Second Outburst of the Yellow Symbiotic Star LT Delphini

Abstract

We present the results of our photoelectric U BV observations of the yellow symbiotic star LT Del over 2010-2018. The binary system LT Del, which consists of a bright K giant and a compact hot star with a temperature of ∼100 000 K, has an orbital period of 476 days. In 2017 the variable experienced a second low-amplitude (∆V ∼ 0. m 7) outburst in the history of its studies whose maximum occurred at an orbital phase of 0.15±0.05. The outburst duration was ∼60 days. The B − V and U − B colors in the outburst became noticeably bluer. A difference in the photometric behavior of the star in the 1994 and 2017 outburst has been detected. In the orbital cycle preceding the 2017 outburst a secondary minimum with a depth of 0. m 15 and 0. m 20 appeared in the B and V light curves, respectively, whose cause is discussed. The phase light and color curves are presented and explained; the position of the star on the color-color diagram is interpreted. We have estimated the parameters of the cool and hot components of the system based on the distance determination from Gaia DR2.

Spectrum of the Yellow Symbiotic Star LT Delphini before, during, and after the 2017 Outburst

“…Data on the observational material are contained in Table 1, while Fig. 1 shows the times of spectroscopic observations together with the U light curve from Ikonnikova et al (2019). Table 1 lists the orbital phases calculated with the following linear elements: JD(Min) = 2445930 + 476. d 0E (Arkhipova et al 2011).…”

“…The orbital period of the system is P orb = 476. d 0 (Arkhipova et al 2011). Over the entire history of its observations LT Del has experienced two low-amplitude outbursts, in 1994 and 2017, the photometric behavior in which was described by Arkhipova et al (1995a) and Ikonnikova et al (2019), respectively.…”