Stars and brown dwarfs in the σ Orionis cluster (Research Note) III. OSIRIS/GTC low-resolution spectroscopy of variable sources

Context. Although many studies have been performed so far, there are still dozens of low-mass stars and brown dwarfs in the young σ Orionis open cluster without detailed spectroscopic characterisation. Aims. We look for unknown strong accretors and disc hosts that were undetected in previous surveys. Methods. We collected low-resolution spectroscopy (R ~ 700) of ten low-mass stars and brown dwarfs in σ Orionis with OSIRIS at the Gran Telescopio Canarias under very poor weather conditions. These objects display variability in the optical, infrared, Hα, and/or X-rays on time scales of hours to years. We complemented our spectra with optical and near-/mid-infrared photometry. Results. For seven targets, we detected lithium in absorption, identified Hα, the calcium doublet, and forbidden lines in emission, and/or determined spectral types for the first time. We characterise in detail a faint, T Tauri-like brown dwarf with an 18 h-period variability in the optical and a large Hα equivalent width of –125  ±  15 Å, as well as two M1-type, X-ray-flaring, low-mass stars, one with a warm disc and forbidden emission lines, the other with a previously unknown cold disc with a large inner hole. Conclusions. New unrevealed strong accretors and disc hosts, even below the substellar limit, await discovery among the list of known σ Orionis stars and brown dwarfs that are variable in the optical and have no detailed spectroscopic characterisation yet.

Salt springs from Romania exploited during early archaeological times : a new candidate for World Heritage

Our approach emphasizes on the importance of the first forms of salt springs exploitation meant to obtain recrystallized salt for the development of prehistoric human communities within the continental inlands of Europe. Although it does not compare with the monumental dimension of World Heritage, the exploitation of some salt springs in Eastern Romania goes back around 8 millennia; they may be the oldest such exploitations in the world, as proven by 14C calibrated data. What differentiates Romanian salt springs from other famous similar areas in Europe is the continuity of exploitation and utilization of natural brine. Actually, these resilient behaviours explain the creation of a whole and complex universe of salt, which also represents a unique point of reference within the intangible World Heritage. It is through this association in variable proportions between tangible (non-monumental) and intangible that these salt springs comprising the oldest traces of salt exploitation can be considered elements of World Heritage. Today, important personalities in the fields of archaeology, anthropology and history posit that salt is a major reference for the development of the entire umanity. Obviously, the breakthrough of this idea requires awareness efforts targeting, on one hand, local communities in those areas wand, on the other, national and international scientific and cultural environments concerned with the World Heritage. In this context, a proper motivation is the fact that the last two decades have witnessed an intensification of research on salt, which turned this topic one of the major themes within European archaeology and ethno-archaeology. In terms of local community awareness concerning the importance of salt springs in the economic development of a (micro) area over time, it is worth underlining mostly the specialists’ efforts of presenting this topic in the media. Moreover, the impact of a recent initiative of the two museums in the area (Piatra Neamț and Târgu Neamț)—establishing distinct sections that represent, by using museum-inspired means, both archaeological vestiges and traditional practices of natural brine exploitation and utilization—will prove its extent in time. Certain local authorities and private entrepreneurs have pinpointed that valorising tourist areas comprising the oldest traces of salt exploitation in Romania is an imminent issue. The greatest challenge is finding a balance between the civilization improvements (upgraded access roads, upgrading operating areas, etc.) and thep rotection of still-alive traditional practices of salt exploitation and use, within rural areas. Certain local authorities and private entrepreneurs have pinpointed that valorising tourist areas comprising the oldest traces of salt exploitation in Romania will become, sooner or later, an imminent issue. The greatest challenge is finding a balance between the civilization improvements (upgraded access roads, upgrading operating areas, etc.) and the protection of still-alive traditional practices of salt exploitation and use, within rural areas.

On neutron stars in ƒ (R) theories: Small radii, large masses and large energy emitted in a merger

In the context of ƒ (R) gravity theories, we show that the apparent mass of a neutron star as seen from an observer at infinity is numerically calculable but requires careful matching, first at the star’s edge, between interior and exterior solutions, none of them being totally Schwarzschild-like but presenting instead small oscillations of the curvature scalar R; and second at large radii, where the Newtonian potential is used to identify the mass of the neutron star. We find that for the same equation of state, this mass definition is always larger than its general relativistic counterpart. We exemplify this with quadratic R^2 and Hu-Sawicki-like modifications of the standard General Relativity action. Therefore, the finding of two-solar mass neutron stars basically imposes no constraint on stable ƒ (R) theories. However, star radii are in general smaller than in General Relativity, which can give an observational handle on such classes of models at the astrophysical level. Both larger masses and smaller matter radii are due to much of the apparent effective energy residing in the outer metric for scalar-tensor theories. Finally, because the ƒ (R) neutron star masses can be much larger than General Relativity counterparts, the total energy available for radiating gravitational waves could be of order several solar masses, and thus a merger of these stars constitutes an interesting wave source.

Incidence of debris discs around FGK stars in the solar neighbourhood

Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their counterparts in the solar system are the asteroid and Edgeworth-Kuiper belts. Aims. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. Methods. The full sample of 177 FGK stars with d ≤ 20 pc proposed for the DUst around NEarby Stars (DUNES) survey is presented. Herschel/PACS observations at 100 and 160 μm were obtained, and were complemented in some cases with data at 70 μm and at 250, 350, and 500 μm SPIRE photometry. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the Disc Emission via a Bias-free Reconnaissance in IR and Sub-mm (DEBRIS) consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age. Results. The subsample of 105 stars with d ≤ 15 pc containing 23 F, 33 G, and 49 K stars is complete for F stars, almost complete for G stars, and contains a substantial number of K stars from which we draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type are 0.26^+0.21_-0.14 (6 objects with excesses out of 23 F stars), 0.21^+0.17_-0.11 (7 out of 33 G stars), and 0.20^+0.14_-0.09 (10 out of 49 K stars); the fraction for all three spectral types together is 0.22^+0.08_-0.07 (23 out of 105 stars). The uncertainties correspond to a 95% confidence level. The medians of the upper limits of L_dust/L_∗ for each spectral type are 7.8 × 10^-7 (F), 1.4 × 10^-6 (G), and 2.2 × 10^-6 (K); the lowest values are around 4.0 × 10^-7. The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts.