Asteroseismogyrometry of low-mass red giants. I. The SOLA inversion method

Hot subdwarfs (sdO/Bs) are the helium-burning cores of red giants, which have lost almost all of their hydrogen envelope. This mass loss is often triggered by common envelope interactions with close stellar or even substellar companions. Cool companions like late-type stars or brown dwarfs are detectable via characteristic light-curve variations like reflection effects and often also eclipses. To search for such objects, we obtained multi-band light curves of 26 close sdO/B binary candidates from the MUCHFUSS project with the BUSCA instrument. We discovered a new eclipsing reflection effect system (P = 0.168938 d) with a low-mass M dwarf companion (0.116 M⊙). Three more reflection effect binaries found in the course of the campaign have already been published; two of them are eclipsing systems, and in one system only showing the reflection effect but no eclipses, the sdB primary is found to be pulsating. Amongst the targets without reflection effect a new long-period sdB pulsator was discovered and irregular light variations were found in two sdO stars. The found light variations allowed us to constrain the fraction of reflection effect binaries and the substellar companion fraction around sdB stars. The minimum fraction of reflection effect systems amongst the close sdB binaries might be greater than 15% and the fraction of close substellar companions in sdB binaries may be as high as 8.0%. This would result in a close substellar companion fraction to sdB stars of about 3%. This fraction is much higher than the fraction of brown dwarfs around possible progenitor systems, which are solar-type stars with substellar companions around 1 AU, as well as close binary white dwarfs with brown dwarf companions. This might suggest that common envelope interactions with substellar objects are preferentially followed by a hot subdwarf phase.

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Tidal Interactions between Binary Stars Can Drive Lithium Production in Low-mass Red Giants

The Astrophysical Journal ◽

10.3847/1538-4357/ab27bf ◽

2019 ◽

Vol 880 (2) ◽

pp. 125 ◽

Cited By ~ 14

Author(s):

Andrew R. Casey ◽

Anna Y. Q. Ho ◽

Melissa Ness ◽

David W. Hogg ◽

Hans-Walter Rix ◽

...

Keyword(s):

Binary Stars ◽

Red Giants ◽

Tidal Interactions ◽

Low Mass

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Planetary nebulae after common-envelope phases initiated by low-mass red giants

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stt1422 ◽

2013 ◽

Vol 435 (3) ◽

pp. 2048-2059 ◽

Cited By ~ 22

Author(s):

Philip D. Hall ◽

Christopher A. Tout ◽

Robert G. Izzard ◽

Denise Keller

Keyword(s):

Planetary Nebulae ◽

Red Giants ◽

Common Envelope ◽

Low Mass

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Compulsory Deep Mixing of3He and CNO Isotopes in the Envelopes of Low‐Mass Red Giants

The Astrophysical Journal ◽

10.1086/529024 ◽

2008 ◽

Vol 677 (1) ◽

pp. 581-592 ◽

Cited By ~ 95

Author(s):

Peter P. Eggleton ◽

David S. P. Dearborn ◽

John C. Lattanzio

Keyword(s):

Red Giants ◽

Deep Mixing ◽

Low Mass

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Active red giants: Close binaries versus single rapid rotators

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037781 ◽

2020 ◽

Vol 639 ◽

pp. A63

Author(s):

Patrick Gaulme ◽

Jason Jackiewicz ◽

Federico Spada ◽

Drew Chojnowski ◽

Benoît Mosser ◽

...

Keyword(s):

Angular Momentum ◽

Close Binaries ◽

Red Giants ◽

Intermediate Mass ◽

Giant Stars ◽

Rotational Modulation ◽

Low Mass ◽

Red Giant Stars ◽

Red Giant ◽

Giant Branch Stars

Oscillating red-giant stars have provided a wealth of asteroseismic information regarding their interiors and evolutionary states, which enables detailed studies of the Milky Way. The objective of this work is to determine what fraction of red-giant stars shows photometric rotational modulation, and understand its origin. One of the underlying questions is the role of close binarity in this population, which relies on the fact that red giants in short-period binary systems (less than 150 days or so) have been observed to display strong rotational modulation. We selected a sample of about 4500 relatively bright red giants observed by Kepler, and show that about 370 of them (∼8%) display rotational modulation. Almost all have oscillation amplitudes below the median of the sample, while 30 of them are not oscillating at all. Of the 85 of these red giants with rotational modulation chosen for follow-up radial-velocity observation and analysis, 34 show clear evidence of spectroscopic binarity. Surprisingly, 26 of the 30 nonoscillators are in this group of binaries. On the contrary, about 85% of the active red giants with detectable oscillations are not part of close binaries. With the help of the stellar masses and evolutionary states computed from the oscillation properties, we shed light on the origin of their activity. It appears that low-mass red-giant branch stars tend to be magnetically inactive, while intermediate-mass ones tend to be highly active. The opposite trends are true for helium-core burning (red clump) stars, whereby the lower-mass clump stars are comparatively more active and the higher-mass ones are less active. In other words, we find that low-mass red-giant branch stars gain angular momentum as they evolve to clump stars, while higher-mass ones lose angular momentum. The trend observed with low-mass stars leads to possible scenarios of planet engulfment or other merging events during the shell-burning phase. Regarding intermediate-mass stars, the rotation periods that we measured are long with respect to theoretical expectations reported in the literature, which reinforces the existence of an unidentified sink of angular momentum after the main sequence. This article establishes strong links between rotational modulation, tidal interactions, (surface) magnetic fields, and oscillation suppression. There is a wealth of physics to be studied in these targets that is not available in the Sun.

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Period spacings in red giants

Astronomy and Astrophysics ◽

10.1051/0004-6361/201832777 ◽

2018 ◽

Vol 618 ◽

pp. A109 ◽

Cited By ~ 19

Author(s):

B. Mosser ◽

C. Gehan ◽

K. Belkacem ◽

R. Samadi ◽

E. Michel ◽

...

Keyword(s):

Asymptotic Expansion ◽

Stellar Evolution ◽

Mixed Mode ◽

Frequency Resolution ◽

Red Giants ◽

Large Set ◽

Low Mass Stars ◽

Mode Pattern ◽

Low Mass ◽

Mixed Modes

Context. Oscillation modes with a mixed character, as observed in evolved low-mass stars, are highly sensitive to the physical properties of the innermost regions. Measuring their properties is therefore extremely important to probe the core, but requires some care, due to the complexity of the mixed-mode pattern. Aims. The aim of this work is to provide a consistent description of the mixed-mode pattern of low-mass stars, based on the asymptotic expansion. We also study the variation of the gravity offset εg with stellar evolution. Methods. We revisit previous works about mixed modes in red giants and empirically test how period spacings, rotational splittings, mixed-mode widths, and heights can be estimated in a consistent view, based on the properties of the mode inertia ratios. Results. From the asymptotic fit of the mixed-mode pattern of a large set of red giants at various evolutionary stages, we derive unbiased and precise asymptotic parameters. As the asymptotic expansion of gravity modes is verified with a precision close to the frequency resolution for stars on the red giant branch (10−4 in relative values), we can derive accurate values of the asymptotic parameters. We decipher the complex pattern in a rapidly rotating star, and explain how asymmetrical splittings can be inferred. We also revisit the stellar inclinations in two open clusters, NGC 6819 and NGC 6791: our results show that the stellar inclinations in these clusters do not have privileged orientation in the sky. The variation of the asymptotic gravity offset with stellar evolution is investigated in detail. We also derive generic properties that explain under which conditions mixed modes can be observed.

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Low-Mass Red Giants: Helium or Nitrogen Flash?

The Astrophysical Journal ◽

10.1086/149550 ◽

1968 ◽

Vol 152 ◽

pp. 345 ◽

Cited By ~ 4

Author(s):

Peter P. Eggleton

Keyword(s):

Red Giants ◽

Low Mass

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A new method to identify subclasses among AGB stars using Gaia and 2MASS photometry

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833615 ◽

2018 ◽

Vol 616 ◽

pp. L13 ◽

Cited By ~ 17

Author(s):

T. Lebzelter ◽

N. Mowlavi ◽

P. Marigo ◽

G. Pastorelli ◽

M. Trabucchi ◽

...

Keyword(s):

Stellar Population ◽

Chemical Properties ◽

Large Magellanic Cloud ◽

Asymptotic Giant Branch ◽

Red Giants ◽

Agb Stars ◽

Population Synthesis ◽

Stellar Population Synthesis ◽

Low Mass ◽

Spectral Ranges

Aims. We explore the wealth of high-quality photometric data provided by data release 2 (DR2) of the Gaia mission for long-period variables (LPVs) in the Large Magellanic Cloud (LMC). Our goal is to identify stars of various types and masses along the asymptotic giant branch. Methods. For this endeavour, we developed a new multi-band approach combining Wesenheit functions WRP,BP−RP and WKs,J−Ks in the Gaia BP, RP, and 2MASS J, Ks spectral ranges, respectively, and use a new diagram, (WRP,BP−RP − WKs,J−Ks) versus Ks, to distinguish between different kinds of stars in our sample of LPVs. We used stellar population synthesis models to validate our approach. Results. We demonstrate the ability of the new diagram to discriminate between O- and C-rich objects, and to identify low-mass, intermediate-mass, and massive O-rich red giants, as well as extreme C-rich stars. Stellar evolution and population synthesis models guide the interpretation of the results, highlighting the diagnostic power of the new tool to discriminate between stellar initial masses, chemical properties, and evolutionary stages.

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