Thick Dust Shell Research Articles

OH/IR star color mimics

view Abstract Citations (36) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS OH/IR Star Color Mimics Lewis, B. M. Abstract IR color criteria are used to select potential OH/IR stars from the IRAS Point Source Catalog. These OH/IR star color mimics, despite often having thick and demonstrably O-rich dust shells to shield their molecules against interstellar UV, have no 1612 MHz masers. The most likely reason for this is that these stars have degenerate companions, which collect an accretion disk from the red giant wind, which in turn provides them with a local source of UV to dissociate molecules from within their dust shells. In some cases this self-generated UV is sufficient to excise all molecules from a shell, as happens with symbiotic novae; in some cases it merely reduces their number and the ability of a shell to support a maser. It is suggested that D-type symbiotic stars can be identified among sources with thick opaque dust shells by a persistent absence of appropriate masers: these are the OH/IR color mimics. Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/171713 Bibcode: 1992ApJ...396..251L Keywords: Hydroxyl Emission; Infrared Stars; Interstellar Masers; Stellar Color; Stellar Envelopes; Astronomical Catalogs; Companion Stars; Infrared Astronomy Satellite; Astrophysics; INFRARED: STARS; MASERS; STARS: CIRCUMSTELLAR MATTER full text sources ADS | data products SIMBAD (340)

Note on a remarkable object close to the star LSS 3027

The star LSS 3027, embedded in a parabolic-shaped dark nebula, and several stars in its vicinity, were observed photometrically and spectroscopically to investigate whether they are candidates of very young Herbig Ae/Be stars. Only the star LSS 3027B is without doubt a very good candidate. Its Hα-emission is very strong, indicating an extended atmosphere around the star, and its near- and far-infrared spectral energy distributions exhibit a strong excess radiation, which most probably is caused by a thick dust shell in the close environment of the star. The ratio of total to selective extinction is found to be equal toR-5.1. It can be shown that it is embedded in the dark cloud mentioned above. From its properties we believe that the remarkable object LSS 3027B is a typical Hergig Be pre-Main-Sequence object.

Far-infrared data for symbiotic stars. II - The IRAS survey observations

IRAS survey data for all known symbiotic binaries are reported. S type systems have 25 micron excesses much larger than those of single red giant stars, suggesting that these objects lose mass more rapidly than do normal giants. D type objects have far-IR colors similar to those of Mira variables, implying mass-loss rate of about 10 to the -6th solar masses/yr. The near-IR extinctions of the D types indicate that their Mira components are enshrouded in optically thick dust shells, while their hot companions lie outside the shells. If this interpretation of the data is correct, then the very red near-IR colors of D type symbiotic stars are caused by extreme amounts of dust absorption rather than dust emission. The small group of D prime objects possesses far-IR colors resembling those of compact planetary nebulae or extreme OH/IR stars. It is speculated that these binaries are not symbiotic stars at all, but contain a hot compact star and an exasymptotic branch giant which is in the process of ejecting a planetary nebula shell.

On the possibility of dust condensation in the ejecta of supernova 1987a

We suggest that supernova 1987a may condense dust of substantial visual optical thickness as do many novae. The dust will act as a calorimeter of the photon luminosity of any central engine that is dominant at the time of dust formation. Observations of novae suggest that dust formation may occur when the expanding ejecta reach a temperature of 1000 K. The early luminosity of the supernova may be dominated by radioactivity that is unrelated to the central engine that determines the energy balance for the long-term development of the supernova. We discuss the possibility that a constant luminosity central power source such as a pulsar dominates the luminosity of the supernova ejecta by the time that dust can condense and argue that, if a shell mass of more than a few tenths of one solar mass was ejected, emission from dust may be observable in the thermal infrared spectral region. Maximum dust optical depth should occur by late 1987 or early 1988. If the dust becomes optically thick, the visual light from the supernova may drop precipitously. The characteristics of an optically thick dust shell as a calorimeter of the luminosity of the central engine are discussed and are related to previous observations of dust formation in type II supernovae. It is suggested that dust of several chemical compositions may form at different epochs.

Infrared spectroscopy of IRC + 10420

High resolution 1.6, 3.3 and 4.6 micron spectra were obtained on the F8 supergiant star IRC + 10420 on June 9-11, 1984, using the 4 m Mayall telescope and Fourier Transform Spectrometer at Kitt Peak. IRC + 10420 is distinguished by its hydrogen maser, unusual for stars of this type. The measured column densities of CO led to an estimated isotopic carbon ratio of 7-11, supporting a model of the object as a highly evolved star. The redshift is interpreted as due to scattering by outward-moving circumstellar dust grains. The IR spectrum of IRC + 10420 suggests that the object is a post-asymptotic branch star still surrounded by the thick dust shell it produced while it was much cooler.

Two supergiants in the Large Magellanic Cloud with thick dust shells

Ground-based observations from 0.6 to 20 microns have identified two luminous, evolved stars surrounded by thick dust shells among the Magellanic Cloud sources detected by the Infrared Astronomical Satellite. Their energy distributions resemble those of typical Galactic OH/IR stars, but they have bolometric magnitudes brighter than -9 and the small-amplitude variability of supergiants. One star, IRAS 04553 - 6825, has a special type of M7.5 and a dust shell which absorbs and reradiates roughly 75 percent of the star's luminosity; its radial velocity confirms its LMC membership. The second star, IRAS 05346 - 6949, has an even thicker dust shell, and the central star is not observable.

BN objects ? A class of very young and massive stars

The Becklin-Neugebauer objects are identified by means of a wide range of observable features as a separate class of very young and massive stars surrounded by optically thick dust shells. We show that they evolutionarily connect real protostars to compact H II regions. We give criteria which should be appropriate to segregate the BN objects from compact H II regions. Finally, we describe the structure of a typical BN object.

On the nature of OH/IR stars

OH/IR stars are the infrared counterparts of galactic OH maser sources which show a characteristic double-peaked emission-line profile. Their strong radio emission can be detected at large distances, making them excellent tracers of distribution and kinematics of evolved stars in the Milky Way. The OH maser profile is typical for line emission from an expanding circumstellar shell. The circumstellar shells of OH/IR stars absorb the optical emission of the central star nearly completely and reemit the energy in the infrared. Having luminosities ~ 105 L⊙ and energy distributions peaking around 10μm, they may make a major contribution to the interstellar radiation field beyond 5μm. With mass loss rates of 10-5 to 10-4 M⊙/yr they lose several solar masses in a few hundred thousand years. OH/IR stars are therefore important objects for recycling stellar matter into the interstellar medium.Progress has been made in understanding the nature of OH/IR stars. They are Mira-like large-amplitude variables with periods up to 5 years long. It is proposed that they are stars of intermediate mass (2–10 M⊙) on the asymptotic giant branch (AGB). They have not only larger masses than Mira variables proper, but also longer periods of pulsation and larger mass loss rates. As a result optically thick circumstellar dust shells are formed, which prevent the detection of these more massive Mira-like variables at optical wavelengths. Radial pulsation (Mira variability) is thus thought to occur for all intermediate-mass stars in the course of their evolution on the AGB. In view of their high mass-loss rates, these stars may be key objects in the study of the formation of planetary nebulae.

A Search in the Infrared to Microwave for Astroengineering Activity

Huge space power plants (Dyson Spheres) utilizing most of a star's energy should be detectable as infrared or microwave sources. A recent far infrared all-sky survey has revealed many sources with a spectrum peaking on this region which is characteristic of the thermal emission of the hypothetical Dyson spheres. The possibility of confusing them with thick circumstellar dust shells around evolved red giant stars is discussed. Microwave detection of cool extended Dyson Spheres by all-sky surveys searching for microwave background fluctuations is also considered.

Theory of Dust Formation in R Coronae Borealis Stars

AbstractApplication of the homogeneous nucleation theory to the problem of R CrB stars shows that the radial distance of the inner boundary of the carbon supersaturation region is at about 12 photospheric radii for a stellar effective temperature Te = 6000 K. Formation of an optically thick dust shell becomes possible at mass loss rates Ṁ ≳ 10−6 M⊙/yr. However, the upper limit of this mass loss rate cannot considerably exceed 10−5 M⊙/yr since, at higher Ṁ, the theoretically predicted rate of the visual brightness decline is larger than that derived from observations. Comparison of the theoretically predicted radii of dust grains with those observed in R CrB and RY Sgr shows that the mass loss rate in these stars should be in the range of 1 × 10−7 M⊙/yr to 3 × 10−6 M⊙/yr.

Parameters of very young and massive stars with dust shells

A radiative transfer model for newly formed hot stars surrounded by optically thick dust shells is applied to a number of compact infrared sources. The model parameters are derived by fitting the synthetic spectra to the observed one between 8 and 13 μm and used to derive additional properties of the shells and the embedded stars.

The nature of AFGL 2591 and its associated molecular outflow Infrared and millimeter-wave observations

The results of infrared photometry from 2 to 160 microns of AFGL and CO(12) observations of its associated molecular cloud and high velocity molecular outflow are presented and discussed. The observed solar luminosity is 6.7 x 10(4) at a distance of 2 kpc. The spectrum of AFGL 2591 is interpreted in the context of a model in which a single embedded object is the dominant source of the infrared luminosity. This object is determined to be surrounded by a compact, optically thick dust shell with a temperature in excess of several hundred degrees kelvin. The extinction to this source is estimated to be between 26 and 50 visual magnitudes. The absolute position of the infrared sources at 10 microns was determined to an accuracy of + or - 1 in. This indicates for the first time that the IR source and H2O source are not coincident. The CO(12) observations show the high-velocity molecular flow near AFGL 2591 to be extended, bipolar and roughly centered on the infrared emission. The observations suggest that the red-shifted flow component extends beyond the boundary of the ambient cloud within which AFGL 2591 is embedded. The CO(12) observations also show that AFGL 2591 is embedded in a molecular cloud with an LSR velocity of -5 km/s.

An investigation of T Tauri variability

view Abstract Citations (16) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An investigation of T Tauri variability. Schmelz, J. T. Abstract Three mechanisms have been suggested to account for the photometric variability in T Tauri stars: (1) changes in the effective photospheric spectral type; (2) changes in the optical thickness of the chromosphere; and (3) changes in the optical thickness of the dust shell. The author investigates these processes with color-color diagrams and energy distribution plots of 14 stars located in the Taurus-Auriga dark cloud complex. A relationship between a strong chromosphere and chromospheric variability was found as well as a similar relationship between a thick dust shell and dust shell variability. There is some evidence that the stellar photosphere becomes more stable with increasing age during the T Tauri phase. Publication: The Astronomical Journal Pub Date: January 1984 DOI: 10.1086/113487 Bibcode: 1984AJ.....89..108S Keywords: Chromosphere; Stellar Activity; Stellar Envelopes; Stellar Spectrophotometry; T Tauri Stars; Color-Color Diagram; Optical Thickness; Photosphere; Spectral Energy Distribution; Stellar Color; Stellar Evolution; Stellar Spectra; Variability; Astrophysics; Circumstellar Shells:T Tauri Stars; Stellar Chromospheres:T Tauri Stars; T Tauri Stars:Circumstellar Shells; T Tauri Stars:Stellar Chromospheres; T Tauri Stars:Variations full text sources ADS | data products SIMBAD (15)

OH/IR Stars: Dark Planetary Nebulae?

The OH maser emission from OH/IR stars (type IIb) originates from the thick circumstellar dust shell. As the envelope is expanding and the strongest maser emission comes from the front- and the backside, where the gain pathlength is longest, we can measure a definite phaselag (and hence a diameter) between front- and backside of the shell for all sources that are variable.

10 and 20 micron images of regions of star formation

view Abstract Citations (105) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 10 and 20 micron images of regions of star formation. Hackwell, J. A. ; Grasdalen, G. L. ; Gehrz, R. D. Abstract A fast-mapping technique, developed for use on the Wyoming Infrared Telescope, has been used to make infrared images of regions of star formation. We present 10 μm and 20 μm images of Mon R2, W51 IRS 1 and IRS 2, S140, and NGC 7538; and 5 μm images of W51 IRS 2 and NGC 7538. The maps have angular resolutions approaching the Rayleigh limit of the 2.3 m telescope and have a sensitivity to extended sources which is as good as or better than previous observations of the same regions. The most luminous infrared sources are associated with radio continuum sources and, for these objects, the radio and 20 μm contours bear a remarkable similarity to one another. We also find qualitative support in the cases of W51 IRS 1, and Mon R2 IRS 1, for the blister model of H II regions, in which the exciting source is relatively well separated from the peak radio and infrared emission. The low-luminosity infrared sources are either not coincident with radio emission or are weak radio sources. This is either because the embedded exciting sources are relatively low luminosity late-type stars which emit relatively few ionizing photons or are surrounded by thick dust shells which block the ionizing flux from the interstellar medium. The observational technique also provides highly accurate positional information for the objects observed, and we make positional comparisons between near-infrared objects, radio continuum sources, OH and H2O masers, and far-infrared sources. We conclude that, in several cases, the OH and H2O masers may be at relatively large distances (1017 - 1018 cm) from their primary sources. Publication: The Astrophysical Journal Pub Date: January 1982 DOI: 10.1086/159552 Bibcode: 1982ApJ...252..250H Keywords: Astronomical Maps; Infrared Astronomy; Nebulae; Star Distribution; Star Formation; Angular Resolution; Contours; Hydroxyl Emission; Infrared Imagery; Infrared Telescopes; Radio Sources (Astronomy); Stellar Luminosity; Water Masers; Astrophysics full text sources ADS | data products SIMBAD (22)

Near-Infrared Photometry of Southern Galactic Wolf-Rayet Stars

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.

Infrared mapping of the galactic plane. I - Low-resolution maps between 0 and 320 deg longitude

view Abstract Citations (49) References (1) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared mapping of the galactic plane. I - Low-resolution maps between 0 and 320 deg longitude Price, S. D. Abstract Sixty percent of the galactic plane was surveyed in broad spectral bands centered at 11 and 20 μm during the AFGL infrared sky survey experiments. Smaller areas were covered at 4 and 27 μm. A number of discrete, extended sources were observed, most of which are associated with H II regions. A diffuse infrared background emission was also observed at longitudes less than 90° from the galactic center. This emission peaks on the galactic plane with roughly constant intensity between 10° and 30° longitude and declines at greater longitudes. The 4-, 11-, and 20- μm peak intensities along the emission ridge are 1.4 × 10-10, 8 × 10-11, and 2 × 10-11 W cm-2 μm-1 sr-1, respectively. A constant 11- to 20-μm intensity ratio of 4 was measured along the galactic plane out to 65 longitude from the center. A smaller ratio of 2 to 2.5 was found in both the Cygnus X region and in the direction of the Sagittarius-Carina spiral arm. The 4-μm observations can be accounted for by the superposition of K and M giants along the line of sight. A significant fraction of the diffuse 11- and 20-μm emission is due to stars embedded in thick dust shells. Discrete, compact H II regions appear to contribute little to the diffuse background. Publication: The Astronomical Journal Pub Date: February 1981 DOI: 10.1086/112873 Bibcode: 1981AJ.....86..193P Keywords: Astronomical Maps; Emission Spectra; Galactic Radiation; Infrared Astronomy; Infrared Radiation; Spectral Bands; Background Radiation; Brightness Temperature; Diffuse Radiation; Giant Stars; Hydrogen Clouds; Late Stars; M Stars; Stellar Radiation; Astrophysics full text sources ADS | Related Materials (3) Part 2: 1982AJ.....87..131P Part 3: 1982AJ.....87.1288T Part 4: 1985AJ.....90.1812L

Simultaneous far-infrared, near-infrared, and radio observations of OH/IR stars

Simultaneous far-infrared, near-infrared, and radio observations have been made of five infrared stars which show OH maser emission at 1612 MHz. These stars have very thick circumstellar dust shells and are not seen optically. The data permit a direct comparison of the far-infrared and maser emission from these sources, which strongly supports the hypothesis that the maser emission is pumped by 35 micron photons. A comparison with data obtained at earlier epochs suggests that the maser emission is saturated. The infrared and radio data are used together with estimates of the source distances to determine the luminosities and mass loss rates for these objects. The luminosities lie in the range 2000-30,000 solar luminosities and are consistent with either Mira variable or M supergiant classifications for the underlying stars. The estimated mass loss rates lie between 0.000005-0.00007 solar mass/year.

A simple estimate of the effect of a thick circumstellar dust shell on photospheric line profiles

A star embedded in an optically thick dust shell may have a line spectrum that closely resembles the spectrum of some unobscured star - e.g., VY CMa (shrouded) and alpha Her (unshrouded). This implies that the surface temperatures of the two stars are nearly equal, but since one is subject to a back-warming effect, the distributions of temperature with depth in the respective atmospheres are dissimilar. We find that effective temperatures and absorption line strengths are reduced in shrouded stars below the values that obtain in their unshrouded counterparts. For models of VY CMa that yield grain temperatures at the inner boundary of the dust shell consistent with observation, the reduction in strength of a visible line is small (not more than 15%). Thus the spectral distribution of alpha Her may be used as an approximation to that of VY CMa before it is scattered by the dust, without the necessity of first correcting for back warming.

Spectrophotometry of OH 26.5+0.6 from 2 to 40 microns

Airborne and ground-based observations show that OH 26.5+0.6 has strong 10 micrometers and weak 18 micrometers silicate absorptions superposed on an overall energy distribution much like a blackbody. The flux level, color temperature, and depth of the 10 micrometers absorption have varied during two years of observations. A model of the source as a late-type variable star that has ejected an optically thick dust shell is suggested; the mass-loss rate implied is greater than about 0.00001 solar masses per year. The fact that significant flux from the source is observed between 4 and 7 micrometers is evidence that oxygen-rich dust has significant opacity in that wavelength range.