A Spatially Resolved Inner Hole in the Disk around GM Aurigae

We present 0.3 arcsec resolution observations of the disk around GM Aurigae with the Submillimeter Array (SMA) at a wavelength of 860 um and with the Plateau de Bure Interferometer at a wavelength of 1.3 mm. These observations probe the distribution of disk material on spatial scales commensurate with the size of the inner hole predicted by models of the spectral energy distribution. The data clearly indicate a sharp decrease in millimeter optical depth at the disk center, consistent with a deficit of material at distances less than ~20 AU from the star. We refine the accretion disk model of Calvet et al. (2005) based on the unresolved spectral energy distribution (SED) and demonstrate that it reproduces well the spatially resolved millimeter continuum data at both available wavelengths. We also present complementary SMA observations of CO J=3-2 and J=2-1 emission from the disk at 2" resolution. The observed CO morphology is consistent with the continuum model prediction, with two significant deviations: (1) the emission displays a larger CO J=3-2/J=2-1 line ratio than predicted, which may indicate additional heating of gas in the upper disk layers; and (2) the position angle of the kinematic rotation pattern differs by 11 +/- 2 degrees from that measured at smaller scales from the dust continuum, which may indicate the presence of a warp. We note that photoevaporation, grain growth, and binarity are unlikely mechanisms for inducing the observed sharp decrease in opacity or surface density at the disk center. The inner hole plausibly results from the dynamical influence of a planet on the disk material. Warping induced by a planet could also potentially explain the difference in position angle between the continuum and CO data sets.

💡 Research Summary

The authors present high‑resolution (≈0.3″) interferometric observations of the protoplanetary disk around the T Tauri star GM Aurigae using the Submillimeter Array (SMA) at 860 µm and the Plateau de Bure Interferometer (PdBI) at 1.3 mm. Both continuum images reveal a pronounced central depression in millimeter optical depth, indicating a sharp drop in material density inside a radius of roughly 20 AU. This “inner hole” had previously been inferred only from spectral energy distribution (SED) modeling; the new data provide the first spatially resolved confirmation of its size and sharpness.

To interpret the data, the authors refine the radiative‑transfer disk model originally presented by Calvet et al. (2005). The revised model reduces the dust surface density inside the cavity to a level that yields an optical depth ≲10⁻³ at millimeter wavelengths, while preserving a steep density gradient at the cavity edge. The model simultaneously reproduces the observed brightness profiles at both 860 µm and 1.3 mm, demonstrating that a single, self‑consistent density and temperature structure can account for the multi‑wavelength continuum data.

Complementary observations of CO J = 3‑2 and CO J = 2‑1 line emission were obtained with the SMA at ≈2″ resolution. The CO maps trace a rotating disk consistent with Keplerian motion, and the overall morphology matches the predictions of the continuum‑based model. However, two notable discrepancies emerge: (1) the observed CO J = 3‑2 / J = 2‑1 line ratio is significantly higher than the model predicts, suggesting that the gas in the upper layers of the disk is hotter than assumed, perhaps due to additional heating by stellar UV/X‑ray radiation or by turbulent dissipation; (2) the position angle (PA) of the CO velocity gradient differs by 11° ± 2° from the PA derived from the dust continuum, implying a possible warp or misalignment between the gas‑rich outer disk and the dust‑dominated inner regions.

The authors evaluate several mechanisms that could generate a sharp inner cavity: (i) photoevaporation, (ii) grain growth leading to reduced opacity, (iii) the presence of a close stellar companion, and (iv) dynamical clearing by an embedded planet. Photoevaporation can produce cavities but cannot simultaneously explain the relatively high accretion rate onto the star and the residual gas observed inside the cavity. Grain growth alone would not produce the observed abrupt drop in millimeter optical depth nor the CO line characteristics. No evidence for a stellar binary is found in radial‑velocity or imaging data. By contrast, a planet with a mass of order 1–10 M_Jup located near the cavity edge can open a gap through tidal torques, depleting both dust and gas interior to its orbit while allowing a modest amount of material to leak across the gap, consistent with the measured accretion rate. Moreover, a massive planet inclined relative to the disk plane can induce a warp, naturally accounting for the PA offset between the CO and dust emission.

In summary, the paper provides compelling observational evidence for a well‑defined, ≈20 AU inner hole in the GM Aur disk and demonstrates that a planet‑driven clearing scenario, possibly accompanied by a warp, offers the most coherent explanation for the combined continuum and CO data. The work underscores the power of spatially resolved millimeter interferometry for probing disk sub‑structures and sets the stage for future ALMA observations that could directly detect the putative planet, map the three‑dimensional geometry of the warp, and further constrain the heating mechanisms operating in the disk’s surface layers.