Colliding Particles in Highly Turbulent Flows

We discuss relative velocities and the collision rate of small particles suspended in a highly turbulent fluid. In the limit where the viscous damping is very weak, we estimate the relative velocities using the Kolmogorov cascade principle.Comment: 5 pages, no figures, v2 contains additional result

Large dust particles in disks around T Tauri stars

We present 7-mm continuum observations of 14 low-mass pre-main-sequence stars in the Taurus-Auriga star-forming region obtained with the Very Large Array with ~1.5" resolution and ~0.3 mJy rms sensitivity. For 10 objects, the circumstellar emission has been spatially resolved. The large outer disk radii derived suggest that the emission at this wavelength is mostly optically thin. The millimetre spectral energy distributions are characterised by spectral indices alpha = 2.3 to 3.2. After accounting for contribution from free-free emission and corrections for optical depth, we determine dust opacity indices beta in the range 0.5 to 1.6, which suggest that millimetre-sized dust aggregates are present in the circumstellar disks. Four of the sources with beta > 1 may be consistent with submicron-sized dust as found in the interstellar medium. Our findings indicate that dust grain growth to millimetre-sized particles is completed within less than 1 Myr for the majority of circumstellar disks.Comment: 11 pages, 4 figure

Reflected Shock Tunnel Noise Measurement by Focused Differential Interferometry

A series of experiments is conducted where a quantitative non-intrusive optical technique is used to investigate disturbances in the free-stream of T5, the free-piston driven reflected shock tunnel at Caltech. The optical technique, focused laser differential interferometry (FLDI), measures fluctuations in density. In the test matrix, reservoir enthalpy is varied while the reservoir pressure is held fixed. The results show the perturbations in density are not a strong function of the reservoir enthalpy. During one experiment, exceptional levels of noise were detected; this unique result is attributed to non-ideal operation of the shock tunnel. The data indicate that rms density fluctuations of less than 0.75% are achievable with attention to tunnel cleanliness. In addition, the spectral content of density fluctuation does not change throughout the test time

Vertical structure models of T Tauri and Herbig Ae/Be disks

In this paper we present detailed models of the vertical structure (temperature and density) of passive irradiated circumstellar disks around T Tauri and Herbig Ae/Be stars. In contrast to earlier work, we use full frequency- and angle-dependent radiative transfer instead of the usual moment equations. We find that this improvement of the radiative transfer has strong influence on the resulting vertical structure of the disk, with differences in temperature as large as 70 %. However, the spectral energy distribution (SED) is only mildly affected by this change. In fact, the SED compares reasonably well with that of improved versions of the Chiang & Goldreich (CG) model. This shows that the latter is a reasonable model for the SED, in spite of its simplicity. It also shows that from the SED alone, little can be learned about the vertical structure of a passive circumstellar disk. The molecular line emission from these disks is more sensitive to the vertical temperature and density structure, and we show as an example how the intensity and profiles of various CO lines depend on the adopted disk model. The models presented in this paper can also serve as the basis of theoretical studies of e.g. dust coagulation and settling in disks.Comment: 12 pages, 15 figures, accepted for publication in A&

An Infrared Emission Line Galaxy at z = 2.43

An object discovered during an infrared survey of the field near the quasar B2 0149+33 has an emission line at 2.25$\mu$m that we interpret as H$\alpha$ at a redshift of 2.43. The K-band image shows two compact components 10 kpc apart surrounded by more extended emission over ~20 kpc. The H$\alpha$ emission appears to be extended over ~15 kpc (2") in a coarsely sampled (0".8/pixel) image. The star formation rate may be as high as 250 - 1000 M$_\odot$ yr$^{-1}$, depending on the extinction. Alternatively, the line may be powered by an active nucleus, although the probability of serendipitously discovering an AGN in the survey volume is only ~0.02. The increasing number of similar objects reported in the literature indicate that they may be an important, unstudied population in the high redshift universe.Comment: ApJ in press, 21 pages, 2 figures. Also available at http://www.mpia-hd.mpg.de/MPIA/Projects/STARS/preprints.htm

Warm Molecular Layers in Protoplanetary Disks

We have investigated molecular distributions in protoplanetary disks, adopting a disk model with a temperature gradient in the vertical direction. The model produces sufficiently high abundances of gaseous CO and HCO+ to account for line observations of T Tauri stars using a sticking probability of unity and without assuming any non-thermal desorption. In regions of radius R > 10 AU, with which we are concerned, the temperature increases with increasing height from the midplane. In a warm intermediate layer, there are significant amounts of gaseous molecules owing to thermal desorption and efficient shielding of ultraviolet radiation by the flared disk. The column densities of HCN, CN, CS, H2CO, HNC and HCO+ obtained from our model are in good agreement with the observations of DM Tau, but are smaller than those of LkCa15. Molecular line profiles from our disk models are calculated using a 2-dimensional non-local-thermal-equilibrium (NLTE) molecular-line radiative transfer code for a direct comparison with observations. Deuterated species are included in our chemical model. The molecular D/H ratios in the model are in reasonable agreement with those observed in protoplanetary disks.Comment: 11 pages, Latex (aa.cls), to be published in Astronomy and Astrophysic

Investigating the meaning of 'good' or 'very good' patient evaluations of care in English general practice: A mixed methods study

This is the final version. Available from the publisher via the DOI in this record.The data set is available on request from the authors: please email Jenni Burt ( [email protected]) for details.Objective: To examine concordance between responses to patient experience survey items evaluating doctors' interpersonal skills, and subsequent patient interview accounts of their experiences of care. Design: Mixed methods study integrating data from patient questionnaires completed immediately after a video-recorded face-to-face consultation with a general practitioner (GP) and subsequent interviews with the same patients which included playback of the recording. Setting: 12 general practices in rural, urban and inner city locations in six areas in England. Participants: 50 patients (66% female, aged 19-96 years) consulting face-to-face with 32 participating GPs. Main outcome measures: Positive responses to interpersonal skills items in a postconsultation questionnaire ('good' and 'very good') were compared with experiences reported during subsequent video elicitation interview (categorised as positive, negative or neutral by independent clinical raters) when reviewing that aspect of care. Results: We extracted 230 textual statements from 50 interview transcripts which related to the evaluation of GPs' interpersonal skills. Raters classified 70.9% (n=163) of these statements as positive, 19.6% (n=45) neutral and 9.6% (n=22) negative. Comments made by individual patients during interviews did not always express the same sentiment as their responses to the questionnaire. Where questionnaire responses indicated that interpersonal skills were 'very good', 84.6% of interview statements concerning that item were classified as positive. However, where patients rated interpersonal skills as 'good', only 41.9% of interview statements were classified as positive, and 18.9% as negative. Conclusions: Positive responses on patient experience questionnaires can mask important negative experiences which patients describe in subsequent interviews. The interpretation of absolute patient experience scores in feedback and public reporting should be done with caution, and clinicians should not be complacent following receipt of 'good' feedback. Relative scores are more easily interpretable when used to compare the performance of providers.NHS Cambridgeshire and Peterborough CCGNational Institute for Health Researc

Infrared emission-line galaxies associated with damped Lyman-alpha and strong metal absorber redshifts

Eighteen candidates for emission line galaxies were discovered in a narrow-band infrared survey that targeted the redshifts of damped Lyman-alpha or metal lines in the spectra of quasars. The presence of emission lines is inferred from the photometric magnitudes in narrow and broad band interference filters, corresponding to H-alpha at redshifts of 0.89 (6 objects) and 2.4 (10 objects), and [OII] at a redshift of 2.3 (2 objects). Most of the candidates are small resolved objects, compatible with galaxies at the redshifts of the absorbers. Because a similar survey targeted at the redshifts of quasars themselves uncovered only one emission-line galaxy in a larger volume, the results imply substantial clustering of young galaxies or formation within filaments or sheets whose locations are indicated by the redshifts of strong absorption along the lines of sight to more distant quasars.Comment: 12 pages including 3 figures, accepted for publication in ApJ Let

Critical Protoplanetary Core Masses in Protoplanetary Disks and the Formation of Short-Period Giant Planets

We study a solid protoplanetary core of 1-10 earth masses migrating through a disk. We suppose the core luminosity is generated as a result of planetesimal accretion and calculate the structure of the gaseous envelope assuming equilibrium. This is a good approximation when the core mass is less than the critical value, M_{crit}, above which rapid gas accretion begins. We model the structure of the protoplanetary nebula as an accretion disk with constant \alpha. We present analytic fits for the steady state relation between disk surface density and mass accretion rate as a function of radius r. We calculate M_{crit} as a function of r, gas accretion rate through the disk, and planetesimal accretion rate onto the core \dot{M}. For a fixed \dot{M}, M_{crit} increases inwards, and it decreases with \dot{M}. We find that \dot{M} onto cores migrating inwards in a time 10^3-10^5 yr at 1 AU is sufficient to prevent the attainment of M_{crit} during the migration process. Only at small radii where planetesimals no longer exist can M_{crit} be attained. At small radii, the runaway gas accretion phase may become longer than the disk lifetime if the core mass is too small. However, massive cores can be built-up through the merger of additional incoming cores on a timescale shorter than for in situ formation. Therefore, feeding zone depletion in the neighborhood of a fixed orbit may be avoided. Accordingly, we suggest that giant planets may begin to form early in the life of the protostellar disk at small radii, on a timescale that may be significantly shorter than for in situ formation. (abridged)Comment: 24 pages (including 9 figures), LaTeX, uses emulateapj.sty, to be published in ApJ, also available at http://www.ucolick.org/~ct/home.htm