LL Aquarii
An Algol-type eclipsing binary with a solar-twin secondary.
LL Aquarii, abbreviated LL Aqr, is an eclipsing binary star system located in the equatorial constellation of Aquarius. At its brightest, the combined apparent visual magnitude is 9.23, making it invisible to the unaided eye. Parallax measurements place it roughly 447 light-years from the Sun, and it is moving closer with a heliocentric radial velocity of about −10 km/s.
The system was identified in 1996 as an Algol-type eclipsing binary through photometric data from the Hipparcos space observatory, and it received the variable-star designation LL Aqr in 1999. It is a detached, double-lined spectroscopic binary. A more detailed light curve from 2004 revealed an eccentric orbit with a period of 20.1784 days. During the primary eclipse, the system dims to magnitude 9.86, while the secondary eclipse reaches magnitude 9.59. The first orbital elements, published in 2008, show an eccentricity of 0.3095 and a mass ratio of 0.86. Stellar models suggest both stars are near the midpoint of their main-sequence lifetimes.
The primary component, more massive, has a stellar classification of F9 V, an F-type main-sequence star. It is 19.5% more massive than the Sun and 32% larger in radius, radiating 2.15 times the Sun’s luminosity from a photosphere at about 6,080 K. Its projected rotational velocity is 3.5±0.5 km/s, and its metallicity is very close to solar.
The secondary component is considered a solar twin, with properties similar to the Sun. It is a G-type main-sequence star, classified as G3 V. Neither star shows signs of stellar activity; both rotate slowly and emit no detectable X-rays. Tidal effects between the two are negligible, and their orbital separation is 40.7 times the Sun’s radius.
- Apparent visual magnitude (peak)
- 9.23
- Distance
- approximately 447 light years
- Heliocentric radial velocity
- −10 km/s
- Orbital period
- 20.1784 days
- Orbital eccentricity
- 0.3095
- Mass ratio
- 0.86
- Primary spectral type
- F9 V
- Secondary spectral type
- G3 V
Lore & Background
In 1996, LL Aquarii was identified as an Algol-type eclipsing binary from photometric observations by the Hipparcos space observatory, and it received its variable-star designation in 1999. The system is a detached double-lined spectroscopic binary. A more extensive light curve in 2004 revealed an eccentric orbit with a period of 20.1784 days. During primary eclipse, the system dims to magnitude 9.86, and during secondary eclipse to 9.59. The first orbital elements were published in 2008, showing an eccentricity of 0.3095 and a mass ratio of 0.86. Stellar models indicate both stars are near the midpoint of their main-sequence lifetimes.
The primary component is an F9 V star with 19.5% more mass than the Sun and a 32% larger radius, radiating 2.15 times the Sun's luminosity at an effective temperature of about 6,080 K. It has a projected rotational velocity of 3.5±0.5 km/s and near-solar metallicity. The secondary is a G3 V star considered a solar twin, with Sun-like properties. Neither star shows signs of stellar activity, being slowly rotating and not emitting X-rays. Tidal effects are negligible, with an orbital separation of 40.7 solar radii.
Reader's Guide
LL Aquarii's significance lies in its well-characterized binary nature, providing a benchmark for studying detached eclipsing binaries with eccentric orbits. The system's orbital elements, including a precise eccentricity of 0.3095 and mass ratio of 0.86, were established in 2008, offering constraints for stellar evolution models. The primary star's F9 V classification and the secondary's status as a solar twin (G3 V) allow comparative studies of main-sequence stars at similar evolutionary stages. The lack of stellar activity and negligible tidal effects make it a clean laboratory for binary dynamics. Its discovery via Hipparcos and subsequent photometric and spectroscopic analyses highlight the value of space-based surveys in identifying faint variable stars. The system's distance of about 447 light years and radial velocity of −10 km/s place it within the local Galactic neighborhood, contributing to our understanding of binary star populations in the solar vicinity.
Did You Know?
- The system has an orbital eccentricity of 0.3095 and a period of 20.1784 days.
- The secondary component is a G3 V solar twin with Sun-like properties.
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