LOFAR observations of decameter carbon radio recombination lines towards Cassiopeia A

Abstract

We present a study of carbon radio recombination lines towards Cassiopeia A using LO-FAR observations in the frequency range 10-33 MHz. Individual carbon α lines are detected in absorption against the continuum at frequencies as low as 16 MHz. Stacking several Cα lines we obtain detections in the 11-16 MHz range. These are the highest signal-to-noise measurements at these frequencies. The peak optical depth of the Cα lines changes considerably over the 11-33 MHz range with the peak optical depth decreasing from 4 × 10 −3 at 33 MHz to 2 × 10 −3 at 11 MHz, while the line width increases from 20 km s −1 to ∼ 150 km s −1 . The combined change in peak optical depth and line width results in a roughly constant integrated optical depth. We interpret this as carbon atoms close to local thermodynamic equilibrium.In this work we focus on how the 11-33 MHz carbon radio recombination lines can be used to determine the gas physical conditions. We find that the ratio of the carbon radio recombination lines to that of the 158 µm [CII] fine-structure line is a good thermometer, while the ratio between low frequency carbon radio recombination lines provides a good barometer. By combining the temperature and pressure constraints with those derived from the line width we are able to constrain the gas properties (electron temperature and density) and radiation field intensity. Given the 1σ uncertainties in our measurements these are; T e ≈ 68-98 K, n e ≈ 0.02-0.035 cm −3 and T r,100 ≈ 1500-1650 K. Despite challenging RFI and ionospheric conditions, our work demonstrates that observations of carbon radio recombination lines in the 10-33 MHz range can provide insight into the gas conditions.

Mapping low-frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54, and 43 MHz

“…The gas properties derived from the CRRL analysis seem to bridge the gap between the atomic gas traced by the 21 cm line of HI and the molecular gas traced by the CO lines in the millimimeter (Oonk et al 2017;Salas et al 2017). From spatially unresolved observations of the 21 cm-HI line in this direction Davies & Matthews (1972) derived a temperature of 140±40 K for the two most prominent Perseus arm absorption features at −47 and −38 km s −1 .…”

“…In order to model the gas properties we assume that the radiation field the gas is immersed in is constant. Since we are studying gas on scales of 1 pc and the gas is at a distance of 220 pc from Cas A (e.g., Kantharia et al 1998;Salas et al 2017), the possible contribution of Cas A to the radiation field (Stepkin et al 2007) will change by a negligible amount over the observed structure. Additionally, there are no other known strong, low-frequency, discrete radiation sources in the field.…”

“…The projected distance on the plane of the sky between the peak of the C268α optical depth and the peak of the 12 CO(2-1) emission is 1.3 ± 0.6 . If we assume that the Perseus arm gas is at a distance of 3.16 ± 0.02 kpc from Earth in the direction of Cas A (Choi et al 2014;Salas et al 2017), then this corresponds to ∼ 1.2 ± 0.5 pc in the plane of the sky. CO will be sufficiently shielded from photo-dissociating photons when A FUV ∼ 1.…”

A high-resolution study of carbon radio recombination lines towards Cassiopeia A

“…The Perseus Arm component centered at -47 km s −1 has been treated in the literature as a single component gas cloud (e.g. Kantharia et al 1998b;Salas et al 2017) whereas the maps presented in figure 4 show evidence for significant spatial and spectral sub-structure. To investigate further the nature of these components we split the emission over the face of Cas A into three parts along the lines shown in the right panel of figure 5 covering (i) The western part of the bridge, (ii) The eastern part of the bridge, (iii) Emission north of the bridge.…”

Low-Frequency Carbon Recombination Lines in the Orion Molecular Cloud Complex

“…Observations of CRRLs have spanned frequencies of 10 to over 5 000 MHz (Payne et al 1994;Roshi, Goss, & Jeyakumar 2014;Salas et al 2017). Peters et al (2010) summarised the information about known Galactic low-frequency CRRLs and, using a modified version of the radiometer equation, calculated the approximate amount of observing time required in order for the new generation of low-frequency telescopes to detect them.…”