Fully Sampled Maps of Ices and Silicates in Front of Cepheus A East with Spitzer

We report the first fully sampled maps of the distribution of interstellar CO2 ices, H2O ices and total hydrogen nuclei, as inferred from the 9.7 micron silicate feature, toward the star-forming region Cepheus A East with the IRS instrument onboard the Spitzer Space Telescope. We find that the column density distributions for these solid state features all peak at, and are distributed around, the location of HW2, the protostar believed to power one of the outflows observed in this star-forming region. A correlation between the column density distributions of CO2 and water ice with that of total hydrogen indicates that the solid state features we mapped mostly arise from the same molecular clumps along the probed sight lines. We therefore derive average CO2 ice and water ice abundances with respect to the total hydrogen column density of X(CO2)_ice~1.9x10^-5 and X(H2O)_ice~7.5x10^-5. Within errors, the abundances for both ices are relatively constant over the mapped region exhibiting both ice absorptions. The fraction of CO2 ice with respect to H2O ice is also relatively constant at a value of 22% over that mapped region. A clear triple-peaked structure is seen in the CO2 ice profiles. Fits to those profiles using current laboratory ice analogs suggest the presence of both a low-temperature polar ice mixture and a high-temperature methanol-rich ice mixture along the probed sightlines. Our results further indicate that thermal processing of these ices occurred throughout the sampled region.Comment: 26 pages, 8 figures, accepted for publication in Ap

Semiconductor quantum dot - a quantum light source of multicolor photons with tunable statistics

We investigate the intensity correlation properties of single photons emitted from an optically excited single semiconductor quantum dot. The second order temporal coherence function of the photons emitted at various wavelengths is measured as a function of the excitation power. We show experimentally and theoretically, for the first time, that a quantum dot is not only a source of correlated non-classical monochromatic photons but is also a source of correlated non-classical \emph{multicolor} photons with tunable correlation properties. We found that the emitted photon statistics can be varied by the excitation rate from a sub-Poissonian one, where the photons are temporally antibunched, to super-Poissonian, where they are temporally bunched.Comment: 4 pages, 2 figure

Optical spectroscopy of single quantum dots at tunable positive, neutral and negative charge states

We report on the observation of photoluminescence from positive, neutral and negative charge states of single semiconductor quantum dots. For this purpose we designed a structure enabling optical injection of a controlled unequal number of negative electrons and positive holes into an isolated InGaAs quantum dot embedded in a GaAs matrix. Thereby, we optically produced the charge states -3, -2, -1, 0, +1 and +2. The injected carriers form confined collective 'artificial atoms and molecules' states in the quantum dot. We resolve spectrally and temporally the photoluminescence from an optically excited quantum dot and use it to identify collective states, which contain charge of one type, coupled to few charges of the other type. These states can be viewed as the artificial analog of charged atoms such as H$^{-}$, H$^{-2}$, H$^{-3}$, and charged molecules such as H$_{2}^{+}$ and H$_{3}^{+2}$. Unlike higher dimensionality systems, where negative or positive charging always results in reduction of the emission energy due to electron-hole pair recombination, in our dots, negative charging reduces the emission energy, relative to the charge-neutral case, while positive charging increases it. Pseudopotential model calculations reveal that the enhanced spatial localization of the hole-wavefunction, relative to that of the electron in these dots, is the reason for this effect.Comment: 5 figure

Fast spectroscopic variations on rapidly-rotating, cool dwarfs. 3: Masses of circumstellar absorbing clouds on AB Doradus

New time-resolved H alpha, Ca II H and K and Mg II h and k spectra of the rapidly-rotating K0 dwarf star AB Doradus (= HD 36705). The transient absorption features seen in the H alpha line are also present in the Ca II and Mg II resonance lines. New techniques are developed for measuring the average strength of the line absorption along lines of sight intersecting the cloud. These techniques also give a measure of the projected cloud area. The strength of the resonance line absorption provides useful new constraints on the column densities, projected surface areas, temperatures and internal turbulent velocity dispersions of the circumstellar clouds producing the absorption features. At any given time the star appears to be surrounded by at least 6 to 10 clouds with masses in the range 2 to 6 x 10(exp 17) g. The clouds appear to have turbulent internal velocity dispersions of order 3 to 20 km/s, comparable with the random velocities of discrete filamentary structures in solar quiescent prominences. Night-to-night changes in the amount of Ca II resonance line absorption can be explained by changes in the amplitude of turbulent motions in the clouds. The corresponding changes in the total energy of the internal motions are of order 10(exp 29) erg per cloud. Changes of this magnitude could easily be activated by the frequent energetic (approximately 10(exp 34) erg) x ray flares seen on this star

Composition of Ices in Low-Mass Extrasolar Planets

We study the formation conditions of icy planetesimals in protoplanetary disks in order to determine the composition of ices in small and cold extrasolar planets. Assuming that ices are formed from hydrates, clathrates, and pure condensates, we calculate their mass fractions with respect to the total quantity of ices included in planetesimals, for a grid of disk models. We find that the composition of ices weakly depends on the adopted disk thermodynamic conditions, and is rather influenced by the initial composition of the gas phase. The use of a plausible range of molecular abundance ratios and the variation of the relative elemental carbon over oxygen ratio in the gas phase of protoplanetary disks, allow us to apply our model to a wide range of planetary systems. Our results can thus be used to constrain the icy/volatile phase composition of cold planets evidenced by microlensing surveys, hypothetical ocean-planets and carbon planets, which could be detected by Corot or Kepler.Comment: Accepted for publication in The Astrophysical Journa

Ice chemistry in embedded young stellar objects in the Large Magellanic Cloud

We present spectroscopic observations of a sample of 15 embedded young stellar objects (YSOs) in the Large Magellanic Cloud (LMC). These observations were obtained with the Spitzer Infrared Spectrograph (IRS) as part of the SAGE-Spec Legacy program. We analyze the two prominent ice bands in the IRS spectral range: the bending mode of CO_2 ice at 15.2 micron and the ice band between 5 and 7 micron that includes contributions from the bending mode of water ice at 6 micron amongst other ice species. The 5-7 micron band is difficult to identify in our LMC sample due to the conspicuous presence of PAH emission superimposed onto the ice spectra. We identify water ice in the spectra of two sources; the spectrum of one of those sources also exhibits the 6.8 micron ice feature attributed to ammonium and methanol. We model the CO_2 band in detail, using the combination of laboratory ice profiles available in the literature. We find that a significant fraction (> 50%) of CO_2 ice is locked in a water-rich component, consistent with what is observed for Galactic sources. The majority of the sources in the LMC also require a pure-CO_2 contribution to the ice profile, evidence of thermal processing. There is a suggestion that CO_2 production might be enhanced in the LMC, but the size of the available sample precludes firmer conclusions. We place our results in the context of the star formation environment in the LMC.Comment: Minor corrections to Table 2. Accepted for publication in ApJ, 66 pages, 9 figures (some in color), 4 table

Analysis of Mars Analogue Soil Samples Using Solid-Phase Microextraction, Organic Solvent Extraction and Gas Chromatography/Mass Spectrometry

Polycyclic aromatic hydrocarbons (PAHs) are robust and abundant molecules in extraterrestrial environments. They are found ubiquitously in the interstellar medium and have been identified in extracts of meteorites collected on Earth. PAHs are important target molecules for planetary exploration missions that investigate the organic inventory of planets, moons and small bodies. This study is part of an interdisciplinary preparation phase to search for organic molecules and life on Mars. We have investigated PAH compounds in desert soils to determine their composition, distribution and stability. Soil samples (Mars analogue soils) were collected at desert areas of Utah in the vicinity of the Mars Desert Research Station (MDRS), in the Arequipa region in Peru and from the Jutland region of Denmark. The aim of this study was to optimize the solid-phase microextraction (SPME) method for fast screening and determination of PAHs in soil samples. This method minimizes sample handling and preserves the chemical integrity of the sample. Complementary liquid extraction was used to obtain information on five- and six-ring PAH compounds. The measured concentrations of PAHs are, in general, very low, ranging from 1 to 60 ng g(sup -1). The texture of soils is mostly sandy loam with few samples being 100% silt. Collected soils are moderately basic with pH values of 8-9 except for the Salten Skov soil, which is slightly acidic. Although the diverse and variable microbial populations of the samples at the sample sites might have affected the levels and variety of PAHs detected, SPME appears to be a rapid, viable field sampling technique with implications for use on planetary missions

Radiative Lifetimes of Single Excitons in Semiconductor Quantum Dots- Manifestation of the Spatial Coherence Effect

Using time correlated single photon counting combined with temperature dependent diffraction limited confocal photoluminescence spectroscopy we accurately determine, for the first time, the intrinsic radiative lifetime of single excitons confined within semiconductor quantum dots. Their lifetime is one (two) orders of magnitude longer than the intrinsic radiative lifetime of single excitons confined in semiconductor quantum wires (wells) of comparable confining dimensions. We quantitatively explain this long radiative time in terms of the reduced spatial coherence between the confined exciton dipole moment and the radiation electromagnetic field.Comment: 4 pages, 3 figure