arXiv subjects
Publications and source records attributed to R. Ibata.
We present nine newly discovered globular clusters in the outer halo of M31, found by a semi-automated procedure from a INT Wide Field Camera survey of the region. The sample includes a candidate at the yet largest known projected galactocentric radius from M31.
Low-mass and brown dwarfs have recently been found as wide companions to many nearby stars, formerly believed to be single. Wide binaries are usually found as common proper motion pairs. Sometimes, more than two objects share the same large proper motion, identifying them as nearby systems. We have found a third, low-mass component to a known wide binary at a distance of about 21pc, consisting of a red and a white dwarf (LHS4039 and LHS4040; about 150AU separation). The new companion, APMPMJ2354-3316C separated by about 2200AU, was classified as M8.5 dwarf. In recent spectroscopic observations it shows a very strong $H_α$ emission line and blue continuum. Comparing this event to flares in late-type M dwarfs, we find some similarity with LHS2397a, a nearby M8 dwarf which is so far the only known example of a low-mass star with a tight brown dwarf companion (separation of less than 4AU). The level of the activity as measured by $L_{H_α}/L_{bol}$ is comparable to that of the M9.5 dwarf 2MASSWJ0149+29 both during the flare and in quiescence.
The possible association of Galactic globular clusters with the Sgr Stream is shortly reviewed at the light of the most recent observations of the Stream. >= 20 % of the Galactic globular clusters with R_{GC}>= 10 Kpc are found to lie within or very nearby to the Sgr Stream as traced over the whole sky by the M giants from the 2MASS catalogue, in agreement with the results by Bellazzini, Ferraro and Ibata (2003a). Another possible association of outer halo globulars (NGC 1904, NGC 1851, NGC 2808, and NGC 2298) is also noted and briefly discussed.
We use infrared Color Magnitude Diagrams from the 2-Micron All-Sky Survey (2MASS) to search for stars belonging to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) around selected Galactic globular clusters. Statistically significant detections are presented for the cases of Pal 12 and NGC 4147, strongly supporting the idea that these clusters are associated with the Sgr Stream and that they were previous members of the Sgr dSph galaxy.
We report the discovery of a wide pair (93 arcsec angular separation) of extremely cool ($T_{eff}<4000$ K) white dwarfs with a very large common proper motion ($\sim1.9$ arcsec/yr). The objects were discovered in a high proper motion survey in the poorly investigated southern sky region with $δ<-60^{\circ}$ using SuperCOSMOS Sky Survey (SSS) data. Both objects, SSSPM J2231-7514 and SSSPM J2231-7515, show featureless optical spectra. Fits of black-body models to the spectra yield effective temperatures of 3810 K and 3600 K, respectively for the bright (V=16.60) and faint (V=16.87) component. Both degenerates are much brighter than other recent discoveries of cool white dwarfs with comparable effective temperatures and/or $B_J-R$ colours. Therefore, they should be relatively nearby objects. The comparison with other cool white dwarfs and a photometric distance determination yield distance estimates between 9 pc and 14 pc. The latter seems to be more realistic, since the good agreement of the proper motion of both components within the errors of about 8 mas/yr and the angular separation between the two stars support a distance of about 15 pc with relatively small masses of the components. With smaller distance we should be able to measure a differential proper motion due to orbital motion if the orbital plane is not strongly inclined and the present orbital velocity vector is not close to the line of sight. The space velocity based on that distance and assumptions on radial velocity makes the new pair of extremely cool white dwarfs some of the probably oldest members of the Galactic disk population, although the possibility that these objects are part of a Galactic halo dark matter component can also not yet be ruled out.
We report the discovery of two well-defined tidal tails emerging from the sparse remote globular cluster Palomar 5. These tails stretch out symmetrically to both sides of the cluster in the direction of constant Galactic latitude and subtend an angle of 2.6 degrees on the sky. The tails have been detected in commissioning data of the Sloan Digital Sky Survey (SDSS), providing deep five-color photometry in a 2.5 degrees wide band along the equator. The stars in the tails make up a substantial part (~1/3) of the current total population of cluster stars in the magnitude interval 19.5 < i* < 22.0. This reveals that the cluster is subject to heavy mass loss. The orientation of the tails provides an important key for the determination of the cluster's Galactic orbit.
It has recently been suggested that the stars in a vertical extension of the red clump feature seen in LMC color-magnitude diagrams could belong to a tidal stream of material located in front of that galaxy. If this claim is correct, this foreground concentration of stars could contribute significantly to the rate of gravitational microlensing events observed in the LMC microlensing experiments. Here we present radial velocity measurements of stars in this so-called ``vertical red clump'' (VRC) population. The observed stellar sample, it transpires, has typical LMC kinematics. It is shown that it is improbable that an intervening tidal stream should have the same distribution of radial velocities as the LMC, which is consistent with an earlier study that showed that the VRC feature is more likely a young stellar population in the main body of that galaxy. However, the kinematic data do not discriminate against the possibility that the VRC is an LMC halo population.