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How are the smallest beasts of the stellar zoo born?

In the same way as a jungle harbours several species of birds and mammals, the stellar (or almost stellar) zoo also offers a variety of objects with different sizes, masses, temperatures, ages, and other physical properties.

On the one hand, there are huge massive stars that easily overshadow one as the Sun. On the other, there are less graceful, but still very interesting inhabitants: small low-mass stars or objects that come out of the stellar classification. These last objects are called ‘brown dwarfs’. Brown dwarfs do not have enough mass (more than 0.075 times the mass of the Sun) to shine by hydrogen burning, thus they differ from ordinary low-mass stars and are halfway between stars and planets. For astronomers, how these ‘failed stars’ are formed still remains as a mystery.

While low-mass stars – such as our Sun – grow up from dense interstellar cores of gas with masses slightly above the final mass of the star, this is less clear for the case of brown dwarfs. This is because brown dwarfs require small low-mass cores of interstellar materials to exist. Such teeny-cores are not predicted by theoretical studies, unless ingredients like turbulence are taken into account.

This problem led astronomers to suggest that brown dwarfs form differently from normal stars, perhaps by dynamical ejection from the parent clump of gas. In this way, dwarfs could remain ‘failed’ before they have completed the more usual course of accretion. Thus, the mystery of brown dwarf formation is left to the final word of the observations.

We have recently seen a new object, named IC348-SMM2E, located in the Perseus molecular cloud , which has been characterized using observations of six different telescopes, covering wavelengths from radio up to optical, and including both ground-based and space telescopes. These observations are not easy because cores of gas surrounding young brown dwarfs are extremely faint. However, the compilation of this large data-set has allowed us to determine the physical properties of the object, such as mass, temperature, and luminosity of the parent dense core. Moreover, researchers detected a bipolar outflow (blue and red contours in the top-right panel of the figure), a well-known fingerprint of young star formation.

Palau, Girart, Estalella 2012, Suplement /La Terra a l'Univers/, Història Natural dels Països Catalans, Enciclopèdia Catalana
Palau, Girart, Estalella 2012, Suplement /La Terra a l’Univers/, Història Natural dels Països Catalans, Enciclopèdia Catalana

Among the properties which make IC348-SMM2E a unique object are its extremely young age, its associated bipolar ejection of outflowing gas, and an elongated disk-like rotating structure perpendicular to the outflow, which are all properties typically seen in the youngest stars (and are sketched in the bottom-right panel of the figure).

More quantitatively, the mass outflow rate, the strength of the bipolar outflow, and the luminosity of the protostellar radiation, have correspondingly smaller values compared to the formation of ordinary stars at the earliest phases. Therefore, this work presents important evidence that brown dwarfs form as a scaled-down version of low-mass stars.

In the coming years, interferometric observations with the new instrument ALMA in Chile – observing microwaves – will allow researchers to resolve the morphology of IC348-SMM2E and other young brown dwarfs. We are on the verge of revealing the inner-most parts and the early life of these little stellar beasts.

Featured image: Pair of eclipsing brown dwarfs by NASA, ESA, and A. Feild (STScI). Public Domain via Wikimedia Commons

Recent Comments

  1. Philip Rushton

    Fascinating. Thank you very much. Have astronomers established a ratio of the percentage of formation of brown dwarves in the galaxy compared to other stars?

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