Tau Ceti

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For the computer game, see Tau Ceti (computer game).
Tau Ceti
Location of Tau Ceti
Tau Ceti (highlighted center) in the southern part of the constellation Cetus.
Observation data
Epoch J2000      Equinox J2000
Constellation Cetus
Right ascension 01h 44m 04.0829s[1]
Declination −15° 56′ 14.928″[1]
Apparent magnitude (V) 3.50[1]
Characteristics
Spectral type G8 V[1]
U−B color index +0.22[1]
B−V color index +0.72[1]
Variable type None
Astrometry
Radial velocity (Rv)-16.4[1] km/s
Proper motion (μ) RA: −1721.94[1] mas/yr
Dec.: 854.17[1] mas/yr
Parallax (π)274.18 ± 0.80 mas[1]
Distance11.90 ± 0.03 ly
(3.65 ± 0.01 pc)
Absolute magnitude (MV)5.68
Details
Mass0.81 M☉
Radius0.83[2] R☉
Luminosity0.59 L☉
Surface gravity (log g)4.4[3] cgs
Temperature5,344 ± 50[4] K
Metallicity22–74%[3][5]
Rotation34 days[6]
Age~1.0 × 1010[2] years
Other designations
Durre Menthor, 52 Ceti, HD 10700, HR 509, BD-16°295, GCTP 365.00, GJ 71, LHS 146, LTT 935, LFT 159, SAO 147986, LPM 84, FK5 59, HIP 8102.[1]

Tau Ceti (τ Cet / τ Ceti) is a star in the constellation Cetus that is similar to the Sun in mass and spectral type and relatively nearby. Tau Ceti is a "metal-deficient" star and therefore is thought to be less likely to have rocky planets around it. However, observations have detected over 10 times as much dust surrounding Tau Ceti as is present in the solar system. No companions have yet been detected through astrometric or radial velocity measurements. (But this only excludes gas giants in relatively close orbits, such as a Hot Jupiter.) Tau Ceti can be seen with the unaided eye as a faint third-magnitude star.

The star Tau Ceti does not have a widely-recognized traditional name, as do many of the more prominent stars. As seen from Tau Ceti, the Sun is a 2nd magnitude star in the constellation Boötes.[a]

Motion

The proper motion of a star is the amount of motion of a star across the celestial sphere. It is determined by comparing the position of a star relative to the more distant background objects. In the case of Tau Ceti, this star is considered to be a high proper motion star. In spite of this, however, Tau Ceti only has an annual traverse of slightly more than an arc second. It will require several thousands years before the location of this star shifts by more than a degree.

A high proper motion is an indicator of close proximity to the Sun.[7] Nearby stars can traverse an angle of arc across the sky more rapidly than the distant, background stars. Hence stars with a high proper motion are good candidates for parallax studies. In the case of Tau Ceti, the parallax measurements indicate a distance of slightly less than 12 light-years. This places it among the nearest star systems to the Sun, and the next-closest spectral class-G star after Alpha Centauri A.[8]

The radial velocity of a star is the motion toward or away from the Sun. Unlike proper motion, a star's radial velocity can not be directly observed, but must be determined through measurement of the spectrum. Due to the Doppler shift, the absorption lines in the spectrum of a star will be shifted slightly toward the red (or longer wavelengths) if the star is moving away from us, or toward blue (or shorter wavelengths) when it moves toward us. In the case of Tau Ceti, the radial velocity is about -17 km/s, with the negative value indicating that it is moving toward the Sun.[9]

The distance to Tau Ceti, along with its proper motion and radial velocity, allow the motion of the star through space to be calculated. The space velocity relative to the Sun is about 37 km/s.[b] This result can then be used to compute an orbital path of Tau Ceti through the Milky Way galaxy. It has a mean galacto-centric distance of 9.7 kpc (32,000 light-years) and an orbital eccentricity of 0.22.[10]

Physical properties

The Sun (left) is both larger and somewhat hotter than the less active Tau Ceti (right).

Most of what is known about the physical properties of Tau Ceti has been determined through spectroscopy measurements. By comparing the spectrum to computed models of stellar evolution, the age, mass, radius and luminosity of Tau Ceti can be estimated. However, by using a device called an astronomical interferometer, fairly accurate measurements of the radius of the star can be made directly. The interferometer uses a long baseline to measure angles much smaller than can be resolved with a conventional telescope. Through this means, the radius of Tau Ceti has been measured as 81.6 ± 1.3% of the solar radius. This is about the size that is expected for a star with somewhat lower mass than the Sun.[2]

The rotation period for Tau Ceti was measured by periodic variations in the classic H and K absorption lines of singly-ionized Calcium, or Ca II. These lines are closely associated with surface magnetic activity,[11] so the period of variation measures the time required for the activity sites to complete a full rotation about the star. By this means the rotation period for Tau Ceti is estimated to be 34 days.[6]

Due to the Doppler effect, the rotation rate of a star effects the width of the absorption lines in the spectrum. (Light from the side of the star moving away from the observer will be shifted to a longer wavelength, light from the side moving away from the observer will be shifted toward a shorter wavelength.) So by analyzing the width of these lines, the rotational velocity of a star can be estimated. The projected rotation velocity for Tau Ceti is:

.

where veq is the velocity at the equator and i is the inclination angle of the rotation axis to the line of sight. For a typical G8 star, the rotation velocity is about 2.5 km/s. The relatively low rotational velocity measurements may indicate that Tau Ceti is being viewed from nearly the direction of its pole.[12][13]

The chemical composition of a star provides important clues to its evolutionary history, including the age at which it formed. The interstellar medium of dust and gas from which stars form is primarily composed of Hydrogen and Helium with trace amounts of heavier elements. As nearby stars continually evolve and die, they seed the interstellar medium with an increasing portion of heavier elements. Thus younger stars will tend to have a higher portion of heavy elements in their atmospheres than do the older stars. These heavy elements are termed metals by astronomers and the portion of heavy elements is the metallicity.[14]

The amount of metallicity in a star is given in terms of the ratio of Iron (Fe), an easily observed heavy element, to Hydrogen. In the case of Tau Ceti, the atmospheric metallicity is roughly:

or somewhat lower than the solar abundance. (However, past measurements have varied from -0.13 to -0.60.)[3][5] This bracket notation is the logarithm of the relative iron abundance as compared to the Sun.

A negative value for the metallicity means a lower abundance of Iron, which indicates this star is probably older than the Sun. The estimated age of Tau Ceti is about 10 Gyr, or significantly older than the Sun (4.57 Gyr) and a substantial portion of the age of the visible Universe. However, computed age estimates for Tau Ceti can range from 4,400–12,000 Myr, depending on the model adopted.[2]

Besides rotation, another factor that can widen the absorption features in the spectrum of a star is pressure-broadening. (See spectral line.) The presence of nearby particles will affect the radiation emitted by an individual particle. So the line width is dependent on the surface pressure of the star, which in turn is determined by the temperature and surface gravity. This technique was used to determine the surface gravity of Tau Ceti. The log g, or logarithm of the star's surface gravity, is about 4.4—very close to the log g = 4.44 for the Sun.[3]

The chromosphere of Tau Ceti, which is the portion of a star's atmosphere just above the light-emitting photosphere, currently displays little or no magnetic activity. This indicates a relatively stable star with only a low level of periodic magnetic variation.[15] Alternatively it has been suggested that the star could be in a low-activity state analogous to a Maunder minimum.[16][17] The last is a name that was given to a historical period when the appearance of sunspots became exceedingly rare on the Sun's surface, and is associated with the Little Ice Age in Europe.

Debris disc around star

In 2004 a team of UK astronomers led by Jane Greaves discovered that Tau Ceti has more than ten times the amount of cometary and asteroidal material orbiting it than does our Sun. (However the dust belt around Tau Ceti is only one-twentieth as dense as the belt around Epsilon Eridani.) This was determined by measuring the disc of cold dust orbiting the star produced by collisions between such small bodies.

This result puts a damper on the possibility of complex life in this system, as planets there would suffer from large impact events roughly ten times more frequently than Earth. However, it is possible that a large Jupiter-sized gas giant could deflect comets and asteroids.[18]

The debris disk was discovered by measuring the amount of radiation emitted by the system in the far infrared portion of the spectrum. The disk forms a symmetric feature that is centered on the star, and the outer radius averages 55 astronomical units. (An astronomical unit, or AU, is the average distance from the Earth to the Sun.) The lack of infrared radiation from the warmer parts of the disk near Tau Ceti imply an inner cut-off at a radius of 10 A.U. By comparison, the Solar System's Kuiper belt extends from 30 to 50 A.U. To be maintained over a long period of time, this ring of dust must be constantly replentished through collisions by larger bodies.[18]

Discoveries of extrasolar planets have shown a correlation between the presence of such planets and a relatively high metallicity of the parent star. This suggests that stars with lower metallicity have a reduced chance of possessing planets.[19] But this new evidence does increase the likelihood that one or more rocky planets orbit the star.

A survey of nearby stars by the Hubble Space Telescope's Wide Field and Planetary Camera was completed in 1999. This program included a search for faint companions to Tau Ceti, but none were discovered to limits of the telescope's resolving power.[20] However this does not rule out the possibility of a smaller, Earth-like planet in orbit around the star.

As the luminosity of Tau Ceti equal to only 55% of the Sun,[10] a terrestrial planet would need to orbit this star at a distance of approximately 0.7 A.U. in order match the solar-illumination level of the Earth. This is slightly less than the average distance between the planet Venus and the Sun. The bulk of the debris disk appears to be orbiting Tau Ceti at a distance of 35–50 AU, or well outside the orbit of this hypothetical Earth-like planet. In this respect the dust belt around Tau Ceti may correspond to the Kuiper belt that lies outside the orbit of Neptune in the solar system.[18]

SETI

Project Ozma was a 1960 program that was intended to "search for extraterrestrial intelligence", or SETI, by examining selected stars for indications of artificial radio signals. It was run by the astronomer Frank Drake, who selected Tau Ceti and Epsilon Eridani as the initial targets for the search. These stars were chosen because they were located near the solar system, and because they were physically similar to the Sun. However, no artificial signals were found despite 200 hours of observations.[21] Subsequent radio searches of this star system have also turned up negative.

See also

Notes

  1. ^ From Tau Ceti the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=13h 44m 04s, Dec=15° 56′ 14″, which is located near Tau Boötis. The absolute magnitude of the Sun is 4.8, so, at a distance of 3.64 parsecs, the Sun would have an apparent magnitude .
  2. ^ The space velocity components are: U = +18; V = +29, and W = +13. This yields a net space velocity of  km/s.

References

  1. ^ a b c d e f g h i j k "SIMBAD Query Result: HD 10700 -- High proper-motion Star". Centre de Données astronomiques de Strasbourg. Retrieved 2007-08-14.
  2. ^ a b c d E. Di Folco, F. Thévenin, P. Kervella, A. Domiciano de Souza, V. Coudé du Foresto, D. Ségransan, P. Morel (2004). "VLTI near-IR interferometric observations of Vega-Like Stars". Astronomy and Astrophysics. 426: 601–617. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^ a b c d G. Cayrel de Strobel, B. Hauck, P. Francois, F. Thevenin, E. Friel, M. Mermilliod, S. Borde (1991). "A catalogue of Fe/H determinations - 1991 edition". Astronomy and Astrophysics Supplement Series. 95 (2): 273–336. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Santos, N. C.; Israelian, G.; García López, R. J.; Mayor, M.; Rebolo, R.; Randich, S.; Ecuvillon, A.; Domínguez Cerdeña, C. (2004). "Are beryllium abundances anomalous in stars with giant planets?". Astronomy and Astrophysics. 427: 1085–1096. Retrieved 2007-02-26.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ a b C. Flynn, O. Morell (1997). "Metallicities and kinematics of G and K dwarfs". Monthly Notices of the Royal Astronomical Society. 286 (3): 617–625. Retrieved 2007-08-14.
  6. ^ a b S. Baliunas, D. Sokoloff, W. Soon (1996). "Magnetic Field and Rotation in Lower Main-Sequence Stars: an Empirical Time-dependent Magnetic Bode's Relation?". Astrophysical Journal Letters. 457: L99. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ Reid, Neill (February 23, 2002). "Meeting the neighbours: NStars and 2MASS". Space Telescope Science Institute. Retrieved 2006-12-11.
  8. ^ Henry, Todd J. (October 1, 2006). "The One Hundred Nearest Star Systems". Research Consortium on Nearby Stars. Retrieved 2006-12-11.
  9. ^ R.P. Butler, G.W. Marcy, E. Williams, C. McCarthy, P. Dosanjh, S.S. Vogt (1996). "Attaining Doppler Precision of 3 M s-1". Publications of the Astronomical Society of the Pacific. 108: 500. Retrieved 2006-12-11.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ a b G. F. Porto de Mello, E. F. del Peloso, L. Ghezzi (2006). "Astrobiologically interesting stars within 10 parsecs of the Sun". Astrobiology. 6 (2): 308–331. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ "H-K Project: Overview of Chromospheric Activity". Mount Wilson Observatory. Retrieved 2006-11-15.
  12. ^ D. F. Gray, S. L. Baliunas (1994). "The activity cycle of tau Ceti". Astrophysical Journal. 427 (2): 1042–1047.
  13. ^ Jeffrey C. Hall, G. W. Lockwood, Erika L. Gibb (1995). "Activity cycles in cool stars. 1: Observation and analysis methods and case studies of four well-observed examples". Astrophysical Journal. 442 (2): 778–793.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. ^ G. Carraro, Y. K. Ng, L. Portinari (1999). "Age Metallicity Relation and Star Formation History of the Galactic Disk". Monthly Notices of the Royal Astronomical Society. 296 (4): 1045–1056. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. ^ P. Frick, S. L. Baliunas, D. Galyagin, D. Sokoloff, W. Soon (1997). "Wavelet Analysis of Stellar Chromospheric Activity Variations". The Astrophysical Journal. 483 (1): 426–434. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. ^ P. G. Judge, S. H. Saar (July 18, 1995). "The outer solar atmosphere during the Maunder Minimum: A stellar perspective". High Altitude Observatory. Retrieved 2007-08-14. {{cite web}}: Check date values in: |date= (help)
  17. ^ Philip G. Judge, Steven H. Saar, Mats Carlsson, and Thomas R. Ayres (2004). "A Comparison of the Outer Atmosphere of the "Flat Activity" Star τ Ceti (G8 V) with the Sun (G2 V) and α Centauri A (G2 V)". The Astrophysical Journal. 609 (1): 392–406. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  18. ^ a b c J. S. Greaves, M. C. Wyatt, W. S. Holland, W. R. F. Dent (2004). "The debris disc around tau Ceti: a massive analogue to the Kuiper Belt". Monthly Notices of the Royal Astronomical Society. 351 (3): L54–L58. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  19. ^ G. Gonzalez (March 17-21, 1997). "The Stellar Metallicity - Planet Connection". ASP Conference Series. Retrieved 2006-11-08. {{cite web}}: Check date values in: |date= (help)
  20. ^ D. J. Schroeder, D. A. Golimowski, R. A. Brukardt, C. J. Burrows, J. J. Caldwell, W. G. Fastie, H. C. Ford, B. Hesman, I. Kletskin, J. E. Krist, P. Royle, R. A. Zubrowski (2000). "A Search for Faint Companions to Nearby Stars Using the Wide Field Planetary Camera 2". Astronomical Journal. 119 (2): 906–922. Retrieved 2007-08-14.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  21. ^ Alexander, Amir (2006). "The Search for Extraterrestrial Intelligence, A Short History". The Planetary Society. Retrieved 2006-11-08.

External links

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