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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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 _⊙.     

Episodic absorption in the outflow of the old nova V603 Aquilae

We report on the time-dependent behaviour of ultraviolet spectral lines in Hubble Space Telescope Goddard High-Resolution Spectrograph data of the classical nova V603 Aql. In particular, episodic blueshifted absorption (extending to ∼−2500 km s⁻¹) is present, with a variability time-scale down to ∼1 min. The data provide a rare opportunity to study the rapid evolution of absorption structures that may be associated with accretion-disc winds in cataclysmic variables. At least three absorption events are recorded (at blueward velocities only) over ∼5 h, each lasting ∼10–15 min. The derived velocity, acceleration and optical depth properties provide an empirical picture of stochastically variable structures in the outflow, with no evidence for short-term (less than ∼1 h) cyclic or modulated behaviour in the overall absorption properties. In contrast, the emission components of the ultraviolet resonance lines are very stable in velocity and strength in this low-inclination system. On at least two occasions there is an intriguing short-term 'flare' in the ultraviolet continuum flux (of up to ∼40 per cent). Though there is no clear one-to-one relation in these data between the continuum fluctuations and the occurrence of the absorption events, the time-scales for the two variable phenomena are essentially the same. The irregular absorption episodes in the ultraviolet data of V603 Aql presently defy a clear physical interpretation. Their overall characteristics are discussed in the context of instabilities in radiation-pressure-driven disc winds.     

Are Sakurai’s object and Nova Aquilae 1919 recurrent outbursts of classical novae?

It is proposed to interpret the outbursts of Nova Aquilae 1919 (V605 Aql) and the nova-like outburst of Sakurai’s object in 1996 as recurrent outbursts of classical novae of the DQ Her and RR Pic types with recurrence periods of about 1100 and 50,000 years, respectively, and some characteristics of these stars are derived on that basis. Only now has V605 Aql returned to the quiet state, and it has an absolute visual magnitude no fainter than 8^m−9^m. The orbital inclination of the prenova’s binary system is about 90°. The absolute visual magnitude of the prenova of V4334 Sgr is no brighter than 3^m. It is quite possible that the orbital inclination of the prenova’s binary system is close to 0°. The expansion velocities of the shells are 30–40 km/sec in both cases. The structure of the ejection is conserved in the recurrent outbursts. Translated from Astrofizika, Vol. 42, No. 4, pp. 563–570, October–December, 1999.     

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.     

An analysis of early spectroscopic observations and the light curve of the nova V1663 Aql (Nova Aql 2005)

I report the results of a spectroscopic follow‐up of the nova V1663 Aql performed at the Loiano telescope. The emission lines in the spectra have been identified to study the early evolution. The spectra measured some weeks after the maximum show Balmer lines, several Fe multiplets and low ionization lines. The presence of Fe multiplets confirms the original classification of V1663 Aql as a Fe II nova. I have built the light curve using the observations of IAU circulars, VSNET, AFOEV, ASAS databases to estimate the parameters of the nova: time and magnitude of maximum, and the decline rates by two or three magnitudes. The parameters have been used to deduce a distance of V1663 Aql in the range 7.3÷11.3 kpc. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The spectroscopic evolution of V2467 Cyg (Nova Cygni 2007) in the first months after the outburst

We present the spectroscopy of nova V2467 Cyg acquired at the Loiano Observatory, Italy, during the first six months after the outburst. We have used the optical spectroscopy to study the physical properties of the ejected material and the photometry to estimate the nova distance. V2467 Cyg is a fast nova, with decline rates by two or three magnitudes of 7.6 and 14.6 days respectively. The light curve exhibited oscillations during the transition stage. The nova achieved an absolute magnitude at maximum in the range –8.5… –9.1. The distance is in the range 2.6… 3.6 kpc. V2467 Cyg showed an early appearance of forbidden lines during the transition stage. Its evolution is similar to the behavior of V1494 Aql (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The continuing saga of Sakurai's object (V4334 Sgr): dust production and helium line emission

We report further UKIRT spectroscopic observations of Sakurai's object (V4334 Sgr) made in 1999 April/May in the 1–4.75 μm range, and find that the emission is dominated by amorphous carbon at T _d~600 K. The estimated maximum grain size is 0.6 μm, and the mass lower limit is 1.7±0.2×10⁻⁸ M_⊙ to 8.9±0.6×10⁻⁷ M_⊙ for distances of 1.1–8 kpc. For 3.8 kpc the mass is 2.0±0.1×10⁻⁷ M_⊙.
We also report strong He  i emission at 1.083 μm, in contrast to the strong absorption in this line in 1998. We conclude that the excitation is collisional, and is probably caused by a wind, consistent with the P Cygni profile observed by Eyres et al. in 1998.     

Infrared photometry of Sakurai’s object (V4334 Sgr) in 2000

The infrared photometric observations of V4334 Sgr in 2000 are presented. They show that a gradual, but nonmonotonic increase in the optical depth of its dust envelope, which was formed early in 1997, had continued until the mid-summer. In July 1999 and July 2000, τ(1.25 µm)≈7.7 and 11.3, respectively. From July through October 2000, the optical depth decreased appreciably. From October 1998 (the first deep minimum of visual brightness) until now, the amplitude of the bolometric-magnitude variations in V4334 Sgr is $ \sim 0^m .5$ . The relation between the bolometric and L magnitudes (m _bol, L) can be fitted by a linear function, m _bol = 1.25L + 4.04. In the dust-envelope model chosen, the percentage of large (a _gr=0.2–0.3 µm) dust grains by particle number increased by a factor of ～4. In the summer of 2000, their fraction by mass was ～78%, and they mainly contributed to the optical depth of the dust envelope. No appreciable correlation between optical depth and bolometric flux was observed.     

Spectroscopic observations of the classical symbiotic star V1413 AQL at the phase of its transition to quiescence

We present the results of our spectroscopic and photometric observations of the classical symbiotic star V1413 Aql in 2003–2007, which cover the transition phase of its hot component from activity to quiescence. Various quiescence criteria for the hot component of the system are analyzed. Judging by its photometric characteristics, the system had not yet returned to quiescence by 2007, although a fairly strong He II λ4686 Å line observed previously during quiescence in 1993 appeared in its spectrum. We model the continuum energy distribution for V1413 Aql based on a standard three-component model and a model with an accretion disk. Analysis of the forbidden [O III] and [Ne III] lines shows that the neon abundance in V1413 Aql may be enhanced with respect to the oxygen abundance. The cool component of V1413 Aql has been found to be a variable M5-type red giant, with the variability amplitude at λ = 7500 Å being at least 2 ^m .     

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.