309 research outputs found
The Galactic Disk Distribution of Planetary Nebulae with Warm Dust Emission Features: II
Can the distribution of warm-dust compositions in IR-bright galactic disk PNe
be linked to the underlying stellar population? The PNe with warm dust emission
represent a homogeneous population, which is presumably young and minimally
affected by a possible dependence of PN lifetime on progenitor mass. The sample
in paper I thus allows testing the predictions of single star evolution,
through a comparison with synthetic distributions and under the assumption that
tip-of-the-AGB and PN statistics are similar. We construct a schematic model
for AGB evolution (adapted from Groenewegen & de Jong 1993), whose
free-parameters are calibrated with the luminosity function (LF) of C stars in
the LMC, the initial-final mass relation, and the range of PN compositions. The
observed metallicity gradient and distribution of star forming regions with
galactocentric radius (Bronfman et al. 2000) allow us to synthesise the
galactic disk PN progenitor population. We find the fraction of O-rich PNe,
f(O), is a tight constraint on AGB parameters. For our best model, a minimum PN
progenitor mass Mmin=1Msun predicts that about 50% of all young PNe should be
O-rich, compared to an observed fraction of 22%; thus Mmin=1.2Msun, at a 2sigma
confidence level. By contrast, current AGB models for single stars can account
neither for the continuous range of N enrichment (Leisy & Dennefeld 1996), nor
for the observation that the majority of very C-rich PNe have Peimbert type I
(paper I). f(O) is thus an observable much easier to model. The decrease in
f(O) with galactocentric radius, as reported in paper I, is a strong property
of the synthetic distribution, independent of Mmin. This trend reflects the
sensitivity of the surface temperature of AGB stars and of the core mass at the
first thermal pulse to the galactic metallicity gradient.Comment: accepted by MNRA
Multi-GPU maximum entropy image synthesis for radio astronomy
The maximum entropy method (MEM) is a well known deconvolution technique in
radio-interferometry. This method solves a non-linear optimization problem with
an entropy regularization term. Other heuristics such as CLEAN are faster but
highly user dependent. Nevertheless, MEM has the following advantages: it is
unsupervised, it has a statistical basis, it has a better resolution and better
image quality under certain conditions. This work presents a high performance
GPU version of non-gridding MEM, which is tested using real and simulated data.
We propose a single-GPU and a multi-GPU implementation for single and
multi-spectral data, respectively. We also make use of the Peer-to-Peer and
Unified Virtual Addressing features of newer GPUs which allows to exploit
transparently and efficiently multiple GPUs. Several ALMA data sets are used to
demonstrate the effectiveness in imaging and to evaluate GPU performance. The
results show that a speedup from 1000 to 5000 times faster than a sequential
version can be achieved, depending on data and image size. This allows to
reconstruct the HD142527 CO(6-5) short baseline data set in 2.1 minutes,
instead of 2.5 days that takes a sequential version on CPU.Comment: 11 pages, 13 figure
Evidence for a circumplanetary disk around protoplanet PDS 70 b
We present the first observational evidence for a circumplanetary disk around
the protoplanet PDS~70~b, based on a new spectrum in the band acquired with
VLT/SINFONI. We tested three hypotheses to explain the spectrum: Atmospheric
emission from the planet with either (1) a single value of extinction or (2)
variable extinction, and (3) a combined atmospheric and circumplanetary disk
model. Goodness-of-fit indicators favour the third option, suggesting
circumplanetary material contributing excess thermal emission --- most
prominent at m. Inferred accretion rates (-- yr) are compatible with observational
constraints based on the H and Br lines. For the planet, we
derive an effective temperature of 1500--1600 K, surface gravity , radius , mass , and possible thick clouds.
Models with variable extinction lead to slightly worse fits. However, the
amplitude (mag) and timescale of variation
(~years) required for the extinction would also suggest
circumplanetary material.Comment: 8 pages, 2 figures, 1 table. This is a pre-copyedited,
author-produced PDF of an article accepted for publication in ApJL on 2019
May 1
HD 169142 in the eyes of ZIMPOL/SPHERE
We present new data of the protoplanetary disc surrounding the Herbig Ae/Be
star HD 169142 obtained in the very broad-band (VBB) with the Zurich imaging
polarimeter (ZIMPOL), a sub-system of the Spectro-Polarimetric High-contrast
Exoplanet REsearch instrument (SPHERE) at the Very Large Telescope (VLT). Our
Polarimetric Differential Imaging (PDI) observations probe the disc as close as
0.03" (3.5au) to the star and are able to trace the disc out to ~1.08"
(~126au). We find an inner hole, a bright ring bearing substructures around
0.18" (21au), and an elliptically shaped gap stretching from 0.25" to 0.47"
(29-55au). Outside of 0.47", the surface brightness drops off, discontinued
only by a narrow annular brightness minimum at ~0.63"-0.74" (74-87au). These
observations confirm features found in less-well resolved data as well as
reveal yet undetected indications for planet-disc interactions, such as
small-scale structures, star-disk offsets, and potentially moving shadows.Comment: Accepted for publication in MNRA
Morphological analysis of the cm-wave continuum in the dark cloud LDN1622
The spectral energy distribution of the dark cloud LDN1622, as measured by
Finkbeiner using WMAP data, drops above 30GHz and is suggestive of a Boltzmann
cutoff in grain rotation frequencies, characteristic of spinning dust emission.
LDN1622 is conspicuous in the 31 GHz image we obtained with the Cosmic
Background Imager, which is the first cm-wave resolved image of a dark cloud.
The 31GHz emission follows the emission traced by the four IRAS bands. The
normalised cross-correlation of the 31 GHz image with the IRAS images is higher
by 6.6sigma for the 12um and 25um bands than for the 60um and 100um bands:
C(12+25) = 0.76+/-0.02 and C(60+100) = 0.64+/-0.01.
The mid-IR -- cm-wave correlation in LDN 1622 is evidence for very small
grain (VSG) or continuum emission at 26-36GHz from a hot molecular phase. In
dark clouds and their photon-dominated regions (PDRs) the 12um and 25um
emission is attributed to stochastic heating of the VSGs. The mid-IR and
cm-wave dust emissions arise in a limb-brightened shell coincident with the PDR
of LDN1622, where the incident UV radiation from the Ori OB1b association heats
and charges the grains, as required for spinning dust.Comment: accepted for publication in ApJ - the complete article with
uncompressed figures may be downloaded from
http://www.das.uchile.cl/~simon/ftp/l1622.pd
Dust masses of disks around 8 Brown Dwarfs and Very Low-Mass Stars in Upper Sco OB1 and Ophiuchus
We present the results of ALMA band 7 observations of dust and CO gas in the
disks around 7 objects with spectral types ranging between M5.5 and M7.5 in
Upper Scorpius OB1, and one M3 star in Ophiuchus. We detect unresolved
continuum emission in all but one source, and the CO J=3-2 line in two
sources. We constrain the dust and gas content of these systems using a grid of
models calculated with the radiative transfer code MCFOST, and find disk dust
masses between 0.1 and 1 M, suggesting that the stellar mass / disk
mass correlation can be extrapolated for brown dwarfs with masses as low as
0.05 M. The one disk in Upper Sco in which we detect CO emission, 2MASS
J15555600, is also the disk with warmest inner disk as traced by its H - [4.5]
photometric color. Using our radiative transfer grid, we extend the correlation
between stellar luminosity and mass-averaged disk dust temperature originally
derived for stellar mass objects to the brown dwarf regime to , applicable to spectral types
of M5 and later. This is slightly shallower than the relation for earlier
spectral type objects and yields warmer low-mass disks. The two prescriptions
cross at 0.27 L, corresponding to masses between 0.1 and 0.2 M
depending on age.Comment: 9 pages,6 figures, accepted to ApJ on 26/01/201
The inner environment of Z~CMa: High-Contrast Imaging Polarimetry with NaCo
Context. Z\,CMa is a binary composed of an embedded Herbig Be and an FU Ori
class star separated by au. Observational evidence indicate a complex
environment in which each star has a circumstellar disk and drives a jet, and
the whole system is embedded in a large dusty envelope.
Aims. We aim to probe the circumbinary environment of Z\,CMa in the inner 400
au in scattered light.
Methods. We use high contrast imaging polarimetry with VLT/NaCo at and
bands.
Results. The central binary is resolved in both bands. The polarized images
show three bright and complex structures: a common dust envelope, a sharp
extended feature previously reported in direct light, and an intriguing bright
clump located 0\farcs3 south of the binary, which appears spatially connected
to the sharp extended feature.
Conclusions.We detect orbital motion when compared to previous observations,
and report a new outburst driven by the Herbig star. Our observations reveal
the complex inner environment of Z\,CMa with unprecedented detail and contrast.Comment: Accepted for publication in A&A Letter
Bayesian Image Reconstruction Based on Voronoi Diagrams
We present a Bayesian Voronoi image reconstruction technique (VIR) for
interferometric data. Bayesian analysis applied to the inverse problem allows
us to derive the a-posteriori probability of a novel parameterization of
interferometric images. We use a variable Voronoi diagram as our model in place
of the usual fixed pixel grid. A quantization of the intensity field allows us
to calculate the likelihood function and a-priori probabilities. The Voronoi
image is optimized including the number of polygons as free parameters. We
apply our algorithm to deconvolve simulated interferometric data. Residuals,
restored images and chi^2 values are used to compare our reconstructions with
fixed grid models. VIR has the advantage of modeling the image with few
parameters, obtaining a better image from a Bayesian point of view.Comment: 27 pages, 10 figures, to be published in APJ, 672, 127
An excess of emission in the dark cloud LDN 1111 with the Arcminute Microkelvin Imager
We present observations of the Lynds' dark nebula LDN 1111 made at microwave
frequencies between 14.6 and 17.2 GHz with the Arcminute Microkelvin Imager
(AMI). We find emission in this frequency band in excess of a thermal
free--free spectrum extrapolated from data at 1.4 GHz with matched uv-coverage.
This excess is > 15 sigma above the predicted emission. We fit the measured
spectrum using the spinning dust model of Drain & Lazarian (1998a) and find the
best fitting model parameters agree well with those derived from Scuba data for
this object by Visser et al. (2001).Comment: accepted MNRA
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