# Gaia 17bpi: An FU Ori Type Outburst

## TL;DR

Gaia 17bpi is a newly identified FU Ori-type outbursting young star, with a multi-year, wavelength-dependent brightening and distinctive spectroscopic features, providing new insights into star formation and accretion processes.

## Contribution

This study documents the first observed wavelength-dependent outburst onset in a young star, combining optical and infrared data with spectroscopic analysis.

## Key findings

- Outburst started between October and December 2014.
- Optical and mid-infrared lightcurves show correlated brightening.
- Spectroscopy reveals characteristic FU Ori features across multiple wavelengths.

## Abstract

We report on the source Gaia 17bpi and identify it as a new, ongoing FU Ori type outburst, associated with a young stellar object. The optical lightcurve from Gaia exhibited a 3.5 mag rise with the source appearing to plateau in mid/late 2018. Mid-infrared observations from NEOWISE also show a $>$3 mag rise that occurred in two stages, with the second one coincident with the optical brightening, and the first one preceding the optical brightening by $\sim$1.5 years. We model the outburst as having started between October and December of 2014. This wavelength-dependent aspect of young star accretion-driven outbursts has never been documented before. Both the mid-infrared and the optical colors of the object become bluer as the outburst proceeds. Optical spectroscopic characteristics in the outburst phase include: a GK-type absorption spectrum, strong wind/outflow in e.g. Mgb, NaD, H$\alpha$, KI, OI, and CaII profiles, and detection of LiI 6707 \AA. The infrared spectrum in the outburst phase is similar to that of an M-type spectrum, notably exhibiting prominent $H_2O$ and $^{12}$CO (2-0) bandhead absorption in the K-band, and likely HeI wind in the Y-band. The new FU Ori source Gaia 17bpi is associated with a little-studied dark cloud in the galactic plane, located at a distance of 1.27 kpc.