We present images of C110$alpha$ and H110$alpha$ radio recombination line (RRL) emission at 4.8 GHz and images of H166$alpha$, C166$alpha$ and X166$alpha$ RRL emission at 1.4 GHz, observed toward the starforming region NGC 2024. The 1.4 GHz image wit
h angular resolution $sim$ 70arcsec is obtained using VLA data. The 4.8 GHz image with angular resolution $sim$ 17arcsec is obtained by combining VLA and GBT data. The similarity of the LSR velocity (10.3 kms) of the C110$alpha$ line to that of lines observed from molecular material located at the far side of the HII region suggests that the photo dissociation region (PDR) responsible for C110$alpha$ line emission is at the far side. The LSR velocity of C166$alpha$ is 8.8 kms. This velocity is comparable with the velocity of molecular absorption lines observed from the foreground gas, suggesting that the PDR is at the near side of the HII region. Non-LTE models for carbon line forming regions are presented. Typical properties of the foreground PDR are $T_{PDR} sim 100$ K, $n_e^{PDR} sim 5$ cmthree, $n_H sim 1.7 times 10^4$ cmthree, path length $l sim 0.06$ pc and those of the far side PDR are $T_{PDR} sim$ 200 K, $n_e^{PDR} sim$ 50 cmthree, $n_H sim 1.7 times 10^5$ cmthree, $l sim$ 0.03 pc. Our modeling indicates that the far side PDR is located within the HII region. We estimate magnetic field strength in the foreground PDR to be 60 $mu$G and that in the far side PDR to be 220 $mu$G. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024.
A compact steep spectrum radio source (J0535-0452) is located in the sky coincident with a bright optical rim in the HII region NGC1977. J0535-0452 is observed to be $leq 100$ mas in angular size at 8.44 GHz. The spectrum for the radio source is stee
p and straight with a spectral index of -1.3 between 330 and 8440 MHz. No 2 mu m IR counter part for the source is detected. These characteristics indicate that the source may be either a rare high redshift radio galaxy or a millisecond pulsar (MSP). Here we investigate whether the steep spectrum source is a millisecond pulsar.The optical rim is believed to be the interface between the HII region and the adjacent molecular cloud. If the compact source is a millisecond pulsar, it would have eluded detection in previous pulsar surveys because of the extreme scattering due to the HII region--molecular cloud interface. The limits obtained on the angular broadening along with the distance to the scattering screen are used to estimate the pulse broadening. The pulse broadening is shown to be less than a few msec at frequencies $gtsim$ 5 GHz. We therefore searched for pulsed emission from J0535-0452 at 14.8 and 4.8 GHz with the Green Bank Telescope (GBT). No pulsed emission is detected to 55 and 30 mu Jy level at 4.8 and 14.8 GHz. Based on the parameter space explored by our pulsar search algorithm, we conclude that, if J0535-0452 is a pulsar, then it could only be a binary MSP of orbital period $ltsim$ 5 hrs.
There is now unequivocal evidence that the jets in FR I radio galaxies are initially relativistic, decelerating flows. On the assumption that they are axisymmetric and intrinsically symmetrical (a good approximation close to the nucleus), we can make
models of their geometry, velocity, emissivity and field structure whose parameters can be determined by fitting to deep VLA observations. Mass entrainment - either from stellar mass loss within the jet volume or via a boundary layer at the jet surface - is the most likely cause for deceleration. This idea is quantitatively consistent with the velocity field and geometry inferred from kinematic modelling and the external gas density and pressure profiles derived from X-ray observations. The jets must initially be very light, perhaps with an electron-positron composition.
We present an analysis of the magnetic-field fluctuations in the magnetoionic medium in front of the radio galaxy 3C 31 derived from rotation-measure (RM) fits to high-resolution polarization images. We first show that the Faraday rotation must be du
e primarily to a foreground medium. We determine the RM structure functions for different parts of the source and infer that the simplest form for the power spectrum is a power law with a high-frequency cutoff. We also present three-dimensional simulations of RM produced by a tangled magnetic field in the hot plasma surrounding 3C 31, and show that the observed RM distribution is consistent with a spherical plasma distribution in which the radio source has produced a cavity.
