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Register a new userImaging of a Transitional Disk Gap in Reflected Light: Indications of Planet Formation Around the Young Solar Analog LkCa 15
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C. Thalmann
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We present H- and Ks-band imaging data resolving the gap in the transitional disk around LkCa 15, revealing the surrounding nebulosity. We detect sharp elliptical contours delimiting the nebulosity on the inside as well as the outside, consistent with the shape, size, ellipticity, and orientation of starlight reflected from the far-side disk wall, whereas the near-side wall is shielded from view by the disks optically thick bulk. We note that forward-scattering of starlight on the near-side disk surface could provide an alternate interpretation of the nebulosity. In either case, this discovery provides confirmation of the disk geometry that has been proposed to explain the spectral energy distributions (SED) of such systems, comprising an optically thick outer disk with an inner truncation radius of ~46 AU enclosing a largely evacuated gap. Our data show an offset of the nebulosity contours along the major axis, likely corresponding to a physical pericenter offset of the disk gap. This reinforces the leading theory that dynamical clearing by at least one orbiting body is the cause of the gap. Based on evolutionary models, our high-contrast imagery imposes an upper limit of 21 Jupiter masses on companions at separations outside of 0.1 and of 13 Jupiter masses outside of 0.2. Thus, we find that a planetary system around LkCa 15 is the most likely explanation for the disk architecture.
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We present four new epochs of Ks-band images of the young pre-transitional disk around LkCa 15, and perform extensive forward modeling to derive the physical parameters of the disk. We find indications of strongly anisotropic scattering (Henyey-Greenstein g = 0.67 [-0.11,+0.18]) and a significantly tapered gap edge (round wall), but see no evidence that the inner disk, whose existence is predicted by the spectral energy distribution, shadows the outer regions of the disk visible in our images. We marginally confirm the existence of an offset between the disk center and the star along the line of nodes; however, the magnitude of this offset (x = 27 [-20,+19] mas) is notably lower than that found in our earlier H-band images (Thalmann et al. 2010). Intriguingly, we also find, at high significance, an offset of y = 69 [-25, +49] mas perpendicular to the line of nodes. If confirmed by future observations, this would imply a highly elliptical -- or otherwise asymmetric -- disk gap with an effective eccentricity of e = ~0.3. Such asymmetry would most likely be the result of dynamical sculpting by one or more unseen planets in the system. Finally, we find that the bright arc of scattered light we see in direct imaging observations originates from the near side of the disk, and appears brighter than the far side because of strong forward scattering.
LkCa 15 hosts a pre-transitional disk as well as at least one accreting protoplanet orbiting in its gap. Previous disk observations have focused mainly on the outer disk, which is cleared inward of ~50 au. The planet candidates, on the other hand, reside at orbital radii around 15 au, where disk observations have been unreliable until recently. Here we present new J-band imaging polarimetry of LkCa 15 with SPHERE IRDIS, yielding the most accurate and detailed scattered-light images of the disk to date down to the planet-hosting inner regions. We find what appear to be persistent asymmetric structures in the scattering material at the location of the planet candidates, which could be responsible at least for parts of the signals measured with sparse-aperture masking. These images further allow us to trace the gap edge in scattered light at all position angles and search the inner and outer disks for morphological substructure. The outer disk appears smooth with slight azimuthal variations in polarized surface brightness, which may be due to shadowing from the inner disk or a two-peaked polarized phase function. We find that the near-side gap edge revealed by polarimetry matches the sharp crescent seen in previous ADI imaging very well. Finally, the ratio of polarized disk to stellar flux is more than six times larger in J-band than in the RI bands.
With the legacy of Spitzer and current advances in (sub)mm astronomy, a large number of transitional disks has been identified which are believed to contain gaps or have developped large inner holes, some filled with dust. This may indicate that complex geometries may be a key feature in disk evolution that has to be understood and modeled correctly. The disk around LkCa 15 is such a disk, with a gap ranging from ~5 - 50 AU, as identified by Espaillat et al. (2007) using 1+1D radiative transfer modelling. To fit the SED, they propose 2 possible scenarios for the inner (<5 AU) disk - optically thick or optically thin - and one scenario for the outer disk. We use the gapped disk of LkCa 15 as a showcase to illustrate the importance of 2D radiative transfer in transitional disks, by showing how the vertical dust distribution in dust-filled inner holes determines not only the radial optical depth but also the outer disk geometry. We use MCMax, a 2D radiative transfer code with a self-consistent vertical structure, to model the SED. We identify two possible geometries for the inner and outer disk, that are both different from those in Espaillat et al. (2007). An inner disk in hydrostatic equilibrium reprocesses enough starlight to fit the near infrared flux, but also casts a shadow on the inner rim of the outer disk. This requires the outer disk scale height to be large enough to rise out of the shadow. An optically thin inner disk does not cast such a shadow, and the SED can be fitted with a smaller outer disk scale height. For the dust in the inner regions to become optically thin however, the scale height would have to be so much larger than its hydrostatic equilibrium value that it effectively becomes a dust shell. It is currently unclear if a physical mechanism exists which could provide for such a configuration.
(Abridged) The detection of forming planets in disks around young stars remains elusive, and state-of-the-art observational techniques provide somewhat ambiguous results. It has been reported that the pre-transitional T Tauri star LkCa 15 could host three planets; candidate planet b is in the process of formation, as inferred from its H$alpha$ emission. However, a more recent work casts doubts on the planetary nature of the previous detections. We have observed LkCa 15 with ISIS/WHT. The spectrographs slit was oriented towards the last reported position of LkCa 15 b (parallel direction) and 90degr from that (perpendicular). The photocenter and full width half maximum (FWHM) of the Gaussians fitting the spatial distribution at H$alpha$ and the adjacent continuum were measured. A well-known binary (GU CMa) was used as a calibrator to test the spectro-astrometric performance of ISIS/WHT, recovering consistent photocenter and FWHM signals. However, the photocenter shift predicted for LkCa 15 b is not detected, but the FWHM in H$alpha$ is broader than in the continuum for both slit positions. Our simulations show that the photocenter and FWHM observations cannot be explained simultaneously by an accreting planet. In turn, both spectro-astrometric observations are naturally reproduced from a roughly symmetric Halpha emitting region centered on the star and extent comparable to the orbit originally attributed to the planet at several au. The extended H$alpha$ emission around LkCa 15 could be related to a variable disk wind, but additional multi-epoch data and detailed modeling are necessary to understand its physical nature. Spectro-astrometry in H$alpha$ is able to test the presence of accreting planets and can be used as a complementary technique to survey planet formation in circumstellar disks.
We present CARMA 1.3 mm continuum observations of the T Tauri star LkCa 15,which resolve the circumstellar dust continuum emission on angular scales between 0.2-3 arcsec, corresponding to 28-420 AU at the distance of the star. The observations resolve the inner gap in the dust emission and reveal an asymmetric dust distribution in the outer disk. (Abridge) We calculate that 90% of the dust emission arises from an azimuthally symmetric ring that contains about 5x10^{-4} M_sun of dust. A low surface-brightness tail that extends to the northwest out to a radius of about 300 AU contains the remaining 10% of the observed continuum emission. The ring is modeled with a rather flat surface density profile between 40 and 120 AU, while the inner cavity is consistent with either a sharp drop of the 1.3 mm dust optical depth at about 42 AU or a smooth inward decrease between 3 and 85 AU. (Abridge). Within 40 AU, the observations constrain the amount of dust between 10^{-6} and 7 Earth masses, where the minimum and maximum limits are set by the near-IR SED modeling and by the mm-wave observations of the dust emission respectively. In addition, we confirm the discrepancy in the outer disk radius inferred from the dust and gas, which corresponds to 150 AU and 900 AU respectively. We cannot reconcile this difference by adopting an exponentially tapered surface density profile as suggested for other systems, but we instead suggest that the gas surface density in the outer disk decreases less steeply than that predicted by model fits to the dust continuum emission. The lack of continuum emission at radii lager than 120 AU suggests a drop of at least a factor of 5 in the dust-to-gas ratio, or in the dust opacity. We show that a sharp dust opacity drop of this magnitude is consistent with a radial variation of the grain size distribution as predicted by existing grain growth models.
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