117 research outputs found
Trumpler 20 - an old and rich open cluster
We show that the open cluster Trumpler 20, contrary to the earlier findings, is actually an old Galactic open cluster. New CCD photometry and high-resolution spectroscopy are used to derive the main parameters of this cluster. At [Fe/H]=â0.11 for a single red giant star, the metallicity is slightly subsolar. The best fit to the colour-magnitude diagrams is achieved using a 1.3-Gyr isochrone with convective overshoot. The cluster appears to have a significant reddening at E(BâV) = 0.46 (for B0 spectral type), although for red giants this high reddening yields the colour temperature exceeding the spectroscopic Teff by about 200 K. Trumpler 20 is a very rich open cluster, containing at least 700 members brighter than MV=+4. It may extend over the field of view available in our study at 20 Ă 20 arcmin
Trumpler 20 - an old and rich open cluster
We show that the open cluster Trumpler 20, contrary to the earlier findings,
is actually an old Galactic open cluster. New CCD photometry and
high-resolution spectroscopy are used to derive the main parameters of this
cluster. At [Fe/H]=-0.11 for a single red giant star, the metallicity is
slightly subsolar. The best fit to the color-magnitude diagrams is achieved
using a 1.3 Gyr isochrone with convective overshoot. The cluster appears to
have a significant reddening at E(B-V)=0.46 (for B0 spectral type), although
for red giants this high reddening yields the color temperature exceeding the
spectroscopic T_eff by about 200 K. Trumpler 20 is a very rich open cluster,
containing at least 700 members brighter than M_V=+4. It may extend over the
field-of-view available in our study at 20'x20'.Comment: 7 pages, 5 figures; accepted for publication in MNRA
Astrometric Control of the Inertiality of the Hipparcos Catalog
Based on the most complete list of the results of an individual comparison of
the proper motions for stars of various programs common to the Hipparcos
catalog, each of which is an independent realization of the inertial reference
frame with regard to stellar proper motions, we redetermined the vector
of residual rotation of the ICRS system relative to the extragalactic
reference frame. The equatorial components of this vector were found to be the
following: mas yr,
mas yr, and mas yr.Comment: 8 pages, 1 figur
High-precision astrometry with VVV. I. An independent reduction pipeline for VIRCAM@VISTA
We present a new reduction pipeline for the VIRCAM@VISTA detector and
describe the method developed to obtain high-precision astrometry with the
VISTA Variables in the V\'ia L\'actea (VVV) data set. We derive an accurate
geometric-distortion correction using as calibration field the globular cluster
NGC 5139, and showed that we are able to reach a relative astrometric precision
of about 8 mas per coordinate per exposure for well-measured stars over a field
of view of more than 1 square degree. This geometric-distortion correction is
made available to the community. As a test bed, we chose a field centered
around the globular cluster NGC 6656 from the VVV archive and computed proper
motions for the stars within. With 45 epochs spread over four years, we show
that we are able to achieve a precision of 1.4 mas/yr and to isolate each
population observed in the field (cluster, Bulge and Disk) using proper
motions. We used proper-motion-selected field stars to measure the motion
difference between Galactic disk and bulge stars. Our proper-motion
measurements are consistent with UCAC4 and PPMXL, though our errors are much
smaller. Models have still difficulties in reproducing the observations in this
highly-reddened Galactic regions.Comment: 11 pages, 10 figures (some in low res), 1 table. Accepted for
publication in MNRAS on March 25, 2015. The FORTRAN routine will be soon made
available at http://groups.dfa.unipd.it/ESPG/ , and via email request to the
first autho
How many Hipparcos Variability-Induced Movers are genuine binaries?
Hipparcos observations of some variable stars, and especially of long-period
(e.g. Mira) variables, reveal a motion of the photocenter correlated with the
brightness variation ({variability-induced mover -- VIM), suggesting the
presence of a binary companion. A re-analysis of the Hipparcos photometric and
astrometric data does not confirm the VIM solution for 62 among the 288 VIM
objects (21%) in the Hipparcos catalogue. Most of these 288 VIMs are
long-period (e.g. Mira) variables (LPV). The effect of a revised chromaticity
correction, which accounts for the color variations along the light cycle, was
then investigated. It is based on `instantaneous' color indices derived
from Hipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged as VIM
in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM after this
improved chromaticity correction is applied. This dramatic decrease in the
number of VIM solutions is not surprising, since the chromaticity correction
applied by the Hipparcos reduction consortia was based on a fixed color.
Astrophysical considerations lead us to adopt a more stringent criterion for
accepting a VIM solution (first-kind risk of 0.27% instead of 10% as in the
Hipparcos catalogue). With this more severe criterion, only 27 LPV stars remain
VIM, thus rejecting 161 of the 188 (86%) of the LPVs defined as VIMs in the
Hipparcos catalogue.Comment: 9 pages, 4 figures, accepted in A&
The investigation of absolute proper motions of the XPM Catalogue
The XPM-1.0 is the regular version of the XPM catalogue. In comparison with
XPM the astrometric catalogue of about 280 millions stars covering entire sky
from -90 to +90 degrees in declination and in the magnitude range 10^m<B<22^m
is something improved. The general procedure steps were followed as for XPM,
but some of them are now performed on a more sophisticated level. The XPM-1.0
catalogue contains star positions, proper motions, 2MASS and USNO photometry of
about 280 millions of the sources. We present some investigations of the
absolute proper motions of XPM-1.0 catalogue and also the important information
for the users of the catalogue. Unlike previous version, the XPM-1.0 contains
the proper motions over the whole sky without gaps. In the fields, which cover
the zone of avoidance or which contain less than of 25 galaxies a quasi
absolute calibration was performed. The proper motion errors are varying from 3
to 10 mas/yr, depending on a specific field. The zero-point of the absolute
proper motion frame (the absolute calibration) was specified with more than 1
million galaxies from 2MASS and USNO-A2.0. The mean formal error of absolute
calibration is less than 1 mas/yr.Comment: 11 pages, 9 figures, accepte
Chromosome maps of young LMC clusters: An additional case of coeval multiple populations
Recent studies have revealed that the Multiple Populations (MPs) phenomenon
does not occur only in ancient and massive Galactic globular clusters (GCs),
but it is also observed in external galaxies, where GCs sample a wide age range
with respect to the Milky Way. However, for a long time, it was unclear whether
we were looking at the same phenomenon in different environments or not. The
first evidence that the MPs phenomenon is the same regardless of cluster age
and host galaxy came out recently when an intermediate-age cluster from the
Small Magellanic Cloud, Lindsay 1, and a Galactic GC have been directly
compared. By complementing those data with new images from the Hubble Space
Telescope (HST), we extend the comparison to two clusters of different ages:
NGC 2121 (2.5Gyr) and NGC 1783 (1.5Gyr), from the Large Magellanic
Cloud. We find a clear correlation between the RGB width in the pseudo-colour
and the age of the cluster itself, with the older
cluster having larger and vice-versa.
Unfortunately, the values cannot be directly linked to the N-abundance
variations within the clusters before properly taking account the effect of the
first dredge-up. Such HST data also allow us to explore whether multiple
star-formation episodes occurred within NGC 2121. The two populations are
indistinguishable, with an age difference of only 612 Myr and an initial
Helium spread of 0.02 or lower. This confirms our previous results, putting
serious constraints on any model proposed to explain the origin of the chemical
anomalies in GCs.Comment: 12 pages, 7 figures, 1 table. Accepted for publication in MNRA
The Distance to the Hyades Cluster Based on Hubble Space Telescope Fine Guidance Sensor Parallaxes
Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) 3 of seven Hyades members in six ïŹelds of view have been analyzed along with their proper motions to determine the distance to the cluster. Knowledge of the convergent point and mean proper motion of the Hyades is critical to the derivation of the distance to the center of the cluster. Depending on the choice of the proper-motion system, the derived cluster center distance varies by 9%. Adopting a reference distance of 46.1 pc or m ÏȘ M Ï 3.32, which is derived from the ground-based parallaxes in the General Catalogue of Trigonometric Stellar Parallaxes (1995 edition), the FK5/PPM proper-motion system yields a distance 4% larger, while the Hanson system yields a distance 2% smaller. The HST FGS parallaxes reported here yield either a 14% or 5% larger distance, depending on the choice of the proper-motion system. Orbital parallaxes (Torres et al.) yield an average distance 4% larger than the reference distance. The variation in the distance derived from the HST data illustrates the importance of the proper-motion system and the individual proper motions to the derivation of the distance to the Hyades center; therefore, a full utilization of the HST FGS parallaxes awaits the establishment of an accurate and consistent proper-motion system
Chromosome maps of young LMC clusters: An additional case of coeval multiple populations
Recent studies have revealed that the Multiple Populations (MPs) phenomenon does not occur only in ancient and massive Galactic globular clusters (GCs), but it is also observed in external galaxies, where GCs sample a wide age range with respect to the Milky Way. However, for a long time, it was unclear whether we were looking at the same phenomenon in different environments or not. The first evidence that the MPs phenomenon is the same regardless of cluster age and host galaxy came out recently when an intermediate-age cluster from the Small Magellanic Cloud, Lindsay 1, and a Galactic GC have been directly compared. By complementing those data with new images from the Hubble Space Telescope (HST), we extend the comparison to two clusters of different ages: NGC 2121 (2.5Gyr) and NGC 1783 (1.5Gyr), from the Large Magellanic Cloud. We find a clear correlation between the RGB width in the pseudo-colour and the age of the cluster itself, with the older cluster having larger and vice-versa. Unfortunately, the values cannot be directly linked to the N-abundance variations within the clusters before properly taking account the effect of the first dredge-up. Such HST data also allow us to explore whether multiple star-formation episodes occurred within NGC 2121. The two populations are indistinguishable, with an age difference of only 612 Myr and an initial Helium spread of 0.02 or lower. This confirms our previous results, putting serious constraints on any model proposed to explain the origin of the chemical anomalies in GCs
- âŠ