M. M. Montgomery

The Volatile Composition of Comet 17P/Holmes after Its Extraordinary Outburst

The volatile abundances in comet 17P/Holmes were measured on three dates (UT 2007 October 27.6 and 31.3 and November 2.3) using high-dispersion (λ/Δ λ ~ 2.5 × 104) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory and CSHELL at the NASA Infrared Telescope Facility. Compared to other comets, the relative gas production rates in the coma show an enhancement of C2H6, HCN, and C2H2 with respect to H2O, by factors of ~2-3. CH3OH was also detected with an abundance relative to H2O that is similar to or perhaps slightly enhanced compared to the values observed in other comets. The apparent enrichment of some volatiles in the coma of 17P/Holmes does not necessarily imply an unusual composition for its nucleus because fractionation effects may be important at the relatively large heliocentric distance (Rh = 2.45 AU) at which our observations were performed. Rotational temperatures were determined for H2O, HCN, C2H6, and C2H2 in the coma on UT October 27.6 and found to be between 60 and 80 K. We used lines in both the ν5 and ν7 bands to obtain the best constraints yet achieved for the rotational temperature of C2H6. The spatial distributions of all measured volatiles in the coma are consistent with each other and suggest at most only a minor contribution from sublimating icy grains within our aperture. The overall gas production rate declined by approximately a factor of 7 between UT October 27.6 and November 2.3 with no significant change measured in the relative production rates of C2H6 and H2O during this time.

A Common Source of Accretion Disk Tilt

Many different system types retrogradely precess, and retrograde precession could be from a tidal torque by the secondary on a misaligned accretion disk. However, a source that causes and maintains disk tilt is unknown. In this work, we show that accretion disks can tilt due to a force called lift. Lift results from differing gas stream supersonic speeds over and under an accretion disk. Because lift acts at the disks center of pressure, a torque is applied around a rotation axis passing through the disks center of mass. The disk responds to lift by pitching around the disks line of nodes. If the gas stream flow ebbs, then lift also ebbs and the disk attempts to return to its original orientation. To first approximation, lift does not depend on magnetic fields or radiation sources but does depend on the mass and the surface area of the disk. Also, for disk tilt to be initiated, a minimum mass transfer rate must be exceeded. For example, a 10?11 M ? disk around a 0.8 M ? compact central object requires a mass transfer rate greater than ? 8 ? 10?11 M ? yr?1, a value well below the known mass transfer rates in cataclysmic variable dwarf novae systems that retrogradely precess and exhibit negative superhumps in their light curves and a value well below mass transfer rates in protostellar-forming systems.

INTERMEDIATE RESOLUTION NEAR-INFRARED SPECTROSCOPY OF 36 LATE M DWARFS

We present observations of 36 late M dwarfs obtained with the Keck II/NIRSPEC in the J band at a resolution of {approx}20,000. We have measured projected rotational velocities, absolute radial velocities, and pseudo-equivalent widths of atomic lines. Twelve of our targets did not have previous measurements in the literature. For the other 24 targets, we confirm previously reported measurements. We find that 13 stars from our sample have v sin i below our measurement threshold (12 km s{sup -1}) whereas four of our targets are fast rotators (v sin i > 30 km s{sup -1}). As fast rotation causes spectral features to be washed out, stars with low projected rotational velocities are sought for radial velocity surveys. At our intermediate spectral resolution, we have confirmed the identification of neutral atomic lines reported in McLean et al. We also calculated pseudo-equivalent widths of 12 atomic lines. Our results confirm that the pseudo-equivalent width of K I lines is strongly dependent on spectral types. We observe that the pseudo-equivalent width of Fe I and Mn I lines remains fairly constant with later spectral type. We suggest that these lines are particularly suitable for deriving metallicities for late M dwarfs.

TILT, WARP, AND SIMULTANEOUS PRECESSIONS IN DISKS

Warps are suspected in disks around massive compact objects. However, the proposed warping source?non-axisymmetric radiation pressure?does not apply to white dwarfs. In this Letter, we report the first smoothed particle hydrodynamic simulations of accretion disks in SU UMa-type systems that naturally tilt, warp, and simultaneously precess in the prograde and retrograde directions using white dwarf V344 Lyrae in the Kepler field as our model. After ~79?days in V344 Lyrae, the disk angular momentum L d becomes misaligned to the orbital angular momentum L o . As the gas stream remains normal to L o , hydrodynamics (e.g., the lift force) is a likely source to disk tilt. In addition to tilt, the outer disk annuli cyclically change shape from circular to highly eccentric due to tidal torques by the secondary star. The effect of simultaneous prograde and retrograde precession is a warp of the colder, denser midplane as seen along the disk rim. The simulated rate of apsidal advance to nodal regression per orbit nearly matches the observed ratio in V344 Lyrae.

Search for radial velocity variations in eight M-dwarfs with NIRSPEC/Keck II

Context. Radial velocity (RV) measurements from near-infrared spectra have become a potentially powerful tool to search for planets around cool stars and sub-stellar objects. As part of a large survey to characterize M-dwarfs using NIRSPEC at Keck II, we obtained spectra of eight late M-dwarfs (spectral types M5.0–M8.0) during two or more observing epochs per target. These spectra were taken with intermediate spectral resolving powers (R ∼ 20 000) in the J-band. Aims. We search for relative RV variability in these late M-dwarfs and test the NIRSPEC capability of detecting short-period brown dwarf and massive planetary companions around low-mass stars in the J-band (≈1.25 μm). Additionally, we reanalyzed the data of the M8-type star vB10 (one of our targets), which had been presented in another article. Methods. To achieve a precise RV measurement stability, the NIRSPEC spectra were self-calibrated by making use of the telluric absorption lines, which are present in the observed spectra and were used as a long-term stable reference. In the modeling process a multi-parameter χ 2 -optimization was employed to generate an accurate description of the observation. The telluric lines allowed us to model the instrumental profile of the spectrograph and the determination of the Doppler shift of the stellar absorption lines. Results. For the entire M-dwarf sample, we found no evidence of relative RV variations induced by a short-period brown dwarf or massive planetary companion. The typical RV precision of the measurements is between 180 and 300 m s −1 ,w hich is sufficient to detect hot Neptunes around M-dwarfs. Moreover, we found that the spurious RV shift detected previously of the star VB10 was caused by asymmetries in the instrumental profile between different observing epochs, which were not taken into account in the previous analysis.

Infrared radial velocities of vB 10

We present radial velocities of the M8V-type, very low-mass star vB 10 obtained at four different epochs between 2001 and 2008. We use high-resolution (R ∼ 20 000) near-infrared (J-band) spectra taken with the nirspec instrument on the Keck II telescope. Our data suggest that vB 10 shows radial velocity variability with an amplitude of ∼ 1k m s −1 , a result that is consistent with the recent finding of a massive planet companion around the star. More velocity measurements and a better sampling of the orbital phase are required to precisely constrain the orbital parameters and the individual masses of the pair.

Radial Velocities of Accreting White Dwarfs

In this work we generate theoretical radial velocities (RVs) of accreting white dwarfs. We compare theoretical RVs with observed RVs to gain insight into which cataclysmic variable (CV) components may contribute to the curves of observed RVs. We present preliminary results.