Object of the Week, December 11, 2022 - A nebula that will make you jump

[Clear Skies]

Two weeks ago I was downloading a bunch of DSS images to piece together a crude mosaic for the extremely large emission nebula Sharpless 2-245 - the "Fishhook Nebula", that lives on the border of the constellations of Taurus & Eridanus and that is part of the (Orion)-Eridanus Bubble (http://www.deepskyforum.com/showthread.php?37-The-Eridanus-Bubble-the-Western-counterpart-of-Barnard-s-Loop).

As the POSS2 Red image loaded, centered on position 04:08:00 -04d20m00s, I noticed a relatively large and bright smudge at bottom left. An image artefact, for sure. I mean... c'mon... I've downloaded a gazillion DSS images over the years and I know an artefact when I see one.

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But there's always that minute chance that such a funny smear is in fact a real object. Better sure than sorry, so I centered it in Aladin. Sure enough, there is was, on the Red image (https://aladin.unistra.fr/AladinLite/?target=04%2010%2008.201-04%2058%2037.99&fov=1.00&survey=CDS%2FP%2FDSS2%2Fred). I then clicked SDSS (https://aladin.unistra.fr/AladinLite/?target=04%2010%2008.201-04%2058%2037.99&fov=1.00&survey=CDS%2FP%2FSDSS9%2Fcolor) and my reaction was akin to Neo in the Matrix (https://www.youtube.com/watch?v=yOOHOT3z3dM): Jesus Chr.., that thing is real!! I pretty much jumped from my seat.

It has to be, by far, the strangest blob I have ever encountered. But is it known..?

As it turns out, it is: it's the "Criss-Cross (https://simbad.u-strasbg.fr/simbad/sim-id?Ident=NAME+Criss-Cross+Nebula&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id) Nebula". Hence the jump (https://www.youtube.com/watch?v=010KyIQjkTk).

There's not too much information available about this blob. A few hits on Google and a few research papers: https://articles.adsabs.harvard.edu/pdf/1997A%26A...324.1165Z & https://arxiv.org/pdf/astro-ph/0702194.pdf.

The jury is still out on the nature of this object. An emission nebula? Supernova remnant? An interstellar cloud of matter excited by a supernova shockwave? If the latter is the case, it would be very similar to the well-known segment H, a.k.a. the "Southeastern Knot (https://articles.adsabs.harvard.edu/pdf/1995ApJ...444..787G)" in the Veil Nebula supernova remnant in Cygnus (https://clearskies.eu/csog/downloads/dsfootw2022#38). The discovery of this object in Eridanus was relatively recent, although it apparently took a few years before it was published in the papers linked to above.

While it is in reach of amateur equipment, some aperture will be required. It's very faint on the POSS2 Blue (https://archive.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_blue&r=04+10+08.21&d=-04+58+37.9&e=J2000&h=30&w=30&f=gif&c=none&fov=NONE&v3=) image (left) and not too bright on the Red (https://archive.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_red&r=04+10+08.21&d=-04+58+37.9&e=J2000&h=30&w=30&f=gif&c=none&fov=NONE&v3=) image (center), either. Size of this nebulosity is about 6' x 3'. The best image available is the DECaLS (https://www.legacysurvey.org/viewer?ra=62.5593&dec=-4.9771&zoom=12&layer=decals-dr7) one (right). A post on facebook (https://m.facebook.com/RADECAstronomy/posts/2334134853512876) tells us it can also be called the "Bra Nebula". I think that is fitting (https://youtu.be/-UJ9K8lMxPA?t=9).

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The position is 04:10:08 -04d59m00s. That's 11 minutes of arc west-southwest of the G0 mag. 7.5 star SAO131003. Otherwise, there isn't really anything going on in the vicinity. Nearest bright star is Omicron-1 Eridani, shining bright yellow at mag. 4.1 but 1.9 degrees to the south-southeast. Nearest bright DSO is the galaxy pair NGC1516A & B, no less than 3.9 degrees to the south-southwest.

As always, there is CSOG guide for the OOTW. Click here to download (https://clearskies.eu/csog/downloads/dsfootw2022#50).

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Now get out there and see if this nebula is within reach. Be sure to report back..!

[Clear Skies]

A follow up on this OOTW... or rather on the post title:

Late October I finally got my hands on a copy of Sven Cederblad's book "Studies of Bright Diffuse Galactic Nebulae (https://twitter.com/clearskies_eu/status/1585605623055618049?s=61&t=qK3r0T1-kU6_9GpvDOd4sw)". Found it on the website of a Swedish online antiquarian. Had been looking for it for only a decade.

The book contained a newspaper clipping.

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I only translated the text today. It reads:

Sportsmen became doctors.

At the doctoral promotion in Lund, mentioned in yesterday's issue, there were two famous sportsmen, who were promoted, allsvenska handball player Sven Sjöstedt to medical dr. and MAI's seven-meter long jump man Sven Cederblad to PhD. Here they are seen in joyful hop over a bench. You can read about the promotion in Uppsala on page 9.

"allsvenska" is the name for the second division (https://en.wikipedia.org/wiki/Allsvenskan_(men%27s_handball)) in Swedish handball, MAI (https://www.mai.se) is the track & field association.

So there you have it: as I noted, nebulae really do make you jump. I rest my case.

[ScottH]

I took a look at the Criss-Cross nebula last evening (2/11/23) using my 16-inch f/4.5 dob. I used 70x and tried my NPB, O-III, and H-Beta filters on it (didn't even attempt to see it unfiltered). I couldn't see anything with either the O-III and H-Beta filters. Only with the NPB filter did I wonder if I was seeing something. I've logged it as a negative observation, however.

This morning, I read the two research papers that Victor had found published on the nebula. The oldest one, published in 1997 by Zanin & Weinberger, made two interesting comments. The first was that "The 'Criss-Cross Nebula', detected by us long ago during systematic searches on the Palomar Sky Survey but never published...". The second was "Interestingly, there is no trace of [O III] ?5007 ?A."

Scott H.