Lunch Talk (2019)

Hung<br> Ashton: This talk combines two projects that uses the Outer Solar System Origins Survey (OSSOS) dataset to search for unusual small bodies. The first search looked for objects beyond 300 au using an uncommon search technique, we use an algorithm to identify objects that appear stationary on the timescale of hours but not present days/weeks/months before and after. After visually scanning all the candidates no slow moving objects were discovered. From the null detection and using a survey simulator, we obtain a model-dependent 95% upper limit of 1000 on the number of `planetary objects', with absolute magnitudes in the range -3 < H < 2, in the distant Solar System. In the second search we looked for interstellar objects (IS0s) using a slightly altered version of the TNO detection software used by OSSOS. No ISOs were discovered. However, we have found bound objects traveling at motions (when observed only over short arcs) that an ISO in the outer Solar System could have, and we show how previous moving-object surveys may have detected ISOs but incorrectly assigned them to bound orbits.

In cosmological simulations, it is found that the degree of fragmentation of the Population III (Pop III) protostellar disk varies strongly from halo to halo. This is to be expected if turbulence within the cloud plays a role in determining when and where fragmentation actually occurs. It is also reasonable to expect that the amount of angular momentum present at small-scales will have a large impact on Population III protostellar disk formation and evolution. We conducted a numerical study with the Voronoi moving-mesh AREPO in which we examined how different levels of initial subsonic turbulence and rotation within the star-forming cloud affect the outcome of Pop III star formation in terms of the total number of objects formed, the overall mass spectrum and the individual accretion history. We used controlled initial conditions which examine the collapse of a gas cloud with pre-defined levels of turbulence or rotation within a small computational box. This allowed us to draw conclusions on the specific role, strength and importance of the particular physical parameters applied here. In this talk, I will present some of our results and discuss the relevance of our numerical approach in which we have performed several realizations per combination of initial conditions and have averaged over the realizations instead of just considering single, individual runs.

Galaxy clusters are the largest gravitaionally bound and virialized objects in the Universe. They are continuously growing through mergers between smaller galaxy clusters. RXJ 1347.5 - 1145 (z = 0.451) is one of the most luminous X-ray galaxy clusters, which means that this cluster is one of the most massive galaxy clusters. Our team carried out the ALMA observation of the Sunyaev-Zel'dovich effect (SZE) on this cluster. This observation enable us to look into the SZE image of the core of galaxy clusters owing to the angular resolution of ALMA. Combining the high-resolution data of Chandra X-ray, we find that the residual X-ray image after removing the global emission shows a clear dipolar pattern characteristic of gas sloshing, whereas we find no significant residual in the SZE image. We then estimate the equation of state of perturbations in the gas from the X-ray and SZE residual images. The inferred velocity is 420 +310/-420 km/s, which is much lower than the adiabatic sound speed of the intracluster medium in the core. We thus conclude that the perturbation is nearly isobaric, and the gas sloshing motion is consistent with being in pressure equilibrium. I will present this result and mention about the other feature in the X-ray and the SZE images of RXJ 1347.5 - 1145.

In this work we explore the long-term dynamical evolution of a bias-free orbital representation of the Kuiper belt, the so-called L7 synthetic model from CFEPS, under the gravitational influence of the Sun and the four giant planets, as well as of the 34 largest known TNOs, those with visual absolute magnitude H_V<4, among which are the four objects currently classified as dwarf planets by the IAU (Pluto, Eris, Haumea, and Makemake); however, in this work we indistinctively call Dwarf Planets (DPs) to all the objects in our 34 members set. Over 1 Gyr time-scales, we analyzed the secular influence of the DPs over Kuiper belt particles and their contribution to the injection rate of new visible Jupiter Family Comets (JFCs), as they make their way from the Scattering, Resonant, and Classical populations to the inner Solar System. We found that DPs effectively increase the number of new visible JFCs originating from the Classical and Resonant populations in almost 20%,when compared with the number of comets produced by the giant planets alone. For the Scattering population the increment is marginal but noticeable. Given the rate of escapes from the Kuiper belt enhanced by DPs, 20% less objects in the Classical and Resonant populations are required to supply the number of JFCs currently observed. Considering recent estimates of the injection rate of new comets required to maintain the population of JFCs in steady state, the maximum numbers of 2 to 10 km sized objects required to supply the 100% of new JFCs are ∼5.7×10^7,∼9.6×10^7, and∼2.8×10^8 for the Scattering, Resonant, and Classical populations, respectively. By comparing with the number of objects in the same size range expected to be present on each of these populations from recent cratering record predictions on the surface of 2014 MU69, we provide some estimates of the fractional contribution of each population to new visible JFCs. Additionally, we found that the Plutinos are the most important source of comets originally coming from a resonant configuration, where the presence of Pluto and the large plutinos Orcus, 2003 AZ84, and Ixion, are important in increasing the number of unstable 3:2 resonators. On the other hand, the twotinos and the populations in the 5:3 and 5:2 mean motion resonances with Neptune all supply a comparable amount of new visible JFCs.

Planetary nebulae, the descendants of low- and intermediate-mass stars, have characteristic onion-like ionization structure, with the highest ionization species closer to the central star. This is true for all planetary nebulae, but HuBi 1, which shows an inverted ionization inner shell. There is a reason for this oddity, a peculiar stellar evolution of its central star, which makes HuBi 1 the missing link of the population of cool C-rich central stars of planetary nebulae.

GPUs have been rising in popularity in scientific computing, but what programming them actually involves? I will introduce you to the basic practical methods of GPU programming using Nvidia CUDA API - with some reflections on the lessons learned during the continuing development of the Astaroth code for computing MHD problems with high-order finite differences.

When a black hole accretes plasmas at a very low accretion rate, an advection-dominated accretion flow (ADAF) is formed. In an ADAF, relativistic electrons emit soft gamma-rays via Bremsstrahlung. Some portion of these MeV photons collide with each other to materialize as electron-positron pairs in the magnetosphere. Such pairs efficiently screen the electric field along the magnetic field lines, when the accretion rate is typically greater than 0.03-0.3% of the Eddington rate. However, when the accretion rate becomes smaller than this value, the number density of the created pairs becomes less than the rotationally induced Goldreich-Julian density. In such a charge-starved magnetosphere, an electric field arises along the magnetic field lines to accelerate charged leptons into ultra-relativistic energies, leading to an efficient GeV and TeV emissions via the curvature and the inverse-Compton processes, spending a fraction of the extracted hole's rotational energy. In the talk, I apply this black-hole gap model to stellar-mass and super-massive black holes and demonstrate that TeV emission is detectable from galactic X-ray binaries during their quiescent state and nearby low-luminosity active galactic nuclei with the Cherenkov Telescope Array, a very near-future gamma-ray observational facility for international joint use.

X-ray spectroscopy techniques provide a powerful method to study the properties of interstellar dust (ID). The extinction features in the spectra of these sources provide information about the crystallinity, chemical composition of the grains and the grain size distribution (Rogantini 2018, Zeegers 2017). The X-ray band is especially suitable to study silicates - one of the main components of ID -since it contains the absorption edges of Si, Mg, O and Fe. The X-rays also provide the advantage that gas and dust can be studied simultaneously and, in this way, provide accurate measurements of the elemental abundance and dust to gas ratio in a variety of regions in the Galaxy. In order to derive the dust properties, it is necessary to acquire a database of detailed extinction cross sections models, that reflects the contents of the dust in the interstellar medium. We present the extinction profiles of a set of newly acquired measurements of 15 dust analogues at the Soleil Synchrotron facility in Paris, where we focus on silicates and the Si-K edge in particular, which is modelled with unprecedented accuracy. Here we also present our results of 9 different lines of sight toward the Galactic plane and give a detailed mapping of the chemical properties of the dust, unveiling the dust nature toward the central region of the Galaxy.

Haumea’s collisional family of water-ice-rich objects is the only minor planet family yet known beyond Neptune. These objects have a common parent body, the dwarf planet Haumea, identified in the Kuiper belt. The Haumea family members have solar colors and a limited range of semi-major axes (a), eccentricities (e) and inclinations (i), unless modified by resonance with Neptune. They have flat phase curves with high albedo surfaces and strong water ice spectral features. A compelling explanation for these surface properties is that the Haumea family members are fragments of Haumea’s icy mantle, ejected during a collision approximately 3.5±2 Gyr ago. The mass and number of fragments of different sizes in Haumea’s family constrain its origin. Here we report the discovery of 1-3 new Haumea family members in the Outer Solar Systems Origins Survey (OSSOS) Ensemble Surveys and determine that the family contains 450(+720,-390) members with absolute magnitudes H < 9.5, ejection velocities ∆v <160 m s−1, and an implied ejected mass from the formation event of 3% the mass of Haumea, suggesting a graze and merge formation scenario and excluding a catastrophic collision

Since the Event Horizon Telescope press release on April 10, the image of the black hole shadow has received a lot of public attention, thanks to the various broadcast, written, online and social media. We will share some of the interesting and light-hearted articles, memes and anecdotes that have appeared, demonstrating how this iconic image has impacted popular culture in just a little more than a week. We also delve into some of the darker and more unfortunate effects this public attention has had on people and science in general.

Simulations generally show that non-self-gravitating clouds have a lognormal column density probability distribution function (PDF), while self-gravitating clouds with active star formation develop a distinct power-law tail at high column density. The transition point in the column density PDF between the lognormal and the power-law portion defines the boundary beyond which the self-gravity becomes dynamically important. In this talk, I will discuss the effect of strong magnetic fields and turbulence on the evolution of the column density PDFs. I have used three-dimensional magnetohydrodynamic (MHD) simulations with non-ideal effects to study different initial conditions and the way they affect the shape of the PDF. Our results are interesting, including a distinct signature on how a combination of strong magnetic fields and turbulence influence the power-law slope and the transitional column density between the lognormal and the power-law portion of the column density PDFs.

Modern cosmological simulations successfully demonstrate that the hierarchical assembly of dark matter halos provided the gravitational wells that nurse the primordial gases to form stars and galaxies inside them. One of the most significant objects in the universe, the first galaxies contained the first groups of stars residing in the dark matter halos are naturally regarded as the fundation of early Universe. To understand the formation of the first galaxies, we use an adaptive mesh refinement (AMR) cosmological code, ENZO to simulate the formation and evolution of the first galaxies. We first model an isolated galaxy by considering much microphysics such as gas dynamics, self-gravity, star formation, stellar feedback, and primordial gas cooling. To examine the effect of Pop III supernovae feedback to the first galaxy formation, we set up the initial temperature, density, and metallicity distributions of Pop III supernova bubbles by assuming different initial PopIII IMF. Our results suggest that star formation in the first galaxies is sensitive to yields and energetics of the first supernovae. Therefore, our study can provide a channel to correlate the populations of the first stars and supernovae to star formation inside these first galaxies which may be soon observed by the James Webb Space Telescopes.

In the last decade, a population of planetary-mass companions on ultra-wide orbits (PMCs; M < 20 Jupiter masses, orbit > 100 AU) has been discovered in direct imaging surveys. Their existence is surprising and challenges star/planet formation theories. Probing the accretion disks of young PMCs may provide a new avenue for understanding the origin of PMCs since these disks regulate angular momentum and entropy as PMCs grow. In this talk, I will summarize our ALMA Bands 6 and 7 observations of PMC disks.

The spectroscopic measurement of stellar mass using stellar evolutionary models is considered reliable for main-sequence stars, but shows significant deviations for pre-main-sequence stars when using different models, due to different underlying assumptions in the models. The aim of our work is to obtain stellar mass measurements of young stellar objects, independent from evolutionary models, with a method based on the Keplerian rotation in circumstellar disks, and compare these against the spectroscopically determined stellar masses. Our method uses the Keplerian velocity pattern in a protoplanetary disk to align its molecular line spectra at different positions in the disk and stack them. The enhanced S/N ratio in the resulting stacked spectrum allows measurements of the rotational velocity to derive the stellar mass even for faint sources with short integration time. We analysed the 13CO, C18O (2-1) and (3-2) and CN data of 32 young stars in the Lupus Clouds obtained with ALMA with integration time of approximately one minute per source using this method. We obtained the mass of 31 stars and found that for a star in the mass range of 0.6 ≤ M* ≤ 1.2 Msun, the stellar mass tends to be underestimated by 20-50% using evolutionary models.

Gas accretion plays a crucial role in galaxy formation and evolution. Indeed, the growth in the stellar masses of galaxies over cosmic time occurs mainly through the accretion of gas from the environment, and no local star-forming galaxies can continue forming stars without replenishing their gas. As the inflow brings metal-poor gas into a galaxy, it lowers the metallicity in the local ISM, meaning that the metallicity distribution is a signpost to recent gas accretion. Therefore, we searched for regions with anomalously low metallicity in the IFU survey MaNGA. I will present how such regions are distributed within mergers, close pairs, and isolated galaxies, and how gas accretion happens in these systems. I will also discuss the lifetime of such regions, the balance between gas accretion and star formation rates, and the implication for galaxy evolution.

Physical processes including gravity, magnetic fields, and turbulence drive the dynamical and the chemical evolution during star formation. Magnetic fields regulate the transportation of angular momentum, determining the efficiency of infall and the growth of disks. While several numerical simulations investigate the dynamical evolution from starless cores to disk formation with different treatments of magnetic fields, observations of the kinematics of starless and protostellar cores are often challenging due to the low temperature and complex velocity structure, respectively. Thus, direct measurements of kinematics at different scales of protostellar envelopes will test models of the star and disk formation. The most direct probe of infall is the redshifted absorption against the central continuum source from optically thick emission, such as HCO+ and HCN. Our ALMA observations show such redshifted absorption toward an isolated embedded protostar, BHR 71, indicative of infall. We use 3D radiative transfer (LIME & Hyperion) to model both the line profiles and the SED to constrain the infalling envelope. The serendipitous discovery of the emission of complex organic molecules (COMs) reveals the “hot corino” nature of BHR 71. The emission of COMs shows clear signatures of rotation, tracing the kinematics at 50–100 AU and hinting an unresolved disk with an outer radius of 14 AU. While the chemical abundance of molecules is the major uncertainty for interpreting the underlying kinematics, observations of multiple transitions of the same molecule along with resolved emission of COMs can empirically trace the kinematics as a function of radius down to the disk-forming region to constrain the model of infall and disk formation.

This is a public talk I gave at Helsinki last month. I will briefly explain a few basic information about black hole, and then explain why we saw the black hole in M87, but not in the Milky Way. Since this talk was for the general public, I did not use any equations, so everyone is welcome to join, if you are interested in.

The discovery of a binary neutron star merger (NSM) through both its gravitational wave and electromagnetic emission has revealed these events to be key sites of r-process nucleosynthesis. However, whether NSMs are truly the dominant sites producing r-process elements to account for the measured abundances in the solar system and in metal-poor stars, and what isotopes do NSMs exactly produce are still undergoing debates. In this talk, I will discuss our recent work which demonstrates that it can be possible to detect the nuclear decay gamma-ray lines from the last ~O(10) NSM remnants in the Milky Way, if the next-generation gamma-ray telescopes can reach a line-sensitivity down to ~ 10^-8 to 10^-6 photons per centimeter square per second. I will also discuss possible implications from such detection to the above debating issues.

The dynamical interactions between a multiple system of protostars and the protoplanetary disk around them can trigger eccentricity growth and warping of the disk. GW Ori is a hierarchical triple system at a distance of 400 parsec. Two of the stars compose a spectroscopic binary with a separation of ~1 AU, and a tertiary component was detected by near-infrared interferometry at a projected distance of ~8 AU. The system harbours a circumtriple protoplanetary disk, one of the only few of its kind, with dust extending to ~400 AU, and gas extending to ~1300 AU. Here we present ALMA observations of the dust continuum and molecular gas emission of the circumtriple disk. We discover three dust rings in the disk, which have different inclinations and centroids. We also find a twist in the CO first moment map. These observations strongly indicate that the GW Ori circumtriple disk has misaligned and eccentric rings, and we interpret them as evidence of ongoing dynamical interactions between the triple stars and the circumtriple disk. In addition, the observations hint at the presence of a companion at ~120 AU, which opens a deep gap and effectively breaks the gas disk at that radius.

Through a large set of numerical N-body simulations of the Solar System, we have explored the long term dynamical evolution (up to 1 Gyr) of the 34 largest objects in the Kuiper belt, those with absolute magnitude, H, less than 4, as listed in the Minor Planet Center as of late 2018. Included in our set are the four trans-Neptunian objects currently classified as “dwarf planets” by the International Astronomical Union (namely Pluto, Eris, Haumea, and Makemake), however, throughout our work, we call all the 34 massive trans-Neptunian objects in our set dwarf planets (DPs). All of our simulations included the Sun (with the added mass of all the interior planets, the Moon and Ceres), the four giant planets, from Jupiter to Neptune, as well as the 34 DPs as massive objects. We explored the regular and chaotic behavior of the DPs. We found that most DPs are regular and well behaved (i.e. non-chaotic). Nevertheless, some interesting cases arise, the most notable one being Haumea, which evolves in a strongly chaotic manner. Some irregular behaviors can be attributed to poorly constrained orbits, as is the case for the recently discovered dwarf planet candidate 2015 RR245, while others seem to be located in highly unstable regions, as is the case of Haumea and 2007 OR10, and to a lesser extent Orcus and 2007 UK126.

The main phase of stellar growth may be characterized by prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, until recently studies of accretion variability had been restricted to the end stages of accretion. Our long-term East Asia Observatory JCMT/SCUBA2 sub-mm monitoring program of eight nearby star forming regions, the first large sub-mm variability program, is measuring accretion variability of protostars to understand the physics of the disk instabilities that drive this variability. To date, we have identified that ~10% of embedded sources as variable at the ~5%/year level, including one quasi-periodic variable. We have also detected what may be the brightest radio flare ever detected from a cool star. I will discuss our results and possible future directions for this science.

he detection of galactic outflows is ubiquitous among starburst galaxies. These galaxies provide ideal condition to frequently produce supernovae over the course of their starburst period, which eject masses and energies out of their galactic disks. In this talk, I will explain how we can study the properties of galactic outflows using both analytical models and numerical simulation. I will compare the results from both approaches and discuss the advantages and disadvantages of each approach. I will then summarise the talk by exploring the importance of galactic outflows for studying and understanding our Universe.

The previous SMA 0.87 mm and CARMA 1.3 mm surveys of dust polarization toward protostars (Galametz et al. 2018; Hull et al. 2017) have shown that polarization percentage in general decreases rapidly with total intensity. They also hinted with a dichotomy that the projected polarization E-segments are either parallel or perpendicular to the outflow axes. Based on the JVLA 18-48 GHz and ALMA 230 and 345 GHz polarization observations towards the specific Class 0 YSO, NGC1333 IRAS4A, we conjectured a unified interpretation of these facts based on aligned dust grains with B-field. At 345 GHz, we found that the radial polarization percentage profiles of IRAS4A1 and IRAS4A2 may be consistent with centrally illuminated collapsing cores, which have an hourglass shaped B-field topology, r^-2 volume density profile, and an approximately consistent grain alignment efficiency. The modestly high optical depth leads to the depolarization at the center of IRAS4A2. The much higher optical depth at the center of IRAS4A1 eventually leads to a 90 degree flip of polarization position angles at 230 and 345 GHz with respect to those observed at 18-48 GHz. At 230 and 345 GHz, what we detected were the linear polarization due to dust extinction instead of dust emission. We conclude that the 230/345 GHz polarization observations towards Class 0 YSOs indeed trace B-field, although any dichotomy related to how E-field is aligned with certain prefer axes (e.g., outflow, filament) may be related to the optical thickness of the selected samples.

Astronomy on Tap (AoT) started in 2014 in New York City by Meg Schwamb with the goal of bringing astronomy research to the general public. The events include a mix of talks, astronomy news updates, quizzes, and a chance for the audience to ask questions of professional astronomers. We launched our monthly English Astronomy on Tap event in March 2019, and have also had two events in Chinese. These events have been a great way of connecting informally with members of the public who are interested in Astronomy. Astronomy on Tap Taipei is organized by Sascha Zeegers, Rosemary Pike, Jonty Marshall, and Paula Granados and 酒吧裡的天文學 is organized by Wen-Ping Lo, and other PhD students. We rely on ASIAA and visiting astronomers to volunteer to give short talks at our events, and the speakers can choose anything astronomy related for their talk. We will discuss the Astronomy on Tap idea and give a shortened version of a typical AoT event.

In the conventional picture of a core-collapse supernova, the iron core collapses into a neutron star or a black hole, and a shockwave unbinds the star. In rare cases, accretion onto a rapidly rotating black hole acts as an "engine" that launches a jet. If that jet tunnels through the star, breaks out, and is pointed at the Earth, we detect a long-duration gamma-ray burst (GRB). There have been thousands of GRBs discovered, almost always by high-energy satellites. However, recent discoveries by optical surveys hint at diverse outcomes that are invisible to GRB satellites, such as jets that are choked inside the star. With the Zwicky Transient Facility (ZTF) we are conducting a systematic exploration of the broader landscape of engine-driven explosions, of which traditional GRBs are just one manifestation. I will show how unexpected arrivals to the landscape (such as AT2018cow) complicate the picture, revealing that some engine-driven explosions take place in dense circumstellar material that was likely ejected in eruptive pre-explosion mass-loss episodes.

Young star clusters (YSCs) with resolved stellar populations are well suited objects for studying and constraining star and star-cluster formation. In most cases, the (pre-main-sequence) stellar populations found in the YSCs are largely coeval with an intrinsic age spread of up to 1 Myr. Such observations can be understood as the YSCs having formed in one burst and star formation having been truncated by stellar feedback. The observed presence of an age spread of up to several Myr, reported for some YSCs, may be explainable as arising from older stars being captured in the forming YSC. The recent discovery by Beccari et al. (2017) that the colour-magnitude diagram of the Orion Nebula Cluster (ONC) contains three well defined populations appears to shatter this model. The only currently viable explanation is that the three sequences are stars with an age difference of about 1Myr. The implication is that the ONC formed in three bursts, with star formation still on-going in the last burst. I will present new observational results focusing on the three populations in the ONC and the binary content of the ONC using OmegaCAM photometry in combination with Gaia DR2 position and proper motion data. Then I will introduce a theoretical model which may explain these observations by an interplay between stellar feedback and cluster dynamics. I will conclude with putting the multiple populations in the ONC into the general observational context of YSCs. The final part of my talk will emphasise the role of gas filaments in star-formation and present the new discovery of stellar relic filaments that are promising probes of later stages of filamentary star-formation.

Understanding various demographic issues is one useful way to improve the working environment and create a motivational atmosphere, which ultimately help improve the work efficiency. In this session I plan to discuss the results of a recent study that tracks a group of astronomy and astrophysics PhD graduates from the US programs for a bit more than a decade, which provides some insights into the current situations about jobs and the gender gap. In light of this study I would like to share some thoughts about ways to make the best use of the (post-)PhD career and also hope to trigger discussions about the issues of gender and careeer in the local astronomical community in Taiwan.

The ALMA proposal review process is one of the most critical functions carried out by the JAO, as it sets the observing program that will be carried out on ALMA. The JAO is committed to making the proposal review process as fair and transparent as possible to ensure that scientific merit is the primary determinant of proposal rank. In this talk, I will analyze the results of the proposal review process in Cycles 0-7 to search for potential biases. Proposal rankings are analyzed with respect to the experience level of a Principal Investigator (PI) in submitting ALMA proposals, regional affiliation (Chile, East Asia, Europe, North America, or Other), and gender. The systematics in the rankings are identified and potential causes will be discussed. I will also discuss upcoming changes in the proposal review process. To further reduce biases in the proposal review process, ALMA will implement dual-anonymous review in Cycle 8 where the reviewers will no longer know the identities of the investigator teams. Finally, I will discuss possible changes for the proposal review process in future cycles. This will be an opportunity to provide input on how to evolve the proposal review process.

In future galaxy surveys such as the PFS survey using the Subaru Telescope, we will be able to measure not only the density fluctuation but also the velocity field up to very large scales. In order to see if the velocity information can be used to extract cosmological information, we measure the velocity correlation function of halos from N-body simulations compare it to the theoretical prediction. We find non-negligible discrepancies between them even on baryon acoustic oscillation scales due to the finite observational volume effect. We are now in the process to fit this discrepancy to understand the observational result and explain its physical meaning.