Enigmatic eruption: the strange case of V838 Monocerotis

Science News,  Oct 14, 2006  by Ron Cowen

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An erupting star near the outskirts of the Milky Way has become one of the most puzzling objects in the galaxy. The star's outburst has set aglow a never-before-seen array of dust eddies, shells, and spirals--a cosmic portrait reminiscent of the whirling patterns of Van Gogh's "Starry Night." But even as astronomers marvel at the artistic display generated by the distant star, dubbed V838 Monocerotis, they're stumped by its behavior.

"We've never seen anything like this," says Howard Bond of the Space Telescope Science Institute in Baltimore.

Although the erupting star briefly became one of the brightest stars in the galaxy, it has now dimmed and become one of the coolest. Moreover, instead of hurling into space its outer layers of material, as happens in a common type of stellar explosion, the star swelled to a gargantuan diameter as large as the average distance between the sun and Saturn.

Theorists have come up with several explanations for the star's extraordinary behavior: a thermonuclear explosion on the star's surface, a collision with an unseen companion, the swallowing of one or more closely orbiting planets. But none of these models fits exactly the fireworks observed. As astronomers work to solve the puzzle of V838 Monocerotis, they're applying their breadth of knowledge about the evolution of stars, and they're confronting what they still don't understand about stars and how they erupt.

PUZZLING PORTRAIT Few astronomers paid much attention in January 2002 when amateur astronomer Nicholas J. Brown of Quinns Rocks, Australia discovered V838 Monocerotis as a brightening star. Within days, the star faded, fitting the pattern of a common type of outburst--called nova from the Latin word for "new star"--that astronomers had documented many times before.

Novas occur when a white dwarf--the dense cinder of a sunlike star--steals matter from a puffier companion star. When the material piling onto the surface of the white dwarf reaches a critical density, it sparks a thermonuclear explosion. The outburst is observed as a jump in brightness over a wide range of wavelengths. Novas can recur as often as every few years as the dwarf accumulates additional material from its partner.

Astronomer Sumner Starrfield of Arizona State University in Tempe says the nova detected by Brown seemed typical. "I don't think anyone was doing anything extraordinary to follow it" because it seemed so run-of-the-mill, Starrfield says.

But just a month after its discovery, the blue-tinged star had a sudden resurgence. It grew about 30 times brighter in a single day, outshining nearly every other star in the Milky Way. After fading in mid-February 2002, it bloomed a third and final time in March of that year, ballooning to 500 times its original width before slowly fading. It also dropped in temperature from 6,000 kelvins, slightly hotter than the sun's visible surface, to 2,000 K and took on a reddish hue.

That's in striking contrast to a classic nova in which a star's outer layers, cast off during an explosion, gradually expand and thin to reveal the blue, still-hot white dwarf that remains behind. The dwarf emits X rays and ultraviolet radiation, neither of which was detected from V838 Monocerotis.

This erupting star "does not resemble any nova that has been studied," says Starrfield. By monitoring the intensity of individual wavelengths of the infrared light from the outburst, astronomers have determined that V838 Monocerotis is a binary star system. It consists of a blue supergiant star and a cool star. The supergiant is just a bystander, the cool star is the one that ballooned so dramatically.

V838 Monocerotis is "the prototype of a new class of star," assert Israeli astrophysicist Alon Retter and his colleagues in a recent review article. Retter, Bond, and several other astronomers described their theories and observations of V838 Monocerotis at a conference in La Palma, Spain, in May and have recently posted the reviews online (www.sciencenews.org/articles/20061014/bob9ref.asp).

MYRIAD MODELS Astronomers have come up with several theories to explain the bizarre properties of V838 Monocerotis. Retter and his colleagues have suggested that the ballooning star engulfed a trio of unseen planets in rapid succession. Each of the three peaks in the star's outbursts might have corresponded to the consumption of a planet, Retter says. He and his team calculate that the energy unleashed by planet swallowing--the point when the planet's gravitational energy is absorbed by the parent star--would account for the observed brightening of the star.

Each planet would have to have been a so-called hot Jupiter, a body as massive as our solar system's giant planet but that circles the star at only about a tenth of the distance between Mercury and the sun.

Astronomers have discovered more than two dozen such planets closely orbiting other stars over the past decade, although they don't know whether any planet circles V838 Monocerotis.