271 research outputs found
Synchrotron emission in molecular cloud cores: the SKA view
Understanding the role of magnetic fields in star-forming regions is of
fundamental importance. In the near future, the exceptional sensitivity of SKA
will offer a unique opportunity to evaluate the magnetic field strength in
molecular clouds and cloud cores through synchrotron emission observations. The
most recent Voyager 1 data, together with Galactic synchrotron emission and
Alpha Magnetic Spectrometer data, constrain the flux of interstellar cosmic-ray
electrons between MeV and GeV, in particular in the
energy range relevant for synchrotron emission in molecular cloud cores at SKA
frequencies. Synchrotron radiation is entirely due to primary cosmic-ray
electrons, the relativistic flux of secondary leptons being completely
negligible. We explore the capability of SKA in detecting synchrotron emission
in two starless molecular cloud cores in the southern hemisphere, B68 and FeSt
1-457, and we find that it will be possible to reach signal-to-noise ratios of
the order of at the lowest frequencies observable by SKA ( MHz)
with one hour of integration.Comment: 5 pages, 4 figures, accepted by Astronomy & Astrophysic
The Resounding Universe
En 1977, la NASA lanzó dos cohetes: Voyager 1 y 2. Oficialmente diseñadas para estudiar los sistemas planetarios de Júpiter y Saturno, las dos sondas aún siguen recolectando una gran cantidad de datos y el 25 de Agosto 2012, Voyager 1 entró en la región inexplorada del espacio conocida como el espacio interestelar. Tanto la Voyager 1 como la 2 llevan consigo un disco fonográfico, el Disco de Oro de Voyager que contiene imágenes, así como ejemplos musicales designados para cualquier forma de vida extraterrestre. La inmensidad del cielo y las estrellas brillantes han inspirado a la humanidad desde tiempos inmemoriales y numerosos científicos fueron apasionados de música o incluso músicos ellos mismos: Galileo, Herschel, Einstein… El objetivo de este artículo es recalcar la conexión peculiar y fascinante entre música y astronomía
Interstellar dust charging in dense molecular clouds: cosmic ray effects
The local cosmic-ray (CR) spectra are calculated for typical characteristic
regions of a cold dense molecular cloud, to investigate two so far neglected
mechanisms of dust charging: collection of suprathermal CR electrons and
protons by grains, and photoelectric emission from grains due to the UV
radiation generated by CRs. The two mechanisms add to the conventional charging
by ambient plasma, produced in the cloud by CRs. We show that the CR-induced
photoemission can dramatically modify the charge distribution function for
submicron grains. We demonstrate the importance of the obtained results for
dust coagulation: While the charging by ambient plasma alone leads to a strong
Coulomb repulsion between grains and inhibits their further coagulation, the
combination with the photoemission provides optimum conditions for the growth
of large dust aggregates in a certain region of the cloud, corresponding to the
densities between cm and
cm. The charging effect of CR is of generic nature, and therefore is
expected to operate not only in dense molecular clouds but also in the upper
layers and the outer parts of protoplanetary discs.Comment: accepted by Ap
Cosmic-ray acceleration in young protostars
The main signature of the interaction between cosmic rays and molecular
clouds is the high ionisation degree. This decreases towards the densest parts
of a cloud, where star formation is expected, because of energy losses and
magnetic effects. However recent observations hint to high levels of ionisation
in protostellar systems, therefore leading to an apparent contradiction that
could be explained by the presence of energetic particles accelerated within
young protostars. Our modelling consists of a set of conditions that has to be
satisfied in order to have an efficient particle acceleration through the
diffusive shock acceleration mechanism. We find that jet shocks can be strong
accelerators of protons which can be boosted up to relativistic energies.
Another possibly efficient acceleration site is located at protostellar
surfaces, where shocks caused by impacting material during the collapse phase
are strong enough to accelerate protons. Our results demonstrate the
possibility of accelerating particles during the early phase of a
proto-Solar-like system and can be used as an argument to support available
observations. The existence of an internal source of energetic particles can
have a strong and unforeseen impact on the star and planet formation process as
well as on the formation of pre-biotic molecules.Comment: Accepted by Astronomy and Astrophysic
Cosmic-ray ionisation in circumstellar discs
Galactic cosmic rays are a ubiquitous source of ionisation of the
interstellar gas, competing with UV and X-ray photons as well as natural
radioactivity in determining the fractional abundance of electrons, ions and
charged dust grains in molecular clouds and circumstellar discs. We model the
propagation of different components of Galactic cosmic rays versus the column
density of the gas. Our study is focussed on the propagation at high densities,
above a few g cm, especially relevant for the inner regions of
collapsing clouds and circumstellar discs. The propagation of primary and
secondary CR particles (protons and heavier nuclei, electrons, positrons, and
photons) is computed in the continuous slowing down approximation, diffusion
approximation, or catastrophic approximation, by adopting a matching procedure
for the different transport regimes. A choice of the proper regime depends on
the nature of the dominant loss process, modelled as continuous or
catastrophic. The CR ionisation rate is determined by CR protons and their
secondary electrons below g cm and by electron/positron
pairs created by photon decay above g cm. We show that a
proper description of the particle transport is essential to compute the
ionisation rate in the latter case, since the electron/positron differential
fluxes depend sensitively on the fluxes of both protons and photons. Our
results show that the CR ionisation rate in high-density environments, like,
e.g., the inner parts of collapsing molecular clouds or the mid-plane of
circumstellar discs, is larger than previously assumed. It does not decline
exponentially with increasing column density, but follows a more complex
behaviour due to the interplay of different processes governing the generation
and propagation of secondary particles.Comment: 19 pages, 11 figures, accepted by A&
Cosmic-ray propagation at small scale: a support for protostellar disc formation
As long as magnetic fields remain frozen into the gas, the magnetic braking
prevents the formation of protostellar discs. This condition is subordinate to
the ionisation fraction characterising the inmost parts of a collapsing cloud.
The ionisation level is established by the number and the energy of the cosmic
rays able to reach these regions. Adopting the method developed in our previous
studies, we computed how cosmic rays are attenuated as a function of column
density and magnetic field strength. We applied our formalism to low- and
high-mass star formation models obtained by numerical simulations of
gravitational collapse that include rotation and turbulence. In general, we
found that the decoupling between gas and magnetic fields, condition allowing
the collapse to go ahead, occurs only when the cosmic-ray attenuation is taken
into account with respect to a calculation in which the cosmic-ray ionisation
rate is kept constant. We also found that the extent of the decoupling zone
also depends on the dust grain size distribution and is larger if large grains
(of radius about 0.1 microns) are formed by compression and coagulation during
cloud collapse. The decoupling region disappears for the high-mass case due to
magnetic field diffusion that is caused by turbulence and that is not included
in the low-mass models. We infer that a simultaneous study of the cosmic-ray
propagation during the cloud's collapse may lead to values of the gas
resistivity in the innermost few hundred AU around a forming protostar that is
higher than generally assumed.Comment: 8 pages, CRISM 2014 conference proceeding
Production of atomic hydrogen by cosmic rays in dark clouds
The presence of small amounts of atomic hydrogen, detected as absorption dips
in the 21 cm line spectrum, is a well-known characteristic of dark clouds. The
abundance of hydrogen atoms measured in the densest regions of molecular clouds
can be only explained by the dissociation of H due to cosmic rays. We want
to assess the role of Galactic cosmic rays in the formation of atomic hydrogen,
by using recent developments in the characterisation of the low-energy spectra
of cosmic rays and advances in the modelling of their propagation in molecular
clouds. We model the attenuation of the interstellar cosmic rays entering a
cloud and compute the dissociation rate of molecular hydrogen due to collisions
with cosmic-ray protons and electrons as well as fast hydrogen atoms. We
compare our results with the available observations. 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 cm. 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, as the one 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. 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.Comment: 7 pages, 7 figures, accepted by Astronomy and Astrophysic
Magnetic Mirroring and Focusing of Cosmic Rays
We study the combined impact of magnetic mirroring and focusing on the
ionization by cosmic rays (CRs) in dense molecular clouds and circumstellar
disks. We show that for effective column densities of up to
cm (where ionization is the main mechanism of energy losses by CRs) the
two effects practically cancel each other out, provided the magnetic field
strength has a single peak along field lines. In this case the ionization rate
at a given location is controlled solely by attenuation of interstellar CRs due
to energy losses. The situation is very different in the presence of magnetic
pockets -- local minima of the field strength, where the CR density and thus
ionization can be reduced drastically. We obtain simple analytical expressions
allowing accurate calculation of the ionization rate in these regions.Comment: Accepted to Ap
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