387 research outputs found
The Rotation Of The Deep Solar Layers
From the analysis of low-order GOLF+MDI sectoral modes and LOWL data (l > 3),
we derive the solar radial rotation profile assuming no latitudinal dependance
in the solar core. These low-order acoustic modes contain the most
statistically significant information about rotation of the deepest solar
layers and should be least influenced by internal variability associated with
the solar dynamo. After correction of the sectoral splittings for their
contamination by the rotation of the higher latitudes, we obtain a flat
rotation profile down to 0.2 solar radius.Comment: accepted in ApJ Letters 5 pages, 2 figure
Evidence for polar jets as precursors of polar plume formation
Observations from the Hinode/XRT telescope and STEREO/SECCHI/EUVI are
utilized to study polar coronal jets and plumes. The study focuses on the
temporal evolution of both structures and their relationship. The data sample,
spanning April 7-8 2007, shows that over 90% of the 28 observed jet events are
associated with polar plumes. EUV images (STEREO/SECCHI) show plume haze rising
from the location of approximately 70% of the polar X-ray (Hinode/XRT) and EUV
jets, with the plume haze appearing minutes to hours after the jet was
observed. The remaining jets occurred in areas where plume material previously
existed causing a brightness enhancement of the latter after the jet event.
Short-lived, jet-like events and small transient bright points are seen (one at
a time) at different locations within the base of pre-existing long-lived
plumes. X-ray images also show instances (at least two events) of
collimated-thin jets rapidly evolving into significantly wider plume-like
structures that are followed by the delayed appearance of plume haze in the
EUV. These observations provide evidence that X-ray jets are precursors of
polar plumes, and in some cases cause brightenings of plumes. Possible
mechanisms to explain the observed jet and plume relationship are discussed.Comment: 10 pages, 4 figures, accepted as APJ Lette
A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds
We present the results of spectroscopy of four bright proplyds in the Orion
Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of
the proplyd spectra from the confusing nebular radiation, the emission line
profiles are compared with those predicted by realistic dynamic/photoionization
models of the objects. The spectral line widths show a clear correlation with
ionization potential, which is consistent with the free expansion of a
transonic, ionization-stratified, photoevaporating flow. Fitting models of such
a flow simultaneously to our spectra and HST emission line imaging provides
direct measurements of the proplyd size, ionized density and outflow velocity.
These measurements confirm that the ionization front in the proplyds is
approximately D-critical and provide the most accurate and robust estimate to
date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are
found for our spectroscopic sample, although extrapolating our results to a
larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the
proplyds as a whole. In view of the reported limits on the masses of the
circumstellar disks within the proplyds, the length of time that they can have
been exposed to ionizing radiation should not greatly exceed 10,000 years - a
factor of 30 less than the mean age of the proplyd stars. We review the various
mechanisms that have been proposed to explain this situation, and conclude that
none can plausibly work unless the disk masses are revised upwards by a
substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in
The Astronomical Journal (scheduled November 1999
MERLIN radio detection of an interaction zone within a binary Orion proplyd system
Presented here are high angular resolution MERLIN 5 GHz (6 cm) continuum
observations of the binary proplyd system, LV 1 in the Orion nebula, which
consists of proplyd 168--326SE and its binary proplyd companion 168--326NW
(separation 0.4 arcsec). Accurate astrometric alignment allows a detailed
comparison between these data and published HST PC Halpha and [Oiii] images.
Thermal radio sources coincide with the two proplyds and originate in the
ionized photoevaporating flows seen in the optical emission lines. Flow
velocities of approx 50 km/s from the ionized proplyd surfaces and \geq 100
km/s from a possible micro-jet have been detected using the Manchester Echelle
spectrometer.
A third radio source is found to coincide with a region of extended, high
excitation, optical line emission that lies between the binary proplyds
168--326SE/326NW . This is modelled as a bowshock due to the collision of the
photoevaporating flows from the two proplyds. Both a thermal and a non-thermal
origin for the radio emission in this collision zone are considered.Comment: 23 pages, 9 figures, accepted by Ap
Can Protostellar Jets Drive Supersonic Turbulence in Molecular Clouds?
Jets and outflows from young stellar objects are proposed candidates to drive
supersonic turbulence in molecular clouds. Here, we present the results from
multi-dimensional jet simulations where we investigate in detail the energy and
momentum deposition from jets into their surrounding environment and quantify
the character of the excited turbulence with velocity probability density
functions. Our study include jet--clump interaction, transient jets, and
magnetised jets. We find that collimated supersonic jets do not excite
supersonic motions far from the vicinity of the jet. Supersonic fluctuations
are damped quickly and do not spread into the parent cloud. Instead subsonic,
non-compressional modes occupy most of the excited volume. This is a generic
feature which can not be fully circumvented by overdense jets or magnetic
fields. Nevertheless, jets are able to leave strong imprints in their cloud
structure and can disrupt dense clumps. Our results question the ability of
collimated jets to sustain supersonic turbulence in molecular clouds.Comment: 33 pages, 18 figures, accepted by ApJ, version with high resolution
figures at:
http://www.ita.uni-heidelberg.de/~banerjee/publications/jet_paper.pd
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Small, Low-energy, Dispersive Solar Energetic Particle Events Observed by Parker Solar Probe
The Energetic Particle InstrumentâLow Energy (EPI-Lo) experiment has detected several weak, low-energy (~30â300 keV nucleonâ»Âč) solar energetic particle (SEP) events during its first two closest approaches to the Sun, providing a unique opportunity to explore the sources of low-energy particle acceleration. As part of the Parker Solar Probe (PSP) Integrated Science Investigation of the Sun (ISâIS) suite, EPI-Lo was designed to investigate the physics of energetic particles; however, in the special lowest-energy "time-of-flight only" product used in this study, it also responds to solar photons in a subset of approximately sunward-looking apertures lacking special light-attenuating foils. During the first three perihelia, in a frame rotating with the Sun, PSP undergoes retrograde motion, covering a 17° heliographic longitudinal range three times during the course of the ~11-day perihelion passes, permitting a unique spatial and temporal study into the location, correlation, and persistence of previously unmeasurable SEPs. We examine the signatures of these SEPs (during the first PSP perihelion pass only) and the connection to possible solar sources using remote observations from the Solar Dynamics Observatory (SDO), the Solar TErrestrial RElations Observatory (STEREO), and the ground-based Global Oscillation Network Group (GONG). The orientation of the Sun relative to STEREO, SDO, and GONG makes such identifications challenging, but we do have several candidates, including an equatorial coronal hole at a Carrington longitude of ~335°. To analyze observations from EPI-Lo, which is a new type of particle instrument, we examine instrumental effects and provide a preliminary separation of the ion signal from the photon background
The three dimensional dynamic structure of the inner Orion Nebula
The three dimensional structure of the brightest part of the Orion Nebula is
assessed in the light of published and new data. We find that the widely
accepted model of a concave blister of ionized material needs to be altered in
the southwest direction from the Trapezium, where we find that the Orion-S
feature is a separate cloud of very optically thick molecules within the body
of ionized gas, which is probably the location of the multiple embedded sources
that produce the outflows that define the Orion-S star formation region.
Evidence for this cloud comes from the presence of H2CO lines in absorption in
the radio continuum and discrepancies in the extinction derived from
radio-optical and optical only emission. We present an equilibrium Cloudy model
of the Orion-S cloud, which successfully reproduces many observed properties of
this feature. We also report the discovery of an open-sided shell of [O III]
surrounding the Trapezium stars, revealed through emission line ratio images
and the onset of radiation shadows beyond some proplyds. We show that the
observed properties of the shell are consistent with it being a stationary
structure, produced by shock interactions between the ambient nebular gas and
the high-velocity wind from theta^1 Ori C. We examine the implications of the
recently published evidence for a large blueshifted velocity of theta^1 Ori C
with respect to the Orion Molecular Cloud, which could mean that this star has
only recently begun to photoionize the Orion Nebula. We show that current
observations of the Nebula do not rule out such a possibility, so long as the
ionization front has propagated into a pre-existing low-density region. In
addition, a young age for the Nebula would help explain the presence of nearby
proplyds with a short mass-loss timescale to photoablation.Comment: AJ in press, 18 pages, 7 figures (2 in full color
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