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A Low-Mass Pre Main Sequence Eclipsing Binary System

ING web news release
27th June, 2007

Astronomers have been able to obtain accurate empirical measurements of the masses and radii of the component stars of a new low mass eclipsing binary system. The star system, called JW 380, comprises of a pair of 0.26, 0.15 solar-mass pre main sequence stars and it is located in the very young Orion Nebula Cluster (ONC).

Figure 1. Colour composition of a Wide Field Camera image of the Orion Nebula. Credit: Jonathan Irwin [ JPEG ].

Eclipsing binaries are a special class of spectroscopic binaries, systems where the components are so close together that they cannot be separated by imaging. The stars orbit each other with a period of 5.3 days, and are separated by only 9.4 solar radii, or 0.04 AU (where 1 AU is the distance from the Earth to the Sun), which is a tenth of the distance from Mercury to the Sun. In an eclipsing binary, the system is edge-on as seen from the Earth, so one star passes in front of the other every half orbit, causing the total light emitted to dim in a periodic fashion. By measuring the shape of these eclipses in a light curve of the system, astronomers can derive the radii of the stars in the binary.

Figure 2. Eclipses in the light curve of JW 380.

This is the second eclipsing binary identified in the ONC. The first was announced during 2006 and contains two brown dwarfs, or "failed stars", systems which are too cool to burn hydrogen into helium. JW 380 contains instead two M-dwarf stars of 3-4 times heavier mass. By identifying eclipsing binaries spanning a range of masses and ages, it is possible to map the evolution of stellar radius with age empirically, which it can then be compared to models of stellar evolution to help understand how stars evolve.

The binary system JW 380 was identified using time-series imaging observations taken with the Wide Field Camera on the Isaac Newton Telescope as part of the Monitor project, a large-scale survey searching for eclipsing binary and transiting extrasolar planet systems in a number of young open star clusters. Follow-up spectroscopic observations obtained using the FLAMES fibre-fed spectrograph on the ESO VLT UT2 and the NOAO Phoenix spectrograph on Gemini South were used to derive the radial velocities and hence component masses of the stars, as well as to confirm the system's membership in the ONC.

The observations confirm the predictions of theoretical models that low-mass stars are "bloated" at these extremely young ages (having measured radii of 1.2 and 0.9 solar radii), because they are still contracting in the stages between birth and arrival onto the main sequence. Comparing the measured radii with the model predictions indicates an age of 2-3 Myr, consistent with existing estimates for the ONC.

A third component in the system is also visible in the spectroscopic observations. This may be physically associated to the binary, making a triple star system with the third component in a wide orbit. This will be the subject of future follow-up observations to derive its orbital parameters.

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Last modified: 22 December 2010