CCD UBVRi photometry of the final helium flash object V4334 Sgr (Sakurai's Object), carried out during 1997 - 1999, is presented, and the light curve from its pre-discovery rise to the dust obscuration phase is constructed. The optical light curve can be divided into four sections, the rise to maximum, the maximum, the dust onset, and the massive dust shell phase. The color indices show a general increase with time, first because of the photospheric expansion and cooling, and later because of the dust forming events. The energy distributions for the years 1996 - 1999 show that an increasing part of the energy is radiated at infrared wavelengths. In 1996, the infrared excess is likely caused by free-free radiation in the stellar wind. Starting from 1997 or 1998 at the latest, carbon dust grains are responsible for the more and more dramatic decrease of optical radiation and the growing infrared excess. Its photometric behavior in 1998 - 1999 mimics the ``red declines'' of R CrB variables, the amplitude, however, is more extreme than any fading ever observed in an R CrB star. Evidence is given that a complete dust shell has formed around V4334 Sgr. It therefore shows similarities with dust-forming classical novae, although evolving ~20 times more slowly. Its luminosity increased by a factor 4 between 1996 and 1998. A comparison of time scales of the final helium flash objects FG Sge, V605 Aql and V4334 Sgr shows that the observed photometric and spectroscopic features are similar, while V4334 Sgr is the most rapidly evolving object to date.

The rise and fall of V4334 Sgr (Sakurai's Object)

Context: In 1996, Sakurai's object (V4334 Sgr) suddenly brightened in the center of a faint Planetary Nebula (PN). This very rare event was interpreted as being the reignition of a hot white dwarf that caused a rapid evolution back to the cool giant phase. From 1998 on, a copious amount of dust has formed continuously, screening out the star that remained embedded in this expanding high optical-depth envelope. Aims: We present observations that we use to study the morphology of the circumstellar dust to investigate the hypothesis that Sakurai's Object is surrounded by a thick spherical envelope of dust. Methods: We acquired unprecedented, high angular-resolution, spectro-interferometric observations, with the mid-IR interferometer MIDI/VLTI, which resolved the dust envelope of Sakurai's object. Results: We report the discovery of a unexpectedly compact (30 × 40 milliarcsec, 105 × 140 AU assuming a distance of 3.5 kpc), highly inclined, dust disk. We used Monte Carlo radiative-transfer simulations of a stratified disk to constrain its geometric and physical parameters, although such a model is only a rough approximation of the rapidly evolving dust structure. Even though the fits are not fully satisfactory, some useful and robust constraints can be inferred. The disk inclination is estimated to be 75° ± 3° with a large scale height of 47 ± 7 AU. The dust mass of the disk is estimated to be 6 × 10-5~Mȯ. The major axis of the disk (132° ± 3°) is aligned with an asymmetry seen in the old PN which was re-investigated as part of this study. This implies that the mechanism responsible for shaping the dust envelope surrounding Sakurai's object was already at work when the old PN formed. Based on observations made with the Very Large Telescope Interferometer at Paranal Observatory under program 079.D-0415. Reduced visibilities and differential phases are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/493/L17

/pdf/a-dense-disk-of-dust-around-the-born-again-sakurai-s-object-17luh57tby.pdf

A dense disk of dust around the born-again Sakurai's object

In 1996, Sakurai's object (V4334 Sgr) suddenly brightened in the centre of a faint Planetary Nebula (PN). This very rare event was interpreted as the reignition of a hot white dwarf that caused a rapid evolution back to the cool giant phase. From 1998 on, a copious amount of dust has formed continuously, screening out the star which has remained embedded in this expanding high optical depth envelope. The new observations, reported here, are used to study the morphology of the circumstellar dust in order to investigate the hypothesis that Sakurai's Object is surrounded by a thick spherical envelope of dust. We have obtained unprecedented, high-angular resolution spectro-interferometric observations, taken with the mid-IR interferometer MIDI/VLTI, which resolve the dust envelope of Sakurai's object. We report the discovery of a unexpectedly compact (30 x 40 milliarcsec, 105 x 140 AU assuming a distance of 3.5 kpc), highly inclined, dust disk. We used Monte Carlo radiative-transfer simulations of a stratified disk to constrain its geometric and physical parameters, although such a model is only a rough approximation of the rapidly evolving dust structure. Even though the fits are not fully satisfactory, some useful and robust constraints can be inferred. The disk inclination is estimated to be 75+/-3 degree with a large scale height of 47+/-7 AU. The dust mass of the disk is estimated to be 6 10^{-5} solar mass. The major axis of the disk (132+/-3 degree) is aligned with an asymmetry seen in the old PN that was re-investigated as part of this study. This implies that the mechanism responsible for shaping the dust envelope surrounding Sakurai's object was already at work when the old PN formed.

/pdf/a-dense-disk-of-dust-around-the-born-again-sakurai-s-object-1uj0dbgvrl.pdf

We present new spectroscopic observations of the He I $\lambda$10830 line in R Coronae Borealis (RCB) stars which provide the first strong evidence that most, if not all, RCB stars have winds. It has long been suggested that when dust forms around an RCB star, radiation pressure accelerates the dust away from the star, dragging the gas along with it. The new spectra show that nine of the ten stars observed have P-Cygni or asymmetric blue-shifted profiles in the He I $\lambda$10830 line. In all cases, the He I line indicates a mass outflow - with a range of intensity and velocity. Around the RCB stars, it is likely that this state is populated by collisional excitation rather than photoionization/recombination. The line profiles have been modeled with an SEI code to derive the optical depth and the velocity field of the helium gas. The results show that the typical RCB wind has a steep acceleration with a terminal velocity of \Vinf = 200-350 \kms and a column density of N $\sim10^{12}$ cm$^{-2}$ in the He I $\lambda$10830 line. There is a possible relationship between the lightcurve of an RCB star and its He I $\lambda$10830 profile. Stars which have gone hundreds of days with no dust-formation episodes tend to have weaker He I features. The unusual RCB star, V854 Cen, does not follow this trend, showing little or no He I absorption despite high mass-loss activity. The He I $\lambda$10830 line in R CrB itself, which has been observed at four epochs between 1978 and 2001, seems to show a P-Cygni or asymmetric blue-shifted profile at all times whether it is in decline or at maximum light.

http://arxiv.org/pdf/astro-ph/0306099.pdf

Winds in R Coronae Borealis Stars

Aims. We investigated the cloud of ejecta resulting from the mass loss associated with the final helium shell flash in V605 Aql. Methods. V605 Aql was imaged at high spatial resolution in both optical emission lines and the infrared continuum using HST/WFPC2 and Gemini/Hokupa' a+QUIRC, respectively. The He I 10 830 A spectrum was also observed. Results. The obscuring circumstellar shell, whose effects were first seen in 1923, is shown to be a disk with extended structure, including knots. The morphology of the V605 Aql circumstellar shell is discussed as an 80 year old in an evolutionary sequence consisting of recent, as well as much older, final flash objects. Conclusions. The obscuration of V605 Aql by dust marked the emergence of the hot white dwarf remnant from an optically thick pseudo-photosphere. This white dwarf drives a 2500 km s -1 wind principally in one plane resulting in a circumstellar disk. Where the wind encounters the circumstellar disk, He I 10 830 A emission is created, and hot, ∼ 1500 K, grains are generated. Grains exit the disk at 350 K, and the accompanying gas then expands at ∼ 140 km s -1 . The strong concentration of the mass loss to the disk suggests the white dwarf is now rotating rapidly. There is convincing evidence in the literature that this process is also seen in V4334 Sgr and is still going on in the old final flash objects A30 and A78.

https://www.aanda.org/articles/aa/pdf/2008/09/aa7738-07.pdf

Imaging ejecta from the final flash star V605 Aquilae

Strong helium 10 830-Aabsorption in Sakurai's object (V4334 Sgr)

planetary nebula nucleus (PNN) undergoing a final helium shell-flash. First detected on 1998 March 18, the profile of the He i feature is P Cygni-like. The absorption depth has increased in three subsequent spectra in 1998. If this is owing to a wind, the profile indicates a wind velocity of ,670 ^ 50 km s. The strong C i 10 690-AE line seen prior to the appearance of the helium feature has disappeared; however Sr ii and CN absorption features remain present. We tentatively identify several new features as Si i. Taken together with other observations we suggest that the data are consistent with Sakurai's Object entering a phase in which it seems to have become a member of the R Coronae Borealis-type class of

/pdf/strong-helium-10-830-a-absorption-in-sakurai-s-object-v4334-2muu9l0cx0.pdf

Strong helium 10 830-a absorption in sakurai's object (v4334 sgr)

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Strong helium 10 830-Aabsorption in Sakurai's object (V4334 Sgr)

Strong helium 10 830-a absorption in sakurai's object (v4334 sgr)