Supernova Cause Revealed A Millennium Later
September 28, 2012

Cause of Rare Supernova Found A Millennium Later

April Flowers for - Your Universe Online

Just over 1,000 years ago, one of the brightest stellar events ever recorded in history occurred. Between April 30 and May 1, 1006, a supernova was widely observed by a number of civilizations across the globe. Now, more than a thousand years later, a research team led by the University of Barcelona, the Instituto de Astrofisica de Canarias (IAC) and the Instituto de Ciencia de Materiales de Madrid (CSIC) has determined that SN 1006 was probably the result of the merger of two white dwarf stars.

Ancient astronomers around the world observed the supernova that is now known as SN 1006. Chinese astronomers, among others, highlighted the fact that the astronomical event was visible for three years. An Egyptian doctor and astronomer, Ali ibn Ridwan (988-1061), made the most explicit record of the event, noting that the supernova was about three times brighter than Venus and  almost a quarter as bright as the Moon.

Pilar Ruiz-Lapuente, a researcher at the Institute of Cosmos Sciences (ICCUB) and the Instituto of Fundamental Physics (IFF-CSIC), explains, “In this work the existing stars in the area have been studied, regarding distance and possible contamination by elements of the supernova, and the results show that there is no star that could be considered the progenitor of this explosion.”

IAC researcher Jonay González Hernández adds, “We have conducted an exhaustive exploration of the area around where the explosion of the supernova of 1006 occurred and have found nothing, which invites us to think that this event was probably the result of a collision and merger of two white dwarf stars of similar mass”.

A supernova like SN 1006 occurs in binary systems, which consist of two astronomical objects bound together by their gravitational pull. There are two possible combinations for binary systems. In the first, a white dwarf is paired with a normal stellar companion that contributes the necessary matter for it to reach critical mass. This is known as the Chandrasekhar limit, where critical mass is 1.4 times the mass of the Sun. Once this limit is reached, the stars explode in a supernova. The second combination is a system with two white dwarf stars that eventually merge to create the supernova.

According to Ruiz-Lapuente, “This new result, together with others previous, suggests that the merger of white dwarfs could be a common pathway that leads to these violent thermonuclear explosions.”

Supernovae explosions happen between stars that are in the last stages of their lives. This explosion produces a huge release of energy that expels massive amounts of matter at an extremely high velocity. A supernova of the type that occurred in 1006 is caused by a thermonuclear explosion when the white dwarf's mass reaches the Chandrasekhar limit. The explosion is so intense that it most likely expels all its matter, thus leaving behind no stellar remnant of the explosion.

The research team was able to conclude that this supernova was the result of two white dwarfs thanks to one final clue: This supernova, about 7,000 light years from Earth, had no stellar companion for the white dwarf progenitor. An explosion produced by the merger of two white dwarfs leaves no trace, however, expect for the supernova remnant that can be studied centuries later.

SN 1006 is one of only four historical supernovae of this type that have been recorded in the Milky Way.

The study, published in Nature, used a UVES high-resolution spectrograph installed in one of four European VLT telescopes belonging to the European Southern Observatory (ESO) in Chile, to observe the stars around the site of the explosion. The spectroscopic and photometric data were analyzed by Joney González Hernández.

“Analysis of the stars in the area of the explosion discarded them as possible companions of the progenitor star of the supernova of 1006,” said Hernández.

Different types of stars in the area were analyzed: giants, subgiants and dwarfs.

According to Hernández: “Only four giant stars are found at the same distance as the remnant of the supernova of 1006, some 7,000 light years from Earth, but the numerical simulations do not predict a companion of these characteristics. The appearance of a possible stellar companion, even a thousand years after the violent impact of an explosion of this type, would not be that of a normal giant star”.

White dwarfs are at the end stage of their lives with a mass of less than 1.4 times that of the Sun. They have exhausted their energy sources and are gradually cooling down. The vast majority of stars in the Milky Way, including our Sun, will end their life cycles as white dwarfs.

The merger of stars of this type is consistent with existing stellar models from the theoretical point of view of the study.

In 2004, the research team identified another star as the stellar companion of the supernova of 1572.

Ruiz-Lapuente comments, “Then, we explored another region near the centre of the remnant of Tycho´s supernova and found a subgiant star of similar temperature to the Sun, which could be the companion to the progenitor star of the supernova of 1572. In this new study, our intention was to look for the companion of the supernova of 1006, but, to our surprise, we did not find one.”

Ruiz-Lapuente began the study of Type Ia supernovae with this method of searching for progenitor stars in 1997. To date, five supernovae have been located and studied. A companion has only been found in the case of Tycho Brahe's 1572 supernova.

“We intend to continue studying supernovae remnants to determine the frequency of the merger of white dwarfs as a pathway. The next will be Kepler´s supernova of 1604”.