We present high-resolution (R ~ 50, 000) observations of near-IR transitions of CO and CN of the five known hydrogen-deficient carbon (HdC) stars and four R Coronae Borealis (RCB) stars. We perform an abundance analysis of these stars by using spectrum synthesis and state-of-the-art MARCS model atmospheres for cool hydrogen-deficient stars. Our analysis confirms reports by Clayton and colleagues that those HdC stars exhibiting CO lines in their spectrum and the cool RCB star S Aps are strongly enriched in 18O (with 16O/18O ratios ranging from 0.3 to 16). Nitrogen and carbon are in the form of 14N and 12C, respectively. Elemental abundances for CNO are obtained from C I, C2, CN, and CO lines. Difficulties in deriving the carbon abundance are discussed. Abundances of Na from Na I lines and S from S I lines are obtained. Elemental and isotopic CNO abundances suggest that HdC and RCB stars may be related objects, and that they probably formed from a merger of an He white dwarf with a C-O white dwarf.

/pdf/cno-abundances-of-hydrogen-deficient-carbon-and-r-coronae-2sne61gl2p.pdf

Cno abundances of hydrogen-deficient carbon and r coronae borealis stars: a view of the nucleosynthesis in a white dwarf merger

The R Coronae Borealis (RCB) stars are hydrogen-deficient, variable stars that are most likely the result of He-CO WD mergers. They display extremely low oxygen isotopic ratios, {sup 16}O/{sup 18}O {approx_equal} 1-10, {sup 12}C/{sup 13}C {>=} 100, and enhancements up to 2.6 dex in F and in s-process elements from Zn to La, compared to solar. These abundances provide stringent constraints on the physical processes during and after the double-degenerate merger. As shown previously, O-isotopic ratios observed in RCB stars cannot result from the dynamic double-degenerate merger phase, and we now investigate the role of the long-term one-dimensional spherical post-merger evolution and nucleosynthesis based on realistic hydrodynamic merger progenitor models. We adopt a model for extra envelope mixing to represent processes driven by rotation originating in the dynamical merger. Comprehensive nucleosynthesis post-processing simulations for these stellar evolution models reproduce, for the first time, the full range of the observed abundances for almost all the elements measured in RCB stars: {sup 16}O/{sup 18}O ratios between 9 and 15, C-isotopic ratios above 100, and {approx}1.4-2.35 dex F enhancements, along with enrichments in s-process elements. The nucleosynthesis processes in our models constrain the length and temperature in the dynamic merger shell-of-fire feature asmore » well as the envelope mixing in the post-merger phase. s-process elements originate either in the shell-of-fire merger feature or during the post-merger evolution, but the contribution from the asymptotic giant branch progenitors is negligible. The post-merger envelope mixing must eventually cease {approx}10{sup 6} yr after the dynamic merger phase before the star enters the RCB phase.« less

/pdf/reproducing-the-observed-abundances-in-rcb-and-hdc-stars-2zj48ol7yl.pdf

Reproducing the Observed Abundances in RCB and HdC Stars with Post-double-degenerate Merger Models—Constraints on Merger and Post-merger Simulations and Physics Processes

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 an observation of the very late thermal pulse object V4334 Sgr (Sakurai’s Object) with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. The emission from 5‐ 38 μm is dominated by the still-cooling dust shell. A number of features are seen in absorption against the dust shell, which we attribute to HCN and polyyne molecules. We use these features to determine the 12 C/ 13 C ratio for the absorbing gas to be ∼3.2 +3.2; this implies that, despite the H-content of the molecules, the hydrocarbon-bearing gas must have originated in material produced in the very late thermal pulse. We see no evidence of emission lines, despite the recently-reported optical and radio observations that suggest the effective temperature of the stellar remnant is rising.

The Spitzer Infrared Spectrometer view of V4334 Sgr (Sakurai's Object)

Infrared spectroscopy and photometry have revealed the remarkable evolution 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 to absorption and emission by newly formed dust located outside the photosphere. In addition, a strong and broad helium 1.083 $\mu$m P Cygni line developed, signifying the acceleration of an outer envelope of material to speeds as high as 1000 km s$^{-1}$. At the same time the photosphere of the central star remained quiescent. By 1999 the photosphere was virtually completely obscured by the dust and the helium emission line was the only detectable spectral feature remaining in the 1-5 $\mu$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.

The Infrared Evolution of Sakurai's Object

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)

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 μm 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 μm band. In 2000 emission by dust has become even more dominant,as the envelope continues to expand and cool and the helium line weakens.

We present UKIRT UIST spectra of Sakurai’s Object (= V4334 Sgr) showing CO fundamentalband absorption features around 4.7 µm. The line centres are at a heliocentric radial velocity of −170 ± 30 km s −1 . The number and relative strengths of the lines indicate a CO gas temperature of 400 ± 100 K and a CO column density of 7 +3 −2 × 10 17 cm −2 . The gas was moving away from the central star at an average speed of ∼290 ± 30 km s −1 in 2003 September. The lines appeared sometime between mid-1999 (well after the opaque dust shell formed) and mid-2000, and may have been somewhat more blueshifted initially than they are now. The observed CO velocity and temperature indicate the continued presence of a fast wind in the object, previously seen in the He I 1.083-µm line beginning just prior to massive dust formation, and more recently in atomic and ionized lines. The dust continuum is consistent with a temperature of 350 ± 30 K, indicating continued cooling of the shell. The similar CO temperature suggests that the bulk of the CO absorption occurs just outside the dust continuum surface.

/pdf/warm-high-velocity-co-in-the-wind-of-sakurai-s-object-v4334-3vtjdo03l1.pdf

Warm high‐velocity CO in the wind of Sakurai's Object (= V4334 Sgr)

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The Infrared Evolution of Sakurai's Object

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

The Infrared Evolution of Sakurai's Object

Warm high‐velocity CO in the wind of Sakurai's Object (= V4334 Sgr)