Long-term Simultaneous Monitoring Observations of SiO and H2O Masers toward the Mira Variable WX Serpentis

TL;DR

This long-term study of WX Serpentis reveals correlated maser intensity variations with optical light, distinct formation regions for SiO and H2O masers, and evidence of asymmetric outflows during the Mira variable phase.

Contribution

First simultaneous long-term monitoring of SiO and H2O masers in WX Serpentis, showing their correlated variability and different formation regions, indicating complex outflow dynamics.

Findings

H2O maser flared in 2019, indicating episodic activity.

Strong correlation between maser intensities and optical light curve.

H2O maser shows outward motion and asymmetric outflow.

Abstract

We present the results from long-term simultaneous monitoring observations of SiO and H2O masers toward the Mira variable star WX Serpentis. This study has been conducted with 21m single-dish radio telescopes of the Korean VLBI Network from 2009 June to 2021 June. Five maser lines were considered: SiO v=1, 2, J=1-0; SiO v=1, J=2-1, 3-2, and H2O 6(1,6)-5(2,3) transitions, with the SiO maser lines distributed near the stellar velocity and the H2O maser exhibiting an asymmetric line profile with five to six peaked components. Intense H2O maser emissions suddenly appeared in 2019 September, indicating flaring. The intensity variations of SiO and H2O masers are strongly correlated with the optical light curve (OLC) of the central star, with individual phase lags; the phase lag of the H2O maser relative to the OLC is larger than that of the SiO masers. The consequent phase difference between the SiO masers and the H2O maser likely indicates that their formation regions and main driving mechanisms are different from each other. The SiO masers in WX Ser exhibit a dominant single-peak velocity distribution, similar to other Mira variable stars. However, the H2O maser displays distinct morphological features, showing a radial acceleration and preferential intensity dominance at blueshifted velocities. This suggests that the H2O maser clouds of WX Ser are moving outward, thereby developing an asymmetric outflow owing to nonuniform material ejection from the stellar atmosphere. The findings confirm that an initial asymmetric outflow structure emerged during the thermally pulsing asymptotic giant branch phase, specifically in the Mira variable star stage.