New Pipeline for Efficient Analysis of Large Spectroscopic Samples of OB Stars

TL;DR

Large OB-star samples require homogeneous, non-LTE analyses that account for spherical winds and model uncertainties, which are not easily scalable with traditional methods. The paper presents a grid-based FASTWIND pipeline with explicit error budgeting and a chi-squared fitting approach to enable automated analysis of hundreds to thousands of spectra. Application to VFTS, XShootU, and BLOeM shows general agreement with literature Teff and log g, but nebular contamination, binarity, and OBe disk emission pose challenges for automated fits. The pipeline is integrated into the 4MOST/VISTA workflow, enabling scalable OB-star parameter estimation for current and future multiplexed spectroscopic surveys, while UV fitting and detailed abundances still require targeted star-by-star analyses.

Abstract

We present a new spectroscopic pipeline designed to analyse large numbers of hot massive stars homogeneously. The pipeline has been developed to utilise large grids of FASTWIND non-LTE, line blanketed models in which spherical geometry is adopted, and uniquely incorporates model errors. The pipeline has been applied to three contemporary datasets involving Very Large Telescope spectroscopy of OB stars in the Magellanic Clouds, namely the VLT FLAMES Tarantula Survey (VFTS), XShooting-ULLYSES (XShootU) and Binaries at Low Metallicity (BLOeM). We find satisfactory agreement with previous detailed temperatures and surface gravities, although strong nebular contamination, binarity and disk emission from OBe stars are problematic for automatic pipelines, requiring visual inspection of fits. The tool has been incorporated into the pipeline for the VISTA/4MOST pipeline.

Figures (3)

• Figure 1: Left: Spectroscopic fit (red) to observations (blue) for VFTS 76 Evans+2011 from Bestenlehner+2024. The grey shaded region is the square root of the diagonal elements of the error uncertainty matrix. Right: Probabilities heatmap of pipeline solution to VFTS 076 in the $\log g - T_{\rm eff}$ plane. Contours indicate 2D confidence intervals of 39.4, 86.5 and 98.9 per cent.
• Figure 2: Left: Pipeline-derived Hertzsprung-Russell diagram of XShootU OB stars in the LMC (blue circles) and SMC (green triangles), together with LMC evolutionary tracks for main sequence stars from Brott+2011 and Kohler+2015; Right: Pipeline mass-loss rates of LMC (blue circles) and SMC (green triangles) XShootU OB stars versus luminosity from Bestenlehner+2025a, in which stars with weak wlnds are indicated with open symbols.
• Figure 3: Left: Pipeline-derived Hertzsprung-Russell diagram of BLOeM OB stars in the SMC, colour coded by luminosity class, with SMC evolutionary tracks from Schootemeijer+2019. Right: Comparison between effective temperatures and surface gravities of BLOeM OB stars (Kiel diagram). Open symbols are single according to analysis of the initial 9 epoch dataset, filled symbols are multiple.