STScI Annual Report 2002: A Living Mission

20 science essays
The Lessons of Deuterium
One way for astronomers to ‘observe’ astrophysical
processes is to study isotopes, which are nuclear variations
of an element with the same number of protons but
differing numbers of neutrons. Isotopic abundances can
reveal the special conditions and processes that created
or destroyed the nuclei of atoms in the Big Bang, inside
stars, by cosmic ray collisions, and in other astrophysical
environments. Using high-resolution spectroscopy, such
as with Hubble’s former Goddard High Resolution
Spectrograph (GHRS) or current Space Telescope Imaging
Spectrograph (STIS) or the Far Ultraviolet Spectroscopic
Explorer (FUSE), astronomers can observe signature spectral
lines of isotopes located in interstellar or intergalactic
space; then, they can use models of spectral line formation
to derive abundances. In this way, we have developed
an understanding of the original creation of light elements
in the Big Bang and of the synthesis of heavy elements
inside stars.
Although hundreds of isotopes surround us today,
only seven nuclear variations of four light elements
survived the Big Bang in significant quantities: 1 H, 4 He, 2 H
(deuterium, D), 3 He, 7 Li, 7 Be, and 6 Li. (Standard nomenclature
identifies an isotope by a preceding superscript to
the element’s chemical symbol giving the ‘mass number’
or total count of nucleons—protons and neutrons.) No
heavier elements survived because no isotope with mass
number 5 or 8 is stable, which created a bottleneck for the
reactions underway during the Big Bang. Only later, after
the first stars formed out of the original isotopes, were the
heavier elements (metals) created by nucleosynthesis in
the new and necessary temperature and pressure conditions
of the stellar interior. Mass loss from mature stars
has continuously enriched space with metals since that
time, giving rise to later generations of stars, planets, and
life itself.
Deuterium is simple and special, and its cosmological
story is unique. It is a fragile nucleus, consisting of a
proton and a neutron, and has a small binding energy,
which means that it is easily destroyed inside stars.
Because we believe it has no significant source other
than the Big Bang itself, the abundance of deuterium
should decrease steadily everywhere over the age of
the universe, as matter is processed through stars.
Scott D. Friedman
Among the seven original isotopes, deuterium is first
in importance as a probe of the Big Bang. This is due both
to its simple evolutionary path of steady disappearance
since its formation and to its special diagnostic value.
While the primordial abundance of any of the light original
isotopes provides constraints for important cosmological
parameters, deuterium is the most sensitive of all.
Measuring the primordial value of D/H—the ratio of the
deuterium to hydrogen abundance—is the best way to
estimate the baryon-to-photon ratio in the early universe
and, from it, to determine Ω B, the fraction of the critical
density contributed by baryons. The critical density—a
benchmark of cosmology—is the dividing line between
an open (forever expanding) and closed (eventually
contracting) universe.
Astronomers detect H and D by observing Lymanseries
absorption lines caused by interstellar or intergalactic
atoms along sight lines toward background light sources,
either stars or quasars. At rest wavelengths, these lines
occur in the far ultraviolet portion of the spectrum, from
1216 Å (Lyman α) to 912 Å (Lyman limit). The deuterium
lines are slightly shifted towards shorter wavelengths, by
82 km s -1 or 0.33 Å at Lyman α. For sight lines without
excessively complicated component structure, the D and H
lines can be measured separately, yielding the D/H ratio.
Since the same stellar processes that destroy deuterium
also manufacture metals, which are scattered through the
interstellar medium (ISM) by stellar explosions and winds,
astronomers expect to find an anticorrelation between the
D/H ratio and metallicity, usually measured as the ratio of
the oxygen or silicon to hydrogen abundance. Observation
of such an anticorrelation would provide strong support
for our ideas of the source of deuterium and general
principles of galactic chemical evolution.
The first measurements of D/H in the ISM were made
with the Copernicus satellite in the 1970s. The best of
these sampled sight lines toward targets out to a distance
of about 200 pc. The data hinted at the possibility of spatial
variability in D/H, but they were also consistent with the
single value of D/H = 1.5 x 10 -5 .
GHRS made the most accurate ISM measurement,
D/H = (1.46 ± 0.05) x 10 -5 , observing the cool star HR1099,
36 pc away. Hubble measurements toward other targets,
all closer than 80 pc, gave similar results. Since Hubble’s
short wavelength limit is about 1150 Å, the only transition
of the H and D lines available is Lyman α. This limits
Hubble deuterium measurements to relatively short sight
lines with modest H column densities, in order that the