[Paper Review] The G9.62+0.19-F Hot Molecular Core - The infrared view on very young massive stars

H. Linz, B. Stecklum+3 more|ArXiv.org|Jun 30, 2004

Astrophysics and Star Formation Studies77 references13 citations

TL;DR

This study presents the first direct infrared detection of a hot molecular core (HMC) at 3.8 μm using high-resolution near- and mid-infrared imaging from the VLT and other telescopes. It reveals complex substructure in the G9.62+0.19–F HMC, identifies a luminous, embedded object (F4) associated with the core via outflow-driven cavity clearing, and demonstrates that infrared emission from HMCs is detectable only under favorable asymmetric extinction conditions due to outflow cavities.

ABSTRACT

(abridged) We present the results of an extensive infrared study of the massive star-forming region G9.62+0.19. The data cover information from broad- and narrow-band filters in the wavelength range from 1 to 19 micrometer and are obtained with ESO's infrared cameras ISAAC and TIMMI2 and with SpectroCam-10 (Mt. Palomar). The high sensitivity and resolution provided by these facilities revealed intriguing new details of this star-forming region and especially about the embedded hot molecular core (HMC) - component F. We analyse the newly found infrared sub-structure of four objects in this HMC region. While one of these objects (F2) is probably a foreground field star, the nature of the brightest object in the near-infrared there (F1) remains somewhat enigmatic. Our new astrometry proves that this object is not coincident with the peak of the molecular line emission of the HMC, but displaced by 1.7 arcsecs (nearly 10000 AU on a linear scale). We estimate this object to be an additional embedded object with a dense dust shell. Very near the HMC location we find L' band emission which strongly rises in flux towards longer wavelengths. We presume that this emission (F4) arises from the envelope of the HMC which is known to be associated with a molecular outflow roughly aligned along the line of sight. Thus, the clearing effect of this outflow causes strong deviations from spherical symmetry which might allow infrared emission from the HMC to escape through the outflow cavities. This presents the first direct detection of an HMC at a wavelength as short as 3.8 micron. At 11.7 and 18.75 micron, the HMC counterpart F4 ultimately proves to be the most luminous IR source within the G9.62+0.19-F region.

Motivation & Objective

• To investigate the infrared morphology and physical conditions of the massive, embedded hot molecular core (HMC) G9.62+0.19–F in the early stages of high-mass star formation.
• To resolve the nature of multiple infrared sources within the HMC region, particularly the bright near-infrared object F1 and the fainter, long-wavelength-emitting object F4.
• To determine whether the HMC is detectable in the near-infrared and to understand the role of circumstellar extinction and outflow cavities in enabling infrared visibility.
• To clarify why ultracompact H II regions (UCH II s) D and E in the same region remain undetected in infrared data despite strong radio emission.

Proposed method

• High-sensitivity, high-angular-resolution near- and mid-infrared imaging was conducted using ISAAC on the VLT, TIMMI2 at La Silla, and SpectroCam-10 at Mount Palomar across 1–19 μm.
• Narrow-band imaging in Brγ and H2 emission lines, combined with K-band imaging polarimetry, was used to identify scattered light and emission regions.
• Astrometric analysis compared the positions of infrared sources with the peak of molecular line emission (NH3 and CH3CN) to assess spatial coincidence and physical association.
• Radiative transfer modeling and SED fitting were applied to the brightest infrared source (F4) to constrain its luminosity, temperature, and dust shell properties.
• High-resolution radio data were re-analyzed to estimate line-of-sight extinction toward UCH II regions D and E, explaining their infrared non-detection.
• The presence of a molecular outflow aligned with the line of sight was used to interpret the asymmetric infrared emission and cavity formation.

Experimental results

Research questions

• RQ1 Is the hot molecular core G9.62+0.19–F detectable in the near-infrared, and if so, under what conditions?
• RQ2 What is the nature of the bright near-infrared source F1, and why is it offset from the HMC's molecular peak?
• RQ3 Why is the HMC's infrared emission most prominent at longer wavelengths (11.7 μm and 18.75 μm), and what physical mechanism enables this?
• RQ4 Why are the ultracompact H II regions D and E not visible in the infrared despite strong radio emission?
• RQ5 How do outflow cavities and asymmetric extinction affect the detectability and morphology of infrared emission from massive young stellar objects?

Key findings

• The HMC G9.62+0.19–F was detected directly at 3.8 μm for the first time, marking the first infrared detection of a hot molecular core at such a short wavelength.
• Object F4, located near the HMC's molecular peak and showing strong long-wavelength emission, is identified as the most luminous infrared source in the region at 11.7 μm and 18.75 μm.
• The near-infrared source F1 is not coincident with the HMC's molecular peak, being displaced by 1.7 arcseconds (~10,000 AU), and is best explained as a deeply embedded object with a dense dust shell.
• The diffuse K-band emission (2.16 μm) is attributed to scattered light, supported by polarimetry, and likely originates from the HMC's outflow cone, which clears a path for infrared radiation.
• The non-detection of UCH II regions D and E in the infrared is consistent with high extinction estimates derived from high-resolution radio molecular line data, explaining their invisibility despite strong radio emission.
• The complex infrared appearance of the region is dominated not only by the intrinsic SED of YSOs but also by the asymmetric distribution of circumstellar material and outflow cavities, which enable emission to escape.