A Four-Star Lightweight

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An important part of exoplanet studies is the attempt to understand how planets and solar systems form. New measurements of the lowest-mass quadruple star system ever discovered are now confirming an intriguing theory: in addition to other channels, large gas planets may form in the same way that stars do.

Formation Channels

Exoplanets have been found in an enormous variety of configurations, from hot Jupiters only 0.01 AU away from their host star, to planetary-mass companions that orbit at a whopping distance of 1,000 AU.

Formation of these gas giants could occur via a number of different theorized pathways, such as growth from rocky cores close to host star, or fragmentation from instabilities far out in the protoplanetary disk. But given that the line between giant planets and brown dwarfs is somewhat fuzzy, another theory has come under consideration as well: could gas giants form out of the collapse and fragmentation of a molecular cloud, in the same way that stars form?

In a recent study, Brendan Bowler and Lynne Hillenbrand (California Institute of Technology) argue that one star system, 2M0441+2301 AabBab, might actually be evidence that this channel works. 2M0441+2301 AabBab is a young (less than 3 million years old) quadruple system in the Taurus star-forming region, previously identified through imaging. Since photometry alone isn’t enough to be sure of the masses of the components, Bowler and Hillenbrand used the OSIRIS instrument on the Keck I telescope to obtain the first resolved spectra of each component of this system, verifying the system’s intriguing properties.

Pair of Pairs

Near-IR spectra of 2M0441+2301 Aa, Ab, Ba, and Bb. The insets shows the unresolved 2MASS image of the system and the Keck/NIRC2 images of each binary subsystem. Click for a better look! [Bowler&Hillenbrand 2015]

Near-IR spectra of 2M0441+2301 Aa, Ab, Ba, and Bb. The insets shows the unresolved 2MASS image of the system and the Keck/NIRC2 images of each binary subsystem. Click for a better look! [Bowler&Hillenbrand 2015]

2M0441+2301 AabBab is what’s known as a hierarchical quadruple system: it consists of a pair of close-binary star systems that orbit each other at an enormous distance of at least 1,800 AU — which means that, if the system is only a few million years old, the binary pairs have orbited each other no more than ~20 times.

The authors’ measurements show that the first binary pair (labeled Aab, where Aa and Ab are the two stars) consists of a 200 MJup low-mass star and a 35 MJup brown dwarf. The second binary pair (Bab) consists of a 19 MJup brown dwarf and a ~10 MJup companion. This gives 2M0441+2301 AabBab a total mass of only ~0.26 solar masses, making it the lowest-mass quadruple system yet discovered.

The hierarchical structure of this system strongly suggests that it formed from the collapse and fragmentation of a molecular cloud core. What makes this system especially interesting is the span of masses involved. The low mass of the companion in Bab indicates that it’s possible to form planetary-mass companions from a cloud-fragmentation pathway — which suggests that this may also be legitimate channel to consider for the formation of massive exoplanets.

Note: article edited to more accurately reflect the specific contributions of this study.

Citation

Brendan P. Bowler and Lynne A. Hillenbrand 2015 ApJ 811 L30. doi:10.1088/2041-8205/811/2/L30

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