OGLE-2002-BLG-360: A dusty anomaly among red nova remnants

¹ Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
² Astronomy & Astrophysics Department, University of California, San Diego, La Jolla, CA 92093-0424, USA
³ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí Franquès 1, 08028 Barcelona, Spain
⁴ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), c. Martí i Franquès, 1, 08028 Barcelona, Spain
⁵ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
⁶ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA

^★ Corresponding authors: thomas@ncac.torun.pl; tomkam@ncac.torun.pl

Received: 25 February 2025
Accepted: 24 May 2025

Abstract

Context. OGLE-2002-BLG-360 is an example of a Galactic red nova, the aftermath of a non-compact stellar merger. The dusty nature of the material surrounding the merger remnant makes observations of this particular source difficult, meaning the properties of the central star and its surrounding environment are poorly understood.

Aims. We aim to establish the characteristics of the merger remnant, as well as those of the dusty environment and its structure. We attempt to establish similarities with other Galactic red novae and argue how such an environment may have formed.

Methods. We use infrared and sub-millimetre observations to construct the spectral energy distribution (SED) between 2 μm and 1.27 mm for an epoch 15–21 years after the red nova eruption. We used the radiative transfer codes DUSTY and RADMC-3D to model the SED and retrieve the physical properties of both the central star and the surrounding dust.

Results. We show that the SED is best replicated by a spherically symmetric model consisting of an M-type supergiant surrounded by 0.012 M_⊙ of dust concentrated within two spherical shells. The dust in the outer shell extends out to a maximum distance of 9500 AU from the central source, whilst the inner shell extends out to 1000 AU. The dust composition is dominated by iron grains (58%), but also contains olivine silicates (25%) and alumina dust (17%), which are both required to reproduce the profile of the observed 10 μm absorption feature.

Conclusions. The inner shell likely originates from merger and post-merger ejecta, whilst the outer shell consists of material lost much earlier, before the merger event occurred. Evolution of the SED indicates continued dust formation within the expanding inner shell, which may be analogous to winds of red supergiants. The object is extremely dusty compared to other Galactic red nova remnants.