Jan 10: Dr. Patrick Hartigan
The ultimate goal of the project was to uncover a timeline for disk evolution, assessing e.g. how
disk masses and sizes change with star (= disk) age. I will briefly discuss this second aspect of the
study, and how (again) measuring masses (of the disks now) is a real challenge.
Title: Organizational Meeting
Abstract: The organizational meeting to set speaker schedules.
Jan 24: Dr. Stephane Guilloteau [Univ. Bordeaux, France]
Title: Disks around young stars: Weighing the stars and their disks
Abstract: Astronomers deduce the masses and ages of stars by their positions on Hertzsprung-Russell diagrams (HRDs) relative to models of stellar evolution and their isochrones. This procedure is regarded as reliable for stars on the main sequence but is much more uncertain for young stars. I will report on direct (dynamical) mass measurements of young stars in using the IRAM millimeter array (now NOEMA) and ALMA to attempt to test and validate evolutionary models. Comparison of the dynamical masses with those predicted from other stars characteristics (effective temperature, spectral type and luminosity) by evolutionary models reveal many discrepancies. Potential causes will be briefly discussed. Strategies to improve our knowledge in this area will be presented.
Title: H- Opacity and Water Dissociation in the Dayside Atmosphere of the Very Hot Gas Giant WASP-18 b
Abstract: I will discuss this paper by Arcangeli et al. 2018 which looks at the atmosphere of hot Jupiter WASP-18 b. H- opacity is considered when they model the atmosphere, and they do not detect water.
Title: Effective Data Visualization
Abstract: Research presentations in physics and astronomy often require visualization of complex data in a way that is simultaneously accessible, informative, and appealing. In this presentation, I will review the most salient guidelines for effective visualization and briefly explain some of the reasoning behind them. The presentation will focus on visual cues that you can harness to help the audience better understand your data and will also provide formatting recommendations specific to scientific data presentation.
Title: Observing Galaxy Mergers at the Epoch of Reionization
Abstract: The galaxies with photometric redshifts observed in a close angular proximity might be either projection coincidences, strongly lensed images of the same galaxy, or separate galaxies that are in a stage of merging. We search for the groups of galaxies in the Hubble Ultra Deep Field (HUDF09) in z ~ 7 and z ~ 8 drop-out samples. We find no close pairs among 50 galaxies in the z ~ 7 sample, while in the z ~ 8 sample we find that 6 out of 22 galaxies have a companion within ~1'' (3 pairs). Adopting a numerical simulation and performing forward modeling, we show that even though mergers are unlikely to have such a high fraction, the projection coincidences and the strong lensing are even less likely mechanisms to account for all of three pairs. Alternatively, there is a possibility of the contamination in the drop-out catalog from lower redshifts, which potentially can account for all of the groups. Finally, we make projection on the sensitivity to mergers of the James Webb Space Telescope (JWST), and discuss the possible applications of the high-redshift merging galaxies for decreasing cosmic variance effects on the luminosity function and for improving the accuracy of photometric redshifts in general.
Title: No Large Population of Unbound or Wide-orbit Jupiter-mass Planets
Abstract: Planet formation theories predict that some planets may be ejected from their parent systems as result of dynamical interactions and other processes. Unbound planets can also be formed through gravitational collapse, in a way similar to that in which stars form. A handful of free-floating planetary-mass objects have been discovered by infrared surveys of young stellar clusters and star-forming regions as well as wide-field surveys, but these studies are incomplete for objects below five Jupiter masses. Gravitational microlensing is the only method capable of exploring the entire population of free-floating planets down to Mars-mass objects, because the microlensing signal does not depend on the brightness of the lensing object. A characteristic timescale of microlensing events depends on the mass of the lens: the less massive the lens, the shorter the microlensing event. A previous analysis of 474 microlensing events found an excess of ten very short events (1–2 days)—more than known stellar populations would suggest—indicating the existence of a large population of unbound or wide-orbit Jupiter-mass planets (reported to be almost twice as common as main-sequence stars). These results, however, do not match predictions of planet-formation theories and surveys of young clusters. Here we analyse a sample of microlensing events six times larger than that of ref. 11 discovered during the years 2010–15. Although our survey has very high sensitivity (detection efficiency) to short-timescale (1–2 days) microlensing events, we found no excess of events with timescales in this range, with a 95 per cent upper limit on the frequency of Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star. We detected a few possible ultrashort-timescale events (with timescales of less than half a day), which may indicate the existence of Earth-mass and super-Earth-mass free-floating planets, as predicted by planet-formation theories.
Feb 14: Dr. Patrick Hartigan [Rice]
Title: Observations of Entrainment in the HH 444 Stellar Jet
Abstract: The canonical model of stellar jets is that they are driven by some sort of magnetically-focused wind from accreting protostellar disks. The boundary between the jet and the ambient gas is of particular interest in these models, both from the standpoint of constraining which radii launch winds (with implications for exoplanets), and because any entrained material can affect estimates of mass loss rates. In this talk I will present new spatially-resolved high-resolution spectra of multiple emission lines from HH 444, one of the best examples of this phenomenon. Combining the emission lines together makes it possible to extract the ionization fraction, temperature, and density as a function of the position and radial velocity in the flow. When compared with archival data, there is clear evidence for entrainment along individual bow shocks in the jet.
Title: Analysis of Astronomical Interface Shapes
Abstract: Images of astrophysical nebulae often show convoluted interfaces produced as ultraviolet radiation from massive stars dissociates and ionizes gas within ambient molecular clouds. The shapes of these interfaces contain important information unattainable in any other way, including the presence of fluid dynamical instabilities of a given scale length, and density enhancements on small scales that provide clues as to the mass distribution of dense cores and the presence of turbulence or magnetic fields. This talk will consider some ways to extract characteristic size scales from interface shapes, and will explore a possible new method to address this general problem. This type of analysis will become more important once JWST returns its first infrared images of nebular objects, as these images should resolve the boundaries between clouds and ionized gas with unprecedented clarity.
Feb 21: Dr. Michael Johnson [York Univ.]
Title: Mapping the Universe on the largest scales with the Sunyaev Zel’dovich effect
Abstract: The Sunyaev Zel’dovich (SZ) effect, temperature/polarization anisotropies and spectral distortions in the cosmic microwave background (CMB) induced by scattering from free electrons in the post-reionization Universe, is a primary target of current and future CMB instruments. Increasingly precise measurements of the SZ effect promise to greatly inform our understanding of the distribution of baryons in the cosmos. In addition, when combined with the next generation of galaxy surveys, the SZ effect has enormous potential to improve our understanding of fundamental cosmology. In this talk I will demonstrate how cross correlations between galaxy surveys and low-noise high-resolution observations of the CMB can be used to reconstruct the velocity and density fields on scales which are currently measured using the primary CMB anisotropies. I will then discuss how such a reconstruction could be used to resolve long-standing ‘anomalies' in the primary CMB, constrain the properties of dark energy, and improve constraints on primordial non-gaussianity.
Feb 28: Dr. George Younes [George Washington Univ.]
Title: The Strongest Magnets in the Universe
Abstract: Magnetars represent the most extreme manifestation of the neutron star population. Their seconds-long rotational periods and large spindown rates imply surface dipole magnetic fields up to 1.0E15 G, the strongest ever measured in the Universe (million-billion times that of Earth), and young ages (thousands of years). They are detected mainly in our own Galaxy as bright X-ray emitters with luminosities exceeding their rotational energy losses. Magnetars are the most variable sources in the neutron star zoo on time-scales ranging from milliseconds to years. On milliseconds to seconds time-scale, they show a unique and peculiar bursting behavior of hard X-ray/soft gamma-ray radiation with luminosities reaching upward of 1.0E47 erg/s/cm2. Following these bursting episodes, magnetars enter a period of enhanced high energy radiation where their X-ray luminosities increase by as many as 3 orders of magnitude compared to their quiescent persistent emission. During this time, their rotational properties can show drastic changes either in the form of timing noise or glitches. They recover back to their usual self within months to years. In this talk, I will give an overview of the magnetar field through recent exciting discoveries such as the unexpected magnetar-like behavior of a rotation powered pulsar, the discovery of the first wind nebula around a magnetar, a property normally associated to rotation powered pulsars, and the status of the strongest, most burst prolific of magnetars, SGR 1806-20, 13 and a half years after it emitted the brightest flash ever detected on Earth. Finally, I will conclude by looking at what the future holds for this rapidly growing field of research.
Mar 7: Dr. Mario Flock [JPL]
Title: Closing the gap between simulations and observations of Protoplanetary Disks
Abstract: In this talk I will give an overview over my latest results to compare current observations with latest 3D radiation MHD simulations of turbulent protoplanetary disks. I will focus on the dust thermal emission in protoplanetary disks, covering the outer regions which emit in the sub/mm. I will present comparisons between the expected dust scale height from simulations and observations and how we could use this results to learn about the gas disk dynamics. I will also show the possibility to explain mm polarization by dust grains which are aligned by the magnetic field. Finally I will draw conclusion on the results and discuss about the possibility to observe MRI activity with current telescope facilities.
Mar 21: Dr. Andrew Smith [UMD]
Title: Gamma-Ray Astronomy at the Highest Energies
Abstract: The High Altitude Water Cherenkov (HAWC) Observatory is TeV gamma-ray detector located in the mountains of central Mexico. The HAWC instrument consists of an array of 300 water tanks instrumented with large-area photomultiplier tubes used to detect the secondary particles from showers produced when high-energy particles impact the atmosphere. Using a new and novel technique, the hadronic backgrounds can be efficiently identified and rejected giving HAWC an unprecedented sensitivity to very high energy gamma rays. In this talk, I will review the scientific challenges of TeV gamma-ray astronomy and present results from the first 3 years of the operation of HAWC, including contributions to the understanding numerous galactic and extragalactic gamma-ray sources, observations that shed new light on the search for dark matter, and detail potential for observing gamma-ray transients coincident with gravitational wave events.
Mar 28: Dr. Jamie Tayar [OSU]
Title: Asteroseismic Tests of Stellar Models
Abstract: Stellar models are one of the pillars of astrophysics, impacting our understanding of everything from planets to galaxies. While these models do well for stars similar to the sun, it has become clear that they are inconsistent with measurements of evolved stars. I will discuss recent efforts using a combination of asteroseismology and the APOGEE spectroscopic survey to identify discrepancies and improve the physics of stellar models. I will begin with a discussion of metallicity dependent temperature offsets between models and data, and how to resolve them with metallicity dependent parameterizations of convection. I will then discuss how these models and data sets can be used to improve our understanding of rotation, mixing, and chemical evolution. Finally, I will discuss the prospects for further improvement to our understanding of stellar physics with the upcoming *TESS* mission.
Apr 4: Dr. Jin Koda [Stony Brook Univ.]
Title: Probing the Molecular Interstellar Medium with Radio Interferometers
Abstract: I will explain the concept of radio interferometers and present some results derived with interferometers on how the interstellar medium (ISM) evolves in the Milky Way (MW) and nearby galaxies. In particular, I will focus on the giant concentrations of molecular gas, namely molecular clouds (GMCs). On large scales, the standard textbooks suggest that GMCs form in galactic spiral arms as the atomic gas is compressed through spiral arm passage. Their formation immediately results in a cascade of turbulence from large to small scales and leads to star formation. However, the observations with radio interferometers are altering this standard view. They are detecting GMCs before their entrance to the spiral arms. Instead, small GMCs coagulate and become larger GMCs in spiral arms, which are then fragmented back into smaller GMCs after the spiral arm passages. We now need to understand how star formation is triggered in the pre-existing GMCs (or suppressed in some GMCs). Radio interferometers can also show extremely small structures within GMCs in the MW by measuring molecular absorption against QSOs. Such observations are revealing molecular "droplets" in GMCs, perhaps the structures at the dissipation scale of turbulent energy cascade. I will stress that future studies of ISM evolution should investigate connections between large and small scales to fully understand ISM evolution and star formation.
Title: A Density Cusp of Quiescent X-ray Binaries in the Central Parsec of the Galaxy
Abstract: The existence of a density cusp localized increase in number of stellar-mass black holes near a supermassive black hole is a fundamental prediction of galactic stellar dynamics. The best place to detect such a cusp is in the Galactic Centre, where the nearest supermassive black hole, Sagittarius A*, resides. As many as 20,000 black holes are predicted to settle into the central parsec of the Galaxy as a result of dynamical friction; however, so far no density cusp of black holes has been detected. Low-mass X-ray binary systems that contain a stellar-mass black hole are natural tracers of isolated black holes. Here we report observations of a dozen quiescent X-ray binaries in a density cusp within one parsec of Sagittarius A*. The lower-energy emission spectra that we observed in these binaries is distinct from the higher-energy spectra associated with the population of accreting white dwarfs that dominates the central eight parsecs of the Galaxy. The properties of these X-ray binaries, in particular their spatial distribution and luminosity function, suggest the existence of hundreds of binary systems in the central parsec of the Galaxy and many more isolated black holes. We cannot rule out a contribution to the observed emission from a population (of up to about one-half the number of X-ray binaries) of rotationally powered, millisecond pulsars. The spatial distribution of the binary systems is a relic of their formation history, either in the stellar disk around Sagittarius A* or through in-fall from globular clusters, and constrains the number density of sources in the modelling of gravitational waves from massive stellar remnants, such as neutron stars and black holes.
Title: The Effect of X-ray Dust Scattering on a Bright Burst from the Magnetar 1E 1547.0–5408
Abstract: A bright burst, followed by an X-ray tail lasting ∼10 ks, was detected during an XMM–Newton observation of the magnetar 1E 1547.0−5408 carried out on 2009 February 3. The burst, also observed by Swift/BAT, had a spectrum well fitted by the sum of two blackbodies with temperatures of ∼4 and 10 keV and a fluence in the 0.3–150 keV energy range of ∼10−5 erg cm−2. The X-ray tail had a fluence of ∼4 × 10−8 erg cm−2. Thanks to the knowledge of the distances and relative optical depths of three dust clouds between us and 1E 1547.0−5408, we show that most of the X-rays in the tail can be explained by dust scattering of the burst emission, except for the first ∼20–30 s. We point out that other X-ray tails observed after strong magnetar bursts may contain a non-negligible contribution due to dust scattering.
Apr 18: Jason Ling
Title: A Tale of Two Galaxies - The Light vs. the Dark (Matter) Abstract: Inferred from numerous observational techniques, Dark Matter is estimated to be approximately four times as abundant as regular, baryonic matter in our Universe. Through its gravitational interactions with normal matter, Dark Matter plays a crucial role in galaxy dynamics, interactions, and evolution. In this talk, we will explore curious examples of local Dark Matter content around two unique galaxies. In recently published work, the galaxy known as Dragonfly 44 was estimated to have nearly 98% of its mass content attributed to Dark Matter, while the galaxy NGC1054-DF2 is consistent with absolutely no Dark Matter. I will present these two cases and discuss the implications of such drastically different results.
The ultimate goal of the project was to uncover a timeline for disk evolution, assessing e.g. how disk masses and sizes change with star (= disk) age. I will briefly discuss this second aspect of the study, and how (again) measuring masses (of the disks now) is a real challenge.