288 research outputs found
GLOBAL VERY LONG BASELINE INTERFEROMETRY OBSERVATIONS OF THE 6.0 GHz HYDROXYL MASERS IN ONSALA 1
We present global very long baseline interferometry observations of the first excited-state hydroxyl (OH) masers in the massive star-forming region Onsala 1 (ON 1). The 29 masers detected are nearly all from the 6035 MHz transition and nearly all are identifiable as Zeeman pair components. The 6030 and 6035 MHz masers are coincident with previously published positions of ground-state masers to within a few milliarcseconds, and the magnetic fields deduced from Zeeman splitting are comparable. The 6.0 GHz masers in ON 1 are always found in close spatial association with 1665 MHz OH masers, in contrast to the situation in the massive star-forming region W3(OH), suggesting that extreme high density OH maser sites (excited-state masers with no accompanying ground-state maser, as seen in W3(OH)) are absent from ON 1. The large magnetic field strength among the northern, blueshifted masers is confirmed. The northern masers may trace an outflow or be associated with an exciting source separate from the other masers, or the relative velocities of the northern and southern masers may be indicative of expansion and rotation. High angular resolution observations of nonmasing material will be required in order to understand the complex maser distribution in ON 1.National Science Foundation (U.S.
EVLA Observations of OH Masers in ON 1
This Letter reports on initial Expanded Very Large Array (EVLA) observations
of the 6035 MHz masers in ON 1. The EVLA data are of good quality, lending
confidence in the new receiver system. Nineteen maser features, including six
Zeeman pairs, are detected. The overall distribution of 6035 MHz OH masers is
similar to that of the 1665 MHz OH masers. The spatial resolution is sufficient
to unambiguously determine that the magnetic field is strong (~ -10 mG) at the
location of the blueshifted masers in the north, consistent with Zeeman
splitting detected in 13441 MHz OH masers in the same velocity range. Left and
right circularly polarized ground-state features dominate in different regions
in the north of the source, which may be due to a combination of magnetic field
and velocity gradients. The combined distribution of all OH masers toward the
south is suggestive of a shock structure of the sort previously seen in W3(OH).Comment: 4 pages using emulateapj.cls including 2 tables and 2 color figure
Persistent Asymmetric Structure of Sagittarius A^* on Event Horizon Scales
The Galactic Center black hole Sagittarius A^* (Sgr A^*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A^* obtained with the EHT on a total of 13 observing nights over four years. Closure phases, which are the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180° rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A^* that is not obscured by refraction due to interstellar electrons along the line of sight
Event-Horizon-Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way with the Inner Stellar Disk
Observations of the black hole in the center of the Milky Way with the Event
Horizon Telescope at 1.3 mm have revealed a size of the emitting region that is
smaller than the size of the black-hole shadow. This can be reconciled with the
spectral properties of the source, if the accretion flow is seen at a
relatively high inclination (50-60 degrees). Such an inclination makes the
angular momentum of the flow, and perhaps of the black hole, nearly aligned
with the angular momenta of the orbits of stars that lie within 3 arcsec from
the black hole. We discuss the implications of such an alignment for the
properties of the black hole and of its accretion flow. We argue that future
Event-Horizon-Telescope observations will not only refine the inclination of
Sgr A* but also measure precisely its orientation on the plane of the sky.Comment: To appear in the Astrophysical Journa
Magnetic Field Clumping in Massive Star-Forming Regions as Determined from Excited-State OH Absorption and Maser Emission
We have observed six high-mass star-forming regions in the 2 Pi 3/2, J = 7/2
lines of OH using the GBT in order to investigate whether the magnetic field,
and hence the density, measured in absorption differs from that implied by
maser Zeeman splitting. We detect absorption in both the 13441 and 13434 MHz
main lines in all six sources. Zeeman splitting in the F = 3-3 absorption line
in W3(OH) implies a line-of-sight magnetic field strength of 3.0 +/- 0.3 mG.
This is significantly less than full magnetic field strengths detected from OH
maser Zeeman splitting, suggesting that OH maser regions may be denser than the
non-masing OH material by a factor of several. Zeeman splitting is not detected
in other sources, but we are able to place upper limits on B_parallel of 1.2 mG
in G10.624-0.385 and 2.9 mG in K3-50. These results are consistent with a
density enhancement of the masers, but other explanations for the lower
magnetic field in absorption compared to maser emission are possible for these
two sources. Absorption in one or both of the 13442 and 13433 MHz satellite
lines is also seen in four sources. This is the very first detection of the 2
Pi 3/2, J = 7/2 satellite lines. Ratios of satellite-line to main-line
absorption suggest enhancement of the satellite lines from local thermodynamic
equilibrium values. Masers are seen in the F = 4-4 and 3-3 transitions of
W3(OH) and the 4-4 transition of ON 1. A previously undetected 4-4 maser is
seen near -44.85 km/s in W3(OH).Comment: 29 pages, 22 figures, scheduled to be published in 20 April 2005
issue of ApJ (vol 623, no 2
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