Master Thesis defense by Majken Ellegaard Christensen – Niels Bohr Institute - University of Copenhagen

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Master Thesis defense by Majken Ellegaard Christensen

Modeling formaldehyde emission toward the high-mass star forming region G327.3-0.6

Abstract
In contrast to the formation of low-mass stars, the formation of high-mass stars is poorly  understood. There are a number of reasons for this: high-mass stars are rarer and therefore on average further away, their evolution is faster which makes it difficult to detect their intermediate stages, they are generally found in clusters and they strongly interact with their environment. Additionally, the envelopes of young high-mass stars are chemically and physically complex regions. In the inner hottest core many complex organics are detected, that arise from evaporation of icy grain mantles, but also in the more extended regions many different molecules are detected. A number of species are present in both regions such as H2CO (formaldehyde). This species is extremely abundant and have a plethora of emission lines that arise from many different energy levels. That makes H2CO very suitable to constrain the physical and chemical structure of the source.

G327.3-0.6 is a high-mass star forming region visible from the Southern sky. In 2008 a line survey was made with the APEX radio telescope in the frequency range 213-853 GHz. In this work I have studied the physical structure of G327.3-0.6 and the chemistry within this region. I have used the line survey to identify rotational transitions of H2CO and its isotopologues: H213CO, H2C18O and HDCO. I have made rotation diagrams to estimate the rotational temperature and column density of the species. Afterwards, I modeled the radiative transfer using a 3D radiative transfer code.

I found that the envelope of G327.3-0.6 appears to be collapsing in a way similar to what is seen in the envelopes around low-mass protostars. I also found that there appears to be different abundances in the three regions of the envelope: an outer region where the temperature is below the H2CO evaporation temperature (<40 K), a middle region with temperatures above the H2CO evaporation temperature (40-100 K) and an inner region with temperatures above the ice evaporation temperature (>100 K). The abundances were found to be in agreement with previously investigated high-mass and low-mass sources in the literature.

The isotope ratios and D/H ratio were found to generally be lower than what is measured in other high-mass and low-mass sources. The low D/H ratio may imply that there are no cold gas-phase reactions taking place, which is in contrast to what is seen in low-mass prestellar cores. The ortho/para ratio of H2CO was found to be unity in the outer region and increase toward the central core. This supports the formation to happen on cold dust grain surfaces. Additionally, I found the upper limit central core mass to be 180 Msun at 1.9". The source shows sign of not being spherically symmetric. This may imply that the central region consists of several cores or the presence of a disk.
Advisor: Jes K. Jørgensen
Co-advisor: Suzanne E. Bisschop