247 research outputs found
Astrophysical constraints on the proton-to-electron mass ratio with FAST
That the laws of physics are the same at all times and places throughout the
Universe is one of the basic assumptions of physics. Astronomical observations
provide the only means to test this basic assumption on cosmological time and
distance scales. The possibility of variations in the dimensionless physical
constant {\mu} - the proton-to-electron mass ratio, can be tested by comparing
astronomical measurements of the rest frequency of certain spectral lines at
radio wavelengths with laboratory determinations. Different types of molecular
transitions have different dependencies on {\mu} and so observations of two or
more spectral lines towards the same astronomical source can be used to test
whether there is any evidence for either temporal or spatial changes in the
physical fundamental constants. {\mu} will change if the relative strength of
the strong nuclear force compared to the electromagnetic force varies.
Theoretical studies have shown that the rotational transitions of some
molecules which have transitions in the frequency range that will be covered by
FAST (e.g., CH3OH, OH and CH) are sensitive to changes in {\mu}. A number of
studies looking for possible variations in {\mu} have been undertaken with
existing telescopes, however, the greater sensitivity of FAST means it will
open new opportunities to significantly improve upon measurements made to date.
In this paper, we discuss which molecular transitions and sources (both in the
Galaxy and external galaxies) are likely targets for providing improved
constraints on {\mu} with FAST
Evolution of HII regions in hierarchically structured molecular clouds
We present observations of the H91 recombination line emission
towards a sample of nine HII regions associated with 6.7-GHz methanol masers,
and report arcsecond-scale emission around compact cores. We derive physical
parameters for our sources, and find that although simple hydrostatic models of
region evolution reproduce the observed region sizes, they significantly
underestimate emission measures. We argue that these findings are consistent
with young source ages in our sample, and can be explained by existence of
density gradients in the ionised gas.Comment: 11 pages, 6 figures; accepted for publication in MNRA
Detection of HCN maser emission in NGC253
We report the detection of maser emission from the transition of
HCN at 36.4~GHz towards the nearby starburst galaxy NGC253. This is the
first detection of maser emission from this transition in either a Galactic or
extragalactic source. The HCN maser emission has a brightness temperature
in excess of 2500 K and is offset from the center of the galaxy by
approximately 18 arcsec (300 pc), but close to a previously reported class~I
methanol maser. Both the HCN and methanol masers appear to arise near the
interface between the galactic bar and the central molecular zone, where it is
thought that molecular gas is being transported inwards, producing a region of
extensive low-velocity shocks.Comment: Accepted for publication in ApJ Letters, 7 pages, 3 figure
The first high-resolution observations of 37.7-, 38.3- and 38.5-GHz methanol masers
We have used the Australia Telescope Compact Array (ATCA) to undertake the
first high angular resolution observations of 37.7-GHz ()
methanol masers towards a sample of eleven high-mass star formation regions
which host strong 6.7-GHz methanol masers. The 37.7-GHz methanol sites are
coincident to within the astrometric uncertainty (0.4 arcseconds) with the
6.7-GHz methanol masers associated with the same star formation region.
However, spatial and spectral comparison of the 6.7- and 37.7-GHz maser
emission within individual sources shows that the 37.7-GHz masers are less
often, or to a lesser degree co-spatial than are the 12.2-GHz and 6.7-GHz
masers. We also made sensitive, high angular resolution observations of the
38.3- and 38.5-GHz class II methanol transitions ( and
, respectively) and the 36.2-GHz () class
I methanol transition towards the same sample of eleven sources. The 37.7-,
38.3- and 38.5-GHz methanol masers are unresolved in the current observations,
which implies a lower limit on the brightness temperature of the strongest
masers of more than K. We detected the 38.3-GHz methanol transition
towards 7 sources, 5 of which are new detections and detected the 38.5-GHz
transition towards 6 sources, 4 of which are new detections. We detected
36.2-GHz class I methanol masers towards all eleven sources, 6 of these are new
detections for this transition, of which 4 sources do not have previously
reported class I methanol masers from any transition.Comment: Accepted for publication in MNRAS, 34 pages, 20 figure
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