An international group of astrophysicists, led
by MPE scientists Marta Obolentseva, Alexei Ivlev, Kedron Silsbee, and
Paola Caselli, have revisited the long-standing problem of evaluating
the rate at which cosmic rays ionize gas in the interstellar medium. By
combining available observational data for diffuse molecular clouds with
novel developments in understanding the dust and gas distribution in
these regions and applying numerical modeling, the scientists were able
to compute the cosmic-ray ionization rate (or its upper limit) for a
dozen nearby clouds. They showed that earlier estimates were a factor of
ten too high.
Galactic cosmic rays (CRs) play a crucial role in the evolution of
molecular clouds, governing multiple physical and chemical processes
that accompany practically all stages of star formation. The impact of
CRs on these processes is quantified in terms of the CR ionization rate
(CRIR), which is the number of ionization events produced by CRs per gas
molecule in unit time. The value of this fundamentally important
parameter has been debated by the star formation community for over half
a century. The principal difficulty here originates from the fact that
the CRIR in the interstellar medium is determined by a relatively small
population of non-relativistic CRs. In contrast to the well-constrained
ultra-relativistic population, there are no robust direct methods to
detect such “low-energy” particles in space – nor can they be measured
on Earth, because of their efficient exclusion from the heliosphere by
the Solar wind.
The only direct method to measure the CR energy
spectra and thus to derive the CRIR would be to use spacecraft that are
able to reach beyond the heliosphere. Such measurements have indeed been
performed a decade ago by the Voyager probes 1 and 2 when they crossed
the outmost edge of the heliosphere – the heliopause. Nevertheless, this
unique direct sampling of CRs still represents the very local
interstellar medium in the immediate proximity of the Sun, at a distance
of only about 120 au.
For this reason, indirect methods have been
widely used to estimate the CRIR in numerous molecular clouds
surrounding us in the Milky Way. Such methods typically rely on
measuring light absorption due to specific ions produced by CRs,
accumulated along the line of sight that connects the observer to the
background star (acting as the emission source). Much attention has been
given to absorption observations of H
3+ ions (molecular hydrogen, H
2,
with an extra proton attached), often considered the most reliable
method to measure the CRIR in diffuse molecular clouds – thanks to the
particularly simple formation and destruction routes of these ions and
the fact that they involve the most abundant molecule in the universe, H
2.
It turned out, however, that typical CRIR values inferred from these
measurements are more than a factor of ten higher than those derived
from the Voyager data!
This dramatic discrepancy has been a major
puzzle in the cosmic-ray community over the last decade. At the same
time, the so-called 3D dust extinction maps have changed our
understanding of the three-dimensional dust and gas distribution in the
surrounding molecular clouds. These maps have been constructed using
distances to over a billion stars, based on parallax measurements by the
Gaia satellite. Recently, the high-resolution maps developed by our
neighbors at MPA in the group of Dr. Torsten Enßlin reached sufficient
accuracy to allow a reconstruction of the gas distribution down to
parsec scales. This breakthrough made it possible to identify the
individual clouds where H
3+ absorption actually
occurred in each observation, and thus to pinpoint precisely the
positions of individual CRIR measurements in 3D space.
Motivated by this staggering development, the scientists revisited the analysis of available H
3+
observations. They performed 3D simulations of the identified clouds,
with the CRIR being the only unconstrained parameter of the model. By
comparing their results with observations, this made it possible for the
first time to self-consistently reconstruct the physical structure of
the individual clouds and derive the respective CRIR.
“One of the astonishing outcomes of our analysis is that the re-evaluated values of
CRIR are an order of magnitude lower than the previous estimates, which
actually brings our results into agreement with the CR spectrum
measured by the Voyager probes”, says Alexei Ivlev, one of the main
authors of the study. “While we of course cannot claim the very local
Voyager spectrum to be representative of a typical Galactic spectrum of
CRs, it is certainly no longer an outlier – as it has been considered
for many years.”
“In addition to the impressive results on the
CRIR, this work represents a major step forward in the realism of
astrochemical modeling. These are the first simulations to incorporate
the actual gas density distribution. I anticipate that combining
astrochemical simulations with accurate determinations of the density
structure and the radiation field will result in many more exciting
advances in the coming years”, Kedron Silsbee adds.
The work that has discovered the drastic reduction in the CRIR also led to a
remarkable “byproduct” discovery: it was found that all earlier
estimates of the gas density in diffuse molecular clouds, where the H
3+
measurements are typically conducted, strongly exceed the values
derived from the extinction maps. In order to identify the origin of
this discrepancy, one of the group’s collaborators, Prof. David Neufeld
from the Johns Hopkins University, has revisited the method commonly
used to estimate gas densities. This method is based on observations of
excited rotational states of molecular carbon (C
2) and therefore depends on the rates of C
2
excitation in collisions with gas molecules. It turned out that the
rates assumed for the earlier estimates were considerably lower than the
accurate values obtained recently, with the result that the inferred
densities were too high. In the companion paper led by
David Neufeld, the scientists presented revised gas densities that are now in good agreement with those from the extinction maps.
“Initially, I had been quite skeptical of the lower density estimates
that emerged from the dust extinction maps, because they were
inconsistent with what we thought we knew. But when I looked more
closely at the methods used previously to evaluate the gas density from
observations of C
2, I found that they had yielded density estimates that were far too high”, says Neufeld.
Ultimately, the dramatically reduced gas densities as well as the reduction in the
CRIR have profound and diverse implications. Not only does this affect
the chemical composition of diffuse and translucent molecular clouds,
but also changes the evolution of their physical structure, which
finally has a broad impact on the initial stages of star formation.
“CRs are fundamental ingredients for the dynamical evolution of interstellar
molecular clouds, where stars and planets form, and for chemical
evolution in space, where the precursors of pre-biotic molecules form.
It is thus crucial for astrophysics and astrochemistry to know the CRIR,
making this one of our long-standing scientific goals”, says Paola
Caselli, director at the Center for Astrochemical studies at MPE. “I am
very proud that our study, also involving scientists from MPA and
international colleagues in a truly interdisciplinary effort, has
achieved this goal”, she adds.
These studies also have important
consequences for all available CRIR measurements utilizing various
ionization tracers. The presented results show that a careful
re-evaluation of previously published estimates of CRIR in molecular
clouds would be useful, in particular by considering the recent
revolutionary changes in our understanding of the diffuse gas
distribution in the Milky Way.
