Document Type

Dissertation

Degree

Doctor of Philosophy

Major

Physics

Date of Defense

11-10-2020

Graduate Advisor

Bruce A. Wilking

Co-Advisor

Alexey Yamilov

Committee

J. Serena Kim

Erika Gibb

Shun Saito

Abstract

The W4 HII region is home to a large number of O and B-type stars as well as the young open cluster IC 1805. OB-type produce intense photoionizing radiation and stellar winds that can induce star formation in surrounding molecular clouds and sometimes carve out large regions of decreased particle density called superbubbles. During the star formation process, young stellar objects (YSOs) are initially surrounded by circumstellar disks made of gas and dust which naturally dissipate after several million years. The dissipation of the disk is thought to occur for a variety of reasons, including photoevaporation by the encompassed protostar and the formation of planets. Photoionizing radiation can affect a YSO that is in close proximity to the OB stars by ablating the outer regions of a disk which cause it to dissipate faster and can disrupt the planet forming process. The goals of this study are to contribute additional evidence for an earlier generation of star formation in the W4 HII region and to characterize the evolutionary status of disk-bearing YSOs. This was done by identifying members of IC 1805 that met several youth criteria. An average extinction of 2.7 ± 0.5 mag and median age of 2.2-2.8 Myr was found for the cluster members. A group of 15 YSOs are candidates for an earlier generation, with a mean age of 8.5 ± 2.8 Myr. Several YSOs located near bright-rimmed clouds support the theory that the OB stars are driving the expansion of the superbubble by their young ages. A total of 74 disk-bearing YSOs were identified with dereddened spectral energy distributions, and 51 were fit with a simple reprocessing disk model. Examination of the disk modeling parameters showed that most disks identified in this survey are in evolved stages. A small number of disk-bearing sources located within projected distances of . 0.7 pc of an O- or B-star showed evidence that their outer disks may be influenced by external photoionizing radiation.

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