Dehnen, Walter; Semczuk, Marcin; Schönrich, Ralph; (2023) A twisted and precessing Cepheid warp in the outer Milky Way disc. Monthly Notices of the Royal Astronomical Society , 523 (1) pp. 1556-1564. 10.1093/mnras/stad1502.

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Abstract

We examine the Galactic warp in a sample of all classical Cepheids with Gaia DR3 radial velocity. In each radial bin, we determine (1) the inclined plane normal to the mean orbital angular momentum of the stars and (2) that best fitting their positions. We find no warping inside R ≈ 11 kpc; for larger R the disc is increasingly inclined, reaching i ∼ 3○ at R ≥ 14 kpc. With larger R the azimuth of the warp’s ascending node shifts from ϕlon ≈ −15○ at 11 kpc by about 14○/kpc in the direction of Galactic rotation, implying a leading spiral of nodes, the general behaviour of warped galaxies. From the method of fitting planes to the positions we also obtain ${\dot{\varphi }}_{\mathrm{lon}}$ and find prograde precession of ${\dot{\varphi }}_{\mathrm{lon}}\sim 12 {\rm km\,s^{-1}\,kpc^{-1}}$ at 12 kpc decreasing to ∼6 km s−1 kpc−1 at 14 kpc and beyond. This would unwind the leading spiral of nodes in ∼100 Myr, suggesting that our instantaneous measurements of ${\dot{\varphi }}_{\mathrm{lon}}$ reflect transient behaviour. This is consistent with existing simulations, which show oscillations in ${\dot{\varphi }}_{\mathrm{lon}}$ overlaying a long-term retrograde differential precession which generates the leading spiral of nodes.

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