Long-term ultraviolet variability of Taurus T Tauri stars from the IUE archive

Abstract

Ultraviolet (UV) radiation from Classical T Tauri stars, generated primarily by magnetospheric accretion, plays a fundamental role in circumstellar disk evolution. However, its long-term variability remains poorly characterized due to the scarcity of homogeneous time-series observations. We investigate the variability of the near-UV continuum and the Mg II resonance doublet to assess whether both diagnostics trace the same underlying accretion variability. We analyzed the complete archive of long-wavelength International Ultraviolet Explorer (IUE) observations of T Tauri stars in Taurus-Auriga, comprising 475 low-dispersion spectra of 36 objects collected over 18 years. We characterized continuum variability using three windows between 2600 and 3100 Angstroms and examined continuum-subtracted Mg II fluxes, UV-V colors, and Lomb-Scargle periodograms to investigate correlations and characteristic timescales. Continuum fluxes between 2675 and 3055 Angstroms are strongly correlated, with variability dominated by changes in continuum level rather than spectral shape. Near-UV and quasi-simultaneous optical fluxes are significantly correlated in six of the seven best-sampled sources, although their relative variability differs. The Mg II-continuum relationship shows substantial object-to-object diversity, ranging from correlated variations with different fractional responses to decoupled or anti-correlated behavior, indicating no universal temporal response. Lomb-Scargle periodograms reveal formally significant features in some well-sampled objects, including near 10 and 33 days. However, irregular and clustered sampling prevents their identification as reliable periodic or characteristic timescales.