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 &gt; 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 (&lt; 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 &gt; 5−8×103 Lʘ. 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

Sekeráš, M.

Skopal, A.

Tarasova, T. N.

Tomov, N. A.

Tomova, M. T.

Wolf, M.

Acta Polytechnica CTU Proceedings

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

277Acta Polytechnica CTU Proceedings 2(1): 277–281, 2015277doi: 10.14311/APP.2015.02.0277What Powers the 2006 Outburst of the Symbiotic Star BF Cygni?A. Skopal1, M. Sekera´sˇ1, N. A. Tomov2, M. T. Tomova2, T. N. Tarasova3, M. Wolf41Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranska´ Lomnica, Slovakia2Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 4700 Smolyan, Bulgaria3Crimean Astrophysical Observatory, 298409 Nauchny, Crimea, Russia4Astronomical Institute, Charles University Prague, 180 00 Praha 8, V Holesˇovicˇka´ch 2, Czech RepublicCorresponding author: skopal@ta3.skAbstractBF Cygni is a classical symbiotic binary. Its optical light curve occasionally shows outbursts of the Z And-type, whosenature is not well understood. During the 2006 August, BF Cyg underwent the recent outburst, and continues its activephase to the present. The aim of this contribution is to determine the fundamental parameters of the hot componentin the binary during the active phase. For this purpose we used a high- and low-resolution optical spectroscopy andthe multicolour UBV RCIC photometry. Our photometric monitoring revealed that a high level of the star’s brightnesslasts for unusually long time of > 7 years. A sharp violet-shifted absorption component and broad emission wings in theHα profile developed during the whole active phase. From 2009, our spectra revealed a bipolar ejection from the whitedwarf (WD). Modelling the spectral energy distribution (SED) of the low-resolution spectra showed simultaneous presenceof 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, sustainedfor the time of years, can be understood as a result of an enhanced transient accretion rate throughout a large disk, leadingalso to formation of collimated ejection from the WD.Keywords: binaries: symbiotic - optical - spectroscopy - photometry - individual: BF Cyg.1 IntroductionSymbiotic stars (SSs) are the largest interacting binarysystems with known orbital periods in order of years.They consist of a cool giant and a WD accreting fromthe giant’s wind. Accretion process heats up the WDto 1 − 2 × 105 K and makes it as luminous as a fewtimes 103 L, whose photons ionize a large fraction ofthe neutral giant’s wind, giving rise to nebular emission.As a result the spectrum of SSs consists of three basiccomponents of radiation – two stellar and one nebular.If a symbiotic system releases its energy approximatelyat a constant rate and the temperature, it conforms theso-called quiescent phase. The stage, when the systembrightens up in the optical by a few magnitudes and/orshows signatures of a mass-outflow, is named an activephase.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(Mu¨rset & Schmid, 1999) and a hot luminous compactobject (Mikolajewska et al., 1989). Its eclipsing naturewas revealed by optical photometry of Skopal (1992).Historical light curve of BF Cyg is characterized by aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaslow, symbiotic-nova-type outburst (1895–1960), withsuperposed 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 cooledto ∼ 20 000 K, and its source expanded to ∼ 7R hav-ing a luminosity of ∼ 10 000L. (see also Skopal,2005). The line spectrum showed violet-shifted ab-sorption lines indicating a mass outflow from the hotcomponent 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 McKeeveret al. (2011) indicated appearance of strong P-Cyg typeof HI, HeI line profiles from the beginning of the out-burst. Photometric observations indicated a high levelof the star’s brightness for much longer time than wasobserved for the previous, 1989-92, active phase (Sivieroet al. 2012; Skopal et al., 2012). Recently, Skopal et al.(2013) reported an evidence of highly-collimated bipo-lar ejection from BF Cyg.277A. Skopal et al. 9 10 11 12 13 47000  48000  49000  50000  51000  52000  53000  54000  55000  56000 1990  1995  2000  2005  2010Ma gn it ud eJulian date - 2 400 000BF CygQ U I E S C E N T   P H A S EA C T I V E   P H A S EACT.  PHASEVBUFigure 1: The UBV light curves of BF Cyg from 1986 to the present. They cover the last, 1989-93, and thepresent, 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 currentactive phase of BF Cyg, with the aim to determine fun-damental parameters of the hot component. We pointthe problem of its high luminosity, which sustains for along time of years.2 ObservationsBroad-band photoelectric UBV and CCD UBV RCICphotometry of BF Cyg was carried out by 0.6-m tele-scopes at the Skalnate´ Pleso and Stara´ Lesna´ observa-tories of Astronomical Institute of the Slovak Academyof Sciences (see Skopal et al. 2012 for details). Thedata are plotted in Fig. 1.The high-resolution spectroscopy was carried out bythe single dispersion slit spectrograph mounted at thecoude´ focus of the 2-m RCC telescope of the Rozhen Na-tional Astronomical Observatory and at the OndrˇejovObservatory.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 withEB−V = 0.35 and the resulting parameters were scaledto a distance of 3.8 kpc (e.g. Skopal, 2005).3 Analysis and Results3.1 Modelling the SED in the opticalAssuming that the optical continuum consists of thethree basic radiative components of radiation (seeSect. 1), the resulting flux in the continuum, F (λ), canbe 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 andFG(λ) represents the flux from the giant. For effectivetemperatures, T effWD ∼ 5 000− 10 000 K, an atmosphericmodel, Fλ(T effWD), is needed to fit the radiation of thewarm pseudophotosphere. Otherwise, a simple black-body radiation is satisfactory. The nebular radiationin the continuum can be approximated by processes ofrecombination and thermal bremsstrahlung in the hy-drogen/helium plasma for Case B. Finally, radiationfrom 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 effWD) + kNελ(Te) + θ2GFλ(T effG ), (2)where θWD = RWD/d and θG = RG/d are angular radiiof the WD pseudophotosphere and the giant, respec-tively. The factor kN (= the observed emission measurein cm−5) scales the volume emission coefficient ελ(Te)of the nebular continuum to observations. Constantelectron temperature, Te, throughout the nebula is as-sumed. Physical parameters of the model spectrum (2),θWD, θG, TeffWD, TeffG , kN and Te, are given by the solu-tion of Eq. (2), which corresponds to a minimum ofthe reduced χ2 function. The SED-fitting analysis wasdescribed by Skopal (2005) and Skopal et al., (2011).3.2 Physical parametersExample of a low-resolution (3400 – 7000 A˚) spectrum,taken around a brightness maximum (23/10/2008), isdepicted in Fig. 2. The model SED shows that thespectrum is dominated by the radiation of the warmWD pseudophotosphere (denoted as the warm stellarcomponent (WSC) by Skopal et al., 2011) and the neb-ular continuum. The light from the giant becomes moresignificant for λ > 6 600 A˚.278What Powers the 2006 Outburst of the Symbiotic Star BF Cygni?The WSC is produced by a source withT effWD ∼ 8 500 K, the effective radius of ∼ 25R andthe luminosity of ∼ 3000L. The nebular compo-nent was characterized with a high emission measureof EM = 4pid2 × kN ∼ 2.6 × 1061(d/3.8 kpc)2 cm−3,radiated at Te ∼ 30 000 K, which correspond to theluminosity LN ∼ 5100L. Thus the lower limit of thetotal hot component luminosity was ∼ 8 100L, be-cause only a fraction of the burning WD radiation canbe converted to the WSC and the nebular emission. 0 1 2 3.6  3.7  3.8  3.9l og ( Fl u x)  [e r g c m-2  s-1  A-1 ]  +  13log(wavelength) [A]BF Cyg23/10/2008He IHαHβHγHδHe IObs.FG(λ)FWD(λ)FN(λ)SEDFigure 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 hererepresent a graphic form of Eq. (1) with the same de-notation in keys.3.3 A disk-like shape of the WDpseudophotosphereThe shape the WD pseudophotosphere cannot be spher-ical, because of the simultaneous presence of the strongnebular emission in the spectrum. If it were a sphere,its radiation would not be capable of giving rise to theobserved nebular emission. On the other hand, thepresence of the strong nebular emission in the spec-trum constrains the presence of a hot ionizing source inthe 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 diskunder a high inclination, its outer rim simulates thewarm photosphere (producing the WSC), while the ma-terial above/below the disk is ionized by the hot centralsource and thus converts a fraction of its radiation tothe nebular emission (see Skopal 2005 and Skopal et al.2011 in detail).3.4 Collimated mass ejectionFigure 3 shows evolution of the Hα profiles from the2006 August eruption to 2013 April. The broad wingsexpanding to ∼ ±2000 km s−1 in Hα were present inall spectra. Significant variations were observed mainlyat/around the line cores. (i) During the whole activephase, a sharp absorption developed on the blue side ofthe 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 2012September, the satellite components were placed nearlysymmetrically with respect to the Hα central emission. 1 2 3 4 5 6 7-2000 -1000  0  1000  2000l og ( Fl u x)  + c on st .Heliocentric radial velocity [km s-1]30/08/200605/06/200902/09/2012BF Cyg Hαdd/mm/yyyy+5.0+4.1+2.5+1.0const.03/11/2012[ NI I ]F eI I24/04/2013Figure 3: Evolution in the Hα line profile alongthe outburst. The filled curves represent the jetemission components (Sect. 3.4). Fluxes are in10−13 erg cm−2 s−1 A˚−1.Their radial velocities of ± ∼ 370 km s−1 and fluxesof ∼ 1.4× 10−11 erg cm−2 s−1 were around a maximum(see Skopal et al., 2013 in detail). (iv) The presenceof satellite components and their properties were un-stable in the spectrum. During two months after their279A. Skopal et al.best pronounced stage (on 02/09/2012), they practi-cally disappeared on 2012 November 3rd. However, in2013 April, they re-appeared again (Fig. 3).The relatively small width of the (well measured)satellite components (FWHM ∼ 245 km s−1) and theirradial velocities suggest that these emissions were pro-duced by radiation of a highly collimated ejection bythe central star.4 Concluding RemarksAccording to the elements of the spectoscopic orbit(Fekel et al., 2001), the mass of the WD in BF Cygis as low as ∼ 0.55 − 0.6M. During the quiescentphase, the luminosity of the hot component was esti-mated to ∼ 10 000L for d = 3.8 kpc (e.g. Mikola-jewska et al., 1989). This quantity suggests that thesource of such the energy output is caused by a stablehydrogen burning on the WD surface at the accretionrate of ∼ 1.4×10−7M yr−1 for the 0.55−0.6M WD(e.g. Shen & Bildsten, 2007). During active phase, theluminosity of the hot component can be > 10 000L(e.g. Cassatella et al., 1992), however, with difficultiesof 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-lossrate from the WD is evidenced by the broad Hα wingswith a violet-shifted absorption component, and (iii)an enhancement of the accretion rate onto the WDis required by the satellite emission components. Thepresence of bipolar jets confirms the presence of a diskaround the accretor during the outburst. These ob-servational properties are consistent with evolution ofburning WDs in the H-R diagram, when the accretionrate increases above the stable burning regime. Theaccretion at ≈ 2 × 10−7M yr−1 throughout the diskduring the outburst can sustain the high luminosity ofthe burning WD at ≈ 10 000L (see Fig. 2 of Shen &Bildsten, 2007).The case of the current BF Cyg active phase leadus to a speculation that the simultaneous presence ofthe enhanced mass outflow and mass infall during someactive phases of SSs can reflect a new type of the ac-cretion process, which can sustain a high luminosity oftheir hot components for a long time of years. Similarproperties with mass outflow/infall and jets were alsoobserved during the 1977–1984 active phase of CH Cyg(e.g. Skopal et al. 2002). It is of interest to note thatthe enhanced mass outflow, sometimes followed withjet-like components, and emergence of a warm pseu-dophotosphere simulated by the irradiated disk are alsoobserved during optical high states of supersoft X-raysources (e.g. Southwell et al. 1996; Hutchings et al.2002; Hachisu & Kato, 2003).AcknowledgementThis research was supported by the Slovak-BulgarianResearch 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 ProgramMSM0021620860 of the Ministry of Education of theCzech Republic and by a grant of the Slovak Academyof 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, 1003doi: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, MNRAS366, 675doi:10.1111/j.1365-2966.2005.09895.xdoi: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] Mu¨rset, U., Schmid, H. M.: 1999, A&AS 137, 473doi: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. 3780doi:10.1093/mnras/292.3.703[16] Skopal, A.: 2005, A&A 440, 995doi: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, 1109doi:10.1002/asna.201111655[19] Skopal, A., et al.: 2011, A&A 536, id. A27[20] Skopal, A., et al.: 2012, Astron. Nachr. 333, 242doi:10.1086/177931280What 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, 1065DISCUSSIONDMITRY BISIKALO: If you have a wind from theWD during the quiescence, how can you accumulatematter to form a huge accretion disk?AUGUSTIN SKOPAL: The presence of a wind fromthe WD is indicated even during quiescent phase by,for example, the broad Hα wings. The presence of alarge disk-like formation during active phases is obser-vationally confirmed. However, its creation is not wellunderstood yet.281

What Powers the 2006 Outburst of the Symbiotic Star BF Cygni?

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