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

Abstract

Methanol, the simplest complex organic molecule found in space, is considered a key compound necessary 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. We investigate the presence of methanol in the protoplanetary disk around the HL Tau protostar, motivated by the detection of spatially resolved warm water emission. Given the similar volatility of methanol and water, thermally desorbed gas-phase methanol is expected to emit from the same region of the HL Tau disk where water vapour has been observed. Accordingly, we selected and imaged the most promising ALMA archival observations to search for rotational methanol lines. We found no methanol emission in the analysed archival datasets. Assuming optically thin emission and 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 (~ 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 < 1.4x 10^(-3) at 200 K), which is, on average, an order of magnitude lower than the values measured for other Young Stellar Objects and Solar System comets. We argue that the most likely explanation for the methanol non-detection 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 due to a different chemical evolution compared to the other sources.

Figures (8)

• Figure 1: The left panel shows a rotational diagram built from the methanol non-detections. In the legend, each transition is named after its quantum numbers. For the lines that are covered in multiple programs, the year of the program is also provided. The right panel displays an exclusion diagram highlighting the values of excitation temperature and total column density compatible with all the $3\sigma$ (presented in the left panel) and $5\sigma$ upper limits on the total CH_3OH column density.
• Figure 2: CH3OH/H2O column density ratio found in the literature for a wide range of clouds, YSOs and Solar System comets (see Sect \ref{['sec:meth_to_water']} for the references). The values for molecular clouds, LYSOs and MYSOs reflect the ice composition, while the ratio was measured in the gas-phase for protoplanetary disks and comets. We used open symbols for the protoplanetary disks in which the upper/lower limit on the methanol-to-water column density ratio was estimated combining measurements taken at different wavelength ranges. The upward triangles for the HD 100546 and the IRS 48 disks indicate a lower limit, whereas the downward triangles for DG Tau A and HL Tau indicate a upper limit. The values for HL Tau are taken dividing the $3\sigma$ upper limit on the $N_{\ce{CH3OH}}$ for the three different assumed excitation temperature in Table \ref{['tab:meth_col_dens']} for the water column density measured by Facchini_2024.
• Figure 3: Ratio between the methanol and the average hydrogen column density versus the Band 7 continuum flux integrated within the methanol snowline (left panel) and versus the mass enclosed inside $r_{snow}$ (right panel). For the HL Tau disk, the $3\sigma$ upper limit on the $N_{\ce{CH3OH}}$ is presented, assuming excitation temperature of 168 K. The low methanol abundance upper limit in the HL Tau disk is likely motivated by the different dust optical depth and by the different stellar luminosity in respect of the other disks in our sample.
• Figure 4: Methanol-to-sulfur monoxide line flux ratio as a function of the stellar luminosity. In addition to HL Tau, we took into consideration all the protoplanetary disks in which SO and CH3OH are both detected and emitting from a similar region of the disk enclosed within the water snowline.
• Figure 5: Integrated spectrum from a 1$"$ circular region centered on HL Tau and extracted from the non-continuum subtracted data cube. The orange and purple line differentiate the spectra extracted in two different execution blocks of the same spectral window. The bold lines were computed performing a moving average over ten consecutive channels to highlight the variations on a scale of $\sim$ 0.003 Jy. The black dotted lines indicate the region of the spectrum where the two covered and non-detected methanol $3_{(-1,2)}-2_{(-0,2)}$ E and $6_{(1,5)}-6_{(0,6)}$ A line fall.