11 research outputs found
Towards a fully consistent Milky Way disc model - II. The local disc model and SDSS data of the NGP region
We have used the self-consistent vertical disc models of the solar
neighbourhood presented in Just & Jahreiss (2010), which are based on different
star formation histories (SFR) and fit the local kinematics of main sequence
stars equally well, to predict star counts towards the North Galactic Pole
(NGP). We combined these four different models with the local main sequence in
the filter system of the SDSS and predicted the star counts in the NGP field
with b>80deg. All models fit the Hess diagrams in the F-K dwarf regime better
than 20 percent and the star number densities in the solar neighbourhood are
consistent with the observed values. The chi^2 analysis shows that model A is
clearly preferred with systematic deviations of a few percent only. The SFR of
model A is characterised by a maximum at an age of 10Gyr and a decline by a
factor of four to the present day value of 1.4Msun/pc^2/Gyr. The thick disc can
be modelled very well by an old isothermal simple stellar population. The
density profile can be approximated by a sech^(alpha_t) function. We found a
power law index alpha_t=1.16 and a scale height of 800pc corresponding to a
vertical velocity dispersion of 45.3km/s. About 6 percent of the stars in the
solar neighbourhood are thick disc stars.Comment: 11 pages, 12 figures, accepted by MNRAS. The qualtity of figures 10
and 12 are much better in the .ps version than the .pdf versio
Rocky Planetesimals as the Origin of Metals in DZ Stars
{Abridged}. An analysis of the calcium and hydrogen abundances, Galactic
positions and kinematics of 146 DZ stars from the Sloan Digital Sky Survey
demonstrates that interaction with the interstellar medium cannot account for
their externally polluted atmospheres. The calcium-to-hydrogen ratios for the
37 DZA stars are dominated by super-solar values, as are the lower limits for
the remaining 109 DZ stars. All together their metal-contaminated convective
envelopes contain 10^{20+-2} g of calcium, commensurate with the masses of
calcium inferred for large asteroids. It is probable that these stars are
contaminated by circumstellar matter; the rocky remains of terrestrial
planetary systems. In this picture, two predictions emerge: 1) at least 3.5% of
all main sequence A- and F-type stars build terrestrial planets; and 2) the DZA
stars are externally polluted by both metals and hydrogen, and hence constrain
the frequency and mass of water-rich, extrasolar planetesimals.Comment: Accepted to MNRA
Galactic structure from the Calar Alto Deep Imaging Survey (CADIS)
We used 1627 faint (15.5< R<23) stars in five fields of the Calar Alto Deep
Imaging Survey (CADIS) to estimate the structure parameters of the Galaxy. The
results were derived by applying two complementary methods: first by fitting
the density distribution function to the measured density of stars
perpendicular to the Galactic plane, and second by modelling the observed
colors and apparent magnitudes of the stars in the field, using Monte Carlo
simulations. The best-fitting model of the Galaxy is then determined by
minimising the C-statistic, a modified chisquared. Our model includes a double
exponential for the stellar disk with scaleheights h_1 and h_2 and a power law
halo with exponent alpha. 24480 different parameter combinations have been
simulated. Both methods yield consistent results: the best fitting parameter
combination is alpha=3.0 (or alpha=2.5, if we allow for a flattening of the
halo with an axial ratio of (c/a)=0.6), h_1=300 pc, h_2=900 pc, and the
contribution of thick disk stars to the disk stars in the solar neighbourhood
is found to be between 4 and 10%.Comment: 16 pages, 10 figures, submitted to Astronomy and Astrophysic
Candidate microlensing events from M31 observations with the Loiano telescope
Microlensing observations towards M31 are a powerful tool for the study of
the dark matter population in the form of MACHOs both in the Galaxy and the M31
halos, a still unresolved issue, as well as for the analysis of the
characteristics of the M31 luminous populations. In this work we present the
second year results of our pixel lensing campaign carried out towards M31 using
the 152 cm Cassini telescope in Loiano. We have established an automatic
pipeline for the detection and the characterisation of microlensing variations.
We have carried out a complete simulation of the experiment and evaluated the
expected signal, including an analysis of the efficiency of our pipeline. As a
result, we select 1-2 candidate microlensing events (according to different
selection criteria). This output is in agreement with the expected rate of M31
self-lensing events. However, the statistics are still too low to draw
definitive conclusions on MACHO lensing.Comment: 12 pages, 6 figures, 5 tables - Accepted for publication in The
Astrophysical Journa
Dynamical mass determination for the very low mass stars LHS 1070 B and C
We followed the relative orbit of the close pair of very late-type
stars in the nearby triple system LHS 1070 during the past seven
years. These observations are used to derive the astrometric elements
of the orbit.
Using the period of years and assuming the
distance of 7.39 pc given by van Altena ([CITE])
to be exactly correct, the combined mass of components B and C is
0.138 ± 0.003 .
Otherwise, this mass scales
with the third power of the distance, which presently is not known
to better than 9% . The dynamical mass would equal the combined mass of
derived from the theoretical mass-luminosity
relation of Baraffe et al. ([CITE]) and Chabrier et al.
([CITE]) for a 5%
larger distance of 7.78 pc. A critical comparison with the
theoretical relations therefore has to await more precise parallax
determinations for this comparatively faint system with visual
magnitude .
The wide orbit of component A around the center of mass of components
B and C cannot yet be determined from our measurements. However, we
can show that it is probably co-planar with the close pair orbit and should
have a period in the range 60-1000 years