Doubt on the form of energy-momentum tensor in the context of thin shells and traversable wormholes

In reference $$[1]$$ the author constructed a junction condition to an external Schwarzschild with cosmological constant and a traversable morris-thorne wormhole. The form of the energy-momentum tensor at the surface $$S$$ (and at the radius $$r=a$$ of junction) as given by:

$$T_{\hat{\mu}\hat{\nu}} = t_{\hat{\mu}\hat{\nu}}\delta(\hat{r}-\hat{a}) \tag{1}$$

where $$\hat{r} = \sqrt{g_{rr}}r$$ is the proper distant through the thin shell.

So, my doubt is:

Why the energy-momentum tensor have the form of $$(1)$$?

$$* * *$$

$$[1]$$ LOBO.F.S.N, et al; Morris -Thorne wormholes with a cosmological constant, https://arxiv.org/abs/gr-qc/0302049.