The Infrared Evolution of Sakurai's Object

Infrared spectroscopy and photometry have revealed the remarkableevolution of Sakurai's Object from 1996 to the present. A cooling,carbon-rich photospheric spectrum was observable from 1996 to 1998.Considerable changes occured in 1998 as the continuum reddened due toabsorption and emission by newly formed dust located outside thephotosphere. In addition, a strong and broad helium 1.083 m P Cygniline developed, signifying the acceleration of an outer envelope ofmaterial to speeds as high as 1000 km s^-1. At the same time thephotosphere of the central star remained quiescent. By 1999 thephotosphere was virtually completely obscured by the dust and the heliumemission line was the only detectable spectral feature remaining in the1–5 m band. In 2000 emission by dust has become even more dominant,as the envelope continues to expand and cool and the helium line weakens.     

Modelling the dust around Sakurai's Object

We follow the evolution of the infrared excess and the dustshell of Sakurai's Object by modelling the dust emission. The optical depth,dust temperature, and shell thickness parameters from the models are presented.Fits of DUSTY models to infrared spectra between 1997 May and 1999September illustrate the development of the dust from small grains to a rangeof grain sizes with an almost constant 60% graphite – 40% amorphous carbonmixture. The size of the infrared emitting region continues to growand may now be big enough to resolve. This may help to answerquestions such as the morphology of the dust shell, and whether the dustshell is `blobby' or smooth.     

Modeling the evolution of Sakurai's Object

Sakurai's Object is a born again AGBstar of the very late thermal pulse flavor. In thiscontribution I will discuss new models of stellar evolution andnucleosynthesis models of this phase. Two most intriguing properties ofSakurai's Object have so far not been understood theoretically: the peculiar chemical appearance, in particular the high lithiumabundance, and the short time scale of only a few years on which thetransition from the dwarf configuration into the born again giantappearance has occurred. A new nucleosynthesis mode of hot hydrogen-deficient ³He burning can explain the extraordinarylithium abundance. During the thermal pulse ³He is ingested fromthe envelope together with the protons into the hot He-flashconvection zone. The first network calculations show that, due to thelarge ¹²C abundance protons are captured rather by carbon, thandestroy newly formed ⁷Be and ultimately ⁷Li. Moreover, the shortevolution time scale has been reproduced by making the assumption that the convective efficiency forelement mixing is smaller by two to three orders of magnitude thanpredicted by the mixing-length theory. As a result the main energygeneration from fast convective proton capture will occur at a largermass coordinate, closer to the surface and the expansion to the giantstate is accelerated to a few years, in excellent agreement with thebehavior of Sakurai's Object. This result represents an independent empiricalconstraint on the poorly known efficiency of element mixing inconvective zones of the stellar interior.     

Sakurai's Object: the web page Open Discussion

At the end of the workshop the contributors bounced some ideas aroundand shared their thoughts about maximizing the advancement thata study of Sakurai's Object can provide. In particular it was resolvedto set up a web site http://astro.uibk.ac.at/sakurai, witha finding chart and a record of data obtained, and to maintain theoriginal workshop web site at http://www.astro.keele.ac.uk/sakurai. Observers and theoreticians are urged to consult,and to contribute to, this resource.     

Theoretical-Observational Comparisons and Details in the Evolution of Sakurai's Object

We create a grid of evolutionary models which include models thatexhibit born-again behavior; that is a very late helium pulse. Ourmodels include metalicities between Z = 0.001 and Z = 0.020. Massloss is varied beginning at the peak of the last thermal pulse onthe AGB. By doing this, we determine the range of helium mass atAGB departure that later produces a very late helium flash. Wepresent a direct comparison between our models, Sakurai's Object andFG Sge. Based on our comparisons, we make an observable predictionfor the future of Sakurai's Object: We expect it to increase intemperature and decrease slightly in luminosity within the next 20to 70 years and then to cool and brighten a second time with alonger time scale of roughly 200 to 500 years. It will become asFG Sge is now.     

Near Infrared Observations of Sakurai's Object during 1998–99

We present new near-infrared observations of Sakurai's Object obtainedduring 1998–99 when this final helium shell flash object was in the dustcondensation phase. The infrared colours have reddened compared to earlierepochs, indicating increased dust condensation. The infrared spectrareveal all the features of a carbon star superposed on a dust continuum.     

Infrared Spectroscopy of V4334 Sgr (Sakurai's Object)

IR spectroscopy and photometry in the 0.8–2.4 and 3–14 mregions are reported for seven dates between March 21 1998 and July20 2000 UT. The shorter wavelength region displays a smooth continuum increasing to longerwavelengths that is indicative of the Wien tail of a Planck function. Only theHe I 1.0830 line is present early and it shows a P-Cygni profile which laterdisappears. The long wave spectra show a smooth continuum between 3 and 13m that was well fit by a gray body at 1000 K. A weak, unidentifiedemission feature appears between 8 and 10 m.     

Optical photometry of V4334 Sgr (Sakurai's Object)

The optical photometric evolution of the final helium flash object V4334 Sgr from 1994 to 2001 is described. The rise to optical maximum (1994–1996) is characterized by a continuous increase of colour indices, indicating a slowly expanding, cooling pseudo-photosphere. This photosphere became stationary in 1997. In the following years, the object underwent brightnessdeclines of increasing strength, which are similar in character to the `reddeclines' of RCB stars. The fading of V4334 Sgr is more dramatic than anybrightness decline of an RCB star: at present, only 10^-5 of the visual light reaches the observer. Most short-wavelength radiation is absorbed by a dust envelopethat completely surrounds the star, and is re-radiated in the infrared. The sparse optical data of 2000–2001 show that the obscuration has not increased in strength any more. The light curve of V4334 Sgr is similar to that of the final helium flash object V605 Aql which erupted in 1919.     

V4334 Sgr (Sakurai's Object): The Distance Problem

The central star V4334 Sgr (Sakurai's Object) of the planetary nebula PN G010.4+04.4 underwent in 1995–1996 the rare event of a very late helium flash. It represents only one out of two such events during the era of modern astronomy (the other event was V605 Aql = Nova Aql 1919). All the other prominent objects of that type originate from events occurring several thousands of years ago (e.g. A30, A78). Thus it is of special interest for stellar evolution theory to model the detailed observations obtained during the last four years. Those models depend essentially on basic stellar parameters like effective temperature, surface gravity and stellar radius. Most of them depend strongly on the assumed distance to the object. Some models may give some constraints on this parameter, but most of them depend on the assumption as input parameter. Hence to determine a reliable distance is of considerable significance. This should be obtained through models that give us lower and upper boundaries, or through means which are independent of models. The detailed review, by using every kind of determination available up to now, leads to a Galactic foreground extinction of E _B–V =0^m75 ±0.05 and a distance of D = 2.0_-0.6 ^+1.0 kpc.     

The radio emission from Sakurai's Object (beyond 1 mm)

Observations of V4334 Sgr have been made with the JCMT, MERLIN, ATCAand the VLA. Searches with JCMT for CO in and around the PNassociated with V4334 Sgr has provided upper limits for the peakemission of 20 mK and the line-flux of 0.17 K-km s^-1. Thisgives an upper limit on the CO mass of 3 × 10^-7D² M at D kpc. VLA observations havedetected 2.7 ± 0.1 mJy of emission from the PN, which appears to beclumpy and extends to a radius of 17 arcsec. This indicates anoptically thick nebula, and suggests a range of distances of 1.83 to4.96 kpc for a reasonable range of masses for optically thick PN.Comparison with estimated total H emission gives E(B-V) =0.8 ±0.1. MERLIN observations do not detect the wind indicated tobe present in IR observations prior to dust obscuration. TheAustralia Telescope Compact Array did not detect OH maser emissionfrom either the main- or satellite-lines. Future VLA observationswill increase integration times and uv-coverage to significantlyimprove the radio imaging. MERLIN target-of-opportunity time isretained in order to attempt to image any fast wind as it develops.An independent estimate of the PN mass would allow the use of theradio observations to determine the distance; conversely anindependent distance measurement would lead to a PN mass estimate.The author urges the acquisition of an accurate measurement of thetotal H emission from the PN.     

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

We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around  4.7 μm  , we determine the excitation conditions in the line-forming region. We find  ¹²C/¹³C = 3.5^+2.0_−1.5  , 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 × 10⁻⁶≤ M _CO≤ 2.7 × 10⁻⁶ M_⊙  of CO ejecta outside the dust, forming a high-velocity wind of  500 ± 80 km s⁻¹  . 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.     

HST Observations of Sakurai's Object (V4334 Sgr) and Related Stars

We describe our ongoing program of HST observations of Sakurai's Object(V4334 Sgr). Direct WFPC2 imaging from August 1996 through August 2000 revealsno transient features (such as light echoes), and documents the decline of thestar to below 24th visual magnitude in 2000. The surrounding planetary nebulahas shown no changes from 1996 through 2000. There are no obvious peculiarfeatures (such as blobs or knots) in the immediate vicinity of the star. Wealso have in place a target-of-opportunity program to obtain UV spectra withHST in the event that the star begins to retrace its evolution back to highsurface temperature. We also present older HST FOC imaging of V605 Aql. The central object is aresolved nebula that emits in [O III] (but not in hydrogen), whose 0.6diameter is consistent with a dust cloud ejected during the 1919 outburst. Thecentral star itself is not seen due to its being embedded in the nebula.Several other central stars (including H 3-75, IC 2120, and Abell 14) havelate-type nuclei and no evidence for hot companions. They may be furthercandidates for born-again red-giant nuclei.     

Modeling of atmospheres and spectral energy distributions of R CrB and V4334 Sgr (Sakurai's Object)

Procedures and results of computations of model atmospheres andspectral energy distributions of R CrB, V4334 Sgr and relatedobjects are discussed. The sequence of SEDs computed for a gridof model atmospheres with 4000 < T_eff < 7000, 0 < logg < 1 provides a unique possibility to analyze the temporalchanges of physical parameters of V 4334 Sgr and related objectsin the framework of a self-consistent approach.     

Infrared spectroscopy of Sakurai's object

We present near (ground-based) and far ( ISO ) infrared spectroscopy of Sakurai's object. As in the case of the optical spectrum, between 1996 and 1997 April the near-infrared spectrum underwent a dramatic change to later spectral type, and there is some evidence that the spectrum continued to evolve during 1997. Molecular features of carbon-bearing molecules (CN, C₂, CO) — corresponding to those seen in cool carbon stars — are now prominent in the 1–2.5 μ m range, and the ¹²C/¹³C ratio is low. The ISO data demonstrate the presence of hot circumstellar dust at a temperature of ∼ 680 K. If the dust shell is optically thin, the dust mass is ∼ 2.8 × 10⁻⁸ M⊙.     

Possible detection of V4334 Sgr (Sakurai's Object) at 450 and 850 μm

We report the possible detection of V4334 Sgr (Sakurai's Object) at 450 and 850 μm with SCUBA on the James Clerk Maxwell Telescope. The submillimetre photometry, combined with a  1–5 μm  spectrum and  8–10 μm  photometry obtained nearly contemporaneously, suggests that the submillimetre emission originates in material ejected during the 1995 event. The dust mass is a  few×10^-7 M_⊙  , the average mass-loss in the form of dust is  few×10^-8 M_⊙ yr^-1  , and the integrated luminosity is  log( L /L_⊙)=3.66  for a distance of 2 kpc. The ejected shell had angular diameter ∼55 mas in 2001 August, and should by now be resolvable in the mid-infrared by  8–10 m  class telescopes.     

Sakurai's object: the ionized nebula at radio wavelengths

Sakurai's object (V4334 Sgr) is a planetary nebula nucleus which is undergoing its final helium shell flash. This is the first of these rare and important events to be observable with non-optical instruments. We report the first radio detection, using a short (2-h) observation with the Very Large Array (VLA) at 4.86 GHz. The radio emission structure is coincident with the 34-arcsec diameter planetary nebula seen in optical emission lines. We find a statistical distance ∼ 3.8 ± 0.6 kpc, with a range of 1.9 <  D  < 5.3 kpc, depending on the planetary nebula (PN) mass. While we have no direct evidence for a new (post-flash) stellar wind, we estimate an upper limit to the mass-loss rate due to any such wind of 1.7 × 10⁻⁷ M⊙ yr⁻¹. The number of emitting knots in the radio-visible nebula indicates an electron density of ∼ 2 × 10⁸ m⁻³ in those knots, and a total emitting ionized mass of ∼ 0.15 M⊙, at an assumed distance of 3.8 kpc. The radio flux density indicates an Hβ flux of ∼ 6 × 10⁻¹⁶ W m⁻², suggesting an extinction E ( B  −  V ) ∼ 1.15, comparable with reddening estimates in the direction of V4334 Sgr.