L. D. Matthews

THE PERFORMANCE AND CALIBRATION OF WFPC2 ON THE HUBBLE SPACE TELESCOPE

The WFPC2 was installed in the Hubble Space Telescope (HST) in 1993 December. Since then, the instrument has been providing high-quality images. A significant amount of calibration data has been collected to aid in the understanding of the on-orbit performance of the instrument. Generally, the behavior of the camera is similar to its performance during the system-level thermal vacuum test at JPL in 1993 May. Surprises were a significant charge-transfer-efficiency (CTE) problem and a significant growth rate in hot pixels at the original operating temperature of the CCDs (-76 °C). The operating temperature of the WFPC2 CCDs was changed to -88 °C on 1994 April 23, and significant improvements in CTE and hot pixels are seen at this temperature. In this paper we describe the on-orbit performance of the WFPC2. We discuss the optical and thermal history, the instrument throughput and stability, the PSF, the effects of undersampling on photometry, the properties of cosmic rays observed on-orbit, and the geometric distortion in the camera. We present the best techniques for the reduction of WFPC2 data, and describe the construction of calibration products including superbiases, superdarks, and fiat fields.

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

A FEATURE MOVIE OF SiO EMISSION 20-100 AU FROM THE MASSIVE YOUNG STELLAR OBJECT ORION SOURCE I

We present multi-epoch Very Long Baseline Array imaging of the 28SiO v = 1 and v = 2, J = 1-0 maser emission toward the massive young stellar object (YSO) Orion Source I. Both SiO transitions were observed simultaneously with an angular resolution of ~0.5 mas (~0.2 AU for d = 414 pc) and a spectral resolution of ~0.2 km s–1. Here we explore the global properties and kinematics of the emission through two 19-epoch animated movies spanning 21 months (from 2001 March 19 to 2002 December 10). These movies provide the most detailed view to date of the dynamics and temporal evolution of molecular material within ~20-100 AU of a massive (8 M ☉) YSO. As in previous studies, we find that the bulk of the SiO masers surrounding Source I lie in an -shaped locus; the emission in the south and east arms is predominantly blueshifted, and emission in the north and west is predominantly redshifted. In addition, bridges of intermediate-velocity emission are observed connecting the red and blue sides of the emission distribution. We have measured proper motions of over 1000 individual maser features and found that these motions are characterized by a combination of radially outward migrations along the four main maser-emitting arms and motions tangent to the intermediate-velocity bridges. We interpret the SiO masers as arising from a wide-angle bipolar wind emanating from a rotating, edge-on disk. The detection of maser features along extended, curved filaments suggests that magnetic fields may play a role in launching and/or shaping the wind. Our observations appear to support a picture in which stars with masses as high as at least 8 M ☉ form via disk-mediated accretion. However, we cannot yet rule out that the Source I disk may have been formed or altered following a recent close encounter.

A MULTI-EPOCH STUDY OF THE RADIO CONTINUUM EMISSION OF ORION SOURCE I: CONSTRAINTS ON THE DISK EVOLUTION OF A MASSIVE YSO AND THE DYNAMICAL HISTORY OF ORION BN/KL

We present new λ7 mm continuum observations of Orion BN/KL with the Very Large Array. We resolve the emission from the young stellar objects radio Source I and BN at several epochs. Radio Source I is highly elongated northwest-southeast, and remarkably stable in flux density, position angle, and overall morphology over nearly a decade. This favors the extended emission component arising from an ionized edge-on disk rather than an outwardly propagating jet. We have measured the proper motions of Source I and BN for the first time at 43 GHz. We confirm that both sources are moving at high speed (12 and 26 km s–1, respectively) approximately in opposite directions, as previously inferred from measurements at lower frequencies. We discuss dynamical scenarios that can explain the large motions of both BN and Source I and the presence of disks around both. Our new measurements support the hypothesis that a close (~50 AU) dynamical interaction occurred around 500 years ago between Source I and BN as proposed by Gomez et al. From the dynamics of encounter, we argue that Source I today is likely to be a binary with a total mass on the order of 20 M ☉ and that it probably existed as a softer binary before the close encounter. This enables preservation of the original accretion disk, though truncated to its present radius of ~50 AU. N-body numerical simulations show that the dynamical interaction between a binary of 20 M ☉ total mass (Source I) and a single star of 10 M ☉ mass (BN) may lead to the ejection of both and binary hardening. The gravitational energy released in the process would be large enough to power the wide-angle, high-velocity flow traced by H2 and CO emission in the BN/KL nebula. Assuming that the proposed dynamical history is correct, the smaller mass for Source I recently estimated from SiO maser dynamics (7 M ☉) by Matthews et al., suggests that non-gravitational forces (e.g., magnetic) must play an important role in the circumstellar gas dynamics.

High-Latitude H I in the Low Surface Brightness Galaxy UGC 7321

From the analysis of sensitive H i 21 cm line observations, we find evidence for vertically extended H i emission ( z jj d2:4 kpc) in the edge-on, low surface brightness spiral galaxy UGC 7321. Three-dimensional modeling suggests that the H i disk of UGC 7321 is both warped and flared, but that neither effect can fully reproduce the spatial distribution and kinematics of the highest z-height gas. We are able to model the highlatitude emission as an additional H i component in the form of a ‘‘ thick disk ’’ or ‘‘ halo ’’ with a FWHM of � 3.3 kpc. We find tentative evidence that the vertically extended gas declines in rotational velocity as a function of z, although we are unable to completely rule out models with constant Vz ðÞ . In spite of the low star formation rate of UGC 7321, energy from supernovae may be sufficient to sustain this high-latitude gas. However, alternative origins for this material, such as slow, sustained infall, cannot yet be excluded. Subject headings: galaxies: halos — galaxies: individual (UGC 7321) — galaxies: ISM — galaxies: spiral

An HST Survey of the mid-UV Morphology of Nearby Galaxies

(Abbreviated) We present an imaging survey of 37 nearby galaxies observed with HST/WFPC2 in the mid-UV F300W filter and in F814W. 11 galaxies were also imaged in F255W. These galaxies were selected to be detectable with WFPC2 in one orbit, and cover a wide range of Hubble types and inclinations. The mid-UV spans the gap between our groundbased optical/NIR images and far-UV images available from the Astro/UIT missions. Our first qualitative results are: (1) Early-type galaxies show a significant decrease in surface brightness going from the red to the mid-UV, and in some cases the presence of dust lanes. Some galaxies would be classified different when viewed in the mid-UV, some become dominated by a blue nuclear feature or point source. (2) Half of the mid-type spiral and star-forming galaxies appear as a later morphological type in the mid-UV, as Astro/UIT also found in the far-UV. Some- times these differences are dramatic. The mid-UV images show a considerable range in the scale and surface brightness of individual star-forming regions. Almost all mid-type spirals have their small bulges bi-sected by a dust-lane. (3) Most of the heterogeneous subset of late-type, irregular, peculiar, and merging galaxies display F300W morphologies that are similar to those seen in F814W, but with differences due to recognizable dust features absorbing the bluer light, and due to UV-bright hot stars, star-clusters, and star-forming ridges. In the rest-frame mid-UV, early- to mid-type galaxies are more likely to be misclassified as later types than vice versa. This morphological K-correction explains only part of the excess faint blue galaxies seen in deep HST fields.We present a systematic imaging survey of 37 nearby galaxies observed with the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC2) in the mid-UV F300W filter, centered at 2930 A, as well as in the I-band (F814W) filter at 8230 A. Eleven of these galaxies were also imaged in the F255W filter, centered at 2550 A. Our sample is carefully selected to include galaxies of sufficiently small radius and high predicted mid-UV surface brightness to be detectable with WFPC2 in one orbit and covers a wide range of Hubble types and inclinations. The mid-UV (2000-3200 A) spans the gap between ground-based UBVR(IJHK) images, which are available or were acquired for the current study, and far-UV images available from the Astro/UIT missions for 15 galaxies in our sample. The first qualitative results from our study are as follows:

High-Latitude HI in the Low Surface Brightness Galaxy UGC7321

From the analysis of sensitive H i 21 cm line observations, we find evidence for vertically extended H i emission ( z jj d2:4 kpc) in the edge-on, low surface brightness spiral galaxy UGC 7321. Three-dimensional modeling suggests that the H i disk of UGC 7321 is both warped and flared, but that neither effect can fully reproduce the spatial distribution and kinematics of the highest z-height gas. We are able to model the highlatitude emission as an additional H i component in the form of a ‘‘ thick disk ’’ or ‘‘ halo ’’ with a FWHM of � 3.3 kpc. We find tentative evidence that the vertically extended gas declines in rotational velocity as a function of z, although we are unable to completely rule out models with constant Vz ðÞ . In spite of the low star formation rate of UGC 7321, energy from supernovae may be sufficient to sustain this high-latitude gas. However, alternative origins for this material, such as slow, sustained infall, cannot yet be excluded. Subject headings: galaxies: halos — galaxies: individual (UGC 7321) — galaxies: ISM — galaxies: spiral

Maser Emission from SiO Isotopologues Traces the Innermost 100 AU Around Radio Source I in Orion Becklin–Neugebauer/Kleinmann-Low

We have used the Very Large Array at 7 mm wavelength to image five rotational transitions (J = 1-0) from three SiO isotopologues toward Orion Becklin-Neugebauer/Kleinmann-Low (BN/KL): 28SiO v = 0, 1, 2; and 29SiO and 30SiO v = 0. For the first time, we have mapped the 29SiO and 30SiO J = 1-0 emission, established the maser nature of the emission, and confirmed association with the deeply embedded high-mass young stellar object commonly denoted radio Source I. The 28SiO v = 0 maser emission shows a bipolar structure that extends over ~700 AU along a northeast-southwest axis, and we propose that it traces a bipolar outflow driven by Source I. The high-brightness isotopic SiO maser emission imaged with a 02 resolution has a more compact distribution, generally similar to that of the 28SiO v = 1, 2 emission, and it probably traces bulk gas flows in a region of diameter 100 AU centered on Source I. On small scales of 10 AU, however, compact 29SiO/30SiO v = 0 and 28SiO v = 1, 2 emission features may be offset from one another in position and line-of-sight velocity. From a radiative transfer analysis based on a large velocity gradient pumping model, we derive similar temperatures and densities for the optimum excitation of both 29SiO/30SiO v = 0 and 28SiO v = 1, 2 masers, significantly higher than required for 28SiO v = 0 maser excitation. In order to account for the small-scale differences among the isotopologues (v = 0) and the main species (v = 1, 2), follow-up radiative transfer modeling that incorporates nonlocal line overlap among transitions of all SiO isotopic species may be required.

AN INFRARED NEBULA ASSOCIATED WITH δ CEPHEI: EVIDENCE OF MASS LOSS?

United States. Dept. of Defense. ASSURE Program (National Science Foundation (U.S.). Research Experience for Undergraduates (Program). Grant 0754568)

Discovery of a detached H i gas shell surrounding α Orionis

We report the detection of the HI line at 21 cm in the direction of alpha Ori with the Nancay Radiotelescope and with the Very Large Array. The observations confirm the previous detection of HI emission centered on alpha Ori, but additionally reveal for the first time a quasi-stationary detached shell of neutral atomic hydrogen ~4 arcmin. in diameter (0.24 pc at a distance of 200 pc). The detached shell appears elongated in a direction opposite to the stars space motion. A simple model shows that this detached atomic gas shell can result from the collision of the stellar wind from alpha Ori with the local interstellar medium (ISM). It implies that alpha Ori has been losing matter at a rate of ~ 1.2x10^-6 solar masses per year for the past 8x10^4 years. In addition, we report the detection of atomic hydrogen associated with the far-infrared arc located 6 arcmin. north-east of alpha Ori, that has been suggested to trace the bow shock resulting from the motion of the star through the ISM. We report also the detection by the Galaxy Evolution Explorer (GALEX) of a far-UV counterpart to this arc.