W-shaped dips in M-dwarf lightcurves

Planet Hunters TESS users have identified two very interesting lightcurves of M-dwarf stars that show repeating patterns that are difficult to explain with well-known astrophysical phenomena such as star spots, eclipsing binaries or stellar pulsations.

wstars1

Phase folded lightcurves of two M-dwarf stars as observed by TESS.

Both of these lightcurves exhibit two dips, a shallow one and a deeper one, which could be explained as the primary and secondary eclipses of an eclipsing binary. However, in addition to these ‘eclipses’, we can also see a w-shaped dip after or after the deeper eclipse. Both lightcurves repeat with a period of less than 1 day and the patterns remain stable over the course of the available TESS observations, as shown in the phase folded figure below. The target stars are both M-dwarfs, with temperatures of around 3000 K and radii of around 0.3 Solar radii.

Similar lightcurves had previously been seen in the Kepler data, see Stauffer et al. 2017 (https://iopscience.iop.org/article/10.3847/1538-3881/aa5eb9/pdf) for more detail. This paper suggests that the pattern could be caused by clouds of dust orbiting around one of the stars. Could that also explain the w-shaped dips in the TESS data, or are we seeing an entirely different phenomenon here?

In order to study these systems in more detail we want to see if we can identify more of them in the TESS data.

If you see any of these please tag a member of the researcher team or use the hashtag #wstars to help us find and study these elusive systems.   

wstars2

Sections of the full TESS  lightcurves of the M-dwarf stars.

About Nora Eisner

Project leader of the Zooniverse citizen science project Planet Hunters TESS and PhD Student at the University of Oxford.

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