R. Van Lieshout

An interferometric study of the Fomalhaut inner debris disk - III. Detailed models of the exozodiacal disk and its origin

Context. Debris disks are thought to be extrasolar analogs to the solar system planetesimal belts. The star Fomalhaut harbors a cold debris belt at 140 AU comparable to the Edgeworth-Kuiper belt, as well as evidence of a warm dust component, unresolved by singledish telescopes, which is suspected of being a bright analog to the solar system’s zodiacal dust. Aims. Interferometric observations obtained with the VLTI/VINCI instrument and the Keck Interferometer Nuller have identified nearand mid-infrared excesses attributed respectively to hot and warm exozodiacal dust residing in the inner few AU of the Fomalhaut environment. We aim to characterize the properties of this double inner dust belt and to unveil its origin. Methods. We performed parametric modeling of the exozodiacal disk (“exozodi”) using the GRaTeR radiative transfer code to reproduce the interferometric data, complemented by mid- to far-infrared photometric measurements from Spitzer and Herschel �� . A detailed treatment of sublimation temperatures was introduced to explore the hot population at the size-dependent sublimation rim. We then used an analytical approach to successively testing several source mechanisms for the dust and suspected parent bodies. Results. A good fit to the multiwavelength data is found by two distinct dust populations: (1) a population of very small (0.01 to 0.5 μm), hence unbound, hot dust grains confined in a narrow region (∼0.1–0.3 AU) at the sublimation rim of carbonaceous material; (2) a population of bound grains at ∼2 AU that is protected from sublimation and has a higher mass despite its fainter flux level. We propose that the hot dust is produced by the release of small carbon grains following the disruption of dust aggregates that originate in the warm component. A mechanism, such as gas braking, is required to further confine the small grains for a long enough time. In situ dust production could hardly be ensured for the age of the star, so we conclude that the observed amount of dust is triggered by intense dynamical activity. Conclusions. Fomalhaut may be representative of exozodis that are currently being surveyed at near and mid-infrared wavelengths worldwide. We propose a framework for reconciling the “hot exozodi phenomenon” with theoretical constraints: the hot component of Fomalhaut is likely the “tip of the iceberg” since it could originate in the more massive, but fainter, warm dust component residing near the ice line. This inner disk exhibits interesting morphology and can be considered a prime target for future exoplanet research.

Gamma radiation following the decay of 124Sb and 124I)

Synopsis The gamma radiation following the decay of 60 days 124Sb and 4.1 days 1241 has been investigated with a scintillation spectrometer. A new gamma ray of 1900 keV has been found in the decay of 124Sb. Gamma rays of 605, 644, 725, 890, 970, 1055, 1320, 1370, 1450, 1510, 1695, 1900, 2090, 2260 and 2745 keV have been found in the decay of 124I. Relative intensities of the gamma rays are reported (table I and II), and a tentative level scheme for 124Te is proposed.

Dusty tails of evaporating exoplanets I. Constraints on the dust composition

Context. Recently, two exoplanet candidates have been discovered, KIC 12557548b and KOI-2700b, whose transit profiles show evidence of a comet-like tail of dust trailing the planet, thought to be fed by the evaporation of the planet’s surface. Aims. We aim to put constraints on the composition of the dust ejected by these objects from the shape of their transit light curves. Methods. We derive a semi-analytical expression for the attenuation of the dust cross-section in the tail, incorporating the sublimation of dust grains as well as their drift away from the planet. This expression shows that the length of the tail is highly sensitive to the sublimation properties of the dust material. We compute tail lengths for several possible dust compositions, and compare these to observational estimates of the tail lengths of KIC 12557548b and KOI-2700b, inferred from their light curves. Results. The observed tail lengths are consistent with dust grains composed of corundum (Al2O3) or iron-rich silicate minerals (e.g., fayalite, Fe2SiO4). Pure iron and carbonaceous compositions are not favoured. In addition, we estimate dust mass loss rates of 1:7 0:5 M Gyr 1 for KIC 12557548b, and >0:007 M Gyr 1 (1 lower limit) for KOI-2700b.

Aspects of the decay of 27Mg and of 89Zr

The relative intensities of the 1.015 and 0.834 MeV gamma rays from 27Mg were determined to be (42 ± 2) : 100. The intensity of the positons from 79 hour 89Zr relative to the 0.915 MeV gamma ray, (44.5 ± 1.5 : 100), shows that ϵβ+ = 3.54 ± ± 0.14, in accord with the theoretical value. A new gamma ray of 1.75 ± 0.02 MeV was observed.

Near-infrared emission from sublimating dust in collisionally active debris disks

Context. Hot exozodiacal dust is thought to be responsible for excess near-infrared (NIR) emission emanating from the innermost parts of some debris disks. The origin of this dust, however, is still a matter of debate. Aims. We test whether hot exozodiacal dust can be supplied from an exterior parent belt by Poynting-Robertson (P-R) drag, paying special attention to the pile-up of dust that occurs owing to the interplay of P-R drag and dust sublimation. Specifically, we investigate whether pile-ups still occur when collisions are taken into account, and if they can explain the observed NIR excess. Methods. We computed the steady-state distribution of dust in the inner disk by solving the continuity equation. First, we derived an analytical solution under a number of simplifying assumptions. Second, we developed a numerical debris disk model that for the first time treats the complex interaction of collisions, P-R drag, and sublimation in a self-consistent way. From the resulting dust distributions, we generated thermal emission spectra and compare these to observed excess NIR fluxes. Results. We confirm that P-R drag always supplies a small amount of dust to the sublimation zone, but find that a fully consistent treatment yields a maximum amount of dust that is about 7 times lower than that given by analytical estimates. The NIR excess due to this material is much less (≲10-3 for A-type stars with parent belts at ≳1 AU) than the values derived from interferometric observations (~10-2). Pile-up of dust still occurs when collisions are considered, but its effect on the NIR flux is insignificant. Finally, the cross-section in the innermost regions is clearly dominated by barely bound grains.

The level scheme of 112Cd

Abstract The level scheme of 112Cd has been investigated by studying the gamma ray spectrum from the decay of 112Ag which excites levels in this nucleide. A source of 112Pd112Ag in equilibrium was used. Gamma rays of 615, 690, 785, 855, 980, 1090, 1210, 1310, 1400, 1490, 1620, 1810, 1930, 2110, 2240, 2300, 2520, 2540, 2680, 2710, 2830, 2950, 3100 and 3280 keV were found to follow the decay of 112Ag. The summing spectrum was examined using a well type NaI(Tl) crystal with the source in different geometries; summing peaks of 3560, 3300, 3130, 2830, 2710, 2450, 2230, and 2050 keV were observed. Gamma-gamma coincidences were performed using a conventional fast-slow coincidence circuit. The results of these coincidence measurements are listed in table I. A level scheme for 112Cd, based on the intensity values of the various gamma rays following the decay of 112Ag and on the summing and gamma-gamma coincidence measurements is presented in fig. 6.

Nuclear levels in 60Ni from the decay of 60Cu

Synopsis Radiations from 60 Cu have been investigated by means of a single channel NaI-scintillation spectrometer and a magnetic lens type β -ray spectrometer. Moreover, γ — γ coincidences using two single channel scintillation spectrometers have been measured. Three positon groups and eight γ -rays were found (see {xgtable I and III}). A decay scheme is proposed in which probable spin and parity assignments are given to four new levels in 60 Ni at 2.18 ± 0.02; 3.11 ± 0.03; 3.52 ± 0.05 and 3.98 ± 0.04 MeV. From the decay characteristics a spin 2 + could be assigned unambiguously to the 60 Cu ground state. The total mass-difference of 6.23 ± 0.02 MeV between 60 Cu and 60 Ni fits well into a linear plot of mass-differences of Cu-Ni and Cu-Zn isobars as a function of the mass number A . A discussion of some extrapolations from those plots to yet uncertain decay characteristics of nuclides in this region is added. A description of the components of the scintillation spectrometer and the two-fold coincidence circuit is given.

Measurement of the radiations from 131I and 131Xe∗ with a lens type beta-ray spectrometer

Abstract A description is given of the construction of a lens type spectrometer with an annular exit slit. An experimental method for the adjustment of this slit is described. The instrument has a transmission of one per cent. and a line width of 2, 3 or 4 per cent for sources of 3, 6 or 9 mm diameter. The influence of radiator thickness on the shape of photoelectron lines has been studied. The beta and gamma ray spectra of 8.0 d 131 I have been investigated as well as the conversion spectrum of 131 Xe∗. Photoelectrons ejected from Pb and U by gamma rays of 80, 284, 364 and 636.5 k e V have been found. Internal conversion electrons from all these gamma rays are present. In sources which have aged for some days also conversion electrons from the 163 k e V gamma ray of 12 d 131 Xe∗ appear. These conversion electrons have also been observed in the conversion spectrum of Xe gas extracted from a 131 I sample. In the beta spectrum of 131 I two components of maximum energies 606 and 334 k e V are easily found, both yielding straight Kurie plots. In addition a very weak third component has been detected which is assumed to lead to the metastable Xe level. Also a very weak internal conversion line is found at 690 k e V corresponding to a hitherto unknown gamma ray of 724 k e V. The level scheme of Metzger and Deutsch is confirmed. A consistent spin and parity assignment to the levels is possible, the 606 k e V beta transition being allowed despite its high ft -value of 4.5 × 10 6 .

A new technetium activity: 4.5 hour 94Tcm

Abstract A 4.5 ± 0.2 hour technetium activity has been produced by bombarding molybdenum, enriched in mass 94, with 16 MeV deuterons. It decays by a 1000 ± 50 keV position branch to a level at 2440 keV in 94Mo, which then emits a cascade of 860, 710 and 870 keV gamma rays. Comparison with the decay of 52 minute 94Tc, which takes place by position emission (2.41 MeV), accompanied by gamma rays of 870, 1530, 1870, 2410, 2730 and 3200 keV, makes the assignment of the 4.5 hour activity to mass 94 unambiguous.

On the level structure of 96Mo

Abstract The level structure of 96Mo was studied through the decay of 4.3± 0.1day 96Tc and of 23.5± 0.5 hour 96Nb. Only the 770 keV transition from the first excited state and the 840 keV stop-over gamma ray from the second excited state seem to be common to these two decays. Three additional higher excited states are populated from 96Tc and four different ones from 96Nb. The various experimental results are cumulated in a tentative decay scheme, the main features being in good aggreement with those proposed in earlier studies.