Lacaille 8760 / AX Mic

VLTI, ESO




Larger and jumbo images.




Lacaille 8760 (Gl or GJ 825) is somewhat
larger and brighter than Alpha Centauri C
(Proxima) and Kapteyn's Star but also
smaller than Sol (more from ESO).

Medialab, © ESA 2002



Larger illustration of
the Darwin Mission.



Astronomers have identified
Lacaille 8760 as a prime target
for NASA's optical SIM and the
ESA's infrared Darwin missions,
now indefinitely postponed.

The Star

This spectral and luminosity type of this cool and dim, main sequence dwarf has been classed as orange as K7 and red as M2 Ve (Hawley et al, 1996). Lacaille 8760 may have around 60 percent of Sol's mass (RECONS), about 66 to 72 percent of its diameter ((Pasinetti-Fracassini et al, 2001; Johnson and Wright, 1983, page 699; and Claude H. Lacy, 1977, page 482), and 2.8 to 3.5 percent of its visual luminosity and 8 percent of its bolometric luminosity (NASA Star and Exoplanet Database, derived from the exponential formula of Kenneth R. Lang, 1980), and is at least half as abundant in elements heavier than hydrogen ("metallicity") as Sol. A relatively mild flare star (that erupts less than once per day), Lacaille 8760 has been given the Variable Star designation AX Microscopii (B.R. Pettersen; P.B. Byrne, 1981; and Woolley et al, 1970). Given its relatively sedate flare rate and the high eccentricity (e= 0.20) and inclination (i= 0.06) of its orbit around the galactic center (Allen and Herrera, 1998; P.B. Byrne, 1981; and Woolley et al, 1970), the star appears to be older than Sol's 4.6 billion years. Some alternative names and useful catalogue numbers for this star are: AX Mic, Gl 825, Hip 105090, HD 202560, CD-39 14192, CP-39 8920, SAO 212866, LHS 66, LTT 8438, LFT 1617, LPM 772, and UGP 518.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

Lacaille 8760 is a flare star, like UV Ceti (Luyten
726-8 B) shown flaring at left. UV Ceti is an extreme
example of a flare star that can boost its brightness by
five times in less than a minute, then fall somewhat
slower back down to normal luminosity within two or three
minutes before flaring suddenly again after several hours.

Habitable Zone

Accounting for infrared heating, the distance from Lacaile 8760 where an Earth-type planet would be comfortable with liquid water based on visible light alone is centered around 0.28 AU (SIM summary data). In that distance range from the star, such a planet would have an orbital period shorter an Earth year. According to alternative calculations performed for the NASA Star and Exoplanet Database, their estimate of the HZ would be centered around 0.402 AU, with inner edge of Lacaille 8760's habitable zone at around 0.272 AU from the star, while the outer edge lies farther out at around 0.532 AU. On the other hand, the flares of Lacaille 8760 would probably be a hazard to most surface lifeforms on an Earth-type planet in a water-zone orbit.

Hunt for Substellar Companions

Moreover, as of August 2004, no supporting evidence for a large Jupiter or brown-dwarf-sized object has been found around Lacaille 8760. Despite such less than favorable system characteristics, astrobiologist Maggie Turnbull from the University of Arizona in Tucson included Lacaille 8760 in late September 2003 in a shortlist of 30 stars (screened from the 5,000 or so stars that are estimated to be located within 100 ly of Earth) that were presented to a group of scientists from NASA's space-telescope project, the Terrestrial Planet Finder (TPF), which will search for habitable planets by using visible light with the "signature" of water and/or oxygen from an Earth-type planet and the ESA's Darwin project involving six space telescopes (Astrobiology Magazine), now both indefinitely postponed. The stars examined were selected from a larger list of 17,129 (of which 75 percent are located within around 450 ly, or 140 parsecs, of Sol) that were assembled into a Catalog of Nearby Habitable Stellar Systems (HabCat) by Turnbull and Jill Tarter of the SETI Institute (see: Margaret C. Turnbull, 2002, in pdf). Selection criteria for the 30-star shortlist included: X-ray luminosity, rotation, spectral types or color, kinematics, metallicity, and Strömgren photometry.

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