Towards a better classification of unclear eruptive variables: the cases of V2492 Cyg, V350 Cep, and ASASSN-15qi

TL;DR

This study investigates the long-term photometric behavior of three young eruptive variable stars using archival photographic plates, revealing insights into their activity mechanisms and challenging previous classifications.

Contribution

It provides a detailed analysis of historical photographic data for V2492 Cyg, V350 Cep, and ASASSN-15qi, offering new insights into their variability mechanisms and activity history.

Findings

V2492 Cyg activity levels have changed over decades.

Extinction alone cannot explain all observed variability.

Recent accretion events are linked to outburst episodes.

Abstract

Eruptive variables are young stars that show episodic variations of brightness: EXors/FUors variations are commonly associated with enhanced accretion outbursts occurring at intermittent cadence of months/years (EXors) and decades/centuries (FUors). Variations that can be ascribed to a variable extinction along their line of sight are instead classified as UXors. We aim at investigating the long-term photometric behaviour of three sources classified as eruptive variables. We present data from the archival plates of the Asiago Observatory relative to the fields where the targets are located. For the sake of completeness we have also analysed the Harvard plates of the same regions that cover a much longer historical period, albeit at a lower sensitivity, however we are only able to provide upper limits. A total of 273 Asiago plates were investigated, providing a total of more than 200 magnitudes for the three stars, which cover a period of about 34 yr between 1958 and 1991. We have compared our data with more recently collected literature data. Our plates analysis of V2492 Cyg provides historical upper limits that seem not to be compatible with the level of the activity monitored during the last decade. Therefore, recently observed accretion phenomena could be associated with the outbursting episodes, more than repetitive obscuration. While a pure extinction does not seem the only mechanism responsible for the ASASSN-15qi fluctuations, it can account quite reasonably for the recent V350 Cep variations.