4,867 research outputs found
Searching for new hypercompact HII regions
Hypercompact (HC) HII regions are, by nature, very young HII regions,
associated with the earliest stages of massive star formation. They may
represent the transition phase as an early B-type star grows into an O-type
star. Unfortunately, so few HCHII regions are presently known that their
general attributes and defining characteristics are based on small number
statistics. A larger sample is needed for detailed studies and good statistics.
Class II methanol masers are one of the best indicators of the early stages of
massive star formation. Using the Arecibo Methanol Maser Galactic Plane Survey
- the most sensitive blind survey for 6.7 GHz methanol masers to date - we
selected 24 HCHII region candidates. We made EVLA continuum observations at 3.6
and 1.3 cm to search for HCHII regions associated with these masers. We
identified six potential HCHII regions in our sample based on the presence of
optically thick free-free emission. Overall, we find that 30% of the methanol
masers have an associated centimeter radio continuum source (separation less
than 0.1 pc), which is in general agreement with previous studies.Comment: Accepted for publication in ApJL, in the Special Issue of the EVL
Recruitment of postlarval penaeid prawns in the Vellar estuary, South India
Abstract only.The northern bank of Vellar estuary (Parangipettai, India) is ideal for postlarval penaeid prawn recruitment. The annual recruitment, distribution and the substratum preference of postlarval immigrants at three different stations in the estuary were studied in detail.
Among the postlarvae of Penaeus, P. (Fenneropenaeus) indicus was dominant followed by P. (Penaeus) monodon, P. (P.) semisulcatus, P. (F.) merguiensis and P. (Melicertus) latisulcatus. In Metapenaeus, postlarvae of M. monoceros were abundant followed by M. dobsoni, M. affinis, M. bre-vicornis and M. lysianassa.
Two peaks were observed in the postlarval penaeid prawn population. In P (F.) indicus and P. (P.) monodon, the primary peak occurred from January to April and the secondary peak from July to September. In M. monoceros and M. dobsoni, the primary peak was from March to May and the secondary peak from August to September. The postlarvae of P. (F.) indicus, P. (P.) monodon, M. monoceros and M. dobsoni were available throughout the year while the others were seasonal. The distribution of postlarvae in the estuary is related to the type of substratum, salinity and temperature. The postlarval population declined during the northeast monsoon (November-December) and in peak summer (May-June). Their abundance decreased in the lower salinity areas of the upper reaches of the estuary
The resource-based view within the conversation of strategic management
Includes bibliographical references
Mapping the potential energy surfaces for ring-closing metathesis reactions of prototypical dienes by electronic structure calculations
The potential energy surfaces for ring-closing metathesis reactions of a series of simple alpha,omega-dienes which lead to 5-10 membered ring products, have been explored using density functional theory methods. We have investigated both the conformational aspects of the hydrocarbon chain during the course of the reactions, as well as the stationary structures on the corresponding potential energy surfaces. Extensive conformational searches reveal that the reaction proceeds via the conformation that would be expected for the cycloalkene product, though most unexpectedly, cyclohexene forms via complexes in boat-like conformations. The M06-L density functional has been used to map out the potential energy surfaces, and has identified metallocyclobutane fragmentation as being generally the highest barrier along the pathway. The structural variations along the pathway have been discussed for the reactant hydrocarbons of differing chain length to identify points at which cyclisation events may begin to affect reaction rates. Our study provides an excellent starting point from which to begin to learn about the way RCM reaction outcomes are controlled by diene structur
Resolving distance ambiguities towards 6.7 GHz methanol masers
Distances to most star forming regions are determined using kinematics,
through the assumption that the observed radial velocity arises from the motion
of the source with respect to the Sun resulting from the differential rotation
of Galaxy. The primary challenge associated with the application of this
technique in the inner Galaxy is the kinematic distance ambiguity. In this
work, we aim to resolve the kinematic distance ambiguity towards a sample of
6.7 GHz methanol masers, which are signposts of the early stages of massive
star formation. We measured 21 cm HI absorption spectra using the Very Large
Array in C and CnB configurations. A comparison of the maximum velocity of HI
absorption with the source velocity and tangent point velocity was used to
resolve the kinematic distance ambiguity. We resolved the distance ambiguity
towards 41 sources. Distance determinations that are in conflict with previous
measurements are discussed. The NE2001 spiral arm model is broadly consistent
with the locations of the star forming complexes. We find that the use of
vertical scale height arguments to resolve the distance ambiguity can lead to
erroneous classifications for a significant fraction of sources.Comment: Accepted by Astronomy & Astrophysic
- …