Composite absorbing potentials

The multiple scattering interferences due to the addition of several contiguous potential units are used to construct composite absorbing potentials that absorb at an arbitrary set of incident momenta or for a broad momentum interval.Comment: 9 pages, Revtex, 2 postscript figures. Accepted in Phys. Rev. Let

Raman cooling and heating of two trapped Ba+ ions

We study cooling of the collective vibrational motion of two 138Ba+ ions confined in an electrodynamic trap and irradiated with laser light close to the resonances S_1/2-P_1/2 (493 nm) and P_1/2-D_3/2 (650 nm). The motional state of the ions is monitored by a spatially resolving photo multiplier. Depending on detuning and intensity of the cooling lasers, macroscopically different motional states corresponding to different ion temperatures are observed. We also derive the ions' temperature from detailed analytical calculations of laser cooling taking into account the Zeeman structure of the energy levels involved. The observed motional states perfectly match the calculated temperatures. Significant heating is observed in the vicinity of the dark resonances of the Zeeman-split S_1/2-D_3/2 Raman transitions. Here two-photon processes dominate the interaction between lasers and ions. Parameter regimes of laser light are identified that imply most efficient laser cooling.Comment: 8 pages, 5 figure

Rotational periods of very young brown dwarfs and very low-mass stars in ChaI

We have studied the photometric variability of very young brown dwarfs and very low-mass stars (masses well below 0.2 M_sun) in the ChaI star forming region. We have determined photometric periods in the Gunn i and R band for the three M6.5-M7 type brown dwarf candidates ChaHa2, ChaHa3 and ChaHa6 of 2.2 to 3.4 days. These are the longest photometric periods found for any brown dwarf so far. If interpreted as rotationally induced they correspond to moderately fast rotational velocities, which is fully consistent with their v sini values and their relatively large radii. We have also determined periods for the two M5-M5.5 type very low-mass stars B34 and CHXR78C. In addition to the Gunn i and R band data, we have analysed JHK_s monitoring data of the targets, which have been taken a few weeks earlier and confirm the periods found in the optical data. Upper limits for the errors in the period determination are between 2 and 9 hours. The observed periodic variations of the brown dwarf candidates as well as of the T Tauri stars are interpreted as modulation of the flux at the rotation period by magnetically driven surface features, on the basis of a consistency with v sini values as well as (R-i) color variations typical for spots. Furthermore, the temperatures even for the brown dwarfs in the sample are relatively high (>2800K) because the objects are very young. Therefore, the atmospheric gas should be sufficiently ionized for the formation of spots on one hand and the temperatures are too high for significant dust condensation and hence variabilities due to clouds on the other hand.Comment: 18 pages, 6 figures, accepted for publication in Ap

Keck Adaptive Optics Imaging of Nearby Young Stars: Detection of Close Multiple Systems

Using adaptive optics on the Keck II 10-meter telescope on Mauna Kea, we have surveyed 24 of the nearest young stars known in search of close companions. Our sample includes members of the MBM 12 and TW Hydrae young associations and the classical T Tauri binary UY Aurigae in the Taurus star-forming region. We present relative photometry and accurate astrometry for 10 close multiple systems. The multiplicity frequency in the TW Hydrae and MBM 12 groups are high in comparison to other young regions, though the significance of this result is low because of the small number statistics. We resolve S 18 into a triple system including a tight 63 mas (projected separation of 17 AU at a distance of 275 pc) binary for the first time, with a hierarchical configuration reminiscent of VW Chamaeleontis and T Tauri. Another tight binary in our sample -- TWA 5Aab (54 mas or 3 AU at 55 pc) -- offers the prospect of dynamical mass measurement using astrometric observations within a few years, and thus could be important for testing pre-main sequence evolutionary models. Our observations confirm with 9-sigma confidence that the brown dwarf TWA 5B is bound to TWA 5A. We find that the flux ratio of UY Aur has changed dramatically, by more than a magnitude in the H-band, possibly as a result of variable extinction. With a smaller flux ratio, the system may once again become detectable as an optical binary, as it was at the time of its discovery in 1944. Taken together, our results demonstrate that adaptive optics on large telescopes is a powerful tool for detecting tight companions, and thus exploring the frequency and configurations of close multiple systems.Comment: accepted for publication in The Astronomical Journa

X-ray Properties of Pre--Main-Sequence Stars in the Orion Nebula Cluster with Known Rotation Periods

We re-analyze all archival Chandra/ACIS observations of the Orion Nebula Cluster (ONC) to study the X-ray properties of a large sample of pre--main-sequence (PMS) stars with optically determined rotation periods. Our goal is to elucidate the origins of X-rays in PMS stars by seeking out connections between the X-rays and the mechanisms most likely driving their production--rotation and accretion. In our sample X-ray luminosity is significantly correlated with stellar rotation, in the sense of decreasing Lx/Lbol with more rapid rotation, suggesting that these stars are in the "super-saturated" regime of the rotation-activity relationship. However, we also find that stars with optical rotation periods are significantly biased to high Lx. This is not the result of magnitude bias in the optical rotation-period sample but rather to the diminishingly small amplitude of optical variations in stars with low Lx. Evidently, there exists in the ONC a population of stars whose rotation periods are unknown and that possess lower average X-ray luminosities than those of stars with known rotation periods. These stars may sample the linear regime of the rotation-activity relationship. Accretion also manifests itself in X-rays, though in a somewhat counterintuitive fashion: While stars with spectroscopic signatures of accretion show harder X-ray spectra than non-accretors, they show lower X-ray luminosities and no enhancement of X-ray variability. We interpret these findings in terms of a common origin for the X-ray emission observed from both accreting and non-accreting stars, with the X-rays from accreting stars simply being attenuated by magnetospheric accretion columns. This suggests that X-rays from PMS stars have their origins primarily in chromospheres, not accretion.Comment: Accepted by the Astronomical Journal. 43 pages, 16 figure

Discovery of Two T Dwarf Companions with the Spitzer Space Telescope

We report the discovery of T dwarf companions to the nearby stars HN Peg (G0V, 18.4 pc, ~0.3 Gyr) and HD 3651 (K0V, 11.1 pc, ~7 Gyr). During an ongoing survey of 5'x5' fields surrounding stars in the solar neighborhood with IRAC aboard the Spitzer Space Telescope, we identified these companions as candidate T dwarfs based on their mid-IR colors. Using near-IR spectra obtained with SpeX at the NASA IRTF, we confirm the presence of methane absorption that characterizes T dwarfs and measure spectral types of T2.5+/-0.5 and T7.5+/-0.5 for HN Peg B and HD 3651 B, respectively. By comparing our Spitzer data to images from 2MASS obtained several years earlier, we find that the proper motions of HN Peg B and HD 3651 B are consistent with those of the primaries, confirming their companionship. HN Peg B and HD 3651 B have angular separations of 43.2" and 42.9" from their primaries, which correspond to projected physical separations of 795 and 476 AU, respectively. A comparison of their luminosities to the values predicted by theoretical evolutionary models implies masses of 0.021+/-0.009 and 0.051+/-0.014 Msun for HN Peg B and HD 3651 B. In addition, the models imply an effective temperature for HN Peg B that is significantly lower than the values derived for other T dwarfs at similar spectral types, which is the same behavior reported by Metchev & Hillenbrand for the young late-L dwarf HD 203030 B. Thus, the temperature of the L/T transition appears to depend on surface gravity. Meanwhile, HD 3651 B is the first substellar companion directly imaged around a star that is known to harbor a close-in planet from RV surveys. The discovery of this companion supports the notion that the high eccentricities of close-in planets like the one near HD 3651 may be the result of perturbations by low-mass companions at wide separations.Comment: Astrophysical Journal, in pres

Flaring Up All Over -- Radio Activity in Rapidly-Rotating Late-Type M and L Dwarfs

We present Very Large Array observations of twelve late M and L dwarfs in the Solar neighborhood. The observed sources were chosen to cover a wide range of physical characteristics - spectral type, rotation, age, binarity, and X-ray and H\alpha activity - to determine the role of these properties in the production of radio emission, and hence magnetic fields. Three of the twelve sources, TVLM513-46546, 2MASS J0036159+182110, and BRI0021-0214, were observed to flare and also exhibit persistent emission, indicating that magnetic activity is not quenched at the bottom of the main sequence. The radio emission extends to spectral type L3.5, and there is no apparent decrease in the ratio of flaring luminosities to bolometric luminosities between M8-L3.5. Moreover, contrary to the significant drop in persistent H\alpha activity beyond spectral type M7, the persistent radio activity appears to steadily increase between M3-L3.5. Similarly, the radio emission from BRI0021-0214 violates the phenomenological relations between the radio and X-ray luminosities of coronally active stars, hinting that radio and X-ray activity are also uncorrelated at the bottom of the main sequence. The radio active sources that have measured rotational velocities are rapid rotators, Vsin(i)>30 km/sec, while the upper limits on radio activity in slowly-rotating late M dwarfs (Vsin(i)<10 km/sec) are lower than these detections. These observations provide tantalizing evidence that rapidly-rotating late M and L dwarfs are more likely to be radio active. This possible correlation is puzzling given that the observed radio emission requires sustained magnetic fields of 10-1000 G and densities of 10^12 cm^-3, indicating that the active sources should have slowed down considerably due to magnetic braking.Comment: Accepted to ApJ; Two new figures; Minor text revision

Evaluation of uncomplicated acute respiratory tract infection management in veterans: A national utilization review.

BackgroundAntibiotics are overprescribed for acute respiratory tract infections (ARIs). Guidelines provide criteria to determine which patients should receive antibiotics. We assessed congruence between documentation of ARI diagnostic and treatment practices with guideline recommendations, treatment appropriateness, and outcomes.MethodsA multicenter quality improvement evaluation was conducted in 28 Veterans Affairs facilities. We included visits for pharyngitis, rhinosinusitis, bronchitis, and upper respiratory tract infections (URI-NOS) that occurred during the 2015-2016 winter season. A manual record review identified complicated cases, which were excluded. Data were extracted for visits meeting criteria, followed by analysis of practice patterns, guideline congruence, and outcomes.ResultsOf 5,740 visits, 4,305 met our inclusion criteria: pharyngitis (n = 558), rhinosinusitis (n = 715), bronchitis (n = 1,155), URI-NOS (n = 1,475), or mixed diagnoses (&gt;1 ARI diagnosis) (n = 402). Antibiotics were prescribed in 68% of visits: pharyngitis (69%), rhinosinusitis (89%), bronchitis (86%), URI-NOS (37%), and mixed diagnosis (86%). Streptococcal diagnostic testing was performed in 33% of pharyngitis visits; group A Streptococcus was identified in 3% of visits. Streptococcal tests were ordered less frequently for patients who received antibiotics (28%) than those who did not receive antibiotics 44%; P &lt; .01). Although 68% of visits for rhinosinusitis had documentation of symptoms, only 32% met diagnostic criteria for antibiotics. Overall, 39% of patients with uncomplicated ARIs received appropriate antibiotic management. The proportion of 30-day return visits for ARI care was similar for appropriate (11%) or inappropriate (10%) antibiotic management (P = .22).ConclusionsAntibiotics were prescribed in most uncomplicated ARI visits, indicating substantial overuse. Practice was frequently discordant with guideline diagnostic and treatment recommendations

The Highly Eccentric Pre-Main Sequence Spectroscopic Binary RX J0529.3+1210

The young system RX J0529.3+1210 was initially identified as a single-lined spectroscopic binary. Using high-resolution infrared spectra, acquired with NIRSPEC on Keck II, we measured radial velocities for the secondary. The method of using the infrared regime to convert single-lined spectra into double-lined spectra, and derive the mass ratio for the binary system, has been successfully used for a number of young, low-mass binaries. For RX J0529.3+1210, a long- period(462 days) and highly eccentric(0.88) binary system, we determine the mass ratio to be 0.78+/-0.05 using the infrared double-lined velocity data alone, and 0.73+/-0.23 combining visible light and infrared data in a full orbital solution. The large uncertainty in the latter is the result of the sparse sampling in the infrared and the high eccentricity: the stars do not have a large velocity separation during most of their ~1.3 year orbit. A mass ratio close to unity, consistent with the high end of the one sigma uncertainty for this mass ratio value, is inconsistent with the lack of a visible light detection of the secondary component. We outline several scenarios for a color difference in the two stars, such as one heavily spotted component, higher order multiplicity, or a unique evolutionary stage, favoring detection of only the primary star in visible light, even in a mass ratio ~1 system. However, the evidence points to a lower ratio. Although RX J0529.3+1210 exhibits no excess at near-infrared wavelengths, a small 24 micron excess is detected, consistent with circumbinary dust. The properties of this binary and its membership in Lambda Ori versus a new nearby stellar moving group at ~90 pc are discussed. We speculate on the origin of this unusual system and on the impact of such high eccentricity on the potential for planet formation.Comment: 4 Figure