The dispersal law of young star clusters: free expansion -- A Gaia/eROSITA census of Orion and Sco--Cen
Abstract
We measure how young star clusters disperse over time in the two nearest star-forming complexes. Our sample comprises 44 \texttt{SigMA}-\textit{Gaia}-defined clusters in Orion and 31 in Sco--Cen, with isochronal ages of 2--25\,Myr. We extend each cluster's census with \textit{eROSITA}-selected young stars that share its parallax and motion and lie within the distance they could have traveled ballistically since birth. By 10--30\,Myr, about one in three former cluster stars lies outside the cluster's cataloged boundary. Across both complexes and the full age range, we find that (1) the half-member radius increases linearly with age, rfifty ≃ sigbirth t, (2) the velocity dispersion shows no measurable evolution, and (3) all studied clusters are unbound. These results support free expansion. Young clusters in Orion and Sco-Cen emerge unbound from their birth clouds and coast at their birth velocity dispersion without significant relaxation or a characteristic dispersal timescale. We further show that memberships defined by density contrast preferentially omit the fastest stars, making the same clusters appear to expand more slowly (∝ t0.6), cool with age, and approach equilibrium.
Related
- A census of Dark Energy Survey Year 3 redMaPPer galaxy clusters in a high exposure eROSITA All Sky Survey region
- The Montreal Open Clusters and Associations (MOCA) Database: A Census of Nearby Associations, Open Clusters, and Young Substellar Objects within 500 pc of the Sun
- The XMM Cluster Survey: Cross-Calibration of eROSITA and XMM using Dark Energy Survey Y3 Galaxy Clusters
- The XMM Cluster Survey: Cross-Calibration of eROSITA and XMM using Dark Energy Survey Y3 Galaxy Clusters
- Quantifying the decline in CO(2-1) intensity around PHANGS-HST star clusters: Constraints on molecular gas dispersal, stellar feedback, and cluster formation efficiency
BibTeX
@misc{alves2026dispersal,
title = {{The dispersal law of young star clusters: free expansion -- A Gaia/eROSITA census of Orion and Sco--Cen}},
author = {João Alves and Stefan Meingast and Alena Rottensteiner},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.05179},
eprint = {2609.05179},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.05179},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The dispersal law of young star clusters: free expansion -- A Gaia/eROSITA census of Orion and Sco--Cen
AU - João Alves
AU - Stefan Meingast
AU - Alena Rottensteiner
PY - 2026
DA - 2026/09/04
PB - arXiv
DO - 10.48550/arxiv.2609.05179
UR - https://arxiv.org/abs/2609.05179
AB - We measure how young star clusters disperse over time in the two nearest star-forming complexes. Our sample comprises 44 \texttt{SigMA}-\textit{Gaia}-defined clusters in Orion and 31 in Sco--Cen, with isochronal ages of 2--25\,Myr. We extend each cluster's census with \textit{eROSITA}-selected young stars that share its parallax and motion and lie within the distance they could have traveled ballistically since birth. By 10--30\,Myr, about one in three former cluster stars lies outside the cluster's cataloged boundary. Across both complexes and the full age range, we find that (1) the half-member radius increases linearly with age, rfifty ≃ sigbirth t, (2) the velocity dispersion shows no measurable evolution, and (3) all studied clusters are unbound. These results support free expansion. Young clusters in Orion and Sco-Cen emerge unbound from their birth clouds and coast at their birth velocity dispersion without significant relaxation or a characteristic dispersal timescale. We further show that memberships defined by density contrast preferentially omit the fastest stars, making the same clusters appear to expand more slowly (∝ t^0.6), cool with age, and approach equilibrium.
ER -