VVV-WIT-13: an eruptive young star with cool molecular features

TL;DR

This paper investigates VVV-WIT-13, an infrared eruptive source in the Galactic disk, to determine the eruption mechanism and evolutionary status. It combines multi-epoch near- and mid-infrared photometry, near-infrared spectroscopy, ExoMol-based molecular modeling, and 2D hydrodynamic simulations to interpret the event. The authors report deep AlO absorption during outburst with a best-fit temperature near 600 K, blue-shifted molecular lines indicating an outflow, and a multi-year outburst with a bolometric luminosity around 55 L⊙, consistent with instability in a young star’s accretion disk. A tidally disrupted giant-planet embryo at a few astronomical units is proposed as a plausible trigger, which, if confirmed, would provide rare real-time insight into planet-disk interactions and episodic accretion in early stellar evolution.

Abstract

Here we investigate an infrared eruptive source, identified from the decade-long VISTA Variables in the Via Lactea survey (VVV). We named this target after a group of variable sources discovered by VVV, as VVV-WIT-13, with WIT standing for "What Is This?", due to its unique photometric variation behaviour and the mysterious origin of the outburst. This target exhibited an outburst with a 5.7 mag amplitude in the Ks-band, remained on its brightness plateau for 3.5 years, and then rapidly faded to its pre-eruptive brightness afterwards. We aim to reveal the variable nature and outburst origin of VVV-WIT-13 by presenting our follow-up photometric and spectroscopic observations along with theoretical models. We gathered photometric time series in both near- and mid-infrared wavelengths. We obtained near-infrared spectra during the outburst and decaying stages on XSHOOTER/VLT and FIRE/Magellan, and then fitted the detected molecular absorption features using models from ExoMol. We applied 2D numerical simulations to re-create the observables of the eruptive phenomenon. We observe deep AlO absorption bands in the infrared spectra of VVV-WIT-13, during the outburst stage, along with other more common absorption bands (e.g. CO). Our best-fit model suggests a 600 K temperature of the AlO absorption band. In the decaying stage, the AlO bands disappeared, whilst broad blue-shifted H2 lines arose, a common indicator of stellar wind and outflow. The observational evidence suggests that the CO and TiO features originate from an outflow or a wind environment. We find that VVV-WIT-13 is an eruptive young star with instability occurring in the accretion disk. One favoured theoretical explanation of this event is a disrupted gas clump at a distance of 3 au from the source. If confirmed, this would be the first such event observed in real time.

Figures (14)

• Figure 1: Infrared images of the surrounding environment of VVV-WIT-13 (located in the center and marked by the green circle in all frames). Left:Spitzer IRAC three-colour image (1 $\times$ 1 deg, blue: [4.5], green: [5.8], red: [8.0]), taken in 2004 during the pre-outburst stage, with Galactic longitude (l) and latitude (b). Sources belonging to the G342.1+0.2 YSO group, selected by SPICY, are marked by magenta circles. Upper right: a zoomed-in 5$\arcmin$$\times$ 5$\arcmin$Spitzer IRAC three-colour image, with the equatorial North-up orientation. Middle and bottom right: 1.2$\arcmin$$\times$ 1.2$\arcmin$$W2$-band images from unWISELang2014aMeisner2017, taken in 2017 and 2020.
• Figure 2: Left: infrared light curves ($K_s$, $W1$ and $W2$-band) of VVV-WIT-13. The $W1$-band error bars (orange) are the standard deviation of the magnitude spread in each epoch before binning. The typical error bar in the $K_s$ band ($<$ 0.1 mag) is smaller than the symbol size. A sinusoidal fitting result is presented by the grey line. The spectroscopic epochs are marked by the vertical dashed lines with the same colour scheme as the right panels. Right: near-infrared spectra of VVV-WIT-13 obtained in 2021, 2023 and 2024. Key spectral features are labelled on the plot.
• Figure 3: Near-infrared colour-colour diagram of YSOs from the VVV survey. VVV-WIT-13 is marked by the black circles (filled: quiescent; open: outburst). Other members in the SPICY group G342.1+0.2 are presented by the grey dots. A selection of VVV FUor-type eruptive sources is shown in red. The dashed line represents the synthetic locus of Class II YSOs Meyer1997. The dotted and solid lines are extinction vectors from WangS2019.
• Figure 4: Left: Spectral energy distribution (SED) of VVV-WIT-13 with data obtained from VVV, SOFI, Spitzer, WISE and integrated spectrum. The pre-outburst SED is presented by dots, and the in-outburst SED is shown by squares. Right: In-outburst SED of VVV-WIT-13 dereddened with $A_V$ ranging from 0 to 17 mag. As a comparison, we present the "high mass accretion" SED calculated by Bell1999. We scaled the model to match the SED of VVV-WIT-13, as presented by the dashed line. Error bars smaller than the size of symbols are not presented.
• Figure 5: Upper left: Near-infrared spectrum of VVV-WIT-13 taken during the brightness plateau. The best-fit molecular spectra (AlO, H$_2$O and CO) are presented in the lower left panel, with effective temperature and column density labelled on the plot. The coloured squares mark the spectral regions presented on the right panels. Spectral regions among $J$, $H$, and $K$-bandpasses are shown in grey, which are dominated by water absorption features. Right: zoomed-in views of two regions with molecular absorption features, with radial velocity listed in the heliocentric frame. The theoretical wavelengths of $^{13}$CO absorption bandheads are presented by dashed grey lines.