Disc-binary interactions in depleted post-AGB binaries

Binary post-asymptotic giant branch (post-AGB) stars have orbital periods in the range of 100−2500 days in eccentric orbits. They are surrounded by circumbinary dusty discs. They are the immediate result of unconstrained binary interaction processes. Their observed orbital properties do not correspond to model predictions: Neither the periods nor the high eccentricities are expected. Indeed, many orbits are eccentric despite the strong tidal interaction when the primary had giant dimensions on the red giant branch and AGB. Our goal is to investigate if interactions between a binary and its circumbinary disc during the post-AGB phase can result in their eccentric orbits, while simultaneously explaining the chemical anomaly known as depletion. For this paper, we selected three binaries (EP Lyr, RU Cen, HD 46703) with well-constrained orbits, luminosities, and chemical abundances. We used the MESA code to evolve post-AGB models, while including the accretion of metal-poor gas. This allows us to constrain the evolution of the stars and study the impact of circumbinary discs on the orbital properties of the models. We investigate the effect of torques produced by gas inside the binary cavity and the effect of Lindblad resonances on the orbit, while also including the tidal interaction following the equilibrium tide model. We find that none of our models are able to explain the high orbital eccentricities of the binaries in our sample. The accretion torque does not significantly impact the binary orbit, while Lindblad resonances can pump the eccentricity up to only e ≈ 0.2. At higher eccentricities, the tidal interaction becomes too strong, so the high observed eccentricities cannot be reproduced. However, even if we assume tides to be ineffective, the eccentricities in our models do not exceed ≈0.25. Finally, the orbit of RU Cen is too wide to reproduce with disc-binary interactions by starting from a circular orbit. We conclude that either our knowledge of disc-binary interactions is still incomplete, or the binaries must have left their phase of strong interaction in an eccentric orbit.

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Omega Centauri: weak MgH band in red giants directly traces the helium content

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1081 ◽

2020 ◽

Vol 495 (1) ◽

pp. 383-401

Author(s):

Arumalla B S Reddy

Keyword(s):

Signal To Noise Ratio ◽

Asymptotic Giant Branch ◽

High Spectral Resolution ◽

High Signal ◽

Synthetic Spectrum ◽

Red Giants ◽

Chemical Abundances ◽

Rich Cluster ◽

Red Giant ◽

The Impact

ABSTRACT High spectral resolution and high signal-to-noise ratio optical spectra of red giants in the globular cluster Omega Centauri are analysed for stellar parameters and chemical abundances of 15 elements including helium by either line equivalent widths or synthetic spectrum analyses. The simultaneous abundance analysis of MgH and Mg lines adopting theoretical photospheres and a combination of He/H ratios proved to be the only powerful probe to evaluate helium abundances of red giants cooler than 4400 K, wherein otherwise helium line transitions (He i 10830 and 5876 Å) present for a direct spectral line analysis. The impact of helium-enhanced model photospheres on the resulting abundance ratios is smaller than 0.15 dex, in agreement with past studies. The first indirect spectroscopic helium abundances measured in this paper for the most metal-rich cluster members reveal the discovery of seven He-enhanced giants ($\Delta Y=+0.15 \pm 0.04$), the largest such sample found spectroscopically to date. The average metallicity of −0.79 ± 0.06 dex and abundances for O, Na, Al, Si, Ca, Ti, Ni, Ba, and La are consistent with values found for the red giant branch (RGB-a) and subgiant branch (SGB-a) populations of Omega Centauri, suggesting an evolutionary connection among samples. The He enhancement in giants is associated with larger s-process elemental abundances, which correlate with Al and anticorrelate with O. These results support the formation of He-enhanced, metal-rich population of Omega Centauri out of the interstellar medium enriched with the ejecta of fast rotating massive stars, binaries exploding as supernovae, and asymptotic giant branch (AGB) stars.

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Infrared interferometric imaging of the compact dust disk around the AGB star HR3126 with the bipolar Toby Jug Nebula

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038577 ◽

2020 ◽

Vol 643 ◽

pp. A175

Author(s):

K. Ohnaka ◽

D. Schertl ◽

K.-H. Hofmann ◽

G. Weigelt

Keyword(s):

Gravity Data ◽

Spectral Energy Distribution ◽

Central Star ◽

Asymptotic Giant Branch ◽

Binary Interaction ◽

Spectral Energy ◽

Interferometric Imaging ◽

Long Baseline ◽

Dust Disk ◽

Red Giant

Aims. The asymptotic giant branch (AGB) star HR3126, associated with the arcminute-scale bipolar Toby Jug Nebula, provides a rare opportunity to study the emergence of bipolar structures at the end of the AGB phase. Our goal is to image the central region of HR3126 with high spatial resolution. Methods. We carried out long-baseline interferometric observations with AMBER and GRAVITY (2–2.45 μm) at the Very Large Telescope Interferometer with spectral resolutions of 1500 and 4500, speckle interferometric observations with VLT/NACO (2.24 μm), and imaging with SPHERE-ZIMPOL (0.55 μm) and VISIR (7.9–19.5 μm). Results. The images reconstructed in the continuum at 2.1–2.29 μm from the AMBER+GRAVITY data reveal the central star surrounded by an elliptical ring-like structure with a semimajor and semiminor axis of 5.3 and 3.5 mas, respectively. The ring is interpreted as the inner rim of an equatorial dust disk viewed from an inclination angle of ~50°, and its axis is approximately aligned with the arcminute-scale bipolar nebula. The disk is surprisingly compact, with an inner radius of a mere 3.5 R⋆ (2 au). Our 2-D radiative transfer modeling shows that an optically thick flared disk with silicate grains as large as ~4 μm can simultaneously reproduce the observed continuum images and the spectral energy distribution. The images reconstructed in the CO first overtone bands reveal elongated extended emission around the central star, suggesting the oblateness of the star’s atmosphere or the presence of a CO gas disk inside the dust cavity. The object is unresolved with SPHERE-ZIMPOL, NACO, and VISIR. Conclusions. If the disk formed together with the bipolar nebula, the grain growth from sub-micron to a few microns should have taken place over the nebula’s dynamical age of ~3900 yrs. The non-detection of a companion in the reconstructed images implies that either its 2.2 μm brightness is more than ~30 times lower than that of the red giant or it might have been shredded due to binary interaction.

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The eccentric behaviour of windy binary stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935625 ◽

2019 ◽

Vol 629 ◽

pp. A103 ◽

Cited By ~ 5

Author(s):

M. I. Saladino ◽

O. R. Pols

Keyword(s):

Mass Transfer ◽

Binary Stars ◽

Numerical Models ◽

Orbital Evolution ◽

Small Sample ◽

Asymptotic Giant Branch ◽

Tidal Effects ◽

Orbital Parameters ◽

Orbital Periods ◽

The Impact

Carbon-enhanced metal-poor stars, CH stars, barium stars, and extrinsic S stars, among other classes of chemically peculiar stars, are thought to be the products of the interaction of low- and intermediate-mass binaries, which occurred when the most evolved star was in the asymptotic giant branch (AGB) phase. Binary evolution models predict that because of the large sizes of AGB stars, if the initial orbital periods of such systems are shorter than a few thousand days, their orbits should have circularised due to tidal effects. However, observations of the progeny of AGB binary stars show that many of these objects have substantial eccentricities, up to e ≈ 0.9. In this work we explore the impact of wind mass transfer on the orbital parameters of AGB binary stars by performing numerical simulations in which the AGB wind is modelled using a hydrodynamical code and the dynamics of the stars is evolved using an N-body code. We find that in most models the effect of wind mass transfer contributes to the circularisation of the orbit, but on longer timescales than tidal circularisation if e ≲ 0.4. For relatively low initial wind velocities and pseudo-synchronisation of the donor star, we find a structure resembling wind Roche-lobe overflow as the stars approach periastron. In this case, the interaction between the gas and the star is stronger than when the initial wind velocity is high and the orbit shrinks while the eccentricity decreases. In one of our models wind interaction is found to pump the eccentricity of the orbit on a similar timescale as tidal circularisation. However, since the orbit of this model is shrinking tidal effects will become stronger during the evolution of the system. Although our study is based on a small sample of models, it offers some insight into the orbital evolution of eccentric binary stars interacting via winds. A larger grid of numerical models for different binary parameters is needed to test if a regime exists where hydrodynamical eccentricity pumping can effectively counteract tidal circularisation, and if this can explain the puzzling eccentricities of the descendants of AGB binaries.

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Spectroscopic binaries RV Tauri and DF Cygni

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834956 ◽

2019 ◽

Vol 628 ◽

pp. A40 ◽

Cited By ~ 2

Author(s):

Rajeev Manick ◽

Devika Kamath ◽

Hans Van Winckel ◽

Alain Jorissen ◽

Sanjay Sekaran ◽

...

Keyword(s):

Orbital Period ◽

Asymptotic Giant Branch ◽

Spectroscopic Binaries ◽

Spectral Energy ◽

Pulsation Frequency ◽

Photometric Variability ◽

Orbital Inclination ◽

Orbital Periods ◽

The Impact

Context. Some RV Tauri stars show a long-term photometric variability in their mean magnitudes. DF Cygni (DF Cyg), the only RV Tauri star in the original Kepler field, and the prototype RV Tauri (RV Tau) are two such stars. Aims. The focus of this paper is on two famous but still poorly understood RV Tauri stars: RV Tau and DF Cyg. We aim to confirm their suspected binary nature and derive their orbital elements to investigate the impact of their orbits on the evolution of these systems. This research is embedded in a wider endeavour to study binary evolution of low- and intermediate-mass stars. Methods. The high amplitude pulsations were cleaned from the radial-velocity data to better constrain the orbital motion, allowing us to obtain accurate orbital parameters. We also analysed the photometric time series of both stars using a Lomb-Scargle periodogram. We used Gaia Data Release 2 (DR2) parallaxes in combination with the spectral energy distributions (SEDs) to compute their luminosities. These luminosities were complemented with the ones we computed using a period-luminosity-colour (PLC) relation for RV Tauri stars. The ratio of the circumstellar infrared (IR) flux to the photospheric flux obtained from the SEDs was used to estimate the orbital inclination of each system. Results. DF Cyg and RV Tau are binaries with spectroscopic orbital periods of 784 ± 16 days and 1198 ± 17 days, respectively. These orbital periods are found to be similar to the long-term periodic variability in the photometry, indicating that binarity indeed explains the long-term photometric variability. The SEDs of these systems indicate the presence of a circumbinary disc. Our line of sight grazes the dusty disc, which causes the photometric flux from the star to extinct periodically with the orbital period. Our derived orbital inclinations enabled us to obtain accurate companion masses for DF Cyg and RV Tau, and these were found to be 0.6 ± 0.1 M⊙ and 0.7 ± 0.1 M⊙, respectively. The derived luminosities suggest that RV Tau is a post asymptotic giant branch (post-AGB) binary, while DF Cyg is likely a post red giant branch (post-RGB) binary. Analysis of the Kepler photometry of DF Cyg revealed a power spectrum with side lobes around the fundamental pulsation frequency. This modulation corresponds to the spectroscopic orbital period and hence to the long-term photometric period. Finally we report on the evidence of high velocity absorption features related to the Hα profile in both objects, indicating outflows launched from around the companion.

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Constraining Roche-Lobe Overflow Models Using the Hot-Subdwarf Wide Binary Population

Open Astronomy ◽

10.1515/astro-2017-0443 ◽

2017 ◽

Vol 26 (1) ◽

pp. 275-279

Author(s):

Joris Vos ◽

Maja Vučković

Keyword(s):

Current Model ◽

Interaction Model ◽

Theoretical Models ◽

Roche Lobe ◽

Binary Interaction ◽

Long Term Study ◽

Gravitational Pull ◽

Red Giant ◽

The Impact

AbstractOne of the important issues regarding the final evolution of stars is the impact of binarity. A rich zoo of peculiar, evolved objects are born from the interaction between the loosely bound envelope of a giant, and the gravitational pull of a companion. However, binary interactions are not understood from first principles, and the theoretical models are subject to many assumptions. It is currently agreed upon that hot subdwarf stars can only be formed through binary interaction, either through common envelope ejection or stable Roche-lobe overflow (RLOF) near the tip of the red giant branch (RGB). These systems are therefore an ideal testing ground for binary interaction models. With our long term study of wide hot subdwarf (sdB) binaries we aim to improve our current understanding of stable RLOF on the RGB by comparing the results of binary population synthesis studies with the observed population. In this article we describe the current model and possible improvements, and which observables can be used to test different parts of the interaction model.

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The discovery of a Li/Na-rich giant star in Omega Centauri: formed from the pure ejecta of super-AGB stars?

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834497 ◽

2019 ◽

Vol 623 ◽

pp. A55 ◽

Cited By ~ 2

Author(s):

A. Mucciarelli ◽

L. Monaco ◽

P. Bonifacio ◽

M. Salaris ◽

X. Fu ◽

...

Keyword(s):

Chemical Composition ◽

Stellar System ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Chemical Abundances ◽

Giant Star ◽

Giant Stars ◽

Unique Object ◽

Red Giant ◽

First Time

We report the discovery of two Li-rich giant stars (fainter than the red giant branch (RGB) bump) in the stellar system Omega Centauri using GIRAFFE-FLAMES spectra. These two stars have A(Li) = 1.65 and 2.40 dex and they belong to the main population of the system ([Fe/H] = –1.70 and –1.82, respectively). The most Li-rich of them (#25664) has [Na/Fe] = +0.87 dex, which is ~0.5 dex higher than those measured in the most Na-rich stars of Omega Centauri of similar metallicity. The chemical abundances of Li and Na in #25664 can be qualitatively explained by deep extra mixing efficient within the star during its RGB evolution or by super-asymptotic giant branch (AGB) stars with masses between ~7 and 8 M⊙. In the latter scenario, this Li/Na-rich star could be formed from the pure ejecta of super-AGB stars before the dilution with pristine material occurs, or, alternatively, be part of a binary system having experienced mass transfer from the companion when this latter evolved through the super-AGB phase. In both these cases, the chemical composition of this unique object could allow to look for the first time at the chemical composition of the gas processed in the interior of super-AGB stars.

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Rotation of the Mass Donors in High-mass X-ray Binaries and Symbiotic Stars

Acta Polytechnica CTU Proceedings ◽

10.14311/app.2015.02.0286 ◽

2015 ◽

Vol 2 (1) ◽

pp. 286-290

Author(s):

K. Stoyanov ◽

R. Zamanov

Keyword(s):

Rotational Velocity ◽

High Mass ◽

Symbiotic Stars ◽

Orbital Eccentricity ◽

X Ray ◽

Tidal Interaction ◽

Theoretical Predictions ◽

Orbital Periods ◽

Red Giant ◽

X Ray Binaries

Our aim is to investigate the tidal interaction in High-mass X-ray Binaries and Symbiotic stars in order to determine in which objects the rotation of the mass donors is synchronized or pseudosynchronized with the orbital motion of the compact companion. We find that the Be/X-ray binaries are not synchronized and the orbital periods of the systems are greater than the rotational periods of the mass donors. The giant and supergiant High-mass X-ray binaries and symbiotic stars are close to synchronization. We compare the rotation of mass donors in symbiotics with the projected rotational velocities of field giants and find that the M giants in S-type symbiotics rotate on average 1.5 times faster than the field M giants. We find that the projected rotational velocity of the red giant in symbiotic star MWC 560 is v sin i= 8.2±1.5 km.s−1, and estimate its rotational period to be Prot<>/sub = 144 - 306 days. Using the theoretical predictions of tidal interaction and pseudosynchronization, we estimate the orbital eccentricity e = 0.68 − 0.82.

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The chemical evolution of the dwarf spheroidal galaxy Sextans

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937146 ◽

2020 ◽

Vol 642 ◽

pp. A176 ◽

Cited By ~ 1

Author(s):

R. Theler ◽

P. Jablonka ◽

R. Lucchesi ◽

C. Lardo ◽

P. North ◽

...

Keyword(s):

Star Formation ◽

Galaxy Evolution ◽

Dwarf Galaxy ◽

Small Mass ◽

Asymptotic Giant Branch ◽

Asymptotic Giant Branch Stars ◽

Chemical Abundances ◽

Low Metallicity ◽

Red Giant ◽

Giant Branch Stars

We present our analysis of the FLAMES dataset targeting the central 25′ region of the Sextans dwarf spheroidal galaxy (dSph). This dataset is the third major part of the high-resolution spectroscopic section of the ESO large program 171.B-0588(A) obtained by the Dwarf galaxy Abundances and Radial-velocities Team. Our sample is composed of red giant branch stars down to V ∼ 20.5 mag, the level of the horizontal branch in Sextans, and allows users to address questions related to both stellar nucleosynthesis and galaxy evolution. We provide metallicities for 81 stars, which cover the wide [Fe/H] = −3.2 to −1.5 dex range. The abundances of ten other elements are derived: Mg, Ca, Ti, Sc, Cr, Mn, Co, Ni, Ba, and Eu. Despite its small mass, Sextans is a chemically evolved system, showing evidence of a contribution from core-collapse and Type Ia supernovae as well as low-metallicity asymptotic giant branch stars (AGBs). This new FLAMES sample offers a sufficiently large number of stars with chemical abundances derived with high accuracy to firmly establish the existence of a plateau in [α/Fe] at ∼0.4 dex followed by a decrease above [Fe/H] ∼ −2 dex. These features reveal a close similarity with the Fornax and Sculptor dSphs despite their very different masses and star formation histories, suggesting that these three galaxies had very similar star formation efficiencies in their early formation phases, probably driven by the early accretion of smaller galactic fragments, until the UV-background heating impacted them in different ways. The parallel between the Sculptor and Sextans dSph is also striking when considering Ba and Eu. The same chemical trends can be seen in the metallicity region common to both galaxies, implying similar fractions of SNeIa and low-metallicity AGBs. Finally, as to the iron-peak elements, the decline of [Co/Fe] and [Ni/Fe] above [Fe/H] ∼ −2 implies that the production yields of Ni and Co in SNeIa are lower than that of Fe. The decrease in [Ni/Fe] favours models of SNeIa based on the explosion of double-degenerate sub-Chandrasekhar mass white dwarfs.

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Influence of Planets on Parent Stars: Angular Momentum

Symposium - International Astronomical Union ◽

10.1017/s0074180900182300 ◽

2004 ◽

Vol 219 ◽

pp. 323-332

Author(s):

Noam Soker

Keyword(s):

Angular Momentum ◽

Globular Clusters ◽

Hubble Space Telescope ◽

Mass Loss Rate ◽

Planetary Nebulae ◽

Asymptotic Giant Branch ◽

Tidal Interaction ◽

Evolved Stars ◽

The Galaxy ◽

Red Giant

I review some possible processes by which planets and brown dwarfs can influence the evolution of their parent evolved stars. As sunlike stars evolve on the red giant branch (RGB) and then on the asymptotic giant branch (AGB), they will interact with their close planets (if exist). The interaction starts with tidal interaction: this will lead the planets to deposit most of their angular momentum to the envelope of the giant, and then spiral-in to the envelope. (Too many papers dealing with close planets [less than about 3-6 AU] around evolved stars neglect tidal interaction, hence their results are questionable.) They may spin-up their parent stars by up to several orders of magnitude. The interaction of substellar objects with evolved star may enhance the mass loss rate, mainly in the equatorial plane. Possible outcomes are: (i) Planetary systems interacting with their parent AGB star may lead to the formation of moderate elliptical planetary nebulae. (ii) RGB stars which lose more mass turn to bluer horizontal branch (HB) stars. Therefore, planets may explain the formation of blue HB stars. This may explain the presence of many blue HB stars in many globular clusters (the planets be the second parameter), and some hot HB stars in the galaxy (sdB stars). (The 8.3 days use of the Hubble Space Telescope in search of planets in a globular clusters with no blue HB stars was a wrong move.) (iii) Most known stars with planets will not form planetary nebulae, because they will lose most of their envelope already on the RGB.

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Lithium in red giant stars: Constraining non-standard mixing with large surveys in the Gaia era

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936360 ◽

2020 ◽

Vol 633 ◽

pp. A34 ◽

Cited By ~ 1

Author(s):

C. Charbonnel ◽

N. Lagarde ◽

G. Jasniewicz ◽

P. L. North ◽

M. Shetrone ◽

...

Keyword(s):

Stellar Evolution ◽

Asymptotic Giant Branch ◽

Mixing Processes ◽

Giant Stars ◽

Red Giant Stars ◽

Red Giant ◽

Good Tracer ◽

The Impact ◽

Maximum Likelihood Technique ◽

Evolution Status

Context. Li is extensively known to be a good tracer of non-standard mixing processes occurring in stellar interiors. Aims. We present the results of a new large Li survey in red giant stars and combine it with surveys from the literature to probe the impact of rotation-induced mixing and thermohaline double-diffusive instability along stellar evolution. Methods. We determined the surface Li abundance for a sample of 829 giant stars with accurate Gaia parallaxes for a large sub-sample (810 stars) complemented with accurate HIPPARCOS parallaxes (19 stars). The spectra of our sample of northern and southern giant stars were obtained in three ground-based observatories (Observatoire de Haute-Provence, ESO-La Silla, and the Mc Donald Observatory). We determined the atmospheric parameters (Teff, log(g) and [Fe/H]), and the Li abundance. We used Gaia parallaxes and photometry to determine the luminosity of our objects and we estimated the mass and evolution status of each sample star with a maximum-likelihood technique using stellar evolution models computed with the STAREVOL code. We compared the observed Li behaviour with predictions from stellar models, including rotation and thermohaline mixing. The same approach was used for stars from selected Li surveys from the literature. Results. Rotation-induced mixing accounts nicely for the Li behaviour in stars warmer than about 4200 K, independently of the mass domain. For stars with masses lower than 2 M⊙ thermohaline mixing leads to further Li depletion below the Teff of the RGB bump (about 4000 K), and on the early asymptotic giant branch, as observed. Depending on the definition we adopt, we find between 0.8 and 2.2% of Li-rich giants in our new sample. Conclusions.Gaia puts a new spin on the understanding of mixing processes in stars, and our study confirms the importance of rotation-induced processes and of thermohaline mixing. However asteroseismology is required to definitively pinpoint the actual evolution status of Li-rich giants.

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