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J. C. Lambert

Publications and source records attributed to J. C. Lambert.

6 recordsLinked to original sources
arXiv2026

Simulation tools for realistic high-order wavefront correction with the Roman Coronagraph

The Roman Space Telescope Coronagraph Instrument (Roman CGI) will demonstrate high-contrast imaging from space using coronagraphic masks, deformable mirrors, and high-order wavefront sensing and control (HOWFSC). After the baseline technology demonstration, the Roman Coronagraph Community Participation Program (CPP) will pursue science and engineering studies that require realistic predictions of instrument behaviour, observing efficiency, and wavefront control performance. This paper describes \texttt{corgihowfsc}, a configurable simulation framework for repeatable Roman CGI HOWFSC studies. The framework preserves the Roman ground-in-the-loop (GITL) workflow represented by the NASA \texttt{cgi-howfsc} package, while allowing the images to be generated by the higher-fidelity \texttt{corgisim} model. In this configuration, \texttt{cgi-howfsc} remains the reference implementation for estimation, control, and compact-model Jacobian generation; \texttt{corgisim} can supply more flight-like images for studies of instrument performance and robustness. \texttt{corgihowfsc} also coordinates exposure planning, camera settings, expected iteration timing, and contrast normalisation of the HOWFSC loop through \texttt{cgi-eetc}, calibration-related workflows through \texttt{cgi-coralign}, structured diagnostics, and local or distributed execution. By exposing observing modes, image models, probe choices, estimators, controllers, deformable-mirror settings, and runtime options through reusable configuration files, \texttt{corgihowfsc} enables controlled comparisons between reference compact-model simulations and higher-fidelity HOWFSC studies.

astro-ph.IM↗
arXiv2020

K-Stacker, an algorithm to hack the orbital parameters of planets hidden in high-contrast imaging. First applications to VLT SPHERE multi-epoch observations

Recent high-contrast imaging surveys, looking for planets in young, nearby systems showed evidence of a small number of giant planets at relatively large separation beyond typically 20 au where those surveys are the most sensitive. Access to smaller physical separations between 5 and 20 au is the next step for future planet imagers on 10 m telescopes and ELTs in order to bridge the gap with indirect techniques (radial velocity, transit, astrometry with Gaia). In that context, we recently proposed a new algorithm, Keplerian-Stacker, combining multiple observations acquired at different epochs and taking into account the orbital motion of a potential planet present in the images to boost the ultimate detection limit. We showed that this algorithm is able to find planets in time series of simulated images of SPHERE even when a planet remains undetected at one epoch. Here, we validate the K-Stacker algorithm performances on real SPHERE datasets, to demonstrate its resilience to instrumental speckles and the gain offered in terms of true detection. This will motivate future dedicated multi-epoch observation campaigns in high-contrast imaging to search for planets in emitted and reflected light. Results. We show that K-Stacker achieves high success rate when the SNR of the planet in the stacked image reaches 7. The improvement of the SNR ratio goes as the square root of the total exposure time. During the blind test and the redetection of HD 95086 b, and betaPic b, we highlight the ability of K-Stacker to find orbital solutions consistent with the ones derived by the state of the art MCMC orbital fitting techniques, confirming that in addition to the detection gain, K-Stacker offers the opportunity to characterize the most probable orbital solutions of the exoplanets recovered at low signal to noise.

astro-ph.EP↗
arXiv2016

Forming disk galaxies in wet major mergers. I. Three fiducial examples

Using three fiducial Nbody+SPH simulations, we follow the merging of two disk galaxies with a hot gaseous halo component each, and examine whether the merger remnant can be a spiral galaxy. The stellar progenitor disks are destroyed by violent relaxation during the merging and most of their stars form a classical bulge, while the remaining form a thick disk and its bar. A new stellar disk forms subsequently and gradually in the remnant from the gas accreted mainly from the halo. It is vertically thin and well extended in its equatorial plane. A bar starts forming before the disk is fully in place, contrary to what is assumed in idealised simulations of isolated bar-forming galaxies. It has morphological features such as ansae and boxy/peanut bulges. Stars of different ages populate different parts of the box/peanut. A disky pseudobulge forms also, so that by the end of the simulation, all three types of bulges coexist. The oldest stars are found in the classical bulge, followed by those of the thick disk, then by those in the thin disk. The youngest stars are in the spiral arms and the disky pseudobulge. The disk surface density profiles are of type II (exponential with downbending), and the circular velocity curves are flat and show that the disks are submaximum in these examples: two clearly so and one near-borderline between maximum and submaximum. On average, only roughly between 10 and 20% of the stellar mass is in the classical bulge of the final models, i.e. much less than in previous simulations.

astro-ph.GA↗
arXiv2005

Can bars be destroyed by a central mass concentration? I. Simulations

We study the effect of a central mass concentration (CMC) on the secular evolution of a barred disc galaxy. Unlike previous studies, we use fully self-consistent 3D $N$-body simulations with live haloes, which are known to be important for bar evolution. The CMC is introduced gradually, to avoid transients. In all cases where the mass of the CMC is of the order of, or more than, a few per cent of the mass of the disc, the strength of the bar decreases noticeably. The amount of this decrease depends strongly on the bar type. For the same CMC, bars with exponential surface-density profile, which formed in a disk-dominated galaxy (MD-type bars), can be totally destroyed, while strong bars with a flat surface-density profile, whose evolution is largely due to the halo (MH-type bars), witness only a decrease of their strength. This decrease occurs simultaneously from both the innermost and outermost parts of the bar. The CMC has a stronger effect on the Fourier components of higher azimuthal wave number $m$, leading to fatter and/or less rectangular bars. Furthermore, the CMC changes the side-on outline from peanut-shaped to boxy or, for massive CMCs, to elliptical. Similarly, side-on initially boxy outlines can be destroyed. The CMC also influences the velocity dispersion profiles. Most of the decrease of the bar strength occurs while the mass of the CMC increases and it is accompanied by an increase of the pattern speed. In all our simulations, the mass of the CMC necessary in order to destroy the bar is at least several per cent of the mass of the disc. This argues that observed super-massive black holes are not likely to destroy pre-existing bars.

astro-ph↗
arXiv2001

Relaxation times calculated from angular deflections

In this paper we measure the two-body relaxation time from the angular deflection of test particles launched in a rigid configuration of field particles. We find that centrally concentrated configurations have relaxation times that can be shorter than those of the corresponding homogeneous distributions by an order of magnitude or more. For homogeneous distributions we confirm that the relaxation time is proportional to the number of particles. On the other hand centrally concentrated configurations have a much shallower dependence, particularly for small values of the softening. The relaxation time increases with the inter-particle velocities and with softening. The latter dependence is not very strong, of the order of a factor of two when the softening is increased by an order of magnitude. Finally we show that relaxation times are the same on GRAPE-3 and GRAPE-4, dedicated computer boards with limited and high precision respectively.

astro-ph↗