arXiv subjects

D. Anish Roshi

Publications and source records attributed to D. Anish Roshi.

45 records · Page 3Linked to original sources

arXiv2006

VLA Observations of Carbon 91$α$ Recombination Line Emission in W49 North

We have detected C91$α$ (8.5891 GHz) emission toward 4 ultra-compact \HII regions (\UCHII s; W49G, J, L & C) in the W49 North massive star forming region with the Very Large Array (VLA) at 3\arcsec resolution. No carbon line emission was detected toward \UCHII s W49F, A, O, S and Q at this frequency to a 3$σ$ level of 2 mJy. We also observed the same region in the C75$α$ line (15.3 GHz) with no detection at a 3$σ$ level of 6 mJy with a 1\arcsec.7 beam. Detection of line emission toward these sources add supporting data to the earlier result of \nocite{retal05a}Roshi et al (2005a) that many \UCHII s have an associated photo-dissociation region (PDR). Similarity of the LSR velocities of carbon recombination lines and H$_2$CO absorption toward \UCHII s in W49 North suggests that the PDRs reside in the dense interface zone surrounding these \HII regions. Combining the observed carbon line parameters at 8.6 GHz with the upper limits on line emission at 15.3 GHz, we obtain constraints on the physical properties of the PDRs associated with W49G and J. The upper limit on the number density of hydrogen molecule obtained from carbon line models is $\sim$ $5 \times 10^6$ \cmthree.

astro-ph↗

arXiv2006

An Experimental setup to develop RFI mitigation techniques for radio astronomy

Increasing levels of Radio Frequency Interference (RFI) are a problem for research in radio astronomy. Various techniques to suppress RFI and extract astronomical signals from data affected by interference are being tried out. However, extracting weak astronomical signals in the spectral region affected by RFI remains a technological challenge. In this paper, we describe the construction of an experimental setup at the Raman Research Institute (RRI), Bangalore, India for research in RFI mitigation. We also present some results of tests done on the data collected using this setup. The experimental setup makes use of the 1.42 GHz receiver system of the 10.4 m telescope at RRI. A new reference antenna, its receiver system and a backend for recording digitized voltage together with the 1.42 GHz receiver system form the experimental setup. We present the results of the characterization of the experimental setup. An off-line adaptive filter was successfully implemented and tested using the data obtained with the experimental setup.

astro-ph↗

arXiv2005

Multi-wavelength carbon recombination line observations with the VLA toward an UCHII region in W48: Physical properties and kinematics of neutral material

Using the Very Large Array (VLA) the C76$α$ and C53$α$ recombination lines (RLs) have been detected toward the ultra-compact \HII\ region (UCHII region) G35.20$-$1.74. We also obtained upper limits to the carbon RLs at 6 cm (C110$α$ & C111$α$) and 3.6 cm (C92$α$) wavelengths with the VLA. In addition, continuum images of the W48A complex (which includes G35.20$-$1.74) are made with angular resolutions in the range 14\arcsec to 2\arcsec. Modeling the multi-wavelength line and continuum data has provided the physical properties of the UCHII region and the photodissociation region (PDR) responsible for the carbon RL emission. The gas pressure in the PDR, estimated using the derived physical properties, is at least four times larger than that in the UCHII region. The dominance of stimulated emission of carbon RLs near 2 cm, as implied by our models, is used to study the relative motion of the PDR with respect to the molecular cloud and ionized gas. Our results from the kinematical study are consistent with a pressure-confined UCHII region with the ionizing star moving with respect to the molecular cloud. However, based on the existing data, other models to explain the extended lifetime and morphology of UCHII regions cannot be ruled out.

astro-ph↗

arXiv2005

An 8.5 GHz Arecibo survey of Carbon Recombination Lines toward Ultra-compact \HII regions: Physical properties of dense molecular material

We report here on a survey of carbon recombination lines (RLs) near 8.5 GHz toward 17 ultra-compact \HII regions (\UCHII s). Carbon RLs are detected in 11 directions, indicating the presence of dense photodissociation regions (PDRs) associated with the \UCHII s. In this paper, we show that the carbon RLs provide important, complementary information on the kinematics and physical properties of the ambient medium near \UCHII s. Non-LTE models for the carbon line forming region are developed, assuming that the PDRs surround the \UCHII s, and we constrained the model parameters by multi-frequency RL data. Modeling shows that carbon RL emission near 8.5 GHz is dominated by stimulated emission and hence we preferentially observe the PDR material that is in front of the \UCHII continuum. We find that the relative motion between ionized gas and the associated PDR is about half that estimated earlier, and has an RMS velocity difference of 3.3 \kms. Our models also give estimates for the PDR density and pressure. We found that the neutral density of PDRs is typically $>$ 5 $\times$ 10$^5$ \cmthree and \UCHII s can be embedded in regions with high ambient pressure. Our results are consistent with a pressure confined \HII region model where the stars are moving relative to the cloud core. Other models cannot be ruled out, however. Interestingly, in most cases, the PDR pressure is an order of magnitude larger than the pressure of the ionized gas. Further investigation is needed to understand this large pressure difference.

astro-ph↗

arXiv2005

A search for 53 MHz OH line near G48.4$-$1.4 using the National MST Radar Facility

We present the results of a search for the ground state hyperfine transition of the OH radical near 53 MHz using the National MST Radar Facility at Gadanki, India. The observed position was G48.4$-$1.4 near the Galactic plane. The OH line is not detected. We place a 3$σ$ upper limit for the line flux density at 39 Jy from our observations. We also did not detect recombination lines (RLs) of carbon, which were within the frequency range of our observations. The 3$σ$ upper limit of 20 Jy obtained for the flux density of carbon RL, along with observations at 34.5 and 327 MHz are used to constrain the physical properties of the line forming region. Our upper limit is consistent with the line emission expected from a partially ionized region with electron temperature, density and path lengths in the range 20 -- 300 K, 0.03 -- 0.3 cm$^{-3}$ and 0.1 -- 170 pc respectively.

astro-ph↗

arXiv2003

A new technique to improve RFI suppression in radio interferometers

Radio interferometric observations are less susceptible to radio frequency interference (RFI) than single dish observations. This is primarily due to : (1)fringe-frequency averaging at the correlator output and (2) bandwidth decorrelation of broadband RFI. Here, we propose a new technique to improve RFI suppression of interferometers by replacing the fringe-frequency averaging process with a different filtering process. In the digital implementation of the correlator, such a filter should have cutoff frequencies $< 10^{-6}$ times the frequency at which the baseband signals are sampled. We show that filters with such cutoff frequencies and attenuation $>$ 40 dB at frequencies above the cutoff frequency can be realized using multirate filtering techniques. Simulation of a two element interferometer system with correlator using multirate filters shows that the RFI suppression at the output of the correlator can be improved by 40 dB or more compared to correlators using a simple averaging process.

astro-ph↗

arXiv2002

Multi-frequency GMRT Observations of the HII regions S 201, S 206, and S 209 : Galactic Temperature Gradient

We present radio continuum images of three Galactic HII regions, S 201, S 206, and S 209 near 232, 327, and 610 MHz using the Giant Meterwave Radio Telescope (GMRT). The GMRT has a mix of short and long baselines, therefore, even though the data have high spatial resolution, the maps are still sensitive to diffuse extended emission. We find that all three HII regions have bright cores surrounded by diffuse envelopes. We use the high resolution afforded by the data to estimate the electron temperatures and emission measures of the compact cores of these HII regions. Our estimates of electron temperatures are consistent with a linear increase of electron temperature with Galacto-centric distance for distances up to 18 kpc (the distance to the most distant HII region in our sample).

astro-ph↗

arXiv2002

Carbon Recombination Lines near 327 MHz I: "Diffuse" CII regions in the Galactic Disk

In earlier papers (Roshi & Anantharamaiah 2000, 2001a), we presented extensive surveys (angular resolution -- 2°$\times$ 2°& 2°$\times$ 6\arcmin) of radio recombination lines (RRLs) near 327 MHz in the longitude range $l = $ 332°$\to$ 89°using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between $l = $ 358°$\to$ 20°and in a few positions at other longitudes. This paper presents the observed carbon line parameters in the high-resolution survey and a study of the galactic distribution and angular extent of the line emission observed in the surveys. The carbon lines detected in the surveys arise in ``diffuse'' \CII regions. The \lv diagram and radial distribution constructed from our carbon line data shows similarity with that obtained from hydrogen recombination lines at 3 cm from \HII regions indicating that the distribution of the diffuse \CII regions in the inner Galaxy resembles the distribution of the star-forming regions. We estimated the [CII] 158 micron emission from diffuse \CII regions and find that upto 95 % of the total observed [CII] 158 micron emission can arise in diffuse \CII regions if the temperature of the latter $\sim 80$ K. Our high-resolution survey data shows that the carbon line emitting regions have structures on angular scale $\sim$ 6\arcmin. Analysis of the dual-resolution observations toward a 2°wide field centered at $l = $ 13°.9 and toward the longitude range $l = $ 1°.75 to 6°.75 shows the presence of narrow ($ΔV \le $ 15 \kms) carbon line emitting regions extending over several degrees in $l$ and $b$. The physical size perpendicular to the line-of-sight of an individual diffuse \CII region in these directions is $>$ 200 pc.

astro-ph↗

arXiv2001

Low-frequency Carbon Recombination Lines towards HI self-absorption features in the Galactic Plane

A survey of radio recombination lines (RLs) in the Galactic plane ($l = $ 332°$\to$ 89°) near 327 MHz made using the Ooty Radio Telescope (ORT) has detected carbon RLs from all the positions in the longitude range 0°$< l < $ 20°and from a few positions at other longitudes. The carbon RLs detected in this survey originate from ``diffuse'' \CII regions. Comparison of the \lv diagram and the radial distribution of carbon line emission with those obtained from hydrogen RLs near 3 cm from \HII regions and ``intense'' \CO emission indicates that the distribution of the diffuse \CII regions in the inner Galaxy is similar to that of spiral arm components. Towards several observed positions, the central velocities of the carbon RLs coincide with \HI self-absorption features suggesting an association of diffuse \CII regions with cool \HI. The observed widths of the lines from the two species are, however, different. We discuss possible reasons for the difference in the line widths.

astro-ph↗