Do accretion discs regulate the momentum of young stars? S.P. Littlefair 1,2, T. Naylor 2, R.D. Jeffries 3, B. Burningham 2, E. Saunders 2 1 Dept of Physics & Astronomy, University of Sheffield, 2 School of Physics, University of Exeter, 3 School of Chemistry and Physics, Keele University Abstract: The question of the early evolution of stellar angular momentum is an interesting and essentially unsolved problem. It is well known that young stars rotate well below their break-up speed [1], and this has been explained by disc-locking theories, where magnetic field lines connect the star to the disc, forcing synchronous rotation between the star and disc material at some radius. However, observational and theoretical support for disc locking is still controversial. In this poster we present new observations of the rotation distributions in the young associations IC 348 and Cepheus OB3b, and review the observational support for disc locking. Periods for CTTs from photometric monitoring: The most efficient method of determining large numbers of periods for young stars is via photometric monitoring. Unfortunately, this method has proved insensitive to periods amongst CTTs in the past, largely due to the irregular variability shown by CTTs [2,3]. This bias against CTTs is a major problem, as an obvious test of disc locking is to compare the period distribution of CTTs against WTTs. We have overcome this bias by using a more intensive monitoring strategy than previous studies. For example, in IC 348 we took data on 15 consecutive nights, and repeated the observations many times within a single night. By contrast, the survey of Cohen et al [2] has much lower temporal density, and many nights can elapse between observations. Because CTTs are erratically variable with time-scales of order one night, this variability masks the periodic signal. Our data is much less vulnerable to this effect: the fraction of CTTs to WTTs amongst the periodic stars in our survey is 0.43±0.13, not significantly different from the cluster as a whole. Correlation of period with disc indicators: The most obvious test of disc-locking theory is to look for correlations between rotation period and indicators of disc presence or accretion. Obvious candidates are infrared excess or H α equivalent width. A correlation between rotation rate and infrared excess in the ONC has previously been claimed by Herbst et al(2002). However, both their chosen disc indicator (I-K excess) and rotation rate are strongly correlated with mass [3,4,5,6]. The observed correlation between rotation and I-K excess is most likely a secondary correlation, arising from the primary, underlying correlation between I-K excess and mass. Likewise, we must be cautious when interpreting a lack of correlation – figure 1 shows that our IC 348 data shows no correlation between K-L colour or H α EW. However, disc holes or inclination effects might hide the correlation with L-band excess, and there is no reason why the current H α EW should represent the time averaged accretion rate, as the emission strength of H α is highly variable. Rotational evolution – high mass stars: Figure 3 shows the period distributions of IC 348 and Cepheus OB3b, along with the distributions for the ONC and NGC 2264, taken from the literature. Figure 2 shows that Cep OB3b is slightly older than NGC 2264, with a likely age of 4 Myr. With this in mind, it appears that the ONC, NGC 2264 and Cep OB3b imply an evolutionary sequence in which stars are initially disc-locked at 8-day periods (the ONC) are released at ~ 1 Myr and gradually spin up, to 2 day periods at ~4 Myr (Cep OB3b). A neat evolutionary sequence is spoiled by IC 348, however. Like the ONC, the high mass stars in IC 348 are rotating slowly, with a peak around 8 days. In fact, the stars in IC 348 are rotating more slowly than the stars in the ONC, with a significant absence of rotators below 2 days. The literature age of IC 348 is circa 2 Myr, similar to that of NGC There are two possible conclusions to be drawn, the most obvious being that cluster environment can dominate over disc locking in determining rotation rates. A more radical suggestion is that the literature age for IC 348 is not correct. The cluster Figure 2: De-reddened colour-magnitude diagrams of the periodic variables in Cep OB3b and NGC Isochrones are from Siess et al 2000 [ref]. Credit: Nathan Mayne, Exeter photometric sequence in IC 348 is poorly fit by models [7], and there is some evidence that the age might be nearer 1 Myr [8], in which case it is not surprising that the period distribution is ONC-like. The low mass stars: The low mass stars present a very confusing picture. It is not clear why they show a uni-modal distribution, nor why they are rotating more rapidly than the high mass stars (but see Barnes 2003 [9] ). Furthermore, the low mass stars do not fit into the evolutionary sequence above: the distributions in NGC 2264 and IC 348 are very similar, despite the very different high-mass distributions in these clusters. Worse still, the low mass stars in Cep OB3b (the oldest association shown here) are rotating much more slowly than those in NGC Much more additional work is needed on the low-mass stars, including deeper variability surveys. Conclusions: The observational evidence for disc locking is mixed. The lack of any clear correlation with disc or accretion indicators is worrying, but time variability of H α, variation in disc-holes or inclination and mass effects make firm conclusions difficult. Likewise, the period distributions do not fit a clear evolutionary sequence, although uncertainty in cluster ages may explain this. Future work should include investigating the link between rotation rate and firm disc indicators, such as Spitzer colours, and investigation into alternative, model independent methods of determining cluster ages. Finally, the puzzle of the low-mass stars needs further observational and theoretical study. Figure 1: Rotation period.vs. disc and accretion indicators for IC 348. No correlation is present. Figure 3: Period distributions of IC 348 and Cepheus OB3b compared to distributions for the ONC [3] and NGC 2264 [5]. Also shown are ages for the clusters commonly quoted in the literature. References: [1] Bouvier et al, 1993, A&A, 272, 176 [2] Cohen et al, 2004, AJ, 127, 1602 [3] Herbst et al, 2002, PASP, 114, 1167 [4] Hillenbrand et al, 1998, AJ, 116, 1816 [5] Lamm et al, 2005, A&A, 430, 1005 [6] Littlefair et al, 2005, MNRAS, 358, 341 [7] Mayne et al, in preparation [8] Herbig, 1998, ApJ, 497, 736 [9] Barnes, 2003, ApJ, 586, Myr ONC M > 0.25 M  M < 0.25 M  2 Myr IC348 M > 0.25 M  M < 0.25 M  2 Myr NGC 2264 R-I Myr Cep OB3b V-I 3.7