arXiv subjects
Publications and source records attributed to R. Silvotti.
In this article we present the preliminary results of an observational search for subdwarf B and white dwarf pulsators in the Kepler field of view, performed using the DOLORES camera attached to the 3.6m Telescopio Nazionale Galileo (TNG).
We present time resolved echelle spectra of the planet-hosting subdwarf B pulsator HS 2201 + 2610 and report on our efforts to extract pulsational radial velocity measurements from this data.
Pulsating subdwarf B stars oscillate in short-period $p$-modes or long-period $g$-modes. HS 0702 + 6043 is one of currently three objects known to show characteristics of both types and hence is classified as hybrid pulsator. We briefly present our analysis of the $g$-mode domain of this star, but focus on first results from long-term photometric monitoring in particular of the $p$-mode oscillations. We present a high-resolution frequency spectrum, and report on our efforts to construct a multi-season O--C diagram. Additionally to the standard (although nontrivial) exercise in asteroseismology to probe the instantaneous inner structure of a star, measured changes in the pulsation frequencies as derived from an {O--C} diagram can be compared to theoretical evolutionary timescales. Within the {EXOTIME} program, we also use this same data to search for planetary companions around extreme horizontal branch objects.
In this article we describe the search for white dwarfs (WDs) in the multi-band photometric data of the Capodimonte deep field survey. The WD candidates were selected through the V-R_C vs B-V color-color diagram. For two bright objects, the WD nature has been confirmed spectroscopically, and the atmospheric parameters (Teff and logg) have been determined. We have computed synthetic stellar population models for the observed field and the expected number of white dwarfs agrees with the observations. The possible contamination by turn-off and horizontal branch halo stars has been estimated. The quasar (QSO) contamination has been determined by comparing the number of WD candidates in different color bins with state-of-the-art models and previous observations. The WD space density is measured at different distances from the Sun. The total contamination (non-degenerate stars + QSOs) in our sample is estimated to be around 30%. This work should be considered a small experiment in view of more ambitious projects to be performed in the coming years in larger survey contexts.
PG 1159-035, a pre-white dwarf with T_eff=140,000 K, is the prototype of both two classes: the PG1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. In this work, we used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods and identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M = 0.59 +/- 0.02 solar masses for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at r_c = 0.83 +/- 0.05 stellar radius. From the multiplet splitting, we calculated the rotational period P_rot = 1.3920 +/- 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for l=1 modes and less than 50% for l=2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars.
We present the detection of low-amplitude, long-period g-modes in two individual sdBV stars which are known to be p-mode pulsators. Only few of these hybrid objects, showing both p- and g-modes, are known today. We resolve the g-mode domain in HS0702+6043 and add HS2201+2610 to the list of hybrid pulsators. To discover the low-amplitude g-modes, a filtering algorithm based on wavelet transformations was applied to denoise observational data.
In this article we present the O-C diagram of the hot subdwarf B pulsating star HS2201+2610 after seven years of observations. A secular increase of the main pulsation period, Pdot=(1.3+-0.1)x10**(-12), is inferred from the data. Moreover, a further sinusoidal pattern suggests the presence of a low-mass companion (Msini=~3.5 Mjup), orbiting the hot star at a distance of about 1.7 AU with a period near 1140 days.
Our goal is to recover the Galactic Halo spatial density by means of field stars. To this aim, we apply a new technique to the Capodimonte Deep Field (OACDF, Alcala' et al. 2004), as a pilot study in view of the VLT Survey Telescope (VST) stellar projects. Considering the unique chance to collect deep and wide-field photometry with the VST, our method may represent a useful tool towards a definitive mapping of the Galactic Halo. In the framework of synthetic stellar populations, turn-off stars are used to reconstruct the spatial density. The determination of the space density is achieved by comparing the data with synthetic color-magnitude diagrams (CMDs). The only assumptions involve the IMF, age and metallicity of the synthetic halo population. Stars are randomly placed in the solid angle. The contributions of the various Monte Carlo distributions (with a step of 4 kpc) along the line of sight are simultaneously varied to reproduce the observed CMD. Our result on the space density is consistent with a power-law exponent n~3 over a range of Galactocentric distances from 8 to 40 kpc.
We report the first time-resolved photometric and spectroscopic optical observations of the X-ray source RX J2133.7+5107, identified in the ROSAT survey. A clear persistent optical light pulsation is discovered with fast photometry at a period of P_{omega} =(570.823 +/-0.013) s which we associate with the spin period of an accreting white dwarf. Radial velocity curves of the strong emission lines show modulation with a period of P_{Omega} =(7.193 +/- 0.016) hr, identified as the orbital period. These observations establish that the source is a member of the intermediate polar class (IPs) of magnetic cataclysmic variables. With only 4 IPs with longer orbital periods, RX J2133.7+5107 is among the widest systems. It is a unique IP with an orbital period in the middle of the so-called (6-10)hr IP gap and it shows a significant degree of asynchronism with a ratio P_{omega}/P_{Omega} of 0.02. When attributed to the motion of the white dwarf, the emission lines orbital modulation yields a mass function of f_m = (1.05 +/- 0.21) 10^{-2} Msun which, for a probable inclination i < 45 deg and a white dwarf mass M_{wd} = (0.6-1.0) Msun, corresponds to a secondary mass M_{s} > (0.27-0.37) Msun.
We report the detection of nonradial g-mode oscillations in the DA white dwarfs HS1039+4112 (B=15.9) and HE1429-0343 (B=15.8) from time-series photometry made at the Loiano 1.5 m telescope. The two stars were previously selected as probable pulsators based on two-color photometry and spectral analysis respectively. Following our temperature and surface gravity determinations, HS1039+4112 (Teff=11200 K, logg=8.2) is located near the red edge of the ZZ Ceti instability strip, whereas HE1429-0343 (Teff=11400 K, logg=7.8) falls in the middle of the strip. Both stars show a multi-mode behavior with the main periods at about 850 and 970 s respectively, and relatively large amplitudes (~7% and ~2.5%).
We present results of the multicolour UBVR photometry of the high-amplitude EC14026-type star, Balloon 090100001. The data span over a month and consist of more than a hundred hours of observations. Fourier analysis of these data led us to the detection of at least 30 modes of pulsation of which 22 are independent. The frequencies of 13 detected modes group in three narrow ranges, around 2.8, 3.8 and 4.7 mHz, where the radial fundamental mode, the first and second overtones are likely to occur. Surprisingly, we also detect 9 independent modes in the low-frequency domain, between 0.15 and 0.4 mHz. These modes are typical for pulsations found in PG1716+426-type stars, discovered recently among cool B-type subdwarfs. The modes found in these stars are attributed to the high-order g modes. As both kinds of pulsations are observed in Balloon 090100001, it represents a link between the two classes of pulsating hot subdwarfs. At present, it is probably the most suitable target for testing evolutionary scenarios and internal constitution models of these stars by means of asteroseismology. Three of the modes we discovered form an equidistant frequency triplet which can be explained by invoking rotational splitting of an $\ell$ = 1 mode. The splitting amounts to about 1.58 $μ$Hz, leading to a rotation period of 7.1 $\pm$ 0.1 days.
We present the Capodimonte Deep Field (OACDF), a deep field covering an area of 0.5 square degrees in the B, V, R optical bands plus six medium-band filters in the wavelength range 773--913 nm. The field reaches the following limiting AB magnitudes: B~25.3, V~24.8 and R~25.1 and contains about 50000 extended sources in the AB magnitude range 18 < R < 25 . Hence, it is intermediate between deep pencil beam surveys and very wide but shallow surveys. The main scientific goal of the OACDF is the identification and characterization of early-type field galaxies at different look-back times in order to study different scenarios of galaxy formation. Parallel goals include the search for groups and clusters of galaxies and the search for rare and peculiar objects (gravitational lenses, QSOs, halo White Dwarfs). In this paper we describe the OACDF data reduction, the methods adopted for the extraction of the photometric catalogs, the photometric calibration and the quality assessment of the catalogs by means of galaxy number counts, spectroscopic and photometric redshifts and star colors. We also present the first results of the search for galaxy overdensities. The depth of the OACDF and its relatively large spatial coverage with respect to pencil beam surveys make it a good tool for further studies of galaxy formation and evolution in the redshift range 0--1, as well as for stellar studies.
CSL1 is a peculiar object discovered in the OACDF. Photometric and spectroscopic investigation strongly suggest that it may be the first case of gravitational lensing by cosmic string. In this paper we derive and discuss a statistical excess of a gravitational lens candidates present in OACDF region surrounding CSL1. This excess cannot be explained on the basis of conventional gravitational lens statistic alone, but is compatible with the proposed cosmic string scenario.
CSL-1 (Capodimonte--Sternberg--Lens Candidate, No.1) is an extragalactic double source detected in the OAC-DF (Osservatorio Astronomico di Capodimonte - Deep Field). It can be interpreted either as the chance alignment of two identical galaxies at z=0.46 or as the first case of gravitational lensing by a cosmic string. Extensive modeling shows in fact that cosmic strings are the only type of lens which (at least at low angular resolution) can produce undistorted double images of a background source. We propose an experimentum crucis to disentangle between these two possible explanations. If the lensing by a cosmic string should be confirmed, it would provide the first measurements of energy scale of symmetry breaking and of the energy scale of Grand Unified Theory (GUT).
We present the last pointed observation of AM Her carried out during the life of the BeppoSAX satellite. It was bright at the beginning of the observation, but dropped to the lowest X-ray level ever observed so far. The X-ray emission during the bright period is consistent with accretion occurring onto the main pole of the magnetized white dwarf. The rapid change from the active state to the low deep state indicates a drop by a factor of 17 in the accretion rate and hence that accretion switched-off. The short timescale (less than one hour) of this variation still remains a puzzle. Optical photometry acquired simultaneousy during the low state shows that the white dwarf remains heated, although a weak emission from the accretion stream could be still present. Cyclotron radiation, usually dominating the V and R bands, is negligible thus corroborating the possibility that AM Her was in an off-accretion state. The X-ray emission during the inactive state is consistent with coronal emission from the secondary late type star.
The Capodimonte Deep Field (OACDF) is a multi-colour imaging survey on two 0.5x0.5 square degree fields performed in the BVRI bands and in six medium-band filters (700 - 900 nm) with the Wide Field Imager (WFI) at the ESO 2.2 m telescope at La Silla, Chile. In this contribution the adopted strategies for the OACDF data reduction are discussed. Preliminary scientific results of the survey are also presented.
We present a simple quantitative model that can describe the photometric B and V band light curves of AM Herculis obtained during a high state. The double-humped shape of the V band light curve is dominated by cyclotron emission from a region at the main accreting pole with an area of ~5x10e16 cm^2 and sustaining an inflow of ~0.06 g/cm^2/s. The almost unmodulated B band is dominated by emission from the accretion stream. The contribution of the heated white dwarf to the optical emission is small in the B band, but comparable to that of the accretion stream in the V band. The emission of the secondary star is negligible both in B and in V.
Temporal and spectral analyses from BeppoSAX observations of AM Her performed during both an intermediate and a high state are presented and discussed. Optical observations taken a few days after the X-ray ones are also presented. During the intermediate state observation, the source was in its "normal", one-pole accretion mode. In the high state it switched to an hitherto unobserved atypical "two-pole" accretion mode, with significant soft and hard X-ray emission from both poles. The emission from the second pole is much softer than that from the primary pole, while the soft X-ray excess of the primary pole is fairly modest in this accretion mode. These facts suggest that accretion onto the secondary is mainly due to blobs penetrating deeply in the photosphere, while that on the primary pole is mostly via a more homogeneous column, giving rise to the classical standing shock. A strong X-ray flaring activity is also observed in the soft X-ray band, but not the hard X-ray and optical emissions indicating that flares are due to inhomogeneous blobby-accretion events.