Table 2
Comparison of process timescales at different distances.
| r (au) | τH (s) | τclu (yr) | τad (yr) | τX-ray (yr) |
|---|---|---|---|---|
| 1 | 3.5 ⋅ 10−4 | 5.4 ⋅ 10−3 | 3.4 ⋅ 10−1 | 1.8 ⋅ 10−1 |
| 10 | 3.5 ⋅ 10−1 | 5.4 ⋅ 100 | 3.4 ⋅ 102 | 1.8 ⋅ 101 |
| 100 | 3.5 ⋅ 102 | 5.4 ⋅ 103 | 3.4 ⋅ 105 | 1.8 ⋅ 103 |
| 1000 | 3.5 ⋅ 105 | 5.4 ⋅ 106 | 3.4 ⋅ 108 | 1.8 ⋅ 105 |
Notes. Typical rehydrogenation timescale kH, clustering timescale kclu, adsorption timescale kad, and X-ray absorption timescale for coronene in the photosphere of a classical T Tauri disc (at three disc scale heights). At all distances, rehydrogenation is faster than the absorption of another X-ray, so a PAH is likely to always be completely hydrogenated before it absorbs another X-ray. The cascade-like breakdown of adsorbed PAH clusters by X-rays is always faster than the rebuilding processes in the outer disc, whereas in the inner disc, clustering and adsorption compete against X-ray destruction.
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