New and existing X-ray, UBVJHK(s)W((1-4)), and spectroscopic observations were analyzed to constrain fundamental parameters for M25, NGC 7790, and dust along their sight-lines. The star clusters are of particular importance because they host the classical Cepheids USgr, CF Cas, and the visual binary Cepheids CEa and CEb Cas. Precise results from the multiband analysis, in tandem with a comprehensive determination of the Cepheids' period evolution (dP/dt) from similar to 140 years of observations, helped to resolve concerns raised regarding the clusters and their key Cepheid constituents. Specifically, the distances derived for members of M25 and NGC7790 are 630 +/- 25 pc and 3.40 +/- 0.15 kpc, respectively.

Using FORS2 on the Very Large Telescope, we have astrometrically monitored over a period of two months the two components of the brown dwarf system WISE J104915.57-531906.1, the closest one to the Sun. Our astrometric measurements - with a relative precision at the milli-arcsecond scale - allowed us to detect the orbital motion and derive more precisely the parallax of the system, leading to a distance of 2.020 +/- 0.019 pc. The relative orbital motion of the two objects is found to be perturbed, which leads us to suspect the presence of a substellar companion around one of the two components. We also performed VRIz photometry of the two components and compared this with models. We confirm the flux reversal of the T dwarf.

We present a new Y dwarf, WISE J030449.03-270508.3, confirmed from a candidate sample designed to pick out low-temperature objects from the Wide-field Infrared Survey Explorer (WISE) data base. The new object is typed Y0pec following a visual comparison with spectral standards, and lies at a likely distance of 10-17 pc. Its tangential velocity suggests thin disc membership, but it shows some spectral characteristics that suggest that it may be metal poor and/or older than previously identified Y0 dwarfs. Based on trends seen for warmer late-type T dwarfs, the Y-band flux peak morphology is indicative of sub-solar metallicity, and the enhanced red wing of the J-band flux peak offers evidence for high gravity and/or low metallicity (with associated model trends suggesting an age closer to similar to 10 Gyr and mass in the range 0.02-0.03 M-circle dot). This object may thus be extending the population parameter space of the known Y0 dwarfs.

Recent ground-based near-IR (NIR) studies of stellar clusters in nearby galaxies have suggested that young clusters remain embedded for 7-10 Myr in their progenitor molecular cloud, in conflict with optical-based studies which find that clusters are exposed after 1-3 Myr. Here, we investigate the role that spatial resolution plays in this apparent conflict. We use a recent catalogue of young (< 10 Myr) massive (> 5000M(circle dot)) clusters in the nearby spiral galaxy, M83, along with Hubble Space Telescope (HST) imaging in the optical and NIR, and ground-based NIR imaging, to see how the colours (and hence estimated properties such as age and extinction) are affected by the aperture size employed, in order to simulate studies of differing resolution. We find that the NIR is heavily affected by the resolution, and when aperture sizes > 40 pc are used, all young/blue clusters move redwards in colour space, which results in their appearance as heavily extincted clusters. However, this is due to contamination from nearby sources and nebular emission, and is not an extinction effect. Optical colours are much less affected by resolution. Due to the larger effect of contamination in the NIR, we find that, in some cases, clusters will appear to show NIR excess when large (> 20 pc) apertures are used. Our results explain why few young (< 6 Myr), low-extinction (AV < 1 mag) clusters have been found in recent ground-based NIR studies of cluster populations, while many such clusters have been found in higher resolution HST-based studies. Additionally, resolution effects appear to (at least partially) explain the origin of the NIR excess that has been found in a number of extragalactic young massive clusters.

Context. The abundances of a-elements are a powerful diagnostic of the star formation history and chemical evolution of a galaxy. Sulphur. being moderately volatile, can be reliably measured in the interstellar medium (ISM) of damped Ly-alpha galaxies and extragalactic H-II regions. Measurements in stars of different metallicity in our Galaxy can then be readily compared to the abundances in external galaxies. Such a comparison is not possible for Si or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur is interesting because it probes nucleosynthetic conditions that are very different from those of Oar Mg. In this context measurements in star clusters are a reliable tracers of the Galactic evolution of sulphur.

Passing stars can perturb the Oort Cloud, triggering comet showers and potentially extinction events on Earth. We combine velocity measurements for the recently discovered, nearby, low- mass binary system WISE J072003.20084651.2 (" Scholz's star") to calculate its past trajectory. Integrating the Galactic orbits of this 0.15 Me binary system and the Sun, we find that the binary passed within only 52+ 23 - 14 kAU ( 0.25+ 0.11 - 0.07 pc) of the Sun 70+ 15 - 10 kya ( 1s uncertainties), i. e., within the outer Oort Cloud. This is the closest known encounter of a star to our solar system with a well- constrained distance and velocity. Previous work suggests that flybys within 0.25 pc occur infrequently ( 0.1 Myr- 1). We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is 98%, but the probability of penetrating the dynamically active inner Oort Cloud (< 20 kAU) is 10- 4. While the flyby of this system likely caused negligible impact on the flux of longperiod comets, the recent discovery of this binary highlights that dynamically important Oort Cloud perturbers may be lurking among nearby stars.

Context. The severe crowding towards the Galactic plane suggests that the census of nearby stars in that direction may be incomplete. Recently, Scholz reported a new M9 object at an estimated distance d similar or equal to 7 pc (WISE J072003.20-084651.2; hereafter WISE J0720) at Galactic latitude b = 2.3 degrees.

So far, roughly 40 quasars with redshifts greater than z = 6 have been discovered(1-8). Each quasar contains a black hole with a mass of about one billion solar masses (10(9) M-circle dot)(2,6,7,9-13). The existence of such black holes when the Universe was less than one billion years old presents substantial challenges to theories of the formation and growth of black holes and the coevolution of black holes and galaxies(14). Here we report the discovery of an ultraluminous quasar, SDSS J010013.02+280225.8, at redshift z = 6.30. It has an optical and near-infrared luminosity a few times greater than those of previously known z > 6 quasars. On the basis of the deep absorption trough(15) on the blue side of the Lyman-alpha emission line in the spectrum, we estimate the proper size of the ionized proximity zone associated with the quasar to be about 26 million light years, larger than found with other z > 6.1 quasars with lower luminosities(16). We estimate (on the basis of a near-infrared spectrum) that the black hole has a mass of similar to 1.2 x 10(10) M-circle dot, which is consistent with the 1.3 x 10(10) M-circle dot derived by assuming an Eddington-limited accretion rate.

We describe the new spectroscopic data reduction pipeline for the multi-object MMT/Magellan Infrared Spectrograph. The pipeline is implemented in IDL as a stand-alone package and is publicly available in both stable and development versions. We describe novel algorithms for sky subtraction and correction for telluric absorption. We demonstrate that our sky subtraction technique reaches the Poisson limit set by the photon statistics. Our telluric correction uses a hybrid approach by first computing a correction function from an observed stellar spectrum, and then differentially correcting it using a grid of atmosphere transmission models for the target airmass value. The pipeline provides a sufficient level of performance for real time reduction and thus enables data quality control during observations. We reduce an example dataset to demonstrate the high data reduction quality.

We report the discovery of a substellar companion to 2MASS J02192210-3925225, a young M6. candidate member of the Tucana-Horologium association (30-40 Myr). This L4 gamma companion has been discovered with seeing-limited direct imaging observations; at a 4 '' separation (160 AU) and a modest contrast ratio, it joins the very short list of young low-mass companions amenable to study without the aid of adaptive optics, enabling its characterization with a much wider suite of instruments than is possible for companions uncovered by highcontrast imaging surveys. With a model-dependent mass of 12-15 M-Jup, it straddles the boundary between the planet and brown dwarf mass regimes. We present near-infrared spectroscopy of this companion and compare it to various similar objects uncovered in the last few years. The J0219-3925 system falls in a sparsely populated part of the host mass versus mass ratio diagram for binaries; the dearth of known similar companions may be due to observational biases in previous low-mass companion searches.