Stellar variation

Tau Ceti

Tau Ceti (τ Cet / τ Ceti, ) is a star in the constellation Cetus that is similar to the Sun in mass and spectral type. At just under 12 light years' distance from the Solar System, it is a relatively close star. Tau Ceti is "metal-deficient" and so it is thought to be less likely to host rocky planets. Observations have detected more than 10 times as much dust surrounding Tau Ceti as is present in the Solar System. The star appears stable, with little stellar variation. No companions have yet been detected around Tau Ceti through astrometric or radial velocity measurements, but given current search refinement this only excludes substellar companions such as large brown dwarfs. Because of the debris disk any planet orbiting Tau Ceti would face far more impact events than the Earth. Despite this hurdle to habitability, its "solar analog" (Sun-like) characteristics have led to widespread interest in the star. It has been consistently listed as a target for Search for Extra-Terrestrial Intelligence (SETI) searches, given its stability and similarity to the Sun.

Tau Ceti does not have a widely recognized traditional name, as do many other prominent stars. It can be seen with the unaided eye as a faint third-magnitude star. As seen from Tau Ceti, the Sun would be a third-magnitude star in the constellation Boötes.

Motion

The proper motion of a star is its amount of movement across the celestial sphere, determined by comparing its position relative to more distant background objects. Tau Ceti is considered to be a high-proper-motion star, although it only has an annual traverse of just under two arc seconds. It will require several thousand 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. Nearby stars can traverse an angle of arc across the sky more rapidly than the distant background stars and are good candidates for parallax studies. In the case of Tau Ceti, the parallax measurements indicate a distance of 11.9 light-years. This makes it one of the closest star systems to the Sun, and the next-closest spectral class-G star after Alpha Centauri A.

The radial velocity of a star is its 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 the observer, or toward blue (or shorter wavelengths) when it moves toward the observer. 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.

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. 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 kiloparsecs (32,000 light-years) and an orbital eccentricity of 0.22.

Physical properties

The Tau Ceti system is believed to have only one stellar component. A dim optical companion has been observed, which is possibly gravitationally bound, but it is more than 10 arcseconds distant from the primary. No astrometric or radial velocity perturbations have been deduced, suggesting a star that does not have a large companion in a close orbit, such as a "hot jupiter". Any potentional Gas Giants around Tau Ceti are likely to be located more at Jupiter-Like distances.

Most of what is known about the physical properties of Tau Ceti has been determined through spectroscopic measurements. By comparing the spectrum to computed models of stellar evolution, the age, mass, radius and luminosity of Tau Ceti can be estimated. However, using an astronomical interferometer, fairly accurate measurements of the radius of the star can be made directly. It deploys 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. Earlier interferometric measurements had suggested 77.3 ± 0.4% of the solar radius but with less precision.

Rotation

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, 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. Due to the Doppler effect, the rotation rate of a star affects 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 towards 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:

begin{smallmatrix} v_{eq} cdot sin i approx 1 text{km/s} end{smallmatrix}.

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.

Metallicity

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. The amount of metallicity in a star is given in terms of the ratio of iron (Fe), an easily observed heavy element, to hydrogen. A logarithm of the relative iron abundance is compared to the Sun. In the case of Tau Ceti, the atmospheric metallicity is roughly:

begin{smallmatrix} left [frac{Fe}{H} right ] = -0.50 end{smallmatrix}

or about a third the solar abundance. Past measurements have varied from -0.13 to -0.60.

This lower abundance of iron indicates that Tau Ceti is almost certainly older than Sol: its estimated age is about 10 Gyr compared to 4.57 Gyr for the Sun. Ten billion years represents a substantial portion of the age of the visible universe. However, computed age estimates for Tau Ceti can range from 4.4–12 Gyr, depending on the model adopted.

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.

Luminosity and variability

The luminosity of Tau Ceti is equal to only 55% of the Sun's. A terrestrial planet would need to orbit this star at a distance of just under 0.7 astronomical units (or AU, the average distance from the Earth to the Sun) in order match the solar-illumination level of the Earth. This is slightly less than the average distance between Venus and the Sun.

The chromosphere of Tau Ceti—the portion of a star's atmosphere just above the light-emitting photosphere—currently displays little or no magnetic activity, indicating a stable star. One nine-year study of temperature, granulation, and the chromosphere showed no systematic variations; Ca II emissions around the H and K infrared bands show a possible 11-year cycle, but this is weak relative to the Sun. Alternatively it has been suggested that the star could be in a low-activity state analogous to a Maunder minimum—a historical period, associated with the Little Ice Age in Europe, when sunspots became exceedingly rare on the Sun's surface. Spectral line profiles of Tau Ceti are extremely narrow, indicating low turbulence and observed rotation.

Debris disk

In 2004 a team of UK astronomers led by Jane Greaves discovered that Tau Ceti has more than 10 times the amount of cometary and asteroidal material orbiting it than does our Sun. This was determined by measuring the disk 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 the system, as any planets would suffer from large impact events roughly ten times more frequently than Earth. Greaves noted at the time of her research: "it is likely that [any planets] will experience constant bombardment from asteroids of the kind believed to have wiped out the dinosaurs." It is possible that a large Jupiter-sized gas giant could deflect comets and asteroids.

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 AU. 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 AU. By comparison, the Solar System's Kuiper belt extends from 30 to 50 AU. To be maintained over a long period of time, this ring of dust must be constantly replenished through collisions by larger bodies. The bulk of the disk appears to be orbiting Tau Ceti at a distance of 35–50 AU, well outside the orbit of the habitable zone. At this distance, the dust belt may be analogous to the Kuiper belt that lies outside the orbit of Neptune in the solar system.

Tau Ceti shows that stars need not lose large disks as they age and such a thick belt may not be uncommon among Sun-like stars. Tau Ceti's belt is only one-twentieth as dense as the belt around its young neighbour, Epsilon Eridani. The relative lack of debris around the Sun may be the unusual case: one research team member suggests the Sun may have passed close to another star early in its history and had most of its comets and asteroids stripped away. Stars with large debris disks have altered astronomical thinking about planet formation; debris disk stars, where dust is continually generated by collisions, appear to readily form planets.

Life and planet searches

Principal factors driving research interest in Tau Ceti are its Sun-like characteristics and their implications for possible planets and life. Hall and Lockwood report that "the terms 'solarlike star,' 'solar analog,' and 'solar twin' [are] progressively restrictive descriptions. Tau Ceti fits the second category, given its similar mass and low variability, but relative lack of metals. The similarities have inspired popular culture references for decades, as well as scientific examination.

Tau Ceti was a target of a few radial velocity planetary searches, which have failed to find any periodical variations attributable to planets. The velocity precision reached so far is about 11 m/s measured over a five year time span. This result excludes the presence of hot Jupiters, and probably excludes any planets with minimum mass greater than or equal to Jupiter’s mass and with orbital periods less than 15 years. In addition, a survey of nearby stars by the Hubble Space Telescope's Wide Field and Planetary Camera was completed in 1999, including a search for faint companions to Tau Ceti; none were discovered to limits of the telescope's resolving power.

These searches only excluded larger brown dwarf bodies and giant planets so a smaller, Earth-like planet in orbit around the star is not precluded. If "hot Jupiters" did exist in close orbit they would likely disrupt the star's habitable zone; their exclusion is thus a positive for the possibility of Earth-like planets. General research has shown a positive correlation between the presence of extrasolar planets and a relatively high metal parent star, suggesting that stars with lower metallicity such as Tau Ceti have a reduced chance of possessing planets. The evidence of a thick debris disk increases the likelihood that one or more rocky planets orbit the star, however, even if it suggests a high bombardment scenario. If planets are found, subsequent searches, with telescopes of sufficient resolution, would look for atmospheric water and temperatures suitable for habitability. Primitive life might reveal itself through an atmospheric composition unlikely to be inorganic, just as oxygen on Earth is indicative of life.

SETI and HabCat

The most optimistic search project to date was Project Ozma, which was intended to "search for extraterrestrial intelligence" (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. Both are located near the solar system and are physically similar to the Sun. No artificial signals were found despite 200 hours of observations. Subsequent radio searches of this star system have also turned up negative.

This lack of results has not dampened interest in observing the Tau Ceti system for biosignatures. In 2002, astronomers Margaret Turnbull and Jill Tarter developed the Catalog of Nearby Habitable Systems (HabCat) under the auspices of Project Phoenix, another SETI endeavour. The list contained more than 17,000 theoretically habitable systems, approximately 10% of the original sample. The next year, Turnbull would further refine the list to the 30 most promising systems out of 5,000 within one hundred light-years of the Sun, including Tau Ceti; this will form part of the basis of radio searches with the Allen Telescope Array. She also chose Tau Ceti for a final shortlist of just five stars suitable for searches by the Terrestrial Planet Finder telescope system, commenting that "these are places I'd want to live if God were to put our planet around another star.

See also

Notes

  1. Its name is a Bayer designation: 'Tau' is a Greek letter and 'Ceti' the possessive form of Cetus.
  2. From Tau Ceti the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=, Dec=, 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 begin{smallmatrix} m = M_v + 5cdot((log_{10} 3.64) - 1) = 2.6 end{smallmatrix}.
  3. The net proper motion is given by:
    begin{smallmatrix} mu = sqrt{ {mu_delta}^2 + {mu_alpha}^2 cdot cos^2 delta } = 1907.79,text{mas/y} end{smallmatrix}.
    where mu_alpha and mu_delta are the components of proper motion in the R.A. and Declination, respectively, and delta is the Declination.
  4. The space velocity components are: U = +18; V = +29, and W = +13. This yields a net space velocity of begin{smallmatrix} sqrt{18^2 + 29^2 + 13^2} = 36.5,text{km/s.} end{smallmatrix}
  5. Whether Jupiter actually provides protection to the inner solar system is still unresolved. See, for instance: Jupiter: Friend or Foe?
  6. The star 18 Scorpii, arguably the truest Solar twin, presents a contrastive example to Tau Ceti: its metallicity is in keeping with Sol but its variability is significantly higher.
  7. 75°N is 90° north of latitude 15° S. In practice, atmospheric effects will reduce visibility of the object when it is near the horizon.

References

External links

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