A far-infrared image of the cold pre-stellar cloud L1544 (the cloud is at the lower left, with other clouds of gas and dust nearby). The cloud is about 450 light-years from Earth in the nearest large region of star formation. New studies of the gas motions in the core show that the stellar embryo is slowly collapsing in a manner that agrees well with some models and excludes others. Credit: ESA/Herschel/SPIRE
Stars
like the Sun begin their lives as cold, dense cores of dust and gas
that gradually collapse under the influence of gravity until nuclear
fusion is ignited. Exactly how the critical collapse process occurs in
these embryos, however, is poorly understood, with several competing
ideas having been advanced. Material might just freely fall to the
center, although in more likely scenarios the infall is inhibited by
pressure from warm gas, turbulent motions, magnetic fields, or even
perhaps by some combination of them. It might be possible to
distinguish between these alternative collapse hypotheses by examining
how the core's density varies with radius, but it turns out that (at
least for spherical clouds) the predicted density distributions all look
about the same. The predicted distributions of velocity for the
infalling gas, however, are quite different.
The dust in these cores makes them completely opaque in the optical,
and so studying their behaviors requires techniques at other
wavelengths. One of the most exciting developments in astronomy over
the past decade has been the development of far-infrared and millimeter
wavelength tools for the tasks of identifying pre-stellar cores as such,
and determining their properties. CfA astronomer Eric Keto and two
colleagues used observations of emission lines from water and carbon
monoxide at both wavelength regimes to measure the velocity distribution
of the gas in a pre-stellar, dense core. Each of these gas molecules
traces a sightly different density of gas (the typical value in these
clouds is about one hundred thousand particles per cubic centimeter).
The data clearly prefer the scenario in which the gas temperature is
nearly constant throughout the cloud with just enough total mass present
for gravity to drive slow contraction. Actually, the paper's authors
were the first to advocate and describe just such a possibility, and
these observations of this particular core bring a satisfying
confirmation that no magnetic fields or turbulence is present or needed.
The new results highlight the dramatic modern successes in unraveling
the earliest stages of stellar birth, and the power of new technology.
More cores now need to be measured in order to determine if these
particular conclusions have general validity.
Reference(s):
"The Dynamics of Collapsing Cores and Star Formation," Eric Keto, Paola Caselli, and Jonathan Rawlings, MNRAS 446, 3731, 2015.
