* Astronomy

Title: Dense, Fe-rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?
Authors: K. J. Borkowski, S. P. Hendrick, S. P. Reynolds
(UPDATE)

We present observations of two LMC supernova remnants (SNRs), DEM L238 and DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central emission, surrounded by a faint shell, is present in both remnants. The central emission has an entirely thermal spectrum dominated by strong Fe L-shell lines, with the deduced Fe abundance in excess of solar and not consistent with the LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission originates in gas swept up and heated by the blast wave. A standard Sedov analysis implies about 50 solar masses in both swept-up shells, SNR ages between 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, and subluminous explosions with energies of 3x10^50 ergs. The central Fe-rich supernova ejecta are close to collisional ionisation equilibrium. Their presence is unexpected, because standard Type Ia SNR models predict faint ejecta emission with short ionisation ages. Both SNRs belong to a previously unrecognised class of Type Ia SNRs characterized by bright interior emission. Denser than expected ejecta and/or a dense circumstellar medium around the progenitors are required to explain the presence of Fe-rich ejecta in these SNRs. Substantial amounts of circumstellar gas are more likely to be present in explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar SNRs could be remnants of "prompt'' Type Ia explosions with young (~100 Myr old) progenitors.

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Title: Low-excitation blobs in the Magellanic Clouds
Authors: F. Meynadier, M. Heydari-Malayeri

Aims : We study an unknown, or very poorly known, interstellar HII component in the Magellanic Clouds. This is the first study ever devoted to this class of objects, which we call Low-excitation blobs (LEBs).
Methods : We used low-dispersion spectroscopy carried out at ESO to obtain emission line intensities of Ha, Hb, and (OIII) (4959+5007) for 15 objects in the Large Magellanic Cloud and 14 objects in the Small Magellanic Cloud. Results are displayed in excitation (oiii/Hb ratio) versus Hb luminosity diagrams.
Results : We show the presence of an LEB component in the Magellanic Clouds and study its relationship with the already known class of high-excitation blobs (HEBs). The newly found LEBs are lower excitation counterparts of HEBs and are powered by less massive exciting stars. Further study of LEBs is expected to provide new pieces of information for a better understanding the low mass end of the upper initial mass function in the Magellanic Clouds.

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Title: Dense Fe-Rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?
Authors: K. J. Borkowski, S. P. Hendrick, S. P. Reynolds

We present observations of two LMC supernova remnants (SNRs), DEM L238 and DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central emission, surrounded by a faint shell, is present in both remnants. The central emission has an entirely thermal spectrum dominated by strong Fe L-shell lines, with the deduced Fe abundance in excess of solar and not consistent with the LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission originates in gas swept up and heated by the blast wave. A standard Sedov analysis implies about 50 solar masses in both swept-up shells, SNR ages between 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, and subluminous explosions with energies of 3x10^50 ergs. The central Fe-rich supernova ejecta are close to collisional ionisation equilibrium. Their presence is unexpected, because standard Type Ia SNR models predict faint ejecta emission with short ionisation ages. Both SNRs belong to a previously unrecognized class of Type Ia SNRs characterised by bright interior emission. Denser than expected ejecta and/or a dense circumstellar medium around the progenitors are required to explain the presence of Fe-rich ejecta in these SNRs. Substantial amounts of circumstellar gas are more likely to be present in explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar SNRs could be remnants of ''prompt'' Type Ia explosions with young (~100 Myr old) progenitors.

Read more (561kb, PDF)

The remains of four supernovas in the nearby Large Magellanic Cloud galaxy provide a dazzling display of one of nature's most explosive events. These X-ray images show multimillion degree gas that has been heated by shock waves from the explosions. Moving in a clockwise direction from the upper left to the lower left, the approximate ages of the remnants are 600 years, 1500 years, 10,000 years and 13,000 years, respectively.


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Credit NASA/CXC/SAO

Chandra X-ray spectra provide important clues as to how these stars exploded. The remnants on the upper left, upper right and lower right show a concentration of elements typical of a Type Ia supernova. Such an explosion is triggered when matter is pulled from a companion star onto a white dwarf star and pushes it past the limit of stability. The ensuing thermonuclear explosion completely disrupts the white dwarf star.

In contrast, the supernova that produced the remnant SNR 0453-68.5 left behind a neutron star. This indicates that a Type II supernova occurred when the nuclear power source of a massive star became exhausted and the stellar core collapsed to form a neutron star, while the outer layers of the star were blown away. The rapidly spinning neutron star is ejecting a magnetized wind of extremely high-energy particles, which appears in the image as the elongated, bright blue-white spot at the centre of the remnant.

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