From Disks to Comets: A Multi-frequency Study of Dust from Ophiuchus Molecular Cloud to the Oort Cloud

Introduction

In recent years, the discovery of more exoplanetary systems and interstellar objects has highlighted the growing synergy between exoplanetary science and planetary science. While exoplanetary science offers statistical insights into planetary architectures and formation scenarios, planetary science provides detailed physical models essential for the characterization of exoplanets [1]. Planet formation, whether in our Solar System or around other stars, is strongly influenced by the initial conditions of protoplanetary disks (PPDs)—including masses, sizes, surface densities, and temperature profiles [10]. In addition to that, the subsequent evolution of these PPDs is governed by processes such as radial drift, vertical mixing, and, in particular, grain growth [2].

One of the major challenges in planet formation is understanding how dust grains grow into millimeter- to centimeter-sized particles. Such large grains—found in comets—serve as local analogs to the solids observed in disks around young stars [3] and provide a unique opportunity to investigate early grain growth processes. However, studies of these particles, which also constitute the bulk of the total coma mass [4], remain largely unexplored. Large particles also contribute to the grain size distribution, which plays a critical role in estimating the total dust mass in PPDs. Accurate dust mass estimates are essential to evaluate the potential to form planetesimals, rocky planets, and giant planet cores [5]. However, the current dust mass estimates are highly uncertain and insufficient to explain the observed high incidence of massive exoplanets [6,7].

These challenges—uncertainties in PPD dust masses and limited characterization of large particles—can be addressed through multi-frequency analysis of dust in both protoplanetary disks and comets. Although numerous studies have individually investigated dust properties in each of these environments, comparative analyses integrating exoplanetary and planetary science remain understudied. Such interdisciplinary comparisons of dust properties have the potential to significantly improve our understanding of the processes governing the evolution of planetary systems.

Observations and Methods

As part of the ODISEA project (Ophiuchus DIsk Survey Employing ALMA) [8], we combined the archival ALMA observations across Band 3 (100 GHz), Band 4 (140 GHz), Band 6 (230 GHz), Band 7 (350 GHz), and Band 8 (410 GHz). This multi-frequency approach allows us to constrain dust temperatures, surface densities, and grain size distributions as a function of radius [9]. This is in contrast to the single-frequency approach, which requires assuming a single temperature and optically thin emission. Our method is based on the radiative transfer equation under the plane-parallel slab approximation, which is given by:

I_ν = B_ν (T_d) [1 - exp (-τ₀ (ν/ν₀) ^β)]

where B_ν(T_d) is the Planck function, T_d is the dust temperature, τ₀ is the optical depth of dust at frequency ν₀, and β is the dust opacity spectral index. This framework, therefore, enables more robust dust mass estimates by integrating over the surface density profile.

To extend this analysis to the solar system context, we apply a similar multi-frequency approach to study the dust properties in comets. Using a similar set of ALMA bands, we target the distribution of dust and the presence of large grains in the comae of the exceptionally bright Oort Cloud comets, C/2023 A3 and C/2017 K2. These long-period comets are among the least thermally processed bodies in the Solar System and are also known to be dust-rich, making them one of the best-preserved reservoirs of primitive material from the solar nebula. Our observations aim to constrain the dust mass-loss rate and model the spectral energy distribution (SED) to derive the grain size distribution and dust structure. Additionally, we obtained mid-infrared observations of comet A3 with VLT’s VISIR instrument to investigate the long-standing contradiction of detecting crystalline silicates, formed at high temperatures, in comets that originated in cold environments.

I will present the analysis of dust continuum from both A3 and K2 based on our ALMA observations, alongside key results of their composition from mid-infrared spectral fitting using dust models. In parallel, I will present the statistical results on dust surface density, maximum grain size, and dust temperature profiles in PPDs, emphasizing the comparison between dust masses derived from single- versus multi-frequency analyses. This represents the first comprehensive multi-frequency study of a large sample of 44 Class I and Class II disks, corresponding to the early stages of PPD evolution, within a single molecular cloud. Finally, I will discuss how integrating findings from both cometary and disk environments through consistent multi-frequency analysis helps bridge a critical knowledge gap in our understanding of grain growth, the role of large particles, and the evolution of dust properties across different stages of planetary system formation.

References

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[5] Drazkowska, J., et al. 2023, PPVII, 717
[6] Greaves, J. S., & Rice, W. K. M. 2010, MNRAS, 407, 1981
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[8] Cieza, L. A., et al. 2019, MNRAS, 482, 698
[9] Sierra, A., et al. 2021, ApJ, 257, 14
[10] Williams, J. P., & Cieza, L. A. 2011, ARAA, 49, 67.