Late Thermal Pulse Research Articles

Long- and short-term variability of the possible nascent planetary nebula IRAS 22568+6141: A late thermal pulse?

Long- and short-term variability of the possible nascent planetary nebula IRAS 22568+6141: A late thermal pulse?

Non-thermal radio emission in Sakurai's Object

The very late thermal pulse (VLTP) affects the evolution of sim 20<!PCT!> of 1--8\,$ M_ stars, repeating the last red giant phases within a few years and leading to the formation of a new, but hydrogen-poor, nebula within the old planetary nebula. The strong dust formation in the latter obscures the optical and near-infrared radiation of the star. We aimed to determine the reheating timescale of the central star in Sakurai's Object, which is an important constraint for the poorly understood VLTP evolution. We observed the radio continuum emission of Sakurai's Object for almost 20 years, from 2004 to 2023. Continuous, multi-frequency observations proved to be essential for distinguishing between phases dominated by photoionization and shock ionization. The flux density fluctuates by more than a factor of 40 within months to years. The spectral index remained negative between 2006 and 2017 and has been close to zero since 2019. The emission region has been only barely resolved since 2021. Non-thermal radio emission observed from 2004 to 2017 traces shocks induced by wind interactions due to discrete mass-loss events. Thermal emission dominates from 2019 to 2023 and may indicate photoionization of the nebula by the central star.

Optical tomography of the born-again ejecta of A 58

In a born-again planetary nebula (PN), processed H-deficient material has been injected inside the old, H-rich nebula as a result of a very late thermal pulse (VLTP) event. Long-slit spectra have been used to unveil the chemical and physical differences between these two structures, but the ejection and shaping processes still remain unclear. To peer into the morpho-kinematics of the H-deficient ejecta in the born-again PN A 58, we present the first integral field spectroscopic observations of a born-again PN as obtained with GTC MEGARA. We detect emission from the Hα, He I, [O III], [N II] and [S II] emission lines, which help us unveil the expansion patterns of the different structures. In combination with ALMA and Hubble Space Telescope data we have been able to produce a complete view of the H-deficient ionized and molecular ejecta in A 58. We propose an hourglass structure for the ionized material, which embraces molecular high-velocity polar components and is simultaneously bisected by an expanding toroidal molecular and dusty structure. Our results leverage the role of a companion in shaping the VLTP ejecta in this born-again PN.

Spatially resolved spectroscopic investigation of the born-again planetary nebula A 78

ABSTRACT We present the analysis of spatially resolved spectroscopic observations of the born-again planetary nebula (PN) A 78 that are used to investigate the chemistry and physical properties of its three main morphological components, namely the inner knots, its eye-like structure, and the low surface-brightness outer nebula. The H-poor chemical abundances of the inner knots confirm the born-again nature of A 78, with an N/O abundances ratio consistent with the predictions of very late thermal pulses (VLTP). On the other hand, the high Ne/O is not expected in VLTP events, which prompts a possible different evolutionary path may be involving a binary system. The low N/O ratio and He/H abundances of the outer shell are indicative of a low-mass progenitor, whereas the chemical abundances of the eye-like structure, which results from the interaction between the H-poor born-again material and the outer nebula, evidence their mixture. Unlike previous works, the extinction is found to be inhomogeneous, being much higher towards the H-poor inner knots, where the presence of large amounts of C-rich dust has been previously reported. Dust-rich material seems to diffuse into outer nebular regions, resulting in zones of enhanced extinction.

A closer look at low-mass post-AGB late thermal pulses

ABSTRACT Late thermal pulse (LTP) stellar evolution models experience a helium pulse that occurs following asymptotic giant branch (AGB) departure and causes a rapid looping evolution in the Hertzsprung–Russell (HR) diagram between the AGB and planetary nebula (PN) phase. The transient LTP phases only last decades to centuries while increasing and decreasing in temperature, luminosity, and size over orders of magnitude. LTP objects have often been described in the context of their more dramatic counterparts, very late thermal pulses (VLTPs). LTP stars do not evolve as quickly and do not become as hydrogen deficient as VLTP objects. They do not become conspicuous until after resembling a PN for thousands of years. We present stellar evolution calculations from the AGB to the PN phase for models over a range of metallicities from Z = 0.0015 to 0.03, and for masses 0.90, 1.2, and 2.0 ${\rm M_{\odot }}$. We focus in on our most dense series (1.2 ${\rm M_{\odot }}$, Z = 0.015) and designate a stratification of LTP types based on at what temperature they erupt, which may hint at the progenitor mass. We discuss one type that fits neither an LTP nor VLTP, which may offer an explanation for the star FG Sge. We present the time-scales during which LTP models heat up until reach peak helium-burning luminosity, during the rapid luminosity decline, and during the period of cooling and brightening, and we briefly discuss four LTP candidates.

Pre-explosion Helium Shell Flash in Type Ia Supernovae

I study the possibility that within the frame of the core degenerate (CD) scenario for type Ia supernovae (SNe Ia) the merger process of the core of the asymptotic giant branch (AGB) star and the white dwarf (WD) maintains an envelope mass of ≈0.03 M ⊙ that causes a later helium shell flash. I estimate the number of pre-explosion helium shell flash events to be less than a few per cent of all CD scenario SNe Ia. A helium shell flash while the star moves to the left on the HR diagram as a post-AGB star (late thermal pulse—LTP) or along the WD cooling track (very LTP—VLTP) causes the star to expand and become a “born again” AGB star. Merger remnants exploding while still on the AGB form hydrogen-polluted peculiar SNe Ia, while an explosion inside an inflated born-again star results in an early flux excess in the light curve of the SN Ia. The fraction of systems that might show an early flux excess due to LTP/VLTP is <few × 10−4 of all SNe Ia, much below the observed fraction. In the frame of the CD scenario SNe Ia with early flux excess result from SN ejecta collision with planetary nebula fallback gas, or from mixing of 56Ni to the outer regions of the SN ejecta. Ongoing sky surveys might find about one case per year where LTP/VLTP influences the SN light curve.

Spectral Variability of the Born-again Ejecta in A 58

Born-again planetary nebulae (PNs) allow investigating stellar evolution, dust production, and nebular shocks in human timescales. Here we present an analysis of multiepoch optical spectroscopic observations of the born-again PN A 58 around V605 Aql, which experienced a very late thermal pulse about a century ago. The H-deficient ejecta has experienced a considerable brightening in the time period considered, from 1996 to 2021, with notable changes also in many emission line ratios. Neither the reduction of the extinction caused by the dilution of the ejecta nor the increase of the ionizing photon flux from the central star seem capable to produce these spectral changes, which are instead attributed to shocks in the bipolar H-poor outflow, dissociating molecular material, and propagating through the outer nebula.

Common envelope evolution in born-again planetary nebulae – Shaping the H-deficient ejecta of A 30

ABSTRACT Born-again planetary nebulae (PNe) are extremely rare cases in the evolution of solar-like stars. It is commonly accepted that their central stars (CSPN) experienced a very late thermal pulse (VLTP), ejecting H-deficient material inside the evolved H-rich PN. Given the short duration of this event and the fast subsequent evolution of the CSPN, details of the mass ejection are unknown. We present the first morphokinematic model of the H-deficient material surrounding a born-again PN, namely A 30. New San Pedro Mártir observations with the Manchester Echelle Spectrograph were recently obtained to map the inner region of A 30 which are interpreted by means of the software shape in conjunction with HST WFC3 images. The shape morphokinematic model that best reproduces the observations is composed by a disrupted disc tilted 37° with respect to the line of sight and a pair of orthogonal opposite bipolar ejections. We confirm previous suggestions that the structures closer to the CSPN present the highest expansion velocities, that is, the disrupted disc expands faster than the farther bipolar features. We propose that the current physical structure and abundance discrepancy of the H-deficient clumps around the CSPN of A 30 can be explained by a common envelope phase following the VLTP event. Our proposed scenario is also compared with other known born-again PNe (A 58, A 78, HuBi 1, and the Sakurai’s Object).

Evolution of Hen 3-1357, the Stingray Nebula

ABSTRACTThe spectroscopic evolution of Hen 3-1357, the Stingray Nebula, is presented by analysing data from 1990 to 2021. High-resolution data obtained in 2021 with South African Large Telescope High Resolution Spectrograph and in 2009 with European Southern Observatory-Very Large Telescope UVES spectrograph are used to determine physical conditions and chemical abundances in the nebula. From comparison of these data with data from different epochs it is found that the intensity of highly ionized emission lines has been decreasing with time, while the emission of low-ionization lines has been increasing, confirming that the nebula is recombining, lowering its excitation class, as a consequence of the changes in the central star which in 2002 had an effective temperature of 60 000 K and from then it has been getting colder. The present effective temperature of the central star is about 40 000 K. It has been suggested that the central star has suffered a late thermal pulse and it is returning to the AGB phase. The nebular chemistry of Hen 3-1357 indicates that all the elements, except He and Ne, present subsolar abundances. The comparison of the nebular abundances with the values predicted by stellar nucleosynthesis models at the end of the AGB phase shows that the central star had an initial mass lower than 1.5 M⊙. We estimated the ADF(O+2) to be between 2.6 and 3.5.

Physical conditions and chemical abundances of the variable planetary nebula IC 4997

ABSTRACT The planetary nebula (PN) IC 4997 is one of a few rapidly evolving objects with variable brightness and nebular emission around a hydrogen-deficient star. In this study, we have determined the physical conditions and chemical abundances of this object using the collisionally excited lines (CELs) and optical recombination lines (ORLs) measured from the medium-resolution spectra taken in 2014 July with the FIbre-fed Échelle Spectrograph on the Nordic Optical Telescope at La Palma Observatory. We derived electron densities of ≳ 3 × 104 cm−3 and electron temperatures of ≳ 14 000 K from CELs, whereas cooler temperatures of ∼11 000 and ∼7000 K were obtained from helium and heavy element ORLs, respectively. The elemental abundances deduced from CELs point to a metal-poor progenitor with [O/H] ≲ −0.75, whereas the ORL abundances are slightly above the solar metallicity, [O/H] ≈ 0.15. Our abundance analysis indicates that the abundance discrepancy factors (ADFs ≡ ORLs/CELs) of this PN are relatively large: ADF(O2+) ≳ 8 and ADF(N2+) ≳ 7. Further research is needed to find out how the ADFs and variable emissions are formed in this object and whether they are associated with a binary companion or a very late thermal pulse.

Chemistry and physical properties of the born-again planetary nebula HuBi 1

ABSTRACT The central star of the planetary nebula (PN) HuBi 1 has been recently proposed to have experienced a very late thermal pulse (VLTP), but the dilution of the emission of the recent ejecta by that of the surrounding H-rich old outer shell has so far hindered confirming its suspected H-poor nature. We present here an analysis of the optical properties of the ejecta in the innermost regions of HuBi 1 using MEGARA high-dispersion integral field and OSIRIS intermediate-dispersion long-slit spectroscopic observations obtained with the 10.4-m Gran Telescopio de Canarias. The unprecedented tomographic capability of MEGARA to resolve structures in velocity space allowed us to disentangle for the first time the Hα and Hβ emission of the recent ejecta from that of the outer shell. The recent ejecta is found to have much higher extinction than the outer shell, implying the presence of large amounts of dust. The spatial distribution of the emission from the ejecta and the locus of key line ratios in diagnostic diagrams probe the shock excitation of the inner ejecta in HuBi 1, in stark contrast with the photoionization nature of the H-rich outer shell. The abundances of the recent ejecta have been computed using the mappings v code under a shock scenario. They are found to be consistent with a born-again ejection scenario experienced by the progenitor star, which is thus firmly confirmed as a new ‘born-again’ star.

V4334 Sgr (Sakurai’s Object): still churning out the dust

ABSTRACT We present a 0.8–2.5 µm spectrum of the very late thermal pulse object V4334 Sgr (Sakurai’s Object), obtained in 2020 September. The spectrum displays a continuum that rises strongly to longer wavelengths, and is considerably brighter than the most recent published spectrum obtained 7 yr earlier. At the longer wavelengths, the continuum is well fitted by a blackbody with a temperature of 624 ± 8 K. However, there is excess continuum at the shortest wavelengths that we interpret as being due to hot dust that has very recently formed in an environment with C/O ≃ 2.5. Other possible sources for this excess continuum are discussed – such as the stellar photosphere dimly seen through the dust shell, and light scattered off the inner wall of the dust torus – but these interpretations seem unlikely. Numerous emission lines are present, including those of He i, C i, [C i], and O i. Our observations confirm that emission in the He i 1.083 µm and [C i] 0.9827/0.9852 µm lines is spatially extended. The [C i] line fluxes suggest that the electron density increased by an order of magnitude between 2013 and 2020, and that these two lines may soon disappear from the spectrum. The flux ratio of the 1.083 and 2.058 µm He i lines is consistent with the previously assumed interstellar extinction. The stellar photosphere remains elusive, and the central star may not be as hot as suggested by current evolutionary models.

First Images of the Molecular Gas around a Born-again Star Revealed by ALMA

Abstract Born-again stars allow probing stellar evolution in human timescales and provide the most promising path for the formation of hydrogen-deficient post-asymptotic giant branch objects, but their cold and molecular components remain poorly explored. Here we present ALMA observations of V 605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated with a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of ≈1″ in diameter. The bulk of the molecular emission is interpreted as being produced in a radially expanding disk-like structure with an expansion velocity v exp ∼ 90 km s−1 and an inclination i ≈ 60° with respect to the line of sight. The observations also reveal a compact high-velocity component, v exp ∼ 280 km s−1, that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age τ ≲ 20 yr, which could either be material that is currently being ejected from V 605 Aql, or is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range M dust ∼ 0.2–8 × 10−3 M ⊙, depending on the dust absorption coefficient. The mass of the CO is M CO ≈ 1.1 × 10−5 M ⊙, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a 12C/13C ratio of 5.6 ± 0.6, which is consistent with the single stellar evolution scenario in which the star experienced a very late thermal pulse instead of a nova-like event as previously suggested.

The Missing Link? Discovery of Pulsations in the Nitrogen-rich PG 1159 Star PG 1144+005

Abstract Up to 98% of all single stars will eventually become white dwarfs—stars that link the history and future evolution of the Galaxy, and whose previous evolution is engraved in their interiors. Those interiors can be studied using asteroseismology, utilizing stellar pulsations as seismic waves. The pulsational instability strips of DA and DB white dwarf stars are pure, allowing for the important generalization that their interior structure represents that of all DA and DB white dwarfs. This is not the case for the hottest pulsating white dwarfs, the GW Vir stars: only about 50% of white dwarfs in this domain pulsate. Several explanations for the impurity of the GW Vir instability strip have been proposed, based on different elemental abundances, metallicity, and helium content. Surprisingly, there is a dichotomy that only stars rich in nitrogen, which by itself cannot cause pulsation driving, pulsate—the only previous exception being the nitrogen-rich nonpulsator PG 1144+005. Here, we report the discovery of pulsations in PG 1144+005 based on new observations. We identified four frequency regions: 40, 55, 97, and 112 day−1 with low and variable amplitudes of about 3–6 mmag and therefore confirm the nitrogen dichotomy. As nitrogen is a trace element revealing the previous occurrence of a very late thermal pulse (VLTP) in hot white dwarf stars, we speculate that it is this VLTP that provides the interior structure required to make a GW Vir pulsator.

Formation and fate of the born-again planetary nebula HuBi 1

ABSTRACT We present the first 3D radiation-hydrodynamic simulations on the formation and evolution of born-again planetary nebulae (PNe), with particular emphasis to the case of HuBi 1, the inside-out PN. We use the extensively tested guacho code to simulate the formation of HuBi 1 adopting mass-loss and stellar wind terminal velocity estimates obtained from observations presented by our group. We found that, if the inner shell of HuBi 1 was formed by an explosive very late thermal pulse (VLTP) ejecting material with velocities of ∼300 km s−1, the age of this structure is consistent with that of ≃200 yr derived from multi-epoch narrow-band imaging. Our simulations predict that, as a consequence of the dramatic reduction of the stellar wind velocity and photon ionizing flux during the VLTP, the velocity and pressure structure of the outer H-rich nebula are affected creating turbulent ionized structures surrounding the inner shell. These are indeed detected in Gran Telescopio Canarias MEGARA optical observations. Furthermore, we demonstrate that the current relatively low ionizing photon flux from the central star of HuBi 1 is not able to completely ionize the inner shell, which favours previous suggestions that its excitation is dominated by shocks. Our simulations suggest that the kinetic energy of the H-poor ejecta of HuBi 1 is at least 30 times that of the clumps and filaments in the evolved born-again PNe A 30 and A 78, making it a truly unique VLTP event.

New models for the rapid evolution of the central star of the Stingray Nebula

Abstract We present stellar evolution calculations from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PN) phase for models of initial mass 1.2 M⊙ and 2.0 M⊙ that experience a Late Thermal Pulse (LTP), a helium shell flash that occurs following the AGB and causes a rapid looping evolution between the AGB and PN phase. We use these models to make comparisons to the central star of the Stingray Nebula, V839 Ara (SAO 244567). The central star has been observed to be rapidly evolving (heating) over the last 50 to 60 years and rapidly dimming over the past 20–30 years. It has been reported to belong to the youngest known planetary nebula, now rapidly fading in brightness. In this paper we show that the observed timescales, sudden dimming, and increasing Log(g), can all be explained by LTP models of a specific variety. We provide a possible explanation for the nebular ionization, the 1980’s sudden mass loss episode, the sudden decline in mass loss, and the nebular recombination and fading.

Carbon dust in the evolved born-again planetary nebulae A 30 and A 78

ABSTRACT We present an infrared (IR) characterization of the born-again planetary nebulae (PNe) A 30 and A 78 using IR images and spectra. We demonstrate that the carbon-rich dust in A 30 and A 78 is spatially coincident with the H-poor ejecta and coexists with hot X-ray-emitting gas up to distances of 50 arcsec from the central stars of PNe (CSPNe). Dust forms immediately after the born-again event and survives for 1000 yr in the harsh environment around the CSPN as it is destroyed and pushed away by radiation pressure and dragged by hydrodynamical effects. Spitzer IRS spectral maps showed that the broad spectral features at 6.4 and 8.0 μm, attributed to amorphous carbon formed in H-deficient environments, are associated with the disrupted disc around their CSPN, providing an optimal environment for charge exchange reactions with the stellar wind that produces the soft X-ray emission of these sources. Nebular and dust properties are modelled for A 30 with cloudy taking into account different carbonaceous dust species. Our models predict dust temperatures in the 40–230 K range, five times lower than predicted by previous works. Gas and dust masses for the born-again ejecta in A 30 are estimated to be $M_\mathrm{gas}=4.41^{+0.55}_{-0.14}\times 10^{-3}$ M⊙ and $M_\mathrm{dust}=3.20^{+3.21}_{-2.06}\times 10^{-3}$ M⊙, which can be used to estimate a total ejected mass and mass-loss rate for the born-again event of $7.61^{+3.76}_{-2.20}\times 10^{-3}$ M⊙ and $\dot{M}=(5{\!-\!}60)\times 10^{-5}$ M⊙ yr−1, respectively. Taking into account the carbon trapped into dust grains, we estimate that the C/O mass ratio of the H-poor ejecta of A 30 is larger than 1, which favours the very late thermal pulse model over the alternate hypothesis of a nova-like event.

Discovery of a Fast-expanding Shell in the Inside-out Born-again Planetary Nebula HuBi 1 through High-dispersion Integral Field Spectroscopy

Abstract HuBi 1 has been proposed to be member of the rare class of born-again planetary nebulae (PNe), i.e., its central star experienced a very late thermal pulse and ejected highly processed material at high speeds inside the old hydrogen-rich PN. In this Letter we present GTC MEGARA integral field spectroscopic observations of the innermost regions of HuBi 1 at high spectral resolution ≃16 km s−1 and multi-epoch subarcsecond images obtained ≃12 yr apart. The analysis of these data indicates that the inner regions of HuBi 1 were ejected ≃200 yr ago and expand at velocities ≃300 km s−1, in excellent agreement with the born-again scenario. The unprecedented tomographic capabilities of the GTC MEGARA high-dispersion observations used here reveal that the ejecta in HuBi 1 has a shell-like structure, in contrast to the disrupted disk and jet morphology of the ejecta in other born-again PNe.

The cooling-down central star of the planetary nebula SwSt 1: a late thermal pulse in a massive post-AGB star?

ABSTRACT SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a late [WC]-type central star. Previous studies suggested that the nebular and stellar lines are slowly changing with time. We studied new and archival optical and ultraviolet spectra of the object. The [O iii] 4959 and 5007 Å to H β line flux ratios decreased between about 1976 and 1997/2015. The stellar spectrum also shows changes between these epochs. We modelled the stellar and nebular spectra observed at different epochs. The analyses indicate a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and 1993. We do not detect significant changes between 1993 and 2015. The observations show that the star performed a loop in the H–R diagram. This is possible when a shell source is activated during its post-AGB evolution. We infer that a late thermal pulse (LTP) experienced by a massive post-AGB star can explain the evolution of the central star. Such a star does not expand significantly as the result of the LTP and does not became a born-again red giant. However, the released energy can remove the tiny H envelope of the star.

SDSS J124043.01+671034.68: the partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?

ABSTRACT The white dwarf SDSS J124043.01+671034.68 (SDSS J1240+6710) was previously found to have an oxygen-dominated atmosphere with significant traces of neon, magnesium, and silicon. A possible origin via a violent late thermal pulse or binary interactions has been suggested to explain this very unusual photospheric composition. We report the additional detection of carbon, sodium, and aluminium in far-ultraviolet and optical follow-up spectroscopy. No iron-group elements are detected, with tight upper limits on titanium, iron, cobalt, and nickel, suggesting that the star underwent partial oxygen burning, but failed to ignite silicon burning. Modelling the spectral energy distribution and adopting the distance based on the Gaia parallax, we infer a low white dwarf mass, $M_\mathrm{wd}=0.41\pm 0.05\, \mathrm{M}_\odot$. The large space velocity of SDSS J1240+6710, computed from the Gaia proper motion and its radial velocity, is compatible with a Galactic rest-frame velocity of ≃ 250 km s−1 in the opposite direction with respect to the Galactic rotation, strongly supporting a binary origin of this star. We discuss the properties of SDSS J1240+6710 in the context of the recently identified survivors of thermonuclear supernovae, the D6 and LP 40−365 stars, and conclude that it is unlikely related to either of those two groups. We tentatively suggest that SDSS J1240+6710 is the partially burned remnant of a low-mass white dwarf that underwent a thermonuclear event.