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TNG data help to discover a new class of very dim Supernovae

Supernovae are among the most energetic and violent events in the Universe. They constitute the final tremendous explosions that end the life cycles of some stars. The typical kinetic energies released in these explosions achieve 1051 ergs, while the outher layers of the exploding stars are ejected at velocities as high as 10% of the speed of light. A canonical supernova shines as brightly as billions of stars together, and the total energy released exceeds that of a star like the Sun during its whole life time of 10 billion years.

However, in the last decade it has been discovered that in some cases massive stars produce explosions up to 100 times less energetic than the usual, and characterised by particularly low ejecta velocities and low luminosities. To conform with the dimness of this supernovae, only little amount of radioactive material may be present in their ejecta. In fact, it is this radioactive material (most notably a radioactive Ni isotope, which is always produced in supernova explosions) that makes the supernovae shine through the energy released in its decay.

Theory suggests that the oxygen-neon core of a star of 7-9 solar masses may undergo a collapse which produces little amount of Ni and results in a low-energy supernova. Alternatively, the progenitor star may be much more massive 25-30 solar masses star and form a black hole in the centre of the supernova. Then most of the radioactive material is swallowed by the black hole instead of being ejected.

So far all low energy supernovae were shown to retain, at the time of explosion, their outer hydrogen envelopes. This was somewhat suprising, since in many cases 25-30 solar masses stars loose most of the hydrogen envelope via strong stellar wind, while 7-9 solar masses stars may get rid of the hydrogen envelope through the interaction with a companion star in binary systems.

A team of researchers of the Queen's University in Belfast, the Italian National Institute for Astrophysics (INAF), the Max Planck Institute for Astrophysics (MPA) and several other institutions have now for the first time found a dim supernova in which no hydrogen was detectable. This new event, SN 2008ha, was discovered in the constellation Pegasus, 67 million lightyears away from Earth. Today the analysis and results of the work leading to this discovery are published by Nature.

Italian Telescopio Nazionale Galileo (TNG) situated in Canary Islands, Spain, observed SN 2008ha at various phases of its evolution. While the TNG contribution to the SN 2008ha observations was fundamental (it provided more than half of the observational data), the supernova was also targeted by several other european telescopes, inlcuding the German/Spanish Calar Alto Telescope (Andalucia, Spain), Nordic Optical Telescope (NOT) and Liverpool Telescope (both Canary Islands, Spain) and the Copernico Telescope (Asiago, Italy). The analysis of the obtained observational data indeed show the low-luminousity nature of SN 2008 and prove that no hydrogen was present in the explosion.

SN2008ha

These characteristics of SN 2008ha are important findings for astronomers, and two scenarios of the origin of this supernova are possible: the progenitor may have been a moderately massive star in a binary system, which lost its envelope through the interaction with the companion, or a very massive star which shed its outer layers through stellar winds and formed a black hole upon core collapse.

This discovery has important implications, in particular, in the connection between supernovae and a group of gamma-ray bursts (GRBs), so-called long gamma-ray bursts. Most long gamma-ray bursts are associated with mainly very energetic and luminous supernovae. However, there are at least 2 proven cases of long gamma-ray bursts with no detected bright supernova, so that if any supernova was present it should be extremely dim. An association with the class of dim supernovae was proposed for these bursts, although at that time only hydrogen-rich dim supernovae were known and there was a major caveat, since an extended hydrogen envelope would prohibit the formation of a gamma-ray burst. The discovery of hydrogen-deficient dim SN 2008ha may provide a clue to these cases and complete the link between supernovae and long gamma-ray bursts.

TNG involvement in the SN field has always been relevant. Starting this year a public service for the spectra classification is provided by our staff via our web pages. GELATO (GEneric cLAssification TOol) is developed in the SN group of Padova Observatory and uses observed spectra as templates of comparison. The GELATO web interface is available at https://gelato.tng.iac.es.

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