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A New FU Orionis Accretion Outburst in the W5 HII Region

Lynne A. Hillenbrand, Matthew J. Graham, Mansi M. Kasliwal, Josiah Purdum, Jesper Sollerman, Adolfo S. Carvalho, Michael A. Kuhn, Roger Smith, Michael C. B. Ashley, Nicholas Earley, Russ R. Laher, Tracy X. Chen

TL;DR

This paper reports the discovery of a new FU Orionis-type accretion outburst in the W5 HII region from a pre-outburst Class II YSO, [KAG2008] 13656. The eruption brightened by about $\Delta r \approx -4$ mag and $\Delta J \approx -3$ mag over ~75 days in late 2025, with peak near $\mathrm{MJD} \approx 60980$, identified via the ZTF alert stream. Follow-up optical/NIR spectroscopy with Palomar's SED Machine and Keck/MOSFIRE reveals absorption features and molecular bands ($H\alpha$, $H\beta$, Na I D, $K I$, $H_2O$, $CO$) consistent with a FU Ori disk and outflow; multi-wavelength lightcurves corroborate the outburst across g/r/J/W1/W2 bands. The authors propose naming the source as FUOr-Cas 0258+6112 if the state persists and encourage continued, multi-wavelength monitoring to advance understanding of episodic accretion and disk evolution in distant star-forming environments.

Abstract

We announce a recently detected outburst that is currently only a few months old, and probably of FU Orionis type. The progenitor to the outburst was an emission-line, flat-spectrum SED young stellar object located in the W5 region, though somewhat outside the main star formation action. We present optical, near-infrared, and mid-infrared lightcurves that illustrate the quiescent state of [KAG2008] 13656 and its subsequent$Δr \approx -4$ mag and $ΔJ\approx -3$ mag outburst over $\sim$75 days in late-2025. Follow-up optical and near-infrared spectroscopy confirms the expected features from an FU Ori disk and outflow.

A New FU Orionis Accretion Outburst in the W5 HII Region

TL;DR

This paper reports the discovery of a new FU Orionis-type accretion outburst in the W5 HII region from a pre-outburst Class II YSO, [KAG2008] 13656. The eruption brightened by about mag and mag over ~75 days in late 2025, with peak near , identified via the ZTF alert stream. Follow-up optical/NIR spectroscopy with Palomar's SED Machine and Keck/MOSFIRE reveals absorption features and molecular bands (, , Na I D, , , ) consistent with a FU Ori disk and outflow; multi-wavelength lightcurves corroborate the outburst across g/r/J/W1/W2 bands. The authors propose naming the source as FUOr-Cas 0258+6112 if the state persists and encourage continued, multi-wavelength monitoring to advance understanding of episodic accretion and disk evolution in distant star-forming environments.

Abstract

We announce a recently detected outburst that is currently only a few months old, and probably of FU Orionis type. The progenitor to the outburst was an emission-line, flat-spectrum SED young stellar object located in the W5 region, though somewhat outside the main star formation action. We present optical, near-infrared, and mid-infrared lightcurves that illustrate the quiescent state of [KAG2008] 13656 and its subsequent mag and mag outburst over 75 days in late-2025. Follow-up optical and near-infrared spectroscopy confirms the expected features from an FU Ori disk and outflow.

Paper Structure

This paper contains 3 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Photometric Outburst Detection and Spectroscopic Follow-up
  3. Discussion

Figures (1)

  • Figure 1: Top left: ZTF r-band image (N/E are top/left) of the pre-outburst source and environment. Top right: SEDs assembled for the progenitor source (black points) along with a plausible stellar model (blue line), and for the outbursting source (magenta). Bottom left: Lightcurves in optical ZTF g-band (0.48 $\mu$m), r-band (0.64 $\mu$m); near-infrared Gattini J-band (1.6 $\mu$m); and mid-infrared NEOWISE W1 (3.6 $\mu$m), W2 (4.5 $\mu$m) bands. Bottom right: Optical spectrophotometry highlighting absorption in plausible disk/wind lines.