Emission from Hot Gas in Pre-Main Sequence Objects: The Accretion Shock and the Inner Disk.

High energy (X-ray and ultraviolet) emission traces hot gas produced by energetic phenomena in T Tauri stars. These phenomena include magnetic heating of the stellar atmosphere and magnetospheric accretion of disk gas onto the stellar surface. High energy emission irradiates the planet-forming disk during a key time for the origin of planets, so an understanding of these energetic processes and their evolution is crucial for theories of disk evolution and planet formation In this thesis, I analyze X-ray and ultraviolet observations of young stars to study high energy events and follow their evolution. I confirm that X-ray emission is saturated during the T Tauri phase and suggest that far ultraviolet emission is also saturated at this age, possibly because the same mechanism heats both the chromosphere and corona. I compare accretion diagnostics for a large sample of T Tauri stars to characterize the properties of magnetospheric accretion. For the first time, I use models of the accretion emission which have contributions from multiple accretion hot spots, characterized by varying energy fluxes in the accretion columns. Models of T Tauri magnetospheres and the magnetic footprints on the star physically motivate this multi-component description of accretion. For RECX-11, a source near the final stage of disk depletion, I show that it has a very low mass accretion rate. If theories that predict the circumstellar disk is losing mass at very high rates are correct, the disk of RECX-11 would have a gap or hole in it, which it does not. I also present observations of hot H2 gas in the inner circumstellar disk. I find that the strength of the H2 emission is correlated with the accretion luminosity of the T Tauri star and show that for young stars in which accretion has ceased, there is no H2 left in the inner disk. I show this is true even for non-accreting young stars at 1-3 Myr, indicating rapid disk removal. Observations of circumstellar gas, combined with knowledge of the radiation fields, are crucial for studying disk evolution. Here, I provide timescales for gas depletion and constraints to disk dissipation models.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/96153/1/lingleby_1.pd

The Evolution of Protoplanetary Disks: Probing the Inner Disk of Very Low Accretors

We report FUV, optical, and NIR observations of three T Tauri stars in the Orion OB1b subassociation with H$\alpha$ equivalent widths consistent with low or absent accretion and various degrees of excess flux in the mid-infrared. We aim to search for evidence of gas in the inner disk in HST ACS/SBC spectra, and to probe the accretion flows onto the star using H$\alpha$ and He I $\lambda$10830 in spectra obtained at the Magellan and SOAR telescopes. At the critical age of 5 Myr, the targets are at different stages of disk evolution. One of our targets is clearly accreting, as shown by redshifted absorption at free-fall velocities in the He I line and wide wings in H$\alpha$; however, a marginal detection of FUV H$_2$ suggests that little gas is present in the inner disk, although the spectral energy distribution indicates that small dust still remains close to the star. Another target is surrounded by a transitional disk, with an inner cavity in which little sub-micron dust remains. Still, the inner disk shows substantial amounts of gas, accreting onto the star at a probably low, but uncertain rate. The third target lacks both a He I line or FUV emission, consistent with no accretion or inner gas disk; its very weak IR excess is consistent with a debris disk. Different processes occurring in targets with ages close to the disk dispersal time suggest that the end of accretion phase is reached in diverse ways.Comment: 13 pages, 3 figures. Accepted for publication in The Astrophysical Journa

Far-Ultraviolet H_2 Emission from Circumstellar Disks

We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ~1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex, and 1, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of H_2 to study the evolution of the gas in the inner disk. We find that the H_2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting H_2 in the disk, the fact that the non-accreting sources show no H_2 emission implies that the H_2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600 Å, we estimate that the column density of H_2 left in the inner regions of the debris disks in our sample is less than ~3 × 10^(–6) g cm^(-2), 9 orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU

Are deprivation-specific cancer survival patterns similar according to individual- and area-based measures? A cohort study of patients diagnosed with five malignancies in England & Wales, 2008-2016

Objective: To investigate if measured inequalities in cancer survival differ when using individual- (‘person’) compared to area- (‘place’) based measures of deprivation for three socio-economic dimensions: income, deprivation and occupation Design: Cohort studySetting: Data from the Office for National Statistics (ONS) Longitudinal Study of England and Wales, UK, linked to the National Cancer Registration DatabaseParticipants: Patients diagnosed with cancers of the colorectum, breast, prostate, bladder or with Non-Hodgkin Lymphoma (NHL) during the period 2008-2016Primary and secondary outcome measures: Differentials in net survival between groups defined by individual wage, occupation and education compared to those obtained from corresponding area-level metrics using the English and Welsh Indices of Multiple Deprivation (IMD).Results: Survival was negatively associated with area-based deprivation irrespective of the type analysed, although a trend from least to most deprived was not always observed. Socio-economic differences were present according to individually-measured socio-economic groups although there was an absence of a consistent ‘gradient’ in survival. The magnitude of differentials was similar for area-based and individually-derived measures of deprivation, which was unexpected.Conclusion: These unique data suggest that the socio-economic influence of ‘person’ is different to that of ‘place’ with respect to cancer outcomes. This has implications for health policy aimed at reducing inequalities. Further research could further consider the separate and additional influence of area-based deprivation over individual-level characteristics (contextual effects) as well as investigate the geographic, socio-economic and healthcare related characteristics of areas with poor outcomes in order to inform policy intervention

Accretion Rates for T Tauri Stars Using Nearly Simultaneous Ultraviolet and Optical Spectra

We analyze the accretion properties of 21 low-mass T Tauri stars using a data set of contemporaneous near-UV (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph and the ground-based Small and Medium Aperture Research Telescope System, a unique data set because of the nearly simultaneous broad wavelength coverage. Our data set includes accreting T Tauri stars in Taurus, Chamaeleon I, η Chamaeleon, and the TW Hydra Association. For each source we calculate the accretion rate (Ṁ) by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high-density, low filling factor accretion spots coexist with low-density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near-IR. Comparing our estimates of Ṁ to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for the Ṁ estimates, to produce correlations between accretion indicators (Hβ, Ca II K, C II], and Mg II) and accretion properties obtained simultaneously

Assessment of the concordance between individual- and area-level measures of socio-economic deprivation in a cancer patient cohort in England and Wales

ObjectivesMost research on health inequalities uses aggregated deprivation scores assigned to the small area where the patient lives; however, the concordance between aggregate area-level deprivation measures and personal deprivation experienced by individuals living in the area is poorly understood. Our objective was to examine the agreement between individual and ecological deprivation. We tested the concordance between metrics of income, occupation and education at individual and area levels, and assessed the reliability of area-based deprivation measures to predict individual deprivation circumstances.SettingEngland and WalesParticipantsA cancer patient cohort of 9,547 individuals extracted from the ONS Longitudinal Study.OutcomesWe quantified the concordance between measures of income, occupation and education at individual and area level. In addition, we used ROC curves and the area under the curve (AUC) to assess the reliability of area-based deprivation measures to predict individual deprivation circumstances.ResultsWe found low concordance between individual and area-level indicators of deprivation (Cramer’s V statistics range between 0.07 and 0.20). The most commonly used indicator in health inequalities research, area-based income deprivation, was a poor predictor of individual income status (AUC between 0.56 and 0.59), whereas education and occupation were slightly better predictors (AUC between 0.62 and 0.65). The results were consistent across sexes and across six major cancer types.ConclusionsOur results indicate that ecological deprivation measures capture only part of the relationship between deprivation and health outcomes, especially with respect to income measurement. This has important implications for our understanding of the relationship between deprivation and health, and, as a consequence, healthcare policy. The results have a wide-reaching impact for the way in which we measure and monitor inequalities, and in turn, fund and organise current UK healthcare policy aimed at reducing them