487 research outputs found
VLBA imaging of a periodic 12.2 GHz methanol maser flare in G9.62+0.20E
The class II methanol maser source G9.62+0.20E undergoes periodic flares at
both 6.7 and 12.2 GHz. The flare starting in 2001 October was observed at seven
epochs over three months using the VLBA at 12.2 GHz. High angular resolution
images (beam size 1.7 x 0.6 mas) were obtained, enabling us to observe
changes in 16 individual maser components. It was found that while existing
maser spots increased in flux density, no new spots developed and no changes in
morphology were observed. This rules out any mechanism which disturbs the
masing region itself, implying that the flares are caused by a change in either
the seed or pump photon levels. A time delay of 1--2 weeks was observed between
groups of maser features. These delays can be explained by light travel time
between maser groups. The regularity of the flares can possibly be explained by
a binary system.Comment: 11 pages, accepted for publication in MNRA
Periodic class II methanol masers in G9.62+0.20E
We present the light curves of the 6.7 and 12.2 GHz methanol masers in the
star forming region G9.62+0.20E for a time span of more than 2600 days. The
earlier reported period of 244 days is confirmed. The results of monitoring the
107 GHz methanol maser for two flares are also presented. The results show that
flaring occurs in all three masing transitions. It is shown that the average
flare profiles of the three masing transitions are similar. The 12.2 GHz masers
are the most variable of the three masers with the largest relative amplitude
having a value of 2.4. The flux densities for the different masing transitions
are found to return to the same level during the low phase of the masers,
suggesting that the source of the periodic flaring is situated outside the
masing region, and that the physical conditions in the masing region are
relatively stable. On the basis of the shape of the light curve we excluded
stellar pulsations as the underlying mechanism for the periodicity. It is
argued that a colliding wind binary can account for the observed periodicity
and provide a mechanism to qualitatively explain periodicity in the seed photon
flux and/or the pumping radiation field. It is also argued that the dust
cooling time is too short to explain the decay time of about 100 days of the
maser flare. A further analysis has shown that for the intervals from days 48
to 66 and from days 67 to 135 the decay of the maser light curve can be
interpreted as due to the recombination of a thermal hydrogen plasma with
densities of approximately and respectively.Comment: 11 pages, 9 figuer
KAT-7 Science Verification: Using HI Observations of NGC 3109 to Understand its Kinematics and Mass Distribution
HI observations of the Magellanic-type spiral NGC 3109, obtained with the
seven dish Karoo Array Telescope (KAT-7), are used to analyze its mass
distribution. Our results are compared to what is obtained using VLA data.
KAT-7 is the precursor of the SKA pathfinder MeerKAT, which is under
construction. The short baselines and low system temperature of the telescope
make it sensitive to large scale low surface brightness emission. The new
observations with KAT-7 allow the measurement of the rotation curve of NGC 3109
out to 32', doubling the angular extent of existing measurements. A total HI
mass of 4.6 x 10^8 Msol is derived, 40% more than what was detected by the VLA
observations.
The observationally motivated pseudo-isothermal dark matter (DM) halo model
can reproduce very well the observed rotation curve but the cosmologically
motivated NFW DM model gives a much poorer fit to the data. While having a more
accurate gas distribution has reduced the discrepancy between the observed RC
and the MOdified Newtonian Dynamics (MOND) models, this is done at the expense
of having to use unrealistic mass-to-light ratios for the stellar disk and/or
very large values for the MOND universal constant a0. Different distances or HI
contents cannot reconcile MOND with the observed kinematics, in view of the
small errors on those two quantities. As for many slowly rotating gas-rich
galaxies studied recently, the present result for NGC 3109 continues to pose a
serious challenge to the MOND theory.Comment: 25 pages, 20 figures, accepted for publication in Astronomical
Journa
Evaluatie Opvangbeleid 2005-2008 overwinterende ganzen en smienten. Deelrapport 10. Hebben overwinterende ganzen invloed op de weidevogelstand?
Dit rapport richt zich op de vraag of de sterk toegenomen dichtheden ganzen in bepaalde gebieden mede verantwoordelijk kunnen zijn voor de achteruitgang van de weidevogels ter plekke. Gebruik makend van data verzameld in de periode 1990-2005 in het kader van het weidevogelmeetnet en de watervogeltellingen in ganzentelgebieden werd onderzocht in hoeverre gebieden met hoge dichtheden ganzen overlappen met gebieden met hoge dichtheden weidevogels. De effecten van hoge dichtheden overwinterende ganzen op in Nederland broedende weidevogels lijken verwaarloosbaar of positief. Resultaten van onderzoek van SOVON en Alterr
New Periodic 6.7 GHz Class II Methanol Maser Associated with G358.460-0.391
Eight new class II methanol masers selected from the 6.7 GHz Methanol
Multibeam survey catalogues I and II were monitored at 6.7 GHz with the 26m
Hartebeesthoek Radio Astronomy Observatory (HartRAO) radio telescope for three
years and seven months, from February 2011 to September 2014. The sources were
also observed at 12.2 GHz and two were sufficiently bright to permit
monitoring. One of the eight sources, namely G358.460-0.391, was found to show
periodic variations at 6.7 GHz. The period was determined and tested for
significance using the Lomb-Scargle, epoch-folding and Jurkevich methods, and
by fitting a simple analytic function. The best estimate for the period of the
6.7 GHz class II methanol maser line associated with G358.460-0.391 is 220.0
0.2 day.Comment: 8 pages, 11 figures, accepted for publication in MNRAS. The appendix
of 4 pages (with 16 figures) will be published as online versio
A Strong Jet Signature in the Late-Time Lightcurve of GW170817
We present new 0.6-10 GHz observations of the binary neutron star merger
GW170817 covering the period up to 300 days post-merger, taken with the Karl G.
Jansky Very Large Array, the Australia Telescope Compact Array, the Giant
Metrewave Radio Telescope and the MeerKAT telescope. We use these data to
precisely characterize the decay phase of the late-time radio light curve. We
find that the temporal decay is consistent with a power-law slope of t^-2.2,
and that the transition between the power-law rise and decay is relatively
sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated)
outflow, but is instead the classic signature of a relativistic jet. This
provides strong observational evidence that GW170817 produced a successful jet,
and directly demonstrates the link between binary neutron star mergers and
short-hard GRBs. Using simple analytical arguments, we derive constraints on
the geometry and the jet opening angle of GW170817. These results are
consistent with those from our companion Very Long Baseline Interferometry
(VLBI) paper, reporting superluminal motion in GW170817.Comment: 11 pages, 3 figures, 3 tables. Accepted for publication in ApJ
Letter
On the methanol masers in G9.62+0.20E and G188.95+0.89
A comparison between the observed light curves of periodic masers in
G9.62+0.20E and G188.95+0.89 and the results of a simple colliding-wind binary
model is made to establish whether the flaring and other time-dependent
behaviour of the masers in these two star forming regions can be ascribed to
changes in the environment of the masers or in the continuum emission from
parts of the background \ion{H}{2} region. It is found that the light curves of
widely different shape and amplitude in these two objects can be explained
within the framework of a periodic pulse of ionizing radiation that raises the
electron density in a volume of partially ionized gas against which the masers
are projected. It is also shown that the decay of the 11.405 \kmps maser in
G188.95+0.89 can be explained very well in terms of the recombination of the
ionized gas against which the maser is projected while it would require very
special conditions to explain it in terms of changes in environment of the
maser. We conclude that for G9.62+0.20E and G188.95+0.89 the observed changes
in the masers are most likely due to changes in the background free-free
emission which is amplified by the masers.Comment: 12 pages, 4 figure
How do methanol masers manage to appear in the youngest star vicinities and isolated molecular clumps?
General characteristics of methanol (CH3OH) maser emission are summarized. It
is shown that methanol maser sources are concentrated in the spiral arms. Most
of the methanol maser sources from the Perseus arm are associated with embedded
stellar clusters and a considerable portion is situated close to compact HII
regions. Almost 1/3 of the Perseus Arm sources lie at the edges of optically
identified HII regions which means that massive star formation in the Perseus
Arm is to a great extent triggered by local phenomena. A multiline analysis of
the methanol masers allows us to determine the physical parameters in the
regions of maser formation. Maser modelling shows that class II methanol masers
can be pumped by the radiation of the warm dust as well as by free-free
emission of a hypercompact region hcHII with a turnover frequency exceeding 100
GHz. Methanol masers of both classes can reside in the vicinity of hcHIIs.
Modelling shows that periodic changes of maser fluxes can be reproduced by
variations of the dust temperature by a few percent which may be caused by
variations in the brightness of the central young stellar object reflecting the
character of the accretion process. Sensitive observations have shown that the
masers with low flux densities can still have considerable amplification
factors. The analysis of class I maser surveys allows us to identify four
distinct regimes that differ by the series of their brightest lines.Comment: 8 pages, 4 figures, invited presentation at IAU242 "Astrophysical
Masers and their environments
A European tool for usual intake distribution estimation in relation to data collection by EFSA
The present document has been produced and adopted by the bodies identified above as author(s). In accordance with Article 36 of Regulation (EC) No 178/2002, this task has been carried out exclusively by the author(s) in the context of a grant agreement between the European Food Safety Authority and the author(s). The present document is published complying with the transparency principle to which the Authority is subject. It cannot be considered as an output adopted by the Authority. The European Food Safety Authority reserves its rights, view and position as regards the issues addressed and the conclusions reached in the present document, without prejudice to the rights of the authors
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