View source for Protoplanetary nebula

{{short description|Nebula surrounding a dying star}}
{{distinguish|Protoplanetary disk}}
[[File:Westbrook Nebula.tif|thumb|right|250px|The [[Westbrook Nebula]], a protoplanetary nebula.]]

A '''protoplanetary nebula''' or '''preplanetary nebula'''<ref name="Sahai & Sanchez Contreras et al 2005">{{cite journal
 | last1 = Sahai
 | first1 = Raghvendra
 | last2 = Sánchez Contreras
 | first2 = Carmen
 | last3 = Morris
 | first3 = Mark
 | title = A Starfish Preplanetary Nebula: IRAS 19024+0044
 | journal = The Astrophysical Journal
 | date = 2005
 | volume = 620
 | issue = 2
 | pages = 948–960
 | bibcode = 2005ApJ...620..948S
 | doi = 10.1086/426469
| url = https://trs.jpl.nasa.gov/bitstream/2014/40924/1/04-3294.pdf
 }}</ref> ('''PPN''', plural '''PPNe''') is an [[astronomical object]] which is at the short-lived episode during a [[star]]'s rapid [[stellar evolution|evolution]] between the late [[asymptotic giant branch]] (LAGB){{Ref_label|A|a|none}} phase and the subsequent [[planetary nebula]] (PN) phase.  A PPN emits strongly in [[infrared]] radiation, and is a kind of [[reflection nebula]].  It is the second-from-the-last high-luminosity evolution phase in the life cycle of intermediate-mass stars (1–8 {{Solar mass|link=y}}).<ref name=Kastner2005>{{citation
 | last1 = Kastner
 | first1 = J. H.
 | title = Near-death Transformation: Mass Ejection in Planetary Nebulae and Protoplanetary Nebulae
 | journal = American Astronomical Society Meeting 206, #28.04; Bulletin of the American Astronomical Society
 | date = 2005
 | volume = 37
 | bibcode = 2005AAS...206.2804K
}}</ref>{{rp|469}}

==Naming==
[[File:IRAS 13208-6020.tif|thumb|Protoplanetary nebula [[IRAS 13208-6020]] is formed from material that is shed by a central star.]]

The name '''protoplanetary nebula''' is an unfortunate choice due to the possibility of confusion with the same term being sometimes employed when discussing the unrelated concept of [[protoplanetary disk]]s.  The name '''protoplanetary nebula''' is a consequence of the older term '''planetary nebula''', which was chosen due to early astronomers looking through telescopes and finding a similarity in appearance of planetary nebula to the gas giants such as [[Neptune]] and [[Uranus]].  To avoid any possible confusion, {{Harvnb|Sahai|Sánchez Contreras|Morris|2005}} suggested employing a new term '''preplanetary nebula''' which does not overlap with any other disciplines of astronomy.  They are often referred to as [[post-AGB star]]s, although that category also includes stars that will never ionize their ejected matter.

==Evolution==

===Beginning===
During the late [[asymptotic giant branch]] (LAGB){{Ref_label|A|a|none}} phase, when mass loss reduces the hydrogen envelope's mass to around 10<sup>−2</sup>&nbsp;{{Solar mass|link=y}} for a core mass of 0.60&nbsp;{{Solar mass}}, a star will begin to evolve towards the blue side of the [[Hertzsprung–Russell diagram]].  When the hydrogen envelope has been further reduced to around 10<sup>−3</sup> {{Solar mass}}, the envelope will have been so disrupted that it is believed further significant mass loss is not possible. At this point, the [[effective temperature]] of the star, ''T''<sub>*</sub>, will be around 5,000&nbsp;[[kelvin|K]] and it is defined to be the end of the LAGB and the beginning of the PPN. {{Harv|Davis|Smith|Gledhill|Varricatt|2005}}

===Protoplanetary nebula phase===

[[File:IRAS 20068+4051.jpg|thumb|Protoplanetary nebula known as [[IRAS 20068+4051]] taken by [[Hubble Space Telescope|Hubble's]] Advanced Camera for Surveys.]]

During the ensuing protoplanetary nebula phase, the central star's [[effective temperature]] will continue rising as a result of the envelope's mass loss as a consequence of the hydrogen shell's burning.  During this phase, the central star is still too cool to ionize the slow-moving circumstellar shell ejected during the preceding AGB phase.  However, the star does appear to drive high-velocity, collimated [[stellar wind|winds]] which shape and shock this shell, and almost certainly entrain slow-moving AGB ejecta to produce a fast molecular wind.  Observations and high-resolution imaging studies from 1998 to 2001, demonstrate that the rapidly evolving PPN phase ultimately shapes the morphology of the subsequent PN.  At a point during or soon after the AGB envelope detachment, the envelope shape changes from roughly spherically symmetric to axially symmetric.  The resultant morphologies are [[bipolar nebula|bipolar]], knotty jets and [[Herbig–Haro object|Herbig–Haro]]-like "bow shocks".  These shapes appear even in relatively "young" PPNe. {{Harv|Davis|Smith|Gledhill|Varricatt|2005}}

===End===
The PPN phase continues until the central star reaches around 30,000&nbsp;K and it is hot enough (producing enough [[ultraviolet]] radiation) to ionize the circumstellar nebula (ejected gases) and it becomes a kind of [[emission nebula]] called a Planetary Nebula.  This transition must take place in less than around 10,000 years or else the density of the [[circumstellar envelope]] will fall below the PN formulation density threshold of around 100{{clarification|date=September 2022}} per cm<sup>3</sup> and no PN will result, such a case is sometimes referred to as a 'lazy planetary nebula'. {{Harv|Volk|Kwok|1989}}

==Recent conjectures==
[[File:An interstellar butterfly.jpg|thumb|An interstellar butterfly - protoplanetary nebula [[Roberts 22]]<ref>{{cite news|title=An interstellar butterfly|url=http://www.spacetelescope.org/images/potw1410a/|access-date=11 March 2014|newspaper=ESA / HUBBLE}}</ref>]]
Bujarrabal et al. (2001) <ref>{{cite journal |url=https://ui.adsabs.harvard.edu/abs/2001A%26A...377..868B/abstract|title=Bujarrabal, V., Castro-Carrizo, A., Alcolea, J., S{\'a}nchez Contreras, C.; 2001.; Mass, linear momentum and kinetic energy of bipolar flows in protoplanetary nebulae.; Astronomy and Astrophysics 377, 868–897. doi:10.1051/0004-6361:20011090|bibcode=2001A&A...377..868B |last1=Bujarrabal |first1=V. |last2=Castro-Carrizo |first2=A. |last3=Alcolea |first3=J. |last4=Sánchez Contreras |first4=C. |journal=Astronomy and Astrophysics |date=2001 |volume=377 |page=868 |doi=10.1051/0004-6361:20011090 }}</ref>
found that the "interacting [[stellar wind]]s" model of Kwok
et al. (1978) <ref>{{cite journal |url=https://ui.adsabs.harvard.edu/abs/1978ApJ...219L.125K/abstract|title=Kwok, S., Purton, C.R., Fitzgerald, P.M.; 1978; On the origin of planetary nebulae; The Astrophysical Journal 219, L125–L127. doi:10.1086/182621|bibcode=1978ApJ...219L.125K |last1=Kwok |first1=S. |last2=Purton |first2=C. R. |last3=Fitzgerald |first3=P. M. |journal=The Astrophysical Journal |date=1978 |volume=219 |doi=10.1086/182621 }}</ref> of radiatively-driven winds is insufficient to account for their CO observations of PPN fast winds which imply high momentum and energy inconsistent with that model.  Complementarily, theorists  (Soker & Livio 1994; <ref>{{cite journal |url=https://ui.adsabs.harvard.edu/abs/1994ApJ...421..219S/abstract|title=Soker, N., Livio, M.; 1994; Disks and jets in planetary nebulae; The Astrophysical Journal 421, 219. doi:10.1086/173639
|bibcode=1994ApJ...421..219S
|last1=Soker
|first1=Noam
|last2=Livio
|first2=Mario
|journal=The Astrophysical Journal
|date=1994
|volume=421
|page=219
|doi=10.1086/173639
}}</ref>
Reyes-Ruiz & Lopez 1999;<ref>{{cite journal |url=https://ui.adsabs.harvard.edu/abs/1999ApJ...524..952R/abstract|title=Reyes-Ruiz, M., López, J.A.; 1999; Accretion Disks in Pre-Planetary Nebulae; The Astrophysical Journal 524, 952–960. doi:10.1086/307827|bibcode=1999ApJ...524..952R |last1=Reyes-Ruiz |first1=M. |last2=López |first2=J. A. |journal=The Astrophysical Journal |date=1999 |volume=524 |issue=2 |page=952 |doi=10.1086/307827 }}</ref> Soker & Rappaport 2000;<ref>{{cite journal |url=https://ui.adsabs.harvard.edu/abs/2000ApJ...538..241S/abstract|title=Soker, N., Rappaport, S.; 2000; The Formation of Very Narrow Waist Bipolar Planetary Nebulae; The Astrophysical Journal 538, 241–259. doi:10.1086/309112|bibcode=2000ApJ...538..241S |last1=Soker |first1=Noam |last2=Rappaport |first2=Saul |journal=The Astrophysical Journal |date=2000 |volume=538 |issue=1 |page=241 |doi=10.1086/309112 |arxiv=astro-ph/9911140 }}</ref> Blackman, Frank & Welch 2001<ref name="BFW2001">{{cite journal |url=https://ui.adsabs.harvard.edu/abs/2001ApJ...546..288B/abstract|title=Blackman, E.G., Frank, A., Welch, C.; 2001; Magnetohydrodynamic Stellar and Disk Winds: Application to Planetary Nebulae; The Astrophysical Journal 546, 288–298. doi:10.1086/318253|bibcode=2001ApJ...546..288B |last1=Blackman |first1=Eric G. |last2=Frank |first2=Adam |last3=Welch |first3=Carl |journal=The Astrophysical Journal |date=2001 |volume=546 |issue=1 |page=288 |doi=10.1086/318253 |arxiv=astro-ph/0005288 }}</ref>) investigated whether  [[accretion disk]] scenarios, similar to models used to explain jets from [[active galactic nuclei]] and [[star formation|young star]]s, could account for both the point symmetry and the high degree of collimation seen in many PPN jets.  In such  models applied to the PPN context, the accretion disk forms through binary interactions.  [[Magneto-centrifugal]] launching from the disk surface is then a way to convert gravitational energy into the kinetic energy of a fast wind in these systems.<ref name="BFW2001" />  If the accretion-disk jet paradigm  is correct and [[magneto-hydrodynamics]] (MHD) processes mediate the energetics and collimation of PPN outflows, then they will also determine physics of the shocks in these flows, and this can be confirmed with high-resolution pictures of the emission regions that go with the shocks. {{Harv|Davis|Smith|Gledhill|Varricatt|2005}}

==See also==
* [[Bipolar nebula]]
* [[Bipolar outflow]]
* [[List of protoplanetary nebulae]]
* [[Planetary nebula]]

== Notes ==
{{refbegin}}
<ol type="a">
<li>{{Note_label|A|a|none}}The late asymptotic giant branch begins at the point on the asymptotic giant branch (AGB) where a star is no longer observable in [[visible light]] and becomes an [[infrared]] object. {{Harv|Volk|Kwok|1989}}</li>
</ol>
{{refend}}

== References ==
{{reflist}}
{{refbegin}}
* {{citation
 | last1 = Davis
 | first1 = C. J.
 | last2 = Smith
 | first2 = M. D.
 | last3 = Gledhill
 | first3 = T. M.
 | last4 = Varricatt
 | first4 = W. P.
 | title = Near-infrared echelle spectroscopy of protoplanetary nebulae: probing the fast wind in H<sub>2</sub>
 | journal = [[Monthly Notices of the Royal Astronomical Society]]
 | date = 2005
 | volume = 360
 | issue = 1
 | pages = 104–118
 | bibcode = 2005MNRAS.360..104D
 | doi = 10.1111/j.1365-2966.2005.09018.x
 | doi-access = free
 | arxiv = astro-ph/0503327 }}
* {{citation
 | last1 = Volk
 | first1 = Kevin M.
 | last2 = Kwok
 | first2 = Sun
 | author2-link = Sun Kwok
 | title = Evolution of protoplanetary nebulae
 | journal = The Astrophysical Journal
 | date = July 1, 1989
 | volume = 342
 | pages = 345–363
 | bibcode = 1989ApJ...342..345V
 | doi = 10.1086/167597
}}.
* {{citation
 | last1 = Szczerba
 | first2 = Natasza
 | last2 = Siódmiak
 | first3 = Grażyna
 | last3 = Stasińska
 | first4 = Jerzy
 | last4 = Borkowski
 | title = An evolutive catalogue of Galactic post-AGB and related objects
 | volume = 469
 | issue = 2
 | pages = 799–806
 | date = April 23, 2007
 | journal = Astronomy and Astrophysics
 | url = http://www.ncac.torun.pl/postagb?cat=confirmed&lang=en&branch=postagbworld&id=0&menuoff=1
 | doi = 10.1051/0004-6361:20067035
 | first1 = Ryszard
 | bibcode=2007A&A...469..799S
 | arxiv = astro-ph/0703717 }}.
{{refend}}

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{{DEFAULTSORT:Protoplanetary Nebula}}
[[Category:Protoplanetary nebulae| ]]
[[Category:Nebulae]]
[[Category:Stellar evolution]]