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Scattered light images of transition discs in the near-infrared often shownon-axisymmetric structures in the form of wide-open spiral arms in addition totheir characteristic low-opacity inner gap region. We study self-gravitatingdiscs and investigate the influence of gravitational instability on the shapeand contrast of spiral arms induced by planet-disc interactions.Two-dimensional non-isothermal hydrodynamical simulations including viscousheating and a cooling prescription are combined with three-dimensional dustcontinuum radiative transfer models for direct comparison to observations. Wefind that the resulting contrast between the spirals and the surrounding discin scattered light is by far higher for pressure scale height variations, i.e.thermal perturbations, than for pure surface density variations. Self-gravityeffects suppress any vortex modes and tend to reduce the opening angle ofplanet-induced spirals, making them more tightly wound. If the disc is onlymarginally gravitationally stable with a Toomre parameter around unity, anembedded massive planet (planet-to-star mass ratio of $10^{-2}$) can triggergravitational instability in the outer disc. The spirals created by thisinstability and the density waves launched by the planet can overlap resultingin large-scale, more open spiral arms in the outer disc. The contrast of thesespirals is well above the detection limit of current telescopes.

Scattered light images of spiral arms in marginally gravitationally unstable discs with an embedded planet