Search

We present the discovery and multiwavelength characterization of VVV J1438-6158 AB, a new field wide-binary system consisting of a |$4.6^{+5.5}_{-2.4}~{\rm Gyr}$| and |$T_{\rm eff} = 9500\pm 125~\mathrm{ K}$| DA white dwarf (WD) and a |$T_{\rm eff} = 2400\pm 50~\mathrm{ K}$| M8 ultracool dwarf (UCD). The projected separation of the system is |$a = 1236.73~{\rm au}$| (⁠|$\sim 13.8\, \mathrm{ arcsec}$|⁠), and although along the line of sight towards the Scorpius–Centaurus stellar association, VVV J1438-6158 AB is likely to be a field star, from a kinematic 6D probabilistic analysis. We estimated the physical and dynamical parameters of both components via interpolations with theoretical models and evolutionary tracks, which allowed us to retrieve a mass of |$0.62\pm 0.18~\mathrm{ M}_\odot$| for the WD, and a mass of |$98.5\pm 6.2~\mathrm{ M}_{\rm Jup}$| (⁠|$\sim 0.094\pm 0.006~\mathrm{ M}_\odot$|⁠) for the UCD. The radii of the two components were also estimated at |$0.01309\pm 0.0003~\mathrm{ R}_{\odot }$| and |$1.22\pm 0.05~\mathrm{ R}_{\rm Jup}$|⁠ , respectively. VVV J1438-6158 AB stands out as a benchmark system for comprehending the evolution of WDs and low-mass companions given its status as one of the most widely separated WD+UCD systems known to date, which likely indicates that both components may have evolved independently of each other, and also being characterized by a large-mass ratio (⁠|$q = 0.15\pm 0.04$|⁠), which likely indicates a formation pathway similar to that of stellar binary systems. [ABSTRACT FROM AUTHOR]

Context. The census of star clusters in the inner Milky Way is incomplete because of extinction and crowding. Aims. We embarked on a program to expand the star cluster list in the direction of the inner Milky Way using deep stacks of KS-band images from the VISTA Variables in Via Lactea (VVV) Survey. Methods. We applied an automated two-step procedure to the point-source catalog derived from the deep KS images: first, we identified overdensities of stars, and then we selected only candidate clusters with probable member stars that match an isochrone with a certain age, distance, and extinction on the color-magnitude diagram. Results. This pilot project only investigates the cluster population in part of one VVV tile, that is, b201. We identified nine cluster candidates and estimated their parameters. The new candidates are compact with a typical radius on the sky of ~0.2-0.4 arcmin (~0.4-1.6 pc at their estimated distances). They are located at distances of ~5-14 kpc from the Sun and are subject to moderate extinction of E(B - V) = 0.4-1.0 mag. They are sparse, probably evolved, with typical ages log(t/1 yr) ~9. Based on the locations of the objects inside the Milky Way, we conclude that one of these objects is probably associated with the disk or halo and the remaining objects are associated with the bulge or the halo. Conclusions. The cluster candidates reported here push the VVV Survey cluster detection to the limit. These new objects demonstrate that the VVV survey has the potential to identify thousands of additional cluster candidates. The sub-arcsec angular resolution and the near-infrared wavelength regimen give it a critical advantage over other surveys. [ABSTRACT FROM AUTHOR]

Context. The severe crowding in the direction of the inner Milky Way suggests that the census of stars within a few tens of parsecs in that direction may not be complete. Aims. We searched for new nearby object companions of known high proper motion (HPM) stars located towards the densest regions of the southern Milky Way where the background contamination presented a major problem to previous observations. Methods. The common proper motion (PM) method was used. We inspected the area around 167 known HPM (⩾200 mas yr-1) stars: 67 in the disk and 100 in the bulge. Multi-epoch images were provided by the Two Micron All Sky Survey (2MASS) and the VISTA Variables in Via Lactea (VVV). The VVV is a new on-going ZYJHKS plus multi-epoch KS survey of ~562 deg2 of the Milky Way bulge and inner southern disk. Results. Seven new co-moving companions were discovered around known HPM stars (L 149-77, LHS 2881, L 200-41, LHS 3188, LP 487-4, LHS 5333, and LP 922-16); six known co-moving pairs were recovered (LTT 5140 A + LTT 5140 B, L 412-3 + L 412-4, LP 920-25 + LP 920-26, LTT 6990A + LTT 6990 B, M124.22158.2900 + M124.22158.2910, and GJ 2136 A + GJ 2136 B); a pair of stars that was thought to be co-moving was found to have different proper motions (LTT 7318, LTT 7319); published HPMs of eight stars were not confirmed (C" 1925, C" 1930, C" 1936, CD-60 4613, LP 866-17, OGLEBUL-SC20 625107, OGLEBUL-SC21 298351, and OGLEBUL-SC32 388121); last but not least, spectral types ranging from G8V to M5V were derived from new infrared spectroscopy for seventeen stars, members of the co-moving pairs. Conclusions. The seven newly discovered stars constitute ~4% of the nearby HPM star list, but this is not a firm limit on the HPM star incompleteness because our starting point - the HPM list assembled from the literature - is incomplete itself, missing many nearby HPM M- and L-type objects, and it is contaminated with non-HPM stars. We have demonstrated that the superior sub-arcsec spatial resolution, with respect to previous surveys, allows the VVV to examine in greater detail the binary nature nature of known HPM stars. The ⩾5 yr span of VVV will provide a sufficient baseline for finding new HPM stars from VVV data alone. [ABSTRACT FROM AUTHOR]