Determining the chemical distribution of the galaxy is a tricky business. The ideal method is spectroscopy but since high quality spectroscopy takes bright targets, the number of potential targets is somewhat reduced. Stars seem like logical choices, but due to differential separation during formation, they don’t provide a true description of the interstellar medium. Clouds of gas and dust are the best choice, but must be illuminated by star formation. Another option is to search for newly formed planetary nebulae which are in the process of enriching the interstellar medium.
A new paper does just this, discovering a new planetary nebula in hopes of mapping the chemical abundance of the galaxy. The new nebula is almost the exact opposite direction of the galactic center when viewed from Earth. It lies at a distance of about 13 kpc (42,400 lightyears) from Earth making it one of the most distant planetary nebulae from the galactic center for which a distance has been determined and currently, the furthest with a measured chemical abundance.
The nebula was originally recorded on images taken by the INT Photometric Hα Survey (IPHAS) in 2003 but the automated program for detecting such objects initially missed the nebula due to its relatively large angular size (10 arcseconds). It was subsequently caught on visual inspection of the mosaics. Follow-up spectroscopy was conducted from 2005 to 2010 and reveal that the nebula is quite regular for planetary nebula, containing strong emission from hydrogen, nitrogen, oxygen, and silicon. The rate of expansion combined with its physical size suggests an age of nearly 18,000 years.
This newly discovered nebula provides a rare data point for the chemical abundance for the outer portions of the galaxy. While the galaxy is known to be enriched towards the galactic center, there has been debate about how quickly, if at all, it falls towards the galactic edge where star formation, and thus, enrichment, is less common. While there aren’t enough known nebulae to determine just yet (only four others are known at similar distances), this planetary nebula suggests that the abundance levels off in the galactic outskirts.
The authors also note that this nebula, as well as potentially the others, aren’t native to the Milky Way. They lie near a structure known as the Monoceros Ring, which is a stream of stars believed to be stretched out as the Milky Way devours the Canis Major Dwarf Galaxy.
Question: what exactly is “differential separation”?
Beautiful.
Meaning of “Differential separation”
At the risk of being completely wrong, I would intepret it as follows: When clouds of molecular gas and dust collapse under the influence of gravity to form objects such as stars and planets, the bodies so formed are, as it were, “layered” or differentiated, with the heavier materials going to the centre and lighter ones ending up nearer the surface, (just as the core of the Sun is obviously so much denser than its atmosphere). Because we can observer only mostly the “outer layers” of such differentiated objects, they are not the best candidates for the study in question.
As a comment Joh.
The lead image you show isn’t this newly found planetary nebula, IPHASX J052531.19+281945.1, but the Hubble image of the southern planetary NGC 2818 in Pyxis (09h16.0m-36°38′).
As for saying “differential separation during formation”, I don’t understand this either. I’d assume you mean that the composition of the star is different than the star-bearing medium from which it was born. (The difference being the removal of dust from the nebulosity and the chemical evolution of the ZAMS (zero aged main sequence.)) As the paper says; “Finding new distant PNe will make a crucial contribution to the knowledge of the Galactic abundance gradient, since their rich emission line spectra can yield accurate abundances.”; but importantly adds; “The abundance gradient is a topic that is still under debate…” They also state in the conclusion of this paper; “…the most sensible conclusion comes from “Essentially we have reached a confusion limit regarding the abundance gradient as derived using PNe.”.”
You saying “in hopes of mapping the chemical abundance of the galaxy”, I think is the not quite true or is realised as really able two be discerned. So yes, the possibility this could be useful, but at the moment, the linked paper rightly says; “it is clear that the properties of the outer Galactic disc are still in need of much better observational definition.”
As for this planetary nebulae, they say;
“IACPN [IPHASX J052531.19+281945.1] is a low density planetary nebula with an emission line spectrum typical of a moderate excitation [planetary nebula/ Peimbert] Type II.* This, together with its strong stratification, apparent from He II [Helium II ion] to O I [Oxygen I ion], suggests that it is composed of a moderately massive nebula hosting a relatively hot star. The ionised hydrogen mass of the nebula, derived from its H-Beta flux, electron temperature, radius, and distance, is M[ass]= 0.4 [Solar Masses]” They go on to say “nebular expansion velocity is 17k.ms^-1, [radius] 0.31 pc at the distance of 12.8 kpc… lead to a kinematic age of 17,800 years.”
Of the central star illuminating and exciting the nebulae they say; “However, given the several uncertain parameters involved (central star luminosity and temperature, kinematical vs. real age), no precise estimate of the stellar mass can be derived.”
Thank for this interesting article…
The author, Jon Voisey, never said that it was; I think that he just put up the image of NGC 2818 for illustrative “eye candy” purposes.
Not being mean here, but it is not captioned.
However, if he was chasing eye-candy, he couldn’t have picked a better object.
This is exactly correct IVAN. The nebula in question is rather boring and spherical without much detail. Thus, I went with an image of a more interesting pne. I generally try to choose images that are related in some way, interesting to look at, and haven’t been used for anything else on UT for a good while.
And so should be stated under the images you choose to use..
Jon
To be pedantic. Please read http://hubblesite.org/about_us/copyright.php that includes;
“Material credited to STScI on this site was created, authored, and/or prepared for NASA under Contract NAS5-26555. Unless otherwise specifically stated, no claim to copyright is being asserted by STScI and it may be freely used as in the public domain in accordance with NASA’s contract. However, it is requested that in any subsequent use of this work NASA and STScI be given appropriate acknowledgement …the credit line for an image lists STScI as the source, the image may be freely used as in the public domain as noted above.”
These guys are happy for us to use images for nothing without copyright infringements. All they request is that we acknowledge the source. No bitterness, no criticism, just the right thing to do. (I use images like this all the time, and are more than happy to meet their request. Not doing so, just gives everyone a bad name.)
Also, who is to know if this image is related to this new PNe or not? I wrote what I wrote because I thought Bill saying “beautiful” mightn’t realise it was the object in question.
Truly, I’d have thought that the rest of what I said was far more interesting!
Also this new distant PNe lies in the constellation of Taurus, and lies merely 18 arcmin SW of the bluish 1.7 magnitude star, Beta Tauri. (This is one of the reasons it was missed by earlier surveys.) Observationally, it is amazingly bright, say, compared to most of the southern MASH catalogued PNe.
I’ve been following research on a host of newly discovered objects turned up in the IPHAS survey for several years now [I’m also interested in possible new objects for observation and/or imaging]. A number of newly discovered planetary & emission nebulae have turned up as well as some suspected supernovae remnants. Thanks Jon (& Andrew) for an interesting article (& comments) on this newly discovered PN.