scholarly journals Period spacings in red giants

2018 ◽  
Vol 618 ◽  
pp. A109 ◽  
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.

scholarly journals Why Do Low-Mass Stars Become Red Giants?

2009 ◽  
Vol 26 (3) ◽  
pp. 203-208 ◽  
Author(s):  
Richard J. Stancliffe ◽  
Alessandro Chieffi ◽  
John C. Lattanzio ◽  
Ross P. Church
Keyword(s):  
Molecular Weight ◽  
Stellar Evolution ◽  
Energy Generation ◽  
Red Giants ◽  
Low Mass Stars ◽  
Change In The Mean ◽  
Low Mass ◽  
The Mean ◽  
Red Giant ◽  
Mean Molecular Weight

AbstractWe revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are discussed. We find that while a change in the mean molecular weight is necessary (but not sufficient) for a 1-M⊙ star to become a red giant, this is not the case in a star of 5 M⊙. It therefore seems that there may be more than one way to make a giant.

scholarly journals Red giant stars: from mixed modes to angular momentum

2019 ◽  
Vol 82 ◽  
pp. 189-211
Author(s):  
K. Belkacem
Keyword(s):  
Angular Momentum ◽  
Current Knowledge ◽  
Red Giants ◽  
Low Mass Stars ◽  
Giant Stars ◽  
Physical Mechanisms ◽  
Star Evolution ◽  
Low Mass ◽  
Mixed Modes ◽  
Red Giant

Solar-like oscillations are ubiquitous to low-mass stars from the main-sequence to the red-giant branch as demonstrated by the space-borne missions CoRoT and Kepler. Understanding the physical mechanisms governing their amplitudes as well as their behavior along with the star evolution is a prerequisite for interpreting the wealth of seismic data and for inferring stellar internal structure. In this paper, I discuss our current knowledge of mode amplitudes with particular emphasis on non-radial modes in red giants (hereafter mixed modes). Then, I will show how these modes permit to unveil the rotation of the inner-most layers of low-mass stars and how they put stringent constraints on the redistribution of angular momentum.

scholarly journals Period spacings in red giants

2017 ◽  
Vol 600 ◽  
pp. A1 ◽  
Author(s):  
B. Mosser ◽  
C. Pinçon ◽  
K. Belkacem ◽  
M. Takata ◽  
M. Vrard
Keyword(s):  
Mixed Mode ◽  
Coupling Factor ◽  
Stellar Structure ◽  
New Method ◽  
Frequency Separation ◽  
Red Giants ◽  
Evolved Stars ◽  
Mode Pattern ◽  
Coupling Factors ◽  
Mixed Modes

Context. The power of asteroseismology relies on the capability of global oscillations to infer the stellar structure. For evolved stars, we benefit from unique information directly carried out by mixed modes that probe their radiative cores. This third article of the series devoted to mixed modes in red giants focuses on their coupling factors, which have remained largely unexploited up to now. Aims. With the measurement of coupling factors, we intend to give physical constraints on the regions surrounding the radiative core and the hydrogen-burning shell of subgiants and red giants. Methods. A new method for measuring the coupling factor of mixed modes was implemented, which was derived from the method recently implemented for measuring period spacings. This new method was automated so that it could be applied to a large sample of stars. Results. Coupling factors of mixed modes were measured for thousands of red giants. They show specific variation with mass and evolutionary stage. Weak coupling is observed for the most evolved stars on the red giant branch only; large coupling factors are measured at the transition between subgiants and red giants as well as in the red clump. Conclusions. The measurement of coupling factors in dipole mixed modes provides a new insight into the inner interior structure of evolved stars. While the large frequency separation and the asymptotic period spacings probe the envelope and core, respectively, the coupling factor is directly sensitive to the intermediate region in between and helps determine its extent. Observationally, the determination of the coupling factor is a prior to precise fits of the mixed-mode pattern and can now be used to address further properties of the mixed-mode pattern, such as the signature of buoyancy glitches and core rotation.

scholarly journals Variations of the mixing character of dipolar mixed modes in red giant stars

2020 ◽  
Vol 495 (1) ◽  
pp. 621-636 ◽  
Author(s):  
C Jiang ◽  
M Cunha ◽  
J Christensen-Dalsgaard ◽  
QS Zhang
Keyword(s):  
Asymptotic Expansion ◽  
Stellar Evolution ◽  
Coupling Strength ◽  
Stellar Model ◽  
Quality Data ◽  
Chemical Abundances ◽  
Giant Stars ◽  
Mixing Character ◽  
Mixed Modes ◽  
Red Giant

ABSTRACT Because of the high-quality data of space missions, the detection of mixed modes has become possible in numerous stars. In this work, we investigate how the mixing character of dipolar mixed modes changes with stellar evolution, as well as with frequency within each stellar model. This is achieved by monitoring the variations in the coupling strength and the period spacing of dipolar mixed modes in red-giant models. These parameters are measured by fitting the asymptotic expansion of mixed modes to the model frequencies of a grid of red-giant models with masses between 1.0 and 2.0 M⊙ and three different chemical abundances. The coupling strength and the period spacing decrease with stellar evolution. We find that the slopes of their decreasing trends depend on the radial order of the pressure mode component. A non-negligible increase of the coupling strength with frequency by up to around 40 per cent is found in the observable frequency range for a set of red-giant models. On the contrary, no significant changes of the period spacing with frequency are found. The changes in the mixing character of the modes are in most cases affected by the model mass and metallicity. Buoyancy glitches also have an impact on the mixing character. Significant fluctuations in the estimated coupling strength and period spacing are found for models approaching the luminosity bump, if the glitch impact of the frequencies is not considered in the applied asymptotic expansion.

scholarly journals Angular momentum redistribution by mixed modes in evolved low-mass stars

2015 ◽  
Vol 579 ◽  
pp. A31 ◽  
Author(s):  
K. Belkacem ◽  
J. P. Marques ◽  
M. J. Goupil ◽  
B. Mosser ◽  
T. Sonoi ◽  
...  
Keyword(s):  
Angular Momentum ◽  
Low Mass Stars ◽  
Low Mass ◽  
Mixed Modes

scholarly journals Fu Orionis eruptions and early stellar evolution

1990 ◽  
Vol 137 ◽  
pp. 229-251
Author(s):  
Bo Reipurth
Keyword(s):  
Stellar Evolution ◽  
Accretion Rate ◽  
Circumstellar Disks ◽  
Circumstellar Disk ◽  
Disk Accretion ◽  
Low Mass Stars ◽  
Photometric And Spectroscopic Observations ◽  
T Tauri ◽  
Accretion Model ◽  
Low Mass

The FU Orionis phenomenon has attracted increasing attention in recent years, and is now accepted as a crucial element in the early evolution of low mass stars. The general characteristics of FUors are outlined and individual members of the class are discussed. The discovery of a new FUor, BBW 76, is presented, together with a discussion of photometric and spectroscopic observations of the star. The evidence for circumstellar disks around T Tauri stars is briefly outlined, and the FUor phenomenon is discussed in the context of a disk accretion model. A large increase in the accretion rate through a circumstellar disk makes the disk self-luminous with a luminosity two or more orders larger than that of the star. Massive cool winds rise from FUors, and it is conceivable that they are related to the initiation of Herbig-Haro flows. The FUor phenomenon appears to be repetitive, and newborn low-mass stars may be cycling between the FUor state and the T Tauri state.

scholarly journals Systematic search for stellar pulsators in the eclipsing binaries observed by Kepler

2019 ◽  
Vol 630 ◽  
pp. A106 ◽  
Author(s):  
Patrick Gaulme ◽  
Joyce A. Guzik
Keyword(s):  
Stellar Evolution ◽  
Percent Level ◽  
Eclipsing Binaries ◽  
Light Curves ◽  
Systematic Search ◽  
Red Giants ◽  
Low Mass Stars ◽  
Orbital Periods ◽  
Red Giant ◽  
Stellar Properties

Eclipsing binaries (EBs) are unique targets for measuring precise stellar properties and can be used to constrain stellar evolution models. In particular, it is possible to measure masses and radii of both components of a double-lined spectroscopic EB at the percent level. Since the advent of high-precision photometric space missions (MOST, CoRoT, Kepler, BRITE, TESS), the use of stellar pulsation properties to infer stellar interiors and dynamics constitutes a revolution for studies of low-mass stars. The Kepler mission has led to the discovery of thousands of classical pulsators such as δ Scuti and solar-like oscillators (main sequence and evolved), but also almost 3000 EBs with orbital periods shorter than 1100 days. We report the first systematic search for stellar pulsators in the entire Kepler EB catalog. The focus is mainly aimed at discovering δ Scuti, γ Doradus, red giant, and tidally excited pulsators. We developed a data inspection tool (DIT) that automatically produces a series of plots from the Kepler light curves that allows us to visually identify whether stellar oscillations are present in a given time series. We applied the DIT to the whole Kepler EB database and identified 303 systems whose light curves display oscillations, including 163 new discoveries. A total of 149 stars are flagged as δ Scuti (100 from this paper), 115 as γ Doradus (69 new), 85 as red giants (27 new), and 59 as tidally excited oscillators (29 new). There is some overlap among these groups, as some display several types of oscillations. Despite the likelihood that many of these systems are false positives, for example, when an EB light curve is blended with a pulsator, this catalog gathers a vast sample of systems that are valuable for a better understanding of stellar evolution.

scholarly journals Stellar models of rotating, pre-main sequence low-mass stars with magnetic fields

2013 ◽  
Vol 9 (S302) ◽  
pp. 112-113 ◽  
Author(s):  
Luiz T. S. Mendes ◽  
Natália R. Landin ◽  
Luiz P. R. Vaz
Keyword(s):  
Magnetic Fields ◽  
Stellar Evolution ◽  
Large Scale ◽  
Main Sequence ◽  
Stellar Structure ◽  
Low Mass Stars ◽  
Large Scale Magnetic Field ◽  
Low Mass ◽  
Structure Equations ◽  
Stellar Models

AbstractWe report our present efforts for introducing magnetic fields in the ATON stellar evolution code code, which now evolved to truly modifying the stellar structure equations so that they can incorporate the effects of an imposed, large-scale magnetic field. Preliminary results of such an approach, as applied to low-mass stellar models, are presented and discussed.

scholarly journals Constraints on stellar evolution from white dwarf asteroseismology

2014 ◽  
Vol 9 (S307) ◽  
pp. 211-212
Author(s):  
Agnès Bischoff-Kim
Keyword(s):  
Stellar Evolution ◽  
Reaction Rate ◽  
Time Dependent ◽  
High Mass ◽  
Low Mass Stars ◽  
Capture Reaction ◽  
The Core ◽  
Inert Core ◽  
Core Phase ◽  
Low Mass

AbstractHigh mass and low mass stars follow a similar evolution until the inert core phase that follows the end of the core helium burning stage. In particular, one common phase of stellar evolution is the alpha capture reaction that turns carbon into oxygen in the core. We can obtain constraints on this reaction rate by studying the remnants of low mass stars, as this is the ultimate reaction that occurs in their core. We also present results that allow us to test the time dependent calculations of diffusion in dense interiors.

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