Mystery of a peculiar star SU Lyn resolved by PRL scientists using AstroSat

In a breakthrough, a team of astronomers from the Physical Research Laboratory (PRL), Ahmedabad used the data from Ultra-Violet imaging telescope (UVIT) onboard India’s Astrosat space observatory to resolve the nature of a peculiar star named SU Lyn. They have utilized, for the first time, the UV spectroscopy capability of UVIT and in the process, have thrown new light on the class of stellar objects known as symbiotic stars.

SU Lyn had long been known as an ostensibly unremarkable red giant star – a class of very large and cool stars, which form at the final stages of stellar evolution. However, it was noticed in 2016 that hard X-ray emission was emanating from SU Lyn. This raised the suspicion that the star harboured a hidden, hot companion assumed to be a white dwarf – an end-product when stars of intermediate-mass die. White dwarfs can be as massive as the Sun, yet they have a size similar to the size of the Earth.

The suggestion that SU Lyn could likely host a white dwarf posed a challenge for our understanding of such systems. Binary stellar systems consisting of a white dwarf and a red giant are known as symbiotic systems.  In a symbiotic system, the white dwarf and red giant's interaction gives rise to several complex physical phenomena such as an accretion disk, jets,  ionized symbiotic nebula, interaction of stellar winds to name a few. Due to this, Symbiotics are considered as one of the most intriguing astrophysical laboratories. A schematic picture of a typical symbiotic system and its various constituents are shown in Figure-1. These symbiotic systems have traditionally been identified and characterized by the presence of intense emission lines of several high ionization species observed in their optical spectra using ground-based telescopes. However, the optical spectrum of SU Lyn was devoid of these lines, raising a question mark on its symbiotic nature.

A schematic diagram showing various components of a symbiotic system

A more definite way to establish the presence of a white dwarf is through ultra-violet (UV) observations since white dwarfs are hot and emit radiation mostly in the UV range. UV radiation, however, cannot penetrate the Earth’s atmosphere and can only be detected using space-based UV telescopes and instruments. But at present, there are few UV telescopes in space and UV telescopes with spectroscopic capability are even rarer.

This is where India’s Astrosat space observatory and one of its payloads UVIT – the Ultra-Violet Imaging Telescope – played a crucial role. Instruments onboard the observatory are capable of recording the UV spectrum of stars, a feature that proved extremely useful. The PRL team had been observing SU Lyn since 2016 with various Indian observing facilities and a suite of instruments, most notably with the UVIT. From the ground, the star was observed with the HESP instrument on the IIA-HCT telescope, with the indigenous in-house developed MFOSC-P spectrograph and with the Near-Infrared Camera and Spectrometer on the PRL 1.2 m telescope at Mount Abu.

The Astrosat-UVIT spectrum of SU Lyn with the emission lines identified. Archival spectra of three other symbiotic systems (ER Del, SY Mus and AS 210) are also shown for comparison.

 The Far-UV (1300-1800 Angstroms) spectrum of SU Lyn, obtained with the Astrosat-UVIT instrument, showed emission lines of silicon (Si IV), carbon (C IV), oxygen (OIII), and nitrogen (N III) in a spectrum typical of symbiotic stars (figure-2). The high-resolution optical spectrum also shows the weak presence of few emission lines, which are typically seen in the optical spectrum of symbiotic stars. The UV spectrum, complemented by optical and NIR spectra, thus, confirms the symbiotic nature of SU Lyn. Using a simple theoretical model to fit the UV observations, it was further shown that the white dwarf in SU Lyn is orders of magnitude less luminous (0.16 solar luminosity) compared to a white dwarf in a traditional symbiotic system (~100-1000 solar luminosity). Instead, the symbiotic phenomenon is predominantly powered by the relatively weaker UV radiation from the accretion disk (0.66 solar luminosity) around the white dwarf. This is the reason that the emission lines are weak in the optical spectrum and why the symbiotic nature of SU Lyn could not be established from ground-based observations earlier.

The resolution of the nature of SU Lyn is a significant result for stellar astronomy. There are only a few hundred symbiotic systems known in our Galaxy. This is in contrast with their predicted population of several hundred thousand. The presence of intense emission lines in low-resolution optical spectra has always been the traditional way to identify and discover symbiotic stars. However, these traditional methods would fail to detect the SU Lyn type of Symbiotics. These recent results by the PRL team have firmly established the existence of SU Lyn type symbiotic systems. It is highly probable that many more symbiotic stars like SU Lyn can exist which have so far evaded the detection by conventional methods. And this could be a reason why a smaller than expected number of symbiotic systems have been discovered so far.

It is equally important to note that these results are derived from a lesser-known spectroscopic capability of the UVIT instrument, which is preliminarily designed as an imaging instrument.

Reference :

Vipin Kumar, Mudit K Srivastava, Dipankar P K Banerjee, Vishal Joshi, UV spectroscopy confirms SU Lyn to be a symbiotic star, Monthly Notices of the Royal Astronomical Society: Letters, Volume 500, Issue 1, January 2021, Pages L12–L16; https://doi.org/10.1093/mnrasl/slaa159