Final Helium Flash Research Articles

Evolutionary Status of W Vir Pulsating Variables

Stellar evolution calculations for population II stars with initial composition $Y_0=0.25$, $Z_0=10^{-3}$ and the initial stellar mass $M_0 = 0.82M_\odot$ were carried out from the main sequence to the white dwarf stage. Twelve AGB and post--AGB evolutionary sequences were computed with different values of the parameter in the Blocker mass loss rate formula ($0.01\le\eta_B\le 0.12$). Selected models of evolutionary sequences with masses $M=0.536M_\odot$, $0.530M_\odot$ and $0.526M_\odot$ that experience the loop in the Hertzsprung--Russel diagram due to the final helium flash were used as initial conditions for solution of the equations of hydrodynamics describing radial stellar oscillations. The region of instability to radial fundamental mode pulsations is shown to extend from the asymptotic giant branch to effective temperatures as high as $T_\mathrm{eff}\approx 6\times 10^3$ K. Pulsation periods of hydrodynamic models are in the range from 15 to 50 day and agree with periods of W~Vir pulsating stars. The models of intermediate spectral type fundamental mode pulsators with periods $\Pi > 50$ day locate in the upper part of the Hertzsprung--Russel diagram in the region of semiregular pulsating variables. We conclude that W~Vir pulsating variables are the low--mass post--AGB stars that experience the final helium flash.

Radial Pulsations of Stars at the Stage of the Final Helium Flash

Stellar evolution calculations to the stage of the cooling white dwarf were done for population~I stars with masses on the main sequence $1M_\odot\le M_0\le 1.5M_\odot$. The final helium flash LTP is shown to occur in post--AGB stars with initial masses $1.3M_\odot\le M_0\le 1.32M_\odot$ for the overshooting parameter $f=0.016$. In the case of more effective overshooting ($f=0.018$) the final helium flash occurs at initial masses $1.28M_\odot\le M_0\le 1.3M_\odot$. Fivefold variations of the parameter responsible for the mass loss rate during the post--AGB stage do not affect occurrence of the final helium flash but lead to perceptible changes of the evolutionary time. Selected models of two evolutionary sequences with initial mass $M_0 = 1.3M_\odot$ computed with overshooting parameters $f=0.016$ and $f=0.018$ were used as initial conditions in solution of the equations of hydrodynamics describing radial oscillations of stars on the stage of the final helium flash at effective temperatures less than $10^4$ K. The maximum pulsation period $\Pi=117$ day determined for the evolutionary sequence $M_0=1.3M_\odot$, $f=0.016$ is in a good agreement with observational estimates of the period of FG Sge. The mass, the radius and the effective temperature of the star are $M=0.565M_\odot$, $R=126R_\odot$ and $T_\mathrm{eff}=4445$ K, respectively. At the same time the average period change rate of FG Sge from 1960 to 1990 is nearly three time larger than its theoretical estimate.

The hydrogen-deficient knot of the ‘born-again’ planetary nebula Abell 58 (V605 Aql)

We have analysed deep optical spectra of the 'born-again' planetary nebula Abell 58 and its hydrogen-deficient knot, surrounding V605 Aql, which underwent a nova-like eruption in 1919. Our analysis shows that the extinction towards the central knot is much higher than previously thought, with c(Hβ) = 2.0. The outer nebula is less reddened, with c(Hβ) = 1.04. We find that the outer nebula has a Ne/O ratio higher than the average PN value. The electron temperature we derive for the central knot varies widely depending on the diagnostic used. The [O iii] nebular-to-auroral transition ratio gives a temperature of 20 800 K, while the ratio of the [N ii] nebular and auroral lines gives Te= 15 200 K. The helium line ratios λ5876/ λ4471 and λ6678/λ4471 imply temperatures of 350 and 550 K, respectively. Weakly temperature-sensitive O ii recombination line ratios imply similarly low electron temperatures. Abundances derived from recombination lines are vastly higher than those found from collisionally excited lines, with the abundance discrepancy factor (ADF) for O2+reaching 89 - the second highest known value after that found for the hydrogen-deficient knots in Abell 30. The observed temperature diagnostics and abundances support the idea that, like Abell 30, the knot of Abell 58 contains some very cold ionized material. Although the central star is carbon-rich (C/O > 1), the knot is found to be oxygen-rich, a situation not predicted by the single-star 'born-again' theory of its formation. We compare the known properties of Abell 58 to those of Abell 30, Sakurai's Object and several novae and nova remnants. We argue that the abundances in the ejecta observed in A 30 and A 58 have more in common with neon novae than with Sakurai's Object, which is believed to have undergone a final helium flash. In particular, the C/O ratio of less than unity and the presence of substantial quantities of neon in the ejecta of both Abell 30 and Abell 58 are not predicted by very late thermal pulse models. Journal compilation © 2007 RAS.

V605 Aql: 80 Years after the Final Helium Shell Flash

We have obtained new optical spectra with the ESO/VLT of the final helium flash star, V605 Aql. These spectra indicate that V605 Aql has evolved significantly in only 80 years. It now has a T $_{\rm eff}\sim$ 95,000 K and has abundances similar to those seen in [WC] central stars but not to those of a typical RCB star.

The progenitor of the “born-again” core V605 Aql and the relation to its younger twin V4334 Sgr

We derived the properties of V605 Aql before the final helium flash pulse by studying its surrounding PN A58. Photoionizing models of our spectral data together with a new distance estimate and a closer look at the recombination timescales lead to a consistent model. Comparing our findings with the only hydrogen-poor twin, namely Sakurai's Object, we conclude that these born-again objects have normal PNe core masses. We are able to prove V605 Aql indeed to be, similar to V4332 Sgr, a very late thermal pulse object and to put constraints on the evolutionary time scales for the transition back to the AGB.

The evolution of V4334 Sgr (Sakurai's object) in the Mid-Infrared

Since Sakurai's object (V4334 Sgr) underwent a late helium flash in 1995, its fast evolution has been monitored with great eort. We present TIMMI and TIMMI 2 photometric observations covering a spectral range from 5 to 20m with a total of 8 filters. We found drastic changes in the spectral energy distribution indicating a drop of temperature in the dust envelope from 1000 to 600 K in half a year only. Combined with optical observations reported elsewhere, we conclude that the phase of dust formation has probably stopped in mid-2001 and that the dusty envelope has been expanding only, ever since. An expansion velocity of about 1000 km s 1 is required to explain the rapid cooling. Other optical and near-infrared observations support the existence of such high velocity flows in the envelope of V4334 Sgr. An estimate for the bolometric luminosity of 2000 L is derived for a distance of 1 kpc. Taking into account theoretical models a distance of 2 1 kpc to V4334 Sgr is suggested. As a consequence of its fast final helium flash evolution, Sakurai's object may start to destroy its surrounding dust in the not too distant future.

The light curve of V605 Aql – the `older twin' of Sakurai's Object

`Nova Aquilae No. 4', discovered in 1919, later renamed V605 Aql, was recognized to be a final helium flash object only in the 1980s. Clayton and De Marco (1997) gave a detailed description of the available spectroscopic and photometric material. Here we try to re-analyze the photometric record. The photographic material is still available and has been used to construct a revised light curve. Only fragmentsof the visual observations were published; the remainder appears to be lost.

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.

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.

The Rise and Fall of V4334 Sagittarii (Sakurai’s Object)

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 prediscovery 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 it is evolving similar to 20 times more slowly. Its luminosity increased by a factor 4 between 1996 and 1998. A comparison of timescales 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.

Mysterious eclipses of the central star of NGC 2346

An explanation is offered for the mysterious eclipses of the central star of the planetary nebula NGC 2346. For more than 80 years until November 1981, the star did not vary in brightness, but since that time large-amplitude eclipse events have been observed. It is proposed that this unique phenomenon is caused by dust grain condensation in a clumpy shell ejected by the hot subdwarf in the nucleus. Metals in the ejected matter rapidly condense when it reaches a distance of about 3400 solar radii and form an optically thick dust cloud which occults the central star. Six characteristics of the observations are explained by the model and four predictions are offered by which the model may be tested. It is suggested that the nucleus of NGC 2346 has just started its final helium flash.