A MULTI-WAVELENGTH STUDY OF MASSIVE STAR FORMING REGION G188.95+0.89 IN PROBING THE DYNAMICS OF MASSIVE STAR FORMATION

Abstract

ABSTRACT Massive stars are born in regions that are opaque to optical observations. This is a challenge in understanding the processes that involve their formation. In order to investigate their formations, a multi-wavelength approach in the infrared (IR) to radio wavelengths was employed to probe the environment around the core, where they are formed. To understand the dynamics of massive star formation, we probed the molecular cloud to check the chemical composition, bipolar outflows and detailed measurements of molecular velocity fields. G188.95+089 is the Massive Star Forming Region (MSFR) that was used in our study. The choice of the star-forming region was due to the fact that it is nearby, emits periodic masers and harbours multiple star-forming cores. While there are many tracers that can be utilized to infer the presence of massive star formation, we relied on Class II methanol masers at 6.7 GHz. In executing the multi-wavelength investigation, radio data from the 26 m Hartebeesthoek Radio Astronomy Observatory (HartRAO) dish was used to analyse the observed spectra of 6.7 GHz methanol masers. Interferometric data from the Atacama Large Millimeter/sub-Millimeter Array (ALMA) were used to check molecular line and continuum emissions of the source. Complementary infrared data from WISE, 2MASS, Hi-GAL, GLIMPSE, IRAS and MSX was used to probe the presence of extended sources surrounding the massive star forming region. Results from radio observations showed that the source had five velocity maser features that exhibited average periodicity of 397.6 days and at least two showed evidence of velocity drifts ranging from −2.38 × 10−5kms−1d−1 to +1.88 × 10−5kms−1d−1. One maser feature at 11.45 Km s−1 exhibited a varying spectra with exponential decay as from year 2003 to present. The spectra of the other maser velocity features have varied significantly since detection in 1991. Using ALMA band 6 at ∼ 1.3 mm and band 7 at ∼ 0.93 mm data, we were able to identify eight continuum cores (MM1-MM8) in the source, with masses ranging from 0.40 to 8.20 solar masses. In the ALMA band 7 observations, G188.95+0.89 MM2 was resolved into two continuum cores separated by 0.1 arcsec. The thermal emission of CH3OH (4(2, 2) - 3 (1, 2)) linked with MM2 has a double peak. In MM2, SiO emission has a bow-shock morphology, but high 12CO evidence for an east-west bipolar outflow is provided by emission to the east and west of MM2. SiO emission shows bipolar outflow centered around MM2 core. Using near- and mid-IR color-color diagrams, young sources were detected in this star-forming region. A total of 36 candidate YSOs, were detected within a 60′′ radius of the G188.95+0.89 source. There is an IR cluster made up of nine of these YSOs just outside the UC HII zone. Only the H and Ks bands of the 2MASS data can reveal nine highly red shifted objects. These sources have redder colors than H − K > 2, indicating that the IR cluster is extremely young. It is unlikely that interstellar absorption alone is responsible for the reddening of the vectors; instead, the presence of a circumstellar disc and envelope must account for at least some of the IR excess. Although further investigation is needed, it appears that the velocity drifts were caused by gas falling into the inner radius of the accretion disk surrounding the protostar G188.95+0.89. The variability of 6.7 GHz methanol masers is a confirmation of on going accretion in the source. Although the presence of accretion disks in the source cannot be confirmed with the existing measurements, the identification of outflows is consistent with their existence. The out-flowing material creates shocks when it encounters the quiescent gas of the envelop. The detection of SiO molecular lines is an effective tool for checking for the existence of shocks. The shock waves pushes the gas into ever denser physical states that allow it to cool and fragment more efficiently. We argue that MM2 has a massive multiple (at least binary) of young star objects, but more VLBI observations are needed to confirm that this is indeed the case.