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A. L. Melchior

Publications and source records attributed to A. L. Melchior.

11 recordsLinked to original sources
arXiv2004

Variable stars towards the bulge of M31: the AGAPE catalogue

We present the AGAPE astrometric and photometric catalogue of 1579 variable stars in a 14'x10' field centred on M31. This work is the first survey devoted to variable stars in the bulge of M31. The R magnitudes of the objects and the B-R colours suggest that our sample is dominated by red long-period variable stars (LPV), with a possible overlap with Cepheid-like type II stars. Twelve nova candidates are identified. Correlations with other catalogues suggest that 2 novae could be recurrent novae and provide possible optical counterparts for 2 supersoft X-ray sources candidates observed with Chandra.

astro-ph↗
arXiv2002

AGAPEROS: Searching for variable stars in the LMC Bar II. Temporal and near-IR analysis of Long-Period Variables

We analysed the light curves of a large sample of long period variables in the LMC from the AGAPEROS catalogue. The (non)regularity of the light change is discussed in detail showing that the majority of the light curves cannot be described properly by a single period. We show that semiregular and small amplitude variability do not necessarily correlate as has been assumed in several previous studies. Using near-infrared data from the DENIS survey we correlate the light change with colours and luminosities of the objects. These results are used to compare long period variables in the LMC with LPVs in the Galactic Bulge and in the solar neighborhood.

astro-ph↗
arXiv2001

Chemodynamical evolution of interacting galaxies

We have undertaken numerical simulations of galaxy interactions and mergers, coupling the dynamics with the star formation history and the chemical evolution. The self-gravity of stars and gas is taken into account through a tree-code algorithm, the gas hydrodynamics through SPH, and an empirical law such as a local Schmidt law is used to compute star formation. The gas and stellar metallicity is computed at each position, according to assumed yields, and the dust amount is monitored. At each step the spectra of galaxies are computed, according to simple radiative transfer and dust models. Initial conditions for these simulations will be taken from a large-scale cosmological frame-work. The aim is to build a statistically significant library of merger histories. The first results of the project will be discussed, in particular on predictions about galaxy surveys at high redshift.

astro-ph↗
arXiv2001

Hands-On TAROT: Intercontinental use of the TAROT for Education and Public Outreach

The TAROT telescope has for primary goal the search for the prompt optical counterpart of Cosmic Gamma-Ray Bursts. It is a completely autonomous 25cm telescope installed near Nice (France), able to point any location of the sky within 1-2 seconds. The control, scheduling, and data processing activities are completely automated, so the instrument is completely autonomous. In addition to its un-manned modes, we added recently the possibility to remotely control the telescope, as a request of the "Hands-On Universe" (HOU) program for exchange of time within automatic telescopes for the education and public outreach. To this purpose we developed a simple control interface. A webcam was installed to visualize the telescope. Access to the data is possible through a web interface. The images can be processed by the HOU software, a program specially suited for use within the classroom. We experienced these feature during the open days of the University of California Berkeley and the Astronomy Festival of Fleurance (France). We plan a regular use for an astronomy course of the Museum of Tokyo, as well as for French schools. Not only does Hands-On TAROT gives the general public an access to professional astronomy, but it is also a more general tool to demonstrate the use of a complex automated system, the techniques of data processing and automation. Last but not least, through the use of telescopes located in many countries over the globe, a form of powerful and genuine cooperation between teachers and children from various countries is promoted, with a clear educational goal.

astro-ph↗
arXiv2001

Hands-On Universe: A Global Program for Education and Public Outreach in Astronomy

Hands-On Universe (HOU) is an educational program that enables students to investigate the Universe while applying tools and concepts from science, math, and technology. Using the Internet, HOU participants around the world request observations from an automated telescope, download images from a large image archive, and analyze them with the aid of user-friendly image processing software. This program is developing now in many countries, including the USA, France, Germany, Sweden, Japan, Australia, and others. A network of telescopes has been established among these countries, many of them remotely operated, as shown in the accompanying demo. Using this feature, students in the classroom are able to make night observations during the day, using a telescope placed in another country. An archive of images taken on large telescopes is also accessible, as well as resources for teachers. Students are also dealing with real research projects, e.g. the search for asteroids, which resulted in the discovery of a Kuiper Belt object by high-school students. Not only Hands-On Universe gives the general public an access to professional astronomy, but it is also a more general tool to demonstrate the use of a complex automated system, the techniques of data processing and automation. Last but not least, through the use of telescopes located in many countries over the globe, a form of powerful and genuine cooperation between teachers and children from various countries is promoted, with a clear educational goal.

astro-ph↗
arXiv1998

AgapeZ1: a Large Amplification Microlensing Event or an Odd Variable Star Towards the Inner Bulge of M31

AgapeZ1 is the brightest and the shortest duration microlensing candidate event found in the Agape data. It occured only 42" from the center of M31. Our photometry shows that the half intensity duration of the event6 is 4.8 days and at maximum brightness we measure a stellar magnitude of R=18.0 with B-R=0.80 mag color. A search on HST archives produced a single resolved star within the projected event position error box. Its magnitude is R=22.

astro-ph↗
arXiv1996

AGAPE: a search for dark matter towards M31 by microlensing effects on unresolved stars

M31 is a very tempting target for a microlensing search of compact objects in galactic haloes. It is the nearest large galaxy, it probably has its own dark halo, and its tilted position with respect to the line of sight provides an unmistakable signature of microlensing. However most stars of M31 are not resolved and one has to use the ``pixel method'': monitor the pixels of the image rather than the stars. AGAPE is the implementation of this idea. Data have been collected and treated during two autumns of observation at the 2 metre telescope of Pic du Midi. The process of geometric and photometric alignment, which must be performed before constructing pixel light curves, is described. Seeing variations are minimised by working with large super-pixels (2.1) compared with the average seeing. A high level of stability of pixel fluxes, crucial to the approach, is reached. Fluctuations of super-pixels do not exceed 1.7 times the photon noise which is 0.1% of the intensity for the brightest ones. With such stable data, 10 microlensing events are expected for a full ``standard halo''. With a larger field, a regular and short time sampling and a long lever arm in time, the pixel method will be a very efficient tool to explore the halo of M31.

astro-ph↗
arXiv1996

AGAPE: a Microlensing Search for Dark Matter by Monitoring Pixels

AGAPE is an observational search of massive compact halo objects (MACHOs) in the direction of M31 by means of a novel method: the gravitational microlensing of unresolved stars. The search consists in examining CCD pixel light curves for microlensing features. The high level of temporal stability necessary to detect microlensing events has been achieved, with quiet pixels stable within a factor of two of the photon noise (the brightest ones down to a level of 0.001 mag). The data analysis is still in progress, but hundreds of variable objects (cepheids, novae, ...) have already been found. Among them there are several lightcurves that resemble microlensing events.

astro-ph↗
arXiv1992

Searching for TeV dark matter by atmospheric Cerenkov techniques

There is a growing interest in the possibility that dark matter could be formed of weakly interacting particles with a mass in the 100 GeV - 2 TeV range, and supersymmetric particles are favorite candidates. If they constitute the dark halo of our Galaxy, their mutual annihilations produce energetic gamma rays that could be detected using existing atmospheric uCerenkov techniques.

hep-ph↗