17 research outputs found
Identification of three new proto-Planetary Nebulae exhibiting the unidentified feature at 21 um
Among its great findings, the IRAS mission showed the existence of an
unidentified mid-IR feature around 21 um. Since its discovery, this feature has
been detected in all C-rich proto-PNe of intermediate spectral type (A-G) and -
weakly - in a few PNe and AGB stars, but the nature of its carriers remains
unknown. In this paper, we show the detection of this feature in the spectra of
three new stars transiting from the AGB to the PN stage obtained with the
Spitzer Space Telescope. Following a recent suggestion, we try to model the
SEDs of our targets with amorphous carbon and FeO, which might be responsible
for the unidentified feature. The fit thus obtained is not completely
satisfactory, since the shape of the feature is not well matched. In the
attempt to relate the unidentified feature to other dust features, we retrieved
mid-IR spectra of all the 21-um sources currently known from ISO and Spitzer
on-line archives and noticed a correlation between the flux emitted in the
21-um feature and that emitted at 7 and 11 um (PAH bands and HAC broad
emission). Such a correlation may point to a common nature of the carriers.Comment: Accepted for publication in Ap
Spitzer detection of PAH and silicate features in post-AGB stars and young Planetary Nebulae
We have observed a small sample of hot post-AGB stars with the InfraRed Array
Camera (IRAC) and the InfraRed Spectrograph (IRS) on-board the Spitzer Space
Telescope. The stars were selected from the literature on the basis of their
far-Infrared excess (i.e., post-AGB candidates) and B spectral type (i.e.,
close to the ionization of the envelope). The combination of our IRAC
observations with 2MASS and IRAS catalog data, along with previous radio
observations in the cm range (where available) allowed us to model the SEDs of
our targets and find that in almost all of them at least two shells of dust at
different temperatures must be present, the hot dust component ranging up to
1000 K. In several targets grains larger than 1 micron are needed to match the
far-IR data points. In particular, in IRAS 17423-1755 grains up to 100 micron
must be introduced to match the emission in the mm range.
We obtained IRS spectra to identify the chemistry of the envelopes and found
that more than 1/3 of the sources in our sample have mixed chemistry, showing
both mid-IR bands attributed to Polycyclic Aromatic Hydrocarbons (PAH) and
silicate features. The analysis of the PAH features indicates that these
molecules are located in the outflows, far away from the central stars. We
consider the larger than expected percentage of mixed-chemistry targets as a
selection bias towards stars with a disk or torus around them. Our results
strengthen the current picture of mixed chemistry being due to the spatial
segregation of different dust populations in the envelopes.Comment: 20 pages, 18 figures, accepted for publication on Ap
A massive nebula around the Luminous Blue Variable star RMC143 revealed by ALMA
The luminous blue variable (LBV) RMC143 is located in the outskirts of the
30~Doradus complex, a region rich with interstellar material and hot luminous
stars. We report the sub-millimetre detection of its circumstellar
nebula with ALMA. The observed morphology in the sub-millimetre is different
than previously observed with HST and ATCA in the optical and centimetre
wavelength regimes. The spectral energy distribution (SED) of RMC143 suggests
that two emission mechanisms contribute to the sub-mm emission: optically thin
bremsstrahlung and dust. Both the extinction map and the SED are consistent
with a dusty massive nebula with a dust mass of
(assuming ). To date, RMC143 has the most
dusty LBV nebula observed in the Magellanic Clouds. We have also re-examined
the LBV classification of RMC143 based on VLT/X-shooter spectra obtained in
2015/16 and a review of the publication record. The radiative transfer code
CMFGEN is used to derive its fundamental stellar parameters. We find an
effective temperature of ~K, luminosity of log, and a relatively high mass-loss rate of ~yr. The luminosity is much lower than previously
thought, which implies that the current stellar mass of is
comparable to its nebular mass of (from an assumed
gas-to-dust ratio of 100), suggesting that the star has lost a large fraction
of its initial mass in past LBV eruptions or binary interactions. While the
star may have been hotter in the past, it is currently not hot enough to ionize
its circumstellar nebula. We propose that the nebula is ionized externally by
the hot stars in the 30~Doradus star-forming region.Comment: Paper accepted by A&A on 09/05/2019 and in proof stage. Second
comments by referee are included in this versio
IC 4406: a radio-infrared view
IC 4406 is a large (about 100'' x 30'') southern bipolar planetary nebula,
composed of two elongated lobes extending from a bright central region, where
there is evidence for the presence of a large torus of gas and dust. We show
new observations of this source performed with IRAC (Spitzer Space Telescope)
and the Australia Telescope Compact Array. The radio maps show that the flux
from the ionized gas is concentrated in the bright central region and
originates in a clumpy structure previously observed in H_alpha, while in the
infrared images filaments and clumps can be seen in the extended nebular
envelope, the central region showing toroidal emission. Modeling of the
infrared emission leads to the conclusion that several dust components are
present in the nebula.Comment: 22 pages, 7 figures, accepted for publication in The Astrophysical
Journal; v.2 has changes in both figures and content; preprint forma