Keck Observations in the INfrared of Taurus and $ρ$ Oph Exoplanets And Ultracool dwarfs (KOINTREAU) II: Two Young Bound Companions to Ophiuchus Stars

TL;DR

KOINTREAU II advances directly imaged substellar companions in the young Ophiuchus region using Keck/NIRC2 AO with a pyramid wavefront sensor, confirming two new bound companions around ISO-Oph 96 and 2MASS J16262785-2625152. The authors combine multi-epoch Keck imaging with Gaia DR3 astrometry to establish comoving status and derive photometric spectral-type constraints, deriving bolometric luminosities and mass estimates via the DUSTY models. KOINTREAU-3b is securely in the planetary regime with a mass of $3.4\pm0.7\ M_Jup$, while KOINTREAU-4b straddles the planetary/brown-dwarf boundary with masses of $11.5^{+1.2}_{-1.6}\ M_Jup$ (Oph age) or $15.3^{+0.7}_{-0.8}\ M_Jup$ (USco age). The results underscore the critical dependence of inferred masses on host ages and extinctions in very young regions, and call for spectroscopy to confirm spectral types and refine ages for tighter mass constraints. Overall, KOINTREAU-II demonstrates the continued power of AO imaging in probing substellar companions at wide separations in nearby young regions.

Abstract

We present the second set of discoveries from Keck Observations in the INfrared of Taurus and $ρ$ Oph Exoplanets And Ultracool dwarfs (KOINTREAU), an adaptive optics survey of young stars in the Taurus and $ρ$ Oph star-forming regions using Keck/NIRC2 in conjunction with the Keck infrared pyramid wavefront sensor. We have discovered two faint comoving companions to young stars ISO-Oph 96 and 2MASS J16262785-2625152. The companion to ISO-Oph 96, KOINTREAU-3b, is at a projected separation of 340 au (2.49"). Using our NIRC2 photometry and evolutionary models, and assuming that the companion has the same extinction as its host star, we infer that KOINTREAU-3b has a mass of $3.4\pm0.7$ M$_{\rm Jup}$. The companion to 2MASS J16262785-2625152, KOINTREAU-4b, has a projected separation of 180 au (1.25") and could have a mass of either $11.5^{+1.2}_{-1.6}$ M$_{\rm Jup}$ or $15.3^{+0.7}_{-0.8}$ M$_{\rm Jup}$, depending on whether the host star is a member of $ρ$ Oph or Upper Sco.

Figures (5)

• Figure 1: 6$^{\prime\prime}$ cutouts from our Keck/NIRC2 signal-to-noise maps from our deep-exposure $K$-band image stacks of each candidate companion, oriented North-up East-left. These maps are computed by subtracting the median and normalizing by the robust standard deviation in successive annuli around the host star. The candidate companions are circled.
• Figure 2: Relative astrometry of KOINTREAU- 3b, the companion to ISO-Oph 96, demonstrating that it is physically associated with ISO-Oph 96 and not a background object. The dark 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 cross and gray dashed lines 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.
• Figure 3: Relative astrometry for KOINTREAU- 4b, our observed companion to 2M 1626$-$ 2625, demonstrating that it is physically associated with 2M 1626$-$ 2625 and not a background object. See Figure \ref{['fig:isooph96_astrometry']} caption for full description.
• Figure 4: $J-K$ colour vs. extinction-corrected apparent $K$-band magnitude for our two candidate companions and their host stars, compared with Ophiuchus sources with spectral types K4-L3 from esplin2020 and known Ophiuchus planetary-mass companions (PMCs; left), and K4-L3 Upper Sco sources luhman2020 and PMCs (right). For KOINTREAU- 3b we have no color data and thus indicate the dereddened $K$-band magnitude using a $\pm1\sigma$-tall band. 2MASS literature photometry has been transformed to MKO per skrutskie2006. Ophiuchus PMCs are, from brightest to faintest: ROXs 12 b, kraus2014, kraus2014; SR12 C, kuzuhara2011, kuzuhara2011; ROXs 42B b, kraus2014, kraus2014; CFHTWIR-Oph 98 B, fontanive2020, fontanive2020. Upper Sco PMCs are, from brightest to faintest: USco 1621 B, chinchilla2020, chinchilla2020; USco 1556 B, chinchilla2020, chinchilla2020; GSC 06214-00210B, ireland2011, ireland2011; 1RXS J160929.1-210524B, lafreniere2008, lafreniere2008.
• Figure 5: Extinction-corrected apparent $K$-band magnitude as a function of spectral type for our candidate companions, plotted against K4-L3 $\rho$ Oph sources and PMCs (left), and K4-L3 Upper Sco sources and PMCs (right). The $K$-band magnitudes of our two candidate companions are roughly consistent with $>$L0 for KOINTREAU- 3b and L0$\pm$2 for KOINTREAU- 4b. The lighter region of each band indicates the range of spectral types disfavored by the Gaia non-detection of each companion, calculated by combining $G-K$ color as a function of spectral type pecaut2013 with the observed $K$-band magnitude of each companion and Gaia$G$-magnitude contrast limits brandeker2019. These bands are $\pm1\sigma$ tall. Ophiuchus and Upper Sco PMCs are the same as in Figure \ref{['fig:cmd_byspt']}.