In order to tackle the problems of low-mass end of the initial mass function (IMF) in star-forming regions and the formation mechanisms of brown dwarfs, we have conducted deep infrared surveys of nearby molecular clouds. We have found a significant population of very low-luminosity sources with IR excesses in the Taurus cloud and the Chamaeleon cloud core regions whose extinction corrected J magnitudes are 3 to 8 mag fainter than those of typical T Tauri stars in the same cloud. Some of them are associated with even fainter companions. Follow-up IR spectroscopy has confirmed for the selected sources that their photospheric temperature is around 2000 to 3000 K. Thus, these very low-luminosity young stellar sources are most likely very low-mass T Tauri stars, and some of them might even be young brown dwarfs.

The initial mass function of stars (IMF) at small masses depends on several factors. First, it depends on the mass function of cloud cores in which stars form. Second, there must be a lower limit to the core mass for contraction; very small mass cores may not contract even if they exist. This must affect greatly the IMF near its lower end. Third, not all core matter may become stars; we must determine the stellar mass M* , or the star formation efficiency M*/Mcc, as a function of the mass of the cloud core, Mcc. In this paper we discuss the second and third points.

White dwarf (WD) evolution is essentially a cooling problem in two parts. Degenerate electrons in the core provide the support while non-degenerate C and O ions provide the thermal reservoir (Koester & Chanmugam 1990, Kawaler 1996). The rate of energy loss through the non-degenerate envelope of H and He—segregated by gravitational settling—is meager enough that the vast majority of WDs have not had time to cool to undetectability in the ~10 Gyr age of the Galactic disk. Because WDs are a remarkably homogeneous class with a mass function sharply peaked about 0.6Mʘ, and because the cooling physics is relatively straightforward to calculate, the observed deficit of WDs below Mv = 16.5 [log(L/Lʘ) ≈ -4.5, Teff ʘ 4000 K; see Liebert et al. 1988 (LDM), Oswalt et al. 1996 (OSWH), and Leggett et al. 1997] provides constraints on the age and star-formation history of the Galactic disk (Winget et al. 1987 (WEA97), Wood 1992 (W92), Yuan 1992, Hernanz et al. 1994).

The maximum fractional surface layer masses that can remain following post-AGB evolution are log(qH) ~ —4 and log(qHe) ~ —2 (for a 0.6 Mʘ remnant—the values are inversely correlated with remnant mass), although we know from WD asteroseismology that surface H layer masses range from this value to smaller than logqH ~ —6 (e.g., Fontaine & Wesemael 1997). Some 75% ofWDs in the McCook & Sion (1987) catalog reveal pure-H photospheres (spectral type DA). The remainder are broadly classified as non-DAs, with most showing show pure-He spectra (DB) or continuous spectra (DC), and with ahandful that show mixed compositions (DAB), or C (DQ) or metal lines (DZ). See Bergeron et al. 1997 for a thorough discusion of the chemical evolutionof cool WDs.

The study of cluster white dwarfs (WDs) has been invigorated recently bythe Hubble Space Telescope (HST). Recent WD studies have been motivated by the new and independent cluster distance (Renzini et al. 1996), age (von Hippel et al. 1995; Richer et al. 1997), and stellar evolution (Koester & Reimers 1996) information that cluster WDs can provide. An important byproduct of these studies has been an estimate of the WD mass contribution in open and globular clusters. The cluster WD mass fraction is of importance for understanding the dynamical state and history of star clusters. It also bears an important connection to the WD mass fractions of the Galactic disk and halo. Current evidence indicates that the open clusters (e.g. von Hippel et al. 1996; Reid this volume) have essentially the same luminosity function (LF) as the solar neighborhood population. The case for the halo is less clear, despite the number of very good globular cluster LFs down to nearly 0.1 solar masses (e.g. Cool et al. 1996; Piotto, this volume), as the field halo LF is poorly known. For most clusters dynamical evolution should cause evaporation of the lowest mass members, biasing clusters to have flatter present-day mass functions (PDMFs) than the disk and halo field populations. Dynamical evolution should also allow cluster WDs to escape, though not in the same numbers as the much lower mass main sequence stars. The detailed connection between cluster PDMFs and the field IMF awaits elucidation from observations and the new combined N-body and stellar evolution models (Tout, this volume). Nevertheless, the WD mass fraction of clusters already provides an estimate for the WD mass fraction of the disk and halo field populations. A literature search to collect cluster WDs and a simple interpretive model follow. This is a work in progress and the full details of the literature search and the model will be published elsewhere.

The expression “low luminosity stars” is a descriptive category which spans a wide range of objects, from the oldest stellar remnants in the Galaxy (Wood) through failed stars (Jones, Gould) to the enigmatic MACHOs, discernible not in themselves but only through their effects on others (Bennet). All of these have attracted considerable attention in recent years, and significant progress has been achieved in each case, even if our understanding has failed to keep up with observations. However, it is in the area of brown dwarfs where the most dramatic results have been obtained. The existence of such objects has been predicted theoretically for well over thirty years, but predictions can fail. Thus the discovery of G1229B (Nakajima et al, 1995), an unambiguous substellar-mass object, followed by the detection of lithium in Teide 1 and Calar 3 in the Pleiades (Rebolo et al, 1996) mark a turning point in studies of brown dwarfs. The issue now is not whether they exist, but what are their properties as a class. With that in mind, a number of obvious questions arise.

First, what is the most effective method of finding brown dwarfs? Both brown dwarfs and VLM stars have effective temperatures below 2500K. Hence, many surveys concentrated on searching for objects which were very red at near-infrared wavelengths, and the blue JHK colours exhibited by Gl 229B cameas something of a surprise. This should not have been the case, of course, since Tsuji (1964) predicted the presence of methane in such cool atmospheres well before the term ’brown dwarf’ was invented. Current theory, as summarised in these sessions by Tsuji and Burrows, predicts that, as one moves to lower temperatures, grain formation initially drives (J-K) redward, before grains settle below τ ~ 1 and CH4 absorption sets in. Throughout, however, the optical-to-IR colours, straddling the peak in the emergent flux distribution, become progressively redder (Reid, 1994). Those colours therefore offer the most effective means of identifying both GD 165B-like and Gl 229B-like sources - an argument reinforced by the first results from the IJK DENIS survey (Delfosse et al, 1997).

In a recent survey for faint red stars from a digital stack of Schmidt plates a number of candidate objects were identified. Parallax’s for three of these objects have been reported showing them to have luminosities which interpreted within the available evolutionary models indicate them to be good brown dwarf candidates. Here we examine spectra of these objects and others from the plate stack. Using standard spectral indices we find that for a given spectral type their spectra are more consistent with the Pleiades brown dwarfs (PPL 15, Teide 1 and Calar 3) than with standard late-type M dwarfs. Our interpretation is that this is due to their selection by RF IN colours which at values > 3 preferentially selects objects with relatively low gravities. For late-type M dwarfs and brown dwarfs low gravities are expected to be a reliable indication of youth. We also notice that the stack objects generally have strong FeH absorption for their spectral type. Current model atmospheres suggest that FeH strongly increases in strength toward lower metallicities and lower temperatures. We believe that this is not consistent with the available observational evidence from late-type M dwarfs. It is possible that solid Fe is forming inthe low temperature atmospheres relatively depleting FeH strengths toward lower temperatures. We find some evidence that for dwarfs at low temperatures dust formation is less prevalent in lower gravity objects suggesting that dwarfs at low temperatures stronger FeH may be an indication of youth. In addition to the spectral evidence the three stack objects whose parallax’s have been measured show small tangential velocities which is a further indication of youth.

Photometric parallaxes derived from B, V, R, I photometry of wide binaries with white dwarf components were used to set a firm lower limit to the age of the Galactic disk (see Oswalt et al. 1996 and the article by Wood elsewhere in these proceedings). Here we show the distribution of projected semi-major axes derived from new B, V, R, I data for over 200 wide pairs.

Like clusters, wide binaries are gradually disrupted, primarily by the gentle but cumulative effects of encounters with the denser parts of giant molecular clouds (see Wasserman & Wienberg 1991 for a review). A sharp cut-off in semi-major axes > 0.1 pc was one of the predictions of competing models. Wood & Oswalt (1992) showed that additional orbital expansion occurs in wide pairs which have experienced isotropic post-MS mass loss. Can we see the signatures of these events in the wide binary sample?

Dwarf carbon (dC) stars are believed to be main-sequence stars which have received carbon-rich material from a former AGB carbon star companion. We have computed model atmospheres, synthetic spectra, and colours for dC stars and compared them with observations. Our models are computed with an updated version of the MARCS code (Jørgensen et al. 1992, A&A 261, 263) and include atomic and molecular line blanketing, grain formation, and collision-induced absorption (CIA) processes (Borysow et al. 1997, A&A 324, 185). The models assume hydrostatic equilibrium, which for giant stars (i.e., high luminosity and low gravity) would be inconsistent with the inclusion of dust. Typical gravities of dC stars are, however, considerably larger than the acceleration due to radiation pressure even on dust grains with a relatively high absorption coefficient.

For the cooler solar metallicity models, grain formation can be substantial, but it depends strongly on the assumed type of dust. In a test model of Teff = 2800 K, C/O = 1.17, and log(g) = 5, 1% of the free carbon (i.e., that not bound in CO molecules) condensed when the dust was assumed to be amorphous carbon, whereas as much as 30% condensed when nano-diamonds (of the extra-solar form which are abundant in carbonaceous chondrite meteorites and are believed to have formed in stellar outflows) were allowedfor. This difference in degree of condensation is due to the very small diamond opacity compared to that of amorphous carbon. In both cases, however, the main effect of the dust was a 400 K heating of the surface layers. The dust itself is not clearly visible in the synthetic spectrum, but the heating and correspondingly reduced molecular partial pressures cause a weakeningof spectral features in the 3μm region (due mainly to reduced C2H2 abundance in the upper layers) and reduced CO fundamental and first overtone band intensities.

We have recently carried out an infrared parallax programme on the UKIRT conducted with a 2562 array. Our observations were made in the near-infrared K-band which is ideal for observing red stars and brown dwarf candidates and has the advantage that differential colour refraction is negligible. We find a parallax of πabs = 156.0 ±13.3 mas for the archetypal late-type M dwarf vB 10 and πabs =11.1 ± 7.5 mas for PC 0025+0447 which we conclude is too distant to be a clear case of a brown dwarf and is probably a late-type M dwarf close to the hydrogen-burning limit. Recent improvements in telescopes and instrumentation working in the infrared should enable accuracies of ~ 1 mas to be obtained for future infrared parallax programmes.

Spectra of M dwarfs are rich in atomic and molecular lines. These spectra provide such basicinformation as Teff (or radius), log g (or mass), surface chemical composition, and something more (e.g. activity) if properly interpreted. It is recognized, however, that spectra of M dwarfs are already dimmed by the dust formed in their photospheres (Tsuji et al. 1996a) and this effect, which has been overlooked until recently, should be taken into account in any interpretation and analysis of the spectra of very low mass objects (VLMOs) including late M dwarfs and brown dwarfs.

We compare observations of the binary system CM Draconis with synthetic spectra. Spectroscopic observations from 0.40 to 2.41μm, combined with photometry and the accurately known surface gravity enable us to estimate the temperature and metallicity using detailed spectra synthesis. We find discrepancies between the analysis of the infrared and optical spectrum: while the optical spectral energy distribution (SED) yields a metal-rich solution with Teff = 3000K, the infrared SED yields around 3200K and —0.8 ≤ [M/H] ≤ —0.6 compatible with the high space motion of the system. The low-metallicity characteristics of the infrared SED could be real and is partly supported by the detailed analysis of the atomic lines in the optical region. Although, the known incompleteness of the TiO and H2O line lists in the models may cause substantial systematic errors.

Most of the flare stars (FSs) discovered in that field are very faint and are missing from all previously published lists of Pleiades cluster members. Reliable membership probabilities (MPs) were only determined for one third of these objects while the rest were supposed to be more or less probable members with a few exceptions. A recent proper motion survey of that field (Schilbach et al. 1995) based on plates taken with the Tautenburg Schmidt telescope and extended to an area of 16.5 square degrees provided new cluster MPs and also photometric data even for a number of the lowest luminosity stars (in fact the faintest object measured has V = 18.26). Of the 520 known FSs in the field 437 (85%) were successfully identified, their J2000.0 coordinates, high precision proper motion components and apparent photographic R magnitudes (and for the majority of them their V and B magnitudes too) determined. Based on these data new cluster MPs have been deduced which show that a considerable number (40%) of the so called Pleiades FSs are not members of that cluster at all. This conclusion follows the tendency already found: when we involve more and more faint FSs into the investigations the percentage of non-members monotonically increases (cf. Haro et al. 1982 and references therein).

The Tycho Catalogue contains proper motions and low precision parallaxes with a median precisions of 25 mas, while having high precision B and V magnitudes (median precision of 0.07 and 0.06 respectivly). The number of highparallax stars is quite unexpected, with over 30 previously unreferenced, non-Hipparcos, stars with a parallax >500 mas and over 2000 with a parallax >250 mas. While many of these are probably mistakes, the completeness limit of Tycho and the precise magnitudes provide us with a way to intelligently build a complete picture of the nearby star field. Catalogues of nearby stars provide us with many fundamental parameters in the field of stellar astronomy, but the completness of known stars within say even 8pcs is still underestimated by about 30%, i.e., 50-60 systems (T.J. Henry, CSSS8, ASP Conf Series 64, 1994).

In a new program at Torino we are using the Tycho Catalogue to intelligently build a list of possible candidates for a 1Opc volume. Figure 1 shows that Tycho will contain basically all ZAMS to spectral type M3 within 10 pes, M5 to 4pcs and M6 to 2pcs. A comparison of the photometric and Tycho distances provides a list free of giants and systems where binarity has affected the color. This list is then added to the Torino Parallax program where we expect to provide more precise parallaxes.

As we approach the 21st Century, enormous technical advances are being made in the acquisition of galaxy redshifts. This Joint Discussion offers a perspective on those advances and the associated redshift surveys. It is representative, but by no means all inclusive.

My introduction offers some historical background, against which the new methods and achievements can be measured.

Redshift surveys tells us about the spatial distribution and evolution of galaxies. Yet the study of large-scale structures in the distribution of galaxies goes back further than one might imagine. The first accurate description of the Virgo Supercluster was given by John Herschel in the mid-19thCentury. To paraphrase Sir John, “Virgo is the central condensation of a roughly spherical cluster of nebulae - our system lies outside the denser part of the cluster, but is involved with its outlying members - forming an element of some one of its protuberances or branches”.

Victorian science might have beaten Hubble had more people believed John Herschel - and had he himself more confidence in his interpretation. Few were prepared to accept the ‘Island Universe’ theory that it implied, and strong criticism followed. Moreover, Herschel was very reluctant to go against the beliefs of his revered father, William Herschel, who, though once believing in the ‘Island Universes’, had in later life accepted that all nebulae were somehow gaseous condensations in our system. Consequently, even John Herschel’s textbook still stated the older, incorrect interpretation.

The concept and the present status of the Sloan Digital Sky Survey are described, with emphasis on the instrumentation aspect of the project.

Fundamental to our understanding of the universe is the evolution of structures, from galaxies to clusters of galaxies to large-scale sheets and filaments of galaxies and voids. The investigation of the evolution of large-scale structure not only provides us with the key test of theories of structure formation, but also allows us to measure fundamental cosmological parameters. The CNOC2 (Canadian Network for Observational Cosmology) Field Galaxy Redshift Survey is the first large redshift survey of faint galaxies carried out with the explicit goal of investigating the evolution of large scale structure. This survey also provides the largest redshift and photometric data set currently available for the study of galaxy population and evolution at the moderate redshift range between 0.1 and 0.6. In this paper we describe the scope and technique of the survey, its status, and some preliminary results.

Recent advances in the technology of rotating liquid-mirrors now make feasible the construction of large optical telescopes for dedicated survey programs. Two three-metre-class astronomical telescopes have been built and asix-metre telescope is under construction. These instruments observe in zenith-pointing mode, using drift-scanning CCD cameras to record continuous imaging of a strip of sky typically 20 arcmin wide. This enables them to observe of order 100 square degrees of sky with an integration time of a few minutes per night. Data can be co-added from night to night in order to increase the depth of the survey. Liquid-mirror telescopes are particularly wellsuited to surveys using broad or intermediate bandwidth filters to obtain photometric redshifts and spectral energy distributions for faint galaxies and quasars.

DEEP is a multi-institutional program designed to undertake a major new spectroscopic survey of faint field galaxies with the Keck II 10-m telescope. The scientific goals are broad and include exploring galaxy formation and evolution, mapping the large scale structure at moderate to high redshifts, and constraining the nature and distribution of dark matter and cosmology. Besides the primary goal of securing large numbers of redshifts (10, 000+) to very faint limits of I ~ 23, DEEP intends to acquire spectra of high enough quality and spectral resolution to extract rotation curves, velocity dispersions, age estimates, and chemical abundances for a brighter subset of galaxies. A new imaging spectrograph for Keck called DEIMOS has been specifically designed to achieve these goals and is currently scheduled for completion by the end of 1998. DEIMOS will provide anoverall gain for multi-object spectroscopy of about 7x compared to the current low-resolution spectrograph (LRIS). While awaiting for DEIMOS to be operational, the interim DEEP science programs have been diverse, but largely concentrated on spectroscopy of faint galaxies observed with HST, especially in the “Groth Strip” and Hubble Deep Field (HDF) and its flanking fields. Recent highlights include redshift and kinematic studies of compact galaxies, high redshift (z ~ 3) galaxies, and distantspirals.

This IAU Joint Discussion proposes to address the subject of redshift surveys in the 21st century. This paper, however, deals with two major new redshift surveys that those involved sincerely hope will be completed in the 20th century. Nonetheless, these surveys are relevant to the topic of the meeting, as they clearly foreshadow the scope and style of redshift surveys, if not in the coming millennium, at least in the coming decade.

The surveys are being carried out with the new Two Degree Field (2dF) facility on the Anglo-Australian Telescope (AAT), a 400-fibre multi-object spectrograph with the capability, as described in Section 2, to increase the size of redshift surveys by an order of magnitude over current best efforts. The main scientific goals, survey strategy and some preliminary results from the 2dF Galaxy Redshift Survey are outlined in Section 3, while Section 4 similarly describes the 2dF QSO Redshift Survey. Further information can be found on the WWW at http://www.aao.gov.au/2df/ for the 2dF facility, at http://msowww.anu.edu.au/~colless/2dF/ for thegalaxy survey and at http://www.aao.gov.au/local/www/rs/qso_surv.html for the QSO survey.