Coordinated thermal and optical observations of Trans-Neptunian object (20000) Varuna from Sierra Nevada

We report on coordinated thermal and optical measurements of trans-Neptunian object (20000) Varuna obtained in January-February 2002, respectively from the IRAM 30-m and IAA 1.5 m telescopes. The optical data show a lightcurve with a period of 3.176+/-0.010 hr, a mean V magnitude of 20.37+/-0.08 and a 0.42+/-0.01 magnitude amplitude. They also tentatively indicate that the lightcurve is asymmetric and double-peaked. The thermal observations indicate a 1.12+/-0.41 mJy flux, averaged over the object's rotation. Combining the two datasets, we infer that Varuna has a mean 1060(+180/-220) km diameter and a mean 0.038(+0.022/-0.010) V geometric albedo, in general agreement with an earlier determination using the same technique.Comment: Accepted for publication in Astronomy & Astrophysics (7 pages, including 3 figures

Variable stars in the dwarf irregular galaxy NGC 6822: the photometric catalogue

Deep B,V time-series photometry obtained with the ESO Very Large Telescope has been used to identify variable stars in the dwarf irregular galaxy NGC 6822. We surveyed a 6.8x6.8 arcmin area of the galaxy and detected a total number of 390 candidate variables with the optimal image subtraction technique (Alard 2000). Light curves on a magnitude scale were obtained for 262 of these variables. Differential flux light curves are available for the remaining sample. In this paper we present the photometric catalogue of calibrated light curves and time-series data, along with coordinates and classification of the candidate variables. A detailed description is provided of the procedures used to identify the variable stars and calibrate their differential flux light curves on a magnitude scale.Comment: 22 pages, 19 figures only as JPEG. Revised version with corrected eq. 5. Full text with better resolution .ps figures available upon request from the authors. Uses aa.cls (included), in press on A&A. Table 2 will only be published at the CDS, Appendix A, Tables 4,5,6 will only be available in the electronic edition of the Journa

Rotational period of WD1953-011 - a magnetic white dwarf with a star spot

WD1953-011 is an isolated, cool (7920 +/- 200K, Bergeron, Legget & Ruiz, 2001) magnetic white dwarf (MWD) with a low average field strength (~70kG, Maxted et al. 2000) and a higher than average mass (~0.74 M_sun, Bergeron et al. 2001). Spectroscopic observations taken by Maxted et al. (2000) showed variations of equivalent width in the Balmer lines, unusual in a low field white dwarf. Here we present V band photometry of WD1953-011 taken at 7 epochs over a total of 22 months. All of the datasets show a sinusoidal variation of approximately 2% peak-to-peak amplitude. We propose that these variations are due to a star spot on the MWD, analogous to a sunspot, which is affecting the temperature at the surface, and therefore its photometric magnitude. The variations have a best-fit period over the entire 22 months of 1.4418 days, which we interpret as the rotational period of the WD.Comment: (1) University of Southampton, (2) University of Warwick, (3) University of Nijmegen, (4) Keele University, (5) University of Leicester. 6 pages, 5 figs, accepted MNRA

The continuous period search method and its application to the young solar analogue HD 116956

We formulate an improved time series analysis method for the analysis of photometry of active stars. This new Continuous Period Search (CPS) method is applied to 12 years of V band photometry of the young solar analogue HD 116956 (NQ UMa). The new method is developed from the previous Three Stage Period Analysis (TSPA) method. Our improvements are the use of a sliding window in choosing the modelled datasets, a criterion applied to select the best model for each dataset and the computation of the time scale of change of the light curve. We test the performance of CPS with simulated and real data. The CPS has a much improved time resolution which allows us to better investigate fast evolution of stellar light curves. We can also separate between the cases when the data is best described by periodic and aperiodic (e.g. constant brightness) models. We find, however, that the performance of the CPS has certain limitations. It does not determine the correct model complexity in all cases, especially when the underlying light curve is constant and the number of observations too small. Also the sensitivity in detecting two close light curve minima is limited and it has a certain amount of intrinsic instability in its period estimation. Using the CPS, we find persistent active longitudes in the star HD 116956 and a "flip-flop" event that occurred during the year 1999. Assuming that the surface differential rotation of the star causes observable period variations in the stellar light curve, we determine the differential rotation coefficient to be |k|>0.11. The mean timescale of change of the light curve during the whole 12 year observing period was T_C=44.1 d, which is of the same order as the predicted convective turnover time of the star. We also investigate the presence of activity cycles on the star, but do not find any conclusive evidence supporting them.Comment: 14 pages, 11 figures, 3 table

Cryptotomography: reconstructing 3D Fourier intensities from randomly oriented single-shot diffraction patterns

We reconstructed the 3D Fourier intensity distribution of mono-disperse prolate nano-particles using single-shot 2D coherent diffraction patterns collected at DESY's FLASH facility when a bright, coherent, ultrafast X-ray pulse intercepted individual particles of random, unmeasured orientations. This first experimental demonstration of cryptotomography extended the Expansion-Maximization-Compression (EMC) framework to accommodate unmeasured fluctuations in photon fluence and loss of data due to saturation or background scatter. This work is an important step towards realizing single-shot diffraction imaging of single biomolecules.Comment: 4 pages, 4 figure

OGLE small amplitude red giant variables in the Galactic Bar

Among over 200,000 Galactic Bulge variable stars in the public domain OGLE catalogue, we found over 15,000 red giant variables following two well defined period -- amplitude relations. The periods are in the range 10 < P < 100 days, and amplitudes in the range 0.005 < A < 0.13 mag in I-band. The variables cover a broad range of reddening corrected colours, 1 < (V-I)_0 < 5, and a fairly narrow range of extinction corrected apparent magnitudes, 10.5 < I_0 < 13 . A subset of variables (type A) has a rms scatter of only 0.44 mag. The average magnitudes for these stars are well correlated with the Galactic longitude, and vary from I_{k,0} = 11.82 for l = +8 deg to I_{k,0} = 12.07 for l = -5 deg, clearly indicating that they are located in the Galactic Bar. Most variables have several oscillation periods.Comment: 15 pages, 9 figures (7 in low resolution), submitted to MNRAS. Article in full resolution can be obtained at http://www.astro.princeton.edu/~leyer/wrayetal.p

Study of the cyclotron feature in MXB 0656-072

We have monitored a type II outburst of the Be/X-ray binary MXB 0656−072 in a series of pointed RXTE observations during October through December 2003. The source spectrum shows a cyclotron resonance scattering feature at 32.8 +0.5 −0.4 keV, corresponding to a magnetic field strength of 3.67 +0.06 −0.04 × 10 12 G and is stable through the outburst and over the pulsar spin phase. The pulsar, with an average pulse period of 160.4 ± 0.4s,shows a spin-up of 0.45 s over the duration of the outburst. From optical data, the source distance is estimated to be 3.9 ± 0.1 kpc and this is used to estimate the X-ray luminosity and a theoretical prediction of the pulsar spin-up during the outburst

Core properties of alpha Cen A using asteroseismology

A set of long and nearly continuous observations of alpha Centauri A should allow us to derive an accurate set of asteroseismic constraints to compare to models, and make inferences on the internal structure of our closest stellar neighbour. We intend to improve the knowledge of the interior of alpha Centauri A by determining the nature of its core. We combined the radial velocity time series obtained in May 2001 with three spectrographs in Chile and Australia: CORALIE, UVES, and UCLES. The resulting combined time series has a length of 12.45 days and contains over 10,000 data points and allows to greatly reduce the daily alias peaks in the power spectral window. We detected 44 frequencies that are in good overall agreement with previous studies, and found that 14 of these show possible rotational splittings. New values for the large and small separations have been derived. A comparison with stellar models indicates that the asteroseismic constraints determined in this study allows us to set an upper limit to the amount of convective-core overshooting needed to model stars of mass and metallicity similar to those of alpha Cen A.Comment: 8 pages, 11 figures, A&A accepte

A mid-term astrometric and photometric study of Trans-Neptunian Object (90482) Orcus

From CCD observations of a fixed and large star field that contained the binary TNO Orcus, we have been able to derive high-precision relative astrometry and photometry of the Orcus system with respect to background stars. The RA residuals of an orbital fit to the astrometric data revealed a periodicity of 9.7+-0.3 days, which is what one would expect to be induced by the known Orcus companion. The residuals are also correlated with the theoretical positions of the satellite with regard to the primary. We therefore have revealed the presence of Orcus' satellite in our astrometric measurements. The photocenter motion is much larger than the motion of Orcus around the barycenter, and we show here that detecting some binaries through a carefully devised astrometric technique might be feasible with telescopes of moderate size. We also analyzed the system's mid-term photometry to determine whether the rotation could be tidally locked to the satellite's orbital period. We found that a photometric variability of 9.7+-0.3 days is clear in our data, and is nearly coincident with the orbital period of the satellite. We believe this variability might be induced by the satellite's rotation. There is also a slight hint for an additional small variability in the 10 hr range that was already reported in the literature. This short-term variability would indicate that the primary is not tidally locked and therefore the system would not have reached a double synchronous state. Implications for the basic physical properties of the primary and its satellite are discussed. From angular momentum considerations we suspect that the Orcus satellite might have formed from a rotational fission. This requires that the mass of the satellite would be around 0.09 times that of the primary, close to the value that one derives by using an albedo of 0.12 for the satellite and assuming equal densities for both objects.Comment: in Press at A&

Short-term variability of a sample of 29 trans-Neptunian objects and Centaurs

We present results of 6 years of observations, reduced and analyzed with the same tools in a systematic way. We report completely new data for 15 objects, for 5 objects we present a new analysis of previously published results plus additional data and for 9 objects we present a new analysis of data already published. Lightcurves, possible rotation periods and photometric amplitudes are reported for all of them. The photometric variability is smaller than previously thought: the mean amplitude of our sample is 0.1mag and only around 15% of our sample has a larger variability than 0.15mag. The smaller variability than previously thought seems to be a bias of previous observations. We find a very weak trend of faster spinning objects towards smaller sizes, which appears to be consistent with the fact that the smaller objects are more collisionally evolved, but could also be a specific feature of the Centaurs, the smallest objects in our sample. We also find that the smaller the objects, the larger their amplitude, which is also consistent with the idea that small objects are more collisionally evolved and thus more deformed. Average rotation rates from our work are 7.5h for the whole sample, 7.6h for the TNOs alone and 7.3h for the Centaurs. All of them appear to be somewhat faster than what one can derive from a compilation of the scientific literature and our own results. Maxwellian fits to the rotation rate distribution give mean values of 7.5h (for the whole sample) and 7.3h (for the TNOs only). Assuming hydrostatic equilibrium we can determine densities from our sample under the additional assumption that the lightcurves are dominated by shape effects, which is likely not realistic. The resulting average density is 0.92g/cm^3 which is not far from the density constraint that one can derive from the apparent spin barrier that we observe.Comment: Accepted for publication in A&