Dust Shell Models Research Articles

Infrared Properties of Carbon Stars in Our Galaxy

We explore the characteristics of carbon stars within our Galaxy through a comprehensive analysis of observational data spanning visual and infrared (IR) bands. Leveraging data sets from IRAS, the Infrared Space Observatory, Akari, the Midcourse Space Experiment, the Two Micron All Sky Survey, the Wide-field Infrared Survey Explorer (WISE), Gaia DR3, AAVSO, and the SIMBAD object database, we conduct a detailed comparison between the observational data and theoretical models. To facilitate this comparison, we introduce various IR two-color diagrams (2CDs), IR color–magnitude diagrams (CMDs), and spectral energy distributions (SEDs). We find that the CMDs, which utilize the latest distance and extinction data from Gaia DR3 for a substantial number of carbon stars, are very useful for distinguishing carbon-rich asymptotic giant branch (CAGB) stars from extrinsic carbon stars that are not in the AGB phase. To enhance the accuracy of our analysis, we employ theoretical radiative transfer models for dust shells around CAGB stars. These theoretical dust shell models demonstrate a commendable ability to approximate the observations of CAGB stars across various SEDs, 2CDs, and CMDs. We present the infrared properties of known pulsating variables and explore the infrared variability of the sample stars by analyzing WISE photometric data spanning the last 14 years. Additionally, we present a novel catalog of CAGB stars, offering enhanced reliability and a wealth of additional information.

EmulART: Emulating radiative transfer—a pilot study on autoencoder-based dimensionality reduction for radiative transfer models

Dust is a major component of the interstellar medium. Through scattering, absorption and thermal re-emission, it can profoundly alter astrophysical observations. Models for dust composition and distribution are necessary to better understand and curb their impact on observations. A new approach for serial and computationally inexpensive production of such models is here presented. Traditionally these models are studied with the help of radiative transfer modelling, a critical tool to understand the impact of dust attenuation and reddening on the observed properties of galaxies and active galactic nuclei. Such simulations present, however, an approximately linear computational cost increase with the desired information resolution. Our new efficient model generator proposes a denoising variational autoencoder (or alternatively PCA), for spectral compression, combined with an approximate Bayesian method for spatial inference, to emulate high information radiative transfer models from low information models. For a simple spherical dust shell model with anisotropic illumination, our proposed approach successfully emulates the reference simulation starting from less than 1% of the information. Our emulations of the model at different viewing angles present median residuals below 15% across the spectral dimension and below 48% across spatial and spectral dimensions. EmulART infers estimates for sim 85% of information missing from the input, all within a total running time of around 20 minutes, estimated to be 6times faster than the present target high information resolution simulations, and up to 50times faster when applied to more complicated simulations.

Spatially resolved mid-infrared observations of the circumstellar environment of the born-again object FG Sge

Context. FG Sge has evolved from the hot central star of the young planetary nebula Hen 1–5 to a G–K supergiant in the last 100 yr. It is one of the three born-again objects that have been identified as of yet, and they are considered to have undergone a thermal pulse in the post-asymptotic giant branch evolution. Aims. We present mid-infrared spectro-interferometric observations of FG Sge and probe its dusty environment. Methods. FG Sge was observed with MIDI at the Very Large Telescope Interferometer at baselines of 43 and 46 m between 8 and 13 µm. Results. The circumstellar dust environment of FG Sge was spatially resolved, and the Gaussian fit to the observed visibilities results in a full width at half maximum of ~10.5 mas. The observed mid-infrared visibilities and the spectral energy distribution can be fairly reproduced by optically thick (τV ≈ 8) spherical dust shell models consisting of amorphous carbon with an inner radius rin of ~30 R★ (corresponding to a dust temperature of 1100 ± 100 K). The dust shell is characterized with a steep density profile proportional to r−3.5±0.5 from the inner radius rin to (5–10) × rin, beyond which it changes to r−2. The dust mass is estimated to be ~ 7 × 10−7 M⊙, which translates into an average total mass-loss rate of ~ 9 × 10−6 M⊙ yr−1 as of 2008 with a gas-to-dust ratio of 200 being adopted. In addition, the 8–13 µm spectrum obtained with MIDI with a field of view of 200 mas does not show a signature of the polycyclic aromatic hydrocarbon (PAH) emission, which is in marked contrast to the spectra taken with the Spitzer Space Telescope six and 20 months before the MIDI observations with wide slit widths of 3″.6–10″. This implies that the PAH emission originates from an extended region of the optically thick dust envelope. Conclusions. The dust envelope of FG Sge is much more compact than that of the other born-again stars’ Sakurai’s object and V605 Aql, which might reflect the difference in the evolutionary status. The PAH emission from the extended region of the optically thick dust envelope likely originates from the material ejected before the central star became H-deficient, and it may be excited by the UV radiation from the central star escaping through gaps among dust clumps and/or the bipolar cavity of a disk-like structure.

A New Catalog of Asymptotic Giant Branch Stars in Our Galaxy

Abstract We present a new catalog of 11,209 O-rich asymptotic giant branch (AGB) stars and 7172 C-rich AGB stars in our Galaxy, identifying more AGB stars in the bulge component and considering more visual carbon stars. For each object, we cross-identify the Infrared Astronomical Satellite (IRAS), AKARI, Midcourse Space Experiment, Wide-field Infrared Survey Explorer (WISE), Two-Micron All-Sky Survey, and American Association of Variable Star Observers counterparts. We present the new catalog in two parts: one is based on the IRAS PSC for brighter or more isolated objects; the other one is based on the ALLWISE source catalog for less bright objects or objects in crowded regions. We present various infrared two-color diagrams (2CDs) for the sample stars. We find that the theoretical dust shell models can roughly explain the observations of AGB stars on the various IR 2CDs. We investigate IR properties of SiO and OH maser emission sources in the catalog. For Mira variables in the sample stars, we find that the IR colors get redder for longer pulsation periods. We also study infrared variability of the sample stars using the WISE photometric data in the last 12 yr: the ALLWISE multiepoch data and the Near-Earth Object WISE Reactivation 2021 data release. We generate light curves using the WISE data at W1 and W2 bands and compute the Lomb–Scargle periodograms for all of the sample stars. From the WISE light curves, we have found useful variation parameters for 3710 objects in the catalog, for which periods were either known or unknown in previous works.

Infrared Properties of Asymptotic Giant Branch Stars in Our Galaxy and the Magellanic Clouds

Abstract We investigate the infrared properties of asymptotic giant branch (AGB) stars in our Galaxy and the Magellanic Clouds using various infrared observational data and theoretical models. We use catalogs for the sample of 4996 AGB stars in our Galaxy and about 39,000 AGB stars in the Magellanic Clouds from the available literature. For each object in the sample, we cross-identify the 2MASS, Wide-field Infrared Survey Explorer, and Spitzer counterparts. To compare the physical properties of O- and C-rich AGB stars in our Galaxy and the Magellanic Clouds, we present IR two-color diagrams (2CDs) using various photometric data. We perform radiative transfer model calculations for AGB stars using various possible parameters of central stars and dust shells. Using the dust opacity functions of amorphous silicate and carbon, the theoretical dust shell models can roughly reproduce the observations of AGB stars on various IR 2CDs. Compared with our Galaxy, we find that the Magellanic Clouds are deficient in AGB stars with thick dust shells. Compared with the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC) is more deficient in AGB stars with thick dust shells. This could be because the Magellanic Clouds are more metal-poor than our Galaxy and the LMC is more metal-rich than the SMC. We also present the IR properties of known pulsating variables. Investigating the magnitude distributions at mid-IR (MIR) bands for AGB stars in the Magellanic Clouds, we find that the SMC is more deficient in bright AGB stars at MIR bands compared with the LMC.

The inner dust shell of Betelgeuse detected by polarimetric aperture-masking interferometry

Context. Theory surrounding the origin of the dust-laden winds from evolved stars remains mired in controversy. Characterizing the formation loci and the dust distribution within approximately the first stellar radius above the surface is crucial for understanding the physics that underlie the mass-loss phenomenon. Aims. By exploiting interferometric polarimetry, we derive the fundamental parameters that govern the dust structure at the wind base of a red supergiant. Methods. We present near-infrared aperture-masking observations of Betelgeuse in polarimetric mode obtained with the NACO/SAMPol instrument. We used both parametric models and radiative transfer simulations to predict polarimetric differential visibility data and compared them to SPHERE/ZIMPOL measurements. Results. Using a thin dust shell model, we report the discovery of a dust halo that is located at only 0.5 R⋆ above the photosphere (i.e. an inner radius of the dust halo of 1.5 R⋆). By fitting the data under the assumption of Mie scattering, we estimate the grain size and density for various dust species. By extrapolating to the visible wavelengths using radiative transfer simulations, we compare our model with SPHERE/ZIMPOL data and find that models based on dust mixtures that are dominated by forsterite are most favored. Such a close dusty atmosphere has profound implications for the dust formation mechanisms around red supergiants.

INFRARED TWO-COLOR DIAGRAMS FOR AGB STARS, POST-AGB STARS, AND PLANETARY NEBULAE

We present various infrared two-color diagrams (2CDs) for asymptotic giant branch (AGB) stars, post-AGB stars, and Planetary Nebulae (PNe) and investigate possible evolutionary tracks. We use catalogs from the available literature for the sample of 4903 AGB stars (3373 O-rich; 1168 C-rich; 362 S-type), 660 post-AGB stars (326 post-AGB; 334 pre-PN), and 1510 PNe in our Galaxy. For each object in the catalog, we cross-identify the IRAS, AKARI, Midcourse Space Experiment, and 2MASS counterparts. The IR 2CDs can provide useful information about the structure and evolution of the dust envelopes as well as the central stars. To find possible evolutionary tracks from AGB stars to PNe on the 2CDs, we investigate spectral evolution of post-AGB stars by making simple but reasonable assumptions on the evolution of the central star and dust shell. We perform radiative transfer model calculations for the detached dust shells around evolving central stars in the post-AGB phase. We find that the theoretical dust shell model tracks using dust opacity functions of amorphous silicate and amorphous carbon roughly coincide with the densely populated observed points of AGB stars, post-AGB stars, and PNe on various IR 2CDs. Even though some discrepancies are inevitable, the end points of the theoretical post-AGB model tracks generally converge in the region of the observed points of PNe on most 2CDs.

MASS-LOSS RATES OF OH/IR STARS

We compare mass-loss rates of OH/IR stars obtained from radio observations with those derived from the dust radiative transfer models and IR observations. We collect radio observational data of OH maser and CO line emission sources for a sample of 1533 OH/IR stars listed in Suh & Kwon (2011). For 1259 OH maser, 76 CO(J=1-0), and 55 CO(J=2-1) emission sources, we compile data of the expansion velocity and mass-loss rate. We use a dust radiative transfer model for the dust shell to calculate the mass-loss rate as well as the IR color indices. The observed mass-loss rates are in the range predicted by the theoretical dust shell models corresponding to <TEX>$\dot{M}=10^{-8}M_{\odot}/yr-10^{-4}M_{\odot}/yr$</TEX>. We find that the dust model using a simple mixture of amorphous silicate and amorphous <TEX>$Al_2O_3$</TEX> (20% by mass) grains can explain the observations fairly well. The results indicate that the dust radiative transfer models for IR observations generally agree with the radio observations. For high mass-loss rate OH/IR stars, the mass-loss rates obtained from radio observations are underestimated compared to the mass-loss rates derived from the dust shell models. This could be because photon momentum transfer to the gas shell is not possible for the physical condition of high mass-loss rates. Alternative explanations could be the effects of different dust-to-gas ratios and/or a superwind.

THE ROLE OF THE ACCRETION DISK, DUST, AND JETS IN THE IR EMISSION OF LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI

We use recent high-resolution infrared (IR; 1–20 μm) photometry to examine the origin of the IR emission in low-luminosity active galactic nuclei (LLAGN). The data are compared with published model fits that describe the spectral energy distribution (SED) of LLAGN in terms of an advection-dominated accretion flow, truncated thin accretion disk, and jet. The truncated disk in these models is usually not luminous enough to explain the observed IR emission, and in all cases its spectral shape is much narrower than the broad IR peaks in the data. Synchrotron radiation from the jet appears to be important in very radio-loud nuclei, but the detection of strong silicate emission features in many objects indicates that dust must also contribute. We investigate this point by fitting the IR SED of NGC 3998 using dusty torus and optically thin (τmid-IR ∼ 1) dust shell models. While more detailed modeling is necessary, these initial results suggest that dust may account for the nuclear mid-IR emission of many LLAGN.

The origin of the IR emission of low-luminosity AGN

AbstractTo test recent suggestions that the infrared emission of low-luminosity AGN arises in a truncated thin accretion disk, we compare recent, high-resolution IR data with published SED model fits that include emission from the truncated disk. We also fit the data with clumpy torus and optically thin dust shell models. These comparisons suggest that dust can better account for the IR emission of the objects in question than can the truncated disk. That optically thin models give a good fit to the data may support a scenario in which the torus of the AGN unified model does not persist in low accretion rate AGN.

Dust shell model of the water fountain source IRAS 16342–3814

We investigate the circumstellar dust shell of the water fountain source IRAS 16342-3814. We performed two-dimensional radiative transfer modeling of the dust shell, taking into account previously observed spectral energy distributions (SEDs) and our new $J$-band imaging and $H$- and $K_S$-band imaging polarimetry obtained using the VLT/NACO instrument. Previous observations expect an optically thick torus in the equatorial plane because of a striking bipolar appearance and a large viewing angle of 30 - 40$\degr$. However, models with such a torus as well as a bipolar lobe and an AGB shell cannot fit the SED and the images simultaneously. We find that an additional optically and geometrically thick disk located inside a massive torus solves this problem. The masses of the disk and the torus are estimated to be 0.01 $M_{\sun}$ at the $a_\mathrm{max}=100 \mu$m dust and 1 $M_{\sun}$ at $a_\mathrm{max}=10 \mu$m dust, respectively. We discuss a possible formation scenario for the disk and torus based on a similar mechanism to the equatorial back flow. IRAS 16342-3814 is expected to undergo mass loss at a high rate. The radiation from the central star is shielded by the dust that was ejected in the subsequent mass loss event. As a result, the radiation pressure on dust particles cannot govern the motion of the particles anymore. The mass loss flow can be concentrated in the equatorial plane by help of an interaction, which might be the gravitational attraction by the companion, if it exists in IRAS 16342-3814. A fraction of the ejecta is captured in a circum-companion or circum-binary disk and the remains are escaping from the central star(s) and form the massive torus.

RR Tel: Determination of Dust Properties During Minimum Obscuration

Abstract the ISO infrared spectra and the SAAO long-term JHKL photometry of RR Tel in the epochs during minimum obscuration are studied in order to construct a circumstellar dust model. the spectral energy distribution in the near- and the mid-IR spectral range (1–15 μm) was obtained for an epoch without the pronounced dust obscuration. the DUSTY code was used to solve the radiative transfer through the dust and to determine the circumstellar dust properties of the inner dust regions around the Mira component. Dust temperature, maximum grain size, dust density distribution, mass-loss rate, terminal wind velocity and optical depth are determined. the spectral energy distribution and the long-term JHKL photometry during an epoch of minimum obscuration show almost unattenuated stellar source and strong dust emission which cannot be explained by a single dust shell model. We propose a two-component model consisting of an optically thin circmustellar dust shell and optically thick dust outside the line of sight in some kind of a flattened geometry, which is responsible for most of the observed dust thermal emission.

IR photometry and dust-shell models for two carbon stars

We present JHKLM photometry of the carbon stars ST And and T Lyn acquired in 2000- 2010. Along with brightness variations due to pulsations, changes on timescales of 2000-3000 days are also observed. Our combined light curves can be satisfactorily represented with light elements derived from visual observations, but the maxima are delayed relative to the calculated times. A color-index analysis demonstrates that the dust shell of ST And is fairly weak, and is manifest only episodically, while the presence of hot dust was always detected for T Lyn. These results confirm models of spherically symmetric stellar dust shells based on mean-flux data, supplemented with observations in the intermediate IR from the IRAS and AKARI satellites. The visual optical depth of the relatively cool dust shell of ST And assuming a dust temperature at the inner edge of T1 = 510 Ki s very low:τV =0 .047. The dust shell of T Lyn is considerably hotter (T1 = 940 K), with τV =0 .95. We estimate the mass-loss rate to be 1.8 × 10 −7 M� /year for ST And and 3.7 × 10 −7 M� /year for T Lyn.

Friedmann universe as a limit of dust shell model

We generalize the equations of motion of a shell for a sequence of thin shells and derive the general form of the expansion law of the dust shell model which represents the FRW universe when we take an appropriate limit. The three Friedmann model for the three possible values of k:0,±1 are satisfied.

The mass-loss return from evolved stars to the Large Magellanic Cloud

The total dust return rate from AGB and RSG star outflows is an important constraint to galactic chemical evolution models. However, this requires detailed radiative transfer (RT) modeling of individual stars, which becomes impractical for large data sets. Another approach is to select the best-fit spectral energy distribution (SED) from a grid of dust shell models, allowing for a faster determination of the luminosities and mass-loss rates for entire samples. We have developed the Grid of RSG and AGB ModelS (GRAMS) to measure the mass-loss return from evolved stars. The models span the range of stellar, dust shell and grain properties relevant to evolved stars. In this paper we present the carbon-star grid and compare our results with data of Large Magellanic Cloud (LMC) carbon stars from the SAGE and SAGE-Spec programs. We generate spherically symmetric dust shell models using the 2Dust code, with hydrostatic models for the central stars. We explore five values of the inner radius R_in of the dust shell (1.5, 3, 4.5, 7 and 12 R_star). We use amorphous carbon dust mixed with 10% silicon carbide by mass. The grain sizes follows a KMH distribution. The models span 26 values of 11.3 um optical depth, ranging from 0.001 to 4. For each model, 2Dust calculates the output SED from 0.2 to 200 um. Over 12,000 models have dust temperatures below 1800 K. The GRAMS synthetic photometry is in good agreement with SAGE photometry for LMC carbon-rich and extreme AGB star candidates, as well as spectroscopically confirmed carbon stars from the SAGE-Spec study. Our models reproduce the IRAC colors of most of the extreme AGB star candidates, consistent with the expectation that a majority of these enshrouded stars have carbon-rich dust. Finally, we fit the SEDs of some well-studied carbon stars and compare the resulting luminosities and mass-loss rates with those from previous studies.

Extended dust shell of the carbon star U Hydrae observed with AKARI

Context. Low- to intermediate-mass stars lose a significant fraction of their mass while they are on the asymptotic giant branch (AGB). This mass loss is considered to determine the final stages of their evolution. The material ejected from the stellar photosphere forms a circumstellar envelope in its surroundings. Layers of circumstellar envelope constitute the footprint of mass-loss history. Aims. Our aim is to probe the mass-loss history in the carbon star U Hya in the last ∼10 4 years by investigating the distribution of dust in the circumstellar envelope with high spatial resolution. Methods. We observed U Hya in the far-infrared (FIR) at 65, 90, 140, and 160 μm simultaneously, using the slow scan observing mode of the far-infrared surveyor (FIS) aboard the infrared astronomical satellite AKARI. It produced a map of ∼10 � × 40 � in size in each band. Results. The FIS maps reveal remarkably circular, ring-like emission structure almost centered on the star, showing the presence of a detached, spherical dust shell. A hollow dust shell model gives the inner radius Rin of 101–107 �� [(2.5–2.6) × 10 17 cm], thickness that covers a half of the total dust mass ΔRhm of 16–23 �� [(3.8–5.6) × 10 16 cm], which gives ΔRhm/Rin ∼ 0.2, and the power-law index of the dust opacity distribution of 1.10–1.15. The dust mass in the shell is well-constrained to be (0.9–1.4) × 10 −4 (κ100/25) −1 M� , where κ100 is the dust absorptivity at 100 μm in units of cm 2 g −1 . The dust mass-loss rate at Rin is found to be (1.8–

IR photometry and models of dust shells for two RCB stars

We present JHKLM photometry obtained in 1984–2009 for the RCB stars UV Cas and SU Tau. No major fadings characteristic of RCB stars were detected during the observations of UV Cas, while two events of this kind occurred for SU Tau. The observed flux and color-index variations can be explained with a changing dust concentration in the line of sight, and possibly variations of the stellar temperature. We use the measured fluxes, supplemented with observations in the intermediate IR, to compute spherically symmetric dust-shell models for the stars. The mass-loss rate is estimated to be 1.7 × 10−6M⊙ yr−1 for UV Cas and 4.1 × 10−6M⊙ yr−1 for SU Tau.

Isothermal circumstellar dust shell model for teaching

We introduce a model of radiative transfer in circumstellar dust shells. By assuming that the shell is both isothermal and its thickness is small compared to its radius, the model is simple enough for students to grasp and yet still provides a quantitative description of the relevant physical features. The isothermal model can be used in a classroom setting as an aid to improve student's understanding of absorption and emission phenomena.

A model for multiple isothermal circumstellar dust shells

We have extended our previously described single and double-shell isothermal circumstellar dust shell models to the case of an arbitrary number of independent isothermal shells. We envisage that this model, the code for which is available on-line, may be useful in three ways: (i) as a tool for the quick representation of the spectrum of dust enshrouded objects, (ii) as an initial guide to the choice of parameters to be used in more sophisticated models and (iii) as a teaching aid for a simple application of radiative transfer theory. For illustrative purposes this model has been used to represent the spectral energy distribution of two evolved objects with optically thick circumstellar dust shells, the post asymptotic giant branch (post-AGB) star IRAS 22036+5306 and the proto-planetary nebula (PPN) M1-92. Both of these objects have very broad spectral energy distributions, their flat distributions having proven difficult to model in the past. The satisfactory fit of the models supports the considerable evidence that some evolved objects in particular, may have multiple discrete circumstellar dust shells, possibly ejected from the central star at different epochs and with different grain compositions. In addition, the isothermal nature of the models suggests the possibility that these shells may be quite limited in their temperature range and physical extent.

VLTI monitoring of the dust formation event of the Nova V1280 Scorpii

Context. We present the first high spatial resolution monitoring of th e dust forming nova V1280 Sco performed with the Very Large Telescope Interferometer (VLTI). Aims. These observations aim at improving the distance determination of such events and constraining the mechanisms leading to very effi cient dust formation under the harsh physical conditions encountered in novae ejecta. Methods. Spectra and visibilities were regularly obtained from the onset of the dust formation 23 days after discovery (or 11 days after maximum) till day 145, using the beam-combiner instruments AMBER (near-IR) and MIDI (mid-IR). These interferometric observations are complemented by near-infrared data from the 1.2m Mt. Abu Infrared Observatory, India. The observations are first interpreted with simple uniform and Gaussian models but more complex models, involving a second shell, are necessary to explain the data obtained from t=110d after outburst. This behavior is in accordance with the light curve of V1280 Sco which exhibits a secondary peak around t=106d, followed by a new steep decline, suggesting a new dust forming event. Spherical dust shell models generated with the DUSTY code are also used to investigate the parameters of the main dust shell. Results. Using uniform disk and Gaussian models, these observations allow us to determine an apparent linear expansion rate for the dust shell of 0.35± 0.03 mas day −1 and the approximate time of ejection of the matter in which dust formed as te jec = 10.5± 7d, i.e. close to the maximum brightness. This information, combined with the expansion velocity of 500±100km.s −1 , implies a distance estimate of 1.6±0.4kpc. The sparse uv coverage does not allow to get clear indications of deviation from spherical symmetry. The dust envelope parameters were determined. The dust mass generated was typically 2-8 10 −9 M⊙ day −1 , with a probable peak in production at about 20 days after the detection of dust and another peak shortly after t=110d, when the amount of dust in the shell was estimated as 2.2 10 −7 M⊙. Considering that the dust forming event lasted at least 200-250d, the mass of the ejected material is likely to have exceeded 10 −4 M⊙. The conditions for the formation of multiple shells of dust are also discussed.