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

The jamming elasticity of emulsions stabilized by ionic surfactants

Oil-in-water emulsions composed of colloidal-scale droplets are often stabilized using ionic surfactants that provide a repulsive interaction between neighboring droplet interfaces, thereby inhibiting coalescence. If the droplet volume fraction is raised rapidly by applying an osmotic pressure, the droplets remain disordered, undergo an ergodic–nonergodic transition, and jam. If the applied osmotic pressure approaches the Laplace pressure of the droplets, then the jammed droplets also deform. Because solid friction and entanglements cannot play a role, as they might with solid particulate or microgel dispersions, the shear mechanical response of monodisperse emulsions can provide critical insight into the interplay of entropic, electrostatic, and interfacial forces. Here, we introduce a model that can be used to predict the plateau storage modulus and yield stress of a uniform charge-stabilized emulsion accurately if the droplet radius, interfacial tension, surface potential, Debye screening length, and droplet volume fraction are known

Detection of Molecular Hydrogen Orbiting a "Naked" T Tauri Star

Astronomers have established that for a few million years newborn stars possess disks of orbiting gas and dust. Such disks, which are likely sites of planet formation, appear to disappear once these stars reach ages of 5-10 times 10^6 yr; yet, >= 10^7 yr is thought necessary for giant planet formation. If disks dissipate in less time than is needed for giant planet formation, such planets may be rare and those known around nearby stars would be anomalies. Herein, we report the discovery of H_2 gas orbiting a weak-lined T Tauri star heretofore presumed nearly devoid of circumstellar material. We estimate that a significant amount of H_2 persists in the gas phase, but only a tiny fraction of this mass emits in the near-infrared. We propose that this star possesses an evolved disk that has escaped detection thus far because much of the dust has coagulated into planetesimals. This discovery suggests that the theory that disks are largely absent around such stars should be reconsidered. The widespread presence of such disks would indicate that planetesimals can form quickly and giant planet formation can proceed to completion before the gas in circumstellar disks disperses.Comment: latex 12 pages, including 1 figur

The Initial Mass Function and Disk Frequency of the Rho Ophiuchi Cloud: An Extinction-Limited Sample

We have completed an optical spectroscopic survey of an unbiased, extinction-limited sample of candidate young stars covering 1.3 square degrees of the Rho Ophiuchi star forming region. While infrared, X-ray, and optical surveys of the cloud have identified many young stellar objects (YSOs), these surveys are biased towards particular stages of stellar evolution and are not optimal for studies of the disk frequency and initial mass function.We have obtained over 300 optical spectra to help identify 135 association members based on the presence of H-alpha in emission, lithium absorption, X-ray emission, a mid-infrared excess, a common proper motion, reflection nebulosity, and/or extinction considerations. Spectral types along with R and I band photometry were used to derive effective temperatures and bolometric luminosities for association members to compare with theoretical tracks and isochrones for pre-main-sequence stars. An average age of 3.1 Myr is derived for this population which is intermediate between that of objects embedded in the cloud core of Rho Ophiuchi and low mass stars in the Upper Scorpius subgroup. Consistent with this age we find a circumstellar disk frequency of 27% plus or minus 5%. We also constructed an initial mass function for an extinction-limited sample of 123 YSOs (A_v less than or equal to 8 mag), which is consistent with the field star initial mass function for YSOs with masses > 0.2 M_sun. There may be a deficit of brown dwarfs but this result relies on completeness corrections and requires confirmation.Comment: 46 pages, 7 figures, 4 table

VLT/NACO adaptive optics imaging of the TY CrA system - A fourth stellar component candidate detected

We report the detection of a possible subsolar mass companion to the triple young system TY CrA using the NACO instrument at the VLT UT4 during its commissioning. Assuming for TY CrA a distance similar to that of the close binary system HD 176386, the photometric spectral type of this fourth stellar component candidate is consistent with an ~M4 star. We discuss the dynamical stability of this possible quadruple system as well as the possible location of dusty particles inside or outside the system.Comment: 4 pages, 2 figures postscrip

Budget projections and clinical impact of an immuno-oncology class of treatments: Experience in four EU markets

Background Immunotherapies have revolutionized oncology, but their rapid expansion may potentially put healthcare budgets under strain. We developed an approach to reduce demand uncertainty and inform decision makers and payers of the potential health outcomes and budget impact of the anti-PD-1/PD-L1 class of immuno-oncology (IO) treatments. Methods We used partitioned survival modelling and budget impact analysis to estimate overall survival, progression-free survival, life years gained (LYG), and number of adverse events (AEs), comparing “worlds with and without” anti-PD-1/PD-L1s over five years. The cancer types initially included melanoma, first and second line non-small cell lung cancer (NSCLC), bladder, head and neck, renal cell carcinoma, and triple negative breast cancer [1]. Inputs were based on publicly available data, literature, and expert advice. Results The model [2] estimated budget and health impact of the anti-PD-1/PD-L1s and projected that between 2018−2022 the class [3] would have a manageable economic impact per year, compared to the current standard of care (SOC). The first country adaptations showed that for that period Belgium would save around 11,100 additional life years and avoid 6,100 AEs. Slovenia - 1,470 LYGs and 870 AEs avoided; Austria - respectively 4,200, 3,000; Italy – 19,800, 6,800. For Austria, the class had a projected share of about 4.5 % of the cancer care budget and 0.4 % of the total 2020 healthcare budget. For Belgium, Slovenia, and Italy - respectively 15.1 % and 1.1 %, 12.6 %, 0.6 %, and 6.5 %, 0.5 %. Conclusion The Health Impact Projection (HIP) is a horizon scanning model designed to estimate the potential budget and health impact of the PD-(L)1 inhibitor class at a country level for the next five years. It provides valuable data to payers which they can use to support their reimbursement plans

Young Low-Mass Stars and Brown Dwarfs in IC 348

I present new results from a continuing program to identify and characterize the low-mass stellar and substellar populations in the young cluster IC 348 (1-10~Myr). Optical spectroscopy has revealed young objects with spectral types as late as M8.25. The intrinsic J-H and H-K colors of these sources are dwarf-like, whereas the R-I and I-J colors appear intermediate between the colors of dwarfs and giants. Furthermore, the spectra from 6500 to 9500 A are reproduced well with averages of standard dwarf and giant spectra, suggesting that such averages should be used in the classification of young late-type sources. An H-R diagram is constructed for the low-mass population in IC 348 (K6-M8). The presumably coeval components of the young quadruple system GG~Tau (White et al.) and the locus of stars in IC 348 are used as empirical isochrones to test the theoretical evolutionary models. For the models of Baraffe et al., an adjustment of the temperature scale to progressively warmer temperatures at later M types, intermediate between dwarfs and giants, brings all components of GG~Tau onto the same model isochrone and gives the population of IC 348 a constant age and age spread as a function of mass. When other observational constraints are considered, such as the dynamical masses of GM~Aur, DM~Tau, and GG~Tau~A, the models of Baraffe et al. are the most consistent with observations of young systems. With compatible temperature scales, the models of both D'Antona & Mazzitelli and Baraffe et al. suggest that the hydrogen burning mass limit occurs near M6 at ages of <10 Myr. Thus, several likely brown dwarfs are discovered in this study of IC 348, with masses down to ~20-30 M_J.Comment: 23 pages, 9 figures, accepted to Ap

Optical Spectroscopy of the Surface Population of the rho Ophiuchi Molecular Cloud: The First Wave of Star Formation

We present the results of optical spectroscopy of 139 stars obtained with the Hydra multi-object spectrograph. The objects extend over a 1.3 square degree area surrounding the main cloud of the rho Oph complex. The objects were selected from narrowband images to have H alpha in emission. Using the presence of strong H alpha emission, lithium absorption, location in the Hertzsprung-Russell diagram, or previously reported x-ray emission, we were able to identify 88 objects as young stars associated with the cloud. Strong H alpha emission was confirmed in 39 objects with line widths consistent with their origin in magnetospheric accretion columns. Two of the strongest emission-line objects are young, x-ray emitting brown dwarf candidates with M8 spectral types. Comparisons of the bolometric luminosities and effective temperatures with theoretical models suggest a medianage for this population of 2.1 Myr which is signifcantly older than the ages derived for objects in the cloud core. It appears that these stars formed contemporaneously with low mass stars in the Upper Scorpius subgroup, likely triggered by massive stars in the Upper-Centaurus subgroup.Comment: 35 pages of postscript which includes seven figures (some of which are multi-panel) and four postscript tables. Astronomical Journal (in press