18 research outputs found
Praesepe (NGC 2632) and its tidal tails
Within a 400 pc sphere around the Sun, we search for Praesepe's tidal tails
in the Gaia DR2 dataset. We used a modified convergent-point method to search
for stars with space velocities close to the space velocity of the Praesepe
cluster. We find a clear indication for the existence of Praesepe's tidal
tails, both extending up to 165~pc from the centre of the cluster. A total of
1393 stars populate the cluster and its tails, giving a total mass of 794
M_Sun. We determined a tidal radius of 10.77 pc for the cluster and a tidal
mass of 483 M_Sun. The corresponding half-mass radius is 4.8 pc. We also found
clear indication for mass segregation in the cluster. The tidal tails outside 2
tidal radii are populated by 389 stars. The total contamination of our sample
by field stars lies between 50 to 100 stars or 3.6 to 7.2 per cent. We used an
astrometrically and photometrically clean sub-sample of Gaia DR2 which makes
our Praesepe sample incomplete beyond M_G ~ 12.0 mag, which corresponds to
about 0.25 M_Sun. A comparison with an N-body model of the cluster and its
tails shows remarkably good coincidence. Three new white dwarfs are found in
the tails.Comment: 6 pages, 5 figures. Accepted by Astronomy and Astrophysic
A census of the near-by Pisces-Eridanus stellar stream -- Commonalities with and disparities from the Pleiades
Within a 400~pc sphere around the Sun, we search for members of the
Pisces-Eridanus (Psc-Eri) stellar stream in the Gaia Data Release 2 (DR2) data
set. We compare basic astrophysical characteristics of the stream with those of
the Pleiades. We used a modified convergent-point method to identify stars with
2D - velocities consistent with the space velocity of the Psc-Eri stream and
the Pleiades, respectively. We found 1387 members of the Psc-Eri stream in a G
magnitude range from 5.1 mag to 19.3 mag at distances between 80 and 380 pc
from the Sun. The stream has a nearly cylindrical shape with length and
thickness of about 700 pc and 100 pc, respectively. The total stellar mass
contained in the stream is about 770 M_Sun, and the members are gravitationally
unbound. For the stream we found an age of about 135 Myr. In many astrophysical
properties Psc-Eri is comparable to the open cluster M45 (the Pleiades): in its
age, its luminosity function (LF), its Present-day mass Function (PDMF) as well
as in its total mass. Nonetheless, the two stellar ensembles are completely
unlike in their physical appearance. We cautiously give two possible
explanations for this disagreement: (i) the star-formation efficiency in their
parental molecular clouds was higher for the Pleiades than for Psc-Eri or/and
(ii) the Pleiades had a higher primordial mass segregation immediately after
the expulsion of the molecular gas of the parental cloud.Comment: 10 pages, 6 figure
Hot Stuff for One Year (HSOY) - A 583 million star proper motion catalogue derived from Gaia DR1 and PPMXL
Recently, the first installment of data from ESA's Gaia astrometric satellite
mission (Gaia-DR1) was released, containing positions of more than 1 billion
stars with unprecedented precision, as well as only proper motions and
parallaxes, however only for a subset of 2 million objects. The second release,
due in late 2017 or early 2018, will include those quantities for most objects.
In order to provide a dataset that bridges the time gap between the Gaia-DR1
and Gaia-DR2 releases and partly remedies the lack of proper motions in the
former, HSOY ("Hot Stuff for One Year") was created as a hybrid catalogue
between Gaia-DR1 and ground-based astrometry, featuring proper motions (but no
parallaxes) for a large fraction of the DR1 objects. While not attempting to
compete with future Gaia releases in terms of data quality or number of
objects, the aim of HSOY is to provide improved proper motions partly based on
Gaia data, allowing some studies to be carried out just now or as pilot studies
for later larger projects requiring higher-precision data. The HSOY catalogue
was compiled using the positions taken from Gaia-DR1 combined with the input
data from the PPMXL catalogue, employing the same weighted least-squares
technique that was used to assemble the PPMXL catalogue itself. Results. This
effort resulted in a four-parameter astrometric catalogue containing
583,000,000 objects, with Gaia-DR1 quality positions and proper motions with
precisions from significantly less than 1 mas/yr to 5 mas/yr, depending on the
object's brightness and location on the sky.Comment: 4 pages, 5 figures, accepted for publication in A&A letter
Single stars in the Hyades open cluster. Fiducial sequence for testing stellar and atmospheric models
Age and mass determinations for isolated stellar objects remain
model-dependent. While stellar interior and atmospheric theoretical models are
rapidly evolving, we need a powerful tool to test them. Open clusters are good
candidates for this role. We complement previous studies on the Hyades
multiplicity by Lucky Imaging observations with the AstraLux Norte camera. This
allows us to exclude possible binary and multiple systems with companions
outside 2--7 AU separation and to create a "single-star sequence" for the
Hyades. The sequence encompasses 250 main-sequence stars ranging from A5V to
M6V. Using the "Tool for Astrophysical Data Analysis" (TA-DA), we create
various theoretical isochrones applying different combinations of interior and
atmospheric models. We compare the isochrones with the observed Hyades
single-star sequence on J vs. J - K_s, J vs. J - H and K_s vs. H - K_s
color-magnitude diagrams. As a reference we also compute absolute fluxes and
magnitudes for all stars from X-ray to mid-infrared based on photometric
measurements available in the literature(ROSAT X-ray, GALEX UV, APASS gri,
2MASS JHK_s, and WISE W1 to W).We find that combinations of both PISA and
DARTMOUTH stellar interior models with BT-Settl 2010 atmospheric models
describe the observed sequence well. The full sequence covers the mass range
0.13 to 2.3 Msun, and effective temperatures between 3060 K and 8200 K. Within
the measurement uncertainties, the current generation of models agree well with
the single-star sequence. The primary limitations are the uncertainties in the
measurement of the distance to individual Hyades members, and uncertainties in
the photometry. Additionally, a small (~0.05 mag) systematic offset can be
noted on J vs. J - K and K vs. H - K diagrams - the observed sequence is
shifted to redder colors from the theoretical predictions.Comment: 6 pages, 2 figures, 1 table. The extended version of the table will
be available online soon. Accepted for publication in Astronomy &
Astrophysic
Open star clusters in the Milky Way - Comparison of photometric and trigonometric distance scales based on Gaia TGAS data
The global survey of star clusters in the Milky Way (MWSC) is a comprehensive
list of 3061 objects that provides, among other parameters, distances to
clusters based on isochrone fitting. The Tycho-Gaia Astrometric Solution (TGAS)
catalogue, which is a part of Gaia data release 1 (Gaia DR1), delivers accurate
trigonometric parallax measurements for more than 2 million stars, including
those in star clusters. We compare the open cluster photometric distance scale
with the measurements given by the trigonometric parallaxes from TGAS to
evaluate the consistency between these values. The average parallaxes of
probable cluster members available in TGAS provide the trigonometric distance
scale of open clusters, while the photometric scale is given by the distances
published in the MWSC. Sixty-four clusters are suited for comparison as they
have more than 16 probable members with parallax measurements in TGAS. We
computed the average parallaxes of the probable members and compared these to
the photometric parallaxes derived within the MWSC. We find a good agreement
between the trigonometric TGAS-based and the photometric MWSC-based distance
scales of open clusters, which for distances less than 2.3 kpc coincide at a
level of about 0.1 mas with no dependence on the distance. If at all, there is
a slight systematic offset along the Galactic equator between and
galactic longitude.Comment: Accepted for publication as a Letter in Astronomy and Astrophysics, 4
pages, 4 figure
Reanalysis of nearby open clusters using Gaia DR1/TGAS and HSOY
Open clusters have long been used to gain insights into the structure,
composition, and evolution of the Galaxy. With the large amount of stellar data
available for many clusters in the Gaia era, new techniques must be developed
for analyzing open clusters, as visual inspection of cluster color-magnitude
diagrams is no longer feasible. An automatic tool will be required to analyze
large samples of open clusters. We seek to develop an automatic
isochrone-fitting procedure to consistently determine cluster membership and
the fundamental cluster parameters. Our cluster characterization pipeline first
determined cluster membership with precise astrometry, primarily from TGAS and
HSOY. With initial cluster members established, isochrones were fitted, using a
chi-squared minimization, to the cluster photometry in order to determine
cluster mean distances, ages, and reddening. Cluster membership was also
refined based on the stellar photometry. We used multiband photometry, which
includes ASCC-2.5 BV, 2MASS JHK_s, Gaia G band. We present parameter estimates
for all 24 clusters closer than 333 pc as determined by the Catalogue of Open
Cluster Data and the Milky Way Star Clusters catalog. We find that our
parameters are consistent to those in the Milky Way Star Clusters catalog. We
demonstrate that it is feasible to develop an automated pipeline that
determines cluster parameters and membership reliably. After additional
modifications, our pipeline will be able to use Gaia DR2 as input, leading to
better cluster memberships and more accurate cluster parameters for a much
larger number of clusters.Comment: 21 pages, 16 figures, accepted for publication in A&
A Strategy for Identifying the Grid Stars for the Space Interferometry Mission (SIM)
We present a strategy to identify several thousand stars that are
astrometrically stable at the micro-arcsecond level for use in the SIM (Space
Interferometry Mission) astrometric grid. The requirements on the grid stars
make this a rather challenging task. Taking a variety of considerations into
account we argue for K giants as the best type of stars for the grid, mainly
because they can be located at much larger distances than any other type of
star due to their intrinsic brightness. We show that it is possible to identify
suitable candidate grid K giants from existing astrometric catalogs. However,
double stars have to be eliminated from these candidate grid samples, since
they generally produce much larger astrometric jitter than tolerable for the
grid. The most efficient way to achieve this is probably by means of a radial
velocity survey. To demonstrate the feasibility of this approach, we repeatedly
measured the radial velocities for a pre-selected sample of 86 nearby Hipparcos
K giants with precisions of 5-8 m/s. The distribution of the intrinsic radial
velocity variations for the bona-fide single K giants shows a maximum around 20
m/s, which is small enough not to severely affect the identification of stellar
companions around other K giants. We use the results of our observations as
input parameters for Monte-Carlo simulations on the possible design of a radial
velocity survey of all grid stars. Our favored scenario would result in a grid
which consists to 68% of true single stars and to 32% of double or multiple
stars with periods mostly larger than 200 years, but only 3.6% of all grid
stars would display astrometric jitter larger than 1 microarcsecond. This
contamination level is probably tolerable.Comment: LaTeX, 21 pages, 8 figures, accepted by PASP (February 2001 issue).
Also available at http://beehive.ucsd.edu/ftp/pub/grid/kgiants.htm
Hyades tidal tails revealed by
Aims. Within a 200 pc sphere around the Sun, we search for the Hyades tidal tails in the Gaia DR2 dataset.
Methods. We used a modified convergent-point method to search for stars with space velocities close to the space velocity of the Hyades cluster.
Results. We find a clear indication for the existence of the Hyades tidal tails, a leading tail extending up to 170 pc from the centre of the Hyades with 292 stars (36 contaminants), and a trailing tail up to 70 pc with 237 stars (32 contaminants). A comparison with an N-body model of the cluster and its tails shows remarkably good coincidence. Five white dwarfs are found in the tails
Nine new open clusters within 500 pc from the Sun
Aims. One of the results of the Milky Way Star Clusters (MWSC) survey
was the detection of a slight under-density of old (ca. 1 Gyr) clusters within the nearest
kilo-parsec from the Sun. This under-density may be due to an ineffectiveness in the
detection of larger structures with lower surface brightness. We report on our attempts to
reveal such clusters.
Methods. We derived proper motions from a combination of Tycho-2 with
URAT1, and obtained a mean precision of about 1.4 mas/yr per co-ordinate for 1.3 million
stars north of −20° declination. We cut the sky into narrow proper motion
slices and searched for spatial over-densities of stars in each slice. We then examined
stars from over-densities in optical and near-infrared colour-magnitude diagrams to
determine if they are compatible with isochrones of a cluster. We estimated the field star
contamination using our data and the Besançon Galactic model.
Results. We detected nine hitherto unknown open clusters in the vicinity
of the Sun with ages between 70 Myr and 1 Gyr, and distances between 200 and 500 pc