169 research outputs found
Los convenios eclesiásticos entre las iglesias protestantes y los estados federales de Alemania: su fundamentación jurídica
Shattering by turbulence as a production source of very small grains
The origin of grain size distribution in the interstellar medium is one of
the most fundamental problems in the interstellar physics. In the Milky Way,
smaller grains are more abundant in number, but their origins are not
necessarily specified and quantified. One of the most efficient drivers of
small grain production is interstellar turbulence, in which dust grains can
acquire relative velocities large enough to be shattered. Applying the
framework of shattering developed in previous papers, we show that small (a\la
0.01~\micron) grains reach the abundance level observed in the Milky Way in
yr (i.e. within the grain lifetime) by shattering in warm neutral
medium. We also show that if part of grains experience additional shattering in
warm ionized medium, carbonaceous grains with a\sim 0.01~\micron are
redistributed into smaller sizes. This could explain the relative enhancement
of very small carbonaceous grains with --100 \AA. Our theory also
explains the ubiquitous association between large grains and very small grains
naturally. Some tests for our theory are proposed in terms of the metallicity
dependence.Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letter
Impact of grain size distributions on the dust enrichment in high-redshift quasars
In high-redshift () quasars, a large amount of dust (\textstyle\sim
10^{8} \mathrm{M}_{\sun}) has been observed. In order to explain the large
dust content, we focus on a possibility that grain growth by the accretion of
heavy elements is the dominant dust source. We adopt a chemical evolution model
applicable to nearby galaxies but utilize parameters adequate to high-
quasars. It is assumed that metals and dust are predominantly ejected by Type
II supernovae (SNe). We have found that grain growth strongly depends on the
grain size distribution. If we simply use the size distribution of grains
ejected from SNe, grain growth is inefficient because of the lack of small
grains (i.e.\ small surface-to-volume ratio of the dust grains). However, if we
take small grain production by interstellar shattering into consideration,
grain growth is efficient enough to account for the rich dust abundance in
high- quasars. Our results not only confirm that grain growth is necessary
to explain the large amount of dust in high- quasars, but also demonstrate
that grain size distributions have a critical impact on grain growth
Synthesized grain size distribution in the interstellar medium
We examine a synthetic way of constructing the grain size distribution in the
interstellar medium (ISM). First we formulate a synthetic grain size
distribution composed of three grain size distributions processed with the
following mechanisms that govern the grain size distribution in the Milky Way:
(i) grain growth by accretion and coagulation in dense clouds, (ii) supernova
shock destruction by sputtering in diffuse ISM, and (iii) shattering driven by
turbulence in diffuse ISM. Then, we examine if the observational grain size
distribution in the Milky Way (called MRN) is successfully synthesized or not.
We find that the three components actually synthesize the MRN grain size
distribution in the sense that the deficiency of small grains by (i) and (ii)
is compensated by the production of small grains by (iii). The fraction of each
{contribution} to the total grain processing of (i), (ii), and (iii) (i.e., the
relative importance of the three {contributions} to all grain processing
mechanisms) is 30-50%, 20-40%, and 10-40%, respectively. We also show that the
Milky Way extinction curve is reproduced with the synthetic grain size
distributions.Comment: 10 pages, 6 figures, accepted for publication in Earth, Planets, and
Spac
Evolution of dust grain size distribution by shattering in the interstellar medium: robustness and uncertainty
Shattering of dust grains in the interstellar medium is a viable mechanism of
small grain production in galaxies. We examine the robustness or uncertainty in
the theoretical predictions of shattering. We identify (the critical
pressure above which the deformation destroys the original lattice structures)
as the most important quantity in determining the timescale of small grain
production, and confirm that the same ( is the duration of
shattering) gives the same grain size distribution [, where is the
grain radius] after shattering within a factor of 3. The uncertainty in the
fraction of shocked material that is eventually ejected as fragments causes
uncertainties in by a factor of 1.3 and 1.6 for silicate and
carbonaceous dust, respectively. The size distribution of shattered fragments
have minor effects as long as the power index of the fragment size distribution
is less than ~ 3.5, since the slope of grain size distribution
continuously change by shattering and becomes consistent with . The grain velocities as a function of grain radius can have an
imprint in the grain size distribution especially for carbonaceous dust. We
also show that the formulation of shattering can be simplified without losing
sufficient precision.Comment: 12 pages, 7 figures, Accepted for publication in Earth, Planets, and
Space (Special Issue: Cosmic Dust V
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