We construct a 3D map of the spatial density of OB stars within 500 pc from the Sun using the Hipparcos catalogue and find three large-scale stream-like structures that allow a new view on the solar ...neighbourhood. The spatial coherence of these blue streams and the monotonic age sequence over hundreds of parsecs suggest that they are made of young stars, similar to the young streams that are conspicuous in nearby spiral galaxies. The three streams are 1) the Scorpius to Canis Majoris stream, covering 350 pc and 65 Myr of star formation history; 2) the Vela stream, encompassing at least 150 pc and 25 Myr of star formation history; and 3) the Orion stream, including not only the well-known Orion OB1abcd associations, but also a large previously unreported foreground stellar group lying only 200 pc from the Sun. The map also reveals a remarkable and previously unknown nearby OB association, between the Orion stream and the Taurus molecular clouds, which might be responsible for the observed structure and star formation activity in this cloud complex. This new association also appears to be the birthplace of Betelgeuse, as indicated by the proximity and velocity of the red giant. If this is confirmed, it would solve the long-standing puzzle of the origin of Betelgeuse. The well-known nearby star-forming low-mass clouds, including the nearby T and R associations Lupus, Cha, Oph, CrA, Taurus, Vela R1, and various low-mass cometary clouds in Vela and Orion, appear in this new view of the local neighbourhood to be secondary star formation episodes that most likely were triggered by the feedback from the massive stars in the streams. We also recover well-known star clusters of various ages that are currently cruising through the solar neighbourhood. Finally, we find no evidence of an elliptical structure such as the Gould belt, a structure we suggest is a 2D projection effect, and not a physical ring.
Context. Filamentary structures are common in molecular clouds. Explaining how they fragment to dense cores is a missing step in understanding their role in star formation. Aims. We perform a case ...study of whether low-mass filaments are close to hydrostatic prior to their fragmentation, and whether their fragmentation agrees with gravitational fragmentation models. To accomplish this, we study the ~6.5 pc long Musca molecular cloud, which is an ideal candidate for a filament at an early stage of fragmentation. Methods. We employ dust extinction mapping, in conjunction with near-infrared JHKS-band data from the CTIO/NEWFIRM instrument, and 870 μm dust continuum emission data from the APEX/LABOCA instrument to estimate column densities in Musca. We use the data to identify fragments from the cloud and to determine the radial density distribution of its filamentary part. We compare the cloud’s morphology with 13CO and C18O line emission observed with the APEX/SHeFI instrument. Results. The Musca cloud is pronouncedly fragmented at its ends, but harbors a remarkably well-defined, ~1.6 pc long filament in its center region. The line mass of the filament is 21–31 M⊙ pc-1 and the full width at half maximum (FWHM) 0.07 pc. The radial profile of the filament can be fitted with a Plummer profile, which has the power-index of 2.6 ± 11% and is flatter than that of an infinite hydrostatic filament. The profile can also be fitted with a hydrostatic cylinder truncated by external pressure. These models imply a central density of ~5–10 × 104 cm-3. The fragments in the cloud have a mean separation of ~0.4 pc, in agreement with gravitational fragmentation. These properties, together with the subsonic and velocity-coherent nature of the cloud, suggest a scenario in which an initially hydrostatic cloud is currently gravitationally fragmenting. The fragmentation started a few tenths of a Myr ago from the ends of the cloud, leaving its center still relatively nonfragmented, possibly because of gravitational focusing in a finite geometry.
Aims. We take advantage of the second data release of the Gaia space mission and the state-of-the-art astrometry delivered from very long baseline interferometry observations to revisit the structure ...and kinematics of the nearby Taurus star-forming region. Methods. We apply a hierarchical clustering algorithm for partitioning the stars in our sample into groups (i.e., clusters) that are associated with the various molecular clouds of the complex, and derive the distance and spatial velocity of individual stars and their corresponding molecular clouds. Results. We show that the molecular clouds are located at different distances and confirm the existence of important depth effects in this region reported in previous studies. For example, we find that the L 1495 molecular cloud is located at d = 129.9+0.4−0.3 d = 129 . 9 − 0.3 + 0.4 $ d=129.9^{+0.4}_{-0.3} $ pc, while the filamentary structure connected to it (in the plane of the sky) is at d = 160.0+1.2−1.2 d = 160 . 0 − 1.2 + 1.2 $ d=160.0^{+1.2}_{-1.2} $ pc. We report B 215 and L 1558 as the closest (d = 128.5+1.6−1.6 pc) d = 128 . 5 − 1.6 + 1.6 pc $ \left(d=128.5^{+1.6}_{-1.6}\,\mathrm{pc}\right) $ and most remote (d = 198.1+2.5−2.5 pc) d = 198 . 1 − 2.5 + 2.5 pc $ \left(d=198.1^{+2.5}_{-2.5}\,\mathrm{pc}\right) $ substructures of the complex, respectively. The median inter-cloud distance is 25 pc and the relative motion of the subgroups is on the order of a few km s−1. We find no clear evidence for expansion (or contraction) of the Taurus complex, but signs of the potential effects of a global rotation. Finally, we compare the radial velocity of the stars with the velocity of the underlying 13CO molecular gas and report a mean difference of 0.04 ± 0.12 km s−1 (with rms of 0.63 km s−1) confirming that the stars and the gas are tightly coupled.
The Kepler mission has discovered thousands of planet candidates. Currently, some of them have already been discarded; more than 200 have been confirmed by follow-up observations, and several ...hundreds have been validated. However, the large majority of the candidates are still awaiting for confirmation. Thus, priorities must be established for subsequent radial velocity observations. The motivation of this work is to provide a set of isolated (good) host candidates to be further tested by other techniques that allow confirmation of the planet. As a complementary goal, we aim to identify close companions of the candidates that could have contaminated the light curve of the planet host due to the large pixel size of the Kepler CCD and its typical PSF of around 6 arcsec. Both goals can also provide robust statistics about the multiplicity of the Kepler hosts. We find that 67.2% of the observed Kepler hosts are isolated within our detectability limits, and 32.8% have at least one visual companion at angular separations below 6 arcsec.
In this work, we propose two convolutional neural network classifiers for detecting contaminants in astronomical images. Once trained, our classifiers are able to identify various contaminants, such ...as cosmic rays, hot and bad pixels, persistence effects, satellite or plane trails, residual fringe patterns, nebulous features, saturated pixels, diffraction spikes, and tracking errors in images. They encompass a broad range of ambient conditions, such as seeing, image sampling, detector type, optics, and stellar density. The first classifier, M
AXI
M
ASK
, performs semantic segmentation and generates bad pixel maps for each contaminant, based on the probability that each pixel belongs to a given contaminant class. The second classifier, M
AXI
T
RACK
, classifies entire images and mosaics, by computing the probability for the focal plane to be affected by tracking errors. We gathered training and testing data from real data originating from various modern charged-coupled devices and near-infrared cameras, that are augmented with image simulations. We quantified the performance of both classifiers and show that M
AXI
M
ASK
achieves state-of-the-art performance for the identification of cosmic ray hits. Thanks to a built-in Bayesian update mechanism, both classifiers can be tuned to meet specific science goals in various observational contexts.
Context. The evolution of lithium abundance over a star’s lifetime is indicative of transport processes operating in the stellar interior. Aims. We revisit the relationship between lithium content ...and rotation rate previously reported for cool dwarfs in the Pleiades cluster. Methods. We derive new LiI 670.8 nm equivalent width measurements from high-resolution spectra obtained for low-mass Pleiades members. We combine these new measurements with previously published ones, and use the Kepler K2 rotational periods recently derived for Pleiades cool dwarfs to investigate the lithium-rotation connection in this 125 Myr-old cluster. Results. The new data confirm the correlation between lithium equivalent width and stellar spin rate for a sample of 51 early K-type members of the cluster, where fast rotating stars are systematically lithium-rich compared to slowly rotating ones. The correlation is valid for all stars over the (J–Ks) color range 0.50–0.70 mag, corresponding to a mass range from about 0.75 to 0.90 M⊙, and may extend down to lower masses. Conclusions. We argue that the dispersion in lithium equivalent widths observed for cool dwarfs in the Pleiades cluster reflects an intrinsic scatter in lithium abundances, and suggest that the physical origin of the lithium dispersion pattern is to be found in the pre-main sequence rotational history of solar-type stars.
Context.
The
β
Pictoris moving group is one of the most well-known young associations in the solar neighbourhood and several members are known to host circumstellar discs, planets, and comets. ...Measuring its age precisely is essential to the study of several astrophysical processes, such as planet formation and disc evolution, which are strongly age-dependent.
Aims.
We aim to determine a precise and accurate dynamical traceback age for the
β
Pictoris moving group.
Methods.
Our sample combines the extremely precise
Gaia
DR2 astrometry with ground-based radial velocities measured in an homogeneous manner. We use an updated version of our algorithm to determine dynamical ages. The new approach takes into account a robust estimate of the spatial and kinematic covariance matrices of the association to improve the sample selection process and to perform the traceback analysis.
Results.
We estimate a dynamical age of 18.5
−2.4
+2.0
Myr for the
β
Pictoris moving group. We investigated the spatial substructure of the association at the time of birth and we propose the existence of a core of stars that is more concentrated. We also provide precise radial velocity measurements for 81 members of
β
Pic, including ten stars with the first determinations of their radial velocities.
Conclusions.
Our dynamical traceback age is three times more precise than previous traceback age estimates and, more importantly, for the first time it reconciles the traceback age with the most recent estimates of other dynamical, lithium depletion boundaries and isochronal ages. This has been possible thanks to the excellent astrometric and spectroscopic precisions, the homogeneity of our sample, and the detailed analysis of binaries and membership.
Context.
The high-precision parallax data of the
Gaia
mission allows for significant improvements in the distance determination to stellar clusters and their stars. In order to obtain accurate and ...precise distance determinations, systematics such as parallax spatial correlations need to be accounted for, especially with regard to stars in small sky regions.
Aims.
Our aim is to provide the astrophysical community with a free and open code designed to simultaneously infer cluster parameters (i.e., distance and size) and distances to the cluster stars using
Gaia
parallax measurements. The code includes cluster-oriented prior families and it is specifically designed to deal with the
Gaia
parallax spatial correlations.
Methods.
A Bayesian hierarchical model is created to allow for the inference of both the cluster parameters and distances to its stars.
Results.
Using synthetic data that mimics
Gaia
parallax uncertainties and spatial correlations, we observe that our cluster-oriented prior families result in distance estimates with smaller errors than those obtained with an exponentially decreasing space density prior. In addition, the treatment of the parallax spatial correlations minimizes errors in the estimated cluster size and stellar distances, and avoids the underestimation of uncertainties. Although neglecting the parallax spatial correlations has no impact on the accuracy of cluster distance determinations, it underestimates the uncertainties and may result in measurements that are incompatible with the true value (i.e., falling beyond the 2
σ
uncertainties).
Conclusions.
The combination of prior knowledge with the treatment of
Gaia
parallax spatial correlations produces accurate (error < 10%) and trustworthy estimates (i.e., true values contained within the 2
σ
uncertainties) of cluster distances for clusters up to ∼5 kpc, along with cluster sizes for clusters up to ∼1 kpc.
Context. AA Tau has been monitored for more than 20 years since 1987 and exhibited a nearly constant brightness level of V = 12.5 mag. We report here that in 2011 it suddenly faded, becoming 2 mag ...fainter in the V-band, and has remained in this deep state since then. Aims. We investigate the origin of the sudden dimming of the AA Tau system. Methods. We report on new optical and near-IR photometry and spectroscopy obtained during the fading event. Results. The system appears to be much redder and fainter than it was in the bright state. Moreover, the 8.2 d photometric period continuously observed for more than 20 years is not detected during most of the deep state. The analysis of the system’s brightness and colors suggests that the visual extinction on the line of sight has increased by about 3–4 mag in the deep state. At optical wavelengths, the system appears to be dominated by scattered light, probably originating from the upper surface layers of a highly inclined circumstellar disk. The profiles of the Balmer lines have significantly changed as well, with the disappearance of a central absorption component regularly observed in the bright state. We ascribe this change to the scattering of the system’s spectrum by circumstellar dust. Remarkably, the mass accretion rate in the inner disk and onto the central star has not changed as the system faded. Conclusions. We conclude that the deepening of the AA Tau system is due to a sudden increase of circumstellar dust extinction on the line of sight without concomitant change in the accretion rate. We suggest that the enhanced obscuration may be produced by a nonaxisymmetric overdense region in the disk, located at a distance of 7.7 AU or more, that was recently brought into the line of sight by its Keplerian motion around the central star.
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