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Measurement of a Diameter and Detection of a Component in the Giant lambda Aquarii

Russian version

    June 25, 2016 the star lambda Aquarius fell into the coverage zone of the Moon in the area of the BTA telescope. This made it possible to conduct rare observations of the star using both the method of lunar occultations and speckle interferometry at the BTA. When the star is covered by the Moon, the time Δt, during which the star disappears (or appears on the other side of the disk), depends on its angular diameter, since the speed of the Moon's motion across the sky is known, V=0.55 arcsecond per second. Then for a star of a diameter of, for example, Θ=0.05 arcsecond, the coverage time will be Δt = Θ/V = 91 milliseconds of arc. However, in practice, we face a number of problems, the main ones are the diffraction of light when the star is covered by the Moon, the uncertainty of the profile of the covering lunar edge, and the photon noise when recording the light flux. Nevertheless, using the method of lunar occultations, astronomers manage to measure angular sizes of stars with a spatial resolution of up to 1-2 milliseconds of arc (msa). Speckle interferometry at the 6-m telescope in the visible range, provides resolution not exceeding 20 msa, i.e., an order of magnitude lower than that obtained with the method of lunar occultations.
    The star lam Aqr (HD 216386) is a long-period irregular variable with small amplitude of brightness variation from 3.6 to 3.8 of the visible magnitude, a red giant of the M1/2 III spectral type. The determination of its radius makes it possible to build a model of the star taking into account the new exact distance to it obtained with the GAIA satellite. We performed observations of the coverage of the star using a speckle interferometric camera in the primary focus of the BTA telescope. The detector was the Andor iXon Ultra camera with EMCCD matrix. The frame rate was raised up to 387.6 Hz with a field size of 64x64 elements. The measurements were carried out in the 694/10-nm filter. In this band, we detect the continuum radiation of the star formed by its photosphere. In the neighboring regions of the spectrum, for example, in the I band, the radiation is formed in an extended stellar atmosphere, and the resulting image will have a larger diameter. The observed brightness variability curve of lam Aqr under Moon's coverage was interpreted using a simple model of a homogeneous disk, from which its visible diameter was determined: Θ=7.11±0.02 msa. At parallax of lam Aqr, equal to 8.47 msa, this diameter corresponds to about 90 solar diameters. We note that the error in determining Θ as a result of observations at the BTA appeared to be an order of magnitude smaller than at telescopes of the 1st class. This can be explained by the great light flux compared with that of smaller telescopes and smoothing out the tremor of the star's image caused by atmospheric turbulence.
    An unexpected result, when observing the lam Aqr coverage, was the detection of a weak companion distant from the red giant by about 0.5 arcseconds. We carried out speckle interferometric observations of the system in the 550/20-nm and 694/10-nm filters. The specified angular distance between the components of the pair was 526±1 msa and the brightness difference in the 550/20-nm filter was 4.51±0.03 magnitude. The companion can be referred to the F5V type stars, whose masses are usually about 1.2 solar masses. Assuming that the mass of the primary star is 3.5 solar masses and also assuming the major semi-axis of the visible orbit of the pair is 88 AU (62 AU is the measured projected parallax of 8.47 msa), we obtain an estimate of the orbital period - 380 years. Speckle interferometric observations of the pair's orbital motion for 10 years will allow us to build a visible orbit and, therefore, to specify its total mass.
    The results presented here were reported at the "Physics of Stars: from Collapse to Collapse" International Conference held in October 3-7, 2016, Nizhny Arkhyz, SAO RAS.

Dyachenko V.V., Balega Yu.Yu., Beskakotov A.S., Maksimov A.F., Rastegaev D.A. (SAO RAS), Rickicki A. (National Astronomical Research Institute of Thailand).
Dyachenko V., Richichi A., Balega Y., Beskakotov A., Maksimov A., Rastegaev D., Lunar occultation and speckle interferomerty of lambda Aquarii with the SAO RAS 6-meter telescope, 2016, Stars: from collapse to collapse: Proc. Conf. held at Spec. Astrophys. Observatory, Nizhny Arkhyz, Russia, 3-7 Oct. 2016, ASP Conf. Ser.

Contact - Dyachenko V.
Fig.1. Occultation curve lam Aqr obtained with the 6-m SAO RAS telescope and the restored profile of the disk of this star
Fig.2. Lam Arq images with a detected companion obtained with speckle interferometry