Very deep images of the Galactic globular cluster M4 (NGC 6121) through the F606W and F814W filters were taken in 2001 with the WFPC2 on board the HST. A first published analysis of this data set ...(Richer et al. 2002) produced the result that the age of M4 is 12.7 +/- 0.7 Gyr (Hansen et al. 2002), thus setting a robust lower limit to the age of the universe. In view of the great astronomical importance of getting this number right, we have subjected the same data set to the simplest possible photometric analysis that completely avoids uncertain assumptions about the origin of the detected sources. This analysis clearly reveals both a thin main sequence, from which can be deduced the deepest statistically complete mass function yet determined for a globular cluster, and a white dwarf (WD) sequence extending all the way down to the 5 sigma detection limit at I~27. The WD sequence is abruptly terminated at exactly this limit as expected by detection statistics. Using our most recent theoretical WD models (Prada Moroni & Straniero 2002) to obtain the expected WD sequence for different ages in the observed bandpasses, we find that the data so far obtained do not reach the peak of the WD luminosity function, thence only allowing one to set a lower limit to the age of M4 of ~9 Gyr. Thus, the problem of determining the absolute age of a globular cluster and, therefore, the onset of GC formation with cosmologically significant accuracy remains completely open. Only observations several magnitudes deeper than the limit obtained so far would allow one to approach this objective.
An investigation of the stability of the transfer function of the European Southern Observatory's Very Large Telescope Interferometer has been carried out through observations of Fomalhaut over a ...wide range in hour angle. No significant variation in the transfer function was found for the zenith angle range 5-70 degrees. The projected baseline varied between 139.7 m and 49.8 m during the observations and, as an integral part of the determination of the transfer function, a new accurate limb-darkened angular diameter for Fomalhaut has been established. This has led to improved values for the emergent flux, effective temperature, radius and luminosity.
We present the first mid-IR long baseline interferometric observations of the circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD 52961, were observed using the VLTI/MIDI ...instrument during Science Demonstration Time. Both objects are known binaries for which a stable circumbinary disc is proposed to explain the SED characteristics. This is corroborated by our N-band spectrum showing a crystallinity fraction of more than 50 % for both objects, pointing to a stable environment where dust processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved in the interferometric observations providing an upper limit of 11 mas (or 18 AU at the distance of this object) on the diameter of the dust emission. This confirms the very compact nature of its circumstellar environment. The dust emission around HD 52961 originates from a very small but resolved region, estimated to be ~ 35 mas at 8 micron and ~ 55 mas at 13 micron. These results confirm the disc interpretation of the SED of both stars. In HD 52961, the dust is not homogeneous in its chemical composition: the crystallinity is clearly concentrated in the hotter inner region. Whether this is a result of the formation process of the disc, or due to annealing during the long storage time in the disc is not clear.
We have carried out extensive VLT-FORS1 observations covering a fair fraction of the intermediate metallicity globular cluster NGC 6712 in the V and R bands. We derive accurate colour-magnitude ...diagrammes (CMD) and luminosity functions (LFs) of the cluster main sequence (MS) for four overlapping fields extending from the centre of the cluster out to a radius of ~10', well beyond the nominal tidal radius, and for a control field at ~42' distance. The LFs extend from the cluster turn-off (TO) at M_R 4 to the point at which the incompleteness drops below 50% (corresponding to R 23 or M_R 7.5) for most fields studied. Cluster stars become indistinguishable from field stars at r 5'. The shape of the cluster's LF and its variation with distance from the centre in these ranges are well described by a standard multi-mass static model having the following parameters: core radius r_c=1', half-light radius r_hl=1.8', tidal radius r_t=5.2', concentration ratio c=0.7, and a power-law global mass function (MF) with index alpha 0.9 for masses smaller than 0.8 Msolar, i.e. for all detected MS stars, and alpha -2.35 for evolved objects. The MF obtained in this way is consistent with that found in a preliminary investigation of this cluster with the VLT Test Camera and confirms that this is the only globular cluster known so far for which the global MF drops with decreasing mass below the TO. Possible reasons for this unique characteristic are discussed with the most likely associated with its extreme vulnerability to tidal disruption.
We report here the first observations of a rapidly rotating Be star, \(\alpha\) Eridani, using Earth-rotation synthesis on the Very Large Telescope (VLT) Interferometer. Our measures correspond to a ...\(2a/2b = 1.56\pm0.05\) apparent oblate star, \(2a\) and \(2b\) being the equivalent uniform disc angular diameters in the equatorial and polar direction. Considering the presence of a circumstellar envelope (CSE) we argue that our measurement corresponds to a truly distorted star since \(\alpha\) Eridani exhibited negligible H\(\alpha\) emission during the interferometric observations. In this framework we conclude that the commonly adopted Roche approximation (uniform rotation and centrally condensed mass) should not apply to \(\alpha\) Eridani. This result opens new perspectives to basic astrophysical problems, such as rotationally enhanced mass loss and internal angular momentum distribution. In addition to its intimate relation with magnetism and pulsation, rapid rotation thus provides a key to the Be phenomenon: one of the outstanding non-resolved problems in stellar physics.
The VLT Interferometer Glindemann, Andreas; Abuter, Roberto; Carbognani, Franco ...
Comptes rendus de l'Académie des sciences. Série IV, Physique, astrophysique,
2001, 2001-1-00, Volume:
2, Issue:
1
Journal Article
The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion this year, when the fourth of the 8-m Unit Telescopes will see first light. At the ...same time, the preparation for first fringes of the VLT Interferometer (VLTI) is advancing rapidly with the goal of having the first fringes with two siderostats in the first quarter of 2001. In this article we describe the status of the VLTI and its subsystems, we discuss the planning for first fringes with the different telescopes and instruments. In conclusion, we present an outlook for the future of interferometry with Very Large Telescopes.
La construction de l'Observatoire du Very Large Telescope (VLT) sur le Cerro Paranal (
2635 m) au Nord du Chili se terminera cette année avec la premiere lumiere du quatrième des grands telescopes de
8 m. Simultanément, la préparation des premières franges de l'interféromètre du VLT (VLTI) avance rapidement, avec pour objectif l'obtention des premières franges avec deux sidérostats à l'entrée du nouveau millénaire. Dans cet article, nous décrirons l'état d'avancement du VLTI et de ses sous-systèmes et le calendrier des premières franges avec les différents télescopes et instruments. Enfin, nous présenterons un panorama de l'avenir de l'interférométrie avec les Très Grands Télescopes.
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