[show abstract][hide abstract] ABSTRACT: We present a model for the rotational evolution of a young, solar-mass star
interacting magnetically with an accretion disk. As in a previous paper (Paper
I), the model includes changes in the star's mass and radius as it descends the
Hayashi track, a decreasing accretion rate, and a prescription for the angular
momentum transfer between the star and disk. Paper I concluded that, for the
relatively strong magnetic coupling expected in real systems, additional
processes are necessary to explain the existence of slowly rotating
pre-main-sequence stars. In the present paper, we extend the stellar spin model
to include the effect of a spin-down torque that arises from an
accretion-powered stellar wind. For a range of magnetic field strengths,
accretion rates, initial spin rates, and mass outflow rates, the modeled stars
exhibit rotation periods within the range of 1--10 days in the age range of
1--3 Myr. This range coincides with the bulk of the observed rotation periods,
with the slow rotators corresponding to stars with the lowest accretion rates,
strongest magnetic fields, and/or highest stellar wind mass outflow rates. We
also make a direct, quantitative comparison between the accretion-powered
stellar wind scenario and the two types of disk-locking models (namely the
X-wind and Ghosh & Lamb type models) and identify some remaining theoretical
issues for understanding young star spins.
The Astrophysical Journal 11/2011; 745(1). · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present a model for the rotational evolution of a young, solar mass star interacting with an accretion disk. The model incorporates a description of the angular momentum transfer between the star and disk due to a magnetic connection, and includes changes in the star's mass and radius and a decreasing accretion rate. The model also includes, for the first time in a spin evolution model, the opening of the stellar magnetic field lines, as expected to arise from twisting via star-disk differential rotation. In order to isolate the effect that this has on the star-disk interaction torques, we neglect the influence of torques that may arise from open field regions connected to the star or disk. For a range of magnetic field strengths, accretion rates, and initial spin rates, we compute the stellar spin rates of pre-main-sequence stars as they evolve on the Hayashi track to an age of 3~Myr. How much the field opening affects the spin depends on the strength of the coupling of the magnetic field to the disk. For the relatively strong coupling (i.e., high magnetic Reynolds number) expected in real systems, all models predict spin periods of less than $\sim3$ days, in the age range of 1--3~Myr. Furthermore, these systems typically do not reach an equilibrium spin rate within 3~Myr, so that the spin at any given time depends upon the choice of initial spin rate. This corroborates earlier suggestions that, in order to explain the full range of observed rotation periods of approximately $1$--$10$ days, additional processes, such as the angular momentum loss from powerful stellar winds, are necessary.
The Astrophysical Journal 05/2010; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The evolution of stellar spin rates observed during star formation is not yet understood, due primarily to the fact that it is still not clear which mechanism(s) is responsible for removing angular momentum. Stellar winds may exert significant torques during pre-main-sequence evolution, provided that the mass loss rates are enhanced by several orders of magnitude relative to their main sequence values. This may be possible, if the winds are powered by the accretion process. We present new calculations of the angular momentum loss from enhanced stellar winds and address how this may help our understanding of young star spins. SPM was supported by an appointment to the NASA Postdoctoral Program at Ames Research Center, administered by ORAU through a contract with NASA.
[show abstract][hide abstract] ABSTRACT: During the last decades, the study of rotation in young low mass stars has been one of the more active areas in the field of stellar evolution. Many theoretical efforts have been made to understand the angular momentum evolution and our picture now, reveals the main role of the stellar magnetic field in all pre-main sequence stage (Ghosh & Lamb 1979, ApJ, 234, 296; Cameron & Campbell 1993, A&A, 274, 309; Cameron & Campbell 1995, A&A, 298, 133; Kúker, Henning, & Rúdiger 2003, ApJ, 589, 397; Matt & Pudritz 2005, MNRAS, 356, 167). The mean rotation of most of the cool low mass stars remains roughly constant during the T Tauri stage. This can be explained by the disc locking scenario. This paradigm suggest that star start out as CTTS with periods of 4-14 days, perhaps locked to their disc, and that this disc is eventually lost mainly by accretion. At the current time, it is not clear that this is true for all low mass stars. Some authors have questioned its validity for stars less massive than 0.5 solar masses. Although the reality may eventually turn out to be considerably more complex, a simple consideration of the effects of and limits on disc locking of young low mass stars seems necessary.We have investigated the exchange of angular momentum between a low mass star and an accretion disc during the Hayashi Track (Pinzón, Kúker, & de la Reza 2005, in preparation) and also along the first 100Myr of stellar evolution. The model incorporates changes in the star's moment of inertia, magnetic field strength (Elstner & Rúdiger 2000, A&A, 358, 612), angular momentum loss by a magnetic wind and an exponential decrease of the accretion rate. The lifetime of the accretion disc is a free parameter in our model. The resulting rotation rates are in agreement with observed vsin and photometric periods for young stars belonging to co-moving groups and open young clusters.
[show abstract][hide abstract] ABSTRACT: Presentamos resultados preliminars de un estudio de la evolución de tasas de acreción en algunos miembros jóvenes de los cúmulos abiertos NGC1502(1), NGC884(3), Trumpler37(2), Biurakan2(3) y Berkeley87(1). Las tasas de acreción se calcularon usando fotometría UVI y la relación de Gullbring. Además encontramos que las tasas de acreción de tres miembros de la asociaciónjóven MBM12 (2 Ma~nos) están decuerdo a los valores esperados para CTTS.
Revista Mexicana de Astronomía y Astrofísica. 01/2006;
[show abstract][hide abstract] ABSTRACT: Nearby associations are excellent objects for the study of the initial spin up phase during the PMS evolution. An empirical approach is adopted here to infer their rotations properties and relations to X-ray emission. Three nearby associations are considered. The TW Hya association with an age of 8 Myr, the Beta Pictoris moving group with 12 Myr and a combination of Tucana and Horologium associations (30 Myr). Two low and high rotation modes are considered for each association with stellar masses less than 1.5M and greater than 1.5M respectively. We infer representative equatorial rotation velocities Vo from the observed distribution of projected rotational velocities vsini. A spin up is found for the high rotation mode, whereas in the low rotation mode the Vo do not increase significantly. This insufficient increase of Vo is probably the cause of a decrease of the total mean specific angular momentum for the low mass stars between 8 and 30 Myr. However, for the high mass stars, where a sufficient spin up is present, the specific angular momentum is practically conserved in this same time interval. By supposing that the distribution of masses of these three associations follows a universal mass function, we estimate the number of members of these associations that remain to be detected. The analysis of rotation and stellar masses using luminosity X-rays indicators present similar properties, as the dependence on stellar mass and rotation, at least for the younger associations, to those obtained for T Tauri stars in the ONC. A strong desaturation effect appears at 30 Myr. This effect seems to be provoked by the minimum configuration of the stellar convection layers, attained for the first time for the higher mass stars at 30 Myr. The desaturation appears to be independent of rotation at this stage. Comment: 37 pages, 10 figures, 4 tables. Accepted by the Astronomical Journal
The Astronomical Journal 06/2004; · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Low mass stars as the Sun, have had during their first million years of life a large interaction with the accretion disk resulting in the star formation. This interaction which consists in a magnetic connection between the central star and the disk results in an important braking of the star rotation. Later, the disk is disintegrated and a spin up process appears. Much later, magnetic winds will again produce a spin down which will result in the more or less slow Main Sequence (MS) rotations. Despite the tremendous increase of measured periods of rotation in Pre Main Sequence Stars (PMS) during these recent years and theoretical advances, the physics at the end of the disk lifetime remains obscure. We are tackling this problem by considering first, which will be the response of the star rotation when a disk has decreased its accretion rate to a minimum value; this is expected to happen at the end of disk lifetime, observationally inferred to happen at an age of near 10 Myr. For this, we have used the model of Cameron and Campbell (1993) which enable to change the accretion rate with the time. After reproducing the rotation rates of Cameron and Campbell we have introduced an emerging radiative core in the star expected to be produced at this age, by means of two polytropes, one representing this core and the second, the convective envelope. A new distribution of internal energy of the star is formed that produces a linear decrease (independently of the values of initial accretion rates) of the stellar moment of inertia. Nevertheless always maintaining the global virialized equilibria. We have detected a new spin up process due to this internal stellar effect. Future studies will consider which will be the effect when a clearing mass of the disk is considered.
Bulletin of the Astronomical Society of Brazil. 08/2003;