Rogue Ones: Orbital census of Galactic Cepheids and their Anomalies
Abstract
Classical Cepheids (DCEPs) are excellent standard candles expected to trace the spatial and kinematic distribution of the Galaxy's young and dynamically cold stellar disc. Using the most precise mid-infrared DCEP distances to date combined with Gaia-DR3 astrometry and line-of-sight velocities, we perform a comprehensive 6D dynamical census of the Milky Way's DCEP population. While the vast majority exhibit the expected disc-like kinematics, we identify 18 kinematically anomalous Cepheids. These `rogue' stars reside on highly inclined orbits (three at > 70 degrees), including two in retrograde motion and one with a total velocity of ~480 \kms. Despite their extreme trajectories, their optical light curves are consistent with DCEP classifications. We explore whether these anomalies originate from possible classification systematics or physical processes and find only three of our sources are likely misclassified. Assuming a runaway scenario we derive dynamical ages for the kinematic anomalies, which we find highly consistent with their Cepheid ages. Spectroscopic follow-up would be insightful as one source in particular is exceptionally metal poor ([Fe/H] ~ -1.6 dex), which is highly atypical for a DCEP. Integrating photometric classification with 6D kinematics will help fully characterise the Galaxy's variable star populations.
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BibTeX
@misc{khanna2026rogue,
title = {{Rogue Ones: Orbital census of Galactic Cepheids and their Anomalies}},
author = {Shourya Khanna and Ronald Drimmel and Eloisa Poggio and Dorota M. Skowron and Jie Yu},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.10623},
eprint = {2606.10623},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.10623},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Rogue Ones: Orbital census of Galactic Cepheids and their Anomalies
AU - Shourya Khanna
AU - Ronald Drimmel
AU - Eloisa Poggio
AU - Dorota M. Skowron
AU - Jie Yu
PY - 2026
DA - 2026/09/04
PB - arXiv
DO - 10.48550/arxiv.2606.10623
UR - https://arxiv.org/abs/2606.10623
AB - Classical Cepheids (DCEPs) are excellent standard candles expected to trace the spatial and kinematic distribution of the Galaxy's young and dynamically cold stellar disc. Using the most precise mid-infrared DCEP distances to date combined with Gaia-DR3 astrometry and line-of-sight velocities, we perform a comprehensive 6D dynamical census of the Milky Way's DCEP population. While the vast majority exhibit the expected disc-like kinematics, we identify 18 kinematically anomalous Cepheids. These `rogue' stars reside on highly inclined orbits (three at > 70 degrees), including two in retrograde motion and one with a total velocity of ~480 \kms. Despite their extreme trajectories, their optical light curves are consistent with DCEP classifications. We explore whether these anomalies originate from possible classification systematics or physical processes and find only three of our sources are likely misclassified. Assuming a runaway scenario we derive dynamical ages for the kinematic anomalies, which we find highly consistent with their Cepheid ages. Spectroscopic follow-up would be insightful as one source in particular is exceptionally metal poor ([Fe/H] ~ -1.6 dex), which is highly atypical for a DCEP. Integrating photometric classification with 6D kinematics will help fully characterise the Galaxy's variable star populations.
ER -