Candidate Pre-Mainsequence F Stars with Circumstellar Dust Identified Using Combined 2MASS and uvby Data

We propose a method that uses near-infrared plus uvby photometry to identify potentially extensive circumstellar dusty environment about F and A stars. The method has been applied to a sample of ~900 metal rich reddened F stars with 2MASS and uvby data, suggesting the presence of circumstellar dust emitting in the near infrared for ~70 stars. The log T_e - M_V diagram suggests that most, if not all, of them are likely pre-mainsequence (PMS). They seem to be consistent with being a continuation of the class of Herbig Ae/Be PMS stars into the spectral type F. Their number drops sharply downward of log T_e ~ 3.84 (spectral types later than ~F5), which may provide new clues to the PMS evolution of stars with 1 to 2 solar mass. We present a list of 21 most conspicuous candidate stars with circumstellar dust. About half of them are associated with the extended star-forming region around rho Oph. The brightest of these 21 stars, with V < 7.5, turn out to be IRAS sources, suggesting the presence of heated dust emitting in the far infrared. Also in this list, HD 81270 is reported as a very unusual star moving away from the Galactic plane at a projected speed of 70 km/sec.Comment: 8 pages, 4 figures, 1 table. To appear in ApJ, part 2, v. 570, 2002 May

The Homogeneity of Interstellar Oxygen in the Galactic Disk

We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of O I 1356 and H I Lyman-alpha absorption in 36 sight lines that probe a variety of Galactic disk environments and include paths that range over nearly 4 orders of magnitude in f(H_2), over 2 orders of magnitude in mean sight line density, and that extend up to 6.5 kpc in length. Consequently, we have undertaken the study of gas-phase O/H abundance ratio homogeneity using the current sample and previously published Goddard High-Resolution Spectrograph (GHRS) results. Two distinct trends are identified in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance with increasing mean sight line density and a gap between the mean O/H ratio for sight lines shorter and longer than about 800 pc. The first effect is a smooth transition between two depletion levels associated with large mean density intervals; it is centered near a density of 1.5 cm^-3 and is similar to trends evident in gas-phase abundances of other elements. Paths less dense than the central value exhibit a mean O/H ratio of log_10 (O/H) = -3.41+/-0.01 (or 390+/-10 ppm), which is consistent with averages determined for several long, low-density paths observed by STIS (Andre et al. 2003) and short low-density paths observed by FUSE (Moos et al. 2002). Sight lines of higher mean density exhibit an average O/H value of log_10 (O/H) = -3.55+/-0.02 (284+/-12 ppm). The datapoints for low-density paths are scattered more widely than those for denser sight lines, due to O/H ratios for paths shorter than 800 pc that are generally about 0.10 dex lower than the values for longer ones.Comment: 33 pages, including 8 figures and 4 tables; accepted for publication in ApJ, tentatively in Oct 200

The effective temperature scale of FGK stars. II. Teff : color : [Fe/H] calibrations

We present up-to-date metallicity-dependent temperature vs. color calibrations for main sequence and giant stars based on temperatures derived with the infrared flux method (IRFM). Seventeen colors in the following photometric systems: UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO, Hipparcos-Tycho, and 2MASS, have been calibrated. The spectral types covered range from F0 to K5 (7000 K<Teff<4000 K) with some relations extending below 4000 K or up to 8000 K. Most of the calibrations are valid in the metallicity range -3.5<[Fe/H]<0.4, although some of them extend to as low as [Fe/H]=-4.0. All fits to the data have been performed with more than 100 stars; standard deviations range from 30 K to 120 K. Fits were carefully performed and corrected to eliminate the small systematic errors introduced by the calibration formulae. Tables of colors as a function of Teff and [Fe/H] are provided. (Abridged)Comment: To appear in ApJ. For online tables and figures, see http://webspace.utexas.edu/ir68/tef

Optical Nonlinearities in Bulk GaAs Determined by EL2 Defect

Time-resolved studies of light diffraction on free carrier phase gratings and light absorption in subnanosecond time domain were carried out in two distinct areas of semi-insulating GaAs with high and low growth-defect density. Numerical analysis was performed in order to reveal the role of EL2 defect in carrier generation and transport. The possibility of transient grating technique to study various defect-governed carrier relaxation processes were demonstrated experimentally

Fano resonances in semiconductor superlattices

We report the first observation of Fano resonances in biased semiconductor superlattices: The excitonic Wannier-Stark ladder transitions show asymmetric absorption due to the coupling to the continua of lower transitions. In contrast to other known examples of Fano resonances, the Fano coupling can be continuously tuned in this system by changing the static field across the superlattice. The line shapes and their coupling dependence are in excellent agreement with theory. We also investigate the dephasing dynamics of the resonances and observe an increase in dephasing with increasing Fano coupling

Nonequilibrium carrier dynamics in heavily p-doped GaAs

A non-degenerate four-wave mixing technique has been applied to investigate carrier transport and recombination in heavily C-doped GaAs embedded in a double-heterostructure. The carriers were injected into the 1 µm-thick p-GaAs layer via the 50 nm-thick barrier of AlGaAs:C or InGaP:Si, using the light interference pattern of two picosecond laser pulses at 532 nm. The dependence of the nonequilibrium carrier grating decay time on the grating period allows the determination of minority carrier diffusion coefficients: D = 35 cm2/s for p-GaAs ($p_0 = 2 \times 10^{19}$ cm−3) with AlGaAs barriers and D = 27 cm2/s for p-GaAs ($p_{0} = 1 \times 10^{19}$ cm−3) with InGaP barriers. This increase of electron mobility at the higher doping level was found to be in agreement with the decreasing role of carrier-carrier scattering in heavily-doped p-GaAs. The fast recombination of nonequilibrium carriers in the vicinity of a front barrier layer was evident and more pronounced for an AlGaAs than for an InGaP barrier

Investigation of photoelectric properties of ZnSe:Cr and ZnTe:V:Al by picosecond four-wave mixing technique

Role of deep impurity levels in carrier generation, transport, and recombination were investigated in bulk ZnSe:Cr and ZnTe:V:Al crystals by four-wave mixing technique..