# IRAS 18317-0757: A Cluster of Embedded Massive Stars and Protostars

## TL;DR

This study provides high-resolution multiwavelength observations of IRAS 18317-0757, revealing a complex, fragmented star-forming region with embedded massive stars, protostars, and molecular cores, highlighting ongoing star formation and cloud fragmentation.

## Contribution

First detailed multiwavelength imaging of IRAS 18317-0757, revealing cloud fragmentation, core properties, and star formation activity with new insights into massive star formation processes.

## Key findings

- Detection of a single UCHII region at 2.7 mm consistent with free-free emission.
- Identification of over two dozen virialized C18O cores containing about 40% of molecular gas.
- Evidence of ongoing star formation with multiple B-type protostars forming in the periphery.

## Abstract

We present high-resolution, multiwavelength continuum and molecular-line images of the massive star forming region IRAS 18317-0757. The IR through mm spectral energy distribution can be approximated by a two-temperature model (25 and 63 K) with a total luminosity of approximately log(L/Lsun)=5.2. Previous submm imaging resolved this region into a cluster of 5 dust cores, one of which is associated with the UCHII region G23.96+0.15, and another with an H2O maser. In our new 2.7 mm continuum image, only the UCHII region is detected, with total flux and morphology in good agreement with the free-free emission in VLA cm-wave maps. For the other four objects, the nondetections at 2.7 mm and in the MSX mid-IR bands are consistent with cool dust emission with a temperature of 13-40K and luminosity of 1000-40000 Lsun. By combining single-dish and interferometric data, we have identified over two dozen virialized C18O cores in this region that contain ~40% of the total molecular gas mass present. While the overall extent of the C18O and dust emission is similar, the emission peaks do not correlate well in detail. At least 11 of the 123 stars identified by 2MASS in this region are likely to be within the star-forming cluster. Two stars (both associated with the UCHII region) were previously identified as O stars via IR spectroscopy. Most of the rest of the reddened stars have no obvious correlation with the C18O cores or the dust cores. In summary, our observations indicate that considerable fragmentation of the molecular cloud has taken place during the time required for the UCHII region to form and the O stars to become detectable at IR wavelengths. Additional star formation appears to be ongoing on the periphery of the central region, where up to four B-type (proto)stars have formed among a substantial number of C18O molecular cores.