Modeling Protostars in the Giant H II region RCW-49 using Monte Carlo Radiative Transfer Techniques

Introduction & Background

GLIMPSE

GLIMPSE (the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) is one of six projects that comprise the Spitzer Legacy Science Program. Researchers in the scientific community will be able to use the Spitzer Space Telescope infrared data from the legacy projects, each of which will bring new and valuable information . Specifically, GLIMPSE data will consist of a 220 square degree (galactic longitude: l=10-65 deg latitude:+/- 1 deg) survey of the inner two-thirds of the galactic plane. The survey area, which is rich in star formation regions and molecular gas, will be imaged in 4 bands in the near-to mid-infrared. Astronomers hope to receive information about, among other things, the structure and design of our galaxy as well as current evolutionary processes that are taking place within the galaxy. As a part of my summer research involving protostars, I will be modeling GLIMPSE data from the Giant H II region RCW-49. With an estimated mass and luminosity of 3 x 10 and 1.4 x 10 , respectively, RCW-49 is one of the most massive and luminous star formation regions in the galaxy.

RCW-49 (distance is approximately 4.2 Kpc)

What is a protostar?

Before a star becomes a part of the main sequence, it must first evolve through several stages:

Class 0 Sources A class 0 source is the earliest of the four evolutionary stages of protostar development.During this time(approx. 10,000 years or less), the protostar is deeply embedded in a circumstellar envelope and most of the source mass is contained within the envelope. Although very young, these sources show strong, highly collimated bipolar molecular outflows...

Size:20,000-25,000 AU

Class I Sources The class I stage lasts on the order of a few 100,000 years. The stellar accretion rate has decreased (and will continue to decrease) and the source is now visible in the near-infrared. The mass transfer between the envelope and the developing pre-main-sequence core has resulted in an approximately equal mass between the two. Class I sources also have significantly wider bipolar cavities than Class 0 sources. The wider cavities may be a result of lower envelope accretion rates, less collimated molecular outflows and/or high powered jets.

Size: Approximately 3000-10,000 AU

Class II Sources Class II sources are thought to be T Tauri Stars with accretion disks. They are approximately 1-10 million years old and are the subjects of intense study due to the possibility of developing planetary systems.

Size: 500 AU or less

Class III & ZAMS Sources The class III stage lasts around 10 million years...after the central protostar gains enough mass from the accretion disk it is ready to become a zero age main sequence star. Some ZAMS have debris disks which may be the result of a quickly-formed high mass star that has reached the main sequence before all of the surrounding dust has been cleared away. Surrounding debris disks may eventually lead to the emergence of planetary systems. The legacy project From Molecular Cores to Planet Forming Disks: An SST Legacy Program will further explore the emergence of planetary systems.

The image above shows approximate evolutionary tracks for pre-main sequence stars of various mass.

Images of Protoplanetary Systems(ClassI-II sources)

The above image shows 6 protostar systems as viewed in the infrared. In order of increasing age: DG Tau B, IRAS 04016+2610, IRAS 04302+2247, Haro 6-5B,IRAS 0248+2612 and Coku Tau1. Clicking on the image will provide the source of the image, a paper written by Padgett et al.(Padgett et al., 1999, Disks and Envelopes around Very Young Stars, AJ, 117, 1490).

Images of Protoplanetary Disks (Late ClassII- III sources)

Clicking on an image will provide more information

Each of the above late Class II-III images from the Orion Nebula was captured by the Hubble Space Telescope. Due to its proximity(~1500 ly) and high star formation activity, the Orion Nebula is a valuable source that astronomers can use to see stellar formation processes of many types. The Class I-II sources are actually closer in the Taurus star formation region (~140 pc). The "proplyds" (proto-planetary disks) above will either continue to evolve and possibly form planetary systems or will be destroyed by hot stellar winds and radiation, a phenomenon that can be read about here. An animation showing a virtual flyby through the Orion Nebula can also be found here. In the animation, deeply embedded Class 0-I sources appear as small clouds of dense dust ( Bok Globules) and Class II-III sources are visible as stars with disks.

What is my Research?

As seen in the proplyd pictures above, depending on the stage of evolution, the light from a protostar will appear differently. Most of the stellar light from a deeply embedded protostar is absorbed and reradiated in the infrared by the dense envelope of dust surrounding it. Class III sources, however,are optically thin but faint thermal emission (infrared) from the surrounding disk can be detected with Spitzer. Of course, the strength of a source's emission is dependent on other factors such as temperature, mass and radius, but the general shape of its spectral energy distribution (SED) depends heavily upon the source's environment. My research consists of compiling a grid of 350 protostar models that represent a range of stellar types and evolutionary stages that can be used to identify stellar characteristics and ages of developing protostars. More on the Grid...

Benefits from this Research

RCW-49 alone contains over 300 protostar sources. Spitzer will reveal hundreds of star formation regions in the galaxy thereby uncovering thousands of protostars. Modeling such a large number of sources requires such a grid paired with model fitting programs.
There are very few observations of high mass protostar formation. High mass stellar formation is different from low mass in that the higher temperatures cause dynamically different interactions with the stellar environment. GLIMPSE will see, for the first time, a large number of developing high mass stars in the Galaxy. The code that produces the models offers a realistic geometric representation of developing protostars which will be undeniably useful in classifying protostars.

The Grid Results Conclusions Useful Links