The observational characteristics of these young objects in their very first evolutionary stages are the fingerprints of the physics involved in their formation process. Although there are general accepted views on the way star-formation occurs and young stellar objects (YSOs) evolve to the main-sequence, there is still a long way to go between the broader theories and an agreed paradigm which can explain in detail the wide range of existing observational properties of YSOs. For example, despite the fact that it is now accepted that the properties of YSOs are governed by magnetic fields and accretion, several questions remain regarding how the star-disc interaction takes place, or how magnetic fields evolve as stars approach the main-sequence. The same is true for the study of discs around YSOs. Their existence has long been predicted, and direct confirmation achieved. However, some of the basic properties, such as the disc masses and mass-accretion rates, are still unknown and largely debated. These constitute fundamental problems that must be solved in order to understand other mechanisms, such as the angular momentum regulation, which allow the young star to evolve without disrupting, as well as the evolution and stability of a potential planet-forming disc.
Despite the ∼Myr timescales involved in the early evolution of young stars, there are physical processes that have timescales comparable to those of human lifetime. Young stars have been found to rotate with periods in the order of days, offering to an observer the opportunity to study different regions of its surface. Cool spots exist on the surface of young stars and, in analogy to sunspots, are regions of lower tempera- ture than the stellar photosphere, which have intense magnetic activity. As the star rotates, the spots cause rotational modulation of the stellar flux. For stars surrounded by circumstellar discs, hot spots (regions with higher temperature than the stellar photosphere, interpreted as the impact points on the stellar surface from disc accretion through magnetic field lines), structural features of the inner disc (as, for example, a warped inner disc), or inhomogeneous distribution of circumstellar material, can cause photometric variability from rotational modulation of the star, or by variations in the accretion rate of material onto the star. These processes represent the potential ideas which were developed to explain the variability observed mainly in optical wavelengths, where most of the photometric monitoring has been done. However, characterisation of these phenomena in the IR regime, more suitable to probe the circumstellar disc temperature regime, is still largely missing. Furthermore, using IR wavelengths it is possible to penetrate into the cloud and detected stars that are not accessible in the optical.
The Rho Ophiuchi Molecular Cloud
The ρ Ophiuchi cluster was monitored with the WFCAM/UKIRT over a total of 14 epochs in the H and K-band, and ∼16000 stars have been detected over the two years of observations. Statistical methods, such as the reduced chi-square and correlation indices, were used in the search for variability. 137 variable objects were found which show timescales of variation, which can go from days to years, amplitude magnitude changes from a few tenths to ∼3 magnitudes, and colour variations of ∼0.2 magnitudes, in average. From the ρ Ophiuchi known population, 128 members have counter-parts in the WFCAM catalogue, and 45% are found to be variable. The variability trends observed are consistent with the existence of cool or/and hot spots on the stellar surface, variations in circumstellar extinction, or structural variations in accretion discs. However, a large fraction of the variable population does not fit into the predicted parameters for near-IR variability, which can be explained by the fact that the variability observed does not reflect a single event, but the net effect of several simultaneous processes. Read more about the results in the original article (Alves de Oliveira et al. 2008).
The Orion Nebula Cluster
In near-IR variability studies of YSOs, it is many times not possible to ascribe a specific physical mechanism to explain the observed variability, in significant part due to the fact that their monitoring data were limited to the JHK bands. Unlike the variability at optical and near-IR wavelengths, mid_IR light curves are most sensitive to features in the circumstellar dust of these YSOs, such as transient heating of the inner disk by a hot spot on the stellar surface or a periodic obscuration of the star by a warp in the inner disk. A large Spitzer program (550 hours) entitled, “Young Stellar Object Variability: Mid Infrared Clues to Accretion Disk Physics” (J. Stauffer, PI) has been started in 2009 to address these questions. Previous studies have demonstrated that discrimination between different physical models for mid-IR variability requires complementary multi-epoch optical/near- IR photometry. For the Spitzer monitoring of the ONC, I was responsible for the WIRCam/CFHT synoptic monitoring. This combination of simultaneous datasets led to some very interesting results, check out the article (Morales-Calderon et al. 2011)