259 research outputs found
Overview of the JWST Program
The James Webb Space Telescope (JWST) is a large aperture (6.5 meter), cryogenic space telescope with a suite of near and mid-infrared instruments covering the wavelength range of 0.6 micron to 28 micron. JWST's primary science goal is to detect and characterize the first galaxies. It will also study the assembly of galaxies, star formation, and the formation of evolution of planetary systems. We will review the expected scientific performance of the observatory, and recent technical progress with the observatory and its complement of instruments
Fomalhaut's Debris Disk and Planet: Constraining the Mass of Formalhaut B from Disk Morphology
Following the optical imaging of exoplanet candidate Fomalhaut b (Fom b), we present a numerical model of how Fomalhaut's debris disk is gravitationally shaped by a single interior planet. The model is simple, adaptable to other debris disks, and can be extended to accommodate multiple planets. If Fom b is the dominant perturber of the belt, then to produce the observed disk morphology it must have a mass M(sub pl) 101.5AU, and an orbital eccentricity e(sub pl) = 0.11 - 0.13. These conclusions are independent of Fom b's photometry. To not disrupt the disk, a greater mass for Fom b demands a smaller orbit farther removed from the disk; thus, future astrometric measurement of Fom b's orbit, combined with our model of planet-disk interaction, can be used to determine the mass more precisely. The inner edge of the debris disk at a approximately equals 133AU lies at the periphery of Fom b's chaotic zone, and the mean disk eccentricity of e approximately equals 0.11 is secularly forced by the planet, supporting predictions made prior to the discovery of Fom b. However, previous mass constraints based on disk morphology rely on several oversimplifications. We explain why our constraint is more reliable. It is based on a global model of the disk that is not restricted to the planet's chaotic zone boundary. Moreover, we screen disk parent bodies for dynamical stability over the system age of approximately 100 Myr, and model them separately from their dust grain progeny; the latter's orbits are strongly affected by radiation pressure and their lifetimes are limited to approximately 0.1 Myr by destructive grain-grain collisions. The single planet model predicts that planet and disk orbits be apsidally aligned. Fomalhaut b's nominal space velocity does not bear this out, but the astrometric uncertainties are difficult to quantify. Even if the apsidal misalignment proves real, our calculated upper mass limit of 3 M(sub J) still holds. Parent bodies are evacuated from mean-motion resonances with Fom b; these empty resonances are akin to the Kirkwood gaps opened by Jupiter. The belt contains at least 3M(sub Earth) of solids that are grinding down to dust, their velocity dispersions stirred so strongly by Fom b that collisions are destructive. Such a large mass in solids is consistent with Fom b having formed in situ
The Low End of the Initial Mass Function in Young LMC Clusters: I. The Case of R136
We report the result of a study in which we have used very deep broadband V
and I WFPC2 images of the R136 cluster in the Large Magellanic Cloud from the
HST archive, to sample the luminosity function below the detection limit of 2.8
Mo previously reached. In these new deeper images, we detect stars down to a
limiting magnitude of m_F555W = 24.7 (~ 1 magnitude deeper than previous
works), and identify a population of red stars evenly distributed in the
surrounding of the R136 cluster. A comparison of our color-magnitude diagram
with recentely computed evolutionary tracks indicates that these red objects
are pre-main sequence stars in the mass range 0.6 - 3 Mo. We construct the
initial mass function (IMF) in the 1.35 - 6.5 Mo range and find that, after
correcting for incompleteness, the IMF shows a definite flattening below ~ 2
Mo. We discuss the implications of this result for the R136 cluster and for our
understanding of starburst galaxies formation and evolution in general.Comment: 29 pages, 6 tables, 11 figures included + 3 external files, accepted
for publication by Ap.
Discovery and Characterization of Transiting SuperEarths Using an All-Sky Transit Survey and Follow-up by the James Webb Space Telescope
Doppler and transit surveys are finding extrasolar planets of ever smaller
mass and radius, and are now sampling the domain of superEarths (1-3 Earth
radii). Recent results from the Doppler surveys suggest that discovery of a
transiting superEarth in the habitable zone of a lower main sequence star may
be possible. We evaluate the prospects for an all-sky transit survey targeted
to the brightest stars, that would find the most favorable cases for
photometric and spectroscopic characterization using the James Webb Space
Telescope (JWST). We use the proposed Transiting Exoplanet Survey Satellite
(TESS) as representative of an all-sky survey. We couple the simulated TESS
yield to a sensitivity model for the MIRI and NIRSpec instruments on JWST. We
focus on the TESS planets with radii between Earth and Neptune. Our simulations
consider secondary eclipse filter photometry using JWST/MIRI, comparing the 11-
and 15-micron bands to measure CO2 absorption in superEarths, as well as
JWST/NIRSpec spectroscopy of water absorption from 1.7-3.0 microns, and CO2
absorption at 4.3-microns. We project that TESS will discover about eight
nearby habitable transiting superEarths. The principal sources of uncertainty
in the prospects for JWST characterization of habitable superEarths are
superEarth frequency and the nature of superEarth atmospheres. Based on our
estimates of these uncertainties, we project that JWST will be able to measure
the temperature, and identify molecular absorptions (water, CO2) in one to four
nearby habitable TESS superEarths.Comment: accepted for PASP; added discussion and figure for habitable planets;
abridged Abstrac
High speed quadrant CCDs for adaptive optics
The Johns Hopkins University is developing an adaptive optics coronagraph for the study of circumstellar material at high resolution. The first generation instrument corrects for image motion, i.e., wavefront tilt, using an image motion sensor coupled to a high speed tip/tilt mirror. The image motion sensor is built around a quadrant CCD which detects offsets from the null position. The performance of this device and present results demonstrating its operation in the laboratory are discussed
Phase light curves for extrasolar Jupiters and Saturns
We predict how a remote observer would see the brightness variations of giant
planets similar to Jupiter and Saturn as they orbit their central stars. We
model the geometry of Jupiter, Saturn and Saturn's rings for varying orbital
and viewing parameters. Scattering properties for the planets and rings at
wavelenghts 0.6-0.7 microns follow Pioneer and Voyager observations, namely,
planets are forward scattering and rings are backward scattering. Images of the
planet with or without rings are simulated and used to calculate the
disk-averaged luminosity varying along the orbit, that is, a light curve is
generated. We find that the different scattering properties of Jupiter and
Saturn (without rings) make a substantial difference in the shape of their
light curves. Saturn-size rings increase the apparent luminosity of the planet
by a factor of 2-3 for a wide range of geometries. Rings produce asymmetric
light curves that are distinct from the light curve of the planet without
rings. If radial velocity data are available for the planet, the effect of the
ring on the light curve can be distinguished from effects due to orbital
eccentricity. Non-ringed planets on eccentric orbits produce light curves with
maxima shifted relative to the position of the maximum planet's phase. Given
radial velocity data, the amount of the shift restricts the planet's unknown
orbital inclination and therefore its mass. Combination of radial velocity data
and a light curve for a non-ringed planet on an eccentric orbit can also be
used to constrain the surface scattering properties of the planet. To summarize
our results for the detectability of exoplanets in reflected light, we present
a chart of light curve amplitudes of non-ringed planets for different
eccentricities, inclinations, and the viewing azimuthal angles of the observer.Comment: 40 pages, 13 figures, submitted to Ap.
Can Life develop in the expanded habitable zones around Red Giant Stars?
We present some new ideas about the possibility of life developing around
sub-giant and red giant stars. Our study concerns the temporal evolution of the
habitable zone. The distance between the star and the habitable zone, as well
as its width, increases with time as a consequence of stellar evolution. The
habitable zone moves outward after the star leaves the main sequence, sweeping
a wider range of distances from the star until the star reaches the tip of the
asymptotic giant branch. If life could form and evolve over time intervals from
to years, then there could be habitable planets with
life around red giant stars. For a 1 M star at the first stages of
its post main-sequence evolution, the temporal transit of the habitable zone is
estimated to be of several 10 years at 2 AU and around 10 years at 9
AU. Under these circumstances life could develop at distances in the range 2-9
AU in the environment of sub-giant or giant stars and in the far distant future
in the environment of our own Solar System. After a star completes its first
ascent along the Red Giant Branch and the He flash takes place, there is an
additional stable period of quiescent He core burning during which there is
another opportunity for life to develop. For a 1 M star there is an
additional years with a stable habitable zone in the region from 7 to 22
AU. Space astronomy missions, such as proposed for the Terrestrial Planet
Finder (TPF) and Darwin should also consider the environments of sub-giants and
red giant stars as potentially interesting sites for understanding the
development of life
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