Keck Observations in the INfrared of Taurus and $ρ$ Oph Exoplanets And Ultracool dwarfs (KOINTREAU) I: A Planetary-Mass Companion and a Disk-Obscured Stellar Companion Discovered in Taurus

TL;DR

KOINTREAU presents the first discoveries from Keck AO imaging of Taurus and ρ Oph, identifying KOINTREAU-1b as a planetary-mass companion with a mass of $10.6^{+2.5}_{-2.3}$ M$_{\rm Jup}$ and KOINTREAU-2b as a young M-dwarf seen in scattered light due to an edge-on disk. Combined imaging, spectroscopy (SpeX and GNIRS), and archival photometry (Pan-STARRS, Spitzer/IRAC) enable robust common proper motion confirmation, spectral typing, extinction estimates, and partial disk characterization. KOINTREAU-1b exhibits a varying near-IR spectrum (M9±2) and possible disk-related variability, while KOINTREAU-2b shows strong He I 1.083 μm emission and subluminosity incompatible with a simple Taurus membership unless grey extinction from an edge-on disk is invoked. A newly derived NIRC2 distortion solution for PyWFS data reveals a 0.118° rotation in position angle relative to prior solutions, informing future astrometric analyses. Overall, the findings expand the Taurus census of planetary-mass companions and disk-obscured young stars, offering important constraints on early substellar evolution and disk geometry.

Abstract

We present the first discoveries from Keck Observations in the INfrared of Taurus and $ρ$ Oph Exoplanets And Ultracool dwarfs (KOINTREAU), an adaptive optics imaging survey of young stars in the Taurus and $ρ$ Oph star-forming regions using the Keck infrared pyramid wavefront sensor (PyWFS). We have found two faint ($Δ$K~7 mag), wide-separation companions to two ~3-Myr-old Taurus members. Relative astrometry for these systems show that both companions are bound to their host stars. We obtained near-infrared spectra of these companions using IRTF/SpeX (R~100) and Gemini/GNIRS (R~1000-2000), and combine these with photometry from our NIRC2 imaging, the Pan-STARRS survey, and Spitzer/IRAC archival imaging to constrain their properties. One companion, KOINTREAU-1b (at a projected separation of 690 au), has an average near-IR spectral type of M9$\pm$2, a gravity classification of VL-G, and a changing spectral type between the SpeX (M7) and GNIRS (L1) observations. We estimate this object's mass to be $10.6^{+2.5}_{-2.3}$ M$_{\rm Jup}$, making KOINTREAU-1b the fifth planetary-mass companion found in Taurus. The other companion, KOINTREAU-2b (projected separation 560 au), has a spectral type of M4.5$\pm$1 but is ~4 magnitudes underluminous relative to other Taurus stars of the same spectral type. We detect exceptionally strong He I 1.083 micron emission from this object, indicative of outflows driven by ongoing accretion, but with a conspicuous lack of accompanying H emission. We conclude that KOINTREAU-2b is a young star obscured by an edge-on disk and observed in scattered light. Finally, we derive a distortion solution for NIRC2 imaging which shows a 0.118° difference in position angle from the previous distortion solution.

Figures (18)

• Figure 1: Example Keck/NIRC2 signal-to-noise maps from our deep-exposure 10$^{\prime\prime}$$K$-band image stacks of each companion. These maps are computed by subtracting the mean and normalizing by the standard deviation in successive annuli around the host star. The companions are circled in green in each case. We note some electronic noise in the lower right quadrant of the shown epoch (December 2023) of KOINTREAU- 2b data, but our measurements are unaffected by this.
• Figure 2: Pan-STARRS $grizy$ 13$^{\prime\prime}$ images of the XEST 13-010 system. The arrow in each image indicates KOINTREAU- 2b.
• Figure 3: The results of our Spitzer/IRAC PSF fitting procedure for XEST 17-036 (top) and XEST 13-010 (bottom), with the positions of each candidate companion indicated with a black arrow. Despite not being visible by eye in the catalog images, fitting and subtracting PSFs of the host star reveals the faint companions detected in our Keck/NIRC2 imaging.
• Figure 4: Relative astrometry of KOINTREAU- 1b. The gray curve shows 1000 draws from a model of the predicted movement of a background object relative to the companion's first epoch detection (the blue 'x' in each panel). The hollow markers indicate where the companion would appear if it were a background object at each epoch, and the filled markers indicate the measured position of the companion in the corresponding epoch. Note that measurements are presented relative to the first epoch, hence first-epoch measurement errors are incorporated into the displayed errorbars for subsequent epochs. Over a baseline of four years, the change in the companion's sky motion, separation and position angle relative to its host star are consistent with zero.
• Figure 5: Relative astrometry of KOINTREAU- 2b, the imaged companion to XEST 13-010, demonstrating that the two are physically associated. See Figure \ref{['fig:xest17astrometry']} for full description.