517 research outputs found
Comparing Tycho-2 Astrometry with UCAC1
The Tycho-2 Catalogue, released in February 2000, is based on the ESA
Hipparcos space mission data and various ground-based catalogs for proper
motions. An external comparison of the Tycho-2 astrometry is presented here
using the first U.S. Naval Observatory CCD Astrograph Catalog (UCAC1). The
UCAC1 data were obtained from observations performed at CTIO between February
1998 and November 1999, using the 206 mm aperture 5-element lens astrograph and
a 4k x 4k CCD. Only small systematic differences in position between Tycho-2
and UCAC1 up to 15 milliarcseconds (mas) are found, mainly as a function of
magnitude. The standard deviations of the distributions of the position
differences are in the 35 to 140 mas range, depending on magnitude. The
observed scatter in the position differences is about 30% larger than expected
from the combined formal, internal errors, also depending on magnitude. The
Tycho-2 Catalogue has the more precise positions for bright stars (V <= 10 mag)
while the UCAC1 positions are significantly better at the faint end (11 mag <=
V <= 12.5 mag) of the magnitude range in common. UCAC1 goes much fainter (to
R=16) than Tycho-2; however complete sky coverage is not expected before mid
2003.Comment: LaTeX, 8 pages, 3 PS figures, accepted by AJ (Aug 2000) see also
http://ad.usno.navy.mil/ad/ucac/ request for UCAC1 CD-ROM: e-mail to
[email protected] request for Tycho-2 CD-ROM: e-mail to
[email protected] or [email protected]
GAIA: Composition, Formation and Evolution of the Galaxy
The GAIA astrometric mission has recently been approved as one of the next
two `cornerstones' of ESA's science programme, with a launch date target of not
later than mid-2012. GAIA will provide positional and radial velocity
measurements with the accuracies needed to produce a stereoscopic and kinematic
census of about one billion stars throughout our Galaxy (and into the Local
Group), amounting to about 1 per cent of the Galactic stellar population.
GAIA's main scientific goal is to clarify the origin and history of our Galaxy,
from a quantitative census of the stellar populations. It will advance
questions such as when the stars in our Galaxy formed, when and how it was
assembled, and its distribution of dark matter. The survey aims for
completeness to V=20 mag, with accuracies of about 10 microarcsec at 15 mag.
Combined with astrophysical information for each star, provided by on-board
multi-colour photometry and (limited) spectroscopy, these data will have the
precision necessary to quantify the early formation, and subsequent dynamical,
chemical and star formation evolution of our Galaxy. Additional products
include detection and orbital classification of tens of thousands of
extra-Solar planetary systems, and a comprehensive survey of some 10^5-10^6
minor bodies in our Solar System, through galaxies in the nearby Universe, to
some 500,000 distant quasars. It will provide a number of stringent new tests
of general relativity and cosmology. The complete satellite system was
evaluated as part of a detailed technology study, including a detailed payload
design, corresponding accuracy assesments, and results from a prototype data
reduction development.Comment: Accepted by A&A: 25 pages, 8 figure
Cosmic Optical Background: the View from Pioneer 10/11
We present the new constraints on the cosmic optical background (COB)
obtained from an analysis of the Pioneer 10/11 Imaging Photopolarimeter (IPP)
data. After careful examination of data quality, the usable measurements free
from the zodiacal light are integrated into sky maps at the blue (~0.44 um) and
red (~0.64 um) bands. Accurate starlight subtraction is achieved by referring
to all-sky star catalogs and a Galactic stellar population synthesis model down
to 32.0 mag. We find that the residual light is separated into two components:
one component shows a clear correlation with thermal 100 um brightness, while
another betrays a constant level in the lowest 100 um brightness region.
Presence of the second component is significant after all the uncertainties and
possible residual light in the Galaxy are taken into account, thus it most
likely has the extragalactic origin (i.e., the COB). The derived COB brightness
is (1.8 +/- 0.9) x 10^(-9) and (1.2 +/- 0.9) x 10^(-9) erg/s/cm2/sr/A at the
blue and red band, respectively, or 7.9 +/- 4.0 and 7.7 +/- 5.8 nW/m2/sr. Based
on a comparison with the integrated brightness of galaxies, we conclude that
the bulk of the COB is comprised of normal galaxies which have already been
resolved by the current deepest observations. There seems to be little room for
contributions of other populations including "first stars" at these
wavelengths. On the other hand, the first component of the IPP residual light
represents the diffuse Galactic light (DGL) - scattered starlight by the
interstellar dust. We derive the mean DGL-to-100 um brightness ratios of 2.1 x
10^(-3) and 4.6 x 10^(-3) at the two bands, which are roughly consistent with
the previous observations toward denser dust regions. Extended red emission in
the diffuse interstellar medium is also confirmed.Comment: Accepted for publication in the Astrophysical Journal; Typos
correcte
From Hipparcos to Gaia
The measurement of the positions, distances, motions and luminosities of
stars represents the foundations of modern astronomical knowledge. Launched at
the end of the eighties, the ESA Hipparcos satellite was the first space
mission dedicated to such measurements. Hipparcos improved position accuracies
by a factor of 100 compared to typical ground-based results and provided
astrometric and photometric multi-epoch observations of 118,000 stars over the
entire sky. The impact of Hipparcos on astrophysics has been extremely valuable
and diverse. Building on this important European success, the ESA Gaia
cornerstone mission promises an even more impressive advance. Compared to
Hipparcos, it will bring a gain of a factor 50 to 100 in position accuracy and
of a factor of 10,000 in star number, collecting photometric,
spectrophotometric and spectroscopic data for one billion celestial objects.
During its 5-year flight, Gaia will measure objects repeatedly, up to a few
hundred times, providing an unprecedented database to study the variability of
all types of celestial objects. Gaia will bring outstanding contributions,
directly or indirectly, to most fields of research in astrophysics, such as the
study of our Galaxy and of its stellar constituents, the search for planets
outside the solar system.Comment: 6 pages. New Horizons in Time Domain Astronomy Proceedings IAU
Symposium No. 285, 2012, E. Griffin, B. Hanisch & R. Seaman, ed
The PPMXL catalog of positions and proper motions on the ICRS. Combining USNO-B1.0 and 2MASS
USNO-B1.0 and 2MASS are the most widely used full-sky surveys. However, 2MASS
has no proper motions at all, and USNO-B1.0 published only relative, not
absolute (i.e. on ICRS) proper motions. We performed a new determination of
mean positions and proper motions on the ICRS system by combining USNO-B1.0 and
2MASS astrometry. This catalog is called PPMXL {VO-access to the catalog is
possible via http://vo.uni-hd.de/ppmxl}, and it aims to be complete from the
brightest stars down to about full-sky. PPMXL contains about 900
million objects, some 410 million with 2MASS photometry, and is the largest
collection of ICRS proper motions at present. As representative for the ICRS we
chose PPMX. The recently released UCAC3 could not be used because we found
plate-dependent distortions in its proper motion system north of -20
declination. UCAC3 served as an intermediate system for . The resulting typical individual mean errors of the proper motions
range from 4 mas/y to more than 10 mas/y depending on observational history.
The mean errors of positions at epoch 2000.0 are 80 to 120 mas, if 2MASS
astrometry could be used, 150 to 300 mas else. We also give correction tables
to convert USNO-B1.0 observations of e.g. minor planets to the ICRS system.Comment: 9 pages, 9 figure
Searching for links between magnetic fields and stellar evolution. II. The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations
The evolution of magnetic fields in Ap stars during the main sequence phase
is presently mostly unconstrained by observation because of the difficulty of
assigning accurate ages to known field Ap stars.
We are carrying out a large survey of magnetic fields in cluster Ap stars
with the goal of obtaining a sample of these stars with well-determined ages.
In this paper we analyse the information available from the survey as it
currently stands.
We select from the available observational sample the stars that are probably
(1) cluster or association members and (2) magnetic Ap stars. For the stars in
this subsample we determine the fundamental parameters T_eff, log(L/L_o), and
M/M_o. With these data and the cluster ages we assign both absolute age and
fractional age (the fraction of the main sequence lifetime completed). For this
purpose we have derived new bolometric corrections for Ap stars.
Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the
TAMS. Statistically for the stars with M > 3 M_o the fields decline with
advancing age approximately as expected from flux conservation together with
increased stellar radius, or perhaps even faster than this rate, on a time
scale of about 3 10^7 yr. In contrast, lower mass stars show no compelling
evidence for field decrease even on a timescale of several times 10^8 yr.
Study of magnetic cluster stars is now a powerful tool for obtaining
constraints on evolution of Ap stars through the main sequence. Enlarging the
sample of known cluster magnetic stars, and obtaining more precise RMS fields,
will help to clarify the results obtained so far. Further field observations
are in progress.Comment: Accepted by Astronomy & Astrophysic
Searching for unknown open clusters in the Tycho-2 catalog
We present 11 new open cluster candidates found in a systematic search for
unknown star clusters using the astrometric and photometric data included in
the Tycho2 catalog. The possible existence of these stellar aggregates is
supported by the analysis of proper motions, color-magnitude diagrams, stellar
density distributions, and by the visual inspection of the Digitized Sky Survey
(DSS) plates. With these tools we were able to determine mean absolute proper
motions as well as preliminary reddenings, distances and ages for the majority
of the candidates. We found that most of them are possibly nearby (closer than
about 600 pc) open clusters never studied before.Comment: 14 pages, 6 figures. Accepted by A&
Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood
Debris discs - analogous to the Asteroid and Kuiper-Edgeworth belts in the
Solar system - have so far mostly been identified and studied in thermal
emission shortward of 100 um. The Herschel space observatory and the SCUBA-2
camera on the James Clerk Maxwell Telescope will allow efficient photometric
surveying at 70 to 850 um, which allow for the detection of cooler discs not
yet discovered, and the measurement of disc masses and temperatures when
combined with shorter wavelength photometry. The SCUBA-2 Unbiased Nearby Stars
(SUNS) survey and the DEBRIS Herschel Open Time Key Project are complimentary
legacy surveys observing samples of ~500 nearby stellar systems. To maximise
the legacy value of these surveys, great care has gone into the target
selection process. This paper describes the target selection process and
presents the target lists of these two surveys.Comment: 67 pages with full tables, 7 figures, accepted to MNRA
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
Kinematic Control of the Inertiality of the System of Tycho-2 and UCAC2 Stellar Proper Motions
Based on the Ogorodnikov-Milne model, we analyze the proper motions of
Tycho-2 and UCAC2 stars. We have established that the model component that
describes the rotation of all stars under consideration around the Galactic y
axis differs significantly from zero at various magnitudes. We interpret this
rotation found using the most distant stars as a residual rotation of the
ICRS/Tycho-2 system relative to the inertial reference frame. For the most
distant ( pc) Tycho-2 and UCAC2 stars, the mean rotation around
the Galactic y axis has been found to be mas yr.
The proper motions of UCAC2 stars with magnitudes in the range are
shown to be distorted appreciably by the magnitude equation in
, which has the strongest effect for northern-sky stars
with a coefficient of mas yr mag. We have detected
no significant effect of the magnitude equation in the proper motions of UCAC2
stars brighter than .Comment: 15 pages, 6 figure
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