Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape observed in young Neptune-sized exoplanets can provide insight into and characterize ...which mechanisms drive this evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 R⊕). It closely orbits a 23 Myr pre-main-sequence M dwarf with an orbital period of 8.46 days. We obtained two visits of AU Mic b at Lyα with Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph. One flare within the
first HST visit is characterized and removed from our search for a planetary transit. We present a nondetection in our first visit, followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow absorbed ∼30% of the star’s Lyα blue wing 2.5 hr before the planet’s white-light transit. We estimate that the highest-velocity escaping material has a column density of 1013.96 cm−2 and is moving 61.26 km s−1 away from the host star. AU Mic b’s large high-energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes, rendering it temporarily unobservable. Our time-variable Lyα transit ahead of AU Mic b could also be explained by an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Lyα observations of this system will confirm and characterize the unique variable nature of its Lyα transit, which, combined with modeling, will tune the importance of stellar wind and photoionization.
Abstract
K2-25b is a Neptune-sized exoplanet (3.45
R
⊕
) that orbits its M4.5 host with a period of 3.48 days. Due to its membership in the Hyades Cluster, the system has a known age (727 ± 75 Myr). ...K2-25b’s youth and its similarities with Gl 436b suggested that K2-25b could be undergoing strong atmospheric escape. We observed two transits of K2-25b at Ly
α
using HST/STIS in order to search for escaping neutral hydrogen. We were unable to detect an exospheric signature, but placed an upper limit of (
R
p
/
R
⋆
)∣
Ly
α
< 0.56 at 95% confidence by fitting the light curve of the Ly
α
red wing, or < 1.20 in the blue wing. We reconstructed the intrinsic Ly
α
profile of K2-25 to determine its Ly
α
flux, and analyzed XMM-Newton observations to determined its X-ray flux. Based on the total X-ray and extreme ultraviolet irradiation of the planet (8763 ± 1049 erg s
−1
cm
−2
), we estimated the maximum energy-limited mass-loss rate of K2-25b to be
10.6
−
6.13
+
15.2
×
10
10
g s
−1
(0.56
M
⊕
per 1 Gyr), five times larger than the similarly estimated mass-loss rate of Gl 436b (2.2 × 10
10
g s
−1
). The photoionization time is about 3 hr, significantly shorter than Gl 436b’s 14 hr. A nondetection of a Ly
α
transit could suggest K2-25b is not significantly losing its atmosphere, or factors of the system are resulting in the mass loss being unobservable (e.g., atmosphere composition or the system’s large high-energy flux). Further observations could provide more stringent constraints.
The Mysterious Affair of the H2 in AU Mic Flagg, Laura; Johns-Krull, Christopher M.; France, Kevin ...
The Astrophysical journal,
07/2022, Letnik:
934, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Molecular hydrogen is the most abundant molecule in the galaxy and plays important roles in planets, their circumstellar environments, and many of their host stars. We have confirmed the presence of ...molecular hydrogen in the AU Mic system using high-resolution FUV spectra from HST-STIS during both quiescence and a flare. AU Mic is a ∼23 Myr M dwarf that hosts a debris disk and at least two planets. We estimate the temperature of the gas at 1000–2000 K, consistent with previous detections. Based on the radial velocities and widths of the H2 line profiles and the response of the H2 lines to a stellar flare, the H2 line emission is likely produced in the star, rather than in the disk or the planet. However, the temperature of this gas is significantly below the temperature of the photosphere (∼3650 K) and the predicted temperature of its starspots (≳2650 K). We discuss the possibility of colder starspots or a cold layer in the photosphere of a pre-main-sequence M dwarf.
Exoplanets can evolve significantly between birth and maturity, as their atmospheres, orbits, and structures are shaped by their environment. Young planets (<1 Gyr) offer an opportunity to probe the ...critical early stages of this evolution, where planets evolve the fastest. However, most of the known young planets orbit prohibitively faint stars. We present the discovery of two planets transiting HD 63433 (TOI 1726, TIC 130181866), a young Sun-like ( ) star. Through kinematics, lithium abundance, and rotation, we confirm that HD 63433 is a member of the Ursa Major moving group (τ = 414 23 Myr). Based on the TESS light curve and updated stellar parameters, we estimate that the planet radii are 2.15 0.10 R⊕ and 2.67 0.12 R⊕, the orbital periods are 7.11 and 20.55 days, and the orbital eccentricities are lower than about 0.2. Using High Accuracy Radial velocity Planet Searcher for the Northern hemisphere velocities, we measure the Rossiter-McLaughlin signal of the inner planet, demonstrating that the orbit is prograde. Since the host star is bright (V = 6.9), both planets are amenable to transmission spectroscopy, radial velocity measurements of their masses, and more precise determination of the stellar obliquity. This system is therefore poised to play an important role in our understanding of planetary system evolution in the first billion years after formation.
The Mysterious Affair of the H 2 in AU Mic Flagg, Laura; Johns-Krull, Christopher M.; France, Kevin ...
The Astrophysical journal,
07/2022, Letnik:
934, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
Molecular hydrogen is the most abundant molecule in the galaxy and plays important roles in planets, their circumstellar environments, and many of their host stars. We have confirmed the ...presence of molecular hydrogen in the AU Mic system using high-resolution FUV spectra from HST-STIS during both quiescence and a flare. AU Mic is a ∼23 Myr M dwarf that hosts a debris disk and at least two planets. We estimate the temperature of the gas at 1000–2000 K, consistent with previous detections. Based on the radial velocities and widths of the H
2
line profiles and the response of the H
2
lines to a stellar flare, the H
2
line emission is likely produced in the star, rather than in the disk or the planet. However, the temperature of this gas is significantly below the temperature of the photosphere (∼3650 K) and the predicted temperature of its starspots (≳2650 K). We discuss the possibility of colder starspots or a cold layer in the photosphere of a pre-main-sequence M dwarf.
Exoplanets can evolve significantly between birth and maturity, as their atmospheres, orbits, and structures are shaped by their environment. Young planets (\(<\)1 Gyr) offer an opportunity to probe ...the critical early stages of this evolution, where planets evolve the fastest. However, most of the known young planets orbit prohibitively faint stars. We present the discovery of two planets transiting HD 63433 (TOI 1726, TIC 130181866), a young Sun-like (\(M_*=0.99\pm0.03\)) star. Through kinematics, lithium abundance, and rotation, we confirm that HD 63433 is a member of the Ursa Major moving group (\(\tau=414\pm23\) Myr). Based on the TESS light curve and updated stellar parameters, we estimate the planet radii are \(2.15\pm0.10R_\oplus\) and \(2.67\pm0.12R_\oplus\), the orbital periods are 7.11 and 20.55 days, and the orbital eccentricities are lower than about 0.2. Using HARPS-N velocities, we measure the Rossiter-McLaughlin signal of the inner planet, demonstrating that the orbit is prograde. Since the host star is bright (V=6.9), both planets are amenable to transmission spectroscopy, radial velocity measurements of their masses, and more precise determination of the stellar obliquity. This system is therefore poised to play an important role in our understanding of planetary system evolution in the first billion years after formation.
Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape observed in young Neptune-sized exoplanets can provide insight into and characterize ...which mechanisms drive this evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 Earth radii). It closely orbits a 23 Myr pre-Main Sequence M dwarf with a period of 8.46 days. We obtained two visits of AU Mic b at Lyman-alpha with HST/STIS. One flare within the first HST visit is characterized and removed from our search for a planetary transit. We present a non-detection in our first visit followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow absorbed about 30% of the star's Lyman-alpha blue-wing 2.5 hours before the planet's white-light transit. We estimate the highest velocity escaping material has a column density of 10^13.96 cm^-2 and is moving 61.26 km/s away from the host star. AU Mic b's large high energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes, rendering it temporarily unobservable. Our time-variable Lyman-alpha transit ahead of AU Mic b could also be explained by an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Lyman-alpha observations of this system will confirm and characterize the unique variable nature of its Lyman-alpha transit, which combined with modeling will tune the importance of stellar wind and photoionization.
Molecular hydrogen is the most abundant molecule in the Galaxy and plays
important roles for planets, their circumstellar environments, and many of
their host stars. We have confirmed the presence of ...molecular hydrogen in the
AU Mic system using high-resolution FUV spectra from HST-STIS during both
quiescence and a flare. AU Mic is a $\sim$23 Myr M dwarf which hosts a debris
disk and at least two planets. We estimate the temperature of the gas at 1000
to 2000 K, consistent with previous detections. Based on the radial velocities
and widths of the H$_2$ line profiles and the response of the H$_2$ lines to a
stellar flare, the H$_2$ line emission is likely produced in the star, rather
than in the disk or the planet. However, the temperature of this gas is
significantly below the temperature of the photosphere ($\sim$3650 K) and the
predicted temperature of its star spots ($\gtrsim$2650 K). We discuss the
possibility of colder star spots or a cold layer in the photosphere of a
pre-main sequence M dwarf.
We review efforts to describe the approach to chiral symmetry restoration in neutron matter from the low-energy realization of QCD, chiral effective field theory.
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