15 Sagittae

Gemini Observatory



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15 Sagittae is located south
of Gamma Sagittae, northwest
of Rho Delphini, and northeast
of Altair.

The Star

15 Sagittae (Sge) is a yellow-orange main sequence dwarf star of spectral and luminosity type G0-1 V (Crepp et al, 2012). The star appears to have 1.08 +/- 0.04 times Sol's mass, 1.115 +/- 0.021 times its diameter, and 1.338 +/- 0.032 times its luminosity. With a metallicity around 1.12 Sol's and higher chromospheric activity than Sol the star appears to be 2.5 +/-1.8 billion years old -- probably younger than Sol's 4.6 billion years (press release), and one analysis of isochrones suggests that the star may only be 2.0 billion years old (Crepp et al, 2012); and Guinan et al, 1999).

Gemini Observatory



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15 Sge has a brown
dwarf companion ("b").

The SIMBAD Astronomical Database identifies 15 Sge as a variable star, and it has been given the New Suspected Variable designation NSV 12757. The star has as many as four visual companions that are not bound to it. Useful catalogue numbers and designations for the star include: 15 Sge, HR 7672, Gl 779, Hip 98819, HD 190406, BD+16 4121, SAO 105635, LHS 3515, LTT 15872, LFT 1517, and Wolf 866.

Keck Observatory



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15 Sge b lies 14 AUs
away from the star.

Brown Dwarf "b"

On January 7, 2002, a team of astronomers (including Michael Liu, Debra Fischer, James Graham, James Lloyd, Geoff Marcy, and R. Paul Butler) announced the discovery of a brown dwarf companion "b" to 15 Sge by direct imaging, at the 199th Meeting of the American Astronomical Society in Washington, DC (Liu et al, 2002; in pdf; and press release and graphics). Hints of this massive substellar object were first detected in high-precision radial velocity data gathered as much as a decade ago from Lick Observatory on Mount Hamilton in Califonia. In June 2001, Liu took high-resolution pictures of 15 Sge using the University of Hawaii's QUIRC camera and Hoku'pa'a adaptive optics (AO) system on the 8.1-meter Gemini North telescope, followed by second and third epoch imaging with the Keck AO system in August and December 2001. With a half year's worth of images, the astronomers determined that the star and the object moved together through the sky with the high common proper motion exhibited by nearby objects in the Solar neighborhood, confirming that the two are physically associated. Spectrum analysis indicated that the companion object had the cool surface temperature characteristic of brown dwarfs (type L 4.5 +/- 1.5).

Jon Lomberg, Gemini Observatory

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15 Sge b has 55 to 78 times Jupiter's mass.

Based on observations through early 2012 and theoretical models of the cooling of very low mass objects, 15 Sge b is estimated to have around 68.7 +2.4/-3.1 Jupiter-masses (Crepp et al, 2012). Thus, its mass equals dozens of times the total mass of all planets in the Solar System. As a result, astronomers believe that it may be too massive to have formed like a planet from the circumstellar disk of gas and dust that enveloped 15 Sge when it was less than a hundred million years old. Rather, it may have developed from a fragment of the molecular cloud that was condensing into 15 Sge itself at an earlier stage of stellar formation.

Jon Lomberg, Gemini Observatory

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15 Sge b's observed separation from its host star is currently 1.5
times greater than Saturn's orbital distance in the Solar System.

Given its high mass of 68.7 times that of Jupiter, the brown dwarf moves around 15 Sge at an average orbital distance (mean semi-axis) of only about 18.3 +0.4/-0.5 AUs (a little short of the orbital distance and Uranus in the Solar System). The brown dwarf has a near edge-on orbital inclination of around 97.3 +0.4/-0.3 degrees from the perspective of an observer on Earth, a period of around 73.3 years (Liu et al, 2002; in pdf; and Crepp et al, 2012).

Hunt for Planetary Objects

Past radial velocity analysis suggests that giant planets of one tenth to 10 times the mass of Jupiter do not exist within 0.1 to four AUs of 15 Sge (Cummings et al, 1999). Moreover, the brown dwarf companion to 15 Sge may eventually prove to have a highly circular orbit that is coplanar with the circumstellar disk so that planets formed in inner orbits around the star. If so, then conditions would be more favorable for the existence of stable orbit for an Earth-like planet (with liquid water) centered around 1.15 AU from around 15 Sge -- between the orbital distances of Earth and Mars in the Solar System. Assuming that 15 Sge has a mass equal to Sol's, then such a planet would have an orbital period of about 450 days, or just over 1.2 years. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.

Brown Dwarfs or Planets?

When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.

© American Scientist



Artwork by Linda Huff
(for Martin et al, 1997)
used with permission.

University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover a nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.

Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.