On the robustness of analysis techniques for molecular detections using high resolution exoplanet spectroscopy. (arXiv:1811.05978v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Cabot_S/0/1/0/all/0/1">Samuel. H. C. Cabot</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Madhusudhan_N/0/1/0/all/0/1">Nikku Madhusudhan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hawker_G/0/1/0/all/0/1">George A. Hawker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gandhi_S/0/1/0/all/0/1">Siddharth Gandhi</a>
High-resolution doppler spectroscopy provides a powerful means for chemical
detections in exoplanetary atmospheres. This approach involves monitoring
hundreds of molecular lines in the planetary spectrum doppler shifted by the
orbital motion of the planet. The molecules are detected by cross-correlating
the observed spectrum of the system with a model planetary spectrum. The method
has led to molecular detections of H2O, CO, and TiO in hot Jupiters using large
ground-based telescopes. Critical to this method, however, is the accurate
removal of the stellar and telluric features from the observed spectrum, also
known as detrending. Previous molecular detections have relied on specific
choices of detrending methods and parameters. However, the robustness of
molecular detections across the different choices has not been investigated in
detail. We conduct a systematic investigation of the effect of detrending
algorithms, parameters, and optimizations on chemical detections using
high-resolution spectroscopy. As a case study, we consider the hot Jupiter HD
189733 b. Using multiple methods, we confirm high-significance detections of
H2O (4.8$sigma$) and CO (4.7$sigma$). Additionally, we report evidence for
HCN at high significance (5.0$sigma$). On the other hand, our results
highlight the need for improved metrics and extended observations for robust
confirmations of such detections. In particular, we show that detection
significances of $gtrsim$ 4$sigma$ can be obtained by optimizing detrending
at incorrect locations in the planetary velocity space; such false positives
can occur in nearly 30% of cases. We discuss approaches to help distinguish
molecular detections from spurious noise.
High-resolution doppler spectroscopy provides a powerful means for chemical
detections in exoplanetary atmospheres. This approach involves monitoring
hundreds of molecular lines in the planetary spectrum doppler shifted by the
orbital motion of the planet. The molecules are detected by cross-correlating
the observed spectrum of the system with a model planetary spectrum. The method
has led to molecular detections of H2O, CO, and TiO in hot Jupiters using large
ground-based telescopes. Critical to this method, however, is the accurate
removal of the stellar and telluric features from the observed spectrum, also
known as detrending. Previous molecular detections have relied on specific
choices of detrending methods and parameters. However, the robustness of
molecular detections across the different choices has not been investigated in
detail. We conduct a systematic investigation of the effect of detrending
algorithms, parameters, and optimizations on chemical detections using
high-resolution spectroscopy. As a case study, we consider the hot Jupiter HD
189733 b. Using multiple methods, we confirm high-significance detections of
H2O (4.8$sigma$) and CO (4.7$sigma$). Additionally, we report evidence for
HCN at high significance (5.0$sigma$). On the other hand, our results
highlight the need for improved metrics and extended observations for robust
confirmations of such detections. In particular, we show that detection
significances of $gtrsim$ 4$sigma$ can be obtained by optimizing detrending
at incorrect locations in the planetary velocity space; such false positives
can occur in nearly 30% of cases. We discuss approaches to help distinguish
molecular detections from spurious noise.
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