EDP Sciences
Free access
Volume 497, Number 1, April I 2009
Page(s) 167 - 176
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361/200811385
Published online 18 February 2009
A&A 497, 167-176 (2009)
DOI: 10.1051/0004-6361/200811385

Observational and theoretical constraints for an H$\alpha$-halo around the Crab nebula

A. Tziamtzis1, M. Schirmer2, 3, P. Lundqvist1, and J. Sollerman1

1  Department of Astronomy, AlbaNova University Center, Stockholm University, 106 91 Stockholm, Sweden
    e-mail: anestis@astro.su.se
2  Isaac Newton Group of Telescopes, Calle Alvarez Abreu 68 2, 38700 Santa Cruz de La Palma, Spain
3  Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received 20 November 2008 / Accepted 5 February 2009

Aims. We searched for a fast moving H$\alpha$ shell around the Crab nebula. Such a shell could account for this supernova remnant's missing mass, and carry enough kinetic energy to make SN 1054 a normal type II event.
Methods. Deep H$\alpha$ images were obtained with WFI at the 2.2 m MPG/ESO telescope and with MOSCA at the 2.56 m NOT. The data are compared with theoretical expectations derived from shell models with ballistic gas motion, constant temperature, constant degree of ionisation, and a power law for the density profile.
Results. We reach a surface brightness limit of 5 $\times$ 10-8 erg s-1 cm-2 sr-1. A halo is detected, but at a much higher surface brightness than our models of recombination emission and dust scattering predict. Only collisional excitation of Ly$\beta$ with partial de-excitation to H$\alpha$ could explain such amplitudes. We show that the halo seen is caused by PSF scattering and thus not related to a real shell. We also investigated the feasibility of a spectroscopic detection of high-velocity H$\alpha$ gas towards the centre of the Crab nebula. Modelling the emission spectra shows that such gas easily evades detection in the complex spectral environment of the H$\alpha$-line.
Conclusions. PSF scattering significantly contaminates our data, preventing a detection of the predicted fast shell. A real halo with observed peak flux of about 2 $\times$ 10-7 erg s-1 cm-2 sr-1 could still be accomodated within our error bars, but our models predict a factor 4 lower surface brightness. Eight meters class telescopes could detect such fluxes unambiguously, provided that a sufficiently accurate PSF model is available. Finally, we note that PSF scattering also affects other research areas where faint haloes are searched for around bright and extended targets.

Key words: ISM: supernova remnants -- stars: supernovae: individual: SN1054

© ESO 2009