J/A+A/634/L4 HST/STIS FUV spectra of K2-18 (dos Santos+, 2020)
The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b.
dos Santos L.A., Ehrenreich D., Bourrier V., Astudillo-Defru N.,
Bonfils X., Forget F., Lovis C., Pepe F., Udry S
<Astron. Astrophys. 634, L4 (2020)>
=2020A&A...634L...4D 2020A&A...634L...4D (SIMBAD/NED BibCode)
ADC_Keywords: Stars, M-type ; Exoplanets ; Spectra, ultraviolet
Keywords: stars: individual: K2-18 - stars: chromosphere -
planets and satellites: atmospheres - ISM: kinematics and dynamics
Abstract:
K2-18 b is a transiting mini-Neptune that orbits a nearby (38pc),
cool M3 dwarf and is located inside its region of temperate
irradiation. We report on the search for hydrogen escape from the
atmosphere K2-18 b using Lyman-α transit spectroscopy with the
Space Telescope Imaging Spectrograph (STIS) instrument installed on
the Hubble Space Telescope (HST). We analyzed the
time-series of fluxes of the stellar Lyman-α emission of K2-18
in both its blue- and redshifted wings. We found that the average
blueshifted emission of K2-18 decreases by 67%±18% during the
transit of the planet compared to the pre-transit emission,
tentatively indicating the presence of H atoms escaping vigorously and
being blown away by radiation pressure. This interpretation is not
definitive because it relies on one partial transit. Based on the
reconstructed Lyman-α emission of K2-18, we estimate an EUV
irradiation in the range 101-102erg/s/cm2 and a total escape
rate on the order of 108g/s. The inferred escape rate suggests that
the planet will lose only a small fraction (<1%) of its mass and
retain its volatile-rich atmosphere during its lifetime. More
observations are needed to rule out stellar variability effects,
confirm the in-transit absorption, and better assess the atmospheric
escape and high-energy environment of K2-18 b.
Description:
HST/STIS spectra of K2-18 using the G140M grating centered on 1222
Angstroem, taken during the transit of the planet K2-18 b in March 9
2018. The data were reduced using the standard STIS pipeline
(stistools); the spectral extraction was performed with user-defined
parameters to minimize instrumental background contamination.
Objects:
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RA (2000) DE Designation(s)
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11 30 14.52 +07 35 18.3 K2-18 = EPIC 201912552
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
orbit1.dat 71 1024 Spectrum taken in orbit 1
orbit2.dat 71 1024 Spectrum taken in orbit 2
orbit3.dat 71 1024 Spectrum taken in orbit 3
orbit4.dat 71 1024 Spectrum taken in orbit 4
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Byte-by-byte Description of file: orbit1.dat orbit2.dat orbit3.dat orbit4.dat
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Bytes Format Units Label Explanations
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1- 23 E23.16 0.1nm lambda Central wavelength of the pixel
25- 47 E23.16 10mW/m2/nm Flux Flux density in the pixel
(in erg/cm2/s/Å)
49- 71 E23.16 10mW/m2/nm e_Flux Flux density uncertainty in the pixel
(in erg/cm2/s/Å)
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Acknowledgements:
Leonardo A. dos Santos, leonardo.dossantos(at)unige.ch
(End) L. dos Santos [Univ. of Geneva, Switzerland], P. Vannier [CDS] 21-Jan-2020