KVN and VERA Array (KaVA)

Large Programs

ESTEMA (Expanded Study on Stellar Masers)

Imai, H.(hiroimai@sci.kagoshima-u.ac.jp), Cho, S.-H.(cho@kasi.re.kr), Asaki, Y., Choi, Y. K., Kim. J. H., Yun, Y. J., Dodson, R. Rioja, M., M. Kino, Oyama,T., Yoon, S.-C., Yoon, D.-H., Kim, D., Oyadomari, M., Burns, R. A., Orosz, G., Nakagawa, A., Chibueze, J. O., Sobolev, A. M., and Nakashima, J.

Proposal ID: EAVN19A-04, EAVN19B-108


ESTEMA aims to publish a database of the largest sample of VLBI images of circumstellar H2O and SiO (43GHz, optionally 86GHz) masers around 80 evolved stars (AGB and post-AGB stars). It enables statistical analyses of the masers among different types of evolved stars observed at a variety of stellar pulsation phases, from microscopic (individual maser spots) to macroscopic (circumstellar envelope, CSE) views.

ESTEMA utilizes the full capabilities of the KaVA system to adopt the techniques of VERA's dual beam astrometry and KVN's source-frequency phase-referencing, in which for each star the relative locations and distributions of different maser lines are directly compared.

Therefore, it enables us to analyze dependence of maser pumping mechanisms on stellar types and pulsation phases, and evolution of asymmetries in stellar mass-loss found through biased H2O maser spatial-kinematic structures in the CSE with respect to the SiO maser location, pinpointing the central star. These information will lead us to understand the basic properties of circumstellar H2O and SiO masers as probes of the physical conditions and dynamics of CSEs.

The ESTEMA outputs will become a legacy reference for planning long-term intensive monitoring campaigns of evolved stars exhibiting high mass-loss rates, with current and forthcoming large radio astronomy facilities including KaVA (extended to EAVN) and ALMA.

Scientific Background

Stars play a major role in the cycle of material and the evolution of atomic abundance in the universe through their formation and copious mass ejection in the final stellar evolution.

There are two major issues in the mass loss process: acceleration of the material ejected from the central star into the circumstellar envelope(CSE), and asymmetry in the stellar mass ejection and the CSE outflows.

While acceleration of dust produced in a C-rich environment has been well explained by radiative pressure from the star, that in an Orich environment has been in debate.

Circumstellar SiO and H2O masers as good probes of the physical conditions and dynamics of CSEs because they are located at the innermost part and the acceleration zone of the CSEs, respectively.

The maser source structures observed in VLBI enable us to reveal three-dimensional velocityfield with high angular resolution. Since maser features are expected to trace giant gas/dust clumps, which would be produced by the convecting cells and in which dust condensation will start, the maser spatiokinematics will have direct link to the important issues mentioned above.

Thanks to VLBI observations, circumstellar masers have revealed the spatio-kinematics of CSEs, become candles for determining trigonometric parallax distances, and determined physical parameters of the stars.

However, there are still ambiguities in the suggestion about the physical conditions of the maser regions and the pumping mechanisms of the masers.

A CSE is a good laboratory of astronomical masers because it is basically an isolated system in the interstellar space and its structure is basically expressed as functions of the distance from the central star. For example, it is still in debate what it reflects physically: periodic passages of shock waves driven by stellar pulsation or periodic change in physical parameters (i.e. temperature) in the maser regions.

This issue will be solved by simultaneously observing the performances of multiple lines of masers: v =0,1,2,3 J = 1→0; 2→1; 3→2 SiO masers at 43, 86, 129 GHz and H2O masers at 22 GHz.

Before we start intensive monitoring of the spatio-kinematics of maser features for a few stellar pulsation cycles (2-10 yr), we can sample the maser data in a short period of VLBI observations from many stars with different stellar pulsation periods and at different pulsation phases.

Scientific Goals
  • ESTEMA aims to find more than 20 stars that will become targets of a future KaVA Large Program, in which each star will be intensively monitored (biweekly - monthly) with KaVA for a few stellar pulsation periods.
  • It will yield the largest sample of CSE masers from 80 stars in VLBI and whose output will be archived in a single database. This is the first statistical analyses of circumstellar H2O and SiO masers with VLBI data.
  • In order to qualitatively and quantitatively interpret the statistical analyses in ESTEMA, we will update the simulation codes of circumstellar SiO masers so as to deal with more maser lines including H2O.
  • Astrometric results from ESTEMA will provide a new list of targets for exploring the period-luminosity relationship of Mira variables with VERA and KaVA.
Observation Plan

ESTEMA was awarded 240 hours in total for observing 80 stars (given in a separate table). We have developed and will adopt an operation design based on the combination of high efficiency observation modes for ESTEMA as shown in Figure.

  1. Snap-shot imaging with KaVA
    The KaVA with baselines of 400-2300 km yields an angular resolution of 1(0.5) mas and can detect maser emission extended up to 10(5) mas at the K-(Q-) band, enabling us to resolve individual maser features and to trace the whole morphology of the maser region.
  2. Source-frequency phase-referencing with KVN (K/Q/W bands)
    The SFPR outputs are used for reliable registration of the KaVA output maps within uncertainty of a few 100 microarcseconds, which may be caused by difference in the prominent structures of the maser spots observed with KVN and KaVA.
  3. Dual-beam astrometry with VERA
    VERA's dual-beam astrometry will measure the absolute coordinates of the masers with an accuracy of ~5 mas.
  4. Ultra-wide band recording with VERA/OCTAVE (optional)
    Ultra-wide band OCTAVE signal recording at a rate of 4 Gbps can cover a total band width of 1 GHz, enabling us to record emission signals of SiO v = 0 and 3 J = 1→0 maser line.
K and Q band in VERA

ESTEMA Source list
Go to the top

Exploring the vicinity of super-massive black hole
with KaVA: Intensive monitoring of M87 and Sgr A*

M. Kino (motoki.kino@nao.ac.jp, NAOJ/Kogakuin Univ.), B.W. Sohn (bwsohn@kasi.re.kr, KASI) on behalf of KaVA AGN Science Working Group*

Proposal ID: EAVN19A-06, EAVN19B-114, EAVN19B-115


Exploring the vicinity of super-massive black holes (SMBHs) is one of the frontiers in astrophysics.

Because of the largeness of angular-sizes of the central SMBHs, Sgr A* is the excellent laboratory for studying gas accretion process onto SMBH and M87 is well known as the best case for investigating plasma outflow ultimately driven by SMBH.

To get better understanding of plasma inflow/outflow physics near SMBHs, here we propose the monitoring programs of Sgr A* and M87.

This program is composed of following three sub-programs, i.e., (i) mapping the jet velocity field in M87 and constraining magnetically-driven-jet paradigm, and (ii) probing the nature accreting plasma onto SMBH by monitoring Sgr A*, and (iii) conducting quasi-simultaneous coherent observations of M87 and Sgr A* with the Event Horizon Telescope (EHT) during its campaign observation periods.

(i) Mapping the jet velocity field (M87)

Understanding the formation mechanism of relativistic jets in active galactic nuclei (AGNs) is one of the holy grails in astrophysics. According to the current leading scenario, AGN jets are driven by magnetic (B) force, and subsequently accelerated through the progressive conversion of the magnetic energy into the kinetic one.

Relativistic MHD models have indicated a slow conversion of magnetic energy into kinetic one in relativistic jets. The nearby radio galaxy M87 is known as the best target to test this scenario because of its proximity and the largeness black hole. One of the goals of this proposal is mapping the velocity field of the M87 jet for testing “B-driven jet” paradigm.

Although mapping the velocity field of M87 has been intensively explored in previous work, the apparent velocities in literatures are significantly different and it looks controversial. In order to get robust measurement of possible super-luminal motion, sufficiently short interval (~2 weeks) is needed.

Our goal is mapping the velocity field of the M87 jet that can give unique constraints on theoretical models of B-field driven jet. From the end of Feb 2016, we have started the dual frequency (i.e., 22 and 43 GHz) monitoring of M87 aiming for quasi-simultaneous detections of same components both at 22 and 43 GHz, which will guarantee a robust conclusion. In Figure 1, one can see the actual image of M87 with KaVA at 22 GHz.

M87 image at 22GHz obtained by KaVA obtained in 2014.
Figure 1. M87 image at 22GHz obtained by KaVA obtained in 2014.

(ii) Probing the nature of accreting plasma onto SMBH (Sgr A*)

Sagittarius A* (Sgr A*) hosts the SMBH with largest angular size and thus Sgr A* is well known as one of the best laboratories to explore mass accretion process onto SMBHs. The G2 encounter event, in particular, provides us the once-in-a-million chance for probing the nature of accreting plasma onto SMBH at the galactic center. Recently, Kawashima and colleagues predict a possible change of B-field strength in Sgr A* after the G2 encounter (Kawashima et al. in prep). According to their numerical simulation, the evolution of the magnetic fields in accreting plasma in Sgr A* after the G2 encounter depends on the collision angle between the G2 and accreting flow onto the central black hole. The timescale of mass accretion via viscosity can be also estimated as a few years.

Our goal is to conduct the monthly monitoring of Sgr A* at 43 GHz to look for imprints of G2 encounter. The enhancement of magnetic energy predicted by Kawashima et al. directly leads to the increase of synchrotron flux densities detected by KaVA. We will systematically continue to measure the B-field strength during the monitoring with a timescale of several years.

(iii) Coherent observation of M87 and Sgr A* with the EHT

The EHT is a project to assemble a VLBI network of mm wavelength dishes that can resolve general relativistic signatures near a SMBH. The EHT is now resolving the regions with the size less than 10 Rs around the SMBHs of Sgr A* and M87. The central goal of the EHT experiment is to resolve regions of space-time where gravity is dominant and to detect the strong gravitational field signatures in Sgr A* and M87 (http://www.eventhorizontelescope.org).

Our goal is to conduct KaVA observation of Sgr A* and M87 at 43 GHz of simultaneous with the EHT during March-April campaign and maximize our scientific output via synergetic observations.

Actual Observation Plan in 2016 & 2017

For promoting potential collaborations with communities dealing with other frequencies and/or wavelength in the future, here we show the actual observation plan of this Large Program in 2016 and 2017 season. For further details, please contact M. Kino (kino at kasi.re.kr) and B.W. Sohn (bwsohn at kasi.re.kr).

plan of AGN LP w/o EHT(2016)

plan of AGN LP w/ EHT(2017)
Go to the top

Understanding high-mass star formation
through KaVA observations of water and methanol masers

HIROTA, Tomoya (tomoya.hirota@nao.ac.jp), KIM, Kee-Tae (ktkim@kasi.re.kr), BYUN, Do-Young, CHIBUEZE, James, HACHISUKA, Kazuya, KIM, Jeong-Sook, KIM, Jong-Soo, KIM, Mikyoung, LIU, Tie, MATSUMOTO, Naoko, MOTOGI, Kazuhito, OH, Chung Sik, SHINO, Nagisa, SUGIYAMA, Koichiro, SUNADA, Kazuyoshi, WU, Yuanwei, and KaVA Star-Formation Science sub-WG

Proposal ID: EAVN19B-104


KaVA Star-Formation Science sub-WG is planning to conduct a systematic observational study of the 22 GHz water (H2O) and 44 GHz class I methanol (CH3OH) masers in high-mass star-forming regions as a four-year KaVA large program. Our sample consists of approximately 100 high-mass young stellar objects (HM-YSOs) in various evolutionary phases, many of which are associated with two or more maser species. The primary science goal is to understand dynami-cal evolution of HM-YSOs and their circumstellar structures by measuring spatial distributions and 3-dimensional velocity fields of multiple maser species. By combining follow-up obser-vations with VERA (distances), JVN/EAVN (6.7 GHz methanol masers), ALMA and JVLA (thermal lines/continuum), and archive data of infrared catalogs, we will provide novel infor-mation on physical properties (density, temperature, size, mass), 3D dynamical structures of disk/jet/out ow/infalling envelope, and relationship between evolutionary phase of HM-YSOs.

1. Introduction

In spite of their importance in evolution of galaxies and interstellar matters, formation processes of high-mass stars have been long-standing issues. Although significant progress in the recent years has suggested disk accretion process, several practical scenarios are still matter of debate (e.g., turbulent core accretion, competitive accretion, and sometimes, merging may occur). Direct observations of detailed circumstellar structures within 103 AU are definitely essential for addressing evolution of individual HM-YSOs and their feedback history that is closely related to overall star-formation process in host protocluster. Furthermore, source-to-source diversities of jet/out ow, disk, and envelope structures, including time-dependent nature, also contain important suggestions on both of initial condition (angular momentum, magnetic field, etc) and evolutionary process in high-mass star formation.

Strong water and methanol masers are known to be associated with HM-YSOs in a wide range of evolu-tionary phases, and hence, have been employed as good probes of VLBI studies of high-mass star-formation. Multiple maser species are complementary with each other for investigating overall 3D structures and dy-namics around host objects by multi-epoch and multi-species VLBI studies. Such well-compiled and time-resolved VLBI dataset is quite unique in ALMA era, providing us quantitative understanding on evolution of HM-YSOs and their circumstellar structures.

2. Science goals

In this project, we plan to carry out a systematic observational study of 22 GHz water and 44 GHz class I methanol masers toward ~100 HM-YSOs by using KaVA. Multi-epoch VLBI observations will provide proper motions of these two maser lines. We will address key issues in high-mass star formation as summarized below.

  • 3D velocity field and spatial structure around various HM-YSOs associated with maser features to investigate dynamical structures of jet/out ow/disk systems.
  • Short-term variation/recurrence of 3D velocity field in high-velocity jet/out ow systems to reveal their launching and mass accretion processes.
  • Statistical relationship between basic properties of central HM-YSOs and their circumstellar structures traced by masers to understand long-term evolution of HM-YSOs.
3. Observing plan

To achieve the above science goals, we will proceed our project along the following 3 steps:

  • First year: Simple snap-shot imaging to study dynamical structure of each maser statistically and to compile a target source list for the further observations.
  • Second year: Monitoring observations to measure internal proper motions in multiple maser lines.
  • Third year and beyond: Absolute proper motion and parallax measurements in phase-referencing mode.

Observing time will be ~200 hours per year but depending on number of detectable target sources and requested monitoring period/interval which will be determined based on the first year results. As follow-up projects, we have been proposing observing time for other telescopes such as VERA, JVN, ALMA, and JVLA, and obtained part of them (e.g. VERA, ALMA cycle 3 filler).

4. Preliminary results

As preparatory studies, we have started imaging test observations of ~10 HM-YSOs associated with the 44 GHz methanol masers. Figure 1 shows the first VLBI detection of the 44 GHz methanol masers (Matsumoto et al. 2014, ApJL, 789, L1), demonstrating the unique capability of KaVA providing relatively short and dense uv coverage. This is also the first refereed journal paper reporting the KaVA scientific observational results.

An example of 44 GHz methanol maser

Figure 1: An example of 44 GHz methanol maser observations of G18.34-1.78 SW with KaVA (Matsumotoet al. 2014 and references therein). (a) Spectra of the 44 GHz methanol maser. (b) Position of the 44 GHz methanol maser (red symbol) superposed on a 1.2 mm continuum map obtained with the MAMBO array on IRAM 30 m telescope. We will study this kind of M-SFRs hosting different evolutionary phase and/or mass of HM-YSOs by using multiple masers.

Source list

KASI logo image

KaVA ( KVN and VERA Array )