Spectro-astrometry of the pre-transitional star LkCa 15 does not reveal an accreting planet but extended Hα emission

Abstract

Context. The detection of forming planets in protoplanetary disks around young stars remains elusive, and state-of-the-art observational techniques provide somewhat ambiguous results. The pre-transitional T Tauri star LkCa 15 is an excellent example. It has been reported that it could host three planets; candidate planet b is in the process of formation, as inferred from its Hα emission. However, a more recent work casts doubts on the planetary nature of the previous detections. Aims. We test the potential of spectro-astrometry in Hα as an alternative observational technique to detect forming planets around young stars, taking LkCa 15 as a reference case Methods. LkCa 15 was observed with the ISIS spectrograph at the 4.2m William Herschel Telescope (WHT). The slit was oriented towards the last reported position of LkCa 15 b (parallel direction) and 90 • from that (perpendicular). The photocenter and full width half maximum (FWHM) of the Gaussians fitting the spatial distribution at Hα 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. Results. A consistent spectro-astrometric signature is recovered for GU CMa. However, the photocenter shift predicted for LkCa 15 b is not detected, but the FWHM in Hα 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, but the lack of photocenter shift alone could still be consistent with an emitting planet with contrast 5.5 mags in Hα or 6 mag in the adjacent continuum. In turn, both spectro-astrometric observations are naturally reproduced from a roughly symmetric Hα emitting region centered on the star and extent comparable to the orbit originally attributed to the planet at several au. Conclusions. The extended Hα 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. Optical spectro-astrometry carried out with mid-size telescopes is capable of probing small-scale structures in relatively faint young stars that are not easily accessible with state-of-the-art instrumentation mounted on larger telescopes. Therefore, spectro-astrometry in Hα is able to test the presence of accreting planets and can be used as a complementary technique to survey planet formation in circumstellar disks.

On the Mass Function, Multiplicity, and Origins of Wide-orbit Giant Planets

“…To our knowledge, this list is complete with respect to companions that have been directly detected in high-contrast imaging with mass estimates in the substellar range, which we verified by cross-checking against the NASA Exoplanet Archive. 9 We excluded planetarymass companions around white dwarf and brown dwarf hosts, and also planet candidates that have been interpreted as potential disk features (e.g., the planet candidates around Fomalhaut: Janson et al 2012, Lawler et al 2015, HD 169142: Ligi et al (2018, and LkCa 15: Thalmann et al 2015, Mendigutía et al 2018). This criteria resulted in a list of 57 companions, whose properties are given in Appendix A.…”

Highly structured disk around the planet host PDS 70 revealed by high-angular resolution observations with ALMA

“…Current direct-imaging infrared surveys reach detection limits of a few Jupiter masses (e.g., Maire et al 2017;Uyama et al 2017), but are often limited by bright and complex disk features. Numerous claims of companion candidates in disks that show asymmetric features are indeed still debated (e.g., HD100546, HD169142, MWC758, LkCa15; see Quanz et al 2015;Currie et al 2015;Follette et al 2017;Rameau et al 2017;Biller et al 2014;Reggiani et al 2014;Ligi et al 2018;Reggiani et al 2018;Kraus & Ireland 2012;Sallum et al 2015;Mendigutía et al 2018) and require confirmation through additional observations at different filter bands, for example.…”

The Disk Substructures at High Angular Resolution Project (DSHARP). IX. A High-definition Study of the HD 163296 Planet-forming Disk

“…Taken at face value, these results tend to suggest the existence of a population of young gas giant planets orbiting at several tens of au from the central star which challenge current planet formation models. Furthermore, the link between planets and rings is supported by the direct detection of possible young planets in the PDS 70 and LkCa 15 systems (Keppler et al 2018;Sallum et al 2015), although the nature of some candidates is debated (Mendigutía et al 2018;Thalmann et al 2016).…”