The outstanding question remained: “What
is this object?” Director’s Discretionary Time
enabled spectroscopy with the Gemini
Multi-Object Spectrograph (GMOS) on Gemini South to provide an answer, and more.
Luhman classified the object as an L8 dwarf,
showing good agreement with a template
spectrum. For ages less than 10 billion years,
the temperature is well below that of the
hydrogen burning limit. Also, considering
the strong lithium absorption, Luhman concludes that the object is a brown dwarf.
As an unexpected bonus, the acquisition
image resolved the source into two components (Figure1). The pair, separated by 1.5
arcseconds, corresponds to 3 astronomical
units at the object’s determined distance.
Examination of earlier, archival images does
not show either source at their present location, arguing that they form a common binary system.
The secondary is only about half a magnitude fainter than the primary, which suggests that it is also a brown dwarf and near
the L/T spectral class transition. Brown dwarf
models are sensitive to age, so a binary system offers robust tests of models and potentially strong constraints on mass, assuming
the objects formed at the same time.
The GMOS observations were obtained on
February 23, 2013, and the full paper will
appear in The Astrophysical Journal Letters.
A preprint is available at:
http://arxiv.org/abs/1303.2401,
as is more information from
http://www.gemini.edu/node/11966.
Light Echoes Show the Asymmetric
Explosion of SN1987A
Observations of light echoes — reflections
of a transient event in the surrounding material — allow astronomers to change perspective. Rather than being effectively fixed
April2013
Figure 2.
Difference image
shows SN1987A light
echoes as positive and
negative (bright and
dark) circular rings.
They appear uniformly
circular because
the echo is reflected
off sheet-like dust
structures. Black boxes
mark the GMOS fields,
and red points show the
spectral locations.
to a viewpoint on Earth, light echoes reveal
the source object from a variety of viewing
angles. Brendan Sinnott (McMaster University) and colleagues used light echoes from
supernova 1987A (SN1987A) to conclude
that this Type II event was asymmetric, with
an elongated 56Ni structure. The strongest
asymmetry they measure is in the Ha line,
and this asymmetry aligns well with the observed axis of ejecta.
The five fields the team observed with GMOS
on Gemini South probe the supernova emission over its first 300 days. Figure 2 shows
the prominent light echos, which appear as
nearly circular rings, along with the slit positions on the GMOS fields.
Variations in spectra obtained at different locations alone do not imply asymmetry in the
supernova emission. The source spectrum itself changes, so the reflected light depends
not only on the dust properties and its distribution but also on the exact region observed. The echo spectra must be compared
to an appropriate isotropic source model,
which is based on the original SN1987A outburst observations. The well-known source
spectrum (SN1987A) is advantageous, then,
because it provides an excellent reference
for isotropic emission scenarios.
The Ha line shows some of the strongest deviations from the isotropic assumption (Fig-
GeminiFocus
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