A multi-epoch, multi-wavelength study of the classical FUor V1515 Cyg approaching quiescence

TL;DR

This study presents multi-epoch, multi-wavelength observations of FUor V1515 Cyg, revealing long-term fading, spectral features indicative of disk cooling, and a decreasing accretion rate, providing insights into its quiescent evolution.

Contribution

It offers the first comprehensive multi-wavelength monitoring of V1515 Cyg's long-term behavior, highlighting spectral and luminosity changes during its transition towards quiescence.

Findings

Long-term fading with variability on weekly and monthly scales.

Spectral evidence of disk cooling and reduced outflow activity.

Significant decrease in accretion disk luminosity from 138 to 45 solar luminosities.

Abstract

Historically, FU Orionis-type stars are low-mass, pre-main sequence stars. The members of this class experience powerful accretion outbursts and remain in an enhanced accretion state for decades or centuries. V1515 Cyg, a classical FUor, started brightening in the 1940s and reached its peak brightness in the late 1970s. Following a sudden decrease in brightness it stayed in a minimum state for a few months, then started a brightening for several years. We present results of our ground-based photometric monitoring complemented with optical/NIR spectroscopic monitoring. Our light curves show a long-term fading with strong variability on weekly and monthly time scales. The optical spectra show P Cygni profiles and broad blue-shifted absorption lines, common properties of FUors. However, V1515 Cyg lacks the P Cygni profile in the Ca II 8498 \r{A} line, a part of the Ca infrared triplet (IRT), formed by an outflowing wind, suggesting that the absorbing gas in the wind is optically thin. The newly obtained near-infrared spectrum shows the strengthening of the CO bandhead and the FeH molecular band, indicating that the disk has become cooler since the last spectroscopic observation in 2015. The current luminosity of the accretion disk dropped from the peak value of 138 $L_{\odot}$ to about 45 $L_{\odot}$, suggesting that the long-term fading is also partly caused by the dropping of the accretion rate.