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Accreting protoplanets in the LkCa 15 transition disk


Exoplanet detections have revolutionized astronomy, offering new insights into solar system architecture and planet demographics. While nearly 1,900 exoplanets have now been discovered and confirmed1, none are still in the process of formation. Transition disks, protoplanetary disks with inner clearings2,3,4 best explained by the influence of accreting planets5, are natural laboratories for the study of planet formation. Some transition disks show evidence for the presence of young planets in the form of disk asymmetries6,7 or infrared sources detected within their clearings, as in the case of LkCa 15 (refs 8, 9). Attempts to observe directly signatures of accretion onto protoplanets have hitherto proven unsuccessful10. Here we report adaptive optics observations of LkCa 15 that probe within the disk clearing. With accurate source positions over multiple epochs spanning 2009–2015, we infer the presence of multiple companions on Keplerian orbits. We directly detect Hα emission from the innermost companion, LkCa 15 b, evincing hot (about 10,000 kelvin) gas falling deep into the potential well of an accreting protoplanet.

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Figure 1: Composite Hα, Ks, and L′ image.
Figure 2: Position evolution.
Figure 3: Spectral energy distributions.


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This work was supported by NSF AAG grant no. 1211329 and NASA OSS grant NNX14AD20G. This material is based upon work supported by the National Science Foundation under grant no. 1228509. This work was performed in part under contract with the California Institute of Technology (Caltech) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant no. DGE-1143953. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors(s) and do not necessarily reflect the views of the National Science Foundation.

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Authors and Affiliations



This work merged two independently acquired and analysed data sets. S.S. led preparation of the manuscript, the orbital fits, and the acquisition and analysis of the LBT data while K.B.F. led the acquisition and analysis of the MagAO data, development of the MagAO SDI pipeline, and drafted MagAO manuscript sections. S.S., K.B.F., J.E., L.C., P.H., A.S., J.M., and K.M. contributed to one or both observing proposals. J.E. modelled circumplanetary disk and hot-start scenarios, developed the NRM mode at LBT, and supervised effort of S.S.; L.C. carried out Hα luminosity calculations and oversaw the MagAO effort. P.H. led LBTI development and support, and helped commission the NRM mode at LBT. K.K. carried out orbital stability analysis. J.M. developed the KLIP code used in MagAO data analysis. P.T. helped develop the NRM mode at LBT. B.M. supervised the effort of K.B.F.; S.S., K.B.F., J.E., L.C., and K.K. contributed key aspects of the manuscript. A.S., V.B., D.D., E.S., and A.V. supported the LBT observations. J.M., K.M., T.R., and A.W. supported the MagAO observations.

Corresponding author

Correspondence to S. Sallum.

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Extended data figures and tables

Extended Data Figure 1 Image reconstructions.

ad, Images reconstructed from closure phases, showing Ks polynomial (a) and LOCI-like (b) calibrations, and L′ polynomial (c) and LOCI-like (d) calibrations. Both calibrations yielded reconstructed images with at least two distinct components. The LOCI-like calibration moved each companion within the position errors derived from the grid χ2 surface.

Extended Data Figure 2 KLIP and ADI Hα SNR maps.

ac, Final KLIP SNR maps for Hα (a), continuum (b) and the difference between the two (ASDI, c). df, Final cADI SNR maps in the same order. Dividing by the radial noise profiles to create these maps should normalize the noise distribution at all radii within the speckle-dominated regime. The presence of dark holes in the maps suggests that we are speckle-dominated out to the AO control radius at r ≈ 20 pixels (white, dashed circles). LkCa 15 b’s separation is 11.6 pixels. The yellow keystones indicate the 2σ range of allowed astrometry for the KLIP ASDI point source (upper right) based on negative simulated planet injection.

Extended Data Figure 3 False positive planet SNR maps.

a, LkCa 15 final ASDI SNR map. b, ASDI SNR map with LkCa 15 b removed. ch, ASDI SNR maps of false positive planets injected at a radius of 11 pixels and contrast of 8 × 10−3 . Recovered parameters for these planets are given in Extended Data Table 2 and were used to determine 1σ astrometric and photometric uncertainties.

Extended Data Figure 4 Hα detection noise statistics.

a, Histogram of noise (non-planet) pixel values in the SNR map within the speckle dominated regime (black line) compared to a Normal distribution (red line). The black arrow denotes the location of the peak SNR value for LkCa 15 b. b, Histogram of the peak values in all noise apertures (see Extended Data Fig. 5) within the control radius (black line) compared to a Normal distribution (red line). The black arrow shows the peak pixel value in the LkCa 15 b aperture.

Extended Data Figure 5 Noise apertures.

Noise apertures (black circles) surrounding LkCa 15 A used to calculate the statistics presented in Extended Data Fig. 4. Colour indicates SNR.

Extended Data Figure 6 LkCa 15 d position angle and separation versus time.

Evolution of position angle and separation (inset) for LkCa 15 d. Green and red points indicate Ks and L′ data, respectively. In both panels, the earliest three points correspond to previously published Keck observations8, and the most recent points show best fits to our data. The coloured error bars are derived using the nonlinear algorithm MPFIT, which significantly underestimates the parameter errors compared to the more robust grid, Δχ2 (black error bars; see Methods). The yellow shaded region spans the position angles and separations allowed at 1σ by the multi-epoch observations, which have semi-major axes between 12.6 and 24.7 au. Solid curves show the best-fit orbit (18.0 au), and dashed curves show an orbit (24.7 au) that is stable for a 0.5 MJ planet exterior to LkCa 15 b and c. Lower mass planets or resonant configurations permit stable orbits for LkCa 15 d at smaller stellocentric radii.

Extended Data Figure 7 Orbital integration results.

a, Stable orbits for LkCa 15 b, c, and d over a 10 Myr integration. b, Osculating eccentricity. The planets are each 0.5 MJ with initial semi-major axes of 12.7, 18.6, and 24.7 au, initial eccentricities of order 10−5, and relative inclinations of <1°. After a 10 Myr integration, the eccentricities of c and d have increased to only a few percent.

Extended Data Table 1 Summary of observations
Extended Data Table 2 False planet injection results

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Sallum, S., Follette, K., Eisner, J. et al. Accreting protoplanets in the LkCa 15 transition disk. Nature 527, 342–344 (2015).

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