HST/NICMOS Observations of Massive Stellar Clusters Near the Galactic Center

We report Hubble Space Telescope (HST) Near-infrared Camera and Multi-object Spectrometer (NICMOS) observations of the Arches and Quintuplet clusters, two extraordinary young clusters near the Galactic Center. For the first time, we have identified main sequence stars in the Galactic Center with initial masses well below 10 Msun. We present the first determination of the initial mass function (IMF) for any population in the Galactic Center, finding an IMF slope which is significantly more positive (Gamma approx -0.65) than the average for young clusters elsewhere in the Galaxy (Gamma approx -1.4). The apparent turnoffs in the color-magnitude diagrams suggest cluster ages which are consistent with the ages implied by the mixture of spectral types in the clusters; we find tau(age) approx 2+/-1 Myr for the Arches cluster, and tau(age) approx 4+/-1 Myr for the Quintuplet. We estimate total cluster masses by adding the masses of observed stars down to the 50% completeness limit, and then extrapolating down to a lower mass cutoff of 1 Msun. Using this method, we find > 10^4 Msun for the total mass of the Arches cluster. Such a determination for the Quintuplet cluster is complicated by the double-valued mass-magnitude relationship for clusters with ages > 3 Myr. We find a lower limit of 6300 Msun for the total cluster mass, and suggest a best estimate of twice this value which accounts for the outlying members of the cluster. Both clusters have masses which place them as the two most massive clusters in the Galaxy.Comment: accepted by ApJ higher resolution versions of figures 1 and 2 can be found at: ftp://quintup.astro.ucla.edu/nicmos1

Inhibition of adenovirus serotype 14 infection by octadecyloxyethyl esters of (S)-[(3-hydroxy-2-phosphonomethoxy)propyl]- nucleosides in vitro.

On September 22, 2008, a physician on Prince of Wales Island, Alaska, notified the Alaska Department of Health and Social Services (ADHSS) of an unusually high number of adult patients with recently diagnosed pneumonia (n = 10), including three persons who required hospitalization and one who died. ADHSS and CDC conducted an investigation to determine the cause and distribution of the outbreak, identify risk factors for hospitalization, and implement control measures. This report summarizes the results of that investigation, which found that the outbreak was caused by adenovirus 14 (Ad14), an emerging adenovirus serotype in the United States that is associated with a higher rate of severe illness compared with other adenoviruses. Among the 46 cases identified in the outbreak from September 1 through October 27, 2008, the most frequently observed characteristics included the following: male (70%), Alaska Native (61%), underlying pulmonary disease (44%), aged > or = 65 years (26%), and current smoker (48%). Patients aged > or = 65 years had a fivefold increased risk for hospitalization. The most commonly reported symptoms were cough (100%), shortness of breath (87%), and fever (74%). Of the 11 hospitalized patients, three required intensive care, and one required mechanical ventilation. One death was reported. Ad14 isolates obtained during the outbreak were identical genetically to those in recent community-acquired outbreaks in the United States which suggests the emergence of a new, and possibly more virulent Ad14 variant. Clinicians should consider Ad14 infection in the differential diagnosis for patients with community-acquired pneumonia, particularly when unexplained clusters of severe respiratory infections are detected

Hot Stars and Cool Clouds: The Photodissociation Region M16

We present high-resolution spectroscopy and images of a photodissociation region (PDR) in M16 obtained during commissioning of NIRSPEC on the Keck II telescope. PDRs play a significant role in regulating star formation, and M16 offers the opportunity to examine the physical processes of a PDR in detail. We simultaneously observe both the molecular and ionized phases of the PDR and resolve the spatial and kinematic differences between them. The most prominent regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars is the primary excitation source, although collisions also preferentially populate the lowest vibrational levels of H2. Variations in density-sensitive emission line ratios demonstrate that the molecular cloud is clumpy, with an average density n = 3x10^5 cm^(-3). We measure the kinetic temperature of the molecular region directly and find T_H2 = 930 K. The observed density, temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the ionized region, n_i=5x10^3 cm^(-3) and T_HII = 9500 K. In the brightest regions of the PDR, the recombination line widths include a non-thermal component, which we attribute to viewing geometry.Comment: 5 pages including 2 Postscript figures. To appear in ApJ Letters, April 200

High Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection

We present new NICMOS/HST infrared images and CGS4/UKIRT Br-alpha (4.05 um) spectroscopy of the Pistol Star and its associated nebula, finding strong evidence to support the hypothesis that the Pistol Nebula was ejected from the Pistol Star. The Pa-alpha NICMOS image shows that the nebula completely surrounds the Pistol Star, although the line intensity is much stronger on its northern and western edges. The Br-alpha spectra show the classical ring-like signature of quasi-spherical expansion, with weak blueshifted emission (V_max approx -60 km/s) and strong redshifted emission (V_max approx +10 km/s), where the velocities are with respect to the velocity of the Pistol Star; further, the redshifted emission appears to be "flattened" in the position-velocity diagram. These data suggest that the nebula was ejected from the star several thousand years ago, with a velocity between the current terminal velocity of the stellar wind (95 km/s) and the present expansion velocity of gas in the outer shell of the nebula (60 km/s). The Pa-alpha image reveals several emission-line stars in the region, including two newly-identified emission-line stars north of the Pistol Star with spectral types earlier than WC8 (T_eff > 50,000 K). The presence of these stars, the morphology of the Pa-alpha emission, and the velocity field in the gas suggest that the side of the nebula furthest from us is approaching, and being ionized by, the hot stars of the Quintuplet, and that the highest velocity redshifted gas has been decelerated by winds from the Quintuplet stars. We also discuss the possibility that the nebular gas might be magnetically confined by the ambient magnetic field delineated by the nearby nonthermal filaments.Comment: Figure 1 is included as a JPG file. Figure 1 and 2 also available at ftp://quintup.astro.ucla.edu/pistol2

Infrared Spectroscopy of a Massive Obscured Star 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 micron) ISO image presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 - 2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1900. The cluster is very young (4 Myr old), massive (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 H II region with n_e ~ 1e4 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 >= 1e5 cm^(-3) on scales of up to 200 pc, radiating L[H_2 1-0 S(1)] = 9600 L_sun.Comment: 4 pages, 5 embedded figures. To appear in proceedings of 33d ESLAB Symposium: Star Formation from the Small to the Large Scale, held in Noordwijk, The Netherlands, Nov. 1999. Also available at http://astro.berkeley.edu/~agilber

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 NIRSPEC, the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10-meter telescope on Mauna Kea, Hawaii. Spectra with a resolving power of R=2500 from 1.135 to 1.360 microns (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 due to pressure broadening. An unidentified feature occurs at 1.22 microns which has a temperature dependence like FeH but has no counterpart in the available FeH opacity data. Because these objects are 3-6 magnitudes brighter in the near-infrared compared to 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 Gl229B or SDSS 1624+0029.Comment: New paper, Latex format, 2 figures, accepted to ApJ Letter

Massive Stars in the Quintuplet Cluster

We present near-infrared photometry and K-band spectra of newly-identified massive stars in the Quintuplet Cluster, one of the three massive clusters projected within 50 pc of the Galactic Center. We find that the cluster contains a variety of massive stars, including more unambiguously identified Wolf-Rayet stars than any cluster in the Galaxy, and over a dozen stars in earlier stages of evolution, i.e., LBV, Ofpe/WN9, and OB supergiants. One newly identified star is the second ``Luminous Blue Variable'' in the cluster, after the ``Pistol Star.'' Given the evolutionary stages of the identified stars, the cluster appears to be about 4 \pm 1 Myr old, assuming coeval formation. The total mass in observed stars is \sim 10^3 \Msun, and the implied mass is \sim 10^4 \Msun, assuming a lower mass cutoff of 1 \Msun and a Salpeter initial mass function. The implied mass density in stars is at least a few thousand \Msun pc^{-3}. The newly-identified stars increase the estimated ionizing flux from this cluster by about an order of magnitude with respect to earlier estimates, to 10^{50.9} photons/s, or roughly what is required to ionize the nearby ``Sickle'' HII region (G0.18 - 0.04). The total luminosity from the massive cluster stars is $\approx 10^{7.5}$ \Lsun, enough to account for the heating of the nearby molecular cloud, M0.20 - 0.033. We propose a picture which integrates most of the major features in this part of the sky, excepting the non-thermal filaments. We compare the cluster to other young massive clusters and globular clusters, finding that it is unique in stellar content and age, except, perhaps, for the young cluster in the central parsec of the Galaxy. In addition, we find that the cluster is comparable to small ``super star clusters.'

High Spectral Resolution Observations of the Massive Stars in the Galactic Center

We present high-resolution near-infrared spectra, obtained with the NIRSPEC spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive stars within the central 25 arcseconds of the Galactic center. We have identified a total of twenty-one emission-line stars, seven of which are new radial velocity detections with five of those being classified as He I emission-line stars for the first time. These stars fall into two categories based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He I doublet absorption lines, and 2) those with broad 2.058 micron He I emission lines. These data have the highest spectral resolution ever obtained for these sources and, as a result, both components of the absorption doublet are separately resolved for the first time. We use these spectral features to measure radial velocities. The majority of the measured radial velocities have relative errors of 20 kms, smaller than those previously obtained with proper-motion or radial velocity measurements for similar stellar samples in the Galactic center. The radial velocities estimated from the He I absorption doublet are more robust than those previously estimated from the 2.058 micron emission line, since they do not suffer from confusion due to emission from the surrounding ISM. Using this velocity information, we agree that the stars are orbiting in a somewhat coherent manner but are not as defined into a disk or disks as previously thought. Finally, multi-epoch radial velocity measurements for IRS 16NE show a change in its velocity presumably due to an unseen stellar companion.Comment: ApJ accepted, 42 pages, 16 figure