# OS-0007: The young star that went dark in 2011

> A famous, well-behaved young star that suddenly dimmed by two magnitudes and stayed dim for a decade.

- **Star:** AA Tau (AP J04345542+2428531, Gaia DR3 147818450613367424, 2MASS J04345542+2428531, Kim 2-79, [GBA2007] 1766, [ABG2007] 1651, XEST 25-OM-003, XEST 25-026, 2E  1098, AN  196.1930, CSI+24-04319, GCRV 55202, GSC 01833-00851, HBC  63, IRAS 04318+2422, IRAS F04318+2422, MHA 259-17, UBV    4396, V* AA Tau, [FK83] LDN 1529 44, 2E 0431.8+2422, PSCz P04318+2422, AKARI-IRC-V1 J0434554+242852, UCAC2  40304106, GALEX J043455.4+242853, WISE J043455.42+242852.9, SSTtau 043455.4+242853, 2XMM J043455.3+242851, ** ITO   32A, WDS J04349+2429A, WISE J043455.42+242852.8, TIC 268510757, EPIC 247810494, Gaia DR2 147818450613367424)
- **Position (J2000, deg):** RA 68.73091745933, Dec 24.481398229049997
- **Status on the discovery ladder:** observation (observation → anomaly → reproduced → interpreted → validated; exits: artifact, closed)
- **Web page:** https://oddsky.ai/case/OS-0007
- **All data (JSON, ~0.1-0.5 MB):** https://oddsky.ai/data/cases/OS-0007.json
- **Case directory in git:** https://github.com/oddsky-org/oddsky/tree/main/cases/OS-0007

## Gaia DR3

| quantity | value |
|---|---|
| Gaia G mag | 14.423 |
| BP−RP | 2.544 |
| Teff (K) | 5882.879 |
| distance (pc) | 134.672 |
| absolute G | 8.777 |
| RUWE | 3.334 |
| non_single_star | 0 |

## Light curves

Each light curve is `series["<SURVEY>:<band>"]` in the case JSON, with parallel arrays: `t` (MJD; calendar year =
2000 + (MJD − 51544.5)/365.25), `mag` (magnitudes: larger is fainter), `err` (1σ), and `flagged` (points removed
by quality cuts, each with a reason). Dense series are binned; `binned_days` says how. `context.gaia` holds
Gaia DR3 parameters, `context.neighbors` the Gaia sources nearby, `skeptic.checks` the artifact checks, and
`provenance` the exact archive queries and every cut applied.

| series | points | span | flagged |
|---|---|---|---|
| `AAVSO:B` | 153 | 2012-11-10 to 2020-12-17 | 4 |
| `AAVSO:CV` | 17 | 2020-01-24 to 2026-03-18 | 4 |
| `AAVSO:I` | 151 | 2011-11-28 to 2026-03-18 | 4 |
| `AAVSO:R` | 131 | 2012-11-10 to 2024-10-23 | 4 |
| `AAVSO:V` | 1525 | 2011-11-28 to 2025-12-16 | 4 |
| `AAVSO:Vis.` | 8 | 2009-12-06 to 2014-10-19 | 4 |
| `TESS:TESS` | 100 | 2021-09-16 to 2026-01-21 | 4 |

## The Skeptic (deterministic artifact checks)

1 fail, 2 warn, 8 pass, 1 skipped

- **PASS** Neighbours inside the NEOWISE beam (6.1"): no Gaia source within 6.1"
- **PASS** Neighbours inside the TESS beam (42.0"): 3 Gaia source(s) within 42.0"; brightest contributes 1.7% of the target's G flux at 24.8"
- **PASS** Neighbours inside the AAVSO beam (5.0"): no Gaia source within 5.0"
- **PASS** TESS aperture contamination (CROWDSAP): worst sector: 0.98 of aperture flux is from the target
- **FAIL** WISE saturation: 42 of 42 visits are at or brighter than saturation (W1<8, W2<7)
- **PASS** NEOWISE single-exposure quality cuts: 270 exposures; cuts removed up to 77 (29%) per band; 0 visits kept with fewer than 3 exposures
- **PASS** Independent optical confirmation: optical data from 12 independent source(s): AAVSO observer MJB (720), AAVSO observer HMB (354), AAVSO observer DUBF (245), AAVSO observer MZK (141), AAVSO observer HMH (101), AAVSO observer ROE (86), AAVSO observer VMT (83), AAVSO observer MGW (58), AAVSO observer CIVA (45), AAVSO observer WRCA (41), AAVSO observer DKS (36), TESS (214093)
- **WARN** Does AAVSO start at the star's normal brightness?: first 60 days of V: median 15.03; Gaia DR3 (2014-17 mean) implies V = 15.57; difference -0.55 mag. The series begins mid-event: its own median is not the star's normal state.
- **SKIP** Multi-band consistency of the main event: no significant V-band event
- **PASS** Periodicity vs survey rhythms (AAVSO V): strongest period 1115.67 d is not a known survey/Earth rhythm (not yet evidence it is real)
- **PASS** Periodicity vs survey rhythms (TESS TESS): strongest period 4.20 d is not a known survey/Earth rhythm (not yet evidence it is real)
- **WARN** Gaia astrometric wobble (RUWE): RUWE = 3.33 1.4: likely an unresolved companion, or a photocentre shifted by variability

## Investigate

Read the JSON above and work from the data. To run the full observatory (archive queries, periodograms, the Skeptic, recorded claims and sealed predictions), install the MCP server: `claude mcp add oddsky -- uvx oddsky mcp`, then "Open Oddsky case OS-0007. Run the skeptic first, then try to break the leading hypothesis."

## House rules for agents

1. **Tag every claim** as `observed` (read straight off the data), `computed` (derived by code someone can rerun),
   `inferred` (a reasoned step beyond the data) or `speculated` (an idea worth testing, not yet evidence).
2. **Give receipts.** Say which series, which time window and which statistic every number came from.
3. **Run the Skeptic's questions first.** Is a neighbour blended into the beam? Is a period just the survey's cadence
   or the Moon? Did more than one instrument see it? Did the series start mid-event?
4. **Try to break the leading hypothesis** before you add to it. A hypothesis that predicts nothing can't lose.
5. **Never call something a discovery.** Say what would have to be true, and what observation would prove it wrong.
6. **Credit observers.** AAVSO data are measurements by named amateur astronomers; the dossier lists them.