Accretion-powered Stellar Winds as a Solution to the Stellar Angular Momentum Problem (original) (raw)

Observational Limits on the Spin-Down Torque of Accretion Powered Stellar Winds

The Astrophysical Journal, 2010

The rotation period of classical T Tauri stars (CTTS) represents a longstanding puzzle. While young low-mass stars show a wide range of rotation periods, many CTTS are slow rotators, spinning at a small fraction of break-up, and their rotation period does not seem to shorten, despite the fact that they are actively accreting and contracting. Matt & Pudritz (2005b) proposed that the spindown torque of a stellar wind powered by a fraction of the accretion energy would be strong enough to balance the spin-up torque due to accretion. Since this model establishes a direct relation between accretion and ejection, the observable stellar parameters (mass, radius, rotation period, magnetic field) and the accretion diagnostics (accretion shock luminosity), can be used to constraint the wind characteristics. In particular, since the accretion energy powers both the stellar wind and the shock emission, we show in this letter how the accretion shock luminosity L UV can provide upper limits to the spin-down efficiency of the stellar wind. It is found that luminous sources with L UV ≥ 0.1L ⊙ and typical dipolar field components < 1 kG do not allow spin equilibrium solutions. Lower luminosity stars (L UV ≪ 0.1L ⊙) are compatible with a zero-torque condition, but the corresponding stellar winds are still very demanding in terms of mass and energy flux. We therefore conclude that accretion powered stellar winds are unlikely to be the sole mechanism to provide an efficient spin-down torque for accreting classical T Tauri stars.

Accretion‐powered Stellar Winds. III. Spin‐Equilibrium Solutions

The Astrophysical Journal, 2008

We compare the stellar wind torque calculated in a previous work (Paper II) to the spin-up and spin-down torques expected to arise from the magnetic interaction between a slowly rotating (∼ 10% of breakup) pre-main-sequence star and its accretion disk. This analysis demonstrates that stellar winds can carry off orders of magnitude more angular momentum than can be transferred to the disk, provided that the mass outflow rates are greater than the solar wind. Thus, the equilibrium spin state is simply characterized by a balance between the angular momentum deposited by accretion and that extracted by a stellar wind. We derive a semi-analytic formula for predicting the equilibrium spin rate as a function only of the ratio ofṀ w /Ṁ a and a dimensionless magnetization parameter, Ψ ≡ B 2 * R 2 * (Ṁ a v esc) −1 , whereṀ w is the stellar wind mass outflow rate,Ṁ a the accretion rate, B * the stellar surface magnetic field strength, R * the stellar radius, and v esc the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of ∼ 10% of breakup speed forṀ w /Ṁ a ∼ 0.1. Finally, the assumption that the stellar wind is driven by a fraction of the accretion power leads to an upper limit to the mass flow ratio ofṀ w /Ṁ a 0.6.

Spin Evolution of Accreting Young Stars. II. Effect of Accretion-Powered Stellar Winds

The Astrophysical Journal, 2012

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 nature of stellar winds in the star-disk interaction

Proceedings of the International Astronomical Union, 2007

Stellar winds may be important for angular momentum transport from accreting T Tauri stars, but the nature of these winds is still not well-constrained. We present some simulation results for hypothetical, hot (∼106K) coronal winds from T Tauri stars, and we calculate the expected emission properties. For the high mass loss rates required to solve the angular momentum problem, we find that the radiative losses will be much greater than can be powered by the accretion process. We place an upper limit to the mass loss rate from accretion-powered coronal winds of ∼ 10−11Myr−1. We conclude that accretion powered stellar winds are still a promising scenario for solving the stellar angular momentum problem, but the winds must be cool (e.g., 104K) and thus are not driven by thermal pressure.

Accretion‐powered Stellar Winds. II. Numerical Solutions for Stellar Wind Torques

The Astrophysical Journal, 2008

In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar winds in torquing down the stars. For this mechanism to be effective, the stellar winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed 2-dimensional (axisymmetric) stellar wind solutions, in order to determine the spin down torque on the star. We discuss wind driving mechanisms and then adopt a Parker-like (thermal pressure driven) wind, modified by rotation, magnetic fields, and enhanced mass loss rate (relative to the sun). We explore a range of parameters relevant for CTTSs, including variations in the stellar mass, radius, spin rate, surface magnetic field strength, the mass loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the stellar wind torque is of sufficient magnitude to be important for spinning down a "typical" CTTS, for a mass loss rate of ∼ 10 −9 M ⊙ yr −1. The winds are wide-angle, self-collimated flows, as expected of magnetic rotator winds with moderately fast rotation. The cases with quadrupolar field produce a much weaker torque than for a dipole with the same surface field strength, demonstrating that magnetic geometry plays a fundamental role in determining the torque. Cases with varying wind acceleration rate show much smaller variations in the torque suggesting that the details of the wind driving are less important. We use our computed results to fit a semi-analytic formula for the effective Alfvén radius in the wind, as well as the torque. This allows for considerable predictive power, and is an improvement over existing approximations.

The Early History of Stellar Spin: the Theory of Accretion onto Young Stellar Objects

EPJ Web of Conferences, 2014

The interaction of the magnetospheres of forming stars with their surrounding protostellar disks results in magnetospheric accretion flow onto the star. How is the associated angular momentum of accreting material channelled? The resolution of this issue is crucial for understanding the origin of the spins of pre main sequence stars. A significant fraction of these rotate very slowly, which indicates that an efficient angular momentum transport mechanism is at work to counteract the strong accretion spin up torques. We review the observational, theoretical, and computational advances in the field and argue that an accretion powered stellar winds together with highly time variable mass ejections from the disk/magnetosphere interface is a likely solution.

New Calculations of Stellar Wind Torques

AIP Conference Proceedings, 2009

Using numerical simulations of magnetized stellar winds, we carry out a parameter study to find the dependence of the stellar wind torque on observable parameters. We find that the power-law dependencies of the torque on parameters is significantly different than what has been used in all spin evolution models to date.

Understanding the Spins of Young Stars

2007

We review the theoretical efforts to understand why pre-main-sequence stars spin much more slowly than expected. The first idea put forward was that massive stellar winds may remove substantial angular momentum. Since then, it has become clear that the magnetic interaction between the stars and their accretion disks explains many of the observed emission properties. The disk locking scenario, which assumes the magnetic star-disk interaction also solves the stellar spin problem, has received the most attention in the literature to date. However, recent considerations suggest that the torques in the star-disk interaction are insufficient for disk locking to explain the slow rotators. This prompts us to revisit stellar winds, and we conclude that stellar winds, working in conjunction with magnetospheric accretion, are a promising candidate for solving the angular momentum problem. We suggest future directions for both observations and theory, to help shed light on this issue.

On Continuum-Driven Winds from Rotating Stars

The Astrophysical Journal, 2012

We study the dynamics of continuum driven winds from rotating stars, and develop an approximate analytical model. We then discuss the evolution of stellar angular momentum, and show that just above the Eddington limit, the winds are sufficiently concentrated towards the poles to spin up the star. A twin-lobe structure of the ejected nebula is seen to be a generic consequence of critical rotation. We find that if the pressure in such stars is sufficiently dominated by radiation, an equatorial ejection of mass will occur during eruptions. These results are then applied to η-Carinae. We show that if it began its life with a high enough angular momentum, the present day wind could have driven the star towards critical rotation, if it is the dominant mode of mass loss. We find that the shape and size of the Homunculus nebula, as given by our model, agree with recent observations. Moreover, the contraction expected due to the sudden increase in luminosity at the onset of the Great Eruption explains the equatorial "skirt" as well.