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I came across a website saying that classical T Tauris have an "EW(Hα) > 5 angstroms".

Does anybody know what "EW" means?

Thanks.

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"EW" stands for Equivalent Width. The equivalent width of a spectral line is essentially the range of continuum one would integrate over to get the same flux as the spectral line.

In your case, this is the Hα line.

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    $\begingroup$ +1, but for future readers, the formula would be nice :) $\endgroup$ – pela Apr 20 '16 at 8:59

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