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19 94Apjs. . .94. .749K the Astrophysical Journal Supplement The Astrophysical Journal Supplement Series, 94:749-788, 1994 October .749K © 1994. The American Astronomical Society. All rights reserved. Printed in U.S.A. .94. 94ApJS. THE LUMINOSITY FUNCTION AT THE END OF THE MAIN SEQUENCE: RESULTS OF A DEEP, LARGE-AREA, 19 CCD SURVEY FOR COOL DWARFS' J. Davy Kirkpatrick2 Steward Observatory, University of Arizona, Tucson, AZ 85721 John T. McGraw and Thomas R. Hess Institute for Astrophysics, University of New Mexico, Albuquerque, NM 87131 AND James Liebert and Donald W. McCarthy, Jr. Steward Observatory, University of Arizona, Tucson, AZ 85721 Received 1993 November 29; accepted 1994 March 24 ABSTRACT The luminosity function at the end of the main sequence is determined from F, R, and / data taken by the CCD/Transit Instrument, a dedicated telescope surveying an 825 wide strip of sky centered at 5 = +28°, thus sampling Galactic latitudes of +90° down to —35°. A selection of 133 objects chosen via R - 7and F — 7 colors has been observed spectroscopically at the 4.5 m Multiple Mirror Telescope to assess contributions by giants and subdwarfs and to verify that the reddest targets are objects of extremely late spectral class. Eighteen dwarfs of type M6 or later have been discovered, with the latest being of type M8.5. Data used for the determination of the luminosity function cover 27.3 deg2 down to a completeness limit of R = 19.0. This luminosity function, computed a F, 7, and bolometric magnitudes, shows an increase at the lowest luminosities, corresponding to spectral types later than M6—an effect suggested in earlier work by Reid & Gilmore and Leggett & Hawkins. When the luminosity function is segregated into north Galactic and south Galactic portions, it is found that the upturn at faint magnitudes exists only in the southern sample. In fact, no dwarfs with Mj > 12.0 are found within the limiting volume of the 19.4 deg2 northern sample, in stark contrast to the smaller 7.9 deg2 area at southerly latitudes where seven such dwarfs are found. This fact, combined with the fact that the Sun is located —10-40 pc north of the midplane, suggests that the latest dwarfs are part of a young population with a scale height much smaller than the 350 pc value generally adopted for other M dwarfs. These objects comprise a young population either because the lower metallicities prevalent at earlier epochs inhibited the formation of late M dwarfs or because the older counterparts of this population have cooled beyond current detection limits. The latter scenario would hold if these late-type M dwarfs are substellar. The luminosity function data together with an empirical derivation of the mass-luminosity relation (from Henry & McCarthy) are used to compute a mass function independent of theory. This mass function increases toward the end of the main sequence, but the observed density of M dwarfs is still insufficient to account for the missing mass. If the increases seen in the luminosity and mass functions are indicative of a large, unseen, substellar population, brown dwarfs may yet add significantly to the mass of the Galaxy. Subject headings: stars: low-mass, brown dwarfs — stars: luminosity function, mass function 1. INTRODUCTION 1.1. The Missing Mass A better knowledge of our own solar neighborhood is essen- Knowledge of the luminosity function for the least luminous tial to a deeper understanding of Galactic structure. Of stars, M stars is an important step in unraveling the mystery of the dwarfs are the most common, but few comprehensive studies missing mass. Using techniques pioneered by Oort (1932, of them have been completed, primarily because of their in- 1960), Bahcall ( 1987, and references therein) determined that trinsic faintness. Yet, it is this faintness which might provide half of the mass in the solar neighborhood must reside in the the answer to one of astronomy’s still unsolved puzzles—that form of unobserved matter. He showed that if these unseen of the Galactic missing mass. In addition, probing fainter and objects have masses below 0.1 A/©, the nearest one would be 1 fainter in search of these objects should ultimately reveal the pc away and would have a proper motion exceeding 1 " per presence of a suspected, though still unconfirmed, population annum. of substellar objects—the so-called brown dwarfs. Recent results, however, have begun to cast doubt upon the reality of this “missing mass.” Bienaymé, Robin, & Crézé 1 ( 1987 ) used a technique based on star counts to obtain a local Observations reported here were obtained with the Multiple Mirror mass density of 0.09-0.12 AT© pc-3, consistent with the ob- Telescope Observatory, a facility operated jointly by the Smithsonian In- _3 stitution and the University of Arizona. served value of 0.10-0.11 A7© pc ( Bahcall 1984). Kuijken & 2 Present address: McDonald Observatory, RLM 15.308, University of Gilmore ( 1989a), using data on K dwarfs near the south Ga- Texas, Austin, TX 78712-1083. lactic pole (SGP), concluded that there is no missing mass in 749 © American Astronomical Society • Provided by the NASA Astrophysics Data System .749K 750 KIRKPATRICK ET AL. Vol. 94 the solar vicinity, with the same result being found when a .94. dwarfs less massive than about 0.06 MQ never reaches this . reanalysis of data on F dwarfs and K giants is used (Kuijken & critical value. As a result, no significant depletion of lithium Gilmore 1989b). Kuijken ( 1991 ) confirmed these results us- should occur in objects of this type. A spectroscopic study of ing a nearby sample of K dwarfs in addition to the SGP six low-luminosity M dwarfs by Magazzù, Martín, & Rebolo 94ApJS. sample. (1993) failed to detect the doublet, suggesting that these dwarfs 19 In response to these claims, Bahcall, Flynn, & Gould ( 1992 ) are more massive than 0.06 M0. used data on K giants at the SGP to produce a local mass Spectroscopic studies of the latest M dwarfs have also begun estimate in which the systematic and random uncertainties are in the infrared. Davidge & Boeshaar (1993) found that the well understood. In a “one-experiment” run, they found that a spectra of an M8.5 and an M9 dwarf were markedly different model having no dark matter is inconsistent with the data at an from the spectra of an M7 and an M8 dwarf over the region 86% confidence level. These authors gave a critical analysis of from 1.5 to 2.4 pm. The most notable difference is that these the Kuijken & Gilmore ( 1989a, b) mass determinations and cooler dwarfs exhibit strong, unidentified absorption features concluded that a more robust analysis of the same data would possibly due to unrecognized polyatomic molecules. Other imply a substantial fraction of missing matter. The inhomo- spectroscopic peculiarities were also noted among these four geneity in the Bienaymé et al. ( 1987 ) star count sample, as well objects. as in the combined K dwarf sample of Kuijken (1991), should Further research cannot be carried out until a larger sample be treated, according to Bahcall et al. (1992), with caution, as of these late-type objects can be identified; less than a dozen this may introduce unwanted systematic errors. objects with types of M8 or later are known (Paper III). As an Clearly, the last has not been written on this subject. If there example, the differences between M dwarfs and GD 165 B will is missing mass, the most likely possibilities are very low lumi- be fully understood and placed in context with current brown nosity degenerates, faint M dwarfs, or brown dwarfs. Although dwarf theory only when other extremely cool objects are dis- the contribution to the local space density by low-luminosity covered and can be studied. white dwarfs has been demonstrated to be negligible (Liebert, Dahn, & Monet 1988), M dwarfs and brown dwarfs of faint absolute magnitude may still lie unrecognized in the immedi- 1.3. Outline of the Paper ate solar vicinity. In this paper, a photometric search is undertaken over a large area of sky to determine the space density of the coolest 1.2. Very Low Mass Stars and Brown Dwarfs dwarfs and of any possibly substellar objects. The CCD/Tran- sit Instrument ( CTI ) used to conduct this search is described in Studies of objects at the end of the main sequence are also § 2. Some of the objects discovered in the CTI databases have vital to determining the observable parameters of the lowest also been targeted spectroscopically in an attempt to assess the mass stars. Once their substellar counterparts are found, re- accuracy of the CTI photometry, particularly near the magni- searchers will be able to compare the two groups of objects. tude limit of the survey, and to identify previously unknown, One of the goals is to see if stars and brown dwarfs can be very late M dwarfs in the solar neighborhood. The selection of distinguished by a parameter more easily measured than the spectroscopic targets and the follow-up observations are dis- mass. cussed in § 3. Some of the spectroscopic groundwork for this investigation Using this complementary spectroscopic information to- is already in place. Spectroscopy of a comprehensive set of gether with photometry of all stars in the CTI databases, a late-type stars has already been acquired; specifically, the sig- luminosity function for M dwarfs is produced. No other recent nature of K5 to M9 dwarfs between 6300 and 9000 Â was photometric determination of the faint end of the stellar lumi- addressed in Kirkpatrick, Henry, & McCarthy (1991, hereafter nosity function has obtained spectra to check the majority of Paper I). These spectra were extended to 1.5 /an and fitted to a targets found in the lowest luminosity bins (as well as a sam- set of theoretical spectra to determine the M dwarf tempera- pling of stars found in the higher luminosity bins).
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