Planetary Nebulae: Observations of the Early Phases

Planetary Nebulae: Observations of the Early Phases
I have always associated the name Bob O’Dell with the Helix Nebula, the planetary nebula featured in this background. A little over four years ago, I had the pleasure of working with Bob on this image of the Helix Nebula which we took with the Hubble Space Telescope during the Leonids meteor shower which was thought to endanger HST if it faced into, or even near, the stream. When released to the public, this HST image took on a life of its own and is known in the internet culture as the “Eye of God.” My esteemed colleague, You-Hua Chu, will talk to you about the Helix and other planetary nebula. My talk will cover the major evolutionary factors that create a planetary nebula from an intermediate mass star, even though it will be admittedly biased towards covering my own research. Margaret Meixner STScI

This H-R diagram illustrates the late stages of stellar evolution for stars between solar masses and outlines the evolutionary path of a star that becomes a planetary nebula. As these stars ascend the asymptotic giant branch or AGB they loose mass at tremendous rates from 10^-7 to 10^-3 Solar masses per year- that is orders of magnitude higher than the solar wind. These winds are thought to begin as a gentle breeze, which I will call the AGB wind, and then the mass loss rates increase in a superwind phase at the end of the AGB. These massive stellar winds enshroud the star in a dusty, molecular rich cocoon and they appear to us like IRC In this picture, its extensive circumstellar shell is illuminated by the interstellar radiation field. This massive wind exhausts the outer envelope of the star and drives the star from the AGB through the short-lived Proto-planetary nebula or PPN phase. The Egg Nebula, shown here in its reflection nebulosity, is the best studied PPN. During the ~1000 years of the PPN phase, the ejected circumstellar environment under goes a transformation. We have morphological transformations occuring turning circumstellar shells from spherically symmetric to axisymmetric geometries. We also have a radiative evolution of the nebula. The hardening radiation field of the central star first photo-dissociates and then photo-ionizes the initially molecular envelope. At some, unknown point during the PPN phase, a fast wind with high velocities will develop and shock the circumstellar molecular gas. When the central star becomes hot enough to photoionize its circumstellar environment, the object appears as a planetary nebula, or PN, such as the Helix nebula. Most of the work I will present focuses on PPN because we have known the least about these objects and because the PPN circumstellar shell offers us a fossil record of the entire AGB mass loss history in a pristine form.

At this stage in his career, Bob O’Dell, or Mr
At this stage in his career, Bob O’Dell, or Mr. Helix, is fittingly where the Helix Nebula would be, still quite luminous, but perhaps about to turn the corner into the fading glory of his later years. This H-R diagram illustrates the late stages of stellar evolution for stars between solar masses. As these stars ascend the asymptotic giant branch or AGB they loose mass at tremendous rates from 10^-7 to 10^-3 Solar masses per year- that is orders of magnitude higher than the solar wind. These winds are thought to begin as a gentle breeze, which I will call the AGB wind, and then the mass loss rates increase in a superwind phase at the end of the AGB. These massive stellar winds enshroud the star in a dusty, molecular rich cocoon and they appear to us like IRC In this picture, its extensive circumstellar shell is illuminated by the interstellar radiation field. This massive wind exhausts the outer envelope of the star and drives the star from the AGB through the short-lived Proto-planetary nebula or PPN phase. The Egg Nebula, shown here in its reflection nebulosity, is the best studied PPN. During the ~1000 years of the PPN phase, the ejected circumstellar environment under goes a transformation. We have morphological transformations occuring turning circumstellar shells from spherically symmetric to axisymmetric geometries. The hardening radiation field of the central star first photo-dissociates and then photo-ionizes the initially molecular envelope. At some, unknown point during the PPN phase, a fast wind with high velocities will develop and shock the circumstellar molecular gas. When the central star becomes hot enough to photoionize its circumstellar environment, the object appears as a planetary nebula, or PN, such as the Helix nebula. Most of the work I will present focuses on PPN because we have known the least about these objects and because the PPN circumstellar shell offers us a fossil record of the entire AGB mass loss history in a pristine form.

Motivations for CO J=1-0 Best tracer of molecular gas mass
AGB mass loss is predominantly molecular Kinematics in proto-planetary nebulae (0.5 km/s) Toughest molecule w.r.t. photodissociation => probes outer regions There have been a large number of single dish CO surveys of evolved stars and millimeter interferometry in CO of these sources. Why do we use CO? First it is the best tracer of molecular gas mass because the H2 molecule which is the most abundant does not have a dipole moment and does not shine at the low temperatures and densities found in the evolved stars. So if you want to study the mass or mass loss rate, then you need to image it in CO 2. Second, the AGB mass loss is predominantly molecular 3. To study the kinematics of ppn, for which we do not have ionized gas lines. The velocity resolution for CO is exquisite. As with all molecules, the radius of CO emission is determined by photodissociation by the ISRF; however,CO is the toughest molecule and thus it probes the outer regions the best CO

Observations Sample of 13 evolved stars: AGB, PPN, PN
Millimeter interferometry with the BIMA array Combined with single dish maps from the NRAO 12 m Full synthesis maps provide Spectral resolution 1 km/s, coverage 128 km/s 0.5" to 200" scale sizes

Vexp V0 Vexp Vexp Vexp Jy V0+Vexp V0-Vexp V0 V0-Vexp V0 V0+Vexp
I will be showing a number of channel maps for this data set and this slide shows you how to interpret them.. V0+Vexp V0+Vexp V0-Vexp V0

IRC+10216: BIMA & NRAO 12 m CO J=1-0 (Fong, Meixner & Shah 2003)
This combined single dish and BIMA data set on IRC reveals the simple signature for expansion. a much larger envelope than the HCN because CO is much more resilient to photodissociation.

IRC+10216 If you integrate over the angular extent of the envelope, the line profile would appear like a perfect parabola. A position-velocity slice in any direction of the envelope would show this ellipsoidal shape expected for a spherical expansion. Fong, Meixner et al. 2006

Egg Nebula / AFGL 2688 Fong, Meixner et al. 2006
The line profile also differs showing high velocity wings and a self-absorption dip on the blue-shifted wing. The position velocity cuts along the bipolar outflow axes show the twisted shape of a bipolar outflow superposed on an expanding envelope. Fong, Meixner et al. 2006

Egg Nebula / AFGL 2688 Fong, Meixner et al. 2006
The Egg Nebula a.k.a. AFGL 2688 is the best studied proto-planetary nebula, and is a carbon rich, DUPLEX PPN. The channel maps here are similar to the two previous objects in that we see evidence for an expanding envelope. However, in contrast to the two previous objects we also see clear evidence for quadrapolar of higher velocities than the simple expansion. Fong, Meixner et al. 2006

NGC 7027 The line profile of all the flux (bottom) shows the expected parabolic profile of an expanding, AGB remnant envelope and the top line profile shows what you get when you look only at the central core region. The position velocity diagrams differ from the previous objects in that there is a central hole, and that the inner shell is an expanding ellipsoid, not a sphere. Like the Egg Nebula, there are high velocity gas showing a bipolar out flow form. Fong, Meixner et al. 2006

NGC 7027 NGC 7027 is one of the best studied, carbon rich, young planetary nebulae. These channel maps reveal an ellipsoidal inner shell, surrounded by a spherical outer shell, epanding. At the higher velocities we find evidence for higher velocity, collimated gas. Fong, Meixner et al. 2006

BIMA CO Survey: Fong, Meixner, Sutton, Zalucha and Welch 2006
After I left Berkeley and started as an assistant professor at the University of Illinois, they expanded the BIMA array to upto 10 dishes and it became possible to do surveys, such as the one pursued by my student David Fong, myself and others, including Jack. This HR diagram shows the properties of the 10 stars that we surveyed with BIMA along with the many others we culled from the literature all of which reported the spatial distribution of the molecular gas in these sources. The overall theme for this project was to investigate the radiative and dynamical evolution of the molecular envelope of evolved stars. One of the results showed that there are three types of kinematic signatures that we called spherical, disk sources or structured outflow sources. The Spherial sources reveal simple expansion, much like IRC The Disk sources appear to have gravitationally bound, keplerian disks like the Red Rectangle. The Structured outflow sources have simple expansion signatures, punctuated by structured bipolar outflows that appear to be shaping the nebula, such as AFGL 718.

BIMA survey: Radiative evolution with time
Fong, Meixner et al. 2006

This cartoon of a planetary nebula shows the physical processes that can convert the molecular gas into atomic gas. On the interior, the central star emits UV radiation which photoionizes the inner edge creating an HII region, behind that only FUV radiation penetrates creating a photodissociation region or PDR which has neutral atomic gas. Surrounding the PDR is the untouched molecular AGB or superwind circumstellar shells. In the outer regions, the insterstellar radiation field can photodissociate the envelope creating an atomic zone. In addition to these radiative processes, there is the dynamical process of the central star’s fast wind shocking the circumstellar envelope which can also convert molecular to atomic gas. Meixner 2001

NGC 7027 BIMA: Fong et al. 2001 WIYN/NIRIM H , [FeII], Br g
The planetary nebula, NGC 7027, reveals exactly this type of structure. The BIMA image shows the outer molecular envelope. The interior structure is shown by this set of images taken with my near-infrared camera, NIRIM, at the WIYN telescope. The central star is surrounded by ionized gas shown by the Br-Gamma line, and the photodissociation region is shown by the [FeII] and the molecular hydrogen line. H , [FeII], Br g 2 Meixner 2001

To study the neutral atomic gas component in evolved stars, we used the Infrared Space Observatory’s Short wavelength spectrometer, SWS, and Long wavelength spectrometer, LWS, to detect the far-infrared atomic fine structure lines which are the dominant cooling lines of PDRs and shocked-gas regions. These lines include SiII 35 um, CII 158 um, OI 63 and 146 um. Both the large LWS beam, which is essentially this entire field of view and the SWS beam outlined here in red covers the central region. (Fong, Meixner et al. 2001)

This H-R diagram shows our sample and some results
This H-R diagram shows our sample and some results. We have an equal number of carbon rich (C-rich) sources shown in squares and oxygen rich (O-rich) sources shown in circles. The red line shows an evolutionary track for a star destined to become a 0.6 Msun white dwarf. The open symbols are undetected sources. The filled sources have been detected in at least one far-infrared line. Besides unusual sources, like Betelgeuse, which has an active chromosphere, and Mira, which has a hot companion, AGB stars are undetected in these lines which means that the interstellar radiation field is not an important contributor to these lines. In fact, we do not start detecting these lines until the central star reaches a temperature of K shown by the Red Rectangle. All stars with higher effective temperatures are detected. This trend is consistent with PDRs being the dominant process. (Fong,Meixner et al. 2001)

Our ISO study included fabry-perot spectra to derive kinematics for the atomic gas. This plot shows the linewidths of the FIR atomic lines compared to the CO line widths. The CO line widths represent the velocity of the expanding circumstellar shell. If the FIR atomic lines arise in PDRs they should have comparable line widths to the CO lines where as if these FIR lines arise in shocked-gas regions their line widths should be significantly larger. This black line represents a one-to-one correspondance for the line widths and the fact that most of the data cluster around this line indicates a PDR origin. The line fit to the NGC 7027 OI line shows such a PDR line. There was one source, AFGL 618, which showed evidence for a PDR and a shocked gas component. The line core is best fit by a PDR and the broad line intense wings arise in the shocked gas region. (Fong,Meixner et al. 2001)

Object C-rich: AFGL 618 0.01 0.006 0.006 IRAS 21282 0.1 0.04 0.04
NGC NGC O-rich: M Hb NGC This table lists the objects in order of increasing evolutionary state, top is carbon rich and bottom is oxygen rich. This column shows the neutral atomic gas mass, this column shows the ratio of this mass to the total mass of the system which includes atomic, ionized and molecular gas mass. The last column shows the … (Meixner et al. 2002)

Spherical AGB Shell: IRC+10216
Axisymmetric PN: NGC 7027 I will now shift gears and talk about the cirumstellar dust shells of proto-planetary nebulae. This image shows the morphological transformation between the spherically symmetric AGB shells and the distinctly axisymmetric planetary nebula. We have imaged PPN dust shells to understand when this transformation takes places. But in a larger view, what we are really trying to do is probe the mass loss history of these objects. Left is a deep exposure optical image of an AGB star, IRC , showing spherically symmetric nature of AGB mass loss by the concentric circumstellar shells. Right is a composite HST image of a PN, NGC 7027, showing the circumstellar environment of more evolved star. This image shows remnants of spherical AGB wind shells in the exterior. Besides those, there is something that is not spherically symmetric in the interior. This axisymmetric structure must have emerged sometime between the AGB phase and the PN phase, which is the PPN phase. Therefore, by observing PPN shells we can figure out how shell morphology evolves at these evolutionary stages. Moreover, the PPN phase is actually an ideal phase in which to study such issues. B + V Deep Exposure FoV ~ AU (Mauron & Huggins 1999) HST Two-Color Composite FoV ~ AU (Bond et al. 1996)

Probing the Mass Loss History
B-G type Post-AGB Dust Shell Rmin Rmax Back in time The PPN dust shell offers us a fossil record of the AGB mass loss history. As the star looses mass it coasts away into the ISM. Assuming a constant velocity, the material that is located furthest from the central star was lost further back in time than the material close to the central star. (Ueta 2002)

Probing the Mass Loss History
B-G type Post-AGB Rmin Rmax PPN shells preserve AGB mass loss history dust grain properties In fact, PPN shells PRESERVE the AGB mass loss history and dust grain property at the time when they were created. So, by observing PPN shells we can reconstruct the AGB mass loss history that tells us how the shell morphology evolved. Then, how do we observe PPN shells? (Ueta 2002)

Observing PPN Dust Shells
B-G type Post-AGB Say, this is a dust grain in the shell, and mostly optical radiation from the central star hits the grain. (Ueta 2002)

Absorption Scattering Such radiation is either scattered or absorbed by the grain. (Ueta 2002)

Optical Reflection Nebulosity! Scattering If scattering happens, we recognize the PPN shells as optical reflection nebulosities. On the other hand, if absorption happens such absorbed emission heats the grain which emits thermal radiation, and we recognize the PPN shells as thermal emission regions. So, there are basically two ways to observe the PPN shells and we used both methods. Thermal Emission Region! Re-emission (Ueta 2002)

Mid-IR PPN Dual Morphology
Toroidal PPNs Large Emission Core Two Emission Peaks as Dust Torus Core/elliptical PPNs Small Emission Core Large Emission Halo Here are some examples. The first type, toroidal type PPNs, are characterized by the large emission core that show two emission peaks that represent limb-brightened edges of a dust torus oriented close to edge-on. The other type, core/elliptical type PPNs, are characterized by a small emission core surrounded by relatively larger emission halo. Meixner et al. 1999

Optical PPN Dual Morphology
Star Obvious Low-level Elongated PPNs D Ust Prominent Longitudinally E Xtended PPNs Here are some examples. The first type, star obvious low-level elongated or SOLE PPNs, are characterized by faint, smooth, and elliptically elongated nebulosity surrounding the brightly visible central star. On the other hand, the second type, dust prominent longitudinally extended or DUPLEX PPNs are characterized by classic bipolar lobes separated by a dust lane. Now, if we combine the two surveys what do we find? Ueta, Meixner, Bobrowsky 2000

Morphological Dichotomy
Toroidal SOLE Optically Thin Shell Core/elliptical DUPLEX Here’s what we found. Interestingly, the toroidal mid-IR type is always associated with SOLE optical type and the core/elliptical mid-IR type is always associated with DUPLEX optical type. And, the most reasonable way to explain both mid-IR and optical morphologies plus this dichotomy is the optical depth of the dust shell. In SOLE type, the mid-IR images, indicated by the contours, show the presence of an edge-on dust torus, and, optical depth of these dust shells is low enough that the central star is prominently visible because of the radiation scattered THROUGH the dust torus. In DUPLEX type, however, optical depth of the shell is so large that the shell completely or partially obscures the central star creating the dust lane, and we can not see the mid-IR structure as well. Optically Thick Shell Ueta, Meixner, Bobrowsky 2000

Optical Depth & Morphology
This one density function can exlain both types of PPN morphologies by changing the equatorial optical depth of the dust shells. When the optical depth of the shell is low, scattered radiation from the central star can go all directions. Then the shell appears as an elliptical nebula reflecting the spheroidal dust distribution of the mid-shell. On the other hand,, when optical depth of the shell is very high, scattered radiation from the central star can ONLY go through the biconical openings of the innermost dust torus. In this case, we see classic bipolar nebulosity. Toroidal-SOLE PPNs Core/elliptical-DUPLEX PPNs Ueta, Meixner, Bobrowsky 2000

Numerical Models: Examples
DUPLEX: IRAS17150 SOLE: IRAS 17436 Let me show you some results. We picked one representative object from each morphological group, IRAS for SOLE and IRAS for DUPLEX. Both are oxygen-rich and oriented close to edge-on, and thus chemistry and inclination angle would not affect the morphology too much. both O-rich & nearly edge-on Meixner et al. (2002)

Model SEDs SOLE: IRAS 17436 DUPLEX: IRAS 17150 Meixner et al. (2002)
These are the best-fit model spectral energy distributions. The models fit the observations well for a wide range of wavelength, reproducing characteristic features such as this 43um water ice emission feature for SOLE PPN and the 9.8um silicate absorption for DUPLEX PPN. Meixner et al. (2002)

Model Images SOLE: IRAS 17436 DUPLEX: IRAS 17150 Meixner et al. (2002)
This is the edge-on orientation. Now, for the SOLE case, the model reproduces elliptically elongated optical nebula with the bright central star as well as two-peak structure in the mid-IR emission region. For the DUPLEX case, classic bipolar nebula and slightly elongated, compact mid-IR emission region are nicely reproduced. Meixner et al. (2002)

Model Parameters ˙ ˙ IRAS 17436 IRAS 17150 L*(L), T*(K) 2800, 7000
27000, 5200 Rmin(cm) 11.2 x1015 9.7 x1015 Inclination (deg) ~ 80 ~ 82 Tdust(K) 110 220 9.8m, equator 0.5 12.0 equator/ pole 9 160 Mshell(M) 0.7 4.9 MAGB (M/yr) 4.1 x10-5 1.9 x10-4 MSW (M/yr) 4.2 x10-5 30.0 x10-4 Here are some of the parameters involved. The models show that DUPLEX PPN, IRAS has very optically thick, equatorially enhanced shell. Shell mass is 70 times larger and thus mass loss rates are higher. These results indicate that DUPLEX PPN evolved out of higher mass progenitors, as previously suggested. ˙ ˙ Meixner et al. (2002)

Dust Properties: IRAS 17436 IRAS 17150 0.2 0.001 10 200
Amorphous silicates 86.5% 100% Crystalline enstatite 6% 0% Crystalline forsterite 5% Water ice 2.5% One of the unanticipated results was the difference in dust composition. Meixner et al. (2002)

Three kinematic signatures
Simple expansion, SOLE PPN: IRC+10216, IRAS , HD179821, HD , HD 56126, IRAS , OH Expansion + collimated outflows, DUPLEX PPN: AFGL 2688, NGC 7027, AFGL 618 Expansion + rotation, New class, Disk PPN: Mira, 89 Her, Red Rectangle Fong, Meixner et al. 2006

Expansion, SOLE PPN: HD Fong, Meixner et al. 2006

Collimated outflows, DUPLEX PPN: Egg Nebula / AFGL 2688
Comparing our BIMA data (left) with the HST H_2 data (right), we find that the shocked H_2 line emission is located exactly where the fast, collimated outflows, shown here in blue and red contours, collide with the slower moving AGB wind. Thompson et al. 1997 Fong, Meixner et al. 2006

Collimated outflows, Young PN: NGC 7027
The central hole in NGC 7027 is created primarily by the advanding photo-dissociation and photo-ionization fronts. The nebula has an onion layered structure with the central star surrounded by a layer of ionized gas, a layer of warm atomic gas (green), a layer of flouresced H_2 line emission and finally an outer layer of molecular gas as shown by the BIMA data on the left and WIYN data on the right. Contours reveal two high velocity collimated outflows. A 3rd bipolar out flow is in the plane of the sky and is responsible for the blowouts observed in the H_2 emission. Fong, Meixner et al. 2006

Red Rectangle, Rotating Disk source: Bujarrabal et al. 2005

Red Rectangle, Disk source: Bujarrabal et al. 2005
Model comparisons: -Keplerian rotation -central mass of 0.9 solar masses

BIMA CO Survey: Fong, Meixner, Sutton, Zalucha and Welch 2006
After I left Berkeley and started as an assistant professor at the University of Illinois, they expanded the BIMA array to upto 10 dishes and it became possible to do surveys, such as the one pursued by my student David Fong, myself and others, including Jack. This HR diagram shows the properties of the 10 stars that we surveyed with BIMA along with the many others we culled from the literature all of which reported the spatial distribution of the molecular gas in these sources. The overall theme for this project was to investigate the radiative and dynamical evolution of the molecular envelope of evolved stars. One of the results showed that there are three types of kinematic signatures that we called spherical, disk sources or structured outflow sources. The Spherial sources reveal simple expansion, much like IRC The Disk sources appear to have gravitationally bound, keplerian disks like the Red Rectangle. The Structured outflow sources have simple expansion signatures, punctuated by structured bipolar outflows that appear to be shaping the nebula, such as AFGL 718.

Summary The hardening of the radiation field (not shocks) dominates the transformation of the molecular gas envelope, formed on the AGB, into the ionized gas. The morphological transformation occurred at the tip of the AGB with further shaping from (collimated) outflows in the proto-planetary nebula phase. Three types of proto-planetary nebulae, in order of lowest to highest mass circumstellar shell: rotating disk, simple expansion, collimated outflows

Extra slides

Post-Asymptotic Giant Branch Object: HD 56126
This multi-wavelength panorama of HD 56126, reveal the content and morphology of the ejected circumstellar matter from an intermediate mass star., AGB stars shed their mass in molecular winds, The CO emission reveals the location of the molecular gas Gemini mid-IR thermal emission, HST optical, scattered light. Gas and Dust are well mixed. Meixner et al. (2004)

HD 56126 Radiative Transfer Gas Model: 0.06 M Meixner et al. (2004)

Post-Asymptotic Giant Branch Object: HD 56126
Radiative Transfer Dust Model (2-Dust, Ueta & Meixner 2003): 7.810-4 M Mgas/Mdust = 75 Meixner et al. (2004)

Radial Profile Dave Fong subtracted a Gaussian fit to each channel map to reveal the underlying intensity profile of the data to reveal a series of molecular arcs surrounding IRC

Molecular Arcs

Composite Sketch If you project the location of all these arcs onto the 2 dimensional plane of the sky, you find a complex arc structure that is quite similar to the

Dust Arcs (Huggins & Mauron 2000)
Deep optical images of the Dust arcs surrounding IRC These dust arcs have puzzled people because their apparent periodicity is ~100 years, which is a difficult timescale to explain with evolved stars. (Huggins & Mauron 2000)

Parabolic Profiles What we’ve revealed with the CO studies that this time scale is more like a few thousand years, which has more plausible explanations such thermal pulses occuring in the stellar interiors.

Planetary Nebulae: Observations of the Early Phases

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