Core Flash Research Articles

Discovery of hot subdwarfs covered with helium-burning ash

Abstract Helium-rich subdwarf O stars (sdOs) are hot compact stars in a pre-white dwarf evolutionary state. Most of them have effective temperatures and surface gravities in the range Teff = 40 000–50 000 K and log g = 5.5–6.0. Their atmospheres are helium dominated. If present at all, C, N, and O are trace elements. The abundance patterns are explained in terms of nucleosynthesis during single star evolution (late helium core flash) or a binary He-core white dwarf merger. Here we announce the discovery of two hot hydrogen-deficient sdOs (PG1654+322 and PG1528+025) that exhibit unusually strong carbon and oxygen lines. A non-LTE model atmosphere analysis of spectra obtained with the Large Binocular Telescope and by the LAMOST survey reveals astonishingly high abundances of C ($\approx 20{{\ \rm per\ cent}}$) and O ($\approx 20{{\ \rm per\ cent}}$) and that the two stars are located close to the helium main sequence. Both establish a new spectroscopic class of hot H-deficient subdwarfs (CO-sdO) and can be identified as the remnants of a He-core white dwarf that accreted matter of a merging low-mass CO-core white dwarf. We conclude that the CO-sdOs represent an alternative evolutionary channel creating PG1159 stars besides the evolution of single stars that experience a late helium-shell flash.

Non-local thermodynamic equilibrium spectral analysis of five hot, hydrogen-deficient pre-white dwarfs

Hot, compact, hydrogen-deficient pre-white dwarfs (pre-WDs) with effective temperatures ofTeff> 70 000 K and a surface gravity of 5.0 < logg< 7.0 are rather rare objects despite recent and ongoing surveys. It is believed that they are the outcome of either single star evolution (late helium-shell flash or late helium-core flash) or binary star evolution (double WD merger). Their study is interesting because the surface elemental abundances reflect the physics of thermonuclear flashes and merger events. Spectroscopically they are divided in three different classes, namely PG1159, O(He), or He-sdO. We present a spectroscopic analysis of five such stars that turned out to have atmospheric parameters in the rangeTeff= 70 000–80 000 K and logg= 5.2–6.3. The three investigated He-sdOs have a relatively high hydrogen mass fraction (10%) that is unexplained by both single (He core flash) and binary evolution (He-WD merger) scenarios. The O(He) star JL 9 is probably a binary helium-WD merger, but its hydrogen content (6%) is also at odds with merger models. We found that RL 104 is the ‘coolest’ (Teff= 80 000 K) member of the PG1159 class in a pre-WD stage. Its optical spectrum is remarkable because it exhibits C IVlines involving Rydberg states with principal quantum numbers up ton= 22. Its rather low mass (0.48−0.02+0.03 M⊙) is difficult to reconcile with the common evolutionary scenario for PG1159 stars due to it being the outcome of a (very) late He-shell flash. The same mass-problem faces a merger model of a close He-sdO plus CO WD binary that predicts PG1159-like abundances. Perhaps RL 104 originates from a very late He-shell flash in a CO/He WD formed by a merger of two low-mass He-WDs.

A Helium-flash-induced Mixing Event Can Explain the Lithium Abundances of Red Clump Stars

Abstract Observations demonstrate that the surface abundance of in low-mass stars changes dramatically between the tip of the red giant branch and the red clump. This naturally suggests an association with the helium core flash, which occurs between these two stages. Using stellar evolution models and a simple, ad hoc mixing prescription, we demonstrate that the enhancement can be explained by a brief chemical mixing event that occurs at the time of the first, strongest He subflash. The amount of already present above the H-burning shell just before the flash, once it mixes into the cooler envelope and undergoes an electron capture converting it to , is sufficient to explain the observed abundance at the red clump. We suggest that the excitation of internal gravity waves by the vigorous turbulent convection during the flash may provide a physical mechanism that can induce such mixing.

Asteroseismic signatures of the helium core flash

All evolved stars with masses $M_\star\lesssim 2M_\odot$ undergo a helium(He)-core flash at the end of their first stage as a giant star. Although theoretically predicted more than 50 years ago, this core-flash phase has yet to be observationally probed. We show here that gravity modes (g modes) stochastically excited by He-flash driven convection are able to reach the stellar surface, and induce periodic photometric variabilities in hot-subdwarf stars with amplitudes of the order of a few mmag. As such they can now be detected by space-based photometry with the Transiting Exoplanet Survey Satellite (TESS) in relatively bright stars (e.g. magnitudes $I_C\lesssim 13$). The range of predicted periods spans from a few thousand seconds to tens of thousand seconds, depending on the details of the excitation region. In addition, we find that stochastically excited pulsations reproduce the pulsations observed in a couple of He-rich hot subdwarf stars. These stars, and in particular the future TESS target Feige 46, are the most promising candidates to probe the He-core flash for the first time.

LITHIUM-RICH GIANTS IN GLOBULAR CLUSTERS*

ABSTRACT Although red giants deplete lithium on their surfaces, some giants are Li-rich. Intermediate-mass asymptotic giant branch (AGB) stars can generate Li through the Cameron–Fowler conveyor, but the existence of Li-rich, low-mass red giant branch (RGB) stars is puzzling. Globular clusters are the best sites to examine this phenomenon because it is straightforward to determine membership in the cluster and to identify the evolutionary state of each star. In 72 hours of Keck/DEIMOS exposures in 25 clusters, we found four Li-rich RGB and two Li-rich AGB stars. There were 1696 RGB and 125 AGB stars with measurements or upper limits consistent with normal abundances of Li. Hence, the frequency of Li-richness in globular clusters is (0.2 ± 0.1)% for the RGB, (1.6 ± 1.1)% for the AGB, and (0.3 ± 0.1)% for all giants. Because the Li-rich RGB stars are on the lower RGB, Li self-generation mechanisms proposed to occur at the luminosity function bump or He core flash cannot explain these four lower RGB stars. We propose the following origin for Li enrichment: (1) All luminous giants experience a brief phase of Li enrichment at the He core flash. (2) All post-RGB stars with binary companions on the lower RGB will engage in mass transfer. This scenario predicts that 0.1% of lower RGB stars will appear Li-rich due to mass transfer from a recently Li-enhanced companion. This frequency is at the lower end of our confidence interval.

EXCITATION OF GRAVITY WAVES BY FINGERING CONVECTION, AND THE FORMATION OF COMPOSITIONAL STAIRCASES IN STELLAR INTERIORS

Fingering convection (or thermohaline convection) is a weak yet important kind of mixing that occurs in stably-stratified stellar radiation zones in the presence of an inverse mean-molecular-weight gradient. Brown et al. (2013) recently proposed a new model for mixing by fingering convection, which contains no free parameter, and was found to fit the results of direct numerical simulations in almost all cases. Notably, however, they found that mixing was substantially enhanced above their predicted values in the few cases where large-scale gravity waves, followed by thermo-compositional layering, grew spontaneously from the fingering convection. This effect is well-known in the oceanographic context, and is attributed to the excitation of the so-called "collective instability". In this work, we build on the results of Brown et al. (2013) and of Traxler et al. (2011b) to determine the conditions under which the collective instability may be expected. We find that it is only relevant in stellar regions which have a relatively large Prandtl number (the ratio of the kinematic viscosity to the thermal diffusivity), $O(10^{-3})$ or larger. This implies that the collective instability cannot occur in main sequence stars, where the Prandtl number is always much smaller than this (except in the outer layers of surface convection zones where fingering is irrelevant anyway). It could in principle be excited in regions of high electron degeneracy, during He core flash, or in the interiors of white dwarfs. We discuss the implications of our findings for these objects, both from a theoretical and from an observational point of view.

ROTATION OF GIANT STARS

The internal rotation of post-main sequence stars is investigated, in response to the convective pumping of angular momentum toward the stellar core, combined with a tight magnetic coupling between core and envelope. The spin evolution is calculated using model stars of initial mass 1, 1.5 and $5\,M_\odot$, taking into account mass loss on the giant branches. We also include the deposition of orbital angular momentum from a sub-stellar companion, as influenced by tidal drag along with the excitation of orbital eccentricity by a fluctuating gravitational quadrupole moment. A range of angular velocity profiles $\Omega(r)$ is considered in the envelope, extending from solid rotation to constant specific angular momentum. We focus on the back reaction of the Coriolis force, and the threshold for dynamo action in the inner envelope. Quantitative agreement with measurements of core rotation in subgiants and post-He core flash stars by Kepler is obtained with a two-layer angular velocity profile: uniform specific angular momentum where the Coriolis parameter ${\rm Co} \equiv \Omega \tau_{\rm con} \lesssim 1$ (here $\tau_{\rm con}$ is the convective time); and $\Omega(r) \propto r^{-1}$ where ${\rm Co} \gtrsim 1$. The inner profile is interpreted in terms of a balance between the Coriolis force and angular pressure gradients driven by radially extended convective plumes. Inward angular momentum pumping reduces the surface rotation of subgiants, and the need for a rejuvenated magnetic wind torque. The co-evolution of internal magnetic fields and rotation is considered in Kissin & Thompson, along with the breaking of the rotational coupling between core and envelope due to heavy mass loss.

A Kansei-Oriented Approach for Product Form Design

The growing users’ awareness in practicing individual life style have led to diverse living products in business markets. Hence, how to effectively meet the emotional or kansei needs of target users become crucial issues for product designers. This article aims to propose a revised Kansei Form Composition (KFC) to more effectively analyze and develop product styling. The updated approach not only integrates kansei thinking and Morphological Analysis but also employs factor weights and flash cards. During the analyzing stages, the new KFC applies visual images as interfaces to induce latent demands of the target interviewers. Related kansei adjectives and form elements are selected as main design factors. Furthermore, weights of the factors are identified through the interviewees’ feedbacks. Core flash cards of design factors are submitted. The revised approach suggests the core flash cards as feasible inspiration tools to the designers in the idea development stages. By using the new KFC, timer form designs for LOHOS involvers are demonstrated. The authenticity and applicability is verified that the revised KFC can catch the preference of user needs more effectively.

ACOUSTIC SIGNATURES OF THE HELIUM CORE FLASH

All evolved stars with masses M <2 solar masses undergo an initiating off-center helium core flash in their 0.48 solar mass He core as they ascend the red giant branch (RGB). This off-center flash is the first of a few successive helium shell subflashes that remove the core electron degeneracy over 2 Myrs, converting the object into a He burning star. Though characterized by Thomas over 40 years ago, this core flash phase has yet to be observationally probed. Using the Modules for Experiments in Stellar Astrophysics (MESA) code, we show that red giant asteroseismology enabled by space-based photometry (i.e. Kepler and CoRoT) can probe these stars during the flash. The rapid (< 100,000 years) contraction of the red giant envelope after the initiating flash dramatically improves the coupling of the p-modes to the core g-modes, making the detection of l=1 mixed modes possible for these 2 Myrs. This duration implies that 1 in 35 stars near the red clump in the HR diagram will be in their core flash phase. During this time, the star has a g-mode period spacing of Delta P_g = 70-100 seconds, lower than the Delta P_g=250 seconds of He burning stars in the red clump, but higher than the RGB stars at the same luminosity. This places them in an underpopulated part of the large frequency spacing (Delta nu) vs. Delta P_g diagram that should ease their identification amongst the thousands of observed red giants.

Modeling He-rich subdwarfs through the hot-flasher scenario

We present 1D numerical simulations aimed at studying the hot-flasher scenario for the formation of He-rich subdwarf stars. Sequences were calculated for a wide range of metallicities and with the He core flash at different points of the post-RGB evolution (i.e. different remnant masses). We followed the complete evolution from the ZAMS, through the hot-flasher event, and to the subdwarf stage for all kinds of hot-flashers. This allows us to present a homogeneous set of abundances for different metallicities and all flavors of hot-flashers. We extend the scope of our work by analyzing the effects in the predicted surface abundances of some standard assumptions in convective mixing and the effects of element diffusion. We find that the hot-flasher scenario is a viable explanation for the formation of He-sdO stars. Our results also show that element diffusion may produce the transformation of (post hot-flasher) He-rich atmospheres into He-deficient ones. If this is so, then the hot-flasher scenario is able to reproduce both the observed properties and distribution of He-sdO stars.

The Mystery of CEMPs+r Stars and the Dual Core-Flash Neutron Superburst

AbstractCarbon-enhanced metal-poor (CEMPs+r) stars show large enhancements of elements produced both by the slow and the rapid neutron capture processes (the s and r process, respectively) and represent a relatively large fraction, 30% to 50%, of the CEMP population. Many scenarios have been proposed to explain this peculiar chemical composition and most of them involve a binary companion producing the s-process elements during its Asymptotic Giant Branch (AGB) phase. The problem is that none of the proposed explanations appears to be able to account for all observational constraints, hence, alternatives are needed to be put forward and investigated. In this spirit, we propose a new scenario for the formation of CEMPs+r stars based on S. W. Campbell's finding that during the ‘dual core flash’ in low-mass stars of extremely low metallicity, when protons are ingested in the He-flash convective zone, a ‘neutron superburst’ is produced. Further calculations are needed to verify if this neutron superburst could make the r-process component observed in CEMPs+r, as well as their Fe abundances. The s-process component would then be produced during the following AGB phase.

Modeling He-rich subdwarfs through the hot-flasher scenario

We present 1D numerical simulations aimed at studying the hot-flasher scenario for the formation of He-rich subdwarf stars. Sequences were calculated for a wide range of metallicities and physical assumptions, such as the stellar mass at the moment of the helium core flash. This allows us to study the two previously proposed flavors of the hot-flasher scenario (“deep” and “shallow” mixing cases) and to identify a third transition type. Our sequences are calculated by solving simultaneously the mixing and burning equations within a diffusive convection picture, and in the context of standard mixing length theory. We are able to follow chemical evolution during deep-mixing events in which hydrogen is burned violently, and therefore able to present a homogeneous set of abundances for different metallicities and varieties of hot-flashers. We extend the scope of our work by analyzing the effects of non-standard assumptions, such as the effect of chemical gradients, extra-mixing at convective boundaries, possible reduction in convective velocities, or the interplay between difussion and mass loss. Particular emphasis is placed on the predicted surface properties of the models. We find that the hot-flasher scenario is a viable explanation for the formation and surface properties of He-sdO stars. Our results also show that, during the early He-core burning stage, element diffusion may produce the transformation of (post hot-flasher) He-rich atmospheres into He-deficient ones. If this is so, then we find that He-sdO stars should be the progenitors of some of the hottest sdB stars.

“Hot Helium Flashers” – The Road to Extreme Horizontal Branch Stars

AbstractObservational and theoretical investigations, performed especially over the last two decades, have strongly attributed the far-UV upturn phenomenon to low-mass, small-envelope, He-burning stars in Extreme Horizontal Branch (EHB) and subsequent evolutionary phases.Using our new stellar evolution code – a code that follows through complete evolutionary tracks, Pre-MS to cooling WD – without any interruption or intervention, we are able to produce a wide array of EHB stars, lying at bluer (Teff ≥ 20,000 K) and less luminous positions on HRD, and also closely examine their post-HB evolution until the final cooling as White Dwarfs.HB morphology is a complex multiple parameter problem. Two leading players, which seem to possess the ability to affect considerably positions of HB, are those of: 1.Helium abundance, and 2.mass-loss efficiency on the first giant branch. We focus here on the latter; thus, EHB stars are produced in our calculations by increasing the mass-loss rate on the RGB, to a state where prior to reaching core He flash conditions, only a very small H-rich envelope remains. The core flash takes place at hotter positions on the HRD, sometimes while already descending on the WD cooling curve. We show preliminary results for a range of initial masses (MZAMS = 0.8 − 1.1 M⊙) and for metallicities covering both populations I and II (Z = 0.01 − 0.001). The [M,Z] combinations have been chosen such that the masses would be above and close to typical MS turnoff masses (e.g. the estimation of MTO ≃ 0.85 for NGC 2808), and also so that the ages at HB are of order of 10 ± 5 Gyr.

A Small Step on the Long Road to Understanding the R Stars: CNO Cycling in Candidate R Star Progenitors

AbstractRecent work has proposed that a merger event between a red-giant and a He white dwarf may be responsible for the production of R stars (Izzard, Jeffery &amp; Lattanzio 2007). We investigate the proposed evolution and nucleosynthesis of such a model. We simulate the hypothesized late ignition of the core flash by increasing neutrino losses until ignition occurs sufficiently far from the centre that the subsequent evolution produces carbon dredge-up to the extent that the post-flash object is a carbon star. Detailed nucleosynthesis is performed within this approximation and we show that the overall properties are broadly consistent with the observations. Details will depend on the dynamics of the merger event.

Evolution of Low‐Mass Population III Stars

We present the evolutionary models of metal-free stars in the mass range from 0.8 to 1.2 Msun with up-to-date input physics. The evolution is followed to the onset of hydrogen mixing into a convection, driven by the helium flash at red giant or asymptotic giant branch phase. The models of mass M >= 0.9 Msun undergo the central hydrogen flash, triggered by the carbon production due to the 3-alpha reactions. We find that the border of the off-center and central ignition of helium core flash falls between 1.1 and 1.2 Msun; the models of mass M <= 1.1 Msun experience the hydrogen mixing at the tip of red giant branch while the models of M = 1.2 Msun during the helium shell flashes on the asymptotic giant branch. The equation of state for the Coulomb liquid region, where electron conduction and radiation compete, is shown to be important since it affects the thermal state in the helium core and influences the red giant branch evolution. It is also found that the non-resonant term of 3-alpha reactios plays an important role, although it has negligible effect in the evolution of stars of younger populations. We compare our models with the computations by several other sets of authors, to confirm the good agreement except for one study which finds the helium ignition much closer to the center with consequences important for subsequent evolution.

Evolution and Nucleosynthesis of Primordial Low‐Mass Stars

We discuss in detail the evolutionary properties of low-mass stars (M ≤ 1 M☉) having metallicity lower than Z = 10-6 from the pre-main sequence up to (almost) the end of the early asymptotic giant branch phase. We also discuss the possibility that the large [C/Fe], [N/Fe] observed on the surface of the most iron-poor star currently known, HE 0107-5240, could be attributed to the autopollution induced by the penetration of the He convective shell into the H-rich mantle during the He core flash of a low-mass, very low metallicity star. On the basis of a quite detailed analysis, we conclude that the autopollution scenario cannot be responsible for the observed chemical composition of HE 0107-5240.

Spectroscopic analyses of the blue hook stars in NGC 2808: A more stringent test of the late hot flasher scenario

Recent UV observations of the globular cluster NGC 2808 (Brown et al. 2001) show a significant population of hot stars fainter than the zero-age horizontal branch (blue hook stars), which cannot be explained by canonical stellar evolution. Their results suggest that stars which experience unusually large mass loss on the red giant branch and which subsequently undergo the helium core flash while descending the white dwarf cooling curve could populate this region. Theory predicts that these late hot flashers should show higher temperatures than the hottest canonical horizontal branch stars and should have helium- and carbon-rich atmospheres. As a test of this late hot flasher scenario, we have obtained and analysed medium resolution spectra of a sample of blue hook stars in NGC 2808 to derive their atmospheric parameters. Using the same procedures, we have also re-analyzed our earlier spectra of the blue hook stars in ω Cen (Moehler et al. 2002) for comparison with the present results for NGC 2808. The blue hook stars in these two clusters are both hotter (T eff ≥ 35 000 K) and more helium-rich than canonical extreme horizontal branch stars in agreement with the late hot flasher scenario. Moreover, we find indications for carbon enhancement in the three most helium-enriched stars in NGC 2808. However, the blue hook stars still show some hydrogen in their atmospheres, perhaps indicating that some residual hydrogen survives a late hot flash and then later diffuses to the surface during the horizontal branch phase. We note that the presence of blue hook stars apparently depends mostly on the total mass of the globular cluster and not so much on its horizontal branch morphology.

The Y 2 Stellar Evolutionary Tracks

We present a database of the latest stellar models of the $Y^2$ (Yonsei-Yale) collaboration. This database contains the stellar evolutionary tracks from the pre-main-sequence birthline to the helium core flash that were used to construct the $Y^2$ isochrones. We also provide a simple interpolation routine that generates stellar tracks for given sets of parameters (metallicity, mass, and $\alpha$-enhancement).

Spectroscopic analyses of the “blue hook” stars inωCentauri: A test of the late hot flasher scenario

(Omega) Cen contains the largest population of very hot horizontal branch (HB) stars known in a globular cluster. Recent UV observations (Whitney et al. 1998; D'Cruz et al. 2000) show a significant population of hot stars below the zero-age horizontal branch (blue stars), which cannot be explained by canonical stellar evolution. Stars which suffer unusually large mass loss on the red giant branch and thus experience the helium core flash while descending the white dwarf cooling curve could populate this region. Theory predicts that these late hot flashers should show higher temperatures than the hottest canonical HB stars and should have helium- and carbon-rich atmospheres. We obtained and analysed medium resolution spectra of a sample of blue hook stars to derive their atmospheric parameters. The blue hook stars are indeed both hotter (T(sub eff)>35,000 K) and more helium-rich than classical extreme HB stars. In addition we find indications for a large enhancement of the carbon abundance relative to the cluster abundance.

A peculiar metal-rich star, HD 135485

Local thermodynamic equilibrium (LTE) absolute and differential abundances are presented for a peculiar metal-rich B-type star, HD 135485. These suggest that HD 135485 has a general enrichment of ∼0.5 dex in all the metals observed (C, N, O, Ne, Mg, Al, Si, P, S, Cl, Ar, Sc, Ti, Cr, Mn, Fe and Sr), except for nickel. The helium enhancement and hence hydrogen deficiency can account for ≤ 0.2 dex of this enhancement of metals, with the additional enhancement probably being representative of the progenitor gas. However, some of the metals appear to have greater enhancements, which may have occurred during the star's evolution. The significantly larger nitrogen abundance coupled with a modest helium enhancement observed in HD 135485 indicates that carbon–nitrogen (CN) processed material has possibly contaminated the stellar surface. Neon and carbon enhancements may indicate that helium core flashes have also occurred in HD 135485. Some of the iron-group elements (viz. Mn and Ni) appear to have similar abundance patterns to that of silicon Ap stars, but it is uncertain how these abundance patterns formed if they were not present in the progenitor gas. From a kinematical investigation it is unclear whether this star formed in a metal-rich region as implied by its chemical composition. From its position in the Hertzsprung–Russell diagram, HD 135485 would appear to be an evolved star lying close to or on the horizontal branch.