A package that aims at bringing models of cosmic dust to Python to make working with them easier.
Note: very early development at the moment so it does not yet do too many useful things and the API will change.
- Clone the repo:
git clone git@gitlab.mpcdf.mpg.de:mglatzle/cosmic_dustbox.git cd cosmic_dustboxpython setup.py install
Note: cosmic_dustbox uses astropy.units throughout.
Evaluating the Weingartner & Draine 2001 size distributions for carbonaceous and silicate grains for R_V=3.1 and b_C=6.0e-5 in case 'A' at a range of grain sizes between 1 nm and 1 micron:
>>> import astropy.units as u
>>> import cosmic_dustbox as cb
>>> import numpy as np
>>> sd_car, sd_sil = cb.sdist.WD01(3.1, 6.0, 'A')
>>> sizes = np.logspace(-9, -6, num=10)*u.m
>>> sd_car(sizes)
<Quantity [7.39939133e+00, 9.02469859e-02, 1.94996300e-02, 1.07158763e-03,
1.18483502e-04, 1.30850238e-05, 1.24648409e-06, 9.43835731e-08,
2.41782877e-09, 2.43258206e-15] 1 / m>
>>> sd_sil(sizes)
<Quantity [7.86318728e+000, 6.70676535e-001, 5.73425242e-002,
4.92807748e-003, 4.28146066e-004, 3.80362296e-005,
3.52990312e-006, 3.09011411e-007, 7.19725776e-020,
9.29346597e-263] 1 / m>
In 1930, after confirming that distant stars in the Milky Way are dimmed more strongly than one would expect from geometry, Trumpler conjectured the existence of "dust particles of various sizes" in the interstellar space of our Galaxy. Countless observations have since then confirmed this and the "interstellar extinction" remains one of the observational pillars of the study of cosmic dust. It was later discovered that there is dust also in interplanetary space and that it is important for planet formation, there is dust in other galaxies, even far away galaxies, and maybe there also is dust in intergalactic space. It was confirmed that dust grains do indeed come in different sizes and, moreover, in different shapes and different chemical compositions. In short, while many things have been understood, it also became apparent that cosmic dust is very complicated. The goal of this software package is to make work with models of cosmic dust on the computer easier while still providing control over all the involved intricacies. An ambitious goal for sure, but we'll try our best.
To introduce the objects central to how we describe cosmic dust, assume we are interested in the dust in a certain environment. Assume then that we cut some volume out of said environment. The volume should, of course, be representative of the environment. So if we are interested in the dust in the Solar System as a whole, a cubic meter of space somewhere between Earth and Mars will be too small and a box the size of the Milky Way will be too large. Suppose we collected all of the dust grains contained in this volume. We would like to be able to describe this collection with a reasonable degree of accuracy without having to store information on every single grain. If we investigated the properties of single grains, at first the diversity would probably be overwhelming since hardly any two grains would be exactly alike. But after looking at enough grains, we would start discovering similarities. Grains that have similar structure, grains that look like larger or smaller versions of other grains, grains that have similar chemical composition and so on. So if we are fine with some level of abstraction and generalization, our grain collection could be represented like this:
with color-coded chemical composition. How do we go about categorizing these grains so that we can think in these categories without worrying about individual grains? Two obvious properties to group by are the chemical composition and the shape:
The "blueprint grain", which we can rescale to obtain any of the grains in one of the groups, is referred to as a grain species. So one of our groups is simply a grain species combined with a size distribution and we call it a grain population. If we think back to the abstract representation of our collection of grains, it is not very different from one of the groups. It simply has more than one grain species, each with its associated size distribution. So to keep it simple, we will also call this a grain population.
So, to recap, what we need to describe a collection of grains are:
- grain species
- size distributions
- grain populations
This approach can be refined as needed by simply splitting grain species. In
the extreme, we would have one species per grain and each species would have a
delta-peak size distribution and we would be back to the overwhelming diversity
we started out from. With this package, we aim to translate the approach to
software. The size distribution class in sdist.py is already in a somewhat
advanced state so feel free to check it out.
"Physics of the Interstellar and Intergalactic Medium" by B. Draine provides a good introduction (and much more than that) to cosmic dust.