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官方頁面: CSI: Astrophysics; solving a 400 year old mystery

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By Matt Ford | Published: December 04, 2008 - 11:51AM CT

By 1572, Danish nobleman Tycho Brahe was an already accomplished astronomer, having spent a number of years making detailed observations of the movement of planets and stars. However, on November 11th of that year, he saw something that he had not seen previously: a new, very bright star appeared suddenly in the constellation Cassiopeia. He published a small book the next year titled De Stella Nova—coining the term nova for a new star—describing his observations of what modern astronomers now refer to as supernova SN 1572.

Multiwavelength image of SN 1572 remnant
Image credit: Max-Planck-Institut für Astronomie

Not only was this a stunning discovery in its own right, but Tycho's calculation based on the parallax of the star put it at a distance well beyond the Moon. This observation flew in the face of the Aristotelian perspective, which held that the heavens beyond the sphere of the moon were immutable. Tycho's discovery was one of the events that precipitated the rejection of the nearly 2000 year old Aristotelian notion of the heavens, a change that ultimately brought about our modern understanding.

While it's easy to observe the remnants of SN 1572, it has been difficult to determine what type of supernova it was. Over the years, it has been postulated that it was a Type Ia supernovae, one that might have been standard, subluminous, or superluminous. Others have hypothesized that it was a core-collapse (type Ib or II-L) supernova. Without direct spectroscopic data, no conclusive determination has been possible. A letter appearing in this week's edition of Nature describes attempts to determine what happened to the progenitor star of SN 1572 and solve the mystery that began over 400 years in the past.

Direct measurements of the light that was given off when the star exploded would seem to be difficult to obtain 400 years after the fact. Even though the source of the light is long gone, however, the light itself still survives due to scattering, absorption, and re-emission in the dust that exists in the intergalactic medium near the remnant. This light echo allows researchers to conduct a "spectroscopic post mortem" on the light that came from SN 1572 all those years ago.

The authors used spectral templates, obtained from observations of well studied modern supernovae, to match their observations of SN 1572 with overluminous, normal, and sub-luminous type Ia supernovas. They found that the sub- and overluminous spectra have features that simply do not match the remnants of SN 1572. They conclude that what Tycho observed belongs to the common variety of normal type Ia supernovae.

Using their data, the researchers were able to place it at a distance of 3.8^+1.5_-1.1 kpc from Earth. This is further out than the normally quoted range of 2.3-2.8 kpc, but it is consistent with the proposed distance to the remaining companion star. If this calculation is verified through other measurements, it would provide conclusive evidence that SN 1572 existed in a single-degenerate scenario, where the exploding star accreted matter from this remaining companion prior to going supernova.

The researchers also observed an oddity in the spectrum—an uncommon adsorption feature of Ca^II—that is similar to a feature observed in SN 2001el. They conclude that SN 1572 has an asphericity, much like SN 2001el, that resulted from either the explosion or the accretion of matter from its neighbor.

Nature, 2008. DOI: 10.1038/nature07608

* Thanks to NI readers ralphpe and ArsEitje. I stole their comments on a previous article to use for the title