Carolyn Brinkworth

Detection of a period decrease in NN Ser with ULTRACAM: evidence for strong magnetic braking or an unseen companion

We present results of high time resolution photometry of the eclipsing pre-cataclysmic variable NN Ser. NN Ser is a white dwarf/M dwarf binary with a very low-mass secondary star (∼0.2 M ○. ). We observed 13 primary eclipses of NN Ser using the high-speed CCD camera ULTRACAM and derived times of mid-eclipse, from fitting of light-curve models, with uncertainties as low as 0.06 s. The data show that the period of the binary is decreasing, with an average rate of P = (9.06 ± 0.06) x 10 -12 s s -1 , which has increased to a rate of P = (2.85 ± 0.15) x 10 -11 s s -1 over the last 2 yr. These rates of period change appear difficult to reconcile with any models of orbital period change. If the observed period change reflects an angular momentum loss, the average loss rate (J = 1.4 ± 0.6 x 10 35 erg) is consistent with the loss rates (via magnetic stellar wind braking) used in standard models of close binary evolution, which were derived from observations of much more massive cool stars. Observations of low-mass stars such as NN Sers secondary predict rates of ∼ 100 times lower than we observe. The alternatives are either magnetic activity-driven changes in the quadrupole moment of the secondary star (Applegates mechanism) or a light traveltime effect caused by the presence of a third body in a long (∼ decades) orbit around the binary. We show that Applegates mechanism fails by an order of magnitude on energetic grounds, but that the presence of a third body with mass 0.0043 < M 3 < 0,18 M ○. and orbital period 30 < P 3 < 285 yr could account for the observed changes in the timings of NN Sers mid-eclipses. We conclude that we have either observed a genuine angular momentum loss for NN Ser, in which case our observations pose serious difficulties for the theory of close binary evolution, or we have detected a previously unseen low-mass companion to the binary.

CONSTRAINTS ON THE LIFETIMES OF DISKS RESULTING FROM TIDALLY DESTROYED ROCKY PLANETARY BODIES

Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infrared excesses in the spectral energy distributions of HE 0110–5630, GD 61, and HE 1349–2305. All three of these stars have helium-dominated atmospheres, and their infrared emissions are consistent with warm dust produced by the tidal destruction of (minor) planetary bodies. This study brings the number of metal-polluted, helium and hydrogen atmosphere white dwarfs surveyed with IRAC to 53 and 38, respectively. It also nearly doubles the number of metal-polluted helium-rich white dwarfs found to have closely orbiting dust by Spitzer. From the increased statistics for both atmospheric types with circumstellar dust, we derive a typical disk lifetime of log [t disk(yr)] = 5.6 ± 1.1 (ranging from 3 × 104 to 5 × 106 yr). This assumes a relatively constant rate of accretion over the timescale where dust persists, which is uncertain. We find that the fraction of highly metal-polluted helium-rich white dwarfs that have an infrared excess detected by Spitzer is only 23%, compared to 48% for metal-polluted hydrogen-rich white dwarfs, and we conclude from this difference that the typical lifetime of dusty disks is somewhat shorter than the diffusion timescales of helium-rich white dwarf. We also find evidence for higher time-averaged accretion rates onto helium-rich stars compared to the instantaneous accretion rates onto hydrogen-rich stars; this is an indication that our picture of evolved star-planetary system interactions is incomplete. We discuss some speculative scenarios that can explain the observations.

A Dusty Component to the Gaseous Debris Disk Around the White Dwarf SDSS J1228+1040

We present Infrared Spectrometer And Array Camera (ISAAC) spectroscopy and ISAAC, UKIDSS, and Spitzer Space Telescope broadband photometry of SDSS J1228+1040—a white dwarf for which evidence of a gaseous metal-rich circumstellar disk has previously been found from optical emission lines. The data show a clear excess in the near- and mid-infrared (IR), providing compelling evidence for the presence of dust in addition to the previously identified gaseous debris disk around the star. The IR excess can be modeled in terms of an optically thick but geometrically thin disk. We find that the inner disk temperatures must be relatively high (~1700 K) in order to fit the spectral energy distribution in the near-IR. These data provide the first evidence for the coexistence of both gas and dust in a disk around a white dwarf, and show that their presence is possible even around moderately hot (~22,000 K) stars.

POSSIBLE SIGNS OF WATER AND DIFFERENTIATION IN A ROCKY EXOPLANETARY BODY

Spitzer observations reveal the presence of warm debris from a tidally destroyed rocky and possibly icy planetary body orbiting the white dwarf GD 61. Ultraviolet and optical spectroscopy of the metal-contaminated stellar photosphere reveal traces of hydrogen, oxygen, magnesium, silicon, iron, and calcium. The nominal ratios of these elements indicate an excess of oxygen relative to that expected from rock-forming metal oxides, and thus it is possible that water was accreted together with the terrestrial-like debris. Iron is found to be deficient relative to magnesium and silicon, suggesting the material may have originated as the outer layers of a differentiated parent body, as is widely accepted for the Moon.

A Spitzer Space Telescope Study of the Debris Disks around four SDSS White Dwarfs

We present Spitzer Space Telescope data of four isolated white dwarfs that were previously known to harbor circumstellar gaseous disks. Infrared Array Camera photometry shows a significant infrared excess in all of the systems, SDSS0738+1835, SDSS0845+2257, SDSS1043+0855, and SDSS1617+1620, indicative of a dusty extension to those disks. The 4.5 μm excesses seen in SDSS0738, SDSS0845, and SDSS1617 are 7.5, 5.7, and 4.5 times the white dwarf contribution, respectively. In contrast, in SDSS1043, the measured flux density at 4.5 μm is only 1.7 times the white dwarf contribution. We compare the measured IR excesses in the systems to models of geometrically thin, optically thick disks, and find that we are able to match the measured spectral energy distributions to within 3σ of the uncertainties, although disks with unfeasibly hot inner dust temperatures generally provide a better fit than those below the dust sublimation temperature. Possible explanations for the dearth of dust around SDSS1043+0855 are briefly discussed. Including our previous study of SDSS1228+1040, all five white dwarfs with gaseous debris disks have significant amounts of dust around them. It is evident that gas and dust can coexist around these relatively warm, relatively young white dwarfs.

SPITZER SPACE TELESCOPE OBSERVATIONS OF MAGNETIC CATACLYSMIC VARIABLES: POSSIBILITIES FOR THE PRESENCE OF DUST IN POLARS

We present Spitzer photometry of six short-period polars, EF Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the Spitzer IRAC (3.6-8.0 μm) data with the 2MASS JHK_s photometry to construct the SEDs of these systems from the near- to mid-IR (1.235-8 μm). We find that five out of the six polars have flux densities in the mid-IR that are substantially in excess of the values expected from the stellar components alone. We have modeled the observed SEDs with a combination of contributions from the white dwarf, secondary star, and either cyclotron emission or a cool, circumbinary dust disk to fill in the long-wavelength excess. We find that a circumbinary dust disk is the most likely cause of the 8 μm excess in all cases, but we have been unable to rule out the specific (but unlikely) case of completely optically thin cyclotron emission as the source of the observed 8 μm flux density. While both model components can generate enough flux at 8 μm, neither dust nor cyclotron emission alone can match the excess above the stellar components at all wavelengths. A model combining both cyclotron and dust contributions, possibly with some accretion-generated flux in the near-IR, is probably required, but our observed SEDs are not sufficiently well sampled to constrain such a complicated model. If the 8 μm flux density is caused by the presence of a circumbinary dust disk, then our estimates of the masses of these disks are many orders of magnitude below the mass required to affect CV evolution.

MEASURING THE ROTATIONAL PERIODS OF ISOLATED MAGNETIC WHITE DWARFS

We present time-series photometry of 30 isolated magnetic white dwarfs, surveyed with the Jacobus Kapteyn Telescope between 2002 August and 2003 May. We find that 9 were untestable due to varying comparison stars, but of the remaining 21, 5 (24%) are variable with reliably derived periods, while a further 9 (43%) are seen to vary during our study, but we were unable to derive the period. We interpret the variability to be the result of rotation of the objects. We find no correlation between rotation period and mass, temperature, magnetic field, or age. We have found variability in 9 targets with low magnetic field strengths and temperatures low enough for partially convective atmospheres, which we highlight as candidates for polarimetry to search for starspots. Most interestingly, we have found variability in one target, PG1658+441, which has a fully radiative atmosphere in which conventional starspots cannot form, but a magnetic field strength that is too low to cause magnetic dichroism. The source of variability in this target remains a mystery.

First Spitzer Space Telescope Observations of Magnetic Cataclysmic Variables: Evidence of Excess Emission at 3-8 μm

We present the first observations of magnetic cataclysmic variables using the Spitzer Space Telescope. We used the Infrared Array Camera to obtain photometry of the Polars EF Eri, GG Leo, V347 Pav, and RX J0154.0-5947 at 3.6, 4.5, 5.8, and 8.0 μm, respectively. In all of our targets, we detect excess mid-infrared emission over that expected from the component stars alone. We explore the origin of this IR excess by examining bremsstrahlung, cyclotron emission, circumbinary dust, and L/T brown dwarf secondary stars. Bremsstrahlung and cyclotron emission appear unlikely to be significant contributors to the observed fluxes. At present, the most likely candidate for the excess emission is dust that is probably located in a circumbinary disk with an inner temperature near 800 K. However, a simple dust disk plus any reasonable low-mass or brown dwarf-like secondary star is unable to fully explain the observed flux densities in the 3-8 μm region.

OBSERVATIONS OF V592 CASSIOPEIAE WITH THE SPITZER SPACE TELESCOPE—DUST IN THE MID-INFRARED

We present the ultraviolet-optical-infrared spectral energy distribution of the low inclination novalike cataclysmic variable (CV) V592 Cassiopeiae, including new mid-infrared observations from 3.5 to 24 μm obtained with the Spitzer Space Telescope. At wavelengths shortward of 8 μm, the spectral energy distribution of V592 Cas is dominated by the steady state accretion disk, but there is flux density in excess of the summed stellar components and accretion disk at longer wavelengths. Reproducing the observed spectral energy distribution from ultraviolet to mid-infrared wavelengths can be accomplished by including a circumbinary disk composed of cool dust, with a maximum inner edge temperature of ≈500 K. The total mass of circumbinary dust in V592 Cas (~10^(21)g) is similar to that found from recent studies of infrared excess in magnetic CVs, and is too small to have a significant effect on the long-term secular evolution of the cataclysmic variable. The existence of circumbinary dust in V592 Cas is possibly linked to the presence of a wind outflow in this system, which can provide the necessary raw materials to replenish the circumbinary disk on relatively short timescales, and/or could be a remnant from the common envelope phase early in the formation history of the system.

The Mid-Infrared Spectrum of the Short Orbital Period Polar EF Eridani from the Spitzer Space Telescope

We present the first mid-infrared (5.5-14.5 μm) spectrum of a highly magnetic cataclysmic variable, EF Eridani, obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The spectrum displays a relatively flat, featureless continuum. A spectral energy distribution model consisting of a 9500 K white dwarf, an L5 secondary star, cyclotron emission corresponding to a B ≈13 MG white dwarf magnetic field, and an optically thin circumbinary dust disk is in reasonable agreement with the extant Two Micron All Sky Survey (2MASS), Infrared Array Camera (IRAC), and IRS observations of EF Eri. Cyclotron emission is ruled out as a dominant contributor to the infrared flux density at wavelengths ≳3 μm. The spectral energy distribution longward of ~5 μm is dominated by dust emission. Even longer wavelength observations would test the models prediction of a continuing gradual decline in the circumbinary disk-dominated region of the spectral energy distribution.