Observations of Low Mass Stars in Clusters: Some Constraints and Puzzles for Stellar Evolution Theory

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.

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

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314001847 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 112-113 ◽

Cited By ~ 1

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.

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Constraints on stellar evolution from white dwarf asteroseismology

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314006747 ◽

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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Why Do Low-Mass Stars Become Red Giants?

Publications of the Astronomical Society of Australia ◽

10.1071/as08060 ◽

2009 ◽

Vol 26 (3) ◽

pp. 203-208 ◽

Cited By ~ 17

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.

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The EBLM Project

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834539 ◽

2019 ◽

Vol 625 ◽

pp. A150 ◽

Cited By ~ 7

Author(s):

Alexander von Boetticher ◽

Amaury H. M. J. Triaud ◽

Didier Queloz ◽

Sam Gill ◽

Pierre F. L. Maxted ◽

...

Keyword(s):

Physical Properties ◽

Stellar Evolution ◽

Binary Systems ◽

Close Binary ◽

Low Mass Stars ◽

Orbital Parameters ◽

Eclipse Observations ◽

Low Mass ◽

Orbital Periods ◽

Host Stars

Measurements of the physical properties of stars at the lower end of the main sequence are scarce. In this context we report masses, radii and surface gravities of ten very-low-mass stars in eclipsing binary systems, with orbital periods of the order of several days. The objects probe the stellar mass-radius relation in the fully convective regime, M⋆ ≲ 0.35 M⊙, down to the hydrogen burning mass-limit, MHB ∼ 0.07 M⊙. The stars were detected by the WASP survey for transiting extra-solar planets, as low-mass, eclipsing companions orbiting more massive, F- and G-type host stars. We use eclipse observations of the host stars, performed with the TRAPPIST, Leonhard Euler and SPECULOOS telescopes, and radial velocities of the host stars obtained with the CORALIE spectrograph, to determine the physical properties of the low-mass companions. Surface gravities of the low-mass companions are derived from the eclipse and orbital parameters of each system. Spectroscopic measurements of the host star effective temperature and metallicity are used to infer the host star mass and age from stellar evolution models for solar-type stars. Masses and radii of the low-mass companions are then derived from the eclipse and orbital parameters of the binary systems. The objects are compared to stellar evolution models for low-mass stars, to test for an effect of the stellar metallicity and orbital period on the radius of low-mass stars in close binary systems. Measurements are found to be in good agreement with stellar evolution models; a systematic inflation of the radius of low-mass stars with respect to model predictions is limited to 1.6 ± 1.2%, in the fully convective low-mass regime. The sample of ten objects indicates a scaling of the radius of low-mass stars with the host star metallicity. No correlation between stellar radii and the orbital periods of the binary systems is determined. A combined analysis with thirteen comparable objects from the literature is consistent with this result.

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Effect of mass gain on stellar evolution

International Astronomical Union Colloquium ◽

10.1017/s0252921100095038 ◽

1981 ◽

Vol 59 ◽

pp. 361-371

Author(s):

R. Ebert ◽

H. Zinnecker

Keyword(s):

Stellar Evolution ◽

Main Sequence ◽

Accretion Rate ◽

Mass Gain ◽

Point Mass ◽

Sequence Evolution ◽

Main Sequence Stars ◽

Low Mass Stars ◽

Low Mass ◽

Cloud Material

AbstractIn this paper we present a fully hydrodynamical treatment of the stationary isothermal accretion problem onto a moving gravitating point mass. The derivation is purely analytical. We find that the accretion rate is more than a factor of 50 higher than the accretion rate derived from the partially non-hydrodynamical treatment by Hoyle and Lyttleton (1939) or Bondi and Hoyle (1944). This result may have some bearing on the evolutionary tracks of young pre-Main Sequence stars still embedded in their parent protocluster cloud. We discuss the work by Federova (1979) who investigated the pre-Main Sequence evolution of degenerate low mass ‘stars’ with strong accretion of protocluster cloud material. We suggest that the stars which lie below the Main Sequence in young clusters could strongly accrete matter at the pre-Main Sequence stage.

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Stellar evolution with turbulent diffusion mixing in low mass stars and 12C/13C ratio in giants of the first ascending branch

Symposium - International Astronomical Union ◽

10.1017/s0074180900031417 ◽

1984 ◽

Vol 105 ◽

pp. 525-527

Author(s):

O. Bienaymé ◽

A. Maeder ◽

E. Schatzman

Keyword(s):

Molecular Weight ◽

Stellar Evolution ◽

Main Sequence ◽

Turbulent Transport ◽

Chemical Species ◽

Life Time ◽

Low Mass Stars ◽

Low Mass ◽

Red Giant ◽

Mean Molecular Weight

We consider stellar evolution in low mass stars (1–3 Mo) near the main sequence with the hypothesis that mild turbulence is present within the all star. Turbulent transport of the elements is modeled by diffusion equations where the diffusion coefficient is chosen to be D = R✶eν where ν is the kinematical viscosity and R✶e is a Reynolds number. We consider the effects of the growth of the gradient of the mean molecular weight on turbulence. The main consequences of diffusion on stellar evolution are (1) an increase of the life time near the main sequence and (2) a change of the radial distributions of chemical species (12C, 13C, 14N, 160) (figure 1). The inhibition of the turbulence, when the gradient of mean molecular weight reaches a certain critical value, allows the evolution towards the red giant branch. When stars evolve towards the giant branch, chemical species are dredged up to the surface. At this stage models with and without diffusion, predict substantially different surface abundances (in particular the 12C/13C and C/N ratios). Comparison between models and the available data on giants during the first dredge-up show that abundance anomalies can be explained if turbulent mixing is present during the main sequence phase (figure 2).

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A closer look at the transition between fully convective and partly radiative low-mass stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834062 ◽

2018 ◽

Vol 619 ◽

pp. A177 ◽

Cited By ~ 2

Author(s):

Isabelle Baraffe ◽

Gilles Chabrier

Keyword(s):

Stellar Evolution ◽

Structural Changes ◽

Main Sequence ◽

Energy Transport ◽

M Dwarfs ◽

Low Mass Stars ◽

Mixing Processes ◽

Nuclear Burning ◽

Low Mass ◽

M Dwarf

Recently, an analysis of Gaia Data Release 2 revealed a gap in the mid-M dwarf main sequence. The authors suggested the feature is linked to the onset of full convection in M dwarfs. Following the announcement of this discovery, an explanation has been proposed based on standard stellar evolution models. In this paper we re-examine this explanation. We confirm that nuclear burning and mixing processes of 3He provide the best explanation for the observed feature. We also find that a change in the energy transport from convection to radiation does not induce structural changes that could be visible. Regarding the very details of the process, however, we disagree with the details of the published explanation and propose an alternative.

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