118098446
2 micron Spectroscopy within 0.3 arcseconds of SgrA
We present moderate (R~$\approx$~2,700) and high resolution (R~$\approx$~22,000) 2.0$-$2.4 \micron\ spectroscopy of the central 0.1 square arcseconds of the Galaxy obtained with NIRSPEC, the facility near-infrared spectrometer for the Keck II telescope. The composite spectra do not have any features attributable to the brightest stars in the central cluster, i.e.\ after background subtraction, W$_{\rm ^{12}CO(2-0)}$~$<$~2~\AA. This stringent limit leads us to conclude that the majority, if not all, of the stars are hotter than typical red giants. Coupled with previously reported photometry, we conclude that the sources are likely OB main sequence stars. In addition, the continuum slope in the composite spectrum is bluer than that of a red giant and is similar to that of the nearby hot star, IRS16NW. It is unlikely that they are late-type giants stripped of their outer envelopes because such sources would be much fainter than those observed. Given their inferred youth ($\tau_{\rm age}$~$<$~20~\Myr), we suggest the possibility that the stars have formed within 0.1 pc of the supermassive black hole. We find a newly-identified broad-line component (V$_{\rm FWHM}$ $\approx$ 1,000 \kms) to the 2.2178 \micron\ [\ion{Fe}{3}] line located within a few arcseconds of Sgr~A$^*$. A similar component is not seen in the Br-$\gamma$ emission.
10816402
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.
2811837
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 Observatory's 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 Neptune's 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.
11159984
The formation and evolution of massive stellar clusters in IC4662
We present a multiwavelength study of the formation of massive stellar clusters, their emergence from cocoons of gas and dust, and their feedback on surrounding matter. Using data that span from radio to optical wavelengths, including Spitzer and Hubble Space Telescope ACS observations, we examine the population of young star clusters in the central starburst region of the irregular Wolf–Rayet galaxy IC4662. We model the radio-to-infrared (IR) spectral energy distributions of embedded clusters to determine the properties of their Hii regions and dust cocoons (sizes, masses, densities, temperatures), and use near-IR and optical data with mid-IR spectroscopy to constrain the properties of the embedded clusters themselves (mass, age, extinction, excitation, abundance). The two massive star-formation regions in IC4662 are excited by stellar populations with ages of ∼4 Myr and masses of ∼3×105 M⊙ (assuming a Kroupa initial mass function). They have high excitation and subsolar abundances, and they may actually be comprised of several massive clusters rather than the single monolithic massive compact objects known as ‘super star clusters’ (SSCs). Mid-IR spectra reveal that these clusters have very high extinction values, AV∼20–25 mag, and that the dust in IC4662 is well mixed with the emitting gas, not in a foreground screen.
10711751
Near-IR Spectroscopy and Population Synthesis of Super Star Clusters in NGC 1569
We present H- and K-band NIRSPEC spectroscopy of super star clusters (SSCs) in the irregular starburst galaxy NGC 1569, obtained at the Keck Observatory. We fit these photospheric spectra to NextGen model atmospheres to obtain effective spectral types of clusters, and find that the information in both H- and K-band spectra is necessary to remove degeneracy in the fits. The light of SSC B is unambiguously dominated by K0 supergiants (Teff = 4400 ± 100 K, log g = 0.5 ± 0.5). The double cluster SSC A has higher Teff (G5) and less tightly constrained surface gravity (log g = 1.3 ± 1.3), consistent with a mixed stellar population dominated by blue Wolf-Rayet stars and red supergiants. We predict the time evolution of infrared spectra of SSCs using Starburst99 population synthesis models coupled with empirical stellar spectral libraries (at solar metallicity). The resulting model sequence allows us to assign ages of 15–18 Myr for SSC B and 18–21 Myr for SSC A.
18103252
K-band spectroscopy of an obscured massive stellar cluster in the Antennae galaxies (NGC 4038/4039) with NIRSPEC
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039) with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the highest surface brightness peak in the mid-IR (12-18 um) ISO image presented by Mirabel et al (1998). We obtained high signal-to-noise 2.03-2.45 um spectra of the nucleus and the obscured star cluster at R = 1900. 
The cluster is very young (age ~ 4 Myr), massive (M ~ 16E6 M_sun), and compact (density ~ 115 M_sun pc^(-3) within a 32 pc half-light radius), assuming a Salpeter IMF (0.1-100 M_sun). Its hot stars have a radiation field characterized by T_eff ~ 39,000 K, and they ionize a compact HII region with n_e ~ 10^4 cm^(-3). The stars are deeply embedded in gas and dust (A_V = 9-10 mag), and their strong FUV field powers a clumpy photodissociation region with densities n_H > 10^5 cm^(-3) on scales of ~ 200 pc, radiating L{H_2 1-0 S(1)}= 9600 L_sun.
73612545
The First Stray Light Corrected EUV Images of Solar Coronal Holes
Coronal holes are the source regions of the fast solar wind, which fills most of the solar system volume near the cycle minimum. Removing stray light from extreme ultraviolet (EUV) images of the Sun's corona is of high astrophysical importance, as it is required to make meaningful determinations of temperatures and densities of coronal holes. EUV images tend to be dominated by the component of the stray light due to the long-range scatter caused by microroughness of telescope mirror surfaces, and this component has proven very difficult to measure in pre-flight characterization. In-flight characterization heretofore has proven elusive due to the fact that the detected image is simultaneously nonlinear in two unknown functions: the stray light pattern and the true image which would be seen by an ideal telescope. Using a constrained blind deconvolution technique that takes advantage of known zeros in the true image provided by a fortuitous lunar transit, we have removed the stray light from solar images seen by the EUVI instrument on STEREO-B in all four filter bands (171, 195, 284, and 304 \AA). Uncertainty measures of the stray light corrected images, which include the systematic error due to misestimation of the scatter, are provided. It is shown that in EUVI, stray light contributes up to 70% of the emission in coronal holes seen on the solar disk, which has dramatic consequences for diagnostics of temperature and density and therefore estimates of key plasma parameters such as the plasma $\beta$\ and ion-electron collision rates.
14364805
Dissipative structures in a nonlinear dynamo
This article considers magnetic field generation by a fluid flow in a system referred to as the Archontis dynamo: a steady nonlinear MHD state is driven by a prescribed body force. The field and flow become almost equal and dissipation is concentrated in cigar-like structures centred on straight-line separatrices. Numerical scaling laws for energy and dissipation are given that extend previous calculations to smaller diffusivities. The symmetries of the dynamo are set out, together with their implications for the structure of field and flow along the separatrices. The scaling of the cigar-like dissipative regions, as the square root of the diffusivities, is explained by approximations near the separatrices. Rigorous results on the existence and smoothness of solutions to the steady, forced MHD equations are given.
50907532
INTERSTELLAR SOLID HYDROGEN
We consider the possibility that solid molecular hydrogen is present in interstellar space. If so cosmic-rays and energetic photons cause ionization in the solid leading to the formation of H +. This ion is not produced by gas-phase reactions and its radiative transitions therefore provide a signature of solid H2 in the astrophysical context. The vibrational transitions of H + are yet to be observed in the laboratory, but we have characterized them in a quantumtheoretical treatment of the molecule; our calculations include anharmonic corrections, which are large. Here we report on those calculations and compare our results with astronomical data. In addition to the H + isotopomer, we focus on the deuterated species (HD) + which is expected to dominate at low ionization rates as a result of isotopic condensation reactions. We can reliably predict the frequencies of the fundamental bands for five modes of vibration. For (HD) + all of these are found to lie close to some of the strongest of the pervasive mid-infrared astronomical emission bands, making it difficult to exclude hydrogen precipitates on observational grounds. By the same token these results suggest that (HD) + could be the carrier of the observed bands. We consider this possibility within the broader picture of interstellar medium photo-processes and we conclude that solid hydrogen may indeed be abundant in astrophysical environments.