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

## TL;DR

This study presents spectroscopic observations of the yellow symbiotic star LT Del during 2010-2018, highlighting a 2017 outburst with significant spectral and temperature changes, revealing insights into its outburst behavior.

## Contribution

First detailed spectroscopic analysis of LT Del's 2017 outburst, showing temperature increase and spectral line flux changes, expanding understanding of yellow symbiotic star outbursts.

## Key findings

- Emission line fluxes increased 5-6 times during outburst
- HeII 4686 line flux increased tenfold
- Star's temperature rose to approximately 130,000 K in 2017

## Abstract

LT Del is a yellow symbiotic system that consists of a bright K3-type giant and a hot subdwarf with a temperature of about 10$^5$ K. We present the results of our spectroscopic observations of LT Del over the period 2010-2018. In 2017 the star experienced a second low-amplitude ($\Delta V\sim0.^{m}7$) outburst in the history of its studies. The emission spectrum of the star represented in the optical range by hydrogen, neutral and ionized helium lines underwent significant changes in the outburst. The fluxes in the HI and HeI emission lines increased by a factor of 5-6, the HeII $\lambda$4686 line grew by a factor of 10. According to our estimates, in the 2017 outburst, the temperature of the exciting star rose to $T_{\text{hot}}\sim$130~000 K, while during the first 1994 outburst the change in temperature was insignificant. This suggests cool and hot outbursts of LT Del by analogy with similar events of another yellow symbiotic star, AG Dra.