1,807 research outputs found
Young red supergiants and the near infrared light appearance of disk galaxies
Disk galaxies often show prominent nonaxisymmetric features at near-infrared
wavelengths. Such features may indicate variations in the surface density of
stellar mass, contributions from young red supergiants in star forming regions,
or substantial dust obscuration. To distinguish among these possibilities, we
have searched for spatial variations in the 2.3 micron photometric CO index
within the disks of three nearby galaxies (NGC 278, NGC 2649, & NGC 5713). This
index measures the strength of the absorption bands of molecular CO in stellar
atmospheres, and is strong in cool, low surface-gravity stars, reaching the
largest values for red supergiants. We observe significant spatial CO index
variations in two galaxies (NGC 278 & NGC 5713), indicating that the dominant
stellar population in the near-infrared is not everywhere the same. Central CO
index peaks are present in two galaxies; these could be due to either
metallicity gradients or recent star formation activity. In addition,
significant azimuthal CO index variations are seen in NGC 278. Because strong
azimuthal metallicity gradients are physically implausible in disk galaxies,
these features are most naturally explained by the presence of a young stellar
population. The fraction of 2 micron light due to young stellar populations in
star forming regions can be calculated from our data. Overall, young stellar
populations can contribute ~3% of a (normal) galaxy's near infrared flux.
Locally, this fraction may rise to ~33%. Thus, young stars do not dominate the
total near infrared flux, but can be locally dominant in star forming regions,
and can bias estimates of spiral arm amplitude or other nonaxisymmetric
structures in galaxies' mass distributions.Comment: 28 pages including 3 postscript figures. A fourth figure is in jpeg
format. Uses AASTeX. Accepted for publication in The Astronomical Journa
1-1.4 Micron Spectral Atlas of Stars
We present a catalog of J-band (1.08 um to 1.35 um) stellar spectra at low
resolution (R ~ 400). The targets consist of 105 stars ranging in spectral type
from O9.5 to M7 and luminosity classes I through V. The relatively featureless
spectra of hot stars, earlier than A4, can be used to remove the atmospheric
features which dominate ground-based J-band spectroscopy. We measure equivalent
widths for three absorption lines and nine blended features which we identify
in the spectra. Using detailed comparison with higher resolution spectra, we
demonstrate that low resolution data can be used for stellar classification,
since several features depend on the effective temperature and gravity. For
example The CN index (1.096 - 1.104 um) decreases with temperature, but the
strength of a blended feature at 1.28 um (consisting of primarily P beta)
increases. The slope of a star's spectrum can also be used to estimate its
effective temperature. The luminosity class of a star correlates with the ratio
of the Mg I (1.1831 um) line to a blend of several species at 1.16 um. Using
these indicators, a star can be classified to within several subclasses.
Fifteen stars with particularly high and low metal abundances are included in
the catalog and some spectral dependence on metal abundance is also found.Comment: 35 pages, 10 figures (3a-e are in gif format. For complete high
resolution figures, go to http://www.astro.ucla.edu/~malkan/newjspec/) ;
Accepted for published in ApJS; For associated spectra files, see
http://www.astro.ucla.edu/~malkan/newjspec
The Dusty Starburst Nucleus of M33
We have thoroughly characterized the ultraviolet to near-infrared (0.15 - 2.2
micron) spectral energy distribution (SED) of the central parsec of the M33
nucleus through new infrared photometry and optical/near-infrared spectroscopy,
combined with ultraviolet/optical observations from the literature and the HST
archive. The SED shows evidence for a significant level of attenuation, which
we model through a Monte Carlo radiative transfer code as a shell of clumpy
Milky Way-type dust (tau_V ~ 2 +/- 1). The discovery of Milky Way-type dust
(with a strong 2175 A bump) internal to the M33 nucleus is different from
previous work which has found SMC-like dust (no bump) near starburst regions.
The amount by which dust can be processed may be related to the mass and age of
the starburst as well as the extent to which the dust can shield itself. Our
starburst models include the effects of this dust and can fit the SED if the
nucleus was the site of a moderate (~10^8 L_sun at 10 Myrs) episode of coeval
star formation about 70 Myrs ago. This result is quite different from previous
studies which resorted to multiple stellar populations (between 2 and 7)
attenuated by either no or very little internal dust. The M33 nuclear starburst
is remarkably similar to an older version (70 Myr versus 10 Myr) of the
ultra-compact starburst in the center of the Milky Way.Comment: 29 pages, 9 embedded figures, ApJ, in pres
Increase of precuneus metabolism correlates with reduction of PTSD symptoms after EMDR therapy in military veterans: an 18F-FDG PET study during virtual reality exposure to war
International audienc
Evaluation of the LEP Centre-of-Mass Energy Above the W-Pair Production Threshold
Knowledge of the centre-of-mass energy at LEP2 is of primary importance to
set the absolute energy scale for the measurement of the W-boson mass. The beam
energy above 80 GeV is derived from continuous measurements of the magnetic
bending field by 16 NMR probes situated in a number of the LEP dipoles. The
relationship between the fields measured by the probes and the beam energy is
calibrated against precise measurements of the average beam energy between 41
and 55 GeV made using the resonant depolarisation technique. The linearity of
the relationship is tested by comparing the fields measured by the probes with
the total bending field measured by a flux loop. This test results in the
largest contribution to the systematic uncertainty. Several further corrections
are applied to derive the the centre-of-mass energies at each interaction
point. In addition the centre-of-mass energy spread is evaluated. The beam
energy has been determined with a precision of 25 MeV for the data taken in
1997, corresponding to a relative precision of 2.7x10^{-4}. This is small in
comparison to the present uncertainty on the W mass measurement at LEP.
However, the ultimate statistical precision on the W mass with the full LEP2
data sample should be around 25 MeV, and a smaller uncertainty on the beam
energy is desirable. Prospects for improvements are outlined.Comment: 24 pages, 10 figures, Latex, epsfig; replaced by version accepted by
European Physical Journal
The Nature of Starburst Activity in M82
We present new evolutionary synthesis models of M82 based mainly on
observations consisting of near-infrared integral field spectroscopy and
mid-infrared spectroscopy. The models incorporate stellar evolution, spectral
synthesis, and photoionization modeling, and are optimized for 1-45 micron
observations of starburst galaxies. The data allow us to model the starburst
regions on scales as small as 25 pc. We investigate the initial mass function
(IMF) of the stars and constrain quantitatively the spatial and temporal
evolution of starburst activity in M82. We find a typical decay timescale for
individual burst sites of a few million years. The data are consistent with the
formation of very massive stars (> 50-100 Msun) and require a flattening of the
starburst IMF below a few solar masses assuming a Salpeter slope at higher
masses. Our results are well matched by a scenario in which the global
starburst activity in M82 occurred in two successive episodes each lasting a
few million years, peaking about 10 and 5 Myr ago. The first episode took place
throughout the central regions of M82 and was particularly intense at the
nucleus while the second episode occurred predominantly in a circumnuclear ring
and along the stellar bar. We interpret this sequence as resulting from the
gravitational interaction M82 and its neighbour M81, and subsequent bar-driven
evolution. The short burst duration on all spatial scales indicates strong
negative feedback effects of starburst activity, both locally and globally.
Simple energetics considerations suggest the collective mechanical energy
released by massive stars was able to rapidly inhibit star formation after the
onset of each episode.Comment: 48 pages, incl. 16 Postscript figures; accepted for publication in
the Astrophysical Journa
Profile scaling in decay of nanostructures
The flattening of a crystal cone below its roughening transition is studied
by means of a step flow model. Numerical and analytical analyses show that the
height profile, h(r,t), obeys the scaling scenario dh/dr = F(r t^{-1/4}). The
scaling function is flat at radii r<R(t) \sim t^{1/4}. We find a one parameter
family of solutions for the scaling function, and propose a selection criterion
for the unique solution the system reaches.Comment: 4 pages, RevTex, 3 eps figure
A Medium-Resolution Near-Infrared Spectral Library of Late Type Stars: I
We present an empirical infrared spectral library of medium resolution
(R~2000-3000) H (1.6 micron) and K (2.2 micron) band spectra of 218 red stars,
spanning a range of [Fe/H] from ~-2.2 to ~+0.3. The sample includes Galactic
disk stars, bulge stars from Baade's window, and red giants from Galactic
globular clusters. We report the values of 19 indices covering 12 spectral
features measured from the spectra in the library. Finally, we derive
calibrations to estimate the effective temperature, and diagnostic
relationships to determine the luminosity classes of individual stars from
near-infrared spectra.
This paper is part of a larger effort aimed at building a near-IR spectral
library to be incorporated in population synthesis models, as well as, at
testing synthetic stellar spectra.Comment: 34 pages, 12 figures; accepted for publication at ApJS; the spectra
are available from the authors upon reques
Near-Infrared Classification Spectroscopy: H-band Spectra of Fundamental MK Standards
We present a catalogue of H-band spectra for 85 stars of approximately solar
abundance observed at a resolving power of 3000 with the KPNO Mayall 4m FTS.
The atlas covers spectral types O7-M5 and luminosity classes I-V as defined on
the MK system. We identify both atomic and molecular indices and line-ratios
which are temperature and luminosity sensitive allowing spectral classification
to be carried out in the H-band. The line ratios permit spectral classification
in the presence of continuum excess emission, which is commonly found in
pre-main sequence and evolved stars. We demonstrate that with spectra of R =
1000 obtained at SNR > 50 it is possible to derive spectral types within +- 2
subclasses for late-type stars. These data are available electronically through
the Astronomical Data Center in addition to being served on the World-Wide-Web.Comment: To appear in the November 20, 1998 issue of ApJ (Volume 508, #1
Near Infrared Spectroscopy of High Redshift Active Galactic Nuclei. I. A Metallicity-Accretion Rate Relationship
We present new near infrared spectroscopic measurements of the H_beta region
for a sample of 29 luminous high redshift quasars. We have measured the width
of H_beta in those sources, and added archival H_beta width measurements, to
create a sample of 92 active galactic nuclei (AGNs) for which H_beta width and
rest-frame UV measurements of N V \lambda 1240 and C IV \lambda 1549
emission-lines are available. Our sample spans six orders of magnitude in
luminosity and includes 31 radio-loud AGNs. It also includes 10 narrow-line
Seyfert 1 galaxies and one broad absorption-line quasar. We find that
metallicity, indicated by the N V/C IV line ratio, is primarily correlated with
accretion rate, which is a function of luminosity and H_beta line-width. This
may imply an intimate relation between starburst, responsible for the metal
enrichment of the nuclear gas, and AGN fueling, represented by the accretion
rate. The correlation of metallicity with luminosity, or black hole (BH) mass,
is weaker in contrast with recent results which were based on measurements of
the width of C IV. We argue that using C IV as a proxy to H_beta in estimating
M_BH might be problematic and lead to spurious BH mass and accretion rate
estimates in individual sources. We discuss the potential implications of our
new result in the framework of the starburst-AGN connection and theories of BH
growth.Comment: 11 pages (emulateapj), 8 figures. Accepted by Ap
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