From: Amara Graps (Amara.Graps@mpi-hd.mpg.de)
Date: Fri Dec 10 1999 - 01:56:11 MST
This long post is in response to something that Robert wrote right
after Thanksgiving. I needed these weeks to answer the whole thing
piecewise (grabbing pieces of time here and there) and while I looked
some things up in the literature.
From: "Robert J. Bradbury" <bradbury@www.aeiveos.com> Fri, 26 Nov 1999:
>On Fri, 26 Nov 1999, Amara Graps wrote:
>> The ISO observations of the amount of dust in our Galaxy being off by
>> about a factor 100 from what the scientists count (currently) as dust
>> sources is a problem.
>Amara, I presume the ISO observations to which you refer are the
>measurements of IR from diffuse background dust.
Well, thermal emission, and I don't know the source(s) of that
emission.
I'll back up and say why I posted that message originally, a couple of
weeks ago, and what I've learned since then.
The talk I heard at MPI-K several weeks ago was about supernovae as
large dust sources, and the talk ws given by an MPI-K reseracher named
Richard Tuffs. Tuffs presented some equations, models, and ISOPHOT
observational data that said that ISO observations of the amount of
dust in our Galaxy was off by about a factor 100 from what the
scientists count (currently) as dust ources. And his tentative
conclusion was that supernovae are much larger dust producers than
originally thought.
I've not spoken with Tuffs and I've had no luck finding any papers by
him on this specific topic (it's too new, I think), so I don't have
very many details. When I spoke on the morning bus with a couple of
people who are in his group, they told me that it's an "old problem"
that has existed since IRAS and with other infrared measurements and
that ISO has simply confirmed it.
So I went searching through a few IRAS results papers, and I didn't
see any research results that stated this "off by a factor of 100"
problem. But then I'm not actively working in the field now, so I
probably don't know the right references.
[I found this useful paper: "Dust Energetics in the Gas Phases of the
Interstellar Medium: The Origin of the Galactic Large-scale
Far-infrared Emission Observed by IRAS", ApJ 336, 762-779, 1989 Jan
15. It gave some global properties of our Galaxy from the IRAS
far-infrared perspective, but it didn't address the problem of this
current discussion.]
I'm a bit befuddled about this. I've worked a little bit in the past
in infrared astronomy, but not during the last 7 years, so I'm not
aware of the latest research work. What I do know is that ISO suffers
from pretty bad calibration problems (ISOPHOT errors, for example can
easily be ~30%), and I know that the galaxy folks up the hill at the
other Max Planck Institut (fuer Astronomie, home of ISOPHOT) are as
befuddled as I am about Tuffs' results, and so they're interested in
communicating more with his group to find out more. So this is
definitely "research in progress" or perhaps "controversial" and you
should probably contact Tuffs directly to get more info (since I don't
have time now to pursue this).
And this paper might be useful for you (I've not looked it up yet)
G.T. Soifer et al., _Ann Rev of Astro & Astrophyics_ 1987, 25, pg.
187 where IRAS observations of galaxies, including their dust emission
is reviewed by Soifer.
> A few questions:
> - Is this IR *known* to be primarily from the local dust cloud,
> the rest of the galaxy, or some combination of both?
No, definitely not local. These are galactic observations: in our own
galaxy (as much as we can), and in other galaxies. According to my
friends on the bus, this "factor 100" problem is observable in other
galaxies.
> - If from the local cloud, what are the estimates for the
> temperature of the cloud as a whole?
According to Priscilla Frisch's paper: "Dust in the Local Interstellar
Wind", the cloud surrounding the solar system is a fragment of a cloud
complex sweeping past the Sun from the center of "Loop I" (which I
have no idea what that is...). The temperature of the interstellar
cloud surrounding the Solar System is 6900K and the local cloud has a
magnetic field of 1.5-6 microGauss. (Frisch gives a reference for that
temperature as Flynn, G., Vallerga, J., Dalaudier, F., and Gladstone,
G.R., 1998, J. Geophysical Res. 103, 648.)
Note that this is really warm! In the diffuse interstellar medium, and
in HII molecular clouds, the mean temperature is about 10-20K. In the
cores of molecular clouds, the temperatures are about 100K.
The interstellar medium is really a multiphase place. The diffuse ISM
is assumed to consist of spherical dusty gas clouds (radius ~2 pc,
mainly neutral atomic hydrogen) embedded in thin hot (T_g=450,000K)
ionized matter called the "coronal gas" produced by supernova
remnants. The clouds are assumed to consist of cold (T_g=80K) dense
cores and warm (T_g=8000K), less dense envelopes of neutral gas which
is ionized at the outer edge. The clouds are in relatively stable
configurations between these phases. Dust contained in the diffuse
clouds have a dust-to-gas mass ratio of about 0.01. (Dorschner, 2000)
Star formation begins in molecular clouds. Molecular clouds are
roughly factor 10^2-10^3 denser and, as a rule, larger than the clouds
of the diffuse ISM. Giant molecular clouds with diameters exceeding
100 pc and masses of the order 10^6 M_solar are the biggest individual
objects in the Galaxy. Molecular clouds exhibit internal structure.
Cold cores with diameters of 0.5-3 pc and densities of 10^4-10^6
(H-atoms) cm^{-3} are the preferential places, where star formation
sets in. (Dorschner, 2000)
> - Does the article you mention (or other sources) breakdown the dust
> abundance into actual particle masses and *densities*?
> [The densities are important because they impact on the speed
> at which interstellar travel can occur. The current literature
> seems fairly conflicting on whether interstellar probes, particularly
> nanoprobes lacking extensive shielding, can survive gas or dust impacts
> at speeds like 0.1c.]
You mean number densities, I think (in the interplanetary dust
business, we often use a dust grain density of ~2.5 gr/cm^3 in our
model calculations, but I don't think that's what you mean).
It would not be possible to break down the interstellar/interplanetary
dust abundances simply. On the larger scale, the dust in our galaxy
has several distinguishable populations, which are typical of the
environment in when the grains formed or in which they were strongly
modified. Such as:
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)
(these four came from Dorchner, 1986, pg. 487)
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.)
In between the stars, you don't only have dust. You have a plasma,
which is ionized gas in which all or many of the atoms have lost one
or several electrons, or in which molecules were disrupted and atoms
freed. Plasma is the most widespread state of matter in nature and is
sometimes called the "fourth state of matter".
Plasma plays a really dominant role in the Universe. Stars consist of
entirely. Our interplanetary space has a plasma that is dominated by
the Sun's solar wind. Interstellar space is filled with a rarefied
plasma. Intergalactic space is also filled with plasma, but at a much
lower density.
A plasma is influenced by magnetic fields. Even though the fields in
space are weak, they are strong enough, given the low density of the
plasma, to exert large forces on the plasma. Let's assume that the
magnetic field in interstellar space is 10^{-5} Gauss (the Earth's
magnetic field is about 1/2 Gauss) and that the plasma particles have
a temperature (i.e. thermal motion, which can be quite "hot" even
though space is "cold") of 10^4K. If the mag field was not present,
then the atoms would travel about 10^15 cm (equals the mean free path,
which is ~diameter of Pluto's orbit) before encountering another
atom. However with the mag field, the atom (proton say), would move
about 10^7 cm (250 km) before encountering another particle. So the
feeble mag field reduced the freedom of motion of the charged particle
by a factor on the order of millions or billions of times.
On average there is about one atom in every cubic centimeter in
interstellar space. The number density in intergalactic space is about
10^{-6} atoms per cm^3 or about one atom per m^3. (My reference for
these numbers is Alfven)
Small (~10 microns radius and smaller) dust particles are also
influenced (dominated) by the plasma and the magnetic field. The dust
particles are usually charged. The electrostatic potential of dust
grains depends not only on the physical properties of the grain, but
also on plasma parameters such as bulk velocity, number density and
temperature. Dust particles are interesting "tracers" of their
environment, because they are carry and transfer charges between
themselves and the plasma, at the same time of being strongly
influenced by the plasma. (This last paragraph tells you something
about my current research.)
> - After a star forms within a dust cloud, how long does it typically
> remain in the dust cloud, i.e. does it ever orbit out of or
> blow the dust cloud away?
The cloud gets blown away. After 10^5-10^7 yrs (Blitz 1993) a cloud
dissolves by star formation. Stellar winds that accompany a protostar
or early star like a T-Tauri star blows out alot of the gas and extra
dust.
>Also, could you provide a little interpretation of the gas/dust
>ratio value? Is this this an atom[molecule]/dust grain # abundance
>or is it a mass ratio?
I'd have to read more of this paper. I was going to take it on the
plane with me. I really don't know how to interpret Frisch's
Ratio_gas/dust of 94, 551, etc. I am still a "newbie" when it comes to
interstellar medium theories and processes. She gives the ratio from
densities of dust from Galileo/Ulysses in-situ measurements of
interstellar dust grains and the densities of gas from the local
interstellar gas medium. So an "R g/d of 94" equals (n_Hm_H +
n_Hem_He) / N_gal-ulysses where n_H, m_H is the number of H atoms,
mass of H atoms etc. in the gas, and N_gal-ulysses is the
density of interstellar dust = 8.5x10^{-27} gr/cm^3 from the
Galileo/Ulysses interstellar dust measurements.
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 (Helou, 1989).
>It is worth noting that none of the [primary] papers that I have
>seen regarding interstellar colonization and/or the Fermi Paradox
>seems to have been concerned with the hazards posed by interstellar
>gas & dust. I would expect that the gas/dust ratio in the galaxy
>should over time decline. The gas, if dominated by uncharged
>molecules, could not easily be repelled by magnetic methods.
Most of the Universe has a magnetic field, and most of the universe is
in a plasma state ...
>On the other hand if much of the dust carries a charge
>it might be more easily pushed out of the way.
Yes, much of the dust carries a charge.
>So it may be that interstellar travel
>is easy early in galactic history but gets progressively more difficult.
>(Of course the argument could be reversed if the gas hitting you is like
>raindrops, and the dust hitting you is like cannonballs).
I need to think about this...
Amara
P.S. I have to drop this topic for a while as I'm soon beginning a
long travel (a conference and then a badly needed vacation) for most
of the next 3-4 weeks.
-----------
References
-----------
Alfven, H. "Plasma Physics" chapter, in _World-Antiworlds: Antimatter
in Cosmology_, 1966.
Blitz, L. 1993. Giant molecular clouds. In Protostars & Planets III
eds. E. H. Levy and J. I. Lunine (Tucson: Univ. Arizona Press), pp.
125--161}
Dorschner, Johann, "Properties of Interstellar Dust," _Physics,
Chemistry and Dynamics of Interplanetary Dust_, Gustafson, Bo A.S. and
Martha Hanner, ed., , ASP Conference series, Vol 104, 1996, p 487-506.
Dorschner, Johann, "Interstellar Dust and Circumstellar Dust Disks",
from the book: _Interplanetary Dust_, Gruen, E., Gustafson, B.A.,
Dermott, S., ed., in preparation, 1999-2000.
Frisch, P. et al 1999, "Dust in the Local Interstellar Wind,"
ApJ 525, 492-516, Nov 1, 1999.
Helou, G., chapter: "Galactic and Extragalactic Dust," in book:
Allamandola, Lou and A.G.G.M Tielens eds., _IAU Symposium 135:
Interstellar Dust_, 1989, Kluwer Press.
--
***************************************************************
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://galileo.mpi-hd.mpg.de/~graps
***************************************************************
"Never fight an inanimate object." - P. J. O'Rourke
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