551 research outputs found
PhoSim-NIRCam: Photon-by-photon image simulations of the James Webb Space Telescope's Near-Infrared Camera
Recent instrumentation projects have allocated resources to develop codes for
simulating astronomical images. Novel physics-based models are essential for
understanding telescope, instrument, and environmental systematics in
observations. A deep understanding of these systematics is especially important
in the context of weak gravitational lensing, galaxy morphology, and other
sensitive measurements. In this work, we present an adaptation of a
physics-based ab initio image simulator: The Photon Simulator (PhoSim). We
modify PhoSim for use with the Near-Infrared Camera (NIRCam) -- the primary
imaging instrument aboard the James Webb Space Telescope (JWST). This photon
Monte Carlo code replicates the observational catalog, telescope and camera
optics, detector physics, and readout modes/electronics. Importantly,
PhoSim-NIRCam simulates both geometric aberration and diffraction across the
field of view. Full field- and wavelength-dependent point spread functions are
presented. Simulated images of an extragalactic field are presented. Extensive
validation is planned during in-orbit commissioning
Disentangling the Origin and Heating Mechanism of Supernova Dust: Late-Time Spitzer Spectroscopy of the Type IIn SN 2005ip
This paper presents late-time near-infrared and {\it Spitzer} mid-infrared
photometric and spectroscopic observations of warm dust in the Type IIn SN
2005ip in NGC 2906. The spectra show evidence for two dust components with
different temperatures. Spanning the peak of the thermal emission, these
observations provide strong constraints on the dust mass, temperature, and
luminosity, which serve as critical diagnostics for disentangling the origin
and heating mechanism of each component. The results suggest the warmer dust
has a mass of \msolar, originates from newly formed
dust in the ejecta, or possibly the cool, dense shell, and is continuously
heated by the circumstellar interaction. By contrast, the cooler component
likely originates from a circumstellar shock echo that forms from the heating
of a large, pre-existing dust shell ~\msolar~by the late-time
circumstellar interaction. The progenitor wind velocity derived from the blue
edge of the He 1 1.083 \micron~P Cygni profile indicates a progenitor eruption
likely formed this dust shell 100 years prior to the supernova explosion,
which is consistent with a Luminous Blue Variable (LBV) progenitor star.Comment: 12 pages, 10 figures, Accepted to Ap
Variations of the 10 um Silicate Features in the Actively Accreting T Tauri Stars: DG Tau and XZ Tau
Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we
observed multiple epochs of 11 actively accreting T Tauri stars in the nearby
Taurus-Auriga star forming region. In total, 88 low-resolution mid-infrared
spectra were collected over 1.5 years in Cycles 2 and 3. The results of this
multi-epoch survey show that the 10 um silicate complex in the spectra of two
sources - DG Tau and XZ Tau - undergoes significant variations with the
silicate feature growing both weaker and stronger over month- and year-long
timescales. Shorter timescale variations on day- to week-long timescales were
not detected within the measured flux errors. The time resolution coverage of
this data set is inadequate for determining if the variations are periodic.
Pure emission compositional models of the silicate complex in each epoch of the
DG Tau and XZ Tau spectra provide poor fits to the observed silicate features.
These results agree with those of previous groups that attempted to fit only
single-epoch observations of these sources. Simple two-temperature, two-slab
models with similar compositions successfully reproduce the observed variations
in the silicate features. These models hint at a self-absorption origin of the
diminution of the silicate complex instead of a compositional change in the
population of emitting dust grains. We discuss several scenarios for producing
such variability including disk shadowing, vertical mixing, variations in disk
heating, and disk wind events associated with accretion outbursts.Comment: 6 pages, emulate apj format, accepted for publication in ApJ Letter
Limitations of the Odds Ratio in Gauging the Performance of a Diagnostic or Prognostic Marker
A marker that is strongly associated with outcome (or disease) is often assumed to be effective for classifying individuals according to their current or future outcome. However, for this to be true, the associated odds ratio must be of a magnitude rarely seen in epidemiological studies. An illustration of the relationship between odds ratios and receiver operating characteristic (ROC) curves shows, for example, that a marker with an odds ratio as high as 3 is in fact a very poor classification tool. If a marker identifies 10 percent of controls as positive (false positives) and has an odds ratio of 3, then it will only correctly identify 25 percent of cases as positive (true positives). Moreover, the authors illustrate that a single measure of association such as an odds ratio does not meaningfully describe a marker’s ability to classify subjects. Appropriate statistical methods for assessing and reporting the classification power of a marker are described. The serious pitfalls of using more traditional methods based on parameters in logistic regression models are illustrated
High Spatial Resolution Thermal-Infrared Spectroscopy with ALES: Resolved Spectra of the Benchmark Brown Dwarf Binary HD 130948BC
We present 2.9-4.1 micron integral field spectroscopy of the L4+L4 brown
dwarf binary HD 130948BC, obtained with the Arizona Lenslets for Exoplanet
Spectroscopy (ALES) mode of the Large Binocular Telescope Interferometer
(LBTI). The HD 130948 system is a hierarchical triple system, in which the G2V
primary is joined by two co-orbiting brown dwarfs. By combining the age of the
system with the dynamical masses and luminosities of the substellar companions,
we can test evolutionary models of cool brown dwarfs and extra-solar giant
planets. Previous near-infrared studies suggest a disagreement between HD
130948BC luminosities and those derived from evolutionary models. We obtained
spatially-resolved, low-resolution (R~20) L-band spectra of HD 130948B and C to
extend the wavelength coverage into the thermal infrared. Jointly using JHK
photometry and ALES L-band spectra for HD 130948BC, we derive atmospheric
parameters that are consistent with parameters derived from evolutionary
models. We leverage the consistency of these atmospheric quantities to favor a
younger age (0.50 \pm 0.07 Gyr) of the system compared to the older age (0.79
\pm 0.22 Gyr) determined with gyrochronology in order to address the luminosity
discrepancy.Comment: 17 pages, 9 figures, Accepted to Ap
Hunting for planets in the HL Tau disk
Recent ALMA images of HL Tau show gaps in the dusty disk that may be caused
by planetary bodies. Given the young age of this system, if confirmed, this
finding would imply very short timescales for planet formation, probably in a
gravitationally unstable disk. To test this scenario, we searched for young
planets by means of direct imaging in the L'-band using the Large Binocular
Telescope Interferometer mid-infrared camera. At the location of two prominent
dips in the dust distribution at ~70AU (~0.5") from the central star we reach a
contrast level of ~7.5mag. We did not detect any point source at the location
of the rings. Using evolutionary models we derive upper limits of ~10-15MJup at
<=0.5-1Ma for the possible planets. With these sensitivity limits we should
have been able to detect companions sufficiently massive to open full gaps in
the disk. The structures detected at mm-wavelengths could be gaps in the
distributions of large grains on the disk midplane, caused by planets not
massive enough to fully open gaps. Future ALMA observations of the molecular
gas density profile and kinematics as well as higher contrast infrared
observations may be able to provide a definitive answer.Comment: Accepted for publication on ApJ Letter
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