arXiv subjects

P. J. Armitage

Publications and source records attributed to P. J. Armitage.

40 records · Page 3Linked to original sources

arXiv1995

Magnetic braking of T Tauri stars

We construct models for the rotation rates of T Tauri stars whose spin is regulated by magnetic linkage between the star and a surrounding accretion disc. Our models utilise a time-dependent disc code to follow the accretion process and include the effects of pre-main-sequence stellar evolution. We find that the initial disc mass controls the evolution of the star-disc system. For sufficiently massive discs, a stellar field of $\sim$ 1 kG is able to regulate the spin rate to the observed values during the classical T Tauri phase. The field then acts to expel the disc and the star spins up at constant angular momentum as a weak-line system. Lower mass discs are ejected at an early epoch and fail to brake the star significantly. We extend the model to close binary systems, and find that the removal of angular momentum from the disc by the secondary significantly prolongs the inner disc lifetime. Such systems should therefore be relatively slow rotators. We also discuss the implications of our model for the spectral energy distributions and variability of T Tauri stars.

astro-ph↗

arXiv1995

Dynamo driven accretion discs and dwarf nova eruptions

We explore the consequences of a magnetic dynamo origin for the viscosity in accretion discs, for the structure and evolution of discs in dwarf nova systems. We propose that the rapid cooling that sets in at the end of a dwarf nova eruption acts to inhibit the Balbus-Hawley instability, and thereby to quench dynamo action and so reduce disc viscosity. We demonstrate that a modified disc instability model can reproduce the basic properties of dwarf nova eruptions, as well as some properties of quiescent discs. We also discuss some observational consequences of our model.

astro-ph↗

arXiv1995

Magnetic cycles and photometric variability of T Tauri stars

We consider the effects of stellar magnetic cycles on disc accretion in T Tauri stars where the inner region of the disc is disrupted by the stellar field. Using time-dependent disc models, we show that order unity variations in field strength can strongly modulate the accretion flow for cycle time-scales of years to decades. Photometric variability is expected to be greatest in the ultraviolet, with smaller but still significant variations probable in the near-infrared. We suggest that variable stellar magnetic fields may contribute to the long-time-scale photometric variability of T Tauri stars. The long-term variability of T Tauri stars may therefore provide a clue to the nature of magnetic dynamo cycles in these stars.

astro-ph↗

arXiv1995

Magnetically modulated accretion in T Tauri stars

We examine how accretion on to T Tauri stars may be modulated by a time-dependent `magnetic gate' where the inner edge of the accretion disc is disrupted by a varying stellar field. We show that magnetic field variations on time-scales shorter than 10^5 yr can modulate the accretion flow, thus providing a possible mechanism both for the marked photometric variability of T Tauri stars and for the possible conversion of T Tauri stars between classical and weak line status. We thus suggest that archival data relating to the spectrophotometric variability of T Tauri stars may provide an indirect record of magnetic activity cycles in low-mass pre-main-sequence stars.

astro-ph↗