weakline

Weakline is a term used in spectroscopy and astronomy to describe a spectral line whose strength is small compared with the surrounding continuum. The strength of a line is commonly measured by its equivalent width, depth, or flux relative to the continuum. A weak line can arise in absorption or emission and may be influenced by factors such as chemical abundance, temperature, density, and instrumental sensitivity.

In stellar astrophysics, the phrase appears most often in reference to weak-line T Tauri stars (WTTS). WTTS are young, low-mass stars that show weak or absent emission lines, particularly hydrogen alpha, indicating minimal ongoing accretion from a circumstellar disk. Their spectra are dominated by photospheric absorption lines and resemble those of main-sequence stars, though they can exhibit strong magnetic activity and X-ray emission. WTTS are contrasted with classical T Tauri stars (CTTS), which display strong emission lines due to active accretion.

In the study of active galactic nuclei and quasars, weak-line quasars (WLQs) describe quasars with unusually weak high-ionization emission lines relative to their continuum. WLQs challenge standard models of broad-line regions and ionizing continua, and various explanations have been proposed, including atypical accretion physics, shielding gas, or orientation effects. WLQs are relatively rare and identified primarily through rest-frame ultraviolet spectroscopy.

Outside astronomy, weak lines in laboratory and atmospheric spectroscopy can result from low elemental abundance, low oscillator strength, high temperatures, diluted line formation regions, or instrumental limitations. The term is context-dependent, and there is no universal threshold for what constitutes a “weak” line. See also spectral line, equivalent width, T Tauri star, weak-line quasar.