MWC 930 is a star just ~2{deg} above the Galactic plane whose nature is not clear and that has not been studied in detail so far. While a post-Asymptotic Giant Branch (AGB) classification was proposed in the past, studies of its optical spectrum and
photometry pointed toward strong variability, therefore the object was reclassified as a Luminous Blue Variable (LBV) candidate. LBVs typically undergo phases of strong mass loss in the form of eruptions that can create shells of ejecta around the star. Our goal is to search for the presence of such a circumstellar nebula in MWC 930 and investigate its properties. To do so, we make use of space-based infrared data from our Spitzer campaign performed with the InfraRed Array Camera (IRAC) and the InfraRed Spectrograph (IRS) as well as data from optical and infrared (IR) surveys. In our Spitzer images, we clearly detect an extended shell around MWC 930 at wavelengths longer than 5 um. The mid-infrared spectrum is dominated by the central star and mostly shows forbidden lines of [FeII], with an underlying continuum that decreases with wavelength up to ~15 um and then inverts its slope, displaying a second peak around 60 um, evidence for cold dust grains formed in a past eruption. By modeling the SED, we identify two central components, besides the star and the outer shell. These extra sources of radiation are interpreted as material close to the central star, maybe due to a recent ejection. Features of C-bearing molecules or grains are not detected.
Maser lines of OH, H2 O, and SiO are commonly observed in O-rich AGB stars, but their presence after the end of the Asymptotic Giant Branch (AGB) phase is linked to non-spherical mass-loss processes. IRAS 15452-5459 is a post-AGB star with an hourgla
ss nebula whose maser lines are quite peculiar. We observed all of the three maser species with the Australia Telescope Compact Array with angular resolutions of 6 , 0.6 , 0.3 , and 1.7 at 18 cm, 13 mm, 7 mm, and 3 mm, respectively. While double peaks are routinely seen in OH and water masers and interpreted as due to expanding envelopes, only very few sources display SiO lines with a similar spectral profile. Our observations confirm the detection of the double peak of SiO at 86 GHz; the same spectral shape is seen in the lower-J maser at 43 GHz. A double peak is also detected in the water line, which covers the same velocity range as the SiO masers. Thermally excited lines of SiO are detected at 7 and 3 mm and span the same velocity range as the maser lines of this species. Although observations at higher angular resolution are desirable to further investigate the spatial distributions of the maser spots, the current data allow us to conclude that the SiO masers are distributed in an hourglass shape and are likely due to the sputtering of dust grains caused by shock propagation. The complex OH profile would instead be due to emission from the fast outflow and an orthogonal structure.
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 PN
e 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.
Searching for variability, we have observed a sample of hot post-AGB stars and young Planetary Nebulae candidates with the Very Large Array at 4.8, 8.4, and 22.4 GHz. The sources had been previously detected in the radio continuum, which is a proof t
hat the central stars have started ionising their circumstellar envelopes and an increase in radio flux with time can be expected as a result of the progression of the ionisation front. Such a behaviour has been found in IRAS 18062+2410, whose radio modelling has allowed us to determine that its ionised mass has increased from 10^{-4} to 3.3 10^{-4} M_sun in 8 years and its envelope has become optically thin at lower frequencies. Different temporal behaviours have been found for three other sources. IRAS 17423-1755 has shown a possibly periodic pattern and an inversion of its radio spectral index, as expected from a varying stellar wind. We estimate that the radio flux arises from a very compact region around the central star (10^{15} cm) with an electron density of 2 10^6 cm^{-3}. IRAS 22568+6141 and 17516-2525 have decreased their radio flux densities of about 10% per year over 4 years. While a linear increase of the flux density with time points out to the progression of the ionisation front in the envelope, decreases as well as quasi-periodic patterns may indicate the presence of unstable stellar winds/jets or thick dusty envelopes absorbing ionising photons.
IC 4406 is a large (about 100 x 30) southern bipolar planetary nebula, composed by two elongated lobes, extending from a bright central region, where there is evidence for the presence of a large torus of gas and dust. In this poster we show new obse
rvations of this source performed with IRAC (Spitzer Space Telescope) and the Australia Telescope Compact Array. Although the possibility for faint extended emission to be missing in the radio maps cannot be ruled out, flux from the ionized gas appears to be concentrated in the bright central region. Comparing ATCA to IRAC images, it seems that, like in other planetary nebulae, ionized and neutral components spatially co-exist in IC 4406.
Observing objects in transition from pre- to young Planetary Nebula (PN), when the central star radiation starts to excite the envelope, can help us to understand the evolution of the circumstellar ejecta and their shaping mechanism/s. In our project
we have selected a sample of hot post-AGB stars as Transition Phase candidates. Radio observations have led to detect free-free radiation from an ionized shell in about half of our targets, providing us with two sub-samples of ionized and non ionized Transition Objects. We are now using IRAC and IRS on the Spitzer Space Telescope to determine if extended emission is present (IRAC) and to study our targets chemistry (IRS). In particular, by comparing spectra from the two sub-samples, the IRS observations will enable us to check how the presence of an ionization front effects the circumstellar envelope. The IRAC measurements, combined with previous ones in the literature, will give us information on the extent and physical conditions of the dust components.
