The rapidly rotating Ross 154 – by Mikko Tuomi

Perhaps the least known star in the Red Dots campaign, Ross 154, is a rapidly rotating M dwarf star that shows elevated activity levels and and flares on its surface. This makes the Red Dots campaign targets more diverse than they otherwise would be – rapid rotation is typically interpreted as a sign of young age of such stellar objects as the rotation period is thought to gradually increase due to magnetic friction resulting in old M dwarfs with slow rotation rates such as Proxima Centauri whose rotation period has been estimated to be 83 days¹. Indeed, Ross 154 has been estimated to have an age of less than one billion years².

The rotation of Ross 154 induces a clear photometric cycle of 2.87 days in our All Sky Automated Survey V-band observations but the brightness of the star also varies with another cycle of 740 days that we interpret to be caused by the star’s activity cycle (Fig. 1). Knowing these two “fundamental” cycles helps interpreting any and all periodicities in the radial velocity data because periodicities that are independent from both rotation and magnetic and/or activity cycles could correspond to planets orbiting the star.

Due to its young age, Ross 154 is an active star and its radial velocities obtained by four independent spectrographs have elevated noise levels due to the star’s active surface. These high-precision spectrographs are HARPS (3.6m telescope, La Silla, Chile), HIRES (Keck telescope, Mauna Kea, Hawaii), PFS (Magellan 6.5m telescope, Las Campanas, Chile), and UCLES (Anglo-Australian Telescope, Siding Springs Observatory, Australia) but their precision is limited by stellar activity that induces radial velocity noise of some 10-20 m/s in the data, depending on the instrument.

Although reasonably noisy due to elevated activity in comparison to the typical radial velocity noise in M dwarf data of 2-4 m/s³, Ross 154 appears to have a moderately significant periodicity in its velocity data set caused by the stellar rotation and the co-rotation of starspots on the stellar surface (Fig. 2).

The Ross 154 radial velocities also show hints of additional periodic signals (Fig. 3), although they are not significantly present in the data. These are likely caused by aliasing and differential rotation of the star but the interpretation is difficult because there is not enough data to rule out alternative solutions present as local probability maxima in Figs. 2 and 3. The current data set is severely limited because the most precise HARPS data set contains only eight velocities. Red Dots campaign will increase this number roughly by a factor of ten, making it possible to search for signals of planets orbiting Ross 154.

References

1. Anglada-Escudé G. et al. “A Terrestrial Candidate in a Temperate Orbit Around Proxima Centauri”, Nature, 536, 437 (2016).
2. Johns-Krull, C. M. & Valenti, J. A. “Detection of Strong Magnetic Fields on M Dwarfs”, The Astrophysical Journal, 459, L95 (1996).
3. Butler R. P. et al. “The LCES HIRES/Keck Precision Radial Velocity Exoplanet Survey”, The Astronomical journal, 153, 208 (2017).