Resolving star spots on WASP-85 A using high-resolution transit spectroscopy
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2025
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| author | Kunovac, Vedad Cegla, Heather Chakraborty, Hritam Lagae, Cis Brown, David J. A. Freckelton, Alix Gill, Samuel López-Morales, Mercedes McCormac, James Mortier, Annelies Timmermans, Mathilde Wilson, Thomas G. Allart, Romain Bryant, Edward M. Burleigh, Matthew R. Doyle, Lauren Gillen, Edward Jenkins, James S. Lafarga, Marina Lendl, Monika Oshagh, Mahmoud Panwar, Vatsal Pedersen, Peter P. Triaud, Amaury West, Richard G. Wheatley, Peter J. |
| author_facet | Kunovac, Vedad Cegla, Heather Chakraborty, Hritam Lagae, Cis Brown, David J. A. Freckelton, Alix Gill, Samuel López-Morales, Mercedes McCormac, James Mortier, Annelies Timmermans, Mathilde Wilson, Thomas G. Allart, Romain Bryant, Edward M. Burleigh, Matthew R. Doyle, Lauren Gillen, Edward Jenkins, James S. Lafarga, Marina Lendl, Monika Oshagh, Mahmoud Panwar, Vatsal Pedersen, Peter P. Triaud, Amaury West, Richard G. Wheatley, Peter J. |
| contents | Stellar surface inhomogeneities such as spots and faculae introduce Doppler variations that challenge exoplanet detection via the radial velocity method. While their impact on disc-integrated spectra is well established, detailed studies of the underlying local line profiles have so far been limited to the Sun. We present an observational campaign targeting the active star WASP-85 A during transits of its hot Jupiter companion. The transits span two stellar rotation periods, allowing us to probe the evolution of active regions. From ground-based photometry we identify seven active regions, six containing dark spots. Using simultaneous ESPRESSO transit spectroscopy, we spatially resolve these regions on the stellar surface by using the planet as a probe. We detect significant bisector shape changes, line broadening, and net redshifts during spot occultations, with velocity shifts of 108-333 m/s (mean uncertainty 50 m/s). The observed broadening is consistent with the Zeeman effect, implying magnetic field strengths (Stokes $I$) $B$ = 2.7-4.4 kG (mean uncertainty 0.6 kG), comparable to solar umbrae. Combined with our photometric spot model, this yields lower limits to the disc-integrated field $Bf = 16 \pm 3$ G and $61 \pm 9$ G for the two hemispheres probed -- at least three times higher than Sun-as-a-star values. We also measure centre-to-limb variations in FWHM, line depth, equivalent width, and convective blueshift, which broadly agree with solar observations and 3D MHD models. This work demonstrates a new way to characterise the surfaces of exoplanet host stars, paving the way for future analyses incorporating synthetic line profiles from 3D MHD simulations. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_16881 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Resolving star spots on WASP-85 A using high-resolution transit spectroscopy Kunovac, Vedad Cegla, Heather Chakraborty, Hritam Lagae, Cis Brown, David J. A. Freckelton, Alix Gill, Samuel López-Morales, Mercedes McCormac, James Mortier, Annelies Timmermans, Mathilde Wilson, Thomas G. Allart, Romain Bryant, Edward M. Burleigh, Matthew R. Doyle, Lauren Gillen, Edward Jenkins, James S. Lafarga, Marina Lendl, Monika Oshagh, Mahmoud Panwar, Vatsal Pedersen, Peter P. Triaud, Amaury West, Richard G. Wheatley, Peter J. Earth and Planetary Astrophysics Solar and Stellar Astrophysics Stellar surface inhomogeneities such as spots and faculae introduce Doppler variations that challenge exoplanet detection via the radial velocity method. While their impact on disc-integrated spectra is well established, detailed studies of the underlying local line profiles have so far been limited to the Sun. We present an observational campaign targeting the active star WASP-85 A during transits of its hot Jupiter companion. The transits span two stellar rotation periods, allowing us to probe the evolution of active regions. From ground-based photometry we identify seven active regions, six containing dark spots. Using simultaneous ESPRESSO transit spectroscopy, we spatially resolve these regions on the stellar surface by using the planet as a probe. We detect significant bisector shape changes, line broadening, and net redshifts during spot occultations, with velocity shifts of 108-333 m/s (mean uncertainty 50 m/s). The observed broadening is consistent with the Zeeman effect, implying magnetic field strengths (Stokes $I$) $B$ = 2.7-4.4 kG (mean uncertainty 0.6 kG), comparable to solar umbrae. Combined with our photometric spot model, this yields lower limits to the disc-integrated field $Bf = 16 \pm 3$ G and $61 \pm 9$ G for the two hemispheres probed -- at least three times higher than Sun-as-a-star values. We also measure centre-to-limb variations in FWHM, line depth, equivalent width, and convective blueshift, which broadly agree with solar observations and 3D MHD models. This work demonstrates a new way to characterise the surfaces of exoplanet host stars, paving the way for future analyses incorporating synthetic line profiles from 3D MHD simulations. |
| title | Resolving star spots on WASP-85 A using high-resolution transit spectroscopy |
| topic | Earth and Planetary Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2510.16881 |