C. Eswaraiah

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.

Young stellar population and ongoing star formation in the H ii complex Sh2-252

In this paper, an extensive survey of the star-forming complex Sh2-252 has been undertaken with an aim to explore its hidden young stellar population as well as to understand the structure and star formation history for the first time. This complex is composed of five prominent embedded clusters associated with the subregions A, C, E, NGC 2175s and Teu 136. We used Two Micron All Sky Survey-near-infrared and Spitzer-Infrared Array Camera, Multiband Imaging Photometer for Spitzer photometry to identify and classify the young stellar objects (YSOs) by their infrared (IR) excess emission. Using the IR colour–colour criteria, we identified 577 YSOs, of which, 163 are Class I, 400 are Class II and 14 are transition disc YSOs, suggesting a moderately rich number of YSOs in this complex. Spatial distribution of the candidate YSOs shows that they are mostly clustered around the subregions in the western half of the complex, suggesting enhanced star formation activity towards its west. Using the spectral energy distribution and optical colour–magnitude diagram-based age analyses, we derived probable evolutionary status of the subregions of Sh2-252. Our analysis shows that the region A is the youngest (∼0.5 Myr), the regions B, C and E are of similar evolutionary stage (∼1–2 Myr) and the clusters NGC 2175s and Teu 136 are slightly evolved (∼2–3 Myr). Morphology of the region in the 1.1 mm map shows a semicircular shaped molecular shell composed of several clumps and YSOs bordering the western ionization front of Sh2-252. Our analyses suggest that next generation star formation is currently under way along this border and that possibly fragmentation of the matter collected during the expansion of the H ii region as one of the major processes is responsible for such stars. We observed the densest concentration of YSOs (mostly Class I, ∼0.5 Myr) at the western outskirts of the complex, within a molecular clump associated with water and methanol masers and we suggest that it is indeed a site of cluster formation at a very early evolutionary stage, sandwiched between the two relatively evolved CH ii regions A and B.

A study of the starless dark cloud LDN 1570: Distance, dust properties, and magnetic field geometry

Aims. We aim to map the magnetic field geometry and to study the dust properties of the starless cloud, L1570, using multi-wavelength optical polarimetry and photometry of the stars projected on the cloud. Methods. The direction of the magnetic field component parallel to the plane of the sky of a cloud can be obtained using polarimetry of the stars projected on and located behind the cloud. It is believed that the unpolarized light from the stars background to the cloud undergoes selective extinction while passing through non-spherical dust grains that are aligned with their minor axes parallel to the cloud magnetic field. The emerging light becomes partially plane polarized. The observed polarization vectors trace the direction of the projected magnetic field of the cloud. We made R-band imaging polarimetry of the stars projected on a cloud, L1570, to trace the magnetic field orientation. We also made multi-wavelength polarimetric and photometric observations to constrain the properties of dust in L1570. Results. We estimated a distance of 394 ± 70 pc to the cloud using 2MASS JHKs colors. Using the values of the Serkowski parameters, σ1, � , λmax, and the position of the stars on the near-infrared color‐color diagram, we identified 13 stars that could possibly have intrinsic polarization and/or rotation in their polarization angles. One star, 2MASS J06075075+1934177, which is a B4Ve spectral type, shows diffuse interstellar bands in the spectrum in addition to the Hα line in emission. There is an indication for slightly bigger dust grains toward L1570 on the basis of the dust grain size-indicators such as λmax and RV values. The magnetic field lines are found to be parallel to the cloud structures seen in the 250 μm images (also in the 8 μm and 12 μm shadow images) of L1570. Based on the magnetic field geometry, the cloud structure, and the complex velocity structure, we conclude that L1570 is in the process of formation due to the converging flow material mediated by the magnetic field lines. A structure function analysis showed that in the L1570 cloud region the large-scale magnetic fields are stronger than the turbulent component of the magnetic fields. The estimated magnetic field strengths suggest that the L1570 cloud region is subcritical and hence could be strongly supported by the magnetic field lines.

Broad-band optical polarimetric studies towards the Galactic young star cluster Berkeley 59

We present multiwavelength optical linear polarimetric observations of 69 stars towards the young open cluster Berkeley 59. The observations reveal the presence of three dust layers located at distances of ∼300, ∼500 and ∼700 pc. The dust layers produce a total polarization P V ∼ 5.5 per cent. The mean values of polarization and polarization angles due to the dust layers are found to increase systematically with distance. We show that polarimetry in combination with the (U−B)–(B−V) colour–colour diagram yields a better identification of cluster members. The polarization measurements suggest that the polarization due the intracluster medium is ∼2.2 per cent. An anomalous reddening law exists for the cluster region, indicating a relatively larger grain size than that in the diffuse interstellar medium. The spatial variation of polarization and colour excess E(B−V) are found to increase with radial distance from the cluster centre, whereas θV and λmax are found to decrease with increasing radial distance from the cluster centre. About 40 per cent of cluster members show the signatures of either intrinsic polarization or rotation in their polarization angles. There is an indication that the starlight of the cluster members might have been depolarized because of non-uniform alignment of dust grains in the foreground dust layers and in the intracluster medium.

Optical and near‐infrared survey of the stellar contents associated with the star‐forming complex Sh2‐252

We present the analyses of the stellar contents associated with the extended Hii region Sh2-252 using deep optical UBVRI photometry, slit and slitless spectroscopy along with the near-infrared (NIR) data from Two-Micron All-Sky Survey (2MASS) for an area similar to 1 x 1 deg2. We have studied the sub-regions of Sh2-252, which includes four compact-H?ii (CH ii ) regions, namely A, B, C and E, and two clusters, NGC 2175s and Teutsch 136 (Teu 136). Of the 15 spectroscopically observed bright stars, eight have been identified as massive members of spectral class earlier than B3. From the spectrophotometric analyses, we derived the average distance of the region as 2.4 +/- 0.2 kpc, and the reddening E(B - V) of the massive members is found to vary between 0.35 and 2.1mag. We found that NGC 2175s and Teu 136, located towards the eastern edge of the complex, are the sub-clusters of Sh2-252. The stellar surface density distribution in K band shows clustering associated with the regions A, C, E, NGC 2175s and Teu 136. We have also identified the candidate ionizing sources of the CH?ii regions. 61 H alpha emission sources are identified using slitless spectroscopy. The distribution of the Ha emission sources and candidate young stellar objects (YSOs) with IR excess on the V/(V - I) colourmagnitude diagram (CMD) shows that a majority of them have approximate ages between 0.1 and 5Myr and masses in the range of 0.32.5 M circle dot. The optical CMDs of the candidate pre-main-sequence (PMS) sources in the individual regions also show an age spread of 0.15?Myr for each of them. We calculated the K-band luminosity functions (KLFs) for the sub-regions A, C, E, NGC 2175s and Teu 136. Within errors, the KLFs for all the sub-regions are found to be similar and comparable to that of young clusters of age \textless5?Myr. We also estimated the mass function of the PMS sample of the individual regions in the mass range of 0.32.5?M circle dot In general, the slopes of the MFs of all the sub-regions are found comparable to the Salpeter value.

Multiwavelength studies of H II region NGC 2467

We present the multiwavelength studies of the H II region Sh2-311 to explore the effects of massive stars on low-mass star formation. In this study we have used optical (UBVI) data from ESO 2.2m Wide Field Imager (WFI), Near-Infrared (NIR) (JHKs) data from CTIO 4m Blanco Telescope and archival Spitzer 8μm data. Based on stellar density contours and dust distribution we have divided the complex into three regions i.e., Haffner 19 (H19), Haffner 18 (H18) and NGC 2467. Using the UBVI data we have estimated the basic parameters of these regions. We have constructed the (J – H)/(H – Ks) color-color diagram and a J/(J – H) color-magnitude diagram to identify young stellar objects (YSOs) and to estimate their masses. Spatial distribution of the YSOs indicates that most of them are distributed at the periphery of the H II region and ionizing star may be responsible for the triggering of star formation at the periphery of the H II region.

Photometric and polarimetric studies towards NGC 1931

We investigated the dust properties and magnetic field orientation towards the star cluster NGC 1931. Cluster members have been identified using both optical color-color as well as QV - UV stokes plane diagrams. Thus identified probable cluster members have been used to estimate the distance to the cluster NGC 1931 which is found to be 2.3±0.3 kpc. Few young stellar objects with possible intrinsic polarization have been identified. One-dimensional surface stellar density contours suggest two clustering in the NGC 1931 region. The reddening E(B - V) is found to be variable between 0.50 to 0.90 mag. Polarimetric and photometric results indicate for the presence of slightly bigger dust grains within the intra-cluster medium. Although our previous work (Eswaraiah et al. 2011) reveals uniform dust grain alignment in the foreground medium towards the direction of NGC 1931, the polarization efficiency is found to be less than that of the general diffuse interstellar medium. This could be due to the normal extinc...