Abstract
Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto stars via funnel flows, which follow stellar magnetic field lines and shock on the stellar surfaces1–3, leaving hot spots with density gradients4–6. Previous work has provided observational evidence of varying density in hot spots7, but these observations were not sensitive to the radial density distribution. Attempts have been made to measure this distribution using X-ray observations8–10; however, X-ray emission traces only a fraction of the hot spot11,12 and also coronal emission13,14. Here we report periodic ultraviolet and optical light curves of the accreting star GM Aurigae, which have a time lag of about one day between their peaks. The periodicity arises because the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of a magnetospheric accretion model, this finding indicates the presence of a radial density gradient in a hot spot on the stellar surface, because regions of the hot spot with different densities have different temperatures and therefore emit radiation at different wavelengths.
Highlights
We present results from our multiwavelength variability study, including Neil Gehrels Swift Observatory (Swift) X-ray and near-ultraviolet (NUV) fluxes, Hubble Space Telescope (HST) NUV spectra, Las Cumbres Observatory Global Telescope (LCOGT) u′g′r′i′ photometry, Transiting Exoplanet Survey Satellite (TESS) photometry and CHIRON Hα spectra taken contemporaneously over roughly 35 days
Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto stars via funnel flows, which follow stellar magnetic field lines and shock on the stellar surfaces[1,2,3], leaving hot spots with density gradients[4,5,6]
There is a dip in all the light curves on December 23–25 (Fig. 1, grey shaded boxes). This is followed by the disappearance of the UV peaks in the light curves; the optical peaks remain, but appear to be lower than those before the dip in the light curves (Fig. 1). We propose that this combination of features in the light curves is due to structure in the hot spot
Summary
The time lag between the peaks in the UV and optical light curves and the eventual disappearance of the UV peaks is the first observational evidence of a density gradient in a hot spot on a stellar surface. Accretion-shock modelling confirms that the high-density region of the hot spot dominates the UV emission and that the low-density region of the hot spot dominates the optical emission (Methods) These models consist of a vertical accretion column close to the surface of the star that is constrained by the stellar magnetic field and divided into three subregions: the pre-shock zone, the post-shock zone and the heated photosphere below the shock[22]. The coverage of the low-density region is 10%–20% and the coverage of the high-density region is 0.1% (Fig. 2, Extended Data Table 2, Methods) The sizes of these accretion hot spots are roughly consistent with those measured from Zeeman Doppler imaging of young stars[24].
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