Red Giant Evolution Research Articles

Fundamental properties of red giants from Kepler and Gaia DR3 data: diagnostic potential of the mass–metallicity diagram

ABSTRACT For solar-like oscillators, the asteroseismic relations connect the stellar mean density and surface gravity to observable properties of the oscillations. Since the space missions CoRoT and Kepler, the asteroseismic relations have been massively employed out of their theoretical domain of validity, that is the main-sequence solar-type stars, in particular for red giant stars. However, despite a surprisingly good performance of the asteroseismic relation with red giant stars, they are biased and attempts to parametrize the departure of the relations for evolved stars have been the subject of many recent efforts. In this study, we propose a new parametrization of the asteroseismic relations for red giants of radii up to ${\sim } 50\, \mathrm{R}_\odot$, based on a calibration performed with Gaia parallax data. We find that the asteroseismic scaling relations depend on metallicity and effective temperature. For this solution, we obtain a parallax offset of about −0.026 ± 0.001 mas, and radius ranges from 3.8 to 53.2 R⊙. Most stars have masses that range from 0.9 to 1.7 M⊙, with the star count peaking around 1.2 M⊙. We also use data of the eclipsing binaries with solar-like oscillating components for calibration and discuss the diagnostic potential of the mass–metallicity diagram for our understanding of the evolution of red giants and the chemodynamics of the Galactic disc. Using this diagram, we identify stars that lost and gained mass and estimate the minimum change in mass.

Renormalization group effects in astrophobic axion models

It has been recently pointed out that in certain axion models it is possible to suppress simultaneously both the axion couplings to nucleons and electrons, realizing the so-called astrophobic axion scenarios, wherein the tight bounds from SN1987A and from stellar evolution of red giants and white dwarfs are greatly relaxed. So far, however, the conditions for realizing astrophobia have only been set out in tree-level analyses. Here we study whether these conditions can still be consistently implemented once renormalization group effects are included in the running of axion couplings. We find that axion astrophobia keeps holding, albeit within fairly different parameter space regions, and we provide analytical insights into this result. Given that astrophobic axion models generally feature flavor-violating axion couplings, we also assess the impact of renormalization group effects on axion-mediated flavor-violating observables.

Connection between the long secondary period and red giant evolution

Aims.The mechanism behind the long secondary period (LSP) observed in pulsating red giants remains unknown. In this work, I investigate the connection between the red giant branch and the asymptotic giant branch evolution and the appearance of the LSP – a phenomenon that has been observed in a significant proportion of red giants.Methods.For the purposes of this work, I used the OGLE-III sample of the OSARG variables in the Large Magellanic Cloud. I constructed density maps in the period-luminosity as well as color-magnitude planes for the stars showing an LSP and compared them to the remaining giants. I also fit the spectral energy distribution to test whether an additional source of reddening is present in the LSP stars.Results.Here, I posit the hypothesis that the LSP phenomenon may be related to a transition between the different pulsation period-luminosity sequences. I also show that an overabundance of the stars showing LSP can be observed around the tip of the red giant branch, and much more prominently, at the upper part of the asymptotic giant branch. The main over-density region appears to be slightly fainter and redder than the bulk of the asymptotic giant branch. It also seems to correspond to the area of the Hertzsprung-Russell diagram where stable winds and high mass loss are present.Conclusions.The LSP can possibly be shown to be a recurring phenomenon that appears and disappears at various points of the red giant evolution. The LSP stars appear to be more reddened than other giants, which suggests the intrinsic nature of the reddening is likely to be related to large dust emission. The analysis appears to confirm the hypothesis that there is a relation between the mass loss due to the presence of strong stellar wind and the appearance of an LSP.

Connection between the Long Secondary Period phenomenon and the red giant evolution

The mechanism behind the Long Secondary Period (LSP) observed in pulsating red giants still remains unknown. In this work, I investigate the connection between the Red Giant Branch and Asymptotic Giant Branch evolution and the appearance of the LSP - the phenomenon observed in a large fraction the red giants. I use the OGLE-III sample of the OSARG variables in the Large Magellanic Cloud. I construct the density maps in the period-luminosity as well as color-magnitude planes for the stars showing LSP and compare them to the remaining giants. I also fit the spectral energy distribution to test whether an additional source of reddening is present in the LSP stars. I post a hypothesis that the LSP phenomenon may be related to a transition between the different pulsation period-luminosity sequences. I also show that an overabundance of the stars showing Long Period Variables can be observed around the Tip of the Red Giant Branch, and much more prominently, at the upper part of the Asymptotic Giant Branch. The main over-density region appears to be slightly fainter and redder than the bulk of the Asymptotic Giant Branch. It also seems to correspond to the area of the Hertzsprung-Russell diagram where stable winds and high mass loss are present. The LSP can possibly be a recurring phenomenon appearing and disappearing in various points of the red giant evolution. The LSP stars appear to be more reddened than other giants, which suggests the intrinsic nature of the reddening, likely related to large dust emission. The analysis seems to confirms the hypothesis that there is a relation between the mass loss due to the presence of strong stellar wind and the appearance of LSP.

Isotopic ratios in the red giant component of the recurrent nova T Coronae Borealis

ABSTRACT We report the determination of abundances and isotopic ratios for C, O, and Si in the photosphere of the red giant (RG) component of the recurrent nova (RN) T Coronae Borealis from new 2.284–2.402 μm and 3.985–4.155 μm spectroscopy. Abundances and isotopic ratios in the photosphere may be affected by (i) processes in the RG interior which are brought to the surface during dredge-up and (ii) contamination of the RG, either during the common envelope phase of the binary evolution or by material synthesized in RN eruptions, or a combination of the two. We find that the abundances of C, O, and Si are reasonably consistent with the expected composition of an RG after first dredge-up, as is the 16O/17O ratio. The 28Si/29Si ratio is found to be 8.6 ± 3.0, and that for 28Si/30Si is 21.5 ± 3.0. The 12C/13C ratio (10 ± 2) is somewhat lower than expected for first dredge-up. The 16O/18O ratio (41 ± 3) is highly inconsistent with that expected either from RG evolution (∼550) or from contamination of the RG by the products of a nova thermonuclear runaway. In particular, the C and O isotopic ratios taken in combination are a puzzle. We urge confirmation of our results using spectroscopy at high resolution. We also encourage a thorough theoretical study of the effects on the secondary star in an RN system of contamination by ejecta having anomalous abundances and isotopic ratios.

Determining the size of the helium core of KIC 9970396 using asteroseismology: a red giant approaching the red giant bump

ABSTRACT Mixed modes are less affected by the surface effect and carry internal information of a star. Asteroseismic modelling of mixed modes can constrain the size of the helium core of a star. Previous work has shown that the evolution stage of KIC 9970396 may be near the red giant bump phase, which is one of the most interesting regions in the red giant evolution stage. It is known that there is a clear relation between the core and mass luminosity, following homological principles on the low-luminosity giant branch. Our aim in this paper is to determine the size of the helium core and constrain the internal structure of the star. Our approach is first to identify the most p-dominated frequencies in the observational mixed modes. The calculated frequencies are then fitted to the observational frequencies, which we choose not to correct for surface effect offsets. We have computed a grid of theoretical models to fit the l = 1 modes of KIC 9970396. The size of the helium core of KIC 9970396 is determined to be MHe = 0.229 ± 0.001 M⊙ and RHe = 0.03055 ± 0.00015 R⊙. The best-fitting model shows that KIC 9970396 is undergoing a stage that is between the first dredge-up event and the red giant bump. Compared with the results of our model, the previous core–mass luminosity relation overestimates the helium core mass by 0.009 M⊙ when the stellar luminosity of a red giant star is known.

An AGB Star with a Thick Circumstellar Shell

The Asymptotic Giant Branch is the terminal phase of red giant evolution with timescales of millions of years and a total mass lost from the star that is a significant fraction of the initial mass. Investigation of one of these stars, WISEA J173046.10–344455.5, a kpc in the direction of the center of the Galaxy, reveals a cool oxygen rich star with a dust shell of blackbody temperature 1305 K.

Axion-electron decoupling in nucleophobic axion models

The strongest upper bounds on the axion mass come from astrophysical observations like the neutrino burst duration of SN1987A, which depends on the axion couplings to nucleons, or the white-dwarf cooling rates and red-giant evolution, which involve the axion-electron coupling. It has been recently argued that in variants of DFSZ models with generation-dependent Peccei-Quinn charges an approximate axion-nucleon decoupling can occur, strongly relaxing the SN1987A bound. However, as in standard DFSZ models, the axion remains in general coupled to electrons, unless an ad hoc cancellation is engineered. Here we show that axion-electron decoupling can be implemented without extra tunings in DFSZ-like models with three Higgs doublets. Remarkably, the numerical value of the quark mass ratio $m_u/m_d\sim 1/2$ is crucial to open up this possibility.

HD 101584: circumstellar characteristics and evolutionary status

Context. There is growing evidence that red giant evolution is often affected by an interplay with a nearby companion, in some cases taking the form of a common-envelope evolution. Aims. We have performed a study of the characteristics of the circumstellar environment of the binary object HD 101584, that provides information on a likely evolutionary scenario. Methods. We have obtained and analysed ALMA observations, complemented with observations using APEX, of a large number of molecular lines. An analysis of the spectral energy distribution has also been performed. Results. Emissions from 12 molecular species (not counting isotopologues) have been observed, and most of them mapped with angular resolutions in the range 0.′′1–0.′′6. Four circumstellar components are identified: (i) a central compact source of size ≈0.′′15, (ii) an expanding equatorial density enhancement (a flattened density distribution in the plane of the orbit) of size ≈3′′, (iii) a bipolar high-velocity outflow (≈150 km s−1), and (iv) an hourglass structure. The outflow is directed almost along the line of sight. There is evidence of a second bipolar outflow. The mass of the circumstellar gas is ≈0.5 [D/1 kpc]2 M⊙, about half of it lies in the equatorial density enhancement. The dust mass is ≈0.01 [D/1 kpc]2 M⊙, and a substantial fraction of this is in the form of large-sized, up to 1 mm, grains. The estimated kinetic age of the outflow is ≈770 [D/1 kpc] yr. The kinetic energy and the scalar momentum of the accelerated gas are estimated to be 7 × 1045 [D/1 kpc]2 erg and 1039 [D/1 kpc]2 g cm s−1, respectively. Conclusions. We provide good evidence that the binary system HD 101584 is in a post-common-envelope-evolution phase, that ended before a stellar merger. Isotope ratios combined with stellar mass estimates suggest that the primary star’s evolution was terminated already on the first red giant branch (RGB). Most of the energy required to drive the outflowing gas was probably released when material fell towards the companion.

Red giant evolution in modified gravity

In this paper, we study the chameleon profile in inhomogeneous density distributions and find that the fifth force in thin shell near the surface is weaker from what expected in homogeneous density distributions. Also, we check the validity of quasi-static approximation for the chameleon scalar field in the astrophysical time scales. We have investigated the rolling down behavior of the scalar field on its effective potential inside a one solar mass red giant star by using MESA code. We have found that the scalar field is fast enough to follow the minimum of the potential. This adiabatic behavior reduces the fifth force and extends the screened regions to lower densities where the field has smaller mass and was expected to be unscreened. As a consequence, the star evolution is similar to what expected from standard general relativity. In addition, considering the stability of star, an approximate constraint on the coupling constant β is found.

Searching for chemical relics of first stars with LAMOST and Subaru

AbstractWe report progresses of a joint project on searching for extremely metal-poor (EMP) stars based on LAMOST survey and Subaru follow-up observation. Follow-up high-resolution snapshot spectra have been obtained for 70 objects, resulting in 42 EMP stars. A number of chemically interesting objects have already been identified, including (1) Two UMP (ultra metal-poor) stars with [Fe/H] ~ −4.0. One of them is the second UMP turnoff star with Li detection. (2) A super Li-rich (A(Li) ~ 3.1) EMP giant. This is the most metal-poor and extreme example of Li enhancement in giants known to date, and will shed light on Li production during the evolution of red giants. (3) A few EMP stars showing extreme overabundance in heavy elements. Detailed abundances of these extreme objects and statistics obtained by the large sample of EMP stars will provide important constraints on the Galactic halo formation.

STRUCTURAL GLITCHES NEAR THE CORES OF RED GIANTS REVEALED BY OSCILLATIONS IN G-MODE PERIOD SPACINGS FROM STELLAR MODELS

With recent advances in asteroseismology it is now possible to peer into the cores of red giants, potentially providing a way to study processes such as nuclear burning and mixing through their imprint as sharp structural variations -- glitches -- in the stellar cores. Here we show how such core glitches can affect the oscillations we observe in red giants. We derive an analytical expression describing the expected frequency pattern in the presence of a glitch. This formulation also accounts for the coupling between acoustic and gravity waves. From an extensive set of canonical stellar models we find glitch-induced variation in the period spacing and inertia of non-radial modes during several phases of red-giant evolution. Significant changes are seen in the appearance of mode amplitude and frequency patterns in asteroseismic diagrams such as the power spectrum and the \'echelle diagram. Interestingly, along the red-giant branch glitch-induced variation occurs only at the luminosity bump, potentially providing a direct seismic indicator of stars in that particular evolution stage. Similarly, we find the variation at only certain post-helium-ignition evolution stages, namely, in the early phases of helium-core burning and at the beginning of helium-shell burning signifying the asymptotic-giant-branch bump. Based on our results, we note that assuming stars to be glitch-free, while they are not, can result in an incorrect estimate of the period spacing. We further note that including diffusion and mixing beyond classical Schwarzschild, could affect the characteristics of the glitches, potentially providing a way to study these physical processes.

The pulsation modes, masses and evolution of luminous red giants

The period-luminosity sequences and the multiple periods of luminous red giant stars are examined using the OGLE III catalogue of long-period variables in the Large Magellanic Cloud. It is shown that the period ratios in individual multimode stars are systematically different from the ratios of the periods at a given luminosity of different period-luminosity sequences. This leads to the conclusion that the masses of stars at the same luminosity on the different period-luminosity sequences are different. An evolutionary scenario is used to show that the masses of stars on adjacent sequences differ by about 16-26% at a given luminosity, with the shorter period sequence being more massive. The mass is also shown to vary across each sequence by a similar percentage, with the mass increasing to shorter periods. On one sequence, sequence B, the mass distribution is shown to be bimodal. It is shown that the small amplitude variables on sequences A', A and B pulsate in radial and nonradial modes of angular degree l=0, 1 and 2, with the l=1 mode being the most common. The stars on sequences C' and C are predominantly radial pulsators (l=0). Matching period ratios to pulsation models shows that the radial pulsation modes associated with sequences A', A, B, C' and C are the 4th, 3rd, 2nd and 1st overtones and the fundamental mode, respectively.

Understanding angular momentum transport in red giants: the case of KIC 7341231

Context. Thanks to recent asteroseismic observations, it has been possible to infer the radial differential rotation profile of subgiants and red giants. Aims. We want to reproduce through modeling the observed rotation profile of the early red giant KIC 7341231 and constrain the physical mechanisms responsible for angular momentum transport in stellar interiors. Methods. We compute models of KIC 7341231 including a treatment of shellular rotation and we compare the rotation profiles obtained with the one derived by Deheuvels et al. (2012). We then modify some modeling parameters in order to quantify their effect on the obtained rotation profile. Moreover, we mimic a powerful angular momentum transport during the Main Sequence and study its effect on the evolution of the rotation profile during the subgiant and red giant phases. Results. We show that meridional circulation and shear mixing alone produce a rotation profile for KIC 7341231 too steep compared to the observed one. An additional mechanism is then needed to increase the internal transport of angular momentum. We find that this undetermined mechanism has to be efficient not only during the Main Sequence but also during the much quicker subgiant phase. Moreover, we point out the importance of studying the whole rotational history of a star in order to explain its rotation profile during the red giant evolution.

Red Giant evolution and specific problems

In spite of the great effort made in the last decades to improve our understanding of stellar evolution, significant uncertainties still remain due to our poor knowledge of some complex physical processes that still require an empirical calibration, such as the efficiency of convective heat transport and interior mixing. Here we will review the impact of these uncertainties on the evolution of red giant stars.

Lithium abundances along the red giant branch: FLAMES-GIRAFFE spectra of a large sample of low-mass bulge stars

Context: A small number of K-type giants on the red giant branch (RGB) is known to be very rich in lithium (Li). This fact is not accounted for by standard stellar evolution theory. The exact phase and mechanism of Li enrichment is still a matter of debate. Aims: Our goal is to probe the abundance of Li along the RGB, from its base to the tip, to confine Li-rich phases that are supposed to occur on the RGB. Methods: For this end, we obtained medium-resolution spectra with the FLAMES spectrograph at the VLT in GIRAFFE mode for a large sample of 401 low-mass RGB stars located in the Galactic bulge. The Li abundance was measured in the stars with a detectable Li 670.8 nm line by means of spectral synthesis with COMARCS model atmospheres. A new 2MASS (J-K) - Teff calibration from COMARCS models is presented in the Appendix. Results: Thirty-one stars with a detectable Li line were identified, three of which are Li-rich according to the usual criterion ($\log\epsilon({\rm Li})>1.5$). The stars are distributed all along the RGB, not concentrated in any particular phase of the red giant evolution (e.g. the luminosity bump or the red clump). The three Li-rich stars are clearly brighter than the luminosity bump and red clump, and do not show any signs of enhanced mass loss. Conclusions: We conclude that the Li enrichment mechanism cannot be restricted to a clearly defined phase of the RGB evolution of low-mass stars ($M\sim1M_{\sun}$), contrary to earlier suggestions from disk field stars.

ChemInform Abstract: Nuclear Astrophysics: the Unfinished Quest for the Origin of the Elements

AbstractReview: 551 refs.

The magnitude difference between the main sequence turn off and the red giant branch bump in Galactic globular clusters

We present new measurements of the magnitude of the main sequence turn off and the red giant branch bump in the luminosity function of a sample of Galactic globular clusters with updated estimates of [Fe/H] and [α/Fe], employing photometric data collected with the Advanced Camera for Survey onboard the HST. We compare measured and predicted values of the magnitude difference between these two features, a rarely employed diagnostic of the internal structure of low-mass stars at the beginning of their red giant evolution. Our analysis discloses a clear discrepancy between theory and observations, because the theoretical red giant branch bump magnitudes are too bright by on average ~0.2 mag. This corroborates results from the more widely studied magnitude difference between horizontal branch and red giant bump, avoiding the well known problems associated with determining the horizontal branch level from colour-magnitude diagrams and with uncertainties in the luminosity of horizontal branch stellar models. We briefly discuss several potential solutions of this discrepancy.

THE SURVIVAL OF WATER WITHIN EXTRASOLAR MINOR PLANETS

We compute that extrasolar minor planets can retain much of their internal H_2O during their host star's red giant evolution. The eventual accretion of a water-rich body or bodies onto a helium white dwarf might supply an observable amount of atmospheric hydrogen, as seems likely for GD 362. More generally, if hydrogen pollution in helium white dwarfs typically results from accretion of large parent bodies rather than interstellar gas as previously supposed, then H_2O probably constitutes at least 10% of the aggregate mass of extrasolar minor planets. One observational test of this possibility is to examine the atmospheres of externally-polluted white dwarfs for oxygen in excess of that likely contributed by oxides such as SiO_2. The relatively high oxygen abundance previously reported in GD 378 plausibly but not uniquely can be explained by accretion of an H_2O-rich parent body or bodies. Future ultraviolet observations of white dwarf pollutions can serve to investigate the hypothesis that environments with liquid water that are suitable habitats for extremophiles are widespread in the Milky Way.

Red-giant seismic properties analyzed with CoRoT

The CoRoT 5-month long observation runs give us the opportunity to analyze a large variety of red-giant stars and to derive fundamental stellar parameters from their asteroseismic properties. We perform an analysis of more than 4\,600 CoRoT light curves to extract as much information as possible. We take into account the characteristics of the star sample and of the method in order to provide asteroseismic results as unbiased as possible. We also study and compare the properties of red giants of two opposite regions of the Galaxy. We analyze the time series with the envelope autocorrelation function in order to extract precise asteroseismic parameters with reliable error bars. We examine first the mean large frequency separation of solar-like oscillations and the frequency of maximum seismic amplitude, then the parameters of the excess power envelope. With the additional information of the effective temperature, we derive the stellar mass and radius. We have identified more than 1\,800 red giants among the 4\,600 light curves and have obtained accurate distributions of the stellar parameters for about 930 targets. We were able to reliably measure the mass and radius of several hundred red giants. We have derived precise information on the stellar population distribution and on the red clump. Comparison between the stars observed in two different fields shows that the stellar asteroseismic properties are globally similar, but with different characteristics for red-clump stars. This study shows the efficiency of statistical asteroseismology: validating scaling relations allows us to infer fundamental stellar parameters, to derive precise information on the red-giant evolution and interior structure and to analyze and compare stellar populations from different fields.