scholarly journals Relating grain size distributions in circumstellar discs to the spectral index at millimetre wavelengths

2020 ◽  
Vol 641 ◽  
pp. A75
Author(s):  
T. Löhne
Keyword(s):  
Size Dependence ◽  
Thermal Emission ◽  
Circumstellar Dust ◽  
Spectral Energy ◽  
Size Distributions ◽  
Energy Distributions ◽  
Erosion Processes ◽  
Grain Size Distributions ◽  
Small Grains ◽  
Insufficient Knowledge

The excess emission seen in spectral energy distributions (SEDs) is commonly used to infer the properties of the emitting circumstellar dust in protoplanetary and debris discs. Most notably, dust size distributions and details of the collision physics are derived from SED slopes at long wavelengths. This paper reviews the approximations that are commonly used and contrasts them with numerical results for the thermal emission. The inferred size distribution indexes p are shown to be greater and more sensitive to the observed sub(mm) spectral indexes, αmm, than previously considered. This effect results from aspects of the transition from small grains with volumetric absorption to bigger grains that absorb and emit near to their surface, controlled by both the real and the imaginary part of the refractive index. The steeper size distributions indicate stronger size-dependence of material strengths or impact velocities or, otherwise, less efficient transport or erosion processes. Strong uncertainties remain because of insufficient knowledge of the material composition, porosity, and optical properties at long wavelengths.

scholarly journals Far-infrared Excesses in Classical Be Stars

2000 ◽  
Vol 175 ◽  
pp. 484-487 ◽  
Author(s):  
A.S. Miroshnichenko ◽  
K.S. Bjorkman
Keyword(s):  
High Resolution ◽  
Far Infrared ◽  
Infrared Emission ◽  
Circumstellar Dust ◽  
Spectral Energy ◽  
Be Stars ◽  
Spectral Energy Distributions ◽  
Energy Distributions ◽  
Rule Out

AbstractA sample of Be stars with large far-infrared excesses detected by IRAS is selected and their high-resolution IRAS images are examined. The far-infrared emission from most of them is marginally extended and is associated with the optical sources. Modeling of the stars’ spectral energy distributions allows us to rule out the infrared cirrus nature of the excesses. We suggest that the excesses are more likely due to circumstellar dust, possibly formed early in the evolution of the star.

Near-Infrared Photometry of Southern Galactic Wolf-Rayet Stars

1982 ◽  
Vol 99 ◽  
pp. 67-72
Author(s):  
C. Sterken ◽  
C. de Loore
Keyword(s):  
Infrared Radiation ◽  
Interstellar Dust ◽  
Near Infrared ◽  
Thermal Emission ◽  
Dust Emission ◽  
Circumstellar Dust ◽  
Infrared Photometry ◽  
Energy Distributions ◽  
Circumstellar Shell

Allen et al. (1972) observed 40 WR stars in the |1.6µ| and |2.2µ| bands, and found evidence of interstellar dust emission in the WC9 stars Ve 2–45, AS320 and HD 313643. Hackwell et al. (1974) reported 2.3 to 23 µ photometry of 19 WR stars and concluded that the excess infrared radiation from the Wolf-Rayet stars (except for the WC9 stars) could be explained by free-free emission from a hot circumstellar shell. Gehrz and Hackwell (1974) found from 2.3 to 23 µ photometry that three out of four WC stars appear to be embedded in thick circumstellar dust (graphite) shells, and concluded that WC9 stars may form a distinct Wolf-Rayet class. Cohen et al. (1975) derived energy distributions of 23 Wolf-Rayet stars from 3µ-11µ scanner spectrophotometry and infrared photometry, and concluded that WN stars show only free-free emission whereas only WC stars show dust. The excesses in WC9 stars are interpreted as thermal emission by graphite grains.

scholarly journals Interferometry meets the third and fourth dimensions in galaxies

2014 ◽  
Vol 10 (S309) ◽  
pp. 1-10
Author(s):  
Virginia Trimble
Keyword(s):  
Radio Astronomy ◽  
Earth Rotation ◽  
Milky Way ◽  
Thermal Emission ◽  
Spectral Energy ◽  
Radio Sources ◽  
Energy Distributions ◽  
The Third ◽  
Grey Scale ◽  
Paraboloid Of Revolution

AbstractRadio astronomy began with one array (Jansky's) and one paraboloid of revolution (Reber's) as collecting areas and has now reached the point where a large number of facilities are arrays of paraboloids, each of which would have looked enormous to Reber in 1932. In the process, interferometry has contributed to the counting of radio sources, establishing superluminal velocities in AGN jets, mapping of sources from the bipolar cow shape on up to full grey-scale and colored images, determining spectral energy distributions requiring non-thermal emission processes, and much else. The process has not been free of competition and controversy, at least partly because it is just a little difficult to understand how earth-rotation, aperture-synthesis interferometry works. Some very important results, for instance the mapping of HI in the Milky Way to reveal spiral arms, warping, and flaring, actually came from single moderate-sized paraboloids. The entry of China into the radio astronomy community has given large (40-110 meter) paraboloids a new lease on life.

scholarly journals Longslit Optical Spectroscopy of Powerful Far-Infrared Galaxies

1989 ◽  
Vol 134 ◽  
pp. 414-415
Author(s):  
Lee Armus ◽  
Timothy M. Heckman ◽  
George K. Miley
Keyword(s):  
Direct Evidence ◽  
Thermal Emission ◽  
Large Range ◽  
Far Infrared ◽  
Spectral Energy ◽  
Infrared Galaxies ◽  
Energy Distributions ◽  
Hii Region ◽  
The Galaxy ◽  
Infrared Spectral

It has been known since the IRAS mission that there exist galaxies with far-infrared luminosities of 1011–1012Lo, and LFTR/LB = 10–100. Through extensive modelling and observations of HII-region/molecular cloud complexes in the Galaxy, this infrared radiation is believed to be thermal emission from heated dust grains (c.f. review by Stein and Soifer 1983). While starburst models are consistent with the data over a large range in wavelength, direct evidence for sizeable populations of young stars is scarce, and in many cases the presence of an active nucleus either cannot be ruled out, or is required on the basis of energy considerations. In order to better understand the energy source responsible for heating the dust, we have undertaken a spectroscopic survey of galaxies chosen to have far-infrared spectral energy distributions similar to the prototypical class members Arp 220, NGC 6240, NGC 3690, and Mrk 231. It was required that between 25μ and 60μ, α ≤ −1.5, and that between 60μ and 100μ, α ≥ −0.5, where Sv α vα.

scholarly journals Polarized emission by aligned grains in the Mie regime: Application to protoplanetary disks observed by ALMA

2020 ◽  
Vol 634 ◽  
pp. L15 ◽  
Author(s):  
V. Guillet ◽  
J. M. Girart ◽  
A. J. Maury ◽  
F. O. Alves
Keyword(s):  
Magnetic Field ◽  
Grain Size ◽  
Thermal Emission ◽  
Protoplanetary Disks ◽  
Polarization Vector ◽  
Size Distributions ◽  
Negative Polarization ◽  
Polarized Emission ◽  
Grain Size Distributions ◽  
The Magnetic Field

Context. The azimuthal polarization patterns observed in some protoplanetary disks by the Atacama Large Millimetre Array (ALMA) at millimeter wavelengths have raised doubts about whether they are truly produced by dust grains that are aligned with the magnetic field lines. These conclusions were based on the calculations of dust polarized emission in the Rayleigh regime, that is, for grain sizes that are much smaller than the wavelength. However, the grain size in such disks is typically estimated to be in the range of 0.1−1 mm from independent observations. Aims. We study the dust polarization properties of aligned grains in emission in the Mie regime, that is, when the mean grain size approaches the wavelength. Methods. By using the T-MATRIX and DustEM codes, we computed the spectral dependence of the polarization fraction in emission for grains in perfect spinning alignment for various grain size distributions. We restricted our study to weakly-elongated oblate and prolate grains of astrosilicate composition that have a mean size ranging from 10 μm to 1 mm. Results. In the submillimeter and millimeter wavelength range, the polarization by B-field aligned grains becomes negative for grains larger than ∼250 μm, meaning that the polarization vector becomes parallel to the B-field. The transition from the positive to the negative polarization occurs at a wavelength of λ ∼ 1 mm. The regime of negative polarization does not exist for grains that are smaller than ∼100 μm. Conclusions. When using realistic grain size distributions for disks with grains up to the submillimeter sizes, the polarization direction of thermal emission by aligned grains is shown to be parallel to the direction of the magnetic field over a significant fraction of the wavelengths typically used to observe young protoplanetary disks. This property may explain the peculiar azimuthal orientation of the polarization vectors in some of the disks observed with ALMA and attest to the conserved ability of dust polarized emission to trace the magnetic field in disks.

scholarly journals The evolutionary nature of RV Tauri stars in the SMC and LMC

2018 ◽  
Vol 618 ◽  
pp. A21 ◽  
Author(s):  
Rajeev Manick ◽  
Hans Van Winckel ◽  
Devika Kamath ◽  
Sanjay Sekaran ◽  
Katrien Kolenberg
Keyword(s):  
Time Series ◽  
Systematic Study ◽  
Orbital Motion ◽  
Circumstellar Dust ◽  
Spectral Energy ◽  
Agb Stars ◽  
Energy Distributions ◽  
The Galaxy ◽  
Red Giant ◽  
Lower Luminosity

Context. Based on their stellar parameters and the presence of a mid-IR excess due to circumstellar dust, RV Tauri stars have been classified as post-AGB stars. Our recent studies, however, reveal diverse spectral energy distributions (SEDs) among RV Tauri stars, suggesting they may occupy other evolutionary channels as well. Aims. The aim of this paper is to present the diverse SED characteristics of RV Tauri stars and investigate their evolutionary nature as a function of their SEDs. Methods. We carried out a systematic study of RV Tauri stars in the SMC and LMC because of their known distances and hence luminosities. Their SEDs were classified into three groups: dusty (disc-type), non-dusty (non-IR), and uncertain. A period-luminosity-colour (PLC) relation was calibrated. The luminosities from the PLC were complemented with those found using their SEDs and the stars were placed on a Hertzsprung-Russell diagram (HRD). I-band time series were used to search for period changes via (O−C) analyses to identify period changes. Results. The four main results from this study are: (1) RV Tauri stars with a clear IR excess have disc-type SEDs, which indicates that the dust is trapped in a stable disc. Given the strong link between disc-type SEDs and binarity in the Galaxy, we postulate that these are binaries as well. These cover a range of luminosities and we argue that the more luminous binaries are post-AGB stars while the lower luminosity binaries are likely post-red giant branch (post-RGB) stars. (2) Two of these objects have variable mean brightness with periods of 916 and 850 days, respectively, caused by variable extinction during orbital motion. (3) Non-dusty RV Tauri stars and objects with an uncertain SED evolve such that the circumstellar dust has dispersed. If they are single stars, they are post-AGB objects of low initial mass (<1.25 M⊙), while if they are binaries, the low-luminosity portion of the sample are likely post-RGB stars. (4) We find that RV Tauri stars with dust are on average more luminous than the rest of the sample.

scholarly journals Variable dust emission by WC type Wolf–Rayet stars observed in the NEOWISE-R survey

2019 ◽  
Vol 488 (1) ◽  
pp. 1282-1300 ◽  
Author(s):  
P M Williams
Keyword(s):  
Dust Emission ◽  
Large Magellanic Cloud ◽  
Variable Stars ◽  
Circumstellar Dust ◽  
Spectral Energy ◽  
Dust Formation ◽  
Energy Distributions ◽  
Optical Photometry ◽  
The Galaxy ◽  
Infrared Spectral

ABSTRACT Photometry at 3.4 and 4.6 ${\mu m}$ of 128 Population I WC type Wolf–Rayet stars in the Galaxy and 12 in the Large Magellanic Cloud (LMC) observed in the WISE NEOWISE-R survey was searched for evidence of circumstellar dust emission and its variation. Infrared spectral energy distributions (SEDs) were assembled, making use of archival r, i, Z, and Y photometry to determine reddening and stellar wind levels for the WC stars found in recent IR surveys and lacking optical photometry. From their SEDs, 10 apparently non-variable stars were newly identified as dust makers, including three, WR 102-22, WR 110-10, and WR 124-10, having subtype earlier than WC8–9, the first such stars to show this phenomenon. The 11 stars found to show variable dust emission include six new episodic dust makers, WR 47c, WR 75-11, WR 91-1, WR 122-14, and WR 125-1 in the Galaxy and HD 38030 in the LMC. Of previously known dust makers, NEOWISE-R photometry of WR 19 captured its rise to maximum in 2018 confirming the 10.1-yr period, that of WR 125 the beginning of a new episode of dust formation suggesting a period near 28.3 yr. while that of HD 36402 covered almost a whole period and forced revision of it to 5.1 yr.

scholarly journals Evolution of dust porosity through coagulation and shattering in the interstellar medium

2021 ◽  
Author(s):  
Hiroyuki Hirashita ◽  
Vladimir B Il’in ◽  
Laurent Pagani ◽  
Charléne Lefévre
Keyword(s):  
Grain Size ◽  
Interstellar Medium ◽  
Size Distribution ◽  
Grain Size Distribution ◽  
Collision Energy ◽  
Size Distributions ◽  
Curve Shape ◽  
Grain Size Distributions ◽  
Small Grains ◽  
Porous Grains

Abstract The properties of interstellar grains, such as grain size distribution and grain porosity, are affected by interstellar processing, in particular, coagulation and shattering, which take place in the dense and diffuse interstellar medium (ISM), respectively. In this paper, we formulate and calculate the evolution of grain size distribution and grain porosity through shattering and coagulation. For coagulation, we treat the grain evolution depending on the collision energy. Shattering is treated as a mechanism of forming small compact fragments. The balance between these processes are determined by the dense-gas mass fraction ηdense, which determines the time fraction of coagulation relative to shattering. We find that the interplay between shattering supplying small grains and coagulation forming porous grains from shattered grains is fundamentally important in creating and maintaining porosity. The porosity rises to 0.7–0.9 (or the filling factor 0.3–0.1) around grain radii $a\sim 0.1~{\rm \mu m}$. We also find that, in the case of ηdense = 0.1 (very efficient shattering with weak coagulation) porosity significantly enhances coagulation, creating fluffy submicron grains with filling factors lower than 0.1. The porosity enhances the extinction by 10–20 per cent at all wavelengths for amorphous carbon and at ultraviolet wavelengths for silicate. The extinction curve shape of silicate becomes steeper if we take porosity into account. We conclude that the interplay between shattering and coagulation is essential in creating porous grains in the interstellar medium and that the resulting porosity can impact the grain size distributions and extinction curves.

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