Extended stellar systems in the solar neighborhood - II. Discovery of a nearby 120{\deg} stellar stream in Gaia DR2

We report the discovery of a large, dynamically cold, coeval stellar stream that is currently traversing the immediate solar neighborhood at a distance of only 100 pc. The structure was identified in a wavelet decomposition of the 3D velocity space of all stars within 300 pc to the Sun. Its members form a highly elongated structure with a length of at least 400 pc, while its vertical extent measures only about 50 pc. Stars in the stream are not isotropically distributed but instead form two parallel lanes with individual local overdensities, that may correspond to a remnant core of a tidally disrupted cluster or OB association. Its members follow a very well-defined main sequence in the observational Hertzsprung-Russel diagram and also show a remarkably low 3D velocity dispersion of only 1.3 km s$^{-1}$. These findings strongly suggest a common origin as a single coeval stellar population. An extrapolation of the present-day mass function indicates a total mass of at least 2000 M$_\odot$, making it larger than most currently known clusters or associations in the solar neighborhood. We estimated the stream's age to be around 1 Gyr based on a comparison with a set of isochrones and giant stars in our member selection and find a mean metallicity of $\left[ \mathrm{Fe/H} \right] = -0.04$. This structure may very well represent the Galactic disk counterpart to the prominent stellar streams observed in the Milky Way halo. As such, it constitutes a new valuable probe to constrain the Galaxy's mass distribution.Comment: accepted for publication in Astronomy & Astrophysics on 17 Jan 201

Extended stellar systems in the solar neighborhood - III. Like ships in the night: the Coma Berenices neighbor moving group

We report the discovery of a kinematically cold group of stars, located in the immediate neighborhood of the well-known star cluster Coma Berenices (Mel 111). The new group identified in tangential velocity space as measured by Gaia contains at least 177 coeval members distributed in two subgroups, and appears as a flattened structure parallel to the plane, stretching for about 50 pc. More remarkably, the new group, which appears to have formed about 300 Myr later than Mel 111 in a different part of the Galaxy, will share essentially the same volume with the older cluster when the centers of both groups will be at their closest in 13 Myr. This will result in the mixing of two unrelated populations with different metallicities. The phase of cohabitation for these two groups is about 20-30 Myr, after which the two populations will drift apart. We estimate that temporal cohabitation of such populations is not a rare event in the disk of the Milky Way, and of the order of once per Galactic revolution. Our study also unveils the tidal tails of the Mel 111 cluster.Comment: Accepted by A&A on March 12, 2019; A paper with similar content (arXiv:1902.01404) has been submitted a few days earlier. Our work was done independently and we obtain similar results, albeit with different technique

The Age Distribution of Stellar Orbit Space Clumps

The orbit distribution of young stars in the Galactic disk is highly structured, from well-defined clusters to streams of stars that may be widely dispersed across the sky, but are compact in orbital action-angle space. The age distribution of such groups can constrain the timescales over which co-natal groups of stars disperse into the `field'. Gaia data have proven powerful to identify such groups in action-angle space, but the resulting member samples are often too small and have too narrow a CMD coverage to allow robust age determinations. Here, we develop and illustrate a new approach that can estimate robust stellar population ages for such groups of stars. This first entails projecting the predetermined action-angle distribution into the 5D space of positions, parallaxes and proper motions, where much larger samples of likely members can be identified over a much wider range of the CMD. It then entails isochrone fitting that accounts for a) widely varying distances and reddenings; b) outliers and binaries; c) sparsely populated main sequence turn-offs, by incorporating the age information of the low-mass main sequence; and d) the possible presence of an intrinsic age spread in the stellar population. When we apply this approach to 92 nearby stellar groups identified in 6D orbit space, we find that they are predominately young ($\lesssim 1$ Gyr), mono-age populations. Many groups are established (known) localized clusters with possible tidal tails, others tend to be widely dispersed and manifestly unbound. This new age-dating tool offers a stringent approach to understanding on which orbits stars form in the solar neighborhood and how quickly they disperse into the field.Comment: Accepted for publication in Ap

VISIONS:the VISTA Star Formation Atlas I. Survey overview

VISIONS is an ESO public survey of five nearby (d < 500 pc) star-forming molecular cloud complexes that are canonically associated with the constellations of Chamaeleon, Corona Australis, Lupus, Ophiuchus, and Orion. The survey was carried out with the Visible and Infrared Survey Telescope for Astronomy (VISTA), using the VISTA Infrared Camera (VIRCAM), and collected data in the near-infrared passbands J (1.25 μm), H (1.65 μm), and KS (2.15 μm). With a total on-sky exposure time of 49.4h VISIONS covers an area of 650 deg2, it is designed to build an infrared legacy archive with a structure and content similar to the Two Micron All Sky Survey (2MASS) for the screened star-forming regions. Taking place between April 2017 and March 2022, the observations yielded approximately 1.15 million images, which comprise 19 TB of raw data. The observations undertaken within the survey are grouped into three different subsurveys. First, the wide subsurvey comprises shallow, large-scale observations and it has revisited the star-forming complexes six times over the course of its execution. Second, the deep subsurvey of dedicated high-sensitivity observations has collected data on areas with the largest amounts of dust extinction. Third, the control subsurvey includes observations of areas of low-to-negligible dust extinction. Using this strategy, the VISIONS observation program offers multi-epoch position measurements, with the ability to access deeply embedded objects, and it provides a baseline for statistical comparisons and sample completeness – all at the same time. In particular, VISIONS is designed to measure the proper motions of point sources, with a precision of 1 mas yr−1 or better, when complemented with data from the VISTA Hemisphere Survey (VHS). In this way, VISIONS can provide proper motions of complete ensembles of embedded and low-mass objects, including sources inaccessible to the optical ESA Gaia mission. VISIONS will enable the community to address a variety of research topics from a more informed perspective, including the 3D distribution and motion of embedded stars and the nearby interstellar medium, the identification and characterization of young stellar objects, the formation and evolution of embedded stellar clusters and their initial mass function, as well as the characteristics of interstellar dust and the reddening law

VISIONS:the VISTA Star Formation Atlas I. Survey overview

VISIONS is an ESO public survey of five nearby (d < 500 pc) star-forming molecular cloud complexes that are canonically associated with the constellations of Chamaeleon, Corona Australis, Lupus, Ophiuchus, and Orion. The survey was carried out with the Visible and Infrared Survey Telescope for Astronomy (VISTA), using the VISTA Infrared Camera (VIRCAM), and collected data in the near-infrared passbands J (1.25 μm), H (1.65 μm), and KS (2.15 μm). With a total on-sky exposure time of 49.4h VISIONS covers an area of 650 deg2, it is designed to build an infrared legacy archive with a structure and content similar to the Two Micron All Sky Survey (2MASS) for the screened star-forming regions. Taking place between April 2017 and March 2022, the observations yielded approximately 1.15 million images, which comprise 19 TB of raw data. The observations undertaken within the survey are grouped into three different subsurveys. First, the wide subsurvey comprises shallow, large-scale observations and it has revisited the star-forming complexes six times over the course of its execution. Second, the deep subsurvey of dedicated high-sensitivity observations has collected data on areas with the largest amounts of dust extinction. Third, the control subsurvey includes observations of areas of low-to-negligible dust extinction. Using this strategy, the VISIONS observation program offers multi-epoch position measurements, with the ability to access deeply embedded objects, and it provides a baseline for statistical comparisons and sample completeness – all at the same time. In particular, VISIONS is designed to measure the proper motions of point sources, with a precision of 1 mas yr−1 or better, when complemented with data from the VISTA Hemisphere Survey (VHS). In this way, VISIONS can provide proper motions of complete ensembles of embedded and low-mass objects, including sources inaccessible to the optical ESA Gaia mission. VISIONS will enable the community to address a variety of research topics from a more informed perspective, including the 3D distribution and motion of embedded stars and the nearby interstellar medium, the identification and characterization of young stellar objects, the formation and evolution of embedded stellar clusters and their initial mass function, as well as the characteristics of interstellar dust and the reddening law

VISIONS: The VISTA Star Formation Atlas -- I. Survey overview

© The Authors 2023. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0).VISIONS is an ESO public survey of five nearby (d < 500 pc) star-forming molecular cloud complexes that are canonically associated with the constellations of Chamaeleon, Corona Australis, Lupus, Ophiuchus, and Orion. The survey was carried out with VISTA, using VIRCAM, and collected data in the near-infrared passbands J, H, and Ks. With a total on-sky exposure time of 49.4 h VISIONS covers an area of 650 deg$^2$, and it was designed to build an infrared legacy archive similar to that of 2MASS. Taking place between April 2017 and March 2022, the observations yielded approximately 1.15 million images, which comprise 19 TB of raw data. The observations are grouped into three different subsurveys: The wide subsurvey comprises shallow, large-scale observations and has visited the star-forming complexes six times over the course of its execution. The deep subsurvey of dedicated high-sensitivity observations has collected data on the areas with the largest amounts of dust extinction. The control subsurvey includes observations of areas of low-to-negligible dust extinction. Using this strategy, the VISIONS survey offers multi-epoch position measurements, is able to access deeply embedded objects, and provides a baseline for statistical comparisons and sample completeness. In particular, VISIONS is designed to measure the proper motions of point sources with a precision of 1 mas/yr or better, when complemented with data from VHS. Hence, VISIONS can provide proper motions for sources inaccessible to Gaia. VISIONS will enable addressing a range of topics, including the 3D distribution and motion of embedded stars and the nearby interstellar medium, the identification and characterization of young stellar objects, the formation and evolution of embedded stellar clusters and their initial mass function, as well as the characteristics of interstellar dust and the reddening law.Peer reviewe

Like ships that pass in the night: Coma Berenices and its neighbor moving group

Offene Sternhaufen und Assoziationen sind hervorragende Indikatoren zur Untersuchung der Entstehung, Entwicklung und Struktur der galaktischen Scheibe. Aufgrund ihrer geringen Geschwindigkeitsdispersion erscheinen offene Sternhaufen als kleinräumige Geschwindigkeitsstrukturen in der Milchstraße. Ihre Untersuchung liefert Einblicke in die dynamische Entwicklung von Sternhaufen in Gezeitenfeldern, in die Auflösung von Sternhaufen und Masseverlust, in die Zusammensetzung der galaktischen Feldsternpopulation und in die Massenverteilung der galaktischen Scheibe. Der zweite Gaia Datenrelease liefert astronomische Daten mit einer nie dagewesenen astrometrischen Genauigkeit. Die genauen Positions- und Geschwindigkeitsmessungen ermöglichen neue Einblicke in die Eigenschaften von Sternpopulationen in der galaktischen Scheibe und tragen dazu bei unser Wissen über die Milchstraße zu revolutionieren. Diese Arbeit untersucht den bekannten Sternhaufen Coma Berenices und eine uns bisher unbekannte Gruppe von Sternen, die in dessen unmittelbarer Nähe liegt und ein sehr ähnliches Geschwindigkeitsprofil aufweist. Die neu entdeckte Gruppe wurde im Tangentialgeschwindigkeitsraum identifiziert, enthält mindestens 177 Sterne und erstreckt sich über 50 Parsec. Sie erscheint als abgeflachte Struktur parallel zur galaktischen Scheibe und ist etwa 300 Myr später als Coma Berenices in einem anderen Teil der Galaxie entstanden. Bemerkenswerterweise liegen die Zentren beider Gruppen in 13 Millionen Jahren so nahe, dass beide Populationen dasselbe galaktische Volumen einnehmen werden. Dies führt zu einer temporären Vermischung zweier nicht verwandter Populationen mit unterschiedlichen Metallizitäten. Nach der Phase des Zusammenlebens von etwa 20-30 Millionen Jahren werden die beiden Populationen auseinanderdriften. Wir schätzen, dass das Zusammentreffen solcher Populationen in der galaktischen Scheibe kein seltenes Ereignis ist und in der Größenordnung von einmal pro galaktischer Revolution liegt. Die Arbeit enthüllt auch die Gezeitenschweife von Coma Berenices, ein Effekt der Sternhaufen-Auflösung durch das Gezeitenfeld der Milchstraße.Open star clusters and associations are excellent tracers to study the formation, evolution, and structure of the Galactic disk. A key characteristic of open clusters is their small velocity dispersion. Therefore, they appear as small-scale velocity structures in the Milky Way, and their investigation delivers insights into e.g. the dynamical evolution of star clusters in tidal fields, star cluster disruption and mass loss, the build-up of the Galactic field population, and the mass distribution of the Galactic disk. The second Gaia data release provides astrometric data with a precision we have never had before. Its accurate position and velocity measurements enable new insights into the properties of stellar populations in the Galactic disk and contribute greatly to revolutionizing our knowledge about the Milky Way. This thesis studies the well-known open cluster Coma Berenices and a previously unknown moving group of stars in its velocity and spatial neighborhood. The new group identified in tangential velocity space as measured by Gaia contains at least 177 coeval members distributed in two subgroups, and appears as a flattened structure parallel to the plane, stretching for about 50 pc. More remarkably, the new group, which appears to have formed about 300 Myr later than Coma Berenices in a different part of the Galaxy, will share essentially the same volume with the older cluster when the centers of both groups will be at their closest in 13 Myr. This will result in the mixing of two unrelated populations with different metallicities. The phase of cohabitation for these two groups is about 20 - 30 Myr, after which the two populations will drift apart. We estimate that temporal cohabitation of such populations is not a rare event in the disk of the Milky Way, and of the order of once per Galactic revolution. Our study also unveils the tidal tails of the Coma Berenices cluster, an effect of cluster disruption due to the tidal field of the Milky Way

The Age Distribution of Stellar Orbit Space Clumps

The orbit distribution of young stars in the Galactic disk is highly structured, from well-defined clusters to streams of stars that may be widely dispersed across the sky, but are compact in orbital action-angle space. The age distribution of such groups can constrain the timescales over which conatal groups of stars disperse into the “field.” Gaia data have proven powerful in identifying such groups in action-angle space, but the resulting member samples are often too small and have too narrow a color–magnitude diagram (CMD) coverage to allow robust age determinations. Here, we develop and illustrate a new approach that can estimate robust stellar population ages for such groups of stars. This first entails projecting the predetermined action-angle distribution into the 5D space of positions, parallaxes, and proper motions, where much larger samples of likely members can be identified over a much wider range of the CMD. It then entails isochrone fitting that accounts for: (a) widely varying distances and reddenings; (b) outliers and binaries; (c) sparsely populated main-sequence turnoffs, by incorporating the age information of the low-mass main sequence; and (d) the possible presence of an intrinsic age spread in the stellar population. When we apply this approach to 92 nearby stellar groups identified in 6D orbit space, we find that they are predominantly young (≲1 Gyr), mono-age populations. Many groups are established (known) localized clusters with possible tidal tails, while others tend to be widely dispersed and manifestly unbound. This new age-dating tool offers a stringent approach to understanding on which orbits stars form in the solar neighborhood and how quickly they disperse into the field

VISIONS: the VISTA Star Formation Atlas

The VISIONS public survey provides large-scale, multi-epoch imaging of five nearby star-forming regions at sub-arcsecond resolution in the near-infrared. All data collected within the program and provided by the European Southern Observatory (ESO) science archive are processed with a custom end-to-end pipeline infrastructure to provide science-ready images and source catalogs. The data reduction environment has been specifically developed for the purpose of mitigating several shortcomings of the bona fide data products processed with software provided by the Cambridge Astronomical Survey Unit (CASU), such as spatially variable astrometric and photometric biases of up to 100 mas and 0.1 mag, respectively. At the same time, the resolution of co-added images is up to 20% higher compared to the same products from the CASU processing environment. Most pipeline modules are written in Python and make extensive use of C extension libraries for numeric computations, thereby simultaneously providing accessibility, robustness, and high performance. The astrometric calibration is performed relative to the Gaia reference frame, and fluxes are calibrated with respect to the source magnitudes provided in the Two Micron All Sky Survey (2MASS). For bright sources, absolute astrometric errors are typically on the order of 10–15 mas and fluxes are determined with sub-percent precision. Moreover, the calibration with respect to 2MASS photometry is largely free of color terms. The pipeline produces data that are compliant with the ESO Phase 3 regulations and furthermore provides curated source catalogs that are structured similarly to those provided by the 2MASS survey