Search

Numerical simulations are used to explore how gravitational interactions within young multiple star systems may determine the binary properties of brown dwarfs. We compare different scenarios for cluster formation and decay and find that brown dwarf binaries, although possible, generally have a low frequency. We also discuss the frequencies of brown dwarf companions to normal stars expected from these models.

Up to now, most planet search projects have concentrated on G and K stars. In order to considerably widen the view, we have stated a survey for planets of old, nearby brown dwarfs and very low-mass stars. Using UVES, we have observed 26 brown dwarfs and very low-mass stars. As it turned out these objects are very inactive and thus highly suitable for such a project. For 19 objects, we can exclude a planet with the mass of 3 MJ, and a period of 100 days or less with a probability of more than 60%. For these objects, we can also exclude Pegasi-planets with a high probability. For another 4 objects, we can exclude at least a brown dwarf companion. One object is a double line spectroscopic binary, and one object shows significant radial-velocity variations that can not be caused by a normal stellar-spot. This object either has a planetary-mass companion, or the variations are caused by surface structures that are quite different from normal star-spots.

In spite of all the attempts to find them, no one has yet detected any brown dwarf in a globular cluster. Although powerful instruments such as the VLT and Advanced Camera could further push the frontiers of this search, globular clusters will probably hold tight to their secrets for a while longer. Nonetheless, the search for very low mass stars in globular clusters has taught us a lot about their original mass distribution (IMF) and its evolution in time. I shall review the results of an investigation carried out over what is presently the largest, most homogeneous sample, and discuss the reasons suggesting that: 1. dynamical evolution (internal and external) has reshaped the cluster mass function over time, but the imprint of the IMF is still visible; 2. the IMF appears to vary very little from cluster to cluster; 3. the most likely functional form of the IMF is that of a power law that rises to a peak at ˜ 0.3 M⊙ and tapers off at smaller masses.

We present results from the most complex hydrodynamical star formation calculation performed to date. It follows the collapse and fragmentation of a large-scale turbulent molecular cloud to form dozens of stars and brown dwarfs. It resolves all fragmentation down to the opacity limit, binary stars with separations as small as 1 AU, and circumstellar disks with radii down to ≈ 10 AU. In this proceedings, we examine the formation mechanism of the brown dwarfs and compare the initial mass function and the properties of the brown dwarfs with observations.

We present the results of a high spatial resolution ultraviolet, optical and near-infrared survey of 44 young binary stars in Taurus-Auriga with separations of 10–1000 AU. The observations were carried out using the Hubble Space Telescope and NASA's IRTF. The binary star properties corroborate our previous work that suggests fragmentation is the dominant binary star formation mechanism. Of particular interest, we find that the components of binary systems are more coeval than randomly paired single T Tauri stars. Several important conclusions are drawn regarding the evolution of circumstellar material in binary systems. The mass accretion rates for primary stars are similar to single stars, which suggests that a companion as close as 10 AU has little effect on the mass accretion rate. These accretion rates, if constant, require replenishment of the inner circumstellar disks for at least the closest (≲ 100 AU) binary systems. On average, circumprimary disks appear to survive longer and accrete at a higher rate than circumsecondary disks do. This suggests that circumprimary disks are being preferentially replenished, possibly from a circumbinary reservoir with low angular momentum relative to the binary. The relative T Tauri types and the binary mass ratios tentatively suggest that systems with separations ≲ 200 AU share a common circumbinary reservoir. The higher mass accretion rates of primary stars relative to secondary stars is most likely due to their larger relative mass.

Analysis of a sample of giant Herbig-Haro flow sources shows that 79%–86% are binaries or higher order multiples. This represents the youngest sample of stars studied so far for binarity. A stellar dynamics jet hypothesis is proposed in which the dynamical decay of triple or multiple systems leads to giant outflow activity. Close triple approaches will cause serious perturbations and probably direct collisions among individual circumstellar disks, with a consequent burst of outflow activity, which can produce giant HH bow shocks. As one component is ejected, the two remaining stars and their small truncated disks form a closer bound pair with high eccentricity. Gas streams from a circumbinary disk feed the stars and this as well as other dynamical effects cause the binary orbit to shrink. As the stellar components gradually spiral towards each other, accretion and outflow becomes cyclic, modulated on an orbital time scale. The resulting HH flows can be read as a fossil record of the evolution of orbital motions of the newly formed binary as it shrinks from a typical separation of 100 AU or more to 10 AU or less. After a triple disintegration event, both components (star and close binary) leave their nascent envelope, and while one component becomes visible as a T Tauri star, the other will be obscured for a while by the envelope and will appear as a bright near-infrared object, thus explaining the socalled IRC binaries which are infrequently found in star forming regions.

We review recent imaging results on the circumstellar matter of young binaries obtained using the Hubble Space Telescope (HST) and by PUEO, a ground-based adaptive optics (AO) system operating at CFHT. In the area of circumbinary disks, new results for GG Tau help form a more complete picture of this prototype system. However, HST images of the UY Aur ring indicate a more complex system than first thought. A new example of a circumstellar disk in a multiple star system has been found in HV Tau, joining HK Tau in this category. Examples of envelopes, outflow cavities, and jets are shown for several young binaries. About half of the nearby young binaries in our survey possess some kind of local nebulosity at optical or near-IR wavelengths.

We review correlations of spectral accretion diagnostics in young binary star systems. Hydrogen emission lines at visual (Hα) and near-infrared (Brγ) wavelengths indicate the presence of active accretion flows onto young stars. We examine the simultaneity of this process in both components of close binaries (separations 100–1500 AU) and find that active accretion is not a random process in these systems: the two stars are typically both active or both inactive. Only 19% of our sample consists of mixed pairs in which just one component displays evidence of accretion, usually the primary. In systems with two active stars, the equivalent width and in particular the line flux of the primary star is generally dominant, indicating stronger accretion activity. We discuss accretion and disk dissipation processes in close binaries and propose that a circumbinary envelope model accounts for the correlated evolution of circumstellar disks around young binaries.

The pre–main-sequence spectroscopic binary AK Sco is surrounded by a circumbinary disk. Yet while this disk must be somewhat disturbed by the binary at its center, there is no evidence from current data that it has a cleared central hole like those seen in some young binaries, nor that accretion from the outer disk has been cut off. Indeed, most of its spectral and photometric diagnostics are indistinguishable from those of young single stars. Why doesn't this system show any evidence of disturbance of its disk? It may be that the center of the disk is indeed largely cleared, but that a small amount of dust in this region supplies the observed near-infrared excess.

We present near-infrared images of the circumbinary disk surrounding the pre–main-sequence binary star, GG Tau A, obtained with NICMOS aboard the Hubble Space Telescope. These images have a SNR ∼25 times higher than previous ground-based measurements, which allows the spatially resolved disk to be explored in detail. The geometry of the ring implies that the circumbinary disk is not intrinsically circular, possibly due to interactions with the central binary star. Overall, the circumbinary disk is redder than the central binary, with the amount of red excess increasing with wavelength. Significant variations in color over the 13 arcsec2 covered by the disk are also observed, raising the possibility that disk inhomogeneities are present.

I review the possible formation mechanisms of close binary stars. The formation of close binary systems is problematic in that there is no theory that does not encounter significant difficulties or unknowns. Fission does not appear to occur in stars. Capture is unlikely to form many close binary systems except possibly amongst massive stars. Fragmentation can form close binary systems but these need to accrete the majority of their eventual mass. Furthermore, there appears to be a limited window in initial conditions that may preclude forming sufficient systems in this way. Possible alternatives include the orbital migration of a binary due to its circumbinary disk and the disintegration of a non-hierarchical multiple system.

Detailed radiative transfer calculations are an essential tool for the analysis of observations related to fragmented molecular cloud cores, protobinaries, and circumbinary and circumstellar disks. In this contribution, recent progress in line and continuum radiative transfer models will be reviewed.

We report on an effort to identify further members of the dwarf carbon class and new members of a putative dwarf S group in the general field through determination of proper motions of catalogued stars. Examination of nearly 1500 C stars and over 300 S stars reveals some interesting false alarms but no new dwarf members of these classes.

Deep BVI CCD photometry of dwarf galaxy DD0 210 obtained in this study shows that the resolved stellar population in this galaxy consists of a dominant old red giant population and some young stars. We have estimated the distance to this galaxy using the I-band magnitude of the tip of the red giant branch, obtaining a value of 1030±50 kpc. This result combined with the velocity of DDO 210 shows that DDO 210 is a member of the Local Group.

Classical studies of accreting white dwarfs have assumed spherical symmetry. However, it is believed that in close binary systems the transfered matter carries angular momentum to spin up the accreting star. Here, we present preliminary results of CO white dwarf models which accrete helium rich matter with effects of rotation considered, in the context of the Sub-Chandrasekhar mass scenario for Type Ia supernovae.

We present new infrared photometry of Pleiades and Praesepe sources, and define the cluster sequences in colour magnitude and 2-colour diagrams. We identify non-members and unresolved binaries, and discuss the clusters binary fractions.

We present results of the first large (> 100 stars) infrared coronographic search for substellar companions to nearby stars. The search consisted of two surveys of stars chosen for their youth and proximity to Earth: 1.) a 178 star infrared survey at Steward and Lick Observatories, with optical followup from Keck Observatory, capable of detecting companions with masses greater than 30MJ, orbiting between about 75 and 300 AU, 2.) a 102 star survey using the Keck telescope, capable of detecting extrasolar brown dwarfs and planets typically more massive than 10 MJ, orbiting between about 75 and 300 AU.This research resulted in the discovery of one brown dwarf companion, zero planets and 23 double stars. The frequency of brown dwarf companions (of any mass) to G, K and M stars orbiting between 75 and 300 AU is measured to be 1 ± 1 %. The frequency of massive (> 30 MJ) brown dwarf companions is found to be 0.6 ± 0.6 %. The frequency of giant planets with masses larger than 10 MJ, between 75 and 300 AU, is measured here for the first time to be no more than about 3 %.

We have started a program to search for Brown Dwarf (BD) companions to nearby stars (5 ≤ d ≤ 10 pc) on long period orbits (up to 2000 AU) using wide field optical/infrared imaging. Our goals in surveying a volume-limited sample of stars are: 1) to determine the binary fraction of widely separated BDs and help constrain the BD formation processes, 2) to search for very cold BDs, 3) to provide a better determination of the age and luminosity of a BD sample. Observations are currently conducted at three observatories, CFHT (3.6m), CTIO (4m) and Observatoire du mont Mégantic (1.6m) and should yield a sample of ~100 star systems to a detection limit of J ≈ 20.

We present high resolution optical spectra obtained with the Keck I telescope of low mass T Tauri stars and brown dwarfs in the Taurus star forming region. Based on the inferred photospheric and circumstellar properties, we conclude that objects in Taurus with masses as low as 50 Jupiters form and evolve in the same way as higher-mass T Tauri stars, but with smaller disks and shorter disk lifetimes.

The proposed Eclipse Discovery mission is an optical space telescope designed to provide a thousandfold reduction in scattered light near bright stars in comparison to any Hubble Space Telescope instrument. A survey of 500 single stars within 15 pc can detect companions with absolute z magnitude of 22 at separations > 10 AU in most of the targets. Spectrophotometry of CH4 and H2O bands between 0.8-1.0 μm can be used to derive the effective temperatures of the objects. The ECLIPSE brown dwarf survey would directly measure the luminosity function of brown dwarf companions down to ~20 Jupiter masses, providing a crucial comparison with field objects.

We have investigated the nature of P Cygni-like features in the high resolution IUE spectra of the symbiotic variables BF Cyg and EG And by looking for orbital phase-dependent variations in the structure and velocity of the C IV (1550 Å) resonance doublet in 10 archival IUE high-dispersion spectra of these systems and by comparing their profiles with P Cygni-like features seen in IUE high-dispersion spectra of other symbiotic stars. For BF Cyg, we find maximal P Cygni absorption equivalent widths and terminal velocities of ~ 900 mÅ and ~ −240 km s−1 respectively, full emission velocity widths at the 10% intensity level of 300–400 km s−1, depending on phase, Φ, variations in the width of the P Cygni absorption, shortward velocity shifts of the C IV emission line in all but two orbital phases, relative to the C IV velocity at Φ = 0.01, and absorption sub-structure within the C IV P Cygni lines which coincide in velocity from image to image. In EG And the C IV P Cygni absorption equivalent width is, on average, 300 mÅ with a terminal velocity of −217 km s−1 (which includes the systemic radial velocity contribution of -93 km s−1). The presence in one image, of a strong absorption feature at He II (1640), having a precise velocity coincidence with the C IV P Cygni absorption component, suggests that the absorbing material responsible for both line features is associated with outflow and P Cygni self-absorption from the vicinity of the hot component. In both BF Cyg and EG And, the structure and behavior of the P Cygni profiles may rule out their origin in the red giant wind or in an expanding circumbinary shell. Our results support the hot component wind interpretation of Mikolajewska et al. (1989). Motivated by the low-mass white dwarf shown to be present in BF Cyg (Mikolajewska et al. 1989), we have carried out quasi-static evolutionary model calculations with accretion onto a 0.55 M⊙ white dwarf accreting at the rate 2 × 10−8 M⊙ yr−1. The sequence of 3800 quasi-static models, the first to be reported for a low-mass hot white dwarf (Mwd < 0.8 M⊙) accreting at a high rate, yields an unexpected outburst behavior in response to accretion at that rate, when compared to evolutionary sequences with accretion onto higher mass white dwarfs. A series of three major thermonuclear shell flashes and one lower energy shell flash is revealed with the timescale during outburst and the inter-outburst timescale substantially shorter than expected for a 0.55 M⊙ white dwarf. Preliminary results on other sequences are also reported. Implications for the observations of symbiotic systems thought to contain low-mass white dwarfs, are discussed.

Observational evidence of transient flares which produce conspicuous changes in the total brightness and in the spectra of dwarf stars is limited, at present, to a small number of faint late-type stars. The outbursts take place with extraordinary rapidity and the duration is usually only a few minutes or hours. Accompanying spectral changes such as veiled and fuzzy absorption lines, greatly increased intensity of the bright lines of hydrogen and helium, and the appearance of an emission continuum shortward of λ 3750, have been reported.

Basic physical characteristics of nova-like binary systems are similar to W UMa systems. Externally, however, these systems are different – nova-like systems contain a white dwarf as a component, whereas both components of W UMa systems are near the main sequence. The hypothesis is suggested of the origin of contact W UMa systems by means of the fission of the rapidly rotating isothermal helium core of an evolved giant star. The contraction of the more massive component leads to the formation of a white dwarf and consequently to the transformation of a W UMa type system to a nova-like system.Submitted to Astrophysics and Space Science.