Between the stars (was: Why Would Aliens Hide? )

Amara Graps (
Fri, 10 Dec 1999 09:56:11 +0100

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" <> 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...


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.


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 *  
      "Never fight an inanimate object." - P. J. O'Rourke