A Detailed View of the Circumstellar Environment and Disk of the Forming O-star AFGL 4176

Abstract

We present a detailed analysis of the disk and circumstellar environment of the forming O-type star AFGL 4176 mm1, placing results from the Atacama Large Millimeter/submillimeter Array (ALMA) into context with multiwavelength data. With ALMA, we detect seventeen 1.2 mm continuum sources within 5 ′′ (21,000 au) of AFGL 4176 mm1. We find that mm1 has a spectral index of 3.4±0.2 across the ALMA band, with >87% of its 1.2 mm continuum emission from dust. The source mm2, projected 4200 au from mm1, may be a companion or a blueshifted knot in a jet. We also explore the morphological differences between the molecular lines detected with ALMA, finding 203 lines from 25 molecules, which we categorize into several morphological types. Our results show that AFGL 4176 mm1 provides an example of a forming O-star with a large and chemically complex disk, which is mainly traced by nitrogen-bearing molecules. Lines that show strong emission on the blueshifted side of the disk are predominantly oxygen-bearing, which we suggest are tracing a disk accretion shock. The molecules C 34 S, H 2 CS and CH 3 CCN trace a slow wide-angle wind or dense structures in the outflow cavity walls. With the Australia Telescope Compact Array (ATCA), we detect a compact continuum source (<2000 × 760 au) at 1.2 cm, associated with mm1, of which >96% is from ionized gas. The ATCA NH 3 (1,1) and (2,2) emission traces a large-scale (r∼0.5 pc) rotating toroid with the disk source mm1 in the blueshifted part of this structure offset to the northwest.

Physical and chemical structure of high-mass star-forming regions

“…An important finding is the chemical segregation of O-and N-bearing species (e.g., Rodgers & Charnley 2001). This has been observed and studied toward many bright HMCs such as Orion-KL (Caselli et al 1993;Feng et al 2015); W3 H2O and W3 OH (Wyrowski et al 1999;Qin et al 2015); AFGL 2591 (Jiménez-Serra et al 2012;Gieser et al 2019); NGC7538 IRS9, W3 IRS5 and AFGL 490 (Fayolle et al 2015); G35.20-0.74N (Allen et al 2017); SgrB2(N) (Bonfand et al 2017;Mills et al 2018); or AFGL 4176 (Johnston et al 2020).…”

ALMA observations of the Extended Green Object G19.01−0.03 – I. A Keplerian disc in a massive protostellar system

“…Interestingly, comparison of observed MYSOs with candidate discs in the literature (e.g. Table 7 of Johnston et al 2020) suggests that there is not a monotonic relationship between luminosity and enclosed stellar mass (though luminosity estimates are affected by distance uncertainties, in particular for sources without maser parallax distances; see also Ilee et al 2016). Notably, a third of the sources tabulated by Johnston et al (2020) are EGOs: G11.92−0.61 MM1, G23.01−0.41, G328.2551−0.5321 (EGO G328.25−0.53) and IRAS 16547−4247 (EGO G343.12−0.06) (Ilee et al 2018;Sanna et al 2019;Csengeri et al 2018;Zapata et al 2019, respectively).…”

“…Table 7 of Johnston et al 2020) suggests that there is not a monotonic relationship between luminosity and enclosed stellar mass (though luminosity estimates are affected by distance uncertainties, in particular for sources without maser parallax distances; see also Ilee et al 2016). Notably, a third of the sources tabulated by Johnston et al (2020) are EGOs: G11.92−0.61 MM1, G23.01−0.41, G328.2551−0.5321 (EGO G328.25−0.53) and IRAS 16547−4247 (EGO G343.12−0.06) (Ilee et al 2018;Sanna et al 2019;Csengeri et al 2018;Zapata et al 2019, respectively). While this is a small subsample, we note that even when considering only EGOs -with similar MIR evidence for active outflows and so ongoing accretionthere does not appear to be a monotonic relation between luminosity and enclosed mass, with G11.92−0.61 MM1 having the lowest luminosity (∼10 4 L , Cyganowski et al 2011a;Moscadelli et al 2016) and the highest stellar mass (>30 M , compared to ≤20M for the other sources: Ilee et al 2018;Johnston et al 2020).…”

“…Cesaroni et al 2007;Sánchez-Monge et al 2013;Cesaroni et al 2014;Beltrán et al 2014;Beltrán & de Wit 2016;Girart et al 2017;Añez-López et al 2020;Jiménez-Serra et al 2020), with relatively few candidate discs observed towards proto-O stars, e.g. AFGL 2591VLA 3 (Jiménez-Serra et al 2012, NGC 6334 I(N) (Hunter et al 2014), IRAS 16547-4247 (Zapata et al 2015(Zapata et al , 2019, AFGL 4176 (Johnston et al 2015(Johnston et al , 2020, AFGL 2136 (Maud et al 2018(Maud et al , 2019, G11.92-0.61 MM1 (Ilee et al 2016(Ilee et al , 2018, G023.01-0.41 (Sanna et al 2019), G345.50+0.35 M, G345.50+0.35 S and G29.96-0.02 (Cesaroni et al 2017), with central protostellar masses ∼10−45 M and luminosities of (0.1 − 5.8) × 10 5 L .…”

Disk fragmentation in high-mass star formation

“…However, recent advancements in interferometry, allowing for sub-arcsecond resolution at (sub-)millimeter wavelengths, have led to a turning point for high-mass star formation studies, as a growing number of disks around O-and B-type stars are detected (e.g. Patel et al 2005;Sánchez-Monge et al 2013;Johnston et al 2015;Ilee et al 2016;Cesaroni et al 2017;Csen-geri et al 2018;Ginsburg et al 2018;Ilee et al 2018a;Girart et al 2018;Moscadelli et al 2019;Maud et al 2019;Johnston et al 2020b;Añez-López et al 2020). A disk-mediated accretion process seems to be a necessity for the formation of higher-mass objects.…”