Multi-wavelength ALMA Imaging of HD 34282: Dust-trapping Signatures of a Vortex Candidate

Abstract

Azimuthal arcs in millimeter continuum emission from protoplanetary disks are often attributed to dust-trapping vortices, but definitive observational confirmation of vortices remains lacking. We present sub-0.1" resolution ALMA continuum observations of the HD 34282 disk at 0.9, 1.3, 2.1, and 3.1 mm. These observations resolve a bright azimuthal arc superposed on a compact double-gap, triple-ring morphology, most clearly at shorter wavelengths, and enable us to probe the physical origin of the arc. It exhibits a lower spectral index than the surrounding rings, consistent with enhanced grain growth and/or higher dust surface density of a dust-trapping vortex. Its azimuthal width decreases with increasing wavelength, consistent with tighter confinement of larger grains, or lower optical depths at longer wavelengths. These observations probe dust with Stokes numbers St < 0.03. Vortex models predict negligible peak shifts in this regime, consistent with the 1.3 to 3.1 mm data. At 0.9 mm, however, the arc peak is offset by 15 +/- 4 degree in the direction of disk rotation relative to longer wavelengths, and the near-side ring emission is locally dimmer compared to the far-side, likely reflecting optical-depth or temperature effects. These observations are consistent with azimuthal dust trapping, potentially associated with a vortex-induced pressure maximum.

Figures (10)

• Figure 1: ALMA continuum images at 3.1 mm (a), 2.1 mm (b), 1.3 mm (c), and 0.9 mm (d). The synthesized beam size (peak SNR) is 0.095 $\times$ 0.081$"$ (60), 0.081 $\times$ 0.049$"$ (80), 0.056 $\times$ 0.051$"$ (55), and 0.059 $\times$ 0.050$"$ (133) from panel (a) to (d). The fits files are available in the online article.
• Figure 2: Normalized azimuthal brightness profiles at $r_\mathrm{arc}$ (Table \ref{['table:bestfit']}) after deprojection using the geometry in Appendix \ref{['app:galario']}. The disk-plane azimuth $\theta$ is measured from the red-shifted major axis and increases counterclockwise. Solid curves show profiles from the native-resolution images; dashed curves are from images convolved to a common $0.095 \times 0.081"$ beam (Band 3). The azimuthal resolution, $\Delta\theta_{\rm beam,common}$, is defined as the tangential FWHM of the deprojected common beam at the arc peak. The black arrow marks clockwise disk rotation de_Boer_2021. The 0.9 mm profile shows a sharp peak with a leading-side shoulder (pink arrows) and asymmetric baselines (orange arrows).
• Figure 3: Residual map generated by subtracting the frank model from the data at 3.1 mm (a), 2.1 mm (b), 1.3 mm (c), and 0.9 mm (d). The synthesized beam is indicated by the ellipse in the lower left corner. The arc radius $r_{\rm arc}$ is indicated by the dashed gray line. At 0.9 mm, a faint arc-like positive residual is visible at $r \sim 0.17"$ (panel d; arrow).
• Figure 4: Top: Residual maps (Figure \ref{['fig:res_frank']}) convolved to a common Band 3 beam ($0.095 \times 0.081"$) and deprojected for comparison across 3.1, 2.1, 1.3, and 0.9 mm (left to right). The horizontal dashed line marks $r_{\rm arc}$. Bottom: Azimuthal profiles of the top panels at $r_{\rm arc}$ (Table \ref{['table:bestfit']}). Cyan shows the raw residual $I_{\rm res}$, green is the opposite-sector background $I_{\rm bg}$, and blue is the background-corrected profile $I_{\rm corr}$; the shaded region indicates $\pm3\sigma$ noise. The orange marker marks the peak $\theta_{\rm peak,arc}$ with $3\sigma$ uncertainty. The horizontal dotted line indicates the half-maximum level in $I_{\rm corr}$, and the black bar (top left) the effective azimuthal beam $\Delta\theta_{\mathrm{beam,common}}$ at the arc peak. The arc is fully resolved in the azimuthal direction at all wavelengths.
• Figure 5: Radial profiles of the normalized residual intensity, $I_{\mathrm{res}}(r,\theta_{\mathrm{peak,arc}})$, extracted at the azimuthal peak of the arc for all four wavelengths, measured from the native-resolution images. The effective beam FWHM at each wavelength, $\Delta r_{\mathrm{beam}}$, is listed in the upper right. The FWHM of the arc, $\Delta r_\mathrm{FWHM,arc}$, is listed in the upper left corner. The arc is only marginally resolved in the radial direction at 3.1 mm ($\Delta r_\mathrm{FWHM,arc}=1.5\Delta r_{\mathrm{beam}}$), but is better resolved at shorter wavelengths ($\Delta r_\mathrm{FWHM,arc}>2\Delta r_{\mathrm{beam}}$).