Indications of Rapid Dust Formation in the Inner Region of a Protoplanetary Disk

We report a significant increase in mid-infrared emission $\leq10$ $μ$m in a transitional disk. The 2024 JWST/MIRI observation of the disk around CVSO 1942 reveals flux increase by a factor of two at $λ\leq10$ $μ$m, compared to the near photospheric flux level observed with Spitzer/IRS in 2005. No significant change in flux at $\gtrsim15$ $μ$m is detected in the spectra. Comparing the MIRI/MRS spectrum and NEOWISE photometry, we found that this $\leq10$ $μ$m flux increase occurs on a timescale of 2 weeks and is consistent with the presence of warm (1,400 K), optically thick, large ($\gtrsim1$ $μ$m) dust grains near the dust sublimation radius. We propose that this rapid dust appearance may indicate in situ dust formation, possibly from planetesimal collisions in the inner disk.

💡 Research Summary

The authors present a multi‑epoch, multi‑wavelength study of the transitional disk around the low‑mass T Tauri star CVSO 1942, focusing on a dramatic increase in mid‑infrared (MIR) emission detected with JWST/MIRI in 2024. By comparing the new MIRI/MRS spectrum (4.9–27 µm) to archival Spitzer/IRS spectra from 2005 (low‑resolution) and 2008 (high‑resolution), they find that the flux at wavelengths ≤10 µm has risen by a factor of two, while the flux at ≥15 µm remains essentially unchanged. This selective brightening indicates the emergence of a new, hot (≈1,400 K) emitting component located near the dust sublimation radius, without a corresponding change in the outer disk (which extends to ~49 au).

The authors supplement the spectroscopic data with photometric monitoring from WISE/NEOWISE (3.4 and 4.6 µm), ZTF (optical g and r), and TESS (high‑cadence optical). The WISE light curve shows that the MIR excess appeared within a two‑week interval, as the last NEOWISE measurement (Feb 25 2024) still matched the photospheric level, while the JWST observation (Mar 10 2024) already displayed the full excess. Optical variability is modest (≤0.1 mag) and periodic with a 4.688‑day rotation period, indicating that stellar activity is not driving the MIR burst.

To quantify the new component, the authors fit the excess with a single‑temperature blackbody, exploring temperatures from 1,200 K to 2,000 K. The best fit is obtained for T≈1,400 K, consistent with emission from the dust sublimation front. Using the fitted solid angle (Ω≈1.2 × 10⁻¹⁹ sr) and the Gaia distance (404 pc), they derive an emitting area of A≈1.9 × 10²³ cm². Assuming an opacity κ≈200 cm² g⁻¹ at 6 µm (appropriate for large (≤1 mm) grains), the minimum dust mass required is M_d ≳ 9.7 × 10²⁰ g (≈4.9 × 10⁻¹³ M_⊙). This is comparable to, or exceeds, the total dust mass previously inferred for the optically thin inner region (≈2 × 10⁻¹² M_⊙), implying a substantial, localized increase in surface density over a very short timescale.

Spectral analysis shows that the 10 µm silicate feature, which traces sub‑micron grains, is unchanged between the 2005 and 2024 spectra. Therefore, the excess is dominated by large grains (≥1 µm) that are optically thick (τ ≳ 1) at MIR wavelengths, rather than a wholesale increase in small dust. The authors also model Hα line profiles using magnetospheric accretion models, finding that the mass accretion rate (Ṁ≈1.3 ± 0.4 × 10⁻¹⁰ M_⊙ yr⁻¹) has remained within a factor of two over the past 15 years. This stability suggests that the MIR burst is not directly linked to a sudden accretion outburst.

The paper discusses possible mechanisms for such rapid dust formation. The leading hypothesis is in‑situ generation of large grains through collisions of planetesimals or larger bodies within the inner disk, which could release copious dust on week‑long timescales. Alternative explanations include magnetorotational instability‑driven bursts that trigger rapid condensation of gas‑phase refractory material, or a sudden change in the vertical structure of the inner rim that exposes previously hidden dust. The unchanged silicate feature argues against a sudden influx of small grains, favoring scenarios that preferentially produce large particles.

Overall, the study demonstrates that JWST/MIRI can capture short‑timescale variability in the inner regions of protoplanetary disks, a regime previously inaccessible. The authors advocate for systematic, high‑cadence MIR monitoring of transitional disks to disentangle the relative roles of collisional cascades, disk instabilities, and accretion variability in shaping the early stages of planet formation.