[en] Calibration is one of the long-standing problems in optical interferometric measurements, particularly with long baselines which demand stars with angular sizes on the milliarcsecond scale and no detectable companions. While systems of calibrators have been generally established for the near-infrared in the bright source regime (K ∼< 3 mag), modern large interferometers are sensitive to significantly fainter magnitudes. We aim to provide a list of sources found to be unresolved from direct observations with high angular resolution and dynamic range, which can be used to choose interferometric calibrators. To this purpose, we have used a large number of lunar occultations recorded with the ISAAC instrument at the Very Large Telescope to select sources found to be unresolved and without close companions. An algorithm has been used to determine the limiting angular resolution achieved for each source, taking into account a noise model built from occulted and unocculted portions of the light curves. We have obtained upper limits on the angular sizes of 556 sources, with magnitudes ranging from K_s ≈ 4 to 10, with a median of 7.2 mag. The upper limits on possible undetected companions (within ≈0.''5) range from K_s ≈ 8 to 13, with a median of 11.5 mag. One-third of the sources have angular sizes ≤1 mas, and two-thirds have sizes ≤2 mas. This list of unresolved sources matches well the capabilities of current large interferometric facilities. We also provide available cross-identifications, magnitudes, spectral types, and other auxiliary information. A fraction of the sources are found to be potentially variable. The list covers parts of the Galactic Bulge and in particular the vicinity of the Galactic Center, where extinction is very significant and traditional lists of calibrators are often insufficient.

[en] We report on 25 subarcsecond binaries, detected for the first time by means of lunar occultations in the near-infrared (near-IR) as part of a long-term program using the ISAAC instrument at the ESO Very Large Telescope. The primaries have magnitudes in the range K = 3.8-10.4, and the companions in the range K = 6.4-12.1. The magnitude differences have a median value of 2.8, with the largest being 5.4. The projected separations are in the range 6-748 mas and with a median of 18 mas, or about three times less than the diffraction limit of the telescope. Among our binary detections are a pre-main-sequence star and an enigmatic Mira-like variable previously suspected to have a companion. Additionally, we quote an accurate first-time near-IR detection of a previously known wider binary. We discuss our findings on an individual basis as far as made possible by the available literature, and we examine them from a statistical point of view. We derive a typical frequency of binarity among field stars of ≈10%, in the resolution and sensitivity range afforded by the technique (≈0.''003 to ≈0.''5, and K ≈ 12 mag, respectively). This is in line with previous results using the same technique but we point out interesting differences that we can trace up to sensitivity, time sampling, and average distance of the targets. Finally, we discuss the prospects for further follow-up studies

[en] We present deep 3500–10000 Å spectra of H ii regions and planetary nebulae (PNe) in the starburst irregular galaxy NGC 4449, acquired with the Multi Object Double Spectrograph at the Large Binocular Telescope. Using the “direct” method, we derived the abundance of He, N, O, Ne, Ar, and S in six H ii regions and in four PNe in NGC 4449. This is the first case of PNe studied in a starburst irregular outside the Local Group. Our H ii region and PN sample extends over a galactocentric distance range of ≈2 kpc and spans ≈0.2 dex in oxygen abundance, with average values of $12+\mathrm{l}\mathrm{o}\mathrm{g}(\mathrm{O}/\mathrm{H})=8.37\pm <!--\; ??\; -->0.05$ and 8.3 ± 0.1 for H ii regions and PNe, respectively. PNe and H ii regions exhibit similar oxygen abundances in the galactocentric distance range of overlap, while PNe appear more than ∼1 dex enhanced in nitrogen with respect to H ii regions. The latter result is the natural consequence of N being mostly synthesized in intermediate-mass stars and brought to the stellar surface during dredge-up episodes. On the other hand, the similarity in O abundance between H ii regions and PNe suggests that NGC 4449’s interstellar medium has been poorly enriched in α-elements since the progenitors of the PNe were formed. Finally, our data reveal the presence of a negative oxygen gradient for both H ii regions and PNe, while nitrogen does not exhibit any significant radial trend. We ascribe the (unexpected) nitrogen behavior to local N enrichment by the conspicuous Wolf-Rayet population in NGC 4449.

[en] We report on 13 subarcsecond binaries, detected by means of lunar occultations in the near-infrared at the ESO Very Large Telescope (VLT). They are all first-time detections except for the visual binary HD 158122, which we resolved for the first time in the near-infrared. The primaries have magnitudes in the range K = 4.5-10.0, and companions in the range K = 6.8-11.1. The magnitude differences have a median value of 2.4, with the largest being 4.6. The projected separations are in the range of 4-168 mas, with a median of 13 mas. We discuss and compare our results with the available literature. With this paper, we conclude the mining for binary star detections in the 1226 occultations recorded at the VLT with the ISAAC instrument. We expect that the majority of these binaries may be unresolvable by adaptive optics on current telescopes, and they might be challenging for long-baseline interferometry. However, they constitute an interesting sample for future larger telescopes and for astrometric missions such as GAIA.

[en] V1118 Ori is an eruptive variable belonging to the EXor class of pre-main-sequence stars whose episodic outbursts are attributed to disk accretion events. Since 2006, V1118 Ori has been in the longest quiescence stage ever observed between two subsequent outbursts in its recent history. We present near-infrared photometry of V1118 Ori carried out over the last eight years, along with complete spectroscopic coverage from 0.35 to 2.5 μm. Long term sampling of V1118 Ori in quiescence has never been performed, and hence we can benefit from the current circumstance by determining the lowest values (i.e., the zeroes) of the parameters to be used as reference for evaluating the physical changes typical of more active phases. A quiescence mass accretion rate of 1–3 × 10⁻⁹ $M_{}$ yr⁻¹ can be derived and the difference from previous determinations is discussed. Based on line emission and IR color analysis, a visual extinction of 1–2 mag is consistently derived, confirming that V1118 Ori (at least in quiescence) is a low-extinction T Tauri star with a bolometric luminosity of about 2.1 $L_{}$. An anti-correlation exists between the equivalent width of the emission lines and the underlying continuum. We searched the literature to evaluate whether or not such behavior is a common feature for the whole class. The anti-correlation is clearly recognizable for all of the available EXors in the optical range (Hβ and Hα lines); however, this is not as evident in the infrared (Paβ and Brγ lines). The observed anti-correlation supports the accretion-driven mechanism as the most likely to account for continuum variations.

[en] We report the LBT/MODS1 spectroscopic confirmation of two images of faint Lyα emitters at z = 6.4 behind the Frontier Fields galaxy cluster MACSJ0717.5+3745. A wide range of lens models suggests that the two images are highly magnified, with a strong lower limit of μ > 5. These are the faintest z > 6 candidates spectroscopically confirmed to date. These may also be multiple images of the same z = 6.4 source as supported by their similar intrinsic properties, but the lens models are inconclusive regarding this interpretation. To be cautious, we derive the physical properties of each image individually. Thanks to the high magnification, the observed near-infrared (restframe ultraviolet) part of the spectral energy distributions and Lyα lines are well detected with S/N(m ₁₅₀₀) ≳ 10 and S/N(Lyα) ≅ 10-15. Adopting μ > 5, the absolute magnitudes, M ₁₅₀₀, and Lyα fluxes are fainter than –18.7 and 2.8 × 10^–18 erg s^–1 cm^–2, respectively. We find a very steep ultraviolet spectral slope β = –3.0 ± 0.5 (F _λ = λ^β), implying that these are very young, dust-free, and low metallicity objects, made of standard stellar populations or even extremely metal poor stars (age ≲ 30 Myr, E(B – V) = 0 and metallicity 0.0-0.2 Z/Z _☉). The objects are compact (<1 kpc²) and with a stellar mass M _* < 10⁸ M _☉. The very steep β, the presence of the Lyα line, and the intrinsic FWHM (<300 km s^–1) of these newborn objects do not exclude a possible leakage of ionizing radiation. We discuss the possibility that such faint galaxies may resemble those responsible for cosmic reionization