Talk:NGC 2818

Open clusters
"It is often stated that a further problem to its being regarded as part of the open cluster, is "that open clusters have relatively poor gravitational cohesion and tend to disperse over periods of the order of hundreds of millions of years, whereas stars that end up as planetary nebulae, are usually long-lived with ages measured in billions of years." However, this is not correct. First, open clusters disperse on far earlier time scales than hundreds of millions of years. Indeed, it is estimated that less than ~10% of the clusters formed in molecular clouds are able to reach ages beyond 10 million years. Second, Planetary Nebula found in the field range from lifetimes of ~40 Myr to billions of years. These planetary nebulae are the remnants of dissolved open clusters. The reasons why planetary nebulae have yet to be discovered as members of open clusters is a statistical one."


 * I'm confused. The bulk of (open?) clusters have lifetimes smaller than 10 million years and the remainder carry on for anything up to a few hundred million years, whereas the stars producing planetary nebulae live from some 40 Myr to billions of years. Just how are planetary nebulae linked to open clusters if they are the "remnants of dissolved (?dispersed) open clusters"? Secondly, from the age brackets given it would appear extremely unlikely that planetary nebulae will be discovered as members of open clusters, since only the tailings of the two groups overlap in time. Is this what is meant by "statistical" difficulties? Rotational (talk) 14:54, 2 February 2009 (UTC)


 * I refer you to the papers by Lada & Lada (2003) and that of Majaess et. al (2007), it may help alleviate the confusion.


 * My confusion arises from a lack of logical continuity in your contributions to the article. The concerns I have raised above are those which I feel any reasonably intelligent reader would have on going through this essay. Having clarity in one's own mind on a topic is not the same thing as framing sentences which lucidly mirror one's thoughts. A paragraph often calls for repeated readings and rewrites to put across an idea. Good, articulate science writers are rare, but are worth their weight in gold. My writing skills are certainly not above average, but my ability to spot logical anomalies is. This is not meant to sound patronising and I apologise if it does, but I'm sure you have valuable insights to bring to these pages if you could just occasionally shift your perspective to that of the reader. Rotational (talk) 19:38, 3 February 2009 (UTC)


 * I apologize to have caused some friction. I thought you would enjoy reading those papers which have the answers to your questions and much more. Indeed, I hardly contributed anything to the article, I merely cropped what was  incorrect. I posted the above comments in an attempt to convince you to stop reposting the assertion you were making, yet I quickly realized there is a big divide and I was hoping that referring you to the papers would help clear things up for you granted they are inherently more comprehensive (they contain graphs, etc.) Perhaps after reading those articles you could articulate what is in them to readers, as your writing skills reveal years of academic practice.  I look forward to reading what you post.  —Preceding unsigned comment added by 142.177.56.140 (talk) 22:29, 3 February 2009 (UTC)


 * ":I'm confused. The bulk of (open?) clusters have lifetimes smaller than 10 million years and the remainder carry on for anything up to a few hundred million years, whereas the stars producing planetary nebulae live from some 40 Myr to billions of years." The most probabilistic open cluster to be found is of age 100 million years.  The issue is that there is, as Lada & Lada (2003) termed: a high infant-mortality rate among open clusters.  Namely that it is extremely rare for a cluster to survive beyond an age of 10 million years.  So although open clusters of age 100 Myr currently outnumber their younger counterparts, the difference should be much larger.  Indeed, it is estimated that less than ~10% of young star clusters reach ages beyond 10 million years, so again, it is quite a miracle that a cluster makes it to an age of 100 Myr.  Many factors influence the survival rate of clusters, one of which, interestingly, is their distance from Galactic center (tidal disruption).  For example, old open clusters are rare within the solar circle.  The question then becomes, what happens to these dissolved open clusters?  Well, the constituents of these dissolved clusters are what make up the field.  "Just how are planetary nebulae linked to open clusters if they are the "remnants of dissolved (?dispersed) open clusters"? Secondly, from the age brackets given it would appear extremely unlikely that planetary nebulae will be discovered as members of open clusters, since only the tailings of the two groups overlap in time."  Ok, so PNe are believed to form from main-sequence stars of masses 8 -> 1 solar masses.  This corresponds to clusters of about 40 Myr to about X Gyrs.   There are many open clusters within that range.  We also find PNe in the field that have the signatures of more massive progenitors (a few solar masses), it follows that it is an issue of statistics as to why we have yet to discover a PN in an OC.  Mainly, the numbers of open clusters within the right age bracket are on the order of hundreds, but the lifetime of the PN phase for more massive progenitors is really short!  On the order of thousands of years.  Basically, a variety of factors (e.g., the number of cluster members) conspire and the statistics are not high enough for us to "catch one in the act".  There are quantitative constructs to estimate the probabilities, but I enjoy the following estimate.  Currently, let us say there are 10-20 known Cepheids which are members of open clusters.  Cepheids, in certain instances, are the progenitors of PNe.  If the Cepheid lifetime is perhaps 10x that of a PNe, then finding a PN as a cluster member would indeed be a rarity.


 * The spiel now looks much better. However, I wonder if the details on the open cluster part should not be moved to the appropriate section under the planetary nebula (section "Membership in Clusters") wikitab, where it would be more pertinent? Namely, the following sections "Partly because of their small total mass, open clusters have relatively poor gravitational cohesion. They are consequently also sensitive to external gravitational influences and tend to disperse after a relatively short time together with the age of most open clusters being some 10 million years, and under exceptional, unperturbed conditions remaining together for up to 100 million years.  Theoretical models predict that planetary nebulae can form from main-sequence stars of between 8 and 1 solar masses, which puts their age at 40 million years and older. Although there are a few hundred known open clusters within that age range, a variety of reasons limit the chances of finding a member of an open cluster in a planetary nebula phase. One such reason is that the planetary nebula phase for more massive stars belonging to younger clusters is on the order of thousands of years - a blink of the eye in cosmic terms. To date, no true association has been established between open clusters and planetary nebulae. [3]"
 * I feel that some background material on the evolution of open clusters is not altogether out of place, however if you feel that Open Clusters or Planetary Nebulae might benefit from the paragraph, then copy it across. ciao Rotational (talk) 21:21, 10 February 2009 (UTC)


 * Ok great, cheers. —Preceding unsigned comment added by 142.177.159.25 (talk) 12:43, 16 February 2009 (UTC)

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