Stellar streams
Stellar streams, encounters and observed projections
A brief encounter can change orbital clocks long after the force has faded. Start with the physical picture, derive the force and projection, then test both numerical accuracy and real-data model quality.
L1
Why does a stellar stream develop a gap?
A flyby changes orbital periods. An early pile-up can become a much later sparse region, without uniquely identifying dark matter.
L1
How tides turn a cluster into a stellar stream
Derive a circular-orbit tidal scale, then use phase space, actions and frequency differences to understand why a thin stream contains many orbits.
L2
From a flyby force to a much later stream gap
Derive a Plummer flyby kick, separate the impulse limit from a finite-duration encounter, and follow the resulting changes in orbital clocks.
L3
Project a stellar stream before fitting it
Define sky coordinates, proper motion and solar reflex, then use known truth and held-out data to separate accurate integration from a good stream model.
L3
Recompute a conditional GD-1 projected-orbit fit
Run independent stream dynamics, acquire fixed author products, and retain nonconvergence, coverage failures and a poor real-data orbit comparison.
L4
Read the 2025 GD-1 thin-stream and cocoon paper
Follow selection, projections, mixture widths and background alternatives in a fixed DESI paper, separating a broad component from its formation history.
Sources and calculation scope
- Erkal & Belokurov (2015), Forensics of Subhalo-Stream Encounters, arXiv:1412.6035v2
All 14 pages actually read. Sections 2–3 and equations 1–23 derive the short-encounter circular-stream model; sections 4–6 compare simulations and physical degeneracies. Printed pages 3/9 actually viewed. The imposed warm ensemble is not the paper’s tidal-release calculation.
- Valluri et al. (2025), GD-1 Stellar Stream and Cocoon in the DESI Early Data Release, arXiv:2407.06336v2
Fixed 30 January 2025 v2, 25 pages fully read; original pages 7/12/14/21 actually viewed. Sections 3–5, Figures 2/7–12, Table 3 and Appendix A are distinguished from our conditional orbit exercise. DOI 10.3847/1538-4357/ada690; no original paper, figure or observed row redistributed.
- Gala v1.12.0: fixed official GD1Koposov10 coordinate source
MIT source, fixed commit bebac7d728478c5122a568e12175ef894d1c1516. Literal rounded R and transpose reverse, normalized direction/proper-motion definitions; optional reader fetches and hashes the exact source without installing Gala.
- Sandbox Physics standalone stellar-stream scientific reader v1
Own Python integrates 25 finite cases anew, checks an independent high-order reference, half steps, analytic limits and saved-record counts. Default zero network. Optional exact-source galpy exercise retains poor fit/nonconvergence/no-coverage failures. No App/Engine, completed tracks, originals or observed-derived figures in the ZIP.