A Young Population of Brown Dwarfs

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. The straightforward interpretation of their position in a colour magnitude diagram suggests that they are metal poor by 0.5-1.0 dex. However using standard spectral indices their spectra seem 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 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 have constructed preliminary models for the number of brown dwarfs expected from the digital stack survey. We find that most of the objects expected are brown dwarfs less than 1 Gyr old and because of this it is important to make such models using a time-dependent scale height

A New Population of Brown Dwarfs

A large area deep survey for brown dwarfs from digitally stacked 1.2m Schmidt plates has revealed a population of very red stars (R-I>3). On the basis of their colours, near IR spectra and their luminosity estimates from proper motions, these objects appeared to be Brown Dwarfs. First measurements of the parallaxes of 8 of the reddest objects at the 1.5 danish telescope confirmed this, and showed that four of them have K-band luminosities some mags below the bottom of the main sequence. We present here those results

Variability in active galactic nuclei: confrontation of models with observations

The variability of active galactic nuclei (AGN) has long held the promise of shedding light on their detailed structure, and possibly other astrophysical phenonema. Different emission mechanisms lead to different patterns of variability in flux which are in principle easily distinguishable. Recent predictions for the expected spectrum of variations for various models are now in such a form that they can be compared with the observed statistical properties of AGN light curves from large scale monitoring programmes. In this paper, we use the results of a long term monitoring programme of a large sample of quasars and Seyfert galaxies, as well as individual light curves from the literature, to distinguish between the various model predictions. The results favour a model based on accretion disc instability over the starburst model where the variation comes from a succession of supernova bursts, but it also appears that much of the observed variation in quasars is due to gravitational microlensing.Comment: 12 pages including 10 figures. Accepted for publication in MNRA

Disparate MgII Absorption Statistics towards Quasars and Gamma-Ray Bursts : A Possible Explanation

We examine the recent report by Prochter et al. (2006) that gamma-ray burst (GRB) sight lines have a much higher incidence of strong MgII absorption than quasar sight lines. We propose that the discrepancy is due to the different beam sizes of GRBs and quasars, and that the intervening MgII systems are clumpy with the dense part of each cloudlet of a similar size as the quasars, i.e. < 10^16 cm, but bigger than GRBs. We also discuss observational predictions of our proposed model. Most notably, in some cases the intervening MgII absorbers in GRB spectra should be seen varying, and quasars with smaller sizes should show an increased rate of strong MgII absorbers. In fact, our prediction of variable MgII lines in the GRB spectra has been now confirmed by Hao et al. (2007), who observed intervening FeII and MgII lines at z=1.48 to be strongly variable in the multi-epoch spectra of z=4.05 GRB060206.Comment: 12 pages, 2 figures; substantially revised model calculation; accepted for publication in Astrophysics & Space Science as a Lette

On the anomalous large-scale flows in the Universe

Recent combined analyses of the CMB and galaxy cluster data reveal unexpectedly large and anisotropic peculiar velocity fields at large scales. We study cosmic models with included vorticity, acceleration and total angular momentum of the Universe in order to understand the phenomenon. The Zeldovich model is used to mimic the low redshift evolution of the angular momentum. Solving coupled evolution equations of the second kind for density-contrast in corrected Ellis-Bruni covariant and gauge-invariant formalism one can properly normalize and evaluate integrated Sachs-Wolfe effect and peculiar velocity field. The theoretical results compared to the observations favor a much larger matter content of the Universe than that of the concordance model. Large-scale flows appear anisotropic with dominant components placed in the plane perpendicular to the axis of vorticity(rotation). The integrated Sachs-Wolfe term has negative contribution to the CMB fluctuations for the negative cosmological constant and it can explain the observed small power of the CMB TT spectrum at large scales. The rate of the expansion of the Universe can be substantially affected by the angular momentum if its magnitude is large enough.Comment: 13 pages, 6 tables, 4 figures, 36 references; version to appear in Eur. Phys. J.

Cosmological Applications of Gravitational Lensing

The last decade has seen an enormous increase of activity in the field of gravitational lensing, mainly driven by improvements of observational capabilities. I will review the basics of gravitational lens theory, just enough to understand the rest of this contribution, and will then concentrate on several of the main applications in cosmology. Cluster lensing, and weak lensing, will constitute the main part of this review.Comment: 26 pages, including 2 figures (a third figure can be obtained from the author by request) gziped and uuencoded postscript file; to be published in Proceedings of the Laredo Advanced Summer School, Sept. 9

The Automated Detection of Variable Objects on Schmidt Plates

This paper will review progress on the automated detection of variable objects on Schmidt plates, describe the various populations of variables which have been discovered, and summarise some of the most interesting results to date. So far, the searches have been based on sequences of a dozen or more sky limited UK 1.2m Schmidt plates taken on timescales from one day to several years. The plates were measured on the COSMOS measuring machine at the Royal Observatory, Edinburgh which typically detects between 100,000 and 200,000 images in the central 16 deg2 of each plate. The measures for each field are combined, and calibrated using deep electronographic sequences, to give data sets of some 100,000 objects, complete in the magnitude range B = 13–21. The procedure is described in some detail by Hawkins (1983a) and results in a sequence of magnitudes accurate to about 0.1m for each object in the measured area. Field effects across the plates are allowed for, and an extensive set of diagnostics allows the distribution of errors as a function of magnitude, and on each plate, to be monitored.</jats:p

Quasar Variability: New Surveys and New Models

AbstractIn this paper results from a monitoring programme of a large sample of quasars comprising regular yearly observations over a period of 23 years are presented. Structure functions of the light curves are calculated and compared with predictions for models of quasar variability of current interest. These include recently published models of varibility from accretion disk instability, variability from starbursts or supernovae, and variations caused by the microlensing effect of compact bodies along the line of sight. The analysis favours the accretion disk model for low luminosity AGN, but suggests that the variations of more luminous quasars are dominated by microlensing.</jats:p