Gaia20dsk: A new MNor discovered by the GLORIOUS collaboration

TL;DR

Gaia20dsk is identified as an MNor-like eruptive YSO through a coordinated analysis of Gaia/ATLAS/WISE photometry and a high-quality X-SHOOTER spectrum. The authors quantify accretion by both SED integration and empirical line diagnostics, obtaining $L_{ m acc}$ values on the order of $${8-26}$$ $L_\odot$ and $\dot M_{ m acc}$ in the $10^{-7}$ to $10^{-6}\,M_\\odot\,{ m yr}^{-1}$ range, depending on the assumed distance of $d_{ m NGC} \approx 1.65$ kpc or $d_{ m BJ} \approx 2.54$ kpc. The source shows two major optical bursts lasting years, a weaker IR response, and emission-line features (e.g., H$ obreak\alpha$, CaII triplet, Pa lines) plus outflow indicators like [SII]/[NII] and H$_2$, placing Gaia20dsk among MNors rather than classical EXors or FUors. Collectively, these results position Gaia20dsk as a moderately luminous, Class I/FS eruptive YSO with substantial ongoing accretion, contributing to the growing MNor–EXor–FUor continuum in the early stages of stellar evolution; the inferred $\dot M_{ m acc}$ values imply a significant, accretion-dominated luminosity during bursts, with practical implications for time-domain studies of YSO evolution in embedded environments.

Abstract

Context. Among known young stellar objects (YSOs), those exhibiting the most dramatic increases in brightness due to sudden increase in mass accretion rate are eruptive young stars. Gaia20dsk is one of the Gaia-alerted young star candidates that has displayed a double, nonperiodic brightening resembling that of other young eruptive stars. Aims. The goal of this work is to determine the physical and accretion properties of Gaia20dsk to confirm its classification as an eruptive young star. Methods. We combined publicly available optical and near-infrared (NIR) photometry with our X-shooter optical/NIR spectrum. In our analysis, we examined the optical and IR light curves from the bursts, reviewing the color-magnitude diagrams across different bands, reporting the detection of emission lines, and providing estimates of the star's accretion rates during the burst. Results. The optical light curve shows two major and one brief brightening events with a maximum amplitude of ~1.8 mag in the last five years. A classification based on spectral index indicates that Gaia20dsk is a flat-spectrum star. The X-shooter spectrum exhibit emission lines characteristic of accreting low-to-intermediate-mass young stars, displaying features typical of MNor-type objects. The mass accretion rate is between (0.5-1.8)*10^{-6} M_sun/yr. Conclusions. Gaia20dsk is an eruptive YSO that exhibits photometric features similar to those of MNors, including its characteristic brightening amplitude and burst duration, along with similar spectroscopic features and accretion rates.

Figures (26)

• Figure 1: Far left: Spatial distribution of clusters from Hunt in the vicinity of Gaia20dsk (shown with a star), overlaid on the DECaPS dust map Zucker2025. The arrows indicate the proper motions of the cluster members. Left center: Distance distribution of the cluster members. The thick solid and dashed lines show the $d_{\rm BJ}$ distance of Gaia20dsk and its uncertainties. Right center: Proper motion distribution of the cluster members and Gaia20dsk, shown with error bars. Far right: Cumulative reddening as a function of distance toward Gaia20dsk, based on the data from Zucker2025. The red dash-dotted line indicates the most likely distance to Gaia20dsk, while the black solid and dashed lines represent $d_{\rm BJ}$ and its uncertainties.
• Figure 2: Gaia20dsk light curve. Gaia $G$ photometry is represented by black dots, with error bars similar in size to or smaller than the data points. ATLAS $o$ weekly median photometry is shown in orange. The WISE $W1$ and $W2$ bands are represented by blue and red dots, shifted by 9.5 mag for clarity. The dashed vertical line indicates the date of the X-SHOOTER measurements. The three triangles at the bottom of the plot mark the first major burst, which began in 2019 October (green); a short brightening event around 2022 February (blue). A second major burst that started in 2023 February (brown).
• Figure 3: X-SHOOTER spectrum of Gaia20dsk. The optical part is shown in gray, while the IR range is shown in red. The spectrum is not corrected for extinction. The noisy part below 7500 Å is not shown.
• Figure 4: Top panel: Segment of the spectra of our target Gaia20dsk (in blue) and the best-fitting photosphere HD188262 (shown in magenta). See Sect. \ref{['par:remove']} for details. Middle panel: Example of one of the CaII lines before correction with the best-fitting photosphere. The colors indicate the same sources as in the top panel. Bottom panel: Photosphere-subtracted accretion-tracing line.
• Figure 5: Gaia20dsk SED. Different colored dots denote different surveys, as explained in the legend. The synthetic photometry in $I$, $J$, $H$, and $K_{\rm s}$ bands represented by the cyan color were computed from the X-SHOOTER spectrum (labeled X-S s.p.). The spectrum itself is plotted with reduced resolution for clarity. The points are not corrected for extinction. The sizes of the error bars are comparable to or smaller than the symbol size.