X-raying the Sco-Cen OB association: The low-mass stellar population revealed by eROSITA

Abstract

We present the results of the first X-ray all-sky survey (eRASS1) performed by the eROSITA instrument on board the Spectrum-Roentgen-Gamma observatory of the Sco-Cen OB association. Bona fide Sco-Cen member stars are young and are therefore expected to emit X-rays at the saturation level. The sensitivity limit of eRASS1 makes these stars detectable down to about a tenth of a solar mass. By cross-correlating the eRASS1 source catalog with the Gaia EDR3 catalog, we arrive at a complete identification of the stellar (i.e., coronal) source content of eROSITA in the Sco-Cen association, and in particular obtain for the first time a 3D view of the detected stellar X-ray sources. Focusing on the low-mass population and placing the optical counterparts identified in this way in a color-magnitude diagram, we can isolate the young stars out of the detected X-ray sources and obtain age estimates of the various Sco-Cen populations. A joint analysis of the 2D and 3D space motions, the latter being available only for a smaller subset of the detected stellar X-ray sources, reveals that the space motions of the selected population show a high degree of parallelism, but there is also an additional population of young, X-ray emitting and essentially cospatial stars that appears to be more diffuse in velocity space. Its nature is currently unclear. We argue that with our procedures, an identification of almost the whole stellar content of the Sco-Cen association will become possible once the final Gaia and eROSITA catalogs are available by the end of this decade. We furthermore call into question any source population classification scheme that relies on purely kinematic selection criteria.