We present homogeneous, standardized UBV(RI)C photometry for over 700 nearby stars selected on the basis of Hipparcos parallaxes. Additionally, we list JHK photometry for about half of these stars, ...as well as L photometry for 86 of the brightest. A number of stars with peculiar colours or anomalous locations in various colour–magnitude diagrams are discussed.
We present a detailed spectroscopic analysis of the luminous blue variable (LBV) AG Carinae (AG Car) during the last two visual minimum phases of its S-Dor cycle (1985-1990 and 2000-2003). The ...analysis reveals an overabundance of He, N, and Na, and a depletion of H, C, and O, on the surface of the AG Car, indicating the presence of a CNO-processed material. Furthermore, the ratio N/O is higher on the stellar surface than in the nebula. We found that the minimum phases of AG Car are not equal to each other, since we derived a noticeable difference between the maximum effective temperature achieved during 1985-1990 (22, 800 K) and 2000-2001 (17,000 K). Significant differences between the wind parameters in these two epochs were also noticed. While the wind terminal velocity was 300 km s-1 in 1985-1990, it was as low as 105 km s-1 in 2001. The mass-loss rate, however, was lower from 1985-1990 (1.5 X 10-5 M yr-1) than from 2000-2001 (3.7 X 10-5 M yr-1). We found that the wind of AG Car is significantly clumped (f 0.10-0.25) and that clumps must be formed deep in the wind. We derived a bolometric luminosity of 1.5 X 106 L during both minimum phases which, contrary to the common assumption, decreases to 1.0 X 106 L as the star moves toward the maximum flux in the V band. Assuming that the decrease in the bolometric luminosity of AG Car is due to the energy used to expand the outer layers of the star, we found that the expanding layers contain roughly 0.6-2 M . Such an amount of mass is an order of magnitude lower than the nebular mass around AG Car, but is comparable to the nebular mass found around lower-luminosity LBVs and to that of the Little Homunculus of Eta Car. If such a large amount of mass is indeed involved in the S Dor-type variability, we speculate that such instability could be a failed Giant Eruption, with several solar masses never becoming unbound from the star.
Context. The influence of binarity on the late stages of stellar evolution remains an open issue. Aims. While the first binary post-AGB stars were serendipitously discovered, 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 exhibit a broad dust excess starting either at H or K, pointing to the presence of a gravitationally bound dusty disc in the system. We initiated an extensive multiwavelength study of those systems and here 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 ratio time-series data, we constructed dedicated autocorrelation masks based on high signal-to-noise ratio spectra, used in our published chemical studies. The radial velocity variations were analysed in detail to differentiate between pulsational variability and variability caused by 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 ranging from 120 to 1800 days. Five systems have non-circular orbits. The mass functions range from 0.004 to 0.57 $M_{\odot}$ and the companions are probably unevolved objects of (very) low initial mass. We argue that these binaries must have evolved through a phase of strong 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 have formed during this strong interaction phase. The eccentric orbits of these highly evolved objects remain poorly understood. In one object, the line-of-sight grazes 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.
We describe briefly the properties of the recently completed Southern African Large Telescope (SALT), along with its first light imager SALTICAM. Using this instrument, we present 4.3 h of high-speed ...unfiltered photometric observations of the eclipsing polar SDSS J015543.40+002807.2 with time-resolution as short as 112 ms, the highest-quality observations of this kind of any polar to date. The system was observed during its high-luminosity state. Two accreting poles are clearly seen in the eclipse light curve. The binary system parameters have been constrained: the white dwarf mass is at the low end of the range expected for cataclysmic variables. Correlations between the positions of the accretion regions on or near the surface of the white dwarf and the binary system parameters were established. The sizes of the accretion regions and their relative movement from eclipse to eclipse were estimated: they are typically 4°–7° depending on the mass of the white dwarf. The potential of these observations will only fully be realized when low-state data of the same kind are obtained and the contact phases of the eclipse of the white dwarf are measured.
We present results from the first year of a campaign to characterize and monitor the optical turbulence profile at the South African Astronomical Observatory's Sutherland observing station in South ...Africa. A Multi Aperture Scintillation Sensor Differential Image Motion Monitor (MASS-DIMM) was commissioned in 2010 March to provide continuous monitoring of the seeing conditions. Over the first month of the campaign, a Slope Detection And Ranging (SLODAR) from Durham University was also installed, allowing an independent verification of the performance of the MASS-DIMM device. After the first year of data collection, the overall median seeing value is found to be 1.32 arcsec as measured at ground level. The ground layer which includes all layers below 1 km accounts for 84 per cent of the turbulence, while the free atmosphere above 1 km accounts for 16 per cent with a median value of 0.41 arcsec. The median isoplanatic angle value is 1.92 arcsec, which is similar to other major astronomical sites. The median coherence time, calculated from corrected MASS measurements, is 2.85 ms. The seeing conditions at the site do show a strong correlation with wind direction, with bad seeing conditions being associated with winds from the south-east.
Improved SAAO–2MASS photometry transformations Koen, C.; Marang, F.; Kilkenny, D. ...
Monthly notices of the Royal Astronomical Society,
October 2007, Letnik:
380, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Near-infrared photometry of 599 stars is used to calculate transformations from the South African Astronomical Observatory (SAAO) JHK system to the Two-Micron All-Sky Survey (2MASS) JHKS system. Both ...several-term formal regression relations and simplified transformations are presented. Inverse transformations (i.e. 2MASS to SAAO) are also given. The presence of non-linearities in some colour terms is highlighted.
The mysterious eruption of V838 Mon Munari, U.; Henden, A.; Kiyota, S. ...
Astronomy and astrophysics (Berlin),
07/2002, Letnik:
389, Številka:
2
Journal Article
Recenzirano
Odprti dostop
V838 Mon is marking one of the most mysterious stellar outbursts on record. The spectral energy distribution of the progenitor resembles an under-luminous F main sequence star (at $V=15.6$ mag), that ...erupted into a cool supergiant following a complex and multi-maxima lightcurve (peaking at $V=6.7$ mag). The outburst spectrum show BaII, LiI and lines of several $s-$elements, with wide P-Cyg profiles and a moderate and retracing emission in the Balmer lines. A light-echo discovered expanding around the object helped to constrain the distance ($d=790 \pm 30$ pc), providing MV=+4.45 in quiescence and MV=-4.35 at optical maximum (somewhat dependent on the still uncertain $E_{B-V}=0.5$ reddening). The general outburst trend is toward lower temperatures and larger luminosities, and continuing so at the time of writing. The object properties conflict with a classification within already existing categories: the progenitor was not on a post-AGB track and thus the similarities with the born-again AGB stars FG Sge, V605 Aql and Sakurai's object are limited to the cool giant spectrum at maximum; the cool spectrum, the moderate wind velocity (500 km s-1 and progressively reducing) and the monotonic decreasing of the low ionization condition argues against a classical nova scenario. The closest similarity is with a star that erupted into an M-type supergiant discovered in M 31 by Rich et al. (CITE), that became however much brighter by peaking at MV=-9.95, and with V4332 Sgr that too erupted into an M-type giant (Martini et al. CITE) and that attained a lower luminosity, closer to that of V838 Mon. M 31-RedVar, V4332 Sgr and V838 Mon could be all manifestations of a new class of astronomical objects.
On the nature of the cool component of MWC 560 Gromadzki, M.; Mikołajewska, J.; Whitelock, P. A. ...
Astronomy and astrophysics (Berlin),
02/2007, Letnik:
463, Številka:
2
Journal Article
Recenzirano
Odprti dostop
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 practically 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 sufficient to power the observed activity of the hot companion.
The post-outburst photometric behaviour of V838 Mon Crause, Lisa A.; Lawson, Warrick A.; Kilkenny, David ...
Monthly notices of the Royal Astronomical Society,
05/2003, Letnik:
341, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The unusual eruptive variable discovered in Monoceros in 2002 January underwent dramatic photometric and spectroscopic changes in the months prior to its 2002 June–August conjunction with the Sun. ...Optical and infrared (IR) photometry obtained at the SAAO between 2002 January and June (JD 245 2280–440) is presented here in an analysis of the post-outburst behaviour of the star. The light curve indicated that three eruptions took place in 2002 January, February and March. South African Astronomical Observatory (SAAO) echelle spectra obtained in the week prior to the March maximum indicated the ejection of a new shell of material. JHKL photometry obtained during 2002 April showed the development of an IR excess owing to the formation of a dust shell. The shell appears to be largely responsible for the rapid fade in the optical flux during 2002 April–May (ΔV ã 6 mag within 3 weeks). Blueing of the optical colours during the decline is probably due either to the revealing of an emission-line region surrounding V838 Mon, or to the unveiling of the progenitor or a spatially close early-type star.
PDF
