493 research outputs found
Soft X-ray Absorption by High-Redshift Intergalactic Helium
The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has
been measured with HST in four quasars over the last few years, over the
redshift range 2.4 < z < 3.2. These observations have indicated that the HeII
reionization may not have been completed until z\simeq 2.8, and that large
fluctuations in the intensity of the HeII-ionizing background were present
before this epoch. The detailed history of HeII reionization at higher
redshifts is, however, model-dependent and difficult to determine from these
observations, because the IGM can be completely optically thick to Lya photons
when only a small fraction of the helium remains as HeII. In addition, finding
quasars in which the HeII Lya absorption can be observed becomes increasingly
difficult at higher redshift, owing to the large abundance of hydrogen Lyman
limit systems.
It is pointed out here that HeII in the IGM should also cause detectable
continuum absorption in the soft X-rays. The spectrum of a high-redshift source
seen behind the IGM when most of the helium was HeII should recover from the
HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic
absorption will generally be stronger, but not by a large factor; the
intergalactic HeII absorption can be detected as an excess over the expected
Galactic absorption from the 21cm HI column density. In principle, this method
allows a direct determination of the fraction of helium that was singly ionized
as a function of redshift, if the measurement is done on a large sample of
high-redshift sources over a range of redshift.Comment: accepted to The Astrophysical Journal Letter
Are we far from testing general relativity with the transiting extrasolar planet HD 209458b `Osiris'?
In this paper we investigate the possibility of measuring the general
relativistic gravitoelectric contribution P^(GE) to the orbital period P of the
transiting exoplanet HD 209458b 'Osiris'. It turns out that the predicted
magnitude of such an effect is \sim 0.1 s, while the most recent determinations
of the orbital period of HD 209458b with the photometric transit method are
accurate to \sim 0.01 s. The present analysis shows that the major limiting
factor is the \sim 1 m s^-1 sensitivity in the measurement of the projected
semiamplitude of the star's radial velocity K. Indeed, it affects the
determination of the mass m of the planet which, in turn, induces a systematic
error in the Keplerian period P^(0) of \sim 8 s. It is of crucial importance
because P^(0) should be subtracted from the measured period in order to extract
the relativistic correction. The present-day uncertainty in does not yet
make necessary the inclusion of relativistic corrections in the data-reduction
process of the determination of the system's parameters. The present situation
could change only if improvements of one-two orders of magnitude in the
ground-based Doppler spectroscopy technique occurred.Comment: LaTex2e, 11 pages, 18 references, no figures, no tables. Section 5
improved. Small corrections. To appear in New Astronom
High-cadence spectroscopy of M-dwarfs â II. Searching for stellar pulsations with HARPS
Stellar oscillations appear all across the HertzsprungâRussell diagram. Recent theoretical studies support their existence also in the atmosphere of M dwarfs. These studies predict for them short periodicities ranging from 20 min to 3 h. Our Cool Tiny Beats (CTB) programme aims at finding these oscillations for the very first time. With this goal, CTB explores the short time domain of M dwarfs using radial velocity data from the High Accuracy Radial velocity Planet Searcher (HARPS)-European Southern Observatory and HARPS-N high-precision spectrographs. Here we present the results for the two most long-term stable targets observed to date with CTB, GJ 588 and GJ 699 (i.e. Barnard's star). In the first part of this work we detail the correction of several instrumental effects. These corrections are especially relevant when searching for subnight signals. Results show no significant signals in the range where M dwarfs pulsations were predicted. However, we estimate that stellar pulsations with amplitudes larger than âŒ0.5âmâsâ1 can be detected with a 90 per cent completeness with our observations. This result, along with the excess of power regions detected in the periodograms, opens the possibility of non-resolved very low amplitude pulsation signals. Next generation more precise instrumentation would be required to detect such oscillations. However, the possibility of detecting pulsating M-dwarf stars with larger amplitudes is feasible due to the short size of the analysed sample. This motivates the need for completeness of the CTB survey
Self-shielding Effects on the Column Density Distribution of Damped Lyman Alpha Systems
We calculate the column density distribution of damped Lyman alpha systems,
modeled as spherical isothermal gaseous halos ionized by the external cosmic
background. The effects of self-shielding introduce a hump in this
distribution, at a column density N_{HI} \sim 1.6x10^{17} X^{-1} cm^{-2}, where
X is the neutral fraction at the radius where self-shielding starts being
important. The most recent compilation of the column density distribution by
Storrie-Lombardi & Wolfe shows marginal evidence for the detection of this
feature due to self-shielding, suggesting a value X \sim 10^{-3}. Assuming a
photoionization rate \Gamma \sim 10^{-12} s^{-1} from the external ionizing
background, the radius where self-shielding occurs is inferred to be about
3.8kpc. If damped Lyman alpha systems consist of a clumpy medium, this should
be interpreted as the typical size of the gas clumps in the region where they
become self-shielding. Clumps of this size with typical column densities N_H
\sim 3x10^{20} cm^{-2} would be in hydrostatic equilibrium at the
characteristic photoionization temperature \sim 10^4 K if they do not contain
dark matter. Since this size is similar to the overall radius of damped \lya
systems in Cold Dark Matter models, where all halos are assumed to contain
similar gas clouds producing damped absorbers, this suggests that the gas in
damped absorbers is in fact not highly clumped.Comment: 9 pages, 3 eps figures, references added, Fig.2 modified, the
inferred size of the clouds increases a little, accepted for publication in
ApJ Letter
Weak Lensing by Large-Scale Structure: A Dark Matter Halo Approach
Weak gravitational lensing observations probe the spectrum and evolution of
density fluctuations and the cosmological parameters which govern them but are
currently limited to small fields and subject to selection biases. We show how
the expected signal from large-scale structure arises from the contributions
from and correlations between individual halos. We determine the convergence
power spectrum as a function of the maximum halo mass and so provide the means
to interpret results from surveys that lack high mass halos either through
selection criteria or small fields. Since shot noise from rare massive halos is
mainly responsible for the sample variance below 10', our method should aid our
ability to extract cosmological information from small fields.Comment: 4 ApJ pages, 3 figures; submitted to ApJ Letter
On rapid migration and accretion within disks around supermassive black holes
Galactic nuclei should contain a cluster of stars and compact objects in the
vicinity of the central supermassive black hole due to stellar evolution, minor
mergers and gravitational dynamical friction. By analogy with protoplanetary
migration, nuclear cluster objects (NCOs) can migrate in the accretion disks
that power active galactic nuclei by exchanging angular momentum with disk gas.
Here we show that an individual NCO undergoing runaway outward migration
comparable to Type III protoplanetary migration can generate an accretion rate
corresponding to Seyfert AGN or quasar luminosities. Multiple migrating NCOs in
an AGN disk can dominate traditional viscous disk accretion and at large disk
radii, ensemble NCO migration and accretion could provide sufficient heating to
prevent the gravitational instability from consuming disk gas in star
formation. The magnitude and energy of the X-ray soft excess observed at
~0.1-1keV in Seyfert AGN could be explained by a small population of
~10^{2}-10^{3} accreting stellar mass black holes or a few ULXs. NCO migration
and accretion in AGN disks are therefore extremely important mechanisms to add
to realistic models of AGN disks.Comment: 6 pages, 2 figures, MNRAS Letters (accepted
Lack of Transit Timing Variations of OGLE-TR-111b: A re-analysis with six new epochs
We present six new transits of the exoplanet OGLE-TR-111b observed with the
Magellan Telescopes in Chile between April 2008 and March 2009. We combine
these new transits with five previously published transit epochs for this
planet between 2005 and 2006 to extend the analysis of transit timing
variations reported for this system. We derive a new planetary radius value of
1.019 +/- 0.026 R_J, which is intermediate to the previously reported radii of
1.067 +/- 0.054 R_J (Winn et al. 2007) and 0.922 +/- 0.057 R_J (Diaz et al.
2008). We also examine the transit timing variation and duration change claims
of Diaz et al. (2008). Our analysis of all eleven transit epochs does not
reveal any points with deviations larger than 2 sigma, and most points are well
within 1 sigma. Although the transit duration nominally decreases over the four
year span of the data, systematic errors in the photometry can account for this
result. Therefore, there is no compelling evidence for either a timing or a
duration variation in this system. Numerical integrations place an upper limit
of about 1 M_E on the mass of a potential second planet in a 2:1 mean-motion
resonance with OGLE-TR-111b.Comment: 28 pages, 7 tables, 6 figures. Accepted by Ap
The CRIRES Search for Planets Around the Lowest-Mass Stars. I. High-Precision Near-Infrared Radial Velocities with an Ammonia Gas Cell
Radial velocities measured from near-infrared spectra are a potentially
powerful tool to search for planets around cool stars and sub-stellar objects.
However, no technique currently exists that yields near-infrared radial
velocity precision comparable to that routinely obtained in the visible. We
describe a method for measuring high-precision relative radial velocities of
these stars from K-band spectra. The method makes use of a glass cell filled
with ammonia gas to calibrate the spectrograph response similar to the "iodine
cell" technique that has been used very successfully in the visible. Stellar
spectra are obtained through the ammonia cell and modeled as the product of a
Doppler-shifted template spectrum of the object and a spectrum of the cell,
convolved with a variable instrumental profile model. A complicating factor is
that a significant number of telluric absorption lines are present in the
spectral regions containing useful stellar and ammonia lines. The telluric
lines are modeled simultaneously as well using spectrum synthesis with a
time-resolved model of the atmosphere over the observatory. The free parameters
in the complete model are the wavelength scale of the spectrum, the
instrumental profile, adjustments to the water and methane abundances in the
atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift.
Tests of the method based on the analysis of hundreds of spectra obtained for
late M dwarfs over six months demonstrate that precisions of ~5 m/s are
obtainable over long timescales, and precisions of better than 3 m/s can be
obtained over timescales up to a week. The obtained precision is comparable to
the predicted photon-limited errors, but primarily limited over long timescales
by the imperfect modeling of the telluric lines.Comment: Accepted for publication in Ap
Cooling Flows and Metallicity Gradients in Clusters of Galaxies
The X-ray emission by hot gas at the centers of clusters of galaxies is
commonly modeled assuming the existence of steady-state, inhomogeneous cooling
flows. We derive the metallicity profiles of the intracluster medium expected
from such models. The inflowing gas is chemically enriched by type Ia
supernovae and stellar mass loss in the outer parts of the central galaxy,
which may give rise to a substantial metallicity gradient. The amplitude of the
expected metallicity enhancement towards the cluster center is proportional to
the ratio of the central galaxy luminosity to the mass inflow rate. The
metallicity of the hotter phases is expected to be higher than that of the
colder, denser phases. The metallicity profile expected for the Centaurus
cluster is in good agreement with the metallicity gradient recently inferred
from ASCA measurements (Fukazawa et al. 1994). However, current data do not
rule out alternative models where cooling is balanced by some heat source. The
metallicity gradient does not need to be present in all clusters, depending on
the recent merging history of the gas around the central cluster galaxy, and on
the ratio of the stellar mass in the central galaxy to the gas mass in the
cooling flow.Comment: uuencoded postscript, 8 pages of text + 2 figures, accepted by The
Astrophysical Journal (Letters
A Cold Front in A3667: Hydrodynamics and Magnetic Field in the Intracluster Medium
This conference presentation discusses a Chandra observation of the cold
front in Abell 3667. We first review our earlier results which include a
measurement of the front velocity, M~1, using the ratio of exterior and
interior gas pressures; observations of the hydrodynamic effects expected for a
transonic front motion (weak bow shock and gas compression near the leading
edge of the front); direct observation of the suppressed diffusion across the
front, and estimate of the magnetic field strength near the front from
suppression of the Kelvin-Helmholtz instabilities.
The new results include using the 2-dimensional brightness distribution
inside the cold front (a) to show that the front is stable and (b) to map the
mass distribution in the gas cloud. This analysis confirms the existence of a
dark matter subcluster traveling with the front. We also fix an algebraic error
in our published calculations for the growth rate of the KH instability and
discuss an additional effect which could stabilize the front against the
small-scale perturbations. These updates only strengthen our conclusions
regarding the importance of the magnetic fields for the front dynamics.Comment: Shortened version of the paper published in Astronomy Letters; based
on talk at conference "High Energy Astrophysics 2001", Moscow, Dec 200
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