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The Unusual Distributions of Ionized Material and Molecular Hydrogen in NGC 6881: Signposts of Multiple Events of Bipolar Ejection in a Planetary Nebula
The planetary nebula NGC 6881 displays in the optical a quadrupolarmorphology consisting of two pairs of highly collimated bipolar lobesaligned along different directions. An additional bipolar ejection isrevealed by the hydrogen molecular emission, but its wide hourglassmorphology is very different from that of the ionized material. Toinvestigate in detail the spatial distribution of molecular hydrogen andionized material within NGC 6881, and to determine the prevalentexcitation mechanism of the H2 emission, we have obtained newnear-IR Brγ and H2 and optical Hα and [N II]images, as well as intermediate resolution JHK spectra. Theseobservations confirm the association of the H2 bipolar lobesto NGC 6881 and find that the prevalent excitation mechanism iscollisional. The detailed morphology and very different collimationdegree of the H2 and ionized bipolar lobes of NGC 6881 notonly imply that multiple bipolar ejections have occurred in this nebula,but also that the dominant shaping agent is different for each bipolarejection: a bipolar stellar wind most likely produced the H2lobes, while highly collimated outflows are carving out the ionizedlobes into the thick circumstellar envelope. The asymmetry between thesoutheast and northwest H2 bipolar lobes suggests theinteraction of the nebula with an inhomogeneous interstellar medium. Wefind evidence that places NGC 6881 in the H II region Sh 2-109 along theOrion local spiral arm.
| Multiple H2 protostellar jets in the bright-rimmed globule IC 1396-N
We present near infrared images of the bright rimmed globule IC 1396-Nin the H_2 2.12 mu m narrow band filter as well as in broad band J, H,and K, filters. We detected several chains of collimated H_2 knotsinside the globule, having different luminosities but similarorientations in the sky. Most of the knots are associated with peaks ofhigh velocity CO emission, indicating that they trace shocked regionsalong collimated stellar jets. From the H2 knots morphologyand orientation, we identify at least three different jets: one of themis driven by the young protostar associated with IRAS 21391+5802, whichrepresents the most luminous object in the region, while we were able toidentify only one of the two other driving sources by means of nearinfrared photometry. Our photometry reveals the existence of a clusterof young embedded sources located in a south-north line which followsthe distribution of the high density gas and testifies for a highlyefficient star formation activity through all the globule. Based onobservations collected with the AZT-24 Telescope at Campo Imperatore, ajoint project between the Astronomical Observatories of Roma (OAR,Italy), Teramo (OACT, Italy) and Pulkovo (OAP, Russia).
| Northern JHK Standard Stars for Array Detectors
We report J, H, and K photometry of 86 stars in 40 fields in theNorthern Hemisphere. The fields are smaller than or comparable to a 4' x4' field of view and are roughly uniformly distributed over the sky,making them suitable for a homogeneous broadband calibration network fornear-infrared panoramic detectors. K magnitudes range from 8.5 to 14 andJ-K colors, from -0.2 to 1.2. The photometry is derived from a total of3899 reduced images; each star has been measured, on average, 26.0 timesper filter on 5.5 nights. Typical errors on the photometry are ~0.012mag.
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