Accurate proper motions of the protostellar binary system L 1551 IRS 5

We present an extensive astrometric study of the protostellar binary system L 1551 IRS 5, utilizing nearly four decades of interferometric observations obtained between 1983 and 2022 with the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA). We focus on observations with sufficient angular resolution to separate the two protostars (L 1551 IRS 5 N and S) in the system and derive accurate absolute proper motions for the two sources, as well as the relative proper motion between them. The absolute proper motion is dominated by the solar motion with only a modest contribution from L 1551 IRS 5's peculiar velocity, as expected for a young stellar object. The relative proper motions enable us to constrain the orbit and derive a total mass of $0.96\pm 0.17$ M$_\odot$ for the system. While the emission of both sources at wavelengths shorter than about 1.3 cm is compact, the emission at longer wavelengths ($\lambda \gtrsim 2$ cm) is often affected by a free-free contribution from nearby shock features. The results presented here demonstrate that, when appropriate care is taken to combine the observations, interferometric data collected with different facilities, at different frequencies, and with different gain calibrators can be combined to obtain accurate astrometry. Observations of L 1551 IRS 5 over the next several decades with the VLA, ALMA, and eventually the ngVLA and SKA ought to improve its dynamical mass measurement down to an accuracy of a few per cent. Similar observations of other young multiple systems have the unique potential to provide dynamical mass estimates for the youngest known stellar objects.