29 research outputs found
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
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
PRIMORDIAL HELIUM ABUNDANCE BY RRL: NEW RESULT
New results of the Primordial Helium abundance (Yp) measurement by radio recombination lines (RRL) observations from galactic HII regions are presented. The RRL observations were carried out with two Parabolas: RT32(λ = 13.5 mm and 3.5 cm, Medicina, Italy) and RT22 (λ = 8 and 13.5 mm, Pushchino, Russia). The current value was obtained on the base of five source observations with the accurate correction for both the ionization structure and dust influence. At present, the obtained Yp = 25.74(±1.0)value seems to be higher than that suggested by optical data, allowing the presence of unknown light particles, and is in agreement with the result from cosmic microwave background asymmetry
Helium abundance and ionization structure in the Orion nebula: radio recombination lines observations
Helium abundance and ionization structure in the Orion nebula: radio recombination lines observations
Results of the Ori