An Unequal Pair of ‘Identical Twin’ Stars

Figure 1: Artist’s impression of an eclipsing binary system.

In 2008, a team of astronomers announced the discovery of a pair of newborn stellar twins residing in the Orion Nebula. The stellar twins exist as a binary star system, identified as Par 1802. Both stars formed at around the same time from the same natal material, and each star in the binary has a mass of 0.41 ± 0.01 solar masses, identical to within 2 percent. As such, they are expected to possess identical physical attributes and are virtually ‘identical twins’. However, in the study, the team reported that these twin stars have surface temperatures differing b ~300 K and luminosities differing by ~50 percent. Furthermore, the sizes of both stars differ by 5 to 10 percent.

Physical parameters of Par 1802:
	Primary Component			Secondary Component
Mass (Sun = 1)	0.414 ± 0.015			0.406 ± 0.014
Surface Temperature (K)	3,945 ± 15			3,655 ± 15
Luminosity (Sun = 1)	0.72 ± 0.11			0.46 ± 0.12
Radius (Sun = 1)	1.82 ± 0.05			1.69 ± 0.05

Figure 2: Light curve of Par 1802 - an eclipsing binary system with a period of 4.67 days. The ratio of eclipse depths provides a direct measure of the ratio of surface temperatures, with the deeper eclipse corresponding to the eclipse of the hotter component (primary) by the cooler component (secondary). (K.G. Stassun et al., 2008)

Par 1802 is a very young equal-mass eclipsing binary system and its estimated age is ~1 million years. In an eclipsing binary system, both stars periodically eclipse each other as they circle around their common centre of mass. The variation in combined brightness when one component eclipses the other can reveal a lot about Par 1802. Here, the equal-mass stars of Par 1802 clearly show unequal surface temperatures, luminosities and sizes. This can be explained by stellar evolution models for young stars with ~0.4 times the Sun’s mass. The models predict that such stars undergo a brief period of rapid evolution at an age of ~1 million years. As a result, the warmer, more luminous and larger star (primary component) can be interpreted as being slightly younger than its companion. An age gap of only a few hundred thousand years is sufficient.

Figure 3: Comparison of the observed physical properties of Par 1802 with theoretical predictions. The measured properties of the primary and secondary components of Par 1802 are shown as green and red symbols, respectively. (K.G. Stassun et al., 2008)

Considering that stars with ~0.4 times the Sun’s mass evolve most rapidly during the first few million years after formation and that such stars can live for many billions of years, an age difference of only a few hundred thousand years in an equal-mass binary system is observationally detectable only during the first few million years of its evolution. At later times, the physical signs of unequal ages become less observable. Par 1802 is an example of how birth order in ‘identical twin’ stars, with a lag of only a few hundred thousand years, can manifest itself as observable physical differences between the two stars - at least when they are very young.

Reference:

K.G. Stassun et al., “Surprising dissimilarities in a newly formed pair of ‘identical twin’ stars”, Nature, 450, 1979-1082 (19 June 2008)