Turbulent flow in the convective envelope generates resonant sound waves trapped in the stellar interiors and observed as non-radial oscillations of the Sun and Sun-like stars. Investigating the observed mode frequencies of the oscillations allows us to improve our knowledge on the internal structure and dynamics of the Sun and Sun-like stars. In this contribution comprising two parts, I summarize basics of helioseismology and discuss effects of stellar magnetic activity on the global seismic parameters. We report statistical behaviors of the global seismic parameters and their implications found in recent analysis we carried out recently. We briefly conclude by discussing issues not to be mentioned here.
Astrophysical sources of very-high-energy (VHE) gamma-ray radiation provide unique information about astrophysical particle acceleration
and cosmic-ray production. In particular, compact binary systems, composed of a compact object (a neutron star or black hole) in orbit
with a massive stellar companion, provide an ideal environment for VHE gamma-ray production.
They are not only powerful particle accelerators, but they also exhibit periodic emission that makes them excellent astrophysical laboratories.
However, only a handful of binary systems have ever been observed in VHE gamma rays.
Partly, this is because VHE gamma-ray binaries appear to be very rare, and part is due to observational bias.
Most instruments operating at TeV are pointed and must allocate time to observing many kinds of objects.
The High Altitude Water Cherenkov (HAWC) Observatory, on the other hand, has high uptime (duty cycle >95%)
and a wide field of view (2 sr), making it well-suited for observing transient and time-varying sources such as binaries.
HAWC is also currently the only detector that is sensitive to gamma-ray photons above 10 TeV. SS 433 is a known microquasar
that has two jets ("east" and "west") terminating in radio lobes of a surrounding supernova remnant, W 50.
The recent observation of SS 433 with HAWC marked the first direct evidence of gamma-ray emission from the jets of a microquasar.
Using HAWC data, we have measured a VHE flux of e1 in the east lobe and w1 in the west lobe with a combined
post-trial statistical significance of 5.4 sigma.
From the International Year of Astronomy in 2009 - The Universe Yours to Discover; to IAU 100 Years Celebrations in 2019 - Under One Sky,
the IAU Office for Astronomy Outreach (IAU OAO) the IAU engages the public in astronomy through access to astronomical information
and communication of the science of astronomy.
The IAU Office for Astronomy Outreach work focuses on building bridges between the IAU and the global astronomy community
of outreach professionals, educators, amateur and professional astronomers, and the public.
Through international cooperation, we envision to make astronomy a science that is accessible to all.
Hydrogen- and helium-ionizing photons from stars and quasars can propagate through the intergalactic medium (IGM)
and change the chemical and kinematical properties of IGM through ionization and heating.
This process, the cosmic reionization, is believed to have started with the birth of first stars and ended
when all the gas in the IGM got ionized due to the plethora of ionizing photons.
Continuous percolation of individual H II regions generate H II regions in cosmological scales,
and this enables us to probe the process itself and the collective properties of radiation sources
through observations targeting large-scale phenomena.
We review our own reionization models that include first stars at high redshift, z~30-15,
and show the current constraint on such models made by the Planck (cosmic microwave background: CMB)
and EDGES (Experiment to Detect the Global EoR Signature, on 21-cm).
We also present how improved CMB and 21-cm observations, e.g. LiteBIRD (Lite (Light) satellite for the studies of B-mode polarization
and Inflation from cosmic background Radiation Detection) and SKA (Square Kilometre Array), can further tighten the constraint
on reionization models.
Molecular clouds form out of the surrounding diffuse gas through the conversion of atomic (HI)
to molecular hydrogen (H2), and previous Galactic and extragalactic observations have suggested
that this HI-to-H2 transition is a major bottleneck process toward star formation.
Among two main flavors of HI, the cold and warm neutral medium (CNM and WNM),
the cold component is expected to play a critical role.
For example, the CNM is far more effective at forming and shielding H2 thanks to its higher density.
In addition, numerical simulations have found that CNM structures have physical properties that resemble those
of molecular clouds, implying that the initial conditions for star formation are set before the gas becomes molecular.
Despite its vital importance for the HI-to-H2 transition and ensuing star formation,
however, the CNM in and around molecular clouds remains largely unexplored.
In this talk, I will describe my attempts for the last few years to understand the roles of the CNM in the formation
and evolution of molecular clouds and present some preliminary results from these attempts.
태양계는 46억년의 긴 나이를 가지고 있다. 따라서 현재의 태양계가 수억년 이상 별 변화없이 비슷한 상태를 유지하고 있으리라,
태양계의 천체들도 수십억년 전에 형성되었으리라 생각하기 쉽다.
적어도 태양계의 행성간 공간에 퍼져 있는 티끌의 경우에 이는 확실하게 사실이 아니다.
행성간 티끌은 보통 짧으면 수년, 길어도 수십만년 정도밖에는 태양계에 머무르지 못한다.
그럼에도 태양계에는 현재 행성간 티끌이 풍부하게 존재한다. 이는 결국 태양계의 다양한 천체에서 끊임없이 티끌이 공급되고 있다는 뜻이 된다.
그간의 연구로 티끌을 공급하는 천체도, 티끌이 방출되는 기작도 매우 다양함이 알려졌다.
바꾸어 말하면 행성간 티끌의 연구를 통하여 특정 천체의 활동성(티끌 방출)을 설명하기 위해 제시된 모형이 얼마나 현실적인지,
그렇다면 그 현상은 얼마나 흔한지, 비슷한 천체가 태양계에는 얼마나 어떻게 분포할지를 모두 살펴볼 수 있다.
본 강연에서는 행성간 티끌을 매개로 소행성, 혜성, 해왕성 바깥 천체 등 태양계 소천체의 분포와 진화,
이들이 보이는 활동성을 간략히 설명할 것이며, 행성간 티끌의 기원, 혜성 기원 티끌의 진화,
소행성에서의 티끌 방출 기작에 대한 자체 연구도 간단히 소개하려 한다.
태양은 11년 주기로 다양한 플라즈마 현상들을 보여주고 있다.
극대기의 태양은 대규모의 폭발과 물질분출 현상을 일으켜 지구주변의 환경에 급격한 변화를 초래하지만,
극소기 동안에도 작지만 끊임없는 활동으로 기후 변화 등 장기간에 걸친 지구환경변화에 영향을 준다.
최근, 우리는 다양한 파장의 지상 망원경과 우주 망원경을 통해 태양을 자세히 관측하고 좀 더 잘 이해하게 되었다.
이번 강연에서는 지난 두 차례의 태양주기 동안 수행했던 조용한 태양과 역동적인 태양에 대한 나의 다양한 연구결과들을 정리하고,
새로운 태양주기를 맞아 현재 진행하고 있는 연구를 소개하고자 한다.
우주의 물질 분포를 정확히 측정하는 것은 관측 우주론에서 중요한 요소이다.
이것은 우주 모형과 은하를 포함한 구조 형성 시나리오를 검증하는 데 중요한 도구로 활용될 수 있기 때문이다.
본 강연에서는 외부 은하 탐사를 통해 알게 된 우주 속 우리의 위치(cosmic address)를 먼저 살펴본다.
그리고, 우주의 물질 분포 측정을 위해 진행된 분광 탐사에 대해서 간략히 살펴보고,
국내에서 진행되고 있는 다천체 분광기 개발을 통한 가까운 우주 분광 탐사 계획(K-SPEC)을 소개하고자 한다.
* 특이사항: 이번 강연은 학부생 분들도 쉽게 이해할 수 있도록 준비되었습니다.
Neutral (HI) gas clouds associated with galaxies are responsible for fuelling the star-formation in the universe.
These stars further inject metals back into the neutral gas clouds both within and outside the galaxy.
The circle continues with metals, radiation and dust from stars influencing the neutral to molecular gas (HI-H2) transition
which further influence the next generation of star-formation. Further, gas abundance and kinematics are influenced
by larger factors such as galactic interactions and mergers.
In my talk, I will focus primarily on my study of high column density neutral gas clouds (Extremely strong Damped Lyman-alpha absorbers, or ESDLAs)
that are observed as absorption signature along the line-of-sight (LOS) of a quasar.
I will further look at the HI-H2 transition and interesting results relevant to diffuse molecular gas.
I will also discuss comparisons between low and high star-forming environment and talk about different samples and techniques
that can be used to study them.
I would like to give my intellectual journey that led to the involvements in gravitational wave science.
My PhD thesis was about the dynamical evolution of dense star clusters including the effects of the binaries and stellar collisions.
The black holes are interesting objects in view of the stellar dynamics in many aspects.
The observational evidence for the supermassive black hole was growing rapidly in late 1980s as the high resolution spectroscopic observations became available,
and the advancement of the speckle interferometry toward the Galactic Center.
My research area expanded to the origin of the supermassive black holes and their interactions with surrounding stars.
In 2000, I was invited to a LISA symposium held in Golm, Germany, where I was first exposed to the gravitational wave experiments.
I felt that the gravitational wave would become an important tool for astronomy and formed a gravitational-wave study group in late 2003.
From 2004, the Korean Gravitational Wave Group (KGWG) organized summer schools every year under the sponsorship
of Asian-Pacific Center for Theoretical Physics (APCTP) and Korea Institute for Science and Technology Information (KISTI) focusing on the general relativity
and gravitational waves and educated ourselves on these topics.
In 2009, KGWG became a member of the LIGO Scientific Collaboration.
While working on the LIGO collaboration, I also continued to study the roles of the black holes in star clusters and found that the black hole merger events
could be more abundant than neutron star binary coalescence in LIGO observations.
The gravitational wave community in Korea is growing and KASI now pursues gravitational wave experiments by developing the measurement techniques
that could enhance the sensitivity significantly.
I will close my talk by giving prospects of gravitational wave astrophysics.