Light curves of stars and exoplanets: estimating inclination, obliquity and albedo
Author
dc.contributor.author
Cowan, Nicolas B.
Author
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Fuentes, Pablo A.
es_CL
Author
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Haggard, Hal M.
es_CL
Admission date
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2014-02-11T14:54:28Z
Available date
dc.date.available
2014-02-11T14:54:28Z
Publication date
dc.date.issued
2013
Cita de ítem
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MNRAS 434, 2465–2479 (2013)
en_US
Identifier
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doi:10.1093/mnras/stt1191
Identifier
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https://repositorio.uchile.cl/handle/2250/126380
General note
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Artículo de publicación ISI
en_US
Abstract
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Distant stars and planets will remain spatially unresolved for the foreseeable future. It is
nonetheless possible to infer aspects of their brightness markings and viewing geometries
by analysing disc-integrated rotational and orbital brightness variations. We compute the
harmonic light curves, Fm
l (t ), resulting from spherical harmonic maps of intensity or albedo,
Y m
l (θ,φ), where l and m are the total and longitudinal orders. It has long been known that many
non-zero maps have no light curve signature, e.g. odd l>1 belong to the nullspace of harmonic
thermal light curves. We show that the remaining harmonic light curves exhibit a predictable
inclination dependence. Notably, odd m > 1 are present in an inclined light curve, but not
seen by an equatorial observer. We therefore suggest that the Fourier spectrum of a thermal
light curve may be sufficient to determine the orbital inclination of non-transiting short-period
planets, the rotational inclination of stars and brown dwarfs, and the obliquity of directly
imaged planets. In the best-case scenario of a nearly edge-on geometry, measuring the m = 3
mode of a star’s rotational light curve to within a factor of 2 provides an inclination estimate
good to ±6◦, assuming that stars have randomly distributed spots. Alternatively, if stars have
brightness maps perfectly symmetric about the equator, their light curves will have no m = 3 power, regardless of orientation. In general, inclination estimates will remain qualitative
until detailed hydrodynamic simulations and/or occultation maps can be used as a calibrator.
We further derive harmonic reflected light curves for tidally locked planets; these are higherorder
versions of the well-known Lambert phase curve. We show that a non-uniform planet
may have an apparent albedo 25 per cent lower than its intrinsic albedo, even if it exhibits
precisely Lambertian phase variations. Finally, we provide low-order analytic expressions for
harmonic light curves that can be used for fitting observed photometry; as a general rule,
edge-on solutions cannot simply be scaled by sin i to mimic inclined light curves.
en_US
Lenguage
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en
en_US
Publisher
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Oxford University Press on behalf of the Royal Astronomical Society