61 research outputs found
Evolutionary variations of solar luminosity
The Theoretical arguments for a 30% increase in the solar luminosity over the past 4.7 billion years are reviewed. A scaling argument shows that this increase can be predicted without detailed numerical calculations. The magnitude of the increase is independent of nuclear reaction rates, as long as conversion of hydrogen to helium provides the basic energy source of the Sun. The effect of the solar luminosity increase on the terrestrial climate is briefly considered. It appears unlikely that an enhanced greenhouse effect, due to reduced gases (NH3, CH4), can account for the long-term paleoclimatic trends
The evolution of rotating stars. 1: Method and exploratory calculations for a 7 solar mass star
A method was developed which allows us to study the evolution of rotating stars beyond the main sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7 solar mass star, rotating according to these different cases, were computed from the ZAMS to the double shell source stage. Each sequence was begun with a (typical) equatorial velocity of 210 km/sec. On the main sequence, the effects of rotation are of minor importance. As the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution
Influence of magnetic pressure on stellar structure: A Mechanism for solar variability
A physical mechanism is proposed that couples the Sun's dynamo magnetic field to its gravitational potential energy. The mechanism involves the isotropic field pressure resulting in a lifting force on the convective envelope, thereby raising its potential energy. Decay of the field due to solar activity allows the envelop to subside and releases this energy, which can augment the otherwise steady solar luminosity. Equations are developed and applied to the Sun for several field configurations. The best estimate model suggests that uniform luminosity variations as large as 0.02% for half a sunspot cycle may occur. Brief temporal variations or the rotation of spatial structures could allow larger excursions in the energy released
Thermal perturbation of the Sun
Thermal perturbations of the solar convection zone can be modeled (to the first order) by perturbing the mixing length parameter alpha (equal to the ratio of the mixing length to the pressure scale height) used in the standard mixing length theory of convection. Results of such an analysis are presented and discussed in relation to recent work by others
A model for the X-ray nova A0620-00
The model involves a white dwarf accreting mass from a late-type subgiant companion. The transient behavior of the X-ray source is explained by the instability to mass loss of the companion (as in Algol-type binaries). The brightening, spectrum, and decay timescale of the optical counterpart are explained in terms of re-emission of X-radiation intercepted by the subgiant. It was concluded that A0620-00 can provide an excellent test case for numerical models of stellar atmospheres irradiated by an external X-ray flux
Physics of rotation in stellar models
In these lecture notes, we present the equations presently used in stellar
interior models in order to compute the effects of axial rotation. We discuss
the hypotheses made. We suggest that the effects of rotation might play a key
role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by
Springe
Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars
(Abridged) We present a detailed analysis of the main physical processes
responsible for the transport of angular momentum and chemical species in the
radiative regions of rotating stars. We focus on cases where meridional
circulation and shear-induced turbulence only are included in the simulations.
Our analysis is based on a 2-D representation of the secular hydrodynamics,
which is treated using expansions in spherical harmonics. We present a full
reconstruction of the meridional circulation and of the associated fluctuations
of temperature and mean molecular weight along with diagnosis for the transport
of angular momentum, heat and chemicals. In the present paper these tools are
used to validate the analysis of two main sequence stellar models of 1.5 and 20
Msun for which the hydrodynamics has been previously extensively studied in the
literature. We obtain a clear visualization and a precise estimation of the
different terms entering the angular momentum and heat transport equations in
radiative zones. This enables us to corroborate the main results obtained over
the past decade by Zahn, Maeder, and collaborators concerning the secular
hydrodynamics of such objects. We focus on the meridional circulation driven by
angular momentum losses and structural readjustements. We confirm
quantitatively for the first time through detailed computations and separation
of the various components that the advection of entropy by this circulation is
very well balanced by the barotropic effects and the thermal relaxation during
most of the main sequence evolution. This enables us to derive simplifications
for the thermal relaxation on this phase. The meridional currents in turn
advect heat and generate temperature fluctuations that induce differential
rotation through thermal wind thus closing the transport loop.Comment: 16 pages, 18 figures. Accepted for publication in A&
Presupernova Structure of Massive Stars
Issues concerning the structure and evolution of core collapse progenitor
stars are discussed with an emphasis on interior evolution. We describe a
program designed to investigate the transport and mixing processes associated
with stellar turbulence, arguably the greatest source of uncertainty in
progenitor structure, besides mass loss, at the time of core collapse. An
effort to use precision observations of stellar parameters to constrain
theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory
Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics
and Space Science, submitted for peer review: 7 pages, 3 figure
YREC: The Yale Rotating Stellar Evolution Code
The stellar evolution code YREC is outlined with emphasis on its applications
to helio- and asteroseismology. The procedure for calculating calibrated solar
and stellar models is described. Other features of the code such as a non-local
treatment of convective core overshoot, and the implementation of a
parametrized description of turbulence in stellar models, are considered in
some detail. The code has been extensively used for other astrophysical
applications, some of which are briefly mentioned at the end of the paper.Comment: 10 pages, 2 figures, ApSS accepte
New Rotation Periods in the Open Cluster NGC 1039 (M 34), and a Derivation of its Gyrochronology Age
Employing photometric rotation periods for solar-type stars in NGC 1039 [M
34], a young, nearby open cluster, we use its mass-dependent rotation period
distribution to derive the cluster's age in a distance independent way, i.e.,
the so-called gyrochronology method. We present an analysis of 55 new rotation
periods,using light curves derived from differential photometry, for solar type
stars in M 34. We also exploit the results of a recently-completed,
standardized, homogeneous BVIc CCD survey of the cluster in order to establish
photometric cluster membership and assign B-V colours to each photometric
variable. We describe a methodology for establishing the gyrochronology age for
an ensemble of solar-type stars. Empirical relations between rotation period,
photometric colour and stellar age (gyrochronology) are used to determine the
age of M 34. Based on its position in a colour-period diagram, each M 34 member
is designated as being either a solid-body rotator (interface or I-star), a
differentially rotating star (convective or C-star) or an object which is in
some transitory state in between the two (gap or g-star). Fitting the period
and photometric colour of each I-sequence star in the cluster, we derive the
cluster's mean gyrochronology age.
47/55 of the photometric variables lie along the loci of the cluster main
sequence in V/B-V and V/V-I space. We are further able to confirm kinematic
membership of the cluster for half of the periodic variables [21/55], employing
results from an on-going radial velocity survey of the cluster. For each
cluster member identified as an I-sequence object in the colour-period diagram,
we derive its individual gyrochronology age, where the mean gyro age of M 34 is
found to be 193 +/- 9 Myr, formally consistent (within the errors) with that
derived using several distance-dependent, photometric isochrone methods (250
+/- 67 Myr).Comment: accepted for publication in Astronomy & Astrophysic
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