Observations Of Cas Research Articles

Features of Secular Changes in the Flux Density of the Cas a Supernova Remnant, from Centimeter to Decameter Wavelengths

The purpose of this work is to summarize a large amount of observational data about the secular decrease of Cas A flux in the radio range, as an indicator of physical processes both in the source itself and as a consequence of the influence of the propagation medium. This paper presents results of observations Cas A and Cyg A on radio telescopes of International Radio Astronomy Center (Latvia), and URAN-4 phased array antenna (Institute of Radio Astronomy NAS of Ukraine). According to URAN-4 observations, there were seasonal–diurnal changes in Cas A/Cyg A flux ratios due to the effects of solar activity on the ionosphere, and there may be no secular decrease in Cas A flux density, or a weak tendency to decrease it. The significant influence of the ionosphere makes it difficult to use Cyg A as a reference source in the decameter radio range. In the centimeter radio range, there were episodic intra-day variations at the level 8–10% of Cas A averaged flux. Additionally, in the period January–February 2021, Cas A flux was about 1.7 times that of Cyg A. Taking into account the observed complex type of secular decrease in Cas A flux against the background of changes in space weather variations, further observations of Cas A were planned at radio observatories in Latvia and Ukraine.

PERIODIC AND SPORADIC VARIATIONS IN THE SPECTRAL FLUX DENSITY OF THE CAS A SUPERNOVA REMNANT

Purpose: Based on the long-term study data in all radio spectrum ranges, the nature of deviations of spectral flux density of the Cas A supernova remnant from the tendency of its secular decrease is considered. The aim of this work is determining the presence of quasiperiodic variations and sporadic changes in the Cas A spectral radiation flux density depending on frequency. Design/methodology/approach: The main database is using the published results obtained with the method of absolute measurements of the Cas A spectral radiation flux density in a wide range from millimeter to decimeter wavelengths, as well as the results obtained with the method of relative measurements of the ratio of the flux densities of the Cas A supernova remnant and radio galaxy Cyg A, this latter being used as a reference source in the meter wavelength range. For making comparison with the aforesaid data obtained with different methods, the results of a long-term monitoring (since 1987) of the variation of the ratio of the spectral flux densities of Cas A and Cyg A made with the URAN-4 radio telescope of the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine at 25 MHz were used. Findings: As a result of the analysis of the observed data for the Cas A radiation flux density in the entire radio wavelength range, the existence of quasiperiodic variations in the range from millimeter to meter wavelengths within 2 to 9 years has been noted. The reason for the detected quasiperiodic variations can be the processes in the Cas A remnant itself. In the decameter wavelength range, according to monitoring data obtained with the URAN-4 radio telescope, the seasonal-diurnal and long-term variations are noted, being associated with changes in the ionosphere state in the solar activity cycle, with some weak appearance of the secular decrease of the Cas A radiation flux. The presence of sporadic variations in the ratio of the spectral flux densities of Cas A and Cyg A is associated with the effect of the increased solar activity. For explaining the lowering of the Cas A spectral flux density to the Cyg A level and maintaining the excess of the Cas A flux at decameter waves, the quasi-simultaneous observations made with radiotelescopes for different wavelength ranges will be required. Conclusions: The evolution of the Cas A supernova remnant remains the focus of interest of current research efforts. An intriguing moment was the discovery of a point X-ray source in the center of radio source Cas A as a possible supernova remnant. The role of this source in the Cas A radio flux secular decrease and in its variations needs to be clarified. A detailed analysis of the long-term data and making quasi-simultaneous observations will allow to reveal the processes occurring in the radio source itself and to determine the influence of the ionosphere state on the results of measurements. A joint program is suggested for observations of Cas A and Cyg A flux variations with the RT-32 and RT-16 radio telescopes, the LOFAR element of the Ventspils International Radio Astronomy Center (Latvia), and the URAN, UTR-2 and GURT radio telescopes of the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine. Key words: Cas A; secular radiation flux decrease; supernova remnant; ionosphere; solar activity

TINY SCALE OPACITY FLUCTUATIONS FROM VLBA, MERLIN, AND VLA OBSERVATIONS OF H I ABSORPTION TOWARD 3C 138

The structure function of opacity fluctuations is a useful statistical tool to study tiny scale structures of neutral hydrogen. Here we present high resolution observation of HI absorption towards 3C 138, and estimate the structure function of opacity fluctuations from the combined VLA, MERLIN and VLBA data. The angular scales probed in this work are ~ 10-200 milliarcsec (about 5-100 AU). The structure function in this range is found to be well represented by a power law S_tau(x) ~ x^{beta} with index beta ~ 0.33 +/- 0.07 corresponding to a power spectrum P_tau(U) ~ U^{-2.33}. This is slightly shallower than the earlier reported power law index of ~ 2.5-3.0 at ~ 1000 AU to few pc scales. The amplitude of the derived structure function is a factor of ~ 20-60 times higher than the extrapolated amplitude from observation of Cas A at larger scales. On the other hand, extrapolating the AU scale structure function for 3C 138 predicts the observed structure function for Cas A at the pc scale correctly. These results clearly establish that the atomic gas has significantly more structures in AU scales than expected from earlier pc scale observations. Some plausible reasons are identified and discussed here to explain these results. The observational evidence of a shallower slope and the presence of rich small scale structures may have implications for the current understanding of the interstellar turbulence.

FORMATION AND EVOLUTION OF DUST IN TYPE IIb SUPERNOVAE WITH APPLICATION TO THE CASSIOPEIA A SUPERNOVA REMNANT

We investigate the formation of dust grains in the ejecta of a SN IIb and their evolution in the shocked gas in the SNR by considering the uniform and power-law density structures for the CSM. Based on these calculations, we also simulate the time evolution of thermal emission from the shock-heated dust in the SNR and compare the results with the observations of Cas A SNR. We find that the total mass of dust formed in the SN IIb is as large as 0.167 M_sun but the average radius of dust is smaller than 0.01 mum and is significantly different from those in SNe II-P with the massive H-envelope. In the explosion with the small-mass H-envelope, the expanding He core undergoes little deceleration, so that the gas density in the He core is too low for large-sized grains to form. In addition, the low-mass H-envelope of the SN IIb leads to the early arrival of the reverse shock at the dust-forming region. If the CSM is more or less spherical, therefore, the newly formed grains would be completely destroyed in the relatively dense shocked gas for the CSM density of n_H > 0.1 cm^{-3}. However, the actual CSM is likely to be non-spherical, so that a part of grains could be ejected into the ISM without being shocked. We demonstrate that the time evolution of the SED by thermal dust emission is sensitive to the ambient gas density and structure that affects the passage of the reverse shock into the ejecta. Thus, the SED evolution well reflects the evolution of dust through erosion by sputtering and stochastic heating. For Cas A, we consider the CSM produced by the steady mass loss of ~8x10^{-5} M_sun/yr during the supergiant phase. Then we find the infrared SED of Cas A is reasonably reproduced by thermal emission from the newly formed dust of 0.08 M_sun, which consists of 0.008 M_sun shocked warm dust and 0.072 M_sun unshocked cold dust.

The hydrodynamics of the supernova remnant Cassiopeia A

<i>Aims. <i/>There are large differences in the proposed progenitor models for the Cas A Supernova remnant (SNR). One of these differences is the presence or absence of a Wolf-Rayet (WR) phase of the progenitor star. The mass loss history of the progenitor star strongly affects the shape of the SNR. In this paper we investigate whether the progenitor star of Cas A had a WR phase or not and how long it may have lasted. <i>Methods. <i/>We performed two-dimensional multi-species hydrodynamical simulations of the CSM around the progenitor star for several WR life times, each followed by the interaction of the supernova ejecta with the CSM. We then looked at the influence of the length of the WR phase and compared the results of the simulations with the observations of Cas A.<i>Results. <i/>The difference in the structure of the CSM, for models with different WR life times, has a strong impact on the resulting SNR. With an increasing WR life time the reverse shock velocity of the SNR decreases and the range of observed velocities in the in increases shocked material. Furthermore, if a WR phase occurs, the remainders of the WR shell will be visible in the resulting SNR.<i>Conclusions. <i/>Comparing our results with the observations suggests that the progenitor star of Cas A did not have a WR phase. We also find that the quasi-stationary flocculi (QSF) in Cas A are not consistent with the clumps from a WR shell that have been shocked and accelerated by the interaction with the SN ejecta. We can also conclude that for an SN explosion taking place in a CSM that is shaped by the wind during a short (<i>≤<i/> 15 000 yr) WR phase, the clumps from the WR shell will be visible inside the SNR.

An Absolute Flux Density Measurement of the Supernova Remnant Cassiopeia A at 32 GH[CLC]z[/CLC

We report 32 GHz absolute flux density measurements of the supernova remnant Cas A, with an accuracy of 2.5%. The measurements were made with the 1.5 m telescope at the Owens Valley Radio Observatory. The antenna gain had been measured by NIST in May 1990 to be 0.505 ± 0.007 mK Jy-1. Our observations of Cas A in May 1998 yield Scas,1998 = 194 ± 5 Jy. We also report absolute flux density measurements of 3C 48, 3C 147, 3C 286, Jupiter, Saturn, and Mars.

Imaging of spectral features of Cas A with MECS on board BeppoSAX

We present BeppoSAX spatially resolved observations of Cas A. The angular resolution of the MECS instrument allowed to spatially image some spectral characteristics and features of the SNR related with the temperature distribution in the source: fluxes in several energy bands, hardness ratio, and Fe line intensity have been mapped with arcmin angular resolution. Beside the astrophysical interest, these results give also information on the capability of BeppoSAX of observing extended sources.

A more direct measure of supernova rates in starburst galaxies

We determine ages for young supernova remnants in the starburst galaxies M82 and NGC 253 by applying Chevalier's model for radio emission from supernova blast waves expanding into the ejecta of their precursor stars. Absolute ages are determined by calibrating the model with radio observations of Cas A. We derive supernova rates of 0.10 and 0.08/yr for M82 and NGC 253, respectively. Assuming L (sub FIR) to be proportional to the supernova rate, we find r(sub SN) approximately equal 2 x 10(exp -12) x L(sub FIR), solar yr(exp -1) for these archetypal starburst galaxies. This approach is unique in that the supernova rate is derived from direct observation of supernova remnants rather than from star formation rates and an assumed initial mass function (IMF). We suggest that the approach presented here can be used to derive star-formation rates that are more directly related to observable quantities than those derived by other methods. We find that the supernova rate, far infrared (FIR) luminosity, and dynamical mass of the M82 starburst place few constraints on the initial mass function (IMF) slope and mass limits.

On the secular decrease of the radio flux from cas a at low frequencies

Abstract Since 1987, systematic observations of Cas A (3C461) and Cyg A (3C405) (as a comparison source) have been carried out at 25 MHz (radiotelescope URAN-4). The result indicates that the problem of the secular decrease of Cas A radio flux at low frequencies can be solved only when the season—day effect, determined by observation conditions, has been taken into account.

Spectra of Cassiopeia A. II - Interpretation

The observations of Cas A presented in Paper I are discussed in the context of shock wave emission models computed by Raymond. The models for the brightest quasi-stationary flocculi indicate an overabundance of He and N relative to H by an order of magnitude. This implies that the gas was originally part of the H-burning core and that presupernova mass loss penetrated this region. The abundances of the fast-moving material support the hypothesis that this is uncontaminated material from the core of a massive star. Relative to O, we have abundance estimates for S, Ar, Ca, Fe, and Ne and have upper limits for H, He, N, Mg, and C. These results are compared to the stellar evolution calculations of Arnett and Schramm, and there is an indication that the presupernova star was very massive. Future searches for lines of He, Mg, and C will be of interest because they are predicted to be near the present detection limit.

Interstellar HDCO - Upper limit and significance as an indicator of interstellar molecule reactions

Upper limits to the strength of the HDCO (1$sub 11$-1$sub 10$) transition at 5346 MHz are obtained from observations of Cas A, Cloud 4, NGC 2264, DR 21, Orion B, and Sgr B2. The observations are interpreted to yield an upper limit to the abundance ratio HDCO/H$sub 2$CO that is most likely)about 1/ 170. We show that likely pure gas-phase processes (those employing CH$sub 3$+O and CH$sub 3$$sup +$+O) which have been proposed to form interstellar formaldehyde must also produce an enhanced HDCO/H$sub 2$CO ratio. Our observations indicate that the required enhancement probably does not occur. Formaldehyde is observed widely throughout the Galaxy and apparently requires a more complicated formation process than the simplest molecules. If gas reactions to produce formaldehyde cannot be found, it seems unlikely that the more complex interstellar molecules are produced purely by gas-phase reactions under the low temperature-density conditions where they are observed. (AIP)

Spectral Indices of 60 Discrete Sources.

Flux densities of 60 discrete sources were measured with a 60-ft reflector and maser radiometer at 1422.4 Mc at Harvard Observatory. The sources have galactic latitudes numerically greater than 10 deg, and were selected from the 85.5-Mc survey by Mills, Slee, and Hill and from the 159-Mc Third Cambridge Survey of Edge et al. The observations consisted of at least two scans in right ascension and two scans in declination across each source; thus the position at 1422.4 Mc could be compared to that reported at lower frequency. In only three cases was the difference in positions greater than 7 minutes of arc. Calibration was obtained from observations of Cas A and the adoption of Findlay's flux density. Spectral indices obtained for these sources have a narrower range than has been previously reported and seem independent of flux density. An explanation is given for the apparent correlation between flux density and spectral index found by other observers. The sources that are identified with galaxies in the Shapley-Ames catalogue have indices confined to the central part of the spectral index distribution, while those identified with more distant anonymous galaxies have a wider distribution of indices. This result is in conflict with recent findings of Heeschen. The sources listed as definitely extended by Mills, Slee, and Hill also have indices near the center of the distribution. If sources have a constant index with respect to frequency and if either the present observations or those at 960 Mc by Harris and Roberts are substantially correct, then the Third Cambridge flux densities are consistently low or the Mills-Slee-Hill flux densities are consistantly high. This applies even to sources whose angular sizes, measured by Moffet at 960 Mc suggest that an error due to angular size was not made in the observations at 159 Mc. The research program at Harvard is sponsored by the National Science Foundation.