277 Acta Polytechnica CTU Proceedings 2(1): 277–281, 2015 277 doi: 10.14311/APP.2015.02.0277 What Powers the 2006 Outburst of the Symbiotic Star BF Cygni? A. Skopal1, M. Sekeráš1, N. A. Tomov2, M. T. Tomova2, T. N. Tarasova3, M. Wolf4 1Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia 2Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 4700 Smolyan, Bulgaria 3Crimean Astrophysical Observatory, 298409 Nauchny, Crimea, Russia 4Astronomical Institute, Charles University Prague, 180 00 Praha 8, V Holešovičkách 2, Czech Republic Corresponding author: skopal@ta3.sk Abstract BF Cygni is a classical symbiotic binary. Its optical light curve occasionally shows outbursts of the Z And-type, whose nature is not well understood. During the 2006 August, BF Cyg underwent the recent outburst, and continues its active phase to the present. The aim of this contribution is to determine the fundamental parameters of the hot component in the binary during the active phase. For this purpose we used a high- and low-resolution optical spectroscopy and the multicolour UBV RCIC photometry. Our photometric monitoring revealed that a high level of the star’s brightness lasts for unusually long time of > 7 years. A sharp violet-shifted absorption component and broad emission wings in the Hα profile developed during the whole active phase. From 2009, our spectra revealed a bipolar ejection from the white dwarf (WD). Modelling the spectral energy distribution (SED) of the low-resolution spectra showed simultaneous presence of a warm (< 10 000 K) disk-like pseudophotosphere and a strong nebular component of radiation (emission measure of ∼ 1061 cm−3). The luminosity of the hot active object was estimated to > 5 − 8 × 103 L�. Such high luminosity, sustained for the time of years, can be understood as a result of an enhanced transient accretion rate throughout a large disk, leading also to formation of collimated ejection from the WD. Keywords: binaries: symbiotic - optical - spectroscopy - photometry - individual: BF Cyg. 1 Introduction Symbiotic stars (SSs) are the largest interacting binary systems with known orbital periods in order of years. They consist of a cool giant and a WD accreting from the giant’s wind. Accretion process heats up the WD to 1 − 2 × 105 K and makes it as luminous as a few times 103 L�, whose photons ionize a large fraction of the neutral giant’s wind, giving rise to nebular emission. As a result the spectrum of SSs consists of three basic components of radiation – two stellar and one nebular. If a symbiotic system releases its energy approximately at a constant rate and the temperature, it conforms the so-called quiescent phase. The stage, when the system brightens up in the optical by a few magnitudes and/or shows signatures of a mass-outflow, is named an active phase. BF Cyg is an eclipsing symbiotic binary with an or- bital period of 757.2 d (e.g. Pucinskas 1970; Fekel et al. 2001). The binary consists of a late-type M5 III giant (Mürset & Schmid, 1999) and a hot luminous compact object (Mikolajewska et al., 1989). Its eclipsing nature was revealed by optical photometry of Skopal (1992). Historical light curve of BF Cyg is characterized by a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa slow, symbiotic-nova-type outburst (1895–1960), with superposed eruptions of the Z And-type (1920, 1989, 2006) and short-term flares (e.g. Skopal et al., 1997; Leibowitz & Formiggini, 2006). The last, 1989 Z And- type eruption was described by Cassatella et al. (1992), Skopal et al. (1997). The UV/optical continuum cooled to ∼ 20 000 K, and its source expanded to ∼ 7 R� hav- ing a luminosity of ∼ 10 000 L�. (see also Skopal, 2005). The line spectrum showed violet-shifted ab- sorption lines indicating a mass outflow from the hot component at 100–500 km s−1. The recent, 2006 outburst of BF Cyg was first re- ported by Munari et al. (2006). Spectroscopic observa- tions by Sitko et al. (2006), Iijima (2006) and McKeever et al. (2011) indicated appearance of strong P-Cyg type of HI, HeI line profiles from the beginning of the out- burst. Photometric observations indicated a high level of the star’s brightness for much longer time than was observed for the previous, 1989-92, active phase (Siviero et al. 2012; Skopal et al., 2012). Recently, Skopal et al. (2013) reported an evidence of highly-collimated bipo- lar ejection from BF Cyg. 277 http://dx.doi.org/10.14311/APP.2015.02.0277 A. Skopal et al. 9 10 11 12 13 47000 48000 49000 50000 51000 52000 53000 54000 55000 56000 1990 1995 2000 2005 2010 M ag ni tu de Julian date - 2 400 000 BF Cyg Q U I E S C E N T P H A S E A C T I V E P H A S EACT. PHASE V B U Figure 1: The UBV light curves of BF Cyg from 1986 to the present. They cover the last, 1989-93, and the present, 2006-13, active phases. During the quiescent phase (∼ 1994 − 2006), the star was by 2–3 mag fainter, being characterized with the wave-like orbitally-related variation. In this contribution we analyze our optical spec- troscopy and UBV RCIC photometry from the current active phase of BF Cyg, with the aim to determine fun- damental parameters of the hot component. We point the problem of its high luminosity, which sustains for a long time of years. 2 Observations Broad-band photoelectric UBV and CCD UBV RCIC photometry of BF Cyg was carried out by 0.6-m tele- scopes at the Skalnaté Pleso and Stará Lesná observa- tories of Astronomical Institute of the Slovak Academy of Sciences (see Skopal et al. 2012 for details). The data are plotted in Fig. 1. The high-resolution spectroscopy was carried out by the single dispersion slit spectrograph mounted at the coudé focus of the 2-m RCC telescope of the Rozhen Na- tional Astronomical Observatory and at the Ondřejov Observatory. The low-resolution (R ∼ 1000) spectroscopic obser- vations were secured by the 2.6-m Shajn telescope, op- erated by the Crimean Astrophysical Observatory. Spectroscopic observations were dereddened with EB−V = 0.35 and the resulting parameters were scaled to a distance of 3.8 kpc (e.g. Skopal, 2005). 3 Analysis and Results 3.1 Modelling the SED in the optical Assuming that the optical continuum consists of the three basic radiative components of radiation (see Sect. 1), the resulting flux in the continuum, F(λ), can be expressed as their superposition, F(λ) = FWD(λ) + FN(λ) + FG(λ), (1) where FWD(λ) is the flux from the WD’s pseudopho- tosphere, FN(λ) is the flux from thermal plasma and FG(λ) represents the flux from the giant. For effective temperatures, T effWD ∼ 5 000 − 10 000 K, an atmospheric model, Fλ(T effWD), is needed to fit the radiation of the warm pseudophotosphere. Otherwise, a simple black- body radiation is satisfactory. The nebular radiation in the continuum can be approximated by processes of recombination and thermal bremsstrahlung in the hy- drogen/helium plasma for Case B. Finally, radiation from the giant is represented by an appropriate syn- thetic spectrum, Fλ(T effG ). Then Eq. (1) can be ex- pressed as, F(λ) = θ2WDFλ(T eff WD) + kNελ(Te) + θ 2 GFλ(T eff G ), (2) where θWD = RWD/d and θG = RG/d are angular radii of the WD pseudophotosphere and the giant, respec- tively. The factor kN (= the observed emission measure in cm−5) scales the volume emission coefficient ελ(Te) of the nebular continuum to observations. Constant electron temperature, Te, throughout the nebula is as- sumed. Physical parameters of the model spectrum (2), θWD, θG, T eff WD, T eff G , kN and Te, are given by the solu- tion of Eq. (2), which corresponds to a minimum of the reduced χ2 function. The SED-fitting analysis was described by Skopal (2005) and Skopal et al., (2011). 3.2 Physical parameters Example of a low-resolution (3400 – 7000 Å) spectrum, taken around a brightness maximum (23/10/2008), is depicted in Fig. 2. The model SED shows that the spectrum is dominated by the radiation of the warm WD pseudophotosphere (denoted as the warm stellar component (WSC) by Skopal et al., 2011) and the neb- ular continuum. The light from the giant becomes more significant for λ > 6 600 Å. 278 What Powers the 2006 Outburst of the Symbiotic Star BF Cygni? The WSC is produced by a source with T effWD ∼ 8 500 K, the effective radius of ∼ 25 R� and the luminosity of ∼ 3000 L�. The nebular compo- nent was characterized with a high emission measure of EM = 4πd2 × kN ∼ 2.6 × 1061(d/3.8 kpc)2 cm−3, radiated at Te ∼ 30 000 K, which correspond to the luminosity LN ∼ 5100 L�. Thus the lower limit of the total hot component luminosity was ∼ 8 100 L�, be- cause only a fraction of the burning WD radiation can be converted to the WSC and the nebular emission. 0 1 2 3.6 3.7 3.8 3.9 lo g( F lu x) [ er g cm -2 s -1 A -1 ] + 1 3 log(wavelength) [A] BF Cyg23/10/2008 H e I H α H β H γ H δ H e I Obs. FG(λ) FWD(λ) FN(λ) SED Figure 2: An example of the low-resolution spectrum (gray line) and its model (heavy solid line) taken dur- ing the 2006-13 active phase of BF Cyg, on 23/10/2008. The model SED and its components of radiation here represent a graphic form of Eq. (1) with the same de- notation in keys. 3.3 A disk-like shape of the WD pseudophotosphere The shape the WD pseudophotosphere cannot be spher- ical, because of the simultaneous presence of the strong nebular emission in the spectrum. If it were a sphere, its radiation would not be capable of giving rise to the observed nebular emission. On the other hand, the presence of the strong nebular emission in the spec- trum constrains the presence of a hot ionizing source in the system. This type of the spectrum (called as two- temperature type) suggests that the WD pseudophoto- sphere has a form of a disk. When viewing the disk under a high inclination, its outer rim simulates the warm photosphere (producing the WSC), while the ma- terial above/below the disk is ionized by the hot central source and thus converts a fraction of its radiation to the nebular emission (see Skopal 2005 and Skopal et al. 2011 in detail). 3.4 Collimated mass ejection Figure 3 shows evolution of the Hα profiles from the 2006 August eruption to 2013 April. The broad wings expanding to ∼ ±2000 km s−1 in Hα were present in all spectra. Significant variations were observed mainly at/around the line cores. (i) During the whole active phase, a sharp absorption developed on the blue side of the profile. (ii) During 2009, additional satellite emis- sion components appeared at the position of a few times ±100 km s−1 to the Hα emission core. (iii) During 2012 September, the satellite components were placed nearly symmetrically with respect to the Hα central emission. 1 2 3 4 5 6 7 -2000 -1000 0 1000 2000 lo g( F lu x) + c on st . Heliocentric radial velocity [km s-1] 30/08/2006 05/06/2009 02/09/2012 BF Cyg Hα dd/mm/yyyy +5.0 +4.1 +2.5 +1.0 const. 03/11/2012 [N II ] F e II 24/04/2013 Figure 3: Evolution in the Hα line profile along the outburst. The filled curves represent the jet emission components (Sect. 3.4). Fluxes are in 10−13 erg cm−2 s−1 Å−1. Their radial velocities of ± ∼ 370 km s−1 and fluxes of ∼ 1.4 × 10−11 erg cm−2 s−1 were around a maximum (see Skopal et al., 2013 in detail). (iv) The presence of satellite components and their properties were un- stable in the spectrum. During two months after their 279 A. Skopal et al. best pronounced stage (on 02/09/2012), they practi- cally disappeared on 2012 November 3rd. However, in 2013 April, they re-appeared again (Fig. 3). The relatively small width of the (well measured) satellite components (FWHM ∼ 245 km s−1) and their radial velocities suggest that these emissions were pro- duced by radiation of a highly collimated ejection by the central star. 4 Concluding Remarks According to the elements of the spectoscopic orbit (Fekel et al., 2001), the mass of the WD in BF Cyg is as low as ∼ 0.55 − 0.6 M�. During the quiescent phase, the luminosity of the hot component was esti- mated to ∼ 10 000 L� for d = 3.8 kpc (e.g. Mikola- jewska et al., 1989). This quantity suggests that the source of such the energy output is caused by a stable hydrogen burning on the WD surface at the accretion rate of ∼ 1.4×10−7 M� yr−1 for the 0.55−0.6 M� WD (e.g. Shen & Bildsten, 2007). During active phase, the luminosity of the hot component can be > 10 000 L� (e.g. Cassatella et al., 1992), however, with difficulties of its precise determination as mentioned in Sect. 3.2. In addition, (i) a significant extension and thus cool- ing of the WD pseudophotosphere is indicated by mod- elling the SED (Sect. 3.2), (ii) an enhanced mass-loss rate from the WD is evidenced by the broad Hα wings with a violet-shifted absorption component, and (iii) an enhancement of the accretion rate onto the WD is required by the satellite emission components. The presence of bipolar jets confirms the presence of a disk around the accretor during the outburst. These ob- servational properties are consistent with evolution of burning WDs in the H-R diagram, when the accretion rate increases above the stable burning regime. The accretion at ≈ 2 × 10−7 M� yr−1 throughout the disk during the outburst can sustain the high luminosity of the burning WD at ≈ 10 000 L� (see Fig. 2 of Shen & Bildsten, 2007). The case of the current BF Cyg active phase lead us to a speculation that the simultaneous presence of the enhanced mass outflow and mass infall during some active phases of SSs can reflect a new type of the ac- cretion process, which can sustain a high luminosity of their hot components for a long time of years. Similar properties with mass outflow/infall and jets were also observed during the 1977–1984 active phase of CH Cyg (e.g. Skopal et al. 2002). It is of interest to note that the enhanced mass outflow, sometimes followed with jet-like components, and emergence of a warm pseu- dophotosphere simulated by the irradiated disk are also observed during optical high states of supersoft X-ray sources (e.g. Southwell et al. 1996; Hutchings et al. 2002; Hachisu & Kato, 2003). Acknowledgement This research was supported by the Slovak-Bulgarian Research and Development Cooperation project SK- BG-0015-10, by the grant BSTC No. 01/14 Bulgaria- Slovakia, by the grant DO 02-85 of Bulgarian Sci- entific Research Fund, by the Research Program MSM0021620860 of the Ministry of Education of the Czech Republic and by a grant of the Slovak Academy of Sciences VEGA No. 2/0002/13. References [1] Cassatella A., et al.: 1992, A&A 258, 368 [2] Fekel, F. C., et al.: 2001, AJ 121, 2219 [3] Hachisu, I., Kato, M.: 2003, ApJ, 588, 1003 doi:10.1086/374303 [4] Hutchings et al.: 2002, AJ, 124, 2833 [5] Iijima, T.: 2006, CBET No. 633 [6] Leibowitz, E. M., Formiggini, L.: 2006, MNRAS 366, 675 doi:10.1111/j.1365-2966.2005.09895.x doi:10.1086/662076 [7] McKeever, J., et al.: 2011, PASP 123, 1062 [8] Mikolajewska, J., et al.: 1989, AJ 98, 1427 [9] Munari, U., et al.: 2006, CBET No. 596 [10] Mürset, U., Schmid, H. M.: 1999, A&AS 137, 473 doi:10.1086/513457 [11] Pucinskas, A.: 1970, Bull. Vilnius Univ. Astron. Obs. No. 27, 24 [12] Shen, K. J., Bildsten, L.: 2007, ApJ 660, 1444 [13] Sitko, M. L., et al.: 2006, IAUC No. 8746 [14] Siviero, A. et al.: 2012, Baltic Astron. 21, 188 [15] Skopal A.: 1992, IBVS No. 3780 doi:10.1093/mnras/292.3.703 [16] Skopal, A.: 2005, A&A 440, 995 doi:10.1046/j.1365-8711.2002.05715.x [17] Skopal, A., et al.: 1997, MNRAS 292, 703 [18] Skopal, A., et al.: 2002, MNRAS 335, 1109 doi:10.1002/asna.201111655 [19] Skopal, A., et al.: 2011, A&A 536, id. A27 [20] Skopal, A., et al.: 2012, Astron. Nachr. 333, 242 doi:10.1086/177931 280 http://dx.doi.org/10.1086/374303 http://dx.doi.org/10.1111/j.1365-2966.2005.09895.x http://dx.doi.org/10.1086/662076 http://dx.doi.org/10.1086/513457 http://dx.doi.org/10.1093/mnras/292.3.703 http://dx.doi.org/10.1046/j.1365-8711.2002.05715.x http://dx.doi.org/10.1002/asna.201111655 http://dx.doi.org/10.1086/177931 What Powers the 2006 Outburst of the Symbiotic Star BF Cygni? [21] Skopal, A., et al.: 2013, A&A 551, id. L10 [22] Southwell et al.: 1996, ApJ, 470, 1065 DISCUSSION DMITRY BISIKALO: If you have a wind from the WD during the quiescence, how can you accumulate matter to form a huge accretion disk? AUGUSTIN SKOPAL: The presence of a wind from the WD is indicated even during quiescent phase by, for example, the broad Hα wings. The presence of a large disk-like formation during active phases is obser- vationally confirmed. However, its creation is not well understood yet. 281 Introduction Observations Analysis and Results Modelling the SED in the optical Physical parameters A disk-like shape of the WD pseudophotosphere Collimated mass ejection Concluding Remarks