Issue |
A&A
Volume 689, September 2024
|
|
---|---|---|
Article Number | A48 | |
Number of page(s) | 21 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/202450107 | |
Published online | 30 August 2024 |
High-energy spectra of LTT 1445A and GJ 486 reveal flares and activity
1
Department of Space Research and Space Technology, Technical University of Denmark,
Elektrovej 328, 2800 Kgs.
Lyngby,
Denmark
e-mail: hdiamondlowe@space.dtu.dk
2
Department of Astronomy, University of Michigan,
1085 S. University Ave., 323 West Hall,
Ann Arbor,
MI
48109,
USA
3
Exoplanets and Stellar Astrophysics Lab, NASA Goddard Space Flight Center,
Greenbelt,
MD
20771,
USA
4
Center for Astrophysics and Space Astronomy, University of Colorado,
389 UCB,
Boulder,
CO
80309,
USA
5
Department of Astrophysical and Planetary Sciences, University of Colorado,
2000 Colorado Avenue,
Boulder,
CO
80309,
USA
6
Center for Astrophysics | Harvard & Smithsonian,
60 Garden Street,
Cambridge,
MA
02138,
USA
7
Bridgewater State University,
131 Summer St.,
Bridgewater,
MA
02325,
USA
8
Laboratory for Atmospheric and Space Physics,
1234 Innovation Drive,
Boulder,
CO
80303,
USA
9
Max Planck Institute for Astronomy,
Königstuhl 17,
69117
Heidelberg,
Germany
10
Department of Astronomy, The University of Texas at Austin,
Austin,
TX
78712,
USA
11
Department of Astronomy & Astrophysics, University of Chicago,
Chicago,
IL
60637,
USA
12
School of Information and Physical Sciences, University of Newcastle,
Callaghan,
NSW,
Australia
13
Southwest Research Institute,
6220 Culebra Rd.,
San Antonio,
TX
78238,
USA
14
Department of Physics and Astronomy, Vanderbilt University,
Nashville,
TN
37235,
USA
15
Department of Astronomy, University of Maryland,
4296 Stadium Drive,
College Park,
MD
20742,
USA
16
Leiden Observatory, Leiden University,
PO Box 9513,
2300 RA
Leiden,
The Netherlands
17
SRON Netherlands Institute for Space Research,
Niels Bohrweg 4,
2333 CA
Leiden,
The Netherlands
18
Hamburger Sternwarte,
Gojenbergsweg 112,
21039
Hamburg,
Germany
Received:
24
March
2024
Accepted:
28
June
2024
The high-energy radiative output, from the X-ray to the ultraviolet, of exoplanet host stars drives photochemical reactions and mass loss in the upper regions of planetary atmospheres. In order to place constraints on the atmospheric properties of the three closest terrestrial exoplanets transiting M dwarfs, we observe the high-energy spectra of the host stars LTT 1445A and GJ 486 in the X-ray with XMM-Newton and Chandra and in the ultraviolet with HST/COS and STIS. We combine these observations with estimates of extreme-ultraviolet flux, reconstructions of the Lyα lines, and stellar models at optical and infrared wavelengths to produce panchromatic spectra from 1 Å to 20 µm for each star. While LTT 1445Ab, LTT 1445Ac, and GJ 486b do not possess primordial hydrogen-dominated atmospheres, we calculate that they are able to retain pure CO2 atmospheres if starting with 10, 15, and 50% of Earth’s total CO2 budget, respectively, in the presence of their host stars’ stellar wind. We use age-activity relationships to place lower limits of 2.2 and 6.6 Gyr on the ages of the host stars LTT 1445A and GJ 486. Despite both LTT 1445A and GJ 486 appearing inactive at optical wavelengths, we detect flares at ultraviolet and X-ray wavelengths for both stars. In particular, GJ 486 exhibits two far-ultraviolet flares with absolute energies of 1029.5 and 1030.1 erg (equivalent durations of 4357 ± 96 and 19 724 ± 169 s) occurring 3 h apart. Based on the timing of the observations, we suggest that these high-energy flares are related and indicative of heightened flaring activity that lasts for a period of days, but our interpretations are limited by sparse time-sampling. Consistent high-energy monitoring is needed to determine the duration and extent of high-energy activity on individual M dwarfs and the population as a whole.
Key words: planets and satellites: terrestrial planets / stars: activity / stars: flare / stars: low-mass
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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