377 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
Interstellar 12C/13C from CH+ absorption lines: Results from an extended survey
The 12C/13C isotope ratio in the interstellar medium (ISM), and its evolution
with time, is an important tracer of stellar yields. Spatial variations of this
ratio can be used to study mixing in the ISM. We want to determine this ratio
and its spatial variations in the local ISM from CH+ absorption lines in the
optical towards early-type stars. The aim is to determine the average value for
the local ISM and study possible spatial variations. We observed a large number
of early-type stars with Feros to extend the sample of suitable target stars
for CH+ isotope studies. The best suited targets were observed with Uves with
higher signal-to-noise ratio and spectral resolution to determine the isotope
ratio from the interstellar CH+ lines. This study significantly expands the
number of 13CH+ detections. We find an average ratio of = 76.27 +- 1.94 or,
for f = 1/R, = (120.46 +- 3.02) 10^{-4}. The scatter in f is 6.3
sigma(). This findings strengthens the case for chemical inhomogeneity in
the local ISM, with important implications for the mixing in the ISM. Given the
large scatter, the present-day value in the ISM is not significantly larger
than the solar value, which corresponds to the local value 4.5 Gyr ago.Comment: 11 pages, 16 figures, 2 tables, A&A submitte
Atomic Gas in Debris Discs
We have conducted a search for optical circumstellar absorption lines in the
spectra of 16 debris disc host stars. None of the stars in our sample showed
signs of emission line activity in either H, Ca II or Na I,
confirming their more evolved nature. Four stars were found to exhibit narrow
absorption features near the cores of the photospheric Ca II and Na I D lines
(when Na I D data were available). We analyse the characteristics of these
spectral features to determine whether they are of circumstellar or
interstellar origins. The strongest evidence for circumstellar gas is seen in
the spectrum of HD110058, which is known to host a debris disc observed close
to edge-on. This is consistent with a recent ALMA detection of molecular gas in
this debris disc, which shows many similarities to the Pictoris system.Comment: Accepted 13/12/2016. Received 2/12/2016; Deposited on 22/11/2016. -
13 Pages, 9 Figures - MNRAS Advance Access published December 15, 201
A New Component in the Radio Continua of PNe
A byproduct of experiments designed to map the CMB is the recent detection of a new component of foreground galactic emission. The anomalous foreground at 10–30 GHz, unexplained by traditional emission mechanisms, correlates with 100 mum dust emission, and is thus presumably due to dust.Is the anomalous foreground ubiquitous in the Galaxy? I will present evidence obtained with the CBI and SIMBA+SEST supporting the existence of the new component in the ISM at large, and in specific objects, in the form of a 31 GHz excess over free-free emission in PNe
Modelling the spinning dust emission from LDN 1780
We study the anomalous microwave emission (AME) in the Lynds Dark Nebula
(LDN) 1780 on two angular scales. Using available ancillary data at an angular
resolution of 1 degree, we construct an SED between 0.408 GHz to 2997 GHz. We
show that there is a significant amount of AME at these angular scales and the
excess is compatible with a physical spinning dust model. We find that LDN 1780
is one of the clearest examples of AME on 1 degree scales. We detected AME with
a significance > 20. We also find at these angular scales that the
location of the peak of the emission at frequencies between 23-70 GHz differs
from the one on the 90-3000 GHz map. In order to investigate the origin of the
AME in this cloud, we use data obtained with the Combined Array for Research in
Millimeter-wave Astronomy (CARMA) that provides 2 arcmin resolution at 30 GHz.
We study the connection between the radio and IR emissions using morphological
correlations. The best correlation is found to be with MIPS 70m, which
traces warm dust (T50K). Finally, we study the difference in radio
emissivity between two locations within the cloud. We measured a factor
of difference in 30 GHz emissivity. We show that this variation can
be explained, using the spinning dust model, by a variation on the dust grain
size distribution across the cloud, particularly changing the carbon fraction
and hence the amount of PAHs.Comment: 14 pages, 11 figures, submitted to MNRA
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
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