1,605 research outputs found
Optical Monitoring of BL Lacertae Object S5 0716+714 with a Novel Multi-Peak Interference Filter
We at first introduce a novel photometric system, which consists of a Schmidt
telescope, an objective prism, a CCD camera, and, especially, a multi-peak
interference filter. The multi-peak interference filter enables light in multi
passbands to pass through it simultaneously. The light in different passbands
is differentially refracted by the objective prism and is focused on the CCD
separately, so we have multi "images" for each object on the CCD frames. This
system enables us to monitor blazars exactly simultaneously in multi wavebands
on a single telescope, and to accurately trace the color change during the
variation. We used this novel system to monitor the BL Lacertae object S5
0716+714 during 2006 January and February and achieved a very high temporal
resolution. The object was very bright and very active during this period. Two
strong flares were observed, with variation amplitudes of about 0.8 and 0.6
mags in the band, respectively. Strong bluer-when-brighter correlations
were found for both internight and intranight variations. No apparent time lag
was observed between the - and -band variations, and the observed
bluer-when-brighter chromatism may be mainly attributed to the larger variation
amplitude at shorter wavelength. In addition to the bluer-when-brighter trend,
the object also showed a bluer color when it was more active. The observed
variability and its color behaviors are consistent with the shock-in-jet model.Comment: 30 pages, 22 figures, accepted by A
Particle acceleration in thick parallel shocks with high compression ratio
We report studies on first-order Fermi acceleration in parallel modified
shock waves with a large scattering center compression ratio expected from
turbulence transmission models. Using a Monte Carlo technique we have modeled
particle acceleration in shocks with a velocity ranging from nonrelativistic to
ultrarelativistic and a thickness extending from nearly steplike to very wide
structures exceeding the particle diffusion length by orders of magnitude. The
nonrelativistic diffusion approximation is found to be surprisingly accurate in
predicting the spectral index of a thick shock with large compression ratio
even in the cases involving relativistic shock speeds.Comment: 4 pages, 2 figures, accepted to A&
Modelling the spectral evolution of classical double radio sources
The spectral evolution of powerful double radio galaxies (FR II's) is thought
to be determined by the acceleration of electrons at the termination shock of
the jet, their transport through the bright head region into the lobes and the
production of the radio emission by synchrotron radiation in the lobes. Models
presented to date incorporate some of these processes in prescribing the
electron distribution which enters the lobes. We have extended these models to
include a description of electron acceleration at the relativistic termination
shock and a selection of transport models for the head region. These are
coupled to the evolution of the electron spectrum in the lobes under the
influence of losses due to adiabatic expansion, by inverse Compton scattering
on the cosmic background radiation and by synchrotron radiation. The
evolutionary tracks predicted by this model are compared to observation using
the power/source-size (P-D) diagram. We find that the simplest scenario, in
which accelerated particles suffer adiabatic losses in the head region which
become more severe as the source expands produces P-D-tracks which conflict
with observation, because the power is predicted to decline too steeply with
increasing size. Agreement with observation can be found by assuming that
adiabatic losses are compensated during transport between the termination shock
and the lobe by a re-acceleration process distributed throughout the head
region.Comment: 14 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Turbulence transmission in parallel modified shocks using ray tracing
We apply a semi-classical approach of handling waves as quasiparticle gas in
a slowly varying flow -- analogous to ray tracing -- to calculate the Alfven
wave transmission parameters, the resulting cross-helicity of the waves and the
scattering-centre compression ratio, for cases where the shock thickness is
large enough for the turbulent waves in the plasma to see the transition of the
background flow parameters as smooth and slowly varying. For nonrelativistic
shocks the wave transmission produces similar effects on the downstream
turbulence and the scattering-centre compression ratio as does the transmission
through a step shock: the downstream Alfven waves propagate predominantly
towards the shock in the local plasma frame and, thus, the scattering-centre
compression ratio is larger than the gas compression ratio. For thick
relativistic shocks, however, we find qualitative differences with respect to
the step-shock case: for low-Alfvenic-Mach-number shocks the downstream waves
propagate predominantly away from the shock, and the scattering-centre
compression ratio is lower than that of the gas. Thus, when taken into account,
the Alfven wave transmission can decrease the efficiency of the first-order
Fermi acceleration in a thick relativistic shock.Comment: 6 pages, 5 figures, accepted to A&
A Herschel study of the properties of starless cores in the Polaris Flare dark cloud region using PACS and SPIRE
The Polaris Flare cloud region contains a great deal of extended emission. It
is at high declination and high Galactic latitude. It was previously seen
strongly in IRAS Cirrus emission at 100 microns. We have detected it with both
PACS and SPIRE on Herschel. We see filamentary and low-level structure. We
identify the five densest cores within this structure. We present the results
of a temperature, mass and density analysis of these cores. We compare their
observed masses to their virial masses, and see that in all cases the observed
masses lie close to the lower end of the range of estimated virial masses.
Therefore, we cannot say whether they are gravitationally bound prestellar
cores. Nevertheless, these are the best candidates to be potentialprestellar
cores in the Polaris cloud region.Comment: 5 pages, 2 figures, accepted by A&
Far-infrared observations of a massive cluster forming in the Monoceros R2 filament hub
We present far-infrared observations of Monoceros R2 (a giant molecular cloud at approximately 830 pc distance, containing several sites of active star formation), as observed at 70 μm, 160 μm, 250 μm, 350 μm, and 500 μm by the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on the Herschel Space Observatory as part of the Herschel imaging survey of OB young stellar objects (HOBYS) Key programme. The Herschel data are complemented by SCUBA-2 data in the submillimetre range, and WISE and Spitzer data in the mid-infrared. In addition, C18O data from the IRAM 30-m Telescope are presented, and used for kinematic information. Sources were extracted from the maps with getsources, and from the fluxes measured, spectral energy distributions were constructed, allowing measurements of source mass and dust temperature. Of 177 Herschel sources robustly detected in the region (a detection with high signal-to-noise and low axis ratio at multiple wavelengths), including protostars and starless cores, 29 are found in a filamentary hub at the centre of the region (a little over 1% of the observed area). These objects are on average smaller, more massive, and more luminous than those in the surrounding regions (which together suggest that they are at a later stage of evolution), a result that cannot be explained entirely by selection effects. These results suggest a picture in which the hub may have begun star formation at a point significantly earlier than the outer regions, possibly forming as a result of feedback from earlier star formation. Furthermore, the hub may be sustaining its star formation by accreting material from the surrounding filaments
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Orthographic facilitation in oral vocabulary acquisition
An experiment investigated whether exposure to orthography facilitates oral vocabulary learning. A total of 58 typically developing children aged 8-9 years were taught 12 nonwords. Children were trained to associate novel phonological forms with pictures of novel objects. Pictures were used as referents to represent novel word meanings. For half of the nonwords children were additionally exposed to orthography, although they were not alerted to its presence, nor were they instructed to use it. After this training phase a nonword-picture matching posttest was used to assess learning of nonword meaning, and a spelling posttest was used to assess learning of nonword orthography. Children showed robust learning for novel spelling patterns after incidental exposure to orthography. Further, we observed stronger learning for nonword-referent pairings trained with orthography. The degree of orthographic facilitation observed in posttests was related to children's reading levels, with more advanced readers showing more benefit from the presence of orthography
Herschel Observations of a Potential Core Forming Clump: Perseus B1-E
We present continuum observations of the Perseus B1-E region from the
Herschel Gould Belt Survey. These Herschel data reveal a loose grouping of
substructures at 160 - 500 micron not seen in previous submillimetre
observations. We measure temperature and column density from these data and
select the nine densest and coolest substructures for follow-up spectral line
observations with the Green Bank Telescope. We find that the B1-E clump has a
mass of ~ 100 solar masses and appears to be gravitationally bound.
Furthermore, of the nine substructures examined here, one substructure (B1-E2)
appears to be itself bound. The substructures are typically less than a Jeans
length from their nearest neighbour and thus, may interact on a timescale of ~
1 Myr. We propose that B1-E may be forming a first generation of dense cores,
which could provide important constraints on the initial conditions of
prestellar core formation. Our results suggest that B1-E may be influenced by a
strong, localized magnetic field, but further observations are still required.Comment: 14 pages, 8 figures, published in A&A: Minor calibration correctio
The Herschel view of the on-going star formation in the Vela-C molecular cloud
As part of the Herschel guaranteed time key program 'HOBYS', we present the
photometric survey of the star forming region Vela-C, one of the nearest sites
of low-to-high-mass star formation in the Galactic plane. Vela-C has been
observed with PACS and SPIRE in parallel mode between 70 um and 500 um over an
area of about 3 square degrees. A photometric catalogue has been extracted from
the detections in each band, using a threshold of 5 sigma over the local
background. Out of this catalogue we have selected a robust sub-sample of 268
sources, of which 75% are cloud clumps and 25% are cores. Their Spectral Energy
Distributions (SEDs) have been fitted with a modified black body function. We
classify 48 sources as protostellar and 218 as starless. For two further
sources, we do not provide a secure classification, but suggest they are Class
0 protostars.
From SED fitting we have derived key physical parameters. Protostellar
sources are in general warmer and more compact than starless sources. Both
these evidences can be ascribed to the presence of an internal source(s) of
moderate heating, which also causes a temperature gradient and hence a more
peaked intensity distribution. Moreover, the reduced dimensions of protostellar
sources may indicate that they will not fragment further. A virial analysis of
the starless sources gives an upper limit of 90% for the sources
gravitationally bound and therefore prestellar. We fit a power law N(logM) prop
M^-1.1 to the linear portion of the mass distribution of prestellar sources.
This is in between that typical of CO clumps and those of cores in nearby
star-forming regions. We interpret this as a result of the inhomogeneity of our
sample, which is composed of comparable fractions of clumps and cores.Comment: 9 pages, 7 figures, accepted by A&
The Pipe Nebula as seen with Herschel: Formation of filamentary structures by large-scale compression ?
A growing body of evidence indicates that the formation of filaments in
interstellar clouds is a key component of the star formation process. In this
paper, we present new Herschel PACS and SPIRE observations of the B59 and Stem
regions in the Pipe Nebula complex, revealing a rich, organized network of
filaments. The asymmetric column density profiles observed for several
filaments, along with the bow-like edge of B59, indicates that the Pipe Nebula
is being compressed from its western side, most likely by the winds from the
nearby Sco OB2 association. We suggest that this compressive flow has
contributed to the formation of some of the observed filamentary structures. In
B59, the only region of the entire Pipe complex showing star formation
activity, the same compressive flow has likely enhanced the initial column
density of the clump, allowing it to become globally gravitationally unstable.
Although more speculative, we propose that gravity has also been responsible
for shaping the converging filamentary pattern observed in B59. While the
question of the relative impact of large-scale compression and gravity remains
open in B59, large-scale compression appears to be a plausible mechanism for
the initial formation of filamentary structures in the rest of the complexComment: 9 pages, 9 figures, accepted for publication in A&
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