arXiv:0807.1473v1 [astro-ph] 9 Jul 2008 07 eefe ae I). spe Paper hereafter line 2007, and comprehen- continuum di a in of attempted tra have modeling we self-consistent paper and recent sive a In SSs. cussed at (Angeloni models specific composite a,b,c). within by 2007 nebulae al. distinguished dust be and could gas 199 SSs Contini of (e.g. network t spectra the in continuum Moreover, and for line accounted the e therefore of coupled modeling was The ranges. photoionization spectral and energy shocks high the p in observations, the ularly explain & finally Kenny th could throughout 1998, systems waves al. symbiotic shock of et network Li complex A wi 1987, 2005). i Willson colliding Taylor the & through to (Girard mass led models 1995) observa- lose (CW) al The et 1997). (Nussbaumer both winds Leahy actually portant & that (Li observa- evidence ranges for tional and spectral 1990) new Vogel in & tions model opti (Nussbaumer detailed first emission the the for line inadequate explain of became alone. to but WD able data, radio the fairly and was by scenario played simple was the g This source cool through photoionizing the configuration of The portion wind. photoionized binary the from a arising of emission light the in spec complex X-rays. interac- of to radio mutual origin from the whose wavelengths, at all giant, generally at is cool recorded star, processes a accretion hot and via compact (WD), tion a dwarf of white interpre composed a are systems they Nowadays binary (1919). as Merril spectroscopicall by as objects introduced culiar were (SSs) stars Symbiotic Introduction 1. edo Send oebr2,2018 20, November srnm Astrophysics & Astronomy HCgi(HCg saogttems tde e tl dis- still yet studied most the amongst is Cyg) (CH Cygni CH spectra symbiotic the explain to attempted models first The n Vlnsaoefo h euadwsra fteexpandin the words. of Key downstream nebula the from arose lines UV and 1 yn hthdnvrbe bevdpreviously. observed been never had that Cygni 2 eevd;accepted ; Received Results. Methods. Aims. Context. tm n otsokpoosa togsokfront. shock strong a at protons post-shock and atoms ff rn eussto requests print iatmnod srnma nvriyo aoa iood Vicolo Padova, e-mail: of University Astronomia, di Dipartimento colo hsc n srnm,TlAi nvriy Tel- University, Tel-Aviv e-mail: Astronomy, and Physics of School ff rn pcswti h Wfaeok(otn tal et (Contini framework CW the within epochs erent nti okw netgt h rgno hsaoaosbroa anomalous this of origin the investigate we work this In ehv on httebodLy broad the that found have We n18,a h n fteatv hs 9718,abod(400 broad a 1977-1986, phase active the of end the at 1985, In oof.neoiuidi,[email protected] [email protected], [email protected] esgetanwitrrtto ftebodLy broad the of interpretation new a suggest We iais yboi tr:idvda:C Cyg CH individual: stars: - symbiotic binaries: .Contini M. : I h ra Ly broad The II. aucitn.CHCyg no. manuscript h yboi trC Cygni CH star symbiotic The .Contini M. α ieoiiae rmtebatwv rae yteoutburst, the by created wave blast the from originated line α msinln xlie yshocks by explained line emission 1 , 2 .Angeloni R. , ff α c of ect artic- pe- y ae nteter fcag rnfrratosbtenamb between reactions transfer charge of theory the on based ABSTRACT iant ing ted cal m- 7). tra nd he vv 97 Israel 69978 Aviv, l’sevtro2 -52 aoa Italy Padova, I-35122 2, ell’Osservatorio c- e r hc ntecliigwn scenario. wind colliding the in shock g Ly d usadn eaiu a o enrpae uigtesub- the during repeated been not has behaviour outstanding ig ugsigepninvlcte pt 4000 redward-shift to strong up a velocities with expansion 2), and suggesting phase 1 wing, active (Figs. appeared the Ly Å, broad of 20 and as end strong the a towards 1985), Then, (January recorded. was Cyg u hoissgetdt xli h opst Ly composite the pr explain the to and suggested trend theories observational the ous phase review active briefly we the s 2 during Sect. for outburst In WD accounting the accompany interpretation that alternative waves an suggest and Cyg paper. CH this in of presented model is overall and The amplified alone. been model has be CW feat not the could spectral it by peculiar because plained I, very Paper broad, This in a disregarded 1985). however, was, of Hack & appearance (Selvelli the charac- tra was remarkable spectrum A Ly 1985 ranges. strong w the optical variations of and spectral UV teristic and the 1987) radio in bipolar (Solf observed end, appeared the Towards jets 1986. optical until lasted that burst h eut yHn uye 20,hratrH0)aotth about HS07) app hereafter (2007, we Ly Sunyaev medium, & circumstellar Heng by the results with the in outburst 3. the the (Sect. on of phase theory action active (1982) 1977-1986 Chevalier’s the adopting of Consequently, end re the the and at scenario picture colliding-wind ing the describe we 3 Sect. In uigteotus tre n17,n Ly no 1977, in started outburst the During .TebodLy broad The 2. rema concluding while 5. 4, Sect. Sect. in follow in appears discussion A 3.2). 1 α interpretations α , edsusteoii fteLy the of origin the discuss We out- powerful a underwent Cyg CH 1977, in particular, In 2 ra iefraina ihvlct hc rns(Sect. fronts shock high-velocity at formation line broad line. n .Rafanelli P. and 0 ms km α msinln Fg ) ee vdn npeiu spec- previous in evident never 1), (Fig. line emission − 1 Ly ) α α ieapae ntesmitcsse CH system symbiotic the in appeared line ie bevtosadformer and observations line: 1 hl h otmoayoptical contemporary the while α α ieapaac n1985 in appearance line msinln,a wide as line, emission α msinfo CH from emission ethydrogen ient

c α ms km ieprofile. line S 2018 ESO − 1 This . hock sult- evi- and ter- ure ex- ere rks 1). ed ly e . 2 Contini et al.: Broad Lyα emission line from CH Cygni

Fig. 2. Evolution of the Lyα profile in the high-resolution IUE spectra, from 1980 to 1995. On the left, the observation date are indicated; the g marks the position of the narrow Lyα geocoronal emission (adapted from Skopal et al. 1998).

Fig. 1. IUE spectra of CH Cyg, showing the appearance of a strong and unusually broad Lyα in 1985, at the end of the 1977- 86 active phase (adapted from: MSH88, top and central panels - former emission has usually been attributed either to the atmo- Selvelli & Hack 1985, bottom panel) sphere of the (Selvelli & Hack 1985) or to some cir- cumbinary material (Skopal et al. 1998), the latter, i.e. the vari- able part of the profile that extends up to a high velocity (∼ 2200 km s−1), has been explainedas a fast outflowin the vicinity of the hot star. An asymmetric high-velocity outflow (2000 km s−1) was sequent evolution throughout the active and quiescent phases. also indicated by the broad Balmer lines occurring in the spec- For instance, during the 1990-91 and 1995 quiescent stages, the trum on short time scales (Iijima et al. 1994). Such velocities Lyα virtually disappeared, together with the hot UV and op- and variabilities are similar to those characteristic of the shock tical continuum, while during the active phase 1992-1995 the between the stars created by collision of the winds (Paper I). Lyα line was similar in profile (Fig. 2), but 2-3 times weaker Therefore, they are less anomalous than the broad (4000 km s−1) than in 1985 (Skopal et al 1998). Lyα line observed at the end of the 1977-1986 active phase. The standard Lyα profile has been decomposed into two - A mechanism for explaining this broadening was proposed stable and variable - components (Skopal et al. 1998). While the by MSH88 who claimed that, according to Johansson & Jordan Contini et al.: Broad Lyα emission line from CH Cygni 3

(1984), the Lyα line with an enhanced red wing, typical of line HeII, OIII] (Selvelli & Hack 1985, MSH88). Forbidden opti- formation in an acceleration outflow, may be broadened by scat- cal lines cannot survive the densities of 108-109 cm−3 that are tering by a high opacity. However, a stellar origin is difficult characteristic of the shock between the stars. They must have to reconcile with the sudden appearance of the broad Lyα line. been emitted from the nebula downstream of the shock expand- MSH88 then proposed a formation region displaced from the ing outward throughout the external region of the system, which orbital plane, connected with outflowing material, while the in accordance with the colliding wind model (Girard & Willson Balmer lines might originate in an accretion disk. 1987), is located on the side of the WD circumstellar region op- posite the red giant. Consequently, the Lyα line, appearing at 3. Theoretical scenarios this very , was emitted from high-velocity gas outflowing throughout the extended hemispherical region on the side of the We would like to explain the exceptionally broad Lyα line emis- WD less disturbed by the dynamicaleffects of the red giantwind. sion in the frame of the shock-front network in CH Cyg. The re- sults of the wind-collision model (Paper I) led to a detailed phys- ical and morphologicalpicture of the emitting nebulae within the 3.2. The blast wave from the outburst SS. In particular it was demonstrated that the spectra depend on the system phase. In January 1985, at the end of the active phase We adopt MSH88 suggestion that the broad Lyα emission region that started in 1977, the UV and optical line ratios revealed that is connected with the outflow material. we were facing the circumstellar mediumof the WD oppositethe Following Chevalier (1982), we consider the interaction of red giant. In this region, the dynamical consequences of the WD the outburst with circumstellar matter on the assumption that is outburst can be compared with those of a supernova explosion, built up by a steady wind. If the ambient density is described by −s −3 though on a different scale. Following Chevalier (1982), 8 ρ ∝ r (ρ= 1.4 mH n, where n is the density in number cm after the burst, the expanding blast wave reached a relative large and mH the mass of the H atom), the steady wind corresponds radius and the velocity of the shock front is in the range of those to s=2. The interaction of the freely expanding matter with the characteristic of broad Lyα line emission by charge-transfer re- surrounding medium gives rise to a high-energy density region actions (HS07). bounded by shock waves. Two shock fronts develop, one pro- ceeding inward in the high-density region, the other expanding outward in the circumstellar medium. 3.1. Results of the colliding-wind model The case of uniform expansion gas is described by s=2 and Collision of the winds (Girard & Willson 1987, Kenny & Taylor γ=5/3. For s=2 the radius of the outer shock is given by the 1/3 2/3 2005) from the two-component stars leads to two main shocks: Primakoff solutions: RBW =(3 E/2 π A) t (Chevalier 1982, −2 the head-on shock between the stars, facing the WD, and the Eq. 5), where E is the total energy, ρ0 = A R (ρ0= 1.4 mH n0, head-on-back shock expanding from the system outwards (men- where n0 is the pre-shock density of the gas upstream), and t is tioned in Paper I as the reverse and expanding shocks, respec- the time elapsed from the burst. tively). This equation is valid for times longer than the time of According to the colliding wind model, during the active change t = 0.677 M3/2/A E1/2, between that of the interaction −1 s phase, the broad Hβ lines with FW0M of 400-1200 km s were of freely expanding matter with the surrounding medium and the emitted downstream of the head-on shock between the stars following one, i.e. when the flow tends toward the self-similar (Angeloni et al. 2007a). Although this shock is actually a stand- solution for a point explosion in a power-low density profile ing shock, it may be accelerated throughout the decreasing den- (Sedov 1959). Here M stands for the ejected mass. sity of the atmosphere by the massive wind from the red gi- The velocity V = dR/dt is constrained by the observed ant. The broad Balmer lines emitted downstream are particu- s FW0M of the Lyα line 8 years after the burst. After some al- larly strong because the intensity of permitted lines depends on gebra we obtain n =1.45 10−47 E. The mass ejected by the WD the temperature of the star T and on the ionization coefficient 0 ∗ in the CH Cyg system duringthe 1977-86outburst is about a few U. T can reach more than 100,000 K during the outburst and ∗ (2-3) 10−6 M (Taylor & Seaquist 1985). The total energy is half U is relatively high (> 1) because the emitting gas is close to ⊙ thermal and half kinetic (Chevalier 1982). The high velocity ob- the hot star. The broad lines decline both in intensity and width served after January 1985 presumes that the velocity of the ejecta during quiescence because T becomes ≤ 30,000 K and the ∗ was ∼ 14,000 km s−1at t=1 . If all the ejecta had a velocity high velocity wind slows down. In a shock dominated regime, of 14,000 km s−1, the associated kinetic energy would amount to Rayleigh-Taylor (R-T), Richtmyer-Meshkov (R-M), and Kelvin- 6 1045 erg, the total energy E=1.2 1046 erg, and consequently n Helmholtz (K-H) instabilities at the shock fronts lead to frag- 0 = 0.2 cm−3. This in turn gives R =2.25 1017 cm. mentation. Adopting a filling factor between 0.001 and 1 (Paper BW I), a maximum geometrical thickness of the filaments ∼ 1014 cm, To check this result we calculate for instance, n0 close to the −1 = 13 and an average velocity of 600 km s during outburst, the vari- WD (n0WD ) at a radiusR0WD 2 10 cm. We choose the distance ability time scale is between ≤ 1 hour and ≤ 1 month. The time that was found for the standing shock between the stars (Paper scale increases during quiescence depending on the velocity de- I) by modeling the continuum SED observed in May 1985. crease. Although the interbinary region shows mixing of the winds from The sudden appearance of the broad (4000 km s−1) Lyα line the stars, the standing shock facing the WD reflects the composi- tion of the WD (e.g. Contini 1997). From n =n R2 /R2 looks quite anomalous. Such a broad line could not even come 0WD 0BW BW 0WD −3 7 −3 from the nebula downstream of the expanding shock, which cor- where n0BW = 0.2 cm , we find n0WD = 2.5 10 cm in good −1 7 −3 responds to Vs≤ 150 km s (Paper I). Interestingly, the broad agreement with the preshock density n0 =5 10 cm adopted Lyα line appeared at about the same time as the line spectrum to explain the continuum SED at that epoch (Paper I, Table 1). 17 observed after November 1984, when the outburst was almost Constraining ts within a maximum radius of ∼ 2. 10 cm and −3 over. The spectrum revealed [OIII] 4363 and [OIII] 5007 lines, adopting n0=0.2 cm , we find ts= 5.2 years, confirming that as well as high ionization UV emission lines, e.g. NV, CIV, SiIV, Chevalier’s model is valid in this case. 4 Contini et al.: Broad Lyα emission line from CH Cygni

We can now adopt the theory of HS07, who treated the blast Interestingly,the energies calculated from the temperature(∼ wave shocks in the SNR case. Their results show that broad 2.94 109 K) downstream of this strong shock front correspond to Balmer and Lyman lines are produced by charge transfer reac- ∼ 250 keV, not far from the gamma-ray energy range, while en- 8 −1 tions between the post-shock protons and the ambient atoms. A ergies of ∼ 21 keV (2.4 10 K) correspond to Vs=4000 km s . population of post-shock atoms follows with a broad velocity We suggest that the observations of symbiotic star outbursts be- −1 distribution (broad neutrals). For Vs≥ 500 km s the broad neu- yond hard X-rays might lead to interesting results. 1/2 trals can produce Lyα that is blue- or red- shifted by resonance The escape velocity of the WD is vesc =(2G MWD/RWD) 13 1/2 with the stationary atoms, hence providing an escaping way for ∼ 1.6 10 /R , adopting a mass MWD ∼ 1M⊙ . We recover a −1 WD the protons. For shocks with Vs≥ 4000 km s , the velocity of 14,000 km s−1similar to what is predicted at early ratio ΓLyα/Hα is ≥ 10 (HS07 Fig. 1), so the Lyα line would be times for the Lyα line FW0M profile, adopting a WD radius RWD strong. ∼ 1.36 108 cm in the CH Cygni system.

4. Discussion 5. Concluding remarks As previously mentioned (Sect. 3.1), the broad Lyα appeared at In 1985, at the end of the active phase 1977-1986,a broad (4000 about the same phase as the expanding shock within the collid- km s−1) Lyα line was observed that had never been present in ing wind scenario. The radius of the expanding nebula Rexp ∼8 previous spectra from the symbiotic system CH Cygni. 1016 cm results from modeling the observed spectral energy dis- We have noticed that the broad Lyα line appeared contem- tribution of the continuum (Paper I). Notice that the radius of poraneously with the optical-UV spectrum emitted downstream the blast wave RBW calculated in Sect. 3.2, is about three times of the shock created by collision of the winds from the stars. −1 that of Rexp, while the physical parameters, Vs=4000 km s and This shock expands throughout the extended circumbinary re- −3 −1 5 n0=0.2 cm of the blast wave, and Vs=150 km s and n0=10 gion located on the side of the WD circumstellar region opposite cm−3 of the expanding shock, are very different. This is not sur- to the red giant; as a result, the Lyα line originates somewhere in prising, considering that the blast wave stems from the outburst, the hemispherical region opposite the WD, where the dynamical while the expanding shock derives from the wind collision. The consequences of the burst are less affected by the red giant wind. two shock fronts will hardly interfere even in the orbital plane, The overall situation is similar to that of an SN explosion. This because R-T and K-H instabilities lead to fragmentation of mat- suggests that the broad Lyα emission line and the other optical ter at the shock fronts with filling factors < 0.01 (Paper I; Contini and UV lines observed at the same phase could be emitted from & Formiggini 2001). different shocked nebulae. We consider that the broad Lyα line A velocity of ∼ 4000 km s−1was evident from the Lyα line stems from the WD outburst. profile after January 1985 in CH Cyg. The other UV and optical Applying the theory developed by Chevalier (1982) for Type lines showed narrow FWHM profiles. In contrast, UV lines in- II supernovae to the interaction of the WD outburst with circum- dicating expansions of 3000-4000 km s−1were observed after the stellar matter, we have found that the expanding blast wave had 17 1985 eruption of the recurrent nova RS Ophiuci (Snijders 1987; reached a radius RBW ∼ 2.25 10 cm 8 years after the burst for Shore et al. 1996) and also optical lines after the last outburst a shock velocity of 4000 km s−1. We then applied the theory de- in 2006 (Bode et al. 2007 and references therein) showed high- veloped by HS07 for high-velocity shock fronts in SNR, namely, velocity expansion. The UV and optical lines strong enough to the broad Lyα line is produced by charge transfer reactions be- be observed can be emitted downstream of a strong shock if the tween the blast wave post-shock protons and the ambient pre- −1 preshock density is high enough to speed up the cooling rate (∝ shock atoms. For shocks with Vs≥ 4000 km s , the luminosity n2) downstream. Consequently the temperature will drop below ratio ΓLyα/Hα is ≥ 10 (HS07), so the observed Lyα line is strong. 106 K, leading to recombination. However, the densities are con- The energy involved with the outburst is E ∼ 1.2 1046 erg, strained by the critical densities for collisional deexcitation of and the ambient density consistent with a velocity of 4000 6 −3 −1 −3 the different ions, which are relatively low (< 10 cm ) for for- km s is n0 ∼ 0.2 cm . Higher velocities of about 14,000 bidden lines. In order to emit strong enough forbidden lines (e.g. km s−1predicted by the Sedov solution at early times, lead to −1 [OIII]), a shock with Vs=4000 km s should propagate through- temperatures in the downstream region of such a strong-shock 4 −3 out a medium with n0 ≤ 10 cm The lines are emitted from front, corresponding to emission in the near gamma-ray fre- the gas beyond the temperature drop characteristic of shock- quency range. Such emission was not observed at that time, first dominated regimes downstream, at a distance from the shock because of technical inadequacy and also because, during the front of ≥ 7 1016 cm. Lower densities correspond to stronger active phase, the WD circumstellar region opposite the red giant forbidden lines at larger distances from the shock front. Strong only beaome visible in 1985 when the broad Lya appeared. The [FeVII]6087, [FeX] 6375, and even [FeXI] 6986 are predicted velocities had already decreased to about 4000 km s−1. Actually, at such high Vs. The UV lines are generally permitted or semi- the SEDs in the VBU range duringthe 1977-86period,presented forbidden, therefore they can also be emitted at higher densities. in PaperI (Fig.6), showthatthe black bodyflux fromthe hotstar The OVI 1034 and CIV 1500 lines will be particularly strong. does not appear. −3 These predictions imply a preshock magnetic field of B0=10 According to previous results (Paper I), the hard X-rays are Gauss. emitted from a small region between the two component stars, Such densities are not unusual in the circumstellar medium, while the soft X-ray are emitted from the extended circumbinary as they were found by modeling the [OII] and [NII] lines dur- region.The results of this papersuggest that the hardest radiation ing the active phase 1998-2001 of CH Cyg (Paper I). The high (∼ 250 keV) comes from the WD circumstellar region close to preshock density would indicate that the shock is interacting the WD on the opposite side of the giant star, while hard X-rays with mass lost by either the progenitor star or by the SS in a could also be observed at distances ≤ 0.08 pc from CH Cyg. previous burst. However, such high velocities were never seen in Finally, the complex light curves of CH Cyg (Eyres et al. the line profiles of CH Cyg, except for the broad Lyα in 1985. 2002) show that the nebulae created by collision of the winds Contini et al.: Broad Lyα emission line from CH Cygni 5 and the dusty shells ejected from the red giant, expanding out- ward beyond the binary system, lead to temporary obscuration (Paper I). We suggest that the shock front at a relatively large radius corresponding to the blast wave may also contribute to obscuration episodes.

Acknowledgements. We are very grateful to the referee for constructive criticism and to Dina Prialnik for many helpful comments. We also thank P. Selvelli and A. Skopal for the kind permission to include some figures adapted from their papers.

References Angeloni, R., Contini, M., Ciroi, S., & Rafanelli, P. 2007a, AJ, 134, 205 Angeloni, R., Contini, M., Ciroi, S., & Rafanelli, P. 2007b, A&A, 471, 825 Angeloni, R., Contini, M., Ciroi, S., & Rafanelli, P. 2007c, A&A, 472, 497 Bode, M.F. et al. 2007, ApJ, 665, L63 Chevalier, R. A. 1982, ApJL, 259, L85 Contini, M. 1997, ApJ, 483, 887 Contini, M. & Formiggini, L. 2001 A&A, 375, 579 Contini, M., Angeloni, R., & Rafanelli, P. 2007, A&A, submitted, Paper I Eyres, S. P. S., et al. 2002, MNRAS, 335, 526 Girard, T., Willson, L.A. 1987, A&A, 183, 247 Heng, K., & Sunyaev, R. 2007, ArXiv e-prints, 710, arXiv:0710.4282 Iijima, T., Strafella, F., Sabbadin, F., & Bianchini, A. 1994, A&A, 283, 919 Johansson, S. & Jordan, C. 1984, MNRAS, 210, 239 Kenny, H.T. & Taylor, A.R. 2005, ApJ, 619, 527 Li, P. S., & Leahy, D. A. 1997, ApJ, 484, 424 Li, P. S., Thronson, H. A., & Kwok, S. 1998, 1997 Pacific Rim Conference on Stellar Astrophysics, 138, 191 Merrill, P. W. 1919, PASP, 31, 305 Mikolajewska, J., Selvelli, P.L., & Hack, M. 1988, A&A 198, 150 Nussbaumer, H., & Vogel, M. 1990, Astronomische Gesellschaft Abstract Series, 4, 19 Nussbaumer, H., Schmutz, W., & Vogel, M. 1995, A&A, 293, L13 Sedov, L.I. 1959, Similarity and Dimensional Methods (New York:Academic Press) Selvelli, P. L., & Hack, M. 1985, Recent Results on Cataclysmic Variables. The Importance of IUE and Exosat Results on Cataclysmic Variables and Low- Mass X-Ray Binaries, 236, 207 Shore, S. N., Kenyon, S. J., Starrfield, S., & Sonneborn, G. 1996, ApJ, 456, 717 Skopal, A., Bode, M. F., Lloyd, H. M., & Drechsel, H. 1998, A&A, 331, 224 Snijders, M. A. J. 1987, Ap&SS, 130, 243 Solf, J. 1987, A&A, 180, 207 Taylor, A.R. & Seaquist, E.R. 1985, IAU circular No. 4055