🔭 DRAFT RESEARCH PROPOSAL · ASTROMETRY · HST + Gaia DR4/DR5
Extending the Häberle et al. (2024) oMEGACat baseline to detect orbital accelerations of the seven fast-moving stars — providing independent IMBH mass constraints between now and ELT first light (~2028) · Working draft · April 2026
The Häberle et al. (2024) discovery used ~15 years of HST observations (oMEGACat I–II). This baseline established velocities but was too short to detect orbital curvature (accelerations) for most of the seven fast stars. Each additional year of HST monitoring increases the acceleration baseline and reduces the uncertainty by (N_epochs)^(3/2) — rapid improvement with continued observations.
The critical window is 2024–2028 — before ELT first light. HST extensions and Gaia DR4/DR5 proper motions during this period establish the longest possible ground-truth baseline for ELT astrometry calibration and can independently detect accelerations for the highest-velocity stars.
Gaia DR4 (expected 2025) and DR5 (2030+) will provide proper motions for millions of OC stars with ~10–20 µas/yr precision — transforming the outer reference frame. While Gaia cannot resolve the innermost arcseconds, it provides the critical astrometric anchor for HST and later ELT measurements. Combining HST inner-core measurements with Gaia outer-field proper motions yields a self-consistent absolute proper-motion frame.
| Parameter | Value |
|---|---|
| Primary instrument | HST WFC3/UVIS (F814W); existing oMEGACat programs as baseline |
| Observation cadence | 1–2 HST epochs per year (2025–2028) |
| Exposure per epoch | ~4–6 orbits in central mosaic (comparable to oMEGACat epochs) |
| Astrometric precision | ~0.02 mas/yr per epoch (matching oMEGACat precision) |
| Gaia integration | Gaia DR4 proper motions for stars r > 10″ as outer reference frame |
| ELT handoff | Final HST epoch ~2027–2028 establishes calibration baseline for MICADO |
| Total HST time | ~8–12 orbits/year × 3 years = ~36 orbits |
| Outcome | Science value |
|---|---|
| Acceleration detected for ≥3 stars at ≥3σ | IMBH mass constraint ±30–50%; orbital geometry constraint begins |
| Orbital curvature detected for 1 star (fastest) | First direct Keplerian orbit segment for any star near an IMBH candidate |
| Discovery of additional fast-moving stars | Expanded sample improves mass constraint; confirms or modifies Bañares-Hernández tension |
| Null: no acceleration detected | Constrains minimum orbital period; limits central mass distribution radial extent |
| Year | Milestone | Deliverable |
|---|---|---|
| 2025 | HST GO proposal; Gaia DR4 integration into oMEGACat framework | Updated oMEGACat reference frame |
| 2026 | New HST epochs; acceleration upper limits or detection candidates | Internal report |
| 2027 | Multi-epoch acceleration analysis; orbital fitting for fastest stars | Conference results; Paper #1 |
| 2028 | Final HST epoch; full baseline analysis; ELT handoff package | Paper #2; ELT MICADO calibration dataset |
| Item | Cost (USD) |
|---|---|
| Postdoc / graduate RA (3 years) | 195,000 |
| HST time (no direct cost via TAC) | 0 |
| HPC (astrometric pipeline, N-body fitting) | 15,000 |
| Travel + publications | 15,000 |
| Total | ~$225,000 (3 yr) |