Tag Archives: magnetic activity

Magnetic activity on planet-host stars

One interesting question is whether close-in hot-Jupiter planets have an effect on the magnetic activity of the host star. There have been suggestions that star–planet interactions might increase magnetic activity on the star, or that tidal interactions might decrease it. Further, if mass lost from planets forms an absorbing cloud around the star, then it might reduce observable signs of magnetic activity, even if it doesn’t affect the magnetic activity itself.

A new paper by Daniel Staab et al, led by the Open University, investigates the issue by looking for markers of magnetic activity in spectra of host stars WASP-43, WASP-51, WASP-72 and WASP-103. In the following plot, RHK is a marker of magnetic activity, plotted against the temperature (B–V) of the star. The green dots are a large sample of field stars, while the four WASP host stars are labelled in red.

Chromospheric activity on planet-host stars.

The result is that at least one planet-host, WASP-43, has an abnormally high degree of magnetic activity, while another one, WASP-72, has abnormally low magnetic activity. Staab et al conclude that there is no single factor explaining the differences, and that more than one effect might be in play. As often, a much larger sample of data is needed to investigate the issue.

Exoplanet cloudiness from transit lightcurves?

An interesting new paper by von Paris et al has explored the effect of the cloudiness of a planet on transit lightcurves. If a planet were cloudy on one limb, but clear on the other limb, then that could make the transit slightly asymmetric. The authors show that, in principle, this effect could be detectable with good-enough quality lightcurves.

An apparent shift in the transit:

Shifted transit

Would then lead to residuals, relative to a “perfect” transit, looking like:

traresids

The authors then claim a possible detection of such an effect in the hot Jupiter HAT-P-7b.

This might open up a new way of exploring the atmospheres of exoplanets. Whether this can ever be done reliably, however, is debatable. A big assumption in the authors’ simulations is that the star being transited is uniform. However, we know that stars are usually magnetically active and so are patchy. Star spots and bright patches on the star are likely to have a greater effect on the transit profile than the cloudiness of the planet’s atmosphere. Still, the effect is worth exploring, particularly for planets transiting magnetically quiet stars.

Radial velocities of the Sun as an exoplanet host star

The main way of measuring the mass of an extra-solar planet is to record the motion of the host star, caused by the gravitational tug of the planet as it orbits. One can do that by measuring the Doppler shift (radial velocity or RV) of the spectrum of the host star.

However, as a planet gets smaller or further from its star, the tug gets smaller, and so the radial-velocity signal decreases. At some point it gets smaller than the intrinsic variations in spectral lines caused by the magnetic activity of the star. Whether one can account for this will limit our ability to prove the existence of small planets in wide orbits.

Radial velocity of the Sun, bounced off the asteroid Vesta

A team lead by Raphaëlle Haywood, of the University of St. Andrews, and now at Harvard, had the idea of treating our own Sun as a star, by looking at the RV signal in sunlight bounced off the asteroid Vesta. They could then compare the RV signal to images of the magnetic activity on our Sun.

Magnetic activity on the Sun.

Magnetic activity across the Sun’s disc

The spectral lines from each region of the Sun’s disc will depend on the local magnetic activity, but the RV measurement bounced off Vesta would be from light averaged over the whole disc of the Sun, just as we’d record from a star.

The results are shown in the plot below. The top panel shows the variations in the measured RV signal, in metres per second. The second panel shows the magnetic flux aggregated across the Sun’s disc, in Gauss. The third panel shows the fraction of the Sun’s disc filled by magnetic activity (Sun spots).

Radial velocity variations of our Sun

Thus a Sun-like star can show intrinsic RV variability at a level of metres per second, and this will cause a problem for detecting the small RV signals of low-mass planets in wide orbits. For example our Earth produces motion in our Sun of only 0.1 metre per second. Unless there are stars much less magnetically active than our Sun, it is going to be hard to obtain an accuracy sufficient to detect the RV signal of an Earth-like planet in an Earth-like orbit.

The authors note, though, a strong correlation between the RV signal and the total magnetic activity. Thus it might be possible to decorrelate against magnetic activity to provide a way of correcting RV signals for this effect, and so dig out smaller signals caused by planets.