EROS Experiments Research Articles

White dwarfs, red dwarfs and halo dark matter

The nature of the microlensing events observed by the MACHO team towards the LMC still remains controversial. Low-mass substellar objects and stars with masses larger than ∼ 1M⊙ have been ruled out, while stars of ∼ 0.5 M⊙ are the most probable candidates. This means that the microlenses should be either red or white dwarfs. Consequently, we assess jointly the relative contributions of both types of stars to the mass budget of the Galactic halo. We use a Monte Carlo code that incorporates up-to-date evolutionary sequences of both red dwarfs and white dwarfs as well as detailed descriptions of both our Galaxy and the LMC and we compare the synthetic populations obtained with our simulator with the results obtained by the MACHO and EROS experiments. We find that the contribution of the red dwarf population is not enough to explain the number of events measured by the MACHO team. Even though, the optical depth obtained in our simulations almost doubles that obtained when taking into account the white dwarf population alone. Finally, we also find that the contribution to the halo dark matter of the entire population under study is smaller than 10%, at the 95% confidence level.

The contribution of red dwarfs and white dwarfs to the halo dark matter

Context. The nature of the several microlensing events observed by the MACHO team towards the LMC still remains controversial. Low-mass substellar objects and stars with masses larger than ∼1 Mhave been ruled out as major components of a MACHO Galactic halo, while stars of half solar masses are the most probable candidates. Aims. We assess jointly the relative contributions of both red dwarfs and white dwarfs to the mass budget of the Galactic halo. Methods. We use a Monte Carlo simulator that incorporates up-to-date evolutionary sequences of both red dwarfs and white dwarfs as well as detailed descriptions of both our Galaxy and the LMC. We explore a complete mass range between 0.08 and 1 Mof possible microlensing candidates and we compare the synthetic populations obtained with our simulator with the results obtained by the MACHO and EROS experiments. Results. The contribution of the red-dwarf population is insufficient to explain the number of events claimed by the MACHO team; this is even though the optical depth measured for the white-dwarf population alone, increases by a factor of two, after the addition of the red-dwarf population. Conclusions. We find that the contribution to the halo dark matter of the entire population under study, is smaller than 10%, at the 95% confidence level.

Entrepreneurship as foresight: A complex social network perspective on organisational foresight

This paper addresses the relationship between foresight and entrepreneurship. It characterises the foresight inherent in entrepreneurial activity as situated within particular discourses, or communities of practice (CoPs) in a range of structures. The argument is based on the use of complexity theory (CT) to provide insight into the dynamics of entrepreneurial activity. Complexity theories of entrepreneurship are grounded in empirical studies of entrepreneurial action. A multilevel model (MM) characterising emergent structures within the entrepreneurial domain is presented as an extension of existing CT, with entrepreneurship characterised as the practice of foresight relating to that structure. The model is grounded in two case studies of entrepreneurial ventures in high velocity business environments, airline services, and sound system accessories. An empirical model—EROS—Experiments, Reflexivity, Organising Domains, and Sensitivity—is developed to reflect entrepreneurial processes at the level of the individual, the firm, and inter-firm connections—and the interactions between them. The notion of the CoP is used to deepen the analyses, examining how the layers of the MM are constituted through the foresight inherent in entrepreneurial activity over time. As well as presenting a coherent theoretical understanding of the entrepreneurial landscape, there are practical implications for policy-makers and educators wishing to improve entrepreneurial foresight.

Monte Carlo simulations of the halo white dwarf population

The interpretation of microlensing results towards the Large Magellanic Cloud (LMC) still remains controversial. While white dwarfs have been proposed to explain these results and, hence, to contribute significantly to the mass budget of our Galaxy, there are also several constraints on the role played by white dwarfs. In this paper we analyze self-consistently and simultaneously four different results, namely, the local halo white dwarf luminosity function, the microlensing results reported by the MACHO team towards the LMC, the results of Hubble Deep Field (HDF) and the results of the EROS experiment, for several initial mass functions and halo ages. We find that the proposed log-normal initial mass functions do not contribute to solve the problem posed by the observed microlensing events and, moreover, they overproduce white dwarfs when compared to the results of the HDF and of the EROS survey. We also find that the contribution of hydrogen-rich white dwarfs to the dynamical mass of the halo of the Galaxy cannot be more than ∼4%.

Microlensing searches with EROS

A review of the searches for microlensing undertaken by the EROS experiment towards the large (LMC) and the small (SMC) Magellanic clouds is presented in this paper. Although about 75 million star-years have been analyzed by EROS2 towards the LMC, and 25 million star-years towards the SMC, 8 new candidates only have been discovered. The discrepancy between the average event durations observed towards each cloud hints to a different population causing the microlensing events in each case. The low number of events is best presented as an exclusion limit on the halo mass fraction composed of dark objects. These results, combined with those from the previous phase of the EROS experiment, now exclude the entire range from 10 -7 to 20 M ⊙) from a significant contribution to a dark standard isothermal and isotropic halo.

Optimal Microlensing Observations

One of the major limitations of microlensing observations toward the Large Magellanic Cloud (LMC) is the low rate of event detection. What can be done to improve this rate? Is it better to invest telescope time in more frequent observations of the inner high surface-brightness fields, or in covering new, less populated outer fields? How would a factor 2 improvement in CCD sensitivity affect the detection efficiency? Would a series of major (factor 2--4) upgrades in telescope aperture, seeing, sky brightness, camera size, and detector efficiency increase the event rate by a huge factor, or only marginally? I develop a simplified framework to address these questions. With observational resources fixed at the level of the MACHO and EROS experiments, the biggest improvement (factor ~2) would come by reducing the time spent on the inner ~25 deg^2 and applying it to the outer ~100 deg^2. By combining this change with the characteristics of a good medium-size telescope (2.5 m mirror, point spread function, thinned CCD chips, 1 deg^2 camera, and dark sky), it should be possible to increase the detection of LMC events to more than 100 per year (assuming current estimates of the optical depth apply to the entire LMC).

Recent Results on Cepheids in the Magellanic Clouds from the EROS Survey

Between October 1996 and February 1997 a specific campaign of the EROS experiment was devoted to Cepheids in central regions of the LMC and the SMC. The Period–Luminosity (PL) relations were determined in the EROS photometric system, and a significant slope change of the period–luminosity relation has been observed for the SMC fundamental mode Cepheids with periods shorter than 2 days.

Microlensing evidence from eros

Two possible gravitational microlensing events have been detected on stars of the Large Magellanic Cloud by the EROS experiment. The time scales of the observed events imply that masses of the lensing objects lie between few hundredths and one solar mass. No candidate has been found in the analysis of data taken in 91–93 by the EROS programme using a CCD camera installed on a 40cm telescope devoted to the search of low mass compact halo objects.

A 20cm 2 CCD mosaic camera for a dark matter search part II : Electronics, data acquisition and performance

To summarize, the readout and the control system of the CCD mosaic camera are running since December 1991 at the La Silla Observatory (ESO). The overall performance of the camera has been good. About 12000 pictures (data and flat-fields) have been successfully registered up to now. We will report in the near future preliminary scientific results of the EROS experiment.