Zombie Star!

White Dwarf stars are the bizarre relics of smaller stars like our own Sun that have perished after having consumed their necessary supplies of nuclear fuel. First the small Sun-like star swells to hideous proportions, to become a monstrous, bloated Red Giant star, that ultimately puffs its outer gaseous layers into interstellar Space, leaving behind only the ghostlike White Dwarf–its former core. But something else happens when the small Sun-like star dwells in a binary system with a “still-living” sister star, in which case strange and ghastly things can occur. As the Dwarf sips up material from its sister star–and victim–it can reach critical mass and blow itself to pieces in a Type Ia supernova blast, leaving absolutely nothing, nothing, nothing at all behind. However, in August 2014, a team of astronomers using NASA’s venerable Hubble Space Telescope (HST), announced that they have–for the first time–detected a star system that later produced a bizarre “zombie star” after an unusually weak supernova explosion of this type.

Examining archived HST images taken before the supernova blast, the astronomers say that they have spotted the blue sister companion star of the ghastly White Dwarf. The White Dwarf had slowly, relentlessly sipped up fuel from its blue stellar sister, eventually triggering a runaway nuclear reaction in the dead star, resulting in a weak supernova explosion.

This particular supernova belongs to a recently identified class of stellar blast termed Type Iax. These exploding small stars are less energetic and considerably dimmer than normal Type Ia supernovae, which also ignite as the result of exploding White Dwarfs in doomed binary systems. At first, astronomers thought these weaker stellar explosions were unique Type Ia supernovae. However, so far, they have not detected more than 30 of these faint runts of the supernova litter, which occur at one-fifth the rate of normal Type Ia supernovae.

“Astronomers have been searching for decades for the progenitors of Type Ia’s. Type Ia’s are important because they’re used to measure vast cosmic distances and the expansion of the Universe. But we have very few constraints on how any White Dwarf explodes. The similarities between Type Iax’s and normal Type Ia’s make understanding Type Iax progenitors important, especially because no Type Ia progenitor has been conclusively identified. This discovery shows us one way that you can get a White Dwarf explosion,” explained Dr. Saurabh Jha in a n August 6, 2014 HUBBLESITE Press Release. Dr. Jha is of Rutgers University in Piscataway, New Jersey.

The team’s study appears in the August 7, 2014 edition of the journal Nature.

Supernovae Runts

When a large, massive star has finally burned up its necessary supply of hydrogen fuel, it may “die” a violent, fiery, explosive supernova death. Supernovae are the most powerful stellar blasts known, and they are visible all the way to the very edge of the visible Universe. The visible Universe is that relatively small region of the unimaginably vast Cosmos that we are able to observe–the rest of it resides beyond the reach of our visibility, because the light emanating from those objects dwelling in those very, very remote regions has not had sufficient time to reach us since the Big Bang birth of the Universe almost 14 billion years ago. The speed of light sets something of a cosmological speed limit. No known signal in the Universe can travel faster than light.

When a heavy, large star perishes in a supernova conflagration, it usually leaves behind a small relic as testimony to its former stellar existence–an extremely weird, very dense stellar corpse termed a neutron star, or an even more bizarre entity called a stellar mass black hole.

All stars, both heavy and light, “live” out the best years of their stellar “lives” on the hydrogen-burning main-sequence. Stars must maintain a very delicate balance between two warring forces–gravity and radiation pressure–from the time they are born until they die. The radiation pressure of a star on the main sequence pushes all of its material outward and away from the star, and it keeps this seething, fiery, roiling ball of gas blissfully bouncy against the heartless squeeze of its own powerful gravity that tries to crush it–pulling all of its material in. A star’s radiation pressure is the result of nuclear fusion, which begins with the burning of hydrogen into helium. Helium is the second-lightest atomic element in the Universe. This process, termed stellar nucleosynthesis, keeps fusing heavier atomic elements out of lighter one. Literally all of the atomic elements heavier than helium (metals, in astronomical terminology) were manufactured in the nuclear-fusing cores of our Universe’s multitude of dazzling stars–or in their explosive supernovae “deaths”, which produce the heaviest atomic elements of all, such as gold and uranium.

When a very massive main-sequence star, weighing a hefty eight solar masses– or even more–has succeeded in fusing its entire necessary supply of hydrogen fuel, it is doomed. The heavy star, at this tragic point, cannot hold its own against the relentless crush of its own weight, and gravity wins this very ancient war–and the massive star goes supernova.

Supernovae usually blast the elderly star to shreds, violently tossing its incandescent, multicolored layers of beautiful gases out into interstellar Space. This violent event occurs when the iron heart of the heavy star attains the truly impressive weight of 1.4 solar-masses. This triggers the doomed star’s ultimate end, which is characterized by that brilliant stellar blast, its grand finale.

Supernovae are usually divided into two main classes–although it is really much more complicated than this. The first of the two primary classes, Type II supernovae, ignite when the heart of a massive star weighing in at 8 to 100 times solar-mass, runs out of its necessary supply of hydrogen fuel and collapses into an unimaginably dense chunk in the smallest fraction of a second–tossing luminous radiation out into the Space between stars. The second class, termed Type Ia supernovae, are triggered when a White Dwarf star has perished after sipping up too much mass from a sister companion star–or, alternatively, after two White Dwarfs collide into each other.

White Dwarfs are the ghostly remains of smaller stars, like our own Sun. Stars that are like our Sun perish much more peacefully than their more massive stellar kin. When a small Sun-like star has at last burned its necessary supply of hydrogen fuel, it has reached the end of the road. White Dwarfs are normally surrounded by glimmering, incandescent, multicolored, and famously beautiful shells of gases (planetary nebulae). This is the fate of stars like our Sun–at least when they are solitary stars. When these small stars dwell in a close binary system with a sister star–that is still on the hydrogen-burning main sequence–it is a party waiting to happen. The fireworks begin when the White Dwarf sips up material from its main-sequence sister star, gulping down more and more and more of its material, until it can swallow no more–and it goes critical. The White Dwarf pays for its sinister feast by going supernova–just like the big guys–and blasting itself to smithereens. This results in a Type Ia supernova.

But over a decade ago, astrophysicists began to notice that some supernovae appeared similar to normal Type Ia events, but were quite a bit fainter. Some of these bewildering stellar blasts emitted a mere 1% of the peak luminosity of normal, familiar Type Ia explosions.

Zombie Star!

The weak Type Iax supernova, named SN 2012Z, was spotted in the Lick Observatory Supernova Search in January 2012. In a stroke of good luck, HST’s Advanced Camera for Surveys also observed the supernova’s host galaxy, NGC 1309 in 2005, 2006, and 2010–before the supernova fireworks had begun. NGC 1309 is located about 110 million light-years from Earth. Curtis McCully, a graduate student at Rutgers and lead author of the team’s paper, reprocessed the pre-supernova images and spotted an object in the supernova’s position. “I was very surprised to see anything at the supernova’s location. We expected that the progenitor system would be too faint to see, like previous searches for normal Type Ia supernova progenitors. It is exciting when nature suprises us,” McCully said in the August 6, 2014 HUBBLESITE Press Release.

The likelihood that the object the team spotted represents merely a chance alignment independent of the supernova is less than 1%. After observing the mysterious object’s colors and computer simulations illustrating possible Type Iax progenitor systems, the astronomers determined that what they were observing was probably the light of a star that had lost its outer hydrogen envelope.

“Back in 2009, when we were just starting to understand this class, we predicted that these supernovae were produced by a White Dwarf and helium star binary system. There’s still a little uncertainty with this Hubble study, but it is essentially validation of our claim,” team member Dr. Ryan Foley noted in the August 6, 2014 Press Release. Dr. Foley is of the University of Illinois at Urbana-Champaign, who helped identify Type Iax supernovae as a new class,

The team of astronomers think that one possible scenario explaining this oddball stellar system predicts that a “seesaw” game occurs between the stars in the binary system, with each star contributing some of its mass to the other. The stars originally weighed approximately seven and four solar-masses, respectively. The more massive seven-solar-mass star evolved with relative speed, throwing its hydrogen and helium onto its smaller stellar sister. At this point, the formerly heavy star managed to lose quite a bit of weight, slimming down to a relatively petite one solar mass–and is left with a carbon and oxygen core. In fact, it has undergone a sea change into a White Dwarf! The sister star, which started out with about four solar masses, has now gained weight–courtesy of its once more obese sister–and begins to evolve speedily, growing ever larger and larger and larger until it engulfs the White Dwarf. The outer layers of this combined star are hurled out, leaving behind the White Dwarf and the two-solar-mass helium heart of the sister star. The White Dwarf continues to swallow matter from its sister–until it grows unstable and blows itself up in a mini-superova–hurling out approximately 50% of a solar-mass of starstuff.

Unlike the explosion of a more common Type Ia supernova, which rips the White Dwarf to pieces, the blast associated with a Type Iax is believed to leave, in its tragic wake, a very badly battered White Dwarf–the living dead of the stellar zoo. Because this traumatized dead star comes back to life even as it explodes, astronomers have playfully nicknamed it a zombie star.

The astronomers admit that they can’t completely rule out some alternative explanations. For example, there is still the lingering possibility that it really was merely a solitary, massive star that went supernova. In order to settle those uncertainties and confirm their hypothesis, the team plans to use the HST again in 2015 to study the area when the supernova’s light has become faint enough to reveal any possible zombie star and its helium sister.

The team has already observed the aftermath of one Type Iax supernova explosion. HST images taken of SN 2008ha in January 2013, more than four years after its fatal blast, reveal a strange object lurking in the supernova’s location. The object could be either the zombie star or its helium sister. Based on the mysterious object’s colors, the astronomers suggest in a separate paper that the star is the helium sister, weighing in at over three times that of our Sun. It is quite a bit less luminous and redder than the SN 2012Z progenitor system. These findings are published in the August 11, 2014 issue of The Astrophysical Journal.

SN 2012Z is one of the more powerful Type Iax supernovae and SN 2008ha is one of the weakest of the class, showing that Type Iax systems are very diverse. And perhaps that diversity is related to how each of these stars explodes. Because these supernovae don’t destroy the White Dwarf completely, we surmise that some of these explosions eject a little bit and some eject a whole lot,” explained Dr. Foley, that study’s lead author, to the press on August 6, 2014.

The astronomers hope that their new discovery will aid in the development of improved models for these White Dwarf blasts, as well as provide a more complete understanding of the relationship between normal Type Ia supernovae and Type Iax–and their progenitors.

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