Mystery Object Neither Star Nor Brown Dwarf
NOTE TO EDITORS: Extensive artwork is available at
www.gemini.edu/EFEriImages
Astronomers using the Gemini North and Keck II telescopes have peered
inside a violent binary star system to find that one of the interacting
stars has lost so much mass to its partner that it has regressed to a
strange, inert body resembling no known star type.
Unable to sustain nuclear fusion at its core and doomed to orbit with
its much more energetic white dwarf partner for millions of years, the
dead star is essentially a new, indeterminate type of stellar object.
“Like the classic line about the aggrieved partner in a romantic
relationship, the smaller donor star gave, and gave, and gave some more
until it had nothing left to give,” says Steve B. Howell, an astronomer
with Wisconsin-Indiana-Yale-NOAO (WIYN) telescope and the National Optical
Astronomy Observatory, Tucson, AZ. “Now the donor star has reached a dead
end – it is far too massive to be considered a super-planet, its composition
does not match known brown dwarfs, and it is far too low in mass to be a
star. There’s no true category for an object in such limbo.”
The binary system, known as EF Eridanus (abbreviated EF Eri), is located
300 light-years from Earth in the constellation Eridanus. EF Eri consists
of a faint white dwarf star with about 60 percent of the mass of the Sun
and the donor object of unknown type, which has an estimated bulk of only
1/20th of a solar mass.
Howell and Thomas E. Harrison of New Mexico State University made high-
precision infrared measurements of the binary star system using the
spectrographic capabilities of the Near Infrared Imager (NIRI) on the
Gemini North telescope and NIRSPEC on Keck II, both on Mauna Kea in Hawaii,
in December 2002 and September 2003, respectively. Supporting observations
were made with the 2.1-meter telescope at Kitt Peak National Observatory
near Tucson in September 2002.
EF Eri is a type of binary star system known as magnetic cataclysmic
variables.
This class of systems may produce many more of these ‘dead’ objects than
scientists have realized, says Harrison, co-author of a paper on the
discovery
to be published in the October 20 issue of the Astrophysical Journal.
“These types of systems are not generally accounted for within the usual
census figures of star systems in a typical galaxy,” Harrison says.
“They certainly should be considered more carefully.”
The white dwarf in EF Eri is a compressed, burnt-out remnant of a
solar-type star that is now about the same diameter as the Earth, though
it still emits copious amounts of visible light. Howell and Harrison
observed EF Eri in the infrared because infrared light from the pair is
naturally dominated by heat and longer wavelength emissions from the
secondary object.
The scientific detective work to deduce the components of this binary
system was complicated greatly by the cyclotron radiation emitted as free
electrons spiral down the powerful magnetic field lines of the white
dwarf. The white dwarf’s magnetic field is about 14 million times as
powerful as the Sun’s. The resulting cyclotron radiation is emitted
primarily in the infrared part of the spectrum.
“In our initial spectroscopy of EF Eri, we noted that some parts of the
infrared continuum light became about 2-3 times brighter for a time period,
then went away. This brightening repeated every orbit, and thus had to
have an origin within the binary,” Howell explains. “We first thought the
brightness change resulted from the difference between a heated side and a
cooler side of the donor object, but further observations with Gemini and
Keck instead pointed to cyclotron radiation. We ‘see’ this additional
infrared component at the phases which occur when the radiation is beamed
in our direction, and we do not see it when the beaming points in other
directions.”
The 81-minute orbital period of the two objects was probably four or five
hours when the mass transfer process began about five billion years ago.
Originally, the secondary object may also have been similar in size to the
Sun, with perhaps 50-100 percent of a solar mass.
“When this interactive process of mass transfer from the secondary star to
the white dwarf begin, and why it stopped, both remain unknown to us,”
Howell says. During this process, repeated outbursts and novae explosions
were very likely. The physics of the process also caused the two objects
to spiral closer to each other. Today, the two objects orbit each other at
about the same separation as the distance from the Earth to the Moon. The
donor object has regressed to a body with a diameter roughly equal to the
planet Jupiter.
The combined observing power of the Gemini 8-meter and Keck 10-meter
telescopes and their large primary mirrors, which were essential to this
research, Howell says, makes it clear that neither spectral features of
the donor nor its composition match any known type of brown dwarf or
planet.
Derek Homeier (University of Georgia) created a series of computer models
that attempt to replicate the conditions at EF Eri, but even the best of
these do not match perfectly.
The shape of the spectra indicate a very cool object (about 1,700 degrees
Kelvin, equivalent to a cool brown dwarf), yet they do not have the same
detailed shape or key features of brown dwarf spectra. The coolest normal
stars (very low mass M-type stars) are about 2,500 degrees K, and Jupiter
is 124 degrees K. The close-in “hot Jupiter” exoplanets detected
indirectly by other astronomers using their gravitational effect on their
parent stars are estimated to be 1,000-1,600 degrees K.
There is a small chance that the EF Eri system could have originally
consisted of the progenitor of the present-day white dwarf star and some
sort of “super -planet” that survived the evolution of the white dwarf to
result in the system observed now, but this is considered unlikely.
“There are about 15 other known binary systems out there that may be
similar to EF Eri, but none has been studied enough to tell,” Howell says.
“We are working on some of them right now, and trying to improve our
models to better match the infrared spectra.”
Co-authors of this paper on EF Eri are Paula Szkody of the University of
Washington in Seattle, and Joni Johnson and Heather Osborne of New Mexico
State.
The WIYN 3.5-meter telescope is located at Kitt Peak National Observatory,
55 miles southwest of Tucson, AZ. Kitt Peak National Observatory is part
of the National Optical Astronomy Observatory, which is operated by the
Association of Universities for Research in Astronomy (AURA), Inc., under
a cooperative agreement with the National Science Foundation (NSF).
The national research agencies that form the Gemini Observatory partnership
include: the US National Science Foundation (NSF), the UK Particle Physics
and Astronomy Research Council (PPARC), the Canadian National Research
Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y
Tecnológica (CONICYT), the Australian Research Council (ARC), the
Argentinean Consejo Nacional de Investigaciones Científicas y Técnicas
(CONICET) and the Brazilian Conselho Nacional de Desenvolvimento
Científico e Tecnológico (CNPq). The Observatory is managed by AURA under
a cooperative agreement with the NSF.
The W.M. Keck Observatory is operated by the California Association for
Research in Astronomy (CARA), a scientific partnership of the California
Institute of Technology, the University of California, and the National
Aeronautics and Space Administration.
