Context. Evidence exists in the 125-Myr Pleiades cluster, and more recently in the 5-Myr NGC 2264 cluster, to show that rotation plays a key role in lithium (Li) depletion processes among low-mass stars. Fast rotators appear to be less Li-depleted than equal-mass slow rotators. Aims. We intend to explore the existence of a Li depletion-rotation connection among the β Pictoris members at an age of about 24 Myr, and to use this correlation either to confirm or to improve age estimates based on the lithium depletion boundary (LDB) modeling. Methods. We photometrically monitored all the known members of the β Pictoris association with at least one lithium equivalent width (Li EW) measurement from the literature. Results. We measured the rotation periods of 30 members for the first time and retrieved the rotation periods for another 36 members from the literature, building a catalogue of 66 members with a measured rotation period and Li EW. Conclusions. We find that in the 0.3 < M < 0.8 M range, there is a strong correlation between rotation and Li EW. For higher mass stars, no significant correlation is found. For very low-mass stars in the Li depletion onset, at about 0.1 M , there are too few data to infer a significant correlation. The observed Li EWs are compared with those predicted by the Dartmouth stellar evolutionary models that incorporate the effects of magnetic fields. After decorrelating the Li EW from the rotation period, we find that the hot side of the LDB is well fitted by Li EW values that correspond to an age of 25 ± 3 Myr, which is in good agreement with independent estimates from the literature.

We present multiwavelength linear polarimetric observations for 44 stars of the NGC 1893 young open cluster region along with V-band polarimetric observations of stars of four other open clusters located between l ∼ 160 • and 175 • . We found evidence for the presence of two dust layers located at a distance of ∼170 and ∼360 pc. The dust layers produce a polarization P V ∼ 2.2 per cent. It is evident from the clusters studied in this work that, in the Galactic longitude range from l ∼ 160 • to 175 • and within the Galactic plane (|b| < 2 • ), the polarization angles remain almost constant, with a mean of ∼163 • and a dispersion of 6 • . The small dispersion in polarization angle could be due to the presence of a uniform dust layer beyond 1 kpc. Present observations reveal that in the case of NGC 1893, the foreground two dust layers, in addition to the intracluster medium, seem to be responsible for the polarization effects. It is also found that towards the direction of NGC 1893, the dust layer that exists between 2 and 3 kpc has a negligible contribution towards the total observed polarization. The weighted mean for percentage of polarization (P max ) and the wavelength at maximum polarization (λ max ) are found to be 2.59 ± 0.02 per cent and 0.55 ± 0.01 µm, respectively. The estimated mean value of λ max indicates that the average size of the dust grains within the cluster is similar to that in the general interstellar medium. The spatial variation of the polarization is found to decrease towards the outer region of the cluster. In this work, we support the notion, as has already been shown in previous studies, that polarimetry, in combination with the (U − B)-(B − V) colour-colour diagram, is a useful tool for identifying non-members in a cluster.

We present the BVR broad-band polarimetric observations of 51 stars belonging to the young open cluster IC 1805. Along with the photometric data from the literature, we have modelled and subtracted the foreground dust contribution from the maximum polarization (P max ) and colour excess [E (B − V)]. The mean value of the P max for intracluster medium and the foreground are found to be 5.008 ± 0.005 and 4.865 ± 0.022 per cent, respectively. Moreover, the mean value of the wavelength of maximum polarization (λ max ) for intracluster medium is 0.541 ± 0.003 m, which is quite similar as the general interstellar medium (ISM). The resulting intracluster dust component is found to have negligible polarization efficiency as compared to interstellar dust. Some of the observed stars in IC 1805 have shown the indication of intrinsic polarization in their measurements.

We present new B, V and R linear polarimetric observations for 61 stars towards the region of the young open cluster NGC 654. In this study we found evidence for the presence of at least two layers of dust along the line of sight to the cluster. The distances to the two dust layers are estimated to be ∼ 200 pc and ∼ 1 kpc which are located much closer to the Sun than the cluster (∼ 2.4 kpc). Both the dust layers have their local magnetic field orientation nearly parallel to the direction of the Galactic plane. The foreground dust layer is found to have a ring morphology with the central hole coinciding with the center of the cluster. The foreground dust grains are suggested to be mainly responsible for both the observed differential reddening and the polarization towards the cluster.

Since 2004, we have been engaged in a long-term observing programme to monitor young stellar objects (YSOs) in the Orion Nebula Cluster (ONC). We have collected about 2000 frames in V, R and I broad-band filters on more than 200 nights distributed over five consecutive observing seasons. The high-quality and time-extended photometric data give us an opportunity to address various phenomena associated with young stars. The prime motivations of this project are (i) to explore various manifestations of stellar magnetic activity in very young low-mass stars, (ii) to search for new pre-main-sequence eclipsing binaries and (iii) to look for any EXor and FUor-like transient activities associated with YSOs. Since this is the first paper on this programme, we give a detailed description of the science drivers, the observation and the data reduction strategies as well. In addition to these, we also present a large number of new periodic variables detected from our first 5 yr of time-series photometric data. Our study reveals that about 72 per cent of classical T Tauri stars (CTTS) in our field of view are periodic, whereas only 32 per cent of weak-lined T Tauri stars (WTTS) are periodic. This indicates that inhomogeneity patterns on the surface of CTTS of the ONC stars are much more stable than on WTTS. From our multiyear monitoring campaign, we found that the photometric surveys based on single season are incapable of identifying all periodic variables. And any study on evolution of angular momentum based on single-season surveys must be carried out with caution.

We report the results of optical polarimetric observations of a Bok globule CB34 to study magnetic field structure on large scales (10 5 − 10 6 AU), which is combined with archival sub-mm observations to characterize the magnetic field structure of CB34 on small scales (10 4 −10 5 AU). The optical polarization measurements indicate that the magnetic field in the globule is constrained to a maximum radius of 10 5 AU around the core, out to densities not smaller than 10 4 cm −3 . Our study is mainly concentrated on two submillimeter cores C1 and C2 of CB34. The direction of magnetic field of core C2 is found to be nearly perpendicular to the CO outflow direction of the globule. The magnetic field of core C1 is almost aligned with the minor axis of the core which is typical for magnetically dominated star formation models. The mean value of offset between the minor axis of core C2 and the outflow direction is found to be 14• which suggests that the direction of the outflow is almost aligned with the minor axis of core C2. The magnetic field strength in the plane-ofsky for cores C1 and C2 is estimated to be ≈ 34µG and ≈ 70µG.

We present BVR polarimetric study of the cool active star LO Pegasi (LO Peg) for the first time. LO Peg was found to be highly polarized among the cool active stars. Our observations yield average values of polarization in LO Peg: PB= 0.387 ± 0.004 per cent, θB= 88°± 1°; PV= 0.351 ± 0.004 per cent, θV= 91°± 1° and PR= 0.335 ± 0.003 per cent, θR= 91°± 1°. Both the degree of polarization and the position angle are found to be variable. The semi‐amplitude of the polarization variability in B, V and R bands is found to be 0.18 ± 0.02, 0.13 ± 0.01 and 0.10 ± 0.02 per cent, respectively. We suggest that the levels of polarization observed in LO Peg could be the result of scattering of an anisotropic stellar radiation field by an optically thin circumstellar envelope or scattering of the stellar radiation by prominence‐like structures.

We present multiwavelength linear polarimetric observations of 104 stars towards the region of young open cluster NGC 6823. The polarization towards NGC 6823 is dominated by foreground dust grains and we found evidence for the presence of several layers of dust towards the line of sight. The first layer of dust is approximately located within 200 pc towards the cluster, which is much closer to the Sun than the cluster itself (∼2.1 kpc). The radial distribution of the position angles for the member stars is found to show a systematic change, while the polarization is found to reduce towards the outer parts of the cluster and the average position angle of the coronal region of the cluster is very close to the inclination of the Galactic parallel (∼32°). The size distribution of the grains within NGC 6823 is similar to those in the general interstellar medium. The patchy distribution of foreground dust grains is suggested to be mainly responsible for both differential reddening and polarization towards NGC 6823. The majority of the observed stars do not show evidence of intrinsic polarization in their light.