We report the discovery of an extremely red planetary-mass companion to 2MASS J22362452+4751425, a approximate0.6 M sub(middot in circle) late-K dwarf likely belonging to the ~120 Myr AB Doradus ...moving group. 2M2236+4751 b was identified in multi-epoch NIRC2 adaptive optics imaging at Keck Observatory at a separation of 3".7, or 230 + or - 20 AU in projection at the kinematic distance of 63 + or - 5 pc to its host star. Assuming membership in the AB Dor group, as suggested from its kinematics, the inferred mass of 2M2236+4751 b is 11-14 M sub(Jup). Follow-up Keck/OSIRIS K-band spectroscopy of the companion reveals strong CO absorption similar to other faint red L dwarfs and lacks signs of methane absorption, despite having an effective temperature of approximate900-1200 K. With a (J-K) sub(MKO) color of 2.69 + or - 0.12 mag, the near-infrared slope of 2M2236+4751 b is redder than all of the HR 8799 planets and instead resembles the approximate23 Myr isolated planetary-mass object PSO J318.5-22, implying that similarly thick photospheric clouds can persist in the atmospheres of giant planets at ages beyond 100 Myr. In near-infrared color-magnitude diagrams, 2M2236+4751 b is located at the tip of the red L dwarf sequence and appears to define the "elbow" of the AB Dor substellar isochrone separating low-gravity L dwarfs from the cooler young T dwarf track. 2M2236+4751 b is the reddest substellar companion to a star and will be a valuable benchmark to study the shared atmospheric properties of young low-mass brown dwarfs and extrasolar giant planets.
ABSTRACT
We present the results of a multiwavelength Professional–Amateur campaign to study the behaviour of flares from the active M1.5V star binary CR Draconis. CR Dra was observed with Transiting ...Exoplanet Survey Satellite (TESS) 20-s photometry, Swift near-ultraviolet (NUV) grism spectroscopy and with ground-based optical photometry and spectroscopy from a global collaboration of amateur astronomers. We detected 14 flares with TESS and Swift simultaneously, one of which also had simultaneous ground-based photometry and spectroscopy. We used the simultaneous two-colour optical and NUV observations to characterize the temperature evolution of the flare and test the accuracy of using optical data to predict NUV emission. We measured a peak temperature of $7100^{+150}_{-130}$ K for this flare, cooler than the typically assumed 9000 K blackbody model used by flare studies. We also found that the 9000 K blackbody overestimated the NUV flux for other flares in our sample, which we attributed to our Swift observations occurring during flare decays, highlighting the phase-dependence for the accuracy of flare models.
The ultraviolet (UV) emission from the most numerous stars in the universe, M dwarfs, impacts the formation, chemistry, atmospheric stability, and surface habitability of their planets. We have ...analyzed the spectral evolution of UV emission from M0-M2.5 (0.3-0.6 M ) stars as a function of age, rotation, and Rossby number using Hubble Space Telescope observations of Tucana-Horologium (40 Myr), Hyades (650 Myr), and field (2-9 Gyr) objects. The quiescent surface flux of their C ii, C iii, C iv, He ii, N v, Si iii, and Si iv emission lines, formed in the stellar transition region, remains elevated at a constant level for 240 30 Myr before declining by 2.1 orders of magnitude to an age of 10 Gyr. The Mg ii and far-UV pseudocontinuum emission, formed in the stellar chromosphere, exhibits more gradual evolution with age, declining by 1.3 and 1.7 orders of magnitude, respectively. The youngest stars exhibit a scatter of 0.1 dex in far-UV line and pseudocontinuum flux attributable only to rotational modulation, long-term activity cycles, or an unknown source of variability. Saturation-decay fits to these data can predict an M0-M2.5 star's quiescent emission in UV lines and the far-UV pseudocontinuum with an accuracy of 0.2-0.3 dex, the most accurate means presently available. Predictions of UV emission will be useful for studying exoplanetary atmospheric evolution and the destruction and abiotic production of biologically relevant molecules and interpreting infrared and optical planetary spectra measured with observatories like the James Webb Space Telescope.
The chaotic wind of WR 40 as probed by BRITE Ramiaramanantsoa, Tahina; Ignace, Richard; Moffat, Anthony F J ...
Monthly notices of the Royal Astronomical Society,
12/2019, Volume:
490, Issue:
4
Journal Article
Peer reviewed
Open access
ABSTRACT
Among Wolf–Rayet stars, those of subtype WN8 are the intrinsically most variable. We have explored the long-term photometric variability of the brightest known WN8 star, WR 40, through four ...contiguous months of time-resolved, single-passband optical photometry with the BRIght Target Explorer nanosatellite mission. The Fourier transform of the observed light curve reveals that the strong light variability exhibited by WR 40 is dominated by many randomly triggered, transient, low-frequency signals. We establish a model in which the whole wind consists of stochastic clumps following an outflow visibility promptly rising to peak brightness upon clump emergence from the optically thick pseudo-photosphere in the wind, followed by a gradual decay according to the right-half of a Gaussian. Free electrons in each clump scatter continuum light from the star. We explore a scenario where the clump size follows a power-law distribution, and another one with an ensemble of clumps of constant size. Both scenarios yield simulated light curves morphologically resembling the observed light curve remarkably well, indicating that one cannot uniquely constrain the details of clump size distribution with only a photometric light curve. Nevertheless, independent evidence favours a negative-index power law, as seen in many other astrophysical turbulent media.
Abstract
Efforts to discover and characterize habitable zone planets have primarily focused on Sun-like stars and M dwarfs. K stars, however, provide an appealing compromise between these two ...alternatives that has been relatively unexplored. Understanding the ultraviolet (UV) environment around such stars is critical to our understanding of their planets, as the UV can drastically alter the photochemistry of a planet’s atmosphere. Here we present near-UV and far-UV Hubble Space Telescope's Cosmic Origins Spectrograph observations of 39 K stars at three distinct ages: 40 Myr, 650 Myr, and ≈5 Gyr. We find that the K star (0.6–0.8
M
⊙
) UV flux remains constant beyond 650 Myr before falling off by an order of magnitude by field age. This is distinct from early M stars (0.3–0.6
M
⊙
), which begin to decline after only a few hundred megayears. However, the rotation–UV activity relation for K stars is nearly identical to that of early M stars. These results may be a consequence of the spin-down stalling effect recently reported for K dwarfs, in which the spin-down of K stars halts for over a gigayear when their rotation periods reach ≈10 days, rather than the continuous spin-down that G stars experience. These results imply that exoplanets orbiting K dwarfs may experience a stronger UV environment than thought, weakening the case for K stars as hosts of potential “super-habitable” planets.
We present the discovery of a wide (67 AU) substellar companion to the nearby (21 pc) young solar-metallicity M1 dwarf CD--35 2722, a member of the 100 Myr AB Doradus association. Two epochs of ...astrometry from the NICI Planet-Finding Campaign confirm that CD--35 2722 B is physically associated with the primary star. Near-IR spectra indicate a spectral type of L4?1 with a moderately low surface gravity, making it one of the coolest young companions found to date. The absorption lines and near-IR continuum shape of CD--35 2722 B agree especially well the dusty field L4.5 dwarf 2MASS J22244381--0158521, while the near-IR colors and absolute magnitudes match those of the 5 Myr old L4 planetary-mass companion, 1RXS J160929.1--210524 b. Overall, CD--35 2722 B appears to be an intermediate-age benchmark for L dwarfs, with a less peaked H-band continuum than the youngest objects and near-IR absorption lines comparable to field objects. We fit Ames-Dusty model atmospheres to the near-IR spectra and find T eff= 1700-1900 K and log(g)= 4.5 ? 0.5. The spectra also show that the radial velocities of components A and B agree to within ?10 km s--1, further confirming their physical association. Using the age and bolometric luminosity of CD--35 2722 B, we derive a mass of 31 ? 8 M Jup from the Lyon/Dusty evolutionary models. Altogether, young late-M to mid-L type companions appear to be overluminous for their near-IR spectral type compared with field objects, in contrast to the underluminosity of young late-L and early-T dwarfs.
Abstract
We present millimeter, optical, and soft X-ray observations of a stellar flare with an energy squarely in the regime of typical X1 solar flares. The flare was observed from Proxima Cen on ...2019 May 6 as part of a larger multi-wavelength flare monitoring campaign and was captured by Chandra, the Las Cumbres Observatory Global Telescope, the Iréné du Pont Telescope at Las Campanas Observatory, and the Atacama Large Millimeter Array. Millimeter emission appears to be a common occurrence in small stellar flares that had gone undetected until recently, making it difficult to interpret these events within the current multi-wavelength picture of the flaring process. The May 6 event is the smallest stellar millimeter flare detected to date. We compare the relationship between the soft X-ray and millimeter emission to that observed in solar flares. The X-ray and optical flare energies of 10
30.3 ± 0.2
and 10
28.9 ± 0.1
erg, respectively, the coronal temperature of
T
= 11.0 ± 2.1 MK, and the emission measure of 9.5 ± 2.2 × 10
49
cm
−3
are consistent with M-X class solar flares. We find the soft X-ray and millimeter emission during quiescence are consistent with the Güdel–Benz relation, but not during the flare. The millimeter luminosity is >100× higher than that of an equivalent X1 solar flare and lasts only seconds instead of minutes as seen for solar flares.
We combine new high resolution imaging and spectroscopy from Keck/NIRC2, Discovery Channel Telescope/ DSSI, and Keck/HIRES with published astrometry and radial velocities to measure individual masses ...and orbital elements of the GJ 3305 AB system, a young (~20 Myr) M+M binary (unresolved spectral type M0) member of the beta Pictoris moving group comoving with the imaged exoplanet host 51 Eri. We measure a total system mass of 1.11 + or - 0.04 M sub(middot in circle), a period of 29.03 + or - 0.50 year, a semimajor axis of 9.78 + or - 0.14 AU, and an eccentricity of 0.19 + or -0.02. The primary component has a dynamical mass of 0.67 + or - 0.05 M sub(middot in circle) and the secondary has a mass of 0.44 + or -0.05 M sub(middot in circle). The recently updated BHAC15 models are consistent with the masses of both stars to within 1.5sigma. Given the observed masses the models predict an age of the GJ 3305 AB system of 37 + or - 9 Myr. Based on the observed system architecture and our dynamical mass measurement, it is unlikely that the orbit of 51 Eri b has been significantly altered by the Kozai-Lidov mechanism.
Abstract
The discovery of habitable zone (HZ) planets around low-mass stars has highlighted the need for a comprehensive understanding of the radiation environments in which such planets reside. Of ...particular importance is knowledge of the far-ultraviolet (FUV) radiation, as low-mass stars are typically much more active than solar-type stars and the proximity of their HZs can be one-tenth the distance. The vast majority of the flux emitted by low-mass stars at FUV wavelengths occurs in the Ly
α
line at 1216 Å. However, measuring a low-mass star’s Ly
α
emission directly is almost always impossible because of the contaminating effects of interstellar hydrogen and geocoronal airglow. We observed Ross 825 (K3) and Ross 1044 (M0), two stars with exceptional radial velocities, with the Space Telescope Imaging Spectrograph on board the
Hubble Space Telescope
. Their radial velocities resulted in significant line shifts, allowing for a more complete view of their Ly
α
line profiles. We provide an updated relation between effective temperature and Ly
α
flux using
Gaia
DR2 astrometry as well as updated, model-independent relationships between Ly
α
flux and UV flux measurements from the
Galaxy Evolution Explorer
(
GALEX
) for low-mass stars. These new relations, in combination with
GALEX
's considerable spatial coverage, provide substantial predictive power for the Ly
α
environments for thousands of nearby, low-mass stars.
Abstract
GJ 9827 is a bright star hosting a planetary system with three transiting planets. As a multiplanet system with planets that sprawl within the boundaries of the radius gap between ...terrestrial and gaseous planets, GJ 9827 is an optimal target to study the evolution of the atmospheres of close-in planets with a common evolutionary history and their dependence from stellar irradiation. Here we report on the Hubble Space Telescope (HST) and CARMENES transit observations of GJ 9827 planets b and d. We performed a stellar and ISM characterization from the ultraviolet HST spectra, obtaining fluxes for Ly
α
and Mg
ii
of
F
(Ly
α
) = (5.42
) × 10
−13
erg cm
−2
s
−1
and
F
(Mg
ii
) = (5.64 ± 0.24) × 10
−14
erg cm
−2
s
−1
. We also investigated a possible absorption signature in Ly
α
in the atmosphere of GJ 9827b during a transit event from HST spectra, as well as H
α
and He
i
signature for the atmosphere of GJ 9827b and d from CARMENES spectra. We found no evidence of an extended atmosphere in either of the planets. This result is also supported by our analytical estimations of mass loss based on the measured radiation fields for all three planets of this system, which led to a mass-loss rate of 0.4, 0.3, and 0.1 planetary masses per Gyr for GJ 9827b, c, and d, respectively. These values indicate that the planets could have lost their volatiles quickly in their evolution and probably do not retain an atmosphere at the current stage.
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