2025A&A...701A.167H

Context. Water masers are common in star-forming regions (SFRs), with the 22.235 GHz transition widely detected in both high- and low-mass protostars. In contrast, (sub)millimeter water maser transitions remain poorly studied, especially in low-mass SFRs, due to atmospheric limitations and a lack of systematic surveys.
Aims. We searched for millimeter water masers in a sample of low-mass SFRs previously known to exhibit 22 GHz emission. Specifically, we targeted the 3_1, 3_ - 2_2, 0_, 10_2, 9_ - 9_3, 6_, and 5_1, 5_ - 4_2, 2_ transitions at 183.3, 321.2, and 325.2 GHz, respectively. We also examined their potential as probes of evolutionary stage by comparing them with previously reported Class I methanol masers (MMs).
Methods. We used the SEPIA 180 and 345 receivers on the APEX 12 m telescope to carry out the observations. To assess the evolutionary stage of each source, we modeled their spectral energy distributions using archival data and used the derived dust temperatures as proxies of ages. We then compared the occurrence of water and MMs across the sample.
Results. We detected 183.3 GHz water masers in 5 out of 18 sources. Among these, Serpens FIRS 1 also exhibits the 321.2 GHz transition line, and IRAS 16293-2422 shows all three targeted transitions (at 183.3, 321.2, and 325.2 GHz). The 325.2 GHz transition was detected only in IRAS 16293-2422. Despite excellent observing conditions, detection rates drop with increasing frequency, reflecting both intrinsic line weakness and variability. Notably, the brightest (sub)millimeter masers can reach flux densities comparable to those of the 22 GHz line. Comparisons of velocity profiles show that different transitions often trace distinct gas components. Water masers generally appear at earlier or comparable evolutionary stages than MMs, suggesting no universal maser-based age sequence.
Conclusions. Our results demonstrate the detectability of submillimeter water in low-mass SFRs, although their occurrence is rare. Velocity overlap between some centimeter and millimeter components suggests partial spatial coincidence, but many features appear uniquely in one frequency regime, indicating that different transitions often trace distinct gas regions with varying physical conditions.