2,825 research outputs found
RELATIVE SENSITIVITY OF SELECTED DISTRIBUTED LAG ESTIMATORS TO MEASUREMENT AND SPECIFICATION ERROR
Research Methods/ Statistical Methods,
The Mid-Infrared Extinction Law in the Ophiuchus, Perseus, and Serpens Molecular Clouds
We compute the mid-infrared extinction law from 3.6-24 microns in three
molecular clouds: Ophiuchus, Perseus, and Serpens, by combining data from the
"Cores to Disks" Spitzer Legacy Science program with deep JHKs imaging. Using a
new technique, we are able to calculate the line-of-sight extinction law
towards each background star in our fields. With these line-of-sight
measurements, we create, for the first time, maps of the chi-squared deviation
of the data from two extinction law models. Because our chi-squared maps have
the same spatial resolution as our extinction maps, we can directly observe the
changing extinction law as a function of the total column density. In the
Spitzer IRAC bands, 3.6-8 microns, we see evidence for grain growth. Below
, our extinction law is well-fit by the Weingartner & Draine
(2001) diffuse interstellar medium dust model. As the extinction
increases, our law gradually flattens, and for , the data are
more consistent with the Weingartner & Draine model that uses
larger maximum dust grain sizes. At 24 microns, our extinction law is 2-4 times
higher than the values predicted by theoretical dust models, but is more
consistent with the observational results of Flaherty et al. (2007). Lastly,
from our chi-squared maps we identify a region in Perseus where the IRAC
extinction law is anomalously high considering its column density. A steeper
near-infrared extinction law than the one we have assumed may partially explain
the IRAC extinction law in this region.Comment: 38 pages, 19 figures in pre-print format. Accepted for publication in
ApJ. A version with full-resolution figures can be found here:
http://peggysue.as.utexas.edu/SIRTF
Repeated structures: image correspondence constraints and 3D structure recovery
Recently, a number of classes of 3D structures have been identified which permit structure recovery and 3D invariants to be measured from a single image of the structure. A large class with this property is the case of repeated structures where a structure (such as a pointset, curve or surface), and a transformed copy of the structure are both observed in a single perspective image. In general the 3D reconstruction is only possible up to a 3D projectivity of space, but smaller ambiguities are possible, depending on the nature of the 3D transformation between the repeated structures. An additional theme of the paper is the development of feature correspondence relations based on the epipolar geometry induced in the image by the repeated structure. In some cases, correspondence is based on projective homologies rather than a true epipolar geometry
Error Propagation in Satellite Multi-image Geometry
This paper describes an investigation of the source of geospatial error in
digital surface models (DSMs) constructed from multiple satellite images. In
this study the uncertainty in surface geometry is separated into two spatial
components; global error that affects the absolute position of the surface, and
local error that varies from surface point to surface point. The global error
component is caused by inaccuracy in the satellite imaging process, mainly due
to uncertainty in the satellite position and orientation (pose) during image
collection. A key result of the investigation is a new algorithm for
determining the absolute geoposition of the DSM that takes into account the
pose covariance of each satellite during image collection. This covariance
information is used to weigh the evidence from each image in the computation of
the global position of the DSM. The use of covariance information significantly
decreases the overall uncertainty in global position. The paper also describes
an approach to the prediction of local error in the DSM surface. The observed
variance in surface position within a single stereo surface reconstruction
defines the local horizontal error. The variance in the fused set of elevations
from multiple stereo pairs at a single DSM location defines the local vertical
error. These accuracy predictions are compared to ground truth provided by
LiDAR scans of the same geographic region of interest.Comment: 15 pages, 27 figure
The impact of shocks on the chemistry of molecular clouds: high resolution images of chemical differentiation along the NGC1333-IRAS2A outflow
This paper presents a detailed study of the chemistry in the outflow
associated with the low-mass protostar NGC1333-IRAS2A down to 3" (650 AU)
scales. Millimeter-wavelength aperture-synthesis observations from the OVRO and
BIMA interferometers and (sub)millimeter single-dish observations from the
Onsala 20m telescope and CSO are presented. The interaction of the highly
collimated protostellar outflow with a molecular condensation ~15000 AU from
the central protostar is clearly traced by molecular species such as HCN, SiO,
SO, CS, and CH3OH. Especially SiO traces a narrow high velocity component at
the interface between the outflow and the molecular condensation.
Multi-transition single-dish observations are used to distinguish the chemistry
of the shock from that of the molecular condensation and to address the
physical conditions therein. Statistical equilibrium calculations reveal
temperatures of 20 and 70 K for the quiescent and shocked components,
respectively, and densities near 10^6 cm^{-3}. Significant abundance
enhancements of two to four orders of magnitude are found in the shocked region
for molecules such as CH3OH, SiO and the sulfur-bearing molecules. HCO+ is seen
only in the aftermath of the shock consistent with models where it is destroyed
through release of H2O from grain mantles in the shock. N2H+ shows narrow
lines, not affected by the outflow but rather probing the ambient cloud.
Differences in abundances of HCN, H2CO and CS are seen between different
outflow regions and are suggested to be related to differences in the atomic
carbon abundance. Compared to the warm inner parts of protostellar envelopes,
higher abundances of in particular CH3OH and SiO are found in the outflows,
which may be related to density differences between the regions.Comment: 18 pages, 13 figures. Accepted for publication in A&
Are young stars always associated with cold massive disks? A CO and millimeter interferometric continuum survey
The results of a combined millimeter-spectral-line and continuum survey of cold far-infrared sources selected to favor embedded young stars in the Galaxy are presented. The spectral-line observations were performed with the 5 meter antenna of the University of Texas Millimeter-Wave Observatory. High resolution continuum observations were obtained with the Owens Valley (OVRO) Millimeter-Wave Interferometer. The goal of the survey was to gain insight into the mass, temperature, and distribution of cold dust which envelopes stars during the earliest stages of their evolution. The first phase of our survey involved 1.2 arcmin resolution observations of CO-12 and CO-13 emission lines toward each source. All but two sources had detectable CO emission. We found that 40% of the sources appear to be associated with star formation as evidenced by the presence of enhanced CO-12 line widths or broad wings. At least five of these objects are associated with bipolar molecular outflows. The second phase of our survey involves high resolution 2.7 mm continuum observations with 3 interferometer baselines ranging from 15 to 55 m in length. Preliminary results indicate that about 25% of the sources in our sample have detectable continuum emission on scales less than 30 arcsec. The high percentage of sources with enhanced CO-12 line widths or broad wings indicates that a significant fraction of our samples, 40%, are likely to be young stars. The lower detection percentage in the continuum observations, 25%, suggest that such objects are not always surrounded by large concentrations of gas and dust. The continuum detection percentage for actual dust emission could be lower than that given above since emission from ionized gas could be responsible for the observed 2.7 mm emission in some objects. To get an understanding of the type of object detected in our survey, a map of one of the survey sources, L1689N, has been made using the OVRO mm interferometer
A λ = 1.3 Millimeter Aperture Synthesis Molecular Line Survey of Orion Kleinmann-Low
We present a 1".3 spatial resolution interferometric spectral line survey of the core of the Orion molecular cloud, obtained with the OVRO millimeter array. Covering 4 GHz bandwidth in total, the survey contains ~100 emission lines from 18 chemical species. The spatial distributions of a number of molecules point to source I near the IRc2 complex as the dominant energy source in the region but do not rule out the presence of additional lower luminosity objects. At arcsecond resolution, the offsets between dust emission and various molecular tracers suggest that the spectacular "hot core" emission in the Orion core arises via the heating and ablation of material from the surfaces of very high density clumps located ≳500 AU from source I and traced by the dust emission. We find no evidence for a strong internal heating source within the hot core condensation(s)
HC3N maps of OMC1
We have made 3.8 sec resolution maps of HC3N (J = 12-11) and 2.7 mm continuum emission in OMC1 using the OVRO mm interferometer. The continuum map, which traces dust column density, shows that the hot core region consists of several clumps, the densest of which lies 3 sec SE of IRc2. HC3N, which traces dense gas, shows the velocity structure in the region. There is no simple pattern of rotation or expansion, nor does the emission resemble a disk centered on IRc2. Since the velocity difference between the hot core and IRc2 and the velocity dispersion in the hot core are comparable with the orbital velocity at a distance of 3 sec. from a 20 M(solar) object, it is possible that the hot core material is bound to IRc2. In the channel at 10.4 km s(-1) V(LSR), we detect strong emission from the source 20 sec NE of IRc2, which confirms indications from continuum and CS (J = 2-1) maps that this is a very dense, possibly protostellar, object. This emission is clearly resolved from the hot core and is elongated north-south, along the direction of the ridge emission. An additional interesting feature in these maps is a compact high velocity source located 4 sec SW of IRc2. This source has a velocity dispersion greather than 20 km/s (FWHM) and is spatially coincident with the zero-offset source seen by Pauls et al. (1983) and a point source in the near IR images taken by Allen et al. (1984). The large localized velocity, dispersion and the highly obscured IR source suggest that this compact source is an outflow from a young stellar companion to IRc2
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