The brightness variations in ground-based
time-series photometry are typically overcome by simultaneous observations of reference stars that are close to the target on the
sky. On the assumption that the reference
stars are not intrinsically variable, and that
the effect of scintillation is a common mode
across a small field-of-view, ground-based differential transit photometry can be obtained
to precisions necessary to probe exoplanet
atmospheres with the transit technique.
Spectroscopy is ultimately needed to resolve
the lines and bands from chemical species
in exoplanet atmospheres. We accomplish
this by performing simultaneous time-series spectroscopy of the transiting planet
host star and a few reference stars of similar
brightness with multi-object slit spectrographs. A key aspect of this approach is the
use of very-wide (12 arcsecond) custom slits,
which are crucial for eliminating light loss
at the slits due to variations in atmospheric
seeing and guiding as a function of time.
The only downsides to this approach are a
loss in spectral resolution over what could
be obtained with slits smaller than the seeing profile, and a higher background. These
factors are not major limitations. We typically have to bin the data to a significantly
lower resolution than is native in the data
(to boost the signal-to-noise ratio) and the
stars we observe are very bright relative to
the background.
History of Ground-based Transit Spectroscopy
The first application of the multi-object transit spectroscopy technique was with
the Focal Reducer and Spectrograph (FORS) instrument on the European Southern
Observatory’s Very Large Telescope (Bean et al., 2010), and we have subsequently
used the technique with MMIRS on Magellan (Bean et al., 2011, and 2013) and now
the Gemini Multi-Object Spectrograph (GMOS) on Gemini North (Stevenson et al.,
2013). Recently, another group has also had success using the technique on GMOS
(Gibson et al., 2013).
January2014 2013 Year in Review
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