First ALMA Maps of Cosmic-Ray Ionization Rate in High-mass Star-forming Regions

Diffuse interstellar clouds show large abundances of H_3^+ which can be maintained only by a high ionization rate of H_2. Cosmic rays are the dominant ionization mechanism in this environment, so the large ionization rate implies a high cosmic-ray flux, and a large amount of energy residing in cosmic rays. In this paper we find that the standard propagated cosmic-ray spectrum predicts an ionization rate much lower than that inferred from H_3^+. Low-energy (~10 MeV) cosmic rays are the most efficient at ionizing hydrogen, but cannot be directly detected; consequently, an otherwise unobservable enhancement of the low-energy cosmic-ray flux offers a plausible explanation for the H_3^+ results. Beyond ionization, cosmic rays also interact with the interstellar medium by spalling atomic nuclei and exciting atomic nuclear states. These processes produce the light elements Li, Be, and B, as well as gamma-ray lines. To test the consequences of an enhanced low-energy cosmic-ray flux, we adopt two physically-motivated cosmic-ray spectra which by construction reproduce the ionization rate inferred in diffuse clouds, and investigate the implications of these spectra on dense cloud ionization rates, light element abundances, gamma-ray fluxes, and energetics. One spectrum proposed here provides an explanation for the high ionization rate seen in diffuse clouds while still appearing to be broadly consistent with other observables, but the shape of this spectrum suggests that supernovae remnants may not be the predominant accelerators of low-energy cosmic rays.

Observations of OH+ are used to infer the interstellar cosmic ray ionization rate in diffuse atomic clouds, thereby constraining the propagation of cosmic rays through and the shielding by interstellar clouds, as well as the low energy cosmic ray spectrum. In regions where the H2-to-H number density ratio is low, dissociative recombination (DR) is the dominant destruction process for OH+ and the DR rate coefficient is important for predicting the OH+ abundance and inferring the cosmic ray ionization rate. We have experimentally studied DR of electronically and vibrationally relaxed OH+ in its lowest rotational levels, using an electron–ion merged-beams setup at the Cryogenic Storage Ring. From these measurements, we have derived a kinetic temperature rate coefficient applicable to diffuse cloud chemical models, i.e., for OH+ in its electronic, vibrational, and rotational ground level. At typical diffuse cloud temperatures, our kinetic temperature rate coefficient is a factor of ∼5 times larger than the previous experimentally derived value and a factor of ∼33 times larger than the value calculated by theory. Our combined experimental and modeling results point to a significant increase for the cosmic ray ionization rate inferred from observations of OH+ and H2O+, corresponding to a geometric mean of (6.6 ± 1.0) × 10−16 s−1, which is more than a factor of 2 larger than the previously inferred values of the cosmic ray ionization rate in diffuse atomic clouds. Combined with observations of diffuse and dense molecular clouds, these findings indicate a greater degree of cosmic ray shielding in interstellar clouds than has been previously inferred.

Low energy cosmic rays are the major ionization agents of molecular clouds. However, it has been shown that, if the cosmic ray spectrum measured by Voyager 1 is representative of the whole Galaxy, the predicted ionization rate in diffuse clouds fails to reproduce data by 1-2 orders of magnitude, implying that an additional source of ionization must exist. One of the solutions proposed to explain this discrepancy is based on the existence of an unknown low energy (in the range 1 keV-1 MeV, not probed by Voyager) cosmic ray component, called carrot when first hypothesized by Reeves and collaborators in the seventies. Here we investigate the energetic required by such scenario. We show that the power needed to maintain such low energy component is comparable of even larger than that needed to explain the entire observed cosmic ray spectrum. Moreover, if the interstellar turbulent magnetic field has to sustain a carrot, through second-order Fermi acceleration, the required turbulence level would be definitely too large compared to the one expected at the scale resonant with such low energy particles. Our study basically rules out all the plausible sources of a cosmic ray carrot, thus making such hidden component unlikely to be an appealing and viable source of ionization in molecular clouds.

Low energy cosmic rays (up to the GeV energy domain) play a crucial role in the physics and chemistry of the densest phase of the interstellar medium. Unlike interstellar ionising radiation, they can penetrate large column densities of gas, and reach molecular cloud cores. By maintaining there a small but not negligible gas ionisation fraction, they dictate the coupling between the plasma and the magnetic field, which in turn affects the dynamical evolution of clouds and impacts on the process of star and planet formation. The cosmic-ray ionisation of molecular hydrogen in interstellar clouds also drives the rich interstellar chemistry revealed by observations of spectral lines in a broad region of the electromagnetic spectrum, spanning from the submillimetre to the visual band. Some recent developments in various branches of astrophysics provide us with an unprecedented view on low energy cosmic rays. Accurate measurements and constraints on the intensity of such particles are now available both for the very local interstellar medium and for distant interstellar clouds. The interpretation of these recent data is currently debated, and the emerging picture calls for a reassessment of the scenario invoked to describe the origin and/or the transport of low energy cosmic rays in the Galaxy.

view Abstract Citations (2) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Multiple ionization by low-energy cosmic rays and the abundance of high ionized interstellar atoms. Watson, W. D. Abstract Calculations are presented and employed in conjunction with available experimental data to estimate cross sections for multiple ionization of atoms by low-energy (MeV/nucleon) cosmic rays. The efficiency of multiple ionization increases rapidly with the charge of the cosmic ray, so that heavier (Z at least 6) nuclei are the dominant contributors for cosmic-ray compositions deduced from higher-energy cosmic rays. Multiple ionization dominates in the production of certain highly ionized atoms by cosmic rays under typical interstellar conditions. N v is considered explicitly, and its production rate by low-energy cosmic rays is estimated to be enhanced by a factor of about 5 to 100. For a low-energy cosmic-ray flux of the proposed intensity for heating and ionizing the interstellar gas, the predicted abundance for N v will be near or above the observed limit if previous calculations are otherwise correct. Publication: The Astrophysical Journal Pub Date: February 1976 DOI: 10.1086/154149 Bibcode: 1976ApJ...204...47W Keywords: Abundance; Atomic Excitations; Cosmic Rays; Gas Ionization; Interstellar Gas; Ion Production Rates; Ionization Cross Sections; Particle Interactions; Radiant Heating; Astrophysics full text sources ADS |

Calculated spectral profiles and galactic distributions are presented for y-ray lines resulting from interactions between low energy cosmic rays and the interstellar gas and dust. Calculated local intensities are also presented for y-ray lines from discrete sources such as supernova remnants and dense interstellar gas clouds. The y-ray lines from excited dust nuclei (which have long mean lifetimes) are sharp, having widths of the order of a few keV; the lines from excited gas nuclei are relatively narrow, having widths of the order of 100 keV; and the lines from excited cosmic ray nuclei are broad, having widths of the order of 1 MeV. The longitudinal distribution of y-ray lines in the galactic plane shows a significant concentration toward the galactic centre, and a rapid falloff beyond I;. 50�. The most intense y-ray lines arise from positron annihilation (0�511 MeV) and the deexcitation of 12C* (4�439 MeV) and 160* (6�131 MeV). In the direction of the galactic centre, these lines have estimated intensities of the order of 10-5 photons cm-2s-1rad- 1, and so they may be resolved from the diffuse y-ray background there by observing with a high resolution Ge(Li) detector. In the direction of several strong discrete sources, the estimated fluxes are generally lower: ~10-6 photons cm-2s-1 for the Crab Nebula and the Vela pulsar, ~10-8 photons cm-2 s-1 for the interstellar dense cloud pOph, but ~10-5 photons cm-2 s-1 for the ring cloud around the galactic centre. The calculated intensities of various other y-ray lines are compared with available experimental data, and their detectability is considered. The implication of the galactic distribution of low energy cosmic rays for the gas density of the interstellar space through which the cosmic rays propagate is also discussed.

Low-energy cosmic rays (LECRs) play a crucial role in the formation of planetary systems, and detecting and reconstructing the properties of early LECRs is essential for understanding the mechanisms of planetary system formation. Given that LECRs interact with the surrounding medium to produce nuclear de-excitation line emissions, which are gamma-ray emissions with energy mainly within 0.1–10 MeV and are unaffected by stellar wind modulation, these emissions can accurately reflect the properties of LECRs. This study introduces an innovative method for using gamma-ray emissions to infer LECR properties. We employed the Parker transport equation to simulate the propagation and spectral evolution of LECRs in a protoplanetary disk and calculated the characteristic gamma-ray emissions resulting from interactions between LECRs and disk material. These gamma-ray emissions encapsulate the spectral information of LECRs, providing a powerful tool to reconstruct the cosmic ray environment at that time. This method, supported by further theoretical developments and observations, will fundamentally enhance our understanding of the impact of CRs on the origin and evolution of planetary systems and address significant scientific questions regarding the cosmic ray environment at the origin of life.

view Abstract Citations (23) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The isotopic composition of the anomalous low-energy cosmic rays Mewaldt, R. A. ; Spalding, J. D. ; Stone, E. C. Abstract Measurements of the isotopes of low energy cosmic ray He, C, N, O and Ne nuclei are analyzed, in concert with previous data, to examine relationships between galactic cosmic rays (GCR) and anomalous cosmic rays (ACR), which could arise from either extragalactic or interstellar sources. A ground-based heavy isotope spectrometer telescope was used for the survey, along with IMP-7 and IMP-8 data and ISEE-3 data. The data were recorded from 1973-78. Spectral and abundance data are provided for each element. Fragmentation was absent in the ACR data, unlike the GCR data. Differences were also observed in the Ne-22/Ne-20 ratios of ACR and GCR. It is suggested, therefore, that the low energy ACR isotopes have an interstellar origin and are useful for studies of the local interstellar medium. Publication: The Astrophysical Journal Pub Date: August 1984 DOI: 10.1086/162324 Bibcode: 1984ApJ...283..450M Keywords: Energy Spectra; Galactic Cosmic Rays; Interstellar Matter; Isotopes; Abundance; Anomalies; Particle Energy; Stellar Winds; Space Radiation full text sources ADS |

Low-energy cosmic rays are a fundamental source of ionization for molecular clouds, influencing their chemical, thermal and dynamical evolution. The purpose of this work is to explore the possibility that a low-energy component of cosmic-rays, not directly measurable from the Earth, can account for the discrepancy between the ionization rate measured in diffuse and dense interstellar clouds. We collect the most recent experimental and theoretical data on the cross sections for the production of H2+ and He+ by electron and proton impact, and we discuss the available constraints on the cosmic-ray fluxes in the local interstellar medium. Starting from different extrapolations at low energies of the demodulated cosmic-ray proton and electron spectra, we compute the propagated spectra in molecular clouds in the continuous slowing-down approximation taking into account all the relevant energy loss processes. The theoretical value of the cosmic-ray ionization rate as a function of the column density of traversed matter is in agreement with the observational data only if either the flux of cosmic-ray electrons or of protons increases at low energies. The most successful models are characterized by a significant (or even dominant) contribution of the electron component to the ionization rate, in agreement with previous suggestions. However, the large spread of cosmic-ray ionization rates inferred from chemical models of molecular cloud cores remains to be explained. Available data combined with simple propagation models support the existence of a low-energy component (below about 100 MeV) of cosmic-ray electrons or protons responsible for the ionization of molecular cloud cores and dense protostellar envelopes.

view Abstract Citations (135) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Light Isotopes, Extinct Radioisotopes, and Gamma-Ray Lines from Low-Energy Cosmic-Ray Interactions Ramaty, Reuven ; Kozlovsky, Benzion ; Lingenfelter, Richard E. Abstract We have carried out detailed calculations of gamma-ray line, light isotope, and extinct radioactive isotope production by low-energy cosmic rays for a broad range of accelerated particle abundances and energy spectra. We first apply these calculations to the gamma-ray line emission observed from the Orion molecular cloud complex. These gamma rays have provided the most direct evidence for the presence of large fluxes of low-energy cosmic rays in the Galaxy. Next we show that the bulk of the Galactic boron, beryllium, and 6Li could have been produced over the age of the Galaxy by interactions of low-energy cosmic rays having typical particle energies around 30 MeV nucleon-1 Such cosmic-ray interactions can account for the observed meteoritic 11B/10B which is higher than the isotopic ratio predicted by relativistic cosmic-ray interactions. Low- energy cosmic rays of similar energies could have also made a significant contribution to the production of 41Ca in meteoritic calcium-aluminum inclusions (CAIs) formed in the early solar system, provided that the cosmic-ray irradiation and CAl formation periods were essentially cotemporal. However, the production of significant amounts of 26Al by low-energy cosmic rays in the early solar system is unlikely because it would overproduce 9Be and 6Li. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176677 Bibcode: 1996ApJ...456..525R Keywords: ACCELERATION OF PARTICLES; ISM: COSMIC RAYS; GALAXY: ABUNDANCES; GAMMA RAYS: THEORY; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; ABUNDANCES; SOLAR SYSTEM: FORMATION; STARS: SUPERNOVAE: GENERAL full text sources ADS |

The production rate of protons of energy less than 1 BeV in interstellar space is calculated from empirical cross sections for proton-proton elastic and inelastic collisions in the kinetic energy range 1 to 30 BeV. Representing the cosmic ray intensity by a simple fit to the observed spectrum in the absence of solar activity the production spectrum is found to decrease steadily from zero energy to a value one- tenth as great at 500 MeV. The integrated production rate of 0-500 MeV protons is 10-26 cm-3 sec-1 from elastic collisions, and the contribution from inelastic collisions is estimated to be of lower order. The production spectrum is combined with the slowing lifetimes of protons in the interstellar gas (Meskys and Milford, Bull. Am. Phys. Soc. 8, 305, 1963) to give the density of these 0-500 MeV protons as 10-li cm-3 with an intensity of 10-i cm-2 sec-1. Recently Hayakawa, Nishimura, and Takayanagi (Publ. Astron. Soc. Japan 13, 184, 1961) have suggested that low-energy cosmic rays contribute appreciably to the heating of interstellar gas clouds, with an assumed low-energy cosmic ray intensity of order 100 cm-2 sec-1. From the above calculations it may be seen that high-energy collisions can not supply any significant fraction of such a hypothesized high intensity. Future measurements of low-energy interstellar cosmic rays will indicate what fraction of these can be attributed to the collision mechanism discussed above, and what fraction must be due to other mechanisms, such as stellar injection and interstellar magnetic field acceleration.

We study the capability of low-energy cosmic rays (CR) to penetrate into diffuse clouds when they move from the hot ionized plasma to a cool cloud embedded in that plasma. The spectrum of CR inside a cloud can be remarkably different from the the one present in the hot interstellar medium because when CRs pass through a dense cloud of matter, they suffer energy losses due to ionization and nuclear interactions. Hence there is a net flux of CRs towards the cloud that can excite Alfven waves. In turn, self-excited Alfven waves enhances the diffusion of CRs near the edge of the cloud, forcing CRs to spend more time in this layer and increasing the amount of energy losses. The final effect is that the flux of CR entering into the cloud is strongly suppressed below an energy threshold whose value depends on ambient parameters. For the first time we use the full kinetic theory to describe this problem, coupling CRs and Alfven waves through the streaming instability, and including the damping of the waves due to ion-neutral friction and the CR energy losses due to ionization and pion production. Differently from previous approaches, this method allow us to describe the full CR spectrum and the magnetic turbulence spectrum in the transition region between hot interstellar medium and the cloud. For the typical size and density of diffuse clouds in the Galaxy, we find that the flux of particles entering the cloud is strongly reduced below E ∼ 10−100 MeV, while for larger energies the CR spectrum remain unaltered.

Low-energy cosmic rays (CRs) are confined by self-generated MHD waves in the mostly neutral interstellar medium. We show that the CR transport equation can be expressed as a continuity equation for the CR number density involving an effective convection velocity. Assuming a balance between wave growth and ion-neutral damping, this equation gives a steady state condition ncr ∝ n up to a critical density for free streaming. This relation naturally accounts for the heretofore unexplained difference in CR ionization rates derived for dense diffuse clouds (McCall et al.) and dark clouds, and predicts large spatial variations in the CR heating rate and pressure.

Recently, the gravitational instability (GI) model of giant planet and brown dwarf formation has been revisited and recast into what is often referred to as the "tidal downsizing" hypothesis. The fragmentation of self-gravitating protostellar discs into gravitationally bound embryos - with masses of a few to tens of Jupiter masses, at semi major axes above 30 - 40 au - is followed by grain sedimentation inside the embryo, radial migration towards the central star and tidal disruption of the embryo's upper layers. The properties of the resultant object depends sensitively on the timescales upon which each process occurs. Therefore, GI followed by tidal downsizing can theoretically produce objects spanning a large mass range, from terrestrial planets to giant planets and brown dwarfs. Whether such objects can be formed in practice, and what proportions of the observed population they would represent, requires a more involved statistical analysis. We present a simple population synthesis model of star and planet formation via GI and tidal downsizing. We couple a semi-analytic model of protostellar disc evolution to analytic calculations of fragmentation, initial embryo mass, grain growth and sedimentation, embryo migration and tidal disruption. While there are key pieces of physics yet to be incorporated, it represents a first step towards a mature statistical model of GI and tidal downsizing as a mode of star and planet formation. We show results from four runs of the population synthesis model, varying the opacity law and the strength of migration, as well as investigating the effect of disc truncation during the fragmentation process. Our early results suggest that GI plus tidal downsizing is not the principal mode of planet formation, but remains an excellent means of forming gas giant planets, brown dwarfs and low mass stars at large semimajor axes. (Abridged)

view Abstract Citations (73) References (86) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Magnetic Turbulence in Interstellar Clouds Myers, P. C. ; Khersonsky, V. K. Abstract Observations of diffuse, dark, and giant molecular clouds and their cores are analyzed to determine properties of their turbulent motions. Estimates of characteristic cloud internal density, external extinction, and external radiation field intensity are used to deduce the electron fraction chie due to both photoionization and cosmic rays. This ionization fraction exceeds that due to cosmic rays alone, by factors approximately 5 for dark cloud cores to approximately 4000 for giant molecular clouds with embedded OB stars. Estimates of characteristic cloud size, density, velocity dispersion, ionization fraction, and magnetic field strength then indicate that four diagnostic numbers exceed unity by a significant factor: the Reynolds number, the magnetic Reynolds number, the Hartmann number, and the 'wave coupling number,' or ratio of cloud size to minimum hydromagnetic wavelength. These results indicate that virtually all observed interstellar clouds have strong coupling between the magnetic field and the neutral gas, through ion-neutral collisions, even if the field is weaker than its equipartition value. This coupling allows energetically significant magnetohydrodynamic (MHD) waves to propagate above cutoff, so that MHD waves, chaotic motions, and clumpy density structure are probably more pervasive in interstellar clouds than would be expected from cosmic-ray ionization alone. This strong coupling implies that the timescale for ambipolar diffusion is at least approximately 107 yr for low-mass cores, and is at least approximately 108 yr for the gas around cores. These timescales may be too long for all of the mass in a low-mass core to condense via ambipolar diffusion. The observed velocity dispersion is strongly correlated with the estimated electron fraction, according to the power law nu approximately chie p, with p approximately = 0.3. This trend, and those already known among velocity dispersion, size, and density, suggest that increasing extinction may influence the structure of cloud density and velocity dispersion by driving a cycle of decreasing ionization, decreasing MHD wave activity, decreasing velocity dispersion, and increasing density. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175434 Bibcode: 1995ApJ...442..186M Keywords: Interstellar Magnetic Fields; Ionization; Magnetic Flux; Magnetohydrodynamic Waves; Molecular Clouds; Turbulence; Abundance; Ambipolar Diffusion; Density Distribution; Photoionization; Reynolds Number; Velocity Distribution; Astrophysics; ISM: CLOUDS; ISM: MAGNETIC FIELDS; ISM: MOLECULES; MAGNETOHYDRODYNAMICS: MHD; TURBULENCE; WAVES full text sources ADS | data products SIMBAD (7)