Thursday, 23 April 2020, 2:44 pmPress Release: University of Canterbury

University of Canterbury (UC) astronomers arepart of an international team that has revealed howexplosions on the surface of a white dwarf star can increaseits brightness by thousands or millions of times making itlook like a new star.

For many yearsastronomers have thought that nuclear fusion of material onthe surface of a white dwarf directly powers all the lightfrom a nova explosion, which happen about 10 times a year inour galaxy.

A nova, or stella nova Latin fornew star is a sudden explosion on the surface of awhite dwarf, which is the hot, burnt-out core of a star. Itproduces an incredible amount of energy and light,increasing the stars brightness by thousands or evenmillions of times. If a nova occurs relatively close toearth it can appear as a new star to the naked eye.

Innew research, a team of international astronomers has shownthat shock waves from the nova explosion, rather thannuclear fusion, cause most of the brightness.

The teamused NASAs space-based telescopes and ground-basedtelescopes, including some at the UCMt John Observatory in Tekapo, to observe a recentnearby nova in the constellation of Carina and proved thatit is indeed shock waves that cause most of the novasbrightness.

Their results are documented in a newpaper called Direct evidence for shock-powered opticalemission in a nova published this month in theinternational journal NatureAstronomy.

UC Associate Professor in Astronomyand Director of the University of Canterbury Mt JohnObservatory KarenPollard, who co-authored the paper, was observing atUCs Mt John Observatory using the McLellan telescope andHERCULES spectrograph a few days after the bright nova inCarina was reported.

I was excited to observe it a new bright novae in the galaxy is an importantopportunity to make a detailed study of the novasproperties and how these change with time. Usingspectroscopy we were able to examine shock-produced emissionand calculate how energetic the shock waves were and howfast the shocked material was moving, shesays.

Elias Aydi, a research associate in MichiganState Universitys (MSU) Department of Physics andAstronomy and lead author of the paper, says the discoveryleads to a new way of understanding the origin of thebrightness of novae and other stellar explosions. Ourfindings present the first direct observational evidence,from unprecedented space observations, that shocks play amajor role in powering these events.

When materialblasts out from the white dwarf, he says it is ejected inmultiple phases and at different speeds. These ejectionscollide with one another and create shocks, which heat theejected material producing much of the light.

Anotherside effect of astronomical shocks are gamma-rays, thehighest-energy kind of electromagnetic radiation. Theastronomers detected bright gamma-rays from the star, knownas nova V906 Carinae (ASASSN-18fv), whose explosion in theconstellation Carina was first detected in March2018.

An optical satellite happened to be looking atthe part of the sky where the nova occurred. Comparing thegamma-ray and optical data, the astronomers noted that everytime there was a fluctuation in gamma-rays, the light fromthe nova fluctuated as well.

The simultaneousfluctuations in both the visual and gamma-ray brightnessconfirmed that both were originating from shocks.

Theresearch team estimates that V906 Car is about 13,000 lightyears from Earth. This means that when the nova was firstdetected in 2018, it had actually happened 13,000 years ago.The new information may also help explain how large amountsof light are generated in other stellar events, includingsupernovae and stellar mergers, when two stars collide withone another. Each nova explosion releases about 10,000 to100,000 times the annual energy output of theSun.

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