The structure and evolution of the spiral arms of our Milky Way are basic but long-standing questions in astronomy. In particular, the lifetime of spiral arms is still a puzzle and has not been well constrained from observations. In this work, we aim to inspect these issues using a large catalogue of open clusters. We compiled a catalogue of 3794 open clusters based on Gaia EDR3. A majority of these clusters have accurately determined parallaxes, proper motions, and radial velocities. The age parameters for these open clusters are collected from references or calculated in this work. In order to understand the nearby spiral structure and its evolution, we analysed the distributions, kinematic properties, vertical distributions, and regressed properties of subsamples of open clusters. We find evidence that the nearby spiral arms are compatible with a long-lived spiral pattern and might have remained approximately stable for the past 80 million years. In particular, the Local Arm, where our Sun is currently located, is also suggested to be long-lived in nature and probably a major arm segment of the Milky Way. The evolutionary characteristics of nearby spiral arms show that the dynamic spiral mechanism might be not prevalent for our Galaxy. Instead, density wave theory is more consistent with the observational properties of open clusters.
Context. The astrometric satellite Gaia is expected to significantly increase our knowledge as to the properties of the Milky Way. The Gaia Early Data Release 3 (EDR3) provides the most precise parallaxes for many OB stars, which can be used to delineate the Galactic spiral structure. Aims. We investigate the local spiral structure with the largest sample of spectroscopically confirmed young OB stars available to date, and we compare it with what was traced by the parallax measurements of masers. Methods. A sample consisting of three different groups of massive young stars, including O–B2 stars, O–B0 stars and O-type stars with parallax accuracies better than 10% was compiled and used in our analysis. Results. The local spiral structures in all four Galactic quadrants within ≈5 kpc of the Sun are clearly delineated in detail. The revealed Galactic spiral pattern outlines a clear sketch of nearby spiral arms, especially in the third and fourth quadrants where the maser parallax data are still absent. These O-type stars densify and extend the spiral structure constructed by using the Very Long Baseline Interferometry maser data alone. The clumped distribution of O-type stars also indicates that the Galaxy spiral structure is inhomogeneous.
The astrometric satellite Gaia recently released part of its third data set, which provides a good opportunity to hunt for more open clusters in the Milky Way. In this work, we conduct a blind search for open clusters in the Galactic disk using a sample-based clustering search method with high spatial resolution, which is especially suited to finding hidden targets. In addition to confirming 1 930 previously known open clusters and 82 known globular clusters, 704 new stellar clusters are proposed as potential open clusters at Galactic latitudes of |b| ≤ 20 • . For each of these new open clusters, we present the coordinates, detailed astrometric parameters, and ages, as well as the radial velocity, if available. Our blind search greatly increases the number of Galactic open clusters as objects of study and shows the incompleteness of the open cluster census across our Galaxy.
We present a survey of molecular outflows across the dark cloud complex in the Cygnus region, based on a 46.75 deg2 field of CO isotopologue data from the Milky Way Imaging Scroll Painting survey. A supervised machine-learning algorithm, the support vector machine, is introduced to accelerate our visual assessment of outflow features in the data cube of 12CO and 13CO J = 1−0 emission. A total of 130 outflow candidates are identified, 77 of which show bipolar structures and 118 are new detections. Spatially, these outflows are located inside dense molecular clouds, and some of them are found in clusters or in elongated linear structures tracing the underlying gas filament morphology. Along the line of sight, 97, 31, and 2 candidates reside in the Local, Perseus, and Outer Arms, respectively. Young stellar objects as outflow drivers are found near most outflows, while 36 candidates show no associated source. The clusters of outflows that we detect are inhomogeneous in their properties; nevertheless, we show that the outflows cannot inject turbulent energy on cloud scales. Instead, at best, they are restricted to affecting the so-called “clump” and “core” scales, and only on short (∼0.3 Myr) estimated timescales. Combined with outflow samples in the literature, our work shows a tight outflow mass–size correlation.
We measured the relative positions between two pairs of compact extragalactic sources (CESs), J1925-2219 and J1923-2104 (C1–C2) and J1925-2219 and J1928-2035 (C1–C3), on 2020 October 23–25 and 2021 February 5 (totaling four epochs), respectively, using the Very Long Baseline Array at 15 GHz. Accounting for the deflection angle dominated by Jupiter, as well as the contributions from the Sun and planets other than Earth, the Moon, and Ganymede (the most massive of the solar system’s moons), our theoretical calculations predict that the dynamical ranges of the relative positions across four epochs in R.A. of the C1–C2 pair and C1–C3 pair are 841.2 and 1127.9 μas, respectively. The formal accuracy in R.A. is about 20 μas, but the error in decl. is poor. The measured standard deviations of the relative positions across the four epochs are 51.0 and 29.7 μas in R.A. for C1–C2 and C1–C3, respectively. These values indicate that the accuracy of the post-Newtonian relativistic parameter, γ, is ∼0.061 for C1–C2 and ∼0.026 for C1–C3. Combining the two CES pairs, the measured value of γ is 0.984 ± 0.037, which is comparable to the latest published results for Jupiter as a gravitational lens, reported by Fomalont & Kopeikin, i.e., 1.01 ± 0.03.
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