* Astronomy

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RE: Planetary nebulae

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Planetary Nebulae

Planetary nebulae (PNs) are among the most beautiful and interesting astronomical objects, but their name is misleading. "Nebula" is Latin for "cloud," and astronomers use it to refer to various kinds of luminous gas clouds. The 18th century French astronomer Charles Messier compiled a catalogue of nebulous celestial objects that could be confused with comets. This catalogue of noncomets included some compact, symmetric gaseous nebulae that, when seen through a small telescope, resembled the disk of Uranus. Immediately after its discovery by William Herschel in 1781, Uranus became a very famous astronomical object, so it must have seemed quite natural to call the recently discovered round nebulae "planetary," in the sense of "Uranus-like." Scientists did not understand what PNs are until the early 20th century, and by that time the name had been in use for so long that it was too late to overcome the power of tradition.
The true nature of PNs became clear when their spectra were observed and correctly interpreted. Some dusty gaseous nebulae are illuminated by a bright nearby star and shine by reflecting the stellar light. But while PNs do have stars in the center, they are not reflection nebulae. Planetary nebulae are much brighter than their central stars, and in each case, the spectra of the star and the nebula are very different.
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Title: Planetary Nebulae Detected in the Spitzer Space Telescope GLIMPSE 3D Legacy Survey
Authors: ong Zhang, Chih-Hao Hsia, Sun Kwok

We used the data from the Spitzer Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) to investigate the mid-infrared (MIR) properties of planetary nebulae (PNs) and PN candidates. In previous studies of GLIMPSE I& II data, we have shown that these MIR data are very useful in distinguishing PNs from other emission-line objects. In the present paper, we focus on the PNs in the field of the GLIMPSE 3D survey, which has a more extensive latitude coverage. We found a total of 90 Macquarie-AAO-Strasbourg (MASH) and MASH II PNs and 101 known PNs to have visible MIR counterparts in the GLIMPSE 3D survey area. The images and photometry of these PNs are presented. Combining the derived IRAC photometry at 3.6, 4.5, 5.8, 8.0 um with the existing photometric measurements from other infrared catalogues, we are able to construct spectral energy distributions (SEDs) of these PNs. Among the most notable objects in this survey is the PN M1-41, whose GLIMPSE 3D image reveals a large bipolar structure of more than 3 arcmin in extent.

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Title: Spitzer Observations of Planetary Nebulae
Authors: You-Hua Chu (U of Illinois)

The Spitzer Space Telescope has three science instruments (IRAC, MIPS, and IRS) that can take images at 3.6, 4.5, 5.8, 8.0, 24, 70, and 160 microns, spectra over 5--38 microns, and spectral energy distribution over 52--100 microns. The Spitzer archive contains targeted imaging observations for more than 100 PNe. Spitzer legacy surveys, particularly the GLIMPSE survey of the Galactic plane, contain additional serendipitous imaging observations of PNe. Spitzer imaging and spectroscopic observations of PNe allow us to investigate atomic/molecular line emission and dust continuum from the nebulae as well as circumstellar dust disks around the central stars. Highlights of Spitzer observations of PNe are reviewed in this paper.

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Title: A Herschel study of Planetary Nebulae
Authors: G. C. Van de Steene, K. M. Exter, P. A. M. van Hoof, T. L. Lim, M. J. Barlow, M. Matsuura, T. Ueta, the MESS Consortium

We present Herschel PACS and SPIRE images of the dust shells around the planetary nebulae NGC 650, NGC 6853, and NGC 6720, as well as images showing the dust temperature in their shells. The latter shows a rich structure, which indicates that internal extinction in the UV is important despite the highly evolved status of the nebulae.

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Dying stars eat comets

When the sun dies, it's not just Earth that will be doomed - the destruction will reach as far as the comets in the outer solar system. That's according to a new explanation of the behaviour of planetary nebulae - bubbles of gas sloughed off by dying stars.
There are two methods for calculating the abundance of elements in planetary nebulae: looking at light emitted when electrons and ionised atoms recombine, or looking at the energy emitted by atoms excited by collisions. Yet they yield very different results, a discrepancy that has baffled astronomers for decades.
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Title: Searching for Faint Planetary Nebulae Using the Digital Sky Survey
Authors: George H. Jacoby, Matthias Kronberger, Dana Patchick, Philipp Teutsch, Jaakko Saloranta, Michael Howell, Richard Crisp, Dave Riddle, Agnés Acker, David J. Frew, Quentin Parker

Recent Halpha surveys such as SHS and IPHAS have improved the completeness of the Galactic planetary nebula (PN) census. We now know of ~3,000 PNe in the Galaxy, but this is far short of most estimates, typically ~25,000 or more for the total population. The size of the Galactic PN population is required to derive an accurate estimate of the chemical enrichment rates of nitrogen, carbon, and helium. In addition, a high PN count (~20,000) is strong evidence that most 1-8 Msun main sequence stars will go through a PN phase, while a low count (<10,000) argues that special conditions (e.g., a close binary interaction) are required to form a PN. We describe a technique for finding hundreds more PNe using the existing data collections of the digital sky surveys, thereby improving the census of Galactic PNe.

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Solved: 30-year-old mystery of 'planetary nebulae'
Astronomers have resolved a 30-year-old argument, ruling out a one-size-fits-all mechanism for shaping some of the most beautiful objects in space -- the "planetary nebulae".

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A planetary nebula is an emission nebula consisting of a glowing shell of gas and plasma formed by certain types of stars when they die. The name originated in the 18th century because of their similarity in appearance to giant planets when viewed through small optical telescopes, and is unrelated to the planets of the solar system. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.

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Title: Spitzer 24 um Images of Planetary Nebulae
Authors: Y.-H. Chu, R.A. Gruendl, M.A. Guerrero, K.Y.L. Su, J. Bilikova, M. Cohen, Q.A. Parker, K. Volk, A. Caulet, W.-P. Chen, J.L. Hora, T. Rauch
(Version v2)

Spitzer MIPS 24 um images were obtained for 36 Galactic planetary nebulae (PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective temperatures of ~100,000 K or higher. Diffuse 24 um emission is detected in 28 of these PNe. The eight non-detections are angularly large PNe with very low H-alpha surface brightnesses. We find three types of correspondence between the 24 um emission and H-alpha line emission of these PNe: six show 24 um emission more extended than H-alpha emission, nine have a similar extent at 24 um and H-alpha, and 13 show diffuse 24 um emission near the center of the H-alpha shell. The sizes and surface brightnesses of these three groups of PNe and the non-detections suggest an evolutionary sequence, with the youngest ones being brightest and the most evolved ones undetected. The 24 um band emission from these PNe is attributed to [O IV] 25.9 um and [Ne V] 24.3 um line emission and dust continuum emission, but the relative contributions of these three components depend on the temperature of the central star and the distribution of gas and dust in the nebula.

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Astronomers at the University of Rochester, home to one of the world's largest groups of planetary nebulae specialists, have announced that low-mass stars and possibly even super-Jupiter-sized planets may be responsible for creating some of the most breathtaking objects in the sky.
The news is ironic because the name "planetary" nebula has always been a misnomer. When these objects were discovered 300 years ago, astronomers couldn't tell what they were and named them for their resemblance to the planet Uranus. But as early as the mid-19th century, astronomers realized these objects are really great clouds of dust emitted by dying stars.
Now, Rochester researchers have found that planets or low-mass stars orbiting these aged stars may indeed be pivotal to the creation of the nebulae's fantastic appearance.

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