light-years), offers an opportunity to measure member
stars’ motions in search of evidence for a central black hole.
Figure 2.
Model contours
overlaid on the GMOS
image show the
location of mixed
dust at various times
after ejection (green
lines, marked in days
since emission) and
the location of dust
of various sizes (red
lines, defined in terms
of the parameter b,
which depends on the
ratio of radiation to
gravitational force).
Most of the emission
is consistent with a
single epoch of ejection
about a year before the
observations.
Impact on a main belt asteroid best explains
the appearance of P/2010 A2 (LINEAR), not
an origin as a true sublimating comet. In addition to the short duration of dust emission,
the velocity distribution of the particles and
the amount of mass ejected are consistent
with this interpretation. An oblique impact is
more likely than a head-on collision, since the
latter would have destroyed the whole body,
which had an extent of 80-90 meters.
Although this object does not add to the
known population of true main belt comets,
this study not only reveals the ongoing processes of the dynamic Solar System, but that
these main belt comets are potentially important as a significant source of the water and
volatile materials found on Earth today. The
complete work is published in Astronomy
and Astrophysics, 537: A69, 2012.
Raminder S. Samra (University
of British Columbia, Canada)
and colleagues have used
the Near-Infrared Imager and
Spectrograph (NIRI) with the
Altair adaptive optics system
on Gemini North for observations in the H and K bands
to measure these proper motions. They made the original observations in
2005, with a subsequent set in 2007 and 2009
(Figure 3). Over the longer time baseline, they
found the proper motion dispersion of the
central stars to be 179 ± 17 microarcseconds
per year.
The search for evidence of a black hole begins with the measurement of proper motion dispersion as a function of distance from
the cluster’s center. In the presence of a black
hole, the dispersion would increase toward
the center. Grouping the data into radial
bins, the team finds that the proper motion
dispersion is instead constant, despite the
small central bin, which is less than 5 arcseconds in radius. Alternatively, comparing the
observations with a model system that in-
Is the Globular Cluster M71
Hiding Something?
Figure 3.
Core of the globular
cluster M71 in the H
band, observed with
NIRI/Altair on Gemini
North. The cluster
center is marked with
a green circle.
22
The dense environments of globular clusters may be the homes of intermediatemass black holes, those with masses of
around several 100 to 104 MSun. These values are intermediate between the stellarmass remnants of supernovae and the supermassive variety at millions to billions
of solar masses in the centers of galaxies.
The globular cluster M71 (also NGC 6838),
at a distance of about 4 kiloparsecs (13,000
GeminiFocus
June2012