1,263 research outputs found
A Multi-Epoch HST Study of the Herbig-Haro Flow from XZ Tauri
We present nine epochs of Hubble Space Telescope optical imaging of the
bipolar outflow from the pre-main sequence binary XZ Tauri. Our data monitors
the system from 1995-2005 and includes emission line images of the flow. The
northern lobe appears to be a succession of bubbles, the outermost of which
expanded ballistically from 1995-1999 but in 2000 began to deform and
decelerate along its forward edge. It reached an extent of 6" from the binary
in 2005. A larger and fainter southern counterbubble was detected for the first
time in deep ACS images from 2004. Traces of shocked emission are seen as far
as 20" south of the binary. The bubble emission nebulosity has a low excitation
overall, as traced by the [S II]/H-alpha line ratio, requiring a nearly
comoving surrounding medium that has been accelerated by previous ejections or
stellar winds.
Within the broad bubbles there are compact emission knots whose alignments
and proper motions indicate that collimated jets are ejected from each binary
component. The jet from the southern component, XZ Tau A, is aligned with the
outflow axis of the bubbles and has tangential knot velocities of 70-200 km/s.
Knots in the northern flow are seen to slow and brighten as they approach the
forward edge of the outermost bubble. The knots in the jet from the other star,
XZ Tau B, have lower velocities of ~100 km/s
High-contrast Imaging from Space: Speckle Nulling in a Low Aberration Regime
High-contrast imaging from space must overcome two major noise sources to
successfully detect a terrestrial planet angularly close to its parent star:
photon noise from diffracted star light, and speckle noise from star light
scattered by instrumentally-generated wavefront perturbation. Coronagraphs
tackle only the photon noise contribution by reducing diffracted star light at
the location of a planet. Speckle noise should be addressed with
adaptative-optics systems. Following the tracks of Malbet, Yu and Shao (1995),
we develop in this paper two analytical methods for wavefront sensing and
control that aims at creating dark holes, i.e. areas of the image plane cleared
out of speckles, assuming an ideal coronagraph and small aberrations. The first
method, speckle field nulling, is a fast FFT-based algorithm that requires the
deformable-mirror influence functions to have identical shapes. The second
method, speckle energy minimization, is more general and provides the optimal
deformable mirror shape via matrix inversion. With a NxN deformable mirror, the
size of matrix to be inverted is either N^2xN^2 in the general case, or only
NxN if influence functions can be written as the tensor product of two
one-dimensional functions. Moreover, speckle energy minimization makes it
possible to trade off some of the dark hole area against an improved contrast.
For both methods, complex wavefront aberrations (amplitude and phase) are
measured using just three images taken with the science camera (no dedicated
wavefront sensing channel is used), therefore there are no non-common path
errors. We assess the theoretical performance of both methods with numerical
simulations, and find that these speckle nulling techniques should be able to
improve the contrast by several orders of magnitude.Comment: 31 pages, 8 figures, 1 table. Accepted for publication in ApJ (should
appear in February 2006
HST PSF simulation using Tiny Tim
Tiny Tim is a program which simulates Hubble Space Telescope imaging camera PSF's. It is portable (written and distributed in C) and is reasonably fast. It can model the WFPC, WFPC 2, FOC, and COSTAR corrected FOC cameras. In addition to aberrations such as defocus and spherical, it also includes WFPC obscuration shifting, mirror zonal error maps, and jitter. The program has been used at a number of sites for deconvolving HST images. Tiny Tim is available via anonymous ftp on stsci.edu in the directory software/tinytim
The development of a comvenient thermal dynamic building model
The present paper describes a method to set up a thermal building model combining relative simplicity with high dynamic accuracy. The models were verified in two Dutch semi-detached dwellings characterized by extreme values of thermal capacity
An improved prism energy analyzer for neutrons
The effects of two improvements of an existing neutron energy analyzer consisting of stacked silicon prism rows are presented. First we tested the effect of coating the back of the prism rows with an absorbing layer to suppress neutron scattering by total reflection and by refraction at small angles. Experiments at HZB showed that this works perfectly. Second the prism rows were bent to shift the transmitted wavelength band to larger wavelengths. At HZB we showed that bending increased the transmission of neutrons with a wavelength of 4.9 . Experiments with a white beam at the EROS reflectometer at LLB showed that bending of the energy analyzing device to a radius of 7.9 m allows to shift the transmitted wavelength band from 0 to 9 to 2 to 1
Identifying Attrition Phases in Survey Data: Applicability and Assessment Study
Background: Although Web-based questionnaires are an efficient, increasingly popular mode of data collection, their utility is often challenged by high participant dropout. Researchers can gain insight into potential causes of high participant dropout by analyzing the dropout patterns.
Objective: This study proposed the application of and assessed the use of user-specified and existing hypothesis testing methods in a novel setting—survey dropout data—to identify phases of higher or lower survey dropout.
Methods: First, we proposed the application of user-specified thresholds to identify abrupt differences in the dropout rate. Second, we proposed the application of 2 existing hypothesis testing methods to detect significant differences in participant dropout. We assessed these methods through a simulation study and through application to a case study, featuring a questionnaire addressing decision-making surrounding cancer screening.
Results: The user-specified method set to a low threshold performed best at accurately detecting phases of high attrition in both the simulation study and test case application, although all proposed methods were too sensitive.
Conclusions: The user-specified method set to a low threshold correctly identified the attrition phases. Hypothesis testing methods, although sensitive at times, were unable to accurately identify the attrition phases. These results strengthen the case for further development of and research surrounding the science of attrition
The Compact UV Nucleus of M33
The most luminous X-ray source in the Local Group is associated with the
nucleus of M33. This source, M33 X-8, appears modulated by ~20% over a ~106 day
period, making it unlikely that the combined emission from unresolved sources
could explain the otherwise persistent ~1e39 erg/s X-ray flux (Dubus et al.
1997, Hernquist et al. 1991). We present here high resolution UV imaging of the
nucleus with the Planetary Camera of the HST undertaken in order to search for
the counterpart to X-8. The nucleus is bluer and more compact than at longer
wavelength images but it is still extended with half of its 3e38 erg/s UV
luminosity coming from the inner 0.14". We cannot distinguish between a
concentrated blue population and emission from a single object.Comment: 3 figures, accepted for publication in ApJ Letter
HST/ACS Images of the GG Tauri Circumbinary Disk
Hubble Space Telescope Advanced Camera for Surveys images of the young binary
GG Tauri and its circumbinary disk in V and I bandpasses were obtained in 2002
and are the most detailed of this system to date. The confirm features
previously seen in the disk including: a "gap" apparently caused by shadowing
from circumstellar material; an asymmetrical distribution of light about the
line of sight on the near edge of the disk; enhanced brightness along the near
edge of the disk due to forward scattering; and a compact reflection nebula
near the secondary star. New features are seen in the ACS images: two short
filaments along the disk; localized but strong variations in disk intensity
("gaplets"); and a "spur" or filament extending from the reflection nebulosity
near the secondary. The back side of the disk is detected in the V band for the
first time. The disk appears redder than the combined light from the stars,
which may be explained by a varied distribution of grain sizes. The brightness
asymmetries along the disk suggest that it is asymmetrically illuminated by the
stars due to extinction by nonuniform circumstellar material or the illuminated
surface of the disk is warped by tidal effects (or perhaps both). Localized,
time-dependent brightness variations in the disk are also seen.Comment: 28 pages, 7 figures, accepted for publication in the Astronomical
Journa
Methods for Evaluating Respondent Attrition in Web-Based Surveys
Background: Electronic surveys are convenient, cost effective, and increasingly popular tools for collecting information. While the online platform allows researchers to recruit and enroll more participants, there is an increased risk of participant dropout in Web-based research. Often, these dropout trends are simply reported, adjusted for, or ignored altogether.
Objective: To propose a conceptual framework that analyzes respondent attrition and demonstrates the utility of these methods with existing survey data.
Methods: First, we suggest visualization of attrition trends using bar charts and survival curves. Next, we propose a generalized linear mixed model (GLMM) to detect or confirm significant attrition points. Finally, we suggest applications of existing statistical methods to investigate the effect of internal survey characteristics and patient characteristics on dropout. In order to apply this framework, we conducted a case study; a seventeen-item Informed Decision-Making (IDM) module addressing how and why patients make decisions about cancer screening.
Results: Using the framework, we were able to find significant attrition points at Questions 4, 6, 7, and 9, and were also able to identify participant responses and characteristics associated with dropout at these points and overall.
Conclusions: When these methods were applied to survey data, significant attrition trends were revealed, both visually and empirically, that can inspire researchers to investigate the factors associated with survey dropout, address whether survey completion is associated with health outcomes, and compare attrition patterns between groups. The framework can be used to extract information beyond simple responses, can be useful during survey development, and can help determine the external validity of survey results
Discovery of an extended debris disk around the F2V star HD 15745
Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we
have discovered dust-scattered light from the debris disk surrounding the F2V
star HD 15745. The circumstellar disk is detected between 2.0" and 7.5" radius,
corresponding to 128 - 480 AU radius. The circumstellar disk morphology is
asymmetric about the star, resembling a fan, and consistent with forward
scattering grains in an optically thin disk with an inclination of ~67 degrees
to our line of sight. The spectral energy distribution and scattered light
morphology can be approximated with a model disk composed of silicate grains
between 60 and 450 AU radius, with a total dust mass of 10E-7 M_sun (0.03
M_earth) representing a narrow grain size distribution (1 - 10 micron).
Galactic space motions are similar to the Castor Moving Group with an age of
~10E+8 yr, although future work is required to determine the age of HD 15745
using other indicators.Comment: 7 pages, 4 figures, ApJ Letters, in pres
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