Discovery of Temperate Latitude Clouds on Titan

Until now, all the clouds imaged in Titan's troposphere have been found at far southern latitudes (60°-90° south). The occurrence and location of these clouds is thought to be the result of convection driven by the maximum annual solar heating of Titan's surface, which occurs at summer solstice (2002 October) in this south polar region. We report the first observations of a new recurring type of tropospheric cloud feature, confined narrowly to ~40° south latitude, which cannot be explained by this simple insolation hypothesis. We propose two classes of formation scenario, one linked to surface geography and the other to seasonally evolving circulation, which will be easily distinguished with continued observations over the next few years

Young red supergiants and the near infrared light appearance of disk galaxies

Disk galaxies often show prominent nonaxisymmetric features at near-infrared wavelengths. Such features may indicate variations in the surface density of stellar mass, contributions from young red supergiants in star forming regions, or substantial dust obscuration. To distinguish among these possibilities, we have searched for spatial variations in the 2.3 micron photometric CO index within the disks of three nearby galaxies (NGC 278, NGC 2649, & NGC 5713). This index measures the strength of the absorption bands of molecular CO in stellar atmospheres, and is strong in cool, low surface-gravity stars, reaching the largest values for red supergiants. We observe significant spatial CO index variations in two galaxies (NGC 278 & NGC 5713), indicating that the dominant stellar population in the near-infrared is not everywhere the same. Central CO index peaks are present in two galaxies; these could be due to either metallicity gradients or recent star formation activity. In addition, significant azimuthal CO index variations are seen in NGC 278. Because strong azimuthal metallicity gradients are physically implausible in disk galaxies, these features are most naturally explained by the presence of a young stellar population. The fraction of 2 micron light due to young stellar populations in star forming regions can be calculated from our data. Overall, young stellar populations can contribute ~3% of a (normal) galaxy's near infrared flux. Locally, this fraction may rise to ~33%. Thus, young stars do not dominate the total near infrared flux, but can be locally dominant in star forming regions, and can bias estimates of spiral arm amplitude or other nonaxisymmetric structures in galaxies' mass distributions.Comment: 28 pages including 3 postscript figures. A fourth figure is in jpeg format. Uses AASTeX. Accepted for publication in The Astronomical Journa

Irradiation Products On Dwarf Planet Makemake

The dark, reddish tinged surfaces of icy bodies in the outer solar system are usually attributed to the long term irradiation of simple hydrocarbons leading to the breaking of C–H bonds, loss of hydrogen, and the production of long carbon chains. While the simple hydrocarbon methane is stable and detected on the most massive bodies in the Kuiper Belt, evidence of active irradiation chemistry is scant except for the presence of ethane on methane-rich Makemake and the possible detections of ethane on more methane-poor Pluto and Quaoar. We have obtained deep high signal-to-noise spectra of Makemake from 1.4 to 2.5 μm in an attempt to trace the radiation chemistry in the outer solar system beyond the initial ethane formation. We present the first astrophysical detection of solid ethylene and evidence for acetylene and high-mass alkanes—all expected products of the continued irradiation of methane, and use these species to map the chemical pathway from methane to long-chain hydrocarbons

Intermediate-mass-ratio-inspirals in the Einstein Telescope: I. Signal-to-noise ratio calculations

The Einstein Telescope (ET) is a proposed third generation ground-based interferometer, for which the target is a sensitivity that is a factor of ten better than Advanced LIGO and a frequency range that extends down to about 1Hz. ET will provide opportunities to test Einstein's theory of relativity in the strong field and will realize precision gravitational wave astronomy with a thousandfold increase in the expected number of events over the advanced ground-based detectors. A design study for ET is currently underway, so it is timely to assess the science that could be done with such an instrument. This paper is the first in a series that will carry out a detailed study of intermediate-mass-ratio inspirals (IMRIs) for ET. In the context of ET, an IMRI is the inspiral of a neutron star or stellar-mass black hole into an intermediate mass black hole (IMBH). In this paper we focus on the development of IMRI waveform models for circular and equatorial inspirals. We consider two approximations for the waveforms, which both incorporate the inspiral, merger and ringdown phases in a consistent way. One approximation, valid for IMBHs of arbitrary spin, uses the transition model of Ori and Thorne [1] to describe the merger, and this is then matched smoothly onto a ringdown waveform. The second approximation uses the Effective One Body (EOB) approach to model the merger phase of the waveform and is valid for non-spinning IMBHs. In this paper, we use both waveform models to compute signal-to-noise ratios (SNRs) for IMRI sources detectable by ET. At a redshift of z=1, we find typical SNRs for IMRI systems with masses 1.4+100 solar masses, 10+100 solar masses, 1.4+500 solar masses and 10+500 solar masses of about 10-25, 40-80, 3-15 and 10-60, respectively. We also find that the two models make predictions for non-spinning inspirals that are consistent to about ten percent.Comment: 27 pages, 9 figures, v3 has an updated reference for consistency with accepted versio

Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope

We have used the Hubble Space Telescope to directly measure the angular size of the large Kuiper belt object 2003 UB313. By carefully calibrating the point spread function of a nearby field star, we measure the size of 2003 UB313 to be 34.3$\pm$1.4 milliarcseconds, corresponding to a diameter of 2400$\pm$100 km or a size $\sim5$% larger than Pluto. The V band geometric albedo of 2003 UB313 is $86\pm7$%. The extremely high albedo is consistent with the frosty methane spectrum, the lack of red coloring, and the lack of observed photometric variation on the surface of 2003 UB313. Methane photolysis should quickly darken the surface of 2003 UB313, but continuous evaporation and redeposition of surface ices appears capable of maintaining the extreme alebdo of this body

Surface Brightness Gradients Produced by the Ring Waves of Star Formation

We compute surface brightness profiles of galactic disks for outwardly propagating waves of star formation with a view to investigate the stellar populations in ring galaxies. We consider two mechanisms which can create outwardly propagating star forming rings in a purely gaseous disk --- a self-induced wave and a density wave. We show that the surface brightness profiles produced by both scenarios of ring formation are similar and are strongly sensitive to the velocity of the wave. The results of our computations are compared with the observational quantities sensitive to the young and old stellar populations in the ring galaxies A0035-335 (the Cartwheel galaxy) and VIIZw466. The best fit to the observed radial H_alpha surface brightness distribution in the Cartwheel galaxy is obtained for a wave velocity of about 90 km/s. The red continuum brightness of the ring can be fully explained by the evolving stars present in the trailing part of the wave. However the red continuum brightness in regions internal to the ring indicates that the wave of star formation propagates in a pre-existing stellar disk in the Cartwheel. The H_alpha and K-band surface brightness profiles in VIIZw466 match the values expected from stellar populations produced by a wave of star formation propagating in a purely gaseous disk very well. We conclude that VIIZw466 is probably experiencing the first event of star formation in the disk.Comment: Uses aas2pp4.sty and epsfig.sty, 15 pages To appear in Astrophysical Journal, March 10, 199

Type Ia Supernova Scenarios and the Hubble Sequence

The dependence of the Type Ia supernova (SN Ia) rate on galaxy type is examined for three currently proposed scenarios: merging of a Chandrasekhar--mass CO white dwarf (WD) with a CO WD companion, explosion of a sub--Chandrasekhar mass CO WD induced by accretion of material from a He star companion, and explosion of a sub--Chandrasekhar CO WD in a symbiotic system. The variation of the SNe Ia rate and explosion characteristics with time is derived, and its correlation with parent population age and galaxy redshift is discussed. Among current scenarios, CO + He star systems should be absent from E galaxies. Explosion of CO WDs in symbiotic systems could account for the SNe Ia rate in these galaxies. The same might be true for the CO + CO WD scenario, depending on the value of the common envelope parameter. A testable prediction of the sub--Chandrasekhar WD model is that the average brightness and kinetic energy of the SN Ia events should increase with redshift for a given Hubble type. Also for this scenario, going along the Hubble sequence from E to Sc galaxies SNe Ia events should be brighter on average and should show larger mean velocities of the ejecta. The observational correlations strongly suggest that the characteristics of the SNe Ia explosion are linked to parent population age. The scenario in which WDs with masses below the Chandrasekhar mass explode appears the most promising one to explain the observed variation of the SN Ia rate with galaxy type together with the luminosity--expansion velocity trend.Comment: 16 pages uuencoded compressed Postscript, 2 figures included. ApJ Letters, in pres

Storms in the tropics of Titan

Methane clouds, lakes and most fluvial features on Saturn's moon Titan have been observed in the moist high latitudes while the tropics have been nearly devoid of convective clouds and have shown an abundance of wind-carved surface features like dunes. The presence of small-scale channels and dry riverbeds near the equator observed by the Huygens probe at latitudes thought incapable of supporting convection (and thus strong rain) has been suggested to be due to geological seepage or other mechanisms not related to precipitation. Here we report the presence of bright, transient, tropospheric clouds in tropical latitudes. We find that the initial pulse of cloud activity generated planetary waves that instigated cloud activity at other latitudes across Titan that had been cloud-free for at least several years. These observations show that convective pulses at one latitude can trigger short-term convection at other latitudes, even those not generally considered capable of supporting convection, and may also explain the presence of methane-carved rivers and channels near the Huygens landing site

Titan imagery with Keck adaptive optics during and after probe entry

We present adaptive optics data from the Keck telescope, taken while the Huygens probe descended through Titan's atmosphere and on the days following touchdown. No probe entry signal was detected. Our observations span a solar phase angle range from 0.05° up to 0.8°, with the Sun in the west. Contrary to expectations, the east side of Titan's stratosphere was usually brightest. Compiling images obtained with Keck and Gemini over the past few years reveals that the east-west asymmetry can be explained by a combination of the solar phase angle effect and an enhancement in the haze density on Titan's morning hemisphere. While stratospheric haze was prominent over the northern hemisphere, tropospheric haze dominated the south, from the south pole up to latitudes of ∼45°S. At 2.1 μm this haze forms a polar cap, while at 1.22 μm it appears in the form of a collar at 60°S. A few small clouds were usually present near the south pole, at altitudes of 30–40 km. Our narrowband J,H,K images of Titan's surface compare extremely well with that obtained by Cassini ISS, down to the small-scale features. The surface contrast between dark and bright areas may be larger at 2 μm than at 1.6 and 1.3 μm, which would imply that the dark areas may be covered by a coarser-grained frost than the bright regions and/or that there is additional 2 μm absorption there