed by the merger of a CO- and aHe-white dwarf (WD), and the FF model involves stellar expansion after a helium-shell flash.The high O/ O ratios found in RCB stars favor the DD model. However a few RCB starsshow Li in their spectra which may instead favor the FF model (Iben et al. 1996; Clayton1996) .There could be as many as 3000 RCB stars in the Galaxy based on the numbers foundin the Large Magellanic Cloud (LMC), but only 55 have been discovered in the Galaxy sofar (Clayton 1996; Alcock et al. 2001; Zaniewski et al. 2005; Tisserand et al. 2008).About forty-five years ago, Hoffmeister (1966) discovered that NSV 11154 was a vari-able star (S 9323 Lyr) and suggested that NSV 11154 is a short periodic variable. NSV11154 was also found to be variable in the ROTSE-I survey with an amplitude of 0.4 magand it was suggested that NSV 11154 is a long period variable (Akerlof et al. 2000; Wils2001). Haussler et al. (2009) examined 562 plates, obtained at Sonnenberg Observatoryduring 1964–1996, and found irregular brightness variations between 13.0 and 17.2 mag. Onthe basis of the lightcurve, they suggested that NSV 11154 may be an RCB star. In thisarticle, we use newly acquired photometry and spectroscopy to attempt to confirm Haussleret al.’s suggestion that NSV 11154 is indeed an RCB star.
2. OBSERVATIONS AND DATA REDUCTION
The UCAC3 coordinates of NSV 11154 are α (2000) 18 h m . s δ (2000)+47 deg 23 ′ . ′′ m pg = 0.17 - 1.09(B - V) (Arp 1961).The transformed data are plotted in Figures 2 and 4. There were 250 additional plates of theNSV 11154 field dating as far back as 1896, available from the Harvard College Observatoryplate archive. These plates have been scanned and photometry was done on the stars aspart of the Digital Access to a Sky Century at Harvard (DASCH) program (Grindlay et al. 3 –2009), from the scanning focused in the Kepler field. Note that NSV 11154 is a few degreesoutside the Kepler field of view, and there are many more Harvard plates covering thisstar not scanned yet. The measured magnitudes in the DASCH database were convertedfrom photographic magnitudes to Kepler Input Catalog (Sloan) g magnitudes (Brown et al.2011). The DASCH data were then converted from g to Johnson V using, V=g ′ + 0.12- 0.56(B - V) (Fukugita et al. 1996). The DASCH data used here came from twelve plateseries from the archive covering the period 1896-1989, and have average uncertainties of 0.15mag (Laycock et al. 2010). This will be further improved with photometric corrections nowbeing optimized (Tang et al. 2011, in preparation). The Sonnenberg plates were obtainedduring 1964-1996 so there is an overlap of roughly twenty years with the DASCH data. TheDASCH photometry is listed in Table 1 and is plotted in Figures 2 and 4.The ROTSE-I Northern Sky Variability Survey (NSVS) detected NSV 11154 as a vari-able (Akerlof et al. 2000). The photometric data were downloaded from the NSVS archive.The ROTSE-I images are unfiltered and Wo´zniak et al. (2004) suggest that m ROT SE is equiv-alent to Johnson V, when it is actually very close to Cousins R (Bernhard et al. 2005). If therelation between m
ROT SE and V T (Wo´zniak et al. 2004) is combined with the equations toconvert from V T to Johnson V , then the conversion is equivalent to V = m ROT SE + (V-R) C = m ROT SE + 0.55. The converted ROTSE-I photometry still seems to have a systematic shiftfrom the actual Johnson V photometry (see below) of ∼ C I C photometry has been obtained with the AAVSO Sonoita Research Ob-servatory (SRO) between 2010 October and 2011 June. The images were obtained with the35cm C14 OTA (SRO35) and the 50cm f/4 Newtonian (SRO50). The images were flat-fieldedand dark subtracted. Aperture photometry was done using DAOPHOT in IRAF. The in-strumental BV magnitudes were transformed to standard magnitudes using a photometricBV sequence of the field (5169jnc) provided by the AAVSO. In particular, two stars, 2MASS18374206+4723474 and 18374814+4724267, were used. These stars are both in the KeplerInput Catalog (Brown et al. 2011). Their Sloan gri magnitudes were transformed to CousinsR and I (Fukugita et al. 1996) and then were used to transform the SRO RI instrumentalmagnitudes of NSV 11154 to standard R C I C magnitudes. The uncertainties are ∼ C I C photometry is listed in Table 3 and plotted in Figures 3 and 4. Inaddition, there is also photometry from 2MASS, AKARI, and IRAS for NSV 11154. Thesedata are tabulated in Table 4. The 2MASS data were obtained during a gap between theSonnenberg and ROTSE-I photometry but the star appears to be at or near maximum light. http://heasarc.nasa.gov/W3Browse/all/tycho2.html
3. DISCUSSION
The historical NSV 11154 lightcurve, seen in Figure 4 from 1896 to 2011, is fragmentary,but several deep declines are apparent. The last decline detected was in 1996, but thecoverage since then has been spotty. No declines are seen in the recent ROTSE-I or SROphotometry. Although the lightcurve data are sparse, NSV 11154 seems to be an active RCBstar having frequent declines. There are at least 13 epochs where NSV 11154 is seen 2 magor more below maximum light. These are listed in Table 5. The characteristic time betweendeclines in RCB stars is typically about 1000 days, but there is a wide range in activityamong the RCB stars (Feast 1986; Jurcsik 1996). From the ROTSE-I and SRO lightcurvesit can be seen that NSV 11154 pulsates with period of ∼
50 days between 1999-2000, and ∼
40 days in 2010-2011. The pulsations have an amplitude of ∼ . absorption bands (Clayton 1996; Alcock et al. 2001). There is no sign of either H β or H γ indicating extreme hydrogen deficiency. The CH band at 4300 ˚A is also absent. Inaddition, the C C band at 4744 ˚A is weak or absent, while the C C band at 4737 ˚Ais very strong indicating a high C to C ratio. This is typical of most RCB stars. NSV11154 lies well out of the Galactic plane at b II = +21 . ◦
8, and so the estimated foregroundextinction is quite small, E(B-V) = 0.07 mag (Schlegel et al. 1998). NSV 11154 has anobserved (B-V)=1.1 mag which is consistent with it being . V = -4 mag (Alcock et al. 2001;Tisserand et al. 2009). Then, if the foreground extinction is A V ∼ ± II ◦ . NSV 11154 lies well away from the extended body of the Sagittarius Dwarf Galaxy(Majewski et al. 2003).Using the photometry in Table 4, an SED for NSV 11154 has been plotted in Figure6. This SED can be fit very well by two blackbodies with temperatures of 4500 and 800 Kcorresponding to the RCB star itself and its circumstellar dust, respectively. The RCB dustshells typically have temperatures range from 600 K to 900 K (Walker 1985).
4. Summary
The suggestion of Haussler et al. (2009), on the basis of the lightcurve, that NSV 11154is an RCB star was correct. The spectrum and colors of NSV 11154 show it to be a cool( . C I C photometry have beencollected giving a historical lightcurve from 1896 to the present. The new photometry showsthat NSV 11154 has a semi-regular pulsation period of 40-50 d with an amplitude of 0.4mag. Although the lightcurve is fragmentary, NSV 11154 has had a number of deep declinesshowing it to be an active RCB star. The star also displays a significant IR excess indicatingthe presence of dust with T ∼
800 K, which is typical of RCB stars.Only 55 other RCB stars are known in the Galaxy, so each addition is important topopulation studies, which will help us to better understand the origins of these mysteriousstars. NSV 11154 lies well above the Galactic plane quite different from most other RCBstars which seem to fit into an old disk or bulge population. This may favor the final flashmodel for this star since the higher stellar density of the Galactic center region is moreconducive to formation of RCB stars by the double degenerate scenario. Despite being veryrare, RCB stars may be a key to understanding the late stages of stellar evolution. Theirmeasured isotopic abundances imply that many RCB stars are produced by the mergersof double degenerate white dwarfs, which may be the low-mass counterparts of the moremassive mergers thought to produce type Ia supernovae. Therefore, knowing the populationof RCB stars in the Galaxy will help determine the frequency of these white dwarf mergers.This paper used data from Digital Access to a Sky Century at Harvard (DASCH)supported by NSF grants AST-0407380 and AST-0909073, as well as by the
Cornel andCynthia K. Sarosdy Fund . 6 –
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This preprint was prepared with the AAS L A TEX macros v5.2. ′ x 5 ′ . The coordinates are J2000.0. North is up and East is to the left. 9 –Fig. 2.— V-band lightcurves of NSV 11154. Top: DASCH plate photometry and upperlimits (arrows). Middle: Sonneberg plate photometry . Bottom: ROTSE-I photometry. 10 –Fig. 3.— SRO lightcurves of NSV 11154. The four panels from the top are B, V, R C and I C plotted vs JD. 11 –Fig. 4.— Historical lightcurve of NSV 11154 from 1896-2011. Symbols are the same as inFigure 2. Several deep declines are detected. 12 – HV 5637NSV 11154 ↓ CH ↓ H γ ↓ C C ↓ H β C ↓↓↓↓↓↓↓↓↓ CN ↓↓↓ ↓↓↓↓↓↓↓↓↓↓ N o r m a li z ed f l u x Fig. 5.— The spectrum of NSV 11154, together with the spectrum of HV 5637 a known coolRCB star, for comparison. Note the absence of hydrogen, the strong CN and C bands, andthe absence of C. The spectrum of NSV 11154 is typical for a cool ( . µ m)0.010.101.00 F l u x ( Jy ) Fig. 6.— The SED of NSV 11154 showing the photometry listed in Table 4 (filled squares).Two blackbodies representing the star and the dust shell, 4500 K (blue line) and 800 K (redline), have been fit to the data. The dashed line is the sum of the two blackbodies. 14 –Table 1. DASCH PhotometryJD V2413740.74 12.72414154.68 12.42414569.61 12.12415150.74 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > a The uncertainties are ∼ σ V σ V σ V σ V σ V σ V σ V C I C · · · · · · · · · C I C C I C · · · · · · · · · C I C a The uncertainties are ∼ σ U 2.50E-02 2.30E-04V 6.28E-02 1.16E-05R C C a Length (d)2415150 · · · · · · ∼ · · · · · · · · · · · · ∼ ∼ > ∼ ∼ > aa