Angular Momentum Loss Rate Research Articles

Angular momentum loss in low-mass stars

The wind models discussed by Mestel (1984) are used here to formulate a general expression for the rate of angular momentum loss by magnetic stellar winds as a function of magnetic field configuration, rotation rate, and stellar model properties. The sensitivity of the rotation velocity to the various wind model parameters, the initial angular momenta, and the time dependence of the angular velocity for each mass is shown. The theoretical results are compared with observational ones, and it is found that the existence of very rapidly rotating stars in young clusters implies that low-mass stars are formed with a large spread of angular momentum. The high efficiency of angular momentum loss through magnetic stellar winds causes the rotation velocity to become less dependent on initial angular momentum J0 with time; by 300 million yur, the rotation velocity becomes independent of J0. This results in a decrease with time in the spread of rotation velocities as a function of stellar mass in young clusters.

Short-term spectral variability in AB Aurigae: Clues for activity in Herbig AE stars. II - The CA II K line

view Abstract Citations (63) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Short-Term Spectral Variability in AB Aurigae: Clues for Activity in Herbig AE Stars. II. The CA II K Line Catala, C. ; Felenbok, P. ; Czarny, J. ; Talavera, A. ; Boesgaard, A. M. Abstract The Herbig Ae star AB Aur (A0ep) was monitored with the CFH 3.6 m and the Observatoire de Haute-Provence 1.52 m telescopes in the Ca II K line, for six nights in 1982 October. Some of the 39 recorded spectra were simultaneous with ultraviolet spectra obtained with IUE. The Ca II K line is variable and modulated with a period of 32±4 hours, corresponding to the estimated rotation period of the star. The difference between the 45 hour period found in the Mg II data (Paper I) and the 32 hour period in the Ca II data is assumed as due to a differential rotation in the envelope of the star. This differential rotation allows an estimate of upper limits for the magnetic field and the Alfvén radius in the envelope of AB Aur, as well as for the rate of angular-momentum loss. Publication: The Astrophysical Journal Pub Date: September 1986 DOI: 10.1086/164551 Bibcode: 1986ApJ...308..791C Keywords: A Stars; Abundance; B Stars; Line Spectra; Stellar Spectra; Stellar Spectrophotometry; Calcium; Emission Spectra; K Lines; Red Shift; Stellar Rotation; Astrophysics; STARS: EMISSION-LINE; STARS: INDIVIDUAL CONSTELLATION NAME: AB AURIGAE; STARS: ROTATION; STARS: SPECTRUM VARIABLES full text sources ADS | data products SIMBAD (1) Related Materials (1) Part 1: 1986ApJ...303..311P

Postthermonuclear runaway angular momentum loss in cataclysmic binaries

view Abstract Citations (40) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Postthermonuclear Runaway Angular Momentum Loss in Cataclysmic Binaries MacDonald, J. Abstract During the common envelope phases that follow hydrogen shell flashes in the white dwarf component of cataclysmic binaries, angular momentum and energy are transferred from the orbit to the common envelope by friction processes. The common envelope is ejected due to the frictional heating, carrying away angular momentum from the binary system. An estimate of the rate of this frictional angular momentum loss (FAML) shows that it can be the dominant angular momentum loss mechanism for cataclysmic binaries with orbital period, P(orb) approximately less than 6 hr. It is suggested that FAML is responsible for the great range in observationally deduced mass transfer rates for systems with similar orbital period. Publication: The Astrophysical Journal Pub Date: June 1986 DOI: 10.1086/164245 Bibcode: 1986ApJ...305..251M Keywords: Angular Momentum; Binary Stars; Cataclysmic Variables; Stellar Envelopes; Stellar Evolution; Stellar Mass Ejection; Gravitational Waves; Magnetic Effects; Mass Transfer; Stellar Orbits; Thermonuclear Reactions; Astrophysics; STARS: BINARIES; STARS: DWARF NOVAE full text sources ADS | data products SIMBAD (4)

Stellar analogs of solar magnetic activity

The techniques and principal results of observational studies of stellar activity are summarized. Both chromospheric and coronal emission clearly track surface magnetic field properties, but it is not well known how the detailed relation between the emission and surface magnetic fields varies with spectral type. For lower Main-Sequence stars of the same spectral type, there is clear evidence of a close relationship between mean activity level and rotation period P rot. There is also less definitive evidence for a similar dependence on convective overturn time τc, such that activity depends on the single parameter Ro = P rot/τc. For single stars, stellar rotation, and magnetic activity both decline smoothly with age. This implies a feedback between angular momentum loss rate and activity level. Temporal variations in mean stellar activity level mimic the solar cycle only for old stars like the Sun, being much more irregular for younger stars. The characteristic timescale of the variations (the ‘cycle period’) appears to depend on Ro for old stars, but shows no clear dependence on either rotation rate or spectral type for younger stars. Further data on mean activity and its variation for a large number of lower Main-Sequence stars should contribute significantly to our understanding of the causes of stellar magnetic activity.

Stellar Winds and Spindown in Red Giants

Recent observations of rotation rates in G and K giants show a sudden drop in the rotation rate at spectral type G5. The observed behavior requires a strong external brake, such as applied by a magnetized stellar wind. Since spectral type G5 coincides with the onset of deep envelope convection in giants, a dynamo mechanism is suggested as the controlling factor. Stellar evolution models have been used to estimate the time scale for the rotational braking, the angular momentum loss rate, and the required magnetic field strengths.

Stellar Winds and Spindown in Solar Type Stars

The angular momentum loss produced by stellar winds is reviewed and a simple model of angular momentum loss with multipole fields is presented in which the field is potential when the flow speed is less than the Alfvén speed, and radial when greater than the Alfvén speed. The simpler the magnetic geometry, the larger is the angular momentum loss rate. This result is used to explain the rotational discontinuity across the Vaughan-Preston gap as being due to a sudden increase in angular momentum loss when the dynamo field switches from a quadropole to a dipole geometry.The evolution of the internal rotation of stars as a result of surface angular momentum loss is considered. In the absence of a magnetic field, differential rotation can drive instabilities which then transport angular momentum out from the interior down the angular velocity gradient. Other instabilities such as that caused by the build up of 3He can also transport angular momentum outwards. If angular momentum is transported by such weak turbulence, it also makes the star more homogeneous than standard evolutionary models and lowers the predicted value of the solar neutrino flux.The recent results on rotational splitting of solar oscillations are considered: these suggest that the inside of the sun is spinning faster than the surface and are compatible with models in which angular momentum is transported by mild turbulence. But data is scarce — and in such circumstances the speculations of the theorist must be viewed with caution!

A lower bound to the rate of mass loss from a circular binary system evolving via emission of gravitational waves and Roche lobe overflow of degenerate component

In a spin-conserved circular binary system which is emitting gravitational waves and losing mass by overflow of the Roche lobe of the less massive and degenerate component, we find that, if the mass exchange occurs on a time scale longer than the thermal time scale, then during the mass exchange phase the orbital period must increase, implying the existence of lower limits both to the rate of mass loss from the system and from the degenerate component and an upper limit to the angular momentum loss rate.

Contact Binary Evolution and Angular Momentum Loss

Some fundamental problems connected with the evolution of W UMa stars are considered. While no generally accepted theory for the evolution of these systems exists, different scenarios lead to single stars on a nuclear or thermal time scale, or even to dwarf novae. The cycling and contact discontinuity models for zero age systems have gained much attention during the last few years. The contact discontinuity hypothesis has been heavily criticized on physical grounds, and the cycling at small mass ratios will probably be too violent leading to overcontact. On the other hand, there is increasing evidence of strong magnetic activity in short period solar type binaries, including W UMa stars (spots, flares, strong chromospheres and coronae etc.). This points to enhanced dynamo action inside rapidly rotating components of solar type close binaries. Extrapolating from single stars one finds that this may efficiently brake the orbital rotation. With an angular momentum loss rate of about 1043 g cm2 s−1 per year corresponding to the thermal time scale of the secondary the scenario, where the angular momentum loss controls the zero age contact evolution, seems at least possible. This scenario needs an (hypothetical) equilibrium process between the degree of contact and magnetic activity, damping the angular momentum loss if the contact becomes too thick, so that marginal contact will be preserved. If the angular momentum loss time scale is longer (comparable to the nuclear time scale of the primary), the system is likely to evolve towards more extreme mass ratios and with less violent cycling. (The complete paper will be published elsewhere.)

The influence of non-uniform solar wind expansion on the angular momentum loss from the Sun

The influence on the rate of angular momentum loss from the Sun of magnetic geometries which are not spherically symmetric is estimated. Departures from spherical symmetry are expected to influence significantly the loss rate by two effects --the presence of closed magnetic field regions with no loss and also the variability in the radial distance to the Alfvenic point, as stressed by Mestel (1968). The loss rate is calculated for an MHD solarwind model with a solar magnetic field whose normal component at the surface is that of a north-- south dipole. In contrast to Mestel's work, where the field was assumed dipolar within a certain surface and radial outside, the coupling between the solar wind and magnetic field is here taken into account exactly. For equivalent boundary conditions at the surface, the resulting field configuration yields an angular momentum loss rate which is only 15% of that for the monopole field normally used in angular momentum loss estimates. If, instead of equating boundary conditions at the Sun, one equates the two losses at the equator to that observed at 1 AU by spacecraft, then the ratio of the total loss for the distended dipole to that for the monopolemore » is about 40%. (auth)« less

Interplanetary Gas. XV. Nonradial Plasma Motions from the Orientations of Ionic Comet Tails

Type I comet tails orientations dispersion attributed to nonradial plasma waves and discontinuities in interplanetary gas, calculating solar angular momentum loss rate