Three-micron spectra of L1527, IRAS  04302, and HK Tau (gray dots with error bars). The thin solid lines in L1527 and IRAS  04302 are the local (3.1–3.7 μm) continua fitted by second order polynomials, while the line in HK Tau is the best-fit absorption of H₂O (Fig. 3). Solid and dashed lines depict the upper limits to the pure CH₄ ice and pure CH₃OH ice absorption depth, respectively.