Large Scale Height Research Articles

Can thick disks originate through minor mergers?

In this study we use numerical N-body/SPH simulations to investigate the vertical stellar distribution of thick disks formed in minor mergers. These disks have a sech-shape vertical surface density profile which needs two sech functions with different scale heights to fit due to an additional stellar excesses at large heights ( z ≳ 4 − 5 kpc). The scale heights of merger-induced thick disks increase with radius but the details of this thickening are sensitive to orbital configurations and gas fraction of the galaxies. The scale height of the stellar excess is constant with radius and largely independent of the initial conditions. The effects of minor mergers is cumulative in the sense that several minor mergers have the same impact of a single minor merger of the same total mass. Scattering by self-gravitating masses (clumps) in unstable gaseous disks can produce thick disks with constant scale height, however, there is not a stellar excess at large scale heights. Most of these results are found to be in good agreement with observations and thus we conclude that minor mergers remains a viable mechanism for the creation of galactic thick disks.

X-RAY AND GAMMA-RAY FLASHES FROM TYPE Ia SUPERNOVAE?

We investigate two potential mechanisms that will produce X-ray and gamma-ray flashes from Type Ia supernovae (SN-Ia). The mechanisms are the breakout of the thermonuclear burning front as it reaches the surface of the white dwarf and the interaction of the rapidly expanding envelope with an accretion disk. Based on the delayed-detonation scenario and detailed radiation-hydro calculation which include nuclear networks, we find that both mechanisms produce ~1 second flashes of high energy radiation with peak luminosities of 10^48 to 10^50 erg/sec with fast rises and exponential declines. The X- and gamma-ray visibility of a SN-Ia will depend strongly on self absorption within the progenitor system, specifically on the properties of the accretion disk and its orientation towards the observer. Such X-ray and gamma-ray flashes could be detected as triggered events by Gamma-Ray Burst (GRB) detectors on satellites, with events in current GRB catalogs. We have searched through the GRB catalogs (for the BATSE, HETE, and Swift experiments) for GRBs that occur at the extrapolated time of explosion and in the correct direction for known Type Ia supernovae with radial velocity of less than 3,000 km/s. For BATSE about 12.9+-3.6 nearby SNe Ia should have been detected, but only 0.8+-0.7 non-coincidental matches have been found. With the HETE and Swift satellites, we expect to see 5.6+-1.3 SN-Ia flashes from known nearby SNe Ia but, yet, no SN-Ia flashes were detected. These place observational limits that the bolometric peak luminosity of SN-Ia Flashes must be less ~10^46 erg/s. We attribute the difference between theory and observational limits to the absorption of the X- and gamma-rays by the accretion disk of large scale height or common envelope that would be smothering the white dwarf.

Altimetry of the Venus cloud tops from the Venus Express observations

Simultaneous observations of Venus by Visible and Infrared Thermal Imaging Spectrometer and Venus Monitoring Camera onboard the Venus Express spacecraft are used to map the cloud top altitude and to relate it to the ultraviolet (UV) markings. The cloud top altitude is retrieved from the depth of CO2 absorption band at 1.6 μm. In low and middle latitudes the cloud top is located at 74 ± 1 km. It decreases poleward of ±50° and reaches 63–69 km in the polar regions. This depression coincides with the eye of the planetary vortex. At the same latitude and hour angle, cloud top can experience fast variations of about 1 km in tens of hours, while larger long‐term variations of several kilometers have been observed only at high latitudes. UV markings correlate with the cloud altimetry, however, the difference between adjacent UV dark and bright regions does not exceed several hundred meters. Surprisingly, CO2 absorption bands are often weaker in the dark UV features, indicating that these clouds may be a few hundred meters higher or have a larger scale height than neighboring clouds. Ultraviolet dark spiral arms, which are often seen at about −70°, correspond to higher altitudes or to the regions with strong latitudinal gradient of the cloud top altitude. Cloud altimetry in the polar region reveals the structure that correlates with the thermal emission maps but is invisible in UV images. This implies that the UV optically thick polar hood is transparent in the near IR.

Circumplanetary disc properties obtained from radiation hydrodynamical simulations of gas accretion by protoplanets

We investigate the properties of circumplanetary discs formed in three-dimensional, self-gravitating radiation hydrodynamical models of gas accretion by protoplanets. We determine disc sizes, scaleheights, and density and temperature profiles for different protoplanet masses, in solar nebulae of differing grain opacities. We find that the analytical prediction of circumplanetary disc radii in an evacuated gap (R_Hill/3) from Quillen & Trilling (1998) yields a good estimate for discs formed by high mass protoplanets. The radial density profiles of the circumplanetary discs may be described by power-laws between r^-2 and r^-3/2. We find no evidence for the ring-like density enhancements that have been found in some previous models of circumplanetary discs. Temperature profiles follow a ~r^-7/10 power-law regardless of protoplanet mass or nebula grain opacity. The discs invariably have large scaleheights (H/r > 0.2), making them thick in comparison with their encompassing circumstellar discs, and they show no flaring.

Seasonal variations of the electron density distribution in the polar region during geomagnetically quiet periods near solar maximum

Meridional electron density distributions above 45° invariant latitude (ILAT) during geomagnetically quiet periods are statistically studied. Electron density data were obtained from plasma waves observed by the Akebono satellite from March 1989 to February 1991 (near solar maximum) in an altitude range of 274–10,500 km. Field‐aligned electron density profiles were fitted by the sum of exponential and power law functions. The transition height, where the power law term equals the exponential term, is highest in the summer (at low solar zenith angle (SZA)) at ∼4000 km and lowest in the winter (at high SZA) at ∼1800 km in a region of ILAT ≥ 70°; this is caused by the larger scale height in the summer (∼550 km) than that in the winter (∼250 km). The largest seasonal variation and SZA dependence of the electron density are found at an altitude of ∼2000 km with a factor of ∼50 (∼104 /cc in the summer, ∼103 /cc in the winter) in the trough, auroral, and polar cap regions. The seasonal variation and SZA dependence are smaller, about a factor of 5–10, above ∼5000 km. Day‐night asymmetries in each season (within a factor of 5) are smaller than the seasonal variation. The scale height is larger in the dayside than in the nightside in each season. These results indicate that photoionization processes in the ionosphere strongly control electron density distributions up to at least ∼5000 km in the trough, auroral, and polar cap regions.

A dense disk of dust around the born-again Sakurai's object

In 1996, Sakurai's object (V4334 Sgr) suddenly brightened in the centre of a faint Planetary Nebula (PN). This very rare event was interpreted as the reignition of a hot white dwarf that caused a rapid evolution back to the cool giant phase. From 1998 on, a copious amount of dust has formed continuously, screening out the star which has remained embedded in this expanding high optical depth envelope. The new observations, reported here, are used to study the morphology of the circumstellar dust in order to investigate the hypothesis that Sakurai's Object is surrounded by a thick spherical envelope of dust. We have obtained unprecedented, high-angular resolution spectro-interferometric observations, taken with the mid-IR interferometer MIDI/VLTI, which resolve the dust envelope of Sakurai's object. We report the discovery of a unexpectedly compact (30 x 40 milliarcsec, 105 x 140 AU assuming a distance of 3.5 kpc), highly inclined, dust disk. We used Monte Carlo radiative-transfer simulations of a stratified disk to constrain its geometric and physical parameters, although such a model is only a rough approximation of the rapidly evolving dust structure. Even though the fits are not fully satisfactory, some useful and robust constraints can be inferred. The disk inclination is estimated to be 75+/-3 degree with a large scale height of 47+/-7 AU. The dust mass of the disk is estimated to be 6 10^{-5} solar mass. The major axis of the disk (132+/-3 degree) is aligned with an asymmetry seen in the old PN that was re-investigated as part of this study. This implies that the mechanism responsible for shaping the dust envelope surrounding Sakurai's object was already at work when the old PN formed.

Emission from Very Small Grains and PAH Molecules in Monte Carlo Radiation Transfer Codes: Application to the Edge‐On Disk of Gomez’s Hamburger

We have modeled optical to far infrared images, photometry, and spectroscopy of the object known as Gomez's Hamburger. We reproduce the images and spectrum with an edge-on disk of mass 0.3M_sun and radius 1600AU, surrounding an A0 III star at a distance of 280pc. Our mass estimate is in excellent agreement with recent CO observations. However, our distance determination is more than an order of magnitude smaller than previous analyses which inaccurately interpreted the optical spectrum. To accurately model the infrared spectrum we have extended our Monte Carlo radiation transfer codes to include emission from polycyclic aromatic hydrocarbon (PAH) molecules and very small grains (VSG). We do this using pre-computed PAH/VSG emissivity files for a wide range of values of the mean intensity of the exciting radiation field. When Monte Carlo energy packets are absorbed by PAHs/VSGs we reprocess them to other wavelengths by sampling from the emissivity files, thus simulating the absorption and re-emission process without reproducing lengthy computations of statistical equilibrium, excitation and de-excitation in the complex many level molecules. Using emissivity lookup tables in our Monte Carlo codes gives the flexibility to use the latest grain physics calculations of PAH/VSG emissivity and opacity that are being continually updated in the light of higher resolution infrared spectra. We find our approach gives a good representation of the observed PAH spectrum from the disk of Gomez's Hamburger. Our models also indicate the PAHs/VSGs in the disk have a larger scaleheight than larger radiative equilibrium grains, providing evidence for dust coagulation and settling to the midplane.

Global properties of the H Idistribution in the outer Milky Way

Aims: We derive the 3-D HI volume density distribution for the Galactic disk out to R = 60 kpc. Methods: Our analysis is based on parameters for the warp and rotation curve derived previously. The data are taken from the Leiden/Argentine/Bonn all sky 21-cm line survey. Results: The Milky Way HI disk is significantly warped but shows a coherent structure out to R = 35 kpc. The radial surface density distribution, the densities in the middle of the warped plane, and the HI scale heights all follow exponential relations. The radial scale length for the surface density distribution of the HI disk is 3.75 kpc. Gas at the outskirts for 40 < R < 60 kpc is described best by a distribution with an exponential radial scale length of 7.5 kpc and a velocity dispersion of 74 km/s. Such a highly turbulent medium fits also well with the average shape of the high velocity profile wings observed at high latitudes. The turbulent pressure gradient of such extra-planar gas is on average in balance with the gravitational forces. About 10% of the Milky Way HI gas is in this state. The large scale HI distribution is lopsided; for R < 15 kpc there is more gas in the south. The HI flaring indicates that this asymmetry is caused by a dark matter wake, located at R = 25 kpc in direction of the Magellanic System. Conclusions: The HI disk is made up of two major components. Most prominent is the normal HI disk which can be traced to R = 35 kpc. This is surrounded by a patchy distribution of highly turbulent gas reaching large scale heights but also large radial distances. At the position of the Sun the exponential scale height in the z direction is 3.9 kpc. This component resembles the anomalous gas discovered previously in some galaxies.

Nearby stars of the Galactic disc and halo – IV

The Milky Way Galaxy has an age of about 13 billion years. Solar-type stars evolve all the long way to the realm of degenerate objects on essentially this time-scale. This, as well as the particular advantage that the Sun offers through reliable differential spectroscopic analyses, render these stars the ideal tracers for the fossil record of our parent spiral. Astrophysics is a science that is known to be notoriously plagued by selection effects. The present work – with a major focus in this fourth contribution on model atmosphere analyses of spectroscopic binaries and multiple star systems – aims at a volume-complete sample of about 300 nearby F-, G-, and K-type stars that particularly avoids any kinematical or chemical pre-selection from the outset. It thereby provides an unbiased record of the local stellar populations – the ancient thick disc and the much younger thin disc. On this base, the detailed individual scrutiny of the long-lived stars of both populations unveils the thick disc as a single-burst component with a local normalization of no less than 20 per cent. This enormous fraction, combined with its much larger scaleheight, implies a mass for the thick disc that is comparable to that of the thin disc. On account of its completely different mass-to-light ratio the thick disc thereby becomes the dark side of the Milky Way, an ideal major source for baryonic dark matter. This massive, ancient population consequently challenges any gradual build-up scenario for our parent spiral. Even more, on the supposition that the Galaxy is not unusual, the thick disc – as it emerges from this unbiased spectroscopic work – particularly challenges the hierarchical cold-dark-matter-dominated formation picture for spiral galaxies in general.

New description of Io's cold plasma torus

Despite more than 25 years of study of the Io plasma torus, its generation, dynamics, and even its spatial structure are still poorly understood, especially in the case of the inner, cold region of the torus. To remedy this lack, we analyzed ground‐based coronagraphic images of the torus in S+ 6371 Å emission. We derived cold torus properties by modeling and removing these images' inherent line‐of‐sight integration and atmospheric blurring, using new deconvolution techniques, obtaining high‐spatial‐resolution estimates of the three‐dimensional (3‐D) S+ distributions. From these 3‐D distributions, we discovered that the cold torus is washer‐shaped, with a roughly constant vertical thickness ≤0.25 Jovian radius (RJ), and a radial width that varies from 0.6 to 0.9 RJ. The cold torus is separated by a 0.1–0.2 RJ‐wide low‐density region, or “gap,” from the “ribbon” region which lies just outside it. The small, approximately constant washer height implies an ion parallel temperature (T∥) of ∼3 eV, compared with a ribbon T∥ that varies from about 20 to 50 eV as a function of Jovian magnetic longitude (λIII). The washer has a distinct inner edge, not seen before, whose jovicentric distance varies with λIII so as to create the variable cold torus width. Thus this inner edge is concentric with neither Jupiter nor the rest of the torus. We also confirm the existence of a tilt between the midplanes of the ribbon and cold torus, with an orientation that cannot be produced by the magnetic mirror force acting on ion temperature anisotropy. The structure and composition of the gap and cold torus are best explained by a model in which a small amount of warm S+ plasma diffuses inwards while radiatively cooling. While still warm, its distribution over a large scale height keeps its density small, forming the gap. After sufficient cooling, it collapses to the centrifugal equator, where its higher density and continued inward diffusion make it more visible as the cold torus washer. However, its low electron temperature (probably ≤ T∥) must be kept from further decline by a hitherto‐unsuspected energy source that powers the observed visible wavelength radiation from the cold torus and fluctuates on timescales less than the plasma diffusion time. The formation of the abrupt cold torus inner edge might indicate the loss there of either this energy source or the plasma itself.

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.

How Do We See a Relativistic Accretion Disk during a Thermal Instability?

We calculated bolometric images of relativistic slim disks during a radiation-pressure-driven thermal instability. When the mass-accretion rate exceeds the critical one, an inner region of the standard accretion disk bursts to change to a slim disk state having a large scale height. That is, the inner region of the disk becomes high temperature, and the thickness of the disk increases due to an increase of the radiation pressure. As a result, we found that the observed image of the disk during the burst strongly depends on the inclination angle. That is, radiation from the innermost disk would be occulted by the disk outer rim for high inclination angles ($i \gtrsim 70^\circ$). We also calculated the spectral energy distribution during a thermal instability. The Wien peak of the spectrum of high inclination angles becomes softer than that of low inclination angles due to the geometrical thickness. From these facts, even if a burst occurs in a black-hole candidate, we may not observe the burst when the inclination angle is large. We may suggest that numerous luminous black-hole candidates are still hidden in our Galaxy.

Very Large Telescope three micron spectra of dust-enshrouded red giants in the Large Magellanic Cloud

We present ESO/VLT spectra in the 2.9−4.1 µm range for a large sample of infrared stars in the Large Magellanic Cloud (LMC), selected on the basis of MSX and 2MASS colours to be extremely dust-enshrouded AGB star candidates. Out of 30 targets, 28 are positively identified as carbon stars, significantly adding to the known population of optically invisible carbon stars in the LMC. We also present spectra for six IR-bright stars in or near three clusters in the LMC, identifying four of them as carbon stars and two as oxygen-rich supergiants. We analyse the molecular bands of C2H2 at 3.1 and 3.8 µm, HCN at 3.57 µm, and sharp absorption features in the 3.70−3.78 µm region that we attribute to C2H2. There is evidence for a generally high abundance of C2H2 in LMC carbon stars, suggestive of high carbon-to-oxygen abundance ratios at the low metallicity in the LMC. The low initial metallicity is also likely to have resulted in less abundant HCN and CS. The sample of IR carbon stars exhibits a range in C2H2:HCN abundance ratio. We do not find strong correlations between the properties of the molecular atmosphere and circumstellar dust envelope, but the observed differences in the strengths and shapes of the absorption bands can be explained by differences in excitation temperature. High mass-loss rates and strong pulsation would then be seen to be associated with a large scale height of the molecular atmosphere.

Structural Parameters of Thin and Thick Disks in Edge-on Disk Galaxies

We analyze the global structure of 34 late-type, edge-on, undisturbed, disk galaxies spanning a wide range of mass. We measure structural parameters for the galaxies using two-dimensional least-squares fitting to our R-band photometry. The fits require both a thick and a thin disk to adequately fit the data. The thick disks have larger scale heights and longer scale lengths than the embedded thin disks by factors of ~2 and ~1.25, respectively. The observed structural parameters agree well with the properties of thick and thin disks derived from star counts in the Milky Way and from resolved stellar populations in nearby galaxies. We find that massive galaxies' luminosities are dominated by the thin disk. However, in low-mass galaxies (Vc 120 km s-1) thick disk stars contribute nearly half the luminosity and dominate the stellar mass. Thus, although low-mass dwarf galaxies appear blue, the majority of their stars are probably quite old. Our data are most easily explained by a formation scenario in which the thick disk is assembled through direct accretion of stellar material from merging satellites while the thin disk is formed from accreted gas. The baryonic fraction in the thin disk therefore constrains the gas richness of the merging pregalactic fragments. If we include the mass in H I as part of the thin disk, the thick disk contains 10% of the baryons in high-mass galaxies and ~25%–30% of the baryons in low-mass galaxies. Our data, therefore, indicate that the fragments were quite gas rich at the time of merging (fgas = 75%–90%). However, because low-mass galaxies have a smaller fraction of baryons in their thin disks, the pregalactic fragments from which they assembled must have been systematically more gas poor. We believe this trend results from increased outflow due to supernova-driven winds in the lower mass pregalactic fragments. We estimate that ~60% of the total baryonic mass in these systems was lost due to outflows. Pushing the episode of significant winds to early times allows the mass-metallicity relationship for disks to be established early, before the main disk is assembled, and obviates the difficulty in driving winds from diffuse disks with low star formation efficiencies. We discuss other implications of this scenario for solving the G dwarf problem, for predicting abundance trends in thick disks, and for removing discrepancies between semianalytic galaxy formation models and the observed colors of low-mass galaxies.

Unidentified γ-ray Sources off the Galactic Plane as Low-Mass Microquasars?

A subset of the unidentified EGRET γ-ray sources with no active galactic nucleus or other conspicuous counterpart appears to be concentrated at medium latitudes. Their long-term variability and their spatial distribution indicate that they are distinct from the more persistent sources associated with the nearby Gould Belt. They exhibit a large scale height of 1.3 ± 0.6 kpc above the Galactic plane. Potential counterparts for these sources include microquasars accreting from a low-mass star and spewing a continuous jet. Detailed calculations have been performed of the jet inverse Compton emission in the radiation fields from the star, the accretion disc, and a hot corona. Different jet Lorentz factors, powers, and aspect angles have been explored. The up-scattered emission from the corona predominates below 100 MeV whereas the disc and stellar contributions are preponderant at higher energies for moderate (∼15∘) and small (∼1∘) aspect angles, respectively. Yet, unlike in the high-mass, brighter versions of these systems, the external Compton emission largely fails to produce the luminosities required for 5 to 10 kpc distant EGRET sources. Synchrotron-self-Compton emission appears as a promising alternative.

The structure of brown dwarf circumstellar discs

We present synthetic spectra for circumstellar disks that are heated by radiation from a central brown dwarf. Under the assumption of vertical hydrostatic equilibrium, our models yield scaleheights for brown dwarf disks in excess of three times those derived for classical T Tauri (CTTS) disks. If the near-IR excess emission observed from brown dwarfs is indeed due to circumstellar disks, then the large scaleheights we find could have a significant impact on the optical and near-IR detectability of such systems. Our radiation transfer calculations show that such highly flared disks around brown dwarfs will result in a large fraction of obscured sources due to extinction of direct starlight by the disk over a wide range of sightlines. The obscured fraction for a 'typical' CTTS is less than 20%. We show that the obscured fraction for brown dwarfs may be double that for CTTS, but this depends on stellar and disk mass. We also comment on possible confusion in identifying brown dwarfs via color-magnitude diagrams: edge-on CTTS display similar colors and magnitudes as a face-on brown dwarf plus disk systems.

The Formation of Dust Lanes: Implications for Galaxy Evolution

From a survey of edge-on disks, we find that disk galaxies show a sharp, mass-dependent transition in the structure of their dusty ISM. In more massive, rapidly rotating disks with Vc > 120 km s-1, we see the well-defined dust lanes traditionally associated with edge-on galaxies. However, in more slowly rotating, lower mass galaxies with Vc < 120 km s-1, we find no dust lanes. Instead, the distribution of dust in these galaxies has a much larger scale height and thus appears more diffuse. Evidence suggests that the change in scale height is due primarily to changes in the turbulent velocities supporting the gas layer rather than to sharp changes in the gas surface density. A detailed analysis of our sample reveals that the decrease in the dust scale height is associated with the onset of disk instabilities, evaluated for a mixed star+gas disk. Specifically, we find that all of the high-mass galaxies with dust lanes are gravitationally unstable and thus are prone to fragmentation and gravitational collapse along spiral arms. Empirically, our data imply that turbulence has lower characteristic velocities in the presence of disk instabilities, leading to smaller gas scale heights and the appearance of narrow dust lanes. The drop in velocity dispersion may be due either to a switch in the driving mechanism for turbulence from supernovae to gravitational instabilities or to a change in the response of the ISM to supernovae after the ISM has collapsed to a dense layer. We hypothesize that the drop in gas scale height may lead to significant increases in the star formation rate when disk instabilities are present. First, the collapse of the gas layer increases the typical gas density, reducing the star formation timescale. Second, the star formation efficiency increases because of lower turbulent velocities. These two effects can combine to produce a sharp increase in the star formation rate with little change in the gas surface density and may therefore provide an explanation for the Kennicutt surface density threshold for star formation. Our data also suggest that star formation will be systematically less efficient in low-mass disks with Vc < 120 km s-1, since these galaxies are stable and lie entirely below the Kennicutt surface density threshold. In these stable systems the effective nucleosynthetic yield is reduced because the star formation timescale becomes longer than the gas accretion timescale, suppressing the metallicity. This effect can possibly produce the observed fall-off in metallicity at rotation speeds Vc < 120 km s-1. Thus, infall provides an equally plausible explanation of the mass-metallicity relation in disks as global outflows driven by supernova winds. The transitions in disk stability, dust structure, and/or star formation efficiency may also be responsible for the observed changes in the slope of the Tully-Fisher relation, in the sharp increase in the thickness of low-mass galaxy disks, and in the onset of bulges in galaxies with Vc 120 km s-1. The latter observation lends support to theories in which bulges in late-type galaxies grow through secular evolution in response to disk instabilities. We include in this paper relationships between the surface density and the vertical stellar velocity dispersion as a function of galaxy rotation speed, which may be useful constraints on galaxy formation models.

Aeronomy of extra-solar giant planets at small orbital distances

One-dimensional aeronomical calculations of the atmospheric structure of extra-solar giant planets in orbits with semi-major axes from 0.01 to 0.1 AU show that the thermospheres are heated to over 10,000 K by the EUV flux from the central star. The high temperatures cause the atmosphere to escape rapidly, implying that the upper thermosphere is cooled primarily by adiabatic expansion. The lower thermosphere is cooled primarily by radiative emissions from H + 3, created by photoionization of H 2 and subsequent ion chemistry. Thermal decomposition of H 2 causes an abrupt change in the composition, from molecular to atomic, near the base of the thermosphere. The composition of the upper thermosphere is determined by the balance between photoionization, advection, and H + recombination. Molecular diffusion and thermal conduction are of minor importance, in part because of large atmospheric scale heights. The energy-limited atmospheric escape rate is approximately proportional to the stellar EUV flux. Although escape rates are large, the atmospheres are stable over time scales of billions of years.

The Gamma‐Ray Blazar Content of the Northern Sky

Using survey data, we have re-evaluated the correlation of flat spectrum radio sources with EGRET sources in the Northern sky. A likelihood analysis incorporating the radio and X-ray properties and the Gamma-ray source localization is used to gauge the reliability of associations and to search for counterparts of previously unidentified EGRET sources. Above |b|=10deg, where the classification is complete, we find that 70% of the Northern EGRET sources have counterparts similar to the bright EGRET blazars. For several of these we identify known blazar counterparts more likely than the earlier proposed 3EG association; for ~20 we have new identifications. Spectroscopic confirmation of these candidates is in progress and we have found flat spectrum radio quasars and BL Lac counterparts with redshifts as high as 4. We also find strong evidence for a set of 28 objects with no plausible counterpart like the known EGRET Blazars. These thus represent either a new extragalactic population or a population of Galactic objects with a large scale height. The survey has been extended into the plane, where we find several new blazar candidates; the bulk of the sources are, however, Galactic. Looking ahead to the GLAST era we predict that several of the present 3EG sources are composite and that higher resolution data will break these into multiple Blazar IDs.

Hubble Space TelescopeWFPC2 Imaging of the Disk and Jet of HV Tauri C

We have obtained Hubble Space Telescope (HST) WFPC2 images of the HV Tauri young triple system. The tertiary star appears as a compact bipolar nebula at visual wavelengths as already known in the near-infrared. New, deeper adaptive optics observations made at the Canada-France-Hawaii Telescope show no point source in the nebula to a limiting magnitude of K > 15. The results therefore confirm that HV Tau C is an optically thick circumstellar disk seen close to edge-on. Clear evidence for small, chromatic dust particles in the outer disk is provided by the color structure of the nebula: the thickness of the central dust lane shrinks by 30% between 0.55 and 2.2 μm. Bipolar jets extending 03-07 perpendicular to the dust lane are seen in HST narrowband [S II] and [O I] images. The continuum images are compared to multiple scattering models, with optimal density model parameters derived through χ2 minimization. A disk density distribution provides a reasonable fit to the K-band image but is unable to reproduce the vertical extent of the nebula at I band without resorting to an unreasonably large scale height. Adding an envelope structure around the disk results in a much better fit to the HST image, and with a physically reasonable disk scale height. Our preferred model has a disk outer radius of 50 AU, inclination of 6°, and scale height of 6.5 AU at r = 50 AU. The thickness of the dark lane establishes a disk mass near 2 × 10-3 M☉ (~2MJup) of dust and gas, if the dust grains have interstellar properties and remain fully mixed vertically. The envelope, with a much smaller mass ~4 × 10-5 M☉, would be very short-lived unless replenished by new material from the star or surrounding medium.