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Home > Public Information > Scientific Highlights > 2004 > WHT Identifies the Companion Star to Type Ia Tycho Brahe's 1572 Supernova



The brightness of type Ia supernovae, and their homogeneity as a class, makes them powerful tools in cosmology, yet little is known about the progenitor systems of these explosions. They are thought to arise when a white dwarf accretes matter from a companion star, is compressed and undergoes a thermonuclear explosion. Unless the companion star is another white dwarf (in which case it should be destroyed by the mass-transfer process itself), it should survive and show distinguishing properties.

Tycho's supernova is one of only two type Ia supernovae observed in our Galaxy, and so provides an opportunity to address observationally the identification of the survi+ving companion.

A team of astronomers has carried out an imaging and spectroscopic survey of the central region of Tycho's supernova remnant, around the position of the explosion, which excluded red giants as the mass donor of the exploding white dwarf. They also found a type G0–G2 star, similar to our Sun in surface temperature and luminosity (but lower surface gravity), moving at more than three times the mean velocity of the stars at that distance, which they claim to be the surviving companion of the supernova.

Centre of Tycho Supernova Remnant
Centre of Tycho's supernova remnant (Tycho SNR). The star marked as 'g' is the one identified as the companion star of Tycho Brahe´s 1572 supernova. Since the supernova explosion in 1572, Tycho G has moved 2.6 arcseconds south on the sky and it is still within a radius of 40 arcseconds centred on Chandra´s measured centre of the X-ray remnant (40 arcseconds is 15% of the innermost radius of Tycho SNR and taken as the error in the determination of the remnant centre).  [ JPEG | TIFF ]

Tycho G is a star of type G0-G2 IV located at the distance of Tycho SNR and it moves in space at 136 km/s, which is a factor of over 3 larger than the mean velocity of the surrounding stars (Tycho G´s metallicity in Fe and Ni are similar to solar values and therefore it can´t be a halo star). Its low surface gravity can also be interpreted as a consequence of mass strip by the impact of the supernova explosion.

Spectra of Tycho G and some candidates
Model fits to observed spectra. The stars shown here are the candidate star for the companion of SN 1572 (Tycho G), a red giant (Tycho A) and main-sequence star (Tycho B) nearest to the distance of SN 1572 and to the SNR X-ray centre. Identifications of the most significant metal lines are given. Spectra were obtained at the William Herschel Telescope with UES and ISIS. The upper panel shows the observed spectrum near Hα. This line is blueshifted, implying a peculiar radial velocity exceeding about 3 times the velocity dispersion for its stellar type. [ JPEG | TIFF ]

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Last modified: 13 December 2010