SAO: Imaging a Multiple Star

[bystander]

Imaging a Multiple Star
Smithsonian Astrophysical Observatory
Weekly Science Update | 2011 Apr 15

Multiple stars - binaries, triplets, or perhaps more stars, that orbit each other - are unique laboratories into the interactions between stars and their early environments. Young stars develop by accreting matter. How and when the accretion stops, and hence what determines a star's final mass, is among the important unsolved puzzles in astronomy. In a multiple star system the accretion is even more complex because it potentially involves material around each star in addition to material around the group. A better understanding of multiple stars, especially those that orbit each other closely and hence affect the accretion more strongly, can shed light on the accretion process.

CfA astronomers Nat Carleton and Marc Lacasse, together with a team of eighteen others, used the Smithsonian's Infrared Optical Telescope Array (IOTA, now retired from operation) on Mt. Hopkins, AZ, to make the first direct optical image of a triple stellar system with an orbit as small as one astronomical unit (one AU is the average distance of the Earth from the sun).

The astronomers were able to measure reliably the parameters of the triple system, called GW Orionis. One star has a mass of 3.6 solar-masses, and it orbits with a 3.1 solar-mass star at a distance of 1.35 AU. A third companion star, previously inferred to exist from studies of the stellar wobble, is also imaged, and orbits the others at a distance of about 8 AU. The system is unusually bright in the near-infrared, suggesting that some accretion onto the system is still continuing, but further work is needed to sort out the answer to this question. The results highlight the power of optical telescope arrays in the investigation of close multiple stars.

First astronomical unit scale image of the GW Ori triple. Direct detection of a new stellar companion. - JP Berger et al

• Astronomy & Astrophysics 529 L1 (May 2011) DOI: 10.1051/0004-6361/201016219
  arXiv.org > astro-ph > arXiv:1103.3888 > 20 Mar 2011

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[Ann]

My software, which uses the Hipparcos catalog to describe the distance to, the luminosity and the color of stars, says that GW Orionis is an unsolved variable, about 1,000 light-years away, with a Hipporcos B magnitude of 11.163 and a Hipporcos V magnitude of 10.026. Its luminosity is given - not by Hipparcos but by my software interpreting Hipparcos - as about 9 times that of the Sun. My software quotes the GCVS catalog to say that the spectral class of the star i G8.5Ve.. To me, this appears to mean that the GCVS catalog thinks that GW Orionis is a main sequence star of spectral class G8.5 and possibly multiple. (I'm not sure about the lowercase "e". Does that mean "emission"?)

I find it interesting that a star that contains two relatively massive components, both more than three times the mass of the Sun, should be so relatively faint (the star may have a combined luminosity of about nine times that of the Sun). It is also interesting that the stars should be so red if they are main sequence stars weighing considerably more than the Sun. Shouldn't such stars be class A stars like Vega?

Could it be that the star is very young and very dust-reddened? The text from Smithsonian Astrophysical Observatory says:

The system is unusually bright in the near-infrared, suggesting that some accretion onto the system is still continuing

Well, it is an interesting star, to be sure. And what an achievement to photograph the components.

Ann