1,630 research outputs found
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
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
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
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
The profile of a decaying crystalline cone
The decay of a crystalline cone below the roughening transition is studied.
We consider local mass transport through surface diffusion, focusing on the two
cases of diffusion limited and attachment-detachment limited step kinetics. In
both cases, we describe the decay kinetics in terms of step flow models.
Numerical simulations of the models indicate that in the attachment-detachment
limited case the system undergoes a step bunching instability if the repulsive
interactions between steps are weak. Such an instability does not occur in the
diffusion limited case. In stable cases the height profile, h(r,t), is flat at
radii r<R(t)\sim t^{1/4}. Outside this flat region the height profile obeys the
scaling scenario \partial h/\partial r = {\cal F}(r t^{-1/4}). A scaling ansatz
for the time-dependent profile of the cone yields analytical values for the
scaling exponents and a differential equation for the scaling function. In the
long time limit this equation provides an exact description of the discrete
step dynamics. It admits a family of solutions and the mechanism responsible
for the selection of a unique scaling function is discussed in detail. Finally
we generalize the model and consider permeable steps by allowing direct adatom
hops between neighboring terraces. We argue that step permeability does not
change the scaling behavior of the system, and its only effect is a
renormalization of some of the parameters.Comment: 25 pages, 18 postscript figure
A 180 Kpc Tidal Tail in the Luminous Infrared Merger Arp 299
We present VLA HI observations and UH88 deep optical B- and R-band
observations of the IR luminous merger Arp 299 (= NGC 3690 + IC 694). These
data reveal a gas-rich, optically faint tidal tail with a length of over 180
kpc. The size of this tidal feature necessitates an old interaction age for the
merger (~750 Myr since first periapse), which is currently experiencing a very
young star burst (~20 Myr). The observations reveal a most remarkable structure
within the tidal tail: it appears to be composed of two parallel filaments
separated by ~20 kpc. One of the filaments is gas rich with little if any
starlight, while the other is gas poor. We believe that this bifurcation
results from a warped disk in one of the progenitors. The quantities and
kinematics of the tidal HI suggest that Arp 299 results from the collision of a
retrograde Sab-Sb galaxy (IC 694) and a prograde Sbc-Sc galaxy (NGC 3690) that
occurred 750 Myr ago and which will merge into a single object in ~60 Myr. We
suggest that the present IR luminous phase in this system is due in part to the
retrograde spin of IC 694. Finally, we discuss the apparent lack of tidal dwarf
galaxies within the tail.Comment: LaTex, 14 pages, 11 figures, 4 tables, uses emulateapj.sty. Accepted
to AJ for July 1999. For version with full-resolution images see
http://www.cv.nrao.edu/~jhibbard/a299/HIpaper/a299HI.htm
Predictive value of dorso-lateral prefrontal connectivity for rTMS response in treatment-resistant depression: A brain perfusion SPECT study
Background: Previous clinical trials have suggested that repetitive transcranial magnetic stimulation (rTMS) has a significant antidepressant effect in patients with treatment resistant depression (TRD). However, results remain heterogeneous with many patients without effective response. / Objective: The aim of this SPECT study was to determine before treatment the predictive value of the connectivity of the stimulated area on further rTMS response in patients with TRD. / Methods: Fifty-eight TRD patients performed a brain perfusion SPECT before high frequency rTMS of the left dorsolateral prefrontal cortex (DLPFC). A voxel based-analysis was achieved to compare connectivity of the left DLPFC in responders and non-responders using inter-regional correlations (pâŻ<âŻ0.005, corrected for cluster volume). A multiple logistic regression model was thereafter used with the goal of establishing a predictive score. / Results: Before rTMS, responders exhibited increased SPECT connectivity between the left DLPFC and the right cerebellum in comparison to non-responders, independently of age, gender, severity of depression, and severity of treatment resistance. The area under the curve for the combination of these two SPECT clusters to predict rTMS response was 0.756 (pâŻ<âŻ0.005). / Conclusions: SPECT connectivity of the left DLPFC predicts rTMS response before treatment
A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar population
diagnostics and is often used to estimate masses of galaxies at low as well as
high redshifts. However, many stellar population models are still relatively
poorly calibrated in this part of the spectrum. To allow an improvement of this
calibration we present a new database of integrated near-infrared JHKs
magnitudes for 75 star clusters in the Magellanic Clouds, using the 2-Micron
All-Sky Survey (2MASS). The majority of the clusters in our sample have robust
age and metallicity estimates from color-magnitude diagrams available in the
literature, and populate a range of ages from 10 Myr to 15 Gyr and a range in
[Fe/H] from -2.17 to +0.01 dex. A comparison with matched star clusters in the
2MASS Extended Source Catalog (XSC) reveals that the XSC only provides a good
fit to the unresolved component of the cluster stellar population. We also
compare our results with the often-cited single-channel JHK photometry of
Persson and collaborators, and find significant differences, especially for
their 30"-diameter apertures up to ~2.5 mag in the K-band, more than 1 mag in
J-K, and up to 0.5 mag in H-K. Using simulations to center apertures based on
maximum light throughput (as performed by Persson et al, we show that these
differences can be attributed to near-IR-bright cluster stars (e.g., Carbon
stars) located away from the true center of the star clusters. The wide age and
metallicity coverage of our integrated JHKs photometry sample constitutes a
fundamental dataset for testing population synthesis model predictions, and for
direct comparison with near-IR observations of distant stellar populations.Comment: AJ August 2006 issue, 67 pages, 8 tables, 17 figure
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