23,920 research outputs found
Comets
Vacuum ultraviolet observations from sounding rockets and satellite observatories of the gaseous comae of several comets are reviewed. The earliest of these led to discovery of the hydrogen envelope extending for millions of km from the nucleus. Subsequent observations of H I Lyman alpha, the OH (0,0 band and the oxygen resonance triplet provided strong evidence for the water-ice model of the cometary nucleus. Several species were discovered in the coma including C, C(+), CO, S, and CS. High resolution spectroscopy and the spatial variation of the observed emissions provide means to elucidate the production and excitation mechanisms of these species. The similarity of the spectra of the half dozen comets observed to date argues for a common, homogeneous composition (with the exception of dust and CO) of the cometary ice and a minimal effect on the neutral species due to molecular collisions in the inner coma
New Measurements of the Radio Photosphere of Mira based on Data from the JVLA and ALMA
We present new measurements of the millimeter wavelength continuum emission
from the long period variable Mira ( Ceti) at frequencies of 46 GHz, 96 GHz,
and 229 GHz (~7 mm, 3 mm, and 1 mm) based on observations obtained
with the Jansky Very Large Array (JVLA) and the Atacama Large
Millimeter/submillimeter Array (ALMA). The measured millimeter flux densities
are consistent with a radio photosphere model derived from previous
observations, where flux density, . The stellar disk
is resolved, and the measurements indicate a decrease in the size of the radio
photosphere at higher frequencies, as expected if the opacity decreases at
shorter wavelengths. The shape of the radio photosphere is found to be slightly
elongated, with a flattening of ~10-20%. The data also reveal evidence for
brightness non-uniformities on the surface of Mira at radio wavelengths. Mira's
hot companion, Mira B was detected at all three observed wavelengths, and we
measure a radius for its radio-emitting surface of
cm. The data presented here highlight the power of the JVLA and ALMA for the
study of the atmospheres of evolved stars.Comment: Accepted to ApJ; 27 pages, 7 figure
STOP - A computer program for supersonic transport trajectory optimization
IBM 7094 digital program using steepest ascent technique for optimizing flight path of supersonic transport aircraf
Spectroscopic signatures related to a sunquake
© 2015. The American Astronomical Society. All rights reserved.. The presence of flare-related acoustic emission (sunquakes (SQs)) in some flares, and only in specific locations within the flaring environment, represents a severe challenge to our current understanding of flare energy transport processes. In an attempt to contribute to understanding the origins of SQs we present a comparison of new spectral observations from Hinode's EUV imaging Spectrometer (EIS) and the Interface Region Imaging Spectrograph (IRIS) of the chromosphere, transition region, and corona above an SQ, and compare them to the spectra observed in a part of the flaring region with no acoustic signature. Evidence for the SQ is determined using both time-distance and acoustic holography methods, and we find that unlike many previous SQ detections, the signal is rather dispersed, but that the time-distance and 6 and 7 mHz sources converge at the same spatial location. We also see some evidence for different evolution at different frequencies, with an earlier peak at 7 mHz than at 6 mHz. Using EIS and IRIS spectroscopic measurements we find that in this location, at the time of the 7 mHz peak the spectral emission is significantly more intense, shows larger velocity shifts and substantially broader profiles than in the location with no SQ, and there is a good correlation between blueshifted, hot coronal, hard X-ray (HXR), and redshifted chromospheric emission, consistent with the idea of a strong downward motion driven by rapid heating by nonthermal electrons and the formation of chromospheric shocks. Exploiting the diagnostic potential of the Mg ii triplet lines, we also find evidence for a single large temperature increase deep in the atmosphere, which is consistent with this scenario. The time of the 6 mHz and time-distance peak signal coincides with a secondary peak in the energy release process, but in this case we find no evidence of HXR emission in the quake location, instead finding very broad spectral lines, strongly shifted to the red, indicating the possible presence of a significant flux of downward propagating Alfvén waves
Unveiling Sources of Heating in the Vicinity of the Orion BN/KL Hot Core as Traced by Highly Excited Inversion Transitions of Ammonia
Using the Expanded Very Large Array, we have mapped the vicinity of the Orion
BN/KL Hot Core with sub-arcsecond angular resolution in seven metastable
inversion transitions of ammonia: (J,K)=(6,6) to (12,12). This emission comes
from levels up to 1500 K above the ground state, enabling identification of
source(s) responsible for heating the region. We used this multi-transition
dataset to produce images of the rotational/kinetic temperature and the column
density of ammonia for ortho and para species separately and on a
position-by-position basis. We find rotational temperature and column density
in the range 160-490 K and (1-4)x10^17 cm^-2, respectively. Our
spatially-resolved images show that the highest (column) density and hottest
gas is found in a northeast-southwest elongated ridge to the southeast of
Source I. We have also measured the ortho-para ratio of ammonia, estimated to
vary in the range 0.9-1.6. Enhancement of ortho with respect to para and the
offset of hot ammonia emission peaks from known (proto)stellar sources provide
evidence that the ammonia molecules have been released from dust grains into
the gas-phase through the passage of shocks and not by stellar radiation. We
propose that the combined effect of Source I's proper motion and its
low-velocity outflow impinging on a pre-existing dense medium is responsible
for the excitation of ammonia and the Orion Hot Core. Finally, we found for the
first time evidence of a slow (5 km/s) and compact (1000 AU) outflow towards
IRc7.Comment: To appear in Astrophysical Journal Letters Special Issue on the EVLA.
8 pages, 4 figure
An HI Imaging Survey of Asymptotic Giant Branch Stars
We present an imaging study of a sample of eight asymptotic giant branch
(AGB) stars in the HI 21-cm line. Using observations from the Very Large Array,
we have unambiguously detected HI emission associated with the extended
circumstellar envelopes of six of the targets. The detected HI masses range
from M_HI ~ 0.015-0.055 M_sun. The HI morphologies and kinematics are diverse,
but in all cases appear to be significantly influenced by the interaction
between the circumstellar envelope and the surrounding medium. Four stars (RX
Lep, Y UMa, Y CVn, and V1942 Sgr) are surrounded by detached HI shells ranging
from 0.36 to 0.76 pc across. We interpret these shells as resulting from
material entrained in a stellar outflow being abruptly slowed at a termination
shock where it meets the local medium. RX Lep and TX Psc, two stars with
moderately high space velocities (V_space>56 km/s), exhibit extended gaseous
wakes (~0.3 and 0.6 pc in the plane of the sky), trailing their motion through
space. The other detected star, R Peg, displays a peculiar "horseshoe-shaped"
HI morphology with emission extended on scales up to ~1.7 pc; in this case, the
circumstellar debris may have been distorted by transverse flows in the local
interstellar medium. We briefly discuss our new results in the context of the
entire sample of evolved stars that has been imaged in HI to date.Comment: Accepted to AJ. A version with full resolution figures is available
at http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_HI_survey.pd
A Documentary of High-Mass Star Formation: Probing the Dynamical Evolution of Orion Source I on 10-100 AU Scales using SiO Masers
A comprehensive picture of high-mass star formation has remained elusive, in
part because examples of high-mass YSOs tend to be relatively distant, deeply
embedded, and confused with other emission sources. These factors have impeded
dynamical investigations within tens of AU of high-mass YSOs--scales that are
critical for probing the interfaces where outflows from accretion disks are
launched and collimated. Using observations of SiO masers obtained with the VLA
and the VLBA, the KaLYPSO project is overcoming these limitations by mapping
the structure and dynamical/temporal evolution of the material 10-1000 AU from
the nearest high-mass YSO: Radio Source I in the Orion BN/KL region. Our data
include ~40 epochs of VLBA observations over a several-year period, allowing us
to track the proper motions of individual SiO maser spots and to monitor
changes in the physical conditions of the emitting material with time.
Ultimately these data will provide 3-D maps of the outflow structure over
approximately 30% of the outflow crossing time. Here we summarize recent
results from the KaLYPSO project, including evidence that high-mass star
formation is occurring via disk-mediated accretion.Comment: 5 pages; to appear in the proceedings of IAU Symposium 242,
Astrophysical Masers and their Environments, ed. J. Chapman & W. Baa
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