Helium Flash Research Articles

The enigmatic He-sdB pulsator LS IV-14°116: new insights from the VLT

The intermediate Helium subdwarf B star LS IV$-$14$^\circ$116 is a unique object showing extremely peculiar atmospheric abundances as well as long-period pulsations that cannot be explained in terms of the usual opacity mechanism. One hypothesis invoked was that a strong magnetic field may be responsible. We discredit this possibility on the basis of FORS2 spectro-polarimetry, which allows us to rule out a mean longitudinal magnetic field down to 300 G. Using the same data, we derive the atmospheric parameters for LS IV$-$14$^\circ$116 to be $T_{\rm eff}$ = 35,150$\pm$111 K, $\log{g}$ = 5.88$\pm$0.02 and $\log{N(\rm He)/N(\rm H)}$ = $-$0.62$\pm$0.01. The high surface gravity in particular is at odds with the theory that LS IV$-$14$^\circ$116 has not yet settled onto the Helium Main Sequence, and that the pulsations are excited by an $\epsilon$ mechanism acting on the Helium-burning shells present after the main Helium flash. Archival UVES spectroscopy reveals LS IV$-$14$^\circ$116 to have a radial velocity of 149.1$\pm$2.1 km/s. Running a full kinematic analysis, we find that it is on a retrograde orbit around the Galactic centre, with a Galactic radial velocity component $U$=13.23$\pm$8.28 km/s and a Galactic rotational velocity component $V$=$-$55.56$\pm$22.13 km/s. This implies that LS IV$-$14$^\circ$116 belongs to the halo population, an intriguing discovery.

ON THE SERENDIPITOUS DISCOVERY OF A Li-RICH GIANT IN THE GLOBULAR CLUSTER NGC 362

We have serendipitously identified the first lithium-rich giant star located close to the red giant branch bump in a globular cluster. Through intermediate-resolution FLAMES spectra we derived a lithium abundance of A(Li)=2.55 (assuming local thermodynamical equilibrium), which is extremely high considering the star's evolutionary stage. Kinematic and photometric analysis confirm the object as a member of the globular cluster NGC 362. This is the fourth Li-rich giant discovered in a globular cluster but the only one known to exist at a luminosity close to the bump magnitude. The three previous detections are clearly more evolved, located close to, or beyond the tip of their red giant branch. Our observations are able to discard the accretion of planets/brown dwarfs, as well as an enhanced mass-loss mechanism as a formation channel for this rare object. Whilst the star sits just above the cluster bump luminosity, its temperature places it towards the blue side of the giant branch in the colour-magnitude diagram. We require further dedicated observations to unambiguously identify the star as a red giant: we are currently unable to confirm whether Li production has occurred at the bump of the luminosity function or if the star is on the pre zero-age horizontal branch. The latter scenario provides the opportunity for the star to have synthesised Li rapidly during the core helium flash or gradually during its red giant branch ascent via some extra mixing process.

The RCB star V854 Centauri is surrounded by a hot dusty shell

Aims : The hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars might be the result of a double-degenerate merger of two white dwarfs (WDs), or a final helium shell flash in a planetary nebula central star. In this context, any information on the geometry of their circumstellar environment and, in particular, the potential detection of elongated structures, is of great importance. Methods : We obtained near-IR observations of V854 Cen with the AMBER recombiner located at the Very Large Telescope Interferometer (VLTI) array with the compact array (B$\leq$35m) in 2013 and the long array (B$\leq$140m) in 2014. At each time, V854 Cen was at maximum light. The $H$- and $K$-band continua were investigated by means of spectrally dependant geometric models. These data were supplemented with mid-IR VISIR/VLT images. Results : A dusty slightly elongated over density is discovered both in the $H$- and $K$-band images. With the compact array, the central star is unresolved ($\Theta\leq2.5$\,mas), but a flattened dusty environment of $8 \times 11$ mas is discovered whose flux increases from about $\sim$20% in the $H$ band to reach about $\sim$50% at 2.3$\micron$, which indicates hot (T$\sim$1500\,K) dust in the close vicinity of the star. The major axis is oriented at a position angle (P.A.) of 126$\pm$29$\deg$. Adding the long-array configuration dataset provides tighter constraints on the star diameter ($\Theta\leq1.0$ mas), a slight increase of the overdensity to $12 \times 15$ mas and a consistent P.A. of 133$\pm$49$\deg$. The closure phases, sensitive to asymmetries, are null and compatible with a centro-symmetric, unperturbed environment excluding point sources at the level of 3% of the total flux in 2013 and 2014. The VISIR images exhibit a flattened aspect ratio at the 15-20% level at larger distances ($\sim$1$\arcsec$) with a position angle of 92$\pm$19$\deg$, marginally consistent with the interferometric observations. Conclusions : This is the first time that a moderately elongated structure has been observed around an RCB star. These observations confirm the numerous suggestions for a bipolar structure proposed for this star in the literature, which were mainly based on polarimetric and spectroscopic observations.

Weak metal lines in optical high-resolution Very Large Telescope and Keck spectra of “cool” PG 1159 stars

PG 1159 stars are very hot (effective temperatures Teff = 75 000–200 000 K), hydrogen-deficient (pre-) white dwarfs. They probably are the result of a late helium-shell flash that laid bare the He, C, and O rich intershell matter of the progenitor Asymptotic Giant Branch (AGB) star. Their chemical surface composition thus allows to conclude on details of AGB-star nucleosynthesis. Due to their very high effective temperatures, detailed spectral analyses are usually completely reliant on ultraviolet observations, except for some species in the hottest PG 1159 stars (Teff > 130 000 K), which do exhibit highly excited lines from the CNO elements and neon (C iv ,N v ,O vi ,N evii-viii) in optical spectra. Particularly problematic are, however, the coolest members of the PG 1159 class that exclusively show C iv lines in the optical. Access to the nitrogen abundance is important to decide which of the late-thermal pulse evolutionary scenarios was experienced by a particular star, while a high oxygen abundance is an important marker that the star could pulsate. In the present paper, we investigate high-resolution high signal-to-noise optical spectra of three “cool” PG 1159 stars (PG 0122+200, PG 2131+066, MCT 0130−1937, Teff = 80 000–95 000 K). With the help of non-LTE model atmospheres and synthetic spectra, we are able to identify a large number of weak CNO lines (C iii ,N iv ,O iii-v) that were not detected before in these stars. They allow abundance determinations and enable us to constrain the effective temperature to high precision through ionization equilibria without the requirement to access the ultraviolet spectral range.

Numerical Experiments for Nuclear Flashes toward Superbursts in an Accreting Neutron Star

We show that the superburst would be originated from thermonuclear burning ignited by accumulated fuels in the deep layers compared to normal X-ray bursts. Two cases are investigated for models related to superbursts by following thermal evolution of a realistic neutron star: helium flash and carbon flash accompanied with many normal bursts. For a helium flash, the burst shows the long duration when the accretion rate is low compared with the observation. The flash could become a superburst if the burning develops to the deflagration and/or detonation. For a carbon flash accompanied with many normal bursts, after successive 2786 normal bursts during 1.81 × 109 s, the temperature reaches the deflagration temperature. This is due to the produced carbon which amount reaches to ≈0.1 in the mass fraction. The flash will develop to dynamical phenomena of the deflagration and/or detonation, which may lead to a superburst.

Evolution of the habitable zone of low-mass stars

We study the temporal evolution of the habitable zone (HZ) of low-mass stars - only due to stellar evolution - and evaluate the related uncertainties. These uncertainties are then compared with those due to the adoption of different climate models. We computed stellar evolutionary tracks from the pre-main sequence phase to the helium flash at the red-giant branch tip for stars with masses in the range [0.70 - 1.10] Msun, metallicity Z in the range [0.005 - 0.04], and various initial helium contents. We evaluated several characteristics of the HZ, such as the distance from the host star at which the habitability is longest, the duration of this habitability, the width of the zone for which the habitability lasts one half of the maximum, and the boundaries of the continuously habitable zone (CHZ) for which the habitability lasts at least 4 Gyr. We developed analytical models, accurate to the percent level or lower, which allowed to obtain these characteristics in dependence on the mass and the chemical composition of the host star. The metallicity of the host star plays a relevant role in determining the HZ. The initial helium content accounts for a variation of the CHZ boundaries as large as 30% and 10% in the inner and outer border. The computed analytical models allow the first systematic study of the variability of the CHZ boundaries that is caused by the uncertainty in the estimated values of mass and metallicity of the host star. An uncertainty range of about 30% in the inner boundary and 15% in the outer one were found. We also verified that these uncertainties are larger than that due to relying on recently revised climatic models, which leads to a CHZ boundaries shift within 5% with respect to those of our reference scenario. We made an on-line tool available that provides both HZ characteristics and interpolated stellar tracks.

OLD PUZZLE, NEW INSIGHTS: A LITHIUM-RICH GIANT QUIETLY BURNING HELIUM IN ITS CORE

About 1% of giant stars have been shown to have large surface Li abundances, which is unexpected according to standard stellar evolution models. Several scenarios for lithium production have been proposed, but it is still unclear why these Li-rich giants exist. A missing piece in this puzzle is the knowledge of the exact stage of evolution of these stars. Using low-and-high-resolution spectroscopic observations, we have undertaken a survey of lithium-rich giants in the Kepler field. In this letter, we report the finding of the first confirmed Li-rich core-helium-burning giant, as revealed by asteroseismic analysis. The evolutionary timescales constrained by its mass suggest that Li-production most likely took place through non-canonical mixing at the RGB-tip, possibly during the helium flash.

Bayesian isochrone fitting and stellar ages

Stellar evolution theory has been extraordinarily successful at explaining the different phases under which stars form, evolve and die. While the strongest constraints have traditionally come from binary stars, the advent of asteroseismology is bringing unique measures in well-characterised stars. For stellar populations in general, however, only photometric measures are usually available, and the comparison with the predictions of stellar evolution theory have mostly been qualitative. For instance, the geometrical shapes of isochrones have been used to infer ages of coeval populations, but without any proper statistical basis. In this chapter we provide a pedagogical review on a Bayesian formalism to make quantitative inferences on the properties of single, binary and small ensembles of stars, including unresolved populations. As an example, we show how stellar evolution theory can be used in a rigorous way as a prior information to measure the ages of stars between the ZAMS and the Helium flash, and their uncertainties, using photometric data only.

EVOLUTION OF THE 1919 EJECTA OF V605 AQUILAE,

New imaging of V605 Aql, was obtained in 2009 with HST/WFPC2, which had a nova-like outburst in 1919, and is located at the center of the planetary nebula (PN), Abell 58. This event has long been ascribed to a final helium shell flash, but it has been suggested recently that it may instead have been an ONe nova. The new images provide an 18 year baseline for the expansion of the ejecta from the 1919 event. In addition, the central star has been directly detected in the visible for the first time since 1923, when it faded from sight due to obscuration by dust. The expansion of the ejecta has a velocity of ~200 km/s, and an angular expansion rate of ~10 mas/yr, consistent with a 1919 ejection. This implies a geometric distance of 4.6 kpc for V605 Aql, consistent with previous estimates. The gas mass in the central knot of ejecta was previously estimated to be 5 x 10^-5 M(Sun). It is estimated that warm dust associated with this gas has a mass of ~10^-5 M(Sun). There is also evidence for a significant amount, 10^-3 M(Sun), of cold (75 K) dust, which may be associated with its PN. The knot ejected in 1919 is asymmetrical and is approximately aligned with the asymmetry of the surrounding PN. Polarimetric imaging was obtained to investigate whether the 2001 spectrum of V605 Aql was obtained primarily in scattered light from dust in the central knot, but the signal-to-noise in the data was insufficient to measure the level of polarization.

The orbital periods of subdwarf B binaries produced by the first stable Roche Lobe overflow channel

Long-orbital-period subdwarf B (sdB) stars with main-sequence companions are believed to be the product of stable Roche Lobe overflow (RLOF), a scenario challenged by recent observations. Here we represent the results of a systematic study of the orbital-period distribution of sdB binaries in this channel using detailed binary evolution calculations. We show that the observed orbital-period distribution of long-period sdB binaries can be well explained by this scenario. Furthermore, we find that, if the progenitors of the sdB stars have initial masses below the helium flash mass, the sdB binaries produced from stable RLOF follow a unique mass -- orbital period relation for a given metallicity $Z$; increasing the orbital period from $\sim 400$ to $\sim 1100$\,d corresponds to increasing the mass of the sdB star from $\sim 0.40$ to $\sim 0.49\,M_\odot$ for $Z=0.02$. We suggest that the longest sdB binaries (with orbital period $> 1100$\,d) could be the result of atmospheric RLOF. The mass -- orbital period relation can be tested observationally if the mass of the sdB star can be determined precisely, e.g.\ from asteroseismology. Using this relation, we revise the orbital period distribution of sdB binaries produced by the first stable RLOF channel for the best fitting model of Han et al (2003), and show that the orbital period has a peak around 830\,d.

SERENDIPITOUS DETECTION OF X-RAY EMISSION FROM THE HOT BORN-AGAIN CENTRAL STAR OF THE PLANETARY NEBULA K 1-16

We report the serendipitous detection of point-like X-ray emission from the hot, PG1159-type central star of the planetary nebula (CSPN) K 1-16 by the XMM-Newton and Chandra X-Ray Observatories. The CSPN lies superimposed on a galaxy cluster that includes an X-ray-bright quasar, but we have successfully isolated the CSPN X-ray emission from the strong diffuse background contributed by the quasar and intracluster gas. We have modeled the XMM-Newton and Chandra X-ray data, taking advantage of the contrasting detection efficiencies of the two observatories to better constrain the low-energy spectral response of Chandra's Advanced CCD Imaging Spectrometer. We find that the CSPN X-ray spectrum is well characterized by the combination of a non-local thermodynamic equilibrium model atmosphere with T ~ 135 kK and a carbon-rich, optically thin thermal plasma with T X ~ 1 MK. These results for X-ray emission from the K 1-16 CSPN, combined with those obtained for other PG1159-type objects, lend support to the born-again scenario for Wolf-Rayet and PG1159 CSPNe, wherein a late helium shell flash dredges up carbon-rich intershell material and ejects this material into the circumstellar environment.

The stratified evolution of a cool star

AbstractA low mass star usually experiences stratification and abundance anomalies during its evolution. A 0.95 M⊙ star with a metallicity Z = 0.004 is followed from the main–sequence to the horizontal branch (HB). On the main sequence the larger effects of stratification may come from accretion as was suggested in relation to metallicity and planet formation. As it evolves through the giant branch, stratification appears around the hydrogen burning shell. It may create hydrodynamic instabilities and be related to abundance anomalies on the giant branch. After the helium flash the star evolves to the HB. If it loses enough mass, it ends up a hot HB star (or in the field an sdB star) with effective temperatures larger than 11000 K. All sdB stars are observed to have approximately solar iron abundance whatever their original metallicity, implying overabundances by factors of up to 100. So should the 0.95 solar mass star. How its internal hydrodynamic properties on the main sequence may influence its fate on the HB is currently uncertain. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

NSV 11749: Symbiotic Nova, Not a Born-Again Red Giant

ABSTRACTNSV 11749 is a little-studied variable star, discovered by W. J. Luyten, which had a long-duration outburst around the year 1903, reaching blue magnitude 12.5 at maximum. Following the outburst, it has apparently been quiescent at about blue magnitude 17 for the past century. It was recently suggested that NSV 11749 may have been a low- or intermediate-mass star that underwent a final helium shell flash, making it temporarily a “born-again” red giant. If so, it would be only the fourth known member of this class, along with V605 Aql, FG Sge, and V4334 Sgr. However, our newly obtained optical and near-IR spectra of the object show that it is instead a symbiotic binary, with strong Balmer and He I-II emission lines, combined with a cool red-giant companion of spectral type M1-2 III. The 1903 outburst was most likely a symbiotic nova event, of which less than a dozen are known at present.

THE HELIUM CONTENT OF GLOBULAR CLUSTERS: NGC 6121 (M4)

He has been proposed as a key element to interpret the observed multiple MS, SGB, and RGB, as well as the complex horizontal branch (HB) morphology. Stars belonging to the bluer part of the HB, are thought to be more He rich (\Delta Y=0.03 or more) and more Na-rich/O-poor than those located in the redder part. This hypothesis was only partially confirmed in NGC 6752, where stars of the redder zero-age HB showed a He content of Y=0.25+-0.01, fully compatible with the primordial He content of the Universe, and were all Na-poor/O-rich. Here we study hot blue HB (BHB) stars in the GC NGC 6121 (M4) to measure their He plus O/Na content. We observed 6 BHB stars using the UVES@VLT2 spectroscopic facility. In addition to He, O, Na, and Fe abundances were estimated. Stars turned out to be all Na-rich and O-poor and to have a homogeneous enhanced He content with a mean value of Y=0.29+-0.01(random)+-0.01(systematic). The high He content of blue HB stars in M4 is also confirmed by the fact that they are brighter than red HB stars (RHB). Theoretical models suggest the BHB stars are He-enhanced by \Delta Y=0.02-0.03 with respect to the RHB stars. The whole sample of stars has a metallicity of [Fe/H]=-1.06+-0.02 (internal error). This is a rare direct measurement of the (primordial) He abundance for stars belonging to the Na-rich/O-poor population of GC stars in a temperature regime where the He content is not altered by sedimentation or extreme mixing as suggested for the hottest, late helium flash HB stars. Our results support theoretical predictions that the Na-rich/O-poor population is also more He-rich than the Na-poor/O-rich generation and that a leading contender for the 2^{nd} parameter is the He abundance.

Grids of stellar models with rotation

[abridged] Many topical astrophysical research areas, such as the properties of planet host stars, the nature of the progenitors of different types of supernovae and gamma ray bursts, and the evolution of galaxies, require complete and homogeneous sets of stellar models at different metallicities in order to be studied during the whole of cosmic history. We present here a first set of models for solar metallicity, where the effects of rotation are accounted for in a homogeneous way. We computed a grid of 48 different stellar evolutionary tracks, both rotating and non-rotating, at Z=0.014, spanning a wide mass range from 0.8 to 120 Msun. For each of the stellar masses considered, electronic tables provide data for 400 stages along the evolutionary track and at each stage, a set of 43 physical data are given. These grids thus provide an extensive and detailed data basis for comparisons with the observations. The rotating models start on the ZAMS with a rotation rate Vini/Vcrit=0.4. The evolution is computed until the end of the central carbon-burning phase, the early AGB phase, or the core helium-flash for, respectively, the massive, intermediate, and both low and very low mass stars. The initial abundances are those deduced by Asplund and collaborators, which best fit the observed abundances of massive stars in the solar neighbourhood. We update both the opacities and nuclear reaction rates, and introduce new prescriptions for the mass-loss rates as stars approach the Eddington and/or the critical velocity. We account for both atomic diffusion and magnetic braking in our low-mass star models. [...]

Effects of a New Triple- Reaction on X-Ray Bursts of a Helium-Accreting Neutron Star

The effects of a new triple-$\alpha$ reaction rate (OKK rate) on the helium flash of a helium accreting neutron star in a binary system have been investigated. Since the ignition points determine the properties of a thermonuclear flash of type I X-ray bursts, we examine the cases of different accretion rates, $dM/dt (\dot{M})$, of helium from $3\times10^{-10} M_{\odot} \rm yr^{-1}$ to $3\times10^{-8} M_{\odot} \rm yr^{-1}$, which could cover the observed accretion rates. We find that for the cases of low accretion rates, nuclear burnings are ignited at the helium layers of rather low densities. As a consequence, helium deflagration would be triggered for all cases of lower accretion rate than $\dot{M}\simeq 3\times10^{-8} M_{\odot} \rm yr^{-1}$. We find that OKK rate could be barely consistent with the available observations of the X-ray bursts on the helium accreting neutron star. However this coincidence is found to depend on the properties of crustal heating and the neutron star model.We suggest that OKK rate would be reduced by a factor of $10^{2-3}$ for $10^8$ K in the range of the observational errors.

A NEW STELLAR MIXING PROCESS OPERATING BELOW SHELL CONVECTION ZONES FOLLOWING OFF-CENTER IGNITION

During most stages of stellar evolution the nuclear burning of lighter to heavier elements results in a radial composition profile which is stabilizing against buoyant acceleration, with light material residing above heavier material. However, under some circumstances, such as off-center ignition, the composition profile resulting from nuclear burning can be destabilizing, and characterized by an outwardly increasing mean molecular weight. The potential for instabilities under these circumstances, and the consequences that they may have on stellar structural evolution, remain largely unexplored. In this paper we study the development and evolution of instabilities associated with unstable composition gradients in regions which are initially stable according to linear Schwarzschild and Ledoux criteria. In particular, we explore the mixing taking place under various conditions with multi-dimensional hydrodynamic convection models based on stellar evolutionary calculations of the core helium flash in a 1.25 \Msun star, the core carbon flash in a 9.3\,\Msun star, and of oxygen shell burning in a star with a mass of 23\,\Msun. The results of our simulations reveal a mixing process associated with regions having outwardly increasing mean molecular weight that reside below convection zones. The mixing is not due to overshooting from the convection zone, nor is it due directly to thermohaline mixing which operates on a timescale several orders of magnitude larger than the simulated flows. Instead, the mixing appears to be due to the presence of a wave field induced in the stable layers residing beneath the convection zone which enhances the mixing rate by many orders of magnitude and allows a thermohaline type mixing process to operate on a dynamical, rather than thermal, timescale. We discuss our results in terms of related laboratory phenomena and associated theoretical developments.

STELLAR EVOLUTION CONSTRAINTS ON THE TRIPLE-α REACTION RATE

We investigate the quantitative constraint on the triple-alpha reaction rate based on stellar evolution theory, motivated by the recent significant revision of the rate proposed by nuclear physics calculations. Targeted stellar models were computed in order to investigate the impact of that rate in the mass range of 0.8 < M / Msun < 25 and in the metallicity range between Z = 0 and Z = 0.02. The revised rate has a significant impact on the evolution of low- and intermediate-mass stars, while its influence on the evolution of massive stars (M >~ 10 Msun) is minimal. We find that employing the revised rate suppresses helium shell flashes on AGB phase for stars in the initial mass range 0.8 < M / Msun < 6, which is contradictory to what is observed. The absence of helium shell flashes is due to the weak temperature dependence of the revised triple-alpha reaction cross section at the temperature involved. In our models, it is suggested that the temperature dependence of the cross section should have at least nu > 10 at T = 1 - 1.2 x 10^8 K where the cross section is proportional to T^{nu}. We also derive the helium ignition curve to estimate the maximum cross section to retain the low-mass first red giants. The semi-analytically derived ignition curves suggest that the reaction rate should be less than ~ 10^{-29} cm^6 s^{-1} mole^{-2} at ~ 10^{7.8} K, which corresponds to about three orders of magnitude larger than that of the NACRE compilation. In an effort to compromise with the revised rates, we calculate and analyze models with enhanced CNO cycle reaction rates to increase the maximum luminosity of the first giant branch. However, it is impossible to reach the typical RGB tip luminosity even if all the reaction rates related to CNO cycles are enhanced by more than ten orders of magnitude.

Two Centuries of Observing R Coronæ Borealis

AbstractR Coronæ Borealis was found to be variable in the year 1783, and was one of the first variable stars to be so identified. Its class, the R Coronæ Borealis (RCB) stars, are rare hydrogen-deficient carbon-rich supergiants. RCB stars undergo massive declines of up to 8 mag due to the formation of carbon dust at irregular intervals. The mechanism of dust formation around RCB stars is not well understood, but the dust is thought to form in or near the atmosphere of the star. Their rarity may stem from the fact that they are in an extremely rapid phase of the evolution, or are in an evolutionary phase that most stars do not undergo. Several evolutionary models have been suggested to account for the RCB stars, including a merger of two white dwarfs (WDs) or a final helium-shell flash (FF) in a PN central star. The large overabundance of 18O found in most of the RCB stars favours the WD merger model, while the presence of Li in the atmospheres of five RCB stars favours the FF one. In particular, the measured isotopic abundances imply that many, if not most, RCB stars are produced by WD mergers, which may be the low-mass counterparts of the more massive mergers thought to produce type Ia supernovæ. Understanding these enigmatic stars depends to a large extent on continuous monitoring to catch their irregular but rapid variations caused by dust formation, their variations due to stellar pulsations, and long-term changes that may occur over centuries. I will use observations of R Coronæ Borealis obtained over 200 years to demonstrate what kinds of monitoring are necessary for these and similar classes of variables.

Multidimensional hydrodynamic simulations of the hydrogen injection flash

The injection of hydrogen into the convection shell powered by helium burning during the core helium flash is commonly encountered during the evolution of metal-free and extremely metal-poor low-mass stars. With specifically designed multidimensional hydrodynamic simulations, we aim to prove that an entropy barrier is no obstacle for the growth of the helium-burning shell convection zone in the helium core of a metal-rich Pop I star, i.e. convection can penetrate into the hydrogen-rich layers for these stars, too. We further study whether this is also possible in one-dimensional stellar evolutionary calculations. Our hydrodynamical simulations show that the helium-burning shell convection zone in the helium core moves across the entropy barrier and reaches the hydrogen-rich layers. This leads to mixing of protons into the hotter layers of the core and to a rapid increase of the nuclear energy production at the upper edge of the helium-burning convection shell - the hydrogen injection flash. As a result a second convection zone appears in the hydrogen-rich layers. Contrary to 1D models, the entropy barrier separating the two convective shells from each other is largely permeable to chemical transport when allowing for multidimensional flow, and consequently, hydrogen is continuously mixed deep into the helium core. We find it difficult to achieve such a behavior in one-dimensional stellar evolutionary calculations.