Modeling the Dust Shell of V4334 Sgr

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 is only one of two such events during the era of modern astronomy (the other event was V605 Aql = Nova Aql 1919). All other prominentobjects of that type originate from events several thousands of years ago (e.g. A30, A78). Hence, only snapshots can be modeled for those objects.V4334 Sgr allows for the first time a dynamic consideration of the formation of the dust shell from the beginning.We present here a model which is able to describe the complete photometric behaviorof the object, including the fine structure dips of the optical light curve during the first two years of the mass loss and the dust formation.     

SAKURAI'S object (V4334 Sgr); The evolution of a final helium flash star

Sakurai's object(V4334 Sgr) in all likelihood is a star undergoing a final helium flash.Since its discovery in 1996 observers have been trying to keep up a monitoring of the amazingly fast changes of its stellar properties;changes occuring on time scales of a few weeks or months. By enabling us to follow the evolution of one object in `real time' it relieves us of the need to painstakingly assemble a consistent understanding from different objects, at the various stages of evolution that they have reached from differing starting conditions.Sakurai's object thereby offers a unique opportunity to study an important but short-lived aspect of regular stellar evolution, the final helium flash.     

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.     

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.     

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.     

Optical Spectroscopy of V4334 Sgr: 1996–2000

Due to a combination of rapid expansion and changes in chemical composition, the optical spectrum of V4334 Sgr changed rapidly between discovery in 1996 Februaryand extinction during 1999. As one of the most rapidly evolving stars ever observed, optical spectroscopy must answer a range of important questions.The development of the optical spectrum and its implications are discussedthrough a review of published literature. A previously unpublished sequence of spectra from near discovery through 1996 demonstrates the thermal evolution of V4334 Sgr, while detailed analysis of an unpublished echelle spectrumobtained in 1996 May raises questions about the model atmospheres used to date.As a result, requirements for future models of the atmosphere of V4334 Sgr are analysed. The future evolution of V4334 Sgr and the dispersion of its dustycocoon are briefly discussed.     

Pulsation–decline relationships in R Coronae Borealis stars

Decline onset times were measured in long-term visual light curves for five R Coronae Borealis (RCB) variable stars. These included RY Sgr and V854 Cen, the two RCB stars previously reported to have a relationship between dust formation events and pulsational variations. Analysis of the decline epochs showed that all decline onsets for a given star obey a linear ephemeris tied to the object's dominant radial pulsation period. Thus, in addition to confirming the pulsation–decline correlation for RY Sgr and V854 Cen, this same behaviour was demonstrated in UW Cen, R CrB and S Aps for the first time. This general result firmly establishes the connection between radial pulsation and mass loss in the RCB stars. The dominant pulsation period of ≈40 d for all of these objects therefore represents the characteristic time-scale on which these stars produce dust.     

Infrared photometry of Sakurai’s object (V4334 Sgr) in 1996–1999

The infrared photometric observations of V4334 Sgr in 1996–1999 are presented. Together with optical data, they have allowed us to accurately estimate the bolometric flux from this star and to investigate the structure of its dust envelope over the above period. The star is shown to have passed through four well-defined stages in these four years as it moved backward along the post-AGB track, and it now appears to have started moving forward after a halt. At the first stage (1996), there was no dust in the star’s envelope. Its visual brightness slightly increased, and it reddened in the entire observed spectral range. The bolometric flux also gradually rose. At the second stage (1997), an optically thick dust envelope condensed around the star, which, however, essentially did not manifest itself at optical wavelengths. The bolometric flux continued to rise through an increase in the star’s infrared brightness alone; the rate of its rise also increased. At the third stage (1998–March 1999), V4334 Sgr entered the R CrB phase. First two shallow minima and then two deep minima were observed at optical wavelengths. The star appreciably reddened during the deep minima. The bolometric flux ceased to rise and began to gradually fall in the second half of 1998. At the fourth stage (since March 1999 up until now), V4334 Sgr has been at a protracted deep minimum, which is atypical of the R CrB stars. The bolometric flux between March and October underwent no significant variations. We describe the structure of the dust envelope around V4334 Sgr since its formation. From June 1997 until July 1998, the optical depth of the dust shell, its inner and outer radii, and its mass increased by factors of ～2.2, ～2.0, 2.3, and ～10, respectively. In July 1998, τ(V)≈2.3, R _{d, in}≈7.4×10¹⁴ cm, R _{d, in}/R _{d, out}≈0.7(R _{d, in}/R _*≈47), and M _dust≈1.6×10^?7 M _⊙.     

High-resolution spectroscopy of V854 Cen in decline – absorption and emission lines of C2 molecules

High-resolution optical spectra of the R Coronae Borealis (RCB) star V854 Centauri in the early stages of a decline show, in addition to the features reported for other RCBs in decline, narrow absorption lines from the C₂ Phillips system. The low rotational temperature, T _rot=1150 K, of the C₂ ground electronic state suggests the cold gas is associated with the developing shroud of carbon dust. These absorption lines were not seen at a fainter magnitude on the rise from minimum light, nor at maximum light. This is the first detection of cold gas around an RCB star.     

CK Vul: reborn perhaps, but not hibernating

It has been claimed that CK Vul (the remnant of Nova Vul 1670) may be the oldest recovered 'old nova' and as such provides evidence in support of the hibernation scenario for classical nova systems. However this interpretation has been challenged. We present 450- and 850-μm photometry of CK Vul which cast further doubt on its old nova status. It displays a large far infrared-submillimetre flux excess, inconsistent with the properties of an old nova. Furthermore, IRAS images show that CK Vul is located in a 'cavity' in the infrared emission, a feature often associated with planetary nebulae. It seems more likely that CK Vul – and hence Nova Vul 1670 – is (like V605 Aql and V4334 Sgr) an evolved star in the throes of a final thermal pulse.     

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.     

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

We report the appearance and evolution during 1998 of strong neutral helium ³S–³P^o absorption at ∼10 830 Å in Sakurai's Object (V4334 Sgr), which is believed to be a 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⁻¹. The strong C  i 10 690-Å 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 stars.     

The born-again (very late thermal pulse) scenario revisited: the mass of the remnants and implications for V4334 Sgr

We present 1D numerical simulations of the very late thermal pulse (VLTP) scenario for a wide range of remnant masses. We show that by taking into account the different possible remnant masses, the observed evolution of V4334 Sgr (a.k.a. Sakurai's object) can be reproduced within the standard 1D mixing length theory (MLT) stellar evolutionary models without the inclusion of any ad hoc reduced mixing efficiency. Our simulations hint at a consistent picture with present observations of V4334 Sgr. From energetics, and within the standard MLT approach, we show that low-mass remnants  ( M ≲ 0.6 M_⊙)  are expected to behave markedly differently from higher mass remnants  ( M ≳ 0.6 M_⊙)  in the sense that the latter remnants are not expected to expand significantly as a result of the violent H-burning that takes place during the VLTP. We also assess the discrepancy in the born-again times obtained by different authors by comparing the energy that can be liberated by H-burning during the VLTP event.     

Photometric and spectroscopic variability of the pre‐main sequence star V1184 Tauri (CB 34V)

We present recent results from optical photometric and spectroscopic observations of the pre‐main sequence star V1184 Tau (CB 34V). The star is associated with the Bok globule CB 34 and was considered as a FUOR candidate in previous studies. Our photometric data obtained from October 2000 to April 2003 show that the stellar brightness varies with an amplitude of about 0.m 5 (I ), but from August 2003 the photometric behavior of the star has changed dramatically. Three deep brightness minima (ΔI ∼ 4^m.2) were observed during the past two years. The analysis of available photometric data suggests that V1184 Tau shows two types of variability produced (1) by rotation of large cool spotted surface and (2) by occultation from circumstellar clouds of dust or from features of a circumstellar disk. The behavior of the V – I index indicates that the star becomes redder towards minimum light, but from a certain turning point (V ∼ 18^m.2) it gets bluer and is fading further. Five medium dispersion optical spectra of V1184 Tau were obtained in the period 2001–2004. Signi.cant changes in the profile and strength of the emission lines in the spectrum of V1184 Tau were found. During minimum light the equivalent width of the Hα emission line increases from 4 Å to 9 Å. The [O I] lines (λλ 6003, 6363 Å) are also seen in emission while the sodium doublet keeps its absorption strength and equivalent width. The possibility to reconstruct the historical light curve of V1184 Tau using photographical plate archives is brie.y discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Supernovae in helium star–compact object binaries: a possible γ-ray burst mechanism

Helium star–compact object binaries, and helium star–neutron star binaries in particular, are widely believed to be the progenitors of the observed double-neutron-star systems. In these, the second neutron star is presumed to be the compact remnant of the helium star supernova. In this paper, the observational implications of such a supernova are discussed, and in particular are explored as a candidate γ-ray burst mechanism. In this scenario, the supernova results in a transient period of rapid accretion on to the compact object, extracting via magnetic torques its rotational energy at highly super-Eddington luminosities in the form of a narrowly beamed, strongly electromagnetically dominated jet. Compton scattering of supernova photons advected within the ejecta, and photons originating at shocks driven into the ejecta by the jet, will cool the jet and can produce the observed prompt emission characteristics, including the peak-inferred isotropic energy relation, X-ray flash characteristics, subpulse light curves, energy-dependent time lags and subpulse broadening, and late time spectral softening. The duration of the burst is limited by the rate of Compton cooling of the jet, eventually creating an optically thick, moderately relativistically expanding fireball that can produce the afterglow emission. If the black hole or neutron star stays bound to a compact remnant, late term light curve variability may be observed as in SN 2003dh.     

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.     

The evolution of a rapidly accreting helium white dwarf to become a low-luminosity helium star

We have examined the evolution of merged low-mass double white dwarfs which become low-luminosity (or high-gravity) extreme helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a helium white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. As the helium-burning shell moves inwards with repeating shell flashes, the effective temperature gradually increases as the star evolves towards the helium main sequence. When the mass interior to the helium‐burning shell is approximately 0.25 M_⊙, the star enters a regime where it is pulsationally unstable. We have obtained radial pulsation periods for these models.
These models have properties very similar to those of the pulsating helium star V652 Her. We have compared the rate of period change of the theoretical models with that observed in V652 Her, as well as with its position on the Hertzsprung–Russell diagram. We conclude that the merger between two helium white dwarfs can produce a star with properties remarkably similar to those observed in at least one extreme helium star, and is a viable model for their evolutionary origin. Such helium stars will evolve to become hot subdwarfs close to the helium main sequence. We also discuss the number of low-luminosity helium stars in the Galaxy expected for our evolution scenario.     

Evolution of the circumstellar shell of pleione (1981–94)

Three spectra of Pleione (1981, 1987–88, 1994), obtained in the blue wavelength range, show strong long-term changes which indicate that the surrounding shell, first rather cool and close to the central star, then expands gradually in the equatorial plane of the star and towards the poles. From 1981 to 1987–88, the spectrum of Pleione is that of the central star, with a cool shell which globally mimics a late-type star. The year 1994 is marked by a pure and stronger Be spectrum, with no shell line.     

The R Coronae Borealis Connection

The R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich supergiants which undergo spectaculardeclines in brightness of up to 8 magnitudes at irregular intervals as dust forms along the line of sight.Understanding the RCB stars is a key test for any theory whichaims to explain hydrogen deficiency in post-Asymptotic Giant Branch (AGB) stars. There are two major evolutionary models for the origin of RCB stars: the Double Degenerate and the Final Helium ShellFlash. In the final flashmodel, there is a close relationship between RCB stars and Planetary Nebulae (PNe). The connection between RCB stars and PNe has recentlybecome stronger, since the central stars of three old PNe (Sakurai's Object, V605 Aql and FG Sge) have had observedoutbursts that transformed them from hot evolved central stars into cool giants with the spectral properties of an RCB star.     

Possible effects of internal rotation in low-mass stars

The influence of internal rotation on the evolution of a 0.85M star is investigated by the construction of model sequences. Rotation is treated by a simple one-dimensional approximation. The calculations assume solid-body rotation on the zero-age Main Sequence, followed by conservation of angular momentum in shells. The 4 cases considered have the initial angular velocities 0,2×10^–4, 6×10^–4, and 8×10^–4/sec. All cases but the last are followed to helium ignition. Compared with the non-rotating case, the rotating models are older at Main-Sequence turnoff, develop fast-spinning central regions on the red-giant branch, and ignite helium at higher surface luminosities and at larger helium-core masses. The increases in the last two quantities are roughly proportional to the square of the initial angular velocity.The 6×10^–4 case is followed through the helium core flash to the zero-age horizontal branch. Under the assumption of spherical symmetry, the non-central ignition of helium leads to a sequence of flashes of decreasing amplitude occurring progressively closer to the center. The flashes are weaker than those encountered in previous studies and do not produce mixing.