Supernova in collision

A team of astronomers from Caltech working on an automated observation system called intermediate Palomar Transient Factory has discovered a new type of Ia supernova that suggests the possibility of two distinct populations for this type of sources.
The Type Ia supernovae are one of the brightest phenomena of the Universe, and are produced when small dense stars called white dwarfs explode with fierce intensity. To their maximum brightness, these supernovae can shine more intensely than an entire galaxy. Although it was observed thousands of supernovae of this type in recent decades, the process by which a white dwarf produces a glow that is still unclear.


Credit: Daniel Kasen, Berkeley Lab/ UC Berkeley

A bit of clarity has begun to have as of May 3, 2014, when a group of astronomers from Caltech working on a robotic observation system called intermediate Palomar Transient Factory (IPTF, a collaboration between several institutions led by Shrinivas Kulkarni, Professor of Astronomy and Planetary Sciences supported by the MacArthur Foundation and director of Caltech Optical Observatories) have discovered a Type Ia supernova identified by the initials iPTF14atg in the galaxy IC 831, about 300 million light-years away from us.
The data collected by the team of IPTF support one of the two competing theories about the origin of Type Ia supernovae, and suggest the possibility that there are two distinct populations of this type of sources. The details are described in a paper to be published in the journal Nature in the number of 21 next May, with first author Yi Cao, a graduate student at Caltech.


Credit: Daniel Kasen, Berkeley Lab/ UC Berkeley

The Type Ia supernovae are known to scientists as “standard candles” because they allow you to measure cosmic distances, because there are good reasons to believe that all have the same absolute brightness, and then depending on how you appear weak estimate their distance. It likes to know that the same lamp placed in a km away appears 100 times weaker if it is 100 meters away from us. This feature is what made the Ia supernovae instruments capable of measuring the increase of expansion of the universe in 1990, providing three scientists the Nobel Prize for Physics in 2011.
There are two competing theories about the origin of these objects, both start from the same scenario: the white dwarf meant to explode part of a pair of stars that orbit a common center of mass. The interaction between these two stars, the theories say that is responsible for the triggering of the explosion of a supernova. What is the nature of this interaction? On this point the theories diverge.
According to one theory, the so-called model “double degenerate”, the companion of the white dwarf next explosion is another white dwarf, and the supernova explosion is triggered when the two objects merge into one another. In the second theory, the model “single degenerate”, the second star is similar to the Sun or a red giant. In this model the gravity of the white dwarf sucks the material of the second star. This process increases the temperature and pressure in the center of the white dwarf until a trigger-type reaction runaway, a particular case of positive feedback during which the system moves in a critical manner from the equilibrium condition, and can end in an explosion.
The difficulty in determining which model is correct is the fact that these supernovae are very rare (occurring about once every few centuries in our galaxy) and that the progenitor star is very weak before occur explosions.
This is where comes in the IPTF. From the top of Mount Palomar in southern California, where the Samuel Oschin Telescope by 1.22 meters, the room completely automated observed in the optical band about 1,000 square degrees of the sky every night (about 1/20 of the sky visible above the horizon) searching for sources transients, or whose brightness changes on time scales ranging from hours to days. Among these sources it is also the type Ia supernovae.


Credit: NASA/CXC/M Weiss

On 3 May last year, the IPTF has collected images of the galaxy known as IC 831 and then sent the data because computers at the National Energy Research Scientific Computing Center analizing them with a machine learning algorithm can distinguish between celestial objects and artifacts. Since this first analysis was performed when it was night in the United States and Europe in broad daylight, the European and Israeli collaborators of the project were the first to examine the results for signs intriguing. After identifying a possible supernova (a signal that was not visible in the pictures taken the previous evening) teams have alerted their American colleagues, including Yi Cao, a graduate student at Caltech and a member IPTF.
Cao and his colleagues then mobilized ground and space telescopes, including NASA’s Swift satellite, which houses a telescope on board to ultraviolet (UV), because it would give a closer look at the young supernova.
“My colleagues and I have spent many sleepless nights to design a system to search for ultraviolet emission from young Type Ia supernovae,” says Cao. “As you can imagine, I am very much impressed when I saw for the first time a bright spot in the position it was in the supernova. I knew that it could be what we hoped to find. ”
The UV radiation has a greater energy than the visible light, which is particularly suitable for observation of very hot objects such as supernovae, although such observations are possible only from space, since the atmosphere absorbs almost all the ultraviolet light from Universe. The telescope aboard Swift has measured a flash of UV radiation that is initially decreased, and then increased again while the supernova lit up. Since a pulse of this type has a short duration, it can be overlooked by surveys looking at the sky less frequently than does the IPTF.
This type of ultraviolet light is consistent with a scenario in which the material ejected by the supernova explosion of an impact against the companion star, generating a shock wave that ignites the surrounding material. In other words, the data are in agreement with the model “single degenerate”.
In 2010, Daniel Kasen, an associate professor of Astronomy and Physics at the University of Berkeley and the Lawrence Berkeley National Laboratory, through theoretical calculations and simulations involved the issue of a similar impulse coming from collisions between supernovae and companion stars. “After my prediction a lot of people tried to catch that signal,” Kasen said. “This is the first time anyone has actually seen. With this observation opens a new way of studying the origins of exploding stars. ”
According to Kulkarni discovery “provides direct evidence of the existence of a companion star in a Type Ia supernova, and it shows that at least some types of these supernovae come from a single scenario degenerate.”
The article in Nature is titled: “Strong ultraviolet pulse from a newborn type Ia supernova.”

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