FU Orionis-type outbursts are rare accretion events in young stars in which the rate at which material falls onto the star climbs to 10⁻⁴ solar masses per year and the luminosity rises by tens of times. Such outbursts last for decades and are thought to play a key role in stellar growth and the chemical evolution of protoplanetary disks. Historically only about a dozen fuors were known, but over the past decade sky surveys (PTF/ZTF, Gaia, ASAS-SN, VVV, NEOWISE) have raised their number to roughly 70. Most of these objects, however, lie farther than 1 kiloparsec, which rules out studying them at high spatial and spectral resolution, and pre-outburst spectra are almost always missing. Against this backdrop PR Ori B stands out: it sits in one of the nearest star-forming regions and has an extensive pre-outburst archive, including high-resolution spectra and ALMA submillimeter data.
The outburst was found while analyzing NEOWISE mid-infrared photometry (3-5 microns), where the PR Ori system showed variability with an amplitude of 1.1 magnitudes. Confirmation came from years of optical observations with the 80 cm RC80 telescope at Konkoly Observatory, data from Gaia (including the Gaia21arx alerts), ASAS-SN, ZTF, and archival images. Near-infrared spectroscopy was carried out on the IRTF with the SpeX instrument in 2023-2025, while high-resolution optical spectra were obtained with the BOES spectrograph (R=45000) and NGPS on the 200-inch Palomar telescope. A mid-infrared spectrum during the outburst was taken on the VLT with the VISIR instrument. To estimate the disk parameters, archival ALMA data (1.3 mm) were used and compared with viscous accretion disk models.
PR Ori B showed two brightness phases: the first (2015-2018) with an amplitude of ΔG=1.6 mag, and the second (from 2019 onward) with a total brightening of 5 magnitudes in the G band. In the mid-infrared the amplitudes were ΔW1=3.6 and ΔW2=2.8 mag. The near-infrared spectrum during the outburst shows deep ¹²CO and H₂O absorption bands, along with a triangular H-band shape, which is characteristic of a viscously heated disk. The optical spectrum displays P Cygni profiles in the Na I D lines and absorption in the infrared Ca II triplet, typical of fuors. The outburst luminosity is estimated at 30-40 solar luminosities, corresponding to an accretion rate of (4-14)×10⁻⁶ solar masses per year. A comparison of the Spitzer/IRS spectra (pre-outburst) and VLT/VISIR spectra (during the outburst) revealed enhanced emission from crystalline forsterite at 11.2 and 11.9 microns, pointing to silicate crystallization in the hot inner disk.
This work sets itself apart from neighboring studies of fuors by using a rare combination: a nearby object with a complete pre-outburst archive and simultaneous coverage from optical to submillimeter wavelengths. Unlike most known fuors, PR Ori B lies at a distance of just 388 parsecs, which makes it possible for the first time to trace in detail the evolution of the disk and wind during an outburst at high angular resolution. The detection of silicate crystallization during the outburst confirms that episodic accretion can be a mechanism for reprocessing dust in protoplanetary disks, something previously inferred only from indirect evidence.
The proximity and brightness of PR Ori B open the door to high-angular-resolution observations with interferometers (such as the VLTI) and future missions, in order to probe the physics of the inner disk and the changes in its structure caused by the increased luminosity. Continued monitoring of the outburst will make it possible to test models of viscous disk evolution and to pin down the link between accretion outbursts and dust crystallization. Studying how the surrounding nebula responds to the outburst radiation is also promising for estimating the geometry and density of the circumstellar environment.
The results will shape our understanding of protoplanetary disk evolution and star formation processes, as well as the interpretation of young-star variability in sky surveys.
Spectroscopic and photometric monitoring of PR Ori B is planned to continue, tracking the decline of the outburst and searching for possible repeat accretion episodes. JWST observations will be carried out to confirm the in-situ formation of crystalline silicates and to study the dust composition.
This work is directly tied to the unsolved problem of what drives FU Orionis outbursts: it is still unclear what triggers the sharp rise in the accretion rate, whether it be gravitational instability in the disk, interaction with a companion, or magnetic processes. The availability of pre-outburst data and the binary nature of PR Ori make it possible to test hypotheses about the outburst trigger. Moreover, the question of how often and how long such events occur in stars of different masses and ages remains open, which matters for assessing their contribution to stellar growth.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This paper reports the discovery of an FU Orionis-type outburst in PR Ori B in the L1641 region, found in NEOWISE data and confirmed by years of optical and infrared observations. The spectra show a transition from a cool late-K star to a viscously heated disk with deep CO and H2O bands, and the outburst luminosity is 30-40 solar luminosities. The object's proximity (388 pc) and the availability of pre-outburst archival data, including high-resolution spectra and ALMA, make it possible to trace the system's evolution in detail.
Where its strength is
A rare chance to study a fuor at a small distance with a full set of pre-outburst observations, which opens the way to a direct comparison of the disk's structure and chemistry before and during the outburst.
Where it could go next
Worth trying
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.52, against a usual 0.64. It looks like few have drilled nearby.
PR Ori B is only the second-brightest known FU Orionis-type outburst in the optical, falling roughly one magnitude short of the prototype of the FU Orionis class. What's more, the PR Ori system was already observed as a variable star back in 1888, and complaints about funding for astronomical observations can be found in publications from that era.
Outbursts of young stars like PR Ori B bring to mind the plot of Stanisław Lem's novel Solaris, in which the planet's ocean responds to the star's radiation: here the star's disk responds to the outburst by reprocessing dust and heating up, which alters the chemistry of the future planetary system.