Stellar cosmic rays as an important source of ionisation in protoplanetary disks: a disk mass dependent process

Abstract We assess the ionising effect of low energy protostellar cosmic rays in protoplanetary disks around a young solar mass star for a wide range of disk parameters. We assume a source of low energy cosmic rays located close to the young star which travel diffusively through the protoplanetary disk. We use observationally inferred values from nearby star-forming regions for the total disk mass and the radial density profile. We investigate the influence of varying the disk mass within the observed scatter for a solar mass star. We find that for a large range of disk masses and density profiles that protoplanetary disks are “optically thin” to low energy (∼3 GeV) cosmic rays. At R ∼ 10 au, for all of the disks that we consider (Mdisk = 6.0 × 10−4 − 2.4 × 10−2M⊙), the ionisation rate due to low energy stellar cosmic rays is larger than that expected from unmodulated galactic cosmic rays. This is in contrast to our previous results which assumed a much denser disk which may be appropriate for a more embedded source. At R ∼ 70 au, the ionisation rate due to stellar cosmic rays dominates in ∼50% of the disks. These are the less massive disks with less steep density profiles. At this radius there is at least an order of magnitude difference in the ionisation rate between the least and most massive disk that we consider. Our results indicate, for a wide range of disk masses, that low energy stellar cosmic rays provide an important source of ionisation at the disk midplane at large radii (∼70 au).

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The role of stellar cosmic rays in protoplanetary disk evolution

10.5194/epsc2020-20 ◽

2020 ◽

Author(s):

Donna Rodgers-Lee ◽

Andrew Taylor ◽

Turlough Downes ◽

Tom Ray

Keyword(s):

Cosmic Rays ◽

Organic Molecules ◽

Protoplanetary Disks ◽

Protoplanetary Disk ◽

X Rays ◽

Solar Mass ◽

Density Profiles ◽

Disk Mass ◽

Solar Type

The role of magnetic fields in the evolution and dispersal of protoplanetary disks remains unclear to date partially due to the uncertainty regarding the sources of ionisation present in protoplanetary disks. Magnetic fields can only influence protoplanetary disk dynamics if the disks are sufficiently ionised. Ionisation due to X-rays, FUV photons and radioactivity is well-studied and generally only leads to high levels of ionisation close to the young star and in the surface layers of protoplanetary disks due to high disk column densities. Here I will instead focus on the importance of stellar cosmic rays which may provide a source of ionisation for the outer regions, and closer to the midplane, of protoplanetary disks. Young solar-type stars are very magnetically active and drive stronger stellar winds in comparison to the present day Sun. The increased magnetic activity of young solar-type stars suggests that they are efficient ~GeV particle accelerators producing so-called stellar cosmic rays. Thus, protoplanetary disks are likely to be bombarded by stellar cosmic rays, influencing their chemical and dynamic evolution. These incident particles are believed to trigger the formation of complex organic molecules. Thus, they are essential to advance our understanding of how organic molecules, the building blocks of life in the Universe, form. Recent ALMA observations have provided a number of tantalising clues as to the possible importance of stellar cosmic rays in protoplanetary disks. On the one hand, chemical modelling of observations of TW Hya&#8217;s protoplanetary disk suggest that the overall ionisation rate is remarkably low. While on the other hand, ALMA observations have been used to infer the presence of significant turbulent motion in DM Tau&#8217;s protoplanetary disk. This turbulent motion is likely driven by the magneto-rotational instability which would require a much higher level of ionisation than was inferred in TW Hya&#8217;s disk for instance. I will discuss the potential influence of stellar cosmic rays in these disks.&#160; More generally, I will present recent results which investigated the propagation, and ionising effect, of stellar cosmic rays in protoplanetary disks around young solar-mass stars. Unlike X-rays and FUV photons, stellar cosmic rays may effectively avoid being attenuated by the high column densities in the inner regions of protoplanetary disks due to their diffusive transport. To construct our disk density profiles, we use observationally inferred values from nearby star-forming regions for the total disk mass and the radial density profile. By varying the disk mass within the observed scatter for a solar-mass star, we find for a large range of disk masses and density profiles that protoplanetary disks are &#8220;optically thin&#8221; to low energy stellar cosmic rays. I will describe how our results indicate, for a wide range of disk masses, that low energy stellar cosmic rays provide an important source of ionisation at the disk midplane at large radii (&#8764;70 au). Finally, I will discuss the type of systems where we expect that stellar cosmic rays are likely to be most influential.&#160;

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A reevaluation of the atmospheric pressure dependence of secondary cosmic-ray neutrons in the context of Cosmic-Ray Neutron Sensing

10.5194/egusphere-egu21-10602 ◽

2021 ◽

Author(s):

Jannis Weimar ◽

Paul Schattan ◽

Martin Schrön ◽

Markus Köhli ◽

Rebecca Gugerli ◽

...

Keyword(s):

Cosmic Rays ◽

Hydrogen Content ◽

Atmospheric Pressure ◽

Cosmic Ray ◽

Galactic Cosmic Rays ◽

Atmospheric Air ◽

Neutron Intensity ◽

The Earth ◽

Wide Range ◽

Primary Cosmic Rays

Secondary cosmic-ray neutrons may be effectively used as a proxy for environmental hydrogen content at the hectare scale. These neutrons are generated mostly in the upper layers of the atmosphere within particle showers induced by galactic cosmic rays and other secondary particles. Below 15 km altitude their intensity declines as primary cosmic rays become less abundant and the generated neutrons are attenuated by the atmospheric air. At the earth surface, the intensity of secondary cosmic-ray neutrons heavily depends on their attenuation within the atmosphere, i.e. the amount of air the neutrons and their precursors pass through. Local atmospheric pressure measurements present an effective means to account for the varying neutron attenuation potential of the atmospheric air column above the neutron sensor. Pressure variations possess the second largest impact on the above-ground epithermal neutron intensity. Thus, using epithermal neutrons to infer environmental hydrogen content requires precise knowledge on how to correct for atmospheric pressure changes.We conducted several short-term field experiments in saturated environments and at different altitudes, i.e. different pressure states to observe the neutron intensity pressure relation over a wide range of pressure values. Moreover, we used long-term measurements above glaciers in order to monitor the local dependence of neutron intensities and pressure in a pressure range typically found in Cosmic-Ray Neutron Sensing. The results are presented along with a broad Monte Carlo simulation campaign using MCNP 6. In these simulations, primary cosmic rays are released above the earth atmosphere at different cut-off rigidities capturing the whole evolution of cosmic-ray neutrons from generation to attenuation and annihilation. The simulated and experimentally derived pressure relation of cosmic-ray neutrons is compared to those of similar studies and assessed in the light of an appropriate atmospheric pressure correction for Cosmic-Ray Neutron Sensing.

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LiBeB Production by Low Energy Galactic Cosmic Rays

The Light Element Abundances - ESO Astrophysics Symposia ◽

10.1007/978-3-540-49169-9_61 ◽

1995 ◽

pp. 382-388 ◽

Cited By ~ 2

Author(s):

Hubert Reeves ◽

Nikos Prantzos

Keyword(s):

Cosmic Rays ◽

Galactic Cosmic Rays ◽

Low Energy

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Ionization of protoplanetary disks by galactic cosmic rays, solar protons, and supernova remnants

Geoscience Frontiers ◽

10.1016/j.gsf.2016.06.010 ◽

2017 ◽

Vol 8 (2) ◽

pp. 247-252 ◽

Cited By ~ 2

Author(s):

Ryuho Kataoka ◽

Tatsuhiko Sato

Keyword(s):

Cosmic Rays ◽

Supernova Remnants ◽

Protoplanetary Disks ◽

Galactic Cosmic Rays

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Low-Energy Break in the Spectrum of Galactic Cosmic Rays

Physical Review Letters ◽

10.1103/physrevlett.108.051105 ◽

2012 ◽

Vol 108 (5) ◽

Cited By ~ 44

Author(s):

A. Neronov ◽

D. V. Semikoz ◽

A. M. Taylor

Keyword(s):

Cosmic Rays ◽

Galactic Cosmic Rays ◽

Low Energy

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The Shklovsky Paradox

Symposium - International Astronomical Union ◽

10.1017/s007418090017041x ◽

1993 ◽

Vol 155 ◽

pp. 179-179

Author(s):

David Buckley ◽

Stephen E. Schneider ◽

David Van Blerkom

Keyword(s):

Estimation Method ◽

High Mass ◽

Constant Density ◽

Solar Mass ◽

Distance Method ◽

Density Profiles ◽

Wide Range ◽

Energy Conserving ◽

Mass Dispersion ◽

Simple Mass

Shklovsky estimated distances to planetary nebulae (PNs) based on an assumed constant ionized mass and the relationship between flux and radius under the assumption of a constant density, fully ionized shell. He found that a mass of ∼0.2 M⊙ yielded the best results. Estimates of the ionized masses of PNs with independently determined distances also rarely exceed a few tenths of a solar mass. This is surprising since many PNs are thought to derive from high mass progenitors (up to 8 M⊙). Recent optical work (Plait and Soker, 1990) and our own computer simulations show that this simple mass estimation method may severely underestimate the total ionized mass. This is because of a halo of lower density ionized material which often contributes only a small fraction of the PN luminosity even though it may contain many times the mass of the dense inner shell. The precipitous drop in surface brightness (both optical and radio) beyond the inner part of the ionized shell also lead to underestimates of the PN's actual ionized radius. Since the evolution of PNs is driven by the expansion of the nebular shell coupled with the evolution of the nucleus (PNN), we ran several simulations using a simple momentum conserving two-wind model as well as employing density profiles derived by more sophisticated energy conserving models, with a wide range of wind parameters and using two different models of PNN evolution. From our simulations (assuming a 4 M⊙ progenitor) we derive an apparent “Shklovsky Mass” – defined as the ionized mass that would be derived from the observationally determined fluxes and radii of our model PNs. While the total ionized masses and Strömgren radii of the model PNs varied widely depending on the PNN and wind parameters, the derived Shklovsky mass consistently remained below one solar mass. This result is almost independent of the total ionized mass or the mass of the progenitor envelope and is fairly insensitive to the wind parameters chosen as input to the models. The observed spread of masses (based on an error analysis of the work of Gathier, 1987) is similar to the mass dispersion in our models for PNs of moderate age. This may explain why the Shklovsky distance method has been found to agree well with kinematic distances (Schneider and Terzian, 1983) even though the fundamental assumptions may be inappropriate for the nebula as a whole.

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Modulation of low-energy galactic cosmic rays over solar maximum (Cycle 20)

Journal of Geophysical Research Atmospheres ◽

10.1029/ja077i034p06881 ◽

1972 ◽

Vol 77 (34) ◽

pp. 6881-6885 ◽

Cited By ~ 15

Author(s):

M. A. I. Van Hollebeke ◽

J. R. Wang ◽

F. B. McDonald

Keyword(s):

Cosmic Rays ◽

Solar Maximum ◽

Galactic Cosmic Rays ◽

Low Energy

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Production of atomic hydrogen by cosmic rays in dark clouds

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834008 ◽

2018 ◽

Vol 619 ◽

pp. A144 ◽

Cited By ~ 12

Author(s):

Marco Padovani ◽

Daniele Galli ◽

Alexei V. Ivlev ◽

Paola Caselli ◽

Andrea Ferrara

Keyword(s):

Cosmic Rays ◽

Atomic Hydrogen ◽

Molecular Hydrogen ◽

Molecular Clouds ◽

Cosmic Ray ◽

Galactic Cosmic Rays ◽

Dissociation Rate ◽

Low Energy ◽

Hydrogen Atoms ◽

Dark Clouds

Context. Small amounts of atomic hydrogen, detected as absorption dips in the 21 cm line spectrum, are a well-known characteristic of dark clouds. The abundance of hydrogen atoms measured in the densest regions of molecular clouds can only be explained by the dissociation of H2 by cosmic rays. Aims. We wish to assess the role of Galactic cosmic rays in the formation of atomic hydrogen, for which we use recent developments in the characterisation of the low-energy spectra of cosmic rays and advances in the modelling of their propagation in molecular clouds. Methods. We modelled the attenuation of the interstellar cosmic rays that enter a cloud and computed the dissociation rate of molecular hydrogen that is due to collisions with cosmic-ray protons and electrons as well as fast hydrogen atoms. We compared our results with the available observations. Results. The cosmic-ray dissociation rate is entirely determined by secondary electrons produced in primary ionisation collisions. These secondary particles constitute the only source of atomic hydrogen at column densities above ~1021 cm−2. We also find that the dissociation rate decreases with column density, while the ratio between the dissociation and ionisation rates varies between about 0.6 and 0.7. From comparison with observations, we conclude that a relatively flat spectrum of interstellar cosmic-ray protons, such as suggested by the most recent Voyager 1 data, can only provide a lower bound for the observed atomic hydrogen fraction. An enhanced spectrum of low-energy protons is needed to explain most of the observations. Conclusions. Our findings show that a careful description of molecular hydrogen dissociation by cosmic rays can explain the abundance of atomic hydrogen in dark clouds. An accurate characterisation of this process at high densities is crucial for understanding the chemical evolution of star-forming regions.

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