Breaking Assumptions on the Excitation Temperatures in Molecular Clouds

How do clouds of neutral hydrogen atoms transform into molecular clouds, the birthplaces of all stars in our galaxy? This is an unanswered question in galactic astronomy that some scientists are investigating using the Arecibo Observatory (AO).

“We often address this question by observing small molecular clouds that are home to the most basic processes that govern the transitions from atoms to molecules,” explained Dr. Allison Smith, a UCF/AO Preeminent Postdoctoral Scholar. “To understand the molecular clouds, we need to know their mass, which we can measure if we know the molecular column density - an observable property of the molecular cloud.”

However, new AO results published in the Monthly Notices of the Royal Astronomical Society show that some of the traditional assumptions used by radio astronomers to derive the column densities may not always be correct.

“This study has provided more evidence that the excitation temperatures can vary between different regions. This should be taken into account when determining the column density for CH, and therefore the column density of the molecular clouds.” Erin Dailey Graduate student at the University of Georgia in Athens

“Excitation temperatures for methylidyne (CH) - a tracer for molecular clouds - are usually assumed to be significantly higher than the background temperature,” explained Erin Dailey, a graduate student at the University of Georgia in Athens and lead author of the publication. “This study has provided more evidence that the excitation temperatures can vary between different regions. This should be taken into account when determining the column density for CH, and therefore the column density of the molecular clouds.”

The team of scientists, which includes Dr. Smith, used the Arecibo Observatory to study the CH 3335-MHz line in otherwise well-characterized nearby molecular clouds to back-out the excitation temperatures. Ms. Dailey and Dr. Smith agreed that the sensitivity needed to complete these measurements meant that AO was the best facility to use for their analysis.

Interpretations that rest on the assumption of a high excitation temperature should always take into account that the assumption may not hold across the molecular cloud. - Dr. Allison Smith UCF/AO Preeminent Postdoctoral Scholar

“Our results show that independent measurements of the excitation temperature are needed for future molecular cloud column density studies,” Dr. Smith responded when asked about the implications of the study. “Or interpretations that rest on the assumption of a high excitation temperature should always take into account that the assumption may not hold across the molecular cloud.”

“There is a lot of fascinating science that we can learn through the exploration of molecular clouds. High-fidelity measurements of the molecules’ column density is the underpinning science,” said Dr. Smith. “Hopefully this work will enable future scientific investigations to be even more accurate!”