Weather in stellar atmosphere revealed by the dynamics of mercury clouds in α Andromedae

Abstract

The formation of long-lasting structures at the surfaces of stars is commonly ascribed to the action of strong magnetic fields. This paradigm is supported by observations of evolving cool spots in the Sun 1 and active late-type stars 2 , and stationary chemical spots in the early-type magnetic stars 3 . However, results of our seven-year monitoring of mercury spots in non-magnetic earlytype star α Andromedae show that the picture of magnetically driven structure formation is fundamentally incomplete. Using an indirect stellar-surface mapping technique, we construct a series of two-dimensional images of starspots and discover a secular evolution of the mercury cloud cover in this star. This remarkable structure-formation process, observed for the first time in any star, is plausibly attributed to a non-equilibrium, dynamical evolution of the heavy-element clouds created by atomic diffusion 4 , and may have the same underlying physics as the weather patterns on terrestrial and giant planets.Since the detection of sunspot magnetic fields 5 , it has become clear that magnetic forces are responsible for the activity and the formation of surface structure in the Sun. Strong magnetic fields, which are generated in the solar interior and brought to the surface by convective motions, inhibit the outward flow of energy, thus creating prominent dark spots with filamentary structure 1 . A very similar relation between magnetic fields and cool surface spots is observed in many active solar-type stars 2 . All these objects possess highly structured magnetic fields, which evolve on timescales from weeks to months. Stars significantly more massive than the Sun do not have an energetic surface convection zone. The lack of mixing in their atmospheres enables diffusive segregation of chemical elements to operate efficiently. The action of the competing forces of downward gravitation acceleration and the outward radiation pressure, collectively referred to as atomic diffusion 6 , leads to selective heavy-element enrichment of stellar atmospheres. This phenomenon of chemical peculiarity is present in most slowly rotating A-and B-type stars. The presence of strong, well-organized magnetic fields in some chemically peculiar (CP) stars alters the efficiency of the atomic transport processes, resulting in the formation of non-uniform distributions of chemical abundances over the surface and with height in their atmospheres. The fields and chemical spots in magnetic CP stars are essentially static, showing no evolutionary changes on the timescales accessible to human observers. Despite a qualitative difference between the magnetic characteristics and related variability patterns of the low-mass solar-type stars and massive early-type objects, an undeniable and seemingly universal link between the presence of magnetic fields and the surface structure formation exists in both groups of stars. A series of recent studies has challenged this picture. A small group of late-B CP stars of mercury-manganese (HgMn) peculiarity type has no convinci...

Dynamical evolution of titanium, strontium, and yttrium spots on the surface of the HgMn star HD 11753

“…We also detected clear differences in the spot configurations obtained from the same lines but for different data sets, which indicates a rather fast dynamical evolution of the abundance distribution with time. Kochukhov et al (2007) discovered mercury clouds in the atmosphere of a HgMn star α And that showed secular changes with a time period of 2−4 years. In our analysis, using two datasets separated by ∼65 days, we reveal that the changes in the chemical spot configuration of HD 11753 appear much faster and can already be detected at a time scale of months.…”

A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy

“…To allow for a more robust normalisation of Hg ii λ3984, the red wing of H was used to define pseudocontinuum regions. (Hubrig et al 2006(Hubrig et al , 2011Savanov et al 2009), 66 Eri A (Makaganiuk et al 2011b), α And A (Adelman et al 2002;Kochukhov et al 2007), and φ Phe (Makaganiuk et al 2012;Korhonen et al 2013). All the stars discussed here are slow or moderately fast rotators with equatorial velocities below 55 km s −1 and rotation periods in the range 2-10 days.…”

“…The data discussed here were obtained over a relatively short time interval, and the possible superposition of secular changes arising from the dynamical evolution of the stellar atmosphere in case of spots at the surface (e.g., Kochukhov et al 2007;Hubrig et al 2010;Korhonen et al 2013) should not be a concern when considering a single dataset. However, it cannot be ruled out that a lack of coherency is encountered when comparing one time series to another (for instance, the HERMES and SARG datasets are separated by ∼4 months).…”

“…Kochukhov et al (2007) first detected mercury clouds with secular evolution on the surface of the bright HgMn star α And. Among the processes invoked by these authors to explain these observations, they considered the time-dependent atomic diffusion scenario proposed by Alecian (1998).…”

The search for magnetic fields in mercury-manganese stars

“…We can also rule out a complex configuration of the magnetic field as it would reveal itself in Stokes V profiles. Hereby, this class of CP stars remains a puzzling example of the chemical spots formation in massive stars, which is possibly related to hydrodynamic instabilities in chemically stratified stellar atmospheres (Kochukhov et al 2007). …”