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

Published online by Cambridge University Press:  09 November 2026

Rakesh PandeyOpen the ORCID record for Rakesh Pandey[Opens in a new window] and
Aina Palau
Rakesh Pandey*
Affiliation:
Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro 8701, Ex-Hda. San José de la Huerta, 58089 Morelia, Michoacán, México
Aina Palau
Affiliation:
Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro 8701, Ex-Hda. San José de la Huerta, 58089 Morelia, Michoacán, México
*
email: pandey.rakesh405@gmail.com
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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 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 $^{13}$CO emission. A hot molecular core and a Keplerian disk are detected in ALMA SO$_2$ transition. Assuming an edge-on disk geometry, the dynamical mass of the central object is estimated to be 14.75$\pm$2.17 M$\odot$.

Keywords

massive stars: protostars

Information

Type
Poster Paper
Information
Proceedings of the International Astronomical Union , Volume 21 , Symposium S402: Massive Stars Across Redshifts in the Era of JWST and Large Surveys , January 2025, pp. 323 - 326
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Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of The International Astronomical Union

1. Introduction

Bloated stars are high-mass young stellar objects (MYSO) in a crucial evolutionary phase of active accretion- when the stellar surface is expanding because of the accretion of matter on to the star (Hosokawa et al. Reference Hosokawa, Yorke and Omukai2010). These stars, unlike typical O-type stars on the main-sequence, have a relatively low effective temperature (consistent with a B-spectral type) and low ionizing flux, and thus are highly deficient in radio emission. This theoretically predicted “bloated” or “swollen” stage in massive star formation remains poorly constrained observationally, with only a few candidates identified so far. Among them, IRAS 19520+2759 (I19520) is the most luminous (L greater than $\gt$ 10 Superscript 5 $^5$ L Subscript circled dot $_\odot$ ) and represents the first O-type bloated star candidate (Palau et al. Reference Palau, Contreras, Sahai, Sánchez-Monge and Rizzo2013).

IRAS 19520+2759 (I19520; RA (J2000) =19h54m05.9s, Dec(J2000)=+28d7m41s) is a MYSO, located at 9 kpc, and detected in optical and Near-infrared (NIR) wavelength (Sánchez Conteras et al. Reference Sánchez Conteras and Sahai2013). The source is classified as “Bloated star candidtae” based on its high luminosity, and relatively low effective temperature estimated from its optical spectrum and lack of centimeter radio emission (Palau et al. Reference Palau, Contreras, Sahai, Sánchez-Monge and Rizzo2013). In our recent investigation, we performed a variability study on I19520 using optical and infrared time series data and found that it shows periodic variability consistent with the pulsational variability predicted for bloated sources (Pandey et al. Reference Pandey, Palau, Serna and Kuiper2025). Interferometric (OVRO) millimeter observations revealed a compact dusty core coincident with the optical source, a powerful collimated CO outflow, and an elongated C Superscript 18 $^{18}$ O structure perpendicular to it (Palau et al. Reference Palau, Contreras, Sahai, Sánchez-Monge and Rizzo2013). However, these observations lacked the angular resolution necessary to probe the inner structure of the system. To overcome these limitations, we conducted high angular resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA; beam size tilde 0.18 double prime times 0.13 double prime $\sim 0.18^{\prime\prime} \times 0.13^{\prime\prime}$ ) and the Expanded Very Large Array (EVLA; beam size tilde 0.41 double prime times 0.37 double prime $\sim 0.41^{\prime\prime} \times 0.37^{\prime\prime}$ ). Our aim is to present the first detailed study of the physical and chemical properties of the dense circumstellar environment of an O-type bloated star candidate.

2. Results and Discussion

Continuum emission and molecular outflows : Figure 1 (a) shows the EVLA K band image (beam size tilde 0.76 double prime times 0.74 double prime $\sim 0.76^{\prime\prime} \times 0.74^{\prime\prime}$ ) tracing the radio continuum emission associated with I19520. The emission exhibits an extended morphology, elongated along the east-west and northeast-southwest directions, and peaks at the position of the optical counterpart of I19520. The spectral index, derived from the EVLA C, K, and Q band fluxes, is 0.54 plus or minus $\pm$ 0.04, consistent with a thermal radio jet.

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 $\times$ (rms = 9 times $\times$ 10 Superscript negative 6 $^{-6}$ Jy beam Superscript negative 1 $^{-1}$ ). (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 $\times$ rms, where rms tilde $\sim$ 3.4 mJy beam Superscript negative 1 $^{-1}$ . (d) Blue- and redshifted Superscript 13 $^{13}$ CO emission (see Section 2) 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.

Four panels showing radio and millimeter observations of a star candidate.

We show the ALMA Band 6 (1.3 mm) continuum (beam size tilde 0.18 double prime times 0.13 double prime $\sim 0.18^{\prime\prime} \times 0.13^{\prime\prime}$ ) in Figure 1 (c). The 1.3 mm continuum map reveals a complex, asymmetric morphology with a bright, compact source at the center (MM1). The peak of this compact emission coincides closely with the optical counterpart (similar to the radio continuum emission), likely tracing the core that harbors the MYSO I19520. In this study, we use Superscript 13 $^{13}$ CO as a tracer of the molecular outflow. Figure 1 (d) presents the Superscript 13 $^{13}$ CO emission integrated over velocity ranges excluding the systemic core interval. The blueshifted lobe spans -35.0 to -22.06 km s Superscript negative 1 $^{-1}$ , and the redshifted lobe minus $-$ 11.68 to minus $-$ 6.0 km s Superscript negative 1 $^{-1}$ . The high-velocity Superscript 13 $^{13}$ CO emission exhibits a clear bipolar morphology, with the blue- and redshifted lobes extending in opposite directions from MM1, oriented roughly east-west (PA almost equals 78 Superscript ring $\approx 78^\circ$ , hereafter Outflow A). Additionally, another high-velocity Superscript 13 $^{13}$ CO bipolar feature is observed along the northeast-southwest direction (PA almost equals 28 Superscript ring $\approx 28^\circ$ , hereafter Outflow B; see Figure 1 (d)), although its blueshifted component is relatively weak.

Hot core and a Keplerian disk: Figure 2 (a) shows intensity-weighted velocity (Moment-1) map of the ALMA SO Subscript 2 $_{2}$ (4–3) emission. The compact emission centered on the continuum source MM1, likely traces the hot-core chemistry, as sulphur-bearing species such as SO Subscript 2 $_{2}$ are commonly enhanced in hot molecular cores (Hamedani Golshan et al. Reference Hamedani Golshan, Sánchez-Monge, Schilke, Sewilo, Moller, Veena and Fuller2024). A clear north-south velocity gradient, roughly perpendicular to outflow A, suggests rotational motion within the hot core. Such rotation is often attributed to toroids or circumstellar disks exhibiting Keplerian rotation (Beltrán et al. 2017). To investigate the kinematics in more detail, we constructed a position–velocity (PV) diagram along the observed gradient (Figure 2 (b)) and fitted the velocity distribution with Keplerian rotation curves following Seifried et al. (Reference Seifried, Sánchez-Monge, Walch and Banerjee2016). The best-fit curve corresponds to a central mass of 14.75 plus or minus $\pm$ 2.17 M Subscript circled dot $_\odot$ , assuming an edge-on geometry, which thus represents a lower limit.

Figure 2.

(a) Moment-1 map of the SO Subscript 2 $_2$ (4-3) transition, with lines indicating the directions of the plausible outflows (see Section 2); the star marks the position of the continuum peak MM1. (b) PV plot for SO Subscript 2 $_2$ (4–3) transition, extracted along the velocity gradient in Moment-1 map, the contours (white) are drawn at 3, 5, 7, 15, 20 times $\times$ (rms=8.6 times $\times$ 10 Superscript negative 4 $10^{-4}$ Jy beam Superscript negative 1 $^{-1}$ ). The point marks the maximum velocity for different radial offsets shown along with the best fitted Keplerian rotation model (blue).

Two panels show data related to the circumstellar environment of a star.

References

Hosokawa, T., Yorke, H. W., Omukai, K., 2010, The Astrophysical Journal, 721, 478 CrossRefGoogle Scholar
Palau, A., Contreras, C. S., Sahai, R., Sánchez-Monge, A., Rizzo, J. R., 2013, MNRAS, 428 CrossRefGoogle Scholar
Pandey, R., Palau, A., Serna, J., Kuiper, R. et al. 2025, MNRAS, 541, 3772–3786 Google Scholar
Sánchez Conteras, C., Sahai, R., et al. 2013, The Astrophysical Journal Supplement Series, 179 Google Scholar
Seifried, D., Sánchez-Monge, A., Walch, S., Banerjee, R., 2016, MNRAS, 459, b1892 CrossRefGoogle Scholar
Beltrán, M. T., de Wit, W. J., 2017, Mem. Soc. Astron. Italiana, 88, 581 Google Scholar
Hamedani Golshan, R., Sánchez-Monge, A., Schilke, P., Sewilo, M., Moller, T., Veena, V. S., Fuller, G. A., 2024, A&A, 688, A3 Google Scholar
Figure 0

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$10−6$^{-6}$ Jy beam−1$^{-1}$). (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$^{-1}$. (d) Blue- and redshifted 13$^{13}$CO emission (see Section 2) 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 1

Figure 2.(a) Moment-1 map of the SO2$_2$ (4-3) transition, with lines indicating the directions of the plausible outflows (see Section 2); the star marks the position of the continuum peak MM1. (b) PV plot for SO2$_2$ (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$10^{-4}$ Jy beam−1$^{-1}$). The point marks the maximum velocity for different radial offsets shown along with the best fitted Keplerian rotation model (blue).