Diverse stages of star formation in the IRAS 18162-2048 region. Emergence of UV Feedback

Abstract

Methods: We obtained adaptive optics-assisted integral field spectroscopy in the near-infrared (IR) $K$ band ($1.93-2.47 \mathrm{μm}$) with VLT/SINFONI, complemented by VLA X and C bands (3$-$6 cm) and ALMA band 3 ($\sim$3.3 mm) observations. Results: The near-IR continuum reveals two infrared sources, IRS 2 and IRS 7, while the main protostellar core IRAS 18162-2048 remains undetected up to $2.47 \mathrm{μm}$. IRS 7 shows a peculiar Hydrogen Recombination Line Br$γ$ profile with a narrow emission component superimposed on a broad absorption feature, consistent with a B2/B3 zero-age main-sequence star. Extended H$_2$ emission exhibits a `sawtooth' pattern in the excitation diagram, characteristic of UV radiation in a PDR rather than shock excitation. The radiative transfer model Cloudy reproduces the H$_2$ ro-vibrational populations for $T_\mathrm{gas}=600$ K and $n_\mathrm{H}=7.9\times10^3 \mathrm{cm^{-3}}$. VLA X and C bands observations reveal a compact radio source previously reported as a stationary condensation (SC) and coincident with IRS 7. For the first time, we detect IRS 7/SC in mm wavelengths. The spectral index in the 3$-$6 cm and 3.3 mm regime is consistent with optically thin free-free emission. Conclusions: Our near-IR and radio observations reveal that IRS 7/SC is a B2/B3 ZAMS star that has begun to photoionise its environment, giving rise to an extended PDR and a compact \ion{H}{ii} region. The coexistence of this source with the deeply embedded protostar IRAS 18162-2048 and other bubble-like structures in the field, suggests a multigenerational star-forming environment. Future \textit{James Webb Space Telescope} observations targeting the H$_2$ pure rotational lines ($3-28 \mathrm{μm}$) and other HRLs less affected by extinction will be essential to characterise the cooler molecular and ionised gas to fully disclose the formation history of the region.

Figures (10)

• Figure 1: Upper left: SINFONI $K$ band continuum view of the IRAS 18162-2048 star-forming region. The black and whites boxes represent the extraction regions for the spectra shown in Figure \ref{['fig:spectra']}. The black crosses represents the positions of IRS 2 and IRS 7 (peak intensity pixel from SINFONI), and the red cross the position of IRAS 18162-2048 (from the VLA data). Upper right: Peak intensity continuum-subtracted map for the Br$\gamma$ emission line. The white crosses represents the positions of IRS 2 and IRS 7, and the red cross the position of IRAS 18162-2048. Lower left: Peak intensity map for the $1-0$ S(1) H$_2$ emission line including the continuum emission. Lower right: Same as lower left but with the continuum subtracted. In all panels, the full FoV of SINFONI is shown.
• Figure 2: Top: Spectra for IRS 2 and IRS 7 as labelled. Bottom: Spectrum extracted at the highest S/N in the main region of H$_2$ emission. Extraction boxes are shown in Figure \ref{['fig:cont_H2_BrG']}. Main H$_2$ emission lines are marked as vertical dashed green lines, Br$\gamma$ as dotted green line, and noisy peaks as red dotted lines and labelled as $+$.
• Figure 3: IRS 7 spectrum zoomed in the $2.07-2.20~\mathrm{\mu m}$ range. A double Gaussian profile (emission + absorption) fit to the Br$\gamma$ line is shown as a red dashed line, whereas the blue dotted line shows the single Gaussian profile fit to the absorption component. Strong emission lines correspond to H$_2$, in particular $2-1$ S(3), $1-0$ S(1), $3-2$ S(4), and $2-1$ S(2), at 2.074, 2.122, 2.128, and 2.154 $\mathrm{\mu m}$, respectively.
• Figure 4: Left: Extinction map obtained from H$_2$$1-0$ Q(3)/$1-0$ S(1) line ratio. Middle: Excitation map from H$_2$ line ratio $1-0$ S(1)/$2-1$ S(1). Right: Radial velocity map using the H$_2$$1-0$ S(1) line. In all panels, gray indicates masked pixels with S/N$<3$.
• Figure 5: Ro-vibrational diagram using the fluxes extracted from the white south box depicted in Figure \ref{['fig:cont_H2_BrG']} bottom left panel and summarised in Table \ref{['tab:observed_lines_IRAS18']}. The blue circles represent the observed H$_2$ data after extinction correction ($A_V=12$ mag), and the pink line the two-temperature component. The plot has been generated using the PDR Toolbox pound2023.