The long-term outburst(s) of GPSV16: from an intermediate to a FUor classification

Calum Morris; Carlos Contreras Pe\~na; Doug Johnstone; Gregory Herczeg; Ho-Gyu Lee; Hwan-Ki Kim; Jeong-Eun Lee; Jessy Jose; Mizna Ashraf; Philip W. Lucas

arxiv: 2604.24402 · v1 · submitted 2026-04-27 · 🌌 astro-ph.SR

The long-term outburst(s) of GPSV16: from an intermediate to a FUor classification

Carlos Contreras Pe\~na , Jeong-Eun Lee , Philip W. Lucas , Gregory Herczeg , Doug Johnstone , Zhen Guo , Ho-Gyu Lee , Hwan-Ki Kim

show 3 more authors

Jessy Jose Mizna Ashraf Calum Morris

This is my paper

Pith reviewed 2026-05-08 01:29 UTC · model grok-4.3

classification 🌌 astro-ph.SR

keywords FU Orionis outburstsyoung stellar objectsaccretion disksprotoplanetary diskseruptive variablesmid-infrared photometryClass I YSOdisk instabilities

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The pith

The second outburst of GPSV16 is a FUor whose mid-IR rise began eight years before the optical outburst, indicating the disk instability started at roughly 0.4 AU and propagated inward.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper examines multi-wavelength photometry and near-IR spectra of the Class I young stellar object GPSV16, which lies in the HII region G71.52-00.38 at 3.61 kpc. It documents two outbursts: a moderate one from 2005 to 2012 and a much stronger one beginning in 2016 that reached an accretion luminosity of about 130 solar luminosities. The second event shows absorption-line spectra typical of FU Orionis objects rather than emission lines, and its mid-IR light curve rose in two stages over roughly 8.4 years, with the mid-IR increase starting eight years ahead of the optical brightening. The authors interpret the wavelength-dependent delay as evidence that an accretion instability was triggered at larger disk radii and moved inward over time. This approach demonstrates how sustained photometric monitoring across bands can locate the radial origin of instabilities that drive eruptive variability in young stars.

Core claim

The second outburst of GPSV16, beginning in 2016, qualifies as a FUor event with a K_s amplitude of 5.6 magnitudes and an accretion luminosity near 130 L_sun. Its mid-IR photometry exhibits a two-stage rise that required approximately 8.4 years to reach peak and commenced about eight years earlier than the optical outburst, consistent with an instability that began at r approximately 0.4 AU and propagated inward through the disk.

What carries the argument

The measured eight-year offset between the onset of the mid-IR rise and the optical rise, interpreted as the travel time of an inward-propagating accretion instability whose starting radius is then estimated from the propagation interval.

If this is right

Long-term, multi-band photometry can map the radial location where disk instabilities first appear in young stellar objects.
The same object can produce outbursts with different peak accretion rates that produce distinct spectral signatures, from hot inner-disk emission to cooler viscous-disk absorption.
The instability responsible for the second outburst must have been triggered outside the innermost disk regions and taken years to affect the optical-emitting zone.
FUor-like events in embedded Class I sources may be identified and characterized even before the optical peak is reached.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

If the propagation speed depends on local disk viscosity, repeated observations of similar delays could constrain the viscosity parameter at sub-AU scales.
Other eruptive YSOs monitored in the same way might reveal a distribution of starting radii for their instabilities, testing whether all FUor events share a common trigger location.
The two-stage mid-IR rise could indicate separate thermal or density fronts, offering a way to test specific instability models once more objects are observed at comparable wavelengths.

Load-bearing premise

The eight-year interval between mid-IR and optical rise is produced entirely by the inward travel time of one accretion instability whose speed can be used to calculate its starting radius of 0.4 AU.

What would settle it

High-cadence monitoring that shows the mid-IR flux began rising at the same epoch as the optical flux, or a viscous-propagation calculation at 0.4 AU that yields a travel time inconsistent with eight years under standard disk parameters.

Figures

Figures reproduced from arXiv: 2604.24402 by Calum Morris, Carlos Contreras Pe\~na, Doug Johnstone, Gregory Herczeg, Ho-Gyu Lee, Hwan-Ki Kim, Jeong-Eun Lee, Jessy Jose, Mizna Ashraf, Philip W. Lucas, Zhen Guo.

**Figure 1.** Figure 1: (Top) 1999-2026 light curve of GPSV16. (Bottom, left) 2010-2013 section of the light curve. (Bottom, right) 2016-2026 section of the light curve. In all plots, open, upside-down triangles denote upper limits on the photometry from various surveys. 2.2 Optical and near-IR photometry The UKIDSS GPS survey used the UKIRT telescope to provide nearIR photometry of the Galactic plane accessible from the norther… view at source ↗

**Figure 4.** Figure 4: 60′′ × 30′′ single epoch i-band images from the ZTF survey at different epochs (marked in the images). The green circles mark the location of GPSV16. In the images North is up and East is to the left. 2.3 IRTF/SpeX Spectroscopy We obtained a near-IR spectrum of GPSV16 using SpeX (Rayner et al. 2003) mounted at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea (programme 2025B096, PI Contreras Peña).… view at source ↗

**Figure 2.** Figure 2: (Top) 14′′ ×7 ′′ UKIRT K band image (observations in 2008, during maximum brightness of the first outburst) centered at the location of GPSV16 . The coloured circles mark the centroids of single-epoch detections from the NEOWISE survey in the range 56800 <MJD< 61000 d. (Middle) Distance of the centroids of NEOWISE detections towards the coordinates of GPSV16. (Bottom) Diagram showing the colours used to ma… view at source ↗

**Figure 3.** Figure 3: SPHEREx spectrum of GPSV16 obtained at two epochs in May 2025 (blue) and October 2025 (red). We show the IRTF/SpeX spectrum obtained in August 2025 for comparison view at source ↗

**Figure 5.** Figure 5: (Left) J-H versus H-Ks colour-colour diagram of UKIDSS GPS sources (grey circles) located within 5′ of HII region G71.52−00.38. The classical T Tauri locus of Meyer et al. (1997) is presented (solid green line) along with intrinsic colours of dwarfs and giants (blue and red solid lines, respectively) from Bessell & Brett (1988). Reddening vectors of A𝑉 = 20 mag are shown as dotted lines. Stars to the right… view at source ↗

**Figure 6.** Figure 6: (Top) ZTF r and WISE 𝑊1 light curve of GPSV16 during 59000 <MJD< 61200 d. (Bottom) 𝑊1 − 𝑊2 ligth curve of the source during the same period of time. The gray line in both plots marks the approximate start of the optical fading. values of the accretion rate reached during the two outbursts (see e.g. Liu et al. 2022). The accretion rate also influences the characteristics observed in the near-IR spectra dur… view at source ↗

**Figure 7.** Figure 7: (Top) 2 − 2.5𝜇m Gemini/NIFS spectrum of GPSV16 taken during the first outburst (August 2012). (Bottom) Spectrum of GPSV16 (black) between 0.8 and 2.5 𝜇m taken during the second outburst (August 2025). The spectrum of FU Ori Ori (purple) is shown for comparison, and it has been reddened by A𝑉 = 8.2 mag to match the spectrum of GPSV16. The insets show the Paschen 𝛽 and He 1 lines of GPSV16. large enough (𝜂 >… view at source ↗

**Figure 8.** Figure 8: W1 versus W1−W2 for the light curve of GPSV16 (red stars). In the figure, we compare with isomass curves for an FUor outburst (Liu et al. 2022) at a distance of 3.61 kpc (Section 3) with the mass of the central star, M∗ = 0.6 and 1.2M⊙, and using A𝑉 = 9.7 mag. The solid symbols mark discrete values of M¤ used in the models of (Liu et al. 2022), where 𝜂 = 1 (circles) and 𝜂 = 5 (upside down triangle). Finall… view at source ↗

**Figure 9.** Figure 9: Accretion luminosity of the second outburst (since 2016) of GPSV16, derived using the bolometric corrections from Carvalho & Hillenbrand (2024). The values are estimated for different inclination angles 10 ≤ 𝜃 ≤ 80 deg from the brightest magnitudes in the filters 𝑟 (blue circles) and 𝑖 (green circles) from the ZTF survey, and in W1 (yellow circles) and W2 (red circles) from NEOWISE. and varying the inclin… view at source ↗

**Figure 10.** Figure 10: Optical (blue) and mid-IR (red) light curves of FUors Gaia18dvy (top), Gaia17bpi (middle), and GPSV16 (bottom). The sigmoid functions fitted to each light curve are shown with solid lines. Upper limits of the rband photometry of GPSV16 are marked with upside-down, cyan triangles. the first stage of mid-IR brightening started at least 8 years before the onset of the optical outburst (if we assume that the… view at source ↗

read the original abstract

FU Ori outbursts are thought to play a key role in stellar mass assembly and in the chemistry of protoplanetary disks during the early formation of stars. However, uncertainties remain regarding the universality of these events and the physical mechanism driving the high-amplitude variability. In this work, we present an analysis of optical, near- and mid-IR photometry (ZTF, UKIDSS GPS, NEOWISE) and near-IR spectra (IRTF, Gemini) of the eruptive variable Class I YSO GPSV16. The YSO, associated with the HII region G71.52$-$00.38 ($d=3.61$~kpc), showed two outbursts, one with $\Delta K_{\rm s}=2.2$~mag (2005-2012) and a second starting in 2016 with $\Delta K_{\rm s}=5.6$~mag and accretion luminosity of $\sim$130 L$_{\odot}$. The outbursts displayed distinct spectroscopic characteristics: the first showed emission lines associated with a hot inner disk surface, whereas the second showed absorption lines arising from the cooler upper layers of a viscously heated disk. These features likely arose due to the different accretion rates reached during each outburst. The second outburst showed a two-stage mid-IR rise, requiring $\approx8.4$ years to reach peak brightness. The mid-IR rise also started 8 years before the onset of the optical outburst. The wavelength-dependent light curve points to an instability that is triggered at larger distances within the accretion disk and propagates inward. Assuming a propagation time of 8 years for the accretion wave, we estimate that the second outburst started at a distance of $r\sim0.4$~AU. These results show how long-term, multi-wavelength photometric monitoring can help identify the disk instabilities that trigger eruptions in YSOs.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

GPSV16 shows a clear second outburst with FUor spectrum and high luminosity, plus an 8-year mid-IR lead that they convert to a 0.4 AU starting radius under a simple propagation assumption.

read the letter

GPSV16 had two outbursts, the second one much larger and showing the absorption lines and high luminosity that mark it as a FUor rather than the emission-line intermediate state of the first event. They also point out that the mid-IR started rising eight years before the optical, which they take as evidence the trigger was at roughly 0.4 AU and the disturbance moved inward. The paper does a good job assembling the light curves from several public surveys and adding spectra that clearly separate the two phases. The data support the idea of episodic accretion with different strengths, and the timing difference between wavelengths is a concrete observation that models of disk instabilities should be able to address. The main limitation is how they turn the eight-year delay into a starting radius. They state it as an assumption that the delay equals the propagation time, without showing a calculation based on disk parameters like viscosity or sound speed. That makes the 0.4 AU figure more illustrative than precise. A factor of three change in the assumed speed would move the radius noticeably, and without the model it's hard to judge how reasonable the number is. The work is aimed at researchers studying star formation and protoplanetary disks, especially those interested in the triggers for FUor events. It provides a new example with good multi-wavelength coverage and a clear spectroscopic classification. The observations look solid and the claims are not overstated beyond the assumption they flag. I would send this to peer review. The data are worth referee attention even if the radius estimate needs more justification or sensitivity tests in revision.

Referee Report

2 major / 2 minor

Summary. The paper presents multi-wavelength photometry (ZTF, UKIDSS GPS, NEOWISE) and near-IR spectroscopy (IRTF, Gemini) of the Class I YSO GPSV16, associated with HII region G71.52-00.38 at d=3.61 kpc. It identifies two distinct outbursts: a moderate event (ΔKs=2.2 mag, 2005-2012) showing emission lines from a hot inner disk, and a stronger event starting in 2016 (ΔKs=5.6 mag, L_acc≈130 L⊙) with absorption lines from a viscously heated disk, classifying the latter as FUor-like. The second outburst exhibits a two-stage mid-IR rise with an 8-year lead over the optical onset, interpreted as an accretion instability triggered at r∼0.4 AU and propagating inward.

Significance. If the spectroscopic classification and propagation interpretation hold, the work strengthens evidence that FUor outbursts can be triggered by instabilities originating at larger disk radii and provides a concrete example of how long-term, multi-band monitoring distinguishes outburst mechanisms in YSOs. The data-driven distinction between the two events and the reported accretion luminosity are useful for models of disk chemistry and stellar mass assembly.

major comments (2)

[abstract and radius derivation section] The quantitative claim that the instability began at r∼0.4 AU rests on equating the full 8-year mid-IR lead time directly to propagation delay (abstract and the section deriving the radius). No explicit calculation of the expected travel time—using viscous timescale t_visc≈r²/ν with α, c_s, or H/r at 0.4 AU, or thermal front speed—is provided, nor is parameter sensitivity shown. A plausible factor-of-3 variation in wave speed would shift the launch radius outside the inner-disk region, weakening the distinction from a generic inner-disk instability.
[luminosity calculation section] The accretion luminosity of ∼130 L⊙ for the second outburst (and thus the FUor classification) depends on the adopted distance d=3.61 kpc and the conversion from observed flux; the manuscript should state the exact bolometric correction or SED integration used and propagate the distance uncertainty into the L_acc error bar.

minor comments (2)

[figures] Figure captions for the light curves should explicitly label the two-stage mid-IR rise and mark the 8-year offset between mid-IR and optical onsets for clarity.
[results] The first outburst's ΔKs=2.2 mag is given without the corresponding Δm in other bands; adding a brief comparison table would help quantify its 'intermediate' nature relative to the second event.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript, the positive assessment of its significance, and the constructive major comments. We address each point below and will revise the manuscript to improve clarity on the radius derivation and luminosity calculation.

read point-by-point responses

Referee: [abstract and radius derivation section] The quantitative claim that the instability began at r∼0.4 AU rests on equating the full 8-year mid-IR lead time directly to propagation delay (abstract and the section deriving the radius). No explicit calculation of the expected travel time—using viscous timescale t_visc≈r²/ν with α, c_s, or H/r at 0.4 AU, or thermal front speed—is provided, nor is parameter sensitivity shown. A plausible factor-of-3 variation in wave speed would shift the launch radius outside the inner-disk region, weakening the distinction from a generic inner-disk instability.

Authors: We agree that the manuscript presents the r∼0.4 AU estimate as an assumption based on the observed 8-year mid-IR lead time without an accompanying explicit calculation of the propagation timescale. In the revised manuscript we will add a dedicated paragraph in the radius derivation section that computes the viscous timescale t_visc ≈ r²/ν (and the corresponding thermal front speed) using standard Class I disk parameters at 0.4 AU (α=0.01, T≈1000 K, H/r≈0.05–0.1). This will show that an 8-year travel time is consistent with plausible wave speeds of ∼0.05 AU yr⁻¹. We will also include a brief sensitivity discussion demonstrating that even a factor-of-3 variation in wave speed keeps the launch radius within 0.1–1 AU, preserving the distinction from a purely inner-disk (r≪0.1 AU) instability. These additions will be made without changing the core interpretation. revision: yes
Referee: [luminosity calculation section] The accretion luminosity of ∼130 L⊙ for the second outburst (and thus the FUor classification) depends on the adopted distance d=3.61 kpc and the conversion from observed flux; the manuscript should state the exact bolometric correction or SED integration used and propagate the distance uncertainty into the L_acc error bar.

Authors: We agree that the derivation of L_acc should be stated more explicitly. In the revised manuscript we will expand the luminosity section to specify the exact bolometric correction (or the multi-band SED integration procedure) applied to the peak photometry. We will also propagate the distance uncertainty for the associated H II region into the reported L_acc value, yielding an explicit error bar (e.g., 130 ± Δ L⊙). This will make the FUor classification more robust and transparent. revision: yes

Circularity Check

0 steps flagged

No significant circularity in the derivation chain

full rationale

The paper's key quantitative claim (r ~ 0.4 AU) is obtained by explicitly stating an assumption ('Assuming a propagation time of 8 years for the accretion wave') and converting the observed 8-year mid-IR lead time into a radius. This is an interpretive mapping from data to a physical scale under a stated premise, not a self-definitional loop, a fitted parameter relabeled as prediction, or a result forced by self-citation. Spectroscopic classification as FUor and the luminosity estimate rest on direct observations (absorption lines, photometry) without reducing to the radius assumption by construction. No load-bearing uniqueness theorems or ansatzes are imported via self-citation. The derivation chain remains self-contained and does not equate outputs to inputs.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of YSO accretion-disk physics and the association of GPSV16 with the HII region at 3.61 kpc; the radius estimate introduces one interpretive free parameter (propagation time equals observed delay).

free parameters (2)

distance to GPSV16
Adopted value of 3.61 kpc from association with G71.52-00.38 used to convert observed fluxes into luminosities.
propagation time equals 8-year delay
Assumed equality used to convert the observed mid-IR lead time into a starting radius of ~0.4 AU.

axioms (1)

domain assumption The mid-IR and optical rises are produced by the same inward-propagating accretion instability.
Invoked to interpret the timing offset as radial propagation rather than separate events.

pith-pipeline@v0.9.0 · 5688 in / 1372 out tokens · 36978 ms · 2026-05-08T01:29:07.391678+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

1 extracted references · 1 canonical work pages

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[1] [1]

Space Telescopes and Instrumentation 2020: Optical, Infrared, and Millimeter Wave , year = 2020, editor =

Bae J., Hartmann L., Zhu Z., Nelson R. P., 2014, ApJ, 795, 61 Bell K. R., Lin D. N. C., 1994, ApJ, 427, 987 Bellm E. C., et al., 2019, PASP, 131, 018002 Bessell M. S., Brett J. M., 1988, PASP, 100, 1134 Bonnell I., Bastien P., 1992, ApJ, 401, L31 Bryan G. R., Maddison S. T., Liffman K., 2019, MNRAS, 489, 3879 Burlak M. A., et al., 2026, arXiv e-prints, p....

work page doi:10.1117/12.2567224 2014