44 research outputs found
Extended stellar systems in the solar neighborhood - I. The tidal tails of the Hyades
We report the discovery of two well-defined tidal tails emerging from the
Hyades star cluster. The tails were detected in Gaia DR2 data by selecting
cluster members in the three-dimensional galactocentric cylindrical velocity
space. The robustness of our member selection is reinforced by the fact that
the sources depict an almost noiseless, coeval stellar main sequence in the
observational Hertzsprung-Russel diagram. The spatial arrangement of the
selected members represents a highly flattened shape with respect to the
direction of movement along the clusters' orbit in the Galaxy. The size of the
entire structure, within the limits of the observations, measures about 200 pc
in its largest extent, while being only about 25 pc thick. This translates to
an on-sky extent of well beyond 100 deg. Intriguingly, a top-down view on the
spatial distribution reveals as distinct S-shape, reminiscent of tidal tails
both observed for globular clusters, as well as modelled for star clusters
bound to the Galactic disk. Even more remarkable, the spatial arrangement, as
well as the velocity dispersion of our source selection is in excellent
agreement with previously published theoretical predictions for the tidal tails
of the Hyades. An investigation into observed signatures of equipartition of
kinetic energy, i.e. mass segregation, remains unsuccessful, most likely due to
the sensitivity limit for radial velocity measurements with Gaia.Comment: accepted by A&A on 29 Nov 2018; A manuscript with similar content
(arXiv:1811.03845) has been submitted to A&A only a few days earlier. The
authors did not know about each other's wor
Estimating Extinction using Unsupervised Machine Learning
Dust extinction is the most robust tracer of the gas distribution in the
interstellar medium, but measuring extinction is limited by the systematic
uncertainties involved in estimating the intrinsic colors to background stars.
In this paper we present a new technique, PNICER, that estimates intrinsic
colors and extinction for individual stars using unsupervised machine learning
algorithms. This new method aims to be free from any priors with respect to the
column density and intrinsic color distribution. It is applicable to any
combination of parameters and works in arbitrary numbers of dimensions.
Furthermore, it is not restricted to color space. Extinction towards single
sources is determined by fitting Gaussian Mixture Models along the extinction
vector to (extinction-free) control field observations. In this way it becomes
possible to describe the extinction for observed sources with probability
densities. PNICER effectively eliminates known biases found in similar methods
and outperforms them in cases of deep observational data where the number of
background galaxies is significant, or when a large number of parameters is
used to break degeneracies in the intrinsic color distributions. This new
method remains computationally competitive, making it possible to correctly
de-redden millions of sources within a matter of seconds. With the
ever-increasing number of large-scale high-sensitivity imaging surveys, PNICER
offers a fast and reliable way to efficiently calculate extinction for
arbitrary parameter combinations without prior information on source
characteristics. PNICER also offers access to the well-established NICER
technique in a simple unified interface and is capable of building extinction
maps including the NICEST correction for cloud substructure. PNICER is offered
to the community as an open-source software solution and is entirely written in
Python.Comment: Accepted for publication in A&A, source code available at
http://smeingast.github.io/PNICER
Pulsating stars in NGC 6231 Frequency analysis and photometric mode identification near the main sequence
We used Johnson UBV photometric CCD observations to identify pulsating and
other variable stars in the young open cluster NGC 6231. The multi-color
information was used to classify pulsating variables, perform frequency
analysis, and - where possible - to compare observed to theoretical amplitude
ratios for mode identification. The data reduction was performed with standard
IRAF tools. Differential light curves have been obtained by identifying a set
of suitable comparison stars and the frequency analysis was then conducted on
the basis of Fourier methods. Our classification of pulsating stars was based
on the time scales and amplitudes of the variability with respect to the
different filters and stellar parameters as calculated from published
Str\"omgren and Geneva photometry. We identified 32 variable stars in the field
of the cluster out of which 21 are confirmed members and twelve are newly
detected variable stars. Ten stars were classified as Slowly Pulsating B (SPB)
stars in NGC 6231 out of which seven are new discoveries. We also analyzed six
previously reported {\beta} Cephei variables in more detail. One of them may be
a hybrid {\beta} Cephei/SPB pulsator. In addition, we investigated five more
previously suspected pulsators of this group which we cannot convincingly
confirm. The remaining eleven variable stars are either not members of NGC 6231
or the membership status is questionable. Among them are three previously known
{\delta} Scuti stars, two newly detected pulsators of this class, one new and
two already known eclipsing binaries, one new SPB variable, one possible
Pre-Main-Sequence (PMS) pulsator and another new variable star for which we
cannot present a classification. With more than 20 main sequence pulsators of
spectral type B, NGC 6231 becomes the open cluster with the largest population
of such pulsating stars known.Comment: 27 pages, 35 figures, 3 Tables, accepted by A&A, abstract excessively
shorted due to character limit
Pulsating and variable stars in the young open star cluster NGC 6231
Diese Arbeit befasst sich mit den veränderlichen und im speziellen mit pulsierenden Sternen in NGC 6231, also Sternen die ü̈ber typische Zeiträume ihre Form und auch physikalischen Eigenschaften ändern. Obwohl sich schon einige Studien in den letzten 30 Jahren mit diesem Thema beschäftigt haben, fehlt immer noch ein vollständiges Bild von Pulsatoren in dem Haufen. Im Zuge dieser neuen Untersuchung wurde ein großer Datensatz, der am Siding Spring Observatory in Australien aufgenommen wurde, untersucht. Die Aufnahmen wurden im speziellen für heiße Pulsatoren des Beta Cephei Typs optimiert, aber aufgrund des großen Dynamikbreichs von modernen CCD Kameras war es auch möglich wesentlich leuchtschwächere Sterne zu erforschen. Die Eigenschaften des Datensatzes erlaubten auch eine Bestimmung des, oder Einschränkung auf eine bestimmte Pulsationsmode die benötigt werden um in folgenden Arbeiten Rückschlüsse auf den Aufbau und die Entwicklung von Sternen zu ziehen.
Um die gesuchten Informationen aus den Aufnahmen zu extrahieren wurde eine speziell entwickelte Technik angewandt die die Erstellung von Lichtkurven für 473 Sterne im Feld von NGC 6231 ermöglichte.
Die Frequenzanalyse der Lichtkurven resultierte in 31 Sternen die Veränderungen in ihrer Helligkeit zeigten. 22 von diesen sind bestätigte Haufenmitglieder. Von den bereits bekannten Beta Cephei Sternen konnten alle bestätigt werden. Von den drei bisher vermuteten SPB Sternen im Haufen wurden alle bestätigt. Hinzu kommen sieben neu entdeckte Sterne dieses Typs. Neben den bereits bekannten Delta Scuti Sternen wurde ein weiterer klassifiziert und untersucht. Des weiteren konnten auch Sterne außerhalb des Haufens untersucht werden, wie etwa ein neu entdeckter Doppelstern
Extended stellar systems in the solar neighborhood -- V. Discovery of coronae of nearby star clusters
In this paper, we present a novel view on the morphology and the dynamical
state of 10 prominent, nearby ( 500 pc), and young (30-300 Myr)
open star clusters with Gaia DR2: Per, Blanco 1, IC 2602, IC 2391,
Messier 39, NGC 2451A, NGC 2516, NGC 2547, Platais 9, and the Pleiades. We
introduce a pioneering member identification method that is informed by cluster
bulk velocities and deconvolves the spatial distribution with a mixture of
Gaussians. Our approach enables inferring the clusters' true spatial
distribution by effectively filtering field star contaminants while at the same
time mitigating the impact of positional errors along the line of sight. This
first application of the method reveals the existence of vast stellar coronae,
extending for 100 pc and surrounding the, by comparison tiny and
compact, cluster cores. The coronae and cores form intertwined, co-eval, and
co-moving extended cluster populations, each encompassing tens of thousands of
cubic parsec and stretching across tens of degrees on the sky. Our analysis
shows that the coronae are gravitationally unbound but largely comprise the
bulk of the populations' stellar mass. Most systems are in a highly dynamic
state, showing evidence of expansion and sometimes simultaneous contraction
along different spatial axes. The velocity field of the extended populations
for the cluster cores appears asymmetric but is aligned along a spatial axis
unique to each cluster. The overall spatial distribution and the kinematic
signature of the populations are largely consistent with the differential
rotation pattern of the Milky Way. This finding underlines the important role
of global Galactic dynamics to the fate of stellar systems. Our results
highlight the complexity of the Milky Way's open cluster population and call
for a new perspective on the characterization and dynamical state of open
clusters.Comment: published in Astronomy & Astrophysics (update 23.07.21: fixed
citation
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. 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,
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 . 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
VISION - Vienna Survey in Orion II. Infrared extinction in Orion A
We have investigated the shape of the extinction curve in the infrared up to
~25 {\mu}m for the Orion A star-forming complex. The basis of this work is
near-infrared data acquired with VISTA, in combination with Pan-STARRS and
mid-infrared Spitzer photometry. We obtain colour excess ratios for eight
passbands by fitting a series of colour-colour diagrams. The fits are performed
using Markov chain Monte Carlo methods, together with a linear model under a
Bayesian formalism. The resulting colour excess ratios are directly interpreted
as a measure of the extinction law. We show that the Orion A molecular cloud is
characterized by flat mid-infrared extinction, similar to many other recently
studied sightlines. Moreover, we find statistically significant evidence that
the extinction law from ~1 {\mu}m to at least ~6 {\mu}m varies across the
cloud. In particular, we find a gradient along galactic longitude, where
regions near the ONC show a different extinction law compared to L1641 and
L1647, the low-mass star-forming sites in the cloud complex. These variations
are of the order of only 3% and are most likely caused by the influence of the
massive stars on their surrounding medium. While the observed general trends in
our measurements are in agreement with model predictions, both well-established
and new dust grain models are not able to fully reproduce our infrared
extinction curve. We also present a new extinction map featuring a resolution
of 1 arcmin and revisit the correlation between extinction and dust optical
depth. This analysis shows that cloud substructure, which is not sampled by
background sources, affects the conversion factor between these two measures.
In conclusion, we argue that specific characteristics of the infrared
extinction law are still not well understood, but Orion A can serve as an
unbiased template for future studies.Comment: accepted by Astronomy & Astrophysic
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 Solar Neighborhood in the Age of Gaia
Most of what we know about the formation of stars, and essentially everything
we know about the formation of planets, comes from observations of our solar
neighborhood within 2 kpc of the Sun. Before 2018, accurate distance
measurements needed to turn the 2D Sky into a faithful 3D physical picture of
the distribution of stars, and the interstellar matter that forms them, were
few and far between. Here, we offer a holistic review of how, since 2018, data
from the Gaia mission are revealing previously unseen and often unexpected 3D
distributions of gas, dust, and young stars in the solar neighborhood. We
summarize how new extinction-based techniques yield 3D dust maps and how the
density structure mapped out offers key context for measuring young stars' 3D
positions from Gaia and VLBI. We discuss how a subset of young stars in Gaia
with measured radial velocities and proper motions is being used to recover 3D
cloud motion and characterize the internal dynamics of individual star-forming
regions. We review relationships between newly-identified clusters and streams
of young stars and the molecular interstellar medium from which they evolve.
The combination of these measures of gas and stars' 3D distribution and 3D
motions provides unprecedented data for comparison with simulations and
reframes our understanding of local star formation in a larger Galactic
context. This new 3D view of our solar neighborhood in the age of Gaia shows
that star-forming regions once thought to be isolated are often connected on
kiloparsec scales, causing us to reconsider models for the arrangement of gas
and young stars in galaxies.Comment: To appear in Protostars and Planets VII; Editors: Shu-ichiro
Inutsuka, Yuri Aikawa, Takayuki Muto, Kengo Tomida, and Motohide Tamur