716 research outputs found
Low-luminosity Extragalactic Water Masers toward M82, M51, and NGC4051
Sub-arcsecond observations using the Very Large Array (VLA) are presented for
low-luminosity water maser in M82, M51, and NGC4051. New maser features have
been detected within the M82 starburst complex. They are largely associated
with star-forming activity, such as optically identified starburst-driven
winds, H II regions, or the early phase of star formation in the galaxy. The
water maser in M51 consists of blueshifted and redshifted features relative to
thesystemic velocity of the galaxy. The redshifted features are measured to the
northwest of the nuclear radio source, while the location of the blueshifted
counterpart is displaced by about 2" from the radio source. A small velocity
gradient closely aligned with the radio jet is detected from the redshifted
features. The redshifted maser most likely amplifies the background radio
continuum jet, while the blueshifted counterpart marks off-nuclear star
formation in the galaxy. All of the detected maser features in the narrow-line
Seyfert 1 galaxy NGC 4051 remain unresolved by new VLA observations. Due to the
low luminosity of the maser, the maser excitation is not directly related to
the active galactic nucleus.Comment: 20 pages, 10 figures. Accepted for publication in AJ (17/Oct/2006
A review of the supply of liquid propellants and other fluids in support of the Space Shuttle Program
In this study, over twenty significant liquid propellants and other fluids were reviewed as to their supply in support of the Space Shuttle Program (SSP), primarily at KSC. The uniqueness of most of the products, either by their application or production characteristics, present a variety of supply issues to contend with. Each, however, is critical to the success of the SSP. It becomes necessary to formulate, and maintain, a logistic approach to assure a continued availability of each product. For convenience, two categories were established. One, labeled limited-availability, represents those products wherein they are single sourced, have production restrictions and/or there has been a history of supply problems. The other, labeled universally-available, is characteristic of those having several sources and/or having little, if any, historical supply problems. This last category was not examined in depth. Through concepts of establishing stockpile inventories, multiple supply contracts, or other arrangements, the supply of liquid propellants and other fluids can be assured
Helium Ionization in the Diffuse Ionized Gas surrounding UCHII regions
We present measurements of the singly ionized helium to hydrogen ratio
() toward diffuse gas surrounding three Ultra-Compact HII
(UCHII ) regions: G10.15-0.34, G23.46-0.20 \& G29.96-0.02. We observe radio
recombination lines (RRLs) of hydrogen and helium near 5 GHz using the GBT to
measure the ratio. The measurements are motivated by the low
helium ionization observed in the warm ionized medium (WIM) and in the inner
Galaxy diffuse ionized regions (DIR). Our data indicate that the helium is not
uniformly ionized in the three observed sources. Helium lines are not detected
toward a few observed positions in sources G10.15-0.34 \& G23.46-0.20 and the
upper limits of the ratio obtained are 0.03 and 0.05
respectively. The selected sources harbor stars of type O6 or hotter as
indicated by helium line detection toward the bright radio continuum emission
from the sources with mean value 0.060.02. Our data
thus show that helium in diffuse gas located a few pc away from the young
massive stars embedded in the observed regions is not fully ionized.We
investigate the origin of the non-uniform helium ionization and rule out the
possibilities : (a) that the helium is doubly ionized in the observed regions
and (b) that the low values are due to additional hydrogen
ionizing radiation produced by accreting low-mass stars (Smith 2014). We find
that selective absorption of ionizing photons by dust can result in low helium
ionization but needs further investigation to develop a self-consistent model
for dust in HII regions.Comment: 43 pages, 11 figures, 5 tables accepted to Ap
Kelvin-Helmholtz Instability in a Weakly Ionized Medium
Ambient interstellar material may become entrained in outflows from massive
stars as a result of shear flow instabilities. We study the linear theory of
the Kelvin - Helmholtz instability, the simplest example of shear flow
instability, in a partially ionized medium. We model the interaction as a two
fluid system (charged and neutral) in a planar geometry. Our principal result
is that for much of the relevant parameter space, neutrals and ions are
sufficiently decoupled that the neutrals are unstable while the ions are held
in place by the magnetic field. Thus, we predict that there should be a
detectably narrower line profile in ionized species tracing the outflow
compared with neutral species since ionized species are not participating in
the turbulent interface with the ambient ISM. Since the magnetic field is
frozen to the plasma, it is not tangled by the turbulence in the boundary
layer.Comment: 21 pages, 4 figure
Interstellar H^+_3: possible detection of the 1_(10)→1_(11) transition of H_2D^+
An interstellar line has been detected in emission at the expected submillimeter wavelength of the 1_(10)→1_(11) transition of H_(2)D^+, the deuterated version of the primary ion (H^(+)_(3)) in the favored ion-molecule reaction scheme for interstellar gas phase chemistry. The strength of the line is in approximate agreement with the theoretically anticipated H_(2)D^+ abundance
VLA Observations of the Infrared Dark Cloud G19.30+0.07
We present Very Large Array observations of ammonia (NH3) (1,1), (2,2), and
CCS (2_1-1_0) emission toward the Infrared Dark Cloud (IRDC) G19.30+0.07 at
~22GHz. The NH3 emission closely follows the 8 micron extinction. The NH3 (1,1)
and (2,2) lines provide diagnostics of the temperature and density structure
within the IRDC, with typical rotation temperatures of ~10 to 20K and NH3
column densities of ~10^15 cm^-2. The estimated total mass of G19.30+0.07 is
~1130 Msun. The cloud comprises four compact NH3 clumps of mass ~30 to 160
Msun. Two coincide with 24 micron emission, indicating heating by protostars,
and show evidence of outflow in the NH3 emission. We report a water maser
associated with a third clump; the fourth clump is apparently starless. A
non-detection of 8.4GHz emission suggests that the IRDC contains no bright HII
regions, and places a limit on the spectral type of an embedded ZAMS star to
early-B or later. From the NH3 emission we find G19.30+0.07 is composed of
three distinct velocity components, or "subclouds." One velocity component
contains the two 24 micron sources and the starless clump, another contains the
clump with the water maser, while the third velocity component is diffuse, with
no significant high-density peaks. The spatial distribution of NH3 and CCS
emission from G19.30+0.07 is highly anti-correlated, with the NH3 predominantly
in the high-density clumps, and the CCS tracing lower-density envelopes around
those clumps. This spatial distribution is consistent with theories of
evolution for chemically young low-mass cores, in which CCS has not yet been
processed to other species and/or depleted in high-density regions.Comment: 29 pages, 9 figures, accepted for publication by ApJ. Please contact
the authors for higher resolution versions of the figure
Magnetic fields at the periphery of UCHII regions from carbon recombination line observations
Several indirect evidences indicate a magnetic origin for the non-thermal
width of spectral lines observed toward molecular clouds. In this letter, I
suggest that the origin of the non-thermal width of carbon recombination lines
(CRLs) observed from photo-dissociation regions (PDRs) near ultra-compact \HII\
regions is magnetic and that the magnitude of the line width is an estimate of
the \alfven speed. The magnetic field strengths estimated based on this
suggestion compare well with those measured toward molecular clouds with
densities similar to PDR densities. I conclude that multi-frequency CRL
observations have the potential to form a new tool to determine the field
strength near star forming regions.Comment: To appear in ApJ Letter
The molecular clump towards the eastern border of SNR G18.8+0.3
The eastern border of the SNR G18.8+0.3, close to an HII regions complex, is
a very interesting region to study the molecular gas that it is probably in
contact with the SNR shock front. We observed the aforementioned region using
the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO
J=3-2, HCO+ J=4-3, and CS J=7-6 lines with an angular resolution of 22". To
complement these observations, we analyzed IR, submillimeter and radio
continuum archival data. In this work, we clearly show that the radio continuum
"protrusion" that was early thought to belong to the SNR is an HII regions
complex deeply embedded in a molecular clump. The new molecular observations
reveal that this dense clump, belonging to an extended molecular cloud that
surrounds the SNR southeast border, is not physically in contact with SNR
G18.8+0.3, suggesting that the SNR shock front have not yet reached it or maybe
they are located at different distances. We found some young stellar objects
embedded in the molecular clump, suggesting that their formation should be
approximately coeval with the SN explosion.Comment: Accepted for publication in A&A (Sept. 7, 2012
SIGGMA: A Survey of Ionized Gas in the Galaxy, Made with the Arecibo Telescope
A Survey of Ionized Gas in the Galaxy, made with the Arecibo telescope
(SIGGMA) uses the Arecibo L-band Feed Array (ALFA) to fully sample the Galactic
plane (30 < l < 75 and -2 < b < 2; 175 < l < 207 and -2 < b < 1) observable
with the telescope in radio recombination lines (RRLs). Processed data sets are
being produced in the form of data cubes of 2 degree (along l) x 4 degree
(along b) x 151 (number of channels), archived and made public. The 151
channels cover a velocity range of 600 km/s and the velocity resolution of the
survey changes from 4.2 km/s to 5.1 km/s from the lowest frequency channel to
the highest frequency channel, respectively.RRL maps with 3.4 arcmin resolution
and line flux density sensitivity of 0.5 mJy will enable us to identify new HII
regions, measure their electron temperatures, study the physics of
photodissociation regions (PDRs) with carbon RRLs, and investigate the origin
of the extended low density medium (ELDM). Twelve Hn{\alpha} lines fall within
the 300 MHz bandpass of ALFA; they are resampled to a common velocity
resolution to improve the signal-to-noise ratio (SN) by a factor of 3 or more
and preserve the line width. SIGGMA will produce the most sensitive fully
sampled RRL survey to date. Here we discuss the observing and data reduction
techniques in detail. A test observation toward the HII region complex
S255/S257 has detected Hn{\alpha} and Cn{\alpha} lines with SN>10
The environment of the infrared dust bubble N65: a mutiwavelength study
AIMS: We investigate the environment of the infrared dust bubble N65 and
search for evidence of triggered star formation in its surroundings. METHODS:
We performed a multiwavelength study of the region around N65 with data taken
from large-scale surveys: Two Micron All Sky Survey, GLIMPSE, MIPSGAL, SCUBA,
and GRS. We analyzed the distribution of the molecular gas and dust in the
environment of N65 and performed infrared photometry and spectral analysis of
point sources to search for young stellar objects and identify the ionizing
star candidates. RESULTS: We found a molecular cloud that appears to be
fragmented into smaller clumps along the N65 PDR. This indicates that the
so-called collect and collapse process may be occurring. Several young stellar
objects are distributed among the molecular clumps. They may represent a second
generation of stars whose formation was triggered by the bubble expanding into
the molecular gas. We dentified O-type stars inside N65, which are the most
reliable ionizing star candidates.Comment: 15 pages, 10 figures, accepted for publication in A&A. Figures
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