D. K. Sahu

Temperate Earth-sized planets transiting a nearby ultracool dwarf star

Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as ‘ultracool dwarfs’. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks, there should be a large but hitherto undetected population of terrestrial planets orbiting them—ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system.

Photometric and spectroscopic evolution of the Type IIP supernova SN 2004et

We present optical photometry and spectroscopy of the normal type IIP supernova SN2004A, which was discovered in the galaxy NGC 6207 on 2004 January 9.84UT. Early observations indicated that the supernova was discovered at about two weeks since explosion. We estimate the distance to NGC 6207 to be 20.35 \pm 4.5 Mpc using the Standard Candle method. Using this distance, we estimate the ejected nickel mass in the explosion to be 0.032 \pm 0.02M\odo\cdot. The plateau luminosity, its duration (about 80 days) and the expansion velocity of the supernova ejecta at the middle of the plateau indicate an explosion energy of 4.7 \pm 2.7 x 10^50 ergs and an ejected envelope mass of 7.2 \pm 2.2M\odot\cdot . The ejected envelope mass implies a main sequence mass of 10 2.5\pm M\odot\cdot for the progenitor.

TYPE Ib SUPERNOVA 2008D ASSOCIATED WITH THE LUMINOUS X-RAY TRANSIENT 080109: AN ENERGETIC EXPLOSION OF A MASSIVE HELIUM STAR

We present a theoretical model for supernova SN 2008D associated with the luminous X-ray transient 080109. The bolometric light curve and optical spectra of the SN are modeled based on the progenitor models and the explosion models obtained from hydrodynamic/nucleosynthetic calculations. We find that SN 2008D is a more energetic explosion than normal core-collapse supernovae, with an ejecta mass of M ej = 5.3 ± 1.0 M ☉ and a kinetic energy of E K = 6.0 ± 2.5 × 1051 erg. The progenitor star of the SN has a 6-8 M ☉ He core with essentially no H envelope (<5 × 10–4 M ☉) prior to the explosion. The main-sequence mass of the progenitor is estimated to be M MS = 20-25 M ☉, with additional systematic uncertainties due to convection, mass loss, rotation, and binary effects. These properties are intermediate between those of normal SNe and hypernovae associated with gamma-ray bursts. The mass of the central remnant is estimated as 1.6-1.8 M ☉, which is near the boundary between neutron star and black hole formation.

ESC observations of SN 2005cf – I. Photometric evolution of a normal Type Ia supernova

We present early-time optical and near-infrared photometry of supernova (SN) 2005cf. The observations, spanning a period from about 12 d before to 3 months after maximum, have been obtained through the coordination of observational efforts of various nodes of the European Supernova Collaboration and including data obtained at the 2-m Himalayan Chandra Telescope. From the observed light curve we deduce that SN 2005cf is a fairly typical SN Ia with a post-maximum decline [� m15(B)true = 1.12] close to the average value and a normal luminosity of MB,max =− 19.39 ± 0.33. Models of the bolometric light curve suggest a synthesized 56 Ni mass of about 0.7 M� . The negligible host galaxy interstellar extinction and its proximity make SN 2005cf a good Type Ia SN template.

The evolution of the peculiar type Ia supernova SN 2005hk over 400 days

UBVRI photometryandmedium-resolution opticalspectroscopyof thepeculiarTypeIasupernova SN2005hkare presentedandanalyzed,coveringthepremaximum phase toaround400daysafter explosion.Thesupernova isfound tobeunderluminouscomparedto‘‘normal’’TypeIasupernovae.Thephotometricandspectroscopicevolutionof SN 2005hk is remarkably similar to the peculiar Type Ia event SN 2002cx. The expansion velocity of the supernova ejecta is found to be lower than normal Type Ia events. The spectra obtained k200 days since explosion do not show the presence of forbidden [Fe ii], [Fe iii], and [Co iii] lines, but are dominated by narrow, permitted Fe ii, NIR Ca ii, and Na i lines with P Cygni profiles. The thermonuclear explosion model with Chandrasekhar mass ejecta and a kinetic energy smaller (EK ¼ 0:3 ;10 51 ergs) than that of canonical Type Ia supernovae is found to well explain the observed bolometric light curve. The mass of 56 Ni synthesized in this explosion is 0.18 M� . The early spectra are successfully modeled with this less energetic model, with some modifications of the abundance distribution. The late spectrum is explained as a combination of a photospheric component and a nebular component. Subject headingg supernovae: general — supernovae: individual (SN 2005hk)

THE UNIQUE TYPE Ib SUPERNOVA 2005bf: A WN STAR EXPLOSION MODEL FOR PECULIAR LIGHT CURVES AND SPECTRA

Observations and modeling for the light curve (LC) and spectra of supernova (SN) 2005bf are reported. This SN showed unique features: the LC had two maxima, and declined rapidly after the second maximum, while the spectra showed strengthening He lines whose velocity increased with time. The double-peaked LC can be reproduced by a double-peaked 56 Ni distribution, with most 56 Ni at low velocity and a small amount at high velocity. The rapid postmaximum decline requires a large fraction of the g-rays to escape from the 56 Ni-dominated region, possibly because of low-density “holes.” The presence of Balmer lines in the spectrum suggests that the He layer of the progenitor was substantially intact. Increasing g-ray deposition in the He layer due to enhanced g-ray escape from the 56 Ni-dominated region may explain both the delayed strengthening and the increasing velocity of the He lines. The SN has massive ejecta (∼6–7 M,), normal kinetic energy [∼(1.0–1.5) # 10 51 ergs], a high peak bolometric luminosity (∼ ergs s 1 ) for an epoch as late as ∼ 40 days, and a large 56 Ni mass 42 5 # 10 (∼0.32 M,). These properties and the presence of a small amount of H suggest that the progenitor was initially massive ( M,) and had lost most of its H envelope, and was possibly a WN star. The double-peaked M ∼ 25–30 56 Ni distribution suggests that the explosion may have formed jets that did not reach the He layer. The properties of SN 2005bf resemble those of the explosion of Cassiopeia A. Subject headings: stars: Wolf-Rayet — supernovae: general — supernovae: individual (Cassiopeia A, SN 2005bf)

Multiwavelength Analysis of the Intriguing GRB 061126: The Reverse Shock Scenario and Magnetization

We present a detailed study of the prompt and afterglow emission from Swift GRB 061126 using BAT, XRT, UVOT data and multicolor optical imaging from 10 ground-based telescopes. GRB 061126 was a long burst (T90 = 191 s) with four overlapping peaks in its γ-ray light curve. The X-ray afterglow, observed from 26 minutes to 20 days after the burst, shows a simple power-law decay with αX = 1.290 ± 0.008. Optical observations presented here cover the time range from 258 s (Faulkes Telescope North) to 15 days (Gemini North) after the burst; the decay rate of the optical afterglow shows a steep-to-shallow transition (from α1 = 1.48 ± 0.06 to α2 = 0.88 ± 0.03) approximately 13 minutes after the burst. We suggest the early, steep component is due to a reverse shock and show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartition value, is a few 10 times larger than in the forward shock in the early afterglow phase. The ejecta might be endowed with primordial magnetic fields at the central engine. The optical light curve implies a late-time break at about 1.5 days after the burst, while there is no evidence of the simultaneous break in the X-ray light curve. We model the broadband emission and show that some afterglow characteristics (the steeper decay in X-ray and the shallow spectral index from optical to X-ray) are difficult to explain in the framework of the standard fireball model. This might imply that the X-ray afterglow is due to an additional emission process, such as late-time central engine activity rather than blast-wave shock emission. The possible chromatic break at 1.5 days after the burst would give support to the additional emission scenario.

iPTF13bvn: THE FIRST EVIDENCE OF A BINARY PROGENITOR FOR A TYPE Ib SUPERNOVA

The recent detection in archival Hubble Space Telescope images of an object at the location of supernova (SN) iPTF13bvn may represent the first direct evidence of the progenitor of a Type Ib SN. The objects photometry was found to be compatible with a Wolf-Rayet pre-SN star mass of ≈11 M ☉. However, based on hydrodynamical models, we show that the progenitor had a pre-SN mass of ≈3.5 M ☉ and that it could not be larger than ≈8 M ☉. We propose an interacting binary system as the SN progenitor and perform evolutionary calculations that are able to self-consistently explain the light curve shape, the absence of hydrogen, and the pre-SN photometry. We further discuss the range of allowed binary systems and predict that the remaining companion is a luminous O-type star of significantly lower flux in the optical than the pre-SN object. A future detection of such a star may be possible and would provide the first robust identification of a progenitor system for a Type Ib SN.

A tale of two GRB-SNe at a common redshift of z=0.54

We present ground-based and Hubble Space Telescope optical observations of the optical transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618, both at a redshift of z= 0.54. For GRB 060729, bumps are seen in the optical light curves (LCs), and the late-time broad-band spectral energy distributions (SEDs) of the OT resemble those of local Type Ic supernovae (SNe). For GRB 090618, the dense sampling of our optical observations has allowed us to detect well-defined bumps in the optical LCs, as well as a change in colour, that are indicative of light coming from a core-collapse SN. The accompanying SNe for both events are individually compared with SN1998bw, a known GRB supernova, and SN1994I, a typical Type Ic supernova without a known GRB counterpart, and in both cases the brightness and temporal evolution more closely resemble SN1998bw. We also exploit our extensive optical and radio data for GRB 090618, as well as the publicly available Swift-XRT data, and discuss the properties of the afterglow at early times. In the context of a simple jet-like model, the afterglow of GRB 090618 is best explained by the presence of a jet-break at t-to > 0.5 d. We then compare the rest-frame, peak V-band absolute magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large sample of local Type Ibc SNe, concluding that, when host extinction is considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of non-GRB/XRF SNe. --------------------------------------------------------------------------------

A tale of two GRB-SNe at a common redshift of z = 0.54

We present ground-based and Hubble Space Telescope optical observations of the optical transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618, both at a redshift of z= 0.54. For GRB 060729, bumps are seen in the optical light curves (LCs), and the late-time broad-band spectral energy distributions (SEDs) of the OT resemble those of local Type Ic supernovae (SNe). For GRB 090618, the dense sampling of our optical observations has allowed us to detect well-defined bumps in the optical LCs, as well as a change in colour, that are indicative of light coming from a core-collapse SN. The accompanying SNe for both events are individually compared with SN1998bw, a known GRB supernova, and SN1994I, a typical Type Ic supernova without a known GRB counterpart, and in both cases the brightness and temporal evolution more closely resemble SN1998bw. We also exploit our extensive optical and radio data for GRB 090618, as well as the publicly available Swift-XRT data, and discuss the properties of the afterglow at early times. In the context of a simple jet-like model, the afterglow of GRB 090618 is best explained by the presence of a jet-break at t-to > 0.5 d. We then compare the rest-frame, peak V-band absolute magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large sample of local Type Ibc SNe, concluding that, when host extinction is considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of non-GRB/XRF SNe. --------------------------------------------------------------------------------