1,194 research outputs found
Methanol Masers as Tracers of Circumstellar Disks
We show that in many methanol maser sources the masers are located in lines,
with a velocity gradient along them which suggests that the masers are situated
in edge-on circumstellar, or protoplanetary, disks. We present VLBI
observations of the methanol maser source G309.92+0.48, in the 12.2 GHz
transition, which confirm previous observations that the masers in this source
lie along a line. We show that such sources are not only linear in space but,
in many cases, also have a linear velocity gradient. We then model these and
other data in both the 6.7 GHz and the 12.2 GHz transition from a number of
star formation regions, and show that the observed spatial and velocity
distribution of methanol masers, and the derived Keplerian masses, are
consistent with a circumstellar disk rotating around an OB star. We consider
this and other hypotheses, and conclude that about half of these methanol
masers are probably located in edge-on circumstellar disks around young stars.
This is of particular significance for studies of circumstellar disks because
of the detailed velocity information available from the masers.Comment: 38 pages, 13 figures accepted by Ap
Temporal Evolution of the Vela Pulsar's Pulse Profile
The mechanisms of emission and changes in rotation frequency ('glitching') of
the Vela pulsar (J0835-4510) are not well understood. Further insight into
these mechanisms can be achieved by long-term studies of integrated pulse
width, timing residuals, and bright pulse rates. We have undertaken an
intensive observing campaign of Vela and collected over 6000 hours of single
pulse data. The data shows that the pulse width changes with time, including
marked jumps in width after micro-glitches (frequency changes). The abundance
of bright pulses also changes after some micro-glitches, but not all. The
secular changes in pulse width have three possible cyclic periods, that match
with X-ray periodicities of a helical jet that are interpreted as free
precession.Comment: 6 pages, 8 figures. Accepted for publication in The Astrophysical
Journa
New class I methanol masers
We review properties of all known collisionally pumped (class I) methanol
maser series based on observations with the Australia Telescope Compact Array
(ATCA) and the Mopra radio telescope. Masers at 36, 84, 44 and 95 GHz are most
widespread, while 9.9, 25, 23.4 and 104 GHz masers are much rarer, tracing the
most energetic shocks. A survey of many southern masers at 36 and 44 GHz
suggests that these two transitions are highly complementary. The 23.4 GHz
maser is a new type of rare class I methanol maser, detected only in two
high-mass star-forming regions, G357.97-0.16 and G343.12-0.06, and showing a
behaviour similar to 9.9, 25 and 104 GHz masers. Interferometric positions
suggest that shocks responsible for class I masers could arise from a range of
phenomena, not merely an outflow scenario. For example, some masers might be
caused by interaction of an expanding HII region with its surrounding molecular
cloud. This has implications for evolutionary sequences incorporating class I
methanol masers if they appear more than once during the evolution of the
star-forming region. We also make predictions for candidate maser transitions
at the ALMA frequency range.Comment: 8 pages, 2 figures, to appear in proceedings for IAUS 287: Cosmic
Masers - from OH to H
High-velocity feature of the class I methanol maser in G309.38-0.13
The Australia Telescope Compact Array (ATCA) has been used to map class I
methanol masers at 36 and 44 GHz in G309.38-0.13. Maser spots are found at nine
locations in an area of 50''x30'', with both transitions reliably detected at
only two locations. The brightest spot is associated with shocked gas traced by
4.5 micron emission. The data allowed us to make a serendipitous discovery of a
high-velocity 36-GHz spectral feature, which is blue-shifted by about 30 km/s
from the peak velocity at this frequency, but spatially located close to
(within a few arcseconds of) the brightest maser spot. We interpret this as
indicating an outflow parallel to the line of sight. Such a high velocity
spread of maser features, which has not been previously reported in the class I
methanol masers associated with a single molecular cloud, suggests that the
outflow most likely interacts with a moving parcel of gas.Comment: 6 pages, 2 figures, accepted by MNRAS Letter
A Survey of the Galactic Plane for 6.7-GHz Methanol Masers I: l = 325.0 - 335.0 ; b = -0.53 - 0.53
We report the results of the first complete survey of an area of the Galactic
Plane for maser emission from the 6.7-GHz transition of methanol. The survey
covers a 10.6-square-degree region of the Galactic Plane in the longitude range
325-335 degrees and latitude range -0.53-0.53 degrees. The survey is sensitive
to masers with a peak flux density greater than approximately 2.6 Jy. The
weakest maser detected has a peak flux density of 2.3 Jy and the strongest a
peak flux density of 425 Jy. We detected a total of 50 distinct masers, 26 of
which are new detections. We show that many 6.7-GHz methanol masers are not
associated with IRAS sources, and that some are associated with sources that
have colours differing from those of a typical ultra-compact HII region
(UCHII). We estimate that the number of UCHII regions in the Galaxy is
significantly more than suggested by IRAS-based estimates, possibly by more
than a factor of two.Comment: 19 pages including 4 figures, using LaTeX formatted with mn.sty,
accepted for publication in MNRA
A bilateral shear layer between two parallel Couette flows
We consider a shear layer of a kind not previously studied to our knowledge.
Contrary to the classical free shear layer, the width of the shear zone does
not vary in the streamwise direction but rather exhibits a lateral variation.
Based on some simplifying assumptions, an analytic solution has been derived
for the new shear layer. These assumptions have been justified by a comparison
with numerical solutions of the full Navier-Stokes equations, which accord with
the analytical solution to better than 1% in the entire domain. An explicit
formula is found for the width of the shear zone as a function of wall-normal
coordinate. This width is independent of wall velocities in the laminar regime.
Preliminary results for a co-current laminar-turbulent shear layer in the same
geometry are also presented. Shear-layer instabilities were then developed and
resulted in an unsteady mixing zone at the interface between the two co-current
streams.Comment: 6 pages, 7 figures. Accepted for publication in Phys. Rev.
Methanol masers : Reliable tracers of the early stages of high-mass star formation
The GLIMPSE and MSX surveys have been used to examine the mid-infrared
properties of a statistically complete sample of 6.7 GHz methanol masers. The
GLIMPSE point sources associated with methanol masers are clearly distinguished
from the majority, typically having extremely red mid-infrared colors, similar
to those expected of low-mass class 0 young stellar objects. The intensity of
the GLIMPSE sources associated with methanol masers is typically 4 magnitudes
brighter at 8.0 micron than at 3.6 micron. Targeted searches towards GLIMPSE
point sources with [3.6]-[4.5] > 1.3 and an 8.0 micron magnitude less than 10
will detect more than 80% of class II methanol masers. Many of the methanol
masers are associated with sources within infrared dark clouds (IRDC) which are
believed to mark regions where high-mass star formation is in its very early
stages. The presence of class II methanol masers in a significant fraction of
IRDC suggests that high-mass star formation is common in these regions.
Different maser species are thought to trace different evolutionary phases of
the high-mass star formation process. Comparison of the properties of the
GLIMPSE sources associated with class II methanol masers and other maser
species shows interesting trends, consistent with class I methanol masers
tracing a generally earlier evolutionary phase and OH masers tracing a later
evolutionary phase.Comment: 45 pages, 19 figures, accepted for publication in Ap
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