The Properties of Pre-Stellar Cores

A false-color infrared image of a young, star-forming dust cloud with several embedded cores (identified in red). A new infrared study of 3218 cores in various stages of development has enabled astronomers to categorize the temperatures, densities, and evolutionary characters of young stellar nurseries. Credit: ASA/Spitzer and P. Myers

Stars like the Sun begin their lives as cold, dense cores of dust and gas that collapse under the influence of gravity until nuclear fusion is ignited. These cores contain hundreds to thousands of solar-masses of material and have gas densities about a thousand times greater than typical interstellar regions (the typical value is about one molecule per cubic centimeter). How the collapse process occurs in these embryos in poorly understood, from the number of stars that form to the factors that determine their ultimate masses, as well as the detailed timescale for stellar birth. Material, for example, might simply fall freely to the center of the core, but in most realistic scenarios the infall is inhibited by pressure from warm gas, turbulent motions, magnetic fields, or some combination of them.

Astronomers are actively studying these issues by observing young stars in the process of being born. The dust in these natal cores (or clumps), however, makes them opaque in the optical, thus requiring observations at other wavelengths, in particular infrared, submillimeter, and radio. In the early stages of star formation, an embryonic star heats the surrounding dust cloud to temperatures between about ten and thirty degrees kelvin before stellar winds and radiation blow the material away and expose the newborn star. CfA astronomers Andres Guzman and Howard Smith, together with their colleagues, have completed an analysis of 3246 star-forming cores, the largest sample ever done. The cold cores themselves were discovered with the APEX submillimeter-wavelength sky survey and then observed in sixteen submillimeter spectral lines; the spectral information enabled the astronomers to determine the distance to each core as well as to probe its chemistry and internal gas motions. The new paper combines these results with far-infrared measurements taken by Herschel Space Observatory surveys. The Herschel data allow the scientists to calculate the dust density, mass, and temperature of each core; the large dataset then permits useful statistical comparisons between cores with varyious parameters.

Sources in the sample fall generically into four categories: quiescent clumps, which have the coldest temperatures (16.8K) and the least infrared emission, protostellar clumps, which are sources with the youngest identifiable stellar objects, ionized hydrogen regions, which are cores within which the stars have ionized some of the surrounding gas, and "photo-dissociation" cores, the warmest of the set, which have dust temperatures around 28K, are slightly more evolved and brighter than the ionized hydrogen cores. Although the groups overlap in their properties, the large sample enables the scientists to conclude that, on average, in the quiescent clumps the dust temperature increases towards the outer regions, whereas the temperatures in protostellar and ionized hydrogen cores increase towards the inner region, consistent with the idea that they are being internally heated. The latter also tend to have dust densities that increase more steeply than the quiescent cores. This study has also identified a population of particularly cold and infrared-dark objects that are probably still in the stages of contraction, or else for some reason have had their star formation aborted. 

The new paper and its catalog are just the beginning: now that the dust in all these cores has been well characterized, astronomers can associate chemistry with dust temperature, for example, and study subgroups that might represent different stellar masses in gestation.

Reference(s): 

"Far-Infrared Dust Temperatures and Column Densities of the MALT90 Molecular Clump Sample," Andrés E. Guzmán, Patricio Sanhueza, Yanett Contreras, Howard A. Smith, James M. Jackson,Sadia Hoq, and Jill M. Rathborne, ApJ 815, 130, 2015.

Source: Smithsonian Astrophysical Observatory (SAO)

Shining Starlight on the Dark Cocoons of Star Birth

This series of images from NASA's Spitzer Space Telescope shows a dark mass of gas and dust, called a core, where new stars and planets will likely spring up. Image credit: NASA/JPL-Caltech/Observatoire de Paris/CNRS. Full image and caption

Astronomers have discovered a new, cosmic phenomenon, termed "coreshine," which is revealing new information about how stars and planets come to be.

The scientists used data from NASA's Spitzer Space Telescope to measure infrared light deflecting off cores -- cold, dark cocoons where young stars and planetary systems are blossoming. This coreshine effect, which occurs when starlight from nearby stars bounces off the cores, reveals information about their age and consistency. In a new paper, to be published Friday, Sept. 24, in the journal Science, the team reports finding coreshine across dozens of dark cores.

"Dark clouds in our Milky Way galaxy, far from Earth, are huge places where new stars are born. But they are shy and hide themselves in a shroud of dust so that we cannot see what happens inside," said Laurent Pagani of the Observatoire de Paris and the Centre National de la Recherche Scientifique, both in France. "We have found a new way to peer into them. They are like ghosts because we see them but we also see through them."

Pagani and his team first observed one case of the coreshine phenomenon in 2009. They were surprised to see that starlight was scattering off a dark core in the form of infrared light that Spitzer could see. They had thought the grains of dust making up the core were too small to deflect the starlight; instead, they expected the sunlight would travel straight through. Their finding told them that the dust grains were bigger than previously thought -- about 1 micron instead of 0.1 micron (a typical human hair is about 100 microns).

That might not sound like a big difference, but it can significantly change astronomers' models of star and planet formation. For one thing, the larger grain size means that planets -- which form as dust circling young stars sticks together -- might take shape more quickly. In other words, the tiny seeds for planet formation may be forming very early on, when a star is still in its pre-embryonic phase.

But this particular object observed in 2009 could have been a fluke. The researchers did not know if what they found was true of other dark clouds -- until now. In the new study, they examine 110 dark cores, and find that about half of them exhibit coreshine.

The finding amounts to a new tool for not only studying the dust making up the dark cores, but also for assessing their age. The more developed star-forming cores will have larger dust grains, so, using this tool, astronomers can better map their ages across our Milky Way galaxy. Coreshine can also help in constructing three-dimensional models of the cores -- the deflected starlight is scattered in a way that is dependent on the cloud structures.

Said Pagani, "We're opening a new window on the realm of dark, star-forming cores."

Other authors are Aurore Bacmann of the Astrophysics Laboratory of Grenoble, France, and Jürgen Steinacker, Amelia Stutz and Thomas Henning of the Max-Planck Institute for Astronomy, Germany. Steinacker is also with the Observatoire de Paris, and Stutz is also with the University of Arizona, Tucson.

The Spitzer measurements are based on data from the mission's public archive, taken before the telescope ran out of its liquid coolant in May 2009 and began its current warm mission.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. For more information about Spitzer, visit http://spitzer.caltech.edu/ and http://www.nasa.gov/spitzer .

Whitney Clavin/Guy Webster 818-354-4673/6278 Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov