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Empirical methods based on correlations between observed magnitudes and color have been employed to characterize stellar variability of young stars (Carpenter et al. 2001, 2002; Alves de Oliveira & Casali 2008). These methods consider variability due to rotational modulation of hot or cool starspots, variable extinction, variable mass accretion and structure changes in the circumstellar environment. Cool starspots are believed to be caused by localized magnetic inhibition of convection energy transport. Hot starspots, on the other hand, result from either surface flaring or heating by mass accretion onto the surface along magnetic field lines. Extinction may occur from asymmetries in an accretion disk or even from isolated dense regions of the parent molecular cloud passing through the line of sight. Variable mass accretion rates can cause the star brightness to vary through the clearing of the inner circumstellar disk. In addition, variability may be caused by energy released as material in

an accretion disk moves toward a star by viscous processes. Finally, these mechanisms are not mutually exclusive and are often seen to exist simultaneously (Herbst et al. 1994).

Each of the above variability mechanisms can be distinguished based on the temporal nature of the variability and correlations between color variability to stellar brightness.1 _The

following set of qualitative observables are developed to classify the observed variability and to connect these variations to physical mechanisms.

• Long lived cool starspots result in periodic variability with periods consistent with the rotational periods of young stars (. 14 days) (Rebull 2001). This variability is often sinusoidal in shape. At the temperature range of most YSOs, the near-IR wavelength regime samples the Rayleigh-Jeans tail of the stellar energy distribution where the contrast between the starspot and surrounding photosphere is small (e.g. (Vrba et al. 1985)). Therefore, the (J-H) and (H-Ks) colors should remain constant

(within photometric errors) as the brightness varies.

• Variability by hot starspots can either result in periodic or irregular variability. Long lived hot starspots caused by accretion onto the stellar surface may result in periodic variability. However it should be noted that accretion induced hot spots may display aperiodic behavior due to a stochastic accretion rate. Variability caused by flares will be aperiodic and will have time scales on the order of hours to days. As with cool starspots, the period of variability will be consistent with the rotational periods of young stars. In both cases the affected photosphere should be hotter than the 1

The correlations between stellar color and brightness are based on models in Carpenter et al. (2001 and references herein).

surrounding surface resulting in the star becoming bluer as the star brightens (Rodon`o & Cutispoto 1988; Panagi & Andrews 1995; Yu & Gan 2006).

• Variable extinction can result in either periodic or long time scale variability. Variabil- ity caused by asymmetries in the inner circumstellar disk, if present, may be periodic with periods from days to weeks. Unlike variability caused by starspots, periodic variable extinction need not appear sinusoidal but present more likely as eclipse-like features. These eclipse-like features are sharp drops or “dips” in the stellar flux with a regularity dependent on the observing cadence. Variability caused by asymmetries in the outer circumstellar disk (>1 AU) will not be periodic within the temporal baseline of this study due to long period of revolution around the host star. This variability and variable extinction from inter cloud material can occur on long time scales, however as the time scale depends on the system geometry, there is no expectation as to its duration. Variable extinction causes the star to redden as the star dims.

• Variability caused by a variable accretion rate within the circumstellar disk is not ex- pected to be periodic. The time scale of variability does place constraints on the physics causing this rate change (e.g. disk viscosity, time variable magnetic field) (Armitage 1995; Mahdavi & Kenyon 1998; Lai 1999; Terquem & Papaloizou 2000; Carpenter et al. 2001). During times of lower accretion rates, the inner disk cools and the inner hole be- comes larger. This, in turn, decreases the contribution of dust reradiation, particularly in the Ks band, to the overall energy budget of the star and circumstellar disk system.

Therefore while the total system flux drops, a larger percentage of emitted radiation is from the star causing the system to become bluer as the system dims. However, if

the inner circumstellar disk edge is dominated by the dust sublimation temperature, a observationally similar effect will result. In this case, an increased accretion rate raises the star’s effective temperature in turn increasing both the distance to the circum- stellar disk inner rim and disk vertical height. The result would be that the system would become brighter as it reddens. Both physical scenarios produce a qualitatively identical result to the observed correlation between brightness and color.

In an attempt to identify the dominant variability mechanism, stars in the variable catalog are placed into subclasses based on the observed shape and time scale of variability. These subclasses are: periodic, long time scale and irregular. These classifications along with the above criterion identified the likely dominant variability mechanism for 53 of the 101 stars in the variable catalog. The type of variability associated with each star is listed in Table 2.2 and each sub class is described in the following subsections. The periodic sub class accounts for 32% of the variable catalog with the majority (88%) lying “on cloud”. Long time scale variables make up 31% of the variable catalog. All LTVs reside “on cloud”. The irregular subclass contains the most members comprising 40% of the variable catalog. Only 68% of irregular variables lie “on cloud”. These subclasses are rough descriptions and are by no means mutually exclusive. For instance, WL 20W and ISO-Oph 126 are placed into both the periodic and long time scale subclasses.

These criteria do not always allow for the dominant variability mechanism to be identified. The main reasons preventing an estimate of the mechanism are: the time scale/period or color correlation is contrary to the above diagnostics, no dominant amplitude variability is clearly evident, or the photometry in J and H is below the completeness limits in each band

resulting in no useful color information. Mechanisms appended with a question mark in Table 2.2 either possess a marginal color correlation via visual inspection, or the diagnostics did not definitively differentiate between proposed mechanisms.