12 research outputs found
Quasar 3C 298: a test-case for meteoritic nanodiamond 3.5 ”m emission
Aims. We calculate the dust emission expected at 3.43 and 3.53 ”m if meteoritic (i.e. hydrogenated) nanodiamonds are responsible for most of the far-UV break observed in quasars.
Methods. We integrate the UV flux that hydrogenated nanodiamonds must absorb to reproduce the far-UV break. Based on laboratory spectra of H-terminated diamond surfaces, we analyse the radiative energy budget and derive theoretically the IR emission profiles expected for possible C-H surface stretch modes of the diamonds.
Results. Using as test case a spectrum of 3C 298 provided by the Spitzer Observatory, we do not find evidence of these emission bands.
Conclusions. While diamonds without surface adsorbates remain a viable candidate for explaining the far-UV break observed in quasars, hydrogenated nanodiamonds appear to be ruled out, as they would give rise to IR emission bands, which have not been observed so far
Quasar 3C298: a test-case for meteoritic nanodiamond 3.5 microns emission
We calculate the dust emission expected at 3.43 and 3.53 microns if
meteoritic (i.e. hydrogenated) nanodiamonds are responsible for most of the
far-UV break observed in quasars.
We integrate the UV flux that hydrogenated nanodiamonds must absorb to
reproduce the far-UV break. Based on laboratory spectra of H-terminated diamond
surfaces, we analyse the radiative energy budget and derive theoretically the
IR emission profiles expected for possible C-H surface stretch modes of the
diamonds.
Using as test case a spectrum of 3C298 provided by the Spitzer Observatory,
we do not find evidence of these emission bands.
While diamonds without surface adsorbates remain a viable candidate for
explaining the far-UV break observed in quasars, hydrogenated nanodiamonds
appear to be ruled out, as they would giverise to IR emission bands, which have
not been observed so far.Comment: To appear in A&A Letters, 4 pages, 2 figure
Energy distribution of individual quasars from far-UV to X-rays: I. Intrinsic UV hardness and dust opacities
Using Chandra and HST archival data, we have studied the individual Spectral
Energy Distribution (SED) of 11 quasars. All UV spectra show a spectral break
around 1100A. 5 X-ray spectra show the presence of a ``soft excess'' and 7
spectra showed an intrinsic absorption. We found that for most quasars a simple
extrapolation of the far-UV powerlaw into the X-ray domain generally lies below
the X-ray data and that the big blue bump and the soft X-ray excess do not
share a common physical origin. We explore the issue of whether the observed
SED might be dust absorbed in the far and near-UV. We fit the UV break,
assuming a powerlaw that is absorbed by cubic nanodiamond dust grains. We then
explore the possibility of a universal SED (with a unique spectral index) by
including further absorption from SMC-like extinction. Using this approach,
satisfactory fits to the spectra can be obtained. The hydrogen column densities
required by either nanodiamonds or amorphous dust models are all consistent,
except for one object, with the columns deduced by our X-ray analysis, provided
that the C depletion is ~0.6. Because dust absorption implies a flux recovery
in the extreme UV (<700A), our modeling opens the possibility that the
intrinsic quasar SED is much harder and more luminous in the extreme UV than
inferred from the near-UV data, as required by photoionization models of the
broad emission line region. We conclude that the intrinsic UV SED must undergo
a sharp turn-over before the X-ray domain.Comment: 49 pages, 11 figures, accepted for publication in Ap
Nanodiamond dust and the energy distribution of quasars
The spectral energy distribution of quasars shows a sharp steepening of the continuum shortward of ~ 1100 A. The steepening could be a result of dust absorption. We present a dust extinction model which considers crystalline carbon grains and compare it with SMC-like dust extinction consisting of a mixture of silicate grains with graphite or amorphous carbon grains. We show that the sharp break seen in individual quasar spectra of intermediate redshif \~ 1-2 can be reproduced by dust absorption provided the extinction curve consists of nanodiamonds, composed of terrestrial cubic diamonds or of diamonds similar to the presolar nanodiamonds found in primitive meteorites
Radiative Energy Budgets in a Microbial Mat Under Different Irradiance and Tidal Conditions
© 2019, Springer Science+Business Media, LLC, part of Springer Nature. Irradiance and temperature variations during tidal cycles modulate microphytobenthic primary production potentially by changing the radiative energy balance of photosynthetic mats between immersion and emersion and thus sediment daily net metabolism. To test the effect of tidal stages on the radiative energy budget, we used microsensor measurements of oxygen, temperature, and scalar irradiance to estimate the radiative energy budget in a coastal photosynthetic microbial mat during immersion (constant water column of 2 cm) and emersion under increasing irradiance. Total absorbed light energy was higher in immersion than emersion, due to a lower reflectance of the microbial mat, while most (> 97%) of the absorbed light energy was dissipated as heat irrespective of tidal conditions. During immersion, the upward heat flux was higher than the downward one, whereas the opposite occurred during emersion. At highest photon irradiance (800 Όmol photon m â2  s â1 ), the sediment temperature increased ~ 2.5 °C after changing the conditions from immersion to emersion. The radiative energy balance showed that less than 1% of the incident light energy (PAR, 400â700 nm) was conserved by photosynthesis under both tidal conditions. At low to moderate incident irradiances, the light use efficiency was similar during the tidal stages. In contrast, we found an ~ 30% reduction in the light use efficiency during emersion as compared to immersion under the highest irradiance likely due to the rapid warming of the sediment during emersion and increased non-photochemical quenching. These changes in the photosynthetic efficiency and radiative energy budget could affect both primary producers and temperature-dependent bacterial activity and consequently daily net metabolism rates having important ecological consequences
STIS optical spectroscopy of the lobes of CRL 618
Context. Many proto-planetary nebulae show collimated structures sometimes showing multiple outflows.
Aims. We present the results of new optical spectroscopic observations (both medium (with a dispersion of 0.56Â Ă
 pixel-1) and low (2.73 and 4.92 Ă
 pixel-1 for the G430L and G750L gratings) spectral resolution) of the lobes of the proto-planetary nebula CRL 618 obtained with the Space Telescope Imaging Spectrograph on board of the Hubble Space Telescope.
Methods. We analyse the density structure and the excitation conditions of the lobes of CRL 618. The spectra obtained at medium spectral resolution (~50 kmâs-1) allow us to quote the fraction of unscattered (intrinsic) Hα emission. We have also obtained de-reddened emission line ratios of several features from the low spectral resolution spectra.
Results. We find that most of the analysed line ratios are reproduced by planar shocks moving through a dense medium (with pre-shock densities  ~104 cm-3) with shock velocities from 30 to 40 kmâs-1 (except the [O III]/HÎČ line ratios which require shock velocities of 80 to 90 kmâs-1). We find that the [S II]-weighted ionization fraction ranges from 0.015 to 0.06. The total densities derived from the electron density and the ionization fraction are  ~105 to 106 cm-3.
Conclusions. We conclude that the spectra of the lobes of CRL 618 can be margially reproduced by steady planeâparallel shock models for shock velocities which are significantly lower than the velocities at which the jet moves outwards (~200Â kmâs-1). These results are consistent with the predictions of a jet with a variable ejection velocity. The mirror-symmetry, the luminosity asymmetry between both lobes and the ejection velocity variability suggest that its central source may host a binary system