78 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
Massive Star Formation in the Molecular Ring Orbiting the Black Hole at the Galactic Center
A ring of dense molecular gas extending 2-7 pc orbits the supermassive black
hole Sgr A* at the center of our Galaxy. Using the Green Bank Telescope, we
detected water maser lines and both narrow (0.35 km/s) and broad (30 - 50 km/s)
methanol emission from the molecular ring. Two of the strongest methanol lines
at 44 GHz are confirmed as masers by interferometric observations. These class
I methanol masers are collisionally excited and are signatures of early phases
of massive star formation in the disk of the Galaxy, suggesting that star
formation in the molecular ring is in its early phase. Close inspection of the
kinematics of the associated molecular clumps in the HCN (J=1-0) line reveals
broad red-shifted wings indicative of disturbance by protostellar outflows from
young (few times 10^4 yr), massive stars embedded in the clumps. The thermal
methanol profile has a similar shape, with a narrow maser line superimposed on
a broad, red-shifted wing. Additional evidence for the presence of young
massive protostars is provided by shocked molecular hydrogen and a number of
striking ionized and molecular linear filaments in the vicinity of methanol
sources suggestive of 0.5-pc scale protostellar jets. Given that the
circumnuclear molecular ring is kinematically unsettled and thus is likely be
the result of a recent capture, the presence of both methanol emission and
broad, red-shifted HCN emission suggests that star formation in the
circumnuclear ring is in its infancy.Comment: 13 pages, 4 figures, ApJ Letters (in press
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
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
Determination of the primordial helium abundance from radio recombination line observations: New data. The source W51
Observations of H and He radio recombination lines in the source W51 have been performed with the RT-22 radio telescope (Pushchino) in two transitions: 56α (8 mm) and 65α (13 mm). We have estimated the spectral line parameters and determined the relative abundance of ionized helium, y + = (9.3 ± 0.35)%. We have carried out a model study of the correction (R) for the ionization structure of HII regions (when passing from the observed y + = N(He+)/N(H+) to the actual y = N(He)/N(H)) as a function of the spectral type of the ionizing star. Hence it follows that it is desirable to choose the sources excited by hot stars of spectral types no later than O6 V to estimate the helium abundance. In this case, the correction is expected to be small and essentially constant, R in the range 1.0-1.05. We have analyzed the correction for the ionization structure of W51, obtained an actual abundance of helium in the range y = (8.9-9.7)%, and determined its primordial abundance Y p (produced during primordial nucleosynthesis in the Universe) in this source. We have made a new estimate of the primordial helium abundance from six Galactic HII regions, where we observed H and He radio recombination lines at different times. The weighted mean Y p = 25.64(±0.70)% has been obtained. On the one hand, this value of Y p does not yet disagree strongly with the conclusions of the standard cosmologicalmodel, but, on the other hand, it admits the existence of at least one unknown light particle in the period of primordial nucleosynthesis outside the scope of the standard cosmological model. One should continue to refine Y p for more reliable conclusions to be reached. © 2013 Pleiades Publishing, Inc
Recommended from our members
Proceedings, Western juniper management short course, October 15-16, 1984, Bend, Oregon
Published 1984. Please look for up-to-date information in the OSU Extension Catalog: http://extension.oregonstate.edu/catalo
Intrauterine Bewegungen, Schlaf-Wachrhythmus und Blutzucker bei gesunden und hypoxischen Lämmern im chronischen Tierexperiment
- …