We compare the properties of warm dust emission from a sample of main-sequence A-type stars (B8-A7) to those of dust around solar-type stars (F5-K0) with similar Spitzer Space Telescope Infrared ...Spectrograph/MIPS data and similar ages. Both samples include stars with sources with infrared spectral energy distributions that show evidence of multiple components. Over the range of stellar types considered, we obtain nearly the same characteristic dust temperatures (~190 K and ~60 K for the inner and outer dust components, respectively)--slightly above the ice evaporation temperature for the inner belts. The warm inner dust temperature is readily explained if populations of small grains are being released by sublimation of ice from icy planetesimals. Evaporation of low-eccentricity icy bodies at ~150 K can deposit particles into an inner/warm belt, where the small grains are heated to T dust~ 190 K. Alternatively, enhanced collisional processing of an asteroid belt-like system of parent planetesimals just interior to the snow line may account for the observed uniformity in dust temperature. The similarity in temperature of the warmer dust across our B8-K0 stellar sample strongly suggests that dust-producing planetesimals are not found at similar radial locations around all stars, but that dust production is favored at a characteristic temperature horizon.
Abstract
RZ Piscium (RZ Psc) is well known in the variable star field because of its numerous irregular optical dips in the past 5 decades, but the nature of the system is heavily debated in the ...literature. We present multiyear infrared monitoring data from Spitzer and WISE to track the activities of the inner debris production, revealing stochastic infrared variability as short as weekly timescales that is consistent with destroying a 90 km sized asteroid every year. ALMA 1.3 mm data combined with spectral energy distribution modeling show that the disk is compact (∼0.1–13 au radially) and lacks cold gas. The disk is found to be highly inclined and has a significant vertical scale height. These observations confirm that RZ Psc hosts a close to edge-on, highly perturbed debris disk possibly due to migration of recently formed giant planets that might be triggered by the low-mass companion RZ Psc B if the planets formed well beyond the snowlines.
We use Hubble Space Telescope (HST) NICMOS continuum and Paa observations to study the near-infrared and star formation properties of a representative sample of 30 local (d 635-75 Mpc) luminous ...infrared galaxies (LIRGs, infrared 8-1000 km luminosities of log L sub(IR) = 11-11.9 L sub( )). The data provide spatial resolutions of 25-50 pc and cover the central 63.3-7.1 kpc regions of these galaxies. About half of the LIRGs show compact (61-2 kpc) Paa emission with a high surface brightness in the form of nuclear emission, rings, and minispirals. The rest of the sample show Paa emission along the disk and the spiral arms extending over scales of 3-7 kpc and larger. About half of the sample contains H II regions with Ha luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24 km and hydrogen recombination (extinction-corrected Paa) luminosity for these LIRGs, and the H II regions in the central part of M51. This relation holds over more than four decades in luminosity, suggesting that the mid-IR emission is a good tracer of the star formation rate (SFR). Analogous to the widely used relation between the SFR and total IR luminosity of R. Kennicutt, we derive an empirical calibration of the SFR in terms of the monochromatic 24 km luminosity that can be used for luminous, dusty galaxies.
We analyze star formation (SF) as a function of stellar mass (M sub(*)) and redshift z in the All-Wavelength Extended Groth Strip International Survey, for star-forming field galaxies with M sub(*) ...10 super(10) M sub((.)) out to z = 1.1. The data indicate that the high specific SF rates (SFRs) of many less massive galaxies do not represent late, irregular or recurrent, starbursts in evolved galaxies. They rather seem to reflect the onset (initial burst) of the dominant SF episode of galaxies, after which SF gradually declines on gigayear timescales to z = 0 and forms the bulk of a galaxy's M sub(*). With decreasing mass, this onset of major SF shifts to decreasing z for an increasing fraction of galaxies (staged galaxy formation). This process may be an important component of the "downsizing" phenomenon. We find that the predominantly gradual decline of SFRs described by Noeske et al. can be reproduced by exponential SF histories (t models), if less massive galaxies have systematically longer e-folding times t, and a later onset of SF (z sub(f)). Our model can provide a first parameterization of SFR as a function of M sub(*) and z, and quantify mass dependences of t and z sub(f) from direct observations of M sub(*) and SFRs up to z> 1. The observed evolution of SF in galaxies can plausibly reflect the dominance of gradual gas exhaustion. The data are also consistent with the history of cosmological accretion onto dark matter halos.
Abstract
V488 Persei is the most extreme debris disk known in terms of the fraction of the stellar luminosity it intercepts and reradiates. The infrared output of its disk is extremely variable, ...similar in this respect to the most variable disk known previously, that around ID8 in NGC 2547. We show that the variations are likely to be due to collisions of large planetesimals (≳100 km in diameter) in a belt being stirred gravitationally by a planetary or low-mass-brown-dwarf member of a planetary system around the star. The dust being produced by the resulting collisions is falling into the star due to drag by the stellar wind. The indicated planetesimal destruction rate is so high that it is unlikely that the current level of activity can persist for much longer than ∼1000–10,000 yr and it may signal a major realignment of the configuration of the planetary system.
Decay of Planetary Debris Disks Rieke, G. H; Su, K. Y. L; Stansberry, J. A ...
The Astrophysical journal,
02/2005, Letnik:
620, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We report new Spitzer 24 km photometry of 76 main-sequence A-type stars. We combine these results with previously reported Spitzer 24 km data and 24 and 25 km photometry from the Infrared Space ...Observatory and the Infrared Astronomy Satellite. The result is a sample of 266 stars with mass close to 2.5 M sub( ), all detected to at least the 67 s level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K - 24 (or 25) km photospheric color and then compared all stars in the sample to that color. Because we have excluded stars with strong emission lines or extended emission (associated with nearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emission more frequently and with higher fractional excess than do the older stars. However, as many as 50% of the younger stars do not show excess emission. The decline in the magnitude of excess emission, for those stars that show it, has a roughly t sub(0)/time dependence, with t sub(0) 6 150 Myr. If anything, stars in binary systems (including Algol-type stars) and l Boo stars show less excess emission than the other members of the sample. Our results indicate that (1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10-60 AU region and (2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion end game, and that individual events often dominate the radiometric properties of a debris system. This latter behavior agrees generally with what we know about the evolution of the solar system, and also with theoretical models of planetary system formation.
► TNO (120347) 2004 SB60 is over 900km in diameter. ► It has the lowest albedo and density of any TNO that big. ► TNO Typhon is confirmed to have a density <0.7g/cc.
We report new Hubble Space ...Telescope and Spitzer Space Telescope results concerning the physical properties of the trans-neptunian object (TNO) binaries (120347) Salacia–Actaea (formerly 2004 SB60), and (42355) Typhon–Echidna (formerly 2002 CR46). The mass of the (120347) Salacia–Actaea system is 4.66±0.22×1020kg. The semi-major axis, period, and eccentricity of the binary orbit are a=5619±87km, P=5.49380±0.00016days, and e=0.0084±0.0076, respectively. In terms of the ratio of the semimajor axis to the radius of the Hill sphere, a/rH, (120347) Salacia–Actaea is the tightest TNO binary system with a known orbit. Based on hybrid Standard Thermal Model (hybrid-STM) fits to the data, the effective diameter and V-band geometric albedo of the system are D=954±109km (making it one of the largest known TNOs), and pV=3.57-0.72+1.03%. Thermophysical models for (120347) Salacia suggest that it probably has a thermal inertia ⩽5Jm−2s−1/2K−1, although we cannot rule out values as high as 30Jm−2s−1/2K−1. Based on the magnitude difference between Salacia and Actaea, δ=2.37±0.06, we estimate their individual diameters to be d1=905±103km and d2=303±35km. The mass density of the components is ρ=1.16-0.36+0.59 g/cm3. Hybrid-STM fits to new Spitzer data for Typhon–Echidna give an effective diameter and V-band geometric albedo for the system of D=157±34km, and pV=6.00-2.08+4.10%. Thermophysical models for (42355) Typhon suggest somewhat lower albedos (probably no higher than about 8.2%, as compared to the hybrid-STM upper limit of 10.1%). Taken together with the previously reported mass, this diameter indicates a density of ρ=0.60-0.29+0.72g/cm3, consistent with the very low densities of most other TNOs smaller than 500km diameter. Both objects must have significant amounts of void space in their interiors, particularly if they contain silicates as well as water–ice (as is expected). The ensemble of binary-TNO densities suggests a trend of increasing density with size, with objects smaller than 400km diameter all having densities less than 1g/cm3, and those with diameters greater than 800km all having densities greater than 1g/cm3. If the eccentricity of the binary orbit of (42355) Typhon–Echidna is not due to recent perturbations, considerations of tidal evolution suggest that (42355) Typhon–Echidna must have a rigidity close to that of solid water ice, otherwise the orbital eccentricity of the system would have been damped by now.
Far-ultraviolet to far-infrared images of the nearby galaxy NGC 5194 (M51a), from a combination of space-based (Spitzer, GALEX, and Hubble Space Telescope) and ground-based data, are used to ...investigate local and global star formation and the impact of dust extinction. The Spitzer data provide unprecedented spatial detail in the infrared, down to sizes 6500 pc at the distance of NGC 5194. The multiwavelength set is used to trace the relatively young stellar populations, the ionized gas, and the dust absorption and emission in H II-emitting knots, over 3 orders of magnitude in wavelength range. As is common in spiral galaxies, dust extinction is high in the center of the galaxy (A sub(V) 6 3.5 mag), but its mean value decreases steadily as a function of galactocentric distance, as derived from both gas emission and stellar continuum properties. In the IR/UV-UV color plane, the NGC 5194 H II knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion (61 dex peak to peak) than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, nonionizing stellar population, up to ages 650-100 Myr. While in starbursts the UV light traces the current star formation rate (SFR), in NGC 5194 it traces a combination of current and recent past SFRs. Possibly, mechanical feedback from supernovae is less effective at removing dust and gas from the star formation volume in normal star-forming galaxies than in starbursts because of the typically lower SFR densities in the former. The application of the starburst opacity curve for recovering the intrinsic UV emission (and deriving SFRs) in local and distant galaxies appears therefore appropriate only for SFR densities 1 M sub( )yr super(-1) kpc super(-2). Unlike the UV emission, the monochromatic 24 km luminosity is an accurate local SFR tracer for the H II knots in NGC 5194, with a peak-to-peak dispersion of less than a factor of 3 relative to hydrogen emission line tracers; this suggests that the 24 km emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 km emission to the SFR varies by a factor of a few from galaxy to galaxy; this variation needs to be understood and carefully quantified before the 24 km luminosity can be used as an SFR tracer for galaxy populations. While also correlated with star formation, the 8 km emission is not directly proportional to the number of ionizing photons; it is overluminous, by up to a factor of 62, relative to the galaxy's average in weakly ionized regions and is underluminous, by up to a factor of 63, in strongly ionized regions. This confirms earlier suggestions that the carriers of the 8 km emission are heated by more than one mechanism.
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