Unveiling the First O-Type Bloated Star Candidate through ALMA and EVLA Observations

TL;DR

The paper presents high-angular-resolution ALMA and EVLA observations of IRAS 19520+2759, a candidate O-type bloated star. They detect a thermal-jet-like radio continuum and a compact 1.3 mm core (MM1) coincident with the optical source. Molecular-line data reveal a large east–west outflow and a secondary outflow, along with a hot core and a Keplerian disk around the central object. Keplerian-rotation modeling of SO2 lines yields a central mass in the range $10$-$15~M_0$ (edge-on), constraining the accretion-active phase of this massive young stellar object. Collectively, the results establish a dense disk+outflow system around an O-type bloated-star candidate, informing theories of early massive-star evolution and bloated phases.

Abstract

We investigate the circumstellar environment of the O-type bloated star candidate IRAS 19520+2759 (I19520) using high-resolution observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Expanded Very Large Array (EVLA). Radio continuum emission traced by the EVLA (C, K, and Q bands) exhibits a spectral index of 0.5, consistent with a thermal jet. ALMA 1.3 mm continuum map reveals a compact source coincident with the optical counterpart of I19520, likely tracing the dense core hosting the central massive young stellar object. A prominent molecular outflow in the east-west direction, along with a possible secondary outflow oriented northeast-southwest, is identified in the $^{13}\mathrm{CO}$ emission. A hot molecular core and a Keplerian disk are detected in several $\mathrm{SO}_2$ transitions. Assuming an edge-on disk geometry, the dynamical mass of the central object is estimated to be in the range of $10$-$15~M_\odot$.

Figures (2)

• Figure 1: (a) EVLA K band image showing the radio continuum emission associated with I19520. Contours are drawn at 3, 5, 13, 20, 30, 50, and 60 $\times$(rms = 9$\times$10$^{-6}$ Jy beam$^{-1}$). (b) Flux density versus frequency plot derived from the EVLA C, K, and Q bands. The slope of the fitted line yields the spectral index of the radio emission. (c) ALMA 1.3 mm continuum image with major continuum peaks are marked with plus symbols. Contours are drawn at 3, 5, 7, 9, 15, 20, 25, 32, 50, and 100$\times$rms, where rms $\sim$ 3.4 mJy beam$^{-1}$. (d) Blue- and redshifted $^{13}$CO emission (see Section \ref{['sec1']}) shown in contours, overlaid with 1.3 mm continuum contours (black). Arrows indicate the directions of the molecular outflows identified in this work. The star symbols in panels (a) and (c) mark the position of the optical counterpart of I19520.
• Figure 2: (a) Moment-1 map of the SO$_2$ (4–3) transition, with lines indicating the directions of the plausible outflows (see Section \ref{['sec1']}); the star marks the position of the continuum peak MM1. (b) PV plot for SO$_2$ (4–3) transition, extracted along the velocity gradient in Moment-1 map, the contours (white) are drawn at 3, 5, 7, 15, 20$\times$(rms=8.6$\times$$10^{-4}$ Jy beam$^{-1}$). The points mark the maximum velocity for different radial offsets shown along with the best fitted Keplerian rotation model (green).