92 research outputs found
Strain field in straight cylindrical shells due to applied forces on an attached shell. Part I: No hole in the intersection region
The results of an experimental stress analysis of the intersection region of two straight cylindrical shells are presented. Two models were used; in the first model the two axes were inclined at 30 degrees while for the second case, this angle was 60 degrees. In each case, the main shell was 6.625 in. in diameter and 0.198 in. thick, while the attached shell was 3.5 in. in diameter and 0.226 in. thick. The intersection region was subjected to in-plane and out-plane moments applied to the attached shell and the measurements were made using foil resistance rosette gauges. These measurements demonstrate that the local stress concentration in the intersection region of the main shell increases with the increase of the acute angle between the axes of the two shells; thus for a given moment loading on the attached shell, the stress concentration will be the largest when the two axes are normal to each other.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
Laboratory and tentative interstellar detection of trans-methyl formate using the publicly available Green Bank Telescope PRIMOS survey
The rotational spectrum of the higher-energy trans conformational isomer of
methyl formate has been assigned for the first time using several pulsed-jet
Fourier transform microwave spectrometers in the 6-60 GHz frequency range. This
species has also been sought toward the Sagittarius B2(N) molecular cloud using
the publicly available PRIMOS survey from the Green Bank Telescope. We detect
seven absorption features in the survey that coincide with laboratory
transitions of trans-methyl formate, from which we derive a column density of
3.1 (+2.6, -1.2) \times 10^13 cm-2 and a rotational temperature of 7.6 \pm 1.5
K. This excitation temperature is significantly lower than that of the more
stable cis conformer in the same source but is consistent with that of other
complex molecular species recently detected in Sgr B2(N). The difference in the
rotational temperatures of the two conformers suggests that they have different
spatial distributions in this source. As the abundance of trans-methyl formate
is far higher than would be expected if the cis and trans conformers are in
thermodynamic equilibrium, processes that could preferentially form
trans-methyl formate in this region are discussed. We also discuss measurements
that could be performed to make this detection more certain. This manuscript
demonstrates how publicly available broadband radio astronomical surveys of
chemically rich molecular clouds can be used in conjunction with laboratory
rotational spectroscopy to search for new molecules in the interstellar medium.Comment: 40 pages, 7 figures, 4 tables; accepted for publication in Ap
37 GHz methanol masers : Horsemen of the Apocalypse for the class II methanol maser phase?
We report the results of a search for class II methanol masers at 37.7, 38.3
and 38.5 GHz towards a sample of 70 high-mass star formation regions. We
primarily searched towards regions known to show emission either from the 107
GHz class II methanol maser transition, or from the 6.035 GHz excited OH
transition. We detected maser emission from 13 sources in the 37.7 GHz
transition, eight of these being new detections. We detected maser emission
from three sources in the 38 GHz transitions, one of which is a new detection.
We find that 37.7 GHz methanol masers are only associated with the most
luminous 6.7 and 12.2 GHz methanol maser sources, which in turn are
hypothesised to be the oldest class II methanol sources. We suggest that the
37.7 GHz methanol masers are associated with a brief evolutionary phase (of
1000-4000 years) prior to the cessation of class II methanol maser activity in
the associated high-mass star formation region.Comment: 14 pages, 4 figures, accepted for publication in Ap
Spectral imaging of the Sagittarius B2 region in multiple 3-mm molecular lines with the Mopra telescope
Using the Mopra telescope, we have undertaken a 3-mm spectral-line imaging
survey of a 5 x 5 arcmin^2 area around Sgr B2. We covered almost the complete
spectral the range from 81.7 to 113.5 GHz, with 2.2 MHz wide spectral channels
or ~ 6 km/s, and have observed 24 lines, with 0.033 MHz wide, or ~ 0.1 km/s
channels. We discuss the distribution of around 50 lines, and present
velocity-integrated emission images for 38 of the lines. In addition, we have
detected around 120 more lines, mostly concentrated at the particularly
spectral line-rich Sgr B2(N) source. There are significant differences in
molecular emission, pointing to both abundance and excitation differences
throughout the region. Seven distinct spatial locations are identified for the
emitting species, including peaks near the prominent star forming cores of Sgr
B2(N), (M) and (S) that are seen in IR-to-radio continuum images. The other
features are a 'North Ridge' and a 'North Cloud' to the north of the Sgr B2
N-M-S cores, a 'South-East Peak' and a 'West Ridge'. The column density, as
evident through C^{18}O, peaks at the Sgr B2(N) and (M) cores, where strong
absorption is also evident in otherwise generally bright lines such as HCO^{+},
HCN and HNC. Most molecules trace a ridge line to the west of the Sgr B2 N-M-S
cores, wrapping around the cores and extending NE to the North Cloud. This is
most clearly evident in the species HC_{3}N, CH_{3}CN, CH_{3}OH and OCS. They
are found to be closer in distribution to the cooler dust traced by the sub-mm
continuum than either the warmer dust seen in the mid-IR or to the radio
continuum. The molecule CN, in contrast, is reasonably uniform over the entire
region mapped, aside from strong absorption at the positions of the Sgr B2(N)
and (M) cores.Comment: 22 pages, 12 figures, 5 tables. MNRAS in press. Version 2 with small
changes after referee's comment
New 9.9-GHz methanol masers
The Australia Telescope Compact Array (ATCA) has been used to make the first
extensive search for the class I methanol masers at 9.9 GHz. In total, 48
regions of high-mass star formation were observed. In addition to masers in
W33-Met (G12.80-0.19) and G343.12-0.06 (IRAS 16547-4247) which have already
been reported in the literature, two new 9.9-GHz masers have been found towards
G331.13-0.24 and G19.61-0.23. We have determined absolute positions (accurate
to roughly a second of arc) for all the detected masers and suggest that some
class I masers may be associated with shocks driven into molecular clouds by
expanding HII regions. Our observations also imply that the evolutionary stage
of a high-mass star forming region when the class I masers are present can
outlast the stage when the class II masers at 6.7 GHz are detectable, and
overlaps significantly with the stage when OH masers are active.Comment: accepted for publication in MNRAS, 14 pages, 3 figures, 4 table
New Molecular Species In Comet C/1995 (Hale-Bopp) Observed with the Caltech Submillimeter Observatory
We present millimeter-wave observations of HNCO, HC3N, SO, NH2CHO, H(13)CN, and H3O(+) in comet C/1995 O1 (Hale-Bopp) obtained in February-April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996 B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H2O is (4-13) x 10(exp -4). HC3N, SO, and NH2CHO are detected for the first time in a comet. The fractional abundance of HC3N based on observations of three rotational lines is (1.9 +/- 0.2) x 10(exp -4). Four transitions of SO are detected and the derived fractional abundance, (2-8) x 10(exp -3), is higher than the upper limits derived from UV observations of previous comets. Observations of NH2CHO imply a fractional abundance of (1-8) x 10(exp -4). H3O(+) is detected for the first time from the ground. The H(13)CN (3-2) transition is also detected and the derived HCN/H(13)CN abundance ratio is 90 +/- 15, consistent with the terrestrial C-13/C-12 ratio. in addition, a number of other molecular species are detected, including HNC, OCS, HCO(+), CO(+), and CN (the last two are first detections in a comet at radio wavelengths)
Oxygen isotopic ratios in galactic clouds along the line of sight towards Sagittarius B2
As an independent check on previous measurements of the isotopic abundance of
oxygen through the Galaxy, we present a detailed analysis of the ground state
rotational lines of 16OH and 18OH in absorption towards the giant molecular
cloud complex, Sagittarius B2. We have modelled the line shapes to separate the
contribution of several galactic clouds along the line of sight and calculate
16OH/18OH ratios for each of these features. The best fitting values are in the
range 320-540, consistent with the previous measurements in the Galactic Disk
but slightly higher than the standard ratio in the Galactic Centre. They do not
show clear evidence for a gradient in the isotopic ratio with galactocentric
distance. The individual 16OH column densities relative to water give ratios of
[H2O/OH]=0.6-1.2, similar in magnitude to galactic clouds in the sight lines
towards W51 and W49. A comparison with CH indicates [OH/CH] ratios higher than
has been previously observed in diffuse clouds. We estimate OH abundances of
10^-7 - 10^-6 in the line of sight features.Comment: 10 pages, 6 figures, accepted for publication in A&
Ground-state ammonia and water in absorption towards Sgr B2
We have used the Odin submillimetre-wave satellite telescope to observe the
ground state transitions of ortho-ammonia and ortho-water, including their 15N,
18O, and 17O isotopologues, towards Sgr B2. The extensive simultaneous velocity
coverage of the observations, >500 km/s, ensures that we can probe the
conditions of both the warm, dense gas of the molecular cloud Sgr B2 near the
Galactic centre, and the more diffuse gas in the Galactic disk clouds along the
line-of-sight. We present ground-state NH3 absorption in seven distinct
velocity features along the line-of-sight towards Sgr B2. We find a nearly
linear correlation between the column densities of NH3 and CS, and a
square-root relation to N2H+. The ammonia abundance in these diffuse Galactic
disk clouds is estimated to be about (0.5-1)e-8, similar to that observed for
diffuse clouds in the outer Galaxy. On the basis of the detection of H218O
absorption in the 3 kpc arm, and the absence of such a feature in the H217O
spectrum, we conclude that the water abundance is around 1e-7, compared to
~1e-8 for NH3. The Sgr B2 molecular cloud itself is seen in absorption in NH3,
15NH3, H2O, H218O, and H217O, with emission superimposed on the absorption in
the main isotopologues. The non-LTE excitation of NH3 in the environment of Sgr
B2 can be explained without invoking an unusually hot (500 K) molecular layer.
A hot layer is similarly not required to explain the line profiles of the
1_{1,0}-1_{0,1} transition from H2O and its isotopologues. The relatively weak
15NH3 absorption in the Sgr B2 molecular cloud indicates a high [14N/15N]
isotopic ratio >600. The abundance ratio of H218O and H217O is found to be
relatively low, 2.5--3. These results together indicate that the dominant
nucleosynthesis process in the Galactic centre is CNO hydrogen burning.Comment: 10 pages, 5 figure
Perspective from a Younger Generation -- The Astro-Spectroscopy of Gisbert Winnewisser
Gisbert Winnewisser's astronomical career was practically coextensive with
the whole development of molecular radio astronomy. Here I would like to pick
out a few of his many contributions, which I, personally, find particularly
interesting and put them in the context of newer results.Comment: 14 pages. (Co)authored by members of the MPIfR (Sub)millimeter
Astronomy Group. To appear in the Proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium "The Dense Interstellar Medium in Galaxies"
eds. S. Pfalzner, C. Kramer, C. Straubmeier, & A. Heithausen (Springer:
Berlin
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