722 research outputs found
Transverse radiation force in a tailored optical fiber
We show, by means of simple model calculations, how a weak laser beam sent
through an optical fiber exerts a transverse radiation force if there is an
azimuthal asymmetry present in the fiber such that one side has a slightly
different refractive index than the other. The refractive index difference
needs only to be very small, of order , in order to produce
an appreciable transverse displacement of order 10 microns. We argue that the
effect has probably already been seen in a recent experiment of She et al.
[Phys. Rev. Lett. 101, 243601 (2008)], and we discuss correspondence between
these observations and the theory presented. The effect could be used to bend
optical fibers in a predictable and controlled manner and we propose that it
could be useful for micron-scale devices.Comment: 4 pages, 3 figures. Accepted for publication as Rapid Communication
in Phys. Rev.
Water masers accompanying OH and methanol masers in star formation regions
The ATCA has been used to measure positions with arcsecond accuracy for 379
masers at the 22-GHz transition of water. The principal observation targets
were 202 OH masers of the variety associated with star formation regions (SFR)s
in the Southern Galactic plane. At a second epoch, most of these targets were
observed again, and new targets of methanol masers were added. Many of the
water masers reported here are new discoveries. Variability in the masers is
often acute, with very few features directly corresponding to those discovered
two decades ago. Within our current observations, less than a year apart,
spectra are often dissimilar, but positions at the later epoch, even when
measured for slightly different features, mostly correspond to the detected
maser site measured earlier, to within the typical extent of the whole site, of
a few arcseconds. The precise water positions show that approximately 79% (160
of 202) of the OH maser sites show coincident water maser emission, the best
estimate yet obtained for this statistic; however, there are many instances
where additional water sites are present offset from the OH target, and
consequently less than half of the water masers coincide with a 1665-MHz
ground-state OH maser counterpart. We explore the differences between the
velocities of peak emission from the three species (OH, methanol and water),
and quantify the typically larger deviations shown by water maser peaks from
systemic velocities. Clusters of two or three distinct but nearby sites, each
showing one or several of the principal molecular masing transitions, are found
to be common. In combination with an investigation of correlations with IR
sources from the GLIMPSE catalogue, these comparative studies allow further
progress in the use of the maser properties to assign relative evolutionary
stages in star formation to individual sites.Comment: 51 pages, 7 figure
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
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
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
On the Possibility of Measuring the Abraham Force using Whispering Gallery Modes
Critical experimental tests of the time-dependent Abraham force in
phenomenological electrodynamics are scarce. In this paper we analyze the
possibility of making use of intensity-modulated whispering gallery modes in a
microresonator for this purpose. Systems of this kind appear attractive, as the
strong concentration of electromagnetic fields near the rim of the resonator
serves to enhance the Abraham torque exerted by the field. We analyze mainly
spherical resonators, although as an introductory step we consider also the
cylinder geometry. The order of magnitude of the Abraham torques are estimated
by inserting reasonable values for the various input parameters. As expected,
the predicted torques turn out to be very small, although probably not beyond
any reach experimentally. Our main idea is essentially a generalization of the
method used by G. B. Walker et al. [Can. J. Phys. 53, 2577] for low-frequency
fields, to the optical case.Comment: 6 pages, no figures. Minor typos corrected, acknowledgment added. To
appear in Phys. Rev.
Weak and Compact Radio Emission in Early High-Mass Star Forming Regions: II. The Nature of the Radio Sources
In this study we analyze 70 radio continuum sources associated with dust
clumps and considered to be candidates for the earliest stages of high-mass
star formation. The detection of these sources was reported by Rosero et al.
(2016), who found most of them to show weak (1 mJy) and
compact (0.6) radio emission. Herein, we
used the observed parameters of these sources to investigate the origin of the
radio continuum emission. We found that at least of these radio
detections are most likely ionized jets associated with high-mass protostars,
but for the most compact sources we cannot discard the scenario that they
represent pressure-confined HII regions. This result is highly relevant for
recent theoretical models based on core accretion that predict the first stages
of ionization from high-mass stars to be in the form of jets. Additionally, we
found that properties such as the radio luminosity as a function of the
bolometric luminosity of ionized jets from low and high-mass stars are
extremely well-correlated. Our data improve upon previous studies by providing
further evidence of a common origin for jets independently of luminosity.Comment: Accepted for publication in the Ap
Weak and Compact Radio Emission in Early High-Mass Star Forming Regions: I. VLA Observations
We present a high sensitivity radio continuum survey at 6 and 1.3cm using
the Karl G. Jansky Very Large Array towards a sample of 58 high-mass star
forming regions. Our sample was chosen from dust clumps within infrared dark
clouds with and without IR sources (CMC-IRs, CMCs, respectively), and hot
molecular cores (HMCs), with no previous, or relatively weak radio continuum
detection at the mJy level. Due to the improvement in the continuum
sensitivity of the VLA, this survey achieved map rms levels of 3-10
Jy beam at sub-arcsecond angular resolution. We extracted 70
centimeter continuum sources associated with 1.2mm dust clumps. Most
sources are weak, compact, and are prime candidates for high-mass protostars.
Detection rates of radio sources associated with the mm dust clumps for CMCs,
CMC-IRs and HMCs are 6, 53 and 100, respectively. This result is
consistent with increasing high-mass star formation activity from CMCs to HMCs.
The radio sources located within HMCs and CMC-IRs occur close to the dust clump
centers with a median offset from it of 12,000AU and 4,000AU,
respectively. We calculated 5 - 25GHz spectral indices using power law fits
and obtain a median value of 0.5 (i.e., flux increasing with frequency),
suggestive of thermal emission from ionized jets. In this paper we describe the
sample, observations, and detections. The analysis and discussion will be
presented in Paper II.Comment: Accepted for publication in the ApJ
Intraday variability of AGNs in the southern hemisphere
Understanding of the spectral and polarimetric characteristics of rapidly scintillating blazars is fundamental in order to describe both the innermost (sub-pc) regions of these compact objects and the interstellar medium responsible for the scintillation. A multi frequency analysis of the intraday variability in PMN J1326-5256, based on the combination of Australia Telescope Compact Array observations with the data from the monitoring projects at the University of Tasmania, will be described. Some implications concerning the structure of compact radio cores and the properties of the interstellar medium will be discussed
6.7 GHz methanol absorption toward the Seyfert 2 galaxy NGC 3079
The detection of the 6.7 GHz line of methanol (CH3OH) is reported for the
first time toward an object beyond the Magellanic Clouds. Using the Effelsberg
100 m telescope, two absorption features were identified toward the Seyfert 2
galaxy NGC 3079. Both components probably originated on lines-of-sight toward
the central region, presumably absorbing the radio continuum of the nuclear
sources A, B, and E of NGC 3079. One absorption feature, at the systemic
velocity, is narrow and may arise from gas not related to the nuclear
environment of the galaxy. The weaker blue-shifted component is wider and may
trace outflowing gas. Total A-type CH3OH column densities are estimated to be
between a few times 10^13 and a few times 10^15 cm^-2. Because of a highly
frequency-dependent continuum background, the overall similarity of HI, OH, and
CH3OH absorption profiles hints at molecular clouds that cover the entire area
occupied by the nuclear radio continuum sources ~ 4 pc.Comment: 4 pages, 1 figure, accepted for publication in A&A Letter
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