Optical spectra of the Very Late Thermal Pulse VLTP object V4334 Sgr have shown a rapidly changing spectrum resulting from shocks in the outflow which created a new bipolar nebula inside the old nebula. We see C II and C III emission lines emerging typical of a WC 1110type star. The strong increase of O III and S III emission lines indicate the possible onset of photoionisation in the new ejecta.
['Astrophysics - Solar and Stellar Astrophysics', 'Astrophysics - Astrophysics of Galaxies', 'Astrophysics - High Energy Astrophysical Phenomena']
Sakurais object a WC star in a new bipolar nebula after a VLTP
{'No Header': 'O. De Marco, A. Zijlstra, R. Szczerba, eds.  \ndoi:10.1017/xxxxx', "Sakurai's object: a [WC] star in a new bipolar nebula after a VLTP": "G. C. Van de Steene 1 , P. A. M. van Hoof 1 , S. Kimeswenger 2 , 3 , M. Hajduk 4 ,D. Tafoya 5 , J. A. Toal'a 6 , A. A. Zijlstra 7 , D. Barr'ıa 8  \n1 Department of Astronomy & Astrophysics, Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium; 2 Universitat Innsbruck, Institut fur Astro- und Teilchenphysik, Technikerstr. 25, 6020 Innsbruck, Austria; 3 Instituto de Astronom'ıa, Universidad Cat'olica del Norte, Av. Angamos 0610, Antofagasta, Chile; 4 Department of Geodesy, Faculty of Geoengineering, University of Warmia and Mazury, ul. Oczapowskiego 2, 10-719 Olsztyn, Poland; 5 Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden; 6 Instituto de Radioastronom'ıa y Astrof'ısica, UNAM, Ant. Carretera a P'atzcuaro 8701, Ex-Hda. San Jos'e de la Huerta, Morelia 58089, Mich., Mexico; 7 Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Manchester, M13 9PL, UK; 8 Facultad de Ingenier'ıa y Arquitectura, Universidad Central de Chile, Av. Francisco de Aguirre 0405, La Serena, Coquimbo, Chile", 'Abstract.': 'Optical spectra of the Very Late Thermal Pulse (VLTP) object V4334 Sgr have shown a rapidly changing spectrum resulting from shocks in the outflow, which created a new bipolar nebula inside the old nebula. We see C II and C III emission lines emerging typical of a [WC 11-10]-type star. The strong increase of [O III] and [S III] emission lines indicate the possible onset of photoionisation in the new ejecta.  \nKeywords. stars: planetary nebulae, VLTP, bipolar', '1. Introduction': "V4334 Sgr (Sakurai's object) is the central star (CS) of an old Planetary Nebula (PN) that underwent a Very Late Thermal Pulse (VLTP) around 1996. The VLTP forced the star to briefly track back to the AGB and experience a second PN formation stage. High resolution ALMA continuum observations have shown the presence of a new bipolar PN inside the old one (Tafoya et al. 2023). We have been observing this object since 2006 to improve our understanding of PN formation and evolution.", '2. Optical Monitoring': 'The first emission lines from the newly ejected gas were discovered in 1998 (Eyres et al. 1999) and 2001 (Kerber et al. 2002). We have been monitoring the changes in the optical emission line spectrum since 2006 using spectra obtained with FORS1&2 at ESO-VLT (Reichel et al. 2022; van Hoof et al. 2007; Van de Steene et al. 2017). The goal of this monitoring program has been to derive the stellar temperature as a function of time in order to compare with stellar evolutionary models (Miller Bertolami et al. 2006; Herwig et al. 2014), which predict a fast reheating to 80 kK on a timescale of decades (Hajduk et al. 2005). Such fast evolution would cause the ejecta to be photoionised rapidly, resulting in observable, dramatic changes in the emission line spectrum.', '3. Evolution of line fluxes': 'From 2001 to 2017 the data show a rapidly evolving spectrum that is completely produced by shocks. These shocks have been the result of the hydrodynamic shaping of the new bipolar nebula forming inside the old nebula. In van Hoof et al. (2007) we argued for the existence of a  \nFigure 1. The emergence of WR lines since 2013 (left), the increase in [O III] emission lines (middle), and the emergence of the C III λ 569.6 nm line (right).  \n<!-- image -->  \nshock that occurred around 1998 and dissipated shortly afterwards based on the optical decline of the line flux values. In 2008 the spectrum changed dramatically. The [OII] λ 732.5 nm lines increased almost tenfold in strength, and weak He I emission appeared. This sudden increase in excitation would have been caused by a second shock. However, unlike the first shock, there has been a gradual increase in line strengths since then (van Hoof et al. 2018). The evolution of the low-ionization forbidden lines is still dominated by these shocks in the most recent spectra. The emergence of a substantial stellar wind may have been the cause of the second shock, which is associated with the bipolar bow-shock features discovered in Hinkle & Joyce (2014). In 2014 there was another dramatic change in the spectrum with the emergence of a complex of emission lines mostly between λ 905.0 and λ 945 nm (Fig.1,left). Many are identified as carbon lines (Williams et al. 2021). The flux evolution of these lines differs markedly from the shock excited helium and forbidden (nebular) lines. This points to a different origin and we assume that these lines are formed in the wind of the central star and are in fact WR features. The emergence of C III lines in the most recent spectra would confirm this assumption (Fig.1, right). The presence of C II and C III lines, but the absence of C IV lines indicate a spectral type [WC 11-10] (Acker & Neiner 2003). This is the first sign of reappearance of the central star after the VLTP. In 2017 the spectra showed the first tentative signs of photoionisation (Fig.1, middle) with a sudden increase in the flux of the [O III] λ 495.9 and λ 500.7 nm lines compared to 2015. This trend has continued in the spectra of 2022 where these lines showed another dramatic increase in strength. For the first time also the [S III] λ 953.1 nm line appeared (Fig.1, left). Its λ 906.9 nm counterpart is blended. However, radio continuum observations have shown only a slight increase of flux density (Hadjuk et al. this volume). It needs to be confirmed whether photoionisation of the new ejecta by the central star has started or this increase is due to strenghtening of the shock.', 'References': 'Acker, A. & Neiner, C., 2003, A&A, 403, 659. Eyres, S. P. S., Smalley, B., Geballe, T. R., et al.,1999, MNRAS, 307, L11 Hajduk, M., Zijlstra, A. A., Herwig, F., et al., 2005, Science, 308, 231 Herwig, F., Woodward, P. R., Lin, P.-H., et al., 2014, ApJL, 792, L3. Hinkle, K. H. & Joyce, R. R. 2014, ApJ, 785, 146 Kerber, F., Pirzkal, N., De Marco, O., et al., 2002, ApJL, 581, L39 Miller Bertolami, M. M., Althaus, L. G., Serenelli, A. M., et al., 2006, A&A, 449, 313 Nakano, S., Sakurai, Y., Hazen, M., et al., 1996, IAU Circ., 6322 Reichel, M., Kimeswenger, S., van Hoof, P. A. M., et al., 2022, ApJ, 939, 103 Tafoya, D., van Hoof, P. A. M., Toal´a, J. A., et al., 2023, A&A, 677, L8 van de Steene, G. C., van Hoof, P. A. M., Kimeswenger, S., et al., 2017, IAUS, 323, 380 van Hoof, P. A. M., Hajduk, M., Zijlstra, A. A., et al., 2007, A&A, 471, L9 van Hoof, P. A. M., Kimeswenger, S., Van de Steene, G., et al., 2018, Galaxies, 6, 79  \nWilliams, R., Manea, C., Margon, B., et al., 2021, ApJ, 906, 31'}