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While quality checking a new motion-aware co-addition of all 12.5 months of WISE data, the source WISE J070720.48+170533.0 was found to have moved 0.9 arc-sec in 6 months. Backtracking this motion allowed us to identify this source as 2MASS J07071961+1705464 and with several entries in the USNO B catalog. An astrometric fit to these archival data gives a proper motion of 1793 +/- 2 mas/yr and a parallax of 35 +/- 42 mas. Photometry from WISE, 2MASS and the POSS can be fit reasonably well by a blackbody with T = 3658 K and an angular radius of 4.36E-11 radians. No clear evidence of H_2 collision-induced absorption is seen in the near-IR. An optical spectrum shows broad deep CaH bands at 638 and 690 nm, broad deep Na D at 598.2 nm, and weak or absent TiO, indicating that this source is an ultra-subdwarf M star with a radial velocity about -21 +/- 18 km/sec relative to the Sun. Given its apparent magnitude, the distance is about 39 +/- 9 pc and the tangential velocity is probably about 330 km/sec, but a more precise parallax is needed to be certain., Comment: Accepted by the Astronomical Journal, 10 pages LaTex with 9 embedded figures. UCAC and CMC data added, fits updated

The Wide-field Infrared Survey Explorer has revealed a T8.5 brown dwarf (WISE J111838.70+312537.9) that exhibits common proper motion with a solar-neighborhood (8 pc) quadruple star system - Xi Ursae Majoris. The angular separation is 8.5 arc-min, and the projected physical separation is about 4000 AU. The sub-solar metallicity and low chromospheric activity of Xi UMa A argue that the system has an age of at least 2 Gyr. The infrared luminosity and color of the brown dwarf suggests the mass of this companion ranges between 14 and 38 Jupiter masses for system ages of 2 and 8 Gyr respectively., Comment: AJ in press, 12 pages LaTeX with 6 figures. More astrometric data and a laser guide star adaptive optics image added

Forty new low mass members with spectral types ranging from M4-M9 have been confirmed in the Orion Molecular Cloud 2/3 region. Through deep, I, z', J, H, K photometry of a 20' x 20' field in OMC 2/3, we selected brown dwarf candidates for follow-up spectroscopy. Low resolution far-red and near-infrared spectra were obtained for the candidates, and 19 young brown dwarfs in the OMC 2/3 region are confirmed. They exhibit spectral types of M6.5-M9, corresponding to approximate masses of 0.075-0.015 M_solar using the evolutionary models of Baraffe et al. (1998). At least one of these bona fide young brown dwarfs has strong Halpha emission, indicating that it is actively accreting. In addition, we confirm 21 new low mass members with spectral types of M4-M6, corresponding to approximate masses of 0.35-0.10 M_solar in OMC 2/3. By comparing pre-main sequence tracks to the positions of the members in the H-R diagram, we find that most of the brown dwarfs are less than 1 Myr, but find a number of low mass stars with inferred ages greater than 3 Myr. The discrepancy in the stellar and substellar ages is due to our selection of only low luminosity sources; however, the presence of such objects implies the presence of an age spread in the OMC 2/3 region. We discuss possible reasons for this apparent age spread., Comment: Accepted for publication in ApJ. 52 pages, 17 figures, 4 tables; main body: 23 pages

During an imaging survey of the Chamaeleon I star-forming region with the ACS aboard HST, we have discovered a candidate substellar companion to the young low-mass star CHXR 73 (~2 Myr, M=0.35 Msun). We measure a projected separation of 1.3+/-0.03" for the companion, CHXR 73 B, which corresponds to 210 AU at the distance of the cluster. A near-infrared spectrum of this source obtained with CorMASS at the Magellan II telescope exhibits strong steam absorption that confirms its late-type nature (>=M9.5). In addition, the gravity-sensitive shapes of the H- and K-band continua demonstrate that CHXR 73 B is a young, pre-main-sequence object rather than a field star. The probability that CHXR 73A and B are unrelated members of Chamaeleon I is ~0.001. We estimate the masses of CHXR 73 B and other known substellar companions in young clusters with a method that is consistent with the dynamical measurements of the eclipsing binary brown dwarf 2M 0535-0546, which consists of a comparison of the bolometric luminosities of the companions to the values predicted by the evolutionary models of Chabrier & coworkers and Burrows & coworkers. We arrive at mass estimates of 0.003-0.004, 0.024+/-0.012, 0.011+0.01/-0.003, and 0.012+0.008/-0.005 Msun for 2M 1207-3932 B, GQ Lup B, DH Tau B, and CHXR 73 B, respectively. Thus, DH Tau B and CHXR 73 B appear to be the least massive companions to stars outside the solar system that have been detected in direct images, and may have masses that are within the range observed for extrasolar planetary companions (M<=0.015 Msun). However, because these two objects (as well as 2M 1207-3932 B) probably did not form within circumstellar disks around their primaries, we suggest that they should be viewed as brown dwarf companions rather than planets., Comment: Astrophysical Journal, in press

Hydrothermal and aqueous alteration can explain some of the exciting results from the MER team s analyses of the martian soil, including the major elements, mobile elements, and the nickel enrichment. Published results from the five lander missions lead to the following conclusions: 1) The soil appears to be globally mixed and basaltic with only small local variations in chemistry. Relative to martian basaltic meteorites and Gusev rocks the soils are depleted in the fluid-mobile element calcium, but only slightly enriched to somewhat depleted in iron oxide. 2) The presence of olivine in the soils based on M ssbauer data argues that the soil is only partly weathered and is more akin to a lunar regolith than a terrestrial soil. 3) The presence of bromine along with sulfur and chlorine in the soils is consistent with addition of a mobile element component to the soil.

Hydrothermal alteration and chemical transport involving impact craters probably occurred on Mars throughout its history. Our studies of alteration products and mobile element transport in ejecta blanket and drill core samples from impact craters show that these processes may have contributed to the surface composition of Mars. Recent work on the Chicxulub Yaxcopoil-1 drill core has provided important information on the relative mobility of many elements that may be relevant to Mars. The Chicxulub impact structure in the Yucatan Peninsula of Mexico and offshore in the Gulf of Mexico is one of the largest impact craters identified on the Earth, has a diameter of 180-200 km, and is associated with the mass extinctions at the K/T boundary. The Yax-1 hole was drilled in 2001 and 2002 on the Yaxcopoil hacienda near Merida on the Yucatan Peninsula. Yax-1 is located just outside of the transient cavity, which explains some of the unusual characteristics of the core stratigraphy. No typical impact melt sheet was encountered in the hole and most of the Yax-1 impactites are breccias. In particular, the impact melt and breccias are only 100 m thick which is surprising taking into account the considerably thicker breccia accumulations towards the center of the structure and farther outside the transient crater encountered by other drill holes.

The role of hydrothermal alteration and chemical transport involving impact craters could have occurred on Mars, the poles of Mercury and the Moon, and other small bodies. We are studying terrestrial craters of various sizes in different environments to better understand aqueous alteration and chemical transport processes. The Lonar crater in India (1.8 km diameter) is particularly interesting being the only impact crater in basalt. In January of 2004, during fieldwork in the ejecta blanket around the rim of the Lonar crater we discovered alteration zones not previously described at this crater. The alteration of the ejecta blanket could represent evidence of localized hydrothermal activity. Such activity is consistent with the presence of large amounts of impact melt in the ejecta blanket. Map of one area on the north rim of the crater containing highly altered zones at least 3 m deep is shown.

The catastrophic Chicxulub event should have generated a large hydrothermal system with volatile element mobilization, producing interesting alteration materials and clays. The Yaxcopoil-1 (YAX) drill hole is located in the annular trough, about 70 km southwest of the crater center, in an area where the impactite layers are relatively thin (approx. 100 m thick). We have analyzed samples from the YAX drill core and from other impact craters including Mistastin and Lonar to determine the nature of alteration and trace element mobilization.

The martian regolith is a globally homogenized product of chemical and aeolian weathering processes. The soil is thought to consist of a rock component, with lesser amounts of mobile elements (Ca, Na, and K) than a presumed protolith, and a salt or mobile element component enriched in sulfur and chlorine. In this study we consider the contributions of hydrothermal processes to the origin of the rock component of the martian soil.

The large energy in terrestrial impacts can create hydrothermal systems and consequently produce hydrothermal alteration materials. In this study we consider the chemistry of impact and volcanic hydrothermal alteration under relatively low water/rock ratios in basaltic or a somewhat more evolved protolith. Our work on the Lonar and Mistastin craters suggests that Fe-rich clays, including Fe-rich saponite can be produced. We postulate that similar alteration materials are produced on Mars and could be a component of the martian soil or regolith, contrary to some earlier studies. The martian regolith is a globally homogenized product of various weathering processes. The soil [1] is thought to consist of a rock component, with lesser amounts of mobile elements (Ca, Na, and K) than a presumed protolith, and a salt or mobile element component enriched in sulfur and chlorine [2, 3]. In this study we consider the contributions of impacts and consequent hydrothermal processes to the rock component of the martian soil.

We have obtained the first high-speed photometry of the eclipsing dwarf nova Z Cha at ultraviolet wavelengths with the Hubble Space Telescope (HST). We observed the eclipse roughly every 4 days over two cycles of the normal eruptions of Z Cha, giving a uniquely complete coverage of its outburst cycle. The accretion disk dominated the ultraviolet light curve of Z Cha at the peak of an eruption; the white dwarf, the bright spot on the edge of the disk, and the boundary layer were all invisible. We were able to obtain an axisymmetric map of the accretion disk at this time only by adopting a flared disk with an opening angle of approximately 8 deg. The run of brightness temperature with radius in the disk at the peak of the eruption was too flat to be consistent with a steady state, optically thick accretion disk. The local rate of mass flow through the disk was approximately 5 x 10(exp -10) solar masses/yr near the center of the disk and approximately 5 x 10(exp -9) solar masses/yr near the outer edge. The white dwarf, the accretion disk, and the boundary layer were all significant contributors to the ultraviolet flux on the descending branches of the eruptions. The temperature of the white dwarf during decline was 18,300 K less than T(sub wd) less than 21,800 K, which is significantly greater than at minimum light. Six days after the maximum of an eruption Z Cha has faded to near minimum light at ultraviolet wavelenghts, but was still approximately 70% brighter at minimum light in the B band. About one-quarter of the excess flux in the B band came from the accretion disk. Thus, the accretion disk faded and became invisible at ultraviolet wavelengths before it faded at optical wavelenghts. The disk did, however, remain optically thick and obscured the lower half of the white dwarf at ultraviolet and possibly at optical wavelenghts for 2 weeks after the eruption ended. By the third week after eruptiuons the eclipse looked like a simple occultation of an unobscured, spherical white dwarf by a dark secondary star. The center of the accretion disk was, therfore, optically thin at ultraviolet wavelenghts and the boundary layer was too faint to be visible.

Photometric and polarimetric observations of both images of the gravitationally lensed quasar Q0957+561 (z(sub em) = 1.41) were obtained in the UV in 1993 with the High Speed Photometer on board the Hubble Space Photometer on board the Hubble Space Telescope. The images exhibited no significant polarization in a bandpass centered on 2770 A (observer's frame); p less than or = 3.2 % (2 sigma upper limit) in each image. The ratio of the flux density in image A to that in image B in late 1993 had a constant valuee, 1.021 +/- 0.008, in four different UV bandpass between 1400 A and 3040 A observer's frame). These results are consistent with the prediction of the gravitation lens interpretation that the photometric ratio of the images measured simultaneously should be independent of frequency. Reprocessed archival spectra of the two images obtained between 1981 and 1983 by the International Ultraviolet Explorer (IUE) show that the photometric ratio of A to B varies between 0.96 and 2.0 in the Ly alpha emission line, and between 0.77 and 1.8 in the O VI lambda 1037 emission line (quasar rest frame). The photometric ratio of A to B at any single epoch is often significantly different in the two emission lines. Accepting the system as a gravitational lens implies that in the quasar the flux in the Ly alpha emsisson line can vary independently of the flux in the 0 IV emission line.

The linear polarization of 3C 345, a superluminal radio source and OVV quasar, was observed in two bandpasses in the ultraviolet (centered at 2160 A and 2770 A) in 1993 April using the High Speed Photometer on the Hubble Space Telescope. The quasar is significantly polarized in the UV (p greater than 5%). Ground-based polarimetry was obtained 11 days later, but a difference in the position angle between the observations in the visible and those in the UV indicate that the magnitude of the polarization of 3C 345 may have changed over that time. If the two observation sets represent the same state of spectral polarization, then the large UV flux implies that either the polarization of the synchrotron continuum must stop decreasing in the UV, or that there is an additional source of polarized flux in the ultraviolet. Only if the UV observations represent a spectral polarization state with the same position angle in the visible seen previously in 3C 345 can the polarized flux be represented by a single power law consistent with the three-component model of Smith et al. This model consists of a polarized synchrotron component, an unpolarized component from the broad-line region, and an unpolarized component attributed to thermal radiation from an optically thick accretion disk. Additional simultaneous polarimetry in the UV and visible will be required to further constrain models of the continuum emission processes in 3C 345 and determine if the UV polarized flux is synchrotron in origin.

In this Letter we present the first high-speed ultraviolet photometry of an active, classical nova, Nova Cygni 1992. The 45 minute observation shows significant evidence for power at frequencies that correspond to periods of about 565 and 900 s. Each of these periods has an amplitude of about 3 mmag. Since this data set is short, we cannot establish the nature of the detected variability and so, we discuss possible physical mechanisms ranging from short-lived phenomena to stable periodic modulations that could result in the observed variations.

An occultation of the star GSC 6323-01396 (V = 11.9) by Saturn's rings was observed with the High-Speed Photometer on the Hubble Space Telescope (HST) on 1991 October 2-3. This occultation occurred when Saturn was near a stationary point, so the apparent motion of Saturn relative to the star was dominated by the HST orbital motion (8 km/s). Data were recorded simultaneously at effective wavelengths of 3200 and 7500 A, with an integration time of 0.15 s. Fifteen segments of occultation data, totaling 6.8 h, were recorded in 13 successive orbits during the 20.0 h interval from UTC 1991 October 2, 19:35 until UTC 1991 October 3, 15:35. Occultations by 43 different features throughout the classical rings were unambiguously identified in the light curve, with a second occultation by 24 of them occurring due to spacecraft orbital parallax during this extremely slow event. Occultation times for features currently presumed circular were measured and employed in a geometrical model for the rings. This model, relating the observed occultation times to feature radii and longitudes, is presented here and is used in a least-squares fit for the pole direction and radius scale of Saturn's ring system.

We present the first high-speed ultraviolet photometry of an oscillating Ap star, HD 60435. After removing known orbital effects related to the Hubble Space Telescope, we confirm the presence of a strong pulsation period at a frequency of 123.70 cycles per day. In addition, we find significant amplitude modulation of this frequency that we suggest could be the result of beating of multiple periodicities. In this context, we suggest evidence for the presence of four additional frequencies at nu = 120.56, 126.55, 149.49, and 221.03 cycles per day. Three of these frequencies correspond well to frequencies detected in optical observations of HD 60435. The fourth, at 149.49 cycles per day, if real, is a potentially new pulsation mode that has not been detected in ground-based observations of this star. The amplitude of the 123 cycles per day pulsation is significantly larger in the ultraviolet than it is in the blue.

Observations of PSR 0531+21 with the High Speed Photometer on the HST in the visible in October 1991 and in the UV in January 1992 are presented. The time resolution of the instrument was 10.74 microsec; the effective time resolution of the light curves folded modulo the pulsar period was 21.5 microsec. The main pulse arrival time is the same in the UV as in the visible and radio to within the accuracy of the establishment of the spacecraft clock, +/- 1.05 ms. The peak of the main pulse is resolved in time. Corrected for reddening, the intensity spectral index of the Crab pulsar from 1680 to 7400 A is 0.11 +/- 0.13. The pulsed flux has an intensity less than 0.9 percent of the peak flux just before the onset of the main pulse. The variations in intensity of individual main and secondary pulses are uncorrelated, even within the same rotational period.

A brief description of the High Speed Photometer (HSP) of the Hubble Space Telescope is given, in particular the HSP light path, detectors, entrance apertures, and filters. The status of HSP testing to date is reported, and problems encountered with the bright earth and the telescope pointing system are described. The calibration effort for the HSP is well under way. Results of internal (instrument coordinate) aperture locations good to 0.05 arcsec and external (telescope coordinate) locations good to 0.02 arcsec are shown. The effects of spacecraft pointing and jitter on HSP photometry are detailed, and a preliminary measurement of spacecraft jitter with HSP is shown. The aperture calibration effort is verified by accurate pointing of a star to different HSP 1.0 arcsec entrance apertures, and photometric performance of the instrument is shown to be accurate to the 2 percent photon noise of the observations. Future science verification and guaranteed observing time programs are listed. Suggestions are made for future space-based photometers.