17,814 research outputs found
NASA scientific and technical publications: A catalog of special publications, reference publications, conference publications, and technical papers, 1989
This catalog lists 190 citations of all NASA Special Publications, NASA Reference Publications, NASA Conference Publications, and NASA Technical Papers that were entered into the NASA scientific and technical information database during accession year 1989. The entries are grouped by subject category. Indexes of subject terms, personal authors, and NASA report numbers are provided
Dynamics of internetwork chromospheric fibrils: Basic properties and MHD kink waves
Using the spectroscopic imaging capabilities of the Swedish Solar Telescope,
we aim to provide the first investigation on the nature and dynamics of
elongated absorption features (fibrils) observed in H in the
internetwork. We observe and identify a number of internetwork fibrils, which
form away from the kilogauss, network magnetic flux, and we provide a synoptic
view on their behaviour. The internetwork fibrils are found to support
wave-like behaviour, which we interpret as Magnetohydrodynamic (MHD) kink
waves. The properties of these waves, that is, amplitude, period, and
propagation speed, are measured from time-distance diagrams and we attempt to
exploit them via magneto-seismology in order to probe the variation of plasma
properties along the wave-guides. We found that the Internetwork (IN) fibrils
appear, disappear, and re-appear on timescales of tens of minutes, suggesting
that they are subject to repeated heating. No clear photospheric footpoints for
the fibrils are found in photospheric magnetograms or H wing images.
However, we suggest that they are magnetised features as the majority of them
show evidence of supporting propagating MHD kink waves, with a modal period of
~s. Additionally, one IN fibril is seen to support a flow directed along
its elongated axis, suggesting a guiding field. The wave motions are found to
propagate at speeds significantly greater than estimates for typical
chromospheric sound speeds. Through their interpretation as kink waves, the
measured speeds provide an estimate for local average Alfv\'en speeds.
Furthermore, the amplitudes of the waves are also found to vary as a function
of distance along the fibrils, which can be interpreted as evidence of
stratification of the plasma in the neighbourhood of the IN fibril.Comment: Accepted Astronomy & Astrophysic
Deconstructing blazars: A different scheme for jet kinematics in flat-spectrum AGN
Recent VLBI studies of the morphology and kinematics of individual BL Lac
objects (S5 1803+784, PKS 0735+178, etc.) have revealed a new paradigm for the
pc-scale jet kinematics of these sources. Unlike the apparent superluminal
outward motions usually observed in blazars, most, if not all, jet components
in these sources appear to be stationary with respect to the core, while
exhibiting strong changes in their position angles. As a result, the jet ridge
lines of these sources evolve substantially, at times forming a wide
channel-flow. We investigate the Caltech-Jodrell Bank flat-spectrum (CJF)
sample of radio-loud active galaxies to study this new kinematic scenario for
flat-spectrum AGN. We develop a number of tools that extract information about
the apparent linear and angular evolution of the CJF jet ridge lines, as well
as their morphology. In this way, we study both radial and non-radial apparent
motions in the CJF jets. We find that approximately half of the sample shows
appreciable apparent jet widths (), with BL Lac jet ridge lines
showing significantly larger apparent widths than both quasars and radio
galaxies. In addition, BL Lac jet ridge lines are found to change their
apparent width more strongly. Finally, BL Lac jet ridge lines show the least
apparent linear evolution, which translates to the smallest apparent expansion
speeds for their components. We find compelling evidence supporting a
substantially different kinematic scenario for flat-spectrum radio-AGN jets and
in particular for BL Lac objects. In addition, we find that variability is
closely related to the properties of a source's jet ridge line. Variable
quasars are found to show "BL Lac like" behavior, compared to their
non-variable counterparts.Comment: 16 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Pilot KaVA monitoring on the M87 jet: confirming the inner jet structure and superluminal motions at sub-pc scales
We report the initial results of our high-cadence monitoring program on the
radio jet in the active galaxy M87, obtained by the KVN and VERA Array (KaVA)
at 22 GHz. This is a pilot study that preceded a larger KaVA-M87 monitoring
program, which is currently ongoing. The pilot monitoring was mostly performed
every two to three weeks from December 2013 to June 2014, at a recording rate
of 1 Gbps, obtaining the data for a total of 10 epochs. We successfully
obtained a sequence of good quality radio maps that revealed the rich structure
of this jet from <~1 mas to 20 mas, corresponding to physical scales
(projected) of ~0.1-2 pc (or ~140-2800 Schwarzschild radii). We detected
superluminal motions at these scales, together with a trend of gradual
acceleration. The first evidence for such fast motions and acceleration near
the jet base were obtained from recent VLBA studies at 43 GHz, and the fact
that very similar kinematics are seen at a different frequency and time with a
different instrument suggests these properties are fundamental characteristics
of this jet. This pilot program demonstrates that KaVA is a powerful VLBI array
for studying the detailed structural evolution of the M87 jet and also other
relativistic jets.Comment: 10 pages, 9 figures, accepted for publication in PAS
FY87 scientific and technical reports, articles, papers, and presentations
The document presents formal NASA technical reports, papers published in technical journals, and presentations by MSFC personnel in FY87. It also includes papers of MSFC contractors. After being announced in STAR, all of the NASA series reports may be obtained from the National Technical Information Service, 5285 Port Royal Road, Springfield, Va. 22161. The information in this report may be of value to the scientific and engineering community in determining what information has been published and what is available
Carbonates in space - The challenge of low temperature data
Carbonates have repeatedly been discussed as possible carriers of stardust
emission bands. However, the band assignments proposed so far were mainly based
on room temperature powder transmission spectra of the respective minerals.
Since very cold calcite grains have been claimed to be present in protostars
and in Planetary Nebulae such as NGC 6302, the changes of their dielectric
functions at low temperatures are relevant from an astronomical point of view.
We have derived the IR optical constants of calcite and dolomite from
reflectance spectra - measured at 300, 200, 100 and 10K - and calculated small
particle spectra for different grain shapes, with the following results: i) The
absorption efficiency factors both of calcite and dolomite are extremely
dependent on the particle shapes. This is due to the high peak values of the
optical constants of CaCO3 and CaMg[CO3]2. ii) The far infrared properties of
calcite and dolomite depend also very significantly on the temperature. Below
200K, a pronounced sharpening and increase in the band strengths of the FIR
resonances occurs. iii) In view of the intrinsic strength and sharpening of the
44 mum band of calcite at 200-100K, the absence of this band -- inferred from
Infrared Space Observatory data -- in PNe requires dust temperatures below 45K.
iv) Calcite grains at such low temperatures can account for the '92' mum band,
while our data rule out dolomite as the carrier of the 60-65 mum band. The
optical constants here presented are publicly available in the electronic
database http://www.astro.uni-jena.de/Laboratory/OCDBComment: 20 pages, 10 figures, accepted by ApJ, corrected typo
Low Power Compact Radio Galaxies at High Angular Resolution
We present sub-arcsecond resolution multi-frequency (8 and 22 GHz)
VLA images of five low power compact (LPC) radio sources, and phase
referenced VLBA images at 1.6 GHz of their nuclear regions. At the VLA
resolution we resolve the structure and identify component positions and flux
densities. The phase referenced VLBA data at 1.6 GHz reveals flat-spectrum,
compact cores (down to a few milliJansky) in four of the five sources. The
absolute astrometry provided by the phase referencing allows us to identify the
center of activity on the VLA images. Moreover, these data reveal rich
structures, including two-sided jets and secondary components. On the basis of
the arcsecond scale structures and of the nuclear properties, we rule out the
presence of strong relativistic effects in our LPCs, which must be
intrinsically small (deprojected linear sizes <~ 10 kpc). Fits of continuous
injection models reveal break frequencies in the GHz domain, and ages in the
range 10^5-10^7 yrs. In LPCs, the outermost edge may be advancing more slowly
than in more powerful sources or could even be stationary; some LPCs might also
have ceased their activity. In general, the properties of LPCs can be related
to a number of reasons, including, but not limited to: youth, frustration, low
kinematic power jets, and short-lived activity in the radio.Comment: 15 pages, 9 .eps figures, accepted by A&
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