Understanding disk evolution and dissipation is essential for studies of planet formation. Transition disks, i.e., disks with large dust cavities and gaps, are promising candidates of active evolution. About two dozen candidates, selected by their Spectral Energy Distribution (SED), have been confirmed to have dust cavities through millimeter interferometric imaging, but this sample is biased toward the brightest disks. The Spitzer surveys of nearby low-mass star-forming regions have resulted in more than 4000 young stellar objects (YSOs). Using color criteria, we selected a sample of ~150 candidates and an additional 40 candidates and known transition disks from the literature. The Spitzer data were complemented by new observations at longer wavelengths, including new JCMT and APEX submillimeter photometry, and WISE and Herschel-PACS mid- and far-infrared photometry. Furthermore, optical spectroscopy was obtained and stellar types were derived for 85% of the sample, including information from the literature. The SEDs were fit to a grid of RADMC-3D disk models with a limited number of parameters: disk mass, inner disk mass, scale height and flaring, and disk cavity radius, where the latter is the main parameter of interest. About 72% of our targets possibly have dust cavities based on the SED. The derived cavity sizes are consistent with imaging/modeling results in the literature, where available. Trends are found with the L_disk_ over L_*_ ratio and stellar mass and a possible connection with exoplanet orbital radii. A comparison with a previous study where color observables are used (Cieza et al., 2010, Cat. J/ApJ/712/925) reveals large overlap between their category of planet-forming disks and our transition disks with cavities. A large number of the new transition disk candidates are suitable for follow-up observations with ALMA.
Ethanimine, a possible precursor of amino acids, is considered an important prebiotic molecule and thus may play important roles in the formation of biological building blocks in the interstellar medium. In addition, its identification in Titan's atmosphere would be important for understanding the abiotic synthesis of organic species. An accurate computational characterization of the molecular structure, energetics, and spectroscopic properties of the E and Z isomers of ethanimine, CH_3_CHNH, has been carried out by means of a composite scheme based on coupled-cluster techniques, which also account for extrapolation to the complete basis-set limit and core-valence correlation correction, combined with density functional theory for the treatment of vibrational anharmonic effects. By combining the computational results with new millimeter-wave measurements up to 300GHz, the rotational spectrum of both isomers can be accurately predicted up to 500GHz. Furthermore, our computations allowed us to revise the infrared spectrum of both E- and Z-CH_3_CHNH, thus predicting all fundamental bands with high accuracy.
High-resolution pure rotational spectra of four alkylnaphthalenes were measured in the range of 6-15GHz using a molecular-beam Fourier-transform microwave spectrometer. Both a- and b-type transitions were observed for 1-methylnaphthalene (1-MN), 1,2-dimethylnaphthalene (1,2-DMN), and 1,3-dimethylnaphthalene (1,3-DMN); only a-type transitions were observed for 2-methylnaphthalene (2-MN). Geometry optimization and vibrational analysis calculations at the B3LYP/6-311++G(d,p) level of theory aided in the assignments of the spectra and the characterization of the structures. Differences between the experimental and predicted rotational constants are small, and they can be attributed in part to low-lying out-of-plane vibrations, which distort the alkylnaphthalenes out of their equilibrium geometries. Splittings of rotational lines due to methyl internal rotation were observed in the spectra of 2-MN, 1,2-DMN, and 1,3-DMN, and allowed for the determination of the barriers to methyl internal rotation, which are compared to values from density functional theory calculations. All four species are moderately polar, so they are candidate species for detection by radio astronomy, by targeting the transition frequencies reported here.
Energy levels and the corresponding transition probabilities as well oscillator strengths and line strengths for allowed (E1) and forbidden (E2, M1) transitions among the lowest 379 levels of fluorine-like Fe XVIII are presented. Calculations were performed using the multiconfigurational Dirac-Fock GRASP code.
Radiative lifetimes from laser-induced fluorescence measurements, accurate to ~+/-5%, are reported for 41 odd-parity levels of HfII (Z=72). The lifetimes are combined with branching fractions measured using Fourier transform spectrometry to determine transition probabilities for 150 lines of HfII. Approximately half of these new transition probabilities overlap with recent independent measurements using a similar approach. The two sets of measurements are found to be in good agreement for lines in common. Our new laboratory data are applied to refine the hafnium photospheric solar abundance and to determine hafnium abundances in 10 metal-poor giant stars with enhanced r-process abundances. For the Sun we derive log{epsilon}(Hf)=0.88+/-0.08 from four lines; the uncertainty is dominated by the weakness of the lines and their blending by other spectral features. Within the uncertainties of our analysis, the r-process-rich stars possess constant Hf/La and Hf/Eu abundance ratios, log{epsilon}(Hf/La)=-0.13+/-0.02 ({sigma}=0.06) and log{epsilon}(Hf/Eu)=+0.04+/-0.02 ({sigma}=0.06). The observed average stellar abundance ratio of Hf/Eu and La/Eu is larger than previous estimates of the solar system r-process-only value, suggesting a somewhat larger contribution from the r-process to the production of Hf and La. The newly determined Hf values could be employed as part of the chronometer pair, Th/Hf, to determine radioactive stellar ages.
New emission branching fraction (BF) measurements for 183 lines of the second spectrum of chromium (Cr II) and new radiative lifetime measurements from laser-induced fluorescence for 8 levels of Cr^+^ are reported. The goals of this study are to improve transition probability measurements in Cr II and reconcile solar and stellar Cr abundance values based on Cr I and Cr II lines. Eighteen spectra from three Fourier Transform Spectrometers supplemented with ultraviolet spectra from a high-resolution echelle spectrometer are used in the BF measurements. Radiative lifetimes from this study and earlier publications are used to convert the BFs into absolute transition probabilities. These new laboratory data are applied to determine the Cr abundance log{epsilon} in the Sun and metal-poor star HD 84937. The mean result in the Sun is <log{epsilon}(CrII)>=5.624+/-0.009 compared to <log{epsilon}(CrI)>=5.644+/-0.006 on a scale with the hydrogen abundance log{epsilon}(H)=12 and with the uncertainty representing only line-to-line scatter. A Saha (ionization balance) test on the photosphere of HD 84937 is also performed, yielding <log{epsilon}(CrII)>=3.417+/-0.006 and <log{epsilon}(CrI, lower level excitation potential E.P.>0eV)>=3.374+/-0.011 for this dwarf star. We find a correlation of Cr with the iron-peak element Ti, suggesting an associated nucleosynthetic production. Four iron-peak elements (Cr along with Ti, V, and Sc) appear to have a similar (or correlated) production history-other iron-peak elements appear not to be associated with Cr.
Radiative lifetimes, accurate to +/-5%, have been measured for 212 odd-parity levels of SmII using laser-induced fluorescence. The lifetimes are combined with branching fractions measured using Fourier transform spectrometry to determine transition probabilities for more than 900 lines of SmII. This work is the largest scale laboratory study to date of SmII transition probabilities using modern methods.
Due to the need of transition probabilities for heavy ions, including those of tellurium, in different fields of physics and in astrophysics, we have investigated theoretically the atomic structure of two selected tellurium ions (Te+ and Te++) for which no theoretical data were available so far. The first transition probabilities have been calculated for the electric dipole (E1) transitions with wavelengths shorter than 1 micrometer in Te II-III. Both the multiconfiguration Dirac-Hartree-Fock (MCDHF) method and the relativistic Hartree-Fock (HFR) approach, in which core-polarization (CPOL) effects were included, have been used for the calculations. Results. The results obtained with these two completely independent methods are in reasonable agreement. As a consequence, the transition probabilities obtained in this work are expected to be reliable. They fill in a gap concerning the radiative parameters in these two ions.
In this paper, we derive the theoretical properties of rovibrational levels belonging to excited B, C, B', and D electronic states of HD. We compute the eigenvalues and eigenfunctions of the nuclear coupled Schroedinger equations using ab initio electronic molecular properties available in the literature. Transition wavenumbers and spontaneous emission probabilities are calculated for all transitions belonging to B-X, C-X, B'-X, and D-X electronic band systems of HD when the upper rotational quantum number is below or equal to 10. We compare our results with available experimental values: the accuracy in the wavenumbers is on the order of 3 reciprocal centimetres, whereas the intensity properties are satisfactorily reproduced. The origin of the remaining discrepancies is analyzed.
Recent radiative lifetime measurements accurate to +/-5% using laser-induced fluorescence (LIF) on 43 even-parity and 15 odd-parity levels of CeII have been combined with new branching fractions measured using a Fourier transform spectrometer (FTS) to determine transition probabilities for 921 lines of CeII. This improved laboratory data set has been used to determine a new solar photospheric Ce abundance, log{epsilon}=1.61+/-0.01 ({sigma}=0.06 from 45 lines), a value in excellent agreement with the recommended meteoritic abundance, log{epsilon}=1.61+/-0.02. Revised Ce abundances have also been derived for the r-process-rich metal-poor giant stars BD+17 3248, CS 22892-052, CS 31082-001, HD 115444, and HD 221170. Between 26 and 40 lines were used for determining the Ce abundance in these five stars, yielding a small statistical uncertainty of {+/-}0.01dex similar to the solar result. The relative abundances in the metal-poor stars of Ce and Eu, a nearly pure r-process element in the Sun, matches r-process-only model predictions for solar system material. This consistent match with small scatter over a wide range of stellar metallicities lends support to these predictions of elemental fractions. A companion paper includes an interpretation of these new precision abundance results for Ce as well as new abundance results and interpretation for Pr, Dy, and Tm.