The spectral energy distribution, variability, and evolution of the high-energy radiation from an M dwarf planet host is crucial in understanding the planet's atmospheric evolution and habitability ...and in interpreting the planet's spectrum. The star's extreme-UV (EUV), far-UV (FUV), and near-UV (NUV) emission can chemically modify, ionize, and erode the atmosphere over time. This makes determining the lifetime exposure of such planets to stellar UV radiation critical for both the evolution of a planet's atmosphere and our potential to characterize it. Using the early M star members of nearby young moving groups, which sample critical ages in planet formation and evolution, we measure the evolution of the GALEX NUV and FUV flux as a function of age. The median UV flux remains at a "saturated" level for a few hundred million years, analogous to that observed for X-ray emission. By the age of the Hyades Cluster (650 Myr), we measure a drop in UV flux by a factor of 2-3 followed by a steep drop from old (several Gyrs) field stars. This decline in activity beyond 300 Myr follows roughly t super(-1). Despite this clear evolution, there remains a wide range, of 1-2 orders of magnitude, in observed emission levels at every age. These UV data supply the much-needed constraints to M dwarf upper-atmosphere models, which will provide empirically motivated EUV predictions and more accurate age-dependent UV spectra as inputs to planetary photochemical models.
Abstract
We present a spectrally and temporally resolved detection of the optical Mg
i
triplet at 7.8
σ
in the extended atmosphere of the ultra-hot Jupiter KELT-9 b, adding to the list of detected ...metal species in the hottest gas giant currently known. Constraints are placed on the density and radial extent of the excited hydrogen envelope using simultaneous observations of H
α
and H
β
under the assumption of a spherically symmetric atmosphere. We find that planetary rotational broadening of
km s
−1
is necessary to reproduce the Balmer line transmission profile shapes, where the model including rotation is strongly preferred over the non-rotating model using a Bayesian information criterion comparison. The time series of both metal line and hydrogen absorption show remarkable structure, suggesting that the atmosphere observed during this transit is dynamic rather than static. We detect a relative emission feature near the end of the transit which exhibits a P-Cygni-like shape, evidence of material moving at ≈50–100 km s
−1
away from the planet. We hypothesize that the in-transit variability and subsequent P-Cygni-like profiles are due to a flaring event that caused the atmosphere to expand, resulting in unbound material being accelerated to high speeds by stellar radiation pressure. Further spectroscopic transit observations will help establish the frequency of such events.
The high energy radiation environment around M dwarf stars strongly impacts the characteristics of close-in exoplanet atmospheres, but these wavelengths are difficult to observe due to geocoronal and ...interstellar contamination. On account of these observational restrictions, a stellar atmosphere model may be used to compute the stellar extreme ultraviolet (EUV; 100-912 ) spectrum. We construct semiempirical nonlocal thermodynamic equilibrium model spectra of the ultracool M8 star TRAPPIST-1 that span EUV to infrared wavelengths (100 −2.5 m) using the atmosphere code PHOENIX. These upper atmosphere models contain prescriptions for the chromosphere and transition region and include newly added partial frequency redistribution capabilities. In the absence of broadband UV spectral observations, we constrain our models using Hubble Space Telescope Lyman observations from TRAPPIST-1 and Galaxy Evolution Explorer UV photometric detections from a set of old M8 stars (>1 Gyr). We find that calibrating the models using both data sets separately yield similar far-ultraviolet and NUV fluxes, and EUV fluxes that range from (1.32-17.4) × 10−14 ergs s−1 cm−2. The results from these models demonstrate that the EUV emission is very sensitive to the temperature structure in the transition region. Our lower activity models predict EUV fluxes similar to previously published estimates derived from semiempirical scaling relationships, while the highest activity model predicts EUV fluxes a factor of 10 higher. Results from this study support the idea that the TRAPPIST-1 habitable zone planets likely do not have much liquid water on their surfaces due to the elevated levels of high energy radiation emitted by the host star.
Low-mass stars are currently the most promising targets for detecting and characterizing habitable planets in the solar neighborhood. However, the ultraviolet (UV) radiation emitted by such stars can ...erode and modify planetary atmospheres over time, drastically affecting their habitability. Thus, knowledge of the UV evolution of low-mass stars is critical for interpreting the evolutionary history of any orbiting planets. Shkolnik & Barman used photometry from the Galaxy Evolution Explorer (GALEX) to show how UV emission evolves for early-type M stars (>0.35 M ). In this paper, we extend their work to include both a larger sample of low-mass stars with known ages as well as M stars with lower masses. We find clear evidence that mid- and late-type M stars (0.08-0.35 M ) do not follow the same UV evolutionary trend as early-Ms. Lower-mass M stars retain high levels of UV activity up to field ages, with only a factor of 4 decrease on average in GALEX NUV and FUV flux density between young (<50 Myr) and old (∼5 Gyr) stars, compared to a factor of 11 and 31 for early-Ms in NUV and FUV, respectively. We also find that the FUV/NUV flux density ratio, which can affect the photochemistry of important planetary biosignatures, is mass- and age-dependent for early-Ms, but remains relatively constant for the mid- and late-type Ms in our sample.
We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (> ~1 M sub(Jup)) around 122 newly identified nearby (<, ~40 pc) young M dwarfs. Half of our ...targets are younger than 135 Myr and 90% are younger than the Hyades (620 Myr). After removing 44 close stellar binaries (implying a stellar companion fraction of >35.4% + or - 4.3% within 100 AU), 27 of which are new or spatially resolved for the first time, our remaining sample of 78 single M dwarfs makes this the largest imaging search for planets around young low-mass stars (0.1-0.6 M sub(middot in circle)) to date. Our H- and K-band coronagraphic observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of 12-14 mag and 9-13 mag at 1", respectively, which correspond to limiting planet masses of 0.5-10 M sub(Jup) at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 + or - 6 M sub(Jup); L0 super(+2) sub(-1); 120 + or - 20 AU), GJ 3629 B (64 super(+30) sub(-23) M sub(Jup); M7.5 + or - 0.5; 6.5 + or - 0.5 AU), 1RXS J034231.8+121622 B (35 + or - 8 M sub(Jup); L0 + or - 1; 19.8 + or - 0.9 AU), and 2MASS J15594729+4403595 B (43 + or - 9 M sub(Jup); M8.0 + or - 0.5; 190 + or - 20 AU). Over 150 candidate planets were identified; we obtained follow-up imaging for 56% of these but all are consistent with background stars. Our null detection of planets enables strong statistical constraints on the occurrence rate of long-period giant planets around single M dwarfs. We infer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13 M sub(Jup) planets between 10-100 AU for hot-start and cold-start (Fortney) evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbor massive gas giants in the 5-13 M sub(Jup) range like those orbiting HR 8799 and beta Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. The frequency of brown dwarf (13-75 M sub(Jup)) companions to single M dwarfs between 10-100 AU is 2.8 super(+2.4) sub(-1.5)%. Altogether we find that giant planets, especially massive ones, are rare in the outskirts of M dwarf planetary systems. Although the first directly imaged planets were found around massive stars, there is currently no statistical evidence for a trend of giant planet frequency with stellar host mass at large separations as predicted by the disk instability model of giant planet formation.
One of the predictions of high-eccentricity planetary migration is that many planets will end up plunging into their host stars. We investigate the consequence of planetary mergers on their stellar ...hosts' spin period. Energy and angular momentum conservation indicate that planet consumption by a star will spin up the star. We find that our proof-of-concept calculations align with the observed bifurcation in the stellar spin-period in young clusters. For example, after a Sun-like star has eaten a Jupiter-mass planet it will spin up by ∼60% (i.e., spin period is reduced by ∼60%), causing an apparent gap in the stellar spin-period between stars that consumed a planet and those that did not. The spun-up star will later spin down due to magnetic braking, consistent with the disappearance of this bifurcation in clusters ( 300 Myr). The agreement between the calculations presented here and the observed spin-period color diagram of stars in young clusters provides circumstantial evidence that planetary accretion onto their host stars is a generic feature of planetary-system evolution.
Using the far-UV (FUV) and near-UV (NUV) photometry from the NASA Galaxy Evolution Explorer (GALEX), we searched for evidence of increased stellar activity due to tidal and/or magnetic star-planet ...interactions (SPI) in the 272 known FGK planetary hosts observed by GALEX. With the increased sensitivity of GALEX, we are able probe systems with lower activity levels and at larger distances than what has been done to date with X-ray satellites. We compared samples of stars with close-in planets (a <0.1 AU) to those with far-out planets (a > 0.5 AU) and looked for correlations of excess activity with other system parameters. This statistical investigation found no clear correlations with a, Mp, or Mp/a, in contrast to some X-ray and Ca II studies. However, there is tentative evidence (at a level of 1,8sigma) that stars with radial-velocity-(RV)-detected close-in planets are more FUV-active than stars with far-out planets, in agreement with several published X-ray and Ca II results. The case is strengthened to a level of significance to 2.3sigma when transit-detected close-in planets are included. This is most likely because the RV-selected sample of stars is significantly less active than the field population of comparable stars, while the transit-selected sample is similarly active. Given the factor of 2-3 scatter in fractional FUV luminosity for a given stellar effective temperature, it is necessary to conduct a time-resolved study of the planet hosts in order to better characterize their UV variability and generate a firmer statistical result.
Chromospherically sensitive atomic lines display different spectra in stellar active regions, spots, and the photosphere, raising the possibility that exoplanet transmission spectra are contaminated ...by the contrast between various portions of the stellar disk. To explore this effect, we performed transit simulations of G-type and K-type stars for the spectral lines Ca ii K at 3933 , Na i 5890 , H i 6563 (H ), and He i 10830 . We find that strong facular emission and large coverage fractions can contribute a non-negligible amount to transmission spectra, especially for H , Ca ii K, and Na i D, while spots and filaments are comparatively unimportant. The amount of contamination depends strongly on the location of the active regions and the intrinsic emission strength. In particular, active regions must be concentrated along the transit chord in order to produce a consistent in-transit signal. Mean absorption signatures in Na i and H , for example, can reach 0.2% and 0.3%, respectively, for transits of active latitudes with line emission similar in strength to moderate solar flares. Transmission spectra of planets transiting active stars, such as HD 189733, are likely contaminated by the contrast effect, although the tight constraints on active region geometry and emission strength make it unlikely that consistent in-transit signatures are due entirely to the contrast effect. He i 10830 is not strongly affected and absorption signatures are likely diluted, rather than enhanced, by stellar activity. He i 10830 should thus be considered a priority for probing extended atmospheres, even in the case of active stars.
Young, low-mass stars in the solar neighborhood are vital for completing the mass function for nearby, young coeval groups, establishing a more complete census for evolutionary studies, and providing ...targets for direct-imaging exoplanet and/or disk studies. We present properties derived from high-resolution optical spectra for 336 candidate young nearby, low-mass stars. These include measurements of radial velocities and age diagnostics such as H and Li λ6707 equivalent widths. Combining our radial velocities with astrometry from Gaia DR2, we provide full 3D kinematics for the entire sample. We combine the measured spectroscopic youth information with additional age diagnostics (e.g., X-ray and UV fluxes, color-magnitude diagram positions) and kinematics to evaluate potential membership in nearby, young moving groups and associations. We identify 77 objects in our sample as bona fide members of 10 different moving groups, 14 of which are completely new members or have had their group membership reassigned. We also reject 44 previously proposed candidate moving group members. Furthermore, we have newly identified or confirmed the youth of numerous additional stars that do not belong to any currently known group and find 69 comoving systems using Gaia DR2 astrometry. We also find evidence that the Carina association is younger than previously thought, with an age similar to the β Pictoris moving group (∼22 Myr).
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