The spin of accreting stars: Dependence on magnetic coupling to the disc

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 09/2004; 356(1). DOI: 10.1111/j.1365-2966.2004.08431.x
Source: arXiv


We formulate a general, steady-state model for the torque on a magnetized star from a surrounding accretion disc. For the first time, we include the opening of dipolar magnetic field lines due to the differential rotation between the star and disc, so the magnetic topology then depends on the strength of the magnetic coupling to the disc. This coupling is determined by the effective slip rate of magnetic field lines that penetrate the diffusive disc. Stronger coupling (i.e., lower slip rate) leads to a more open topology and thus to a weaker magnetic torque on the star from the disc. In the expected strong coupling regime, we find that the spin-down torque on the star is more than an order of magnitude smaller than calculated by previous models. We also use our general approach to examine the equilibrium (`disc-locked') state, in which the net torque on the star is zero. In this state, we show that the stellar spin rate is roughly an order of magnitude faster than predicted by previous models. This challenges the idea that slowly-rotating, accreting protostars are disc locked. Furthermore, when the field is sufficiently open (e.g., for mass accretion rates > 5 x 10^{-9} M_sun / yr, for typical accreting protostars), the star will receive no magnetic spin-down torque from the disc at all. We therefore conclude that protostars must experience a spin-down torque from a source that has not yet been considered in the star-disc torque models--possibly from a stellar wind along the open field lines. Comment: Accepted by MNRAS

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    • "Do global simulations allow asymptotically steady-state discs without large scale transport or do they converge to similar end states with a determinable mix of disk, corona, and jet? A complicating and diversifying feature is the interaction between the disc and the stellar magnetosphere (Ghosh & Lamb 1978; Matt & Pudritz 2005; Perna et al. 2006; Romanova et al. 2012). If the spin of the central object such as a neutron star or black hole acts as a significant source of input energy, then the interior jet or coronae is not entirely sourced by the accretion. "
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    ABSTRACT: Accretion disc theory is far less developed than that of stellar evolution, although a similarly mature phenomenological picture is ultimately desired. While conceptual progress from the interplay of theory and numerical simulations has amplified awareness of the role of magnetic fields in angular momentum transport, there remains a significant gap between the output of magneto-rotational instability (MRI) simulations and the synthesis of lessons learned into improved practical models. If discs are turbulent, then axisymmetric models must be recognized to be sensible only as mean field theories. Such is the case for the wonderfully practical and widely used framework of Shakura-Sunyaev (SS73). This model is most justifiable when the radial angular momentum transport dominates in discs and the transport is assumed to take the form of a local viscosity. However, the importance of large scale fields in coronae and jets and numerical evidence from MRI simulations points to a significant fraction of transport being non-local. We first review how the SS73 viscous closure emerges from a mean field theory and then discuss the reasons the theory must be augmented to include large scale transport. We discuss a number of open opportunities for theory and interpretation of numerical simulations, with the ultimate challenge being a mean field accretion theory that also couples to large scale dynamo theory and self-consistently produces coronae and jets. While there has elsewhere been well-deserved focus toward small scale collisionless plasma processes in the context of transport in low density accretion discs, here we emphasize the importance of large scales as a fundamental frontier. Computational limitations have focused attention toward smaller scales when it comes to transport but hopefully the next generation of global simulations can help inform mean field models.
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    ABSTRACT: Introduction: It is a well known that classical T Tauri stars (CTTS) are slow rotators with periods in a range 2-10 days [1]. A fundamental question is how fast they spin-down from their initial fast rotation, to their present slow rotation and what mechanism is responsible for the spin-down. In this Abstract we emphasize that the early stages of CTTSs evolution are expected to be inthpropeller" regime of accre-tion where the magnetosphere of the star rotates with super-Keplerian velocity and any disk matter which reaches magnetosphere is predicted to be pushed away from the star [2,3]. In this stage the star is expected to spin-down rapidly. However, the theoretical estimates of the spin-down are rather uncertain. For this reason we carried out axisymmetric (2.5D) simulations of disk accretion to a rapidly rotating CTTSs [4-6] with the main goals of (1) estimating the rate of spin-down in the propeller stage of evolution, and (2) studying the nature of the matter flow at this stage. Figure 1. In the strong propeller regime, the disk os-cillates between a "high" state (top plot) and a "low" state (bottom plot). The color background shows the density, black lines are B-field lines, and the arrows are proportional to the flow velocity.
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    ABSTRACT: We present a photometric study of I-band variability in the young cluster IC 348. The main purpose of the study was to identify periodic stars. In all, we find 50 periodic stars, of which 32 were previously unknown. For the first time in IC 348, we discover periods in significant numbers of lower-mass stars (M < 0.25 M⊙) and classical T Tauri stars. This increased sensitivity to periodicities is a result of the enhanced depth and temporal density of our observations, compared with previous studies. The period distribution is at first glance similar to that seen in the Orion nebula cluster (ONC), with the higher-mass stars (M > 0.25 M⊙) showing a bi-modal period distribution concentrated around periods of 2 and 8 d, and the lower-mass stars showing a uni-modal distribution, heavily biased towards fast rotators. Closer inspection of the period distribution shows that the higher-mass stars show a significant dearth of fast rotators, compared to the ONC, whilst the low-mass stars are rotating significantly faster than those in Orion. We find no correlation between rotation period and K–L colour or Hα equivalent width. We also present a discussion of our own IC 348 data in the context of previously published period distributions for the ONC, the Orion flanking fields and NGC 2264. We find that the previously claimed correlation between infrared excess and rotation period in the ONC might, in fact, result from a correlation between infrared excess and mass. We also find a marked difference in period distributions between NGC 2264 and IC 348, which presents a serious challenge to the disc-locking paradigm, given the similarity in ages and disc fractions between the two clusters.
    Monthly Notices of the Royal Astronomical Society 04/2005; 358:341-352. DOI:10.1111/j.1365-2966.2005.08737.x · 5.11 Impact Factor
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