Mystery Object Neither Star Nor Brown Dwarf (Forwarded)

From: Andrew Yee (ayee_at_nova.astro.utoronto.ca)
Date: 10/05/04 

Date: Tue, 05 Oct 2004 18:56:02 -0400

Gemini Observatory
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Tuesday, 05 October 2004

Mystery Object Neither Star Nor Brown Dwarf

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 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.

Gemini North NIRI Observations of EF Eridani

The Near Infrared Imager (NIRI) on Gemini North was used to make the
observations used in this research on EF Eridani. The observations were made
from 7:21 until 9:00 UT on December 24, 2002 in queue scheduling mode. The f/6
camera was used with a four pixel slit and the K-grism to produce K-band spectra
with a resolution of R ~ 780. Individual spectra with exposure times of 120
seconds were obtained at five different positions along the slit with additional
6" offsets. In total, 40 individual observations were obtained in 100 minutes
covering 123% of an orbital cycle of the EF Eri system. Additional spectra were
obtained to remove atmospheric telluric features from the data.

Full paper to be published in the Astrophysical Journal, October 20, 2004. See
preprint of paper here:
      http://arxiv.org/format/astro-ph/0409735

Full Resolution Illustrations Available Here:
 
http://www.gemini.edu/index.php?option=content&task=view&id=73&Itemid=0&limit=1&limitstart=1

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