J/AJ/170/346 MEGARA obs. of Ha & FeI during HD 189733b transit (Ehrich+, 2025)
An Hα transit of HD 189733b to assess stellar activity across the transit
chord close to JWST observations.
Ehrich K.E., Dittmann J.A., Halverson S.P., Camazon-Pinilla A.
<Astron. J., 170, 346 (2025)>
=2025AJ....170..346E 2025AJ....170..346E
ADC_Keywords: Exoplanets; Spectroscopy; Optical; Stars, activity
Keywords: Exoplanets ; Stellar activity ; High resolution spectroscopy ;
Hot Jupiters ; Transmission spectroscopy
Abstract:
Transmission 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 Hα line from
the MEGARA spectrograph on the 10m GTC to examine the activity level
of HD 189733 during a transit. We measure Hα becoming shallower
midtransit by an Hα index of δ=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
Rspot=3.47±0.30R⊕ 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.
Description:
To examine the Hα signal during the transit of HD 189733b, we
acquired 132 individual optical spectra from the Multi-Espectrografo
en GTC de Alta Resolucion para Astronomia (MEGARA) spectrograph,
located on the Gran Telescopio Canarias (GTC) 10.4m telescope in La
Palma, Canary Islands, Spain on 2022 June 11. We used the "HR-R" mode
of MEGARA, which yields a resolving power of approximately R∼20,000
and spans a wavelength range of approximately 6405.61-6797.14Å,
making it ideal for Hα observations at 6562.8Å. Our
observations were taken with the VPH665-HR grating and had an exposure
time of 11s per frame. we found that 4 of the 132 frames had cosmic
rays present in the flux of the Hα feature. We discarded these
frames, therefore using 128 total frames for our analysis.
Objects:
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RA (2000) DE Designation(s)
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20 00 43.71 +22 42 39.0 HD 189733 = TOI-4470
20 00 43.71 +22 42 39.0 HD 189733b = HD 189733b
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 48 128 MEGARA dataset
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See also:
J/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)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 8 F8.5 h Time [-1.981/1.242] Time from t0 (1)
10- 16 F7.5 --- Halpha [0.134/0.141] The Hα index value
18- 24 F7.5 --- e_Halpha [3.1e-4/1.41e-3] Uncertainty in Ha
26- 32 F7.5 --- FeI [0.114/0.126] The Fe I index value
34- 40 F7.5 --- e_FeI [3e-4/1.4e-3] Uncertainty in FeI
42- 48 F7.5 --- Airmass [1.03/1.85] Airmass
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Note (1): where t0 is the time of mid-transit.
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History:
From electronic version of the journal
License: CC-BY-4.0
(End) Prepared by [AAS], Robin Leichtnam [CDS] 07-Sep-2026