23 research outputs found
Herschel Observations and Updated Spectral Energy Distributions of Five Sunlike Stars with Debris Disks
Observations from the Herschel Space Observatory have more than doubled the
number of wide debris disks orbiting Sunlike stars to include over 30 systems
with R > 100 AU. Here we present new Herschel PACS and re-analyzed Spitzer MIPS
photometry of five Sunlike stars with wide debris disks, from Kuiper belt size
to R > 150 AU. The disk surrounding HD 105211 is well resolved, with an angular
extent of >14" along the major axis, and the disks of HD 33636, HD 50554, and
HD 52265 are extended beyond the PACS PSF size (50% of energy enclosed within
radius 4.23"). HD 105211 also has a 24-micron infrared excess that was
previously overlooked because of a poorly constrained photospheric model.
Archival Spitzer IRS observations indicate that the disks have small grains of
minimum radius ~3 microns, though the minimum grain gradius is larger than the
radiation pressure blowout size in all systems. If modeled as
single-temperature blackbodies, the disk temperatures would all be <60 K. Our
radiative transfer models predict actual disk radii approximately twice the
radius of model blackbody disks. We find that the Herschel photometry traces
dust near the source population of planetesimals. The disk luminosities are in
the range 0.00002 <= L/L* <= 0.0002, consistent with collisions in icy
planetesimal belts stirred by Pluto-size dwarf planets.Comment: Accepted for publication in ApJ. 18 pages, including 10 figures and 3
table
Zodiac II: Debris Disk Imaging Potential
Zodiac II is a proposed coronagraph on a balloon-borne platform, for the purpose of observing debris disks around nearby stars. Zodiac II will have a 1.2-m diameter telescope mounted in a balloon-borne gondola capable of arcsecond quality pointing, and with the capability to make long-duration (several week) flights. Zodiac II will have a coronagraph able to make images of debris disks, meaning that its scattered light speckles will be at or below an average contrast level of about 10(exp -7) in three narrow (7 percent) bands centered on the V band, and one broad (20%) one at I band. We will discuss the potential science to be done with Zodiac II
Characterizing the Variability of Stars with Early-release Kepler Data
We present a variability analysis of the early-release first quarter of data publicly released by the Kepler project. Using the stellar parameters from the Kepler Input Catalog, we have separated the sample into 129,000 dwarfs and 17,000 giants and further sub-divided the luminosity classes into temperature bins corresponding approximately to the spectral classes A, F, G, K, and M. Utilizing the inherent sampling and time baseline of the public data set (30 minute sampling and 33.5 day baseline), we have explored the variability of the stellar sample. The overall variability rate of the dwarfs is 25% for the entire sample, but can reach 100% for the brightest groups of stars in the sample. G dwarfs are found to be the most stable with a dispersion floor of σ ~ 0.04 mmag. At the precision of Kepler, >95% of the giant stars are variable with a noise floor of ~0.1 mmag, 0.3 mmag, and 10 mmag for the G giants, K giants, and M giants, respectively. The photometric dispersion of the giants is consistent with acoustic variations of the photosphere; the photometrically derived predicted radial velocity distribution for the K giants is in agreement with the measured radial velocity distribution. We have also briefly explored the variability fraction as a function of data set baseline (1-33 days), at the native 30 minute sampling of the public Kepler data. To within the limitations of the data, we find that the overall variability fractions increase as the data set baseline is increased from 1 day to 33 days, in particular for the most variable stars. The lower mass M dwarf, K dwarf, and G dwarf stars increase their variability more significantly than the higher mass F dwarf and A dwarf stars as the time baseline is increased, indicating that the variability of the lower mass stars is mostly characterized by timescales of weeks while the variability of the higher mass stars is mostly characterized by timescales of days. A study of the distribution of the variability as a function of galactic latitude suggests that sources closer to the galactic plane are more variable. This may be the result of sampling differing populations (i.e., ages) as a function of latitude or may be the result of higher background contamination that is inflating the variability fractions at lower latitudes. A comparison of the M dwarf statistics to the variability of 29 known bright M dwarfs indicates that the M dwarfs are primarily variable on timescales of weeks or longer presumably dominated by spots and binarity. On shorter timescales of hours, which are relevant for planetary transit detection, the stars are significantly less variable, with ~80% having 12 hr dispersions of 0.5 mmag or less
Survey of Nearby FGK Stars at 160 μm with Spitzer
The Spitzer Space Telescope has advanced debris disk science tremendously with a wealth of information on debris disks around nearby A, F, G, K, and M stars at 24 and 70 μm with the MIPS photometer and at 8-34 μm with IRS. Here we present 160 μm observations of a small subset of these stars. At this wavelength, the stellar photospheric emission is negligible and any detected emission corresponds to cold dust in extended Kuiper Belt analogs. However, the Spitzer 160 μm observations are limited in sensitivity by the large beam size which results in significant "noise" due to cirrus and extragalactic confusion. In addition, the 160 μm measurements suffer from the added complication of a light leak next to the star's position whose flux is proportional to the near-infrared flux of the star. We are able to remove the contamination from the leak and report 160 μm measurements or upper limits for 24 stars. Three stars (HD 10647, HD 207129, and HD 115617) have excesses at 160 μm that we use to constrain the properties of the debris disks around them. A more detailed model of the spectral energy distribution of HD 10647 reveals that the 70 and 160 μm emission could be due to small water ice particles at a distance of 100 AU, consistent with Hubble Space Telescope optical imaging of circumstellar material in the system
NASA ExoPAG Study Analysis Group 11: Preparing for the WFIRST Microlensing Survey
NASA's proposed WFIRST-AFTA mission will discover thousands of exoplanets
with separations from the habitable zone out to unbound planets, using the
technique of gravitational microlensing. The Study Analysis Group 11 of the
NASA Exoplanet Program Analysis Group was convened to explore scientific
programs that can be undertaken now, and in the years leading up to WFIRST's
launch, in order to maximize the mission's scientific return and to reduce
technical and scientific risk. This report presents those findings, which
include suggested precursor Hubble Space Telescope observations, a
ground-based, NIR microlensing survey, and other programs to develop and deepen
community scientific expertise prior to the mission.Comment: 35 pages, 5 Figures. A brief overview of the findings is presented in
the Executive Summary (2 pages
Target Selection for the LBTI Exozodi Key Science Program
The Hunt for Observable Signatures of Terrestrial planetary Systems (HOSTS)
on the Large Binocular Telescope Interferometer will survey nearby stars for
faint emission arising from ~300 K dust (exozodiacal dust), and aims to
determine the exozodiacal dust luminosity function. HOSTS results will enable
planning for future space telescopes aimed at direct spectroscopy of habitable
zone terrestrial planets, as well as greater understanding of the evolution of
exozodiacal disks and planetary systems. We lay out here the considerations
that lead to the final HOSTS target list. Our target selection strategy
maximizes the ability of the survey to constrain the exozodi luminosity
function by selecting a combination of stars selected for suitability as
targets of future missions and as sensitive exozodi probes. With a survey of
approximately 50 stars, we show that HOSTS can enable an understanding of the
statistical distribution of warm dust around various types of stars and is
robust to the effects of varying levels of survey sensitivity induced by
weather conditions.Comment: accepted to ApJ
Herschel Observations of Disks around Late-type Stars
A set of twenty late-type (K5-M5) stars were observed with the Herschel Space Observatory at 100 and 160 microns with the goal of searching for far-infrared excesses indicative of the presence of circumstellar disks. Out of this sample, four stars (TYC 7443-1102-1, TYC 9340-437-1, GJ 784 and GJ 707) have infrared excesses above their stellar photospheres at either 100 or 160 μm or both. At 100 microns TYC 9340-437-1 is spatially resolved with a shape that suggests it is surrounded by a face-on disk. The 100 μm excess flux associated with GJ 707 is marginal at around 3σ. The excess flux associated with GJ 784 is most likely due to a background galaxy as the dust radius estimated from the spectral energy fit implies that any associated dust disk should have been resolved in the Herschel images but is not. TYC 7443-1102-1 has been observed with ALMA which resolves the emission at its location into two distinct sources making the Herschel excess most likely also due to a background galaxy. It is worth noting that this star is in the 23 Myr old β Pic association. With a disk luminosity on the order of 10⁻³ L_*, this system is an ideal follow-up target for high-contrast imaging and ALMA