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(IAAC) Re: Obj: NGC 6781 (Snowglobe, PK41-2.1, HIII-743, PNG41.8-2.9)...


In response to my observing log, Tal Mentall notes:
>The SkyAtlas 2000.0 Companion, 2nd Ed. says that central star is 16.8 mag. 


Thanks for looking that up, Tal! If 16.8 is the correct visual magnitude of the
central star, then I'll lay strong odds that what we saw that night was not the
central star! On the other hand there seem to be multiple stars within the ring
of this little PN... Being of a somewhat complex shape (and the star only being
occasionally seen), we might easily have been noting a star that was within the
ring, but off-center. And I guess there's the possibility that the "Companion"
might actually be quoting a magnitude in some other 'band' than v, without its
authors necessarily being aware that their original source was doing so... ?


Jeff Bondono, in the database of his wonderful observing planner software,
'dObjects', gives the central star magnitude that I quoted, "14.95?m". The
references he mentions for this object's data include: Brian Skiff's Webb
Society Quarterly Journal article "Precise Positions for NGC/IC Planetary
Nebulae", and also "SkyCat II" (Sky Catalog 2000.0, volume 2). I'd assume
that the central star magnitude Jeff gave came from SkyCat.

Doug Snyder, on his excellent Planetary Nebulae web site http://blackskies.com,
lists the central star of the "Snowglobe" as *photographic* magnitude 16.3. It
is generally not at all clear to me how a 'p' mag can be converted into a 'v'
mag, (especially when I don't know of any color indices for the star). So...



I looked for other reports of internal stars within N6781, and found these:

Steve Coe using a 17" scope, reports at http://blackskies.com/reports13.htm:
"A mag 13 star is at the NE edge and once or twice I caught a fainter glimmer
of an interior star north of center."

Yann Pothier also in a 17" reports, http://visualdeepsky.org/logs/msg01780.html:
"... [There is a] mag 15 star in the ring to the W (and a little S) and another
just outside to the ENE..."

R. Bartlett in his StarLore site http://www.starlore.net/ngc6781.htm reports:
"You'll need clear dark skies to see this one. Some detail can be seen with
medium scopes but only more powerful scopes will show the magnitude 16.1*
central star. NGC 6781 is expanding at a rate of 12 km per second."

Noted deep-sky sketcher Scott Davis's site http://clifty.com/scott/DSO/6781.html
contains the following interesting but unattributed information:
"NGC-6781... MAGNITUDE OF CENTRAL STAR: +15.5"

Finally, avid CCDer Scott Griswold from Astro. Soc. Greater Hartford (along with
an unnamed accomplice), notes at http://ccdguy.homestead.com/planetaries.html:
"NGC6781: Look at this beauty!  Only 16x45 second images capture this gem and
it's central star.  The nebula glows at mag 11.8, and it's central star at mag
16.3..well within the reach of a six inch scope [using a CCD]!"


Oh, well, it will probably require a more patient look at this amazing little
object, with similar skies and a similar instrument, to finally resolve the
question of the internal star or stars in the "Snowglobe Nebula"! :)

Thanks again for posing an interesting question on this log, Tal.

Clear skies,
Lew Gramer


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