IPHAS discoveries of young stars towards Cyg OB2 and its southern periphery

TL;DR

This study used IPHAS and follow-up spectroscopy to discover over 50 young, Halpha-emitting stars near Cyg OB2, revealing clustering of pre-main sequence stars and insights into star formation triggered by OB stars.

Contribution

The paper reports the discovery of a large population of young stars around Cyg OB2 using combined photometric and spectroscopic data, highlighting new clustering and star formation insights.

Findings

Over 50 Halpha-emitting young stars discovered.

Clustering of T Tauri stars near DR 15 and G79.29+0.46.

Evidence of star formation triggered by OB stars.

Abstract

We report on the discovery of over 50 strong Halpha emitting objects towards the large OB association Cyg OB2 and the HII region DR 15 on its southern periphery. This was achieved using the INT Photometric Halpha Survey of the Northern Galactic Plane (IPHAS), combined with follow-up spectroscopy using the MMT multi-object spectrometer HectoSpec. We present optical spectra, supplemented with optical r', i' and Halpha photometry from IPHAS, and near-infrared J, H, and K photometry from 2MASS. The position of the objects in the (J - H) versus (H - K) diagram strongly suggests most of them are young. Many show CaII IR triplet emission indicating that they are in a pre-main sequence phase of evolution of T Tauri and Herbig Ae nature. Among these, we have uncovered pronounced clustering of T Tauri stars roughly a degree south of the centre of Cyg OB2, in an arc close to the HII region DR 15, and the radio ring nebula G79.29+0.46, for which we discuss its candidacy as a luminous blue variable (LBV). The emission line objects toward Cyg OB2 itself could be the brightest most prominent component of a population of lower mass pre-main sequence stars that has yet to be uncovered. Finally, we discuss the nature of the ongoing star formation in Cyg OB2 and the possibility that the central OB stars have triggered star formation in the periphery.