Typical PN |
Bipolar PN |
I learned a lot this Chilean Summer! In particular, I learned lots about Planetary Nebulae. I really enjoyed it and I am very interested in them now. I had no clue about them before this summer. I set up this page to spread the knowledge I acquired to others in a similar position I was in. I tried to keep jargon and technical terms down to a minimum. I just a few words of caution...
Well... you can click on the links, but they just give you the images. The true purpose of the links is to change the image when you rollover it. It is an attempt to make this page interactive! So try it!
Planetary Nebulae (PNe) were discovered by astronomers in the 1700's. Anything that wasn't a star, planet, or comet appeared as a haze, so these objects were deemed nebulae, the latin word for clouds. The nature of the nebulae was unknown. Some thought they were composed of a luminous fluid while others held the belief that the nebulae were comprised of hundreds of faint stars. In 1785, William Herschel distinguished a set of nebulae that resembled the greenish disk of a planet, he called them Planetary Nebulae. The typical form of nebulae identified by Herschel as PNe were circular with a star at the center. Currently there are many more morphologies. Here are two examples of Planetary Nebulae:
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Typical PN |
Bipolar PN |
Planetary Nebula form at the very late stage of the stellar evolution of most low mass stars (0.8 to 5 Msun). The star sheds mass in the form of expanding spherical shells of gas. As the star loses mass it evolves into a hot white dwarf (WD). The gas is photoionized by the WD and becomes nebulous. The PN will continue to glow until the WD cools to the point where it can no longer sustain photoionization. Then WD and nebula fade away like a dying ember.
Typical PNe are thought to come from stellar winds ejecting mass from a Post AGB star. The material has expansion velocities around 5-25 km/s. The mass is ejected uniformly from the star creating a ring-like pattern. The ring is merely a projection effect-limb brightening of the spherical shell of gas and dust (mass). Well over half of all PNe are this type, however, a significant fraction exhibit bipolarity.
5-15% of all PNe exhibit true bipolar characteristics. They have narrow waists, two lobes, and high expansion velocities. There is more uncertainty in the origin and processes that result in Bipolar Planetary Nebulae. Every imaginable physical process can be made into a model that creates bipolarity. Currently models of Interacting Winds, rapidly rotating stars with strong magnetic fields, and binary nuclei are the strongest. I personally believe that Bipolar PNe have binary systems at their centers. It is difficult to detect binary nuclei because the dust and gas in the center area obscure the central object(s). However, there have been some detections of variability of the center of a few PNe in the IR.
Furthermore, a lot of Bipolar PNe exhibit point and/or plane symmetry as opposed to axisymmetry. Point symmetry means that every blob or bright spot on one lobe can be traced through the center to an identical blob on the other lobe. An explanation of this can be that the mass loss is subject to precession. In which case, a binary system would be a likely choice as source of the precession.
The higher expansion velocities of the material in Bipolar PNe can possibly be attributed to the same process that forms the bipolarity. Thus giving more clues to the progenitors of Bipolar PNe. The velocities of Bipolar PNe are in the neighborhood of 100-200 km/s and some are much much faster!
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Sa 2-237 is the Bipolar Planetary Nebula I have been studying. The image to the left is a false color image comprised of two narrow band filter images. The red color represents the extent of the H-alpha + [NII]658.3nm emission of Sa 2-237, the blue represents the [OIII]500.7nm emission. Strong emission in H-alpha, forbidden [NII], and [OIII] is characteristic of Bipolar PNe. North is toward the top and East is toward the left. The images were acquired on ESO's NTT by Romano Corradi. The star north of the central region is a foreground star. One can immediately identify the point symmetry in the two lobes of Sa 2-237. The two bright regions in the east and west lobes can be connected with a line that goes through the central region. The pinched waist of Sa 2-237 also adds to its classification as a true Bipolar Planetary Nebulae. Spectral data of Bipolar PNe give enormous information. Kinematics of the lobes can be deciphered and the shape can be fitted with models to extract the inclination to the line of sight. The inclination is used to calculate the true deprojected expansion velocities of the material in the lobes. |
The spectra were taken with EMMI on ESO's NTT with an H-alpha filter to select the wavelength region of interest at a slit position angle of 60° to align with the long 'axis' of Sa 2-237. The position angle is chosen to acquire spectra of the fastest moving material, which would logically be at the poles. The three bright lines are, from left to right, [NII] 654.8nm, H-alpha 656.3nm, and [NII] 658.4nm. By Atomic Physics, the two forbidden NII lines' relative intensities are fixed by the factor 3. Usually H-alpha emission dominates nearer the central object, however, further out in the nebula the NII lines are much stronger. This is probably due to density effects. The color was added for effect and has no physical relevance.
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This is the [NII] line on the right of the above spectrum. I have added a blend from blue to red to illustrate the shifts from the rest wavelength. The red areas are redshifted so they are going away from us; the blue areas are blueshifted and coming toward us. Toward the purple regions there are small velocity shifts, in some cases zero. From this information we can infer that the left lobe in the image is going away from us and the lower lobe is coming toward us. The velocities measured from the red and blue shifts should be roughly equal and they give us the expansion velocity of the polar regions. The spectrum was extracted with IRAF packages "twod" and "longslit". The wavelength calibration was performed with thorium lamp spectra through the same H-alpha filter. The velocities determined from the fitting of gaussians to prominent regions that appear in each emission line are presented below. The extreme blue and red shifts were also used to determine the highest velocities. |
The velocites of Bipolar Planetary Nebulae are around 100-200 km/s. By velocity, I mean the speed at which the gas and dust is moving relative to the central object. Similarly to binary star radial velocities, the values for the velocity come with a projection effect due to the inclination of the nebula. However, since the velocities of the gas and dust give Planetary Nebulae their shape, we can model the shape and extract the inclination.
The model we use is the one developed by J. Solf and H. Ulrich to describe the shape of the R Aquarii nebula in an Astronomy and Astrophysics article. The model assumes no acceleration and a velocity that is proportional to the distance of the material from the center. Furthermore, the expansion velocity increases from the equator to the pole. The parameters for the model are the heliocentric systemic velocity, the polar velocity, the equatorial velocity, the inclination, and the age of the nebula divided by the distance. The shape model is written in SuperMongo and plots the image contours. The image is rotated to align the PNe 'axis' with the y-axis of the diagram. Below is the H-alpha image next to the [NII]658.4nm line.
Using well educated guesses we can simultaneously fit the shape and the spectra of Sa 2-237. Since Sa 2-237 clearly exhibits it's bipolarity, we can assume that the inclination is close to 90. The observed velocities from the spectra also give us a roundabout region to begin guessing. The parameters are tweaked until the deprojected expansion velocity matches the polar velocity and the shape and spectra are adaquately fitted. This is tricky because the deprojected expansion velocity depends on the inclination, but the reward is a concrete inclination angle.
The spectral energy distribution(SED) can provide valuable information. By integrating under the curve of the SED we can obtain the bolometric luminosity of Sa 2-237. We gathered the magnitudes and fluxes at given wavelengths from various sources. From these data we calculate the flux lambda then make a SED plot.
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The graph plots the logarithm of the wavelength times the flux and the logarithm of the wavelength. There is a peak in the IR region due to of scattering and re-emission of photons in the IR.
Using the SED of Sa 2-237 and the SED of M 2-9, which has a hard distance, we can compare the two and find a distance for Sa 2-237. This comparative analysis is possible by assuming that all Bipolar PNe are within a certain stage in there life and that the process in each is similar. The choice of a companion is not only governed by the hard distance known to M 2-9 but also by the similarities between the two PNe.
These are the first in-depth observations of Sa 2-237. All the results contained within this section comprise everything that is known about Sa 2-237 as of March 23, 2001.
| Inclination (LoS) |
70° |
| Maximum De-projected Expansion Velocity |
308 km/s |
| Heliocentric Systemic Velocity |
47 km/s |
| Distance |
2.1 kpc |
| Size |
0.37 pc |
| Age |
624 years |
| Bolometric Luminosity |
340 Lsun |
I hope you have enjoyed this page. My advisors, Hugo E. Schwarz and Romano L. M. Corradi, have been instrumental in the development of my knowledge in Planetary Nebula. I previously had no notions of what PNe were, and I had even less knowledge of reducing spectral data.
I find the physical processes in PNe fascinating and hope to continue similar research in the future. New observations that I obtained on Monday, March 19, 2001 will help my goals as well as shed more light on Sa 2-237. The low resolution spectroscopy data will give temperature and density estimates and hopefully more info on the processes governing Bipolar Planetary Nebulae formation. Ciao!
This has been a RUDY! PRODUCTIONS production.
