4,580 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
The Field White Dwarf Mass Distribution
We revisit the properties and astrophysical implications of the field white
dwarf mass distribution in preparation of Gaia applications. Our study is based
on the two samples with the best established completeness and most precise
atmospheric parameters, the volume-complete survey within 20 pc and the Sloan
Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of
the observed mass distributions with Monte Carlo simulations, but find that it
is difficult to constrain independently the initial mass function (IMF), the
initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the
variation of the Galactic disk vertical scale height as a function of stellar
age, and binary evolution. Each of these input ingredients has a moderate
effect on the predicted mass distributions, and we must also take into account
biases owing to unidentified faint objects (20 pc sample), as well as unknown
masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS
sample). Nevertheless, we find that fixed standard assumptions for the above
parameters result in predicted mean masses that are in good qualitative
agreement with the observed values. It suggests that derived masses for both
studied samples are consistent with our current knowledge of stellar and
Galactic evolution. Our simulations overpredict by 40-50% the number of massive
white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a
Salpeter IMF when we account for all biases. Furthermore, we find no evidence
of a population of double white dwarf mergers in the observed mass
distributions.Comment: 15 pages, 16 figures, accepted for publication in MNRA
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
Stormy weather in 3C 196.1: nuclear outbursts and merger events shape the environment of the hybrid radio galaxy 3C 196.1
We present a multi-wavelength analysis based on archival radio, optical and
X-ray data of the complex radio source 3C 196.1, whose host is the brightest
cluster galaxy of a cluster. HST data show H+[N II] emission
aligned with the jet 8.4 GHz radio emission. An H+[N II] filament
coincides with the brightest X-ray emission, the northern hotspot. Analysis of
the X-ray and radio images reveals cavities located at galactic- and cluster-
scales. The galactic-scale cavity is almost devoid of 8.4 GHz radio emission
and the south-western H+[N II] emission is bounded (in projection) by
this cavity. The outer cavity is co-spatial with the peak of 147 MHz radio
emission, and hence we interpret this depression in X-ray surface brightness as
being caused by a buoyantly rising bubble originating from an AGN outburst
280 Myrs ago. A \textit{Chandra} snapshot observation allowed us to
constrain the physical parameters of the cluster, which has a cool core with a
low central temperature 2.8 keV, low central entropy index 13 keV
cm and a short cooling time of 500 Myr, which is of the age
of the Universe at this redshift. By fitting jumps in the X-ray density we
found Mach numbers between 1.4 and 1.6, consistent with a shock origin. We also
found compelling evidence of a past merger, indicated by a morphology
reminiscent of gas sloshing in the X-ray residual image. Finally, we computed
the pressures, enthalpies and jet powers associated with
the cavities: erg,
erg s for the inner cavity and erg,
erg s for the outer cavity.Comment: 14 pages, 4 figures, ApJ accepte
The MURALES survey II. Presentation of MUSE observations of 20 3C low-z radio galaxies and first results
We present observations of a complete sub-sample of 20 radio galaxies from
the Third Cambridge Catalog (3C) with redshift <0.3 obtained from VLT/MUSE
optical integral field spectrograph. These data have been obtained as part of
the survey MURALES (a MUse RAdio Loud Emission line Snapshot survey) with the
main goal of exploring the Active Galactic Nuclei (AGN) feedback process in a
sizeable sample of the most powerful radio sources at low redshift. We present
the data analysis and, for each source, the resulting emission line images and
the 2D gas velocity field. Thanks to their unprecedented depth (the median 3
sigma surface brightness limit in the emission line maps is 6X10^-18 erg s-1
cm-2 arcsec-2, these observations reveal emission line structures extending to
several tens of kiloparsec in most objects. In nine sources the gas velocity
shows ordered rotation, but in the other cases it is highly complex. 3C sources
show a connection between radio morphology and emission line properties.
Whereas, in three of the four Fanaroff and Riley Class I radio galaxies (FRIs),
the line emission regions are compact, ~1 kpc in size; in all but one of the
Class II radiogalaxies FRIIs, we detected large scale structures of ionized gas
with a median extent of 17 kpc. Among the FRIIs, those of high and low
excitation show extended gas structures with similar morphological properties,
suggesting that they both inhabit regions characterized by a rich gaseous
environment on kpc scale.Comment: Accepted for publication in A&
Pulse width biases the balance of excitation and inhibition recruited by transcranial magnetic stimulation
Pulse width biases the balance of excitation and inhibition recruited by transcranial magnetic stimulation
- …