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Constraints on fast ejecta in the Crab supernova remnant from optical spectral lines
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy. Korea Astronomy and Space Science Institute, Republic of Korea.
2012 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 423, no 2, 1571-1582 p.Article in journal (Refereed) Published
Abstract [en]

The low kinetic energy and mass of the Crab supernova remnant challenge our understanding of core-collapse supernova explosions. A possibility is that the Crab nebula is surrounded by a shell of fast ejecta containing the missing kinetic energy and mass. The only direct evidence for such a fast shell comes from an absorption feature in the Crab pulsar spectrum as a result of C iv?1550. The velocities inferred from the C iv line absorption extend to at least similar to 2500 km s-1, which is about twice as fast as the expansion of main shell of the remnant in our direction. We have searched for additional evidence of fast-moving ejecta in the optical spectra obtained with the FORS1 instrument at the European Southern Observatory (ESO) 8.2-m Very Large Telescope (VLT) and with the Andalucia Faint Object Spectrograph and Camera (ALFOSC) at the 2.56-m Nordic Optical Telescope (NOT), with the focus on absorption in Ca ii??3934,3968, and emission components in [O iii] ??4959,5007. The data are compared with the C iv?1550 absorption, and with theoretical expectations derived from shell models with ballistic gas motion, and a power-law density profile of the fast ejecta. Along the line of sight to the pulsar, we find that no gas in the nebula moves faster towards us than approximate to 1400 km s-1. We identify this gas as part of the known main shell of the remnant. This velocity agrees with previous results showing that the Crab nebula is moving slowly in this direction. It is slower than the velocity of 1680 km s-1 used in the models of Sollerman et al. as a minimum velocity of the presumed fast shell of supernova ejecta to account for the C iv line absorption. We find faster moving gas within 310 arcsec north and south of the pulsar, where the fastest gas moving towards us, as traced by [O iii], has a velocity of 16501700 km s-1. The fastest [O iii] emitting gas along the line of sight to the pulsar, on the rear side of the nebula, has a velocity of approximate to+1800 km s-1, which is higher than the velocity previously recorded for that direction. However, neither the [O iii] nor Ca ii lines display any signatures of fast shell ejecta at the velocities inferred from the C iv line absorption. To fully rule out the possibility that a chimney-like structure directed towards us could be responsible for the C iv line absorption, we need deep observations taken with 810-m class telescopes with good spectral resolution. We show that a spectral resolution better than similar to 200 km s-1 is needed to draw any conclusions on emission lines from gas moving towards us, along the line of sight of the pulsar, faster than similar to 1700 km s-1. To probe the fast shell ejecta, new observations from the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) should be substantially more powerful than the previous HST Space Telescope Imaging Spectrograph (STIS) data to fully explore the C iv?1550 absorption-line profile.

Place, publisher, year, edition, pages
2012. Vol. 423, no 2, 1571-1582 p.
Keyword [en]
supernovae: general, supernovae: individual: SN 1054, ISM: supernova remnants
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-79894DOI: 10.1111/j.1365-2966.2012.20979.xISI: 000305070900046OAI: oai:DiVA.org:su-79894DiVA: diva2:551775
Note

AuthorCount:2;

Available from: 2012-09-12 Created: 2012-09-11 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Observations of young core collapse supernova remnants
Open this publication in new window or tab >>Observations of young core collapse supernova remnants
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Studies of young remnants offer an opportunity to test theoretical models of stellar evolution, explosion models and nucleosynthesis, as well as our understanding of the compact objects in the centre of the exploded stars.

The first part of the thesis involves observations of the Crab nebula. We have used photometric and spectroscopic observations to search for a faint halo around the visible nebula, that could carry the missing mass and kinetic energy of the nebula. No halo was found. In the photometric data due to psf contamination, and in the spectroscopic no fast velocity components were present. We have also used optical and IR photometry to check for variation in the emissivity and dynamic structure of the Crab pulsar wind nebula (PWN), to try to understand the nature of plerionic PWNe. There, we measured flux variations up to 20% in the IR and also shifting of the wisps with velocities up to 0.2c. We also showed that the nearby red knot moves in tandem with the Crab pulsar.

The second part of the thesis, deals with photometric &  spectroscopic observations of SN 1987A in the LMC. The aim of the project was to monitor the evolution of the outer rings (ORs) of SN 1987A. The fading of the ORs is consistent with recombination and cooling after the initial flash ionization by the supernova. From the spectroscopic data we measured the density and temperature in the ORs where we found temperatures of ~ 12,000 K for the [N II] gas, and ~ 25,000 K for the [O III]. Finally, from the [O II], and [S II] ratios we estimated electron densities of ~ 1,000 cm-3 and ~ 2,500 cm-3, respectively. From the evolution of Hα, we argue that the highest density in the ORs could be 5,000 cm-3.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2011. 63 p.
Keyword
supernova remnants, Crab, SN 1987A
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-58319 (URN)978-91-7447-319-3 (ISBN)
Public defence
2011-10-05, FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

Available from: 2011-09-13 Created: 2011-05-31 Last updated: 2015-07-29Bibliographically approved

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