[Paper Review] The Diskionary: A Glossary of Terms Commonly Used for Disks and Related Objects, First Edition

Neal J. Evans, Nuria Calvet+7 more|ArXiv.org|Jan 13, 2009

Ancient and Medieval Archaeology Studies7 citations

TL;DR

The Diskionary provides a standardized, consensus-based glossary of terms used in disk astronomy, clarifying observational and physical definitions for disk types, evolutionary stages, and structural components. It resolves terminology inconsistencies from Spitzer-era discoveries by compiling definitions from expert panels, emphasizing context-specific usage over prescriptive naming, with key contributions including distinctions between optically thick/thin, cold/hot, and flared/truncated disks based on SED properties and physical models.

ABSTRACT

Based on a panel discussion at the meeting "New Light on Young Stars: Spitzer's View of Circumstellar Disks", we provide some definitions of common usage of terms describing disks and related objects.

Motivation & Objective

• Address the growing confusion in disk astronomy terminology due to Spitzer Space Telescope's high-sensitivity discoveries of rare disk types.
• Resolve inconsistencies in how terms like 'transitional disk' or 'cold disk' are used across different research communities.
• Create a living reference document that records common usages rather than enforcing a single standard, ensuring clarity through context and definitions.
• Differentiate between observational (SED-based) and physical (structure-based) definitions of disk properties to improve precision in scientific communication.
• Facilitate interdisciplinary collaboration by unifying terminology across observational, theoretical, and modeling communities in disk research.

Proposed method

• Gathered definitions through teleconferences, email exchanges, and live panel discussions at the 2008 Pasadena conference on circumstellar disks.
• Grouped related terms by conceptual themes (e.g., SED classes, disk structure, evolutionary stages) rather than alphabetical order for logical coherence.
• Defined terms using both observational criteria (e.g., SED slope α, T_bol, flux ratios) and physical interpretations (e.g., optical depth, disk truncation).
• Assigned author initials to each definition to track provenance and acknowledge iterative contributions from panel members.
• Included comments and references to original sources to contextualize definitions and highlight debates or ambiguities in usage.
• Distinguished between observational and physical definitions, while acknowledging that many terms straddle both categories.

Experimental results

Research questions

• RQ1 What are the most commonly used terms in disk astronomy, and how do their meanings vary across different research communities?
• RQ2 How can observational SED properties (e.g., α, T_bol, flux ratios) be used to define disk types consistently?
• RQ3 To what extent do physical interpretations of disk structure (e.g., optical depth, flaring, truncation) align with observational classifications?
• RQ4 Why has the proliferation of new disk types (e.g., transitional, cold, anemic disks) led to terminological confusion, and how can it be mitigated?
• RQ5 How should ambiguous or context-dependent terms (e.g., 'flat', 'warm', 'cold') be defined to avoid miscommunication in scientific literature?

Key findings

• The term 'Class 0' is defined observationally by T_bol < 70 K and L_smm / L_bol > 0.5%, with additional criteria for indirect evidence of a central YSO in a spheroidal envelope.
• Class I sources are defined by α ≥ 0.3 in the NIR–MIR SED slope, with physical interpretation as objects dominated by envelope emission.
• Flat SEDs are characterized by −0.3 ≤ α < 0.3 or T_bol between 350 K and 1450 K (with extinction correction), indicating a composite emission from disk and envelope.
• Transitional disks are identified by weak or absent near-IR excess and SEDs suggesting inner disk truncation, with cold disks defined by Sν(30)/Sν(13) > 5 or steep long-wavelength rise.
• Optically thick disks are physically defined by τ > 1 at most radii, often associated with νSν slope ≈ −1 and L_disk / L_star ≥ 0.1, while optically thin disks have τ ≪ 1 and slope ≈ −3.
• The term 'cold disk' is an observational descriptor based on SED shape, not physical temperature, and is used to distinguish disks with inner holes from anemic or transitional disks, though its use is debated due to semantic ambiguity.