Pre-main sequence stars, emission stars and recent star formation in the Cygnus Region

TL;DR

This study investigates recent star formation in the Cygnus region by analyzing pre-main sequence and emission stars across five clusters, revealing ongoing star formation for up to 7 million years and identifying Classical Be stars in young clusters.

Contribution

It combines optical and infrared data to identify pre-main sequence stars and emission stars, estimating star formation durations and challenging the idea that the Be phenomenon is solely evolutionary.

Findings

Star formation has been ongoing in the Cygnus region for up to 7 Myr.

Identified 93 pre-MS stars and 9 emission line stars in five clusters.

Classical Be stars can exist in very young clusters, indicating the phenomenon is not purely evolutionary.

Abstract

The recent star formation history in the Cygnus region is studied using 5 clusters (IC 4996, NGC 6910, Berkeley 87, Biurakan 2 and Berkeley 86). The optical data from the literature are combined with the 2MASS data to identify the pre-main sequence (pre-MS) stars as stars with near IR excess. We identified 93 pre-MS stars and 9 stars with H$_\alpha$ emission spectra. The identified pre-MS stars are used to estimate the turn-on age of the clusters. The duration of star formation was estimated as the difference between the turn-on and the turn-off age. We find that, NGC 6910 and IC 4996 have been forming stars continuously for the last 6 -- 7 Myr, Berkeley 86 and Biurakan 2 for 5 Myr and Berkeley 87 for the last 2 Myr. This indicates that the Cygnus region has been actively forming stars for the last 7 Myr, depending on the location. 9 emission line stars were identified in 4 clusters, using slit-less spectra (Be 87 - 4 stars; Be 86 - 2 stars, NGC 6910 - 2 stars and IC 4996 - 1 star). The individual spectra were obtained and analysed to estimate stellar as well as disk properties. All the emission stars are in the MS, well below the turn-off, in the core hydrogen burning phase. These stars are likely to be Classical Be (CBe) stars. Thus CBe phenomenon can be found in very young MS stars which are just a few (2 -- 7) Myrs old. This is an indication that CBe phenomenon need not be an evolutionary effect.