Transport of angular momentum has been a challenging topic within the stellar evolution community, even more since the recent asteroseismic surveys. All published studies on rotation using asteroseismic observations show a discrepancy between the observed and calculated rotation rates, indicating there is an undetermined process of angular momentum transport active in these stars. We aim to constrain the efficiency of this process by investigating rotation rates of 2.5 M$_{\odot}$ stars. First, we investigated whether the Tayler-Spruit dynamo could be responsible for the extra transport of angular momentum for stars with an initial mass of 2.5 M$_{\odot}$. Then, by computing rotating models including a constant additional artificial viscosity, we determined the efficiency of the missing process of angular momentum transport by comparing the models to the asteroseismic observations of core helium burning stars. Parameter studies were performed to investigate the effect of the stellar evolution code used, initial mass, and evolutionary stage. We evolved our models into the white dwarf phase, and provide a comparison to white dwarf rotation rates. The Tayler-Spruit dynamo is unable to provide enough transport of angular momentum to reach the observed values of the core helium burning stars investigated in this paper. We find that a value for the additional artificial viscosity $ν_{\rm{add}}$ around 10$^7$ cm$^2$ s$^{-1}$ provides enough transport of angular momentum. However, the rotational period of these models is too high in the white dwarf phase to match the white dwarf observations. From this comparison we infer that the efficiency of the missing process must decrease during the core helium burning phase. When excluding the $ν_{\rm{add}}$ during core helium burning phase, we can match the rotational periods of both the core helium burning stars and white dwarfs.
16 Cyg A and B are among the brightest stars observed by Kepler. What makes these stars more interesting is that they are solar analogs. 16 Cyg A and B exhibit solar-like oscillations. In this work we use oscillation frequencies obtained using 2.5 years of Kepler data to determine the current helium abundance of these stars. For this we use the fact that the helium ionization zone leaves a signature on the oscillation frequencies and that this signature can be calibrated to determine the helium abundance of that layer. By calibrating the signature of the helium ionization zone against models o…
We present a summary of an asteroseismic signature of helium ionization reported by Houdek & Gough (2007, 2008, 2009) for low-degree p modes in solar-type stars, and illustrate its applications for asteroseismic diagnoses.
Asteroseismology of non-radial pulsations in Hot B Subdwarfs (sdB stars) offers a unique view into the interior of core-helium-burning stars. Ground-based and space-borne high precision light curves allow for the analysis of pressure and gravity mode pulsations to probe the structure of sdB stars deep into the convective core. As such asteroseismological analysis provides an excellent opportunity to test our understanding of stellar evolution. In light of the newest constraints from asteroseismology of sdB and red clump stars, standard approaches of convective mixing in 1D stellar evolution mo…
Helium burning in the convective cores of horizontal branch and `red clump' stars appears to involve a process of `ingestion' of unburnt helium into the core, the physics of which has not been identified yet. I show here that a limiting factor controlling the growth is the buoyancy of helium entering the denser C+O core. It yields a growth rate which scales directly with the convective luminosity of the core, and agrees with constraints on core size from current asteroseismology.
We have analyzed 18 quarters of long-cadence data of KIC 9145955 provided by \emph{Kepler}, and extracted 61 oscillation frequencies from these high precision photometric data. The oscillation frequencies include 7 $l = 0$ modes, 44 $l = 1$ modes, 7 $l = 2$ modes, and 3 $l = 3$ modes. We identify $l = 0$ modes as p modes and $l = 2$ modes as p-dominated modes. For $l = 1$ modes, all of them are identified as mixed modes. These mixed modes can be used to determine the size of the helium core. We conduct a series of asteroseismic models and the size of the helium core is determined to be $M_{\rm…
Detailed understanding of stellar physics is essential towards a robust determination of stellar properties (e.g. radius, mass, and age). Among the vital input physics used in the modelling of solar-type stars which remain poorly constrained, is the initial helium abundance. To this end, when constructing stellar model grids, the initial helium abundance is estimated either (i) by using the semi-empirical helium-to-heavy element enrichment ratio, (${ΔY}/{ΔZ}$), anchored to the standard Big Bang Nucleosynthesis value or (ii) by setting the initial helium abundance as a free variable. Adopting 3…
Despite the fact that the initial helium abundance is an essential ingredient in modelling solar-type stars, its abundance in these stars remains a poorly constrained observational property. This is because the effective temperature in these stars is not high enough to allow helium ionization, not allowing any conclusions on its abundance when spectroscopic techniques are employed. To this end, stellar modellers resort to estimating the initial helium abundance via a semi-empirical helium-to-heavy element ratio, anchored to the the standard Big Bang nucleosynthesis value. Depending on the choi…
Theoretical predictions for the core helium burning phase of stellar evolution are highly sensitive to the uncertain treatment of mixing at convective boundaries. In the last few years, interest in constraining the uncertain structure of their deep interiors has been renewed by insights from asteroseismology. Recently, Spruit (2015) proposed a limit for the rate of growth of helium-burning convective cores based on the higher buoyancy of material ingested from outside the convective core. In this paper we test the implications of such a limit for stellar models with a range of initial mass and…
Helium core white dwarfs (WDs) with mass M <~ 0.20 Msun undergo several Gyrs of stable hydrogen burning as they evolve. We show that in a certain range of WD and hydrogen envelope masses, these WDs may exhibit g-mode pulsations similar to their passively cooling, more massive carbon/oxygen core counterparts, the ZZ Cetis. Our models with stably burning hydrogen envelopes on helium cores yield g-mode periods and period spacings longer than the canonical ZZ Cetis by nearly a factor of 2. We show that core composition and structure can be probed using seismology since the g-mode eigenfunctions pr…
We present a new, better-constrained asteroseismic analysis of the helium-atmosphere (DB) white dwarf discovered in the field of view of the original Kepler mission. Observations obtained over the course of two years yield at least seven independent modes, two more than were found in the discovery paper for the object. With several triplets and doublets, we are able to fix the $\ell$ and $\rm{m}$ identification of several modes before performing the fitting, greatly reducing the number of assumptions we must make about mode identification. We find a very thin helium layer for this relatively h…
Regions of rapid variation in the internal structure of a star are often referred to as acoustic glitches since they create a characteristic periodic signature in the frequencies of p modes. Here we examine the localized disturbance arising from the helium second ionization zone in red giant branch and clump stars. More specifically, we determine how accurately and precisely the parameters of the ionization zone can be obtained from the oscillation frequencies of stellar models. We use models produced by three different generation codes that not only cover a wide range of stages of evolution a…
The influence of rotational mixing on the global parameters and asteroseismic properties of red giant stars is reviewed. While red giants are generally characterised by low surface rotational velocities, they may have been rotating much more rapidly during the main sequence, so that the rotational history of a star has a large impact on its properties during the red giant phase. For stars massive enough to ignite helium burning in non-degenerate conditions, rotational mixing leads to a significant increase of the stellar luminosity and shifts the location of the core helium burning phase to a …
The helium abundance of stars is a strong driver of evolutionary timescales, however it is difficult to measure in cool stars. We conduct an asteroseismic analysis of NGC 6791, an old, metal rich open cluster that previous studies have indicated also has a high helium abundance. The cluster was observed by Kepler and has unprecedented lightcurves for many of the red giant branch stars in the cluster. Previous asteroseismic studies with Kepler data have constrained the age through grid based modeling of the global asteroseismic parameters ($Δν$ and $ν_{\mathrm{max}}$). However, with the precisi…
Stellar cores may be considered as a nuclear reactor that play important role in injecting new synthesized elements in the interstellar medium Helium burning is an important stage that contribute to the synthesis of key elements such as carbon, through the triple- α process, and oxygen. In the present paper, we introduce a computational method for the fractional model of the nuclear helium burning in stellar cores. The system of fractional differential equations is solved simultaneously using series expansion method. The calculations are performed in the sense of modified Riemann-Liouville fra…
Superbursts from accreting neutron stars probe nuclear reactions at extreme densities ($ρ\approx 10^{9}~g\,cm^{-3}$) and temperatures ($T>10^9~K$). These bursts ($\sim$1000 times more energetic than type I X-ray bursts) are most likely triggered by unstable ignition of carbon in a sea of heavy nuclei made during the rp-process of regular type I X-ray bursts (where the accumulated hydrogen and helium are burned). An open question is the origin of sufficient amounts of carbon, which is largely destroyed during the rp-process in X-ray bursts. We explore carbon production in steady-state burning v…
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 Sat…
The evolution of low- and intermediate mass stars at the onset and during core helium burning is reviewed. Particular emphasis is laid on structural differences, which may allow to identify a star's nature and evolutionary phase in spite of the fact that it is found in a region of the Hertzsprung-Russell-Diagram objects from both mass ranges may populate. Seismic diagnostics which are sensitive to the temperature and density profile at the border of the helium core and outside of it may be the most promising tool.
Core helium burning primary red clump (RC) stars are evolved red giant stars which are excellent standard candles. As such, these stars are routinely used to map the Milky Way or determine the distance to other galaxies among other things. However distinguishing RC stars from their less evolved precursors, namely red giant branch (RGB) stars, is still a difficult challenge and has been deemed the domain of asteroseismology. In this work, we use a sample of 1,676 RGB and RC stars which have both single epoch infrared spectra from the APOGEE survey and asteroseismic parameters and classification…
Overshooting and semiconvection are among the most uncertainties in the evolution of massive stars. Complete mixing over a certain distance beyond the convective boundary (Stothers \& Chin 1985) and an exponentially decaying diffusion outside the convection zone (Herwig 2000) are commonly adopted for the overshoot mixing. Recently, Li (2012, 2017) developed the $k$-$ω$ model, which can be used in both convection zones and overshooting regions. We incorporated it in calculations of $30M_{\odot}$ stellar models. We find that in the main sequence stage, models with the $k$-$ω$ model are almost id…
Current stellar model predictions of adiabatic oscillation frequencies differ significantly from the corresponding observed frequencies due to the non-adiabatic and poorly understood near-surface layers of stars. However, certain combinations of frequencies -- known as frequency ratios -- are largely unaffected by the uncertain physical processes as they are mostly sensitive to the stellar core. Furthermore, the seismic signature of helium ionization provides envelope properties while being almost independent of the outermost layers. We have developed an advanced stellar modelling approach in …
We model the oscillations of the SONG target $μ$ Herculis to estimate the parameters of the star. The $\ell$ = 1 mixed modes of $μ$ Her provide strong constraints on stellar properties. The mass and age given by our asteroseismic modelling are 1.10$^{+0.11}_{-0.06}$ M$_{\odot}$ and 7.55$^{+0.96}_{-0.79}$ Gyr. The initial helium abundance is also constrained at around $Y_{\rm{init}}$ = 0.28, suggesting a ratio in the elements enrichment law ($ΔY/ΔZ$) around 1.3, which is closed to the solar value. The mixing-length parameter converges to about 1.7, which is $\sim$ 10\% lower than the solar valu…
We explore the question of whether the interior state of massive red supergiant supernova progenitors can be effectively probed with asteroseismology. We have computed a suite of ten models with ZAMS masses from 15 to 25 m_sun in intervals of 1 m_sun, including the effects of rotation, with the stellar evolutionary code MESA. We estimate characteristic frequencies and convective luminosities of convective zones at two illustrative stages, core helium burning and off-center convective carbon burning. We also estimate the power that might be delivered to the surface to modulate the luminous outp…
The asteroseismic analysis of white dwarfs allows us to peer below their photospheres and determine their internal structure. At ~ 28,000 K EC20058-5234 is the hottest known pulsating helium atmosphere white dwarf. As such, it constitutes an important link in the evolution of white dwarfs down the cooling track. It is also astrophysically interesting because it is at a temperature where white dwarfs are expected to cool mainly through the emission of plasmon neutrinos. In the present work, we perform an asteroseismic analysis of EC20058-5234 and place the results in the context of stellar evol…
The influence of rotation on the properties of red giants is studied in the context of the asteroseismic modelling of these stars. While red giants exhibit low surface rotational velocities, we find that the rotational history of the star has a large impact on its properties during the red giant phase. In particular, for stars massive enough to ignite He burning in non-degenerate conditions, rotational mixing induces a significant increase of the stellar luminosity and shifts the location of the core helium burning phase to a higher luminosity in the HR diagram. This of course results in a cha…
Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes. Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology…
Aims. We investigate the s-process during the AGB phase of stellar models whose cores are enforced to rotate at rates consistent with asteroseismology observations of their progenitors and successors. Methods. We calculated new 2M$_{\odot}$, Z=0.01 models, rotating at 0, 125, and 250 km/s at the start of main sequence. An artificial, additional viscosity was added to enhance the transport of angular momentum in order to reduce the core rotation rates to be in agreement with asteroseismology observations. We compared rotation rates of our models with observed rotation rates during the MS up to …
Extra modes in helium-core-burning stars probing an infra core cavity
Université PSL（巴黎文理研究大学）；CNRS（法国国家科学研究中心）；Sorbonne Université（索邦大学）；Université Paris Cité（巴黎西岱大学）；CY Cergy Paris Université（塞吉-巴黎大学）；The University of Tokyo（东京大学）；Université Paris-Saclay（巴黎萨克雷大学）；Universidade do Porto（波尔图大学）；Université Côte d’Azur（蔚蓝海岸大学）；Université de Toulouse（图卢兹大学）；University of Bologna（博洛尼亚大学）
Red Clump stars have been found to be enhanced in lithium relative to stars at the tip of the Red Giant Branch (TRGB), which is unexpected in current stellar models. At the TRGB, stars undergo the helium flash, during which helium burning briefly generates roughly $10^9 \, L_\odot$ of power and drives vigorous convection within the star's core. The helium-burning shell excites large fluxes of internal gravity waves. Here we investigate whether or not these waves can deposit enough heat to destabilize the hydrogen-burning shell, generate a convection zone there, and thereby drive the Cameron-Fo…
HD 176465 is a binary system whose both components are solar-like pulsators and whose oscillation frequencies were observed by the \kepler\ mission. In this paper we have modeled the asteroseismic and spectroscopic data of the stars, and have determined their convection-zone helium abundances using the signatures left by the He{\sc ii} ionization zone on the mode frequencies. As expected we find that the components of the binary have the same age within uncertainties ($3.087 \pm 0.580$ Gyr and $3.569 \pm 0.912$ Gyr); they also have the same initial helium abundance (Y$_{\mathrm{init}}$=0.253 $…
Stars between two and three solar masses rotate rapidly on the main sequence, and the detection of slow core and surface rotation in the core-helium burning phase for these stars places strong constraints on their angular momentum transport and loss. From a detailed asteroseismic study of the mixed-dipole mode pattern in a carefully selected, representative sample of stars, we find that slow core rotation rates in the range reported by prior studies are a general phenomenon and not a selection effect. We show that the core rotation rates of these stars decline strongly with decreasing surface …
We present the results of the asteroseismic analysis of the hydrogen-deficient white dwarf PG 0112+104 from the $Kepler$-2 field. Our seismic procedure using the forward method based on physically sound, static models, includes the new core parameterization leading us to reproduce the periods of this star near the precision of the observations. This new fit outperforms current state-of-the-art standards by order of magnitudes. We precisely establish the internal structure and unravel the inner C/O stratification of its core. This opens up interesting perspectives on better constraining key pro…
Blue large-amplitude pulsators (BLAPs) are a newly discovered type of variable star. Their typical pulsation periods are on the order of a few tens of minutes, with relatively large amplitudes of 0.2-0.4 mag in optical bands, and their rates of period changes are on the order of $10^{-7} yr^{-1}$ (both positive and negative). They are extremely rare objects and attempts to explain their origins and internal structures have attracted a great deal of attention. Previous studies have proposed that BLAPs may be pre-white dwarfs, with masses around $0.3M_\odot$, or core-helium-burning stars in the …
A Helium-shell Burning Blue Horizontal Branch Star Produced from Common Envelope Evolution
Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Hebei Normal University（河北师范大学）
Effective values for the key helium burning reaction rates, triple-alpha and 12C(alpha,gamma)16O, are obtained by adjusting their strengths so as to obtain the best match with the solar abundance pattern of isotopes uniquely or predominately made in core collapse supernovae. These effective rates are then used to determine the production of the neutrino isotopes. The use of effective rates considerably reduces the uncertainties in the production factors arising from uncertainties in the helium burning rates, and improves our ability to use the production of 11B to constrain the neutrino emissi…
We consider the blue loops in the Hertzsprung-Russell diagram that occur when intermediate-mass stars begin core helium burning. It has long been known that the excess of helium above the burning shell, the result of the contraction of the convective core during core hydrogen burning, has the effect of making such stars redder and larger than they would be otherwise. The outward motion of the burning shell in mass removes this excess and triggers the loop. Hitherto nobody has attempted to demonstrate why the excess helium has this effect. We consider the effect of the local opacity, which is r…
We have studied the sensitivity of supernova production of the gamma emitting nuclei 26Al, 44Ti and 60Fe to variations of the rates of the triple alpha and 12C(alpha, gamma) reactions. Over a range of twice their experimental uncertainties we find variations in the production of 60Fe by more than a factor of five. Smaller variations, about a factor of two to three, were observed for 26Al and 44Ti. The yields of these isotopes change significantly when the abundances of Lodders (2003) are used instead of those of Anders and Grevesse (1989). These sensitivities will limit conclusions based on a …
The detection of mixed oscillation modes offers a unique insight into the internal structure of core helium burning (CHeB) stars. The stellar structure during CHeB is very uncertain because the growth of the convective core, and/or the development of a semiconvection zone, is critically dependent on the treatment of convective boundaries. In this study we calculate a suite of stellar structure models and their non-radial pulsations to investigate why the predicted asymptotic g-mode $\ell = 1$ period spacing $ΔΠ_1$ is systematically lower than is inferred from Kepler field stars. We find that o…
We study the sensitivity of presupernova evolution and supernova nucleosynthesis yields of massive stars to variations of the helium-burning reaction rates within the range of their uncertainties. We use the current solar abundances from Lodders (2009) for the initial stellar composition. We compute a grid of 12 initial stellar masses and 176 models per stellar mass to explore the effects of independently varying the 12^C(a,g)16^O and 3a reaction rates, denoted R_a12 and R_3a, respectively. The production factors of both the intermediate-mass elements (A=16-40) and the s-only isotopes along th…
Hot subdwarf stars of spectral type B (sdBs) are evolved, core helium-burning objects. The formation of those objects is puzzling, because the progenitor star has to lose almost its entire hydrogen envelope in the red-giant phase. Binary interactions have been invoked, but single sdBs exist as well. We report the discovery of two close hot subdwarf binaries with small radial velocity amplitudes. Follow-up photometry revealed reflection effects originating from cool irradiated companions, but no eclipses. The lower mass limits for the companions of CPD-64$^{\circ}$481 ($0.048\,M_{\rm \odot}$) a…
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 evolut…
Massive stars are key sources of radiative, kinetic, and chemical feedback in the universe. Grids of massive star models computed by different groups each using their own codes, input physics choices and numerical approximations, however, lead to inconsistent results for the same stars. We use three of these 1D codes---GENEC, KEPLER and MESA---to compute non-rotating stellar models of $15~\mathrm{M}_\odot$, $20~\mathrm{M}_\odot$, and $25~\mathrm{M}_\odot$ and compare their nucleosynthesis. We follow the evolution from the main sequence until the end of core helium burning. The GENEC and KEPLER…
Shaping the horizontal branch: The role of envelope mass in the evolution of stripped core-helium-burning stars
Instituto de Física y Astronomía, Universidad de Valparaíso（瓦尔帕莱索大学物理与天文学研究所）；School of Physics and Astronomy, Monash University（蒙纳士大学物理与天文学院）；Australian Research Council Centre of Excellence for Gravitational Wave Discovery（澳大利亚研究委员会引力波发现卓越中心）
CHARA Interferometry and TESS Asteroseismology of the Core-Helium Burning Red Giant $κ$ Cyg
Sydney Institute for Astronomy, School of Physics, University of Sydney（悉尼大学物理学院天体物理研究所）；Institute for Astronomy, University of Hawai‘i（夏威夷大学天文研究所）；Sydney Informatics Hub, Core Research Facilities, University of Sydney（悉尼大学核心研究设施悉尼信息学中心）
Pulsating hot B subdwarf (sdB) stars, which are core-He burning objects, are one of the showcases of asteroseismology. Thanks to the combination of rich pulsation spectra and state-of-the-art modeling tools it is possible to tightly constrain fundamental parameters such as the stellar mass. There are on the contrary very few hotter sdO pulsators, which are in a more advanced evolutionary stage. Some of them are identified in Globular Clusters (GCs), but they are extremely rare in the field. Recently, it was suggested that PB8783, one of the very first sdB pulsators discovered in 1997, may in f…
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 a…
Asteroseismic measurements enable inferences of the underlying stellar structure, such as the density and the speed of sound at various points within the interior of the star. This provides an opportunity to test stellar evolution theory by assessing whether the predicted structure of a star agrees with the measured structure. Thus far, this kind of inverse analysis has only been applied to the Sun and three solar-like main-sequence stars. Here we extend the technique to stars on the subgiant branch, and apply it to one of the best-characterized subgiants of the Kepler mission, HR 7322. The ob…
Stars between two and three solar masses rotate rapidly on the main sequence, and their rotation rates in the core helium burning (secondary clump) phase can therefore be used to test models of angular momentum loss used for gyrochronology in a new regime. Because both their core and surface rotation rates can be measured, these stars can also be used to set strong constraints on angular momentum transport inside stars. We find that they are rotating slower than angular momentum conservation and rigid rotation would predict. Our results are insensitive to the degree of core-envelope coupling b…
Localised modelling error in the near-surface layers of evolutionary stellar models causes the frequencies of their normal modes of oscillation to differ from those of actual stars with matching interior structures. These frequency differences are referred to as the asteroseismic surface term. Global stellar properties estimated via detailed constraints on individual mode frequencies have previously been shown to be robust with respect to different parameterisations of this surface term. It has also been suggested that this may be true of a broader class of nonparametric treatments. We examine…
