Ultra-diffuse galaxies (UDGs) have the sizes of giants but the luminosities of dwarfs. A key to understanding their origins comes from their total masses, but their low surface brightnesses (  m V ( ) 25.0) generally prohibit dynamical studies. Here, we report the first such measurements for a UDG (VCC 1287 in the Virgo cluster), based on its globular cluster system dynamics and size. From seven GCs we measure a mean systemic velocity v sys = 10 M e , yielding a dark-to-stellar mass fraction of ∼3000. We show that this UDG is an outlier in M star -M halo relations, suggesting extreme stochasticity in relatively massive starforming halos in clusters. Finally, we discuss how counting GCs offers an efficient route to determining virial masses for UDGs.

Abstract. We present the discovery of the optical transient of the long-duration gamma-ray burst GRB 000630. The optical transient was detected with the Nordic Optical Telescope 21.1 hours after the burst. At the time of discovery the magnitude of the transient was R = 23.04 ± 0.08. The transient displayed a power-law decline characterized by a decay slope of α = −1.035 ± 0.097. A deep image obtained 25 days after the burst shows no indication of a contribution from a supernova or a host galaxy at the position of the transient. The closest detected galaxy is a R = 24.68 ± 0.15 galaxy 2.0 arcsec north of the transient. The magnitudes of the optical afterglows of GRB 980329, GRB 980613 and GRB 000630 were all R > ∼ 23 less than 24 hours from the burst epoch. We discuss the implications of this for our understanding of GRBs without detected optical transients. We conclude that i) based on the gamma-ray properties of the current sample we cannot conclude that GRBs with no detected OTs belong to another class of GRBs than GRBs with detected OTs and ii) the majority ( > ∼ 75%) of GRBs for which searches for optical afterglow have been unsuccessful are consistent with no detection if they were similar to bursts like GRB 000630 at optical wavelengths.

Abstract. Faint meteors observed with Super-Schmidt cameras are re-examined in order to assess whether their dynamical and photometric behavior can be described by means of the single body theory. Velocities, decelerations and magnitudes are fitted simultaneously to synthetic curves resulting from integration of the appropriate set of differential equations. The parameters determined by this procedure are the ablation coefficient, the shape-density coefficient and the preatmospheric mass of each individual meteoroid. It turns out that 73% of the meteors analyzed here (with magnitudes in the range from +2.5 to −5) are reasonably well described by this theory, suggesting that they did not undergo significant fragmentation during their atmospheric flight. Nevertheless, we identify some systematic differences between observed and theoretical light curves of meteors for which the fit is good. Meteoroid bulk densities are estimated from the retrieved shape-density coefficients. The distributions of individual values are broad, indicating that objects of different densities coexist within the same meteoroid population. The average density is found to be 2400, 1400, and 400 kg m −3 for A-type, B-type and C-type meteoroids, respectively. These results do not confirm the large values determined from quasicontinuous fragmentation models.

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We report the results of a systematic photometric survey of the peripheral regions of a sample of 14 globular clusters in the outer halo of the Milky Way at distances d GC > 25 kpc from the Galactic Centre. The survey is aimed at searching for the remnants of the host satellite galaxies where these clusters could originally have been formed before being accreted on to the Galactic halo. The limiting surface brightness varies within our sample, but reaches μ V, lim = 30-32 mag arcsec −2. For only two globular clusters (NGC 7492 and Whiting 1; already suggested to be associated with the Sagittarius galaxy), we detect extended stellar populations that cannot be associated with either the clusters themselves or with the surrounding Galactic field population. We show that the lack of substructures around globular clusters at these Galactocentric distances is still compatible with the predictions of cosmological simulations whereby in the outer halo the Galactic globular cluster system is built up through hierarchical accretion at early epochs.