Unveiling the mystery behind high-velocity absorption (HVA) events in blue supergiants. I. New observational evidence of HVA events in blue supergiants

Late B- and early A-type supergiants are massive stars at an advanced stage of stellar evolution, located in the transitional region between the main sequence (MS) and the red supergiant (RSG) phases. These stars exhibit diverse forms of spectral variability, including high-velocity absorption (HVA) events observed mainly in the hydrogen Balmer lines, most prominently in H̊m α. HVAs are characterised by substantial increases in line intensity accompanied by pronounced blueward shifts, whose physical origins and driving mechanisms remain poorly understood. This study aims to systematically compile and analyse existing observations of HVA events in late B- and early A-type supergiants, supplemented with new spectral data. The objectives are to characterise the frequency, properties, and temporal evolution of HVA events, identify the stellar parameters associated with their occurrence, and explore the physical processes involved. Through this analysis, we seek to provide new insights into the atmospheric dynamics of massive stars and their evolutionary pathways. We analysed both published and newly acquired high- and medium-resolution spectra of late B- and early A-type supergiants. The dataset comprises more than 12,000 spectra, dominated by the extensively monitored star Rigel, while the remaining objects are represented by smaller datasets ranging from several hundred spectra to only a few epochs per star. Key properties of HVA events, including blue-edge velocities, line-core shifts, and event durations, were measured from the H̊m α line. The photospheric variability during these events was also investigated. The analysis revealed several new HVA events. In all cases, asymmetries were detected in the blue wings of higher Balmer lines. HVA signatures are consistently detected first in the higher Balmer lines before appearing in H̊m α, suggesting a deep photospheric origin. Stronger events lasted longer (up to several months) compared to weaker events (several weeks). Repeated HVA events were identified in the best-sampled stars, with the observed intervals between individual detections ranging from less than one year to several years. However, gaps in the temporal coverage prevent these intervals from being interpreted as characteristic recurrence times. Some HVAs were identified in stars with initial masses between 12--25,M_⊙ and within a narrow effective temperature range of 9,000--13,000, corresponding to spectral types A2 to B7. Our systematic compilation and analysis of HVA events enlarge the observational dataset available for studying this variability in late B- and early A-type supergiants. The newly identified events, together with the homogeneous measurements of their properties, help define the stellar conditions in which HVAs occur. These results place important constraints on their physical origin and provide a basis for future works on the atmospheric dynamics of evolved massive stars.