Degree Type

Dissertation

Date of Award

2015

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

Charles R. Kerton

Abstract

This thesis describes observational studies of four small star-forming H II regions (KR 7, KR 81, KR 120 and KR 140) and star-formation scenario associated with the Young Stellar Objects (YSOs) in each region. In addition to that, we also present an analysis of HCO+ (J=3→2) and H13CO+ (J=3→2) observations of the Massive (M ∼ 20 M⊙) submillimeter/infrared source IRAS 01202+6133 located on the periphery of the H II region. In this research, we improved existing 1-D radiative transfer model for a collapsing core that happens in the early phase − Class I protostar − of star formation.

The molecular gas surrounding an H II region is thought to be a place where star formation can be induced. We selected four small H II region in order to minimize the feedbacks and dynamics from multiple exciting sources. These regions are very young and ionized by the single O or B spectral type stars.

A space based telescope Wide-field Infrared Survey Explorer (WISE) used for identi- fying and classifying the YSOs population surrounding a sample of H II regions.

First, we used WISE data from AllWISE catalog with some constrains such as spatial coordinates, signal-to-noise ratio and contaminations. After we retrieved sources from catalog in each region, we classified YSOs with two different methods; color-color diagram and spectral index (α). Based on the color-color diagram using WISE 3.4 μm, 4.6 μm and 12 μm bands, we classified the YSOs as Class I, Class II and using 3.4 μm, 4.6 μm and 22 μm, we were able to classify Transition Disks and Class III YSOs. 2MASS and WISE combined color-color diagram also used in order to compare the classification only use of WISE color-color diagram. Considering a reddening effect from 2MASS Ks band, the classification from both WISE only and 2MASS, WISE combined color-colordiagram. A spectral index (α) also can be used as classifying YSOs. Based on the WISE magnitude, spectral index (α) can be derived from the flux of mid-infrared observation provides a quick and easy way to classify YSOs. However, this method is less accurate then color-color diagrams because we cannot filter out all the contaminants and lack of data sets. Therefore, color-color diagrams can be used a primary methods to identify and classify YSOs. Based on the spatial distribution and number ratio of YSOs, a sequential star-formation scenario is dominant for KR 7, KR 81 and KR 120. In KR 140 region both a sequential star-formation scenario and a spontaneous star-formation scenario can be used to explain the origin of star-forming scenario.

Next, we observed HCO+ line profile to investigate the infall motion of the protostar in KR 120 region. The HCO+ line profile has a classic blue-asymmetric shape with the optically thin H13CO+ line peaking at the position expected if the HCO+ line arises from a combination of self-absorption and infall motion. We have modified existing analytic radiative transfer models to allow for the fitting of submm line profiles that have both self-absorption features and optically thin wings and applied these models to our HCO+ spectrum of IRAS 01202+6133 in KR 120. We conclude that it is a young Class I YSO with a substantial envelope undergoing slow infall and having some outflow motions. The young age of the H II region rules out a ”collect and collapse” scenario. While we cannot eliminate the possibility that IRAS 01202+6133 formed spontaneously at its current location, considering its early evolutionary state and its proximity to the H II region, we think that the formation of IRAS 01202+6133 was triggered by the expansion of KR 120 (Sh 2-187).

DOI

https://doi.org/10.31274/etd-180810-4396

Copyright Owner

Sung-Ju Kang

Language

en

File Format

application/pdf

File Size

113 pages

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