Astronomers resolve mystery of white dwarf’s mass

This figure illustrates the new orbital solution, plotted
together with all published data in the Washington Double Star
database as well as the heretofore unpublished data in the
recent speckle measurements. In this figure, micrometric
observations are indicated by green plus signs, photographic
measures by purple asterisks, adaptive optics by blue filled
circles, CCD measures by purple triangles and the four new
speckle measures as blue stars. A dot-dash line indicates the
line of nodes, and a curved arrow in the lower right corner
indicates the direction of orbital motion. The scale, in
arcseconds, is given on the left and bottom axis. Finally, the
previous orbit calculation is shown as a dashed ellipse.
Credit: U.S. Naval Observatory

New observations of the white dwarf/red dwarf binary star 40
Eridani BC by astronomers at the U.S. Naval Observatory
(USNO) have revealed new, definitive values for the orbital
period and masses of the components of this interesting
stellar pair. A paper describing the observations and the
results by Dr. Brian Mason, Dr. Bill Hartkopf, and intern
Korie Miles has been accepted for publication in the
Astronomical Journal.

40 Eridani BC (also known as Omicron-2 Eridani) is a well-known
double star that has been observed by many astronomers since
its properties were first accurately measured by William Rutter
Dawes in 1867. It is located about 16 light-years from Earth
and is easily observed in amateur telescopes. Measuring the
period of the component stars as they their center of mass and knowing their
distance allows astronomers to compute their combined masses.
As more observations were recorded over the decades, the
characteristics of the stars’ orbits were computed, allowing a
first determination of the stars’ combined masses. It quickly
became apparent that 40 Eridani BC was an unusual system.

By combining the computed orbits with spectrographic data and
the stars’ nearby location, it was found that the brighter
component was a “white dwarf,” the highly compressed remnant of
a star that has collapsed after exhausting its nuclear fuel.
The fainter component is a “red dwarf,” a low-luminosity,
low-mass star that will feebly shine for hundreds of billions
of years. While may be the most prevalent types
of “normal” stars in the galaxy, white dwarf stars are
comparatively rare. 40 Eridani B is the second-brightest white
dwarf known and is the only one that can easily be seen in
backyard telescopes. It was also the first to have its mass determined by
measuring its , a characteristic of
very dense objects.

Utilizing a technique called “speckle interferometry,” Dr.
Mason and his colleagues observed 40 Eridani BC over the course
of six nights in early 2017 using the USNO’s 66-cm (26-inch)
“Great Equatorial” refractor telescope, purchased in 1873. The
lens on this telescope was used by astronomer Asaph Hall to
discover the moons of Mars, Phobos and Deimos, in 1877.
Re-mounted at its present site in 1893, the telescope has been
used for measuring double since that time.

Prior orbit calculations for 40 Eridani BC yielded a
discrepancy between the mass of the white dwarf component
derived from its orbital motion and that determined by its
gravitational redshift.

“Due to the long period of most visual binaries and the
understandable impatience of calculators,” says Dr. Mason,
“orbits are often calculated when they ‘can’ be and not
necessarily when they ‘should’ be.”

The newly reported observations by Dr. Mason et al. and
archival observations allow a new orbit to be calculated which
resolves that discrepancy. The new observations indicate that
the components of 40 Eridani BC circle each other with a period
of 230.29 +/- 0.68 years, about 20 years less than the previous
determination. The mass of the white dwarf component is now
believed to be 0.573 +/- 0.018 solar masses, about 0.15 solar
mass greater than the previous estimate and closer to the
result obtained by gravitational redshift.

Dr. Mason notes, “Now that the from the orbit matches that from the
gravitational redshift, this source of consternation has gone
away and it is not necessary to invoke other more exotic
solutions to the problem. Patience is a virtue.”

Explore further:

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More information: Binary Star Orbits. V. The Nearby
White Dwarf – Red Dwarf pair 40 Eri BC, Brian D. Mason, William
I. Hartkopf & Korie N. Miles, 2017, to appear in the
Astronomical Journal

Journal reference: Astronomical

Provided by: U.S. Naval Observatory