Devendra Sahu - Academia.edu (original) (raw)
Papers by Devendra Sahu
Monthly Notices of the Royal Astronomical Society
Comet 2I/Borisov is the first true interstellar comet discovered. Here, we present results from o... more Comet 2I/Borisov is the first true interstellar comet discovered. Here, we present results from observational programs at two Indian observatories, 2 m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle (HCT) and 1.2 m telescope at the Mount Abu Infrared Observatory (MIRO). Two epochs of imaging and spectroscopy were carried out at the HCT and three epochs of imaging at MIRO. We found CN to be the dominant molecular emission on both epochs, 2019 November 30 and December 22, at distances of rH = 2.013 and 2.031 au, respectively. The comet was inferred to be relatively depleted in Carbon bearing molecules on the basis of low C2 and C3 abundances. We find the production rate ratio, Q(C2)/Q(CN) = 0.54 ± 0.18, pre-perihelion and Q(C2)/Q(CN) = 0.34 ± 0.12 post-perihelion. This classifies the comet as being moderately depleted in carbon chain molecules. Using the results from spectroscopic observations, we believe the comet to have a chemically heterogeneous surface ...
The Astronomical Journal
We present results of our study of PDS 11 binary system, which belongs to a rare class of isolate... more We present results of our study of PDS 11 binary system, which belongs to a rare class of isolated, high galactic latitude T Tauri stars. Our spectroscopic analysis reveals that PDS 11 is a M2−M2 binary system with both components showing similar Hα emission strength. Both the components appear to be accreting, and are classical T Tauri stars. The lithium doublet Li i λ6708, a signature of youth, is present in the spectrum of PDS 11A, but not in PDS 11B. From the application of lithium depletion boundary age-dating method and a comparison with the Li i λ6708 equivalent width distribution of moving groups, we estimated an age of 10−15 Myr for PDS 11A. Comparison with pre-main sequence evolutionary models indicates that PDS 11A is a 0.4 M ⊙ T Tauri star at a distance of 114−131 pc. PDS 11 system does not appear to be associated with any known star forming regions or moving groups. PDS 11 is a new addition, after TWA 30 and LDS 5606, to the interesting class of old, dusty, wide binary classical T Tauri systems in which both components are actively accreting.
Monthly Notices of the Royal Astronomical Society
We present optical UBVRI photometry and medium resolution spectroscopy of the Type Ib supernova i... more We present optical UBVRI photometry and medium resolution spectroscopy of the Type Ib supernova iPTF13bvn, spanning a phase of ∼−13 d to +71 d with respect to B-band maximum. The post-maximum decline rates indicate a fast decline with m 15 (B) = 1.82. Correcting for a Galactic extinction E(B − V) MW = 0.045 and host galaxy extinction of E(B − V) host = 0.17, the absolute V-band magnitude peaks at M V = −17.23 ± 0.20. The bolometric light curve indicates that ∼0.09 M of 56 Ni was synthesized in the explosion. The earliest spectrum (−13 d) shows the presence of He I 5876 Å feature at a velocity of ∼15 000 km s −1 , which falls rapidly by the time the SN approaches the epoch of B-band maximum. The photospheric velocity near maximum light, as indicated by the Fe II 5169 Å feature, is ∼9000 km s −1. The estimate for the 56 Ni mass, together with the estimates for the ejected mass (M ej) and kinetic energy of the explosion (E k) indicate that iPTF13bvn is a low-luminosity Type Ib supernova, with a lower than average ejected mass and kinetic energy. Our results suggest that the progenitor of iPTF13bvn is inconsistent with a single Wolf-Rayet star.
Ultraviolet observations using the Solar Blind Channel on the Hubble Space Telescope provide ligh... more Ultraviolet observations using the Solar Blind Channel on the Hubble Space Telescope provide light curves and low resolution spectra of three pulsating white dwarfs in the cataclysmic variables SDSS013132.39-090122.3, SDSSJ161033.64-010223.3 and SDSSJ220553.98+115553.7. The UV light curves show enhanced pulsation amplitudes over those from simultaneous and previous optical photometry, while the UV-optical spectra are fit with white dwarf temperatures near 15,000K. These temperatures place the accreting white dwarfs outside the instability zone for non-interacting DAV white dwarfs and show that the instability strip is complex for accreting white dwarfs.
Nature, May 12, 2016
Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as 'u... more 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 da...
Bulletin of the Astronomical Society of India
Bulletin of the Astronomical Society of India
The supernova SN 2002ap was discovered in the outer regions of the nearby spiral M74 on January 2... more The supernova SN 2002ap was discovered in the outer regions of the nearby spiral M74 on January 29.4 UT. Early photometric and spectroscopic observations indicate the supernova belongs to the class of Ic hypernova. Late time (After JD 2452500) light curve decay slopes are similar to that of the hypernovae SN 1997ef and SN 1998bw. We present here the BV RI photometric light curves and colour evolutions of SN 2002ap to investigate the late time nature of the light curve.
ABSTRACT CBET 316 available at Central Bureau for Astronomical Telegrams.
Bulletin of the Astronomical Society of India
Aims. We present multiwavelength observations of the gamma-ray burst GRB 051028 detected by HETE-... more Aims. We present multiwavelength observations of the gamma-ray burst GRB 051028 detected by HETE-2 in order to derive its afterglow emission parameters and to determine the reason for its optical faintness when compared to other events. Methods. Observations were taken in the optical (2.0m Himalayan Chandra Telescope, 1.34m Tautenburg, 4.2m William Herschel Telescope) and in X-rays (Swift/XRT) between 2.7 hours and ∼ 10 days after the onset of the event. Results. The data can be interpreted by collimated emission in a jet with a typical value of p = 2.4 which is moving in an homogeneous interstellar medium and with a cooling frequency ν c still above the X-rays at 0.5 days after the burst onset. GRB 051028 can be classified as a "gray" or "potentially dark" GRB. On the basis of the combined optical and Swift/XRT data, we conclude that the reason for the optical dimness is not extra absorption in the host galaxy, but rather the GRB taking place at high-redshift. We also notice the very striking similarity with the optical lightcurve of GRB 050730, a burst with a spectroscopic redshift of 3.967, although GRB 051028 is ∼ 3 mag fainter. We suggest that the bumps could be explained by multiple energy injection episodes and that the burst is intrinsically faint when compared to the average afterglows detected since 1997. The non-detection of the host galaxy down to R = 25.1 is also consistent with the burst arising at high redshift, compatible with the published pseudo-z of 3.7 ± 1.8.
CBET 790 available at Central Bureau for Astronomical Telegrams.
Bulletin of the Astronomical Society of India
Although well established for BL Lac objects and radio-loud quasars, the occurrence of intra-nigh... more Although well established for BL Lac objects and radio-loud quasars, the occurrence of intra-night optical variability (INOV) in radio-quiet quasars is still debated, primarily since only a handful of INOV events with good statistical significance, albeit small amplitude, have been reported so far. This has motivated us to continue intra-night optical monitoring of bona-fide radio-quiet quasars (RQQs). Here we present the results for a sample of 11 RQQs monitored by us on 19 nights. On 5 of these nights a given RQQ was monitored simultaneously from two well separated observatories. In all, two clear cases and two probable case of INOV were detected. From these data, we estimate an INOV duty cycle of ∼8% for RQQs which would increase to 19% if the 'probable variable' cases are also included. Such comparatively small INOV duty cycles for RQQs, together with the small INOV amplitudes (∼1%), are in accord with the previously deduced characteristics of this phenomenon.
ABSTRACT CBET 992 available at Central Bureau for Astronomical Telegrams.
SN 2004et was observed with the 2-m Himalayan Chandra Telescope (HCT) of the Indian Astronomical ... more SN 2004et was observed with the 2-m Himalayan Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO), Hanle, India, using the Himalaya Faint Object Spectrograph Camera (HFOSC). The photometric monitoring of SN 2004et, in Bessell UBVRI filters, began on ...
Monthly Notices of the Royal Astronomical Society
Comet 2I/Borisov is the first true interstellar comet discovered. Here, we present results from o... more Comet 2I/Borisov is the first true interstellar comet discovered. Here, we present results from observational programs at two Indian observatories, 2 m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle (HCT) and 1.2 m telescope at the Mount Abu Infrared Observatory (MIRO). Two epochs of imaging and spectroscopy were carried out at the HCT and three epochs of imaging at MIRO. We found CN to be the dominant molecular emission on both epochs, 2019 November 30 and December 22, at distances of rH = 2.013 and 2.031 au, respectively. The comet was inferred to be relatively depleted in Carbon bearing molecules on the basis of low C2 and C3 abundances. We find the production rate ratio, Q(C2)/Q(CN) = 0.54 ± 0.18, pre-perihelion and Q(C2)/Q(CN) = 0.34 ± 0.12 post-perihelion. This classifies the comet as being moderately depleted in carbon chain molecules. Using the results from spectroscopic observations, we believe the comet to have a chemically heterogeneous surface ...
The Astronomical Journal
We present results of our study of PDS 11 binary system, which belongs to a rare class of isolate... more We present results of our study of PDS 11 binary system, which belongs to a rare class of isolated, high galactic latitude T Tauri stars. Our spectroscopic analysis reveals that PDS 11 is a M2−M2 binary system with both components showing similar Hα emission strength. Both the components appear to be accreting, and are classical T Tauri stars. The lithium doublet Li i λ6708, a signature of youth, is present in the spectrum of PDS 11A, but not in PDS 11B. From the application of lithium depletion boundary age-dating method and a comparison with the Li i λ6708 equivalent width distribution of moving groups, we estimated an age of 10−15 Myr for PDS 11A. Comparison with pre-main sequence evolutionary models indicates that PDS 11A is a 0.4 M ⊙ T Tauri star at a distance of 114−131 pc. PDS 11 system does not appear to be associated with any known star forming regions or moving groups. PDS 11 is a new addition, after TWA 30 and LDS 5606, to the interesting class of old, dusty, wide binary classical T Tauri systems in which both components are actively accreting.
Monthly Notices of the Royal Astronomical Society
We present optical UBVRI photometry and medium resolution spectroscopy of the Type Ib supernova i... more We present optical UBVRI photometry and medium resolution spectroscopy of the Type Ib supernova iPTF13bvn, spanning a phase of ∼−13 d to +71 d with respect to B-band maximum. The post-maximum decline rates indicate a fast decline with m 15 (B) = 1.82. Correcting for a Galactic extinction E(B − V) MW = 0.045 and host galaxy extinction of E(B − V) host = 0.17, the absolute V-band magnitude peaks at M V = −17.23 ± 0.20. The bolometric light curve indicates that ∼0.09 M of 56 Ni was synthesized in the explosion. The earliest spectrum (−13 d) shows the presence of He I 5876 Å feature at a velocity of ∼15 000 km s −1 , which falls rapidly by the time the SN approaches the epoch of B-band maximum. The photospheric velocity near maximum light, as indicated by the Fe II 5169 Å feature, is ∼9000 km s −1. The estimate for the 56 Ni mass, together with the estimates for the ejected mass (M ej) and kinetic energy of the explosion (E k) indicate that iPTF13bvn is a low-luminosity Type Ib supernova, with a lower than average ejected mass and kinetic energy. Our results suggest that the progenitor of iPTF13bvn is inconsistent with a single Wolf-Rayet star.
Ultraviolet observations using the Solar Blind Channel on the Hubble Space Telescope provide ligh... more Ultraviolet observations using the Solar Blind Channel on the Hubble Space Telescope provide light curves and low resolution spectra of three pulsating white dwarfs in the cataclysmic variables SDSS013132.39-090122.3, SDSSJ161033.64-010223.3 and SDSSJ220553.98+115553.7. The UV light curves show enhanced pulsation amplitudes over those from simultaneous and previous optical photometry, while the UV-optical spectra are fit with white dwarf temperatures near 15,000K. These temperatures place the accreting white dwarfs outside the instability zone for non-interacting DAV white dwarfs and show that the instability strip is complex for accreting white dwarfs.
Nature, May 12, 2016
Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as 'u... more 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 da...
Bulletin of the Astronomical Society of India
Bulletin of the Astronomical Society of India
The supernova SN 2002ap was discovered in the outer regions of the nearby spiral M74 on January 2... more The supernova SN 2002ap was discovered in the outer regions of the nearby spiral M74 on January 29.4 UT. Early photometric and spectroscopic observations indicate the supernova belongs to the class of Ic hypernova. Late time (After JD 2452500) light curve decay slopes are similar to that of the hypernovae SN 1997ef and SN 1998bw. We present here the BV RI photometric light curves and colour evolutions of SN 2002ap to investigate the late time nature of the light curve.
ABSTRACT CBET 316 available at Central Bureau for Astronomical Telegrams.
Bulletin of the Astronomical Society of India
Aims. We present multiwavelength observations of the gamma-ray burst GRB 051028 detected by HETE-... more Aims. We present multiwavelength observations of the gamma-ray burst GRB 051028 detected by HETE-2 in order to derive its afterglow emission parameters and to determine the reason for its optical faintness when compared to other events. Methods. Observations were taken in the optical (2.0m Himalayan Chandra Telescope, 1.34m Tautenburg, 4.2m William Herschel Telescope) and in X-rays (Swift/XRT) between 2.7 hours and ∼ 10 days after the onset of the event. Results. The data can be interpreted by collimated emission in a jet with a typical value of p = 2.4 which is moving in an homogeneous interstellar medium and with a cooling frequency ν c still above the X-rays at 0.5 days after the burst onset. GRB 051028 can be classified as a "gray" or "potentially dark" GRB. On the basis of the combined optical and Swift/XRT data, we conclude that the reason for the optical dimness is not extra absorption in the host galaxy, but rather the GRB taking place at high-redshift. We also notice the very striking similarity with the optical lightcurve of GRB 050730, a burst with a spectroscopic redshift of 3.967, although GRB 051028 is ∼ 3 mag fainter. We suggest that the bumps could be explained by multiple energy injection episodes and that the burst is intrinsically faint when compared to the average afterglows detected since 1997. The non-detection of the host galaxy down to R = 25.1 is also consistent with the burst arising at high redshift, compatible with the published pseudo-z of 3.7 ± 1.8.
CBET 790 available at Central Bureau for Astronomical Telegrams.
Bulletin of the Astronomical Society of India
Although well established for BL Lac objects and radio-loud quasars, the occurrence of intra-nigh... more Although well established for BL Lac objects and radio-loud quasars, the occurrence of intra-night optical variability (INOV) in radio-quiet quasars is still debated, primarily since only a handful of INOV events with good statistical significance, albeit small amplitude, have been reported so far. This has motivated us to continue intra-night optical monitoring of bona-fide radio-quiet quasars (RQQs). Here we present the results for a sample of 11 RQQs monitored by us on 19 nights. On 5 of these nights a given RQQ was monitored simultaneously from two well separated observatories. In all, two clear cases and two probable case of INOV were detected. From these data, we estimate an INOV duty cycle of ∼8% for RQQs which would increase to 19% if the 'probable variable' cases are also included. Such comparatively small INOV duty cycles for RQQs, together with the small INOV amplitudes (∼1%), are in accord with the previously deduced characteristics of this phenomenon.
ABSTRACT CBET 992 available at Central Bureau for Astronomical Telegrams.
SN 2004et was observed with the 2-m Himalayan Chandra Telescope (HCT) of the Indian Astronomical ... more SN 2004et was observed with the 2-m Himalayan Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO), Hanle, India, using the Himalaya Faint Object Spectrograph Camera (HFOSC). The photometric monitoring of SN 2004et, in Bessell UBVRI filters, began on ...