70 research outputs found
Time-resolved Ultraviolet Spectroscopy of the M-dwarf GJ 876 Exoplanetary System
Extrasolar planets orbiting M-stars may represent our best chance to discover
habitable worlds in the coming decade. The ultraviolet spectrum incident upon
both Earth-like and Jovian planets is critically important for proper modeling
of their atmospheric heating and chemistry. In order to provide more realistic
inputs for atmospheric models of planets orbiting low-mass stars, we present
new near- and far-ultraviolet (NUV and FUV) spectroscopy of the M-dwarf
exoplanet host GJ 876 (M4V). Using the COS and STIS spectrographs aboard the
Hubble Space Telescope, we have measured the 1150-3140A spectrum of GJ 876. We
have reconstructed the stellar HI LyA emission line profile, and find that the
integrated LyA flux is roughly equal to the rest of the integrated flux
(1150-1210A + 1220-3140A) in the entire ultraviolet bandpass (F(LyA)/F(FUV+NUV)
~0.7). This ratio is ~ 2500x greater than the solar value. We describe the
ultraviolet line spectrum and report surprisingly strong fluorescent emission
from hot H2 (T(H2) > 2000 K). We show the light-curve of a chromospheric +
transition region flare observed in several far-UV emission lines, with
flare/quiescent flux ratios >= 10. The strong FUV radiation field of an M-star
(and specifically LyA) is important for determining the abundance of O2 -- and
the formation of biomarkers -- in the lower atmospheres of Earth-like planets
in the habitable zones of low-mass stars.Comment: 6 pages, 4 figures. ApJL accepte
The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars
The spectral and temporal behavior of exoplanet host stars is a critical
input to models of the chemistry and evolution of planetary atmospheres. At
present, little observational or theoretical basis exists for understanding the
ultraviolet spectra of M dwarfs, despite their critical importance to
predicting and interpreting the spectra of potentially habitable planets as
they are obtained in the coming decades. Using observations from the Hubble
Space Telescope, we present a study of the UV radiation fields around nearby M
dwarf planet hosts that covers both FUV and NUV wavelengths. The combined
FUV+NUV spectra are publically available in machine-readable format. We find
that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ
832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition
region UV emission. No "UV quiet" M dwarfs are observed. The bright stellar
Ly-alpha emission lines are reconstructed, and we find that the Ly-alpha line
fluxes comprise ~37-75% of the total 1150-3100A flux from most M dwarfs; >
10^{3} times the solar value. The F(FUV)/F(NUV) flux ratio, a driver for
abiotic production of the suggested biomarkers O2 and O3, is shown to be ~0.5-3
for all M dwarfs in our sample, > 10^{3} times the solar ratio. For the four
stars with moderate signal-to-noise COS time-resolved spectra, we find UV
emission line variability with amplitudes of 50-500% on 10^{2} - 10^{3} s
timescales. Finally, we observe relatively bright H2 fluorescent emission from
four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832).
Additional modeling work is needed to differentiate between a stellar
photospheric or possible exoplanetary origin for the hot (T(H2) \approx
2000-4000 K) molecular gas observed in these objects.Comment: ApJ, accepted. 16 pages, 10 figures. On-line data at:
http://cos.colorado.edu/~kevinf/muscles.htm
Metal Depletion and Warm H2 in the Brown Dwarf 2M1207 Accretion Disk
We present new far-ultraviolet observations of the young M8 brown dwarf 2MASS
J12073346-3932539, which is surrounded by an accretion disk. The data were
obtained using the Hubble Space Telescope-Cosmic Origins Spectrograph. Moderate
resolution spectra (R~17,000-18,000) obtained in the 1150-1750 A and 2770-2830
A bandpasses reveal H2 emission excited by HI Ly photons, several
ionization states of carbon (CI - CIV), and hot gas emission lines of HeII and
NV (T ~ 10^4-5 K). Emission from some species that would be found in a typical
thermal plasma at this temperature (SiII, SiIII, SiIV, and MgII) are not
detected. The non-detections indicate that these refractory elements are
depleted into grains, and that accretion shocks dominate the production of the
hot gas observed on 2MASS J12073346-3932539. We use the observed CIV luminosity
to constrain the mass accretion rate in this system. We use the kinematically
broadened H2 profile to confirm that the majority of the molecular emission
arises in the disk, measure the radius of the inner hole of the disk
(R_{hole}~3R_{*}), and constrain the physical conditions of the warm molecular
phase of the disk (T(H2)~2500-4000 K). A second, most likely unresolved H2
component is identified. This feature is either near the stellar surface in the
region of the accretion shock or in a molecular outflow, although the
possibility that this Jovian-like emission arises on the day-side disk of a 6
M_{J} companion (2M1207b) cannot be conclusively ruled out. In general, we find
that this young brown dwarf disk system is a low-mass analog to classical T
Tauri stars that are observed to produce H2 emission from a warm layer in their
disks, such as the well studied TW Hya and DF Tau systems.Comment: ApJ, accepted. 12 pages, 10 figures. 3 tables
A Randomized Comparison of Alternative Formats for Clinical Simulations
Computer-based clinical simulations for medical education vary widely in structure and for mat, yet few studies have examined which formats are optimal for particular educational settings. This study is a randomized comparison of the same simulated case in three formats: a "pedagogic" format offering explicit educational support, a "high-fidelity" format attempting to model clinical reasoning in the real world, and a "problem-solving" format that requires students to express specific diagnostic hypotheses Data were collected from rising third- year medical students using a posttest, attitudinal questionnaire, students' writeups of the case, and log files of students' progress through the simulation. Student performances on all measures differed significantly by format. In general, students using the pedagogic format were more proficient but less efficient. They acquired more information but were able to do proportionately less with it. The results suggest that the format of computer-based simulations is an important educational variable. Key words. medical education, undergraduate; clinical reasoning; computer-assisted instruction. (Med Decis Making 1991;11:265-272
Observations of Mass Loss from the Transiting Exoplanet HD 209458b
Using the new Cosmic Origins Spectrograph (COS) on the {\it Hubble Space
Telescope (HST)}, we obtained moderate-resolution, high signal/noise
ultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit,
both orbital quadratures, and secondary eclipse. We compare transit spectra
with spectra obtained at non-transit phases to identify spectral features due
to the exoplanet's expanding atmosphere. We find that the mean flux decreased
by % for the C II 1334.5323\AA\ and 1335.6854\AA\ lines and by
% for the Si III 1206.500\AA\ line during transit compared to
non-transit times in the velocity interval --50 to +50 km s. Comparison
of the C II and Si III line depths and transit/non-transit line ratios shows
deeper absorption features near --10 and +15 km s and less certain
features near --40 and +30--70 km s, but future observations are needed
to verify this first detection of velocity structure in the expanding
atmosphere of an exoplanet. Our results for the C II lines and the
non-detection of Si IV 1394.76\AA\ absorption are in agreement with
\citet{Vidal-Madjar2004}, but we find absorption during transit in the Si III
line contrary to the earlier result. The % obscuration of the star
during transit is far larger than the 1.5% obscuration by the exoplanet's disk.
Absorption during transit at velocities between --50 and +50 km s in the
C II and Si III lines requires high-velocity ion absorbers, but models that
assume that the absorbers are high-temperature thermal ions are inconsistent
with the COS spectra. Assuming hydrodynamic model values for the gas
temperature and outflow velocity at the limb of the outflow as seen in the C II
lines, we find mass-loss rates in the range (8--40) g s.Comment: 25 pages, 4 figures, Astrophysical Journal in pres
Searching for Far-Ultraviolet Auroral/Dayglow Emission from HD209458b
We present recent observations from the HST-Cosmic Origins Spectrograph aimed
at characterizing the auroral emission from the extrasolar planet HD209458b. We
obtained medium-resolution (R~18-20,000) far-ultraviolet (1150-1700A) spectra
at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar
baseline measurement at secondary eclipse. This analysis includes a catalog of
stellar emission lines and a star-subtracted spectrum of the planet. We present
an emission model for planetary H2 emission, and compare this model to the
planetary spectrum. No unambiguously identifiable atomic or molecular features
are detected, and upper limits are presented for auroral/dayglow line
strengths. An orbital velocity cross-correlation analysis finds a statistically
significant (3.8 sigma) feature at +15 (+/- 20) km/s in the rest frame of the
planet, at 1582 A. This feature is consistent with emission from H2 B-X (2-9)
P(4) (lambda_{rest} = 1581.11 A), however the physical mechanism required to
excite this transition is unclear. We compare limits on relative line strengths
seen in the exoplanet spectrum with models of ultraviolet fluorescence to
constrain the atmospheric column density of neutral hydrogen between the star
and the planetary surface. These results support models of short period
extrasolar giant planets with weak magnetic fields and extended atomic
atmospheres.Comment: Accepted to ApJ. 12 pages, 5 figures, 4 table
The MUSCLES Extension for Atmospheric and Transmission Spectroscopy: UV and X-ray Host-star Observations for JWST ERS & GTO Targets
X-ray through infrared spectral energy distributions (SEDs) are essential for
understanding a star's effect on exoplanet atmospheric composition and
evolution. We present a catalog of panchromatic SEDs, hosted on the Barbara A.
Mikulski Archive for Space Telescopes (MAST), for 11 exoplanet hosting stars
which have guaranteed JWST observation time as part of the ERS or GTO programs
but have no previous UV characterization. The stars in this survey range from
spectral type F4-M6 (0.14-1.57 M), rotation periods of ~4-132 days, and
ages of approximately 0.5-11.4 Gyr. The SEDs are composite spectra using data
from the Chandra X-ray Observatory and XMM-Newton, the Hubble Space Telescope,
BT-Settl stellar atmosphere models, and scaled spectra of proxy stars of
similar spectral type and activity. From our observations, we have measured a
set of UV and X-ray fluxes as indicators of stellar activity level. We compare
the chromospheric and coronal activity indicators of our exoplanet-hosting
stars to the broader population of field stars and find that a majority of our
targets have activity levels lower than the average population of cool stars in
the solar neighborhood. This suggests that using SEDs of stars selected from
exoplanet surveys to compute generic exoplanet atmosphere models may
underestimate the typical host star's UV flux by an order of magnitude or more,
and consequently, that the observed population of exoplanetary atmospheres
receive lower high-energy flux levels than the typical planet in the solar
neighborhood.Comment: 28 pages, 10 figures, 7 tables, accepted for publication in A
The MUSCLES Treasury Survey. II. Intrinsic Lyα and extreme ultraviolet spectra of K and M dwarfs with exoplanets
This work was supported by NASA grants HST-GO-12464.01 and HST-GO-13650.01 to the University of Colorado at Boulder. Sarah Rugheimer would like to acknowledge support from the Simons Foundation (339489, Rugheimer).The ultraviolet (UV) spectral energy distributions (SEDs) of low-mass (K- and M-type) stars play a critical role in the heating and chemistry of exoplanet atmospheres, but are not observationally well-constrained. Direct observations of the intrinsic flux of the Lyα line (the dominant source of UV photons from low-mass stars) are challenging, as interstellar H i absorbs the entire line core for even the closest stars. To address the existing gap in empirical constraints on the UV flux of K and M dwarfs, the MUSCLES Hubble Space Telescope Treasury Survey has obtained UV observations of 11 nearby M and K dwarfs hosting exoplanets. This paper presents the Lyα and extreme-UV spectral reconstructions for the MUSCLES targets. Most targets are optically inactive, but all exhibit significant UV activity. We use a Markov Chain Monte Carlo technique to correct the observed Lyα profiles for interstellar absorption, and we employ empirical relations to compute the extreme-UV SED from the intrinsic Lyα flux in ∼100 bins from 100-1170. The reconstructed Lyα profiles have 300 km s-1 broad cores, while >1% of the total intrinsic Lyα flux is measured in extended wings between 300 and 1200 km s-1. The Lyα surface flux positively correlates with the Mg ii surface flux and negatively correlates with the stellar rotation period. Stars with larger Lyα surface flux also tend to have larger surface flux in ions formed at higher temperatures, but these correlations remain statistically insignificant in our sample of 11 stars. We also present H i column density measurements for 10 new sightlines through the local interstellar medium.Publisher PDFPeer reviewe
OBSERVATIONS OF THE WIND FROM THE TRANSITING EXOPLANET HD 209458b
ABSTRACT Using the new Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we obtained the first moderate resolution, high signal/noise ultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit, both orbital quadratures, and secondary eclipse. We compare transit spectra with spectra obtained at nontransit phases to identify spectral features due to the planet's atmosphere and wind. We find decreased flux by 8 ± 2% in the C II 1334.5323Å and 1335.6854Å lines and in the Si III 1206.500Å line during transit compared to nontransit times in the velocity interval -50 to +50 km s −1 . The 8 ± 2% obscuration of the star during transit is far larger than the 1.5% obscuration by the exoplanet's disk. Absorption during transit at velocities between -50 and +50 km s −1 could be explained by a Roche lobe filled with wind material that is optically thick in the C II and Si III lines or by an extended cometary tail. We identify mass loss from the exoplanet's atmosphere at speeds near 42 ± 4 km s −1 , the escape speed predicted from the planet's mass and radius, with a mass loss rate of 3.2 × 10 11 g s −1
- …