Donald F. Figer

Massive Stars in the Arches Cluster

We present and use new spectra and narrowband images, along with previously published broadband images, of stars in the Arches cluster to extract photometry, astrometry, equivalent width, and velocity information. The data are interpreted with a wind/atmosphere code to determine stellar temperatures, luminosities, mass-loss rates, and abundances. We have doubled the number of known emission-line stars, and we have also made the first spectroscopic identification of the main sequence for any population in the Galactic center. We conclude that the most massive stars are bona fide Wolf-Rayet (W-R) stars; are some of the most massive stars known, having Minit > 100 M☉; and have prodigious winds, > 10-5 M☉ yr-1, that are enriched with helium and nitrogen; with these identifications, the Arches cluster contains about 5% of all known W-R stars in the Galaxy. We find an upper limit to the velocity dispersion of 22 km s-1, implying an upper limit to the cluster mass of 7 × 104 M☉ within a radius of 0.23 pc; we also estimate the bulk heliocentric velocity of the cluster to be vcluster, ☉ ≈ +95 km s-1. Taken together, these results suggest that the Arches cluster was formed in a short, but massive, burst of star formation about 2.5 ± 0.5 Myr ago, from a molecular cloud that is no longer present. The cluster happens to be approaching and ionizing the surface of a background molecular cloud, thus producing the thermal arched filaments. We estimate that the cluster produces 4 × 1051 ionizing photons s-1, more than enough to account for the observed thermal radio flux from the nearby cloud, 3 × 1049 ionizing photons s-1. Commensurately, it produces 107.8 L☉ in total luminosity, providing the heating source for the nearby molecular cloud, Lcloud ≈ 107 L☉. These interactions between a cluster of hot stars and a wayward molecular cloud are similar to those seen in the Quintuplet/Sickle region. The small spread of formation times for the known young clusters in the Galactic center and the relative lack of intermediate-age stars (τage = 107.0-107.3 yr) suggest that the Galactic center has recently been host to a burst of star formation. Finally, we have made new identifications of near-infrared sources that are counterparts to recently identified X-ray and radio sources.

An Extended Star Formation History for the Galactic Center from Hubble Space Telescope NICMOS Observations

We present Hubble Space Telescope (HST) Near-Infrared Camera and Multiobject Spectrometer (NICMOS) observations as evidence that continuous star formation has created much of the central stellar cusp of the Galaxy. The data are the deepest ever obtained for a Galactic center population, being more than 50% complete for mF205W < 19.3, or initial stellar masses 2 M☉. We use Geneva and Padova stellar evolution models to produce synthetic luminosity functions for burst and continuous star formation scenarios, finding that the observations are fitted best by continuous star formation at a rate that is consistent with the recent star formation activity that produced the three massive young clusters in the central 50 pc. Further, it is not possible to fit the observations with ancient burst models, such as would be appropriate for an old population like that in Baades window or NGC 6528.

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*.

A newly discovered young massive star cluster at the far end of the Galactic Bar

We present a near-infrared study of the candidate star cluster Mercer 81, located at the centre of the G338.4+0.1 Hxa0ii region and close to the TeV gamma-ray source HESS 1640–465. Usingu2002Hubble Space Telescope/NICMOS imaging and VLT/ISAAC spectroscopy, we have detected a compact and highly reddened cluster of stars, although the bright stars in the centre of the field are in fact foreground objects. The cluster contains nine stars with strong Pα emission, one of which we identify as a Wolf–Rayet (WR) star, as well as an A-type supergiant. The line-of-sight extinction is very large,u2002AV∼ 45, illustrating the challenges of locating young star clusters in the Galactic plane. From a quantitative analysis of the WR star, we argue for a cluster age of 3.7xa0Myr, and, assuming that all emission-line stars are WR stars, a cluster mass of ≳104xa0M⊙. A kinematic analysis of the cluster’s surrounding Hxa0ii region shows that the cluster is located in the Galactic disc at a distance of 11 ± 2xa0kpc. This places the cluster close to where the far end of the Bar intersects the Norma spiral arm. This cluster, as well as the nearby cluster [DBS2003]179, represents the first detections of active star cluster formation at this side of the Bar, in contrast to the near side which is well known to have recently undergone a ∼106xa0M⊙ starburst episode.

Polarization of the Thermal Emission from the Dust Ring at the Center of the Galaxy

We present new results on the polarization of the far-infrared emission from the dust ring at the center of the Galaxy. Previous far-infrared polarimetry of the region has been confined to the main ridge of emission. The new measurements cover the entire area of the ring. Polarization maps computed for magnetic fields with biaxial symmetry cannot be made to fit the data. Maps computed for axisymmetric fields provide much better fits, but the results show that the ring and field cannot both have ideal symmetry. The dispersion of the measured position angles about the directions computed from the self-similar model of Wardle & Konigl (1990) is comparable to the dispersion about mean values observed in the most orderly dark clouds

Polarization of the far-infrared emission from the thermal filaments of the Galactic center arc

The polarization of the 100 micron continuum emission has been measured at 14 positions in the dense, warm molecular cloud associated with the arched filaments, or the bridge, of the radio arc near the Galactic center. At all positions the percent polarization is found to be quite large, ranging up to 6.5 percent. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains. The directions of the polarization vectors then indicate that the magnetic field is (1) parallel to the long dimension of the thermal radio filaments, and (2) very uniform on scales of 1-10 pc. Of several explanations for the inferred field geometry, the simplest is that it results from the unusually large dynamical shear in the emitting cloud.

Resolved Infrared Spectroscopy of Two High-Redshift Radio Galaxies

We report on the rest-frame optical-line emission from the extended gas in two high-redshift radio galaxies, 4C 48.48 at z = 2.343 and 4C 40.36 at z = 2.265. These are among the first observations to obtain rest-frame optical spectra for distinct knots of gas within a high-redshift radio galaxy. Although the dominant ionization mechanism in both sources is photoionization by the central AGNs, we find evidence of another mechanism within the off-nuclear emission of each source. We measure [O II], [Ne III] and [O III] for several regions in 4C 48.48; these measurements indicate shock ionization in one region. We measure seven lines in 4C 40.36 and compare the on- and off-nuclear emission from the two strongest, Hα and [O III]; we find that [O III]/Hα is at least a factor of 4 weaker off-nucleus, suggesting star formation in the extended region.

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.

Emission Lines in the Near-infrared Spectra of the Infrared Quintuplet Stars in the Galactic Center

We report the detection of a number of emission lines in the 1.0--2.4~

Ultra-Low Background Operation of Near-Infrared Detectors Using Reference Pixels for NGST

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