13 research outputs found
On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae
Photometric variability attributed to cloud phenomena is common in L/T transition brown dwarfs. Recent studies show that such variability may also trace aurorae, suggesting that localized magnetic heating may contribute to observed brown dwarf photometric variability. We assess this potential correlation with a survey of 17 photometrically variable brown dwarfs using the Karl G. Jansky Very Large Array at 4–8 GHz. We detect quiescent and highly circularly polarized flaring emission from one source, 2MASS J17502484-0016151, which we attribute to auroral electron cyclotron maser emission. The detected auroral emission extends throughout the frequency band at ~5–25σ, and we do not detect evidence of a cutoff. Our detection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength of ≥2.9 kG, similar to those of other radio-bright ultracool dwarfs. We show that Hα emission continues to be an accurate tracer of auroral activity in brown dwarfs. Supplementing our study with data from the literature, we calculate the occurrence rates of quiescent emission in L dwarfs with low- and high-amplitude variability and conclude that high-amplitude optical and infrared variability does not trace radio magnetic activity in L dwarfs
On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae
Photometric variability attributed to cloud phenomena is common in L/T
transition brown dwarfs. Recent studies show that such variability may also
trace aurorae, suggesting that localized magnetic heating may contribute to
observed brown dwarf photometric variability. We assess this potential
correlation with a survey of 17 photometrically variable brown dwarfs using the
Karl G. Jansky Very Large Array (VLA) at 4 -- 8 GHz. We detect quiescent and
highly circularly polarized flaring emission from one source, 2MASS
J17502484-0016151, which we attribute to auroral electron cyclotron maser
emission. The detected auroral emission extends throughout the frequency band
at 5 -- 25, and we do not detect evidence of a cutoff. Our
detection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength
of 2.9 kG, similar to those of other radio-bright ultracool dwarfs. We
show that H emission continues to be an accurate tracer of auroral
activity in brown dwarfs. Supplementing our study with data from the
literature, we calculate the occurrence rates of quiescent emission in L dwarfs
with low- and high-amplitude variability and conclude that high amplitude O/IR
variability does not trace radio magnetic activity in L dwarfs.Comment: 26 pages, 5 figures, 6 table
Extending Optical Flare Models to the UV: Results from Comparing of TESS and GALEX Flare Observations For M Dwarfs
The ultraviolet (UV) emission of stellar flares may have a pivotal role in
the habitability of rocky exoplanets around low-mass stars. Previous studies
have used white-light observations to calibrate empirical models which describe
the optical and UV flare emission. However, the accuracy of the UV predictions
of models have previously not been tested. We combined TESS optical and GALEX
UV observations to test the UV predictions of empirical flare models calibrated
using optical flare rates of M stars. We find that the canonical 9000 K
blackbody model used by flare studies underestimates the GALEX NUV energies of
field age M stars by up to a factor of 6.50.7 and the GALEX FUV energies
of fully convective field age M stars by 30.610.0. We calculated energy
correction factors that can be used to bring the UV predictions of flare models
closer in line with observations. We calculated pseudo-continuum flare
temperatures that describe both the white-light and GALEX NUV emission. We
measured a temperature of 10,700 K for flares from fully convective M stars
after accounting for the contribution from UV line emission. We also applied
our correction factors to the results of previous studies of the role of flares
in abiogenesis. Our results show that M stars do not need to be as active as
previously thought in order to provide the NUV flux required for prebiotic
chemistry, however we note that flares will also provide more FUV flux than
previously modelled.Comment: 20 pages, 9 figures, 4 tables. Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
HAZMAT. V. The Ultraviolet and X-ray Evolution of K Stars
Knowing the high-energy radiation environment of a star over a planet's
formation and evolutionary period is critical in determining if that planet is
potentially habitable and if any biosignatures could be detected, as UV
radiation can severely change or destroy a planet's atmosphere. Current efforts
for finding a potentially habitable planet are focused on M stars, yet K stars
may offer more habitable conditions due to decreased stellar activity and more
distant and wider habitable zones (HZ). While M star activity evolution has
been observed photometrically and spectroscopically, there has been no
dedicated investigation of K-star UV evolution. We present the first
comprehensive study of the near-UV, far-UV, and X-ray evolution of K stars. We
used members of young moving groups and clusters ranging in age from 10 - 625
Myr combined with field stars and their archived GALEX UV and ROSAT X-ray data
to determine how the UV and X-ray radiation evolve. We find that the UV and
X-ray flux incident on a HZ planet is 5 - 50 times lower than that of HZ
planets around early-M stars and 50 - 1000 times lower than those around late-M
stars, due to both an intrinsic decrease in K dwarf stellar activity occurring
earlier than for M dwarfs and the more distant location of the K dwarf HZ.Comment: 27 pages, 19 figure
Current Population Statistics Do Not Favor Photoevaporation over Core-Powered Mass Loss as the Dominant Cause of the Exoplanet Radius Gap
We search for evidence of the cause of the exoplanet radius gap, i.e. the
dearth of planets with radii near . If the cause was
photoevaporation, the radius gap should trend with proxies for the early-life
high-energy emission of planet-hosting stars. If, alternatively, the cause was
core-powered mass loss, no such trends should exist. Critically, spurious
trends between the radius gap and stellar properties arise from an underlying
correlation with instellation. After accounting for this underlying
correlation, we find no trends remain between the radius gap and stellar mass
or present-day stellar activity as measured by near-UV emission. We dismiss the
nondetection of a radius gap trend with near-UV emission because present-day
near-UV emission is unlikely to trace early-life high-energy emission, but we
provide a catalog of GALEX near-UV and far-UV emission measurements for general
use. We interpret the nondetection of a radius gap trend with stellar mass by
simulating photoevaporation with mass-dependent evolution of stellar
high-energy emission. The simulation produces an undetectable trend between the
radius gap and stellar mass under realistic sources of error. We conclude that
no evidence, from this analysis or others in the literature, currently exists
that clearly favors either photoevaporation or core powered mass loss as the
primary cause of the exoplanet radius gap. However, repeating this analysis
once the body of well-characterized planets has roughly doubled
could confirm or rule out photoevaporation.Comment: 27 pages, 32 figures, accepted to Ap
Constraining the Physical Properties of Stellar Coronal Mass Ejections with Coronal Dimming: Application to Far Ultraviolet Data of Eridani
Coronal mass ejections (CMEs) are a prominent contributor to solar system
space weather and might have impacted the Sun's early angular momentum
evolution. A signal diagnostic of CMEs on the Sun is coronal dimming: a drop in
coronal emission, tied to the mass of the CME, that is the direct result of
removing emitting plasma from the corona. We present the results of a coronal
dimming analysis of Fe XII 1349 A and Fe XXI 1354 A emission from
Eridani ( Eri), a young K2 dwarf, with archival far-ultraviolet
observations by the Hubble Space Telescope's Cosmic Origins Spectrograph.
Following a flare in February 2015, Eri's Fe XXI emission declined
by %. Although enticing, a scant 3.8 min of preflare observations
allows for the possibility that the Fe XXI decline was the decay of an earlier,
unseen flare. Dimming nondetections following each of three prominent flares
constrain the possible mass of ejected Fe XII-emitting (1 MK) plasma to less
than a few g. This implies that CMEs ejecting this much or more
1 MK plasma occur less than a few times per day on Eri. On the Sun,
g CMEs occur once every few days. For Eri, the mass loss
rate due to CME-ejected 1 MK plasma could be , well below
the star's estimated 30 mass loss rate (wind + CMEs). The
order-of-magnitude formalism we developed for these mass estimates can be
broadly applied to coronal dimming observations of any star.Comment: 27 pages, 22 figures, accepted to Ap
HAZMAT. VIII. A Spectroscopic Analysis of the Ultraviolet Evolution of K Stars: Additional Evidence for K Dwarf Rotational Stalling in the First Gigayear
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
\textit{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 Myr. 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 Gyr when their rotation periods reach
10 d, 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.Comment: 18 pages, 7 figure
Hyades Member K2-136c:The Smallest Planet in an Open Cluster with a Precisely Measured Mass
International audienceK2-136 is a late-K dwarf (0.742 ± 0.039 M ⊙) in the Hyades open cluster with three known, transiting planets and an age of 650 ± 70 Myr. Analyzing K2 photometry, we found that planets K2-136b, c, and d have periods of 8.0, 17.3, and 25.6 days and radii of 1.014 ± 0.050 R ⊕, 3.00 ± 0.13 R ⊕, and 1.565 ± 0.077 R ⊕, respectively. We collected 93 radial velocity (RV) measurements with the High-Accuracy Radial-velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph (Telescopio Nazionale Galileo) and 22 RVs with the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) spectrograph (Very Large Telescope). Analyzing HARPS-N and ESPRESSO data jointly, we found that K2-136c induced a semi-amplitude of 5.49 ± 0.53 m s-1, corresponding to a mass of 18.1 ± 1.9 M ⊕. We also placed 95% upper mass limits on K2-136b and d of 4.3 and 3.0 M ⊕, respectively. Further, we analyzed Hubble Space Telescope and XMM-Newton observations to establish the planetary high-energy environment and investigate possible atmospheric loss. K2-136c is now the smallest planet to have a measured mass in an open cluster and one of the youngest planets ever with a mass measurement. K2-136c has ~75% the radius of Neptune but is similar in mass, yielding a density of g cm-3 (~2-3 times denser than Neptune). Mass estimates for K2-136b (and possibly d) may be feasible with more RV observations, and insights into all three planets' atmospheres through transmission spectroscopy would be challenging but potentially fruitful. This research and future mass measurements of young planets are critical for investigating the compositions and characteristics of small exoplanets at very early stages of their lives and providing insights into how exoplanets evolve with time
Short Period Eclipsing Binaries in the Field of the Young Cluster NGC 2362
In a study of rotation periods in the young (t ~ 5 Myr) cluster NGC 2362 (Hamilton et al. 2009), several new eclipsing systems were discovered. In this poster, we present photometric observations of these systems and separate them into likely eclipsing binaries and interesting puzzles. The binaries are most likely field stars and are not a part of the cluster