Double with screening

A star Double with screening is a star doubles whose two components are successively occulted by the the Moon (or another body of the Solar system) at the time of the relative movement of this body compared to the couple. Thanks to the measurement of the difference in time during the immersion (or emergence) of each component at the lunar edge, this technique allows high a angular resolution, about some thousandth of Seconde of arc.

History

Antarès, the rival of Mars, seems to have the privilege to be the first double with detected screening. The couple could besides have preserved its secrecy a long time, if the chance had not smiled to certain professor Burg with Vienna, on April 13rd 1819: “ With 12:03 mn17,1s, I observed the emergence of a star of 6,7 mag, which, after approximately 5 seconds suddenly seemed to me a star first magnitude… Perhaps that Antarès is a double star, and the small one observed in first is so close to principal star that both, even seen with a good Télescope, do not appear separate. ”. If he were not believed at that time, the assumption of the Réfraction of a “lunar atmosphere” being privileged, the duplicity of Antarès finally was confirmed by O.M. Mitchel on June 13rd 1846 with the very new glasses of Cincinnati. Always according to White, it is John Herschel which emitted, in 1833, the suggestion that measurements of screenings could bring to detection double stars an high-resolution of which were deprived the traditional observations.

Measurements of occulation can appear simple, but by observing the curve of light attentively one can note curious oscillations at the time of the immersion or emergence of the star, which in general require a small historical turning in connection with screening, because inseparable from that of the detection of the doubles.

The observation of stellar screenings is undoubtedly as old as humanity, but one of the oldest mentions which are reported seems to be in the Almageste; the first indication (more or less) usable of the moment of a stellar screening will await on July 5th 1623: it (Spica) is observed with Paris by Ismaël Boulliau, with a telescope, but without clock. Many observations took place then. Certain observers, and not of least, saw a variation speed connect at the time of screenings, and of the questions of optics, even physiological were called upon; it should be recalled that the question will arise a long time (until the beginning of the twentieth century) of the existence or not of an atmosphere on the Moon. The Astronomer Royal Airy suggested that the effect observed was due to the accumulation of rings of lunar light diffraction without obviously being able to prove it. Eddington (1909) seems to be the first to take into account simply the effect of diffraction of the light of star, but the first sufficiently fast experimental measurement to highlight this diffraction will await Arnulf: screening of Régulus, on April 6th, 1933 with the Observatory of Paris, on the telescope of 1m of the site of Meudon.

Indeed, with the first order, the screening of a star by the Moon can be modelled by a diffraction by an edge of screen: the lunar edge plays the part of the half-plane, lit by the light of ad infinitum supposed star. This effect is initially mentioned, not in relation to measurement of double stars, but with respect to that, at least also important at the time like now, of the stellar diameters. MacMahon (1908) had indeed suggested, in a purely geometrical way, to estimate diameters using photographic fast measurements during screenings, but Eddington had highly retorted the month according to whether this measurement would be unusable for the majority of stars because limited by diffraction. Moreover, both argued that the irregularity of lunar surface would limit measurements. Actually, the fringes of diffraction contain the information which the object is not specific, and of many measurements of diameters were possible since, and this without too many significant disturbances due to lunar surface: in 1987, there were 346 measurements diameter for 124 stars.

Intensive observations took place during the years 1970, in particular by D.S. Evans, then in the close relation Infrarouge. It is estimated that approximately 10% of the observations of screening can bring to discovered duplicity of the object. The catalog of the doubles with screening of Mason (1995) contains 772 measurements for 357 systems.

Capacities of the method

Oldest of the techniques of angular high-resolution, this method remains always used thanks to its performances, and is well adapted to the systematic double star discovery, towards which other more expensive techniques can be then inclined. The method of screening has several advantages indeed:

neither atmospheric turbulence (except scintillation), nor the diffraction of the telescope disturbs too measurement: diffraction is carried out upstream, by the Moon, in the vacuum, and telescopes it is only used to collect the light;
the angular resolution is not directly related to the diameter of the instrument, but of course the signal-to-noise report/ratio is; signal-to-noise > 10 is sufficient to detect doubles of separation > 5 thousandth of second of arc and difference in magnitude < 1.5;
the low costs and simplicity in terms of observation and data reduction do not hold this method with professional astronomers having large equipment;
it needs there only little time observation;

but it also has several counterparts:

first of all, only the stars in the vicinity of the ecliptic can be observed: the course of the Moon only covers only 10% of the sky. Screenings by asteroids bring a better cover of the sky but the prediction of screenings is more complex: the positions must be known precisely taking into account the low apparent dimension of the asteroids;
the observation of duplicity is not répétable (verifiable) by this method, except waiting ~18.5 years;
only one measurement of time separating the two events does not suffice for it all alone to obtain the separation and the angle of position of the couple. Measured separation is not the true separation of the couple, but projected separation: one only obtains a unidimensional measurement, perpendicular to the tangent with the lunar limb. Nevertheless, of the measurements made simultaneously in other points of the terrestrial sphere allow to thwart this problem;
the method is limited to a weak difference in magnitude (~3.5) between tight components;

Parameters

The use of stellar screenings aims at solving tight couples. If the couple can be thus solved, and if it is a physical couple, to see with visual Binaire how the masses can possibly be obtained using complementary data (spectroscopic).

The angular velocity of the relative movement of the Moon is around 0.5" /s. If one wants to obtain measurements with high-resolution, it is thus necessary to sample with approximately 1000 measurements a second. The duration of installation being thus very short, this method supports the most brilliant stars.

Once the data obtained, a usual method is to adjust a model with the observations, models containing the following parameters:

diameters of the components,
their separation (projected),
magnitude of each component,
the relative speed of the limb,
the level of the bottom of sky.

In the best of the cases (observation to the immersion and emergence, with a different filter for each), one could thus in theory obtain the diameters, the magnitudes and the colors of the components, as well as the angle of position and separation between them.

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