1,735 research outputs found
Flux calibration of medium-resolution spectra from 300 nm to 2500 nm: Model reference spectra and telluric correction
While the near-infrared wavelength regime is becoming more and more important
for astrophysics there is a marked lack of spectrophotometric standard star
data that would allow the flux calibration of such data. Furthermore, flux
calibrating medium- to high-resolution \'echelle spectroscopy data is
challenging even in the optical wavelength range, because the available flux
standard data are often too coarsely sampled. We will provide standard star
reference data that allow users to derive response curves from 300nm to 2500nm
for spectroscopic data of medium to high resolution, including those taken with
\'echelle spectrographs. In addition we describe a method to correct for
moderate telluric absorption without the need of observing telluric standard
stars. As reference data for the flux standard stars we use theoretical spectra
derived from stellar model atmospheres. We verify that they provide an
appropriate description of the observed standard star spectra by checking for
residuals in line cores and line overlap regions in the ratios of observed
(X-shooter) spectra to model spectra. The finally selected model spectra are
then corrected for remaining mismatches and photometrically calibrated using
independent observations. The correction of telluric absorption is performed
with the help of telluric model spectra.We provide new, finely sampled
reference spectra without telluric absorption for six southern flux standard
stars that allow the users to flux calibrate their data from 300 nm to 2500 nm,
and a method to correct for telluric absorption using atmospheric models.Comment: Reference spectra available at CDS. Published in A&A 568, A9, 201
Strain localization and percolation of stable structure in amorphous solids
Spontaneous strain localization occurs during mechanical tests of a model
amorphous solid simulated using molecular dynamics. The degree of localization
depends upon the extent of structural relaxation prior to mechanical testing.
In the most rapidly quenched samples higher strain rates lead to increased
localization, while the more gradually quenched samples exhibit the opposite
strain rate dependence. This transition coincides with the k-core percolation
of atoms with quasi-crystal-like short range order. The authors infer the
existence of a related microstructural length scale.Comment: 4 pages, 4 figure
The Planetary Mass Companion 2MASS1207-3932 B: Temperature, Mass and Evidence for an Edge-On Disk
We present J-band imaging and H+K-band low-resolution spectroscopy of
2MASS1207-3932 AB, obtained with VLT NACO. For the putative planetary mass
secondary, we find J = 20.0+/-0.2 mag. The HK spectra of both components imply
low gravity, and a dusty atmosphere for the secondary. Comparisons to synthetic
spectra yield Teff_A ~ 2550+/-150K, and Teff_B ~ 1600+/-100K, consistent with
their late-M and mid-to-late L types. For these Teff, and an age of 5-10 Myrs,
evolutionary models imply M_A ~ 24+/-6 M_Jup and M_B ~ 8+/-2 M_Jup. Independent
comparisons of these models to the observed colors, spanning ~I to L', also
yield the same masses and temperatures. Our primary mass agrees with other
recent analyses; however, our secondary mass, while still in the planetary
regime, is 2-3 times larger than claimed previously. This discrepancy can be
traced to the luminosities: while the absolute photometry and Mbol of the
primary agree with theoretical predictions, the secondary is ~ 2.5+/-0.5 mag
fainter than expected in all bands from I to L' and in Mbol. This accounts for
the much lower secondary mass (and temperature) derived earlier. We argue that
this effect is highly unlikely to result from a variety of model-related
problems, and is instead real. This conclusion is bolstered by the absence of
any luminosity problems in either the primary, or in AB Pic B which we also
analyse. We therefore suggest grey extinction in 2M1207B, due to occlusion by
an edge-on circum-secondary disk. This is consistent with the observed
properties of edge-on disks around T Tauri stars, and with the known presence
of a high-inclination evolved disk around the primary. Finally, the system's
implied mass ratio of ~0.3 suggests a binary-like formation scenario.
(abridged)Comment: Accepted by The Astrophysical Journal, 43 pages text + 16 figs + 1
tabl
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
Optical Spectral Signatures of Dusty Starburst Galaxies
We analyse the optical spectral properties of the complete sample of Very
Luminous Infrared Galaxies presented by Wu et al. (1998a,b) and we find a high
fraction (~50 %) of spectra showing both a strong H_delta line in absorption
and relatively modest [OII] emission (e(a) spectra). The e(a) signature has
been proposed as an efficient method to identify dusty starburst galaxies and
we study the star formation activity and the nature of these galaxies, as well
as the effects of dust on their observed properties. We examine their emission
line characteristics, in particular their [OII]/H_alpha ratio, and we find this
to be greatly affected by reddening. A search for AGN spectral signatures
reveals that the e(a)'s are typically HII/LINER galaxies. We compare the star
formation rates derived from the FIR luminosities with the estimates based on
the H_alpha line and find that the values obtained from the optical emission
lines are a factor of 10-70 (H_alpha) and 20-140 ([OII]) lower than the FIR
estimates (50-300 M_sun yr^-1). We then study the morphological properties of
the e(a) galaxies, looking for a near companion or signs of a
merger/interaction. In order to explore the evolution of the e(a) population,
we present an overview of the available observations of e(a)'s in different
environments both at low and high redshift. Finally, we discuss the role of
dust in determining the e(a) spectral properties and we propose a scenario of
selective obscuration in which the extinction decreases with the stellar age.Comment: 26 pages, Latex, including 7 postscript figures, accepted for
publication in the Astrophysical Journa
New Near-Infrared Spectroscopy of the High Redshift Quasar B 1422+231 at z=3.62
We present new near-infrared (rest-frame UV-to-optical) spectra of the high
redshift, gravitationally lensed quasar B 1422+231 (z=3.62). Diagnostic
emission lines of FeII, [OIII]5007, and Hb, commonly used to determine the
excitation, ionization, and chemical abundances of radio-quiet and radio-loud
quasars, were detected. Our new data show that the ratio FeII(UV)/Hb=18.1+-4.6
and FeII(optical)/Hb=2.3+-0.6 are higher than those reported by Kawara et al.
(1996) by factors of 1.6 and 3.3, respectively, although the ration
[OIII]5007/Hb=0.19+-0.02 is nearly the same between the two measurements. The
discrepancy of the line flux ratios between the measurements is likely due to
improved data and fitting procedures rather that to intrinsic variability.
While approximately half of the high-z quasars observed to date have much more
extreme FeII(optical)/Hb ratios, the line ratio measured for B 422+231 are
consistent with the observed range of FeII(optical) ratios of low-z quasars.Comment: 5 pages, 1 table, 4 figures. To appear in The Astronomical Journa
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
The Next Generation Virgo cluster Survey. V. modelling the dynamics of M87 with the Made-to-Measure method
We study the dynamics of the giant elliptical galaxy M87 from the central to
the outermost regions with the made-to-measure (M2M) method. We use a new
catalogue of 922 globular cluster line-of- sight velocities extending to a
projected radius of 180 kpc (equivalent to 25 M87 effective radii), and SAURON
integral field unit data within the central 2.4 kpc. 263 globular clusters,
mainly located beyond 40 kpc, are newly observed by the Next Generation Virgo
Survey (NGVS). For the M2M modelling, the gravitational potential is taken as a
combination of a luminous matter potential with a constant stellar
mass-to-light ratio and a dark matter potential modelled as a logarithmic
potential. Our best dynamical model returns a stellar mass-to-light ratio in
the I band of M/LI = 6.0(+ -0.3) M_sun/L_sun with a dark matter potential scale
velocity of 591(+ -50) km/s and scale radius of 42(+ -10) kpc. We determine the
total mass of M87 within 180 kpc to be (1.5 + - 0.2) 10^13 M_sun. The mass
within 40 kpc is smaller than previous estimates determined using globular
cluster kinematics that did not extend beyond 45 kpc. With our new globular
cluster velocities at much larger radii, we see that globular clusters around
40 kpc show an anomalously large velocity dispersion which affected previous
results. The mass we derived is in good agreement with that inferred from ROSAT
X-ray observation out to 180 kpc. Within 30 kpc our mass is also consistent
with that inferred from Chandra and XMM-Newton X-ray observations, while within
120 kpc it is about 20% smaller. The model velocity dispersion anisotropy beta
parameter for the globular clusters in M87 is small, varying from -0.2 at the
centre to 0.2 at 40 kpc, and gradually decreasing to zero at 120 kpc.Comment: 20 pages, 16 figures, Accepted by Ap
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
VLT/ISAAC H-band spectroscopy of embedded massive YSOs
We have performed intermediate resolution (R = 5000), high signal-to-noise
H-band spectroscopy of a small, initial sample of three massive embedded young
stellar objects (YSOs), using VLT/ISAAC. The sample has been selected from
sources characterised in previous literature as being likely of OB spectral
type, to be unambiguously associated with bright (H < 14) single point sources
in the 2MASS database, and to have no optical counterparts. Of the targets
observed, one object shows a ~B3 spectrum, similar to a main sequence object of
the same spectral type. A second object exhibits weak HeI and H emission,
indicating an early-type source: we detect HeII absorption, which supports a
previous indirect derivation of the spectral type as mid-O. The third object
does not show absorption lines, so no spectral type can de derived. It does,
however, exhibit a rich spectrum of strong, broad emission lines and is likely
to be surrounded by dense circumstellar material and at a very early
evolutionary stage. Our results from this very small sample are in agreement
with those of Kaper et al. (2002), who also find spectra similar to optically
visible main sequence stars, together with emission line objects representing a
very early evolutionary phase, in their much larger sample of K-band spectra.Comment: 10 pages, 14 figures, A&A (accepted
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