🔭 astrophysics

Characterisation of an EXor outburst SPICY 97589

This article characterizes the 2023 outburst of the young stellar object SPICY 97589 as an EXor-like accretion event, identifying a central M3.0 star with a mass of 0.29 $M_\odot$ and an accretion rate that increased by two orders of magnitude during the outburst, thereby providing new insights into the evolution of accretion disks driven by episodic accretion.

Original authors: Aaron Labdon, Rik Claes

Published 2026-05-06

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Aaron Labdon, Rik Claes

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a young star still in its "baby" phase, sitting in a cosmic nursery. This star, named SPICY 97589, is surrounded by a swirling disk of gas and dust, much like a child spinning in circles wrapped in a blanket. Normally, this star gently pulls some of this blanket material toward itself to grow. But sometimes, the star experiences a sudden, massive hunger pang and grabs a huge portion of material all at once. This event is called an accretion outburst.

This article is a detailed performance report on a specific outburst that occurred in 2023, temporarily turning the star into a "superstar" before it calmed down again. Here is what astronomers found, explained simply:

1. The Identity of the Star: A Tiny, Young Baby

Before this event, we knew little about the star's true personality because the outburst was so bright that it obscured the star's face. Once the star calmed down, astronomers used a special tool (a high-tech camera called X-Shooter) to search for the star's "fingerprint" (its spectrum).

What they found: The star is of type M3.0, which is a cool, reddish dwarf star. It is quite small (about 30% of the mass of our Sun) and very young—only about 750,000 years old. To put this in perspective: our Sun is 4.6 billion years old; this star is, on the cosmic timescale, essentially a newborn.
The Analogy: Imagine finding a toddler who suddenly has a beard and looks like a giant during a tantrum. Once the tantrum (the outburst) is over, you realize it was actually just a tiny toddler.

2. The Outburst: A Cosmic "Feast"

The star recently had two major outbursts: one in 2017 and a larger one in 2023.

The 2023 Event: The star suddenly became 2.5 times brighter in visible light than usual. This was not a slow change; it was a rapid rise.
The Cause: The star did not just shine; it actively sucked material from its surrounding disk. Astronomers calculated that during the peak of this event, the star consumed material 100 times faster than during quiet phases.
The Analogy: Imagine a person who normally eats one sandwich per day. Suddenly, for several months, they devour an entire buffet every hour. That is what happened to this star.

3. The Evidence: A Forest of Light

To prove that this was a binge-eating episode and not just a temperature change of the star, astronomers examined the light coming from the star.

The "Forest": During the outburst, the light spectrum looked like a dense forest of trees. These "trees" were bright lines of light (emission lines) from elements such as hydrogen, helium, and calcium.
The Wind: The shape of these light lines told a story about wind. The star blew a strong wind away from its surface, like a hairdryer on "high." As the star calmed down, the wind died down, and the "forest" of light lines disappeared, leaving only a quiet whisper.
The Analogy: It is like a campfire. When the fire rages (outburst), you see bright sparks flying everywhere (strong emission lines) and hear the crackling of the wind. When the fire dies down (quiet phase), you see only the glowing embers and hear a gentle breeze.

4. The Aftermath: A Star That Got Too Full?

Here is a strange twist that astronomers noticed. Normally, a star returns to its normal, pre-outburst brightness after its "feast."

The Anomaly: After SPICY 97589 ended its 2023 outburst, it did not just return to normal; it actually became dimmer than before the outburst.
The Theory: Astronomers suspect the star ate so much and so quickly that it emptied the inner part of its "blanket" (the disk). Without this inner dust, which reflects light or supplies the star, the system became darker. It is like a child eating all the cookies in the jar, leaving the jar empty and the kitchen darker than before.

5. Why This Matters

This star is a textbook example of a variable star of type EXor. These are stars that have short, frequent binge-eating episodes (unlike their cousins, the FUors, which have massive, century-long eating binges).

The Conclusion: SPICY 97589 is one of the youngest EXor stars ever found. It shows us that even very young, low-mass stars can have dramatic, violent growth spurts. It proves that stars do not grow steadily; they grow in bursts and jolts, devouring their surroundings in large gulps.

In summary: The article tells the story of a tiny, newborn star that had a massive, short-lived binge-eating episode in 2023. By examining the light it emitted during and after the binge, astronomers confirmed it was a classic "EXor" outburst, measured exactly how much it ate, and found that the star is now somewhat dimmer because it has exhausted its own food supplies.

Technical Summary: Characterization of an EXor Outburst in SPICY 97589

Problem and Context
Stellar outbursts driven by episodic accretion are considered ubiquitous among young stellar objects (YSOs), yet relatively few such objects have been fully characterized. Although accretion is fundamental to mass transport in astrophysics, observations indicate that it is not a stationary process but rather consists of episodic outbursts ranging from low variability to dramatic eruptions. These are broadly classified into FU Orionis types (FUors), characterized by long-lasting, high-amplitude brightness increases, and EX Lupi types (EXors), characterized by shorter, low-amplitude outbursts that recur every few years. SPICY 97589 (Gaia23bab) was identified in March 2023 as an erupting YSO that had already reached its maximum brightness. Previous preliminary studies (Kuhn et al. 2023; Giannini et al. 2024; Nagy et al. 2025) provided an initial light curve analysis and rough spectral estimates, but a comprehensive spectroscopic characterization of the central star and accretion dynamics during and after the outburst was required to fully understand the nature of the event and its impact on disk evolution.

Methodology
The authors employed multi-band, medium-resolution spectroscopy using the X-Shooter instrument at the European Southern Observatory (ESO). Observations were conducted in two distinct epochs:

Outburst Peak: April 19, 2023, approximately two months after the peak of the 2023 outburst, when the object was still about 2 magnitudes brighter than in its quiescent state.
Quiescent State: July 25, 2024, after the object had returned to a stable, faint state.

The dataset covers the UVB (0.3–0.55 µm), VIS (0.55–1.02 µm), and NIR (0.99–2.47 µm) bands. Data reduction utilized the standard X-Shooter pipeline (v. 3.6.3) for absolute flux calibration and Molecfit (v. 1.5.9) for terrestrial correction.

To characterize the central star, the authors applied the FRAPPE tool to the post-outburst spectrum, as the spectrum during the outburst was too heavily veiled by accretion emission to determine photospheric features. This tool fits a grid of accretion slab models, interpolated Class III stellar templates, and extinction laws. Stellar parameters were further constrained using isochrones from Baraffe et al. (2015).

Accretion rates were derived by measuring the equivalent widths (EW) of 22 emission lines (primarily hydrogen, but also Helium I and Calcium II) in the dereddened, continuum-subtracted spectra. The authors utilized established relationships between line luminosity and mass accretion rate (e.g., Alcalá et al. 2017; Fairlamb et al. 2017). Balmer series lines were largely excluded from the final accretion rate calculation because strong P-Cygni profiles affected their equivalent widths. Light curve analysis combined Gaia G-band and WISE mid-infrared (W1, W2) photometry to model the rise and fall of the 2017 and 2023 outbursts.

Main Results

Stellar Properties: The central star is identified as an M3.0 type object with an effective temperature of 3410 K, an extinction of $A_V = 4.4$ , and a luminosity of $0.41 \pm 0.09 L_{\odot}$ . Based on isochrone fitting, the stellar mass is estimated at $0.29 \pm 0.05 M_{\odot}$ with an age of $0.75 \pm 0.5$ Myr.
Accretion Rates: The mass accretion rate during the 2023 outburst was measured at $\dot{M} = 2.38 \pm 0.58 \times 10^{-7} M_{\odot} \text{yr}^{-1}$ . In the post-outburst quiescent state, the rate dropped to $\dot{M} = 4.24 \pm 0.67 \times 10^{-9} M_{\odot} \text{yr}^{-1}$ . This represents a decrease of two orders of magnitude.
Outburst Characteristics: The 2023 outburst reached a peak of 2.51 magnitudes above the mean quiescent brightness in the Gaia G-band, with a full width at half maximum (FWHM) of 237 days and a total duration of 671 days. This is somewhat stronger and longer than the 2017 outburst (duration of 358 days).
Spectral Evolution: The spectrum during the outburst is dominated by a "forest" of emission lines (Balmer, Paschen, Brackett series, and metallic lines such as Fe, Ca, Mg). Notably, the Balmer lines show P-Cygni profiles with both absorption and emission peaks that are redshifted, indicating an inclined disk wind geometry. In contrast, the quiescent spectrum shows a transition to weak, double-peaked emissions without P-Cygni absorption, indicating a "switching off" of the high-velocity disk wind mechanism.
Post-Outburst Fading: Unlike some other EXors (e.g., EX Lup), which return to their pre-outburst levels, SPICY 97589 remained fainter immediately after the 2023 event than in its pre-outburst state. The authors hypothesize that this may be due to the depletion of the inner disk and warm dust by the outburst.

Significance and Claims
The work confirms SPICY 97589 as an episodic accretor of the EXor type, adding it to the small population of known objects in this class. The study provides the first robust characterization of the central star's parameters (mass, age, spectral type) by utilizing post-outburst spectra to overcome the veiling effects present during the outburst.

The authors claim that the derived accretion rate of $\sim 2.4 \times 10^{-7} M_{\odot} \text{yr}^{-1}$ agrees with other independent measurements of this object taken shortly after the peak, validating the methodology used. The significant decline in accretion rate (by two orders of magnitude) and the specific spectral evolution (from strong P-Cygni profiles to double-peaked emissions) offer insights into the physical mechanisms driving disk winds and the depletion of inner disk material during outbursts.

The work notes that SPICY 97589 is remarkably young ( $\sim 0.75$ Myr) compared to other known EXors (e.g., EX Lup at $\sim 3.5$ Myr), which may challenge the classical view that EXor-like outbursts occur only after the infalling envelope of Class I objects is depleted. However, the authors maintain a modest stance, acknowledging that isochronal ages for individual targets can be unreliable and that the discrepancy between the derived age and the spectral index (which suggests Class II) could be due to modeling uncertainties or the high variability of the object. The work underscores the importance of accretion outbursts for stellar mass buildup and estimates that $\sim 2.3 \times 10^{-8} M_{\odot}$ was accreted during the 2023 event alone.

1. The Identity of the Star: A Tiny, Young Baby

2. The Outburst: A Cosmic "Feast"

3. The Evidence: A Forest of Light

4. The Aftermath: A Star That Got Too Full?

5. Why This Matters

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