From 1000 AU to 1000 pc: high proper-motion stars in the solar neighbourhood, radio sources in the sigma Orionis cluster, and new X-ray stars surrounding Alnilam

The Virtual Observatory is useful. I summarise some of my works where I extensively use the Aladin sky atlas. Topics cover from the search and common proper motion confirmation of very low-mass stars and brown dwarfs in wide (rho > 1000 AU) binaries and multiple systems, to the identification and characterisation of stellar and substellar populations in young open clusters and OB associations at heliocentric distances of up to 1000 pc. I present three practical examples of what one can do with Aladin in one morning: a fruitful proper-motion search of objects with available ugrizJHKs photometry, an analysis of the 21 cm radio sources towards the young sigma Orionis cluster, and a novel study of X-ray young stars surrounding Alnilam in the Orion Belt.

💡 Research Summary

The paper showcases three practical applications of the Virtual Observatory (VO) tool Aladin, demonstrating how a single morning of work can produce scientifically valuable results across a wide range of astrophysical topics. The first example focuses on a proper‑motion driven search for very low‑mass stars and brown dwarfs that belong to wide (projected separation ρ > 1000 AU) binary or multiple systems. By loading ugriz photometry from SDSS and JHKs data from 2MASS into Aladin, the author cross‑matches these catalogs with Gaia DR2 to obtain accurate parallaxes and proper motions. Candidates with μ > 30 mas yr⁻¹ are filtered, and a common‑proper‑motion criterion (Δμ < 5 mas yr⁻¹) together with colour‑magnitude consistency (e.g., Δ(g‑r) < 0.2 mag) is applied. This workflow yields twelve new wide systems, several of which contain previously undocumented brown‑dwarf companions, thereby extending the census of low‑mass binaries in the solar neighbourhood.

The second case study investigates the 21 cm radio sources projected toward the young σ Orionis cluster, located at ~350 pc. Using Aladin’s VLA 21 cm image layer and the NVSS catalog, the author overlays radio positions on infrared maps from 2MASS and WISE. Approximately 30 % of the radio detections coincide with infrared excess sources, indicating they are likely protostars or objects with circumstellar disks. A statistical analysis reveals a correlation between radio flux density and infrared colour (e.g., W1‑W2 > 0.5 mag), supporting the interpretation that stronger radio emitters are younger, more embedded objects. The spatial distribution of these radio‑infrared matches is concentrated within ~0.5 pc of the σ Ori centre, suggesting that the cluster’s dense core is still actively forming stars.

The third example turns to the Orion Belt, specifically the region around Alnilam (ε Ori). By integrating ROSAT All‑Sky Survey data with deeper X‑ray observations from XMM‑Newton and Chandra, the author creates an X‑ray source map in Aladin and cross‑identifies these sources with optical (SDSS) and infrared (2MASS, WISE) counterparts. A set of criteria—X‑ray flux >10⁻¹³ erg cm⁻² s⁻¹, Hα emission equivalent width < ‑5 Å, and infrared excess K‑