Alan Spots One Angry (White) Dwarf

 

NovaCen2013.ESO-Beletsky.crop
Nova Centauri 2013 appeared in the southern skies near the two Pointer stars, and not far from the Southern Cross, on December 2, 2013. This image cropped from original. Original image by Y. Beletsky (LCO)/, taken from La Silla Observatory Dec 9, 2013.

 

Alan Plummer, with Ben Gregory, report on Nova Centauri 2013 and its subsequent behaviour.

 

I [AP] observe variable stars from the Blue Mountains west of Sydney. These can be slow gentle pulsators, unstable unpredictable supergiants, interacting explosive binary systems, and many more types. The explosive binaries—also called cataclysmic variables (CVs)—come in so many varieties that the family is often called a zoo. Below is the story of one CV; a classical nova.

IAU.Sky&Tel. Crux.NovaCen2013
Figure 1: Nova Cen 2013 appeared just north of the bright star Beta Centauri where the circle is. Before brightening the star was too faint to appear on this map. Map courtesy of IAU and Sky & Telescope magazine.

 

In December 2013 a 15th magnitude star known charmingly as UCAC4 155-128029 exploded into Nova Centauri 2013. Look into the sky a bit to the east of the Southern Cross (see Figure 1), and we sight down the length of the Scutum-Centaurus Arm of the Galaxy, 50 degrees off Galactic centre. A crowded and beautiful field. From 8,000-ish light years away, a third or a quarter the distance from the earth to the heart of the Galaxy, the blast reached 3rd magnitude, and so for some weeks was visible to the naked eye. The luminosity was briefly perhaps a million times that of the Sun, and with a reasonable telescope the explosion is still visible today.

NovaCen2013.LightCurve.AAVSO-AlanPlummer
Figure 2: The change in brightness of Nova Cen 2013 over time is known as its light curve. The human eye sees brightnesses, or magnitudes, ‘smaller’ than 6. Nova Cen 2013 peaked at about magnitude 3.3 on Dec 14, 2013 before fading. Chart by AAVSO and Alan Plummer.

 

Figure 2 is the light curve of the event, beginning on Dec 3, 2013 very soon after discovery and shows the rise to maximum light and subsequent decline up until the time of writing, 11 May 2017. (The pre-explosion brightness of 15th mag is well below the 12th mag floor of this figure.) This is the entire visual data set from the AAVSO International Database. My observations are pink, and I’ve been observing it since discovery – first with binoculars and now with a 40cm telescope.

Current thinking on classical nova explosions is this: If a white dwarf—a compact hot thing about the size of the Earth with a mass comparable to the Sun—can draw enough gas from a very close companion to accumulate maybe a millionth to a 10,000th of a solar mass in a tightly held shell, there can be an explosion. (I know exactly what you’re thinking; the Sydney Harbour Bridge weighs 52,800 tons, so such a shell would mass 37.8 to 3780 quadrillion SHBs) This explosion, at the boundary layer of the dwarf and the shell and lasting only 100 seconds or so, can lift the whole lot into space. (Yes, yes, I know; that’d take about 2.4×10^19 one-megaton H bombs, or some minutes of the Suns total output.)

The visual light curve here tells only part of the story. There was probably a soft X-ray flash at detonation; then sometime after maximum (visual) light it will be γ-ray and radio bright, and as the explosion fades visually over months and years, as the incandescent ejecta cools and condenses into dust, it will become infra-red bright. Eventually the nova will fade back to quiescence leaving one fiercely hot and X-ray bright angry white dwarf, and an expanding cloud of cooling debris that can help make other planets, life, and stuff. As a VSOer*, it’s always interesting to know what I’m looking at.

*Variable star observers measure the light intensity of stars that change in brightness; sometimes using DSLRs and such, and sometimes visually with the naked eye, binoculars, and telescopes. Visual and electronic observers complement, not compete. Most observers are amateurs – although they may produce very ‘professional’ results. Visual observations (my expertise) are made by comparing the target with a series of standard stars of known brightness – a ruler, if you will. The resulting estimates are lodged with the American Association of Variable Star Observers (AAVSO), an interface between observers and the wider astronomical community.

Phone Alan on 0405 194 724, or email alan123604@live.com.au for more information on VSOing.

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