The Three‐dimensional Kinematics of Water Masers around the Semiregular Variable RT Virginis

Abstract

We report observations of water masers around the semiregular variable RT Virginis, which have been made with the Very Long Baseline Array of the National Radio Astronomy Observatory at five epochs, each separated by 3 weeks of time. We detected about 60 maser features at each epoch. Overall, 61 features, detected at least twice, were tracked by their radial velocities and proper motions. The three-dimensional maser kinematics exhibited a circumstellar envelope that is expanding roughly spherically with a velocity of '8 km s À1 . Asymmetries in both the spatial and velocity distributions of the maser features were found in the envelope, but they were less significant than those found in other semiregular variables. Systematic radial velocity drifts of individual maser features were found with amplitudes of 2 km s À1 yr À1 . For one maser feature, we found a quadratic position shift with time along a straight line on the sky. This apparent motion indicates an acceleration with an amplitude of 33 km s À1 yr À1 , implying the passage of a shock wave driven by the stellar pulsation of RT Vir. The acceleration motion is likely seen only on the sky plane because of a large velocity gradient formed in the accelerating maser region. We estimated the distance to RT Vir to be about 220 pc on the basis of both the statistical parallax and model-fitting methods for the maser kinematics.

The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

“…Imai et al 2003;Richards et al 2011). The mass-loss rate of the star, found from water maser emission and CO line emission, is around 1 to 5 × 10 −7 M yr −1 (Bowers & Johnston 1994;Olofsson et al 2002;Imai et al 2003).…”

“…Imai et al 2003;Richards et al 2011). The mass-loss rate of the star, found from water maser emission and CO line emission, is around 1 to 5 × 10 −7 M yr −1 (Bowers & Johnston 1994;Olofsson et al 2002;Imai et al 2003). Measurement values of the gas velocity vary from 6.2 km s −1 (González Delgado et al 2003) in the SiO emitting region (∼400 AU away from the star) to 7.8 km s −1 (Olofsson et al 2002) in the outer CO emitting region.…”

Evolved star water maser cloud size determined by star size

“…The maser proper motions were consistent with a combination of bulk proper motion and a spherically expanding flow. Imai et al (2003) monitored RT Vir using the VLBA for 5 epochs in 1998. The tendency for the E and W to be dominated by red-and blue-shifted emission is seen in their results as well as in our Figs.…”

The physics of water masers observable with ALMA and SOFIA: model predictions for evolved stars

“…This is consistent with the predictions of Ireland et al (2008). However, in this context, one would expect to see evidence for shocks close to the inner rim of the water maser shell, but Imai et al (2003) observed a shock-accelerated feature further out in the shell around RT Vir, and VY CMa also shows evidence that suggests shocks far from the star (Section 1.2). It is clear that shocks alone cannot explain water maser variability, since monitoring shows that features throughout the entire 22-GHz shells (perhaps 15/150 AU thick around AGB/RSG stars) vary in concert within weeks to years -much faster than even a highly supersonic shock.…”

“…Individual 22-GHz H2O maser sources show a wide range of values for the expansion velocity, with spectral extents exceeding 40 km s −1 for supergiants and some OH/IR stars, whilst Miras have values between 4 and ∼40 km s −1 . For example, VLBI observations of RT Vir (Imai et al 2003) found an approximate velocity of expansion of 8 km s −1 in the water maser zone. Infra-Red Astronomy Satellite (IRAS) colour-colour diagrams in Kim et al (2014) indicate that H2O masers form at an earlier evolutionary stage than SiO masers, but the H2O masers may also persist into the PPN (proto-planetary nebula) stage, probably in a new form dominated by asymmetric outflow.…”