Abstract

When the magnetic field of a young star interacts with a surrounding accretion disc, radial currents are generated within the disc, which then produce toroidal magnetic disc fields. The interaction between the radial current and toroidal field conspire to produce a Lorentz force that compresses the disc. We suggest that this mechanism may be what is producing the ""light house"" effect seen in young stellar systems such as LRLL 31, where the infrared spectrum of LRLL 31 is observed to undergo unusual temporal changes in infrared flux between 5 and 40 microns. In particular, as the flux between 5 and 8.5 micron increases the flux between 8.5 and 40 micron decreases (Flaherty et al. 2011). A possible explanation for this behaviour is that the young star is surrounded by a transition or pre-transition disc. In such a disc the inner regions are optically thick, as are the outer regions, but the middle disk region is optically thin. As a consequence, when the scale height of the inner disc is compressed then the outer disc is illuminated. Alternatively, when the scale height of the inner disc is not compressed then the outer disc is in shadow. It is by such a mechanism that observed ""seesaw"" behaviour in the infrared spectrum is produced. LRLL 31 is not uniqu e in this regard and recent surveys have found more examples of this behaviour in other young stellar systems. We discuss how our “magnetic scale height” may be producing this effect.

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