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.
In the mid-August Monthly Notices of the Royal Astronomical
Society, Retter and his colleagues argue that the consumption of
even a single hot Jupiter could account for the multistage outburst.
In this scenario, a lone Jupiter-mass planet halts three times as
it plummets into the inflating star, unleashing energy during each
stop. The incoming orb finally comes to rest about a solar radius
from the star’s core.
Monocerotis haven’t had a chance
to mix lithium into their cores and
would have high concentrations of
the element at their surfaces even
without invoking planet swallowing, notes Mark Rushton of Keele
The planet-gorging model is also
one way to account for the lithium
that researchers have observed in the
star. Lithium can barely be detected
in most stars because it mixes readily into a star’s core, where nuclear
reactions destroy it. A freshly
digested planet rich in lithium would
temporarily boost a star’s surface
abundance of the element.
But stars as young as V838
University in England. NEW PORTRAIT — In this latest image of the wispy light
Rushton, Starrfield, and several echo of the star V838 Monocerotis (arrow), taken Sept. 9 by
other researchers contend that the Hubble Space Telescope, several new features have
planet swallowing doesn’t provide emerged. These include the vortex pattern in the upper-left
enough energy to power the erup- portion of the echo and the band of light running through
tion of V838 Monocerotis. “I’m very the star. The latter feature suggests that the illuminated
skeptical [because] the energetics dust was ejected by the star during previous eruptions.
are all wrong,” says Starrfield. “I’ve
been telling this to Alon [Retter] for 3 years, and we’ve never
convinced each other.”
“In principle, a collision of a … star with a planet can lead to
[an outburst], but not as bright as that of V838 Monocerotis,”
assert Noam Soker of the Technion University in Haifa, Israel,
and Romuald Tylenda of the Copernicus Astronomical Center
in Torun, Poland, in a review article recently posted online
( http://xxx.lanl.gov/abs/astro-ph/0606371).
According to Soker and Tylenda, a stellar crash is a more promising model. They propose that a low-mass star, one-tenth to one-third the mass of the sun, collided and merged with V838 Monocerotis, generating the fireworks. A third star might have been part
of this fragile stellar partnership and might have been ejected during the merger. By getting kicked out of the system, this third star
could have brought the two colliding stars together.
An analysis of such a collision suggests that the merger would
rapidly produce a red, puffy star, as required for the V838 Monocerotis outburst, Soker and Tylenda argue. The researchers describe
this latest work in the May Astronomy & Astrophysics.
“All models I know of to explain V838 Monocerotis, beside the stellar-merger model, were invented after the eruption and had to be
twisted to fit some of the properties,” says Soker. Retter counters that
the energy produced by a collision with a low-mass star isn’t much
greater than that generated when a heavy planet is swallowed.
BOND, NASA, ESA
But Starrfield says that he worries that just as planet dining
might produce too little energy, a stellar collision might produce
too much. He suggests yet another theory—that the outburst might
be the result of an unusual nova, in which a thermonuclear explosion is triggered on the surface of a cool star—a white dwarf—
rather than a hot, blue star. The explosion would then merely be
the most recent among a string of cosmic bombs that the star has
detonated over the past few thousand years. But exactly how cool
stars might spark these explosions remains unclear.
A DUSTY RESOLUTION? About 2 months after V838 Monocerotis’ discovery, astronomers awoke to the artistic side of the odd
star: It had begun to show light echoes. Similar to the sound of a
hand clap reverberating from the walls of an empty ballroom, flashes
of light from the star’s outburst scattered off clumps of surrounding dust, setting them aglow like a string of lightbulbs that switch
on sequentially. The light echoes create the illusion of a halo of dust
expanding faster than light, but the dust isn’t actually moving. The
light simply brings different parts of
the dust clouds into view at different
times.
In April 2002, the Hubble Space
Telescope got into the act. Astronauts had installed a new detector
on Hubble, the Advanced Camera
for Surveys, with twice as large a
field of view as Hubble’s previous
workhorse instrument had and
about double its sensitivity.
Soon after the light echoes were
discovered, a team including Bond
and Starrfield asked for time with
the new camera to capture several
images of V838 Monocerotis. Some
people consider those images
among the most stunning that Hubble has ever recorded. The pictures,
as well as ones recorded as recently
as last month, may shed light on the
true nature of the stellar outbursts.
It all depends on whether the illuminated dust came from previous
outbursts of the star or was already
there as part of the interstellar
medium. By observing the light echoes, astronomers plan to
develop a three-dimensional map of the dust and determine the
origin of the material, notes Bond. That, in turn, may provide a clue
about which of the outburst models is correct. The nova model,
for instance, predicts that much of the dust was expelled by the star
in a series of thermonuclear explosions.
The amount of dust in the echoing region provides another way
to discriminate between the models. If the dust is more than 10
times as great as the mass of the sun, it couldn’t have come from
a young star, says Soker. Preliminary estimates suggest that the
region contains 100 to 1,000 solar masses of material, he says.
Measurements of the star’s spin, as well as determinations of
whether V838 Monocerotis sports a dusty disk, could make or break
the stellar-merger model. In that scenario, the collision induces high
angular momentum in V838 Monocerotis. Like the proverbial ice
skater who pulls in her arms to speed her spin, the star rotates faster
as it contracts. Evidence for such fast rotation would support the
merger scenario, notes Soker. The high spin rate created by the
merger would also produce a dense, dusty disk rapidly rotating
around the star. This disk might be revealed as the star shrinks.
Another way to distinguish among the models may be to determine whether the star has a close, surviving companion. The nova
theory predicts that the star has a partner star from which it siphons
material. In contrast, the merger model predicts that a close companion couldn’t have survived the 2002 eruption. It would have
been destroyed during the collision.
Some of the developments that would indicate the origin of the
fireworks from V838 Monocerotis are still several years in the future.
In the meantime, astronomers have some beautiful pictures adorning their walls—and an intriguing stellar mystery. ■
