A nomenclature for individual exocomets

Technical Summary: A Nomenclature for Individual Exocomets

Problem Statement
With recent observational advancements, the study of exocomets has transitioned from an era focused on identifying general signatures to an epoch where individual exocomet detections are analyzed in detail to characterize specific bodies. Currently, the field lacks a systematic nomenclature to uniquely identify these objects. Unlike exoplanets and minor bodies in the Solar System, which possess established naming conventions, exocomets often suffer from multiple, inconsistent designations or a lack of unique identifiers. This absence hinders the ability to systematically catalog, compare, and reference individual exocomets across different studies.

Methodology
The authors propose a comprehensive nomenclature scheme by adapting principles from existing astronomical naming conventions (specifically those for exoplanets and Solar System comets) while addressing the unique constraints of exocometary observations. The methodology involves:

1. Defining Core Principles: Establishing that a nomenclature must uniquely designate individual objects within a set, avoiding ambiguity while remaining specific enough to distinguish different bodies.
2. Evaluating Information Content: Systematically reviewing potential data points to include in the designation:
   • Host Star: Adopting the host star's catalog name to define the system and confirm the object is bound to a stellar host (excluding free-floating interstellar comets).
   • Cometary Nature: Selecting a specific letter to denote the object's nature.
   • Periodicity: Deciding against including periodicity indicators, as most exocomets are not yet observed with sufficient data to determine periods.
   • Observation Date: Utilizing the observation date (year-month-day) as the primary temporal identifier, analogous to gamma-ray burst naming, to distinguish individual events and allow for the identification of multiple observations of the same object.
   • Detection Method/Physical Properties: Explicitly excluding detection methods (e.g., spectroscopy, photometry) and physical properties (e.g., radial velocity) from the core designation to maintain simplicity, reserving these details for cases where the primary designation is insufficient.
3. Addressing Degeneracy: Introducing an optional mechanism to handle multiple exocomets observed on the same day within the same system.
4. Priority Rules for Star Names: Establishing a strict hierarchy for selecting the host star's name to ensure consistency: Bayer designation > Flamsteed Catalog > Henry-Draper Catalog > Observatory/Survey catalogs (e.g., CoRoT, Kepler, TESS).

Key Contributions and Proposed Scheme
The paper proposes a standardized naming convention with the structure: StarName CYYYYMMDD[a|b|...|z].

• StarName: The catalog name of the host star, selected based on the priority order defined above.
• C: A capital letter "C" is adopted to specify the cometary nature. The authors explicitly rejected "E" (for exocomet or evaporating) to avoid confusion with "Falling Evaporating Bodies" (FEB) and to prevent ambiguity with other extrasolar objects like exo-asteroids.
• YYYYMMDD: The date of the observation (year, month, day) in a continuous format. This replaces the half-month/sequence number system used for Solar System comets, which is insufficient for exocomet systems where many objects may be observed in a single half-month.
• [a|b|...|z]: An optional lowercase letter appended immediately after the date to distinguish multiple exocomets observed on the same day within the same system. The order of these letters is left to the discoverers, similar to the designation of planets in multi-planet systems.

Results and Examples
The authors apply this scheme to several archetypal exocomets to demonstrate its utility:

• β Pic C19841113: Designating the first clearly identified exocomet from Ferlet et al. (1987).
• HD 172555 C20110611: Identifying a specific exocomet transiting HD 172555 with a blueshift of -2 km/s.
• β Pic C20091027a through f: Distinguishing six simultaneous exocomets observed in a single HARPS spectrum on October 27, 2009, using the optional suffix letters.
• KIC 846285 C20090805: Designating a string of transiting exocomets (or a single event) observed in Kepler data.
• β Pic C20181102: Naming the deepest and cleanest exocometary transit light curve discovered by TESS.

Significance and Claims
The paper claims that this nomenclature is a necessary step to integrate exocomet studies fully into the broader field of exoplanetology. By providing a unique, systematic identifier, the scheme facilitates the cross-referencing of bodies detected, analyzed, and characterized in different studies. The authors emphasize that this is a proposal intended to be adopted by the International Astronomical Union's Working Group on Small Body Nomenclature (WGSBN). They argue that as new ground-based (e.g., ELT) and space-based (e.g., PLATO) facilities come online, a standardized naming system is essential to grant discovered exocomets "full existence as named astronomical bodies." The authors note that the proposal is open to future improvement but aims to provide a practical framework for the current and near-future era of detailed exocomet characterization.