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Crab Nebula Discovery

See also the news item "Explosive origins for cosmic dust". This work was published in November 2012's edition of The Astrophysical Journal as A Cool Dust Factory in the Crab Nebula: A Herschel Study of the Filaments H. L. Gomez et al. 2012 ApJ 760 96 (also available via arXiv).

MESS

Herschel will study stars in our Galaxy to find out how much gas and cosmic dust are created and expelled during their lifetimes. This will help us understand how successive generation of stars and planets are formed. We already know that elements produced by stars in the past gave rise to our Solar System, and has led to at least one planet rich in molecules and teeming with life.

A three colour image of the core-collapse remnant Cassiopeia A

A three colour image of the core-collapse remnant Cassiopeia A combined from Herschel PACS images at wavelengths of 70 (blue), 100 (green) and 250 microns (red). See how clear the remnant is in blue, this is due to the infrared emission from hot dust (around 100K) but only a tiny amount of dust is required to shine this brightly at this wavelength. The faint green blob in the centre of the remnant is the newly discovered cool dust component (around 30,000 Earth masses). The red structures all over the map show emission from dust in unrelated Galactic clouds in the foreground (these are actually the spiral arm in between us and Cas A). For more details see Barlow et al 2010.

The life-cycle of a star depends on its mass. Stars less than eight times the mass of the Sun live for a long time (300 million to 10 billion years). They puff away their outer layers towards the end of their lives, producing a series of shells around the star. These multiple shells are effectively a fossil record of the stellar mass-loss history. About 90% of stars formed in our Galaxy go through this process, so understanding the evolution and mass loss from these stars is key to understanding how most stars live and die.

Stars more than eight times heavier than the Sun also enrich galaxies with cosmic dust and heavy elements. Near the end of their relatively short lives they eject material in intense winds, sometimes with speeds up to 4 million mph. The enigmatic star eta Carinae is thought to have lost over 80 times the mass of our Sun in the last 1,000 years in the form of gas and dust (Gomez et al 2011). Galactic supernova remnants like Cassiopeia A may be important at polluting galaxies with lots of dust and an important place for forming molecules (Rho et al 2010), but this is still a controversial topic.

As part of MESS, Herschel will look in detail at evolved stars and five galactic supernova remnants. Herschel's photometers will image the dust emission and its spectrometers will search for signatures of molecules and dust. The MESS team, including Gomez, Gear and Hargrave published 9 papers in the Herschel special issue journal and one in Nature led by Leen Decin. These papers included the discovery of a new cool dust component in the Galactic supernova remnant Cassiopeia A (about 30,000 Earth's worth of dust) led by Mike Barlow (Barlow et al 2010).

A three colour image of the Type Ia remnant Tycho

A three colour image of the Type Ia remnant Tycho combined from Herschel PACS images at wavelengths of 70 (blue), 100 (green) and 160 microns (red). Again, notice how bright the remnant is at blue wavelengths but then disappears at the longer wavelengths as in Kepler. For more details see Gomez et al 2012.

Recently, Dr Haley Gomez, published the first comprehensive study of dust formation in the ejecta of Type Ia supernova remnants (Gomez et al 2012). A Type Ia supernova originates from the thermonuclear explosion of a low mass star (a white dwarf) in a binary system with another star. These are the very objects used to probe huge distances as they act like a beacon in the distant Universe - they are extremely bright (so we see them far away) but they are also useful as all Ia explosions are thought to have the same intrinsic energy when they explode. For this reason, Ia explosions were used in a recent study in 1998 which found that that further away supernovae were dimmer than they should be, hence the idea of a mysterious force pushing everything further away from each other than we expected - i.e. dark energy. This study won the Nobel Prize for Physics in 2011 so these astrophysical objects are very important to understand, and the presence of dust around these objects could slightly affect these results.

A three colour image of the Type Ia remnant Kepler

A three colour image of the Type Ia remnant Kepler combined from Herschel PACS images at wavelengths of 70 (blue), 100 (green) and 160 microns (red). Again, the remnant is clearly seen at 70 microns (blue in this image) due to the presence of a tiny amount of infrared emission from hot dust (less than 10 Earth's worth of dust). As with Cas A, the red structures all over the map show emission from dust in unrelated Galactic clouds in the foreground. For more details see Gomez et al 2012.

Dr Gomez found no evidence for significant dust formation in Type Ia supernovae which places stringent constraints on the environments in which dust and molecules can form, and also in the contribution of these exploding binary systems to the dust budget in galaxies. This is a relief to studies which hope that distant Ia sources are not being "dimmed" by dust shells.

The lack of cold dust grains in the ejecta of the two remnants studied by Dr Gomez suggests that Type Ia remnants do not produce substantial quantities of iron-rich dust. When astronomers compare the amount of iron expected in the gas expanding out from the explosion, it's a lot more than we observe using X-ray spectra. Astronomers had assumed this is because a lot of the iron was taken from the gas and turned into a solid dust form, hidden from the spectra. However, we see no evidence for this in the far-infrared emission from dust in these two objects, and this has important consequences for the 'missing' iron mass in supernova ejecta.