Science - 16.08.2019

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SCIENCE

By Stefano Bovino^1 and Daniele Galli^2

T

he reason why astronomers find the

helium hydride ion (HeH+) so intrigu-

ing is simple: This light, highly reactive

molecule with strong acid character

binds together hydrogen and helium,

the first and most abundant elements

in the Universe. The first clear detection of

HeH+ in a nebula, earlier this year, has only

intensified interest in the primordial mole-

cule. On page 676 of this issue, Novotný et al.

( 1 ) studied HeH+ under conditions designed

to mimic those of the early Universe. Their

work offers insight into the chemical compo-

sition of the Universe before formation of the

first stars.

Discovered in the laboratory as early as

1925 ( 2 ), HeH+ has long been suspected to be

present in the interstellar medium. Astrono-

mers engaged in a decades-long search for

HeH+ in interstellar regions that house hot

gas, an environment wherein the simultane-

ous presence of ionized hydrogen and neutral

helium favors HeH+ formation. Unsuccess-

ful attempts have been made in planetary

nebulae (bubbles of hot gas surrounding a

dying star) ( 3 , 4 ), supernova ejecta (material

expanding from star explosion) ( 5 ), and qua-

sars (supermassive galactic objects thought

to contain a black hole) ( 6 ).

This search finally came to an end in April

2019, when HeH+ was unambiguously de-

tected in the hot gas of the planetary nebula

NGC 7027 ( 7 ). In a complementary laboratory

experiment, Novotný et al. collided a beam

of HeH+ with a beam of electrons at a very

low temperature to elucidate the characteris-

tics of HeH+ during the epoch that preceded

formation of the first stars and galaxies (i.e.,

the cosmological “dark ages,” ~400,000 years

after the Big Bang) (see the figure). The low

experimental temperature was used to mimic

conditions of the early Universe.

But when and how did the first molecules

appear in the hot, harsh environment of the

expanding early Universe? Three minutes

after the Big Bang, the Universe consisted

of a mixture of hydrogen, helium, and trace

amounts of deuterium and lithium ions.

These positively charged nuclei progressively

combined with electrons to form neutral at-

oms: first helium and then hydrogen and the

other elements. Because of the expansion of

the Universe, only 99.99% of the hydrogen

ions present after the Big Bang could com-

plete this process. The tiny but crucial left-

over free protons (H+) and electrons (e−) acted

as catalysts for the first gas-phase chemical

reactions among the dominant neutral spe-

cies ( 8 ). This marks the dawn of chemistry,

during which the first-ever molecular bond

was formed to create HeH+.

Compared with the thousands of reac-

tions and the hundreds of species needed to

Despite these developments, the neces-

sary support from experimental data for

the majority of chemical reactions remains

sparse. Important progress was achieved

in two recent laboratory experiments that

solved a long-standing controversy on the

destruction of H 3 + by collisions with elec-

trons ( 11 , 12 ), a crucial reaction for the

chemistry of interstellar clouds. The experi-

ment by Novotný et al. fits perfectly within

these efforts to provide reliable data for

deciphering the chemistry of the Universe.

The authors made use of the newly com-

pleted electrostatic cryogenic ion storage

ring CSR at the Max Planck Institute for

Nuclear Physics, which has made it possible

to measure the efficiency of the destruction

of cold HeH+ by collisions with electrons.

Novotný et al. measured a substantial re-

duction, relative to previous findings ( 13 ),

in the velocity at which this reaction occurs

at low temperatures. This implies a higher

abundance of HeH+ in the early Universe

than previously estimated ( 10 ) and thus a

stronger interaction with the cosmic mi-

crowave background radiation. Further ob-

servational developments might facilitate

detection of HeH+ in the first galaxies, an

important step in reconstructing the chemi-

cal history of the Universe.

Other molecules relevant to astrochemis-

try can now be studied with the new CSR.

These experiments, coupled with newly de-

vised theoretical calculations and other labo-

ratory studies being done worldwide, signal

a bright future for astrochemistry, which is

essential for unraveling complex processes

such as star and planet formation and the

origins of life. j

REFERENCES AND NOTES

1. O. Novotný et al., Science 365 , 676 (2019).


2. T. R. Hogness, E. G. Lunn, Phys. Rev. 26 , 44 (1925).


3. C. Cecchi-Pestellini, A. Dalgarno, Astrophys. J. 413 , 611
   (1993).


4. X. W. Liu et al., Mon. Not. R. Astron. Soc. 290 , L71 (1997).


5. S. Miller et al., Nature 355 , 420 (1992).


6. I. Zinchenko, V. Dubrovich, C. Henkel, Mon. Not. R. Astron.
   Soc. 415 , L78 (2011).


7. R. G ü ste n et al., Nature 568 , 357 (2019).


8. D. Galli, F. Palla, Annu. Rev. Astron. Astrophys. 51 , 163 (2013).


9. D. R. G. Schleicher et al., Astron. Astrophys. 490 , 521 (2008).


10. S. Bovino et al., Astron. Astrophys. 529 , A140 (2011).
    1 1. B. J. McCall et al., Phys. Rev. A 70 , 052716 (2004).


11. H. Kreckel et al., Phys. Rev. Lett. 95 , 263201 (2005).


12. C. Strömholm et al., Phys. Rev. A 54 , 3086 (1996).

ACKNOWLEDGMENTS

We thank D. Schleicher, N. Leigh, and our late colleague F. Palla for

stimulating discussions about early-Universe astrochemistry.

10.1126/science.aay5825

(^1) Departamento de Astronomía, Facultad Ciencias Física y
Matematícas, Universidad de Concepción, Concepción, Chile.
(^2) INAF-Osservatorio Astrofisico di Arcetri, Firenze, Italy.
Email: [email protected]; [email protected]
EARLY UNIVERSE
First molecule still animates astronomers
Helium hydride ion (HeH+)
The quasar specifes an
ideal environment for
HeH+ observation.
Beginning of
“dark ages”
Big Bang
He
H+
Cosmic microwave
background radiation
H
H+
13.8 billion years
Present
Quasar
A study of the helium hydride ion stirs up primordial astrochemistry
INSIGHTS | PERSPECTIVES
Early Universe, first molecules
Hydrogen recombination began after the generation
of cosmic microwave background radiation.
16 AUGUST 2019 • VOL 365 ISSUE 6454 639
model the chemistry of the present-day in-
terstellar medium, an understanding of the
chemistry of the early Universe should be
easily won. Because of the simplicity of the
reactants and the lack of complex phenom-
ena, such as magnetic fields, turbulence,
and stellar radiation, the field of primordial
chemistry has received special attention
and become the test bed for astrochemical
kinetic models that describe the evolution
of chemical species under interstellar me-
dium conditions. The chemistry of helium
has been the focus of several recent investi-
gations ( 9 , 10 ).