456 research outputs found
A non exhaustive bibliography on gravitational lensing
The authors present a non exhaustive bibliography on "gravitational lensing", totalizing more than 1000 titles. Books (related to) and conference proceedings fully devoted to "gravitational lensing" are listed separately
Photometric monitoring of the doubly imaged quasar UM673: possible evidence for chromatic microlensing
We present the results of two-band CCD photometric monitoring of the
gravitationally lensed quasar Q 0142-100 (UM 673).The data, obtained at ESO-La
Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998 -
September 1999) and in the Johnson V-band (October 1998 to December 2001), were
analyzed using three different photometric methods. The light-curves obtained
with all methods show variations, with a peak-to-peak amplitude of 0.14
magnitude in . Although it was not possible to measure the time delay
between the two lensed QSO images, the brighter component displays possible
evidence for microlensing: it becomes bluer as it gets brighter, as expected
under the assumption of differential magnification of a quasar accretion diskComment: Accepted for publication in Astronomy & Astrophysics; 8 pages, 7
figure
Coronagraphic imaging of three weak-line T Tauri stars: evidence of planetary formation around PDS 70
Context.High angular resolution imaging of nearby pre-main sequence stars with ages between 1 and 30 Myr can give valuable information on planet formation mechanisms. This range of ages is thought to correspond to the dissipation of the optically thick dust disks surrounding young stars and to the end of the planet formation.
Aims.This paper presents new observations of three weak-line T Tauri Stars (WTTS) of intermediate ages ranging from 7 to 16 Myr. It aims at increasing the knowledge and sample of circumstellar disks around "old" WTTS.
Methods.We observed three stars with the VLT's NAOS-CONICA adaptive optics system in coronagraphic mode. The four-quadrant phase mask coronagraph was used to improve the dynamic range (by a factor of ~100) while preserving the high angular resolution (inner working angle of 0".15).
Results.One object of our sample (PDS 70), a K5 star, exhibits a brown dwarf companion and a disk in scattered light with a surface brightness power law of r^-2.8, extending from a distance of 14 to 140 AU (assuming a stellar distance of 140 pc) and an integrated luminosity of 16.7 mJy in the K_s-band. The mass of the companion can be estimated to be within a range between 27 and 50 Jupiter masses with an effective temperature of 2750 ± 100K. This object also shows a resolved outflow stretching up to ~550 AU.
Conclusions.This newly detected circumstellar disk shows strong similarities with the disk around TW Hya, and adds to the observed population of "old" TTS surrounded by circumstellar material. Moreover, three clues of planetary formation are brought to light by this study
The Optimal Gravitational Lens Telescope
Given an observed gravitational lens mirage produced by a foreground
deflector (cf. galaxy, quasar, cluster,...), it is possible via numerical lens
inversion to retrieve the real source image, taking full advantage of the
magnifying power of the cosmic lens. This has been achieved in the past for
several remarkable gravitational lens systems. Instead, we propose here to
invert an observed multiply imaged source directly at the telescope using an
ad-hoc optical instrument which is described in the present paper. Compared to
the previous method, this should allow one to detect fainter source features as
well as to use such an optimal gravitational lens telescope to explore even
fainter objects located behind and near the lens. Laboratory and numerical
experiments illustrate this new approach
Low-rank plus sparse decomposition for exoplanet detection in direct-imaging ADI sequences. The LLSG algorithm
Context. Data processing constitutes a critical component of high-contrast exoplanet imaging. Its role is almost as important as the choice of a coronagraph or a wavefront control system, and it is intertwined with the chosen observing strategy. Among the data processing techniques for angular differential imaging (ADI), the most recent is the family of principal component analysis (PCA) based algorithms. It is a widely used statistical tool developed during the first half of the past century. PCA serves, in this case, as a subspace projection technique for constructing a reference point spread function (PSF) that can be subtracted from the science data for boosting the detectability of potential companions present in the data. Unfortunately, when building this reference PSF from the science data itself, PCA comes with certain limitations such as the sensitivity of the lower dimensional orthogonal subspace to non-Gaussian noise.
Aims. Inspired by recent advances in machine learning algorithms such as robust PCA, we aim to propose a localized subspace projection technique that surpasses current PCA-based post-processing algorithms in terms of the detectability of companions at near real-time speed, a quality that will be useful for future direct imaging surveys.
Methods. We used randomized low-rank approximation methods recently proposed in the machine learning literature, coupled with entry-wise thresholding to decompose an ADI image sequence locally into low-rank, sparse, and Gaussian noise components (LLSG). This local three-term decomposition separates the starlight and the associated speckle noise from the planetary signal, which mostly remains in the sparse term. We tested the performance of our new algorithm on a long ADI sequence obtained on β Pictoris with VLT/NACO.
Results. Compared to a standard PCA approach, LLSG decomposition reaches a higher signal-to-noise ratio and has an overall better performance in the receiver operating characteristic space. This three-term decomposition brings a detectability boost compared to the full-frame standard PCA approach, especially in the small inner working angle region where complex speckle noise prevents PCA from discerning true companions from noise
Small Scale Structure at High Redshift: II. Physical Properties of the CIV Absorbing Clouds
Keck HIRES spectra were obtained of the separate images of three
gravitationally lensed QSOs (UM 673, Q1104-1804, and Q1422+2309). We studied
the velocity and column density differences in CIV doublets in each QSO. Unlike
the low ionization gas clouds typical of the interstellar gas in the Galaxy or
damped Ly alpha galaxies, the spatial density distribution of CIV absorbing gas
clouds turns out to be mostly featureless on scales up to a few hundred
parsecs, with column density differences rising to 50 percent or more over
separations beyond a few kpc. Similarly, velocity shear becomes detectable only
over distances larger than a few hundred pc, rising to 70 km/s at a few kpc.
The energy transmitted to the gas is substantially less than in present day
star-forming regions, and the gas is less turbulent on a given spatial scale
than, e.g., local HII regions. The quiescence of CIV clouds, taken with their
probable low density, imply that these objects are not internal to galaxies.
The CIV absorbers could be gas expelled recently to large radii and raining
back onto its parent galaxy, or pre-enriched gas from an earlier (population
III) episode of star formation, falling into the nearest mass concentration.
However, while the metals in the gas may have been formed at higher redshifts,
the residual turbulence in the clouds and the minimum coherence length measured
here imply that the gas was stirred more recently, possibly by star formation
events recurring on a timescale on the order of 10-100 Million years (abstract
abbreviated).Comment: latex file plus 15 postscript figures (45 pages in total); to be
published in the ApJ, June 20, 2001 issu
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