Chemical and Isotopic Homogeneity Between the L Dwarf CD-35 2722 B and its Early M Host Star

TL;DR

CD-35 2722 B, a ~30 M_Jup L dwarf companion to a young AB Doradus member, is analyzed with high-resolution KPIC $K$-band spectroscopy to constrain bulk properties, chemical abundances, and cloud signatures. By retrieving equilibrium-chemistry abundances and testing different $P$–$T$ treatments, the study finds [M/H] for the host of $-0.16\pm0.25$ dex and [M/H] for the companion of $0.27^{+0.14}_{-0.13}$ dex, with C/O ≈ 0.55 and a robust detection of $^{13}$CO in the companion at ~5σ, giving $^{12}$CO/$^{13}$CO ≈ 159. The host and companion show chemical consistency within ~1.5σ for metallicity and ~0.6σ for isotopologue ratios, supporting formation via gravitational instability. The lack of definitive clouds in the $K$-band data highlights the sensitivity limitations of high-resolution near-infrared spectra to aerosols and motivates broader-wavelength observations. Together, the results inform planet–star formation pathways and illustrate the value of isotopologue and metallicity diagnostics in young, wide-separation substellar systems.

Abstract

CD-35 2722 B is an L dwarf companion to the nearby, $\sim 50-200$ Myr old M1 dwarf CD-35 2722 A. We present a detailed analysis of both objects using high-resolution ($R \sim 35,000$) $K$ band spectroscopy from the Keck Planet Imager and Characterizer (KPIC) combined with archival photometry. With a mass of $30^{+5}_{-4} M_{\mathrm{Jup}}$ (planet-to-host mass ratio 0.05) and projected separation of $67\pm4$ AU from its host, CD-35 2722 B likely formed via gravitational instability. We explore whether the chemical composition of the system tells a similar story. Accounting for systematic uncertainties, we find $\mathrm{[M/H]}=-0.16^{+0.03}_{-0.02} \mathrm{(stat)} \pm 0.25 \mathrm{(sys)}$ dex and $^{12}\mathrm{C}/^{13}\mathrm{C}=132^{+20}_{-14}$ for the host, and $\mathrm{[M/H]}=0.27^{+0.07}_{-0.06} (\mathrm{stat}) \pm 0.12 (\mathrm{sys})$ dex, $^{12}\mathrm{CO}/^{13}\mathrm{CO}=159^{+33}_{-24} \mathrm{(stat)}^{+40}_{-33} \mathrm{(sys)}$, and $\mathrm{C/O} = 0.55 \pm 0.01 (\mathrm{stat}) \pm 0.04 (\mathrm{sys})$ for the companion. The chemical compositions for the brown dwarf and host star agree within the $1.5σ$ level, supporting a scenario where CD-35 2722 B formed via gravitational instability. We do not find evidence for clouds on CD-35 2722 B despite it being a photometrically red mid-L dwarf and thus expected to be quite cloudy. We retrieve a temperature structure which is more isothermal than models and investigate its impact on our measurements, finding that constraining the temperature structure to self-consistent models does not significantly impact our retrieved chemical properties. Our observations highlight the need for data from complementary wavelength ranges to verify the presence of aerosols in likely cloudy L dwarfs.

Figures (9)

• Figure 1: Results from evolutionary models, where the mass, radius, $\log g$, and $T_{\mathrm{eff}}$ are self-consistently calculated. We obtain the priors $43\pm 8~M_\mathrm{Jup}$ and $1.25\pm 0.15~R_\mathrm{Jup}$ (black dashed lines) on the mass and radius through visually comparing the Gaussians to the evolutionary model posterior probabilities. Note that the mass estimate is slightly larger than, but consistent with, the value of $31 \pm 8~M_\mathrm{Jup}$ from Wahhaj_2011.
• Figure 2: Cross-correlation function for the brown dwarf CD-35 2722 B. We find a highly significant $>100\sigma$ detection of this companion, which is expected given its brightness Wahhaj_2011 and large separation (2.8") from the host star. The gray line represents the CCF of background flux; its standard deviation is taken to be the CCF noise.
• Figure 3: Best-fit model (red) and spectrum (black, with uncertainties as gray shaded regions) for CD-35 2722 A, using the PHOENIX grid. Residuals have been shifted by $+0.5$ for visual purposes. The corresponding properties are shown in Figure \ref{['fig:host_corner']}. The gaps in the data are due to strong tellurics, which have been masked based on the response function with a transmission threshold of 0.30. The bandheads of $^{12}$CO and $^{13}$CO are shown as vertical dashed lines, in blue and orange respectively. The labels denote the transitions between vibrational states. The spectral feature at 2.307 $\mu$m is due to a linelist mismatch or due to an unidentified species not present in the host star; this highlights a limitation of precomputed grid models. The standard deviation of the residuals (0.018) is comparable to the median errorbar (0.021).
• Figure 4: Joint posterior distribution for best-fit parameters to CD-35 2722 A spectrum. Note that the uncertainties here do not include systematic uncertainties, which inflate the errors for $T_{\mathrm{eff}}$ and $\mathrm{[M/H]}$.
• Figure 5: Posterior distribution of C/O, $\mathrm{[M/H]}$, and $^{12}$CO/$^{13}$CO for select retrievals in Table \ref{['tab:lnz']}. All retrievals use equilibrium chemistry and Gaussian priors on the $P-T$ profile, except for the one shown in pink which fixes the $P-T$ profile and is labeled as such. The uncertainties here are only statistical and do not include systematic uncertainties. All parameters are consistent to within $3\sigma$ across the models except for C/O, which is nominally discrepant at $7\sigma$ between the fixed $P-T$ model and all other models. This discrepancy lowers to $0.9\sigma$ upon considering systematic uncertainties. The cloud-free model with Gaussian $P-T$ priors, represented in black, is the configuration we choose as our fiducial result.