LHS 2397a / LP 732-94

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Melanie Freed, Laird Close, Nick Siegler
Gemini Observatory, University of Arizona,
University of Hawaii/Institute for Astronomy

LHS 2397a is a very dim red dwarf star with
a close brown dwarf companion, separated
by only 3.0 AUs (NOAO press release and
more graphics). (See a Digitized Sky Survey
field image around LHS 2397a at the
Nearby Stars Database.)

System Summary

LHS 2397a is located 46.5 (ly) away from Sol. It lies in the northwest part (11:21:49-13:13.1 C~, ICRS 2000.0) of Constellation Crater, the Cup -- roughly north of Alkes (Alpha Crateris), northwest of Delta Crateris, northeast of Nu Hydrae, and south of Constellation Leo. Also known as Luyten Palomar (LP) 732-94, the relatively high proper motion of this dim star was probably discovered by Willem Jacob Luyten (1899-1994), possibly with Charles T. Kowal of Palomar Observatory (who discovered asteroid 38 Leda and the comet-like Edgeworth-Kuiper object, 2060 Chiron). It is one of 64 very low-mass stars originally identified as by John Gizis of the University of Delaware in the 1990s. On May 21, 2002, a team of astronomers announced the discovery of a very close, brown dwarf companion using direct infrared imaging with adaptive optics (NOAO press release).

Jon Lomberg, Gemini Observatory

Larger image.

The dimness of low-mass stars allows
direct detection of close and even
smaller and dimmer companions (more).

The Star

NASA -- larger image

LHS 2397a is a dim red dwarf star, even dimmer than
Gliese 623 A (M2.5V) or B (M5.8Ve) at lower right.

This is a very red dwarf of spectral and luminosity type M8 V. It may have nine percent or less of Sol's mass, about a tenth of its diameter, and only 2.5 millionth of its brightness. Some alternative useful star catalogue numbers for the star are: N1 951 or GJ 3655, LHS 2397a, LP 732-94, and FS 129.

Brown Dwarf Companion

Jon Lomberg, Gemini Observatory

Larger image.

LHS 2397a has such a
smaller and dimmer,
brown dwarf companion,
which is still much
more massive than
Jupiter or Earth (more).

On May 21, 2002, a team of astronomers (including Laird Close, Nick Siegler, Melanie Freed, and James Liebert from the University of Arizona and its Steward Observatory; Wolfgang Brandner at the European Southern Observatory; and Eduardo Martin and Dan Potter at the Institute for Astronomy, University of Hawaii) announced the discovery of a very dim brown dwarf companion to LHS 2397a at a close distance of around only 3.0 AUs (NOAO press release). Found using the University of Hawaii's Adaptive Optics system Hokupa`a and its QUIRC infrared imager at the Gemini Observatory, the object is a brown dwarf of spectral type L7, with about 38 to 70 times the mass of Jupiter. As one of 12 low-mass stars (with about 1/10th or less of a Solar mass) that were found by May 21, 2002 to have low mass companions, the discovery of this object suggests that these low-mass binary pairs may be quite common. The discovery of so many low-mass pairs was unexpected, because most very low-mass stars and brown dwarfs were thought to be solo objects that wander though space alone after being ejected out of their stellar nurseries during the star formation process.

© John Whatmough (Artwork from Extrasolar Visions, used with permission)
LHS 2397a's companion is somewhat warmer than the brown dwarf companion
to Gliese 229 -- with its own dark satellite, as imagined by Whatmough.

Because the Titanium Oxide (TiO)and Vanadium Oxide (VO) bands which dominate the far-optical portions of late-M dwarf stellar spectra disappear in these cooler, substellar objects, astronomers in the 2MASS Project defined a new spectral class, "L," where metallic oxides are replaced by metallic hydrides and neutral alkali metals as major spectroscopic signatures. As a result, 25 dim objects were typed as L brown dwarfs were typed by 1999 (Kirkpatrick et al, 1999). At least five of the twenty L dwarfs detected by 2MASS exhibit a 6708 angstrom-lithium doublet at low resolution (with the strongest having an equivalent width of 18.5 angstrom), which implies that they are of substellar mass given the presence of lithium for objects that are so cool. 2MASS astronomers then estimated that the density of L brown dwarfs may be roughly around one per 10 square degrees, or about 4,000 L dwarfs in the whole sky. Preliminary calculations for the mass function indicate that L and the even cooler, methane-dominated "T" brown dwarfs -- such as Gliese 229) lead 2MASS astronomers to estimate that there may be twice as many brown dwarfs (L and T types) as stars in the Milky Way Galaxy but that they may sum to only 15 percent of its stellar mass.

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)

Although brown dwarfs lack sufficient mass (at least 75-80 Jupiters) to
ignite core hydrogen fusion, the smallest true stars (red dwarfs) can
have such cool atmospheric temperatures (below 4,000° K) that it is
difficult to distinguish them from brown dwarfs. While Jupiter-class planets
may be much less massive than brown dwarfs, they are about the same
diameter and may contain many of the same atmospheric molecules.

University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover 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.

Closest Neighbors

The following star systems are located within 10 ly of LHS 2397a.

Star System	Spectra & Luminosity	Distance (light-years)
Hip 56157	?	3.4
LP 732-35	M V	5.2
BD-17 3336	K7 V	6.7
SZ Crateris AB	K4-M0 V M V	8.2
HR 4587	G8-K0 IV	8.9
BD-18 3019 AB	M1 V DQ9 /VII	9.2
GJ 1140	DA6 /VII	9.2
L 755-53	M3 V	9.9
* plus bright stars *		. . .
Ross 948 AB	G V M2 V	14
Alkiba AB	F2 V-IV ?	15
Zavijava 3?	F8-A5 V-IV ? ?	15
83 Leonis AB	G6/8-K0 IV K2 V-IV	17
Porrima 3	F0 V ? F0 V	18

Other Information

Up-to-date technical summaries on this star can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS, and the Nearby Stars Database.

Crater, the Cup, ranks among the dimmest of the Classical constellations. Although not as bright as the Constellation's Delta Crateris, Alpha Crateris was given first rank by Bayer; its proper name is Alkes, Arabic for the "Wine Cup." Along with Constellation Corvus, the Crow, Crater rides the back of Hydra, the Water Serpent. For more information about the stars and objects in this constellation, go to Christine Kronberg's Crater. For an illustration, see David Haworth's Crater.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.