An expanding shell of neutral hydrogen surrounding the supernova remnant HB 21

Assousa, G. E.; Erkes, J. W.

doi:10.1086/111493

Cited by 10 publications

(2 citation statements)

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“…It is difficult to detect expanding H i shells, because most known SNRs are located in the Galactic plane, where the Galactic background H i emission causes severe contamination. There had been papers reporting the detection of H i shells with expansion velocities smaller than ∼20 km s −1 , but the association was rather ambiguous (Assousa & Erkes 1973; Knapp & Kerr 1974; Colomb & Dubner 1980, 1982). If the expansion velocity of the shell is very large, larger than the maximum velocity permitted by the Galactic rotation, however, then it could be easily discernable from the background emission.…”

Section: Introductionmentioning

confidence: 99%

Visibility of old supernova remnants in H i 21-cm emission line

Koo

Kang

2004

Monthly Notices of the Royal Astronomical Society

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We estimate the number of old, radiative supernova remnants (SNRs) detectable in H i 21‐cm emission line in the Galaxy. We assume that old SNRs consist of expanding H i shells and that they are visible if the line‐of‐sight velocities are sufficiently outside the velocity range of the Galactic background H i emission. This criterion of visibility makes it possible to calculate the background contamination and to make a comparison with observation. The Galactic disc in our model is filled with atomic gas of moderate (∼0.1 cm−3) density representing the warm neutral interstellar medium. We assume that only Type Ia supernovae produce isolated SNRs with expanding H i shells, or ‘H i SNRs’. According to our result, the contamination due to the Galactic background H i emission limits the number of visible SNRs to ≃270, or ≃9 per cent of the total H i SNRs. They are concentrated along the loci of tangential points. The telescope sensitivity further limits the number. We compare the result with observations to find that the observed number (≤25) of H i SNRs is much less than expected. A plausible explanation is that previous observational studies, which were made towards the SNRs identified mostly in radio continuum, missed most of the H i SNRs because they are too faint to be visible in radio continuum. We propose that the faint, extended H i 21‐cm emission line wings protruding from the Galactic background H i emission in large‐scale (ℓ, v) diagrams could be possible candidates for H i SNRs, although our preliminary result shows that their number is considerably less than expected in the inner Galaxy. We conclude that a possible explanation for the small number of H i SNRs in the inner Galaxy is that the interstellar space there is largely filled with a very tenuous gas as in the three‐phase interstellar medium (ISM) model, not with the warm neutral medium of moderate density.

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Section: Introductionmentioning

confidence: 99%

Visibility of old supernova remnants in H i 21-cm emission line

Koo

Kang

2004

Monthly Notices of the Royal Astronomical Society

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“…12 CO, H i and radio continuum observations indicate that the SNR is interacting with the interstellar material (Tatematsu et al 1990). The 21 cm line observations show an HI shell around the SNR expanding at ∼ 25 km s −1 (Assousa & Erkes 1973). A giant molecular cloud east of the remnant has been observed and the CO emission morphology indicates that HB21 is interacting with the molecular cloud (Tatematsu et al 1990).…”

Section: Hb21mentioning

confidence: 99%

Arecibo-Green Bank-LOFAR Carbon Radio Recombination Line observations toward cold HI Clouds

Roshi

Peters

Emig

et al. 2021

Preprint

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We present results from a search for radio recombination lines in three H i self-absorbing (HISA) clouds at 750 MHz and 321 MHz with the Robert C. Byrd Green Bank Telescope (GBT), and in three Galactic Plane positions at 327 MHz with the Arecibo Telescope. We detect Carbon Recombination Lines (CRRLs) in the direction of DR4 and DR21, as well as in the galactic plane position G34.94+0.0. We additionally detect Hydrogen Recombination Lines (HRRLs) in emission in five of the six sightlines, and a Helium line at 750 MHz towards DR21. Combining our new data with 150 MHz LOFAR detections of CRRL absorption towards DR4 and DR21, we estimate the electron densities of the line forming regions by modeling the line width as a function of frequency. The estimated densities are in the range 1.4 → 6.5 cm −3 towards DR4, for electron temperatures 200 → 20 K. A dual line forming region with densities between 3.5 → 24 cm −3 and 0.008 → 0.3 cm −3 could plausibly explain the observed line width as a function of frequency on the DR21 sightline. The central velocities of the CRRLs compare well with CO emission and HISA lines in these directions. The cloud densities estimated from the CO lines are smaller (at least a factor of 5) than those of the CRRL forming regions. It is likely that the CRRL forming and H i self-absorbing gas is located in a denser, shocked region either at the boundary of or within the CO emitting cloud.

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Observational Properties of Pulsars

Groth

1975

Neutron Stars, Black Holes and Binary X-Ray Sources

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An expanding shell of neutral hydrogen surrounding the supernova remnant HB 21

Cited by 10 publications

References 0 publications

Visibility of old supernova remnants in H i 21-cm emission line

Visibility of old supernova remnants in H i 21-cm emission line

Arecibo-Green Bank-LOFAR Carbon Radio Recombination Line observations toward cold HI Clouds

Observational Properties of Pulsars

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