42 research outputs found
The Sizes of 1720 MHz OH Masers: VLBA and MERLIN Observations of the Supernova Remnants W44 and W28
We have used the NRAO Very Long Baseline Array (VLBA) to image OH(1720 MHz)
masers in the supernova remnants W28 and W44 at a resolution of 40 mas. We also
used MERLIN to observe the same OH(1720 MHz) masers in W44 at a resolution of
290 x 165 mas. All the masers are resolved by these VLBA and MERLIN
observations. The measured sizes range from 50 to 180 mas and yield brightness
temperature estimates from 0.3--20 x 10**8 K. We investigate whether these
measured angular sizes are intrinsic and hence originate as a result of the
physical conditions in the supernova remnant shock, or whether they are scatter
broadened sizes produced by the turbulent ionized gas along the line of sight.
While the current data on the temporal and angular broadening of pulsars,
masers and extragalactic soures toward W44 and W28 can be understood in terms
of scattering, we cannot rule out that these large sizes are intrinsic. Recent
theoretical modeling by Lockett et al. suggests that the physical parameters in
the shocked region are indicative of densities and OH abundances which lead to
estimates of sizes as large as what we measure. If the sizes and structure are
intrinsic, then the OH(1720 MHz) masrs may be more like the OH(1612 MHz) masers
in circumstellar shells than OH masers associated with HII regions. At two
locations in W28 we observe the classical S-shapes in the Stokes V profiles
caused by Zeeman splitting and use it to infer magnetic fields of order 2
milliGauss.Comment: 24 pages, 6 figures, accepted by Ap
A Very High Spectral Resolution Study of Ground-State OH Masers in W3(OH)
We present VLBA observations of the ground-state hydroxyl masers in W3(OH) at 0.02 km s-1 spectral resolution. Over 250 masers are detected, including 56 Zeeman pairs. Lineshapes are predominantly Gaussian or combinations of several Gaussians, with normalized deviations typically of the same magnitude as in masers in other species. Typical FWHM maser linewidths are 0.15 to 0.38 km s-1 and are larger in the 1665 MHz transition than in the other three ground-state transitions. The satellite-line 1612 and 1720 MHz masers show no evidence of sigma^+/-2,3 components. The spatial positions of most masers are seen to vary across the line profile, with many spots showing clear, organized positional gradients. Equivalent line-of-sight velocity gradients in the plane of the sky typically range from 0.01 to 1 km s-1 AU-1 (i.e., positional gradients of 1 to 100 AU (km s-1)-1). Small velocity gradients in the 1667 MHz transition support theoretical predictions that 1667 MHz masers appear in regions with small velocity shifts along the amplification length. Deconvolved maser spot sizes appear to be larger in the line wings but do not support a spherical maser geometry
Excited-state OH Mainline Masers in AU Geminorum and NML Cygni
Excited-state OH maser emission has previously been reported in the
circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum
and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of
the 1665, 1667, and excited-state 4750 MHz mainline OH transitions in AU Gem
and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and
6035 MHz OH mainline transitions in NML Cyg. We detect masers in both mainline
transitions in AU Gem but no excited-state emission in either star. We conclude
that the excited-state OH emission in AU Gem is either a transient phenomenon
(such as for NML Cyg outlined below), or possibly an artifact in the data, and
that the excited state OH emission in NML Cyg was generated by an episode of
enhanced shock between the stellar mass-loss and an outflow of the Cyg OB2
association. With these single exceptions, it therefore appears that
excited-state OH emission indeed should not be predicted nor observable in
evolved stars as part of their normal structure or evolution.Comment: ApJ Letter, accepted, 4 pages, 2 figure
A measurement of the energy spectra of cosmic rays from 20 to 1000 GeV per amu
A group collaboration was made in the development of the Bristol University Gas Spectrometer number 4 (BUGS 4). The BUGS 4 detector is designed to measure the charge spectrum for species between oxygen and the iron peak as a function of energy per nucleon, between 20 and 1000 GeV/amu. It is particularly concerned with energies above 50 GeV/amu. The high energy component is considerably less affected by propagation through the interstellar medium than the lower energy component and is expected to approach the original charge spectrum of the source more closely. This information allows one to unravel the effects of cosmic ray production, acceleration, and propagation. The detector is described in total detail. The method of estimating the charge and energy of a cosmic ray depends on the energy of the particle. Calculations and experiments lead to the expectation of a nearly constant charge resolution of about 0.2 charge units over the whole energy range except 4.5 less than gamma less than 20. In this band, the experiment is insensitive to energy. A balloon flight is planned in 1993
The Sizes of OH (1720 MHz) Supernova Remnant Masers: MERLIN and VLBA Observations of IC443
MERLIN and VLBA observations of the 1720 MHz maser emission from the OH
molecule in the supernova remnant IC443 are presented. Based on MERLIN data
with a resolution of 160 mas, the deconvolved sizes of the maser sources are in
the range 90 to 180 mas (135 to 270 AU). The 12 mas resolution VLBA images show
compact cores with sizes in the range 15 to 55 mas. The maser brightness
temperatures are (2-34)x10^6 K for the MERLIN sources and (5-19)x10^8 K for the
VLBA cores, in agreement with theory. Unlike the Zeeman Stokes V profiles
observed in other OH (1720 MHz) SNR masers, single-handed circular polarization
line profiles are observed in IC443 on all angular scales from 1000 to 10 mas
resolution. For one line component, the observed line width is 0.24+/-0.07
km/s, compared to an estimated Doppler width of 0.49 km/s. This discrepancy in
line widths can be accounted for if the maser emission arises from an elongated
ellipsoidal region of masing gas.Comment: 22 pages, 4 figures, 3 tables, AASTex, accepted to Ap
Phase-Referenced VLBA Observations of OH Masers at 4765 MHz
We report VLBA observations of maser emission from the rotationally excited
doublet Pi 1/2, J=1/2 state of OH at 4765 MHz. We made phase-referenced
observations of W3(OH) at both 4765 MHz and 1720 MHz and found emission in
three fields within a about 2000 AU diameter region and verified that in two of
the three fields, 4765 MHz and 1720 MHz emission arise from the same position
to within about 4 mas (about 5 AU diameter emission regions along an
approximately N-S arc with linear extent about 500 AU. In addition, we carried
out phase-referenced observations of 4765 MHz emission from K3-50. We searched
for the 4765 MHz line in W49 (without phase referencing) and W75N
(phase-referenced to the strongest 4765 MHz maser feature in DR21EX); we were
unable to detect these sources with the VLBA. For 2 1/2 years (including the
dates of the VLBA observations), we carried out monitoring observations of 4765
MHz emission with the VLA. Constraints on models for maser emission at 1720 MHz
and 4765 MHz are derived from the observations. These observations are then
briefly compared with existing models.Comment: 30 pages, 7 figures, to be published in Ap. J. (Dec 10, 2003
New Herbig-Haro Objects and Giant Outflows in Orion
We present the results of a photographic and CCD imaging survey for
Herbig-Haro (HH) objects in the L1630 and L1641 giant molecular clouds in
Orion. The new HH flows were initially identified from a deep H-alpha film from
the recently commissioned AAO/UKST H-alpha Survey of the southern sky. Our
scanned H-alpha and broad band R images highlight both the improved resolution
of the H-alpha survey and the excellent contrast of the H-alpha flux with
respect to the broad band R. Comparative IVN survey images allow us to
distinguish between emission and reflection nebulosity. Our CCD H-alpha, [SII],
continuum and I band images confirm the presence of a parsec-scale HH flow
associated with the Ori I-2 cometary globule and several parsec-scale strings
of HH emission centred on the L1641-N infrared cluster. Several smaller
outflows display one-sided jets. Our results indicate that for declinations
south of -6 degrees in L1641, parsec-scale flows appear to be the major force
in the large-scale movement of optical dust and molecular gas.Comment: 14 pages, Latex using MN style, 21 figures in JPEG format. Higher
resolution figures available from S.L. Mader. Accepted by MNRAS. Email
contact for higher resolution images: [email protected]