[en] This thesis work is dedicated to the problem of the search and the characterisation of dark matter. From an observational point of view, on different length scale, from galactic to the entire Universe, it exists a disagreement between the dynamical and luminous estimation of the mass of astrophysical objects (as galaxies and cluster of galaxies). This is a sort of missing mass problem. This makes necessary, in the framework of the standard cosmological model, the introduction of a dark component of mass, dark meaning that it doesn't emit electromagnetic radiation and whose presence can be detected through its gravitational effects. Even if both theory and observation agree on the need of such a component, a still unanswered question is the nature of such mass component. We are going to address the problem of the dark matter component in galactic haloes, where the observational evidences in this sense (rotation curves) are very robust. To this end we use an original set of data (the MDM data) collected with the aim of studying the dark component in form of MACHOs (Massive Astrophysical Compact Halo Objects) in the halo of our and of the nearby galaxy of Andromeda (M31). This search is based on the gravitational microlensing effect, the deflection of light due to the presence of a massive body (the MACHO) along the line of sight between the observer and a luminous source. This effects shows as a temporal luminosity variation of the source. From this analysis it is possible to estimate the distribution of the mass in form of MACHO in the halo. This search has been carried out using the pixel lensing technique (proposed and developed by the AGAPE collaboration), that allows one to detect luminosity variations of unresolved sources. In the first chapter we recall some basics points about the cosmological model and on the problem of dark matter. We the review the microlensing effect and the pixel lensing technique. Chapter two is devoted to the description of the experimental setup, and to different points linked with the image analysis (calibration, composition). In chapter three we deal with the techniques used to analyse the signal with respect to the problem of the detection of microlensing event. The background noise being given here by variable stars, in particular we study how we can characterize the achromaticity properties of the signal that interest us. We then discuss the results from two simulations: the first one aimed at the study of the selection criteria we use to detect interesting variations, the second a Monte Carlo simulation of the experience. In chapter 4 we consider various aspects of the analysis. Dealing with the chromatic effects, we study some kind of variable sources. We carry out an extension of known microlensing candidates detected by other collaborations. Eventually, we come to the discussion of the results of our selection, where we get five light curves compatible with a microlensing signal. Looking at the simulation, we then draw our conclusion on the problem. Our analysis tends to confirm that only a small fraction of the galactic haloes is in form of MACHO, and in particular to exclude as a major component substellar objects. (author)

[en] Microlensing is the tool of choice for the search and the analysis of compact halo objects (^MACHOs⁾, a still viable class of dark matter candidates at the galactic scale. Different analyses point towards an agreement in excluding dark matter MACHOs of less than about 10^-1 M_o-dot; it remains however an ongoing debate for values in the mass range (0.1- 1) M_o-dot. The more robust constraints, though not all in agreement, come from the observational campaigns towards the Magellanic Clouds (the LMC and the SMC). The analyses towards the nearby galaxy of M31, in the so called 'pixel lensing' regime, have expanded the perspectives in this field of research. In this contribution first we draw a critical view on recent results and then we focus on the pixel lensing analysis towards M31 of the PLAN collaboration.

[en] M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (T _eff) and we measure bolometric luminosities (L _bol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T _eff and L _bol using the Stefan–Boltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of −2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.

[en] The Nancy Grace Roman Space Telescope (Roman) will perform a Galactic Exoplanet Survey (RGES) to discover bound exoplanets with semimajor axes greater than 1 au using gravitational microlensing. Roman will even be sensitive to planetary-mass objects that are not gravitationally bound to any host star. Such free-floating planetary-mass objects (FFPs) will be detected as isolated microlensing events with timescales shorter than a few days. A measurement of the abundance and mass function of FFPs is a powerful diagnostic of the formation and evolution of planetary systems, as well as the physics of the formation of isolated objects via direct collapse. We show that Roman will be sensitive to FFP lenses that have masses from that of Mars (0.1 M _⊕) to gas giants (M ≳ 100 M _⊕) as isolated lensing events with timescales from a few hours to several tens of days, respectively. We investigate the impact of the detection criteria on the survey, especially in the presence of finite-source effects for low-mass lenses. The number of detections will depend on the abundance of such FFPs as a function of mass, which is at present poorly constrained. Assuming that FFPs follow the fiducial mass function of cold, bound planets adapted from Cassan et al., we estimate that Roman will detect ∼250 FFPs with masses down to that of Mars (including ∼60 with masses ≤ M _⊕). We also predict that Roman will improve the upper limits on FFP populations by at least an order of magnitude compared to currently existing constraints.

[en] We report the mass and distance measurements of two single-lens events from the 2017 Spitzer microlensing campaign. The ground-based observations yield the detection of finite-source effects, and the microlens parallaxes are derived from the joint analysis of ground-based observations and Spitzer observations. We find that the lens of OGLE-2017-BLG-1254 is a 0.60 ± 0.03 M _⊙ star with D _LS = 0.53 ± 0.11 kpc, where D _LS is the distance between the lens and the source. The second event, OGLE-2017-BLG-1161, is subject to the known satellite parallax degeneracy, and thus is either a ${0.51}_{-<!--\; ???\; -->0.10}^{+0.12}{M}_{\odot <!--\; ???\; -->}$ star with D _LS = 0.40 ± 0.12 kpc or a ${0.38}_{-<!--\; ???\; -->0.12}^{+0.13}{M}_{\odot <!--\; ???\; -->}$ star with D _LS = 0.53 ± 0.19 kpc. Both of the lenses are therefore isolated stars in the Galactic bulge. By comparing the mass and distance distributions of the eight published Spitzer finite-source events with the expectations from a Galactic model, we find that the Spitzer sample is in agreement with the probability of finite-source effects occurring in single-lens events.

[en] We report the lens mass and distance measurements of the nearby microlensing event TCP J05074264+2447555 (Kojima-1). We measure the microlens parallax vector ${\mathit{\pi <!--\; ??\; -->}}_{\mathrm{E}}$ using Spitzer and ground-based light curves with constraints on the direction of lens-source relative proper motion derived from Very Large Telescope Interferometer (VLTI) GRAVITY observations. Combining this ${\mathit{\pi <!--\; ??\; -->}}_{\mathrm{E}}$ determination with the angular Einstein radius ${\mathrm{\theta <!--\; ??\; -->}}_{\mathrm{E}}$ measured by VLTI-GRAVITY observations, we find that the lens is a star with mass $M_{\mathrm{L}}=0.495\pm <!--\; ??\; -->0.063M_{\odot <!--\; ???\; -->}$ at a distance D _L = 429 ± 21 pc. We find that the blended light basically all comes from the lens. The lens-source proper motion is ${\mathrm{\mu <!--\; ??\; -->}}_{\mathrm{r}\mathrm{e}\mathrm{l},\mathrm{h}\mathrm{e}\mathrm{l}}=26.55\pm <!--\; ??\; -->0.36\mathrm{m}\mathrm{a}\mathrm{s}{\mathrm{y}\mathrm{r}}^{-<!--\; ???\; -->1}$, so with currently available adaptive-optics instruments, the lens and source can be resolved in 2021. This is the first microlensing event whose lens mass is unambiguously measured by interferometry + satellite-parallax observations, which opens a new window for mass measurements of isolated objects such as stellar-mass black holes.