F. Marang

Near-infrared photometry of carbon stars*

Near-infrared, JHKL, photometry of 239 Galactic carbon-rich variable stars is presented and discussed. From these and published data the stars were classified as Mira or non-Mira variables and amplitudes and pulsation periods, ranging from 222 to 948 days for the Miras, were determined for most of them. A comparison of the colour and period relations with those of similar stars in the Large Magellanic Cloud indicates minor differences, which may be the consequence of sample selection effects. Apparent bolometric magnitudes were determined by combining the mean JHKL fluxes with mid-infrared photometry from IRAS and MSX. Then, using the Mira period luminosity relation to set the absolute magnitudes, distances were determined – to greater accuracy than has hitherto been possible for this type of star. Bolometric corrections to the K magnitude were calculated and prescriptions derived for calculating these from various colours. Mass-loss rates were also calculated and compared to values in the literature. Approximately one third of the C-rich Miras and an unknown fraction of the non-Miras exhibit apparently random obscuration events that are reminiscent of the phenomena exhibited by the hydrogen deficient RCB stars. The underlying cause of this is unclear, but it may be that mass loss, and consequently dust formation, is very easily triggered from these very extended atmospheres.

Post-AGB stars with hot circumstellar dust: binarity of the low-amplitude pulsators

Context. The influence of binarity on the late stages of stellar evolut ion. Aims. While the first binary post-AGB stars were serendipitously d iscovered, the distinct characteristics of their Spectral Energy Distribution (SED) allowed us to launch a more systematic search for binaries. We selected post-AGB objects which show a broad dust excess often starting already at H or K, pointing to the presence of a gravitationally bound dusty disc in the system. We started a very extensive multi-wavelength study of those systems and here we report on our radial velocity and photometric monitoring results for six stars of early F type, which are pulsators of small amplitude. Methods. To determine the radial velocity of low signal-to-noise time-series, we constructed dedicated auto-correlation masks based on high signal-to-noise spectra, used in our published chemical studies. The radial velocity variations were subjecte d to detailed analysis to differentiate between pulsational variability and variabilit y due to orbital motion. When available, the photometric monitoring data were used to complement the time series of radial velocity data and to establish the nature of the pulsation. Finally orbital minimalisation was performed to constrain the orbital elements. Results. All of the six objects are binaries, with orbital periods ran ging from 120 to 1800 days. Five systems have non-circular orbits. The mass functions range from 0.004 to 0.57 M⊙ and the companions are likely unevolved objects of (very) low initial mass. We argue that these binaries must have been subject to severe binary interaction when the primary was a cool supergiant. Although the origin of the circumstellar disc is not well understood, the disc is generally believed to be formed during this strong interaction phase. The eccentric orbits of these highly evolved objects remain poorly understood. In one object the line-of-sight is grazi ng the edge of the puffed-up inner rim of the disc. Conclusions. These results corroborate our earlier statement that evolved objects in binary stars create a Keplerian dusty circumbinary disc. With the measured orbits and mass functions we conclude that the circumbinary discs seem to have a major impact on the evolution of a significant fraction of binary systems.

The 2003 shell event in η Carinae

Near-infrared, JHKL, photometry of η Car is reported covering the period 2000-2004. This includes the 2003 shell event which was the subject of an international multiwavelength campaign. The fading that accompanied this event was similar to, although slightly deeper than, that which accompanied the previous one. The period between these events is 2023 ± 3 d and they are strictly periodic. Their cause, as well as that of the quasi-periodic variations and secular brightening, are discussed. It seems possible that all three types of variability are consequences of the binary nature of the star.

The case for asymmetric dust around a C‐rich asymptotic giant branch star

JHKL observations of the mass-losing carbon Mira variable IRAS 15194–5115 (II Lup) extending over about 18 yr are presented and discussed. The pulsation period is 575 d and has remained essentially constant over this time span. The star has undergone an extensive obscuration minimum during this time. This is complex and, like such minima in similar objects (e.g. R For), does not fit the model predictions of a simple long-term periodicity. Together with the high-resolution observations of Lopez et al., the results suggest that the obscuration changes are caused by the formation of dust clouds of limited extent in the line of sight. This is an R Coronae Borealis-type (RCB-type) model. The effective reddening law at J and H is similar to that found for R For.

The symbiotic binary system RX Puppis: a possible recurrent nova with a Mira companion

We present an analysis of photometric and spectroscopic observations of the symbiotic binary system RX Pup with the aims of developing a reliable binary model for the system and identifying mechanisms responsible for its spectacular activity. The binary is composed of a long-period Mira variable surrounded by a thick dust shell and a hot ∼ 0.8M ⊙ white dwarf companion. The hot component produces practically all activity observed in the UV, optical and radio range, while variable obscuration of the Mira by circumstellar dust is responsible for long-term changes in the near-infrared magnitudes. The observations show RX Pup underwent a nova-like eruption during the last three decades. The hot component contracted in radius at roughly constant luminosity from 1975 to 1986, and was the source of a strong stellar wind which prevented it from accreting material lost in the Mira wind. Around 1988/9 the hot component turned over in the HR diagram and by 1991 its luminosity had faded by a factor of ∼ 30 with respect to the maximum plateau value and the hot wind had practically ceased. By 1995 the nova remnant started to accrete material from the Mira wind, as indicated by a general increase in intensity of the optical continuum and H i emission. The quiescent spectrum resembles the quiescent spectra of symbiotic recurrent novae, and its intensity indicates the hot component must accrete as much as ∼ 1 per cent of the Mira wind, which is more or less the amount predicted by Bondi-Hoyle theory. The earliest observational records from the 1890s suggest that another nova-like eruption of RX Pup occurred around 1894.

A search for previously unrecognized metal-poor subdwarfs in the Hipparcos astrometric catalogue

ABSTRA C T We have identified 317 stars included in the Hipparcos astrometric catalogue that have parallaxes measured to a precision of better than 15 per cent, and the location of which in the OMV;OB 2 VUTU diagram implies a metallicity comparable to or less than that of the intermediate-abundance globular cluster M5. We have undertaken an extensive literature search to locate Stromgren, Johnson/Cousins and Walraven photometry for over 120 stars. In addition, we present new UBV(RI )C photometry of 201 of these candidate halo stars, together with similar data for a further 14 known metal-poor subdwarfs. These observations provide the first extensive data set of RCIC photometry of metal-poor, main-sequence stars with well- determined trigonometric parallaxes. Finally, we have obtained intermediate-resolution optical spectroscopy of 175 stars. 47 stars still lack sufficient supplementary observations for population classification; however, we are able to estimate abundances for 270 stars, or over 80 per cent of the sample. The overwhelming majority have near-solar abundance, with their inclusion in the present sample stemming from errors in the colours listed in the Hipparcos catalogue. Only 44 stars show consistent evidence of abundances below âFe=Haa21:0. Nine are additions to the small sample of metal-poor subdwarfs with accurate photometry. We consider briefly the implication of these results for cluster main-sequence fitting.

HST and VLT observations of the symbiotic star Hen 2–147 - Its nebular dynamics, its Mira variable and its distance

Aims. We investigate the dynamics of the nebula around the symbiotic star Hen 2-147, determine its expansion parallax, and compare it with the distance obtained via the period-luminosity relation for its Mira variable. Methods. A combination of multi-epoch HST images and VLT integral field high-resolution spectroscopy is used to study the nebular dynamics both along the line of sight and in the plane of the sky. These observations allow us to build a 3D spatio-kinematical model of the nebula, which, together with the measurement of its apparent expansion in the plane of the sky over a period of 3 years, provides the expansion parallax for the nebula. Additionally, SAAO near-infrared photometry obtained over 25 years is used to determine the Mira pulsation period and derive an independent distance estimation via the period-luminosity relationship for Mira variables. Results. The geometry of the nebula is found to be that of a knotty annulus of ionized gas inclined to the plane of sky and expanding with a velocity of ∼90 km s -1 . A straightforward application of the expansion parallax method provides a distance of 1.5 ± 0.4 kpc, which is a factor of two lower than the distance of 3.0 ± 0.4 kpc obtained from the period-luminosity relationship for the Mira (which has a pulsation period of 373 days). The discrepancy is removed if, instead of expanding matter, we are observing the expansion of a shock front in the plane of the sky. This shock interpretation is further supported by the broadening of the nebular emission lines.

The effect of dust obscuration in RR Telescopii on optical and IR long-term photometry and Fe II emission lines

Aims. Infrared and optical photometric and spectroscopic observations of the symbiotic nova RR Tel are used to study the effects and properties of dust in symbiotic binaries containing a cool Mira component, as well as showing “obscuration events” of increased absorption, which are typical for such Miras. Methods. A set of photometric observations of the symbiotic nova RR Tel in different wavelength bands – visual from 1949 to 2002 and near-infrared (JHKL) from 1975 to 2002 – are presented. The variability due to the normal Mira pulsation was removed from the JHKL data, which were then compared with the American Association of Variable Star Observers’ (AAVSO) visual light curve. The changes of the Fe II emission line fluxes during the 1996-2000 obscuration episode were studied in the optical spectra taken with the Anglo-Australian telescope. Results. We discuss the three periods during which the Mira component was heavily obscured by dust as observed in the different wavelength bands. A change in the correlations of J with other infrared magnitudes was observed with the colour becoming redder after JD 2 446 600. Generally, J−K was comparable, while K −L was larger than typical values for single Miras. A distance estimate of 2.5 kpc, based on the IR data, is given. A larger flux decrease for the permitted than for the forbidden Fe II lines, during the obscuration episode studied, has been found. There is no evidence for other correlations with line properties, in particular with wavelength, which suggests obscuration due to separate optically thick clouds in the outer layers. Conclusions.

On the nature of the cool component of MWC 560

Context. MWC 560 (V694 Mon) is one of the most enigmatic symbiotic system with a very active accretion-powered hot component. Such activity can be supported only by a luminous asymptotic giant branch star, i.e. a Mira or SR variable, with a high mass-loss rate. It is also a very unusual jet source because the jet axis lies practicall y parallel to the line of sight. Aims. The aims of our study are the determination of the evolutionary status of the cool component of MWC 560. Methods. Our methods involve analysis of near-IR JHKL and optical light curves. Results. The cool component of MWC 560 pulsates with a period of∼340 days, and it is probably a red SR variable on the thermally pulsing AGB. The high mass-loss rate expected for such a star is suffi cient to power the observed activity of the hot companion.

Preliminary Results from XCOV 17: PG 1336-018

We present a preliminary analysis of the data collected during the 1999/1 (XCOV17) Whole Earth Telescope (WET) campaign to observe the pulsating subdwarf B star PG 1336-018. Since PG 1336-018 is an HW Vir type eclipsing binary, our hope is to use the known orbital period, assume a tidally locked system, and search for rotationally split modes. To this end, we can already report some success.