ABSTRACT
Several recent studies have shown that the Vela OB2 region hosts a complex constellation of sub-populations with ages in the range 10 to 50 Myr. Such populations might represent the best ...example of the outcome of clustered star formation in giant molecular clouds (GMCs). We use Gaia DR2 data over an area of 40 deg radius around the open cluster Collinder 135 to extend the study of the stellar populations of the Vela OB2 region over an area of several hundreds of parsecs on sky. Detailed clustering algorithms combined with the exquisite astrometric quality of the Gaia catalogue allow us to detect a new cluster named BBJ 1 that shows the same age as NGC 2547 (30 to 35 Myr), but located at a distance of 260 pc from it. Deeper investigation of the region via clustering in 5D parameter space and in the colour–magnitude diagram allows us to detect a filamentary structure of stars that bridges the two clusters. Given the extent in space of such structure (260 pc) and the young age (∼35 Myr), we exclude that such population originates by the same mechanism responsible to create tidal streams around older clusters. Even if we miss a complete picture of the 3D motion of the studied stellar structure because of the lack of accurate radial velocity measurements, we propose that such structure represents the detection of a 35-Myr-old outcome of a mechanism of filamentary star formation in a GMC.
ABSTRACT
We report the discovery of the oldest stellar substructure in the Orion star-forming region (OSFR), the Orion relic filament. The relic filament is physically associated with the OSFR, as ...demonstrated by Gaia DR2 photometry and astrometry, as well as by targeted radial velocity follow-up observations of a bright subsample of proper motion-selected candidate members. Gaia DR2 parallaxes place the Orion relic filament in the more distant part of the OSFR, ∼430 pc from the Sun. Given its age, velocity dispersion, spatial extent, and shape, it is not possible for the Orion relic filament to have formed as a single stellar cluster, even taking into account residual gas expulsion. The relic filament is also too young to be a tidal stream, because Galactic tides act on much longer time-scales, of the order of 100 Myr. It therefore appears likely that the structure formed from a molecular cloud filament similar to Orion A in the OSFR and retained its morphology despite decoupling from its natal gas. Hence, the Orion relic filament bears witness to the short-lived evolutionary phase between gas removal and dispersion owing to shears and tides, and provides crucial new insights into how stars are formed in molecular clouds.
Stars are generally spherical, yet their gaseous envelopes often appear nonspherical when ejected near the end of their lives. This quirk is most notable during the planetary nebula phase, when these ...envelopes become ionized. Interactions among stars in a binary system are suspected to cause the asymmetry. In particular, a precessing accretion disk around a companion is believed to launch point-symmetric jets, as seen in the prototype Fleming 1. Our finding of a post-common-envelope binary nucleus in Fleming 1 confirms that this scenario is highly favorable. Similar binary interactions are therefore likely to explain these kinds of outflows in a large variety of systems.
Abstract
The orbital parameters of binaries at intermediate periods (102–103 d) are difficult to measure with conventional methods and are very incomplete. We have undertaken a new survey, applying ...our pulsation timing method to Kepler light curves of 2224 main-sequence A/F stars and found 341 non-eclipsing binaries. We calculate the orbital parameters for 317 PB1 systems (single-pulsator binaries) and 24 PB2s (double-pulsators), tripling the number of intermediate-mass binaries with full orbital solutions. The method reaches down to small mass ratios q ≈ 0.02 and yields a highly homogeneous sample. We parametrize the mass-ratio distribution using both inversion and Markov-Chain Monte Carlo forward-modelling techniques, and find it to be skewed towards low-mass companions, peaking at q ≈ 0.2. While solar-type primaries exhibit a brown dwarf desert across short and intermediate periods, we find a small but statistically significant (2.6σ) population of extreme-mass-ratio companions (q < 0.1) to our intermediate-mass primaries. Across periods of 100–1500 d and at q > 0.1, we measure the binary fraction of current A/F primaries to be 15.4 per cent ± 1.4 per cent, though we find that a large fraction of the companions (21 per cent ± 6 per cent) are white dwarfs in post-mass-transfer systems with primaries that are now blue stragglers, some of which are the progenitors of Type Ia supernovae, barium stars, symbiotics, and related phenomena. Excluding these white dwarfs, we determine the binary fraction of original A/F primaries to be 13.9 per cent ± 2.1 per cent over the same parameter space. Combining our measurements with those in the literature, we find the binary fraction across these periods is a constant 5 per cent for primaries M1 < 0.8 M⊙, but then increases linearly with log M1, demonstrating that natal discs around more massive protostars M1 ≳ 1 M⊙ become increasingly more prone to fragmentation. Finally, we find the eccentricity distribution of the main-sequence pairs to be much less eccentric than the thermal distribution.
ABSTRACT Using Gaia DR2 data, combined with OmegaCAM ground-based optical photometry from the Accretion Disc with OmegaCAM survey, and detailed radial velocity measurements from ESO-Gaia, we analyse ...in detail a 10 × 5 deg region around the Wolf–Rayet Star γ2 Vel, including the previously known clusters Gamma Vel and NGC2547. Using clustering analysis that considers positions, proper motions, and parallax, we discover six clusters or associations – four of which appear new. Analysis of the colour–magnitude diagram for these clusters shows that four of them formed coevally from the same molecular clouds 10 Myr ago, while NGC 2547 formed together with a newly discovered cluster 30 Myr ago. This study shows the incredible wealth of data provided by Gaia for the study of young stellar clusters.
The technique of transmission spectroscopy allows us to constrain the chemical composition of the atmospheres of transiting exoplanets. It relies on very high signal-to-noise spectroscopic (or ...spectrophotometric) observations and is thus most suited for bright exoplanet host stars. In the era of the Transiting Exoplanet Survey Satellite, Next Generation Space Telescope, and PLAnetary Transits and Oscillations of stars (PLATO), more and more suitable targets, even for mid-sized telescopes, are discovered. Furthermore, a wealth of archival data is available that could become a basis for long-term monitoring of exo-atmospheres. We analyzed archival High Accuracy Radial velocity Planet Searcher (HARPS) spectroscopic time series of four host stars to transiting bloated gas exoplanets, namely WASP-76b, WASP-127b, WASP-166b, and KELT-11b, searching for traces of sodium (sodium doublet), hydrogen (H , Hβ), and lithium (670.8 nm). The archival data sets include spectroscopic time series taken during transits. Comparing in- and out-of-transit spectra we can filter out the stellar lines and investigate the absorption from the planet. Simultaneously, the stellar activity is monitored using the Mg i and Ca i lines. We detect sodium in the atmosphere of WASP-76b at a 7-9 level. Furthermore, we report also at a 4-8 level of significance the detection of sodium in the atmosphere of WASP-127b, confirming earlier results based on low-resolution spectroscopy. The data show no sodium nor any other atom at high confidence levels for WASP-166b nor KELT-11b, hinting at the presence of thick high clouds.
Context. Despite the thorough work of van Leeuwen, the parallax of α Centauri is still far from being carved in stone. Any derivation of the individual masses is therefore uncertain, if not ...questionable. And yet, that does not prevent this system from being used for calibration purpose in several studies. Aims. We aim for more accurate model-free parallax and individual masses of this system. Methods. With HARPS, the radial velocities are not only precise but also accurate. Ten years of HARPS data are enough to derive the complement of the visual orbit for a full 3D orbit of α Cen. Results. We locate α Cen (743 mas) right where Hipparcos had originally put it, i.e. slightly farther away than derived by Söderhjelm. The components are thus a bit more massive than previously thought (1.13 and 0.97 M⊙ for A and B, respectively). These values are now in excellent agreement with the latest asteroseismologic results.
PDF
