The initial multiplicity of stellar systems is highly uncertain. A number of mechanisms have been proposed to explain the origin of binary and multiple star systems, including core fragmentation, disk fragmentation and stellar capture1,2,3. Observations show that protostellar and pre-main-sequence multiplicity is higher than the multiplicity found in field stars4,5,6,7, which suggests that dynamical interactions occur early, splitting up multiple systems and modifying the initial stellar separations8,9. Without direct, high-resolution observations of forming systems, however, it is difficult to determine the true initial multiplicity and the dominant binary formation mechanism. Here we report observations of a wide-separation (greater than 1,000 astronomical units) quadruple system composed of a young protostar and three gravitationally bound dense gas condensations. These condensations are the result of fragmentation of dense gas filaments, and each condensation is expected to form a star on a timescale of 40,000 years. We determine that the closest pair will form a bound binary, while the quadruple stellar system itself is bound but unstable on timescales of 500,000 years (comparable to the lifetime of the embedded protostellar phase10). These observations suggest that filament fragmentation on length scales of about 5,000 astronomical units offers a viable pathway to the formation of multiple systems.
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We thank K. Todorov for discussions and comments that improved the paper. We acknowledge support from the Swiss National Science Foundation project CRSII2_141880 (J.E.P.), NSF CAREER award AST-0845619 (H.G.A.), NSF grant AST-0908159 (A.A.G.), ERC project PALs 320620 (P.C.) and NASA grant NNX09AB89G (T.L.B.). S.S.R.O. acknowledges support from NASA through Hubble Fellowship grant 51311.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. R.J.P. acknowledges support from the Royal Astronomical Society through a research fellowship. This research made use of astrodendro (http://www.dendrograms.org), Astropy, and APLpy (http://aplpy.github.com). The James Clerk Maxwell Telescope is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK, the National Research Council of Canada, and (up to 31 March 2013) the Netherlands Organisation for Scientific Research. Additional funds for the construction of SCUBA-2 were provided by the Canada Foundation for Innovation. The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc.
The authors declare no competing financial interests.
Extended data figures and tables
a, b, Dust continuum emission observed with SCUBA-2 at 450 µm (a) and 850 µm (b). Contour levels are drawn at 3, 6, 9, 12 and 15× rms, where rms is 0.23 mJy per pixel and 0.026 mJy per pixel in the 450 µm and 850 µm maps, respectively. Emission from dust associated with the protostar B5-IRS1 dominates the field, and it makes it difficult to extract the emission from B5-Cond2 and B5-Cond3. B5-Cond1 is clearly detected in the dust continuum emission. Since the dust continuum emission does show the presence of the filaments in low level emission, we conclude they are real column density features. Filled white circles at bottom left corners show the angular resolution of the observations. Blue- and orange-filled circles show the condensation centres, while the filled stars indicate the protostar (B5-IRS1) location.
a, The enclosed condensation mass, derived from NH3(1,1), at different effective radii for each condensation; b, the corresponding virial parameter as a function of effective radius for each condensation. The condensation mass grows rapidly with radius, with a profile similar to one expected for a density distribution of ρ ∝ r−1.5 (dotted line in a) until it is close to the condensation boundary. In comparison, the dashed line shows the expected result in hydrostatic equilibrium (ρ ∝ r−2), which is different to the observed distribution. The virial parameter decreases with radius until it reaches a minimum of ∼1.5, and then it slowly increases until it reaches the condensation boundary. Notice that virial parameters below the horizontal line (α = 2) imply bound condensations. The grey shaded region marks the regime where the effective radius is smaller than the angular resolution of the observations. The circles show the values at the half-mass radius.
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Pineda, J., Offner, S., Parker, R. et al. The formation of a quadruple star system with wide separation. Nature 518, 213–215 (2015). https://doi.org/10.1038/nature14166
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