A multi-wavelength study on the low-mass stellar populations of the southern high-mass star-forming regions RCW 34 and NGC 2626

Abstract

The physical processes of how stars form is well understood and documented. However, the finer details of how high-mass stars (M > 8 Mꙩ) form is not yet fully known. High-mass stars are usually associated with class II 6.7-GHz methanol masers and are always accompanied by lower-mass stars, all forming part of the same cluster. The high extinction caused by the dust in the molecular clouds from which these young open clusters form serves as an observational obstacle in the optical. Two high-mass star-forming regions located in Vela, RCW 34 and NGC 2626, are part of the few clusters that are detectable in the optical and are the focus of this thesis. A deep near-infrared (NIR) imaging study was performed on RCW 34 and NGC 2626, using the 1.4-m Infrared survey facility (IRSF) telescope in Sutherland. A total exposure time of about 3 hours was obtained of a 15.9' x 15.9' and a 7.9' x 7.9' field around RCW 34 and NGC 2626, respectively. From the Near-infrared (NIR) photometry, using the Linear regression with errors and scatter (LinES) optimisation method, a custom extinction law was derived for RCW 34. The same regression method was used for NGC 2626 and is equivalent to the general extinction law used for the Galactic plane. An extinction map of each region was built using the extinctions measured for each field star behind the two star-forming regions, using the NICEST interpolation method. The NIR photometry showed that there are 593 and 39 sources with excess NIR-emission in RCW 34 and NGC 2626, respectively. The excess emission sources in RCW 34 showed that the cluster is larger than stated in the literature, and it spans the entire 15.9' x 15.9' field. Excess emission sources in NGC 2626 confirmed the extent of the cluster and the remnant molecular cloud, as documented in the literature. Optical photometry from the VST/OmegaCAM Photometric Hα survey of the Southern Galactic plane (VPHAS+) survey was obtained for RCW 34 and NGC 2626. For each cluster the sources with Hydrogen Balmer-alpha line (Hα) emission were identified based on the amount of (r'–Hα) colour excess emitted from the source. The optical r'/(r'–i') colour-magnitude diagrams were used to determine the mass and age of cluster members that showed excess NIR- or Hα-emission. Spectroscopy was performed with Southern African large telescope (SALT) on stars with a high likelihood of being Hα-emission sources based on the excess emission in their NIR-colours and optical colours. A total of 92 candidates were identified in RCW 34 and 24 in NGC 2626. Of the candidates in each region, 18 and 11 showed Hα-emission lines in RCW 34 and NGC 2626, respectively. The SALT candidates were selected based on the amount of (Rs–Hα) and (H–Ks) colour excess they showed in the SuperCOSMOS sky survey (SuperCOSMOS) and IRSF photometric systems. The stars with an Hα -emission line were identified as T Tauri stars. Most of the Hα-emission line profiles were noisy, those with the best signal-to-noise profiles were used to determine what the emission source of the Hα-emission line is. Combining the SALT observations with the VPHAS+ photometry showed that sources with an Hα-emission line identified with SALT do not show (r'–H) colour excess. This eliminated the possible use of VPHAS+ photometry in the identification of sources with Hα-emission. The SALT spectra of the star driving the Herbig-Haro object (HH-132) and the methanol maser in NGC 2626 were used to classify the star as a late O or early B star. This may be the first time that optical spectra of a star that caused a maser have been observed in literature. The spectra shows that this young high-mass star is on the main sequence.