# Stars/Infrareds

"In the constellation, Orion's head is represented by the star Lamdba Orionis (fuzzy red dot in [the middle of the image on the right]). When viewed in infrared light, NASA's Wide-field Infrared Survey Explorer, or WISE, shows a giant nebula around Lambda Orionis, inflating Orion's head to huge proportions."
"Lambda Orionis is a hot, massive star that is surrounded by several other hot, massive stars, all of which are creating radiation that excites a ring of dust, creating the "Lambda Orionis molecular ring." Also known as SH 2-264, the Lambda Orionis molecular ring is sometimes called the Meissa ring. In Arabic, the star Lambda Orionis is known as "Meissa" or "Al-Maisan," meaning "the shining one." The Meissa Ring is of interest to astronomers because it contains clusters of young stars and proto-stars, or forming stars, embedded within the clouds. With a diameter of approximately 130 light-years, the Lambda Orionis molecular ring is notable for being one of the largest star-forming regions WISE has seen. This is also the largest single image featured by WISE so far, with an area of the sky approximately 10 by 10 degrees in size, equivalent to a grid of 20 by 20 full moons. Nevertheless, at less than one percent of the whole sky's area, it is just a taste of WISE data."
"The bright blue star in the lower left corner of the image is the star Betelgeuse, which represents one shoulder of the hunter Orion. The name Betelgeuse is actually a corruption of the original Arabic phrase "Yad al-Jauza'," meaning "hand of the giant one." Betelgeuse is well known for being a red supergiant star, yet in WISE's infrared view it appears blue, as do most stars in WISE images. This is because most stars, including Betelgeuse, put out more light in the shortest infrared wavelengths of light captured by WISE, and those shorter wavelengths are presented in WISE images as blue and cyan."
"In visible light, Orion's other shoulder is clearly marked by the variable star Bellatrix. In infrared light, however, Bellatrix is a somewhat unremarkable cyan-colored star in the right side of the image. In Latin, Bellatrix means "female warrior"".
"Also seen in this image are two dark nebulae, Barnard 30 and Barnard 35, which are parts of the Meissa ring that are so dense they block out visible light. Barnard 30 is the bright knob of gas and dust in the top center part of the image. Barnard 35 appears as a hook extending towards the center of the ring just above and to the right of the star Betelgeuse. The bright reddish object seen to in the middle right part of the image is the star HR 1763, which is surrounded by another star-forming region, LBN 867."
"Color in this image represents specific wavelengths of infrared light. Blue and cyan represent 3.4- and 4.6-microns, primarily light emitted by hot stars. Green and red represent 12- and 22-micron light, which is mainly radiation from warm dust."

## S stars

Stars "of spectral type S are characterized by unusual photospheric abundances which imply enrichment of the stellar surface by nucleosynthesis products. Spectroscopically, S stars are identified by bands of ZrO and LaO, replacing the TiO bands found in M stars. The spectra of S stars indicate strong enhancement of s-process elements in the photosphere (an accident of nomenclature - when the S spectral type was introduced, the slow neutron capture process was unknown). Abundance analyses show that in S stars, the C/O ratio is very close to unity [...], which also implies the presence of the products of nucleosynthesis at the stellar surface."
The "extrinsic S stars, includes stars which have elemental abundances which appear to have been altered by mass transfer from a binary companion."
The "intrinsic S stars, includes stars which have high luminosity and lie on the asymptotic giant branch (AGB). They show evidence that their compositional abnormalities are a result of nucleosynthesis and [perhaps] convective mixing to the surface. In particular, a defining characteristic which distinguishes the two types is that the intrinsic S stars contain technetium, while the extrinsic S stars do not."
"Both HCN and SiO have readily observable lines at 3 mm." χ Cyg is at a distance of 170 pc, but parallax measurements put it at D = 144 ± 25 pc (Stein 1991), parallax of 5.53 mas (198 ± 38) pc as of 2007 according to SIMBAD.
"All of these stars are bright at 2 µm and therefore have circumstellar dust [...] In one observing session, we obtained a 5 x 5 cross at HPBW spacings for the star χ Cyg in the CO J = 2-1 line. The data were relatively noisy because of limited integration time and weather conditions but do indicate that the envelope is extended with respect to the 25" telescope beam."
χ Cyg was "detected in the SiO v = 1 J = 2-1 maser emission line [...] χ Cyg has an unusually large dust/gas ratio of 9.0 x 10$^{-3}$ [The dust-to-gas ratio for S stars detected in CO J = 1-0 is] 9.0 x 10$^{-3}$ [...] For one star in our sample namely χ Cyg, the SiO J = 2-1, v = 0 emission has been mapped interferometrically [...] the SiO abundance at the base of the expanding envelope must be ~ 2 x 10$^{-5}$ to explain the observed intensity distribution of the SiO emission. Thus, a substantial fraction (30%-50%) of all silicon atoms are in the form of gas phase SiO at the point where molecules are injected into the stellar wind. As the gas moves away from the star, the SiO is depleted from the gas, presumably by the process of grain formation, such that at radii of several x 10$^{15}$ cm, the SiO gas phase abundance has fallen by > 90%. [...] χ Cyg, which has a relatively low mass-loss rate and hence low envelope opacity to UV photons."

## IK Tauri

In 1965 two extremely cool stars were discovered with, at the time, the temperatures of these extremely red objects estimated to be around 1,000 K.
These were designated after the initials of their discoverers as NML Cygni and NML Tauri.
NML Tauri was identified as a Mira variable in 1967.
The name NML Tauri fell into disuse after the star received its variable star designation of IK Tauri.
IK Tauri varies approximately every 470 days between extreme visual magnitudes of 10.8 and 16.5. It was classified as a Mira variable soon after discovery on the basis of its spectrum showing strong hydrogen emission, and its very large visual amplitude. During each cycle the spectrum of the star also varies, consistently reaching M10 near minimum and only M6-M8 at maximum.
IK Tauri has strong maser emission from its extended atmosphere and circumstellar material.
The circumstellar material is rich in dust, with alumina close to the star and silicates further out, each forms a separate shell, one within twice the star's radius and one more than three times its radius, where the densest region of dust is at 6-8 times IK Tauri's radius.
Although IK Tauri is far below naked eye visibility even at maximum brightness, due to the low temperature and strong extinction at visual wavelengths, in the infrared, it is brighter than prominent stars such as Rigel (K-band magnitude +0.18) and comparable to Sirius (K-band magnitude −1.35).
As a Mira variable, IK Tauri is an asymptotic giant branch (AGB) star, originally around 1.5 M$_☉$.
In theory, it has exhausted its core hydrogen and helium, is not massive enough to ignite its carbon-oxygen core, and is now alternately fusing in concentric hydrogen and helium shells; with the inert core growing and the hydrogen shell nearing the surface, mass loss becomes very high and the star becomes highly obscured visually, an infrared star.

## Two Micron All-Sky Survey

"Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The image is derived from the 2MASS Extended Source Catalog (XSC)—more than 1.5 million galaxies, and the Point Source Catalog (PSC)--nearly 0.5 billion Milky Way stars. The galaxies are color coded by redshift (numbers in parentheses) obtained from the UGC, CfA, Tully NBGC, LCRS, 2dF, 6dFGS, and SDSS surveys (and from various observations compiled by the NASA Extragalactic Database), or photo-metrically deduced from the K band (2.2 μm). Blue/purple are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04) and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center)."
The Two Micron All-Sky Survey, or 2MASS, was an astronomical survey of the whole sky in the infrared and one of the most ambitious such projects.
It took place between 1997 and 2001, in two different locations: at the U.S. Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona, IAU code#G00–G99, G91, and at the Cerro Tololo Inter-American Observatory in Chile, IAU code#I00–I99, I02, each using a 1.3-meter telescope for the Northern and Southern Hemisphere, respectively.
It was conducted in the short-wavelength infrared at distinct frequency bands near 2 microns, from which the photometric survey with its mercury cadmium telluride (HgCdTe) detectors derives its name.
The goals of this survey included:
* Detection of galaxies in the "Zone of Avoidance", a strip of sky obscured in visible light by our own galaxy, the Milky Way.
* Detection of brown dwarfs. 2MASS discovered a total of 173, including 2MASS 0939-2448, 2MASS 0415-0935, 2M1207, and 2MASS J04414489+2301513.
* An extensive survey of low mass stars, the most common type of star both in our own galaxy and others.
* Cataloging of all detected stars and galaxies.
* Infrared measurements from the 2MASS survey have been particularly effective at unveiling previously undiscovered star clusters.
The resulting data and images from the survey are currently in the public domain, and may be accessed online for free by anyone. There is also a list of 2MASS science publications with links to free pre-publication copies of the papers.