Blending-induced beating and emission in the symbiotic star Terz V 2513

TL;DR

Terz V 2513’s apparent light-curve beating arises from blending with a nearby Mira rather than intrinsic multi-periodicity. By combining Gaia DR3 and OGLE-IV photometry with high-resolution imaging and optical/IR spectroscopy (SALT and NTT), the authors disentangle the two sources and identify the true symbiotic counterpart as Gaia DR3 4061345440488592896, a Mira with a pulsation period of $P=161$ days. The infrared spectrum of this star exhibits emission lines and CO features consistent with other symbiotic Miras, and the system is likely a post-nova symbiotic, akin to the symbiotic nova V5590 Sgr. This study illustrates the importance of careful crowded-field analysis and demonstrates how multi-wavelength data fusion can correct misidentifications, improving the classification and understanding of variable stars in dense fields.

Abstract

We present a detailed analysis of Terz V 2513 (=2MASS J17334728-2719266), a poorly studied symbiotic star. Our motivation was a peculiar beating pattern in its light curves from all-sky surveys and our own observations. Using \textit{Gaia} DR3 and OGLE-IV photometry, we show that this variability arises from blending with a nearby, unrelated Mira variable (\textit{Gaia} DR3 406134544052580377 = OGLE-BLG-LPV-241930). Analysis of VPHAS+ and Pan-STARRS imaging, combined with optical and infrared spectroscopy from the Southern African Large Telescope and ESO New Technology Telescope, further reveals that the symbiotic star has been misidentified in the literature. We identify the correct counterpart as \textit{Gaia} DR3 4061345440488592896 (=OGLE-BLG-LPV-241932), a Mira with a 161-day period. Its infrared spectrum displays prominent emission lines and is remarkably similar to those of other symbiotic Miras. Based on our data and previous studies, Terz V 2513 likely experienced a symbiotic nova outburst in the past. This study highlights the importance of careful analysis of survey light curves in crowded fields and demonstrates how combining multi-wavelength photometry, spectroscopy, and high-precision \textit{Gaia} data can disentangle blended sources and accurately determine their nature.

Figures (8)

• Figure 1: Pan-STARRS $r$-band image of the field around Terz V 2513. The displayed field of view is 30$\times$30. The positions of the two stars discussed in the text are indicated.
• Figure 2: Multi-band images of the field around Terz V 2513. Images from VPHAS+ (SDSS $u$, SDSS $g$, SDSS $r$, H$\alpha$, SDSS $i$ bands), Pan-STARRS ($g$, $r$, $i$, $z$, $y$ bands), 2MASS $J$, and VVV $J$-band are shown. The displayed field of view is $\sim$16.5$\times$16.5. The positions of the two stars are indicated in the same way as in Fig. \ref{['fig:panstarrs']}. Images are arranged in order of increasing filter effective wavelength.
• Figure 3: NTT/SofI spectra of both sources. Line colors correspond to those of the respective stars in Fig. \ref{['fig:panstarrs']}. For comparison, the spectrum of the symbiotic nova with a Mira-type donor V5590 Sgr (Nova Sgr 2012b) is shown in green. Major emission and absorption features are labeled in gray. Shaded regions indicate wavelength intervals most affected by telluric absorption.
• Figure 4: SALT/RSS spectrum of Terz V 2513. Light from both sources visible in Fig. \ref{['fig:panstarrs']} is blended in this spectrum, and is represented here by the purple color. The spectrum is not dereddened. Due to limitations in absolute flux calibration arising from the moving pupil design of SALT, the flux is normalized to the median value. The identification of emission lines is shown in gray.
• Figure 5: Photometry of the studied sources. A: Light curves in the $V$ and $I_c$ bands from our observations, along with ATLAS $o$ and $c$ bands. Light from both stars is blended in these data (indicated by the purple symbol borders). B:Gaia DR3 $G$-band light curves of the two sources. Point colors correspond to the star colors in Fig. \ref{['fig:panstarrs']}. C: OGLE-IV $I$-band light curves of both sources. The epochs of the SALT and NTT spectra are marked by vertical lines. Note the different JD ranges shown on the x-axes of each panel.