RSS 1.0Not Refereed
Browse by
Recent Submissions
-
Deconvolving the complex structure of the asteroid beltThe asteroid belt is a unique source of information on some of the most important questions facing solar system science. These questions include the sizes, numbers, types and orbital distributions of the planetesimals that formed the planets, and the identification of those asteroids that are the sources of meteorites and near-Earth asteroids. Answering these questions requires an understanding of the dynamical evolution of the asteroid belt, but this evolution is governed by a complex interplay of mechanisms that include catastrophic disruption, orbital evolution driven by Yarkovsky radiation forces, and chaotic orbital evolution driven by gravitational forces. While the timescales of these loss mechanisms have been calculated using estimates of some critical parameters that include the thermal properties, strengths and mean densities of the asteroids, we argue here that the uncertainties in these parameters are so large that deconvolution of the structure of the asteroid belt must be guided primarily by observational constraints. We argue that observations of the inner asteroid belt indicate that the size-frequency distribution is not close to the equilibrium distribution postulated by Dohnanyi (<xref rid=ref10 ref-type=bibr>1969</xref>). We also discuss the correlations observed between the sizes and the orbital elements of the asteroids. While some of these correlations are significant and informative, others are spurious and may arise from the limitations of the Hierarchical Clustering Method that is currently used to define family membership.
-
The clumped winds of the most massive starsThe core of the cluster R136 in the Large Magellanic Cloud hosts the most massive stars known. The high mass-loss rates of these stars strongly impact their surroundings, as well as the evolution of the stars themselves. To quantify this impact accurate mass-loss rates are needed, however, uncertainty about the degree of inhomogeneity of the winds (`wind clumping'), makes mass-loss measurements uncertain. We combine optical and ultraviolet HST/STIS spectroscopy of 56 stars in the core of R136 in order to put constraints on the wind structure, improving the accuracy of the mass-loss rate measurements. We find that the winds are highly clumped, and use our measured mass-loss rates to test theoretical predictions. Furthermore we find, for the first time, tentative trends in the wind-structure parameters as a function of mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped than those with lower mass-loss rates.
-
Constraining physical processes in pre-supernovae massive star evolutionWhile we have growing numbers of massive star observations, it remains unclear how efficient the key physical processes such as mass loss, convection and rotation are, or indeed how they impact each other. We reconcile this with detailed stellar evolution models, yet these models have their own drawbacks with necessary assumptions for 3-dimensional processes like rotation which need to be adapted into 1-dimensional models. The implementation of empirical mass-loss prescriptions in stellar evolution codes can lead to the extrapolation of base rates to unconstrained evolutionary stages leading to a range of uncertain fates. In short, there remain many free parameters and physical processes which need to be calibrated in order to align our theory better with upcoming observations. We have tested various processes such as mass loss and internal mixing, including rotational mixing and convective overshooting, against multiple observational constraints such as using eclipsing binaries, the Humphreys-Davidson limit, and the final masses of Wolf-Rayet stars, across a range of metallicities. In fact, we developed a method of disentangling the effects of mixing and mass loss in the `Mass-Luminosity Plane' allowing direct calibration of these processes. In all cases, it is important to note that a combined appreciation for both stellar winds and internal mixing are important to reproduce observations.
-
Predictions for the Maximum Masses of Black Holes below the PI BoundaryWhile the initial discovery of GW150914 resulted in the detection of black holes larger than initially expected, it was the GW190521 event which truly challenged astrophysical assumptions about stellar evolution and black hole progenitors, as the components of GW190521 were firmly within the traditional Pair-Instability (PI) mass-gap – a range of masses where no black holes were expected to be created due to PI supernovae (PISN). We investigate the possibility that this merger involved first generation black holes, and that the unexpectedly heavy 85 solar mass BH could be produced from fundamental stellar physics. We present the results of studies involving the stellar evolution code MESA, as we systematically vary several parameters of stellar physics (in particular mixing and mass loss) to test assumptions and build a population of potential black hole progenitors within the traditional PI gap.
-
A resolved view of the impact of massive star formation in the atomic, molecular and ionized gas in the Carina NebulaThe Carina Nebula Complex (CNC) is a spectacular star-forming region located at 2.3 kpc, which is close enough to observe different size scales in detail. With more than 65 O-stars and more than 900 young stellar objects identified it is also the nearest analogue of more extreme star forming regions, such as 30 Doradus. In this talk I will present the results of a major effort to study the relationship between the different gas phases in the Carina region from 100 pc to 0.01 pc using the Australia Telescope Compact Array (ATCA), the Mopra telescope and ALMA. At large scales, CO image combined with far-infrared data from Herschel revealed the overall molecular mass and its distribution across the CNC (Rebolledo et al. 2016). An extremely detailed map of the HI 21-cm line across the whole nebula revealed a complex filamentary structure in the atomic gas, which allowed the identification of regions where phase transition between atomic and molecular gas is happening (Rebolledo et al. 2017). An ATCA 1-3 GHz radio continuum image across the whole Carina region revealed a complete and spectacular view of the ionized gas in the region (Rebolledo et al. 2021). At small scales, ALMA high spatial resolution observations of molecular line tracers and dust showed that the level of stellar feedback effectively influences the fragmentation process in clumps, and provides further evidence for a higher level of turbulence in the material with a higher level of massive stellar feedback (Rebolledo et al. 2020).
-
Pre- and protostellar cores in the 200 brightest Planck compact sourcesThe Planck compact source catalogue provides excellent samples for studying the earliest phases of star formation. Covering all galactic longitudes and latitudes they can give an overview of how star formation varies throughout the Milky Way, enabling a better insight into star formation out of the Galactic plane, where our current understanding is restricted mostly to nearby clouds. We examined the 200 brightest Planck compact sources visible from the northern hemisphere using observations from the James Clerk Maxwell Telescope's SCUBA2 bolometer array. Its high resolution revealed diverse, mostly filamentary structures, and allowed the extraction of point sources from the maps. We classified more than 1500 of these point sources and compiled a catalogue with their positions, sizes, and physical parameters. In our statistical analysis, we investigate properties of star-forming regions at different latitudes, aiming to better understand the flow of interstellar matter in the Galaxy and thus refine our latest view of the Milky Way.
-
How to make an 85 Solar Mass Black HoleWe present in-progress resolution test and parameter space studies for very massive stars using MESA, showcasing current MESA version convergence studies.
-
On the Z-(in)dependence of the Humphreys-Davidson LimitThe temperature independent part of the Humphreys-Davidson (HD) limit sets the boundary for evolutionary channels of massive stars that either end their lives as red supergiants (RSGs) or as the hotter blue supergiants (BSGs) and Wolf-Rayet stars. Recent downward revision of most luminous RSGs the Galaxy below log(L / L<SUB>⊙</SUB>) ≈ 5.5, more in line with the Magellanic Clouds, might hint towards a metallicity (Z)-independent HD limit. We present MESA single star models in the 15-40 M<SUB>⊙</SUB> range and study the different Z-dependent processes that could potentially affect the location of the upper luminosity limit of RSGs.
-
Mass loss implementation and temperature evolution of very massive starsVery massive stars (VMS) dominate the physics of young clusters due to their extreme stellar winds. The mass lost by these stars in their winds determine their evolution, chemical yields and their end fates. In this contribution we study the main-sequence evolution of VMS with a new mass-loss recipe that switches from optically-thin O star winds to optically-thick Wolf-Rayet type winds through the model independent transition mass loss.
-
Near-Infrared [Fe II] and H<SUB>2</SUB> Study of the Galactic Supernova RemnantsWe have searched for near-infrared (NIR) [Fe II] (1.644 μm) and H<SUB>2</SUB> 1-0 S(1) (2.122 μm) emission features associated with Galactic supernova remnants (SNRs) using the narrow-band imaging surveys UWIFE / UWISH2 (UKIRT Widefield Infrared Survey for [Fe II] / H<SUB>2</SUB>). Both surveys cover about 180 square degrees of the first Galactic quadrant (7° < l < 65° -1.3° < b < +1.3°), and a total of 79 SNRs are falling in the survey area. We have found 19 [Fe II]- and 19 H<SUB>2</SUB>-emitting SNRs, giving a detection rate of 24%. Eleven SNRs show both emission features. Some of the SNRs show bright, complex, and interesting structures that have never been reported in previous studies. The brightest SNR in the both emission is W49B, contributing ~70% of the total [Fe II] luminosity of the detected SNRs. The total [Fe II] luminosity, however, is considerably less than what we would expect from the SN rate of our Galaxy.Among the SNRs showing both [Fe II] and H<SUB>2</SUB> emission lines, some SNRs show the “[Fe II]-H<SUB>2</SUB> reversal” phenomenon, i.e., the H<SUB>2</SUB> emission features are detected outside the [Fe II] emission boundary. We carried out high resolution (R~40,000) NIR H- and K-band spectroscopy of the five SNRs showing the [Fe II]-H<SUB>2</SUB> reversal (G11.2-0.3, KES 73, W44, 3C 396, W49B) using IGRINS (Immersion GRating INfrared Spectrograph). Various ro-vibrational H<SUB>2</SUB> lines have been detected, which are used to derive the kinematic distances to the SNRs and to investigate the origin of the H<SUB>2</SUB> emission. The detected H<SUB>2</SUB> lines show broad line width (> 10 km s<SUP>-1</SUP>) and line flux ratios of thermal excitation. We discuss the origin of the extended H<SUB>2</SUB> emission features beyond the the [Fe II] emission boundary.
-
Highly Excited Molecular Hydogren in Shocked Molecular Gas: Line Emission from Newly Reformed H<SUB>2</SUB>?We report high sensitivity K-band spectra of the Herbig-Haro 7 bow shock and selected portions of the energetic outflow in the Orion Molecular Cloud, each a location of bright line emission by shocked molecular hydrogen (H<SUB>2</SUB>). Among the many detected emission lines of H<SUB>2</SUB> are some from highly excited ro-vibrational levels, with upper state energies as high as the dissociation limit near 50,000 K, much higher than previously observed. In all cases the H<SUB>2</SUB> level populations are well fit by a two-temperature model with the vast majority of the H<SUB>2</SUB> at a temperature near 2,000 K but with one to a few percent of the H<SUB>2</SUB> at a temperature near 5,000 K. The existence of the latter H<SUB>2</SUB> appears to be broadly consistent with it having recently reformed following collisional dissociation by the shock; however, the well-defined kinetic temperature of 5,000 K is surprising. The presence of such high temperature H<SUB>2</SUB> appears to be a common characteristic of shock-excited molecular gas.
-
Infrared Line Emission in the Herbig-Haro 7 Bowshock from 5,000 K Molecular Hydrogen in Quasi-bound StatesPrevious spectroscopy of shock-excited H2 line emission in the Herbig-Haro 7 (HH7) bow shock and in the Orion Molecular Cloud (OMC-1) outflow has demonstrated that, although the vast majority of the line emission in those regions comes from gas at T ≈ 2,000 K, as expected for continuous shocks, 1-2% of the H2 is in LTE at a temperature of ~5,000 K. The existence of this latter component of the H2 is not explained by current shock models. One H2 line detected in HH7 is emitted from the quasi-bound v=2 J=29 level, whose energy is ~700 K above the dissociation limit of the ground state of H2. New calculations of the properties of quasi-bound H2 levels have led to the identification of a previously unidentified line in the HH7 spectrum as originating in a second quasi-bound state, v=1, J=31, ~1500 K above the dissociation limit. The existence of this 5,000 K gas and the confirmation that shock-excited H2 in molecular clouds exists in the above highly excited states offer new opportunities to study and more fully understand shock-excitation in molecular clouds.
-
ISM studies towards several PWNePulsar wind Nebulae (PWNe) represent the largest population of TeV sources detected by H.E.S.S. Here, we present a ISM study combining 7mm Mopra observations with Nanten CO(1-0) and SGPS Hi data to understand the ISM environment towards HESS J1809-193, HESS J1026-582 and HESS J1826-130, as well as the morphology of the TeV source gamma-ray emission. Combining the TeV emission with several extended dense regions will then be useful to quantify the distinguish the contribution from PWNe and SNRs towards various HESS TeV sources.
-
The Of?p Stars of the Magellanic Clouds: Are They Strongly Magnetic?All known Galactic Of?p stars have been shown to host strong, organized, magnetic fields. Recently, five Of?p stars have been discovered in the Magellanic Clouds. They posses photometric tep{Naze} and spectroscopic tep{Walborn} variability compatible with the Oblique Rotator Model (ORM). However, their magnetic fields have yet to be directly detected. We have developed an algorithm allowing for the synthesis of photometric observables based on the Analytic Dynamical Magnetosphere (ADM) model of tet{Owocki}. We apply our model to OGLE photometry in order to constrain their magnetic geometries and surface dipole strengths. We predict that the field strengths for some of these candidate extra-Galactic magnetic stars may be within the detection limits of the FORS2 instrument.
-
Fifty Years of Observations of Grw+70(°) 8247Among white dwarfs, Grw+70$^\circ$ 8247 stands as a particularly interesting object for various reasons. It is the first white dwarf in which a magnetic field was discovered. It is one of the stars with the longest rotation period (probably between 100 and 1000 yr). It shows one of the most unusual optical spectra ever seen because spectral lines are produced in presence of one of the strongest magnetic fields ever measured in the universe ($\sim 400$ MG). Most importantly, this star has been the subject of a number of scientific articles co-authored by John Landstreet, from the one reporting the discovery of its magnetic field in 1970, to a very recent paper which presents new spectropolarimetric observations obtained from the William Herschel Telescope. Here I will show how the polarisation spectra of the star have changed during John's career, and what we have learnt from the observations of this star.
-
Thomas Richard (Dick) Carson (1930-2019)Simon Jeffery recalls a dedicated colleague whose work made stellar opacities and pulsations central to models of stellar evolution.
-
Armagh Observatory's meridian marksJohn Butler surveys the extant and vanished meridian marks of Armagh Observatory and explains their significance for 18th and 19th century observations.
-
Starquakes spring stellar surprisesDon Kurtz, Simon Jeffrey and Conny Aerts describe discoveries in the new era of precision asteroseismology.
-
Where past meets futureIreland's historic astronomical observatories at Armagh, Birr and Dunsink are embarking on the process of seeking UNESCO World Heritage Status. Michael Burton examines the deep astronomical heritage that they embody.
