Star Formation Studies at the CSO

Ruisheng Peng

Caltech Submillimeter Observatory

Caltech Submillimeter Observatory operates a 10.4-meter radio-style

telescope equipped with heterodyne receivers that covers most of the

atmospheric windows above Mauna Kea in the sub-millimeter range from

200 GHz to 900 GHz. In addition, there are two facility bolometer

cameras operating at 350 micron and 450 micron, 1.1 mm and 2.1 mm

respectively. These instruments truly bear out the sub-milliliter in

our name.

But why concentrate on the sub-millimeter wavelengths? Reasons are

numerous. To begin with, cold dust grains (5-50 K) in space shines

brightest at these wavelengths; cool and warm gas (10 to a few hundred

K) emits their brightest molecular and atomic lines in this wavelength

range. Also, dust grains effectively absorbs stellar photons in the

shorter wavelengths (UV, visible, and near infrared) and re-emit in

far infrared and sub-millimeter wavelengths, making dust emission an

useful tool to study objects shrouded in dust, such as quasars and

galaxies. Moreover, observations at sub-millimeter are little

hampered by dust extinction, either in the Galactic plane along the

way, or locally around the object of study, so one could look deeper

into such objects such as the center of our own Milky Way Galaxy, or

a newly formed, deeply embedded star.

Then why would anyone be interested in the cool dust and molecular

gas in space? Well, they are everywhere, and they are deeply involved

in the formation and evolutions of a lot of interesting objects:

galaxies, stars, etc. Dust and gas are often well mixed and exist in

the form of molecular clouds. Dust and gas also interact with each

other, creating a quite intricate chemical network which produces

such complex molecules as vinegar, alcohol, as well as abundant supply

of water, mostly in gaseous form. One area of great importance is

that stars form from dust and gas in dense cores in molecular clouds.

So to study how star forms, one needs to start with dust and gas.

Star forms, and star dies. Some of them die a spectacular death, in

the form of violent explosions. The formation of stars, not as

impressive in comparison, also has a few tricks of its own. It is

often accompanied by powerful jets and outflows. It can come in as

single stars, binaries, or in clusters of multiple stars. Some of

them may carry a disk that resembles a primitive planetary disk

similar to the one around the Sun. If the conditions are right, there

could be a Earth-like planet, and more interesting things could follow...

The study of star formation as a field has come a long way. We now

have a good outline of how star forms in isolated dense molecular

cores: it all began by the core collapsing under its own weight. The

collapse starts at the center and gradually propagates out. As the

proto-star in the center accumulates more gas and dust from the

parental envelope through an accretion disk, it starts to develop

outflows, often in the polar direction. The gradual clearing of the

parental envelope by the outflows leaves only the young star and the

residual proto-planetary disk, which would lead to formation of a

mature planetary system. This whole process, starting from core

collapse, could take tens of million years.

There are, however, many unanswered questions. For example, what

triggers the core collapse? How does a proto-planetary disk form? How

do stars form in clusters? What makes a massive dense cloud core to

fragment into smaller pieces? How does an ongoing star forming

process impact on the neighboring dense cores? A detailed

understanding of the physical environments of the star forming regions

holds the key to these questions.

That brings us back to the study of dust and gas. At the CSO, we

use the bolometer cameras to make maps of molecular clouds and star

forming regions. These maps reveal the location and distribution

of dust condensations and visualize how these dense cores relate to

each other. They serve as road maps for further study in other

wavelengths. In addition, one can derive temperature, mass, and

density profiles of the dust core, as well as the properties of dust

grains, such size and composition.

The heterodyne receivers provide us a means to study spectral lines

from various molecular species in molecular clouds. Molecular

spectral lines can tell us a lot of the dense cloud cores: temperature,

density, mass of the gas in the core; how dense cores move relative to

each other, and the general dynamic status of the core, ie. whether it

is expanding, contracting, or rotating, etc.

Combining these information, and comparing between star-forming

cores and non-star-forming cores, one would hope to find the clues to

many of the unanswered questions in star formation. There are

certainly a lot to be done and a lot to be learned in the field of

star formation. And the study of star formation is, I hope you agree,

is a worthy endeavor.