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Astron. Astrophys. 325, 551-558 (1997)

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4. Discussion and conclusions

Our results show that (as expected) the discovery of atmospheric bands of planets through circumstellar dust of a debris disk strongly depends on the dust mass, i. e. the age of the central star. Dust population and disk structure (gaps) also influence the probability of detection of planetary spectral features as well as their shape. From the observational point of view, beam size and the observer's position in respect to the disk determine the contrast of the band features to the spectral background.

  • In all cases, a band detection is most likely for pole-on candidates. As a consequence, extra-solar planets candidates detected indirectly by astrometry (e. g., Lalande 21185, Walker 1996) are preferred objects which should be investigated for spectroscopic features.
  • It is important to know what additional emission component from the debris disk influences the band features most - the thermal dust emission or scattering. For Earth-like planets photons from the thermal dust radiation dominate the whole wavelength range of investigation. Because of the increase of the number of thermal photons with wavelength, the O3 [FORMULA] and [FORMULA] bands ([FORMULA] m) are easier to be detected than the CO2 [FORMULA] band ([FORMULA] m).

    In case of Jupiter-like planets we can find more scattered stellar photons shortward of about [FORMULA] m. Larger wavelengths are again dominated by thermal photons.

  • To get photon fluxes for the O3 bands from an Earth-like planet comparable with the fluxes from the thermally emitting dust, beam sizes smaller than 0.1 AU are necessary for the [FORMULA] [FORMULA] dust disk close to the pole-on view (for the disk with gaps see the following point). For the [FORMULA] [FORMULA] dust disk the beam size has to be even smaller.

    In case of the Jupiter-like planet, beam sizes in the range of about 1 AU would allow to detect a CO2 band within a [FORMULA] [FORMULA] dust disk close to the pole-on view (see also the following point). For edge-on configurations, this beam size has to be reduced by half (a quarter in area). Going to the [FORMULA] [FORMULA] dust disk, we need an about three times smaller beam size. Atmospheric features in the range of up to about [FORMULA] m (possible features of CH4 -dominated atmospheres at shorter wavelengths: CH4 [FORMULA] at 7.7 µm, C2 H6 [FORMULA] at 12.2 µm) can be already detected with [FORMULA] 1 AU beams for the [FORMULA] [FORMULA] pole-on dust disk or at any tilt for the [FORMULA] [FORMULA] dust disk.

  • Gaps in the disk can significantly increase the detection probability of planetary spectral features for Jupiter-like planets near their pole-on view (see Fig. 6). In this case, the O3 band of the Earth-like planet can be already discovered in the [FORMULA] [FORMULA] disk using a [FORMULA]  AU beam size. In case of the Jupiter-like planet a detection of atmospheric features seems to be possible with beam sizes of some AU.
  • A method to get planet fluxes with larger beam sizes (still small enough to get well resolved disk images) could be their separation from the asymmetric disk image assuming central symmetry for the disk alone.
  • By the aid of the "large beam size curves" in Figs. 4 and 5 which due to their lower statistical noise give an impression of the course of the photon fluxes with wavelength, we can derive how the band profiles change by the inference from the "background disk light".

    So we predict that the long wavelength wing of the O3 band is raised by the thermal dust emission continuum. The same effect is expected for Jupiter's spectral features around [FORMULA] m (see Fig. 6). In case of core-mantle grains a thermal dust emission feature is expected short before 10 µm (see Fig. 1). This would lead to a bump in the ozone feature of the Earth-like planet.

    The ozone band is also influenced by scattered light. Especially the ice feature of the core-mantle grains can be misinterpreted as O3 band. Bands of Jupiter-like planets are changed by scattered light up to about [FORMULA] m. A strong influence is expected for the CH4 [FORMULA] band at 7.7 µm.

  • Taking into account a correlation between the age of the central star and the circumstellar dust mass (André 1994, [FORMULA] variation), Jupiter-like planets should be detectable by their spectral characteristics up to about [FORMULA] m using beam sizes [FORMULA]  AU around G-type stars with ages larger than [FORMULA]  yr. Earth-like planets can be discovered with 0.1 AU beams around G-type stars older than [FORMULA]  yr.
  • In case of the Jupiter-like planet detected around the M2V star Lalande 21185 a linear dimension of 1 AU corresponds to a beam size of [FORMULA]. The data deduced for the planet (0.9 Jupiter masses, 0.35 to 1.35 Jupiter radii,  2 AU distance from star, 5.8 years orbital period) would allow an observation of a possible CO2 band with a relativly large beam size.
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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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