Limit on overshooting from the convective core in upper main-sequence stars

1990 ◽  
Vol 348 ◽  
pp. L21 ◽  
Author(s):  
Richard B. Stothers ◽  
Chao-Wen Chin
Keyword(s):  
Main Sequence ◽  
Main Sequence Stars ◽  
Convective Core

scholarly journals Discovery of fossil magnetic fields in the intermediate-mass pre-main sequence stars

2008 ◽  
Vol 4 (S259) ◽  
pp. 445-446
Author(s):  
Evelyne Alecian ◽  
Gregg A. Wade ◽  
Claude Catala
Keyword(s):  
Magnetic Fields ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Intermediate Mass ◽  
Lower Mass ◽  
Convective Core

AbstractIt is now well-known that the surface magnetic fields observed in cool, lower-mass stars on the main sequence (MS) are generated by dynamos operating in their convective envelopes. However, higher-mass stars (above 1.5 M⊙) pass their MS lives with a small convective core and a largely radiative envelope. Remarkably, notwithstanding the absence of energetically-important envelope convection, we observe very strong (from 300 G to 30 kG) and organised (mainly dipolar) magnetic fields in a few percent of the A and B-type stars on the MS, the origin of which is not well understood. In this poster we propose that these magnetic fields could be of fossil origin, and we present very strong observational results in favour of this proposal.

scholarly journals Magnetic main sequence stars as progenitors of blue supergiants

2014 ◽  
Vol 9 (S307) ◽  
pp. 391-392
Author(s):  
I. Petermann ◽  
N. Castro ◽  
N. Langer
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Main Sequence ◽  
Mass Range ◽  
Main Sequence Stars ◽  
Core Mass ◽  
Hydrogen Burning ◽  
Sequence Band ◽  
Convective Core ◽  
The Right

AbstractBlue supergiants (BSGs) to the right the main sequence band in the HR diagram can not be reproduced by standard stellar evolution calculations. We investigate whether a reduced convective core mass due to strong internal magnetic fields during the main sequence might be able to recover this population of stars. We perform calculations with a reduced mass of the hydrogen burning convective core of stars in the mass range 3–30 M⊙ in a parametric way, which indeed lead to BSGs. It is expected that these BSGs would still show large scale magnetic fields in the order of 10 G.

scholarly journals First evidence of inertial modes in γ Doradus stars: The core rotation revealed

2020 ◽  
Vol 640 ◽  
pp. A49
Author(s):  
R.-M. Ouazzani ◽  
F. Lignières ◽  
M.-A. Dupret ◽  
S. J. A. J. Salmon ◽  
J. Ballot ◽  
...  
Keyword(s):  
Main Sequence ◽  
Density Stratification ◽  
Uniform Density ◽  
Main Sequence Stars ◽  
Intermediate Mass ◽  
The Core ◽  
Intermediate Mass Stars ◽  
Convective Core ◽  
Coupling Factors ◽  
Core Rotation

The advent of space photometry with CoRoT and Kepler has allowed for the gathering of exquisite and extensive time series for a wealth of main-sequence stars, including γ Doradus stars, whose detailed seismology was not achievable from the ground. γ Doradus stars present an incredibly rich pulsation spectra, with gravito-inertial modes, in some cases supplemented with δ Scuti-like pressure modes – for the hybrid stars – and, in many cases, with Rossby modes. The present paper aims to show that in addition to these modes which have been established in the radiative envelope, pure inertial modes that are trapped in the convective core can be detected in Kepler observations of γ Doradus stars thanks to their resonance with the gravito-inertial modes. We started by using a simplified model of perturbations in a full sphere of uniform density. Under these conditions, the spectrum of pure inertial modes is known from analytical solutions of the so-called Poincaré equation. We then computed coupling factors, which helped select the pure inertial modes which interact best with the surrounding dipolar gravito-inertial modes. Using complete calculations of gravito-inertial modes in realistic models of γ Doradus stars, we are able to show that the pure inertial and gravito-inertial resonances appear as “dips” in the gravito-inertial mode period spacing series at spin parameters that are close to those predicted by the simple model. We find the first evidence of such dips in the Kepler γ Doradus star KIC 5608334. Finally, using complete calculations in isolated convective cores, we find that the spin parameters of the pure inertial and gravito-inertial resonances are also sensitive to the density stratification of the convective core. In conclusion, we have discovered that certain dips in gravito-inertial mode period spacings that have been observed in some Kepler stars are, in fact, signatures of resonances with pure-inertial modes that are trapped in the convective core. This holds the promise that it would be possible to finally access the central conditions, namely, the rotation and density stratification, of intermediate-mass stars in the main sequence.

scholarly journals Rotating convective core excites non-radial pulsations to cause rotational modulations in early-type stars

2020 ◽  
Vol 497 (4) ◽  
pp. 4117-4127
Author(s):  
Umin Lee ◽  
Hideyuki Saio
Keyword(s):  
Main Sequence ◽  
Inertial Frame ◽  
Low Frequency ◽  
Rotation Frequency ◽  
Early Type ◽  
Main Sequence Stars ◽  
Convective Core ◽  
Resonance Coupling ◽  
Early Type Stars ◽  
Radial Pulsations

ABSTRACT We discuss low-frequency g modes excited by resonant couplings with weakly unstable oscillatory convective modes in the rotating convective core in early-type main-sequence stars. Our non-adiabatic pulsation analyses including the effect of Coriolis force for $2\, \mathrm{ M}_\odot$ main-sequence models show that if the convective core rotates slightly faster than the surrounding radiative layers, g modes in the radiative envelope are excited by a resonance coupling. The frequency of the excited g mode in the inertial frame is close to |mΩc| with m and Ωc being the azimuthal order of the g mode and the rotation frequency of the convective core, respectively. These g-mode frequencies are consistent with those of photometric rotational modulations and harmonics observed in many early-type main-sequence stars. In other words, these g modes provide a non-magnetic explanation for the rotational light modulations detected in many early-type main-sequence stars.

scholarly journals Excitation of Non-Radial Oscillations by Overstable Convection in Differentially Rotating Massive Main Sequence Stars

1988 ◽  
Vol 108 ◽  
pp. 97-98
Author(s):  
Umin Lee
Keyword(s):  
Angular Velocity ◽  
Main Sequence ◽  
Inertial Frame ◽  
Resonance Width ◽  
Main Sequence Stars ◽  
Coupled Mode ◽  
Convective Core ◽  
Resonance Coupling ◽  
Convective Mode

SummaryWe investigated overstable convective modes of differentially rotating massive main sequence stars. It is examined that the overstable convective modes in a rapidly rotating convective core may excite envelope non-radial oscillations by the resonance coupling between them. Let us denote by q the ratio of the angular velocity of the rapidly rotating core to that of the envelope. Then, it is found that as the ratio q increases, the order of the non-radial g modes resonantly coupled with the convective mode becomes lower, so that the excited g modes come to have shorter periods in an inertial frame. It is also found that when the separation of the frequencies of the consecutive envelope g modes is wider than the resonance width, there occurs the alternation between the strong and the weak resonances, i.e., the resonantly coupled mode becomes overstable or neutral according to the strength of the resonance.

Is there large convective-core overshooting in Kepler targets KIC 2837475 and 11081729?

2020 ◽  
Vol 497 (4) ◽  
pp. 4042-4050
Author(s):  
Jianwei Zhang
Keyword(s):  
Internal Structure ◽  
Main Sequence ◽  
Normal Values ◽  
Independent Method ◽  
Observational Constraints ◽  
Main Sequence Stars ◽  
Convective Core ◽  
Best Fitting ◽  
Model Independent ◽  
Fitting Model

ABSTRACT The ratio of small-to-large separations r010 has been widely used in helioseismology and asteroseismology to investigate the internal structure of a star, as it is approximately independent of the structure of the outer layers. Several studies have used this tool to constrain the convective-core overshooting of main-sequence stars (i.e. 0.0 ≤ δov ≤ 0.2). This is consistent with the generally accepted values. However, Yang et al. have proposed that there is large convective-core overshooting in the Kepler targets KIC 2837475 and 11081729: 1.2 ≤ δov ≤ 1.6 and 1.7 ≤ δov ≤ 1.8, respectively. These are much larger than the normal values. Thus, the aim of this study is to re-investigate the ratios of the two stars using a model-independent method with the latest p-mode observations. Our results indicate that there is no robustness for including such a large convective-core overshooting while modelling these two stars. In fact, this leads to over-fitting, and the observational constraints of r010 prefer models with a normal convective-core overshooting (i.e. 0.0 ≤ δov ≤ 0.2) as the candidates for the best-fitting model of KIC 2837475 and 11081729.

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