Massive stars in the SDSS-V survey: New O2 stars in the Large Magellanic Cloud

Abstract

{O2 stars define the hottest end of the normal O-star sequence, but their identification requires blue-optical nitrogen diagnostics absent from many surveys.} {We reassess three luminous Large Magellanic Cloud (LMC) sources with multi-epoch Sloan Digital Sky Survey V (SDSS-V) spectroscopy obtained with the Baryon Oscillation Spectroscopic Survey (BOSS) spectrographs: \wsi, \lh, and \sk.} {We measure visit-resolved radial velocities and H, He, and N equivalent widths (EWs), compare robust combinations with LMC O2 templates at a common effective resolution, and use OSTAR2002 spectral energy distribution (SED) fits and a Gaia colour--magnitude diagram (CMD) as broad-band checks.} {The weighted $\log|{\rm EW}(\niv~\lambda4058)/{\rm EW}(\niii~\lambda4640)|$ values are $0.64\pm0.29$, $0.48\pm0.23$, and $0.34\pm0.44$ for \wsi, \lh, and \sk. The detection of \nv~$λ\lambda4604,4620$ supports the O2~If$^\ast$ and O2~V--III classifications adopted for \wsi\ and \lh, respectively. For \sk, the uncertain \niv/\niii\ ratio and non-detection of \nv\ motivate O2--3~V--III. Weak \hei~$\lambda4471$ may indicate unresolved later-type contamination in \wsi\ and \sk. The radial velocities of \sk\ show suggestive but non-significant variability. OSTAR2002 fits over 45--55~kK and the Gaia CMD place all three sources in the luminous very-early-O regime; \wsi\ shows a strong mid-infrared (MIR) excess, \lh\ is nearly photospheric, and \sk\ shows only a modest $3$--$8\,μ$m excess.} {The BOSS spectra add \wsi\ and \lh\ to the LMC very-early-O population and identify \sk\ as an additional O2--3 source. These cases show how incomplete blue coverage and ambiguous catalogue associations can conceal the hottest massive stars.}