Supernova remnants

 

A supernova remnant (SNR) is the remains of a supernova explosion. Due to the tremendous energy released, the largest part of the mass of the star travels into space at supersonic velocities. It is the interaction of the expanding supernova remnant shell with the low density interstellar medium which gives rise to radiation that can be detected by modern experiments, satellites or ground--based telescopes. This shell of gas is traveling for tens of thousands of years, its velocity is continously decreasing until it mixes with the interstellar medium and its surface brightness becomes so low that we cannot detect it anymore. SNRs are very important for understanding our Galaxy. They influence star formation and the recycling of heavy elements throughout the Galaxy and accelerate cosmic rays. In addition in SNR's we have the opportunity to observe processes under conditions that would be impossible to produce in Earth - based laboratories. These involve Atomic physics (e.g. collisional ionization, photoionization, etc.) - Radiative transfer - Hydrodynamics- Magnetic fields.

 

SNR CTB1
(0.3m telescope, Ha+[NII] filter)

Phases in SNR evolution

  • Free expansion (Mswept-up<Mejecta) with Shock velocity > 1,000 km/s, Δt~1,000 yrs

  • Adiabatic expansion (Mswept-up>Mejecta) with Shock velocity < 1000 km/s, Δt~10,000 yrs

  • Radiative expansion (Mswept-up>Mejecta) Shock velocity > 10 km/s, Δt>100,000 yrs

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    Observations of SNRs

    Currently known SNRs
    - Green's catalogue lists 231 SNRs (mainly radio detections; ~1/3 show optical emission)
    - Observable from Skinakas 105 SNRs (>Thirty (30)of these show optical emission)

    Skinakas contribution:

    The 30 cm wide field telescope was used for detection and imaging and the 1.3 m telescope for detailed studies and spectroscopy of SNRs.

    Results:
    - Three (3) new candidate SNRs (totally unknown before) have been detected.
    - Four (4) SNRs have been detected for the first time in the optical
    - Nine (9) SNRs have been imaged for the first time with a CCD and several new structures have been detected. - Five (5) radio-SNRs no optical emission could be detected.

    Main Derived Properties

  • Detailed morphology of individual SNRs in several optical emission lines

  • Flux calibrated images and long - slit spectra

  • Evidence for a highly inhomogenuous and “clumpy” ISM

  • Deduced shock velocities ~ 60 - 140 km/s

  • Determination of the ionization state of the ISM “clouds”

  • Indication for the presence of magnetic fields (~10 μG)

  • Explosion energy estimates (compatible with X--ray observations)

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    Example: SNR G65.3 + 5.7

    snr_g65.jpg (272315 bytes)

    The first CCD mosaics of this supernova remnant in the optical emission lines of [OII] and [OIII] were obtained from Skinakas Observatory. The new images reveal several diffuse and filamentary structures both inside and outside the extent of the remnant. The medium ionization line of [OIII] 5007 A provides the sharpest view to the system, while the remnant appears less filamentary in the emission line of [OII]3727 A. There are significant morphological differences between the two images strongly suggesting the presence of incomplete shock structures. Long and short scale variations are observed in several areas of the remnant, the shortest ones with a length of ~0.6 light-years. Deep long--slit spectra were taken at several different positions to study the physical conditions in more detail. All spectra show that the emission originates from shock heated gas, while the majority of them is characterized by large [OIII]/Hbeta ratios. The absolute Ha flux ranges from 0.5 - 22 x 10^-17 erg/s/cm^2/arcsec^2, while the sulfur line ratios indicate electron densities below ~200 cm^-3. Estimates of the shock velocities and the column densities lie in the range of 90-140 km/s. Finally, the observed variations of the Ha/Hbeta ratios may reflect the presence of intrinsic absorption affecting the optical spectra.
    The estimated distance to this remnant is ~3000 light-years and its age is ~26000 years.