ALMA view on the nature of the compact VLA continuum sources in the massive young stellar object G25.65+1.05

Abstract

This paper presents high-resolution ALMA observations of the massive young stellar object G25.65+1.05, which is known to host water maser super flares. To investigate the nature of compact continuum sources that have been previously identified in this region, we analyzed 1.3 mm dust continuum and molecular line emission. The central millimeter peak MM1 coincides with the centimeter source VLA 2, has a complex molecular spectrum, and is identified as a hot molecular core. Molecular emission of SiO and CH3CN in the vicinity of MM1 reveals kinematics consistent with wide-angle outflow structures and a possible rotating disk in the source. VLA sources 1A, 1B, and 3 are lacking compact millimeter counterparts and trace shocked regions where the outflow interacts with the surrounding material. In particular, VLA 1A, the site of H2O maser super flares, is interpreted as a shock interface that exhibits developed turbulent movements seen in the SiO molecular line. The observed turbulence creates conditions required for H2O maser action, directly linking the nature of VLA 1A to the origin of the H2O maser super flares.

Figures (9)

• Figure 1: Left panel: Dust continuum emission at 1.3 mm in the G25.65+1.05 region observed with ALMA. The presented field size is 20$^{"}\times$20$^{"}$. The vertical colored panel indicates the brightness of emission in Jy/beam. The green contours are set to (3, 5, 9, 13, 17)$\times\sigma$, where $\sigma=1.003\times10^{-3}$ Jy/beam. Right panel: Dust continuum emission at 1.3 mm in the G25.65+1.05 region observed with ALMA in the vicinity of the brightest millimeter continuum peak MM1. The vertical colored panel indicates the brightness of emission in Jy/beam. The green contours are set to (3, 5, 9, 13, 17)$\times\sigma$, where $\sigma=1.003\times10^{-3}$ Jy/beam. Positions of the continuum peaks VLA 1-4 detected at 15 GHz in the work Bayandina2023 are marked by crosses.
• Figure 2: Spectra of the brightest source MM1 in the G25.65+1.05 region in four spectral windows. Vertical lines indicate molecular transitions of CH$_3$OH, SiO, CH$_3$CN, SO, DCN, and some other molecules.
• Figure 3: Spectral channels maps of the SiO (5-4) emission in the G25.65+1.05 region. The reference coordinates of the images are [RA, DEC]=[18:34:20.9, $-$05:59:42.2] and the image size is 10$^{"}\times$10$^{"}$. The velocity range corresponding to the channels is [24.50$-$58.86] km/s.
• Figure 4: Left panel: Integrated (moment 0) map of the SiO (5-4) emission in the vicinity of the brightest millimeter source MM1 in the G25.65+1.05 region. Grey contours show 1.3-mm continuum revealed in ALMA observations in this work, the levels are set to (3, 5, 9, 13, 17)$\times\sigma$, where $\sigma=1.003\times10^{-3}$ Jy/beam. Green contours represent 2-cm continuum observed at the VLA in the work Bayandina2023, the levels are set to (3, 5, 9, 13, 17)$\times\sigma$, where $\sigma=1.76\times10^{-5}$ Jy/beam. Right panel: Moment 2 map of SiO emission in the vicinity of main core MM1. The vertical colored panel indicates the velocity dispersion in the region. Grey contours indicate 1.3-mm continuum revealed in ALMA observations in this work, the levels are set to (3, 5, 9, 13, 17)$\times\sigma$, where $\sigma=1.003\times10^{-3}$ Jy/beam. Green contours show the continuum emission at 15 GHz from the work Bayandina2023, the levels are set to (3, 5, 9, 13, 17)$\times\sigma$, where $\sigma=1.76\times10^{-5}$ Jy/beam.
• Figure 5: Moment 0 maps of CH$_3$CN (12-11) K=3 and K=7 emission at 220.709 and 220.539 GHz, respectively, in the vicinity of the central source MM1. Black contours show 1.3-mm continuum revealed in ALMA observations in this work, the levels are set to (3, 5, 9, 13, 17)$\times\sigma$, where $\sigma=1.003\times10^{-3}$ Jy/beam. The size of the image is 4$^{"}\times$4$^{"}$ and the reference coordinates are [RA, DEC]=[18:34:20.9, $-$05:59:42.2]. The velocity range used for integration is [35.65$-$51.56] km/s.