Kyung-Won Suh

Pulsation Phase-Dependent Dust Shell Models for Oxygen-rich Asymptotic Giant Branch Stars*

We investigate the spectral energy distributions (SEDs) of oxygen-rich asymptotic giant branch (AGB) stars at different pulsation phases using infrared observational data, including data from the Infrared Space Observatory. Comparing the results of detailed radiative transfer model calculations with observations, we explore the changes of the relevant parameters of the dust shells and central stars depending on the pulsation phase. We find that the schemes of the SED changes for low mass-loss rate O-rich AGB (LMOA) stars are quite different from those for high mass-loss rate O-rich AGB (HMOA) stars. For LMOA stars, we find that the dust formation temperature is much lower than 1000 K, the stronger stellar winds produce more dust grains, and the dust shell optical depth increases at the maximum phase. For HMOA stars, the deep silicate absorption features show significant variations depending on the pulsation phase, mainly due to changes in the properties of the dust shells. Considering the dust formation and evaporation processes in HMOA stars, we propose three possible dust models to explain the SED changes. Contrary to previous investigations, we find that the models that do not require dust evaporation at the maximum phase produce SEDs similar to the observations.

Infrared two-colour diagrams for AGB stars using AKARI, MSX, IRAS and NIR data

Using a revised version of the catalogue of asymptotic giant branch (AGB) stars by Suh & Kwon, we present various infrared two-colour diagrams (2CDs) for 3003 O-rich, 1168 C-rich, 362 S-type and 35 silicate carbon stars in our Galaxy. For each object in the new catalogue, we cross-identify the AKARI, Midcourse Space Experiment (MSX) and Two-Micron All-Sky Survey (2MASS) counterparts by finding the nearest object from the position information in the Infrared Astronomical Satellite (IRAS) Point Source Catalog (PSC). For the large sample of AGB stars, we present infrared two-colour diagrams using IRAS (PSC), AKARI (PSC and bright-source catalogue (BSC)), MSX (PSC) and near-infrared (K and L bands, including 2MASS data in the KS band) data for different classes of AGB stars based on the chemistry of the dust shell and/or the central star. The infrared two-colour diagrams (2CDs) of AGB stars can provide useful information about the structure and evolution of the dust envelopes as well as the central stars. On the 2CDs, we plot tracks of the theoretical radiative transfer model results with increasing dust-shell optical depths. Comparing the observations with the theoretical models on the new 2CDs, we find that the basic model tracks roughly coincide with the densely populated observed points. Generally, we can explain the observations of O-rich and C-rich AGB stars on the various 2CDs with theoretical models using dust opacity functions of amorphous silicate, amorphous carbon, SiC and corundum. For O-rich AGB stars, we find that the models using corundum as well as silicate can improve the fit to the observations.

Infrared two-colour diagrams for AGB stars using AKARI, MSX, IRAS and near-infrared data

Using a revised version of the catalogue of asymptotic giant branch (AGB) stars by Suh & Kwon, we present various infrared two-colour diagrams (2CDs) for 3003 O-rich, 1168 C-rich, 362 S-type and 35 silicate carbon stars in our Galaxy. For each object in the new catalogue, we cross-identify the AKARI, Midcourse Space Experiment (MSX) and Two-Micron All-Sky Survey (2MASS) counterparts by finding the nearest object from the position information in the Infrared Astronomical Satellite (IRAS) Point Source Catalog (PSC). For the large sample of AGB stars, we present infrared two-colour diagrams using IRAS (PSC), AKARI (PSC and bright-source catalogue (BSC)), MSX (PSC) and near-infrared (K and L bands, including 2MASS data in the KS band) data for different classes of AGB stars based on the chemistry of the dust shell and/or the central star. The infrared two-colour diagrams (2CDs) of AGB stars can provide useful information about the structure and evolution of the dust envelopes as well as the central stars. On the 2CDs, we plot tracks of the theoretical radiative transfer model results with increasing dust-shell optical depths. Comparing the observations with the theoretical models on the new 2CDs, we find that the basic model tracks roughly coincide with the densely populated observed points. Generally, we can explain the observations of O-rich and C-rich AGB stars on the various 2CDs with theoretical models using dust opacity functions of amorphous silicate, amorphous carbon, SiC and corundum. For O-rich AGB stars, we find that the models using corundum as well as silicate can improve the fit to the observations.

WATER ICE IN HIGH MASS-LOSS RATE OH/IR STARS

We investigate water-ice features in spectral energy distributions (SEDs) of high mass-loss rate OH/IR stars. We use a radiative transfer code which can consider multiple components of dust shells to make model calculations for various dust species including water ice in the OH/IR stars. We find that the model SEDs are sensitively dependent on the location of the water-ice dust shell. For two sample stars (OH 127.8+0.0 and OH 26.5+0.6), we compare the detailed model results with the infrared observational data including the spectral data from the Infrared Space Observatory (ISO). For the two sample stars, we reproduce the crystalline water-ice features (absorption at 3.1 μm and 11.5 μm; emission at 44 and 62 μm) observed by ISO using a separate component of the water-ice dust shell that condensed at about 84-87 K (r ~ 1500-1800 AU) as well as the silicate dust shell that condensed at about 1000 K (r ~ 19-25 AU). For a sample of 1533 OH/IR stars, we present infrared two-color diagrams (2CDs) using the Infrared Astronomical Satellite and AKARI data compared with theoretical model results. We find that the theoretical models clearly show the effects of the crystalline water-ice features (absorption at 11.5 μm and emission at 62 μm) on the 2CDs.

Modeling IR spectra of OH/IR stars at different phases ?

We investigate the spectral energy distributions (SEDs) of OH/IR stars (OH127.8+0.0 and OH26.5+0.6) having thick dust envelopes at different pulsation phases. Using new infrared observational data including the Infrared Space Observatory (ISO) data, we determine the new pulsation parameters. The deep silicate absorption features show significant variations depending on the pulsation phase. The variations are mainly due to changes in the properties of dust envelopes around the OH/IR stars. Comparing the results of detailed radiative model calculations with observations, we explore the changes of the relevant parameters of the envelopes and central stars depending on the pulsation phase. We find that when the central luminosity increases from the minimum to maximum phase, the inner radius of the dust shell increases with velocity faster than the outer shell expansion velocity and the dust shell optical depth decreases. During the phase change from the minimum to maximum, we find that dust formation ceases and about a half of the dust grains in the volume difference should have evaporated. During the phase change from the maximum to minimum, we find that the dust formation should be enhanced because the inner radius is decreasing. In the outer radii of the dust shell, the constant dust winds are easily maintained. We expect that the dust evaporation process driven by pulsation could be a mechanism for crystallizing the dust grains in inner regions of the dust shells around OH/IR stars.

PROPERTIES OF OH, SIO, AND H2O MASER EMISSION IN O-RICH AGB STARS

We investigate the properties of OH, SiO, and H2O maser emission in O-rich AGB stars. We use a sample of 3373 objects, which is an updated version of the list of O-rich AGB stars presented in Suh & Kwon (2011). We divide the 3373 O-rich AGB stars into four different groups based on the maser emission: OH maser sources (1533), SiO sources (1627), H₂O sources (452), and sources with no maser (610). To understand the nature of the maser sources, we present various infrared two-color diagrams (2CDs) using IRAS, 2MASS, and AKARI data. For each group, we compare the positions on various infrared 2CDs with theoretical models. We find that the OH maser sources generally show larger color indices and larger dust optical depths than SiO or H₂O sources. We suggest that the differences of the color indices for different maser sources are due to different mass-loss rates and dust formation processes.

A CATALOG OF AGB STARS IN IRAS PSC

We make a new catalog of AGB stars in our Galaxy from the sources listed in the Infrared Astronomical Satellite (IRAS) point source catalog (PSC) compiling the lists of previous works with verifying processes. We verify the class identification of AGB stars into oxygen-rich and carbon-rich stars using the information from recent investigations. For the large sample of AGB stars, we present infrared two-color diagrams from the observations at near infrared bands and IRAS PSC. On the two-color diagrams, we plot the tracks of theoretical radiative transfer model results with increasing dust shell optical depths. Comparing the observations with the theoretical tracks, we discuss the meaning of the infrared two-color diagrams.

DUST AROUND HERBIG AE/BE STARS

We model dust around Herbig Ae/Be stars using a radiative transfer model for multiple isothermal circumstellar dust shells to reproduce the multiple broad peaks in their spectral energy distributions (SEDs). Using the opacity functions for various types of dust grains at different temperatures, we calculate the radiative transfer model SEDs for multiple dust shells. For eight sample stars, we compare the model results with the observed SEDs including the Infrared Space Observatory (ISO) and AKARI data. We present model parameters for the best fit model SEDs that would be helpful to understand the overall structure of dust envelopes around Herbig Ae/Be stars. We find that at least four separate dust components are required to reproduce the observed SEDs. For all the sample stars, two innermost dust components (a hot component of 1000-1500 K and a warm component of 300-600 K) with amorphous silicate and carbon grains are needed. Crystalline dust grains (corundum, forsterite, olivine, and water ice) are needed for some objects. Some crystalline dust grains exist in cold regions as well as in hot inner shells.

[REVIEW] ASTROPHYSICS OF DUSTY STELLAR WINDS FROM AGB STARS

The main site of dust formation is believed to be the cool envelopes around AGB stars. Nearly all AGB stars can be identified as long-period variables (LPVs) with large amplitude pulsation. Shock waves produce by the strong pulsation and radiation pressure on newly formed dust grains drive dusty stellar winds with high mass-loss rates. IR observations of AGB stars identify various dust species in different physical conditions. Radio observations of gas phase materials are helpful to understand the overall properties of the stellar winds. In this paper, we review (i) classification of AGB stars; (ii) IR two-color diagrams of AGB stars; (iii) pulsation of AGB stars; (iv) dust around AGB stars including dusty stellar winds; (v) dust envelopes around AGB stars; (vi) mass-loss and evolution of AGB stars; and (vii) contribution of AGB dust to galactic environments. We discuss various observational evidences and their theoretical interpretations.

A NEW CATALOG OF SILICATE CARBON STARS

A silicate carbon star is a carbon star which shows circumstellar silicate dust features. We col-lect a sample of 44 silicate carbon stars from the literature and investigate the validity of the classification. For some objects, it is uncertain whether the central star is a carbon star. We confirm that 29 objects are verified silicate carbon stars. We classify the confirmed objects into three subclasses based on the evolution phase of the central star. To investigate the effect of the chemical transition phase from O to C, we use the radiative transfer models for the detached silicate dust shells. The spectral energy distributions and the infrared two-color diagrams of the silicate carbon stars are compared with the theoretical model results. For the chemical transition model without considering the effect of a disk, we find that the life time of the silicate feature is about 50 to 400 years depending on the initial dust optical depth.