US, WASHINGTON (ORDO NEWS) — An international team of astronomers conducted high-resolution optical spectroscopy and X-ray observations of the symbiotic star EF Aquilae. The results of these observations unraveled the nature of this object. New results are reported in an article published May 7 on arXiv.
Astronomers suggest that the symbiotic counterparts show dramatic, episodic changes in the spectra of their light, because one star in the pair is a very hot one, a small star, and the other a cold giant. In general, such systems are important for researchers studying aspects of stellar evolution.
Astronomers divide symbiotic stars (SySt) into two main classes: S-type and D-type. Most of the known SySts are of the S type, in which the near infrared spectra usually prevail in the photosphere of a cold star, and they are indistinguishable from ordinary giants of the late type. D-type symbiotic stars exhibit additional radiation attributed to the thick shells of stardust. SySts of this class experience large amplitude fluctuations due to the presence of World variables (red giants with pulsation periods of more than 100 days and amplitudes of more than one magnitude in the infrared range and 2.5 magnitudes in the visual wavelength range) and other long-period variable stars.
Located about 11,000 light-years from us, EF Aquilae (abbreviated as EF Aql) is a D-type system with a World variable. Although EF Aql was identified as a variable star in 1925, it was confirmed as SySt only in 2016. However, this star has not yet been thoroughly studied, and its nature is still not fully understood.
So astronomers, led by Kirill Stoyanov of the Institute of Astronomy and the National Astronomical Observatory in Sofia, Bulgaria, studied EF Aql in detail. By analyzing data obtained mainly from the Large South African Telescope (SALT), the National Rozhen Astronomical Observatory and NASA’s Neil Gerls Swift Observatory, they received more information about the properties of this system.
“We received high-resolution optical spectroscopy and X-ray observations of the D-type symbiotic star EF Aql,” astronomers wrote in the article.
The study showed that the hot component EF Aql has a temperature of about 55,000 K and a luminosity of about 5.3 solar luminosities. The period EF Aql was determined as 320.4 days, which corresponds to the period of pulsations of a donor star of the World type in the system. The distance to the double was estimated at 10,100 light-years, so the object turned out to be closer to Earth than previously thought.
According to this article, EF Aql is one of the weakest X-ray systems discovered so far. Astronomers noted that quick observations did not detect a double in X-rays, but the system was identified in the ultraviolet range using UVOT (ultraviolet / optical telescope).
The rate of loss of mass of the World donor in EF Aql was calculated as 0.00000025 solar masses per year, which is consistent with the values obtained for other stars of the World in symbiotic systems. This, along with other results of the study, allowed the authors to conclude that EF Aql is a symbiotic star with accretion nutrition without burning the shell.
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