We present a comprehensive multiwavelength study of the star‐forming region NGC 1893 to explore the effects of massive stars on low‐mass star formation. Using near‐infrared colours, slitless spectroscopy and narrow‐band Hα photometry in the cluster region we have identified candidate young stellar objects (YSOs) distributed in a pattern from the cluster to one of the nearby nebulae Sim 129. The V, (V−I) colour–magnitude diagram of the YSOs indicates that majority of these objects have ages between 1 and 5 Myr. The spread in the ages of the YSOs may indicate a non‐coeval star formation in the cluster. The slope of the K‐band luminosity function for the cluster is estimated to be 0.34 ± 0.07, which agrees well with the average value (∼0.4) reported for young clusters. For the entire observed mass range 0.6 < M/M⊙≤ 17.7 the value of the slope of the initial mass function, ‘Γ’, comes out to be −1.27 ± 0.08, which is in agreement with the Salpeter value of −1.35 in the solar neighbourhood. However, the value of ‘Γ’ for pre‐main‐sequence phase stars (mass range 0.6 < M/M⊙≤ 2.0) is found to be −0.88 ± 0.09 which is shallower than the value (−1.71 ± 0.20) obtained for main‐sequence stars having mass range 2.5 < M/M⊙≤ 17.7 indicating a break in the slope of the mass function at ∼2 M⊙. Estimated ‘Γ’ values indicate an effect of mass segregation for main‐sequence stars, in the sense that massive stars are preferentially located towards the cluster centre. The estimated dynamical evolution time is found to be greater than the age of the cluster, therefore, the observed mass segregation in the cluster may be the imprint of the star formation process. There is evidence for triggered star formation in the region, which seems to govern initial morphology of the cluster.

We present $UBVI_c$ CCD photometry of the young open cluster Stock 8 with the aim to study the basic properties and star formation scenario in this region. The radius of the cluster is found to be $\sim 6^{\prime}$ ($\sim 3.6$ pc) and the reddening within the cluster region varies from $E(B-V)=0.40$ to 0.60 mag. The cluster is located at a distance of $2.05 \pm 0.10$ kpc. Using H$\alpha$ slitless spectroscopy and 2MASS NIR data we identified H$\alpha$ emission and NIR excess young stellar objects (YSOs), respectively. The colour-magnitude diagrams of these YSOs reveal that majority of these objects have ages between 1 to 5 Myr indicating a non-coeval star formation in the cluster. Massive stars in the cluster region reveal an average age of $\le$ 2 Myr. In the cluster region ($r \le 6^\prime$) the slope of the mass function (MF), $\Gamma$, in the mass range $\sim 1.0 \le M/M_\odot < 13.4$ can be represented by a power law having a slope of $-1.38\pm0.12$, which agrees well with Salpeter value (-1.35). In the mass range $0.3 \le M/M_\odot < 1.0$, the MF is also found to follow a power law with a shallower slope of $\Gamma = -0.58\pm 0.23$ indicating a break in the slope of the IMF at $\sim 1 M_\odot$. The slope of the $K$-band luminosity function for the cluster ($r \le 6^\prime$) is found to be $0.31\pm0.02$, which is smaller than the average value ($\sim$ 0.4) obtained for embedded star clusters. A significant number of YSOs are distributed along a Nebulous Stream towards the east side of the cluster. A small cluster is embedded in the Nebulous Stream. The YSOs lying in the Nebulous Stream and in the embedded cluster are found to be younger than the stars in the cluster Stock 8. It appears that star formation activity in the Nebulous Stream and embedded cluster may be independent from that of Stock 8.Comment: 48 pages, 29 figures, accepted for publication in MNRA

Emission‐line stars in young open clusters are identified to study their properties, as a function of age, spectral type and evolutionary state. 207 open star clusters were observed using the slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission‐line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20 per cent clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0–10 and 20–30 Myr age bin (∼40 per cent each). Most of the emission stars in our survey belong to Classical Be class (∼92 per cent) while a few are Herbig Be stars (∼6 per cent) and Herbig Ae stars (∼2 per cent). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and 1624 (all 4 Myr old) while NGC 6756 (125–150 Myr) is the oldest cluster to have Classical Be stars. The Classical Be stars are located all along the main sequence (MS) in the optical colour–magnitude diagrams (CMDs) of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn‐off. The distribution of Classical Be stars as a function of spectral type shows peaks at B1–B2 and B6–B7 spectral types. The Be star fraction [N(Be)/N(B+Be)] is found to be less than 10 per cent for most of the clusters and NGC 2345 is found to have the largest fraction (∼26 per cent). Our results indicate there could be two mechanisms responsible for the Classical Be phenomenon. Some are born Classical Be stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to Classical Be stars, within the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20–30 Myr age bin.

The climatological features of the diurnal cycle and its spatial and temporal variability are investigated around the Himalayas using hourly, 0.05°ϫ 0.05°grid, near-surface rainfall data from the Precipitation Radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite during June-July-August (JJA) of 1998-2002. Though sampling errors inherent to TRMM PR measurements around the Himalayas could influence results, PR-observed precipitation features show agreement with previous studies in this region.The analysis of precipitation characteristics presented here is based on two rain-rate thresholds: (a) light rain rate (Յ5 mm h Ϫ1 ), and (b) moderate to heavy rain rate (Ͼ5 mm h Ϫ1 ). The results suggest that afternoon to evening precipitation is noticed as embedded convection within a large region of light rain over the south-facing slopes of the Himalayas. The moderate to heavy conditional rain rate exhibits a relatively stronger diurnal cycle of precipitation in this region. However, this may be biased because of sampling. Almost all the Tibetan Plateau shows light rain activity.The Tibetan Plateau and northern Indian subcontinent regions are characterized by daytime maximum precipitation. From the analysis of near-surface rainfall over the finescale topography, it is observed that daytime (1200-1800 LT) precipitation is concentrated over the ridges and strong ridge-valley gradients with rain appearing over the south-facing slopes of the Himalayas. During midnight-early morning, intense rainfall concentrates over the ridges as well as in river valleys. Precipitation broadening and movement are noticed during this time period.

UBVRI CCD photometry in a wide field around two young open clusters, NGC 663 and 654, has been carried out. Hα and polarimetric observations for the cluster NGC 654 have also been obtained. We use the photometric data to construct colour–colour and colour–magnitude diagrams, from which we can investigate the reddening, age, mass and evolutionary states of the stellar contents of the these clusters. The reddening across the cluster regions is found to be variable. There is evidence for anomalous reddening law in both clusters; however, more infrared and polarimetric data are needed to conclude about the reddening law. Both clusters are situated at about a distance of 2.4 kpc. Star formation in both clusters is found to be a continuous process. In the case of NGC 663, star formation seems to have taken place sequentially, in the sense that formation of low‐mass stars precedes the formation of most massive stars. Whereas, in the case of NGC 654, formation of low‐mass stars did not cease after the formation of most massive stars in the cluster.