Examinando por Autor "Mužić, K."
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Ítem Properties of the solar neighbor WISE J072003.20-084651.2(EDP Sciences, 2015-02) Ivanov, V.D.; Vaisanen, P.; Kniazev, A.Y.; Beletsky, Y.; Mamajek, E.E.; Mužić, K.; Beamín, J.C.; Boffin, H.M.J.; Pourbaix, D.; Gandhi, P.; Gulbis, A.; Monaco, L.; Saviane, I.; Kurtev, R.; Mawet, D.; Borissova, J.; Minniti, D.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 ≃ 7 pc (WISE J072003.20- 084651.2; hereafter WISE J0720) at Galactic latitude b = 2.3°. Aims. Our goals are to determine the physical characteristics of WISE J0720, its kinematic properties, and to address the question of whether it is a binary object, as was suggested in the discovery paper. Methods. Optical and infrared spectroscopy from the Southern African Large Telescope and Magellan, respectively, and spectral energy distribution fitting were used to determine the spectral type of WISE J0720. The measured radial velocity, proper motion, and parallax yielded its Galactic velocities. We also investigated if WISE J0720 may show X-ray activity based on archival data. Results. Our spectra are consistent with spectral type L0 ± 1. We find no evidence for binarity, apart from a minor 2σ level difference in the radial velocities taken at two different epochs. The spatial velocity of WISE J0720 does not connect it to any known moving group; instead, it places the object with high probability in the old thin disk or in the thick disk. The spectral energy distribution fit hints at excess in the 12 μm and 22 μm WISE bands which may be due to a redder companion, but the same excess is visible in other late-type objects, and it more likely implies a shortcoming of the models (e.g., problems with the effective wavelengths of the filters for these extremely cool objects, etc.) rather than a disk or redder companion. The optical spectrum shows some Hα emission, indicative of stellar activity. Archival X-ray observations yield no detection. Conclusions. WISE J0720 is a new member of the solar neighborhood, the third nearest L dwarf. Our data do not support the hypothesis of its binary nature. © ESO 2015.Ítem Temperature constraints on the coldest brown dwarf known: WISE 0855-0714(EDP Sciences, 2014-10) Beamín, J.C.; Ivanov, V.D.; Bayo, A.; Mužić, K.; Boffin, H.M.J.; Allard, F.; Homeier, D.; Minniti, D.; Gromadzki, M.; Kurtev, R.; Lodieu, N.; Martin, E.L.; Mendez, R.A.Context. Nearby isolated planetary mass objects are beginning to be discovered, but their individual properties are poorly constrained because their low surface temperatures and strong molecular self-absorption make them extremely faint. Aims. We aimed to detect the near-infrared emission of the coldest brown dwarf (BD) found so far, WISE0855-0714, located ~2.2 pc away, and to improve its temperature estimate (Teff = 225−260 K) from a comparison with state-of-the-art models of BD atmospheres. Methods. We observed the field containing WISE0855-0714 with HAWK-I at the VLT in the Y band. For BDs with Teff< 500 K theoretical models predict strong signal (or rather less molecular absorption) in this band. Results. WISE0855-0714 was not detected in our Y-band images, thus placing an upper limit on its brightness to Y> 24.4 mag at 3σ level, leading to Y − [ 4.5 ] > 10.5. Combining this limit with previous detections and upper limits at other wavelengths, WISE0855-0714 is confirmed as the reddest BD detected, further supporting its status as the coldest known brown dwarf. We applied spectral energy distribution fitting with collections of models from two independent groups for extremely cool BD atmospheres leading to an effective temperature of Teff< 250 K,Ítem WISE J061213.85-303612.5: A new T-dwarf binary candidate(EDP Sciences, 2015-06) Huélamo, N.; Ivanov V.D., Kurtev R.; Girard, J.H.; Borissova, J.; Mawet, D.; Mužić, K.; Cáceres, C.; Melo, C.H.F.; Sterzik, M.F.; Minniti, D.Context. T and Y dwarfs are among the coolest and least luminous objects detected, and they can help to understand the properties of giant planets. Up to now, there are more than 350 T dwarfs that have been identified thanks to large imaging surveys in the infrared, and their multiplicity properties can shed light on the formation process. Aims. The aim of this work is to look for companions around a sample of seven ultracoool objects. Most of them have been discovered by the WISE observatory and have not been studied before for multiplicity. Methods. We observed a sample six T dwarfs and one L9 dwarf with the Laser Guide Star (LGS) and NAOS-CONICA, the adaptive optics (AO) facility, and the near infrared camera at the ESO Very Large Telescope.We observed all the objects in one or more near-IR filters (JHKs). Results. From the seven observed objects, we have identified a subarcsecond binary system, WISE J0612-3036, composed of two similar components with spectral types of T6. We measure a separation of ρ = 350 ± 5 mas and a position angle of PA = 235 ± 1°. Using the mean absolute magnitudes of T6 dwarfs in the 2MASS JHKs bands, we estimate a distance of d = 31 ± 6 pc and derive a projected separation of ρ ∼ 11 ± 2 au. Another target, WISE J2255-3118, shows a very faint object at 1″. 3 in the Ks image. The object is marginally detected in H, and we derive a near infrared color of H - Ks > 0.1mag. HST/WFC3 public archival data reveals that the companion candidate is an extended source. Together with the derived color, this suggests that the source is most probably a background galaxy. The five other sources are apparently single, with 3-σ sensitivity limits between H = 19-21 for companions at separations >0″.5. Conclusions. WISE 0612-3036 is probably a new T-dwarf binary composed of two T6 dwarfs. As in the case of other late T-dwarf binaries, it shows a mass ratio close to 1, although its projected separation, ∼11 au, is larger than the average (∼5 au). Additional observations are needed to confirm that the system is bound. © 2015 ESO.