J/AJ/170/346ivo://CDS.VizieR/J/AJ/170/346CDSEhrich K.E.Dittmann J.A.Halverson S.P.Camazon-Pinilla A.2026-09-07T15:05:29Z2026-09-07T16:02:07ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frexoplanetsspectroscopyvisible-astronomystellar-activityTransmission spectroscopy allows us to detect molecules in planetary atmospheres, but is subject to contamination from inhomogeneities on the stellar surface. Quantifying the extent of this contamination is essential for accurate measurements of atmospheric composition, as stellar activity can manifest as false atmospheric signals in planetary transmission spectra. We present a study of hot Jupiter HD 189733b, which has over 50hr of JWST observations scheduled or taken, to measure the activity level of the host star at the current epoch. We utilize high-resolution spectra of the H{alpha} line from the MEGARA spectrograph on the 10m GTC to examine the activity level of HD 189733 during a transit. We measure H{alpha} becoming shallower midtransit by an H{alpha} index of {delta}=0.00156+/-0.00026, which suggests that HD 189733b crosses an active region as it transits. We posit this deviation is likely caused by a spot along the transit chord with an approximate radius of R_spot_=3.47+/-0.30R_{Earth}_ becoming occulted during transit. Including an approximation for unocculted spots, we estimate that this spot could result in transit depth variations of ~17ppm at the 4.3um CO2 feature. Since this is comparable to JWST NIRCam Grism mode's noise floor of ~20ppm, it could bias atmospheric studies by altering the inferred depths of the planet's features. Thus, we suggest ground-based high-resolution monitoring of activity indicator species concurrently taken with JWST data when feasible to disentangle stellar activity signals from planetary atmospheric signals during transit.2025AJ....170..346Ehttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/AJ/170/346CatalogResearchIsServedByTAP VizieR generic servicerelated-toJ/ApJS/152/261 : Chromospheric Ca II emission in nearby stars (Wright+, 2004)J/A+A/469/309 : Chromospheric activity in late-type stars (Cincunegui+, 2007)J/A+A/554/A98 : Limb-darkening coefficients for red giants (Neilson+, 2013)J/ApJ/791/55 : HD 189733b in transit light curve (McCullough+, 2014)J/ApJ/795/166 : Extrasolar planet HD 189733b whitelight curve (Crouzet+, 2014)J/MNRAS/472/1618 : Kepler study of starspot lifetimes (Giles+, 2017)J/A+A/619/A96 : HD189733 spectral variability (Kohl+, 2018)J/A+A/652/A28 : Activity indicators across the M dwarf domain (Lafarga+, 2021)J/A+A/671/A130 : CLV curves of 10 spectr. lines taken with SST (Pietrow+, 2023)J/A+A/672/A24 : Geometric albedo of HD 189733b (Krenn+, 2023)J/AJ/169/38 : HD 189733b NIRCam transmission & emission spectra (Zhang+, 2025)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/AJ/170/346https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/AJ/170/346http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/AJ/170/346https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/AJ/170/346https://vizier.iucaa.in/viz-bin/votable?-source=J/AJ/170/346http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/AJ/170/346GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapOpticaldefaultJ/AJ/170/346/table1MEGARA datasetrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintTime[-1.981/1.242] Time from t0 (1)htime.epochdoubleHalpha[0.134/0.141] The H{alpha} index valuespect.index;em.line.Halphadoublee_Halpha[3.1e-4/1.41e-3] Uncertainty in Hastat.error;spect.index;em.line.HalphadoubleFeI[0.114/0.126] The Fe I index valuespect.indexdoublee_FeI[3e-4/1.4e-3] Uncertainty in FeIstat.error;spect.indexdoubleAirmass[1.03/1.85] Airmassobs.airMassdouble