The DA+dMe eclipsing binary EC13471-1258: its cup runneth over...just

EC13471-1258 is a detached eclipsing binary with Porb = 3h37m, comprising a DA white dwarf and a dMe dwarf. Total eclipses of the white dwarf lasting 14 min, and a large amplitude ellipsoidal variation are seen in the light curve. Flares from the dMe star occur regularly. Each star contributes roughly equal amounts of light at 5500 Ang. HST STIS spectra show strong Ly alpha with weak metal lines, and yield Teff = 14220 K, log g = 8.34, Z = 1/30th solar, K = 138 km/s and V sin i = 400 km/s for the white dwarf. Optical spectra yield the spectral type (M3.5-4.0), Teff = 3100 K, Z = solar, K = 266 km/s and V sin i = 140 km/s for the dMe star. The H alpha emission line comprises 2 or more components and implies that very weak mass transfer is occurring. The dynamical solution also implies that the dMe star just fills its Roche lobe. Accurate masses and radii for each star were derived: the dMe values favour the Clemens et al. (1998) mass-radius relation. The large rotational velocity of the white dwarf (400 km/s) suggests that the system has transferred mass in the past so that it is presently a hibernating cataclysmic variable. The metallicity contrast between the component stars provides an opportunity for tests of diffusion theory.Comment: 25 pages, 18 figures, accepted for publication in MNRA

Classical Cepheids: Yet another version of the Baade-Becker-Wesselink method

We propose a new version of the Baade--Becker--Wesselink technique, which allows one to independently determine the colour excess and the intrinsic colour of a radially pulsating star, in addition to its radius, luminosity, and distance. It is considered to be a generalization of the Balona approach. The method also allows the function F(CI) = BC + 10 log (Teff) for the class of pulsating stars considered to be calibrated. We apply this technique to a number of classical Cepheids with very accurate light and radial-velocity curves and with bona fide membership in open clusters (SZ Tau, CF Cas, U Sgr, DL Cas, GY Sge), and find the results to agree well with the reddening estimates of the host open clusters. The new technique can also be applied to other pulsating variables, e.g. RR Lyrae and RV Tauri.Comment: 6 pages, 2 figures, 1 table; Submitted to Astrophysical Bulletin, 201

The effect of two-temperature post-shock accretion flow on the linear polarization pulse in magnetic cataclysmic variables

The temperatures of electrons and ions in the post-shock accretion region of a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass flow rates or for sufficiently weak magnetic fields. At lower mass flow rates or in stronger magnetic fields, efficient cyclotron cooling will cool the electrons faster than the electrons can cool the ions and a two-temperature flow will result. Here we investigate the differences in polarized radiation expected from mCV post-shock accretion columns modeled with one- and two-temperature hydrodynamics. In an mCV model with one accretion region, a magnetic field >~30 MG and a specific mass flow rate of ~0.5 g/cm/cm/s, along with a relatively generic geometric orientation of the system, we find that in the ultraviolet either a single linear polarization pulse per binary orbit or two pulses per binary orbit can be expected, depending on the accretion column hydrodynamic structure (one- or two-temperature) modeled. Under conditions where the physical flow is two-temperature, one pulse per orbit is predicted from a single accretion region where a one-temperature model predicts two pulses. The intensity light curves show similar pulse behavior but there is very little difference between the circular polarization predictions of one- and two-temperature models. Such discrepancies indicate that it is important to model some aspect of two-temperature flow in indirect imaging procedures, like Stokes imaging, especially at the edges of extended accretion regions, were the specific mass flow is low, and especially for ultraviolet data.Comment: Accepted for publication in Astrophysics & Space Scienc

New polarimetric constraints on axion-like particles

We show that the parameter space of axion-like particles can be severly constrained using high-precision measurements of quasar polarisations. Robust limits are derived from the measured bounds on optical circular polarisation and from the distribution of linear polarisations of quasars. As an outlook, this technique can be improved by the observation of objects located behind clusters of galaxies, using upcoming space-borne X-ray polarimeters.Comment: Submitted to JCA

The Initial Conditions to Star Formation: Low Mass Stars at Low Metallicity

We have measured the present accretion rate of roughly 800 low-mass (~1-1.4 Mo) pre-Main Sequence stars in the field of SN 1987A in the Large Magellanic Cloud. The stars with statistically significant Balmer continuum and Halpha excesses are measured to have accretion rates larger than about 1.5x10^{-8} Mo/yr at an age of 12-16 Myrs. For comparison, the time scale for disk dissipation observed in the Galaxy is of the order of 6 Myrs.Comment: 4 pages, 1 figure, to appear in IMF@50, ed. by E. Corbelli, F. Palla, H. Zinnecker (Dordrecht: Kluwer

Stellar population synthesis at the resolution of 2003

We present a new model for computing the spectral evolution of stellar populations at ages between 100,000 yr and 20 Gyr at a resolution of 3 A across the whole wavelength range from 3200 to 9500 A for a wide range of metallicities. These predictions are based on a newly available library of observed stellar spectra. We also compute the spectral evolution across a larger wavelength range, from 91 A to 160 micron, at lower resolution. The model incorporates recent progress in stellar evolution theory and an observationally motivated prescription for thermally-pulsing stars on the asymptotic giant branch. The latter is supported by observations of surface brightness fluctuations in nearby stellar populations. We show that this model reproduces well the observed optical and near-infrared colour-magnitude diagrams of Galactic star clusters of various ages and metallicities. Stochastic fluctuations in the numbers of stars in different evolutionary phases can account for the full range of observed integrated colours of star clusters in the Magellanic Clouds. The model reproduces in detail typical galaxy spectra from the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We exemplify how this type of spectral fit can constrain physical parameters such as the star formation history, metallicity and dust content of galaxies. Our model is the first to enable accurate studies of absorption-line strengths in galaxies containing stars over the full range of ages. Using the highest-quality spectra of the SDSS EDR, we show that this model can reproduce simultaneously the observed strengths of those Lick indices that do not depend strongly on element abundance ratios [abridged].Comment: 35 pages, 22 figures, to appear in MNRAS; version with full resolution figures available at http://www.iap.fr/~charlot/bc2003/pape

Multi-Site Observations of the DAV White Dwarf R 548

The pulsating DA white dwarf R 548 was observed for 46 h in October 1993 in an eight-site campaign. New peaks near the known doublets in the Fourier transform are found

Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programmes

The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source emission cooled from approximately 6 400 K to 2 100 K over a 7-d period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (~2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor

The Hamburg/ESO R-process Enhanced Star survey (HERES). I : Project description and discovery of two stars with strong enhancements of neutron-capture elements

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