Ratio Of Scale Height Research Articles

The impact of turbulent transport efficiency on surface vertical heat fluxes in the Arctic stable boundary layer predicted from similarity theory and machine-learning

Abstract We analyzed 14 days of observations from sonic anemometry and high-resolution fiber optic distributed sensing collected in the stable polar boundary layer (SBL). The study sought to evaluate if and under which conditions the sensible heat flux is related to the temperature gradient. Machine learning methods were employed to identify drivers of and model heat fluxes. We found the recently proposed coupling metric Ω defined as the ratio of the buoyancy length scale and measurement height to delineate physically meaningful transport regimes. The regime transition marks the point where static stability in addition to the vertical turbulence strength control the heat transport, which is rather gradual than abrupt. The maximum downward heat flux is reached when one third of turbulent eddies exceed the opposing buoyancy forces in the SBL. We found evidence that even for large Ω a substantial fraction of the turbulent transport is non-equilibrium. The non-dimensional temperature gradient is better explained by variations in Ω than ζ = zL−1 from Monin-Obukhov Similarity theory. Its continuous organization with Ω across stabilities suggest that the vertical heat transport always remains coupled to the surface, but its efficiency and the resulting flux vary. 43% of the total enthalpy is exchanged during conditions of limited transport efficiency in the very SBL despite the small flux magnitude of ≤ 7 W m−2, which underlines the importance of quantifying the weak surface exchange for polar regions. When predicting sensible heat fluxes using mean quantities from weather stations, the net longwave radiative forcing and the horizontal wind speed are the most important predictors representing stratification and bulk shear.

In LIGO’s Sight? Vigorous Coherent Gravitational Waves from Cooled Collapsar Disks

We present the first numerical study of gravitational waves (GWs) from collapsar disks, using state-of-the-art 3D general relativistic magnetohydrodynamic simulations of collapsing stars. These simulations incorporate a fixed Kerr metric for the central black hole (BH) and employ simplified prescriptions for disk cooling. We find that cooled disks with an expected scale height ratio of H/R ≳ 0.1 at ∼10 gravitational radii induce Rossby instability in compact, high-density rings. The trapped Rossby vortices generate vigorous coherent emission regardless of disk magnetization and BH spin. For BH mass of ∼10 M ⊙, the GW spectrum peaks at ∼100 Hz, with some breadth due to various nonaxisymmetric modes. The spectrum shifts toward lower frequencies as the disk viscously spreads and the circularization radius of the infalling gas increases. Weaker-cooled disks with H/R ≳ 0.3 form a low-density extended structure of spiral arms, resulting in a broader, lower-amplitude spectrum. Assuming an optimistic detection threshold with a matched-filter signal-to-noise ratio of 20 and a rate similar to Type Ib/c supernovae, LIGO–Virgo–KAGRA (LVK) could detect ≲1 event annually, suggesting that GW events may already be hidden in observed data. Third-generation GW detectors could detect dozens to hundreds of collapsar disks annually, depending on the cooling strength and the disk formation rate. The GW amplitudes from collapsar disks are ≳100 times higher with a substantially greater event rate than those from core-collapse supernovae, making them potentially the most promising burst-type GW class for LVK and Cosmic Explorer. This highlights the importance of further exploration and modeling of disk-powered GWs, promising insights into collapsing star physics.

Turbulent Vortex with Moderate Dust Settling Probed by Scattering-induced Polarization in the IRS 48 System

We investigate the crescent-shaped dust trap in the transition disk Oph IRS 48 using well-resolved (sub)millimeter polarimetric observations at ALMA Band 7 (870 μm). The dust polarization map reveals patterns consistent with dust-scattering-induced polarization. There is a relative displacement between the polarized flux and the total flux, which holds the key to understanding the dust scale heights in this system. We model the polarization observations, focusing on the effects of dust scale heights. We find that the interplay between the inclination-induced polarization and the polarization arising from radiation anisotropy in the crescent determines the observed polarization; the anisotropy is controlled by the dust optical depth along the midplane, which is, in turn, determined by the dust scale height in the vertical direction. We find that the dust grains can be neither completely settled nor well mixed with the gas. The completely settled case produces little radial displacement between the total and polarized flux, while the well-mixed case produces an azimuthal pattern in the outer (radial) edge of the crescent that is not observed. Our best model has a gas-to-dust scale height ratio of 2 and can reproduce both the radial displacement and the azimuthal displacement between the total and polarized flux. We infer an effective turbulence α parameter of approximately 0.0001–0.005. The scattering-induced polarization provides insight into a turbulent vortex with a moderate level of dust settling in the IRS 48 system, which is hard to achieve otherwise.

Millimeter gap contrast as a probe for turbulence level in protoplanetary disks

Turbulent motions are believed to regulate angular momentum transport and influence dust evolution in protoplanetary disks. Measuring the strength of turbulence is challenging through gas line observations because of the requirement for high spatial and spectral resolution data, and an exquisite determination of the temperature. In this work, taking the well-known HD 163296 disk as an example, we investigated the contrast of gaps identified in high angular resolution continuum images as a probe for the level of turbulence. With self-consistent radiative transfer models, we simultaneously analyzed the radial brightness profiles along the disk major and minor axes, and the azimuthal brightness profiles of the B67 and B100 rings. By fitting all the gap contrasts measured from these profiles, we constrained the gas-to-dust scale height ratio $\Lambda$ to be $3.0_{-0.8}^{+0.3}$, $1.2_{-0.1}^{+0.1}$ and ${\ge}\,6.5$ for the D48, B67 and B100 regions, respectively. The varying gas-to-dust scale height ratios indicate that the degree of dust settling changes with radius. The inferred values for $\Lambda$ translate into a turbulence level of $\alpha_{\rm turb}\,{<}\,3\times10^{-3}$ in the D48 and B100 regions, which is consistent with previous upper limits set by gas line observations. However, turbulent motions in the B67 ring are strong with $\alpha_{\rm turb}\,{\sim}1.2\,{\times}\,10^{-2}$. Due to the degeneracy between $\Lambda$ and the depth of dust surface density drops, the turbulence strength in the D86 gap region is not constrained.

The distribution of dust in edge-on galaxies: I. The global structure

ABSTRACT In this first paper in a series we present a study of the global dust emission distribution in nearby edge-on spiral galaxies. Our sample consists of 16 angularly large and 13 less spatially resolved galaxies selected from the DustPedia sample. To explore the dust emission distribution, we exploit the Herschel photometry in the range 100–500 $\mu $m. We employ Sérsic and 3D disc models to fit the observed 2D profiles of the galaxies. Both approaches give similar results. Our analysis unequivocally states the case for the presence of extraplanar dust in between 6 and 10 large galaxies. The results reveal that both the disc scale length and height increase as a function of wavelength between 100 and 500 $\mu $m. The dust disc scale height positively correlates with the dust disc scale length, similar to what is observed for the stellar discs. We also find correlations between the scale lengths and scale heights in the near- and far-infrared which suggest that the stellar discs and their dust counterparts are tightly connected. Furthermore, the intrinsic flattening of the dust disc is inversely proportional to the maximum rotation velocity and the dust mass of the galaxy: more massive spiral galaxies host, on average, relatively thinner dust discs. Also, there is a tendency for the dust-to-stellar scale height ratio to decrease with the dust mass and rotation velocity. We conclude that low-mass spiral galaxies host a diffuse, puffed-up dust disc with a thickness similar to that of the stellar disc.

Galaxy flybys: evolution of the bulge, disc, and spiral arms

ABSTRACT Galaxy flybys are as common as mergers in low-redshift Universe and are important for galaxy evolution as they involve the exchange of significant amounts of mass and energy. In this study, we investigate the effect of minor flybys on the bulges, discs, and spiral arms of Milky Way mass galaxies for two types of bulges – classical bulges and boxy/peanut pseudo-bulges. Our N-body simulations comprise of two disc galaxies of mass ratios 10:1 and 5:1, where the discs of the galaxies lie in their orbital plane and the pericentre distance is varied. We performed photometric and kinematic bulge–disc decomposition at regular time-steps and traced the evolution of the disc size, spiral structure, bulge sersic index, bulge mass, and bulge angular momentum. Our results show that the main effect on the discs is disc thickening, which is seen as the increase in the ratio of disc scale height to scale radius. The strength of the spiral structure A2/A0 shows small oscillations about the mean time-varying amplitude in the pseudo-bulge host galaxies. The flyby has no significant effect on non-rotating classical bulge, which shows that these bulges are extremely stable in galaxy interactions. However, the pseudo-bulges become dynamically hotter in flybys indicating that flybys may play an important role in accelerating the rate of secular evolution in disc galaxies. This effect on pseudo-bulges is a result of their rotating nature as part of the bar. Also, flybys do not affect the time and strength of bar buckling.

The Effect of Early Limited Activity after Bipedicular Percutaneous Vertebroplasty to Treat Acute Painful Osteoporotic Vertebral Compression Fractures

Although percutaneous vertebroplasty (PVP) can effectively relieve the pain for patients with acute osteoporotic vertebral compression fractures (OVCFs), many patients still complain of mild back pain in the early postoperative period. The aim of this study was to assess the effect of early limited activity (LA) on prognosis after bipedicular small-cement-volume (i.e., PVP) to treat single-segment acute OVCFs. A prospective study and retrospective observations were performed on 125 patients with a minimum of 1 year of follow-up. A university hospital orthopedics and pathology departments. All patients were allocated into an LA group (n = 64) and an unlimited activity group (ULA group, n = 61). Patients in the LA group were suggested to keep time of off-bed activity < 4 hours per day in the first 3 weeks postoperatively. Patients in the ULA group did not limit activity. The demographic, clinical, and radiologic outcomes were assessed, such as pain intensity Numeric Rating Scale (NRS-11) and vertebral height ratio (i.e., fractured vertebral height/adjacent nonfractured vertebral height). Based on outcomes following surgery, all patients were classified as responders (NRS-11 score 1-day postoperation < 50% of preoperative NRS-11 score) or low responders (NRS-11 score 1-day postoperation >= 50% of preoperative NRS-11 score). The demographic results and complications were similar. In the LA group, NRS-11 scores at 1 and 3 months postoperation respectively were 2.23 ± 0.42 and 1.46 ± 0.40, and corresponding scores respectively were 2.85 ± 0.80 and 1.73 ± 0.77 in the ULA group, and there was a difference in the 2 groups in both time points (P < 0.05). At 12 months postoperation, anterior and middle vertebral height ratio respectively were 78.42% ± 3.52% and 82.37% ± 3.49% in the LA group, which were higher than 76.87% ± 3.68% and 81.10% ± 3.31% in the ULA group (P < 0.05). Thirty-two cases were low responders. Among those, NRS-11 scores at 1 and 3 months postoperation respectively were 2.29 ± 0.45 and 1.53 ± 0.46 in the LA group, which were lower than 3.67 ± 0.80 and 2.56 ± 0.79 in the ULA group (P < 0.05), and at 12 months postoperation, anterior vertebral height ratio was 79.81% ± 3.25% in the LA group and 75.60% ± 3.50% in the ULA group (P < 0.05). First, some patients lacked the results of bone mineral density during follow-up; second, the limited time in our study was chosen from our previous working experience, which may lack an objective basis; third, NRS-11 is solely used as an indicator of clinical outcomes in our study; finally, our next studies can increase the sample size to improve the clinically difference. LA in the early period after PVP can help patients achieve more pain relief postoperatively and maintain better vertebral shape, especially for low responders. Osteoporotic vertebral compression fractures, percutaneous vertebroplasty, Numeric Rating Scale, vertebral height, responders, low responders, limited activity, complications.

The Influence of Accretion Disk Thickness on the Large-scale Magnetic Dynamo

Abstract The evolution of the magnetic field from the large-scale dynamo is considered a central feature of the accretion disk around a black hole. The resulting low-frequency oscillations introduced from the growth and decay of the field strength, along with the change in field orientation, play an integral role in the accretion disk behavior. Despite the importance of this process and how commonly it is invoked to explain variable features, it still remains poorly understood. We present a study of the dynamo using a suite of four global, high-resolution, MHD accretion disk simulations. We systematically vary the scale height ratio and find the large-scale dynamo fails to organize above a scale height ratio of h/r ≳ 0.2. Using spacetime diagrams of the azimuthal magnetic field, we show the large-scale dynamo is well ordered in the thinner accretion disk models, but fails to develop the characteristic “butterfly” pattern when the scale height ratio is increased, a feature which is also reflected in the power spectra. Additionally, we calculate the dynamo α-parameter and generate synthetic light curves. Using an emission proxy, we find the disks have markedly different characters as stochastic photometric fluctuations have a larger amplitude when the dynamo is unordered.

A constant intrinsic thickness for dwarf irregular galaxies?

Abstract We assess the intrinsic thickness parameter, qo, for a sample of dwarf irregular galaxies and compare to larger, more massive spiral galaxy systems. We use optical photometry to determine b/a (minor-to-major axial ratio), and we use the H i kinematic inclination angle to derive qo. We find that qo ranges from 0.1 to 0.8 for the dwarfs. We find a trend in qo with luminosity, indicating that fainter dwarfs are thicker than brighter ones, similar to previous studies. However, we also find a trend in qo with H i kinematic inclination, which indicates that either the radius at which we measure b/a may be too small, or, that the stellar and gas discs have different inclinations. Because we selected only those objects that have nearly identical morphological and kinematic position angles, we find the latter reason unlikely. We find a weak trend in qo with hz/RD (ratio of stellar scaleheight to scalelength), which points to b/a not fully representing the stellar disc distribution. We conclude that a constant qo may not be appropriate for dwarf irregulars.

Transmission spectroscopy of HAT-P-32b with the LBT: confirmation of clouds/hazes in the planetary atmosphere

Spectroscopic observations of a transit event of an extrasolar planet offer the opportunity to study the composition of the planetary atmosphere. We observed a transit of the inflated Hot Jupiter HAT-P-32b with MODS at the LBT to characterize its atmosphere from 3300 to 10000 AA. A time series of target and reference star spectra was binned in two broad-band wavelength channels, from which differential transit light curves were constructed. These broad-band light curves were used to confirm previous transit parameter determinations. To derive the planetary transmission spectrum with a resolution of R ~ 60, we created a chromatic set of 62 narrow-band light curves with an average wavelength width of about 100 AA. The spectrum was corrected for the third-light of a near-by M star, whose spectrum was resolved in the individual exposures. Additionally, we undertook a photometric monitoring campaign of the host star to correct for the influence of starspots. The transmission spectrum of HAT-P-32b shows no pressure-broadened absorption features from Na and K, which is interpreted by the presence of clouds or hazes in the planetary atmosphere. This result is in agreement to previous studies on the same planet. The presence of TiO in gas phase could be ruled out. We find a 2.8 sigma indication of increased absorption in the line core of potassium (KI~7699 AA). No narrow absorption features of Na and Halpha were detected. Furthermore, tentative indications were found for a slope of increasing opacity toward blue wavelengths from the near-IR to the near-UV with an amplitude of two scale heights. If confirmed by follow-up observations, it can be explained by aerosols either causing Mie scattering or causing Rayleigh scattering with an aerosol - gas scale height ratio below unity.

Modeling the plasmasphere to topside ionosphere scale height ratio

A new model of plasmasphere to topside ionosphere scale heights ratio is developed, based on topside electron density ( N e ) profiles deduced from the International Satellites for Ionospheric Studies (ISIS)-1 satellite measurements. The model is able to improve operational algorithms for space weather predictions. The topside ionospheric and plasmaspheric scale heights are determined by the lowest and largest gradients of measured profiles, respectively, converted in d h /dlnNe units. The new model depends on four parameters: the month of the year ( M ), the local time (LT), the geomagnetic latitude (glat), and the ln(O + ) density ( z O) at the O + -H + ion transition height. It is designed to replace the old one-dimensional model of the ratio in the TaD (TSM-assisted Digisonde) profiler. The parameters M , LT, and glat are approximated by trigonometric basis functions, while z O is described by a polynomial. A series of models were produced with different number of coefficients (number of terms) of the basis functions. Comparison between models revealed that those with larger number of coefficients can produce unrealistic extremes of the model curves due to the non-uniform sampling of data along the axes. Further considered is the simplest model approximating M , LT, and glat by simple 24 sinusoidal functions and linearly depending on z O. The model description and its 54 coefficients are given in and can be used by other users for reconstruction of plasmasphere density profiles. The main variation of the ratio along geomagnetic latitude at fixed values of the other model parameters is illustrated in a series of plots.

Climatology of global gravity wave activity and dissipation revealed by SABER/TIMED temperature observations

Gravity wave activity and dissipation in the height range from the low stratosphere to the low thermosphere (25–115 km) covering latitudes between 50°S and 50°N are statistically studied by using 9-year (January 22, 2002–December 31, 2010) SABER/TIMED temperature data. We propose a method to extract realistic gravity wave fluctuations from the temperature profiles and treat square temperature fluctuations as GW activity. Overall, the gravity wave activity generally increases with height. Near the equator (0°–10°), the gravity wave activity shows a quasi-biennial variation in the stratosphere (below 40 km) while from 20° to 30°, it exhibits an annual variation below 40 km; in low latitudes (0°–30°) between the upper stratosphere and the low thermosphere (40–115 km), the gravity wave activity shows a semi-annual variation. In middle latitudes (40°–50°), the gravity wave activity has a clear annual variation below 85 km. In addition, we observe a four-monthly variation with peaks occurring usually in April, August, December in the northern hemisphere and in February, June, October in the southern hemisphere, respectively, above 85 km in middle latitudes, which has been seldom reported in gravity wave activity. In order to study the dissipation of gravity wave propagation, we calculate the gravity wave dissipation ratio, which is defined as the ratio of the gravity wave growth scale height to the atmosphere density scale height. The height variation of the dissipation ratio indicates that strong gravity wave dissipation mainly concentrates in the three height regions: the stratosphere (30–60 km), the mesopause (around 85 km) and the low thermosphere (above 100 km). Besides, gravity wave energy enhancement can be also observed in the background atmosphere.

TIME EVOLUTION OF A VISCOUS PROTOPLANETARY DISK WITH A FREE GEOMETRY: TOWARD A MORE SELF-CONSISTENT PICTURE

Observations of protoplanetary disks show that some characteristics seem recurrent, even in star formation regions that are physically distant such as surface mass density profiles varying as $r^{-1}$, or aspect ratios about 0.03 to 0.23. Accretion rates are also recurrently found around $10^{-8} - 10^{-6}~\mathrm{M_{\odot}~yr^{-1}}$ for disks already evolved (Isella et al., 2009, Andrews et al., 2009, 2010). Several models have been developed in order to recover these properties. However, most of them usually simplify the disk geometry if not its mid-plane temperature. This has major consequences for modeling the disk evolution over million years and consequently planet migration. In the present paper, we develop a viscous evolution hydrodynamical numerical code that determines simultaneously the disk photosphere geometry and the mid-plane temperature. We then compare our results of long-term simulations with similar simulations of disks with a constrained geometry along the Chiang & Goldreich (1997) prescription (dlnH/dlnr = 9/7). We find that the constrained geometry models provide a good approximation of the disk surface density evolution. However, they differ significantly regarding the temperature time evolution. In addition, we find that shadowed regions naturally appear at the transition between viscously dominated and radiation dominated regions that falls in the region of planetary formation. We show that $\chi$ (photosphere height to pressure scale height ratio) cannot be considered as a constant, consistently with Watanabe et al. (2008). Comparisons with observations show that all disk naturally evolve toward a shallow surface density disk ($\Sigma \propto r^{-1}$). The mass flux across the disk stabilizes in about 1 million year typically.

A multilayer model for thermal infrared emission of Saturn’s rings II: Albedo, spins, and vertical mixing of ring particles inferred from Cassini CIRS

Since the Saturn orbit insertion of the Cassini spacecraft in mid-2004, the Cassini composite infrared spectrometer (CIRS) measured temperatures of Saturn’s main rings at various observational geometries. In the present study, we apply our new thermal model (Morishima, R., Salo, H., Ohtsuki, K. [2009]. Icarus 201, 634–654) for fitting to the early phase Cassini data (Spilker, L.J., and 11 colleagues [2006]. Planet. Space Sci. 54, 1167–1176). Our model is based on classical radiative transfer and takes into account the heat transport due to particle motion in the azimuthal and vertical directions. The model assumes a bimodal size distribution consisting of small fast rotators and large slow rotators. We estimated the bolometric Bond albedo, A V, the fraction of fast rotators in cross section, f fast, and the thermal inertia, Γ, by the data fitting at every radius from the inner C ring to the outer A ring. The albedo A V is 0.1–0.4, 0.5–0.7, 0.4, 0.5 for the C ring, the B ring, the Cassini division, and the A ring, respectively. The fraction f fast depends on the ratio of scale height of fast rotators to that of slow rotators, h r. When h r = 1, f fast is roughly half for the entire rings, except for the A ring, where f fast increases from 0.5 to 0.9 with increasing saturnocentric radius. When h r increases from 1 to 3, f fast decreases by 0.2–0.4 for the B and A rings while no change in f fast is seen for the optically thin C ring and Cassini division. The large f fast seen in the outer A ring probably indicates that a large number of small particles detach from large particles in high velocity collisions due to satellite perturbations or self-gravity wakes. The thermal inertia, Γ, is constrained from the efficiency of the vertical heat transport due to particle motion between the lit and unlit faces, and is coupled with the type of vertical motion. We found that in most regions, except for the mid B ring, sinusoidal vertical motion without bouncing is more reasonable than cycloidal motion assuming bouncing at the midplane, because the latter motion gives too large Γ as compared with previous estimations. For the mid B ring, where the optical depth is highest in Saturn’s rings, cycloidal vertical motion is more reasonable than sinusoidal vertical motion which gives too small Γ.

Testing turbulent convection theory in solar models - I. Structure of the solar convection zone

Turbulent convection models (TCMs) based on hydrodynamic moment equations are compared with the classical mixing-length theory (MLT) in solar models. The aim is to test the effects of some physical processes on the structure of the solar convection zone, such as the dissipation, diffusion and anisotropy of turbulence that have been ignored in the MLT. Free parameters introduced by the TCMs are also tested in order to find appropriate values for astrophysical applications. It is found that the TCMs usually give larger convective heat fluxes than the MLT does, and the heat transport efficiency is sensitively related to the dissipation parameters used in the TCMs. As a result of calibrating to the present solar values, our solar models usually have rather smaller values of the mixing length to local pressure scaleheight ratio than the standard solar model. The turbulent diffusion is found to have important effects on the structure of the solar convection zone. It leads to significantly lowered and expanded profiles for the Reynolds correlations, and a larger temperature gradient in the central part of the superadiabatic convection region but a smaller one near the boundaries of the convection zone. It is interesting to note that, due to a careful treatment of turbulence developing towards isotropic state, our non-local TCM results in radially dominated motion in the central part and horizontally dominated motion near the boundaries of the convection zone, just as what has been observed in many 3D numerical simulations. Our solar models with the TCMs give small but meaningful differences in the temperature and sound speed profiles compared with the standard solar model using the MLT.

Modeling the NIR-silhouette massive disk candidate in M 17

Aims. The physical properties of the massive disk candidate in the star-forming region M 17 are analyzed. Methods. Making use of the rare configuration in which the gas and dust structure is seen in silhouette against the background radiation at λ = 2.2 µm, we determine the column density distribution from a high-resolution NAOS/CONICA image. The influence of scattered light on the mass determination is analyzed using 3D radiative transfer calculations. Further upper flux limits derived from observations with the Spitzer telescope at MIR wavelengths are used together with the NACO image to estimate the flux from the central object. For a range of stellar radii, stellar surface temperatures, and dust grain sizes, we apply three different models to account for the observed fluxes. The stability of the disk against self-gravitational forces is analyzed calculating the ratio of the gravitational acceleration by the central object and the disk, and the deviations from a Keplerian profile. Results. We find that the column density is consistent with a central source surrounded by a rotationally symmetric distribution of gas and dust. The extent of the symmetric disk part is about 3000 AU, with a warped point-symmetrical extension beyond that radius, and therefore larger than any circumstellar disk yet detected. The modeling yields a radial density powerlaw exponent of −1.1 indicating a flat radial density distribution, and a large e-folding scale height ratio H/R of about 0.5. The mass of the entire disk estimated from the column density is discussed depending on the assumed distance and the dust model and ranges between 0.02 and 5 M� . We conclude that unless a star is located close to the disk in the foreground, scattered light will have little influence on the mass determination. We present a Spitzer image taken at λ = 7.8 µm with the disk seen in emission and identify polycyclic aromatic hydrocarbon (PAH) emission on the disk surface excited by the nearby massive stars as a possible source. Our 3D radiative transfer calculations for the scattered light image of the central source through an edge-on disk indicate that the elliptical shape seen in the NACO image does not require the assumption of a binary system and that it is consistent with a single object. We derive stellar main sequence masses of several M� ,5 0M� ,o r 10M� , depending on our assumptions that the extinction of the stellar flux is dominated (i) by the outer disk, (ii) by an inner disk comparable to the disks around intermediate-mass stars, or (iii) by an inner disk with dominating hot dust emission. We find that even for a star-disk mass ratio of 1, only the outer parts of the circumstellar disk may be influenced by self-gravity effects due to the large e-folding scale height ratio.

Disk Evolution sincez ∼ 1 in a CDM Universe

Increasingly large populations of disk galaxies are now being observed at increasingly high redshifts, providing new constraints on our knowledge of how such galaxies evolve. Are these observations consistent with a cosmology in which structures form hierarchically? To probe this question, we employ SPH/N-body galaxy-scale simulations of late-type galaxies. We examine the evolution of these simulated disk galaxies from redshift 1 to 0, looking at the mass-size and luminosity-size relations, and the thickness parameter, defined as the ratio of scale height to scale length. The structural parameters of our simulated disks settle down quickly, and after redshift z = 1 the galaxies evolve to become only slightly flatter. Our simulated present-day galaxies are larger, more massive, less bright, and redder than at z = 1. The inside-out nature of the growth of our simulated galaxies reduces, and perhaps eliminates, expectations of evolution in the size-mass relation.

Identification of the 10-μm ammonia ice feature on Jupiter

We present the first detection of NH3 ice in the thermal infrared in Jupiter's atmosphere using Cassini CIRS observations in the 10-μm region obtained on 31 December 2000 and 1 January 2001.We identify a brightness temperature difference αTB(1040cm−1)−TB(1060cm−1) as an indicator of spectrally identifiable NH3 ice, where 1040cm−1 is an adjacent continuum region and 1060cm−1 is the NH3 ice feature. Higher values of α imply a stronger NH3 ice signature in the spectrum. Using midlatitude zonally averaged CIRS spectra, we demonstrate systematic spatial variations in α, with the highest values at the equator and near 23°N.In one CIRS spectral average (covering 22–25°N and 140–240°W), our radiative transfer models are consistent with an optical depth of 0.75±0.25 for NH3 ice particles modeled as randomly oriented 4:1 prolate spheroids (volumeequivalentradius=0.79μm). Particles larger or smaller than 1μm by about a factor of 2 would be unable to duplicate the observed NH3 ice feature at 1060cm−1: absorption due to larger particles is excessively broadened, and absorption due to smaller particles is hidden by NH3 gas absorption at 1067cm−1. We also modeled an average spectrum for a second region on Jupiter (14–17°N and 10–70°W), finding an upper limit of τ=0.2 for the same NH3 ice particle type. The choice of prolate spheroid particles is based on laboratory studies of NH3 ice aerosols, although 1-μm Mie-scattering spheres would also have detectable signatures at 1060cm−1. We model the 1-μmNH3 ice cloud with a particle-to-gas scale height ratio Hp/Hg=1. For both CIRS spectra analyzed, the spectrum at frequencies greater than 1100cm−1 also requires a second cloud with essentially grey absorption, which we modeled using 10-μmNH3 ice spheres distributed with Hp/Hg=1/8 and a cloud base at 790mbar.

Atmospheric Lensing and Oblateness Effects during an Extrasolar Planetary Transit

Future high-precision photometric measurements of transiting extrasolar planets promise to tell us much about the characteristics of these systems. We examine how atmospheric lensing and (projected) planet oblateness/ellipticity modify transit light curves. The large density gradients expected in planet atmospheres can offset the unfavorably large observer lens–to–source lens distance ratio and allow the existence of caustics. Under such conditions of strong lensing, which we quantify with an analytic expression, starlight from all points in the planet’s shadow is refracted into view, producing a characteristic slowing down of the dimming at ingress (vice versa for egress). A search over several parameters, such as the limb-darkening profile, the planet radius, the transit speed, and the transit geometry, cannot produce a nonlensed transit light curve that can mimic a lensed light curve. The fractional change in the diminution of starlight is approximately the ratio of atmospheric scale height to planet radius, expected to be 1% or less. The lensing signal varies strongly with wavelength—caustics are hidden at wave bands where absorption and scattering are strong. Planet oblateness induces an asymmetry to the transit light curve about the point of minimum flux, which varies with the planet orientation with respect to the direction of motion. The fractional asymmetry is at the level of 0.5% for a projected oblateness of 10%, independent of whether or not lensing is important. For favorable ratios of planet radius to stellar radius (i.e., gas giant planets), the above effects are potentially observable with future space-based missions. Such measurements could constrain the planet shape and its atmospheric scale height, density, and refractive coefficient, providing information on its rotation, temperature, and composition. We have examined a large range of planetary system parameter space including the planetary scale height and orbital distance. For HD 209458b, the only currently known transiting extrasolar planet, caustics are absent because of the very small lens-source separation (and a large scale height caused by a high temperature from the small separation). Its oblateness is also expected to be small because of the tidal locking of its rotation to orbital motion. Finally, we provide estimates of other variations to transit light curves that could be of comparable importance—including rings, satellites, stellar oscillations, star spots, and weather. Subject headings: gravitational lensing — planetary systems — stars: atmospheres

Oosterhoff I and II RR Lyrae Variable Masses

AbstractThe anomalously low theoretical masses for double-mode RR Lyrae variable stars obtained from accurately observed period ratios recently have been increased by using stellar models constructed with the new Livermore OPAL opacities. These new models appear to the lowest order radial modes to be less concentrated in density, and they thus produce longer periods. Since the fundamental mode is increased in period more than the first overtone, this period ratio from the model is decreased. Cox (1991) showed that now these pulsation-based masses agree with evolution masses, and all RR Lyrae variable stars show little mass loss from the red giant tip in earlier evolution. This investigation has been validated by two additional papers that use more directly the OPAL opacities rather than simple adjustments to the Stellingwerf (1975ab) fit. These new papers (Kovacs, Buchler, and Marom, 1991, and Kovacs, Buchler, Marom, Iglesias, and Rogers, 1992) also display composition effects for Population II mixtures.My new study uses directly the latest OPAL intermediate coupling opacities for the Oosterhoff I and II (Z=0.0003 and 0.0001, respectively) compositions in frozen-in convection (mixing length/pressure scale height ratio=1.25) models to see if there is any difference between the masses of variable stars in these two globular cluster classes. Derived masses are just over 0.65 and 0.75 M⊙, respectively. The Oosterhoff I mass could be as large as the Oosterhoff II mass only if the iron abundance in the Z mixture is increased by more than 30% over that for the solar mixture.The Sandage effect of higher periods for variables in Oosterhoff II clusters at lower Z than for the Oosterhoff I clusters might be explained with my new masses by having the Oosterhoff II variables born much to the blue of the instability strip and evolving at increasing luminosity to the red. For Oosterhoff I variables, they can be born at their lower mass and higher Z inside the instability strip at lower luminosity, evolving both to the red and blue with little evolutionary luminosity increase before they become red nonvariables.