It might be, although Hubble took some incredibble images in visible of planetary nebulae, so I thought, why not spectra of closer dust disks...

Interesting! So we are sure that there are no optically active ions in the dust, say lanthanides?

Well, this is relatively simple. First such a nebula does not emit light in the visible. Pictures in the visible do show dust which is scattering the light from the central star. Very close to the star you may have some excited ions (hydrogen for example) but this does not represent much. Scattering in itslelf is not very interesting, it just follows a kind of power law spectrum and there is not much to see in termes of spectral features. In the IR, things do change. First you may record some absorption and thus bands linked to water ice or silicates but two problems arise easily: the disk is often optically thick, even in the IR. Thus not much to see. Grains themselves become reasonably large, say more than the wavelngth of the light. Theu just become opaque to their own radiation so that again there is not much to see. Last but not least, these objects are usually faint and difficult to observe.

I agree with you. In order to have emission in visible, in general you need high energy excitation which is usually scattered while in IR you are not seeing atomic but molecular transitions. IR radiation also suffers scattering but strong re-absorption aswell.

Since I am currently working in lanthanides, I was thinking about their unique property to be excited in IR and upconverting light i.e. emitting in visible upon IR excitation (and also in IR). Solid grains surely contain lanthanides but the problem is again scattering. On the other hand, lanthanides exposed directly, closer to the star can be excited both in visible and IR while they can transfer their radiation to farther neighbours in IR range. Those farther ions can again emit in both IR and visible and so on. This way, the energy is a bit more preserved than when you only have downconversion in which radiation is either re-absorbed at the same IR wavelength or re-emitted at lower energies. Would we be able to see anything of that I can not know since I haven't found the data.

Lanthanides?? Aren't they like atomic nos. ~60 - 70 ??

Dust is low-Z metals, CNO, Si & H's attached. You'd have to find a protoplanetary disk made of dust formed from the ejecta of a whole cluster of SNs to get a detectable amount of heavier stuff into the dust. More likely for anything like solar or typical ISM abundances, not enough lanthanides in the whole dust disk to fill a shoebox.

The shortest the wavelength, the thickest (optically thick disk) is. So, information will come only from the edges and will be limited because the dust grains are larger than in the ISM, they scatter a lot and are optically thick to themselves so that not much information is expected. Besides, they are not many instruments available right now for disks that are discovered recently. We need to wait for JWST (MIRI) or SPICA a bit later, the later one being still not yet fully decided.

Allan, the Earth has much more lanthanides than you believe. How was the Earth formed then? Where do the lanthanides we have on Earth come from if not from the dust in protoplanetary disk? On the other hand, it is true that in general the longer wavelength you observe, farther you see into the disk because of scattering and down-conversion; i.e.you need short wavelengths/high energy for atomic ionization and excitation which is later re-emitted at lower energy/longer wavelengths which now excite not atoms but molecules deeper in the disk, farther from the star.

But a very interesting property of lanthanides used a lot lately is their ability of up-conversion, absorbing a couple of photons in IR to emit 1 photon at shorter wavelengths. This way, a part of shorter wavelength emission could be conserved, although of low intensity. The concentrations of lanthanides as impurities in dust that are dealt with in other applications in laboratory experiments can be 0.1mol% or lower which is pretty low concentration for materials of high molar mass. I agree we should wait for the future missions in order to observe that but I believe we can prepare the field performing lab experiments simulating those conditions in the lab.

Why is this interesting to do? Because if we manage to detect them, they can be used as remote sensors for various conditions, as it is started to be applied in the industry.

I am looking for an atomic astrophysics and spectroscopy group who would be willing to participate in this kind of novel experiments.

Well, Martina, be careful not to compare the Earth with dust grains in protoplanetary disks. Dust grains, not processed (yet) by accretion and planetary processes will have, altogether, cosmic abundances for their elements. So look at the cosmic abundances of lanthanides compared to abundant elements in dust grains (say Si, Mg, Fe). You might be disappointed. The Erath's crust has nothing to do with cosmic abundances. Besides, if ther is some light emission (actually certainly line emission at given wavelengths) from some lanthanides within a protoplanetary disk, these phoons will not escape the nebula, they will be reabsorbed. Anyway, I will have a look to lanthanides properties...just not to miss a priori something. Finally, remember a good principle to be used in astrophysics, may be more than anywhere elese: Occam's razor.

a (very) quick survey of the wikipedia shows that Lanthanides are much more abundant in the crust of the Earth than in meteorites (a factor of 100 at least). Reasons are based on geochemical evebts that take place on a planet but not in parent bodies of meteorites.

I agree with you. I know it is a bit far-fetched. Although we should be aware that the intensity of the emission from elements excited in a range of wavelengths is not directly proportional to its abundance.

Some observers look for near-IR line veiling. The hot inner disk will emit continuum light in the near-IR that will then "fill in" the protostar's near-IR spectral lines, lowering the line/continuum ratio. Line veiling measurements are difficult to undertake, though, so there is no large catalog available. We also use visible light to tell us about the disks, even though we don't observe disk emission directly in the visible. For example, measurements of the 10% width of the H-alpha line can be used to figure out the accretion rate onto the star. Measured accretion rates are used to inform disk models.

Thank you Sarah, very interesting! I wish I would find somebody in Europe doing these things!