Biman J. Medhi

Exploring pre-main-sequence variables of the ONC: the new variables

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.

A multiwavelength polarimetric study towards the open cluster NGC 1893

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 λmax 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.

The rotation-lithium depletion correlation in the β Pictoris association and the LDB age determination

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 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.

Optical polarimetric study of open clusters: Distribution of Interstellar matter towards NGC 654

ABSTRACT We present new B,V and R linear polarimetric observations for 61 starstowards the region of the young open cluster NGC 654. In this study we foundevidence for the presence of at least two layers of dust along the line of sight tothe cluster. The distances to the two dust layers are estimated to be ∼ 200 pcand ∼ 1 kpc which are located much closer to the Sun than the cluster (∼ 2.4kpc). Both the dust layers have their local magnetic field orientation nearlyparallel to the direction of the Galactic plane. The foreground dust layer isfound to have a ring morphology with the central hole coinciding with thecenter of the cluster. The foreground dust grains are suggested to be mainlyresponsible for both the observed differential reddening and the polarizationtowards the cluster.Keywords: polarization-dust, extinction-openclustersandassociations:individual:NGC 654 1 INTRODUCTIONThe wavelength dependence of interstellar extinction and polarization provides constraintson the characteristics of interstellar grains. Interstellar polarization strongly varies withwavelength (Serkowski, Mathewson & Ford 1975; Wilking et al. 1980). In particular, thewavelength of maximum interstellar polarization (λ

LO Pegasi: an investigation of multiband optical polarization

We presentBVR 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, θ B = 88 ◦ ± 1 ◦ ; PV = 0.351 ± 0.004 per cent, θ V = 91 ◦ ± 1 ◦ and PR = 0.335 ± 0.003 per cent, θ R = 91 ◦ ± 1 ◦ .B oth 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.

Broad-band optical polarimetric study of IC 1805

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.

Polarization towards the young open cluster NGC 6823

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.

Polarimetric studies of Comet C/2009 P1 (Garradd)

We present the optical imaging polarimetric observations of comet C/2009 P1 (Garradd) at three different phase angles e.g. 28

The β Pictoris association: Catalog of photometric rotational periods of low-mass members and candidate members

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FR Cnc revisited: photometry, polarimetry and spectroscopy

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