Searching for reflected light from $\tau$ Bootis b with high-resolution ground-based spectroscopy: Approaching the $10^{-5}$ contrast barrier

• Main Authors: Hoeijmakers, H. J, Snellen, I. A. G, van Terwisga, S. E
• Format: Journal Article
• Language: English
• Published: 14-11-2017
• Subjects: Physics - Earth and Planetary Astrophysics
• DOI: 10.48550/arxiv.1711.05334

A&A 610, A47 (2018) It is challenging to measure the starlight reflected from exoplanets because
of the extreme contrast with their host stars. For hot Jupiters, this contrast
is in the range of $10^{-6}$ to $10^{-4}$, depending on their albedo, radius
and orbital distance. Searches for reflected light have been performed since
the first hot Jupiters were discovered, but with very limited success because
hot Jupiters tend to have low albedo values due to the general absence of
reflective cloud decks. The aim of this study is to search for reflected light
from $\tau$ Boo b, a hot Jupiter with one of the brightest host stars. Since
its discovery in 1997, it has been the subject of several reflected-light
searches using high-dispersion spectroscopy. Here we aim to combine these data
in to a single meta-analysis. We analysed more than 2,000 archival
high-dispersion spectra obtained with the UVES, ESPaDOnS, NARVAL UES and
HARPS-N spectrographs during various epochs between 1998 and 2013. Each
spectrum was first cleaned of the stellar spectrum and subsequently
cross-correlated with a PHOENIX model spectrum. These were then Doppler shifted
to the planet rest-frame and co-added in time, weighted according to the
expected signal-to-noise of the planet signal. We reach a 3$\sigma$ upper limit
of the planet to star contrast of $1.5 \times 10^{-5}$. Assuming a planet
radius of 1.15 $R_J$, this corresponds to an optical albedo of 0.12 between
400-700 nm. This low albedo is in line with secondary eclipse and phase curve
observations of other hot Jupiters using space-based observatories, as well as
theoretical predictions of their reflective properties.