A Method to Get Objective Spectra of Exoplanets
Nataliya ZUBKO1, Ryosuke UEMURA1, Naoshi BABA1 and Naoshi MURAKAMI2
1) Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Kita 13
Nishi 8, Sapporo 060-8628
2) National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588
Abstract
The main problem in the direct detection of exoplanets is the suppression of starlight. A
high-contrast imaging scheme must be realized for the detection of faint companions. Several
kinds of differential methods have been proposed to improve the detectability of exoplanets. Our
nulling coronagraph with a four-quadrant polarization mask (FQPoM) makes it possible to obtain
not only the direct image of an exoplanet but also its spectrum. We report a method to obtain
planetary objective spectra by conducting a polarization degree analysis of the obtained spectra.
Our simulation experiment shows that a planetary objective spectrum is detectable with a contrast
of 7×10-5 and an angular distance of 4.8 λ/D.
Key words: polarization, image processing – techniques, instrumentation, techniques: high angular
resolution.
1. Introduction
Achieving high-contrast imaging is a necessary condition for the direct detection of
exoplanets. Therefore, the goal is developing methods for the effective suppression of starlight.
We present several experimental results obtained with our FQPoM coronagraph (Baba et al.
2002). In our experiment for removing star light, we use two techniques: destructive interference
of starlight that is realized with an FQPoM coronagraph, and a differential polarization technique
(Baba & Murakami 2003). In addition, for revealing planetary spectra we propose to use
polarization degree analysis of objective spectra (Zubko et al 2007), because the degree of
polarization of light scattered by a planet can exceed the polarization degree of stellar light (Stam
et al. 2004).
2. Experiment
Fig. 1 shows our experimental setup. We simulate stellar and planetary light with Xe
lamps. To make features in the planet spectrum, an artificial absorption line was created with
filter on the planetary signal. By means of optical fibers, light beams are directed to the optical
bench. A set of glass plates make the planet light partially polarized. In our experiments the
degree of polarization of planet light was about 50 %. The planetary and stellar light are
combined with a cube beam splitter, and are then led to the polarization modulator and
coronagraphic optics. Beams passed through different zones of the FQPoM are combined in a
destructive way in the Lyot stop plane, providing the nulling of stellar light. A blazed grating
forms the objective spectra which are detected by an EM CCD camera.
After suppressing starlight with the nulling coronagraph, the residual spectrum of the star
still remains with high intensity in comparison with the exoplanet light. For further suppression of
the starlight, we use a differential technique. By changing the retardation of a liquid-crystal
variable ratarder (LCVR) we take two spectra with 0° and 90° polarizations. The unpolarized
stellar noise can be removed from spectra by subtracting one polarized component from the other;
thus the differential spectra will contain only the polarized component of the planet signal.
Fig. 1 Optical setup
Fig.2 Differential spectrum of objective spectra (left) and the degree of polarization of the differential
spectrum (right). Intensity contrast between the planet and the star is ~ 10-4.
3. Result
Fig. 2 (left) shows the differential spectra of s- and p-polarized components of the
objective spectra. As can be seen from the figure, the planetary spectrum is still weak against the
stellar noise. For extraction of planetary signal an analysis of the degree of polarization is then
applied, because the degree of polarization of light reflected by a planet can significantly exceed
unpolarized stellar light. Fig. 2 (right) shows the degree of polarization of differential spectra.
From this figure it can be seen that planetary signal dominates over stellar noise. Thus, the
analysis of the degree of polarization is effective for the detection of the planet signal, and it can
also be useful for revealing any features in the objective spectrum.
4. Conclusion
In our experiment we achieved a detected intensity contrast between the planet and star of
7×10-5. It is shown that for distinguishing planetary spectrum from stellar noise, the analysis of
the degree of polarization can be useful.
References
Baba, N., Murakami, N., Ishigaki, T., & Hashimoto, N., 2002, Opt. Lett. , 26, 1373
Baba, N., & Murakami, N., 2003, Publ. Astron. Soc. Pacific, 115, 1363
Stam, D.M., Hovenier, J.W., Waters, L.B.F.M., 2004, Astron. Astrophys., 428, 663
Zubko, N., Baba, N., Morisaki, S., Murakami, N., 2007, Opt. Express, 15, 12189