7,137 research outputs found
Kretschmann Scalar for a Kerr-Newman Black Hole
I have derived the Kretschmann scalar for a general black hole of mass m,
angular momentum per unit mass a, and electric charge Q. The Kretschmann scalar
gives the amount of curvature of spacetime, as a function of position near (and
within) a black hole. This allows one to display the "appearance" of the black
hole itself, whether the black hole is merely of stellar mass, or is a
supermassive black hole at the center of an active galaxy. Schwarzschild black
holes, rotating black holes, electrically charged black holes, and rotating
electrically-charged black holes, are all illustrated. Rotating black holes are
discovered to possess a negative curvature that is not analagous to that of a
saddle.Comment: 14 pages including four figure
Gallery of Planetary Nebula Spectra
We present the Gallery of Planetary Nebula Spectra now available at
http://oit.williams.edu/nebulae The website offers high-quality, moderate
resolution (~7-10 A FWHM) spectra of 128 Galactic planetary nebulae from
3600-9600 A, obtained by Kwitter, Henry, and colleagues with the Goldcam
spectrograph at the KPNO 2.1-m or with the RC spectrograph at the CTIO 1.5-m.
The master PN table contains atlas data and an image link. A selected object's
spectrum is displayed in a zoomable window; line identification templates are
provided. In addition to the spectra themselves, the website also contains a
brief discussion of PNe as astronomical objects and as contributors to our
understanding of stellar evolution. We envision that this website, which
concentrates a large amount of data in one place, will be of interest to a
variety of users: researchers might need to check the spectrum of a particular
object of interest; the non-specialist astronomer might simply be interested in
perusing such a collection of spectra; and finally, teachers of introductory
astronomy can use this database to illustrate basic principles of atomic
physics and radiation. To particularly encourage this last use, we have
developed two paper-and-pencil exercises to introduce beginning astronomy
students to the wealth of information that PN spectra contain.Comment: Two pages, two figures. Contributed paper to IAU Symp. 234,
``Planetary Nebulae in our Galaxy and Beyond.'
Voyager investigation of the cosmic diffuse background: Observations of rocket-studied locations with Voyager
Attachments to this final report include 2 papers connected with the Voyager work: 'Voyager Observations of Dust Scattering Near the Coalsack Nebula' and 'Search for the Intergalactic Medium'. An appendix of 12 one-page write-ups prepared in connection with another program, UVISI, is also included. The one-page write-ups are: (1) Sky survey of UV point sources to 600 times fainter than previous (TD-1) survey; (2) Diffuse galactic light: starlight scattered from dust at high galactic latitude; (3) Optical properties of interstellar grains; (4) Fluorescence of molecular hydrogen in the interstellar medium; (5) Line emission from hot interstellar medium and/or hot halo of galaxy; (6) Integrated light of distant galaxies in the ultraviolet; (7) Intergalactic far-ultraviolet radiation field; (8) Radiation from recombining intergalactic medium; (9) Radiation from re-heating of intergalactic medium following recombination; (10) Radiation from radiative decay of dark matter candidates (neutrino, etc.); (11) Reflectivity of the asteroids in the Ultraviolet; and (12) Zodiacal light
Dust near luminous ultraviolet stars
This report describes research activities related to the Infrared Astronomical Satellite (IRAS) sky survey. About 745 luminous stars were examined for the presence of interstellar dust heated by a nearby star. The 'cirrus' discovered by IRAS is thermal radiation from interstellar dust at moderate and high galactic latitudes. The IRAS locates the dust which must (at some level) scatter ultraviolet starlight, although it was expected that thermal emission would be found around virtually every star, most stars shown no detectable emission. And the emission found is not uniform. It is not that the star is embedded in 'an interstellar medium', but rather what is found are discrete clouds that are heated by starlight. An exception is the dearth of clouds near the very hottest stars, implying that the very hottest stars play an active role with respect to destroying or substantially modifying the dust clouds over time. The other possibility is simply that the hottest stars are located in regions lacking in dust, which is counter-intuitive. A bibliography of related journal articles is attached
Diffuse Background Radiation
A new determination of the upper limit to the cosmic diffuse background
radiation, at ~110 nm, of 300 photons s-1 cm-2 sr-1 nm-1, is placed in the
context of diffuse background measurements across the entire electromagnetic
spectrum, including new optical, infrared, visible, and gamma-ray background
measurements. The possibility that observed excess diffuse visible radiation is
due to redshifted cosmological Lyman alpha recomination radiation is explored.
Also, a new standard of units for the display of spectra is advocated.Comment: Nine pages and one figur
Climigration? Population and climate change in Arctic Alaska
Residents of towns and villages in Arctic Alaska live on “the front line of climate change.” Some communities face immediate threats from erosion and flooding associated with thawing permafrost, increasing river flows, and reduced sea ice protection of shorelines. The term climigration, referring to migration caused by climate change, originally was coined for these places. Although initial applications emphasized the need for government relocation policies, it has elsewhere been applied more broadly to encompass unplanned migration as well. Some historical movements have been attributed to climate change, but closer study tends to find multiple causes, making it difficult to quantify the climate contribution. Clearer attribution might come from comparisons of migration rates among places that are similar in most respects, apart from known climatic impacts. We apply this approach using annual 1990–2014 time series on 43 Arctic Alaska towns and villages. Within-community time plots show no indication of enhanced out-migration from the most at-risk communities. More formally, there is no significant difference between net migration rates of at-risk and other places, testing several alternative classifications. Although climigration is not detectable to date, growing risks make either planned or unplanned movements unavoidable in the near future
Dust Scattered Radiation in the Galactic Poles
We have modeled the diffuse background at the Galactic Poles in the
far-ultraviolet (FUV: 1536 \AA) and the near-ultraviolet (NUV: 2316 \AA). The
background is well-fit using a single-scattering dust model with an offset
representing the extragalactic light plus any other contribution to the diffuse
background. We have found a dust albedo of 0.35 -- 0.40 (FUV) and 0.11 -- 0.19
in the NGP () and 0.46 -- 0.56 (FUV) and 0.31 -- 0.33 (NUV) in
the SGP (. The differences in the albedo may reflect changes in
the dust-to-gas ratio over the sky or in the dust distribution. We find offsets
at zero-reddening of 273 -- 286 and 553 -- 581 photons cm s
sr \AA in the FUV and NUV, respectively, in the NGP with similar
values in the SGP.Comment: 5 pages, 7 figure
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