Variability, rotation and magnetic activity in the pre-main sequence stars in NGC 2264

Pre-Main Sequence stars are young stars whose cores are not dense and hot enough to ignite the thermonuclear reactions that will power them for the rest of their evolution. These stars are sorted in three classes: the youngest stars are the class I sources, still embedded in an accreting envelope of gas; class II stars have dispersed their accreting envelope but they may still accrete gas from a surrounding accretion disk, which can eventually evolve in a planetary system; the class III stars have also lost their disks, and they are contracting until their cores have not started the thermonuclear reactions (entering in the “Main Sequence” phase).

 

Pre-main sequence stars are also variable sources. In class III stars this variability is due to the stellar magnetic activity (spots, faculae, flares, prominences, etc…) similar to that of Main Sequence stars, even if these phenomena are more intense in young stars because their magnetic fields are 1000-10000 times stronger. However, given the large distances that separate us from the stars, stellar magnetic phenomena can be directly observed only in the Sun. In the other stars, the magnetic activity can be analyzed only by studying their variability with high temporal resolution. Besides, these studies must be performed with a “multi-band” approach, e.g. comparing simultaneous observations in different bands of the electromagnetic spectrum. This because data in different bands probe phenomena occurring in different regions of the stars. For instance, photospheric and chromospheric phenomena can be observed in optical, while coronal phenomena in X-rays and UV, and phenomena related to accretion disks and envelopes in infrared.

 

This has motivated the “Coordinated Synoptic Investigation of NGC 2264“, an unprecedented project based on simultaneous observations acquired with 15 telescopes of the young (1-2 million years) stellar cluster NGC 2264. The study “CSI2264: Simultaneous optical and X-ray variability in the pre-main sequence stars of NGC2264. II: Photometric variability, magnetic activity, and rotation in class III objects and stars with transition disks” of M. G. Guarcello (INAF- Astronomical Observatory of Palermo) is the fifth study produced by OAPA astronomers in the framework of this project. This study focuses on the optical and X-ray variability observed in 74 stars in the class III phase or in the transition between the class II and class III phases. In 16 targets, the optical and X-ray variability is observed to be anti-correlated, in 11 correlated, while in 17 stars the variability in the two bands is not correlated. The anti-correlated optical and X-ray variability observed in the first group is due to the simultaneous emergence along the line of sight during stellar rotation of coronal active regions (bright in X-rays) and photospheric spots (which reduce the optical brightness of the star); the correlated variability is instead due to the emergence of coronal active regions and networks of faculae (bright in optical). The authors demonstrate that these two groups of stars have different rotational properties. In particular, stars with anti-correlated optical and X-rays variability (spot-dominated) rotate slower than the stars with correlated variability (faculae-dominated). This suggests an important role played by stellar rotation, which is related to the intensity and topology of the magnetic field, to set the dominant magnetic activity in pre-main sequence stars.

 

The figure (link) shows a light curve (i.e. the intensity of the radiation emitted by the star vs. time) in optical (solid line) and X-rays (dots) in one of the stars analyzed in this study. The shaded intervals are those for which no X-ray data are available. The red numbers identify the X-ray observations.

 

by Mario Giuseppe Guarcello  ( follow mguarce)