Establishing Visible Interferometer System Responses: Resolved and Unresolved Calibrators

The propagation of errors through the uniform disk visibility function is examined. Implications of those errors upon measures of absolute visibility through optical and near-infrared interferometers are considered within the context of using calibration stars to establish system visibilities for these instruments. We suggest a simple ratio test to establish empirically whether or not the measured visibilities produced by such an instrument are relative (errors dominated by calibrator angular size prediction error) or absolute (errors dominated by measurement error).Comment: 20 pages, 7 figures, to be published in the PAS

Spitzer 24-micron Time-Series Observations of the Eclipsing M-dwarf Binary GU Bootis

We present a set of {\it Spitzer} 24$\mu$m MIPS time series observations of the M-dwarf eclipsing binary star GU Bo\"otis. Our data cover three secondary eclipses of the system: two consecutive events and an additional eclipse six weeks later. The study's main purpose is the long wavelength (and thus limb darkening-independent) characterization of GU Boo's light curve, allowing for independent verification of the results of previous optical studies. Our results confirm previously obtained system parameters. We further compare GU Boo's measured 24$\mu$m flux density to the value predicted by spectral fitting and find no evidence for circumstellar dust. In addition to GU Boo, we characterize (and show examples of) light curves of other objects in the field of view. Analysis of these light curves serves to characterize the photometric stability and repeatability of {\it Spitzer's} MIPS 24\micron array over short (days) and long (weeks) timescales at flux densities between approximately 300--2,000$\mu$Jy. We find that the light curve root mean square about the median level falls into the 1--4% range for flux densities higher than 1mJy. Finally, we comment on the fluctuations of the 24\micron background on short and long timescales.Comment: ApJ accepted. 10 pages, 12 figure

Spitzer Space Telescope Observations of Circumbinary Dust Disks around Polars

We present Spitzer Space Telescope IRAC photometry of the magnetic cataclysmic variables EF Eri, MR Ser, VV Pup, V834 Cen, GG Leo and V347 Pav. When we combine our results with the 2MASS data, we find that at least five of the polars have flux densities in the mid-IR in excess of the emission expected from the stellar components alone. We are unable to model this mid-IR excess with cyclotron emission, but we can recreate the observed spectral energy distributions with the inclusion of a simple circumbinary dust disk model. Importantly, we find that the masses of our modelled disks are approximately 12 orders of magnitude lower than required to significantly affect CV evolution. The accretion disk-less polars are ideal places to search for these disks, since the luminous accretion disk in most CVs would drown out the faint IR signature of the cooler, dimmer circumbinary disks

Outbursts of EX Hydrae Revisited

We present optical spectroscopy of EX Hya during its 1991 outburst. This outburst is characterised by strong irradiation of the front face of the secondary star by the white dwarf, an overflowing stream which is seen strongly in HeII and by a dip in the light curves, which extends from 0.1-0.6 in the binary and spin phases. Strong irradiation of the accretion curtain and that of the inner regions of the disc led to strong emission of HeII and to the suppression of the Hg and Hb emission. Disc overflow was observed in quiescence in earlier studies, where the overflow stream material was modulated at high velocities close to 1000 km/s. In outburst, the overflowing material is modulated at even higher velocities (~1500 km/s). These are streaming velocities down the field lines close to the white dwarf. Evidence for material collecting near the outer edge of the disc and corotating with the accretion curtain was observed. In decline, this material and the accretion curtain obscured almost all the emission near binary phase 0.4, causing a dip. The dip minimum nearly corresponds with spin pulse minimum. This has provided additional evidence for an extended accretion curtain, and for the corotation of material with the accretion curtain at the outer edge of the disc. From these observations we suggest that a mechanism similar to that of Spruit & Taam, where outbursts result due to the storage and release of matter outside the magnetosphere, triggers the outbursts of EX Hya. This is followed by the irradiation of the secondary star due to accretion induced radiation.Comment: 12 pages, 14 figures, 1 table. Figures 6, 7, 8 and 11 at low resolution. Paper accepted by the Monthly Notices of the Royal Astronomical Societ

Hipparcos red stars in the HpV_{T2} and VI_C systems

For Hipparcos M, S, and C spectral type stars, we provide calibrated instantaneous (epoch) Cousins $V-I$ color indices using newly derived $HpV_{T2}$ photometry. Three new sets of ground-based Cousins $VI$ data have been obtained for more than 170 carbon and red M giants. These datasets in combination with the published sources of $VI$ photometry served to obtain the calibration curves linking Hipparcos/Tycho $Hp-V_{T2}$ with the Cousins $V-I$ index. In total, 321 carbon stars and 4464 M- and S-type stars have new $V-I$ indices. The standard error of the mean $V-I$ is about 0.1 mag or better down to $Hp\approx9$ although it deteriorates rapidly at fainter magnitudes. These $V-I$ indices can be used to verify the published Hipparcos $V-I$ color indices. Thus, we have identified a handful of new cases where, instead of the real target, a random field star has been observed. A considerable fraction of the DMSA/C and DMSA/V solutions for red stars appear not to be warranted. Most likely such spurious solutions may originate from usage of a heavily biased color in the astrometric processing.Comment: 10 figures, 1 electronic table, accepted in A&