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Item Photopolarimetric study of the star-forming clouds CB3, CB25, and CB39(Astronomy & Astrophysics, 2010-09-07) Sen, A.K.; Polcaro, V. F; Dey, I.; et.alThe small compact isolated dark clouds also known as “Bok globules” are believed to be ideal sites for low-mass star formation. Some of these clouds are undergoing gravitational collapse, and the ambient magnetic field plays a key role in collapse dynamics. The background star polarimetry is generally accepted as a good tool to map the magnetic field, which is responsible for the alignment of dichroic grains that produce polarization. Aims. The background star polarization when studied together with extinction is expected to help us to understand various grain properties and the role of polarimetry as a tracer of magnetic field in these star-forming clouds. With this idea, polarization and colour excess E(B − V) values for a set of background stars have been studied together to understand various astrophysical process in some star-forming dark clouds. Methods. Optical photometric observations of the three clouds CB3, CB25, and CB39 were carried out at the 2 m H.C. Telescope, India, to determine the colour excess E(B − V) of the background stars by following a technique adopted by Bernabei & Polacaro (2001, A&A, 371, 123). These three clouds were selected from a set of eight clouds previously observed by us in optical polarimetry (Sen et al. 2000, A&AS, 141, 175). Further independent spectroscopic measurements of a few selected sample stars were recently carried out during February and March 2010 from 1.52 m Cassini Telescope, Loinao, Italy, to confirm the correctness of estimated E(B − V) values obtained by this photometric technique. Results. The colour excess E(B − V) values so obtained were compared with optical polarization values obtained for the same set of stars. It was found that the measured extinction values increase with the increase in percentage polarization for the cloud CB39 and to some extent for CB25. However, for cloud CB31 no such correlation was observed. It is normally expected that the grains causing extinction should also cause polarization of the light from background stars. Any possible deviation from this under different circumstances here has been discussed in the light of the ongoing physical processes in the star-forming clouds.Item A CCD photometric study of the late type contact binary EK Comae Berenices(New Astronomy, 2010-04-30) Deb, Sukanta; Singh, Harinder P; Seshadri, T.R; et.alWe present CCD photometric observations of the W UMa type contact bi- nary EK Comae Berenices using the 2 metre telescope of IUCAA Girawali Observatory, India. The star was classified as a W UMa type binary of subtype-W by Samec et al. (1996). The new V band photometric observa- tions of the star reveal that shape of the light curve has changed significantly from the one observed by Samec et al. (1996). A detailed analysis of the light curve obtained from the high-precision CCD photometric observations of the star indicates that EK Comae Berenices is not a W-type but an A- type totally eclipsing W UMa contact binary. The photometric mass ratio is determined to be 0.349 ± 0.005. A temperature difference of T = 141±10 K between the components and an orbital inclination of i[o] = 89.800±0.075 were obtained for the binary system. Absolute values of masses, radii and luminosities are estimated by means of the standard mass-luminosity relation for zero age main-sequence stars. The star shows O’Connell effect, asymmetries in the light curve shape around the primary and secondary maximum. The observed O’Connell effect is explained by the presence of a hot spot on the primary component.Item SN 2008gz − most likely a normal type IIP event(Mon. Not. R. Astron. Soc., 2011-02-08) Roy, Rupak; Kumar, Brijesh; Moskvitin, Alexander S; et.alWe present BV RI photometric and low-resolution spectroscopic investigation of a type II core-collapse supernova (SN) 2008gz, which occurred in a star forming arm and within a half-light radius (solar metallicity region) of a nearby spiral galaxy NGC 3672. The SN event was detected late, and a detailed investigation of its light curves and spectra spanning 200 days suggest that it is an event of type IIP similar to archetypal SNe 2004et and 1999em. However, in contrast to other events of its class, the SN 2008gz exhibits rarely observed V magnitude drop of 1.5 over the period of a month during plateau to nebular phase. Using 0.21 mag of AV as a lower limit and a distance of 25.5 Mpc, we estimate synthesized 56Ni mass of 0.05 ± 0.01M⊙ and a mid-plateau MV of −16.6 ± 0.2 mag. The photospheric velocity is observed to be higher than that was observed for SN 2004et at similar epochs, indicating explosion energy was comparable to or higher than SN 2004et. Similar trend was also seen for the expansion velocity of H-envelopes. By comparing its properties with other well studied events as well as by using a recent simulation of pre-SN models of Dessart, Livne & Waldman (2010), we infer an explosion energy range of 2−3×1051 erg and this coupled with the observed width of the forbidden [O i] 6300-6364°A line at 275 days after the explosion gives an upper limit for the main-sequence (non-rotating, solar metallicity) progenitor mass of 17M⊙. Our narrow-band Hα observation, taken nearly 560 days after the explosion and the presence of an emission kink at zero velocity in the Doppler corrected spectra of SN indicate that the event took place in a low luminosity star forming Hii region.Item SN 2008in − BRIDGING THE GAP BETWEEN NORMAL AND FAINT SUPERNOVAE OF TYPE IIP(2011-06-10) Roy, Rupak; Kumar, Brijesh Kumar; Benetti, Stefano; et.alWe present optical photometric and low-resolution spectroscopic observations of the Type II plateau supernova (SN) 2008in, which occurred in the outskirts of the nearly face-on spiral galaxy M 61. Photometric data in the X-rays, ultraviolet and near-infrared bands have been used to characterize this event. The SN field was imaged with the ROTSE-IIIb optical telescope about seven days before the explosion. This allowed us to constrain the epoch of the shock breakout to JD = 2454825.6. The duration of the plateau phase, as derived from the photometric monitoring, was 98 days. The spectra of SN 2008in show a striking resemblance to those of the archetypal low-luminosity IIP SNe 1997D and 1999br. A comparison of ejecta kinematics of SN 2008in with the hydrodynamical simulations of Type IIP SNe by Dessart et al. (2010) indicates that it is a less energetic event ( 5 × 1050 erg). However, the light curve indicates that the production of radioactive 56Ni is significantly higher than that in the low-luminosity SNe. Adopting an interstellar absorption along the SN direction of AV 0.3 mag and a distance of 13.2 Mpc, we estimated a synthesized 56Ni mass of 0.015M⊙. Employing semi-analytical formulae (Litvinova & Nadezhin 1985), we derived a pre-SN radius of 126R⊙ , an explosion energy of 5.4 × 1050 erg and a total ejected mass of 16.7M⊙ . The latter indicates that the zero age main-sequence mass of the progenitor did not exceed 20M⊙ . Considering the above properties of SN 2008in and its occurrence in a region of sub-solar metallicity ([O/H] 8.44 dex), it is unlikely that fall-back of the ejecta onto a newly formed black hole occurred in SN 2008in. We therefore favor a low-energy explosion scenario of a relatively compact, moderate-mass progenitor star that generates a neutron star.Item Star Formation Activity in the Galactic H ii Complex S255-S257(2011-06-09) Ojha, D.K.; Samal, M.R; Pandey, A.K.; et.alWe present results on the star-formation activity of an optically obscured region containing an embedded cluster (S255-IR) and molecular gas between two evolved H ii regions S255 and S257. We have studied the complex using optical, near-infrared (NIR) imaging, optical spectroscopy and radio continnum mapping at 15 GHz, along with Spitzer-IRAC results. It is found that the main exciting sources of the evolved H ii regions S255 and S257 and the compact H ii regions associated with S255-IR are of O9.5 - B3 V nature, consistent with previous observations. Our NIR observations reveal 109 likely young stellar object (YSO) candidates in an area of 40.9 40.9 centered on S255-IR, which include 69 new YSO candidates. To see the global star formation, we constructed the V I=V diagram for 51 optically identified IRAC YSOs in an area of 130 130 centered on S255-IR. We suggest that these YSOs have an approximate age between 0.1 - 4 Myr, indicating a non-coeval star formation. Using spectral energy distribution (SED) models, we constrained physical properties and evolutionary status of 31 and 16 YSO candidates outside and inside the gas ridge, respectively. The models suggest that the sources associated within the gas ridge are of younger population (mean age 1.2 Myr) than the sources outside the gas ridge (mean age 2.5 Myr). The positions of the young sources inside the gas ridge at the interface of the H ii regions S255 and S257, favor a site of induced star formationItem THE DISCOVERY AND NATURE OF THE OPTICAL TRANSIENT CSS100217:102913+404220(The Astrophysical Journal, 2011-07-10) Drake, A.J; Djorgovski, S.G; Mahabal, A; et.alWe report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations showed that this transient was coincident with a galaxy at redshift z = 0.147 and reached an apparent magnitude of V ∼ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be MV = −22.7 approximately 45 days after maximum light. Over a period of 287 rest-frame days, this event had an integrated bolometric luminosity of 1.3 × 1052 erg based on time-averaged bolometric corrections of ∼15 from V- and R-band observations. Analysis of the pre-outburst Sloan Digital Sky Survey (SDSS) spectrum of the source shows features consistent with a narrow-line Seyfert 1 galaxy. High-resolution Hubble Space Telescope and Keck follow-up observations show that the event occurred within 150 pc of the nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival Catalina Sky Survey NEO search and SDSS data to investigate the three most likely sources of such an event: (1) an extremely luminous supernova, (2) the tidal disruption of a star by the massive nuclear black hole, and (3) variability of the central active galactic nucleus (AGN). We find that CSS100217 was likely an extremely luminous Type IIn supernova and occurred within the range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity.