Figure 3.18: AOC J162636.81-241900.2; variable member with a brightening of several magnitudes in both H and K bands.
magnitudes, as well as the H−K colour variations, are twice as large as the ones seen among other variables. Although this is in conformity with the scenario just described, it also opens the possibility that more extreme variability behaviours are, after all, common among young stars. Only targeted photometric and spectroscopic monitoring of variable stars of different types, looking for a correlation between colours, magni- tudes, and spectral features would help in isolating the important physical mechanisms.
3.9
Variability properties and their relation with
stellar parameters
Having established the main characteristics of the near-IR variability observed in the list of variables and analysing the possible physical origins, it is important to discuss how these relate to other stellar parameters such as the presence of an IR-excess, or the previous detection of X-ray variability in these stars.
From the 137 variable stars detected, 112 are present in the Spitzer catalogues. In Sect. 3.5, it has been shown that the combination of near- and mid-IR is an excellent tool to identify IR-excesses in young stars, which are an indication of the presence of a surrounding accretion disc. Accretion-related phenomena are observed not only in CTTSs, but also at the substellar regime. Analogously to CTTSs, very low mass stars and brown dwarfs also show accretion-related activity such as jets, variations in the con- tinuum flux, and strong, variable line-emission (Mohanty et al. 2005; Scholz & Eisl¨offel 2005; Whelan et al. 2005).
A relation between the amplitude of the variations and the presence of IR-excess has been studied. Fig. 3.19 shows the peak-to-peak amplitude of the variability for the H-band versus K-band, for all the variable objects with detections in the Spitzer catalogues which showed an IR-excess (filled circles) or not (open squares). There are two main differences between the two groups. First, for the stars which did not show any IR-excess, the amplitudes of the variation do not exceed <0.4 magnitudes in both bands. This is consistent with the idea that cool spots alone can explain the variations seen in these stars, for which indication of circumstellar material has not been found. On the other hand, stars which have shown an IR-excess have amplitudes that can reach up to 1.0 magnitudes. This is more easily shown in Fig. 3.20, where it can be seen that as the H-band amplitudes increase, the corresponding near-IR colours are redder and also the fraction of stars with IR-excess increases. This trend suggests that the presence or absence of an accretion disc could be the main factor in dictating the observed amplitude of the photometric variations. Furthermore, the same relation as been observed between WTTSs and CTTSs in the near-IR study of the Orion Nebula Cluster by Carpenter et al. (2001). Finally, the second striking difference between the two groups in Fig. 3.19, is that most of the stars with IR-excess show a displacement in relation to the group with no IR excess, since for any ΔK, stars with IR-excess often show a higher ΔH than stars without IR-excess. This is in agreement with the existence of hot spots as the cause of variability in stars that show IR-excess, since it provides evidence for the spot temperature. As explained in the previous section, both hot and cool spots lead to amplitude modulation, which is larger at shorter wavelengths. However, given that the temperature of the hot spot is much higher, the increase in amplitude is steeper than for cool spots, which is what is observed in Fig. 3.19. This is easily explained if it is assumed that the star and spot can be described as a single- temperature blackbody. Then, the higher the temperature, the steeper the Planck curve becomes, affecting more shorter than long wavelengths. However, some of the sources in this diagram do not fall into either of these two groups, which seem to be well described by cool and hot spots, and instead show larger variations at the longer wavelengths. The majority of these sources have variability consistent with changes in an accretion disc, which ties well into the scenario described in the previous section.
In Sect. 3.6, 17 from the variable members have been found to have X-ray detec- tions and to show flaring activity or variable X-ray emission. If IR-excess is used as
3.9 Variability properties and their relation with stellar parameters 67
Figure 3.19: H-band versus K-band amplitudes of variability for stars which show an indication of IR-excess (filled circles) and stars that do not have an IR-excess (open squares).
a signature for the presence of an accretion disc, a rough comparison between X-ray activity and accretion can be made. However, some of these variable stars with X-ray variability show IR-excess, while others do not, and there is no clear relation between the two properties. This result agrees with the study of Stassun et al. (2007), where the authors claim they did not find a statistically significant difference in the level of X-ray variability among the accretors as compared to the non-accretors.
Finally, it is important to address the results for the variability properties in brown dwarfs. There are ∼22 brown dwarfs confirmed in ρ Ophiuchi (Wilking et al. 1997; Luhman & Rieke 1999; Cushing et al. 2000; Natta et al. 2002; Wilking et al. 2008) and all of them have counter-parts in the WFCAM catalogues. However, only 5 brown dwarfs are found to be variable according to the criteria used and detection limits. From these, 4 have low amplitudes both in H and K (<0.2) and only one shows high variability with amplitudes 0.5. Caballero et al. (2004) studied photometric vari- ability of 28 young brown dwarfs candidates in σ Orionis (∼3 Myr old) in the I-band and found amplitude variations ranging from less than 0.01 up to ∼0.4 magnitudes.
Figure 3.20: H-band amplitude versus H-K colour for stars which show an indication of IR-excess (filled circles) and stars that do not have an IR-excess (open squares). It can be seen that as the H-band amplitudes increase, the near-IR colours become redder and the fraction of stars with IR-excess increases.
It thus seems likely that the reason why all of the known brown dwarfs in Ophiuchus have not been detected as variables is sensitivity. However, the fact that these 5 BDs were found to be variables is extremely important, since it makes them good targets for a more detailed study and a search for periodic behaviour. In the substellar regime, the average period is seen to decrease with decreasing masses, extending down to the breakup period, the limit of the rotational velocity (Scholz & Eisl¨offel 2005). Stellar evolution is altered in presence of extremely fast rotation, for which the study of these objects is fundamental for evolutionary models of brown dwarfs (Herbst et al. 2007).