Damage mechanics of human cortical bone

Skeletal fragility is an important orthopedic concern including the prevention of osteoporosis, long-term stability of prosthetic implants and stress fractures. Damage in human cortical bone has been implicated as a cause of increased fragility and is thought to initiate bone remodeling. Therefore, characterization of the mechanisms of damage initiation and accumulation in bone is challenging not only from the engineering prospective but also has a potential of revealing new insights in its physiology. The main objectives of this dissertation work were to study the early stages of damage development in human cortical bone and develop a constitutive formulation describing damage behavior.;Laser Scanning Confocal Microscopy was utilized to study damage genesis. It was found that confocal microscopy allowed detection of the early stages of damage development within the lacunae-canalicular network of cortical bone. Based on those observations and a common knowledge about the operation of mechanosensing cells housed within the tissue it was proposed that damage could initiate bone remodeling much earlier than it is currently believed.;In the second portion of this work the evolutionary properties of chosen damage parameters were investigated under 3-point bending loading. The parameters of interest were the stiffness drop and the permanent strain. It was found that shear stresses play profound role in the failure behavior. It was also shown that damage parameter defined as the stiffness drop after successive cycles is a quadratic function of nonlinear strain. In addition a linear relationship was obtained between the permanent and total strain.;The third portion of this work was concerned with the development and validation of a constitutive model for cortical bone based on the continuum damage mechanics. It was demonstrated that bone is a linear viscoelastic material for stress levels below a threshold value. Beyond the threshold it behaves as a viscoelastic damaging material. Subsequently a coupled viscoelasticity-damage formulation was adopted and a model was derived based on thermodynamics of irreversible processes. The model was simplified for one-dimensional uniaxial case and experiments were performed for the model verification

High Resolution Near-Infrared Spectroscopy of FUors and FUor-like stars

We present new high resolution (R=18,000) near-infrared spectroscopic observations of a sample of classical FU Orionis stars (FUors) and other young stars with FUor characteristics that are sources of Herbig-Haro flows. Spectra are presented for the region 2.203 - 2.236 microns which is rich in absorption lines sensitive to both effective temperatures and surface gravities of stars. Both FUors and FUor-like stars show numerous broad and weak unidentified spectral features in this region. Spectra of the 2.280 - 2.300 micron region are also presented, with the 2.2935 micron v=2-0 CO absorption bandhead being clearly the strongest feature seen in the spectra all FUors and Fuor-like stars. A cross-correlation analysis shows that FUor and FUor-like spectra in the 2.203 - 2.236 micron region are not consistent with late-type dwarfs, giants, nor embedded protostars. The cross-correlations also show that the observed FUor-like Herbig-Haro energy sources have spectra that are substantively similar to those of FUors. Both object groups also have similar near-infrared colors. The large line widths and double-peaked nature of the spectra of the FUor-like stars are consistent with the established accretion disk model for FUors, also consistent with their near-infrared colors. It appears that young stars with FUor-like characteristics may be more common than projected from the relatively few known classical FUors.Comment: 21 pages, 4 figures, accepted by The Astronomical Journa

New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS

We carried out a survey of infrared star clusters and stellar groups on the 2MASS J, H and K_s all-sky release Atlas in the Northern and Equatorial Milky Way (350 < l < 360, 0 < l < 230). The search in this zone complements that in the Southern Milky Way (Dutra et al. 2003a). The method concentrates efforts on the directions of known optical and radio nebulae. The present study provides 167 new infrared clusters, stellar groups and candidates. Combining the two studies for the whole Milky Way, 346 infrared clusters, stellar groups and candidates were discovered, whereas 315 objects were previously known. They constitute an important new sample for future detailed studies.Comment: Accepted to Astronomy and Astrophysic

High-resolution polarimetry of Parsamian 21: revealing the structure of an edge-on FU Ori disc

We present the first high spatial resolution near-infrared direct and polarimetric observations of Parsamian 21, obtained with the VLT/NACO instrument. We complemented these measurements with archival infrared observations, such as HST/WFPC2 imaging, HST/NICMOS polarimetry, Spitzer IRAC and MIPS photometry, Spitzer IRS spectroscopy as well as ISO photometry. Our main conclusions are the following: (1) we argue that Parsamian 21 is probably an FU Orionis-type object; (2) Parsamian 21 is not associated with any rich cluster of young stars; (3) our measurements reveal a circumstellar envelope, a polar cavity and an edge-on disc; the disc seems to be geometrically flat and extends from approximately 48 to 360 AU from the star; (4) the SED can be reproduced with a simple model of a circumstellar disc and an envelope; (5) within the framework of an evolutionary sequence of FUors proposed by Green et al. (2006) and Quanz et al. (2007), Parsamian 21 can be classified as an intermediate-aged object.Comment: Accepted for publication in the MNRAS. 16 pages, 18 figures and 5 table

Optical spectroscopy of EX Lupi during quiescence and outburst: Infall, wind, and dynamics in the accretion flow

We explore the accretion mechanisms in EX Lupi, prototype of EXor variables, during its quiescence and outburst phases. We analyse high-resolution optical spectra taken before, during, and after its 2008 outburst. In quiescence and outburst, the star presents many permitted emission lines, including typical CTTS lines and numerous neutral and ionized metallic lines. During the outburst, the number of emission lines increases to over a thousand, with narrow plus broad component structure (NC+BC). The BC profile is highly variable on short timescales (24-72h). An active chromosphere can explain the metallic lines in quiescence and the outburst NC. The dynamics of the BC line profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform accretion column that suffers velocity variations along the line-of-sight on timescales of days. Assuming Keplerian rotation, the emitting region would be located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity profiles of the lines reveal a combination of rotation and infall. Line ratios of ions and neutrals can be reproduced with a temperature of T~6500 K for electron densities of a few times 10$^{12}$cm$^{-3}$ in the line-emitting region. The data confirm that the 2008 outburst was an episode of increased accretion, albeit much stronger than previous EX Lupi and typical EXors outbursts. The line profiles are consistent with the infall/rotation of a non-axisymmetric structure that could be produced by clumpy accretion during the outburst phase. A strong inner disk wind appears in the epochs of higher accretion. The rapid recovery of the system after the outburst and the similarity between the pre-outburst and post-outburst states suggest that the accretion channels are similar during the whole period, and only the accretion rate varies, providing a superb environment for studying the accretion processes.Comment: 15 pages plus 26 pages online material, accepted by A&

Near infrared imaging of NGC2316

In the present paper we present JHK photometric results of the young embedded cluster NGC 2316. We construct the cluster radial profile from which we determine a radius of 0.63 pc. We find 189 $\pm$ 29 cluster members in an extinction limited sub-sample of the survey, 22 $\pm$ 19 of which are possibly substellar. An average extinction of 4.5 visual magnitudes is derived using (H-K) colours of control fields. This extinction is due to the presence of residual parental molecular cloud. NGC 2316 presents 16% source fraction of excess emission which is consistent with other results from clusters with an age of 2-3 Myr. This age is consistent with the distribution of sources in the colour-magnitude diagram when compared to theoretical isochrones, and the overall shape of the cluster KLF. The substellar population of the cluster is similar or smaller than that observed for other embedded clusters and the stellar objects dominate the cluster membership.Comment: 4 pages, 5 figures, accepted for publication in Astronomy & Astrophysics Letters. Full resolution images and paper available from http://www-cfa.harvard.edu/~pteixeir/NGC2316

Near-Infrared Variability in the Orion Nebula Cluster

Using the United Kingdom Infrared Telescope on Mauna Kea, we have carried out a new near-infrared J, H, K monitoring survey of almost a square degree of the star-forming Orion Nebula Cluster with observations on 120 nights over three observing seasons, spanning a total of 894 days. We monitored ~15,000 stars down to J=20 using the WFCAM instrument, and have extracted 1203 significantly variable stars from our data. By studying variability in young stellar objects (YSOs) in the H-K, K color-magnitude diagram, we are able to distinguish between physical mechanisms of variability. Many variables show color behavior indicating either dust-extinction or disk/accretion activity, but we find that when monitored for longer periods of time, a number of stars shift between these two variability mechanisms. Further, we show that the intrinsic timescale of disk/accretion variability in young stars is longer than that of dust-extinction variability. We confirm that variability amplitude is statistically correlated with evolutionary class in all bands and colors. Our investigations of these 1203 variables have revealed 73 periodic AA Tau type variables, many large-amplitude and long-period (P > 15 day) YSOs, including three stars showing widely-spaced periodic brightening events consistent with circumbinary disk activity, and four new eclipsing binaries. These phenomena and others indicate the activity of long-term disk/accretion variability processes taking place in young stars. We have made the light curves and associated data for these 1203 variables available online.Comment: Corrected typo in author nam