This figure illustrates the spatial distribution of gas density obtained from the high-resolution 3D dust extinction map. Shown is the cross section of one of molecular clouds where the CRIR was measured, the line of sight toward a background star is indicated by the dashed line. The individual pixels reflect the 1-parsec spatial resolution provided by the map. © M. Obolontseva et al.
Shown are re-evaluated values of the CRIR (ζH2 , blue bullets) obtained from the analysis of observations toward different stars (indicated by their HD catalogue numbers), and earlier results for these sight lines. The gray curve represents the CRIR derived from the Voyager data. The horizontal axis indicates the gas column density of molecular clouds where the CRIR was measured. © M. Obolontseva et al.
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 H3+ ions (molecular hydrogen, H2, 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, H2. 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 H3+ 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 H3+ 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 H3+ 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 (C2) and therefore depends on the rates of C2 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 C2, 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.
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 H3+ ions (molecular hydrogen, H2, 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, H2. 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 H3+ 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 H3+ 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 H3+ 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 (C2) and therefore depends on the rates of C2 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 C2, 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.
Contact:
Priv. Doz. Dr. habil. Alexei Ivlev
scientist in CAS group
Tel:++49 89 30000-3356
ivlev@mpe.mpg.de
Max Planck Institute for extraterrestrial Physics , Garching
Marta Obolentseva
PhD-student CAS group
marta@mpe.mpg.de
Max Planck Institute for extraterrestrial Physics
Prof. Dr. Paola Caselli
Director of the CAS group at MPE
Tel:+49 89 30000-3400
Fax:+49 89 30000-3399
caselli@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics, Garching
Original Publications
1. M. Obolentseva, A. Ivlev et al.
Re-evaluation of the cosmic-ray ionization rate in diffuse clouds
The Astrophysical Journal (2024), Vol. 973, A142
DOI
2. D. Neufeld, D. Welty, A. Ivlev et al.
The densities in diffuse and translucent molecular clouds: estimates from observations of C2 and from 3-dimensional extinction maps
The Astrophysical Journal 2024, Vol. 973, A143
DOI
Further Information
Cosmic rays in molecular gas
One of the principal aims of the CAS-Theory group is to understand the physics of low-energy CRs in molecular gas, by combining advanced methods of the kinetic theory and plasma physics and applying available observational constraints. More
3. Edenhofer, C. Zucker, P. Fran et al.
A parsec-scale Galactic 3D dust map out to 1.25 kpc from the Sun★
A&A, Vol. 685, A82 (2024)
Source | DOI
JWST sheds Light on the Journey of Cosmic Icy Grains
July 04, 2024
Using the JWST, a team of researchers including Paola Caselli and Michela Giuliano from MPE, have probed deep into dense cloud cores, revealing details of interstellar ice that were previously unobservable. The study focuses on the Chamaeleon I region, using JWST’s NIRCam to measure spectroscopic lines towards hundreds of stars behind the cloud. More
