Sakurai's Object: characterizing the near-infrared CO ejecta between 2003 and 2007

H. L. Worters 1 2 M. T. Rushton 2 S. P. S. Eyres 2 T. R. Geballe 0 A. Evans 3 0 Gemini Observatory , 670 N. A'ohoku Place, Hilo, HI 96720 , USA 1 South African Astronomical Observatory , Observatory, 7935 , South Africa 2 Centre for Astrophysics, University of Central Lancashire , Preston PR1 2HE 3 Astrophysics Group, Keele University , Keele, Staffordshire ST5 5BG A B S T R A C T We present observations of Sakurai's Object obtained at 1-5 m between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around 4.7 m, we determine the excitation conditions in the line-forming region. We find 12C/13C = 3.5+21..05, consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of 2.2 106 MCO 2.7 106 M of CO ejecta outside the dust, forming a high-velocity wind of 500 80 km s1. We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor. 1 I N T R O D U C T I O N Sakurais Object is a highly evolved post-asymptotic giant branch (post-AGB) star that had begun to venture down the white dwarf cooling track when, in 1995, it underwent sudden rebrightening (Nakano et al. 1996) due to a final helium shell flash, or very late thermal pulse (VLTP) (Duerbeck et al. 1997). Since then, Sakurais Object has undergone observable changes on time-scales of weeks to months, providing an instance in which this very brief stage of evolution experienced by 15 per cent of intermediate-mass stars (Iben, Tutukov & Yungelson 1996) could be tracked over a period of only a few years. Several phases of dust production followed the outburst, with a deep optical minimum beginning in early 1999, such that any changes in the central star have since been inferred from radio and infrared observations (Tyne et al. 2000; Hajduk et al. 2005; van Hoof et al. 2007). Subsequent observations and modelling have revealed much about the dust shell formation and the outer regions of the ejecta (e.g. Kimeswenger & Koller 2002; Tyne et al. 2002). Using observations at 2.3 m from 1998, Pavlenko et al. (2004) modelled overtone CO band absorption in the stellar atmosphere to determine a 12C/13C ratio of 4 1, consistent with VLTP nucleosynthesis. In order to follow the development of the ejecta, annual monitoring of the target in the near-infrared has been undertaken. The discovery of fundamental band (4.7 m) lines of CO in the wind was reported by Eyres et al. (2004), based on a low-resolution spectroscopy obtained in 2002 and 2003. Here, we present the analysis of observations of the CO fundamental lines in data obtained between 2003 and 2007, mostly at higher resolution. 2 O B S E RVAT I O N S All data presented here were obtained at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea, Hawaii, using the facility spectrographs CGS4 and UIST. A summary of the observations is given in Table 1. Low-resolution spectra were predominantly obtained with UIST, utilizing the 0.24 and 0.48 arcsec slits, but in one instance (2005 August 3) CGS4 and its 0.61 arcsec slit were used. For all the observations, the telescope was nodded in the standard ABBA pattern. Standard stars were observed just prior to or after Sakurais Object and at airmasses close to those of the target. The spectra of Sakurais Object were ratioed by those of the appropriate calibration stars (Table 1). Flux calibration was performed utilizing photometry of the standard stars and colour corrections based on Tokunaga (2000). Wavelength calibration was obtained from an arc lamp or telluric absorption lines in the spectrum of the calibration star, and was accurate to better than 0.0001 m for the UIST and low-resolution CGS4 spectra. In the case of the CGS4 echelle observations on 2004 June 10, three echelle settings were used with a slit width of 0.9 arcsec to Waveband 1.402.51 500 HIP 86814 F6V 2.913.64 1400 BS 6378 A2IVV 3.624.23 1400 BS 6378 A2IVV 4.385.31 1200 BS 6378 A2IVV 1.402.51 2000 HIP 86814 F6V 2.232.99 2000 HIP 86814 F6V 2.913.64 2000 BS 6378 A2IVV 3.624.23 2000 BS 6378 A2IVV 4.385.31 2000 BS 6496 F5V 4.6784.692 20000 BS 6378 A2IVV 4.7134.727 20000 BS 6378 A2IVV 4.7584.762 20000 BS 6378 A2IVV 4.485.12 2000 BS 6595 F5V 3.624.23 2000 BS 6595 F5V 4.385.31 2000 BS 6595 F5V cover the wavelength range 4.684.77 m at a resolving power of 20 000 (15 km s1). Following data reduction, the three spectra were adjoined after applying small scaling factors to two of the spectra in order to match the continuum levels. Wavelength calibration for these spectra was derived from telluric CO lines and is accurate to 3 km s1. 3 R E S U LT S Fig. 1(a) shows spectra of Sakurais Object in the 15 m region from 2003 to 2007. The continuum can be seen to fade from one year to the next, with a substantial decrease in flux density between the 2004 and 2005 observations. To aid comparison of the continuum shapes, Fig. 1(b) shows the 2005 and 2007 spectra scaled up by a factor of 14. Vertical offsets have also been applied to the 2004, 2005 and 2007 spectra for display purposes. A difference in gradient of the continua can be seen, particularly in the M band, indicating a blackbody peak at increasing wavelength in consecutive years, strongly suggesting cooling of the dust. The only features superimposed upon the continuum are the fundamental absorption lines of CO around 4.7 m. These can be seen in Fig. 1(c); an enlargement of the 12CO P-branch and 13CO R-branch region. The 2004, 2005 and 2007 low-resolution spectra have been convolved with a Gaussian profile of a full width at half-maximum (FWHM) equal to the resolution of the 2003 spectrum. Comparing the spectra, one can see a significant weakening of the CO band from 2003 to 2005. Although individual lines cannot clearly be seen in the 2005 spectrum, the apparent emission bump at 4.67 m, the wavelength of the CO band centre, is a strong indication that weak P- and R-branch absorption lines are still present. The low signal-to-noise ratio of the 2007 spectrum precludes a meaningful estimate of the CO band strength. Fig. 2 is the echelle spectrum from 2004 June, showing 12CO and 13CO in absorption around 4.7 m. The absorption features have non-Gaussian profiles and show remarkably large linewidths. We measure a full width at zero intensity (FWZI) of 400 70 km s1 and FWHM of 180 15 km s1. By measuring the blueshift of absorption features in the echelle spectrum, we can improve upon the 290 30 km s1 absorption maximum found Figure 1. (a) UKIRT spectra of Sakurais Object on 2003 September 8, 2004 June 10, 2005 August 3 and 2007 June 2. Gaps occur in the data due to strong telluric absorption. (b) The same UKIRT spectra shown in (a), with 2005 and 2007 spectra scaled up by a factor of 14 to (...truncated)