The very fast evolution of Sakurai's object

TL;DR

This paper reports on the rapid evolution of Sakurai's object, a star that experienced a late thermal pulse, with detailed spectral observations helping to refine stellar evolutionary models.

Contribution

It provides new high-resolution spectral data and temporal analysis of Sakurai's object, enhancing understanding of its rapid post-thermal pulse evolution.

Findings

Rapid change in optical emission lines since 2001

Temporal evolution of morphological components observed

Constraints on stellar evolution models improved

Abstract

V4334 Sgr (a.k.a. Sakurai's object) is the central star of an old planetary nebula that underwent a very late thermal pulse a few years before its discovery in 1996. We have been monitoring the evolution of the optical emission line spectrum since 2001. The goal is to improve the evolutionary models by constraining them with the temporal evolution of the central star temperature. In addition the high resolution spectral observations obtained by X-shooter and ALMA show the temporal evolution of the different morphological components.

Full text

††volume: 323††journal: Planetary nebulae: Multiwavelength probes of stellar and galactic evolution††editors: X. Liu, L. Stanghellini, & A. Karakas eds.

1 Introduction

V4334 Sgr (a.k.a. Sakurai’s object) is the central star of an old planetary nebula (PN) that underwent a very late thermal pulse (VLTP) a few years before its discovery in 1996 ([Nakano et al. (1996), Nakano et al. 1996]). During the VLTP it ingested its remaining hydrogen rich envelope into the helium burning shell and ejected the processed material shortly afterwards to form a new, hydrogen deficient nebula inside the old PN. The star brightened consideraby to become a very cool (born-again) AGB star with a spectrum resembling a carbon star. After a few years, dust formation started in the new ejecta and the central star became highly obscured, similar to R CrB stars. Emission lines were discovered: first He i 1083 nm in 1998 ([Eyres et al. (1999), Eyres et al. 1999]), later in 2001 also optical forbidden lines from neutral and singly ionized nitrogen, oxygen and sulfur ([Kerber et al. (2002), Kerber et al. 2002]). Sakurai baffled the scientific community with its very fast evolution, much faster than pre-discovery models predicted. The current models ([Lawlor02, Lawlor & MacDonald 2003], [Herwig et al. (2011), Herwig et al. 2011], [Herwig (2014), Herwig et al. 2014], [Miller Bertolami et al. (2006), Miller Bertolami et al. 2006]) can be improved by constraining them with the temporal evolution of the central star temperature.

2 Observations and results

2.1 Optical monitoring

We have been monitoring the evolution of the optical emission line spectrum since 2001 using spectra obtained with FORS at the ESO-VLT (Fig. 1). The goal of this monitoring program is to derive the stellar temperature as a function of time ([van Hoof et al. (2007), van Hoof et al. 2007],

[van Hoof et al. (2008), van Hoof et al. 2008]). From 2001 through 2007 the optical spectrum showed an exponential decline in flux and the level of excitation also dropped. We see this as evidence for a shock that occurred around 1998 and cooled afterwards. A plausible explanation is that this is the fastest material ejected in the VLTP hitting older ejecta. The optical line fluxes started to increase again since 2008. The sudden jump in [O ii] flux in 2008 could point to a second shock as the cause of the change in behavior. The optical spectrum shows new lines which have been emerging since 2013. Some have tentatively been identified as electronic transitions of CN. At least some of these newly emerging lines are formed very close to the central star, possibly in the disk. In the PV diagram of [N ii] 658.3 nm, obtained with X-shooter, we see that the red- and blue-shifted emission come from different regions that are spatially shifted w.r.t. the continuum. This emission line, as well as other optical forbidden lines, originate in the bipolar lobes seen by [Hinkle & Joyce (2014), Hinkle & Joyce (2014)].

2.2 ALMA Observations

The continuum emission is unresolved which indicates that all the dust is in the disk. In the ALMA spectra we detect CO, CN, HC3N, as well as 13C isotopologues of these species. The CO and HC3N (+isotopologues) emission is unresolved, coincides with the position of the central star, and most likely comes from the disk (Fig. 2). The CN and 13CN emission is spatially resolved and coincides with the bipolar lobes seen by [Hinkle & Joyce (2014), Hinkle & Joyce (2014)] (Fig. 2). CN could be formed via shock-induced dissociation of HCN in the lobes.