R. K. S. Yadav

The end of the white dwarf cooling sequence in M 67

In this paper, we present for the first time a proper-motion-selected white dwarf (WD) sample of the old Galactic open cluster M 67, down to the bottom of the WD cooling sequence (CS). The color-magnitude diagram is based on data collected with the LBC-Blue camera at the prime-focus of LBT. As first epoch data, we used CFHT-archive images collected 10 years before LBC data. We measured proper motions of all the identified sources. Proper motions are then used to separate foreground and background objects from the cluster stars, including WDs. Finally, the field-object cleaned WD CS in the V vs. B-I color-magnitude diagram is compared with the models. We confirm that the age derived from the location of the bottom of the WD CS is consistent with the turn off age.

Non-uniform extinction in young open star clusters

The extinction law and the variation of colour excess with position, luminosity and spectral class in young open star clusters NGC 663, NGC 869, NGC 884, NGC 1502, NGC 1893, NGC 2244, NGC 2264, NGC 6611, Tr 14, Tr 15, Tr 16, Coll 228, Tr 37 and Be 86 have been studied. The difference in the minimum and maximum values of E ( B-V ) of cluster members has been considered as a measure of the presence of non-uniform gas and dust inside the clusters. Its value ranges from 0.22 to 1.03 mag in the clusters under study, which indicates that non-uniform extinction is present in all the clusters. It has been noticed for the first time in NGC 1502 and Tr 37. It is also found that the differential colour excess in open clusters, which may be caused by the presence of gas and dust, decreases systematically with the age of clusters, indicating that matter is either used in star formation or blown away by hot stars or both. There is no uniformity in the variation of E ( B-V ) with position or spectral class or luminosity. Except in Tr 14, all clusters show a random spatial distribution of E ( B-V ) , indicating a random distribution of gas and dust inside the clusters. TheE ( B-V) value correlates with both luminosity and spectral class only in the case of Coll 228, Tr 16 and Be 86. The members of these clusters at λ ≥λ R show larger values of colour excess ratios than the normal ones. The value of E( U- V) /E( B-V) for most of the cluster members is close to the normal interstellar value of 1.73. However, the colour excess ratios with E(B-V) at λ ≥λ J are smaller than the normal value for NGC 663, NGC 869, NGC 884 and NGC 1502, while they are larger for NGC 6611, Coll 228, Tr 16 and Tr 14. Thus there is no uniformity in the relationship of extinction properties amongst the clusters under study.

OPTICAL PHOTOMETRIC AND POLARIMETRIC INVESTIGATION OF NGC 1931

We present optical photometric and polarimetric observations of stars toward NGC 1931 with the aim of deriving cluster parameters such as distance, reddening, age, and luminosity/mass function as well as understanding dust properties and star formation in the region. The distance to the cluster is found to be 2.3 {+-} 0.3 kpc and the reddening E(B - V) in the region is found to be variable. The stellar density contours reveal two clusters in the region. The observations suggest a differing reddening law within the cluster region. Polarization efficiency of the dust grains toward the direction of the cluster is found to be less than that for the general diffuse interstellar medium (ISM). The slope of the mass function (-0.98 {+-} 0.22) in the southern region in the mass range of 0.8 < M/M {sub Sun} < 9.8 is found to be shallower in comparison to that in the northern region (-1.26 {+-} 0.23), which is comparable to the Salpeter value (-1.35). The K-band luminosity function (KLF) of the region is found to be comparable to the average value of the slope ({approx}0.4) for young clusters obtained by Lada and Lada; however, the slope of the KLF is steeper inmorexa0» the northern region as compared to the southern region. The region is probably ionized by two B2 main-sequence-type stars. The mean age of the young stellar objects (YSOs) is found to be 2 {+-} 1 Myr, which suggests that the identified YSOs could be younger than the ionizing sources of the region. The morphology of the region, the distribution and ages of the YSOs, and ionizing sources indicate a triggered star formation in the region.«xa0less

Photometry and low resolution spectroscopy of hot post-AGB candidates

We have obtained Johnson U, B, V and Cousins R, I photometry and low resolution spectra of a small sample of hot post-AGB candidates. Using the present data in combination with JHK data from 2MASS, infrared data from the MSX catalog and the IRAS fluxes, we have studied the spectral energy distribution (SED) of these stars. Using the DUSTY code we have estimated the dust temperatures, the distances to the stars, the mass-loss rates, angular radii of the inner boundary of the dust envelopes and dynamical ages from the tip of the AGB. These candidates have also been imaged through a narrow band Hα filter, to search for nebulosity around the central stars. Our Hα images revealed the bipolar morphology of the low excitation PN IRAS 17395-0841 with an angular extent of 2.8 �� . The bipolar lobes of IRAS 17423-1755 in Hα were found to have an angular extent of 3.5 �� (south-east lobe) and 2.2 �� (north-west lobe). The dust envelope characteristics, low resolution spectrum and IRAS colors suggest that IRAS 18313-1738 is similar to the proto-planetary nebula (PPN) HD 51585. The SED of IRAS 17423-1755, IRAS 18313-1738 and IRAS 19127+1717 show a warm dust component (in addition to the cold dust) which may be due to recent and ongoing mass-loss.

A deep UBVRI CCD photometric study of the open clusters Tr 1 and Be 11

We present deep UBVRI charge-coupled device (CCD) photometry for the young open star clusters Tr 1 and Be 11. The CCD data for Be 11 are obtained for the first time. The sample consists of ∼1500 stars reaching down to V ∼ 21 mag. Analysis of the radial distribution of the stellar surface density indicates that radius values for Tr 1 and Be 11 are 2.3 and 1.5 pc, respectively. The interstellar extinction across the face of the imaged clusters region seems to be non-uniform with a mean value of E(B - V) = 0.60 ′ 0.05 and 0.95 ′ 0.05 mag for Tr 1 and Be 11, respectively. A random positional variation of E(B - V) is present in both the clusters. In the cluster Be 11, the reason for the random positional variation may be the apparent association of the H II region (S 213). The 2MASS JHK data in combination with the optical data in the cluster Be 11 yield E(J - K) = 0.40 ′ 0.20 mag and E(V - K) = 2.20 ′ 0.20 mag. Colour excess diagrams indicate a normal interstellar extinction law in the direction of the cluster Be 11. The distances to Tr 1 and Be 11 are estimated as 2.6 ′ 0.10 and 2.2 ′ 0.10 kpc, respectively, while the theoretical stellar evolutionary isochrones fitted to the bright cluster members indicate that the clusters Tr 1 and Be 11 are 40 ′ 10 and 110 ′ 10 Myr old, respectively. The mass functions corrected for both field star contamination and data incompleteness are derived for both clusters. The slopes 1.50 ′ 0.40 and 1.22 ′ 0.24 for Tr 1 and Be 11, respectively, are in agreement with the Salpeter value. Observed mass segregations in both clusters may be the result of dynamical evolutions or the imprint of star formation processes or both.

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.

Intranight optical variability of \gamma-ray loud Narrow Line Seyfert 1 galaxies

The Large Area Telescope (LAT) onboard the {it Fermi} Gamma Ray Space Telescope has detected gamma-ray emission from some Narrow Line Seyfert 1(NLSy1) galaxies. This indicates the presence of relativistic jets in these sources similar to blazars and radio galaxies. In an attempt to have an idea of the intranight optical variability (INOV) characteristics of these gamma-ray loud NLSy1 galaxies, we have carried out optical flux monitoring observations of three NLSy1 galaxies detected by {it Fermi}/LAT: 1H 0323+342, PMN J0948+0022 and PKS 1502+036. These optical monitoring observations showed the presence of rapid optical flux variations in these sources. The intranight differential light curves of these sources have revealed flux variations on time scales of hours with amplitudes of variability > 3 percent for most of the time. On using the {it F}-statistics to classify the variability nature of these sources, we obtained a duty cycle (DC) of INOV of ~85 percent. Alternatively, the more commonly used {it C}-statistics gave a DC of INOV of ~57 percent. Such high DC of INOV are characteristics of the BL Lac class of AGN. The results of our monitoring observations thus indicate that there is similarity in the INOV nature of gamma-ray loud NLSy1 galaxies and BL Lac objects, arguing strongly for the presence of relativistic jets aligned closely to the observers line of sight. Moreover, our dense monitoring observations on some of the nights have led to the clear detection of some mini-flares superimposed on the flux variations during the night over timescales as short as 12 minutes. The detection of short timescale flux variability in the sources studied here is clearly due to stronger time compression leading to the jets in these sources having large Doppler factors, similar to that of the inner jets of TeV blazars.

Investigation of the stellar content in the western part of the Carina nebula

We obtained deep

Apex determination and detection of stellar clumps in the open cluster M 67

UBVRI

Pre-main-sequence population in NGC 1893 region: X-ray properties

H