Doppler shadow analysis reveals surface features of HD 189733 during planetary transits

Star system: HD 189733 A | Distance from the Sun: 64.5 light years

Studying exoplanet atmospheres with transmission spectroscopy can be complicated by features on the star’s surface, which can affect the observed spectra. To better understand these effects, astronomers analyzed the surface of the star HD 189733 by observing two transits of its planet using the ESPRESSO spectrograph. They used a technique called Doppler shadow analysis to probe how the star’s spectral lines change from the center to the edge (limb) of its disk.

The team cross-correlated the observed spectra with selected iron (Fe I) lines and compared the results with solar data and computer simulations. They found that, for most line sets, the depth of the spectral lines increases from the limb to the center of the star, matching predictions from stellar atmosphere models. For one set, the line widths also decrease from limb to center, similar to what is seen on the Sun, though this was not reproduced by the simulations. These findings show that ESPRESSO can measure subtle changes in stellar spectra during planetary transits and highlight the need for improved models to fully explain the observations.

Key findings
- Doppler shadow analysis with ESPRESSO reveals center-to-limb variations in HD 189733’s spectral lines
- Line depths increase from the limb to the center, consistent with model predictions
- For one line set, line widths decrease from limb to center, matching solar observations
- Some observed features are not reproduced by current simulations, suggesting missing physics
- The technique helps disentangle stellar and planetary signals in exoplanet studies

Scientific study

Exploring the surface of HD 189733 via Doppler shadow analysis of planetary transits (external link)