4,875 research outputs found
White Dwarf Cosmochronology in the Solar Neighborhood
The study of the stellar formation history in the solar neighborhood is a
powerful technique to recover information about the early stages and evolution
of the Milky Way. We present a new method which consists of directly probing
the formation history from the nearby stellar remnants. We rely on the volume
complete sample of white dwarfs within 20 pc, where accurate cooling ages and
masses have been determined. The well characterized initial-final mass relation
is employed in order to recover the initial masses (1 < M/Msun < 8) and total
ages for the local degenerate sample. We correct for moderate biases that are
necessary to transform our results to a global stellar formation rate, which
can be compared to similar studies based on the properties of main-sequence
stars in the solar neighborhood. Our method provides precise formation rates
for all ages except in very recent times, and the results suggest an enhanced
formation rate for the solar neighborhood in the last 5 Gyr compared to the
range 5 < Age (Gyr) < 10. Furthermore, the observed total age of ~10 Gyr for
the oldest white dwarfs in the local sample is consistent with the early
seminal studies that have determined the age of the Galactic disk from stellar
remnants. The main shortcoming of our study is the small size of the local
white dwarf sample. However, the presented technique can be applied to larger
samples in the future.Comment: 25 pages, 10 figures, accepted for publication in the Astrophysical
Journa
A Quantitative Analysis of the Available Multicolor Photometry for Rapidly Pulsating Hot B Subdwarfs
We present a quantitative and homogeneous analysis of the broadband
multicolor photometric data sets gathered so far on rapidly pulsating hot B
subdwarf stars. This concerns seven distinct data sets related to six different
stars. Our analysis is carried out within the theoretical framework developed
by Randall et al., which includes full nonadiabatic effects. The goal of this
analysis is partial mode identification, i.e., the determination of the degree
index l of each of the observed pulsation modes. We assume possible values of l
from 0 to 5 in our calculations. For each target star, we compute a specific
model atmosphere and a specific pulsation model using estimates of the
atmospheric parameters coming from time-averaged optical spectroscopy. For
every assumed value of l, we use a formal chi-squared approach to model the
observed amplitude-wavelength distribution of each mode, and we compute a
quality-of-fit Q probability to quantify the derived fit and to discriminate
objectively between the various solutions. We find that no completely
convincing and unambiguous l identification is possible on the basis of the
available data, although partial mode discrimination has been reached for 25
out of the 41 modes studied. A brief statistical study of these results
suggests that a majority of the modes must have l values of 0, 1, and 2, but
also that modes with l = 4 could very well be present while modes with l = 3
appear to be rarer. This is in line with recent results showing that l = 4
modes in rapidly pulsating B subdwarfs have a higher visibility in the optical
domain than modes with l = 3. Although somewhat disappointing in terms of mode
discrimination, our results still suggest that the full potential of multicolor
photometry for l identification in pulsating subdwarfs is within reach.Comment: 59 pages, 18 figures, accepted for publication in the Astrophysical
Journal Supplement Serie
DETERMINATION OF THE THIXOCASTING TEMPERATURES OF AZ91D AND OTHER Mg ALLOYS USING A QUENCHING METHOD
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