Stellar Multiplicity

Stellar multiplicity is an ubiquitous outcome of the star formation process. Characterizing the frequency and main characteristics of multiple systems and their dependencies on primary mass and environment is therefore a powerful tool to probe this process. While early attempts were fraught with selection biases and limited completeness, instrumentation breakthroughs in the last two decades now enable robust analyses. In this review, we summarize our current empirical knowledge of stellar multiplicity for Main Sequence stars and brown dwarfs, as well as among populations of Pre-Main Sequence stars and embedded protostars. Clear trends as a function of both primary mass and stellar evolutionary stage are identified that will serve as a comparison basis for numerical and analytical models of star formation.Comment: original version submitted to ARA&A, final version to appear in vol. 51, comments welcome, 38 pages, 5 figure

A rocky planet transiting a nearby low-mass star

M-dwarf stars -- hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun -- are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star. The nearest such planets known to transit their star are 39 parsecs away, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.Comment: Published in Nature on 12 November 2015, available at http://dx.doi.org/10.1038/nature15762. This is the authors' version of the manuscrip

Brown Dwarfs and Planets Around Solar--Type Stars: Searches by Precise Velocities

: We review eight major searches for both brown dwarfs and extra--solar planets which use precise velocity measurements to detect the reflex motion of the host primary star. Two searches have reported 11 brown dwarf candidates in total, while the searches having highest velocity precision (15 m s \Gamma1 ) have revealed none. We examine this apparent conflict by modelling the occurrence and detectability of brown-- dwarf companions, including their distribution of mass, orbital period and inclination. The velocities constrain the frequency of brown dwarf companions to be no more than 1% within 5 AU of G,K stars. However, the observed distribution of M2 sin i is consistent with two forms of the true companion mass distribution: 1) It is approximately constant at all masses, implying that 1.0% of solar--type stars have a brown dwarf companion within 5 AU, or 2) It rises gently toward low masses, i.e., dN=dM / M \Gamma0:8 , but drops for masses below 0.1 M fi , requiring few or no bro..