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| 2. |
- Dhawan, Suhail, et al.
(författare)
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Nebular spectroscopy of SN 2014J : Detection of stable nickel in near-infrared spectra
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 619
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Tidskriftsartikel (refereegranskat)abstract
- We present near-infrared (NIR) spectroscopy of the nearby supernova 2014J obtained similar to 450 d after explosion. We detect the [Ni II] 1.939 mu m line in the spectra indicating the presence of stable Ni-58 in the ejecta. The stable nickel is not centrally concentrated but rather distributed as the iron. The spectra are dominated by forbidden [Fe II] and [Co II] lines. We used lines, in the NIR spectra, arising from the same upper energy levels to place constraints on the extinction from host galaxy dust. We find that that our data are in agreement with the high A(v) and low R-v found in earlier studies from data near maximum light. Using a Ni-56 mass prior from near maximum light gamma-ray observations, we find 0.053 +/- 0.018 M-circle dot of stable nickel to be present in the ejecta. We find that the iron group features are redshifted from the host galaxy rest frame by similar to 600 km s(-1).
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| 3. |
- Kaplan, D. L., et al.
(författare)
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A Dense Companion to the Short-period Millisecond Pulsar Binary PSR J0636+5128
- 2018
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 864:1
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Tidskriftsartikel (refereegranskat)abstract
- PSR J0636+5128 is a millisecond pulsar in one of the most compact pulsar binaries known, with a 96 minute orbital period. The pulsar mass function suggests a very low mass companion, similar to that seen in so-called black widow binaries. Unlike in most of those, however, no radio eclipses by material driven off from the companion were seen leading to the possibility that the companion was a degenerate remnant of a carbon-oxygen white dwarf. We report the discovery of the optical counterpart of its companion in images taken with the Gemini North and Keck I telescopes. The companion varies between r' = 25 and r' = 23 on the 96 minute orbital period of the binary, caused by irradiation from the pulsar's energetic wind. We modeled the multicolor light curve using parallax constraints from pulsar timing and determine a companion mass of (1.71 +/- 0.23) x 10(-2) M-circle dot,M- a radius of (7.6 +/- 1.4) x 10(-2) R-circle dot, and a mean density of 54 +/- 26 g cm(-3), all for an assumed neutron star mass of 1.4 M-circle dot. This makes the companion to PSR J0636+5128 one of the densest of the black widow systems. Modeling suggests that the composition is not predominantly hydrogen, perhaps due to an origin in an ultracompact X-ray binary.
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| 4. |
- Kotak, Rubina, et al.
(författare)
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A new look at the pulsating DB white dwarf GD 358: Line-of-sight velocity measurements and constraints on model atmospheres
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 397, s. 1043-1055
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Tidskriftsartikel (refereegranskat)abstract
- We report on our findings of the bright, pulsating, helium atmospherewhite dwarf GD 358, based on time-resolved optical spectrophotometry. Weidentify 5 real pulsation modes and at least 6 combination modes atfrequencies consistent with those found in previous observations. Themeasured Doppler shifts from our spectra show variations with amplitudesof up to 5.5 km s-1 at the frequencies inferred from the fluxvariations. We conclude that these are variations in the line-of-sightvelocities associated with the pulsational motion. We use the observedflux and velocity amplitudes and phases to test theoretical predictionswithin the convective driving framework, and compare these with similarobservations of the hydrogen atmosphere white dwarf pulsators (DAVs).The wavelength dependence of the fractional pulsation amplitudes(chromatic amplitudes) allows us to conclude that all five real modesshare the same spherical degree, most likely, l=1. This is consistentwith previous identifications based solely on photometry. We find that ahigh signal-to-noise mean spectrum on its own is not enough to determinethe atmospheric parameters and that there are small but significantdiscrepancies between the observations and model atmospheres. The sourceof these remains to be identified. While we infer Teff =24 kKand log g ~ 8.0 from the mean spectrum, the chromatic amplitudes, whichare a measure of the derivative of the flux with respect to thetemperature, unambiguously favour a higher effective temperature, 27 kK,which is more in line with independent determinations from ultra-violetspectra.The data presented herein were obtained at the W.M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W.M. Keck Foundation.
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| 5. |
- Kotak, Rubina, et al.
(författare)
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A puzzling periodicity in the pulsating DA white dwarf G 117-B15A
- 2004
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 413, s. 301-308
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Tidskriftsartikel (refereegranskat)abstract
- We present time-resolved optical spectrophotometry of the pulsatinghydrogen atmosphere (DA) white dwarf G 117-B15A. We find threeperiodicities in the pulsation spectrum (215 s, 272 s, and 304 s) all ofwhich have been found in earlier studies. By comparing the fractionalwavelength dependence of the pulsation amplitudes (chromatic amplitudes)with models, we confirm a previous report that the strongest mode, at215 s, has ℓ=1. The chromatic amplitude for the 272 s mode is verypuzzling, showing an increase in fractional amplitude with wavelengththat cannot be reproduced by the models for any ℓ at opticalwavelengths. Based on archival HST data, we show that while thebehaviour of the 215 s mode at ultra-violet wavelengths is as expectedfrom models, the weird behaviour of the 272 s periodicity is notrestricted to optical wavelengths in that it fails to show the expectedincrease in fractional amplitude towards shorter wavelengths. We discusspossible causes for the discrepancies found for the 272 s variation, butfind that all are lacking, and conclude that the nature of thisperiodicity remains unclear.Based in part on data obtained at the W.M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W.M. Keck Foundation.
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| 6. |
- Kotak, Rubina, et al.
(författare)
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Pulsating or not? A search for hidden pulsations below the red edge of the ZZ Ceti instability strip
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 391:3, s. 1005-1012
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Tidskriftsartikel (refereegranskat)abstract
- The location of the red edge of the ZZ Ceti instability strip is definedobservationally as being the lowest temperature for which a white dwarfwith a H-rich atmosphere (DA) is known to exhibit periodic brightnessvariations. Whether this cut-off in flux variations is actually due to acessation of pulsation or merely due to the attenuation of anyvariations by the convection zone, rendering them invisible, is notclear. The latter is a theoretical possibility because with decreasingeffective temperature, the emergent flux variations become an eversmaller fraction of the amplitude of the flux variations in theinterior. In contrast to the flux variations, the visibility of thevelocity variations associated with the pulsations is not thought to besimilarly affected. Thus, models imply that were it still pulsating, awhite dwarf just below the observed red edge should show velocityvariations. In order to test this possibility, we used time-resolvedspectra of three DA white dwarfs that do not show photometricvariability, but which have derived temperatures only slightly lowerthan the coolest ZZ Ceti variables. We find that none of our threetargets show significant periodic velocity variations, and set 95%confidence limits on amplitudes of 3.0, 5.2, and 8.8 km s-1.Thus, for two out of our three objects, we can rule out velocityvariations as large as 5.4 km s-1 observed for the strongestmode in the cool white dwarf pulsator ZZ Psc. Inorder to verify our procedures, we also examined similar data of a knownZZ Ceti, HL Tau 76. Applying external informationfrom the light curve, we detect significant velocity variations for thisobject with amplitudes of up to 4 km s-1. Our results suggestthat substantial numbers of pulsators having large velocity amplitudesdo not exist below the observed photometric red edge and that the latterprobably reflects a real termination of pulsations. The data presentedherein were obtained at the W. M. Keck Observatory, which is operated asa scientific partnership among the California Institute of Technology,the University of California and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W. M. Keck Foundation.
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| 7. |
- Kotak, Rubina, et al.
(författare)
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Time-resolved optical spectroscopy of the pulsating DA white dwarf HS 0507+0434B : New constraints on mode identification and pulsation properties
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 388:1, s. 219-234
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, , of the modes, most of which have . The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of km s -1, which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective.
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