James R. Graham

A Survey of Organic Volatile Species in Comet C/1999 H1 (Lee) Using NIRSPEC at the Keck Observatory

The organic volatile composition of the long-period comet C/1999 H1 (Lee) was investigated using the —rst of a new generation of cross-dispersed cryogenic infrared spectrometers (NIRSPEC, at the Keck Observatory atop Mauna Kea, HI). On 1999 August 19¨21 the organics spectral region (2.9¨3.7 km) was completely sampled at both moderate and high dispersion, along with the CO fundamental region (near 4.67 km), revealing emission from water, carbon monoxide, methanol, methane, ethane, acetylene, and hydrogen cyanide. Many new multiplets from OH in the 1¨0 band were seen in prompt emission, and numerous new spectral lines were detected. Several spectral extracts are shown, and global production rates are presented for seven parent volatiles. Carbon monoxide is strongly depleted in comet Lee relative to comets Hyakutake and Hale-Bopp, demonstrating that chemical diversity occurred in the giant

High-Precision Stellar Radial Velocities in the Galactic Center*

We present radial velocities for 85 cool stars projected onto the central parsec of the Galaxy. The majority of these velocities have relative errors of ~1 km s-1, or a factor of ~30-100 smaller than those previously obtained with proper-motion or other radial velocity measurements for a similar stellar sample. The error in a typical individual stellar velocity, including all sources of uncertainty, is 1.7 km s-1. Two similar data sets were obtained 1 month apart, and the total error in the relative velocities is 0.80 km s-1 in the case where an object is common to both data sets. The data are used to characterize the velocity distribution of the old population in the Galactic center. We find that the stars have a Gaussian velocity distribution with a mean heliocentric velocity of -10.1 ± 11.0 km s-1 (blueshifted) and a standard deviation of 100.9 ± 7.7 km s-1; the mean velocity of the sample is consistent with no bulk line-of-sight motion with respect to the local standard of rest. At the 1 σ level, the data are consistent with a symmetric velocity distribution about any arbitrary axis in the plane of the sky. We find evidence for a flattening in the distribution of late-type stars within a radius of ~0.4 pc and infer a volume density distribution of r-1/4 in this region. Finally, we establish a first epoch of radial velocity measurements that can be compared with subsequent epochs to measure small accelerations (1 km s-1 yr-1), corresponding to the magnitude expected over a time span of several years for stars nearest to Sgr A*.

J-Band Infrared Spectroscopy of a Sample of Brown Dwarfs Using NIRSPEC on Keck II.

Near-infrared spectroscopic observations of a sample of very cool, low-mass objects are presented with higher spectral resolution than in any previous studies. Six of the objects are L dwarfs, ranging in spectral class from L2 to L8/9, and the seventh is a methane or T dwarf. These new observations were obtained during commissioning of the near-infrared spectrometer (NIRSPEC), the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10 m telescope on Mauna Kea, Hawaii. Spectra with a resolving power of R approximately 2500 from 1.135 to 1.360 µm (approximately J band) are presented for each source. At this resolution, a rich spectral structure is revealed, much of which is due to blending of unresolved molecular transitions. Strong lines due to neutral potassium (K i) and bands due to iron hydride (FeH) and steam (H2O) change significantly throughout the L sequence. Iron hydride disappears between L5 and L8, the steam bands deepen, and the K i lines gradually become weaker but wider because of pressure broadening. An unidentified feature occurs at 1.22 µm that has a temperature dependence like FeH but has no counterpart in the available FeH opacity data. Because these objects are 3-6 mag brighter in the near-infrared compared with the I band, spectral classification is efficient. One of the objects studied (2MASSW J1523+3014) is the coolest L dwarf discovered so far by the 2 Micron All-Sky Survey (2MASS), but its spectrum is still significantly different from the methane-dominated objects such as Gl 229B or SDSS 1624+0029.

Discovery of an Obscured Broad-Line Region in the High-Redshift Radio Galaxy MRC 2025-218.

This Letter presents infrared spectra taken with the newly commissioned near-infrared spectrometer (NIRSPEC) on the Keck II telescope of the high-redshift radio galaxy MRC 2025-218 (z=2.63). These observations represent the deepest infrared spectra of a radio galaxy to date and have allowed for the detection of Hbeta, [O iii] lambdalambda4959, 5007, [O i] lambda6300, Halpha, [N ii] lambdalambda6548, 6583, and [S ii] lambdalambda6716, 6713. The Halpha emission is very broad (FWHM=9300 km s-1) and luminous (2.6x1044 ergs s-1), and it is very comparable to the line widths and strengths of radio-loud quasars at the same redshift. This strongly supports active galactic nucleus unification models linking radio galaxies and quasars, although we discuss some of the outstanding differences. The line [O iii] lambda5007 is extremely strong and has extended emission with large relative velocities toward the nucleus. We also derive that if the extended emission is due to star formation, each knot has a star formation rate comparable to a Lyman-break galaxy at the same redshift.

NIRSPEC brown dwarf spectroscopic survey

The NIRSPEC Brown Dwarf Spectroscopic Survey is a project to obtain a consistent set of high-quality near-IR spectra for each spectral class and sub-class of low-mass and/or sub- stellar objects to provide a new data base for models of the atmosphere of brown dwarfs and extra-solar giant planets. Most of the current targets are L-dwarfs and T-dwarfs discovered by the 2MASS. The survey is begin performed with the recently-commissioned near-IR spectrometer, NIRSPEC, a 1-5 micrometers cryogenic spectrograph at the WM Keck Observatory on Mauna Kea, using resolving powers of R equals 2,500-25,000. Preliminary results for four sources, three L-dwarfs and one T-dwarf, are presented here. Spectra from 1.13-2.33 micrometers at an average resolution of R equals 2,500 illustrate the development of deep steam bands and the weakening of FeH through the L-sequence, and the emergence of methane bands in the T-dwarfs. Complex detail in the spectra are the result of blending of numerous unresolved molecular transitions.

The Altitude of an Infrared-bright Cloud Feature on Neptune from Near-Infrared Spectroscopy*

We present 2.03–2.30 μm near-infrared spectroscopy of Neptune taken 1999 June 2 (UT) with the W. M. Keck Observatorys near-infrared spectrometer (NIRSPEC) during the commissioning of the instrument. The spectrum is dominated by a bright cloud feature, possibly a storm or upwelling, in the southern hemisphere at approximately 50° south latitude. The spectrum also includes light from a dimmer northern feature at approximately 30° north latitude. We compare our spectra (λ/Δλ ~ 2000) of these two features with a simple model of Neptunes atmosphere. Given our model assumption that the clouds are flat reflecting layers, we find that the top of the bright southern cloud feature sat at a pressure level of 0.14 bars, and thus this cloud did not extend into the stratosphere (P 0.1 bars). A similar analysis of the dimmer northern feature yields a cloud-top pressure of 0.084 ± 0.026 bars. This suggests that the features we observed efficiently transport methane to the base of the stratosphere but do not directly transport methane to the upper stratosphere (P < 10-2 to 10-3 bars), where photolysis occurs. Our observations do not constrain how far these clouds penetrate down into the troposphere. We find that our model fits to the data restrict the fraction of H2 in ortho-para thermodynamic equilibrium to greater than 0.8.

NIRSPEC observations of the galactic center

We report NIRSPEC/Keck observations of the Galactic Center obtained in both low and high resolution modes under excellent seeing conditions. The data were obtained as part of the NIRSPEC commissioning program and will be used to determine: 1) the nature of the stars in the central 0.02 pc, 2) the velocities and accelerations of stars around the central black hole, 3) the velocities of ionized gas in the central parsec, 4) the extent of the main sequence population and star formation history in central parsec, 5) the mass magnitude relation and initial mass function in the Arches cluster, 6) the nature of the MIR sources in the central parsec and Quintuplet clusters, 7) the physical parameters of stellar atmosphere/winds of super luminous stars, and 8) the metallicity in the GC as inferred from observations of red supergiants, red giants, and hot stars. We present a sample of these data, including a high resolution slit scan movie of the central parsec, and show how they can be used to vastly improve the current state of the art in the related science topics. Further, we discuss preliminary results concerning the nature of the central cusp stars and the resultant implications for star formation near a supermassive black hole.