190 research outputs found
Probing micro-arcsec astrometry with NACO
Relative astrometric measurements with a precision far better than 1 mas
(milli-arcsec) are commonly regarded as the domain of interferometry.
Pioneering work by Pravdo & Shaklan (1996), made in the optical, reached a
precision of 150 micro-arcsec in direct imaging but is ultimately limited by
atmospheric turbulence and differential chromatic refraction (DCR) effects.
Neuhaeuser et al. (2006, 2007) demonstrated that AO assisted observations with
NACO in a near-infrared narrow band filter allow measurements with a precision
of ~50 muas (micro-arsec) on a 0.6 arcsec binary within one hour and are
unaffected by DCR effects. This opens new possibilities for astrometric
detections of extrasolar planets and the determination of their true masses. We
discuss here how to improve the measurements and address the necessary
calibrations.Comment: To be published in the proceedings of the 2007 ESO Instrument
Calibration Worksho
Tuning gaps and phases of a two-subband system in a quantizing magnetic field
In this work we study the properties of a two-subband quasi-two-dimensional
electron system in a strong magnetic field when the electron filling factor is
equal to four. When the cyclotron energy is close to the intersubband splitting
the system can be mapped onto a four-level electron system with an effective
filling factor of two. The ground state is either a ferromagnetic state or a
spin-singlet state, depending on the values of the inter-level splitting and
Zeeman energy. The boundaries between these phases are strongly influenced by
the inter-electron interaction. A significant exchange-mediated enhancement of
the excitation gap results in the suppression of the electron-phonon
interaction. The rate of absorption of non-equilibrium phonons is calculated as
a function of Zeeman energy and inter-subband splitting. The phonon absorption
rate has two peaks as a function of intersubband splitting and has a step-like
structure as a function of Zeeman energy
A Bayesian Approach to Calibrating Period-Luminosity Relations of RR Lyrae Stars in the Mid-Infrared
A Bayesian approach to calibrating period-luminosity (PL) relations has
substantial benefits over generic least-squares fits. In particular, the
Bayesian approach takes into account the full prior distribution of the model
parameters, such as the a priori distances, and refits these parameters as part
of the process of settling on the most highly-constrained final fit.
Additionally, the Bayesian approach can naturally ingest data from multiple
wavebands and simultaneously fit the parameters of PL relations for each
waveband in a procedure that constrains the parameter posterior distributions
so as to minimize the scatter of the final fits appropriately in all wavebands.
Here we describe the generalized approach to Bayesian model fitting and then
specialize to a detailed description of applying Bayesian linear model fitting
to the mid-infrared PL relations of RR Lyrae variable stars. For this example
application we quantify the improvement afforded by using a Bayesian model fit.
We also compare distances previously predicted in our example application to
recently published parallax distances measured with the Hubble Space Telescope
and find their agreement to be a vindication of our methodology. Our intent
with this article is to spread awareness of the benefits and applicability of
this Bayesian approach and encourage future PL relation investigations to
consider employing this powerful analysis method.Comment: 6 pages, 1 figure. Accepted for publication in Astrophysics & Space
Science. Following a presentation at the conference The Fundamental Cosmic
Distance Scale: State of the Art and the Gaia Perspective, Naples, May 201
Orbital structure of the GJ876 extrasolar planetary system, based on the latest Keck and HARPS radial velocity data
We use full available array of radial velocity data, including recently
published HARPS and Keck observatory sets, to characterize the orbital
configuration of the planetary system orbiting GJ876. First, we propose and
describe in detail a fast method to fit perturbed orbital configuration, based
on the integration of the sensitivity equations inferred by the equations of
the original -body problem. Further, we find that it is unsatisfactory to
treat the available radial velocity data for GJ876 in the traditional white
noise model, because the actual noise appears autocorrelated (and demonstrates
non-white frequency spectrum). The time scale of this correlation is about a
few days, and the contribution of the correlated noise is about 2 m/s (i.e.,
similar to the level of internal errors in the Keck data). We propose a
variation of the maximum-likelihood algorithm to estimate the orbital
configuration of the system, taking into account the red noise effects. We
show, in particular, that the non-zero orbital eccentricity of the innermost
planet \emph{d}, obtained in previous studies, is likely a result of
misinterpreted red noise in the data. In addition to offsets in some orbital
parameters, the red noise also makes the fit uncertainties systematically
underestimated (while they are treated in the traditional white noise model).
Also, we show that the orbital eccentricity of the outermost planet is actually
ill-determined, although bounded by . Finally, we investigate
possible orbital non-coplanarity of the system, and limit the mutual
inclination between the planets \emph{b} and \emph{c} orbits by
, depending on the angular position of the mutual orbital
nodes.Comment: 36 pages, 11 figures, 3 tables; Accepted to Celestial Mechanics and
Dynamical Astronom
The Large Magellanic Cloud and the Distance Scale
The Magellanic Clouds, especially the Large Magellanic Cloud, are places
where multiple distance indicators can be compared with each other in a
straight-forward manner at considerable precision. We here review the distances
derived from Cepheids, Red Variables, RR Lyraes, Red Clump Stars and Eclipsing
Binaries, and show that the results from these distance indicators generally
agree to within their errors, and the distance modulus to the Large Magellanic
Cloud appears to be defined to 3% with a mean value of 18.48 mag, corresponding
to 49.7 Kpc. The utility of the Magellanic Clouds in constructing and testing
the distance scale will remain as we move into the era of Gaia.Comment: 23 pages, accepted for publication in Astrophysics and Space Science.
From a presentation at the conference The Fundamental Cosmic Distance Scale:
State of the Art and the Gaia Perspective, Naples, May 201
The PL calibration for Milky Way Cepheids and its implications for the distance scale
The rationale behind recent calibrations of the Cepheid PL relation using the
Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent
conclusions regarding a possible change in slope of the PL relation for
short-period and long-period Cepheids are tied to a pathological distribution
of HST calibrators within the instability strip. A recalibration of the
period-luminosity relation is obtained using Galactic Cepheids in open clusters
and groups, the resulting relationship, described by log L/L_sun =
2.415(+-0.035) + 1.148(+-0.044)log P, exhibiting only the moderate scatter
expected from color spread within the instability strip. The relationship is
confirmed by Cepheids with HST parallaxes, although without the need for
Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos
parallaxes, albeit with concerns about the cited precisions of the latter. A
Wesenheit formulation of Wv = -2.259(+-0.083) - 4.185(+-0.103)log P for
Galactic Cepheids is tested successfully using Cepheids in the inner regions of
the galaxy NGC 4258, confirming the independent geometrical distance
established for the galaxy from OH masers. Differences between the extinction
properties of interstellar and extragalactic dust may yet play an important
role in the further calibration of the Cepheid PL relation and its application
to the extragalactic distance scale.Comment: Accepted for Publication (Astrophysics & Space Science
Current-Density Functional Theory of the Response of Solids
The response of an extended periodic system to a homogeneous field (of
wave-vector ) cannot be obtained from a time-dependent density
functional theory (TDDFT) calculation, because the
Runge-Gross theorem does not apply. Time-dependent {\em current}-density
functional theory is needed and demonstrates that one key ingredient missing
from TDDFT is the macroscopic current. In the low-frequency limit, in certain
cases, density polarization functional theory is recovered and a formally exact
expression for the polarization functional is given.Comment: 5 pages, accepted in PR
Building the cosmic distance scale: from Hipparcos to Gaia
Hipparcos, the first ever experiment of global astrometry, was launched by
ESA in 1989 and its results published in 1997 (Perryman et al., Astron.
Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho
catalogues, ESA SP-1200, 1997). A new reduction was later performed using an
improved satellite attitude reconstruction leading to an improved accuracy for
stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys.
439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007).
The Hipparcos Catalogue provided an extended dataset of very accurate
astrometric data (positions, trigonometric parallaxes and proper motions),
enlarging by two orders of magnitude the quantity and quality of distance
determinations and luminosity calibrations. The availability of more than 20000
stars with a trigonometric parallax known to better than 10% opened the way to
a drastic revision of our 3-D knowledge of the solar neighbourhood and to a
renewal of the calibration of many distance indicators and age estimations. The
prospects opened by Gaia, the next ESA cornerstone, planned for launch in June
2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more
dramatic: a billion objects with systematic and quasi simultaneous astrometric,
spectrophotometric and spectroscopic observations, about 150 million stars with
expected distances to better than 10%, all over the Galaxy. All stellar
distance indicators, in very large numbers, will be directly measured,
providing a direct calibration of their luminosity and making possible detailed
studies of the impacts of various effects linked to chemical element
abundances, age or cluster membership. With the help of simulations of the data
expected from Gaia, obtained from the mission simulator developed by DPAC, we
will illustrate what Gaia can provide with some selected examples.Comment: 16 pages, 16 figures, Conference "The Fundamental Cosmic Distance
scale: State of the Art and the Gaia perspective, 3-6 May 2011, INAF,
Osservatorio Astronomico di Capodimonte, Naples. Accepted for publication in
Astrophysics & Space Scienc
The Fueling and Evolution of AGN: Internal and External Triggers
In this chapter, I review the fueling and evolution of active galactic nuclei
(AGN) under the influence of internal and external triggers, namely intrinsic
properties of host galaxies (morphological or Hubble type, color, presence of
bars and other non-axisymmetric features, etc) and external factors such as
environment and interactions. The most daunting challenge in fueling AGN is
arguably the angular momentum problem as even matter located at a radius of a
few hundred pc must lose more than 99.99 % of its specific angular momentum
before it is fit for consumption by a BH. I review mass accretion rates,
angular momentum requirements, the effectiveness of different fueling
mechanisms, and the growth and mass density of black BHs at different epochs. I
discuss connections between the nuclear and larger-scale properties of AGN,
both locally and at intermediate redshifts, outlining some recent results from
the GEMS and GOODS HST surveys.Comment: Invited Review Chapter to appear in LNP Volume on "AGN Physics on All
Scales", Chapter 6, in press. 40 pages, 12 figures. Typo in Eq 5 correcte
Young and Intermediate-age Distance Indicators
Distance measurements beyond geometrical and semi-geometrical methods, rely
mainly on standard candles. As the name suggests, these objects have known
luminosities by virtue of their intrinsic proprieties and play a major role in
our understanding of modern cosmology. The main caveats associated with
standard candles are their absolute calibration, contamination of the sample
from other sources and systematic uncertainties. The absolute calibration
mainly depends on their chemical composition and age. To understand the impact
of these effects on the distance scale, it is essential to develop methods
based on different sample of standard candles. Here we review the fundamental
properties of young and intermediate-age distance indicators such as Cepheids,
Mira variables and Red Clump stars and the recent developments in their
application as distance indicators.Comment: Review article, 63 pages (28 figures), Accepted for publication in
Space Science Reviews (Chapter 3 of a special collection resulting from the
May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space
Age
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