HD 106906 b: A planetary-mass companion outside a massive debris disk

Abstract: We report the discovery of a planetary-mass companion, HD 106906 b, with the new Magellan Adaptive Optics (MagAO) + Clio2 system. The companion is detected with Clio2 in three bands: $J$, $K_S$, and $L^\prime$, and lies at a projected separation of 7.1'' (650 AU). It is confirmed to be comoving with its $13\pm2$ Myr-old F5 host using Hubble Space Telescope/Advanced Camera for Surveys astrometry over a time baseline of 8.3 yr. DUSTY and COND evolutionary models predict the companion's luminosity corresponds to a mass of $11\pm2 M_{Jup}$, making it one of the most widely separated planetary-mass companions known. We classify its Magellan/Folded-Port InfraRed Echellette $J/H/K$ spectrum as L$2.5\pm1$; the triangular $H$-band morphology suggests an intermediate surface gravity. HD 106906 A, a pre-main-sequence Lower Centaurus Crux member, was initially targeted because it hosts a massive debris disk detected via infrared excess emission in unresolved Spitzer imaging and spectroscopy. The disk emission is best fit by a single component at 95 K, corresponding to an inner edge of 15-20 AU and an outer edge of up to 120 AU. If the companion is on an eccentric ($e>0.65$) orbit, it could be interacting with the outer edge of the disk. Close-in, planet-like formation followed by scattering to the current location would likely disrupt the disk and is disfavored. Furthermore, we find no additional companions, though we could detect similar-mass objects at projected separations $>35$ AU. In situ formation in a binary-star-like process is more probable, although the companion-to-primary mass ratio, at $<1\%$, is unusually small.

• The paper presents high-resolution observations with MagAO-Clio2 that reveal HD 106906 b as a ~11 MJup companion at a projected separation of 650 AU, challenging standard planet formation models.
• Advanced photometric, astrometric, and spectroscopic analyses confirm the companion’s comoving status and L-type spectral features, reinforcing its planetary-mass classification.
• The study suggests that the potential eccentric orbit of HD 106906 b could actively shape the outer edges of its host star's massive debris disk, offering new insights into disk-planet interactions.

Overview of "A Planetary-Mass Companion to HD 106906"

The paper "HD 106906 b: A planetary-mass companion outside a massive debris disk" by Bailey et al. discusses the discovery and analysis of a planetary-mass companion to the star HD 106906, designated HD 106906 b. Utilizing the Magellan Adaptive Optics (MagAO) with the Clio2 system, the study provides significant observational data, including photometry, astrometry, and spectroscopy, to characterize this companion. The findings are potentially significant for understanding both planet formation mechanisms and planet-disk interactions at wide separations.

Key Findings

The companion HD 106906 b was detected across three bands ($J$, $K_S$, and $L^\prime$) and is confirmed to be comoving with its host star through precise astrometric measurements. Photometric and spectroscopic analysis places the mass of HD 106906 b at approximately $11 \pm 2~M_{Jup}$, which renders it one of the most widely separated and least massive planetary-mass companions known so far, at a projected separation of 650 AU. Its mass estimate, bolstered by the DUSTY and COND evolutionary models, and its $L$-type spectral classification suggest an intermediate surface gravity, typical for young, planetary-mass objects.

The primary star, HD 106906 A, hosts a substantial debris disk as revealed by infrared excess emission. The disk, characterized by a single temperature component at 95 K, implies a spatial extent from roughly 15 to potentially 120 AU. Given HD 106906 b's wide separation, its potential influence on the disk depends on the orbit being sufficiently eccentric ($e>0.65$).

Implications

The peculiar architecture of the HD 106906 system, featuring a distant, planetary-mass companion and a massive debris disk, has several implications:

1. Formation Theories: The existence at such a wide separation challenges typical planet formation models, as conventional core accretion at this distance is improbable. The companion's formation might be more analogous to binary star formation, albeit with an unusually low companion-to-primary mass ratio of less than 1%.
2. Disk-Planet Interactions: The eccentric orbit postulated for HD 106906 b suggests it may play a role in sculpting the outer edges of the debris disk, akin to other systems where planets influence disk morphology through gravitational interactions. This necessitates considering the gravitational dynamics involved in systems with similar configurations.
3. Prospective Observations: The system presents an intriguing target for further observational campaigns. Utilizing future high-resolution imaging (in both scattered light and at sub-millimeter wavelengths) could yield resolved images of the disk, offering deeper insight into the disk-planet dynamics and formation environment.
4. Statistical Significance in Exoplanet Studies: Adding HD 106906 b to the catalog of directly imaged exoplanets enhances the statistical basis for assessing the population characteristics and diversity of planetary systems, particularly those with wide-orbit companions.

Future Research Directions

The discovery and initial analysis of HD 106906 b pave the way for future studies on several fronts:

• Spectral Analysis: Obtaining higher-resolution and higher signal-to-noise ratio spectra could refine the surface gravity estimates and improve our understanding of the companion's atmosphere and composition.
• Dynamic Studies: Comprehensive multi-epoch astrometric observations could delineate HD 106906 b's orbit, confirming its eccentricity and theorized influence on the debris disk.
• Formation Mechanisms: By contrasting such systems with others, refined models of planetary formation and migration could emerge, particularly examining the role of in situ formation processes for wide-orbit planetary bodies.

In summary, HD 106906 b offers a compelling case study for the complexities inherent in planet formation and the intricate dynamics at play in planetary systems exhibiting substantial separations between components. The findings contribute valuable empirical data to the ongoing exploration of planetary systems beyond our own.