UST Journal Club
21
2023-08
2023-06-14 13:00 ~ 14:00
- Speaker : Jaegun Yoo
- Place : JYS 331-2
- Professor :
Abstract :
Confidence in spacecraft thermal models can be built by tuning their numerous parameters using the results of a thermal-balance test. In such a test, the flight article is placed in a thermal vacuum chamber configured to be as similar to the orbital environment as possible. High power-draw subsystems in the spacecraft are “pulsed” on for a few minutes so that the heat propagation through the system can be measured and conductive values in the model tuned. The thermal model can then be used to make more reliable predictions for the orbital temperatures. The ultimate validation of the model is a comparison of the predictions to the actual on-orbit measured temperatures. This paper describes the procedure, analysis, and results of all of the aforementioned as they apply to the first Miniature X-ray Solar Spectrometer 3U CubeSat. The first Miniature X-ray Solar Spectrometer was deployed from the International Space Station on 16 May 2016 and deorbited on 6 May 2017. Many of the tuned-model parameters are applicable to other CubeSats, and could provide a baseline for programs that do not have the resources to dedicate to detailed thermal modeling and testing. A generally good agreement was found to within a few degrees Celsius, between the thermal model and the actual orbital measurements.
21
2023-08
2023-06-07 13:00 ~ 14:00
- Speaker : Hoang Ngoc Huy Nguyen
- Place : JYS 331-2
- Professor :
Abstract :
Typhoon is one of the major hazards in ocean coastal areas, but traditional techniques are inadequate to monitor typhoons due to limited or high-cost observations, like radio sounding and meteorological radar. Previous studies have found that typhoons can cause ionospheric disturbances, but the relationship and characteristics are still unclear. In this paper, about 400 stations observations of the Global Positioning System (GPS) network in Taiwan are used to extract ionospheric disturbances during multiple typhoons. The detailed characteristics of the ionospheric disturbances are investigated using a fourth-order Butterworth filter following the 2016 Nepartak, 2019 Lekima, 2019 Mitag, and 2020 Hagupit typhoons. The results show that significant ionospheric disturbances were observed during the typhoons, and the larger disturbances are mostly located 400–1200 km far from the typhoon eye. The estimated horizontal propagation velocity of the ionospheric disturbances is about 127–194 m/s. The locations of the ionospheric disturbances between the typhoon eye and the landfall site are related to the typhoon path. The azimuth distribution of the ionospheric disturbance around the typhoon eye is affected by the GPS elevation angles. At 500–700 km from the typhoon eye, the mean ionospheric disturbances are 0.17 TECU (TEC Unit) and 0.15 TECU for super typhoon Nepartak and Lekima, and 0.13 TECU and 0.18 TECU for typhoon Mitag and Hagupit. The higher the intensity of the typhoon is, the greater the magnitude of the ionospheric disturbance is.
21
2023-08
2023-05-24 13:00 ~ 14:00
- Speaker : Madeeha Talha
- Place : JYS 331-2
- Professor :
Abstract :
The Earth’s magnetosphere is the region of space where plasma behavior is dominated by the geomagnetic field. It has a long tail typically extending hundreds of Earth radii (RE) with plentiful open magnetic fluxes threading the magnetopause associated with magnetic reconnection and momentum transfer from the solar wind. The open-flux is greatly reduced when the interplanetary magnetic field points northward, but the extent of the magnetotail remains unknown. Here we report direct observations of an almost complete disappearance of the open-flux polar cap characterized by merging poleward edges of a conjugate horsecollar aurora (HCA) in both hemispheres’ polar ionosphere. The conjugate HCA is generated by particle precipitation due to Kelvin-Helmholtz instability in the dawn and dusk cold dense plasma sheets (CDPS). These CDPS are consist of solar wind plasma captured by a continuous dual-lobe magnetic reconnections, which is further squeezed into the central magnetotail, resulting in a short “calabash-shaped” magnetotail.
21
2023-08
2023-05-17 12:30 ~ 13:30
- Speaker : Nguyen Chau Giang
- Place : JYS 331-2
- Professor :
Abstract :
The polarized dust emission observed in Class 0 protostellar cores at high angular resolution with ALMA has raised several concerns about the grain alignment conditions in these regions. We aim to study the role of the radiation field on the grain alignment mechanisms occurring in the interior (10 micron, which are required to bring the dust grain alignment efficiencies of the synthetic observations up to observed levels. Our radiative transfer calculations show that irradiation plays an important role in the mechanisms that dictate the size range of aligned grains in Class 0 protostars. Regions of the envelope that are preferentially irradiated harbor strong polarized dust emission but can be affected by the rotational disruption of dust grains. Episodes of high luminosity could affect grain alignment and trigger grain disruption mechanisms.
21
2023-08
2023-05-10 12:30 ~ 13:30
- Speaker : Gia Bao Truong Le
- Place : LWC 102
- Professor :
Abstract :
Dust-induced polarization in the interstellar medium (ISM) is due to asymmetric grains aligned with an external reference direction, usually the magnetic field. For both the leading alignment theories, the alignment of the grain's angular momentum with one of its principal axes and the coupling with the magnetic field requires the grain to be paramagnetic. Of the two main components of interstellar dust, silicates are paramagnetic, while carbon dust is diamagnetic. Hence, carbon grains are not expected to align in the ISM. To probe the physics of carbon grain alignment, we have acquired Stratospheric Observatory for Infrared Astronomy/Higch-resolution Airborne Wideband Camera-plus far-infrared photometry and polarimetry of the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10° 216. The dust in such CSEs are fully carbonaceous and thus provide unique laboratories for probing carbon grain alignment. We find a centrosymmetric, radial, polarization pattern, where the polarization fraction is well correlated with the dust temperature. Together with estimates of a low fractional polarization from optical polarization of background stars, we interpret these results to be due to a second-order, direct radiative external alignment of grains without internal alignment. Our results indicate that (pure) carbon dust does not contribute significantly to the observed ISM polarization, consistent with the nondetection of polarization in the 3.4 μm feature due to aliphatic CH bonds on the grain surface.
21
2023-08
2023-05-03 13:00 ~ 14:00
- Speaker : Yeon-ho Choi
- Place : JYS 331-2
- Professor :
Abstract :
Measurements of the atmospheric carbon (C) and oxygen (O) relative to hydrogen (H) in hot Jupiters (relative to their host stars) provide insight into their formation location and subsequent orbital migration1,2. Hot Jupiters that form beyond the major volatile (H2O/CO/CO2) ice lines and subsequently migrate post disk-dissipation are predicted have atmospheric carbon-to-oxygen ratios (C/O) near 1 and subsolar metallicities2, whereas planets that migrate through the disk before dissipation are predicted to be heavily polluted by infalling O-rich icy planetesimals, resulting in C/O
21
2023-08
2023-04-26 13:00 ~ 14:00
- Speaker : Kushal Lodha
- Place : JYS 331-1
- Professor :
Abstract :
The density profiles of dark matter halos are typically modeled using empirical formulas fitted to the density profiles of relaxed halo populations. We present a neural network model that is trained to learn the mapping from the raw density field containing each halo to the dark matter density profile. We show that the model recovers the widely used Navarro-Frenk-White profile out to the virial radius and can additionally describe the variability in the outer profile of the halos. The neural network architecture consists of a supervised encoder-decoder framework, which first compresses the density inputs into a low-dimensional latent representation, and then outputs ρ(r) for any desired value of radius r. The latent representation contains all the information used by the model to predict the density profiles. This allows us to interpret the latent representation by quantifying the mutual information between the representation and the halos’ ground-truth density profiles. A two-dimensional representation is sufficient to accurately model the density profiles up to the virial radius; however, a three-dimensional representation is required to describe the outer profiles beyond the virial radius. The additional dimension in the representation contains information about the infalling material in the outer profiles of dark matter halos, thus discovering the splashback boundary of halos without prior knowledge of the halos’ dynamical history.
21
2023-08
2023-04-05 13:00 ~ 14:00
- Speaker : Hyeon-Woo Jeong
- Place : JYS 331-1
- Professor :
Abstract :
Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree ΠX of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock.
21
2023-08
2023-03-29 12:30 ~ 13:30
- Speaker : Mingyeong Lee
- Place : JYS 331-2
- Professor :
Abstract :
When the temperature and pressure conditions allow it, water ice can deposit as frost on the regolith of planetary surfaces. Frost is an important indicator of the surface physical conditions, and may trigger geological processes by its deposition and sublimation. This works aims to explore, experimentally, the possibility of detecting early stages of frost formation and to characterize its spectrophotometric and spectropolarimetric signatures in visible reflected light. We deposit ice on top of different regolith simulants, measuring the dust temperature, the thickness, and the morphology of the frost through a microscope, while measuring the reflected light at phase angles of 50° and 61°, and the linear polarization at phase angles of 5° and 16°, at three different wavelengths (450, 550, and 750nm). We show that both the spectral slope (in particular between 450–550nm), and the difference of polarization between 450 and 750nm are efficient methods to detect frost layers with thicknesses as low as 10 to 20 µm. Furthermore, we find that the linear polarization at 16° relates to the temperature of the regolith i.e. the type of the deposited ice crystalline structure.
21
2023-08
2023-03-22 13:00 ~ 14:00
- Speaker : William Davison
- Place : JYS 311-2
- Professor :
Abstract :
Measuring time lags between time series or light curves at different wavelengths from a variable or transient source in astronomy is an essential probe of physical mechanisms causing multiwavelength variability. Time lags are typically quantified using discrete correlation functions (DCFs), which are appropriate for linear relationships. However, in variable sources such as X-ray binaries, active galactic nuclei (AGNs), and other accreting systems, the radiative processes and the resulting multiwavelength light curves often have nonlinear relationships. For such systems it is more appropriate to use nonlinear information-theoretic measures of causation such as mutual information, routinely used in other disciplines. We demonstrate with toy models the limitations of using the standard DCF and show improvements when using a discrete mutual information function (DMIF). For nonlinear correlations, the latter accurately and sharply identifies the lag components as opposed to the DCF, which can be erroneous. Following that, we apply the DMIF to the multiwavelength light curves of AGN NGC 4593. We find that X-ray fluxes are leading UVW2 fluxes by ∼0.2 days, closer to model predictions from reprocessing by the accretion disk than the DCF estimate. The uncertainties with the current light curves are too large, though, to rule out negative lags. Additionally, we find another delay component at approximately-1 day, i.e., UVW2 leading x rays, consistent with inward propagating fluctuations in the accretion disk scenario. This is not detected by the DCF. Keeping in mind the nonlinear relation between x ray and UVW2, this is worthy of further theoretical investigation. From both the toy models and real observations, it is clear that the mutual-information-based estimator is highly sensitive to complex nonlinear relations. With sufficiently high temporal resolution and signal-to-noise ratio, we will precisely detect each of the lag features corresponding to these relations.
21
2023-08
2023-03-15 12:30 ~ 13:30
- Speaker : Hyobin Im
- Place : JYS 311-2
- Professor :
Abstract :
The Evolutionary Map of the Universe (EMU) large-area radio continuum survey will detect tens of millions of radio galaxies, giving an opportunity for the detection of previously unknown classes of objects. To maximise the scientific value and make new discoveries, the analysis of this data will need to go beyond simple visual inspection. We propose the coarse-grained complexity, a simple scalar quantity relating to the minimum description length of an image, that can be used to identify unusual structures. The complexity can be computed without reference to the broader sample or existing catalogue data, making the computation efficient on new surveys at very large scales (such as the full EMU survey). We apply our coarse-grained complexity measure to data from the EMU Pilot Survey to detect and confirm anomalous objects in this data set and produce an anomaly catalogue. Rather than work with existing catalogue data using a specific source detection algorithm, we perform a blind scan of the area, computing the complexity using a sliding square aperture. The effectiveness of the complexity measure for identifying anomalous objects is evaluated using crowd-sourced labels generated via the Zooniverse.org platform. We find that the complexity scan identifies unusual sources, such as odd radio circles, by partitioning on complexity. We achieve partitions where 5 per cent of the data is estimated to be 86 per cent complete, and 0.5 per cent is estimated to be 94 per cent pure, with respect to anomalies and use this to produce an anomaly catalogue.
21
2023-08
2023-03-08 13:00 ~ 14:00
- Speaker : Dong-Hyeok Koh
- Place : JYS 311-2
- Professor :
Abstract :
Binary formation is an important aspect of star formation. One possible route for close-in binary formation is disk fragmentation. Recent observations show that small-scale asymmetries (10,500 au). This material connects the outer dense core with the region where asymmetries arise near disk scales. This new structure type, ten times larger than those seen near disk scales, suggests a different interpretation of previous observations: large-scale accretion flows funnel material down to disk scales. These results reveal the under-appreciated importance of the local environment on the formation and evolution of disks in early systems and a possible initial condition for the formation of annular features in young disks.
21
2023-08
2023-03-08 12:00 ~ 13:00
- Speaker : KOH Dong-Hyeok
- Place : JYS 331-1
- Professor :
Binary formation is an important aspect of star formation One possible route for close-in binary formation is disk fragmentation . Recent observations show that small-scale asymmetries ( 10,500 au) . This material connects the outer dense core with the region where asymmetries arise near disk scales. This new structure type, ten times larger than those seen near disk scales, suggests a different interpretation of previous observations: large-scale accretion flows funnel material down to disk scales. These results reveal the under-appreciated importance of the local environment on the formation and evolution of disks in early systems and a possible initial condition for the formation of annular features in young disks.