Hunting for methanol in the water-rich, planet-forming disk around HL Tau

Context. Methanol, the simplest complex organic molecule found in space, is considered a key compound for the formation of chemical species of prebiotic interest. Methanol detections in protoplanetary disks remain scarce, even though it is frequently detected in the material surrounding other young stellar objects (YSOs).Aims. We investigated the presence of methanol in the protoplanetary disk around the HL Tau protostar, motivated by the detection of spatially resolved warm water emission.Methods. Given the similar volatilities of methanol and water, thermally desorbed gas-phase methanol is expected to emit from the same region of the HL Tau disk where water vapor has been observed. Accordingly, we selected and imaged the most promising ALMA archival observations to search for rotational methanol lines.Results. We find no methanol emission in the analyzed archival datasets. Assuming optically thin emission and local thermodynamic equilibrium (LTE), we derive stringent upper limits on the methanol column density for different excitation temperatures: <7.2 x 10(14) cm(-2) at 100 K and <1.8 x 10(15) cm(-2) at 200 K, assuming a circular emitting region with a radius of 17 au (similar to 0.12"). Furthermore, we obtain a stringent upper limit on the methanol-to-water column density ratio (<0.55 x 10(-3) at 100 K and <1.4 x 10(-3) at 200 K), which is, on average, an order of magnitude lower than the values measured for other YSOs and Solar System comets.Conclusions. We argue that the most likely explanation for the methanol nondetection in HL Tau is the presence of optically thick dust in the central region of the disk, which obscures part of the methanol emission. The upper limit on the methanol-to-water ratio in the HL Tau disk is at least an order of magnitude smaller than most clouds, YSOs, and comets, possibly due to radiative transfer and/or excitation effects, or to a different chemical evolution compared to the other sources.