Recent work has demonstrated that high levels of X-ray and UV activity on young M dwarfs may drive rapid atmospheric escape on temperate, terrestrial planets orbiting within the habitable zone. ...However, secondary atmospheres on planets orbiting older, less active M dwarfs may be stable and present more promising candidates for biomarker searches. In order to evaluate the potential habitability of Earth-like planets around old, inactive M dwarfs, we present new Hubble Space Telescope and Chandra X-ray Observatory observations of Barnard's Star (GJ 699), a 10 Gyr old M3.5 dwarf, acquired as part of the Mega-MUSCLES program. Despite the old age and long rotation period of Barnard's Star, we observe two FUV (δ130 5000 s; E130 1029.5 erg each) and one X-ray (EX 1029.2 erg) flares, and we estimate a high-energy flare duty cycle (defined here as the fraction of the time the star is in a flare state) of ∼25%. A publicly available 5 to 10 m spectral energy distribution of GJ 699 is created and used to evaluate the atmospheric stability of a hypothetical, unmagnetized terrestrial planet in the habitable zone (rHZ ∼ 0.1 au). Both thermal and nonthermal escape modeling indicate (1) the quiescent stellar XUV flux does not lead to strong atmospheric escape: atmospheric heating rates are comparable to periods of high solar activity on modern Earth, and (2) the flare environment could drive the atmosphere into a hydrodynamic loss regime at the observed flare duty cycle: sustained exposure to the flare environment of GJ 699 results in the loss of 87 Earth atmospheres Gyr−1 through thermal processes and 3 Earth atmospheres Gyr−1 through ion loss processes. These results suggest that if rocky planet atmospheres can survive the initial ∼5 Gyr of high stellar activity, or if a second-generation atmosphere can be formed or acquired, the flare duty cycle may be the controlling stellar parameter for the stability of Earth-like atmospheres around old M stars.
Abstract
Two close-in planets have been recently found around the M-dwarf flare star AU Microscopii (AU Mic). These Neptune-sized planets (AU Mic b and c) seem to be located very close to the ...so-called “evaporation valley” in the exoplanet population, making this system an important target for studying atmospheric loss on exoplanets. This process, while mainly driven by high-energy stellar radiation, will be strongly mediated by the space environment surrounding the planets. Here we present an investigation of this last area, performing 3D numerical modeling of the quiescent stellar wind from AU Mic, as well as time-dependent simulations describing the evolution of a highly energetic coronal mass ejection (CME) event in this system. Observational constraints on the stellar magnetic field and properties of the eruption are incorporated in our models. We carry out qualitative and quantitative characterizations of the stellar wind, the emerging CMEs, as well as the expected steady and transient conditions along the orbit of both exoplanets. Our results predict extreme space weather for AU Mic and its planets. This includes sub-Alfvénic regions for the large majority of the exoplanet orbits, very high dynamic and magnetic pressure values in quiescence (varying within 10
2
–10
5
times the dynamic pressure experienced by Earth), and an even harsher environment during the passage of any escaping CME associated with the frequent flaring observed in AU Mic. These space weather conditions alone pose an immense challenge for the survival of exoplanetary atmospheres (if any) in this system.
Abstract
Characterizing the fundamental parameters of stars from observations is crucial for studying the stars themselves, their planets, and the galaxy as a whole. Stellar evolution theory ...predicting the properties of stars as a function of stellar age and mass enables translating observables into physical stellar parameters by fitting the observed data to synthetic isochrones. However, the complexity of overlapping evolutionary tracks often makes this task numerically challenging, and with a precision that can be highly variable, depending on the area of the parameter space the observation lies in. This work presents S
tel
N
et
, a Deep Neural Network trained on stellar evolutionary tracks that quickly and accurately predicts mass and age from absolute luminosity and effective temperature for stars with close-to-solar metallicity. The underlying model makes no assumption on the evolutionary stage and includes the pre-main-sequence phase. We use bootstrapping and train many models to quantify the uncertainty of the model. To break the model’s intrinsic degeneracy resulting from overlapping evolutionary paths, we also built a hierarchical model that retrieves realistic posterior probability distributions of the stellar mass and age. We further test and train S
tel
N
et
using a sample of stars with well-determined masses and ages from the literature.
Abstract
Low-mass (≲1.2
M
⊙
) main-sequence stars lose angular momentum over time, leading to a decrease in their magnetic activity. The details of this rotation–activity relation remain poorly ...understood, however. Using observations of members of the ≈700 Myr old Praesepe and Hyades open clusters, we aim to characterize the rotation–activity relation for different tracers of activity at this age. To complement published data, we obtained new optical spectra for 250 Praesepe stars, new X-ray detections for 10, and new rotation periods for 28. These numbers for Hyads are 131, 23, and 137, respectively. The latter increases the number of Hyads with periods by 50%. We used these data to measure the fractional H
α
and X-ray luminosities,
L
H
α
/
L
bol
and
L
X
/
L
bol
, and to calculate Rossby numbers
R
o
. We found that at ≈700 Myr almost all M dwarfs exhibit H
α
emission, with binaries having the same overall color–H
α
equivalent width distribution as single stars. In the
R
o
–
L
H
α
/
L
bol
plane, unsaturated single stars follow a power law with index
β
= −5.9 ± 0.8 for
R
o
> 0.3. In the
R
o
–
L
X
/
L
bol
plane, we see evidence for supersaturation for single stars with
R
o
≲ 0.01, following a power law with index
β
sup
=
0.5
−
0.1
+
0.2
, supporting the hypothesis that the coronae of these stars are being centrifugally stripped. We found that the critical
R
o
value at which activity saturates is smaller for
L
X
/
L
bol
than for
L
H
α
/
L
bol
. Finally, we observed an almost 1:1 relation between
L
H
α
/
L
bol
and
L
X
/
L
bol
, suggesting that both the corona and the chromosphere experience similar magnetic heating.
Abstract
We present a three-dimensional, time-dependent MHD simulation of the short-term interaction between a protoplanetary disk and the stellar corona in a T Tauri system. The simulation includes ...the stellar magnetic field, self-consistent coronal heating and stellar wind acceleration, and a disk rotating at sub-Keplerian velocity to induce accretion. We find that, initially, as the system relaxes from the assumed initial conditions, the inner part of the disk winds around and moves inward and close to the star as expected. However, the self-consistent coronal heating and stellar wind acceleration build up the original state after some time, significantly pushing the disk out beyond 10
R
⋆
. After this initial relaxation period, we do not find clear evidence of a strong, steady accretion flow funneled along coronal field lines, but only weak, sporadic accretion. We produce synthetic coronal X-ray line emission light curves, which show flare-like increases that are not correlated with accretion events nor with heating events. These variations in the line emission flux are the result of compression and expansion due to disk–corona pressure variations. Vertical disk evaporation evolves above and below the disk. However, the disk–stellar wind boundary stays quite stable, and any disk material that reaches the stellar wind region is advected out by the stellar wind.
We present mocassin two-dimensional photoionization and dust radiative transfer models of a prototypical T Tauri disk irradiated by X-rays from the young pre-main-sequence star. The calculations ...demonstrate a layer of hot gas reaching temperatures of image10 super(6) K at small radii and image10 super(4) K at a distance of 1 AU. The gas temperatures decrease sharply with depth, but appear to be completely decoupled from dust temperatures down to a column depth of image cm super(-2). We predict that several fine-structure and forbidden lines of heavy elements, as well as recombination lines of hydrogen and helium, should be observable with current and future instrumentation, although optical lines may be smothered by the stellar spectrum. Predicted line luminosities are given for the brightest collisionally excited lines (down to image10 super(-8) L sub(image)) and for recombination transitions from several levels of H i and He i. The mass-loss rate due to X-ray photoevaporation estimated from our models is of the order of 10 super(-8) M sub(image) yr super(-1), implying a dispersal timescale of a few Myr for a disk of mass 0.027 M sub(image), which is the mass of the disk structure model we employed. We discuss the limitations of our model and highlight the need for further calculations that should include the simultaneous solution of the two-dimensional radiative transfer problem and the one-dimensional hydrostatic equilibrium problem in the polar direction.
Abstract We report the serendipitous discovery of a large edge-on protoplanetary disk in Pan-STARRS (PS1) images. PS1 has five broadband filters designated as g P1 , r P1 , i P1 , z P1 , and y P1 ...with mean wavelengths 4866, 6215, 7545, 8679, and 9633 Å, respectively. The disk’s apparent size in the PS1 images is ≈11″, making this one of the largest known disks on the sky. It is likely a young system, still surrounded by the envelope, which is very faint but still visible in the PS1 images in the northern part (alternatively this structure could be filaments from the disk itself). We use the PS1 magnitudes and other available photometric data to construct the spectral energy distribution of the disk. An optical spectrum indicates that the obscured star is hot, most likely of type late A. We adopt a distance of 300 pc for this object based on Gaia Data Release 3 extinctions. We model the system using the HOCHUNK3D radiative transfer software and find that the system is consistent with a hot star of effective temperature 8000 K surrounded by a disk of size 1650 au and mass 0.2 M ⊙ at inclination 82°.
We have carried out a survey of X-ray emission from stars with giant planets, combining both archival and targeted surveys. Over 230 stars have been currently identified as possessing planets, and ...roughly one-third of these have been detected in X-rays. We carry out detailed statistical analysis on a volume-limited sample of main-sequence star systems with detected planets, comparing subsamples of stars that have close-in planets with stars that have more distant planets. This analysis reveals strong evidence that stars with close-in giant planets are on average more X-ray active by a factor of sim4 than those with planets that are more distant. This result persists for various sample selections. We find that even after accounting for observational sample bias, a significant residual difference still remains. This observational result is consistent with the hypothesis that giant planets in close proximity to the primary stars influence the stellar magnetic activity.
Abstract
The proximity and duration of the tidal disruption event ASASSN-14li led to the discovery of narrow, blueshifted absorption lines in X-rays and UV. The gas seen in X-ray absorption is ...consistent with bound material close to the apocenter of elliptical orbital paths, or with a disk wind similar to those seen in Seyfert-1 active galactic nuclei. We present a new analysis of the deepest high-resolution XMM-Newton and Chandra spectra of ASASSN-14li. Driven by the relative strengths of He-like and H-like charge states, the data require N/C ≥ 2.4, in qualitative agreement with UV spectral results. Flows of the kind seen in the X-ray spectrum of ASASSN-14li were not clearly predicted in simulations of TDEs; this left open the possibility that the observed absorption might be tied to gas released in prior active galactic nucleus (AGN) activity. However, the abundance pattern revealed in this analysis points to a single star rather than a standard AGN accretion flow comprised of myriad gas contributions. The simplest explanation of the data is likely that a moderately massive star (
M
≳ 3
M
⊙
) with significant CNO processing was disrupted. An alternative explanation is that a lower mass star was disrupted that had previously been stripped of its envelope. We discuss the strengths and limitations of our analysis and these interpretations.
Long‐term ecosystem development involves changes in plant community composition and diversity associated with pedogenesis and nutrient availability, but comparable changes in soil microbial ...communities remain poorly understood. In particular, it is unclear whether the diversity of plants and microbes respond to similar abiotic drivers, or become decoupled as resources change over long time‐scales.
We characterized communities of archaea, bacteria and fungi in soils along a 2‐million‐year chronosequence of coastal dunes in a biodiversity hot spot in Western Australia. The chronosequence involves marked changes in soil pH and nutrient availability that drive major shifts in plant community composition and diversity as soils age.
Patterns of α‐diversity for microbial groups differed along the chronosequence. Bacterial α‐diversity was greatest in intermediate‐aged soils; archaeal diversity was greater in young alkaline or intermediate‐aged soils, while fungal α‐diversity—like plant diversity—was greatest in old, strongly weathered soils where phosphorus is the limiting nutrient.
Changes in microbial community composition along the chronosequence were explained primarily by the long‐term decline in soil pH, with a smaller influence of the relative abundance of plant nutrient‐acquisition strategies. However, changes between the prokaryote and fungal communities, and between fungal and plant communities, became increasingly decoupled along the chronosequence, demonstrating that the coordination of change in biological communities by abiotic drivers becomes weaker during long‐term ecosystem development.
Several bacterial taxa, including DA101 (Verrucomicrobia), “Candidatus Solibacter” (Acidobacteria) and Gaiella (Actinobacteria), were particularly abundant on the oldest dunes, indicating that they are adapted to acquire phosphorus from extremely infertile soils. However, we cannot disentangle the influence of phosphorus from the long‐term decline in soil pH along the chronosequence.
Synthesis. These results provide evidence for contrasting patterns of plant and microbial community composition and α‐diversity in response to acidification and nutrient depletion during long‐term pedogenesis.
These results provide evidence for contrasting patterns of plant and microbial community composition and α‐diversity in response to acidification and nutrient depletion during long‐term pedogenesis.
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