5,321 research outputs found
The QSO evolution derived from the HBQS and other complete QSO surveys
An ESO Key programme dedicated to an Homogeneous Bright QSO Survey (HBQS) has
been completed. 327 QSOs (Mb<-23, 0.3<z<2.2) have been selected over 555 deg^2
with 15<B<18.75. For B<16.4 the QSO surface density turns out to be a factor
2.2 higher than what measured by the PG survey, corresponding to a surface
density of 0.013+/-.006 deg^{-2}. If the Edinburgh QSO Survey is included, an
overdensity of a factor 2.5 is observed, corresponding to a density of
0.016+/-0.005 deg^{-2}. In order to derive the QSO optical luminosity function
(LF) we used Monte Carlo simulations that take into account of the selection
criteria, photometric errors and QSO spectral slope distribution. The LF can be
represented with a Pure Luminosity Evolution (L(z)\propto(1+z)^k) of a two
power law both for q_0=0.5 and q_0=0.1. For q_0=0.5 k=3.26, slower than the
previous Boyle's (1992) estimations of k=3.45. A flatter slope beta=-3.72 of
the bright part of the LF is also required. The observed overdensity of bright
QSOs is concentrated at z<0.6. It results that in the range 0.3<z<0.6 the
luminosity function is flatter than observed at higher redshifts. In this
redshift range, for Mb<-25, 32 QSOs are observed instead of 19 expected from
our best-fit PLE model. This feature requires a luminosity dependent luminosity
evolution in order to satisfactorily represent the data in the whole 0.3<z<2.2
interval.Comment: Invited talk in "Wide Field Spectroscopy" (20-24 May 1996, Athens),
eds. M. Kontizas et al. 6 pages and 3 eps figures, LaTex file, uses epfs.sty
and crckapb.sty (included
High-resolution O VI absorption line observations at 1.2 < z < 1.7 in the bright QSO HE 0515-4414
STIS Echelle observations at a resolution of 10 km/s and UVES/VLT
spectroscopy at a resolution of 7 km/s of the luminous QSO HE 0515-4414 (z_em =
1.73, B = 15.0) reveal four intervening O VI absorption systems in the redshift
range 1.2 < z_abs < 1.7 (1.38503, 1.41601, 1.60175, 1.67359). In addition two
associated systems at z = 1.69707 and z = 1.73585 are present. For the first
time high resolution observations allow to measure radial velocities of H I, C
IV and O VI simultaneously in several absorption systems (1.385, 1.674, 1.697)
with the result that significant velocity differences (up to 18 km/s) are
observed between H I and O VI, while smaller differences (up to 5 km/s) are
seen between C IV and O VI. We tentatively conclude that H I, O VI, and C IV
are not formed in the same volumes and that therefore implications on
ionization mechanisms are not possible from observed column density ratios O
VI/H I or O VI/C IV. The number density of O VI absorbers with W_rest > 25 mA
is dN/dz < 10, roughly a factor of 5 less than what has been found by Tripp at
al. (2000) at low redshift. An estimate of the cosmological mass-density of the
O VI-phase yields Omega_b(O VI) = 0.0003 h^{-1}_{75} for [O/H] = -1 and an
assumed ionization fraction O VI/O = 0.2. This corresponds to an increase by
roughly a factor of 15 between z = 1.5 (this work) and the value found by Tripp
et al. (2000) at z = 0.21, if the same oxygen abundance [O/H] = -1 is assumed.
Agreement with the simulations by Dave et al. (2001) can be obtained, if the
oxygen abundance increases by a factor of 3 over the same redshift interval.Comment: 8 pages, 1 figure, accepted for publication in A&
A New Version of Reimers' law of Mass Loss Based on a Physical Approach
We present a new semi-empirical relation for the mass loss of cool stellar
winds, which so far has frequently been described by "Reimers' law".
Originally, this relation was based solely on dimensional scaling arguments
without any physical interpretation. In our approach, the wind is assumed to
result from the spill-over of the extended chromosphere, possibly associated
with the action of waves, especially Alfven waves, which are used as guidance
in the derivation of the new formula. We obtain a relation akin to the original
Reimers law, but which includes two new factors. They reflect how the
chromospheric height depends on gravity and how the mechanical energy flux
depends, mainly, on effective temperature. The new relation is tested and
sensitively calibrated by modelling the blue end of the Horizontal Branch of
globular clusters. The most significant difference from mass loss rates
predicted by the Reimers relation is an increase by up to a factor of 3 for
luminous late-type (super-)giants, in good agreement with observations.Comment: 12 pages, 4 figures, accepted by ApJ Letter
On the orbital period of the magnetic Cataclysmic Variable HS 0922+1333
Context: The object HS 0922+1333 was visited briefly in 2002 in a mini survey
of low accretion rate polars (LARPs) in order to test if they undergo high
luminosity states similar to ordinary polars. On the basis of that short
observation the suspicion arose that the object might be an asynchronous polar
(Tovmassian et al. 2004). The disparity between the presumed orbital and spin
period appeared to be quite unusual. Aims: We performed follow-up observations
of the object to resolve the problem. Methods: New simultaneous spectroscopic
and photometric observations spanning several years allowed measurements of
radial velocities of emission and absorption lines from the secondary star and
brightness variations due to synchrotron emission from the primary. Results:
New observations show that the object is actually synchronous and its orbital
and spin period are equal to 4.04 hours. Conclusions: We identify the source of
confusion of previous observations to be a high velocity component of emission
line arousing from the stream of matter leaving L1 point.Comment: 5 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Static Critical Behavior of the Spin-Freezing Transition in the Geometrically Frustrated Pyrochlore Antiferromagnet Y2Mo2O7
Some frustrated pyrochlore antiferromagnets, such as Y2Mo2O7, show a
spin-freezing transition and magnetic irreversibilities below a temperature Tf
similar to what is observed nonlinear magnetization measurements on Y2Mo2O7
that provide strong evidence that there is an underlying thermodynamic phase
transition at Tf, which is characterized by critical exponents \gamma \approx
2.8 and \beta \approx 0.8. These values are typical of those found in random
spin glasses, despite the fact that the level of random disorder in Y2Mo2O7 is
immeasurably small.Comment: Latex file, calls for 4 encapsulated postscript figures (included).
Submitted to Phys. Rev. Letters
Fluctuations of the intergalactic ionization field at redshift z ~ 2
(Abridged) Aims. To probe the spectral energy distribution (SED) of the
ionizing background radiation at z ~ 2 and to specify the sources contributing
to the intergalactic radiation field. Methods. The spectrum of a bright quasar
HS1103+6416 (zem = 2.19) contains five successive metal-line absorption systems
at zabs = 1.1923, 1.7193, 1.8873, 1.8916, and 1.9410. The systems are optically
thin and reveal multiple lines of different metal ions with the ionization
potentials lying in the extreme ultraviolet (EUV) range (1 Ryd to 0.2 keV). For
each system, the EUV SED of the underlying ionization field is reconstructed by
means of a special technique developed for solving the inverse problem in
spectroscopy. For the zabs = 1.8916 system, the analysis also involves the HeI
resonance lines of the Lyman series and the HeI 504 A continuum, which are seen
for the first time in any cosmic object except the Sun. Results. From one
system to another, the SED of the ionizing continuum changes significantly,
indicating that the intergalactic ionization field at z ~ 2 fluctuates at the
scale of at least Delta_z ~ 0.004. This is consistent with Delta_z ~ 0.01
estimated from HeII and HI Lyman-alpha forest measurements between the
redshifts 2 and 3.Comment: 29 pages, 18 figures, 3 tables, accepted for publication in A\&
Star-forming regions of the Aquila rift cloud complex. I. NH3 tracers of dense molecular cores
(Abridged) Aims. In the present part of our survey we search for ammonia
emitters in the Aquila rift complex which trace the densest regions of
molecular clouds. Methods. From a CO survey carried out with the Delingha 14-m
telescope we selected ~150 targets for observations in other molecular lines.
Here we describe the mapping observations in the NH3(1,1) and (2,2) inversion
lines of the first 49 sources performed with the Effelsberg 100-m telescope.
Results. The NH3(1,1) and (2,2) emission lines are detected in 12 and 7
sources, respectively. Among the newly discovered NH3 sources, our sample
includes the following well-known clouds: the starless core L694-2, the Serpens
cloud Cluster B, the Serpens dark cloud L572, the filamentary dark cloud L673,
the isolated protostellar source B335, and the complex star-forming region
Serpens South. Angular sizes between 40" and 80" (~0.04-0.08 pc) are observed
for compact starless cores but as large as 9' (~0.5 pc) for filamentary dark
clouds. The measured kinetic temperatures of the clouds lie between 9K and 18K.
From NH3 excitation temperatures of 3-8K we determine H2 densities with typical
values of ~(0.4-4) 10^4 cm^-3. The masses of the mapped cores range between
~0.05 and ~0.5M_solar. The relative ammonia abundance, X= [NH3]/[H2], varies
from 10^-7 to 5 10^-7 with the mean = (2.7+/-0.6) 10^-7 (estimated from
spatially resolved cores assuming the filling factor eta = 1). In two clouds,
we observe kinematically split NH3 profiles separated by ~1 km/s. The splitting
is most likely due to bipolar molecular outflows for one of which we determine
an acceleration of <~ 0.03 km/s/yr. A starless core with significant rotational
energy is found to have a higher kinetic temperature than the other ones which
is probably caused by magnetic energy dissipation.Comment: 28 pages, 22 figures, 6 tables, accepted for publication in A&
A hydrodynamic study of the circumstellar envelope of alpha Scorpii
Context: Both the absolute mass-loss rates and the mechanisms that drive the
mass loss of late-type supergiants are still not well known. Binaries such as
alpha Sco provide the most detailed empirical information about the winds of
these stars.
Aims: The goal was to improve the binary technique for the determination of
the mass-loss rate of alpha Sco A by including a realistic density distribution
and velocity field from hydrodynamic and plasma simulations.
Methods: We performed 3D hydrodynamic simulations of the circumstellar
envelope of alpha Sco in combination with plasma simulations accounting for the
heating, ionization, and excitation of the wind by the radiation of alpha Sco
B. These simulations served as the basis for an examination of circumstellar
absorption lines in the spectrum of alpha Sco B as well as of emission lines
from the Antares nebula.
Results: The present model of the extended envelope of alpha Sco reproduces
some of the structures that were observed in the circumstellar absorption lines
in the spectrum of alpha Sco B. Our theoretical density and velocity
distributions of the outflow deviate considerably from a spherically expanding
model, which was used in previous studies. This results in a higher mass-loss
rate of (2 +/- 0.5) x 10^-6 M_sun/yr. The hot H II region around the secondary
star induces an additional acceleration of the wind at large distances from the
primary, which is seen in absorption lines of Ti II and Cr II at -30 km/s.Comment: 12 pages, 14 figures, accepted for publication in A&
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