Characterisation of an EXor outburst SPICY 97589

TL;DR

The paper tackles episodic accretion in very young stars by characterizing the 2023 EXor-type outburst of SPICY 97589 (Gaia23bab) using multi-wavelength, medium-resolution spectroscopy. It combines peak-outburst and post-outburst data to derive the stellar properties (approximately a very young, low-mass M3.0 star with $T_{eff} \approx 3410$ K, $M_* \approx 0.29$ Msun, $L_* \approx 0.41$ Lsun) and to quantify accretion through 22 line tracers, yielding an outburst rate of $\dot M_{out} \approx 2.38 \times 10^{-7}$ Msun/yr and a quiescent rate of $\dot M_{qui} \approx 4.24 \times 10^{-9}$ Msun/yr. The 2023 event was longer and more energetic than the 2017 outburst, and the spectra reveal strong disk winds and evolving hydrogen features, including P-Cygni profiles during outburst and a switch to weaker emission in quiescence. The integrated accreted mass is estimated at ~2.3e-8 Msun over about 671 days, underscoring the role of episodic accretion in assembling stellar mass and driving disk evolution in the earliest stages of YSO development.

Abstract

Stellar outbursts from variable or periodic accretion are thought to be ubiquitous across young stellar populations. However, relatively few outbursting objects have been discovered to date. Here, we present the characterisation of a new EXor-type episodic accretor. We aim to characterise the nature of the 2023 outburst of SPICY 97589/Gaia23bab and characterise the stellar source for the first time, while exploring how an accretion outburst contributes to disk evolution. We employ multi-waveband medium-resolution spectroscopy with UVB-VIS-NIR coverage during the peak of the 2023 outburst and the post-outburst quiescent object. The broad wavelength coverage of the dataset allows for robust measurements of the accretion rate using known line tracers. The addition of quiescent spectra provides a good estimation of stellar parameters of the central star while also informing us on the evolution of the disk during outburst phases. We find the stellar source to be a 3410\,K, M3.0 type star with a luminosity of 0.41 $L_\odot$ and an estimated stellar mass of 0.29 $M_\odot$. We measure the accretion rate of SPICY 97589 to be $\dot M = 2.38\pm0.58\times10^{-7}\,\mathrm{M_\odot yr^{-1}}$. This value is at two orders of magnitude greater than the quiescent accretion rate. Thus, we confirm that the 2023 outburst was driven by an influx of material from the surrounding environment to the central star, an accretion outburst. The spectral fingerprint of emission lines is also characteristic of an outbursting EXor-type source, including variable disk winds.

Figures (6)

• Figure 1: Spectral energy distribution (SED) of SPICY 97589. Circles are the photometry from sources described in Appendix \ref{['AppA']}. All photometry is taken during quiescence, pre-2017 outburst. The blue line is the binned X-Shooter flux calibrated 2023 outburst spectrum. The red line is the binned X-Shooter flux calibrated spectrum post-2023 outburst.
• Figure 2: Best fit of the X-Shooter spectrum of SPICY 97589. The observed spectrum is shown in black. The best fit photospheric template in yellow, and the slab model in green. The best fit is shown in light blue. The blue points indicate the wavelength ranges used to constrain the model.
• Figure 3: Gaia G-band light curve over the past seven years. Highlighted in orange and red are the 2017 and 2023 outbursts, respectively. The black dashed lines are the data of the X-Shooter spectroscopic observations on 2023-04-19 and 2024-07-25. The mean quiescent magnitude is measured as $18.66\pm0.12$.
• Figure 4: WISE light curves in the W1 ($3.35\,\mathrm{\mu m}$) and W2 ($4.6\,\mathrm{\mu m}$) shown in blue and red respectively. For each date of WISE observations, multiple images are taken, and the photometry of each image is averaged to provide the final values. The black dashed line represents the Gaia G band light curve, adjusted in magnitude scale by 7.5mag. Highlighted in orange and red are the 2017 and 2023 outbursts, respectively.
• Figure 5: Continuum-subtracted X-Shooter spectra across the three instrument arms. The three panels from top to bottom show the spectra from the UVB, VIS and NIR arms. In black is the outburst spectra and in red is the quiescent spectra. Highlighted as dashed lines are the prominent emission lines present in the spectra; a full list of the identified lines is presented in Appendix A.