Are internal proper motion measurements a useful 3D constraint?


The Moiré phenomenon, apparent pattern motion that is muuuuuch faster than the actual flow speed.

We commonly use images and kinematic information from spectroscopic observations as constraints for 3D reconstructions of expanding circumstellar nebulae or supernovae. The measurements of the Doppler-velocity are crucial, since in expanding nebulae they can usually be mapped to position along the line of sight (with the caveats that we have discussed in earlier posts and in a recent conference talk).

But kinematic information can also be obtained from measurements of motion in the plane of the sky, yielding the velocity component that is not accessible to spectroscopy. Unfortunately, such proper motion measurements within a nebula are more difficult than their counterparts of the motion of stars on the sky, since the position is more diffuse compared to that of the image of a star.

An second problem is that the measured motion may represent the displacement of a pattern, that has little to do with the actual gas flow (which is what the Doppler-effect delivers in spectroscopic velocity measurements). For instance, such patterns can be shock waves or ionization fronts or combinations of those. Deducing a flow speed that can then be combined in an overall model that includes velocity information from spectroscopy can be tricky. Mellema (2004) has studied the phenomenon to estabilish a quantitative difference between the advancing ionization front and the gas flow.

Another problem may be the superposition of more than one flow due to projection on the sky, creating a mixture of motions that can produce fake patterns, such as the famous Moiré patterns of superimposed systems of lines (see Figure). Mertens & Lobanov (2016) explored how to separate multiple flows in VLBI-observations of relativistic jets from Active Galactic Nuclei (AGN). They used wavelength decomposition to separate a faster smooth flow through the center of the jet (the “spine”), from a more irregular region that is the envelope of the spine.

A similar approach was taken by Riera et al. (2014) to analyze the proper motion in the nebula CRL618 and hydrodynamic simulations that aimed at reproducing the observed proper motion. This approach seemed to have worked well in this object, since it consisted of rather discrete knots that had traveled a substantial angular distance compared to their size.

García-Díaz et al. (2015) compared two different methods to determine the internal proper motion in the planetary nebula NGC 2392 (the “Eskimo Nebula”). They based their analysis on several hundred manually set rectangular regions within Hubble Space Telescope images that where taken with a time separation of about 8 years. Instead of using a wavelet decomposition, they used cross-correlation and least-square methods, similar the one used by Szyszka et al. (2011) for NGC 6302. The difference in the results for the two methods was not huge, but significant if one wants to use proper motion measurements as constraints for 3D-reconstructions.

In observations of filamentary planetary nebulae such as NGC6302 or NGC2392, the issue of superposition of more than one filament (for instance from the front and back walls) is likely to generate locally spurious results. If superimposed filaments move at an angle to each other, non-radial patterns can appear that propagate much faster than the actual flow speed, not unlike the Moiré-phenomenon.

So, until now proper motion analysis in planetary nebulae has been a bit disappointing as a tool for contraining 3D reconstructions. Hopefully new methods can improve this situation. There is a research problem for image process experts, since new approaches to measuring the internal proper motions in astrophysical nebulae need to be introduced.



3D Astrophysics Newsletter


De-projection of the density map shown at different latitudes.

5.2 Today´s new entry:

The 3D structure of the Galactic bulge

M. Zoccali, E. Valenti

Abstract: We review the observational evidences concerning the three-dimensional structure of the Galactic bulge. Although the inner few kpc of our Galaxy are normally referred to as {\it the bulge}, all the observations demonstrate that this region is dominated by a bar, i.e., the bulge is a bar. The bar has a boxy/peanut (X\–shaped) structure in its outer regions, while it seems to become less and less elongated in its innermost region. A thinner and longer structure departing from the main bar has also been found, although the observational evidences that support the scenario of two separate structures has been recently challenged. Metal poor stars ([Fe/H]≲−0.5 dex) trace a different structure, and also have different kinematics.

Journal: Publications of Astronomical Society of Australia, in press
URL of preprint:
Submitted by: Manuela Zoccali

3D Astrophysics Newsletter


Cuts through the structure grid used as the input density distribution for the mocassin model of Abell 14.

5.1 Today´s new entry:

Deciphering the bipolar planetary nebula Abell 14 with 3D ionization and morphological studies

S. Akras , N. Clyne, P. Boumis, H. Monteiro, D. R. Gonçalves , M. P. Redman , S. Williams

Abstract: Abell 14 is a poorly studied object despite being considered a born again planetary nebula. We performed a detailed study of its 3D morphology and ionization structure using the SHAPE and MOCASSIN codes. We found that Abell 14 is a highly evolved, bipolar nebula with a kinematical age of 19,400 yr for a distance of 4 kpc. The high He abundance, and N/O ratio indicate a progenitor of 5 M⊙ that has experienced the third dredge–up and hot bottom burning phases. The stellar parameters of the central source reveal a star at a highly evolved stage near to the white dwarf cooling track, being inconsistent with the born again scenario. The nebula shows unexpectedly strong [N I] λ5200 and [O I] λ6300 emission lines indicating possible shock interactions. Abell 14 appears to be a member of a small group of highly evolved, extreme Type-I PNe. The members of this group lie at the lower-left corner of the PNe regime on the [N II]/Hα vs. [S II]/Hα diagnostic diagram, where shock–excited regions/objects are also placed. The low luminosity of their central stars, in conjunction with the large physical size of the nebulae, result in a very low photo-ionization rate, which can make any contribution of shock interaction easily perceptible, even for small velocities.

Journal: Monthly Notices of the Royal Astronomical Society, in press
URL to preprint:
Submitted by: Stavros Akras


3D Astrophysics Newsletter

Summary and reference updates of recent publications:

4.7 Danehkar, A.; Discovery of Collimated Bipolar Outflows in the Planetary Nebula TH 2-A; The Astrophysical Journal, 815, 1, article 35

4.6 Durech J., Hanus J., Vanco R., Asteroids@home – A BOINC distributed computing project for asteroid shape reconstruction;
URL:  Astronomy and Computing 13 (2015) 80-84

4.5 F.P.A. Vogt, C.I. Owen, L. Verdes-Montenegro, S. Borthakur; Advanced Data Visualization in Astrophysics: the X3D Pathway; The Astrophysical Journal, in press

4.4 K. Gesicki, A. A. Zijlstra, and C. Morisset; 3D pyCloudy modelling of bipolar planetary nebulae: evidence for fast fading of the lobes;  Astronomy & Astrophysics, in press
URL of preprint:

4.3  A. Gómez-Muñoz, M. W. Blanco Cárdenas, R. Vázquez, S. Zavala, P. F. Guillén, S. Ayala;  Morpho-kinematics of the planetary nebula NGC 3242: an analysis beyond its multiple-shell structure;  Monthly Notices of the Royal Astronomical Society, in press
URL of preprint:

4.2 Adam H. Greenberg, Jean-Luc Margot; Improved Algorithms for Radar-Based Reconstruction of Asteroid Shapes; The Astronomical Journal, Volume 150, Issue 4, article id. 114, 10 pp.
URL of preprint:

4.1 Niall Clyne, Stavros Akras, Wolfgang Steffen, Matthew P. Redman, Denise R. Gonçalves, Eamonn Harvey;  A morpho-kinematic and spectroscopic study of the bipolar
nebulae: M 2–9, Mz 3, and Hen 2–104;
URL: Astronomy & Astrophysics, Volume 582, id.A60, 19 pp.