103 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
Star Formation in the Central 400 pc of the Milky Way: Evidence for a Population of Massive YSOs
The central kpc of the Milky Way might be expected to differ significantly
from the rest of the Galaxy with regard to gas dynamics and the formation of
YSOs. We probe this possibility with mid-infrared observations obtained with
IRAC and MIPS on Spitzer and with MSX. We use color-color diagrams and SED fits
to explore the nature of YSO candidates (including objects with 4.5 micron
excesses possibly due to molecular emission). There is an asymmetry in the
distribution of the candidate YSOs, which tend to be found at negative Galactic
longitudes; this behavior contrasts with that of the molecular gas,
approximately 2/3 of which is at positive longitudes. The small scale height of
these objects suggests that they are within the Galactic center region and are
dynamically young. They lie between two layers of infrared dark clouds and may
have originated from these clouds. We identify new sites for this recent star
formation. The methanol masers appear to be associated with young, embedded
YSOs characterized by 4.5 micron excesses. We use the SEDs of these sources to
estimate their physical characteristics. Within the central 400x50 pc
(|l|<1.3\degr and |b|<10') the star formation rate based on the identification
of Stage I evolutionary phase of YSO candidates is about 0.14 solar mass/yr. We
suggest that a recent burst of star formation took place within the last 10^5
years. This suggestion is also consistent with estimates of star formation
rates within the last ~10^7 years showing a peak around 10^5 years ago. Lastly,
we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of
the Galaxy. This implies that star formation does not appear to be dramatically
affected by the extreme physical conditions in the Galactic center region.Comment: 96 pages, ten tables, 35 figures, ApJ (in press), replaced by a
revised versio
A Submillimeter HCN Laser in IRC+10216
We report the detection of a strong submillimeter wavelength HCN laser line
at a frequency near 805 GHz toward the carbon star IRC+10216. This line, the
J=9-8 rotational transition within the (04(0)0) vibrationally excited state, is
one of a series of HCN laser lines that were first detected in the laboratory
in the early days of laser spectroscopy. Since its lower energy level is 4200 K
above the ground state, the laser emission must arise from the inner part of
IRC+10216's circumstellar envelope. To better characterize this environment, we
observed other, thermally emitting, vibrationally excited HCN lines and find
that they, like the laser line, arise in a region of temperature approximately
1000 K that is located within the dust formation radius; this conclusion is
supported by the linewidth of the laser. The (04(0)0), J=9-8 laser might be
chemically pumped and may be the only known laser (or maser) that is excited
both in the laboratory and in space by a similar mechanism.Comment: 11 pages, 3 figure
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 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)
Detection of vibrationally excited methyl formate in W51 e2
Methyl formate in its first torsionally excited state (vt=1 at 131 cm-1) is
detected for the first time toward W51 e2. All transitions from excited methyl
formate within the observed spectral range are actually detected (82
transitions) and no strong lines are missing. The column density of the excited
state is comparable to that of the ground state. For a source size of 7'' we
find that Trot = 104 +/- 14 K and N = 9.4 +4.0/-2.8 x 10^16 cm-2 for the
excited state and Trot = 176 +/- 24 K and N = 1.7 +.2/-.2 x 10^17 cm-2 for the
ground state. Lines from ethyl cyanide in its two first excited states (vt=1,
torsion mode at 212 cm-1) and (vb=1, CCN in-plane bending mode at 206 cm-1) are
also present in the observed spectrum. However blending problems prevent a
precise estimate of its abundance. With regard to the number of lines of
excited methyl formate and ethyl cyanide detected in W51 e2, it appears that
excited states of large molecules certainly account for a large number of
unidentified lines in spectral survey of molecular clouds
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
Quantum Imaging with Incoherently Scattered Light from a Free-Electron Laser
The advent of accelerator-driven free-electron lasers (FEL) has opened new
avenues for high-resolution structure determination via diffraction methods
that go far beyond conventional x-ray crystallography methods. These techniques
rely on coherent scattering processes that require the maintenance of
first-order coherence of the radiation field throughout the imaging procedure.
Here we show that higher-order degrees of coherence, displayed in the intensity
correlations of incoherently scattered x-rays from an FEL, can be used to image
two-dimensional objects with a spatial resolution close to or even below the
Abbe limit. This constitutes a new approach towards structure determination
based on incoherent processes, including Compton scattering, fluorescence
emission or wavefront distortions, generally considered detrimental for imaging
applications. Our method is an extension of the landmark intensity correlation
measurements of Hanbury Brown and Twiss to higher than second-order paving the
way towards determination of structure and dynamics of matter in regimes where
coherent imaging methods have intrinsic limitations
- …