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VLA OH Zeeman Magnetic Field Detections Toward Five Galactic Supernova Remnants

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VLA OH Zeeman Magnetic Field Detections Toward Five Galactic Supernova Remnants. C. L. Brogan 1 , D. A. Frail 2 , W. M. Goss 2 , & T. H. Troland 1. ( 1 University of Kentucky; 2 National Radio Astronomy Observatory).

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VLA OH Zeeman Magnetic Field Detections Toward Five Galactic Supernova Remnants

C. L. Brogan1, D. A. Frail2, W. M. Goss2, & T. H. Troland1

(1 University of Kentucky; 2 National Radio Astronomy Observatory)

Abstract: We have observed the OH (1720 MHz) line in five galactic SNRs with the VLA to measure their magnetic field strengths using the Zeeman effect. Our estimated fields range from 0.2 to 2 mG. The magnetic field strengths are consistent with the hypothesis that ambient molecular cloud magnetic fields are compressed via the SNR shock to the observed values. Magnetic fields of this magnitude exert a considerable influence on the the cloud with the magnetic pressures (10-7 - 10-9 erg cm-3) exceeding the pressure in the ISM or even the thermal pressure of the hot gas interior to the remnant.

  • Properties of OH (1720 MHz) Masers
  • 1720 MHz masers trace molecular clouds - Temperature T ~ 50 - 125 K
  • - Density n ~ (0.1 - 5) x 105 cm-3
  • (Lockett, Gauthier, & Elitzur 1999; Wardle et al. 1999)
  • They are found in ~ 10% of galactic SNRs
  • - Evidence for molecular cloud/SNR interaction
  • (Frail et al. 1996; Green et al. 1997)
  • They must originate in transverse shocks
  • - Velocity coherence
  • (Lockett, Gauthier, & Elitzur 1999; Frail & Mitchell 1999)

Figure 3: Cartoon of SNR/molecular cloud interaction. In this picture, X-rays (from the SNR) and cosmic rays excite the Lyman-Werner bands of H2 which in turn re-radiate in the FUV (see Wardle 1999). This weak FUV flux can then dissociate ~ 1% of the H2O synthesized in the C-type shock to produce the OH abundance needed for maser production in the postshock gas. Comparison of estimates for the pre and postshock densities suggest that the postshock magnetic fields are amplified from their preshock values by simple shock compression (i.e. rps/ro ~ Bps /Bo).

  • Conclusions
  • We have detected magnetic fields between 0.2 and 2
  • mG toward 5 galactic SNRs.
  • The B-fields are strong enough to have a significant
  • impact on the energetics of the postshock gas.
  • The observed field magnitudes can be obtained
  • through shock compression of the molecular cloud
  • => no need to invoke turbulent amplification.
  • OH (1720 MHz) masers are a powerful probe of
  • SNR /Molecular cloud interactions.
  • Role of Magnetic Fields in SNR Shocks
  • Determines amount of shock compression
  • Synchrotron Emission from SNR
  • Compression of field lines can help support the
  • postshock gas
  • Ambipolar diffusion can lengthen cooling time in
  • postshock gas
  • Cosmic Ray Acceleration

Figure 1: Continuum images of the observed SNRs along with a Table of our OH (1720 MHz) maser Zeeman magnetic field detections. Note that field values were calculated using the “thermal” Zeeman equation so that the listed field strengths could overestimate the total field strength by up to a factor of five (e.g. Elitzur 1996). The maser locations in each SNR are marked by plus symbols or crosses.

  • Magnetic Fields From the Zeeman Effect
  • Polarization studies of synchrotron emission only give
  • direction of B.
  • For “thermal” Zeeman splitting < the line width, the field
  • strength along the line of sight (Blos) can be determined.
  • (Troland & Heiles 1982)
  • Stokes V = R - L
  • Stokes I = R + L
  • Z=0.6536 Hz G-1 (1720 MHz OH)
  • Future work
  • High Resolution Molecular Data
  • - Obtain column densities and estimate density
  • - Derive temperatures
  • - Locations of transverse shocks from kinematics
  • VLBI to resolve maser spots
  • - Is there evidence for tangling in the magnetic field lines
  • as suggested by the VLBI results of Claussen et al.
  • (1999) toward W28?

Figure 2: Sample fits of the Zeeman magnetic field strengths for W51. The upper panels show the VLA Stokes I profiles (solid histograms), and the lower panels show the VLA Stokes V profiles (solid histograms) with the fitted derivative of Stokes I shown as smooth dotted curves.

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