September 16, 2009: Himalia and more from Lake Sonoma

Steve Gottlieb

A week ago on September 16th, I met Carter Scholz and Dana Patchick for some mid-week observing at Lake Sonoma. I was a little apprehensive about heading north on 101 during commute hours, but traffic was unusually light and I was up at the observing site (Lone Rock) in 85 minutes from Albany, arriving just before sunset. Carter and Dana pulled into the lot very shortly afterwards and we set up under a clear, dry conditions with a slight breeze that suggested we were in for a good evening.

I had brought along a list of obscure odds and ends to observe as I've finished up all the NGC objects visible from our latitude for this time of year (my main observing program). Dana mentioned early on that he had brought along charts and images to try and track down Himalia, the 6th satellite of Jupiter discovered, and brightest of the outer irregular moons. I had never tried for Himalia before, though I've viewed Triton, Neptune's brightest moon, several times as well as Oberon and Titania, the two brightest moons of Uranus. So, I was looking forward to taking a look later in the evening.

So, where is Himalia and why isn't it better known? Well, the magnitude is currently 14.8 (it varies from 14.6 at best to 15.6), so it's a pretty faint target. Himalia was discovered photographically on Lick plates in 1904. Interestingly, the last moon to be discovered visually was Jupiter's Amalthea, found by Barnard using the Lick's 36-inch refractor in 1892. So, why wasn't Himalia picked up visually also?

It's orbit is far beyond the Galilean satellites, 7 million miles from Jupiter and ventures as far as a full degree from Jupiter from our vantage point. Currently the separation is about 45' and that's not changing rapidly, as Himalia takes 250 days to orbit Jupiter. Dana thought it might be possible to detect motion over a few hours, but it's moving at only 10 arcseconds an hour, so the motion over a few hours is pretty minimal.

Dana brought along a chart of the field that he generated using Guide software as well as a print out of a DSS image of the field to rule out stellar imposters that we might confuse. With Dana's help, it didn't take long to starhop over to the field from Jupiter and we immediately picked out Himalia as a mag 14.8 "star" in the exact position plotted on Guide. The DSS image confirmed the additional "interloper".

It really wasn't a difficult observation with my scope, though of course knowing the precise position was the key. Identifying Himalia was similar to chasing down a faint quasar or other stellar object and making sure you've pinned the right object. If anyone is interesting in duplicating this observation, go to JPL's Horizons web interface at http://ssd.jpl.nasa.gov/horizons.cgi and generate an ephemeris table for Himalia for the evening you plan to observe it. Just a single position should suffice.

I took a look at some sky-plotting software that includes the minor solar system moons and found some disappointing results. Both SkyTools 3 and Guide software plotted the position of Himalia accurately, but Voyager 4 and StarryNight Pro 4.5 were both far off, and would have been useless in finding Himalia.

Here are notes on the three most interesting deep sky objects I also looked at with my 18-inch f/4.3 Starmaster --

vdB 1 = LBN 578
00 10 46.4 +58 46 10
Size: 5'
This relatively bright reflection nebula appeared as a moderately large glow surrounding a bright triangle of stars including mag 8.3 BD+5722 = HD 627 and a pair of mag 8 stars (39" separation) about 1.5' SSW. The glow is perhaps 3' in size and irregularly shaped. Out of curiosity, I tried a broadband DeepSky filter and it darkened the background considerably and sharpened the contrast with the sky around the periphery, so the border was easier to trace. A nearby Herbig-Haro object (HH 164) was visible as a detached, very small faint glow surrounding a dim star 6' NE. Although I've never run across mention of this object before, it's located just 25' SE of mag 2.2 Beta Cas (Caph) in the same low power field and deserves to be on any list of interesting, obscure objects.

Preite-Martinez 1-333
21 40 59.3 +58 58 49
Size: 40" (visual estimate)
This object is not on any list of planetaries you're likely to run across for the simple reason it was verified to be a true planetary in 2009! It had been previously listed as a PN candidate due to its colors, but was verified spectroscopically just recently. I was able to pick it up immediately unfiltered at 175x as a faint, very unevenly lit, 40" disc. A UHC filter gave a good contrast gain and the disc could be held continuously. The outline was well-defined and circular, though clearly brighter on the south rim. At times a brighter spot or knot appeared on the south end, at other times it appeared a larger arc was brighter along the south side. The view through the UHC filter was preferred over the OIII. Like vdB 1, the location is pretty remarkable as it shares the same field with Mu Cephei, Herschel's Garnet Star! The new planetary is located 23' to the NW of the reddish star.

vdB 152 = Ced 201 = LBN 531
22 13.5 +70 15
Size: 6'
This fairly bright reflection nebula surrounds mag 8.8 BD+69 1231 and is a photographic showpiece. It was easily picked up at 175x as an obvious, moderately large glow encasing the bright star. The glow is perhaps 1.5' in diameter, mostly extending to the east of the star and weak or non-existent on the southwest side. The illuminating star is furthest north in a string with two additional mag 8-9 stars 7' SSE and 12' SSE. Besides these striking stars, a bright double star shares the same low power field 15' SW (Struve 2883 = 5.7/7.7 at 15"). Deep images reveal this reflection nebula is the head of a very large cometary nebula with a long "tail" extending to the north. Check out the image at http://www.astrogb.com/vdb152.htm

Steve