14,333 research outputs found
Adaptive Optics Near-Infrared Spectroscopy of the Sgr A* Cluster
We present K-band ~ 2600 spectroscopy of five stars
(K ~ 14 - 16 mag) within 0.''5 of Sgr A*, the radio source associated with the
compact massive object suspected to be a 2.6 x 10 \msun black hole at the
center of our Galaxy. High spatial resolution of ~ 0.''09, and good strehl
ratios of ~ 0.2 achieved with adaptive optics on the 10-meter Keck telescope
make it possible to measure moderate-resolution spectra of these stars
individually for the first time. Two stars (S0-17 and S0-18) are identified as
late-type stars by the detection of CO bandhead absorption in their spectra.
Their absolute K magnitudes and CO bandhead absorption strengths are consistent
with early K giants. Three stars (S0-1, S0-2, and S0-16), with r
0.0075 pc (~ 0.''2) from Sgr A*, lack CO bandhead absorption, confirming the
results of earlier lower spectral and lower spatial resolution observations
that the majority of the stars in the Sgr A* Cluster are early-type stars. The
absolute K magnitudes of the early-type stars suggest that they are late O -
early B main sequence stars of ages 20 Myr. The presence of young stars in
the Sgr A* Cluster, so close to the central supermassive black hole, poses the
intriguing problem of how these stars could have formed, or could have been
brought, within its strong tidal field.Comment: 19 pages, 8 figures, 2 tables. Accepted for publication in Ap
Enterprise Resource Planning and Organizational Knowledge: Patterns of Convergence and Divergence
This paper reports on a qualitative research study to investigate how enterprise resource planning systems impact organizational knowledge. Cognitive mapping methodology was used to capture and analyze the perspectives of senior managers from the IT and user organizations of a major corporation. The results indicate that ERP systems produce effects that make business knowledge become more focused or convergent from the perspective of the organization and more wide-ranging or divergent from the perspective of the individual. Other important effects include changes to the organization\u27s core competencies and changes in the risk profile regarding the loss of organizational knowledge. The research contributes to the knowledge-based view of enterprise systems
The K-band spectrum of the Cataclysmic Variable RXJ 0502.8+1624 (Tau 4)
We present the K-band spectrum of the cataclysmic variable RXJ 0502.8+1624
(Tau 4). The spectrum shows a broad, smooth hump, with no absorption lines from
the secondary star visible. This result indicates that the infrared light of
this system is dominated by cyclotron emission, and, in combination with the
optical spectrum and X-ray properties, suggests that Tau 4 is a polar-type
cataclysmic variable (CV).
The system was chosen for study because the broadband JHK colours of Tau 4
are consistent with an L-type dwarf, suggesting that this system might harbour
an elusive sub-stellar secondary star. The result presented here, along with
the recent discovery of cyclotron emission in the cataclysmic variable EF Eri,
suggests that care must be taken when using the broadband JHK colours of CVs
when targeting searches for sub-stellar secondary starsComment: 4 pages, to appear as research note in A&
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
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.Comment: 4 pages, 4 embedded figures, uses emulateapj.sty. To appear in ApJL.
Also available at http://astro.berkeley.edu/~agilber
Analysis of direct segregated boundary-domain integral equations for variable-coefficient mixed bvps in exterior domains
This is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2013 World Scientific Publishing.Direct segregated systems of boundary-domain integral equations are formulated for the mixed (Dirichlet–Neumann) boundary value problems for a scalar second-order divergent elliptic partial differential equation with a variable coefficient in an exterior three-dimensional domain. The boundary-domain integral equation system equivalence to the original boundary value problems and the Fredholm properties and invertibility of the corresponding boundary-domain integral operators are analyzed in weighted Sobolev spaces suitable for infinite domains. This analysis is based on the corresponding properties of the BVPs in weighted Sobolev spaces that are proved as well.The work was supported by the grant EP/H020497/1 \Mathematical analysis of localised boundary-domain integral equations for BVPs with variable coefficients" of the EPSRC, UK
ISTRAŽIVANJE MOGUĆNOSTI RASTA KOMARČI DRŽANIH U KOMERCIJALNOM RECIRKULACIJSKOM SISTEMU I TRETIRANIH NUSPROIZVODOM INDUSTRIJSKE PROIZVODNJE GOVEĐEG HORMONA RASTA
Gilthead sea bream, maintained in a commercial scale recirculation system, were subjected to three injections (0,5 and 10 µg -1 body weight) with a by-product from the industrial production of recombinant bovine growth hormone (rbGH). Injections were provided at experiment start and at 3 and 6 weeks. Growth performance of animals was evaluated over a period of 8 weeks (n = 171 per treatment). At trial end fish were examined for proximate composition, fillet yield and visceral indices. No differences were recorded in individual growth performance between the three treatment groups (P > 0.05). Examination of protein productive value and protein efficiency ratio indicated approximately 20% of dietary protein was incorporated into animals irrespective of treatment. However incorporation of dietary lipid decreased with increasing dose of rbGH. High dose GH decreased liver weight (P 0,05). Istraživanje vrijednosti produktivnog proteina i količine efikasnosti proteina pokazalo je da je približno 20% proteina bilo inkorporirano u životinje neovisno o tretmanu. No inkorporacija lipida bila je smanjena povećanjem doze hormona rasta (rbGH). Visoka doza hormona rasta smanjila je težinu jetre (P < 0,05) u usporedbi s kontrolom s istodobnim smanjenjem hepatosomatskog indeksa (P < 0,05). Težina i postotak fileta bili su viši u životinja tretiranih dozom od 10 µg g -1 tjelesne težine u usporedbi s niskom dozom rbGH tretiranih riba (P < 0,05)
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