ISO observations of dust formation in Sakurai’s object - Monitoring the mass loss of a very late Helium flash star

We present ISOCAM observations of Sakurai's object (V4334 Sgr) covering the wavelength range of 4 to 15 mu m in seven filters. The photometry shows that in the period from February 1997 to February 1998 the flux over the whole wavelength range has increased by a factor of about ten. Combined with ground-based data we conclude that this increase is the result of mass loss from Sakurai's object and the formation of hot dust around it. Using a spherically symmetric dust radiative transfer model we obtain a quantitative result of a variable and increasing mass loss rate reaching some 10(-7) M_{sun }/yr, a value not uncommon among stars during the Asymptotic Giant Branch (AGB) evolution. This is in agreement with the notion that Sakurai's object is retracing its own evolutionary history as a consequence of a very late Helium flash. We conclusively demonstrate that significant mass loss is associated with such an event and foster the link to the other few known examples of final Helium flashes. Footnote: ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the United Kingdom) and with the participation of ISAS and NASA

`Thermal' SiO radio line emission towards M-type AGB stars: a probe of circumstellar dust formation and dynamics

An extensive radiative transfer analysis of circumstellar SiO `thermal' radio line emission from a large sample of M-type AGB stars has been performed. The sample contains 18 irregulars of type Lb (IRV), 7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12 Miras. New observational material, which contains data in several SiO rotational lines in the ground vibrational state, is presented for the IRV/SRVs. The detection rate was about 60%. SiO fractional abundances have been determined through radiative transfer modelling. The abundance distribution of the IRV/SRV sample has a median value of 8E-6. The high mass loss rate Miras have a bimodal abundance distribution, a low abundance group (on average 4E-7) and a high abundance group (on average 7E-6). The derived SiO abundances are in all cases well below the abundance expected from stellar atmosphere chemistry, on average by a factor of ten. In addition, there is a trend of decreasing SiO abundance with increasing mass loss rate. This is interpreted in terms of depletion of SiO molecules by the formation of silicate grains in the circumstellar envelopes, with an efficiency which increases with the mass loss rate. The SiO and CO radio line profiles differ in shape. In general, the dominating part of the SiO profiles are narrower than the CO profiles, but they have low-intensity wings which cover the full velocity range of the CO profile. This is interpreted (as has been done also by others) as due to the influence of gas acceleration in the region which produces a significant fraction of the SiO line emission. Finally, a number of sources which have peculiar CO line profiles are discussed from the point of view of their SiO line properties

The VMC survey III : Mass-loss rates and luminosities of LMC AGB stars

Context. Asymptotic giant branch (AGB) stars are major contributors to both the chemical enrichment of the interstellar medium and the integrated light of galaxies. Despite its importance, the AGB is one of the least understood phases of stellar evolution. The main difficulties associated with detailed modelling of the AGB are related to the mass-loss process and the 3rd dredge-up efficiency Aims. We provide direct measures of mass-loss rates and luminosities for a complete sample of AGB stars in the Large Magellanic Cloud, disentangling the C- and O-rich stellar populations. Methods. Dust radiative transfer models are presented for all 374 AGB stars candidates in one of the fields observed by the new VISTA survey of the Magellanic Clouds (VMC). Mass-loss rates, luminosities and a classification of C-and O-rich stars are derived by fitting the models to the spectral energy distribution (SED) obtained by combining VMC data with existing optical, near-, and mid-infrared photometry. Results. The classification technique is reliable at a level of - at worst -75% and significantly better for the reddest dusty stars. We classified none of the stars with a relevant mass-loss rate as O-rich, and we can exclude the presence of more than one dusty O-rich star at a similar to 94% level. The bolometric luminosity function we obtained is fully consistent with most of the literature data on the LMC and with the prediction of theoretical models, with a peak of the C-star distribution at M-bol similar or equal to -4.8 mag and no stars brighter than the classical AGB tip, at M-bol = -7.1 mag. Conclusions. This exploratory study shows that our method provides reliable mass-loss rates, luminosities and chemical classifications for all AGB stars. These results offer already important constraints to AGB evolutionary models. Most of our conclusions, especially for the rarer dust-enshrouded extreme AGB stars, are however strongly limited by the relatively small area covered by our study. Forthcoming VMC observations will easily remove this limitation.Peer reviewe

The VMC Survey - XIII : Type II Cepheids in the Large Magellanic Cloud

Date of Acceptance: 27/10/2014The VISTA (Visible and Infrared Survey Telescope for Astronomy) survey of the Magellanic Clouds System (VMC) is collecting deep Ks-band time-series photometry of the pulsating variable stars hosted in the system formed by the two Magellanic Clouds and the Bridge connecting them. In this paper, we have analysed a sample of 130 Large Magellanic Cloud (LMC) Type II Cepheids (T2CEPs) found in tiles with complete or near-complete VMC observations for which identification and optical magnitudes were obtained from the OGLE III (Optical Gravitational Lensing Experiment) survey. We present J and Ks light curves for all 130 pulsators, including 41 BL Her, 62 W Vir (12 pW Vir) and 27 RV Tau variables. We complement our near-infrared photometry with the V magnitudes from the OGLE III survey, allowing us to build a variety of period-luminosity (PL), period-luminosity-colour (PLC) and period-Wesenheit (PW) relationships, including any combination of the V, J, Ks filters and valid for BL Her and W Vir classes. These relationships were calibrated in terms of the LMC distance modulus, while an independent absolute calibration of the PL(Ks) and the PW(Ks, V) was derived on the basis of distances obtained from Hubble Space Telescope parallaxes and Baade-Wesselink technique. When applied to the LMC and to the Galactic globular clusters hosting T2CEPs, these relations seem to show that (1) the two Population II standard candles RR Lyrae and T2CEPs give results in excellent agreement with each other; (2) there is a discrepancy of ~0.1 mag between Population II standard candles and classical Cepheids when the distances are gauged in a similar way for all the quoted pulsators. However, given the uncertainties, this discrepancy is within the formal 1σ uncertainties.Peer reviewedFinal Accepted Versio