From: Amara Graps (Amara.Graps@mpi-hd.mpg.de)
Date: Mon Dec 11 2000 - 15:55:44 MST
Spike Jones wrote:
> Is there a way to calculate an upper bound on the density
> of dust between stars?
>
>Dooooh! Somebody save me from myself. This upper
>bound would require 90% of the matter in the universe
>to be not-hydrogen-or-helium, which is absurd by any
>model Ive heard proposed. I think I recall a good
>argument that the upper bound on the fraction of
>metals in the current epoch is about 1% the mass of
>the hydrogen and helium. So that puts an upper bound
>of about 200 nanograms of interstellarlopers per cubic
>kilometer. Right? Amara? spike
My dear Spike, you are the third person who has asked me that question
in the last several months (the other two being Eric Drexler and
Robert Bradbury). You all have the same goal in mind, I know. Your
approach makes perfect sense to me ... but ..
I think you either need to talk to an interstellar medium specialist
to get an up-to-date and complete answer, or else wait until my thesis
is finished. What I know is pretty basic stuff. I've written it on the
list before, so this will be a repeat.
I understand that if we compare the density of interstellar dust
grains with the density of the interstellar gas, the dust-to-gas
ratio is typically 10^{-2}, which means that a large fraction of
heavy elements must be tied up in dust grains. Most astrophysical
objects such as the Sun, HII regions, etc. seem to have a fairly
standard chemical composition consisting of 73% hydrogen, 25% helium
and 2 per cent of heavier elements by mass. So therefore, the
gas-to-dust (or better the dust-to-gas) ratio can be related
directly to the metallicity and to the history of the interstellar
medium. I don't know very much about how this works though, but G.
Helou at Caltech has written some things about that in the context
of galactic and extragalctic dust. And I don't know more precisely
the numbers for elemental abundances than what I wrote above.
Maybe this paper can say more (I have not read it):
astro-ph/0007314 :
Title: Chemical Abundances in our Galaxy and Other Galaxies
Derived from H II Regions Authors: M. Peimbert, L. Carigi,
A. Peimbert Comments: 10 pages, 2 figures. The evolution of
Galaxies. I- Observational Clues. Conference Proceedings
We discuss the accuracy of the abundance determinations of H
II regions in our Galaxy and other galaxies. We focus on the
main observational constraints derived from abundance
determinations that have implications for models of galactic
chemical evolution: a) the helium to hydrogen abundance
ratio, He/H; b) the oxygen to hydrogen abundance ratio, O/H;
c) the carbon to oxygen abundance ratio, C/O; d) the helium
to oxygen and helium to heavy elements abundance ratios,
Delta Y/ Delta O and Delta Y/ Delta Z; and e) the primordial
helium abundance, Yp. (43kb)
If you spend some time on ADS or the astro-ph using keywords like
"heavy element abundances insterstellar medium", that would be one
good beginning to get more papers like the above. (that's what I would
do to get up to speed on the abundances).
Please note that that there are different dusty regions of space with
different densities and different heavy elemental abundances.
1) stardust
2) dust in the clouds of the diffuse interstellar medium
3) dust in molecular clouds
4) circumstellar dust (young stellar objects or in planetary systems)
I don't know the number densities for all of these, but here are a
couple of rough numbers.
In the diffuse interstellar medium (not in dark clouds) there are
about 10^{-12}- 10^{-13} grains/cm^3 but near a dusty star there will
be far more grains per cm^3. I'm not sure of "how much more" but the
papers on circumstellar dust stars like beta Pic should have this
number. Also, when I calculate the mass of the moon converted into 100
micron radius dust particles of density 3 gr/cm^3 in a spherical
volume of 1AU, I get a number density of 10{-10} cm^3, so that should
be in the ballpark of what you will find in the beta Pic papers. (In
magnetospheres of giant planets like Jupiter and Saturn, the number
density of dust is even greater, especially if you have dust sources
like moons, rings, etc.)
I told both Eric and Robert and this list also about this next paper:
"Dust in the local interstellar wind" (1999)
P. C. Frisch, J. Dorschner, J. Geiss, J. M. Greenberg,
E. Gruen, M. Landgraf, P. Hoppe, A. P. Jones, W. Kraetschmer,
T. J. Linde, G. E. Morfill, W. T. Reach, J. Slavin, J. Svestka,
A. Witt, and G. P. Zank. Astrophysical Journal, 525 , 492-516
Frisch et al's paper does and concludes:
* Elemental abundances and the dust-to-gas ratio are calculated towards
two directions ("column densities") in our local interstellar
environment: towards eps CMa and alpha Sco.
* There is not enough mass "missing" from the gas phase to form the dust
grain population observed by the spacecraft.
* Dust grains are overabundant in the cloud feeding interstellar
grains into the solar system.
* The cloud around the solar system is a weakly depleted cloud with
respect to the refractory elements.
* There is evidence for interstellar shock fronts destroying grains in
the local interstellar cloud (LIC).
* The eps CMA LIC component has no carbon available for the dust, a
manifestation of the "carbon crisis" written in Dwek (1997). The lack
of carbon avail for the interstellar dust grains and the presence of
SiC presolar grains in meteorites may indicate an additional
carbon-rich grain source beyond gas condensation from circumstellar
dust grains.
more pointers:
some interstellar dust references:
http://www.mpi-hd.mpg.de/dustgroup/dune/content/references.htm
Priscilla Frisch's map of our galactic neighborhood:
http://www.mpi-hd.mpg.de/dustgroup/dune/content/map.htm
Oh, yes, Alyssa Goodman has an excellent article in Sky&Telescope
called "Recycling in the Galaxy" (I think that's the title) published
a few months ago. My copy is not at hand, but perhaps she has some
things to say regarding this topic too. Her Web site is
http://cfa-www.harvard.edu/~agoodman/ (I think), and so you might find
some papers on this topic there.
Amara
--
*********************************************************************
Amara Graps | Max-Planck-Institut fuer Kernphysik
Interplanetary Dust Group | Saupfercheckweg 1
+49-6221-516-543 | 69117 Heidelberg, GERMANY
Amara.Graps@mpi-hd.mpg.de * http://www.mpi-hd.mpg.de/dustgroup/~graps
*********************************************************************
"Never fight an inanimate object." - P. J. O'Rourke
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