12 research outputs found
VLA Spectral Line Observations of a Shocked Cold H II Region in G70.7+1.2
We have observed the molecular globule G70.7+1.2 at 1375 MHz using the C configuration of the VLA, and have imaged a peculiar H168α recombination line detected at the Arecibo radio telescope. The narrow width of the recombination line (Δv ~ 3 km s-1) indicates gas cooler than 185 K and suggests that the globule harbors the coldest known H II region. Previous work showed that the recombination line came either from newly ionized gas outside a bow shock produced by supersonic motion of an early-type star through the molecular globule, or from a cold H II region inside the globule. The 20'' angular resolution of the VLA image of G70.7+1.2 was sufficient to resolve the separation between the nonthermal radio-emitting bow shock and the thermal H II region. The spectral line images show that the radio recombination line comes from cold gas near the outer boundary of the bow shock and that the line intensity is enhanced by stimulated amplification of the nonthermal continuum emanating from the bow shock
Star formation in clusters: early sub-clustering in the Serpens core
We present high resolution interferometric and single dish observations of
molecular gas in the Serpens cluster-forming core. Star formation does not
appear to be homogeneous throughout the core, but is localised in spatially-
and kinematically-separated sub-clusters. The stellar (or proto-stellar)
density in each of the sub-clusters is much higher than the mean for the entire
Serpens cluster. This is the first observational evidence for the hierarchical
fragmentation of proto-cluster cores suggested by cluster formation models.Comment: 11 pages, 3 Figures, ApJ Letters in pres
Carbon Recombination Lines from the Galactic Plane at 34.5 & 328 MHz
We present results of a search for carbon recombination lines in the Galaxy
at 34.5 MHz (C) made using the dipole array at Gauribidanur near
Bangalore. Observations made towards 32 directions, led to detections of lines
in absorption at nine positions. Followup observations at 328 MHz
(C) using the Ooty Radio Telescope detected these lines in emission.
A VLA D-array observation of one of the positions at 330 MHz yielded no
detection implying a lower limit of 10' for the angular size of the line
forming region.
The longitude-velocity distribution of the observed carbon lines indicate
that the line forming region are located mainly between 4 kpc and 7 kpc from
the Galactic centre. Combining our results with published carbon recombination
line data near 76 MHz (\nocite{erickson:95} Erickson \et 1995) we obtain
constraints on the physical parameters of the line forming regions. We find
that if the angular size of the line forming regions is , then
the range of parameters that fit the data are: \Te K, \ne \cm3 and pathlengths pc which may correspond to thin
photo-dissociated regions around molecular clouds. On the other hand, if the
line forming regions are in extent, then warmer gas (\Te K) with lower electron densities (\ne \cm3) extending
over several tens of parsecs along the line of sight and possibly associated
with atomic \HI gas can fit the data. Based on the range of derived parameters,
we suggest that the carbon line regions are most likely associated with
photo-dissociation regions.Comment: To appear in Journal of Astrophysics & Astronomy, March 200
EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions
The role of magnetic fields in the formation of high-mass stars is still
under debate, and recent measurements of their orientation and strength by
using polarized maser emissions are contributing new insights. Masers
polarization, in particular of the 6.7-GHz methanol masers, are one of the best
probes of the magnetic field morphologies around massive protostars.
Determining the magnetic field morphology around an increasing number of
massive protostars at milliarcsecond resolution by observing 6.7-GHz methanol
masers is crucial to better understand the role of magnetic fields in massive
star formation.The First EVN Group consists of 4 massive star-forming
complexes: W51, W48, IRAS18556+0138, and W3(OH). These contain well-studied
\hii ~regions from some of which molecular bipolar outflows were also detected
(W51-e2, G35.20-0.74N). Nine of the European VLBI Network antennas were used to
measure the linear polarization and Zeeman-splitting of the 6.7-GHz methanol
masers in the star-forming regions of the First EVN Group. We detected a total
of 154 CH3OH masers, one third of these towards W3(OH). Fractional linear
polarization (1.2-11.5%) was detected towards 55 masers. The linear
polarization vectors are well-ordered in all the massive star-forming regions.
We measured significant Zeeman-splitting in 3 massive star-forming regions
(W51, W48, and W3(OH)) revealing a range of separations -3.5 m/s<\Delta
V_{z}<3.8 m/s with the smallest |\Delta V_{z}|=0.4m/s. We were also able to
compare our magnetic field results with those obtained from submillimeter
wavelength dust observation in W51 and show that the magnetic field at low and
high resolutions are in perfect agreement.Comment: 15 pages, 11 figures, 5 tables, accepted by Astronomy & Astrophysic
A <i>Herschel</i> and BIMA study of the sequential star formation near the W 48A H II region
We present the results of Herschel HOBYS (Herschel imaging survey of OB Young Stellar objects) photometric mapping combined with Berkeley Illinois Maryland Association (BIMA) observations and additional archival data, and perform an in-depth study of the evolutionary phases of the star-forming clumps in W 48A and their surroundings. Age estimates for the compact sources were derived from bolometric luminosities and envelope masses, which were obtained from the dust continuum emission, and agree within an order of magnitude with age estimates from molecular line and radio data. The clumps in W 48A are linearly aligned by age (east-old to west-young): we find a ultra-compact (UC) H II region, a young stellar object (YSO) with class II methanol maser emission, a YSO with a massive outflow and finally the NH2D prestellar cores from Pillai et al. This remarkable positioning reflects the (star) formation history of the region. We find that it is unlikely that the star formation in the W 48A molecular cloud was triggered by the UC H II region and discuss the Aquila supershell expansion as a major influence on the evolution of W 48A. We conclude that the combination of Herschel continuum data with interferometric molecular line and radio continuum data is important to derive trustworthy age estimates and interpret the origin of large-scale structures through kinematic information
Magnetic Quadrupole Decay of (1s2s2p) 4p05/2-(1s)22s2se1/2 Transition of Lithium Isoelectronic Sequence
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