Robert R. Howell

A lunar occultation and direct imaging survey of multiplicity in the Ophiuchus and Taurus star-forming regions

We present an IR lunar occultation and direct imaging search for companions in the Ophiuchus star-forming region and update a similar search of the Taurus region. The search is sensitive to companions in the angular separation range 0.005-10 sec. In Ophiuchus, we surveyed 35 young star targets; this sample contains at least 10 binaries, two triples, and one quadruple. Ten of the companion stars are newly discovered. In Taurus, the survey now includes 47 systems among which there are at least 22 binaries and four triples. Only two companion stars are newly identified because there is strong overlap with prior work. All the triples and quadruple are hierarchical. The observed binary frequency in Ophiuchus, in the 3-1400 AU range of separations, is at least 1.1 +/- 0.3 that of the nearby solar-like stars. This value is a lower bound because we make no corrections for incompleteness. In Taurus, in the same range of separations, the observed binary frequency is at least 1.6 +/- 0.3 that of the nearby solar-like stars. This value extends Ghez et al.s (1993) and Leinerts et al.s (1993) determination of an excess binary frequency to 3 AU separation. We used the weak-line T Tauri star/T Tauri star (WT/TT) type and the K-L color index to distinguish between systems with and without inner disks. We find no convincing difference in the binary frequency or distribution of separations of the systems with and without inner disks. The 1.3 mm continuum emission of the single systems exceeds that of the multiples suggesting that their extensive outer disks are more massive. The specific angular momenta of the binaries overlap those of molecular cloud cores measured by Goodman et al. (1993).

Multiplicity among the young stars in Taurus

Twenty-eight young stars systems in the northern Taurus star-forming region are surveyed for multiplicity. These observations, made by infrared lunar occultation and imaging techniques, can identify binaries in the angular separation range 0.005-10 arcsec. Of the 28 systems, 11 are binaries and two are triples, giving an observed multiplicity of 1.5 stars per system. The binary frequency and distribution of separations are similar to those of the main-sequence F and G spectral-type stars. Stars that have detected companions and those without are indistinguishable in H-alpha equivalent width, indicating that stars with active inner disks are represented in both groups. The multiple systems are in general weaker 1.3 mm continuum emitters than the single stars, indicating that the large-scale disks in these systems are less massive. However, there are important exceptions so the evolution of disks in multiple systems is not clear.

Loki, Io: A periodic volcano

Loki is the most powerful volcano in the Solar System. It has been observed to be in continuous though variable activity since 1979. Synthesis of more than a decade of groundbased data suggests that Loki eruptions are cyclic, with a 540 day period. Application of a simple lava cooling model to temperatures in Loki Patera, and eruption start and end times, implies that brightenings are due to a resurfacing wave propagating across the patera. The data are most consistent with lava lake overturn, but resurfacing by lava flows cannot be ruled out. A porosity gradient in the lake crust could cause lava lake overturn to occur periodically on the timescale observed.

Violent silicate volcanism on Io in 1996

We monitored Ios volcanic thermal emission from 1.7 to 4.8 µm in 1995/96. The data reveal a number of brightenings when Ios thermal emission increased several-fold. We use a model of emission from lava flows to understand observations of two brightenings in 1996. The brightenings were caused by eruptions ≃ 3km² in extent, with eruptive rates of 104-106 m²s−1 and with maximum lava temperatures ≥ 1400 K. These events were probably lava lakes and/or fire fountains.

Ground‐based observations of volcanism on Io in 1999 and early 2000

Ground-based observations of volcanism on Io during the period of the 1999 and early 2000 Galileo close flybys have detected several types of activity, providing information which complements the spacecraft observations. At Loki a brightening began between August 25 and September 9 and continued through February. On August 2 a major outburst was observed near (14°N, 74°W) whose brightness corresponds to an area of approximately 350 km 2 at a temperature of 1100 K. Observations of eruptions in late June (9906A) and in late November (9911A, at Tvashtar) provide temporal and photometric constraints on activity also seen by Galileo. High-resolution adaptive optics images provide further information on the fainter sources distributed across the surface.

Hydrogen Sulfide on IO: Evidence from Telescopic and Laboratory Infrared Spectra

Evidence is reported for hydrogen sulfide (H2S) on Ios surface. An infrared band at 3.915 (� 0.015) micrometers in several ground-based spectra of Io can be accounted for by reflectance from H2S frost deposited on or cocondensed with sulfur dioxide (SO2) frost. Temporal variation in the occurrence and intensity of the band suggests that condensed H2S on Ios surface is transient, implying a similar variation of H2S abundance in Ios atmosphere. The band was observed in full-disk measurements of Io at several orbital longitudes, including once at 24�(∼0.5 hour after Ios reappearance after an eclipse)—but not after another reappearance at 22�—and once at 95�(on Ios leading hemisphere). These results suggest that condensed H2S is sparse and variable but can be widespread on Ios surface. When present, it would not only produce the infrared band but would brighten Ios typical surface at ultraviolet and visible wavelengths.

Discovery of Soft X-Ray Emission from Io, Europa and the Io Plasma Torus

We report the discovery of soft (0.25-2 keV) X-ray emission from the Galilean satellites Io and Europa, probably Ganymede, and from the Io Plasma Torus (IPT). Bombardment by energetic (greater than 10 keV) H, O, and S ions from the region of the IPT seems to be the likely source of the X-ray emission from the Galilean satellites. According to our estimates, fluorescent X-ray emission excited by solar X-rays, even during flares from the active Sun, charge-exchange processes, previously invoked to explain Jupiters X-ray aurora and cometary X-ray emission, and ion stripping by dust grains fail to account for the observed emission. On the other hand, bremsstrahlung emission of soft X-rays from nonthermal electrons in the few hundred to few thousand eV range may account for a substantial fraction of the observed X-ray flux from the IPT.

Discovery of five pre-main-sequence binaries in Taurus

The progress of an infrared K band lunar occultation search for binaries among premain-sequence stars is reported. From the fall of 1986 to the fall of 1989, 13 objects have been observed, all in the Taurus star-forming region. Five new binaries have been discovered: DF Tau, DI Tau, FS Tau (Haro 6-5) A, FV Tau (Haro 6-8), and FW Tau, with projected separations ranging from 0.01 to 0.57 arcsec. Including results reported previously, a total of 31 objects, 21 in Taurus and 10 in Ophiuchus, have been surveyed and nine double systems have been found. The detected binary frequency (0.29) seems consistent with that of the solar-type main-sequence field stars.

Io hot spots: Infrared photometry of satellite occultations

Abstract Infrared photometry of occultations of Io by the other Galilean satellites is used to map Ios active hot spots. Excluding a 15° wide longitude strip centered near 215°W, each point on Ios surface was occulted during at least one of seven occultations observed during the latter half of 1985. The greatest spatial resolution and sensitivity to hot spots occurs near the sub-Earth point, which always lies near the equator, and both spatial resolution and sensitivity decrease away from the sub-Earth point due to foreshortening. Most of the measurements were made with some combination of the UH 2.2 m, UKIRT, IRTF, and CFHT telescopes on Mauna Kea, Hawaii, or with the AAT in Australia, usually with one broad-bandpass infrared filter d centered at 3.8 μm ( L ′), 4.8 μm ( M ), or 8.7 μm used at each telescope. For several of the oscillations, data in more than one bandpass were acquired. A model for the occultation lightcurves is developed and fit to the data to determine the apparent path of the occulting satellite relative to Io. The astrometric results are compared to existing Galilean satellite ephemerides and are used to improve on them. The mean error in the apparent relative position of the occulting satellite is estimated to be 178 km. An occultation of Loki, Ios largest hot spot, measured at 3.8 μm wavelength, shows the region of thermal emission spatially resolved and approximately 200 km in diameter. The location of the emitting area is 308 ± 1°W longitude and 20 ± 3°N latitude, near Loki Patera and the fissure that was the source of two eruptive plumes during the Voyager encounters. A new hot spot is discovered on Ios leading hemisphere at 79 ± 5°W longitude and 22 ± 5°S latitude in the 3.8- and 4.8-μm data from 10 July UT. The region of emission is smaller than ∼20 km in diameter.

The light curve and changes in the circumstellar envelope around IRC+10216

This paper presents results of NIR photometric observations of IRC + 10216 star during the time interval 1965-1990, which were combined with measurements of the circumstellar shell surrounding IRC + 10216 by IR speckle interferometry. Using these data together with published observations, a very precise light curve was constructed, demonstrating the occurrence of long-term changes in the circumstellar shell surrounding IRC + 10216. The observations are discussed in the framework of two alternatives: (1) the occurrence of dynamical changes in the shell and (2) blocking of the direct light from the star to a portion of the shell by intervening material. 25 refs.