A Mid-Infrared Imaging Survey of Embedded Young Stellar Objects in the Rho Ophiuchi Cloud Core

Results of a comprehensive, new, ground-based mid-infrared imaging survey of the young stellar population of the Rho Ophiuchi cloud are presented. Data were acquired at the Palomar 5-m and at the Keck 10-m telescopes with the MIRLIN and LWS instruments, at 0.25 arcsec and 0.25 arcsec resolutions, respectively. Of 172 survey objects, 85 were detected. Among the 22 multiple systems observed, 15 were resolved and their individual component fluxes determined. A plot of the frequency distribution of the detected objects with SED spectral slope shows that YSOs spend ~400,000 yr in the Flat Spectrum phase, clearing out their remnant infall envelopes. Mid-infrared variability is found among a significant fraction of the surveyed objects, and is found to occur for all SED classes with optically thick disks. Large-amplitude near-infrared variability, also found for all SED classes with optically thick disks, seems to occur with somewhat higher frequency at the earlier evolutionary stages. Although a general trend of mid-infrared excess and NIR veiling exists proceeding through SED classes, with Class I objects generally exhibiting K-veilings > 1, Flat Spectrum objects with K-veilings > 0.58, and Class III objects with K-veilings =0, Class II objects exhibit the widest range of K-band veiling values, 0-4.5. However, the highly variable value of veiling that a single source can exhibit in any of the SED classes in which active disk accretion can take place is striking, and is direct observational evidence for highly time-variable accretion activity in disks. Finally, by comparing mid-infrared vs. near-infrared excesses in a subsample with well-determined effective temperatures and extinction values, disk clearing mechanisms are explored. The results are consistent with disk clearing proceeding from the inside-out.Comment: 18 pages + 5 tables + 7 figure

Radial Distribution of Dust Grains Around HR 4796A

We present high-dynamic-range images of circumstellar dust around HR 4796A that were obtained with MIRLIN at the Keck II telescope at lambda = 7.9, 10.3, 12.5 and 24.5 um. We also present a new continuum measurement at 350 um obtained at the Caltech Submillimeter Observatory. Emission is resolved in Keck images at 12.5 and 24.5 um with PSF FWHM's of 0.37" and 0.55", respectively, and confirms the presence of an outer ring centered at 70 AU. Unresolved excess infrared emission is also detected at the stellar position and must originate well within 13 AU of the star. A model of dust emission fit to flux densities at 12.5, 20.8, and 24.5 um indicates dust grains are located 4(+3/-2) AU from the star with effective size, 28+/-6 um, and an associated temperature of 260+/-40 K. We simulate all extant data with a simple model of exozodiacal dust and an outer exo-Kuiper ring. A two-component outer ring is necessary to fit both Keck thermal infrared and HST scattered-light images. Bayesian parameter estimates yield a total cross-sectional area of 0.055 AU^2 for grains roughly 4 AU from the star and an outer-dust disk composed of a narrow large-grain ring embedded within a wider ring of smaller grains. The narrow ring is 14+/-1 AU wide with inner radius 66+/-1 AU and total cross-sectional area 245 AU^2. The outer ring is 80+/-15 AU wide with inner radius 45+/-5 AU and total cross-sectional area 90 AU^2. Dust grains in the narrow ring are about 10 times larger and have lower albedos than those in the wider ring. These properties are consistent with a picture in which radiation pressure dominates the dispersal of an exo-Kuiper belt.Comment: Accepted by Astrophysical Journal (Part1) on September 9, 2004. 13 pages, 10 figures, 2 table

Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum (F.S.) objects in the Perseus, Taurus, Chamaeleon I/II, Rho Ophiuchi, and Serpens dark clouds. We detected 45/48 (94%) of the single sources, 16/16 (100%) of the primary components, and 12/16 (75%) of the secondary/triple components of the binary/multiple objects surveyed. The composite spectral energy distributions (SEDs) for all of our sample sources are either Class I or F.S., and, in 15/16 multiple systems, at least one of the individual components displays a Class I or F.S. spectral index. However, the occurrence of mixed pairings, such as F.S. with Class I, F.S. with Class II, and, in one case, F.S. with Class III, is surprisingly frequent. Such behaviour is not consistent with that of multiple systems among T Tauri stars (TTS), where the companion of a classical TTS also tends to be a classical TTS, although other mixed pairings have been previously observed among Class II objects. Based on an analysis of the spectral indices of the individual binary components, there appears to be a higher proportion of mixed Class I/F.S. systems (65-80%) than that of mixed Classical/Weak-Lined TTS (25-40%), demonstrating that the envelopes of Class I/ F.S. systems are rapidly evolving during this evolutionary phase. We report the discovery of a steep spectral index secondary companion to ISO-ChaI 97, detected for the first time via our mid-infrared observations. In our previous near- infrared imaging survey of binary/multiple Class I/F.S. sources, ISO-ChaI 97 appeared to be single. With a spectral index of Alpha >= 3.9, the secondary component of this system is a member of a rare class of very steep spectral index objects, those with Alpha > 3. Only three such objects have previously been reported, all of which are either Class 0 or Class I.Comment: 31 pages, 4 figures, 6 table

High Resolution Mid-Infrared Imaging of Ultraluminous Infrared Galaxies

Observations of ultraluminous infrared galaxies (ULIRGs) with an achieved resolution approaching the diffraction limit in the mid-infrared from 8 - 25 $\mu$m using the Keck Telescopes are reported. We find extremely compact structures, with spatial scales of $< 0.3''$ (diameter) in six of the seven ULIRGs observed. These compact sources emit between 30% and 100% of the mid-infrared energy from these galaxies. We have utilized the compact mid-infrared structures as a diagnostic of whether an AGN or a compact (100 -- 300 pc) starburst is the primary power source in these ULIRGs. In Markarian 231, the upper limit on the diameter of the 12.5 $\mu$m source, 0.13$''$, shows that the size of the infrared source must increase with increasing wavelength, consistent with AGN models. In IRAS 05189-2524 and IRAS 08572+3915 there is strong evidence that the source size increases with increasing wavelength. This suggests heating by a central source rather than an extended luminosity source, consistent with the optical classification as an AGN. The compact mid-infrared sources seen in the other galaxies cannot be used to distinguish the ultimate luminosity source. If these ULIRGs are powered by compact starbursts, the star formation rates seen in the central few hundred parsecs far exceed the global rates seen in nearby starburst galaxies, and approach the surface brightness of individual clusters in nearby starburst galaxies.Comment: 33pages, 6 tables, 5 figures, Accepted for publication in A

\u3cem\u3eγ\u3c/em\u3e-ray Spectroscopy of \u3csup\u3e166\u3c/sup\u3eHf: X(5) in \u3cem\u3eN\u3c/em\u3e \u3e 90?

Excited states in 166Hf were populated in the β+/∈ decay of 166Ta and studied through off-beam γ –ray spectroscopy at the Yale moving tape collector. New coincidence data found no support for two previously reported excited 0+ states and led to a substantially revised level scheme. Similarities between the revised level scheme of 166Hf and the X(5) critical point symmetry are discussed, and the extent of X(5) behavior in this mass region is explored through the W and Os isotopes. Among X(5) candidates with N \u3e 90, good agreement is observed for most energies and interband B(E2) strengths, while all exhibit similar disagreements with other key observables, in particular, yrast B(E2) values and spacing in the excited K = 0+ sequence

A large sample study of spin relaxation and magnetometric sensitivity of paraffin-coated Cs vapor cells

We have manufactured more than 250 nominally identical paraffin-coated Cs vapor cells (30 mm diameter bulbs) for multi-channel atomic magnetometer applications. We describe our dedicated cell characterization apparatus. For each cell we have determined the intrinsic longitudinal, \sGamma{01}, and transverse, \sGamma{02}, relaxation rates. Our best cell shows \sGamma{01}/2\pi\approx 0.5 Hz, and \sGamma{02}/2\pi\approx 2 Hz. We find a strong correlation of both relaxation rates which we explain in terms of reservoir and spin exchange relaxation. For each cell we have determined the optimal combination of rf and laser powers which yield the highest sensitivity to magnetic field changes. Out of all produced cells, 90% are found to have magnetometric sensitivities in the range of 9 to 30 fTHz. Noise analysis shows that the magnetometers operated with such cells have a sensitivity close to the fundamental photon shot noise limit

To the Continuum and Beyond: Structure of U Nuclei

An experiment was performed at the 88-inch cyclotron at LBNL to investigate the structure of uranium isotopes and concurrently test the so-called surrogate ratio method. A 28 MeV proton beam was used to bombard 236U and 238U targets and the outgoing light ions were detected using the STARS silicon telescope allowing isotopic assignments and the excitation energy of the compound nucleus to be measured. A fission detector was placed at backward angles to give particle-fission coincidences, while the six clover germanium detectors of the LIBERACE array were used for particle-γ coincidences. The (p,d) reaction channels on 236U and 238U targets were used as a surrogate to measure the σ(234U(n,f))/σ(236U(n,f)) cross section ratio. The results give reasonable agreement with literature values over an equivalent neutron energy range between 0 MeV and 6 MeV. Structure results in 235U include a new (3/2−) level at 1035 keV, that is tentatively assigned as the 3/2−[501] Nilsson state. The analogue 3/2−[501] state in 237U may be associated with a previously observed level at 1201 keV, whose spin/parity is restricted to Jπ = 3/2− on the basis of newly observed decays to the ground band

Utilizing (\u3cem\u3ep,d\u3c/em\u3e) and (\u3cem\u3ep,t\u3c/em\u3e) Reactions to Obtain (\u3cem\u3en,f\u3c/em\u3e) Cross Sections in Uranium Nuclei Via the Surrogate-Ratio Method

The surrogate ratio method has been tested for (p,d) and (p,t) reactions on uranium nuclei. 236U and 238U targets were bombarded with 28-MeV protons and the light ion recoils and fission fragments were detected using the Silicon Telescope Array for Reaction Studies detector array at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. The (p,df) reaction channels on 236U and 238U targets were used as a surrogate to determine the σ[236U(n,f)]/σ[234U(n,f)] cross-section ratio. The (p,tf) reaction channels were also measured with the same targets as a surrogate for the σ[235U(n,f)]/σ[(233U(n,f)] ratio. For the (p,df) and (p,tf) surrogate measurements, there is good agreement with accepted (n,f) values over equivalent neutron energy ranges of En=0–7 MeV and En=0–5.5 MeV, respectively. An internal surrogate ratio method comparing the (p,d) and (p,t) reaction channels on a single target is also discussed. The σ[234U(n,f)]/σ[233U(n,f)] and σ[236U(n,f)]/σ[235U(n,f)] cross-section ratios are extracted using this method for the 236U and 238U targets, respectively. The resulting fission cross-section ratios show relatively good agreement with accepted values up to En∼5 MeV

Isomeric Decay of \u3csup\u3e208\u3c/sup\u3eRa

Low-energy excited states of 208Ra were investigated using the 182W(30Si, 4n) reaction at the Wright Nuclear Structure Laboratory of Yale University. Fusion evaporation recoils were selected using the gas-filled spectrometer SASSYER. Delayed γ rays, following isomeric decays, were detected at the focal plane of SASSYER with a small array of three clover Ge detectors. Transitions following a proposed J π = 8+ isomer were observed, and the half-life was measured