Lista de partes críticas del compresor CPT 15:

Star 2004 2006-2007 2008 2009-2010 2010-2011 2011 AA Tau - - - 58 CQ Tau - 24 - - - - CW Tau 12 - - 30 - - DO Tau - 24 - - ∼17 35 Haro 6-13 20 - - - 30 30 IQ Tau - - - - LkHa 327 - 20, 130 - - - - RU Lup - - - - RW Aur 30 30 30 30 30 30

Table 3.6: Periods (expressed in days) of stars in group A and B. Periods are shown for different years or groups of years. LkHa 327 does not belong to either groups, but its period was analysed for reasons explained in the text.

As already seen, all lightcurves in the analysed sample present a certain level of vari- ability, and eight of them have particularly high variations, namely those belonging to the previously mentioned group A and B. It is on these two groups that it was searched for possible periods, in particular long periods, i.e. longer than two weeks. Results are in Table 3.6, where the uncertainty is ±1 day. The entire lightcurves of stars in group A and B are in Appendix B.

It is worth mentioning again that these stars are not strictly periodic, but more often quasi-periodic, which means that periods can be seen only in a certain time interval or that periods of different length are superimposed and often mingled together. In this latter case especially, only tentative periods were often found, but they are not reported in Table 3.6. Short periods below a week are also not included in the Table, but the analysis were

3.4. Time series analysis and period search

performed on three stars to make a comparison with the literature (Section 3.4.2). Further, some stars show different periods in different observing years, or even an increasing periodicity over a few months. In this case more than one period is listed in Table 3.6, separated by a comma.

The most robust results were found for AA Tau, CQ Tau, CW Tau, DO Tau, IQ Tau and RW Aur, but all stars from group A and B are included in the period analysis descrip- tion. Table 3.6 reports in italics LkHa 327. As explained in Section 3.2.1, in LkHa 327 some data points at the end of 2007 were removed because the reliability of variations was not clear. However, those data points had been initially included in the analysis and this star, along with LkHa 326, had belonged to group A. Since some long periods were found in years 2006-2007 (without the subsequently removed trimester), they are reported in Table 3.6 and discussed at the end of this Section.

AA Tau

The lightcurve has data at irregular intervals from 2004 to 2011. The mean magnitude is 12.98±0.36, with a larger scatter in years 2010 and 2011. There are some scattered observations in 2008, but not useful to determine a period. The most interesting feature is the part of the lightcurve between 19th September and 18th November 2011, which can be directly fitted with a sine curve having a period of 58 days, as shown in Fig. 3.8. Obser- vations in earlier years do not show this behaviour. The ample scatter is due to shorter variations, of about 5 days, which are slightly shorter than the typical rotation period of this star: 8.4 days according to Bouvier et al. (1999) or 8.2 according to Artemenko et al. (2012).

Figure 3.8: Lightcurve of AA Tau (not folded) between 19th September and 18th November 2011.

CQ Tau

CQ Tau is a fairly bright star, with a mean magnitude 10.41±0.36. It was observed from 2004 to 2010, with the majority of the observations being between October 2006 and February 2007. In this latter time interval a period of 24 days was found, which is shown in Fig. 3.9.

Figure 3.9: Folded lightcurve for CQ Tau between 9th December 2006 and 19th February 2007.

CW Tau

CW Tau lightcurve has mean magnitude 12.95±0.47 and shows high scatter, over half a magnitude, in some years. It presents, however, irregular variations throughout most of the decade, and a long period of 30 days was found only between 8th November 2009 and 18th January 2010, as shown in Fig. 3.10. In 2004 only short term periods were found,

3.4. Time series analysis and period search

increasing between 5 and 12 days over the year, comparable with the rotation period of 5.95 days found by Xiao et al. (2012) or 8.25 days by Bouvier et al. (1995). Nonetheless, this latter period is more plausibly due to the presence of a close companion, as explained by the authors. The increase in periodicity observed in 2004 might be a combination of rotation period and eclipsing effect.

Figure 3.10: Folded lightcurve for CW Tau between 8th November 2009 and 18th January 2010.

DO Tau

The lightcurve of DO Tau has a mean magnitude 13.28±0.36. The star was observed at irregular intervals between 2004 and 2011. Fig. 3.11 shows two plausible long periods: 24 days, between September 2006 and January 2007, and 35 days between October and November 2011. The scatter visible around the fitted sin curve is due to variations on a short timescale, typically of 6-7 days, which are shorter than the 12.5 days rotation period found by Osterloh et al. (1996). Overall, this star shows a variety of quasi-periodicities of different length over the decade, but no strict long periods apart from the above mentioned ones.

Figure 3.11: Folded lightcurve for DO Tau between 25th September 2006 and 19th January 2007 (left) and between 2nd October 2011 and 20th November 2011 (right).

IQ Tau

IQ Tau, observed from 2004 to 2011, has a fairly faint lightcurve with a mean magnitude 13.42 ±0.41. Overall, the lightcurve has small variations with amplitudes lower than 1.5-2.0 magnitudes, but presents a large scatter up to 3.5 mag from September 2006 to February 2007. No long periods were found over the entire decades, but only short periods below two weeks: 6.5 days in the last trimester of 2010, and 9 or 12 days in other years. Nevertheless, they confirm previous measurements of rotation periods: 6.5 days is in good agreement with the rotation period of 6.25 days found by Bouvier et al. (1995) and 6.9 days by Xiao et al. (2012); the 12 days with the 12.5 days rotation period found by Osterloh et al. (1996). However, since these periods are all shorter than two weeks, they are not included in Table 3.6.

Haro 6-13

Haro 6-13 was observed from July 2004 to November 2011. It is the faintest star amongst those in group A and B, having mean magnitude 16.33±0.47. Nevertheless, the lightcurve is characterised by constant high variations throughout the entire period of observations, where luminosity can vary up to three magnitudes within a single day. From a visual inspection quasi-periodic variations are clearly visible, but a possible periodic pattern of 30 days was found only in the last trimester of 2011, as shown in Fig. 3.12. In the previous years variations are very irregular, apart from a tentative period of about 15 days from July to end of September 2004. However, it is not clear whether this star is contaminated by the light of a field star in the neighbourhood. In that case its variability variability

3.4. Time series analysis and period search

would be dubious. The rotation period is 5 days, according to theνsinimeasurements by White & Hillenbrand (2004).

Figure 3.12: Folded lightcurve for Haro 6-13 between 19th September and 18th November 2011.

RU Lup

For RU Lup data are available only from 2006 to 2008, with some sporadic observations in 2009, 2010 and 2011. Compared to the previous stars, it is brighter, with a mean magnitude 11.41±0.36 and has a fairly stable lightcurve, with greater variations mainly in 2007. Its high pooled sigma, 0.42, is explained through a high scatter during the nights 8-9-10 May 2008, where the magnitude dropped to 10.63±0.42. The lightcurve presents some variations, mainly on a short timescale, but no clear period was found. Percy et al. (2010) measured a period of 230 days, but stated also that this star has an erratic behaviour, with periods changing or evening missing in different intervals. The rotation period for this star is 3.7 days (Hutchinson et al., 1989; Stempels et al., 2007).

RW Aur

RW Aur is a bright star with a mean magnitude 10.56±0.30, and was observed from 2004 to the beginning of 2011, with most of the observations being available until end 2008. This is the only case where a single period of 30 days folded very well over the entire lightcurve, as shown in Fig. 3.13. This period is certainly longer than the rotation period, known to be 5.6 days (Dodin et al., 2011). Percy & Palaniappan (2006) did not find any strong periodic pattern in the lightcurve, but noticed a possible period of 5 days using Fourier analysis, which would be in agreement with the measurement by Dodin et al.

(2011).

Figure 3.13: Lightcurve and folded lightcurve of RW Aur over the entire timescale, between 27th July 2004 and 11th February 2011.

LkHa 327

LkHa 327 was analysed, because it initially belonged to group A, as explained at the beginning of this section. Although it does not classify anymore in the group of high variable stars, it is worth mentioning its periodicity in the last trimester of 2006.

LkHa 327 has mean magnitude 14.20±0.2, and was observed between July 2004 and February 2008. In addition to a 20-day period folded on the lightcurve between November 2006 and January 2007, LkHa 327 presents an interesting long 130 days period from September 2006 to January 2007, visible on the unfolded lightcurve shown on the right hand side of Fig. 3.14. Despite high fluctuations, due to variations on shorter timescales, the peculiarity of this period is that it is directly visible on the lightcurve, without folding it. Other shorter periods include a 7 days period in 2004, between July and September, and a 6-day period between September and November 2006.

3.4. Time series analysis and period search

Figure 3.14: Lightcurve of LkHa 327 between July 2004 and December 2007 (left) and, not folded, between 10th September 2006 and 19th January 2007 (right).