The Very Large Telescope (VLT) of the ESO Observatory on Cerro Paranal (2635 m) in Northern Chile, is composed of four large telescopes, that are meant to be used in an interferometric mode, the VLTI.
On March 17, 2001, the VLT Interferometer observed its first fringes on a star, using two siderostats and the test camera VINCI, built by the Paris-Meudon Observatory. Details on the experiment and about interferometry in general can be found in the references below. Click on the photo, to enlarge its size.
Application to Cepheids
Cepheids are variable supergiant stars : they undergo radial pulsations, and their radius and luminosity oscillate with a period of the order of 5 to 60 days. They are known to obey a tight relation P(L) between their absolute luminosity L and their oscillation period P. Once this relation is calibrated precisely for nearby Cepheids, the absolute luminosity of remote Cepheids is derived from the measured period, and therefore their distance, and by the way also the distance of the objects (clusters, galaxies) in which they are included. Indeed, the distance of a star is easily derived from the comparison of its absolute luminosity to its apparent luminosity. And thus a precise P(L) relation converts the Cepheids into primary distance indicators for extragalactic astronomy. The calibration of the relation P(L) is delicate, however. Even the nearest Cepheids are usually too far away for a direct measurement of their distance (and absolute luminosity). The angular diameter is usually not measurable from a single telescope. In a first method, called the Baade-Wesselink method, the radial velocity (velocimetry) and the color (colorimetry) of the star is measured all along the period. Two times (phases) are chosen in which the star has the same color but different radii and different luminosities. The (common) temperature is evaluated from the (common) color, the ratio of the radii is derived fom the ratio of the apparent luminosities, the difference between the two radii is derived from the integration of the velocity between the two selected times. A simple algebra derives the two values of the radius, the absolute luminosity follows from the knowledge of the radius and temperature (hence the distance). The method is sensitive to the temperature error. A better estimation is obtained, when the angular size of the star is measured with an interferometer. Long baselines are required, since the diameter is of the order of a few milli-arcsecond (mas). The second method relies heavily on long baseline interferometers : with the latter, the angular diameter of the Cepheid variable star is measured at two different phases of its pulsation. This provides a first number X : the amplitude of its angular size variation between these two phases. This first result is usually expressed in milliarcseconds. Complementary observations by radial velocimetry (spectroscopic measurement of the speed of the surface of the star) are then used to determine the linear amplitude of the diameter variation of the Cepheid between the two same phases as above. This second number Y is a linear value, expressed in meters. Using the two values X and Y measured above, it is possible to compute directly the distance in parsecs to the star through the simple equation d[pc] = 9.305 * Y[m] / X[mas].
A second method to obtain the distances to the Cepheids Left : Measurement of the radial velocity (perpendicular to the sky plane) as a function of time Right : Direct measurement of the oscillations of the stellar radius
Because of the turbulence of the atmosphere, which is worse at visible wavelengths, interferometric measurements are easier in the near-infrared bands (near 2 microns). The spatial resolution that can be obtained is proportional to the distance between the interfering telescopes (the baseline). Below are presented observations of the Cepheid Zeta Geminorum with the FLUOR beam combiner, installed at the IOTA interferometer. The mean uniform disk angular diameter was measured to be 1.64 +0.14 -0.16 mas. The distance to zeta Gem is evaluated by the first method above to be 502 +/- 88 pc. FLUOR (Fiber Linked Unit for Optical Recombination) has served as a proto-type of the VINCI instrument, and is one of the three recombination instruments of IOTA (Infrared and Optical Telescope Array), a collaboration with 5 American Institutions, in Arizona.
The measurement was also done last November with the VLTI, with the interferences between ANTU and MELIPAL (distant by a baseline of 102m). The angular diameter was measured more accurately, to be 1.78 +/- 0.02 mas. Pulsational variations were not detected with FLUOR/IOTA, but are expected to be measured with VINCI/VLTI.
Left : Study of the Cepheid Zeta Gem with FLUOR/IOTA in 1999/2000. The pulsation is not detected, but the diameter is measured to be 1.64 +/- 0.16 mas Right : Model of the pulsation curve of Zeta Gem as will be observed with VINCI/VLTI, based on the precision achieved during the commissioning run (by P. Kervella)
During 2002, the VLTI science instruments MIDI and AMBER and the fringe sensor unit FINITO will arrive, and the integration of the Auxiliary Telescopes will start. Once the ATs and the science instruments are functional, regular science operations can start.
References
- P. Kervella, V. Coudé du Foresto, G. Perrin, M.Schoeller, W. A. Traub, M. G. Lacasse, 2001, ``The angular diameter and distance of the Cepheid Zeta Geminorum’’ Astronomy and Astrophysics, 367, 876 (astro-ph/0102359)P. Kervella, Soutenance de thèse, le 14 Novembre 2001 [ Giant Eyes for the VLT Interferometer First Scientific Results with Combined Light Beams from Two 8.2-m Unit Telescopes E "First Light" for the VLT Interferometer - Excellent Fringes From Bright Stars Prove VLTI Concept
Contact
- Pierre Kervella (DESPA, Observatoire de Meudon and ESO-Garching)
- Vincent Coudé du Foresto (DESPA, Observatoire de Meudon)
Dernière modification le 21 décembre 2021