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تسجيل مستخدم جديدSearching for Cool Dust in the Mid-to-Far Infrared: the Mass Loss Histories of The Hypergiants $mu$ Cep, VY CMa, IRC+10420, and $rho$ Cas
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We present mid- and far- IR imaging of four famous hypergiant stars: the red supergiants $mu$ Cep and VY CMa, and the warm hypergiants IRC +10420 and $rho$ Cas. Our 11 to 37 $mu$m SOFIA/FORCAST imaging probes cool dust not detected in visual and near-IR imaging studies. Adaptive optics (AO) 8 - 12 $mu$m imaging of $mu$ Cep and IRC +10420 with MMT/MIRAC reveals extended envelopes that are the likely sources of these stars strong silicate emission features. We find $mu$ Ceps mass-loss rate to have declined by about a factor of 5 over a 13,000 history, ranging from 5 $times$ 10$^{-6}$ down to $sim$1 $times$ 10$^{-6}$ $M_{odot}$ yr$^{-1}$. The morphology of VY CMa indicates a cooler dust component coincident with the highly asymmetric reflection nebulae seen in the visual and near-IR. The lack of cold dust at greater distances around VY CMa indicates its mass-loss history is limited to the last $sim$1200 years, with an average rate of 6 $times$ 10$^{-4}$ $M_{odot}$ yr$^{-1}$. We find two distinct periods in the mass-loss history of IRC +10420 with a high rate of 2 $times$ 10$^{-3}$ $M_{odot}$ yr$^{-1}$ until approximately 2000 yr ago, followed by an order of magnitude decrease in the recent past. We interpret this change as evidence of its evolution beyond the RSG stage. Our new infrared photometry of $rho$ Cas is consistent with emission from the expanding dust shell ejected in its 1946 eruption, with no evidence of newer dust formation from its more recent events.
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Imaging and spectroscopy of the knots, clumps, and extended arcs in the complex ejecta of VY CMa confirm a record of high mass loss events over the past few hundred years. HST/STIS spectroscopy of numerous small knots close to the star allow us to me
asure their radial velocities from the strong K I emission and determine their separate motions, spatial orientations, and time since ejecta. Their ages concentrate around 70, 120, 200 and 250 years ago. A K I emission knot only 50 mas from the star ejected as recently as 1985 -- 1995 may coincide with an H2O maser. Comparison with VY CMas historic light curve from 1800 to the present, shows several knots with ejection times that correspond with extended periods of variability and deep minima. The similarity of this correspondence in VY CMa with the remarkable recent dimming of Betelgeuse and an outflow of gas is apparent. The evidence for similar outflows from the surface of a more typical red supergiant suggests that discrete ejections are more common and surface or convective activity is a major source of mass loss for red supergiants.
We present 2 - 5 micron adaptive optics (AO) imaging and polarimetry of the famous hypergiant stars IRC +10420 and VY Canis Majoris. The imaging polarimetry of IRC +10420 with MMT-Pol at 2.2 micron resolves nebular emission with intrinsic polarizatio
n of 30%, with a high surface brightness indicating optically thick scattering. The relatively uniform distribution of this polarized emission both radially and azimuthally around the star confirms previous studies that place the scattering dust largely in the plane of the sky. Using constraints on scattered light consistent with the polarimetry at 2.2 micron, extrapolation to wavelengths in the 3 - 5 micron band predicts a scattered light component significantly below the nebular flux that is observed in our LBT/LMIRCam 3 - 5 micron AO imaging. Under the assumption this excess emission is thermal, we find a color temperature of ~ 500 K is required, well in excess of the emissivity-modified equilibrium temperature for typical astrophysical dust. The nebular features of VY CMa are found to be highly polarized (up to 60%) at 1.3 micron, again with optically thick scattering required to reproduce the observed surface brightness. This stars peculiar nebular feature dubbed the Southwest Clump is clearly detected in the 3.1 micron polarimetry as well, which, unlike IRC+10420, is consistent with scattered light alone. The high intrinsic polarizations of both hypergiants nebulae are compatible with optically thick scattering for typical dust around evolved dusty stars, where the depolarizing effect of multiple scatters is mitigated by the grains low albedos.
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ew data are used to probe the evolution of the dust shells on a decade time-scale, complementing contemporaneous studies at other wavelengths. Self-consistent, spherically-symmetric models at maximum and minimum light both show the inner radius of the IRC+10216 dust shell to be much larger (150 mas) than that expected from the dust condensation temperature, implying that dust production has slowed or stopped in recent years. Apparently, dust does not form every pulsational cycle (638 days), and these mid-infrared results are consistent with recent near-IR imaging which indicates little or no new dust production in the last three years (Tuthill et al 2000). Spherically symmetric models failed to fit recent VY CMa data, implying that emission from the inner dust shell is highly asymmetric and/or time-variable.
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