A survey for variable young stars with small telescopes: III -- Warm spots on the active star V1598Cyg

TL;DR

This study analyzes multi-band photometric data of the active star V1598Cyg over four years, revealing its rapid rotation, magnetic activity, and a possible six-year magnetic cycle, with detailed spot property estimations.

Contribution

It introduces a grid-based method to estimate star spot temperatures and coverage from multi-band light curves, validated on extensive observational data.

Findings

V1598Cyg is a fast rotator with a 0.8246-day period.

Star spots are warmer than the photosphere, indicating chromospheric plage.

A six-year brightness cycle suggests a magnetic activity cycle.

Abstract

Magnetic spots on low-mass stars can be traced and characterised using multi-band photometric light curves. Here we analyse an extensive data set for one active star, V1598Cyg, a known variable K dwarf which is either pre-main sequence and/or in a close binary system. Our light curve contains 2854 photometric data points, mostly in $V$, $R_c$, $I_c$, but also in $U$, $B$ and $H\alpha$, with a total baseline of about 4yr, obtained with small telescopes as part of the HOYS project. We find that V1598Cyg is a very fast rotator with a period of 0.8246 days and varying amplitudes in all filters, best explained as a signature of strong magnetic activity and spots. We fit the photometric amplitudes in $V$, $R_c$, $I_c$ and use them to estimate spot properties, using a grid-based method that is also propagating uncertainties. We verify the method on a partial data set with high cadence and all five broad-band filters. The method yields spot temperatures and fractional spot coverage with typical uncertainties of 100K and 3-4%, respectively. V1598Cyg consistently exhibits spots that are a few hundred degrees warmer than the photosphere, most likely indicating that the light curve is dominated by chromospheric plage. The spot activity varies over our observing baseline, with a typical time scale of 0.5-1yr, which we interpret as the typical spot lifetime. Combining our light curve with archival data, we find a six year cycle in the average brightness, that is probably a sign of a magnetic activity cycle.