Deneb Algedi (Delta Capricorni) 4?
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H.E. Bond, E. Nelan,
Like Sirius, Delta
Capricorni Aa is a bluish
white star that is much
brighter, hotter, and
massive than our Sun,
Sol. (See Sloan Digital
Sky Survey field images
of Delta Capricorni
Delta Capricorni is located about 38.6 light-years (ly) from Sol. It lies in the northeastern part of (21:47:2.4-16:7:38.2, ICRS 2000.0) of Constellation Capricornus, the Water or Sea Goat -- southwest of Sadalmelik (Alpha Aquarii) and southeast of Sadalsuud (Beta Aquarii) and the Saturn Nebula (NGC 7009), slightly northeast of Theta Capricorni and M30; slightly southeast of Algedi (Alpha1,2 Capricorni) Dabih (Beta1 Capricorni), northwest of Fomalhaut (Alpha Piscis Australis), and west of Skat (Delta Aquarii). In the western hemisphere, it came to be called "Deneb Algedi (or Algiedi)" for the tail of the goat (Richard Hinckley Allen, 1889: page 141) as well as Scheddi or Sheddi (which also used for Gamma Capricorni). The primary star is a spectroscopic double with two additional faint companions at relatively wide separations of 69 and 115+ arcseconds, which were probably first discerned by Sir John Frederick William Herschel (1792–1871). (See an animation of the binary and potentially habitable zone orbits of the Aab system, with a table of basic orbital and physical characteristics.)
Although Star Aa has been classified by some as a yellowish white giant star, it is probably a bluish white subgiant, of spectral and luminosity type A3-F2 IV-IIIm whose exact classification has probably been made particularly difficult by light pollution from its close stellar companion (NASA Stars and Exoplanet Database; Gray et al, 2006; Garrison and Beattie, 1998; Malasan et al, 1989; and Srivastava et al, 1988). Based on a spectral classification as an A-type star, the star should have around 2.0 Solar mass (Batten andFletcher, 1992). Assuming a synchronous orbit with its companion star and an orbital inclination of 72.5 degrees, it should have around 1.9 times Sol's diameter (Batten and Fletcher, 1992). Star Aa probably has around 8.5 to 9.7 times Sol's visual luminosity. The system may be around 600 million years old (Trilling et al, 2006: Table 1, page 42; and Figure 1, page 51).
Although the Delta Capricorni system was "certainly associated" with an IRAS point source (21442-1621) for excess infrared emissions at a separation of 5 arc-seconds, particularly at 12 micrometers (Friedemann et al, 1996: Table 3, page 223), the star does not appear to be particularly "dusty" (Cheng et al, 1992). While early indications of gas streaming and possibly a gaseous disk around the primary comparable in size to the star (R. K. Srivastava, 1988) possibly from mass ejection during flares (Wonnacott et al, 1992), no excess infrared emission from dust particles at 24 and 70 micrometers were detected with the Spitzer Space Telescope (Trilling et al, 2006: Table 1, page 42; and Figure 1, page 51; and Rhee et al, 2007). Although very young stars tend to have circumstellar dust or "debris" disks, such dust grains generally disappear quickly during the formation of the star. Despite their relatively youth, many A-type stars (like Delta Capricorni Aa) are already old enough that most of the dust found around these relatively short-lived are not a remnant of the star formation process but are instead generated through collisions of larger proto-planetary bodies. If located within a few AUs of the central star, this dust is warmed to temperatures that produce near-infrared emission with small grains producing scattered light.
After radial velocity variations were found in 1906 by V.M. Sliper), Star Aa was subsequently determined to have a closely orbiting companion (a bright single-line spectroscopic binary) at a separation of about 0.025 AUs (Budding et al, 2004) with an orbital period slightly over 1.02 days (Malkov et al, 2006; Pourbaix et al, 2004; Wonnacott et al, 1992; Batten and Fletcher, 1992; Malasan et al, 1989; Alan H. Batten, 1961a; and Maryjane Stewart, 1958), and so the system has been classified as an Algol-type eclipsing binary. The diameters of both stars are estimated to be small enough to lie within their Roche lobes (Malasan et al, 1989). "A sinusoidal variation is evident ..., which is indicative of the possible presence of a third body, having a period of nearly 62 years, however, it is yet to be confirmed" (R. K. Srivastava, 1988). Star Aa is possibly also a Delta Scuti-type variable (like Altair and Denebola, which vary in brightness by small amounts over periods lasting only hours and including radial as well as non-radial pulsations). Useful catalogue numbers and designations for the star include: Del Cap A, 49 Del Cap, HR 8322, Gl 837, HD 207098, HIP 107556, BD-16 5943, SAO 164644, FK5 819, LTT 8693, ADS 15314 A, WDS 21470-1608 A, and HJ 3056 A.
An Earth-type planet could have liquid water in a stable orbit centered around 3 AU from Star Aa -- between the orbital distances of the Main Asteroid Belt and Jupiter in the Solar System. Such a planet would have an orbital period around 3.7 years. If there is life on any Earth-type planet orbiting youthful Stars Aa and Ab, it is likely to be primitive single-cell, anaerobic (non-oxygen producing) bacteria under constant bombardment by meteorites and comets as Earth was for the first billion years. Since there is unlikely to be free oxygen in the atmosphere of such a planet, it probably would not have an ozone layer (O3) as Star Aa puts out a lot more hard radiation in ultraviolet wavelengths than Sol. Such a planet would, however, be difficult to detect using present astronomical methods and equipment.
Based on extreme ultraviolet (EUV) observations, Star Ab, the dim eclipsing companion of the primary, is no longer thought to be a white dwarf, stellar remnant. Optical and EUV observations indicate that the object is a mildly "active late-type star" (Wonnacott et al, 1992), possibly a spectral type K or late G star on the main sequence or still contracting towards it (Batten and Fletcher, 1992). Star Ab is probably in synchronous orbit with the primary. It appears to have at least one active star spot (Wonnacott et al, 1992). The star may have around 73 percent of Sol's mass (Batten and Fletcher, 1992). (See an animation of the binary and potentially habitable zone orbits of the Aab system, with a table of basic orbital and physical characteristics.)
The object's visual magnitude of 15.8 at a distance of 38.6 light-years implies a visual luminosity of less than 6/100,000 of Sol's. In 1901, it was separated from the primary by around 820 AUs (Sep(AB)= 69.1"), at a position angle of 94 degrees. Some useful star designations are: BD-16 5943 B, ADS 15314 B, WDS 21470-1608 B, and HJ 3056 B.
The object's visual magnitude of 12.7 at a distance of 38.6 light-years implies a visual luminosity of less than 1/1000 of Sol's . Between 1901 and 1910, it was separated from Star A by around 1,400 AUs (Sep(AC)= 115.2" (at a position angle of 302 degrees) in 1901 and 118.9 in 1910). Some useful star designations are: BD-16 5943 C, ADS 15314 C, WDS 21470-1608 C, and HJ 3056 C.
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Denebola.
|Star System||Spectra &|
|L 715-89||M3 V||3.3|
|GJ 1265||M V||5.8|
|Wolf 918||M3 V||6.6|
|LP 760-3||M6.5 V||7.3|
|Wolf 1561 AB||M4.5 Ve |
|L 714-88||M3.5 V||9.0|
|L 856-54 AB||M3.5 V |
|* plus bright stars *||. . .|
|Psi Capricorni||F4-5 V||16|
|HR 7898||G8-K0 V||16|
Up-to-date technical summaries on this stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARCNS, and SIMBAD; and the NASA Stars and Exoplanet Database. Additional information may be available at Roger Wilcox's Internet Stellar Database, and from www.alcyone.de for Deneb Algedi. New research papers may eventually become available at the SAO/NASA ADS.
Constellation Capricornus represents the Water or Sea Goat. According to Eratosthenes of Cyrene (284-202 BCE), Capricornus represents the son of Aegipan, who lived with the Greek God Zeus on Mount Ida on Crete and accompanied Zeus in battle against the Titans. He found the Horns of the Sea (Conchas) and taught his companions how to use them against the Titans. Zeus subsequently placed Capricornus under the stars in a way that he was depicted as part goat and part fish. For more information and an illustration of the constellation, go to Christine Kronberg's Capricornus. For another illustration, see David Haworth's Capricornus.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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